05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 1 Infrared Observations of Interstellar Ices Adwin Boogert NASA Herschel Science

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 1

Infrared Observations of Interstellar Ices

Adwin BoogertNASA Herschel Science Center

IPAC, CaltechPasadena, CA, USA


ScopeLecture 1 (Monday): What you need to know when planning, reducing, or analyzing infrared spectroscopic observations of dust and ices.

Lecture 2 (Tuesday): Basic physical and chemical information derived from interstellar ice observations. Not discussed: laboratory techniques (see Palumbo lectures) and surface chemistry (see Cuppen lectures).

Lecture 3 (Tuesday): Infrared spectroscopic databases. What's in them and how (not) to use them.

Drylabs (Tuesday): Using databases of interstellar infrared spectra and of laboratory ices. Deriving ice abundances and analyzing ice band profiles.

NOTE: Please download all presentations and drylab tar file:

spider.ipac.caltech.edu/~aboogert/Cuijk/


TopicsBasics

Ice mantle formation Deriving ice column densities and abundances

YSO and background source selectionContinuum determinationVibrational modesThe interstellar ice inventoryIce band profile analysis:

Polar versus apolar ices Amorphous versus crystalline ices Segregation in the ices Grain size and shape effects

Location of icesProcessing of ices by YSOsComplex molecules in ices?


Background Reading

For the basics: Dust in the galactic environment, 2nd ed. by D.C.B. Whittet. Bristol: Institute of Physics (IOP) Publishing, 2003 Series in Astronomy and Astrophysics, ISBN 0750306246.

More advanced: Chapter 10 in “The Physics and Chemistry of the Interstellar Medium”, A. G. G. M. Tielens, ISBN 0521826349. Cambridge, UK: Cambridge University Press, 2005.

Current status of observational ice studies: Oberg et al. 2011, ApJ 740, 109


More realistic grain:

Many molecules (H2, H2O) much more easily formed on grain surfaces. Freeze out <100 K. Keywords: Physisorption, chemisorption, tunneling (see lectures by Cuppen) Interstellar ‘ice’ or ‘dirty ice’: any frozen volatile, e.g. H2O, H2O mixtures, pure CO, but NOT H2.

Basics: Ice Mantle Formation


Basics: Ice Mantle FormationFor gas at number density n, mean speed <v>, mean particle mass <m>, gas-to-grain sticking coefficient S, grain radius a, and grain density :

Grain mantle thickness: Mass growth rate: dm/dt=S**a2*n*<v>*<m> Radius growth rate: da/dt=(dm/dt)/(4**a2*) da/dt=S*n*<v>*<m>/(4*) Mantle thickness independent of grain radius

Dense clouds can have mantles as thick as 0.1 um, and in deeply embedded protostars even more.

Mantle thicker than most grain cores according to MRN grain size distribution

n(a)~a-3.5, amin=0.005 μm, amax=0.25 μm


Col

umn

Den

sity

COH2O

Extinction (AV)

Basics: Ice Mantle FormationGrain temperature and interstellar radiation field inhibit ice formation at low visual extinction (AV): the ice formation thresholdTaurus cloud: H2O ices absent below visual extinction AV~3 and CO ices below AV~7. Difference due to lower Tsub of CO. Variation between clouds due to different conditions

Extinction (AV)


Basics: Ice Column Densities and Abundances

Ice column densities: N=peak*FWHM/Alab

Alab integrated band strength measured in laboratory

A[H2O 3 m]=2.0x10-16 cm/molecule

Order of magnitude in quiescent dense clouds: N(H2O-ice)=1018 cm-2 along absorption 'pencil beam'.

This is ice layer of 0.3 m at 1 g/cm3 in laboratory.

Order of magnitude estimate of NH (for ice abundances): AV=t9.7*18.5 mag (Roche & Aitken 1984)

NH=AV*2.0*1021 cm-2 (Bohlin et al. 1977)

Ice abundance: X(H2O-ice)=N(H2O-ice)/NH~10-4

This is comparable to X(CO-gas)


Observing IcesIces form anywhere T<90 K and

Av>few magn. Visible against

continuum YSO or background star.

H2O

CO2

silicates

H2O

NH4

+

H2O

H2O NH

4+

silicates CO2

Star-forming dense core

Foreground cloud(s)

envelope

outflow

disk

star

Background star


Source SelectionTaking advantage of large scale infrared imaging surveys, YSOs

and background stars can be selected using broad-band 2-25 μm colors. Extinction determined for many background stars, assuming average, intrinsic stellar colors (“NICE” method).

