Elemental Abundances and Mass Densities of Dust and Gas in the Local Interstellar Cloud

ELEMENTAL ABUNDANCES AND MASS DENSITIES OF DUST AND GAS IN THE LOCALINTERSTELLAR CLOUD

Hiroshi Kimura, Ingrid Mann, and Elmar K. Jessberger

Institut fur Planetologie, WestfalischeWilhelms-Universitat, Wilhelm-Klemm-Strasse 10, D-48149Munster, Germany;[email protected], [email protected], [email protected]

Received 2002 July 1; accepted 2002 September 11

ABSTRACT

Observationally derived gas-phase abundances and appropriate assumptions for the total elemental abun-dances of dust and gas determine the elemental composition of dust and the elemental depletion from gas inthe interstellar medium (ISM). In addition to the elemental abundances, the total mass ratio of hydrogenatoms to dust grains per spatial volume is a measure of the interaction between dust and gas in the ISM.Recent remote astronomical observations and in situ measurements provide the opportunity of estimatingthe elemental abundances and the hydrogen gas-to-dust mass ratio of the Local Interstellar Cloud (LIC), inwhich the Sun is currently embedded. We show that the composition of dust in the LIC is similar to that ofcometary dust in the solar system, although the nitrogen abundance remains uncertain. Depletions of ele-ments from the LIC gas are consistent with measurements of warm neutral clouds in the Galactic disk, exceptforMg and Si, which are heavily depleted in the LIC. Remote astronomical observations and in situ measure-ments give essentially the same value for the gas-to-dust mass ratio of the LIC, which is comparable to theaverage value of the diffuse ISM in the Galaxy. This indicates the association of dust with gas in the LIC,which is also inferred from the depletion pattern in the LIC. Neither the depletions of elements nor the gas-to-dust mass ratio show evidence for severe grain destruction that would result from shocks with velocity�1:5� 107 cm s�1 as suggested by a model that postulates the LIC to be a fragment of the expanding Loop Isuperbubble shell. Our results rather favor an alternative model that describes the origin of the LIC as beingone of H i cloudlets expelled from the interaction zone between the Loop I superbubble and the LocalBubble, which encloses the LIC and similar clouds in the solar neighborhood.

Subject headings: dust, extinction — ISM: abundances — ISM: clouds —ISM: individual (Local Interstellar Cloud)

1. INTRODUCTION

Dust and gas are the main constituents of the interstellarmedium (ISM), and hence their interaction is of greatimportance for better understanding the evolution of theISM. For those dust particles that exchange atoms with gas,the composition of interstellar dust can be constrained bymeasuring depletions in the gas phase of the ISM. Namely,an estimate of the grain composition relies on the assump-tion that the missing atoms in gas relative to reference abun-dances of the ISM reside in dust. The depletion of elementshas been derived from absorption spectra of interstellar gasmeasured along lines of sight long enough to contain a num-ber of diffuse interstellar clouds. The elemental abundancesof dust and gas are also used to determine the total massratio of hydrogen atoms to dust grains in a unit spatial vol-ume as an indicator for the formation and destruction ofdust in the ISM. The hydrogen gas-to-dust mass ratio Rg=d

is approximately 100 for the diffuse ISM averaged over longlines of sight passing through a number of interstellar clouds(Spitzer 1954; Knapp &Kerr 1974).

Recent high-resolution and high signal-to-noise ratioobservations of nearby stars with the Goddard High Reso-lution Spectrograph (GHRS) on the Hubble Space Tele-scope (HST) have revealed small-scale structures in thelocal ISM (Lallement et al. 1990). It is found that the Sun iscurrently immersed in a cloud of partially ionized and warmrarefied gas, which is referred to as the Local InterstellarCloud (LIC; Bertin et al. 1993; Redfield & Linsky 2000).The relative motion between the Sun and the LIC provides

the opportunity of identifying the components of the LICthat penetrate into the heliosphere, which is the regionaround the Sun filled with the solar wind plasma. Neutralhelium gas and dust streaming into the heliosphere havedirectly been detected with the Energetic Particle and Inter-stellar Gas Instrument (EPAC/GAS) and Cosmic DustExperiment (DUST) instruments on boardUlysses (Witte etal. 1993; Grun et al. 1994).

On the basis of abundance arguments, Rg=d ¼ 306–394for the LIC has been deduced from remote astronomicalobservations of gas absorption lines toward � CMa (cf.Frisch et al. 1999). The Rg=d value of the LIC can also bederived from in situ measurements of LIC dust and gas inthe heliosphere. The total mass density of interstellar dust inthe heliosphere amounts to �d ¼ ð7:5 28Þ � 10�27 g cm�3

based on the Ulysses and Galileo in situ measurements(Kimura, Mann, & Wehry 1999; Frisch et al. 1999).1 Thetotal mass density of hydrogen gas in the LIC is estimated tobe �g ¼ 4:0� 10�25 g cm�3 from the Ulyssesmeasurementsof interstellar pickup ions (Gloeckler & Geiss 2002). As aresult, one obtains Rg=d ¼ 14–53, which disagrees withRg=d ¼ 306–394 estimated from gas absorption measure-ments. This discrepancy in the Rg=d values has been inter-preted to indicate that interstellar grains detected by in situexperiments are too large to be physically and chemicallycoupled to the gas in the LIC.

1 According to the ApJ policy, we use cgs units throughout this paper.

The Astrophysical Journal, 582:846–858, 2003 January 10

# 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.

846

The gas-to-dust mass ratio derived from gas absorptionmeasurements depends on both the measured column den-sities of each element and the column density of hydrogenatoms. Recently, Gry & Jenkins (2001) have estimated thecolumn density of hydrogen atoms in the LIC toward �CMa using the HST/GHRS observations of sulfur absorp-tion lines and the meteoritic abundance of sulfur relative tohydrogen, assuming no depletion of sulfur. They suggestedthe hydrogen column density for the LIC to be NðHÞ ¼4:7� 1:1� 1017 cm�2, in contrast to NðHÞ ¼ 2:9� 1017

cm�2 that was previously used for the determination of Rg=d

in the LIC. If such a high value of the hydrogen column den-sity is confirmed, then the elemental depletions wouldincrease, lowering the gas-to-dust mass ratio of the LICcompared to Rg=d ¼ 306–394. On the other hand, the massdensity of interstellar dust derived from in situ measure-ments is based on the data measured at heliocentric distan-ces between 1 and 5.4 AU from the Sun.2 As pointed out byMann (1996), a variation in the flux of interstellar dust withdistance from the Sun has to be taken into account for adetailed analysis of the in situ data. When interstellar grainsapproach the Sun, the solar gravitational attraction isexpected to enhance the spatial density of the grains (Ber-taux & Blamont 1976; Mann & Kimura 2000). Previousstudies based on the whole set of the in situ data neglectedthe effect of gravitational focusing and therefore probablyoverestimated the real value of the dust mass density in theLIC. We hence expect that the hydrogen gas-to-dust massratio of the LIC is higher thanRg=d ¼ 14–53.

We here hypothesize that theRg=d of the LIC derived fromgas absorption measurements matches the value estimatedfrom in situ dust measurements (Kimura et al. 2001). Thisimplies that the detected interstellar grains were coupledwith gas in the LIC and is consistent with the fact that theLIC dust and gas components measured in the heliospherehave identical heliocentric velocity (cf. Grun et al. 1994). Tosupport this hypothesis, we reexamine the hydrogen gas-to-dust mass ratio of the LIC based on recent results from gasabsorptionmeasurements through the LIC and on a detailedanalysis of the in situ measurements of interstellar dust. Wefirst improve estimates for the elemental abundances of theLIC with new data of interstellar gas identified as compo-nents of the LIC and estimates for the mass densities of dustand gas with in situ data of interstellar dust and pickup ions.The gas-to-dust mass ratio of the LIC is then derived fromthe elemental abundances and from the mass densities ofdust and gas in the LIC. We finally discuss the compositionof dust, the depletion of gas, and the possible association ofdust with gas in the LIC, as well as limitations of our analy-ses. This gives implications for the origins of the LIC.

2. REMOTE ASTRONOMICAL MEASUREMENTS OFINTERSTELLAR GAS

2.1. Gas AbsorptionMeasurements in Lines of Sight throughthe LIC

The LIC is located inside the Local Bubble, which is filledwith hot, low-density gas on a 100 pc scale and is adjacent tothe Loop I superbubble (Egger, Freyberg, & Morfill 1996).3

In a different scenario, Frisch (1996) claimed that the LIC isa fragment of the Loop I superbubble shell and that theLocal Bubble is an appendix of the Loop I superbubble.The physical and chemical properties seem to be uniformover the LIC, implying a well-mixed gas with a common his-tory (Linsky et al. 2000). We therefore assume that the ele-mental abundances can be better determined by averagingover a number of sight lines. The elemental abundance of anelement is determined by the ratio of its column density tothe column density of hydrogen. The column density of anelement with a specific ionization state can be derived fromobserved absorption features of that element. The compo-nent of the absorption line originating from the LIC is dis-tinguishable by its Doppler shift from the other cloudcomponents in the Local Bubble. First we describe the col-umn densities of the LIC measured withHST along lines ofsight to nearby stars and then average the elemental abun-dances. The distance d, the longitude l, and the latitude b ofeach star are listed in Table 1. Also given are the H i columndensity of the LIC component, the species for which the col-umn densities of the LIC component were measured, andthe references.

