FINAL TECHNICAL REPORT

to the

National Aeronautics and Space Administration

on Grant NAG 5-266

to the Ohio State University Research Foundation

1314 Kirinear Road

Columbus, OH 43212

Principal Investigator: Robert F. Wing

Astronomy Department

Ohio State University

174 West 18th Avenue

Columbus, OH 43210

Co-Investigator: Kenneth G. Carpenter

Astronomy Department

Ohio State University

174 West 18th Avenue

Columbus, OH 43210

in collaboration with: Drs. R. E. Stencel and J. L. Linsky

Joint Institute for Laboratory Astrophysics

University of Colorado

Boulder, CO 80309

Title: "MEASUREMENTS OF CHROMOSPHERIC DENSITIES AND GEOMETRICAL

EXTENSIONS OF LATE-TYPE GIANT AND SUPER-GIANT STARS"

Period covered: 1 September 1982 - 31 August 1983

NASA Technical Officer: Dr. Yogi Kondo

IUE Observatory

Goddard Space Flight Center

Greenbelt, MD 20771
CONTENTS

I. INTRODUCTION

II. SUMMARY OF RESULTS

III. PUBLICATION OF RESULTS .

IV. PROJECT EXPENDITURES
I. INTRODUCTION

The research described in this document is based upon observations

made with the International Ultraviolet Explorer (IUE) satellite. The

work has been a collaborative effort between scientists at the Ohio State

University, the University of Colorado, and the University of Oxford in

England. The subject grant, NAG 5-266, covers only a portion of Ohio

State University's contribution to this work.

Support by NASA for this work began in the Spring of 1981 with the

award of two grants, one to the University of Colorado (P.I.'s: R. E.

Stencel and J. L. Linsky) and one to the Ohio State University (P.I.:

R. F. Wing, and Co-I: K. G. Carpenter). Originally, this had been a

joint proposal by the four investigators under the title, "Measurements

of the Chromospheric Densities and Geometrical Extensions of Late-Type

Giant and Supergiant Stars Using C II Lines as Diagnostics", and the work

has continued as a fully collaborative venture. The Ohio State University

portion of the funding was transmitted as Supplements Nos. 3 and 4 to

NSG 5368, with Robert F. Wing as Principal Investigator.

One year later, in the Spring of 1982, the IUE Observatory approved

a continuation proposal entitled "Further. Measurements of Chromospheric

Densities and Geometrical Extensions of Late-Type Giant and Super Giant

Stars Using C II Line Ratios" by the same four Colorado/Ohio State investigators. We requested that the Ohio State portion of the funding be

transmitted as a further supplement to NSG 5368, but it was sent instead

as a new grant, NAG 5-266. Consequently, this Report is separated from

others describing the same science.
4.

II. SUMMARY OF RESULTS

Multiplet (UV 0.01) of the C II ion has proved to be one of the most

interesting and useful groups of atomic lines that can be studied in the

ultraviolet spectra of late-type stars accessible to the IUE satellite.

Its strongest line, at 2325 A, was listed as the "strongest unidentified

line" in our high-resolution spectra of y Crucis and other cool giant

stars observed during lUE's first year of operation (Wing and Carpenter,

B.A.A.S. 10, 444, 1978). Two years later these lines were identified as

intercombination lines of C II by Brown and Jordan of Oxford and by Stencel

and Linsky of JILA. By analogy with similar transitions in B I, N III,

amd 0 IV, which had been studied in the context of the solar atmosphere,

the lines of the C II multiplet were expected to be useful indicators of

the electron density. Efforts were then made to measure the C II lines

on all suitable existing IUE exposures, to calculate theoretical line

ratios as a function of electron density and temperature, and to compare

the theoretical ratios with observation.

The first results of this study were very encouraging and were reported by Stencel, Linsky, Brown, Jordan, Carpenter, Wing, and Czyzak

(Monthly Notices Roy. Astron. Soc. 196, 47P, 1981). As shown in Figure 1,

the relative strengths of the five lines of C II (UV 0.01) vary from

object to object and in particular are quite different in the low-density

planetary nebula as compared to the late-type giants. Figure 2 shows

that the various line ratios (designated R,, R2, and RO, and defined in

the figure) are calculated to be very sensitive to electron density in

7 9 _3

the range 10 - 10 cm . In the high-density limit the ratios approach

the values observed in the solar chromosphere, while in the low-density

limit they agree with the ratios measured in the planetary nebula NGC 6572.