L 1014; AV=2-35; 20” resol; Huard et al. (ApJ 640, 391, 2006)


Spectra of Spitzer-selected YSO and Background Star

YSO

BackgroundStar

Isolated core L 1014; AV=2-35; Huard

et al. 2006, ApJ 640, 391, 2006


Continuum DeterminationIce features are studies on optical depth scale

Critical step in the analysis of ice bands is continuum determination.This can be done:

Locally, on a limited wavelength range (for single or few features), usually done with polynomial fit

Globally: Physical model of source (can be done for background stars,

rarely feasible for YSOs) Polynomial or spline. Relatively subjective. Fits can be guided by

taking into account models and laboratory spectra of dominant absorbers (H

2O and silicates).

=−ln fluxcontinuum


Continuum Determination Background Stars

H2O

H2O

H2O

silicates

CO2

?

Red: M1 III model and featureless extinction curve at AK=1.5 magn

Green: H2O ice and silicate model added

c2 minimization includes: Spectral type (CO and SiO bands) Stellar models (MARC; Decin et al.) Extinction laws Silicates model L-band spectra (H2O ice) H2O ice model 1-25 mm photometry


Continuum Determination YSOsContinuum determination YSOs much harder because models have many poorly constrained degrees of freedom.


Observing Solid State MoleculesH2O ice has many broad absorption bands:Symmetric stretchAsymmetric stretchBending modeLibration modeCombination modesLattice mode (can be in emission)

etc...

CO:one vibrational modeNo features for species without permanent dipole moment (O2, N2, H2).


Ice Inventory


Ice Inventory


Ice Inventory

[H2O and silicate subtracted!]


[H2O and silicate subtracted!]

Ice Inventory


NH3/CH3OH=4

NH3/CH3OH<0.5

(SVS 4-5)

Ice Inventory


Protostellar luminosity apparently not a dominant factor in ice formation and processing

Low Mass vs High Mass Protostar

Noriega-Crespo et al. ApJS 154, 352 (2004)


CO, incl 13CO few-50%

CO2, incl. 13CO2 15-35%

CH42-4%

CH3OH <8, 30%

[HCOOH] 3-8%

NH3<10, 40%

H2CO <2, 7%

[HCOO-] 0.3%OCS <0.05, 0.2%

[SO2]<=3%

[NH4+] 3-12%

[OCN-] <0.2, 7%

Ice Inventory

'Typical' abundances with respect to H2O ice. Species in brackets somewhat disputed.


Ice Versus Gas Phase

InventoryGas phase molecules detected in space (123 listed here). Currently up to ~150?

Far less ices than gas phase species detected because ices can only be detected by absorption spectroscopy: weakest features (1%) represent column density 0.01*4 [cm-1]/1e-17 [cm/molecule]=4e15 cm-2,

orders of magnitude higher than gas phase detections! www.cv.nrao.edu/~awootten/allmols.html


Ice Band Profiles Analysis

Ice band profiles contain wealth of information because they depend on dipole interactions (bond lengths are modified by attractive or repulsive electric forces). Strong effects caused by:

Ice composition, pure ices versus mixturesMatrix structure: amorphous versus crystalline. Temperature.Grain size and shape: surface charge induced by external light (polarizability)

Comparison with laboratory analogs powerful tool, but at same time fitting is subtle and solutions not unique.

Instead of fitting 1000 lab spectra to the interstellar spectra, best to use trends in peak position versus width to draw general conclusions.


Ice Band ProfilesPolar vs Apolar Ices

Peak position and width of CO ice band depends strongly on what other molecules are present in the ice.

(also note strong dependence on thermal history and grain shape)


• Interstellar CO ice band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures:

– 'polar' H2O:CO

– 'apolar' CO2:CO

– 'apolar' pure CO

(Boogert, Hogerheijde & Blake, ApJ 568,761, 2002)




CO ice profiles vary in different sight-lines, as a result of different sublimation temperatures: ~90 K for H2O-rich and ~18 K for CO-rich. Profile good indicator of thermal processing.Polar/non-polar distinction relevant for outgassing behavior of comets. 'Pockets' of apolar CO may result in sudden sublimation.


Ice Band Profiles: Amorphous vs. Crystalline

Interstellar H2O ices formed in amorphous phase, as evidenced by great width.Crystallization by YSO heat.

Crystallization temperature ~120 K in laboratory, but ~70 K in space due to longer time scales: ~1 hour in lab and 104-105 yr in space:

Time scale ~exp(Ebarrier/T)

[For same reason sublimation temperature in lab (~180 K) higher than in space (~90 K)]


Ice Band ProfilesSegregation

If average bond strength in pure ice is stronger than in a mixture, mixture will segregate into clusters of pure ice as the ice is heated. This is how 'pure CO2' ice is formed in space: CO2, H2O, and CH3OH formation are linked through OH. The characteristic double peak of the 15 mm CO2 bending mode appears when the mixture of CO2 with H2O and/or CH3OH is heated.

