IUE observations of neutral hydrogen and deuterium in the local interstellar medium

Adv. Space Res. Vol. 6, No. 2, pp. 87—90, 1986 Printed in Great Britain. All rights reserved.

0273—1177/86 80.00 + .50 Copyright © COSPAR

IUE OBSERVATIONS OF NEUTRAL HYDROGEN AND DEUTERIUM IN THE LOCAL INTERSTELLAR MEDIUM W. B. Landsman,* J. Murthy,~R. C. Henry,** H. W. Moos,** J. L. Linsky,***~A. Vidal-Madjart and C. Gry~ *s.A. S.C. Tech., Goddard Space Flight Center, Greenbelt MD 20771, U.S.A. **Johns Hopkins University, Baltimore,MD 21218, U.S.A. * * *Joint Institutefor Laboratory Astrophysics, University of

Colorado and National Bureau of Standards, Boulder, CO 803090440, U.S.A. flnstitut d’Astrophysique de Paris, 75014 Paris, France .tESA, P.O. Box 54065, 28080 Madrid, Spain ABSTRACT

Small-aperture, high-dispersion IUE spectra have been obtained of seven late-type stars that, in general, confirm previous Copernicus results concerning the distribution of hydrogen and deuterium in the local interstellar medium. In addition, the IUE Ly Alpha spectra of Altair, and of the Alpha Cen components, suggest that multiple velocity components exist in these two directions. INTRODUCTION The Lyman Alpha emission of nearby late-type stars can be used to probe the distribution of neutral hydrogen and deuterium in the local interstellar medium. These data are uniquely capable of studying the neutral gas within 10 pc of the sun, where a strong density gradient is known to exist /1,2/. The derived HI column densities are needed to plan and interpret future observations in the EUV. The cosmologically significant DIM ratio may be derived, in sightlines that are relatively simple and well-understood. Before its demise in 1980, the Copernicus satellite had been used to obtain reasonable quality, high-dispersion (0.05 A) Lyman Alpha spectra of five late-type stars /1,3,4,5,6/. The IUE has several disadvantages for detailed Lyman Alpha emission studies; its spectral resolution (0.1 A) is about half that of Copernicus, non-statistical fixed pattern noise and ITF uncertainties make an error analysis difficult, and the small aperture must generally be used to prevent contamination from geocoronal and interplanetary diffuse Lyman Alpha. Nevertheless, the IUE does have some advantages over Copernicus for this type of analysis, such as more complete spectral coverage, shorter exposure times, and the possibility for certain targets of achieving a higher signal-to-noise. Results that have been obtained with IUE are summarized in the next section. A more complete discussion may be found in the original papers /7,8,9,10/. RESULTS The spectrum of Procyon (F5 IV-V, d = 3.5 pc) shown in Fig. 1 was obtained by the addition of four large aperture, and one small aperture, IUE images. There is excellent overall agreement with the Copernicus data of Anderson et al. /5/, although none of the interstellar parameters ~erived with IUE data are as strongly constrained. The value of D/H = 1.3 (+1.2, -0.5) x 10 should be considered the most reliable toward a late-type star, because of the lack of profile variability, the relatively high signal to noise, the well-defined deuterium feature, and the lack of evidence for multiple components. Archival small-aperture images of Capella (G5 III + GO III, d

=

13.2 pc) were obtained

3at phases 0.25 (Fig. 1) and 0.75. The derived average hydrogen density 0.016 < n(H) (cm ) < 0.033 is good agreement with the range derived by Dupree et al./31 but somewhat lower than the range determined by McClintock et al. /41. A strong lower limit of D/H > 1.8 x 10 was determined for the data at both phases. The IUE data (Fig. 1) of Eps En (Kl V, d = 3.3 pc) show better overall definition than the Copernicus data /4/, and an improved value could be determined of the1heliocentric interstellar bulk velocity along this sightline of v = 11 ±4.4 km s . A small aperture IUE image (Fig. 1) of 11111099 (KOIV +GOV, d 33 pc) was obtained during velocity conjunction to minimize asymmetries caused by emission from both components. The strong dropoff in average §Staff Member, Quantum Physics Division, National Bureau Standards 87

W. B. Landsman et a!.

88

first discovered by Anderson ~nd Weiler /1/ is confirmed, although the actual derived number density, 0.009 < n(H) cm < 0.016, is found to be somewhat higher /6,10/.