2.1.1. �Aur (Capella)

Capella is a binary system, and its Mg ii and Fe ii profilesshow only one single-absorption component whose mea-sured velocity is identical to the projected velocity of theLIC (Linsky et al. 1993). We use the column densities ofMg ii, Fe ii, and H i derived by Linsky et al. (1995a) fromtheHST/GHRS observations at two different phases of thesystem and those of C ii and C ii* by Wood & Linsky(1997).4 The interstellar absorption of O i in the LIC wasmeasured with HST/GHRS at a low spectral resolution byLinsky et al. (1995b), and the O i column density was laterinferred by Linsky et al. (1995a). The new spectra from theSpace Telescope Imaging Spectrometer (STIS) onHST andFar Ultraviolet Spectroscopic Explorer (FUSE) havebecome available, and we can utilize the most recent valuesfor the C iii, N i, N ii, O i, Al ii, and Si ii column densities(Wood et al. 2002a).

2.1.2. �CMi (Procyon)

Linsky et al. (1995a) divided the absorption profiles ofMg ii, Fe ii, H i, and D imeasured along the Procyon line ofsight from HST/GHRS into two interstellar components.One of the components has been attributed to the LIC, andtherefore we use the Mg ii, Fe ii, and H i column densitiescorresponding to the LIC for estimates of the gas-phaseabundances.

2.1.3. �CMa (Sirius)

Sirius is a binary of Sirius A and Sirius B, which are an A1main-sequence star and a carbon-core white dwarf, respec-tively. Lallement et al. (1994) provided the column densitiesof Mg i, Mg ii, and Fe ii in the LIC with the identification oftwo distinct interstellar clouds in the line of sight toward Sir-ius A based on the HST/GHRS observations. In additionto the two cold clouds, Bertin et al. (1995) included one hotcloud to estimate the H i column density of the LIC alongthe Sirius A sight line from the D i column density assumingthe D i to H i ratio of 1:65� 10�5. However, their results2 The astronomical unit (AU) is the average distance between the Earth

and the Sun’1:5� 1013 cm.3 The parsec (pc) is defined as a distance corresponding to a parallax of

100 ’ 3:1� 1018 cm. 4 The asterisk signifies that the atom is in an excited state.

DUST AND GAS IN LOCAL INTERSTELLAR CLOUD 847

give an H i column density of NðH iÞ ¼ 3:4� 1:0�1017 cm�2 for the total line of sight to Sirius A, as opposedto NðH iÞ ¼ 5:2þ1:4

�1:0 � 1017 cm�2 toward the total Sirius Bsight line reported by Holberg et al. (1998) based on theExtreme Ultraviolet Explorer (EUVE) observations.Recently, Hebrard et al. (1999) have identified no signatureof this third cloud in the Ly� line toward Sirius A and havedetermined the H i and D i column densities for the twoclouds from the HST/GHRS observations at both mediumand high spectral resolutions. Their results show that the D i

to H i ratio amounts to 1:6� 0:4� 10�5 and the H i columndensity for the total sight line to NðH iÞ ¼ 6:5� 1017 cm�2.They have further derived the column densities of C ii, N i,O i, Mg ii, Si ii, Fe ii, and H i for the LIC toward Sirius Aand Sirius B. We adopt the column density of Mg i by Lalle-ment et al. (1994) and the LIC results of Hebrard et al.(1999), in which the interstellar absorption lines of C ii* andSi iiiwere not detected.

2.1.4. �CMa

Gry et al. (1995) analyzed the GHRS data with six clouds,but from the reanalysis with a more complete data set it waslater found that the spectra contain five interstellar clouds(Gry & Jenkins 2001). The column densities of C ii, C ii*,C iv, N i, O i, Mg i, Mg ii, Si ii, Si iii, S ii, and Fe ii in theLIC toward � CMa have been derived from GHRS andIMAPS (Interstellar Medium Absorption Profile Spectro-graph) aboard ORFEUS-SPAS II, but the absorption linesof N v, Si iv, and S iii were not identified within the detec-tion limit. The Si iii component is inconsistent with nonde-

tection of Si iii toward Sirius, whose line of sight is close to�CMa (cf. Hebrard et al. 1999). Because the presence of C iv

and Si iii components is questioned, we do not consider theircontributions to the C and Si column densities (see Wood etal. 2002a). Spectroscopic observations of � CMa with theEUVE satellite suggest the interstellar H i column densityfor the total sight line to be in the range from 7� 1017 to1:2� 1018 cm�2 (Cassinelli et al. 1995). Vallerga & Welsh(1995) estimated NðH iÞ ¼ 8:9þ1:1

�1:0 � 1021 m�2 for the totalline of sight to � CMa from the EUVE data. Gry & Jenkins(2001) attribute 53.4% of the column density of the totalsight line to the column density for the LIC based on thehigh correlation between O i and H i. Therefore, we mayassume the H i column density of the LIC component tobe NðH iÞ ¼ 4:76� 0:74� 1017 cm�2. This agrees withNðH iÞ ¼ 4:4þ1:6

�0:6 � 1017 cm�2 estimated for the LIC com-ponent by Gry & Jenkins (2001) assuming that the abun-dance ratio of O i to H i is 3:16� 10�4. Note that theseresults are opposed to the H i column density of2:0� 1017 cm�2 assumed for the LIC by Frisch et al. (1999).The unsaturated lines of N i, Mg ii, Si ii, and Fe ii indicatethe ratio of the column density for the LIC in the � CMasight line to that in the �CMa sight line to be 1:5� 0:2 (Gry& Jenkins 2001). Since our assumption for the LIC compo-nent of the H i column density provides the ratio for H i tobe 1.2, the column density might be slightly underestimated.

2.1.5. G191-B2B (EG 247)

G191-B2B, which is a hot white dwarf with DA spectraltype, has been observed with HST/GHRS in order to

TABLE 1

Parameters Considered to Determine Absorption by the Local Interstellar Cloud

Star

d

(pc)

l

(deg)

b

(deg)

NðH iÞ(1017 cm�2) Species Reference

�CMa ............... 2.6 227 �9 4.0 � 1.5 C ii, N i, O i, Mg i, Mg ii, Si ii, Fe ii 1, 2

�Eri ................... 3.2 196 �48 7:5þ1:3�1:1 Mg ii, Fe ii 3

61 Cyg A ............ 3.5 82 �6 6:8þ1:2�1:0 Mg ii 4

�CMi ................ 3.5 214 +13 7.5 � 0.2 Mg ii, Fe ii 5

40 Eri A.............. 5.0 201 �38 7:1þ1:8�1:5 Mg ii 4

�Aql.................. 5.1 48 �8.9 18 Mg ii, Fe ii 6, 7

�Gem................ 10.3 192 +23 11þ3�2 Mg ii, Fe ii 8, 3

EP Eri ................ 10.4 192 �58 8:9þ2:3�1:8 Mg ii 9

�Aur ................. 12.9 163 +5 17 � 3 C ii, C ii*, C iii, N i, N ii, O i, Mg ii, Al ii, Si ii, Fe ii 10, 11, 12

� Cas.................. 16.7 118 �3 14þ2�1 Mg ii, Fe ii 8

DXLeo.............. 17.7 201 +46 5:0þ2:1�1:5 Mg ii 9

�Tri................... 19.7 139 �31 12þ3�2 Mg ii, Fe ii 8

V368 Cep ........... 19.7 118 +17 8:9þ2:3�1:8 Mg ii 9

PWAnd............. 21.9 115 �31 11þ3�2 Mg ii 9

V711 Tau ........... 29.0 185 �42 7:9þ1:0�0:9 Mg ii 13

� Cas .................. 30.5 127 �2 18� 3 Mg i, Mg ii 2

�Gem................ 37.5 191 +23 9:9þ2:6�2:0 Mg ii 8

G191-B2B.......... 68.8 156 +7 17þ2�7 C ii, N i, O i, S ii, S iii, Mg ii, Si ii, Si iii, Fe ii 6, 14, 15, 16

Feige 24.............. 74.4 166 �50 12 � 4 C ii, N i, O i, Si ii 17

RE J1032+532... 132 158 +53 42 � 5 C ii, C ii*, N i, O i, Si ii, S ii 18, 19

�CMa ................ 132 240 �11 4.8 � 0.7 C ii, C ii*, N i, O i, Mg i, Mg ii, Si ii, S ii, Fe ii 20, 21

Note.—Distance d, Galactic longitude l, and Galactic latitude b for stars considered to show absorption by the Local InterstellarCloud (LIC). Also given are the neutral hydrogen column density of the LIC component, NðH iÞ, the species for which the columndensity of the LIC component was measured, and the references.

References.—(1) Hebrard et al. 1999; (2) Lallement & Ferlet 1997; (3) Redfield & Linsky 2002; (4)Wood&Linsky 1998; (5) Linsky etal. 1995a; (6) Lallement et al. 1995; (7) W. Landsman 1997, cited in Frisch et al. 1999 as private communication; (8) Dring et al. 1997; (9)Wood et al. 2000; (10) Lallement et al. 1994; (11)Wood& Linsky 1997; (12)Wood et al. 2002a; (13) Piskunov et al. 1997; (14) Lemoine etal. 1996, 2002; (15) Sahu et al. 1999; (16) Vidal-Madjar et al. 1998; (17) Vennes et al. 2000; (18) Barstow et al. 1997; (19) Holberg et al.1999; (20) Gry & Jenkins 2001; (21) Vallerga &Welsh 1995.