Oberg et al., 2009, A&A 505, 183


Ice Band ProfilesGrain Shape and Size Effects

Laboratory and interstellar absorption spectra cannot always be compared directly: Scattering on large (micron sized) grains leads to 3 μm red wing (often observed)Surface modes in small grains may lead to large absorption profile variationsFor ice refractive index m=n+ik, absorption cross section ellipsoidal grain proportional to (Mie theory) (2nk/L2)/[(1/L-1+n2-k2)2+(2nk)2]Resonance for sphere (L=1/3) occurs at k2-n2=2, so at large k (=strong transitions)Important for pure CO, but not for CO diluted in H2O and also not for 13CO.


• First detection of solid 13CO

• High spectral resolution required!

• New insights into nature apolar ices:

– 13CO well fitted with pure CO, but 12CO requires ellipsoidal grains

– Most CO not mixed with CO2

(Boogert, Blake & Tielens, ApJ 577, 271 (2002))

Ice Band ProfilesGrain Shape Effects


Processing of YSO Ices

Bill Saxton (NRAO/AUI/NSF)

Ice processing 'hot topic' in astrobiology. Which processing mechanism (heating, UV light, cosmic rays) is relevant where? Do processed ices survive and how?


Ices formed anywhere T<90 K and extinction large enough.Proper interpretation of ice features, such as the causes of processing history, requires knowledge of location of ices along the line of sight. This can be achieved by complementary gas phase observations, by high spatial resolution spectroscopy, and/or physical models.

Where ARE the Ices?


Spatial distribution CO2 in circumstellar envelope.(Pontoppidan et al. 2008)

(R)~R-1.5

Where ARE the Ices?

Physical conditions vary with radius from the star and vertically in disk. Understanding ice processing of proto-planetary environment requires disk modelling.


Ices on small scales can nowadays be studied using infrared spectrometers behind adaptive optics systems on large (>=8 m) telescopes, approaching diffraction limit (<0.1''). E.g., IRCS at Subaru and CONICA at VLT.

Where ARE the Ices?

Crystalline H2O ice from upper disk layers YLW16A detected by Schegerer et al. (2010)

1X1'' FOV

Edge on disk HV Tau C can be separated by binary component and slit can be oriented at desired angle w.r.t. disk. H2O ice band depth varies on few year time scale (Terada et al. 2007).

slit


Processing Ices YSO Envelopes

Observational evidence for thermal processing of ices near YSOs:

Solid 13CO2 band profile varies toward different protostars…


Processing Ices YSO Envelopes

Observational evidence for thermal processing of ices near YSOs:

Solid 13CO2 band profile varies toward different protostars…

…and laboratory simulated spectra show this is due to CO2:H2O mixture progressively heated by young star (Boogert et al. 2000; Gerakines et al. 1999)


Processing Ices YSO EnvelopesObservational evidence for thermal processing of ices near YSOs:Solid CO2 band profile varies toward different protostars……and laboratory simulated spectra show this is due to CO2:H2O mixture progressively heated by young star (Boogert et al. 2000; Gerakines et al. 1999)H

2O crystallization (Smith et al. 1989)

gas/solid ratio increases (van Dishoeck et al. 1997)Detailed modelling gas phase mm-wave observations (van der Tak et al. 2000)

Little evidence for energetic processing of ices, however......


Complex Molecules?

Greenberg et al. ApJ 455, L177 (1995): launched processed ice sample in earth orbit exposing directly to solar radiation (EURECA experiment). Yellow stuff turned brown: highly carbonaceous residue, also including PAH.

Complex species formed, some are of biological interest:

•POM (polyoxymethylene, -(CH2-O)n-

•HMT (hexamethylenetetramine, C

6H

12N

4)

•Amino acids (glycine)•Urea (H

2NCONH

2)

•PAHs (polycyclic aromatic hydrocarbons)


Complex Molecules?Observational evidence for complex products of UV/CR

bombardment of simple ices is weak at best: Triple peak 3.4 mm band seen only in diffuse medium

and it is not polarized as opposed to silicates/ices: not in processed mantle but separate grains

Perhaps organic residue in 5-8 mm region?


Molecular Evolution: Hot Cores

Formic acidMethyl formate

Formic acidDimethyl ether

Ceccarelli et al. A&A 521, L22 (2010)

High spectral and spatial resolution observations ofHot cores. Trace the ices indirectly, after sublimation.


Future of Astrochemistry is Bright....

Thirty Meter Telescope

Atacama Large MM Array

James Webb Space Telescope

….plus a lot more……

Documents

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 1 Infrared Observations of Interstellar Ices Adwin Boogert NASA Herschel Science