Adv. Space Res. Vol. 6. No. 8. pp. 195--19"7. 1986 Printed in Great Britain. All rights reserved.

0"273--117786 $0.00 + .50 Copyright ~ COSPAR

EVOLUTION OF THE CHROMOSPHERIC AND CORONAL ACTIVITY OF INTERMEDIATE MASS STARS

Alexander Brown

Joint Institute for Laboratory Astrophysics, Universi~ of Colorado and National Bureau of Standards, Boulder, CO 80309-0440, U.S.A.

ABSTRACT

Recent ultraviolet and X-ray observations pertaining to the outer atmospheric structure of intermediate mass (4-6 M~) stars and the evolution of their structure are presented. A distance-llmited (d ~ 200 pc) IUE ultraviolet survey of early K bright giants shows that C IV emission commonly is present. These stars are almost evenly split between stars show- ing hybrld-chromospherie and coronal outer atmospheric structures. EXOSAT observations have been obtained for three hybrid stars, of which only a TrA, the nearest, is detected. The temperature of the emitting plasma is likely to be ~106 K. Observations of six K II stars made with the Einstein satellite show no detections. The general conclusion from the available X-ray data is that early K bright giants are not strong X-ray sources.

INTRODUCTION

Previous studies of the chromospheric and coronal emission from cool stars have shown that the area of the HR diagram occupied by the early K bright giants (luminosity class II) is particularly interesting. Linsky and Haisch /i/ proposed that cool star outer atmospheres fall into two broad categories; coronal stars with atmospheric structures similar to the sun and non-coronal stars with little plasma hotter than a few ×104 K and strong stellar winds. Hartmann et al. /2,3/ showed that a hybrid-chromospheric structure can occur with the presence of both 105 K plasma, indicated by C IV emission, and a strong stellar wind, seen as a P Cygni absorption feature in the Mg II emission line profiles. These hybrid stars are particularly common among early K bright giants /4-6/ and therefore I have in- vestigated the chromospheric and coronal properties of these stars using ultraviolet and X-ray data. This paper presents a preliminary report on my findings, which will be fully presented in two papers to be submitted to The Astrophysical Journal.

IUE SURVEY OF EARLY K BRIGHT GIANTS

In an attempt to study the relative abundance of the different types of outer atmospheric structure among early K bright giants, I undertook a distance-limited survey using the IUE satellite. All stars classified in the Bright Star Catalogue /7/ as bright giants with spectral types KI-K3 were included in the survey to a limiting visual magnitude of 4.5. Stars already known to possess hot companions were excluded. The sample totaled 15 stars and contained 4 previously known hybrid stars. The survey should be complete to a distance of 200 ±45 parsec, assuming absolute magnitudes of M v = -2±0.5 for early K bright giants.

The final spectra for this program were obtained in 1986 May and the results are summarized in Table i. This table shows the properties of the sample, the velocity of the local in- terstellar medium relative to the stellar photosphere, VLISM (important in interpreting Mg II emission line profiles) /8/, details of the IUE exposures and the deduced atmospheric structure. "Hybrid" and "coronal" stars both show C IV but a "'hybrid" star shows a broad shortward shifted P Cygni absorption feature in its Mg II emission lines that is absent for a "coronal" star. The coronal stars have Mg II emission lines that show two interesting properties, namely: the intrinsic emission profiles after allowing for the effect of In- terstellar absorption appear to be asymmetric to the red and there is a definite lack of flux at the shortward edge of the emission, which could he either merely the dark absorp- tion llne core or perhaps high velocity (~250-350 km/s) Mg II absorption, but only further deeper exposures can resolve this question.

195

196 A. Brown

T&BLE I Nearby (d ~ 200 pc) Early-K Bright Giants Studied with IUE

Star HR V Spectral Longest Exposure (min) (mag) Type VLISM Structure

(km/s) SWP-LO LWP-HI

BInd 7986 3.65 K1 II -9 360 70 "Coronal"* @ Her 6695 3.86 K1 lla . . . . . . Hybrid

TrA 6217 1.92 K2 lib-Ilia . . . . . . Hybrid

--- 3017 3.61 K2.5 Ib-lla -3 I0 35 Hybridt Pay 6582 3.62 K2 II -6 30 60 ---

s Oph 6498 4.34 K2 II +6 (300) 195 "Coronal"* --- 999 4.47 K2 II-III +17 30 60 ---

~60ri 1601 4.47 K2 II +6 30 30 ---

a Hya 3748 1.98 K3 II-III +20 400 50 'Coronal"*

I Aur 1577 2.69 K3 II . . . . . . Hybrid y Aql 7525 2.72 K3 II -- . . . . Hybrid

Ara 6461 2.85 K3 Ib-lla -16 30 90 Non-coronal Her 6418 3.16 K3 llab +12 3[0 150 "Coronal"*

q Car 4050 3.40 K3 lla -6 330 120 Hybrid 1 Lac 8498 4.13 K3 II-III +i 419 60 "Coronal"* or

Hybrid

*"Coronal" here refers to stars showing C IV emission but no "double absorption" in the Mg II profiles. tBg-A0 companion present.