848 KIMURA, MANN, & JESSBERGER Vol. 582

measure the D/H ratio in the interstellar gas. Lallement etal. (1995) evaluated the column density of Mg ii with theGHRS data at a high spectral resolution for three interstel-lar components. Lemoine et al. (1996) derived the columndensities of H i, D i, C ii, N i, O i, Si ii, and Si iii at a mediumspectral resolution and those of Mg ii and Fe ii at a highspectral resolution. The Mg ii column density given byLallement et al. (1995) coincides with the value of Lemoineet al. (1996) within the error bars. Vidal-Madjar et al. (1998)estimated the column densities of N i, O i, Si ii, and Si iiifrom the GHRS data at a high spectral resolution bothincluding D i and H i lines and excluding D i and H i lines.The LIC component of Si iii was not identified in the highspectral resolution data, and the upper limit estimated forthe Si iii column density rules out an appreciable presence ofSi iii in the LIC toward G191-B2B. However, the derivedD/H ratio of 1:19þ0:35

�0:25 � 10�5 for the LIC toward G191-B2B disagrees with the commonly accepted value of’1:6� 10�5 (Vidal-Madjar et al. 1998). Analyses of boththe GHRS data and the new STIS data have later turnedout to lead to a D i/H i ratio of 1:60þ0:39

�0:27 � 10�5 for the LICin the line of sight toward G191-B2B (Sahu et al. 1999).Only two interstellar clouds were required for the profile fit-ting of interstellar D i and H i lines, and the H i column den-sity of the total sight line was found to agree with thatderived from the EUVE data (see Barstow, Hubeny, & Hol-berg 1999). Lemoine et al. (2002) have claimed the presenceof three interstellar clouds by analyzing the new STIS dataof H i, D i, C ii, N i, O i, S ii, S iii, Si ii, Si iii, and Fe ii.5

Because their best fits do not allow for estimates of possibleuncertainties, the column densities given by the otherauthors are also considered for our analyses. Vidal-Madjar& Ferlet (2002) discuss a large uncertainty in the determina-tion of the H i column density along the line of sight toG191-B2B. We here use the H i column density of the LICgiven by Sahu et al. (1999) but also that of Lemoine et al.(1996, 2002) in order to take into account its uncertainty.

2.1.6. RE J1032+532 (WD1029+537)

The interstellar absorption lines of C ii, C ii*, N i, O i,Mg ii, Si ii, Si iii, and S ii toward the hot DA white dwarfRE J1032+532 have been identified with HST/STIS (Hol-berg et al. 1999). This sight line contains the LIC and theother cloud, which was observed only in the Si ii componentand is found to have a column density of approximately 7%of the LIC component. The tentative identification of theSi iii absorption line as a LIC component was shown to bedoubtful because the velocity vector of the LIC projectedonto the direction to the star is largely different from themeasured value for the Si iii absorption line, compared withthe other lines. This conclusion is also supported by theabsence of Si iv and C iv absorption lines toward REJ1032+532 within the detection limit. We therefore use thecolumn densities of C ii, C ii*, N i, O i, Si ii, and S ii for theLIC but discard the Si iii column density.6 The H i columndensity for the total sight line has been derived from theextreme-ultraviolet spectra with EUVE and the H i Ly�profile with HST/STIS (Barstow et al. 1997; Holberg et al.

1999). Because both observations give an identical columndensity of H i within the error bars, we use the H i columndensity from the EUVE observations as the H i column den-sity for the LIC.

2.1.7. Feige 24

Two interstellar clouds have been identified in the absorp-tion spectra toward Feige 24 with the HST/STIS observa-tions (Vennes et al. 2000). We use the column densities ofC ii, N i, O i, Si ii, and H i derived for the LIC from the data.The absorption features of the C iv, Si iii, Si iv, and S ii col-umn densities were not found for the LIC.

2.1.8. �Aql (Altair)

On the basis of ground-based observations of Na i andCa ii lines, Ferlet, Lallement, & Vidal-Madjar (1986) foundthat the Altair sight line contains three interstellar clouds.We use the column densities of Mg ii and Fe ii in the LICgiven by Lallement et al. (1995) and H i by W. Landsman(1997).7

2.1.9. V711 Tau (HR 1099)

Piskunov et al. (1997) have reported the identification ofthe LIC in the absorption spectra of Mg ii, H i, and D i

toward lines of sight to HR 1099 and � Cet with HST/GHRS. The absorption spectra toward HR 1099 and � Cetcontain three and two distinct clouds, respectively, but theLIC component along the � Cet sight line rather belongs tothe south Galactic pole cloud because of the anomalouslyhighMg ii/H i ratio in comparison to the LIC value (Linskyet al. 2000). We use theMg ii andH i column densities of theLIC along the line of sight toHR 1099 but not those of � Cetfor our analysis.

2.1.10. � Cas

TheHST/GHRS observations at amedium spectral reso-lution have been analyzed for the estimate of the D/H ratioalong the line of sight to � Cas (Piskunov et al. 1997). TheLIC is found to be the only one in theMg ii spectra, and thiswas confirmed by high spectral resolution spectra (Dring etal. 1997). We adopt the column densities of Mg ii, Fe ii, andH i, which have been derived from the HST/GHRS highspectral resolution observations.

2.1.11. � Cas

The Doppler shift of the Mg ii absorption lines along theline of sight to � Cas observed with HST/GHRS has beenshown to be comparable with the motion of the LIC (Lalle-ment et al. 1995). Lallement & Ferlet (1997) derived theMg i and Mg ii column densities of the LIC from theHST/GHRS measurements. They also estimated the H i columndensity of the LIC toward � Cas by comparison withCapella assuming the homogeneity of the LIC.

2.1.12. �Tri

The HST/GHRS observations have revealed the pres-ence of two interstellar clouds along the line of sight to � Tri(Dring et al. 1997). One of the clouds was identified as theLIC, and the column densities of Mg ii, Fe ii, and H i havebeen estimated for both clouds.5 The column densities of the LIC component estimated in Lemoine et

al. (2002) were not explicitly given in the paper, but they have been providedto us by private communication withM. Lemoine.

6 TheMg ii column density was not given in Holberg et al. (1999). 7 Cited in Frisch et al. (1999) as private communication.

No. 2, 2003 DUST AND GAS IN LOCAL INTERSTELLAR CLOUD 849

2.1.13. � Gem

Dring et al. (1997) have derived the column densities ofMg ii and H i toward the late-type star � Gem from theHST/GHRS high spectral resolution observations. Theyhave found two interstellar clouds in the line of sight to thestar, one of which marginally corresponds to the LIC alongwith a redshift. Although the Doppler shift of the absorp-tion line can also match the projected velocity of the G cloudas pointed out by Linsky &Wood (2000), the line of sight to� Gem is not directed to the G cloud (see Lallement et al.1995; Linsky et al. 2000). Recently, a high spectral resolu-tion spectrum with HST/STIS has provided the columndensity of Fe ii along the � Gem sight line where the Dop-pler shift seems to better agree with the LIC than the Gcloud (Redfield & Linsky 2002).

2.1.14. �Gem

The line of sight to the post–main-sequence binary system� Gem also contains two interstellar clouds as measuredwith HST/GHRS at a high spectral resolution (Dring et al.1997). Because the line of sight to � Gem is only 1� awayfrom the � Gem sight line, the detected two clouds shouldbe identical to those observed toward � Gem. Although theMg ii and H i column densities were measured with a highspectral resolution, their velocities are only marginally con-sistent with the LIC velocity. Since we have no reason foropposing this identification, we shall still adopt the columndensities as the component corresponding to the LIC.

2.1.15. �Eri

The GHRS spectra of Mg ii, H i, and D i along the sightline toward � Eri were fitted only with the component corre-sponding to the LIC at a high spectral resolution (Dring etal. 1997). The presence of a stellar hydrogen wall, which wassuggested from the data analyses, indicates that � Eri islocated inside the LIC. We use the estimate for the H i col-umn density by Dring et al. (1997) but the column densitiesof Mg ii and Fe ii from higher spectral resolution spectragiven in Redfield & Linsky (2002).

2.1.16. 40 Eri A

The Mg ii spectra of theHST/GHRS observations alongthe 40 Eri A line of sight require only one absorbing compo-nent that is attributed to the LIC (Wood & Linsky 1998).The analyses have provided the Mg ii column density and avelocity that is consistent with the projected velocity of theLIC along the 40 Eri A sight line. If the H i column densityis derived from the GHRS Ly� spectra without assumingthe presence of a circumstellar hydrogen wall, then the D i

to H i ratio becomes unreasonably low compared to pre-vious GHRS measurements. Therefore, a circumstellarhydrogen wall was applied to estimate the LIC componentof the H i column density.

2.1.17. EPEri (HD 17925)

Two interstellar components of absorption feature havebeen used to fit the spectra of Mg ii, H i, and D i toward EPEri with HST/GHRS (Wood et al. 2000). One of the com-ponents was attributed to the LIC, whose H i column den-sity coincides with the prediction from the LIC model ofRedfield & Linsky (2000).

2.1.18. PWAnd (HD 1405)

The GHRS spectra of Mg ii, H i, and D i along the PWAnd line of sight are best fitted with two interstellar clouds(Wood et al. 2000). We use the Mg ii and H i column den-sities measured for the LIC where the H i column density isshown to agree with the predicted value from Redfield &Linsky (2000).