Evidently the chromospheres of red giant stars have intermediate densities —

precisely in the range where the C II lines are most sensitive.

Subsequently, we showed that the C II lines can give an indication of

the geometrical size of the emitting region. Since the integrated intensity
1.5

1.0

0.5

vt

£

o

O»

t.

Hi

+

X

o

tu

>

cc

UJ

to

OQ

o

1.5

1.0

0.5

ot Boo K2

a Tau K5 IE

••« Ori M2 lob '

i r

y Cru M2 IH

I

Gru M3 -PLANETARY NEBULA

NGC 6572

1.0

0.5

2321 2325 2329 2321

WAVELENGTH (X)

2325 2329

Figure 1. Emission lines of C II UV multiplet 0.01 observed in several

late-type stars and a planetary nebula. Note that the ratio

2325/2327 is closer to unity in the nebula than in any of the

class III objects, and is intermediate in Alpha Ori. From

Stencel, Linsky, Jordan, Brown, Carpenter, Wing and Czyzak (1981)
6.

- 4

A2324.7

3 R3=X2326.9

-2

log Ne(cm"3)

R2

6

5

4

3

2

PN

A

A

l:ct Boo K2 III 2:a Tau K5 III

3:y Cru M3 II 4:6 Gru

5:a Ori M2 lab M3 II

l

Sun

-a

4

3

2

log Ne(cni3)

Figure 2. Upper diagram: calculated line ratios for three pairs of lines

within the C II UV 0.01 multiplet, plotted against log Ne- Note the

great sensitivity of these ratios to Ne between 107 and 10^ cm" .

The sensitivity to temperature, on the other hand, is small.

Lower diagram: As above, for Te = 10,000 K, except that the A-values

have been adjusted empirically to force better agreement between

calculated and observed solar ratios. The error bars represent

+15%. From Stencel et al. (1981).
7.

in the multiplet is proportional to the number of C II ions, while the

ratios discussed above indicate the electron density, one can combine

these data to determine the path length over which the lines are formed

and hence the thickness of the chromosphere. This calculation is best

done in conjunction with observations of the C II line at 1335 A, since

then the temperature is no longer a free parameter. The important

result which soon emerged from this work is that the chromospheres of

red giant and supergiant stars are greatly extended, on the order of

one stellar radius in thickness, in contrast to the solar chromosphere,

the thickness of which is only a few percent of the solar radius.

Recent work on the C II lines has concentrated on acquisition of

data on additional stars (a slow process, since exposures of several

hours are required to record even the brightest late-type stars adequately

at 2325 A at high resolution) and the reduction of these spectra. We

have also developed an improved treatment of the carbon and hydrogen

ionization equilibria, which permits a more accurate estimate of the

extent of the chromosphere. As reported by Stencel and Carpenter (NASA

CP-2238, p. 243, 1982), a distinction can now be made between the giants

earlier than about K2 III, which like the Sun have thin chromospheres

surrounded by hot coronae, and the cooler giants which have thick chromospheres surrounded by cool circumstellar envelopes.

Clearly, observations of the intercombination multiplet of C II have

added considerably to our understanding of the outer atmospheres of

late-type giant stars.
8.

III. PUBLICATION OF RESULTS

The chief results from this work have been published under grant

NSG 5368 to the Ohio State University and grants NAG 5-82 and NAS 5-23274

to the University of Colorado.

The following paper has been published under the subject grant

NAG 5-266:

"Density Sensitive C II Lines in Cool Giant Stars", by

Robert E. Stencel and Kenneth G. Carpenter, in Advances

in Ultraviolet Astronomy; Four Years of IUE Research,

ed. Y. Kondo, J. M. Mead, and R. D. Chapman, NASA CP-2238,

1982; pp. 243-246.

Reprints of the above paper are submitted herewith.