EXOSAT OBSERVATIONS

In 1985 March EXOSAT observed 3 hybrid stars [= TrA for >2 × 104 sec; i Aur and 6 TrA (G2 Ib-lla) for ~2 x I0 ~ sec]. Only the closest star ~ TFA was detected with a count-rate of 3.3 x 10-3 cts/sec; the upper limits for the outer stars were -1.7 × 10 -3 cts/sec at the

3o level. The observations were made with the LE CMA detectors and thin (3000 %) Lexan

filters.

a TrA is at a distance of -72 pc. The hydrogen column density and velocity were determined

in this llne of sight using 1UE spectra of B stars. The ISM velocity of -7 km/s agrees well with the low velocity absorption seen in the Mg II emission lines. The three B stars used gave the following results:

HD 148740, B6 III, d = 516 pc, log N H = 20.1, log NH(EB_ V) = 20.5

HD 142514, B7 III, d = 205 pc, log N H = 20.2, log NH(EB_v) = 20.5

HD 149671, B7 IV, d = 188 pc, log N H = 20.0, log NH(EB_v) = 20.3

These values give an indication of the appropriate N H value for interpreting the EXOSAT ob- servation of ~ TrA. Values of I x 1020 and 5 × I019-cm-2 were used for this preliminary analysis. The transmittances associated with these two values change sharply over the spectral range of the 3000 Lexan passband. The transmlttances are less than 0.5 at k > 80 % and k > 100 A for log N H = 19.7 and 20.0, respectively.

The detection of a TrA therefore suggests the presence of plasma at T e > 10 6 K with emis-

sion from cooler regions being a less likely explanation.

With log N H = 20.0 the implied volume emission measure is 4 x 10 52 cm-3 at log T e = 6.0 and 2 x 10 52 cm -3 at log T e = 6.2. The equivalent values for log N H = 19.7 are 3.5 × 1052 and 2 × 10 52 , respectlvely. No information exists in the data to confine the temperature of the emitting plasma. (The emissivities of Shull /9/ were used in these calculations.)

EINSTEIN OBSERVATIONS

In collaboration with Drake and Linsky, the X-ray emission from G and K supergiants and giants has been investigated. This sample includes six of the stars in the K II IUE sample [t Aur, ~60ri, ~ Hya, ~ Oph, 0 Her and y Aql]. None of these stars are detected at a 3~ level. For t Aur and e Her a positive but <1.5 o signal is seen:

-L3 -2 -I fx(L Aur) = (1.0±0.7) x i0 ergs cm s

f (e Her) (1.0±0.8) x 10 -13 ergs cm -2 = s x

Chromospheric and Coronal Activity' 197

Therefore from EINSTEIN satellite data there is no evidence for X-ray emission from hybrid stars or any other early K bright giant.

CONCLUSIONS

IUE Survey

a) The presence of C IV in the IUE spectra of all but one of the stars studied with deep SWP low dispersion spectra indicates that 10 5 K plasma is normally present in the outer atmospheres of early K bright giants.

b) Hybrid-chromosphere stars constitute ~50% of the stars in this region of the HR diagram.

c) A significant group of apparently coronal stars exists with C IV emission and intrinsic Mg II emission profiles with stronger red emission components but without the obvious wind absorption feature of the hybrid stars. One of these stars, = Her, was shown by Leggett e_~t al___. /I0/ to be significantly less luminous than the hybrid star y Aql (220 L~ compared with 4700 L@). Therefore the observed atmospheric differences could possibly be a luminosity effect with the coronal stars intermediate in luminosity between bright giants and giants, and correspondingly also of intermediate mass.

d) The evolutionary timescales of stars evolving to the right in this part of the HR dia- gram are much shorter (by a factor of I0) than for the He burning stars evolving to the left. Therefore, nearly all the K bright giants should be evolving to the left and it is unlikely that the direction of evolution can be the dominant factor in causing the observed distribution of hybrid and coronal stars.

X-Ray Observations

a) Early K bright giants are not strong X-ray sources. None were detected by the Einstein satellite and only = TrA by EXOSAT.

b) The emission from = TrA is likely to be relatively soft X-rays. The temperature of the emitting plasma is unlikely to be less than 106 K, but the coronal temperature is probably not very hot or more stars would have been detected by the Einstein satellite.

c) Only future observations with good spectral resolution will be able to resolve the ques- tion of how hot the plasma is in the coronae and upper transition regions of hybrid and other K II stars. Far-EUV and soft X-ray emission lines will provide the required informa- tion.

ACKNOWLEDGMENTS

I thank my colleagues J. L. Linsky, S. A. Drake and M. Van Steenberg for their collabora- tion and permission to describe our joint work as part of this paper. I thank the staff of the Goddard IUE ground station and the EXOSAT ground station for their assistance in acquiring these data. This work was supported by grants NAG5-82, NAG8-572 and NAG8-510 from the National Aeronautics and Space Administration through the University of Colorado. The IUE spectra were analyzed at the IUE Regional Data Analysis Facility in Boulder, which is supported by grant NASAS-26409.

REFERENCES

I. J. L. Linsky and B. M. Haisch, Ap. J. (Letters) 229, L27 (1979). 2. L. Hartmann, A. K. Dupree and J. C. Raymond, Ap. J. (Letters) 236, L143 (1980). 3. L. Hartmann, A. K. Dupree and J. C. Raymond, Ap. J. 246, 193 (1981). 4. D. Reimers, Astr. Ap. 107, 192 (1982). 5. L. Hartmann, C. Jordan, A. Brown and A. K. Dupree, Ap. J. 296, 576 (1985). 6. D. Reimers, Astr. Ap. 136, L5 (1984). 7. D. Hoffleit, Brisht Star Catalogue~ 4th Revised Edition, Yale University Observatory

(1982). 8. S. A. Drake, A. Brown and J. L. Linsky, Ap. J. 284, 774 (1984). 9. J. M. Shull, Ap. J. Suppl. 46, 27 (1981).

I0. S. K. Leggett, C. M. Mountain, M. J. Selby, D. E. Blackwell, A. J. Booth, D. J. Haddock and A. D. Petford, Astr. Ap. 159, 217 (1986).