2.1.19. V368 Cep (HD 220140)

The line of sight to V368 Cep contains only one absorp-tion component that has a velocity consistent with the LIC(Wood et al. 2000). The Mg ii and H i column densities ofthe LIC were derived from theHST/GHRS observations.

2.1.20. DXLeo (HD 82443)

The absorption spectra from the HST/GHRS observa-tions along the sight line to DX Leo have been explainedwith only one interstellar component consistent with theLIC (Wood et al. 2000). Although the H i line might be con-taminated by the H i absorption in the interface regionbetween the interstellar plasma and the solar wind, we adoptthe estimated values for the Mg ii and H i column densitiesof the LIC.

2.1.21. 61 Cyg A

Two interstellar clouds were required to fit the GHRSMg ii spectra toward 61 Cyg A, and the Ly� line was usedto search for evidence for the circumstellar hydrogen wall(Wood & Linsky 1998). Two estimates of H i column den-sity were given, one of which assumes the presence of a cir-cumstellar hydrogen wall and provides a better stellar Ly�profile. We use the values for the Mg ii column density andthat H i column density of the LIC suggested by Wood &Linksy (1998).

2.2. Average Composition of the LIC

2.2.1. Gas-Phase Elemental Abundances

While astronomical measurements of gas absorption linesprovide the abundances of elements relative to neutralhydrogen, the elemental abundances in the gas phase of theLIC must be considered with respect to the sum of neutraland ionized hydrogen abundances. In order to determinethe abundances of elements relative to the total number ofneutral and ionized hydrogen, we need to take into accountthe ionization fraction of hydrogen �H ¼ nðH iiÞ=½nðH iÞþnðH iiÞ�, where nðH iÞ and nðH iiÞ are the number den-sities of neutral and singly ionized hydrogen atoms, respec-tively. The hydrogen ionization fraction in the LIC has beenestimated to be �H ¼ 0:25� 0:07 from in situ measurementsof interstellar pickup ions (Gloeckler & Geiss 2002). Wood& Linsky (1997) have derived �H ¼ 0:45� 0:25 from meas-urements of the C ii and C ii* column densities towardCapella. This value is consistent with the H i and H ii col-umn densities for the line of sight through the LIC to thewhite dwarf RE J1032+532 (Holberg et al. 1999). Althoughboth values agree with each other within the given errorbars, we shall examine two cases for the hydrogen ioniza-tion fraction of �H ¼ 0:25� 0:07 and �H ¼ 0:45� 0:25separately.

It is straightforward to derive the gas-phase abundancesof C, Mg, Al, Si, S, and Fe, since their dominant ionizationspecies in the LIC have been measured (see Slavin & Frisch


2002). The N i column density in the LIC has been measuredfor six lines of sight to date, but a substantial fraction ofnitrogenmay be singly ionized in the LIC. TheFUSE spectratoward Capella, which contain only the LIC component,show that roughly one-half the nitrogen is in the form of N ii

(Wood et al. 2002a). Toward the sight lines where only theN i column densitywasmeasured, we calculate theN columndensity by doubling the N i column density. There are nomeasurements of O ii spectra in the LIC, but oxygen atomsmay in part be ionized in the LIC. The neutral fraction ofoxygen in the LIC is presumably correlated with that ofhydrogen in the LIC as a result of charge exchange interac-tions between hydrogen and oxygen. Therefore, we estimatethe gas-phase abundance of oxygen in the LIC assuming thatthe O/H ratio equals the O i/H i ratio (see Gry & Jenkins2001). Figure 1 shows the column densities of C, N, O, Mg,Al, Si, S, and Fe as a function of the column density of neu-tral hydrogen in logarithmic scales. The oxygen abundanceshown in Figure 1 corresponds to the case for �H ¼ 0:25,while increasing the value by 34.4% provides the case for�H ¼ 0:45. The dotted lines indicate the fitting curves for thedata on the basis of the assumption that the column densityof an element is in direct proportion to the column density ofneutral hydrogen. Hebrard et al. (1999) suggest that the col-umn densities of C ii and O i along the Sirius line of sight areslightly overestimated. The C ii column density of the LICtoward �CMa is the upper limit estimated by the S ii columndensity and the meteoritic abundances of C and S. There-fore, we exclude theC ii andO i column densities to the Siriussight line and the C ii column density to the �CMa sight linefor the fitting procedure.

2.2.2. Elemental Depletions

If dust is associated with gas in the LIC, the depletion ofelements from the gas phase correlates with the elementalcomposition of grains. A comparison between the deple-tions of the LIC and those of diffuse interstellar clouds inour Galaxy may give insight into the properties of the LIC.Figure 2 shows the logarithmic depletions of the LIC, coldneutral interstellar medium (CNM), and warm neutralinterstellar medium (WNM), according to a probable orderof condensation of the elements (from right to left). The log-arithmic depletion is defined as the ratio of the gas-phaseabundance to a reference abundance in logarithmic scales.The solar photospheric abundances have recently beenqualified as an excellent reference for the ISM (Sofia &Meyer 2001; Holweger 2001). Although the elemental abun-dances of the solar photosphere and the LIC may not neces-sarily be the same, we assume the solar photosphericabundances to represent the total compositions of the LIC.A complete table of solar photospheric abundances hasbeen given by Grevesse & Sauval (1998), while the abundan-ces of C, N, O, Ne,Mg, Si, and Fe have been revised by Hol-weger (2001). The photospheric abundances of silicon andiron in the Sun updated by Wedemeyer (2001) and BellotRubio & Borrero (2002), respectively, agree well with thevalues given by Holweger (2001). The solar photosphericabundances by Holweger (2001) are shown to be consistentwith the average composition of young F and G stars in theGalactic disk estimated by Sofia &Meyer (2001). Independ-ently, Asplund (2000), Asplund et al. (2000), and AllendePrieto, Lambert, & Asplund (2001, 2002) have estimated thesolar photospheric abundances of Si, Fe, O, and C. Their

oxygen, silicon, and iron abundances are in accord with thevalues given by Holweger (2001) within the uncertainties,but their carbon abundance is considerably lower than thevalue given by Holweger (2001). In Figure 2, we take twosets of the solar photospheric values: the abundances of C,N, O, Mg, Si, and Fe from Holweger (2001) and the abun-dances of Al and S from Grevesse & Sauval (1998); and theabundances of Si, Fe, O, and C from Asplund (2000),Asplund et al. (2000), Allende Prieto et al. (2001), andAllende Prieto et al. (2002), respectively, the abundances ofN andMg from Holweger (2001), and the abundances of Aland S from Grevesse & Sauval (1998). As a result of the dif-ferent abundance values, the logarithmic depletions of C, O,Si, and Fe in the LIC, CNM, and WNM show uncertaintiesof 0.202, 0.046, 0.026, and 0.002, respectively. The logarith-mic depletions of the LIC in Figure 2 are calculated with�H ¼ 0:25, and therefore those for �H ¼ 0:45 need to beshifted by �0.135. The gas-phase abundances of CNM andWNM are taken fromWelty et al. (1999) and Sembach et al.(2000), respectively. The depletions of C, N, O, S, Fe, andAl in the gas phase show the similarity between the LIC andWNM, but Si and Mg are heavily depleted in the LIC. Fol-lowing the order of condensation, the depletion in the LICbecomes weaker from aluminum to sulfur, but such a trendis not seen in the depletions from oxygen to carbon. Thedepletion pattern from aluminum to sulfur is consistent withcondensation of grains in stellar atmospheres or nebulae,while that from oxygen to carbon can be interpreted asaccretion of these elements in the ISM (see Field 1974).

2.2.3. Dust-Phase Elemental Abundances

Because the depletion pattern implies association of dustwith gas in the LIC, we shall derive the dust-phase abundan-ces from the gas-phase abundances and the solar photo-spheric abundances. Tables 2 and 3 give the estimatedvalues for the dust-phase abundances of elements as well astheir gas-phase abundances and the adopted abundances ofthe solar photosphere. As a result, the hydrogen gas-to-dustmass ratio from Table 2 amounts to Rg=d ¼ 94:4� 20:9 for�H ¼ 0:25� 0:07 and Rg=d ¼ 86:8� 23:7 for �H ¼ 0:45�0:25. From Table 3, we obtain Rg=d ¼ 126� 25 for �H ¼0:25� 0:07 and Rg=d ¼ 113� 34 for �H ¼ 0:45� 0:25.Note that the hydrogen gas-to-dust mass ratio is lower whenwe assume the higher hydrogen ionization fraction.