IV. PROJECT EXPENDITURES

The project budget of $5000. was spent entirely for the salary of

a graduate research assistant, and for the associated fee waivers and

indirect costs. The student worked half-time for 4.5 months and was

primarily involved with the reduction and measurement of the IUE spectra.
from Advances in Ultraviolet Astronomy: Four Years of IUE Research, ed.

Y. Kondo, J. M. Mead, and R. D. Chapman, NASA CP-2238 (1982), p. 243.

DENSITY SENSITIVE C II LINES IN COOL GIANT STARS

Robert E. Stencel

Joint Institut e for Laboratory Astrophysics, University of Colorado and

National Bureau of Standards, Boulder, CO 80309

and

Kenneth G. Carpenter

Department of Astronomy, Ohio State University

Columbus, OH 43210

ABSTRACT

The density sensitivity of the emission liner, within the UV 0.01 multiplet of C II near 2325 A has been examined in additional late type giants and

super.-.lants with deep LU'R high dispersion exposures. The new data support the

original contention based on these line:! that noncoronal red giants possess

ge""?ot ri ca.l !.y extended chromospheres.

Given the complexity of the spectra of late type post main sequence

stars. It Is crucial that spectroscopic methods be established which can unaitihiguously determine basic atmospheric parameters, nuch as the electron density . One such density diagnostic involves the mld-UV 1ntercomblnation lines

of c. II, multiplet UV 0.01 near 2125 A (Stencel et al . 1981). By iterating

het.wt?en three observed line ratios within tVie multiplet and atomic theory, a

SSL-! f-consistent set of atomic parameters which fit the observations was derived. Those lines exhibit a density sensitivity between 107-109 cm~3. In

that study, except for the solar chromosphere which provided the high density

limit , all of the stars considered were late K and M giants and superglants

which arc inferred to lack solar-like corona on the basis of their low upper

'.traits if soft X-ray emission (Ayres e t al. 1981a).

Wo l.ave endeavored during the fourth year of IUE operations to extend the

observational sample across the cool half of th« H-R diagram, particularly to

..•:.;>l<>re the so-called Li risky and Halseh (1979) division between giant stars

wit h and without transition regions (TR) and coronae. To this end, we have

observed throe coronal type stars: Beta Dra (c;2 T.h-11), Beta Gem (KO III) and

'•>'.• 1V>: (KO Up + wd). Ihe latter is an interacting binary and must be cautiously compared with single stars (cf. Schindler et al. 1982), In addition,

we have observed Epslion Go:?. v.C8 Ib) and Epsi.lon Pe;; (K2 Ib) to extend the

•-urvoy of C 11} in the spectra -of noncoronal stars.
OBSERVATIONS

Figure 1 displays the 2320-2330 A region of spectrum in the five new

observations, plus a comparison with the previously observed, high signal-tonoise (S/N), strong C II lines in Alpha Ori (M2 lab). The figure also indicates the exposure times used for these LWR echelle mode observations during

1931. The wavelength scales are those provided by the IUESIPS, and given the

small range of THDA (13±1°C), suggest real velocity shifts. The primary member of the multiplet, near 2325.4 A appears to have been detected in each

case, except perhaps in Beta Dra where a strong continuum has swamped much of

the emission. The ratio of 2325.4 A/2328.1 A (R^ theoretically decreases

with increasing electron density, while the ratios 2325.4 A/2326.9 A (R2) and

2324.7 A/2326.9 A (R3) both increase with increasing electron density. On

this basis we can derive some preliminary density estimates, limited largely

by the low S/N:

Star

P Dra

£ Gem

56 Peg

E Gem

e Peg

Spectral

Type

G2 Ib-II

KO III

KO Ilp+wd

G8 Ib

K2 Ib

Radial

Velocity

-20 km s'1

+3

-24

+11

+5

C II] Ratios

Rl

1.8

2.2

3.0

3.2

2.5

R2

2.3

4.8

2.5

2.3

2.7

R3

1.5

2.0

1.3

0.9

1.2

Log Ne

8.3 ± 1.0

8.7 ± 0.3

8.0 ± 0.5

7.8 ± 0.6

8.0 ± 0.8

Improvements in the use of the C II ratios for accurate density determinations

must await more accurate atomic parameters for C II, as well as higher S/N observations which will become possible with the High Resolution Spectrograph on

the Space Telescope In the mid-1980s.