It may be worthwhile to compare the elemental composi-tion of dust in the LIC with that of cometary dust in thesolar system since primordial interstellar dust is believed tobe closely linked to the material from which comets areformed. The elemental composition of dust in comet Halleywas measured in situ with time of flight mass spectrometersaboard VeGa-1, VeGa-2, and Giotto (Kissel et al. 1986a,1986b). On the other hand, stratospheric collections ofinterplanetary dust particles (IDPs) have provided theopportunity to study the composition of cometary dust in alaboratory. We shall consider the elemental composition ofporous IDPs that are presumed to originate from comets(Jessberger et al. 2001). Figure 3 compares the elementalcomposition of dust in the LIC with those of dust in cometHalley and porous IDPs normalized to the Mg abundances.The Mg-normalized composition of the solar photosphereis denoted by dashed lines taking into account the uncer-tainty in the abundance determination given in Tables 2 and3. The elemental composition of the LIC dust is derived


from the most depleted case (Rg=d ¼ 86:8) from Table 2(�H ¼ 0:45) and the least depleted case (Rg=d ¼ 126) fromTable 3 (�H ¼ 0:25). The Mg-normalized elemental abun-dances of dust in comet Halley, which are taken from Jess-berger, Christoforidis, & Kissel (1988), might be uncertain

by a factor of 2. The average compositions of chondriticporous IDPs except for nitrogen are given by Schramm,Brownlee, & Wheelock (1989), and for the nitrogen abun-dance, we assume the N-to-C abundance ratio of 0.1 esti-mated as an upper limit for one chondritic, anhydrous IDP

4

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ε CMa

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(cm

-2)

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olum

n de

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(cm

-2)

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α CMa

81012

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colu

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cm-2

)

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olum

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4

681012

2

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681013

Fe

colu

mn

dens

ity (

cm-2

)

3 4 5 6 7 8 9

10182 3 4 5


Fig. 1.—Column densities of C, N, O, Mg, Al, Si, S, and Fe in the LIC as a function of the H i column density derived from gas absorption measurements.The O column densities are estimated for the hydrogen ionization fraction of �H ¼ 0:25, while those for �H ¼ 0:45 need to be increased by 34.4%. The C col-umn densities of �CMa and �CMa and the O column density of �CMa are excluded from the curve-fitting procedures.

852 KIMURA, MANN, & JESSBERGER

TABLE 2

Average Elemental Abundances of the Local Interstellar Cloud per 106Hydrogen Atoms

�H ¼ 0:25� 0:07a �H ¼ 0:45� 0:25b

Element AtomicWeight Gas Dust Gas Dust Sun Reference

Carbon ......... 12.011 180 � 47 211 � 108 132 � 68 259 � 119 391þ110�86 1

Nitrogen ....... 14.007 49.7 � 6.3 35.6 � 22.7 36.4 � 16.8 48.9 � 27.6 85:3þ24:8�19:2 1

Oxygen ......... 15.999 284 � 68 261 � 119 284 � 143 261 � 174 545þ107�90 1

Magnesium... 24.305 2.85 � 0.45 31.7 � 4.8 2:09� 0:99 32.4 � 4.9 34:5þ5:1�4:5 1

Aluminium ... 26.982 0.12 � 0.03 2.84 � 0.48 0.09 � 0.04 2.87 � 0.48 2:95þ0:52�0:44 2

Silicon........... 28.086 5.04 � 0.73 29.3 � 3.9 3.69 � 1.73 30.7 � 4.2 34:4þ4:1�3:7 1

Sulfur............ 32.066 11.3 � 1.6 10.0 � 5.6 8.32 � 3.88 13.1 � 6.7 21:4þ6:2�4:8 2

Iron............... 55.845 1.40 � 0.16 26.7 � 5.3 1.03 � 0.47 27.0 � 5.3 28:1þ5:8�4:8 1

a The hydrogen ionization fraction of �H ¼ 0:25� 0:07 results in the hydrogen gas-to-dust mass ratio ofRg=d ¼ 94:4� 20:9.b The hydrogen ionization fraction of �H ¼ 0:45� 0:25 results in the hydrogen gas-to-dust mass ratio ofRg=d ¼ 86:8� 23:7.References.—(1) Holweger 2001; (2) Grevesse & Sauval 1998.

TABLE 3

Same as Table 2 but with a Different Set of Solar Photospheric Abundances

�H ¼ 0:25� 0:07a �H ¼ 0:45� 0:25b

Element AtomicWeight Gas Dust Gas Dust Sun Reference

Carbon ......... 12.011 180 � 47 65.2 � 52.6 132 � 68 113 � 72 245þ24�22 1

Nitrogen ....... 14.007 49.7 � 6.3 35.6 � 22.7 36.4 � 16.8 48.9 � 27.6 85:3þ24:8�19:2 2

Oxygen ......... 15.999 284 � 68 206 � 88 284 � 143 206 � 154 490þ60�53 3

Magnesium... 24.305 2.85 � 0.45 31.7 � 4.8 2.09 � 0.99 32.4 � 4.9 34:5þ5:1�4:5 2

Aluminium ... 26.982 0.12 � 0.03 2.84 � 0.48 0.09 � 0.04 2.87 � 0.48 2:95þ0:52�0:44 4

Silicon........... 28.086 5.04 � 0.73 27.3 � 3.1 3.69 � 1.73 28.7 � 3.4 32:4þ3:1�2:9 5

Sulfur............ 32.066 11.3 � 1.6 10.0 � 5.6 8.32 � 3.88 13.1 � 6.7 21:4þ6:2�4:8 4

Iron............... 55.845 1.40 � 0.16 26.8 � 3.2 1.03 � 0.47 27.2 � 3.3 28:2þ3:4�3:1 6

a The hydrogen ionization fraction of �H ¼ 0:25� 0:07 results in the hydrogen gas-to-dust mass ratio ofRg=d ¼ 126� 25.b The hydrogen ionization fraction of �H ¼ 0:45� 0:25 results in the hydrogen gas-to-dust mass ratio ofRg=d ¼ 113� 34.References.—(1) Allende Prieto et al. 2002; (2) Holweger 2001; (3) Allende Prieto et al. 2001; (4) Grevesse & Sauval 1998; (5)

Asplund 2000; (6) Asplund et al. 2000.

Fig. 2.—Logarithmic depletions of gas in the LIC (L) with respect to thesolar photospheric abundances calculated from the hydrogen ionizationfraction �H ¼ 0:25, and the assumption of �H ¼ 0:45 shifts the logarithmicdepletions by�0.135. The different solar photospheric abundances listed inTable 2 and 3 yield an uncertainty of 0.202, 0.046, 0.026, and 0.002 in thelogarithmic depletions of C, O, Si, and Fe, respectively. The elements alongthe horizontal axis follow a probable order of condensation of the elementsin stellar atmospheres from right to left. For comparison, the logarithmicdepletions in the CNM (C) and WNM (W) in the Galaxy are estimatedfromWelty et al. (1999) and Sembach et al. (2000), respectively.

0.01

0.1

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Abu

ndan

ce n

orm

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ed to

Mg

C N O S Si Mg Fe Al

Element

I

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I

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L

L

L L L

L

H

H

H

H

H

H

H

H

L

L

L

L

L L L

L

Sun

Fig. 3.—Elemental abundances of dust in the LIC (L) normalized to theMg abundance in comparison with those of porous IDPs (I), dust in cometHalley (H), and the solar photosphere. The solar photospheric abundancesare taken from Tables 2 and 3, while the elemental abundances of the LICdust are shown only for the most depleted case (Table 2 with the hydrogenionization fraction �H ¼ 0:45) and the least depleted case (Table 3;�H ¼ 0:25).

that is rich in carbon (Keller et al. 1995). Porous IDPs showlower values than dust in comet Halley for carbon and oxy-gen and most probably for nitrogen. The Mg-normalizedabundances of elements in the LIC dust are similar within afactor of 2 either to Halley’s dust or to IDPs depending onthe adopted abundances of the solar photosphere, althoughnitrogen is less abundant in cometary dust.

3. IN SITU MEASUREMENTS OFINTERSTELLAR DUST

3.1. Dynamics of Interstellar Dust in the Heliosphere

The motion of the Sun relative to the LIC initially produ-ces a monodirectional flux of the LIC dust and gas towardthe Sun. However, the spatial number density and heliocen-tric speed of grains are modified by their interactions withthe solar gravitational field, the solar radiation field, and thesolar and interstellar magnetic fields (Mann & Kimura2000). The Lorentz forces acting on electrically chargedgrains in the magnetic fields are less important for heavygrains that influence the determination of the dust mass den-sity. We shall here consider the effect of the solar gravita-tional and radiation pressure forces on the dynamicalevolutions of interstellar dust in the heliosphere.

3.1.1. Gravitational Focusing

The dynamics of large grains plays an important role ininferring the mass density of dust in the LIC from in situmeasurements of interstellar dust in the solar systembecause, as will be discussed later, the dust mass density ismost probably dominated by large grains. Theoretically, thetrajectory of interstellar dust is focused near the Sun whenthe grains are large enough to have the ratio � of radiationpressure to solar gravity smaller than unity. The trajectoriesof small grains, depending on their properties, may also beaffected by the gravitational focusing if � < 1. This gravita-tional focusing of interstellar dust by the Sun enhances themass density as well as the number density of these grains inthe downstream direction. Note that the gravitationalfocusing increases not only the spatial density but also theheliocentric speed of the grains.

3.1.2. Radiative Repulsion

When the �-ratio of the LIC dust exceeds unity, the grainsare decelerated by the solar radiation pressure, and thus theirheliocentric speed decreases with decreasing heliocentric dis-tances. The radiative repulsion deflects their trajectories andforms a forbidden region that no such grains could reach. Itis worthwhile noting that the number density of grains with� > 1 is strongly enhanced near the edge of the forbiddenregion. However, because of their lower masses comparedwith those of larger grains having � < 1, they do not signi-ficantly contribute to the total mass density of the LICdust.

3.1.3. Expected Variations in Spatial Density andHeliocentric Speed

We can analytically calculate the enhancement factors forthe number density nðrÞ and the heliocentric speed vðrÞ ofinterstellar dust at a position r from the Sun along the trajec-tory of Ulysses. The theoretical estimates for nðrÞ and vðrÞbased on the equations given in Axford (1972) require infor-mation on the downstream direction, initial heliocentricspeed, and �-ratio of interstellar grains. Let the LIC dust

originally flow along the downstream direction of the LICneutral helium, while the �-ratio of the LIC dust is left as aparameter. The downstream of the LIC neutral helium ischaracterized by the ecliptic longitude and latitude of73=9� 0=8 and �5=6� 0=4, respectively, with the heliocen-tric speed of ð2:53� 0:04Þ � 106 cm s�1 (see Witte, Banasz-kiewicz, & Rosenbauer 1996). Figure 4 indicates theexpected variations in nðrÞ, vðrÞ, and the heliocentric dis-tance r ¼ jrj over the time period of theUlysses in situ meas-urements through the end of 1999. The number density andheliocentric speed of grains with � < 1 increases at smallerheliocentric distances, while the effect is stronger for smaller�-values. At the beginning of the mission, the number den-sity reaches up to 6 times the initial value for these grains,but the enhancement factor of the heliocentric speed isalways less than a factor of 2. Dust grains with � > 1 shownoticeable increases in the number density and a decrease inthe heliocentric speed at the edge of their forbidden regions.Unlike the grains with � < 1, the number densities areenhanced at different time spans for the grains with � > 1,strongly depending on their �-values.

3.2. Impact Data Analysis

3.2.1. Identification of the LICDust

In order to derive the mass density of dust in the LICfrom in situ measurements, we have to identify the LIC

5

4

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1r/

r 020001998199619941992

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10

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n d(r)

/nd(

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1.6

1.2

0.8

v d(r)

/vd(

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ß=0 ß=0.5 ß=1.4 ß=1.7

Fig. 4.—Expected changes in the heliocentric speed vðrÞ and numberdensity nðrÞ of interstellar dust at Ulysses’s position r during the in situmeasurements. The speed and the density are normalized to their initial val-ues vð1Þ and nð1Þ, respectively, in the LIC. The solid, dashed, dotted, anddot-dashed curves show the results for the ratio � of radiation pressure tosolar gravity being 0, 0.5, 1.4, and 1.7, respectively. Also shown is the helio-centric distance r of theUlysses spacecraft normalized to r0 ¼ 1 AU.


dust within the data that contain dust impacts related to allkinds of sources. The presence of the interstellar dust fluxhas been suggested by the data from the twin DUST instru-ments on board Ulysses and Galileo (Grun et al. 2001). TheUlysses measurements are more suitable for studying inter-stellar dust because of its out-of-ecliptic trajectory minimiz-ing the contribution of interplanetary dust. We thereforeanalyze the Ulysses/DUST data measured over the timeperiod between 1990 October and 1999 December (Grun etal. 1995; Kruger et al. 1999, 2001). The position and veloc-ity of Ulysses and the pointing direction of the DUSTinstrument at the time of impact have been registered, andthe mass and impact velocity of grains at each impact havebeen estimated from the data. We shall discard a smallnumber of impacts that do not contain information on theimpact direction or that show unacceptably large uncer-tainty in the measured mass and impact speed. We furtherreduce the data set by removing the impacts measured dur-ing the time period when Jupiter dust streams were clearlydetected (Grun et al. 2001). The LIC dust can be identifiedby the velocity vector with respect to the Sun, assumingthat grains initially flow into the solar system from thesame direction as the LIC neutral helium. In order to deter-mine the heliocentric velocity of the detected grains, wepresume that the grains hit the detector from the pointingdirection of the detector. Then impacts may be regarded asLIC grains when the angle between the heliocentric velocityand the downstream direction of the LIC neutral helium isless than 70�, which corresponds to a half-opening angle ofthe detector.

3.2.2. Heliocentric Speed of the LICDust

We average the heliocentric speed of the measured LICgrains over 1–2, 2–3, 3–4, 4–5, or 5–5.4 AU in order to eval-uate its dependence on the heliocentric distance. Figure 5illustrates the increase in the heliocentric speed of the LICdust with decreasing heliocentric distance. This is consistentwith the trend predicted for grains with � < 1. The errorbars in the heliocentric speed are propagated from theuncertainty in the determination of the impact speed, whichis less than a factor of 6. The extrapolation of the best-fittingcurve gives the heliocentric speed of interstellar dust flow tobe vdð1Þ ¼ 2:45þ0:11

�0:12 � 106 cm s�1 in the LIC. This agreeswith the heliocentric speed of ð2:57� 0:05Þ � 106 cm s�1

derived from the Doppler shift measurements of the LIC inCa ii absorption lines and with ð2:53� 0:04Þ � 106 cm s�1

from in situ measurements of the interstellar neutral heliumflow (Lallement & Bertin 1992;Witte et al. 1996).

3.2.3. UpstreamDirection of the LICDust

As a result of the deflection of dust by the gravitationalfocusing and the radiative repulsion, the heliocentric veloc-ity of the LIC dust in the heliosphere is not strictly parallelto the downstream direction of the LIC neutral He particles.This can be seen in the data set of the LIC dust: in Figure 6,we plot as crosses the ecliptic longitude and latitude ofupstream directions simply according to heliocentric veloc-ities of grains measured between 2 and 5.4 AU. The circlemarks the ecliptic longitude of 252� � 5� and the ecliptic lat-itude of 5� � 5�, which enclose the longitude 253=9� 0=8and the latitude 5=6� 0=4 of the upstream direction for theLIC neutral helium. As can be seen in the figure, there areno dust grains with a velocity that is parallel to the expecteddownstream direction.

3.2.4. Mass Density of the LICDust

Although the heliocentric velocity of each dust particlehas a component perpendicular to the downstream direc-tion, we may assume that the average velocity of the LICgrains is parallel to the downstream direction. The massdensity of the LIC dust is the ratio of the total grain mass tothe average volume that is a product of the average helio-centric speed of grains, the average impact area of the detec-tor, and the time period of observations. The averageimpact area is derived from the average impact velocity andthe pointing direction of the detector, where the averageimpact velocity is determined by the average heliocentricvelocity and the Ulysses heliocentric velocity. Note that thetime period of observations does not include the time spanwhen Jupiter dust streams were identified nor the time span

Fig. 5.—Heliocentric speed of dust streaming from the LIC into theheliosphere as a function of heliocentric distance. Open circles with errorbars are derived from the Ulysses in situ data. Dotted curve gives the bestfitting of the data.

80

60

40

20

0

-20

-40

-60

Ecl

iptic

latit

ude

(deg

)

320280240200

Ecliptic longitude (deg)

Fig. 6.—Ecliptic longitude and latitude of the direction of incominginterstellar grains based on their heliocentric velocities measured withUlys-ses. The circle near the center of the figure indicates a probable upstreamdirection of the interstellar dust, which is given by the ecliptic longitude of252� � 5� and the ecliptic latitude of 5� � 5�.


when DUST was not operated. Figure 7 shows the massdensity of the LIC dust (open circles with error bars) derivedfrom the data at different heliocentric distances. The dustmass density increases with decreasing heliocentric distanceas expected for the gravitational focusing. The best fitting(dotted curve) indicates the dust mass density in the LICto be �dð1Þ ¼ 3:7þ4:0

�2:0 � 10�27 g cm�3. Consequently, weobtain the hydrogen gas-to-dust mass ratio ofRg=d ¼ 110þ127

�59 using the hydrogen mass density of the LICdetermined by the Ulysses in situ measurements of interstel-lar pickup ions (i.e., �H ¼ 0:25� 0:07). If the higher valuefor the hydrogen ionization fraction, �H ¼ 0:45� 0:25, isassumed, then the Ulysses/DUST measurements result inRg=d ¼ 149þ215

�88 .

4. DISCUSSION

4.1. Contamination from Large Interplanetary Dust

The mass density of interstellar dust in the solar systemshows evidence for the spatial accumulation of large grainsdue to the gravitational focusing as the heliocentric distancedecreases. It is, however, important to note the difficultyfrom the detection geometry of Ulysses of distinguishinginterstellar and interplanetary grains near the ecliptic planein the heliocentric distance range of 1–2 AU based on theirtrajectories. The in situ dust measurements indicate that themass density averaged over the distances between 1 and 2AU from the Sun is 1 order of magnitude larger than the ini-tial mass density in the LIC (see Fig. 7). As seen in Figure 4,the gravitational focusing of grains with � < 1 enhances themass density at the beginning of the mission between 1 and2 AU. However, we expect that the gravitational focusingalone does not increase the mass density by 1 order of mag-nitude, in particular, when the value is averaged over therange between 1 and 2 AU. The average speed of grains at1–2 AU, which exceeds the Keplerian speed, indicates thatthe majority of grains identified as the LIC dust are indeedinterstellar. Nevertheless, the mass density is sensitive to asmall number of large grains in contrast to the small grains,which are abundant in the data set of interstellar grains.Therefore, the mass density between 1 and 2 AU is overesti-mated by the contamination from large interplanetary dustin the data set. We should, however, emphasize that thisdoes not affect the determination for the mass density ofdust in the LIC.