INFERENCES

The ratios of the 1335 A resonance lines of C II to the interccmbination

lines is sensitive to Texc. Among coronal type giants like Beta Dra and Beta

Gem, the lines of C II near 1335 A are clearly present. Among the noncoronal

giants, the resonance lines of C II are much weaker and often blended with

fluoresced CO emission (Ayres et al. 1981b). These resonance lines are computed to form in the lower TR in the Sun (£20,000 K), but our calculations for

giant stars suggest significant contribution to the line flux from the chromospheres. We are typically finding upper limits to the Texc for noncoronal

giant stars, of 5000-7000 K. Physically, the emission measure suggests that

in order to obtain significant flux from the intercombinatlon lines when the

resonance lines are weak or absent (low Texc), formation in an extended chromosphere is required. Solving for the line emissivity of C II UV 0.01 requires an accurate ionizatlon equilibrium, which is difficult to compute in

the turbulent and partially-ionized chromospheres of red giants. However,

244
because the C II IN 0.01 lines are not self-reversed, we can adopt an optically thin approximation, and the emitting layer thickness (cm) can be expressed

as a function of the observed flux, distance factors and the ionizatlonexcitation populations which involve an exponential of inverse Texc. Thus,

coronal type giants like Beta Dra and Beta Gem which have solar like TeXc are

deduced to have thin C II emitting layers (r « R*). The noncoronal giants

with low Texc ar& deduced to have very large C II emitting layers (r > R^).

Although the calculations require improvements and generalization, it is encouraging that this evidence concurs with parallel data from radio and narrow

band speckle interferometry observations which also point to extended chromospheres among noncoronal giants and supergiants (cf. Stencel 1982). The evidence for a discontinuous change in chromospheric extent between coronal and

noncoronal giants must be a crucial clue to the mechanism of rapid mass loss

(cf. Mullan and Stencel — this volume).

LINES OF Si II UV 0.01

Multiplet UV 0.01 of Si II which occurs between 2330-2350 A also appears

in emission in the spectra of our observed stars. We have examined line ratios In this multiplet to look for any correlation they may exhibit against

the C II derived densities, and find little evidence for such. Presumably

chromospheres can easily populate the upper level of UV multiplet 1 of Si II

(which is only 1.5 eV above the upper level of UV 0.01), in contrast to populating the upper levels of UV 1 of C II, which are 4.0 eV above UV 0.01.

ACKNOWLEDGMENTS

We wish to thank R. F. Wing, J. L. Linsky and C. Jordan for useful

discussions, along with the staff of the IUE Observatory for their generous

assistance. This work was supported in part by NASA grant No. NAG5-82 for

which we are grateful.

REFERENCES

Ayres, T., Linsky, J., Vaiana, G., Golub, L., and Rosner, R. 1981a, Ap. J.,

250, 293.

Ayres, T., Moos, H., and Linsky, J. 1981b, Ap. J., 248, L137.

Linsky, J., and Haisch, B. 1979, Ap. J., 229. L27.

Schindler, M., Stencel, R., Linsky, J., Basri, G., and Helfand, D. 1982, Ap.

J_., submitted.

Stencel, R., Linsky, J., Jordan, C., Brown, A., Carpenter, K., Wing, R., and

Czyzak, S. 1981, M.N.R.A.S., 196, 47p.

Stencel, R. 1982, in Proceedings. Second Cambridge Workshop on Cool Stars,

ed. M. S. Giampapa, S.A.O. Spec. Report, in press.

245
o

x

in

CM

I

£ 3.2

n—r

/3 Dro G2Ib-E

I 35 min.

-| T 1 1 [-

e Gem G8Ib

270 min.

i—i—i—r

- /3 Gem KOm

1 120 min.

56 Peg KOH + wd

JL 270mm.

cc

Ld

cn

OQ

- € Peg K2Ib

1 200 min.

-• a Ori M2Iob

120 min.

2320. 2325. 2330

WAVELENGT H

2325.

- 1.6

1.2

0.8

0.4

0

- 1.6

16.

12.

8.

0

2330.

Figure 1. The C II intersystem lines in several late-type stars.

246