4.2. Deficit of Small Interstellar Dust

A large fraction of the LIC grains smaller than 10�16 g aremost probably deflected from their original flow near theheliopause, which is the interface region between the inter-stellar plasma and the solar wind (Kimura & Mann 1998).Even if they could penetrate the heliopause and could reachUlysseswithout any significant deflection, the DUST instru-ment has a reduced detection efficiency for such small grainsin the heliosphere. The value of the Rg=d for the LIC derivedfrom in situ dust measurements might, therefore, be overes-timated because of the missing small grains in the in situdata. However, it is not straightforward to estimate the totalmass of grains missing in the data without a priori knowingthe size distribution of dust in the LIC. The mass of inter-stellar dust may simply be assumed to originally follow thepower-law size distribution nðmÞdm /m�p dm over the massrange from zero tommax. Then the cutoff of the lower end ofthe mass distribution at the mass mmin results in thedust mass density being estimated as a fraction of1� mmin=mmaxð Þ2�p smaller than the real value in the LIC.The power index of p ¼ 1:55 has been evaluated from theUlysses data in the mass range of 10�12.5 to 10�8.5 g at helio-centric distances from 1 to 5.4 AU (Kimura et al. 1999). Weexpect that the slope of the mass distribution in the LIC willturn out to be steeper than previously estimated from meas-urements in the heliosphere once the gravitational focusingand the radiation repulsion are taken into consideration.Further data analyses are necessary to better understandthe size distribution of grains in the LIC, but this is beyondthe scope of this paper. We shall here assume p ¼ 11=6,which is classically applied for the power-law size distribu-tion of interstellar dust as a result of collisional evolution(see Biermann & Harwit 1980; Draine & Lee 1984). Settingmmax ¼ 10�8:5 g and mmin ¼ 10�12:5 g to estimatethe maximum contribution of missing grains, we obtainRg=d ¼ 85:9þ99:9

�46:9 for the ionization fraction of hydrogen�H ¼ 0:25� 0:07 or Rg=d ¼ 117þ168

�69 for �H ¼ 0:45� 0:25.This estimate constrains the hydrogen gas-to-dust massratio as the lower limit, in contrast to Rg=d ¼ 110þ127

�59 for�H ¼ 0:25� 0:07 or Rg=d ¼ 149þ215

�88 for �H ¼ 0:45� 0:25being the upper limit from in situ measurements. This agreeswith Rg=d ¼ 94:4� 20:9 (Table 2) or 126� 25 (Table 3) for�H ¼ 0:25� 0:07 and Rg=d ¼ 86:8� 23:7 (Table 2) or113� 34 (Table 3) for �H ¼ 0:45� 0:25 derived fromremote astronomical observations of interstellar gasabsorption lines in the LIC. Therefore, once the size distri-bution of missing grains and the solar photospheric abun-dances are known, the values for Rg=d derived from remoteobservations and in situ measurements should converge. Itis worthwhile noting that this value may coincide with thecanonical value of the average ISM, namely, Rg=d � 100(Spitzer 1954; Knapp & Kerr 1974). With our currently bestestimate, it is appropriate to assume that the hydrogen gas-to-dust mass ratio of the LIC amounts to �100 rather than�10–50 or�300–400.

4.3. Dust Composition and Gas Depletion

The elemental composition of dust in the LIC resemblesthat of cometary dust in the solar system, although there is adifference in the nitrogen abundances. The nitrogen abun-dance is a measure of timescales of the grain exposure toultraviolet radiation in the ISM because nitrogen in the dustphase easily returns to the gas phase through ultraviolet

10-27

10-26

10-25M

ass

dens

ity (

g cm

–3)

54321

Heliocentric distance (AU)

ρd(∞) = 3.7+4.0–2.0 ×10–27 g cm–3

Fig. 7.—Mass density of dust streaming from the LIC into the helio-sphere as a function of heliocentric distance. Open circles with error barsare derived from theUlysses in situ data. Dotted curve gives the best fittingof the data.


photoprocessing in the ISM (Greenberg et al. 2000). How-ever, the nitrogen abundance in the solar photosphere mightbe overestimated as suggested by Allende Prieto et al.(2002). By lowering the solar photospheric N abundance,the nitrogen abundance of the LIC dust approaches that ofcometary dust. In addition, the difference in the nitrogenabundance between the LIC dust and cometary dust mightsimply result from an overestimate of its neutral fraction inthe LIC gas. The uncertainty in the N ii column density is solarge that the neutral fraction of nitrogen in the LIC isknown only within the range from 0.276 to 0.881 (Wood etal. 2002a). The neutral fraction of nitrogen in the local ISMis estimated from the FUSE spectra along the total lines ofsight toward WD 2211�495, HZ 43A, WD 0621�376, BD+28�4211, and WD 1634�573 to be 0.180–0.534 (0.334),0.339–0.569 (0.437), less than 0.830, 0.344–0.968, and0.166–0.580 (0.344), respectively, where the numbers inparentheses indicate probable values for the neutral fractionof nitrogen (Hebrard et al. 2002; Kruk et al. 2002; Lehner etal. 2002; Sonneborn et al. 2002; Wood et al. 2002b). On thebasis of initial FUSE spectra, Jenkins et al. (2000) havedetermined the upper limit for the neutral fraction of nitro-gen to be 0.62, 0.44, and 0.78 toward G191-B2B, WD2211�495, and WD 2331�475, respectively. The above-mentioned stars in the local ISM seem to be located withinthe Local Bubble, except for BD +28�4211, which pene-trates a dense neutral gas wall surrounding the Local Bub-ble. Moos et al. (2002) conclude that the Local Bubble is awell-mixed gas because the deuterium abundance seems tohave a single value within the Local Bubble. If the neutralfraction of nitrogen in the LIC gas is as low as one-third, theMg-normalized abundances of dust in the LIC are entirelyin good agreement with those of cometary dust; this wouldindicate the similarity in the composition between interstel-lar dust in the primordial solar nebula and interstellar dustin the local cloud that currently surrounds the Sun. Other-wise, the neutral fraction of nitrogen of �0.5 would mani-fest different stages of evolution between cometary dust andthe LIC dust. Further studies of the N abundance in thesolar photosphere and observations of N ii spectra in theLIC will help to better explain the evolution of dust and gasin the ISM.

4.4. Origin of the Local Interstellar Cloud

The depletions of elements from the LIC gas and the gas-to-dust mass ratio provide information on the origin of theLIC through the evolution of dust and gas (Frisch 2000).The large depletions of elements from the gas are problem-atic for a certain model of the LIC formation that describesthe LIC as a fragment of the expanding Loop I superbubbleshell (Frisch et al. 1999). This model requires grains in theLIC to be destroyed by shock fronts passing through theLIC from the direction of the center of the Loop I superbub-ble. Our results for the hydrogen gas-to-dust mass ratio ofthe LIC based on both gas absorption spectra and dustimpact data do not confirm the previously estimated highvalue Rg=d � 300–400, which was interpreted as evidencefor grain destruction in interstellar shocks with velocity�ð1 2Þ � 107 cm s�1. Interstellar shocks may not destroythe silicate component in the dust if it is covered by anorganic refractory mantle and the grains have a fluffyporous structure (Jones et al. 1994). This implies that theelements forming the organic refractory mantle such as

C, N, and O have returned to the gas phase, but this is notthe case as seen in their depletions (see Fig. 2). In fact, thedepletion pattern can be explained with condensation of Al,Fe, Mg, Si, and S and subsequent accretion of O, N, and Cin the ISM. An alternative model suggests that the LIC issimply one of cloudlets expelled from the interaction zonebetween the Local Bubble and the Loop I superbubble(Breitschwerdt, Freyberg, & Egger 2000). This model doesnot impose shock-induced destruction on the LIC grainsnor the low depletions of elements on the LIC gas. Recently,Franco (2002) has identified a void in the interaction zoneproduced by the detachment of a cloud whose size is similarto the LIC. We should also mention that the problem of theassociation of dust with gas within the LIC, which wasraised by Egger et al. (1996), could be solved with this sce-nario of the LIC formation. Namely, a larger scale andhigher density of the interaction zone, compared with theLIC, may enable grains to have been associated with gas inthe interaction zone prior to the formation of the LIC. Inconclusion, our results suggest that the LIC is not a frag-ment of the expanding Loop I superbubble shell but acloudlet detached from the interaction zone of two bubbles.

5. SUMMARY

We have derived the hydrogen gas-to-dust mass ratio andthe elemental abundances of dust and gas in the LIC fromcolumn densities of the LIC gas measured from satellitesand from masses and impact velocities of the LIC dust mea-sured from spacecraft in the solar system. Remote astro-nomical observations of gas absorption spectra for the LICcomponents result in the hydrogen gas-to-dust mass ratioRg=d � 100. Independently, our analysis of in situ measure-ments of the LIC dust and gas streaming into the helio-sphere, when extrapolated to the LIC, yields Rg=d � 100.We show that a possible contamination of large interplanet-ary dust and deficit of small interstellar dust in the derivedmass density does not play an important role for the deter-mination of the hydrogen gas-to-dust mass ratio in the LIC.The elemental abundances of dust in the LIC are similar tothose of cometary dust, but further investigations of theirnitrogen abundances are necessary to explore the evolutionof dust from the ISM to planetary systems. The elementaldepletions in the LIC are consistent with condensation ofAl, Fe, Mg, Si, and S in stellar atmospheres or nebulae andaccretion of O, N, and C in the ISM. Our results indicate theassociation of dust with gas in the LIC in favor of the LICformation scenario as one of cloudlets detached from theinteraction zone between the Local Bubble and the Loop Isuperbubble.

We thank Brian E. Wood, Jeffrey L. Linsky, CarlosAllende Prieto, Hartmut Holweger, and Ulysses J. Sofia foruseful communications, Martin Lemoine for providing theLIC column densities toward G191-B2B, and HaraldKruger for providing the orbital data of Ulysses. Construc-tive comments from the referee, Jeffrey L. Linsky, havehelped to greatly improve this paper. This research has beensupported by the German Aerospace Center DLR (Deut-schen Zentrum fur Luft-und Raumfahrt) under the project‘‘Kosmischer Staub: Der Kreislauf interstellarer und inter-planetarerMaterie ’’ (RD-RX-50 OO 0101-ZA).


REFERENCES

Allende Prieto, C., Lambert, D. L., & Asplund,M. 2001, ApJ, 556, L63———. 2002, ApJ, 573, L137Asplund,M. 2000, A&A, 359, 755Asplund, M., Nordlund, A., Trampedach, R., & Stein, R. F. 2000, A&A,359, 743

Axford, W. I. 1972, in Solar Wind, ed. C. P. Sonett, P. J. Coleman, Jr., &J.M.Wilcox (NASA SP-308;Washington, DC: NASA), 609

Barstow, M. A., Dobbie, P. D., Holberg, J. B., Hubeny, I., & Lanz, T.1997,MNRAS, 286, 58

Barstow,M. A., Hubeny, I., &Holberg, J. B. 1999,MNRAS, 307, 884Bellot Rubio, L. R., & Borrero, J. M. 2002, A&A, 391, 331Bertaux, J. L., & Blamont, J. E. 1976, Nature, 262, 263Bertin, P., Lallement, R., Ferlet, R., & Vidal-Madjar, A. 1993, J. Geophys.Res., 98, 15193

Bertin, P., Vidal-Madjar, A., Lallement, R., Ferlet, R., & Lemoine, M.1995, A&A, 302, 889

Biermann, P., &Harwit,M. 1980, ApJ, 241, L105Breitschwerdt, D., Freyberg, M. J., & Egger, R. 2000, A&A, 361, 303(erratum 364, 935)

Cassinelli, J. P., et al. 1995, ApJ, 438, 932Draine, B. T., & Lee, H.M. 1984, ApJ, 285, 89Dring, A. R., Linsky, J., Murthy, J., Henry, R. C., Moos, W., Vidal-Madjar, A., Audouze, J., & Landsman,W. 1997, ApJ, 488, 760

Egger, R. J., Freyberg,M. J., &Morfill, G. E. 1996, Space Sci. Rev., 75, 511Ferlet, R., Lallement, R., & Vidal-Madjar, A. 1986, A&A, 163, 204Field, G. B. 1974, ApJ, 187, 453Franco, G. A. P. 2002,MNRAS, 331, 474Frisch, P. C. 1996, Space Sci. Rev., 78, 213———. 2000, J. Geophys. Res., 105, 10279Frisch, P. C., et al. 1999, ApJ, 525, 492Gloeckler, G., & Geiss, J. 2002, in Solar and Galactic Composition, ed.R. F.Wimmer-Schweingruber (NewYork: Springer), 281

Greenberg, J.M., et al. 2000, ApJ, 531, L71Grevesse, N., & Sauval, A. J. 1998, Space Sci. Rev., 85, 161Grun, E., Baguhl, M., Svedhem, H., & Zook, H. A. 2001, in InterplanetaryDust, ed. E. Grun et al. (Heidelberg: Springer), 295

Grun, E., Gustafson, B., Mann, I., Baguhl, M., Morfill, G. E., Staubach,P., Taylor, A., & Zook, H. A. 1994, A&A, 286, 915

Grun, E., et al. 1995, Planet. Space Sci., 43, 971Gry, C., & Jenkins, E. B. 2001, A&A, 367, 617Gry, C., Lemonon, L., Vidal-Madjar, A., Lemoine, M., & Ferlet, R. 1995,A&A, 302, 497

Hebrard, G., Mallouris, C., Ferlet, R., Lemoine, M., Vidal-Madjar, A., &York, D. 1999, A&A, 350, 643

Hebrard, G., et al. 2002, ApJS, 140, 103Holberg, J. B., Barstow, M. A., Bruhweiler, F. C., Cruise, A. M., & Penny,A. J. 1998, ApJ, 497, 935

Holberg, J. B., Bruhweiler, F. C., Barstow, M. A., & Dobbie, P. D. 1999,ApJ, 517, 841

Holweger, H. 2001, in Solar and Galactic Composition, ed. R. F. Wimmer-Schweingruber (NewYork: Springer), 23

Jenkins, E. B., et al. 2000, ApJ, 538, L81Jessberger, E. K., Christoforidis, A., &Kissel, J. 1988, Nature, 332, 691Jessberger, E. K., et al. 2001, in Interplanetary Dust, ed. E. Grun et al.(Heidelberg: Springer), 253

Jones, A. P., Tielens, A. G. G.M., Hollenbach, D. J., &McKee, C. F. 1994,ApJ, 433, 797

Keller, L. P., Thomas, K. L., Bradley, J. P., &McKay, D. S. 1995, Meteor-itics, 30, 526

Kimura, H., &Mann, I. 1998, ApJ, 499, 454Kimura, H., Mann, I., & Jessberger, E. K. 2001, in Proc. Meteoroids 2001Conf., ed. B.Warmbein (ESA SP-495; Noordwijk: ESA), 633

Kimura, H.,Mann, I., &Wehry, A. 1999, Ap&SS, 264, 213

Kissel, J., et al. 1986a, Nature, 321, 280———. 1986b, Nature, 321, 336Knapp, G. R., &Kerr, F. J. 1974, A&A, 35, 361Kruger, H., et al. 1999, Planet. Space Sci., 47, 363———. 2001, Planet. Space Sci., 49, 1303Kruk, J.W., et al. 2002, ApJS, 140, 19Lallement, R., & Bertin, P. 1992, A&A, 266, 479Lallement, R., Bertin, P., Ferlet, R., Vidal-Madjar, A., & Bertaux, J. L.1994, A&A, 286, 898

Lallement, R., & Ferlet, R. 1997, A&A, 324, 1105Lallement, R., Ferlet, R., Lagrange, A. M., Lemoine, M., & Vidal-Madjar,A. 1995, A&A, 304, 461

Lallement, R., Ferlet, R., Vidal-Madjar, A., & Gry, C. 1990, in Physicsof the Outer Heliosphere, ed. S. Grzedzielski & D. E. Page (Oxford:Pergamon), 37

Lehner, N., et al. 2002, ApJS, 140, 81Lemoine, M., Vidal-Madjar, A., Bertin, P., Ferlet, R., Gry, C., &Lallement, R. 1996, A&A, 308, 601

Lemoine,M., et al. 2002, ApJS, 140, 67Linsky, J. L., Diplas, A., Wood, B. E., Brown, A., Ayres, T. R., & Savage,B. D. 1995a, ApJ, 451, 335

Linsky, J. L., Redfield, S., Wood, B. E., & Piskunov, N. 2000, ApJ, 528,756

Linsky, J. L., & Wood, B. E. 2000, in IAU Symp. 198, The Light Elementsand Their Evolution, ed. L. Da Silva, M. Spite, & J. R. De Medeiros(San Francisco: ASP), 141

Linsky, J. L., Wood, B. E., Judge, P., & Brown, A. 1995b, ApJ, 442, 381Linsky, J. L., et al. 1993, ApJ, 402, 694Mann, I. 1996, Space Sci. Rev., 78, 259Mann, I., & Kimura, H. 2000, J. Geophys. Res., 105, 10317Moos, H.W., et al. 2002, ApJS, 140, 3Piskunov, N., Wood, B. E., Linsky, J. L., Dempsey, R. C., & Ayres, T. R.1997, ApJ, 474, 315

Redfield, S., & Linsky, J. L. 2000, ApJ, 534, 825———. 2002, ApJS, 139, 439Sahu,M. S., et al. 1999, ApJ, 523, L159Schramm, L. S., Brownlee, D. E., & Wheelock, M. M. 1989, Meteoritics,24, 99

Sembach, K. R., Howk, J. C., Ryans, R. S. I., & Keenan, F. P. 2000, ApJ,528, 310

Slavin, J. D., & Frisch, P. C. 2002, ApJ, 565, 364Sofia, U. J., &Meyer, D.M. 2001, ApJ, 554, L221 (erratum 558, L147)Sonneborn, G., et al. 2002, ApJS, 140, 51Spitzer, L., Jr. 1954, ApJ, 120, 1Vallerga, J. V., &Welsh, B. Y. 1995, ApJ, 444, 702Vennes, S., Polomski, E. F., Lanz, T., Thorstensen, J. R., Chayer, P., &Gull, T. R. 2000, ApJ, 544, 423

Vidal-Madjar, A., & Ferlet, R. 2002, ApJ, 571, L169Vidal-Madjar, A., et al. 1998, A&A, 338, 694Wedemeyer, S. 2001, A&A, 373, 998Welty, D. E., Hobbs, L. M., Lauroesch, J. T., Morton, D. C., Spitzer, L., &York, D. G. 1999, ApJS, 124, 465

Witte, M., Banaszkiewicz, M., & Rosenbauer, H. 1996, Space Sci. Rev., 78,289

Witte, M., Rosenbauer, H., Banaszkiewicz, M., & Fahr, H. 1993, Adv.Space Res., 13(6), 121

Wood, B. E., Ambruster, C. W., Brown, A., & Linsky, J. L. 2000, ApJ, 542,411

Wood, B. E., & Linsky, J. L. 1997, ApJ, 474, L39———. 1998, ApJ, 492, 788Wood, B. E., Redfield, S., Linsky, J. L., & Sahu, M. S. 2002a, ApJ, 581,1168

Wood, B. E., et al. 2002b, ApJS, 140, 91

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Elemental Abundances and Mass Densities of Dust and Gas in the Local Interstellar Cloud