HI density

~

~A7~

1215

-

1215.4 1215.8 Wavelength (A)

ERI

~O

1216.2

~,/~LPRO0YON~OYON

1215

-

1215.4 1215.8 Wavelength (A)

HR1099

-

1216.2

—

~:

0~~H1IIflhi,//t\~ 1215.2

Figure 1

1215.5 1215.8 Wavelength (A)

1216.1

______________

1214.5

1215 1215.5 1216 1216.5 Wavelength (A)

1217

TUE spectra (error bars) of Capella, Procyon, Eps En, and HR1099 are shown along with a best-fit model (solid lines) to a single-component, thermally broadened interstellar medium.

The TUE spectrum of Altair (A7 IV, d = 5.1 pc) shown in Fig. 2 was obtained by co-addition of three images, but the signal-to-noise is still very poor due to long-wavelength scattered light. (Lyman Alpha emission from Altair was detected with Copernicus, but only at a 2 sigma level /8/.) The significance of the very broad (0.6 A) saturated interstellar core is still being analyzed. However, a likely explanation may be provided in terms of the multiple velocity components recently discovered in high-resolution Ca Il-K line observations toward Altair /11/. Landsman et al. /7/ compared a composite IUE spectrum of Alpha Cen A (G2V, d = 1.3 pc) with two Copernicus spectra taken in 1976 and 1978. The three spectra show good overall agreement, although the interstellar deuterium feature appeared somewhat stronger in the 1978 Copernicus spectrum. A peculiarity of both the Copernicus and IUE spectra was that a much better fit could be 1obtained in a single component model if the deuterium feature was blueshifted 8 km s from its predicted Isotopic shift from the HI bulk velocity. The most likely explanation for this effect is that multiple interstellar velocity components exist even for the short Alpha Cen sightline. Synthetic multicomponent models show that the velocity centroid for DI is defined by the cooler interstellar component, while the velocity centroid of the highly saturated HI profile is defined by the warmer component. Subsequently, Landsman et al. /9/ obtained an IUE spectrum of the Lyman Alpha emission of Alpha Cen B (K1V), which had never been observed with Copernicus. The excellent agreement between the HI absorption profiles observed toward the two Alpha Cen stars, demonstrated that any stellar self-reversal has little effect on the observed profile1 The Alpha Cen B data was not of sufficient quality to address the reality of the 8 km s bulk velocity shift of the DI feature; however the large derived velocity dispersion, b(H) > 13.8 km 5 , supports the suggestion of multiple velocity components.

LUE Observations of HI and DI in the LISM

89

Altair

1214 1214.5

Figure 2

1215 1215.5 1216 1216.5 Wavelength

Composite TUE spectrum of the Lyman Alpha emission of Altair. A is due to contamination from geocoronal Lyman Alpha.

1217

The gap near 1215.7

FUTURE OBSERVATIONS The two key questions to be addressed by future research are the possible existence of variations in D/H, and the nature of the multiple velocity componen~s. Although all observations of late-type stars are consistent with D/H = 2.0 x 10 , the lower D/H values derived toward some hot stars suggest real variations of D/H in the solar neighborhood /12/. However, the present values of D/H should be considered provisional until the multicomponent nature of the local interstellar medium /13/ is more fully investigated. Further progress will require observations at higher spectral resolution, and toward several additional sightlines. The liES guaranteed time observations ~hould make great strides in first area by reobserving most of the stars discussed here at 10 spectral resolution /14/. Suitable latetype stars for probing new sightlines may be selected by using the TUE archives to obtain low-dispersion Lyman Alpha flux measurements. In the current observing year, we plan to obtain small-aperture, high-dispersion images of the strong Lyman Alpha sources Alpha Tn (F6IV, d = 17.5 pc) and Sig Gem (KOIIT, d = 45 pc). REFERENCES 1.

R. C. Anderson and E. J. Weller, Copernicus observations of interstellar matter in the direction of HR 1099, Astrophys. J. 224, 132 (1978)

2.

F. C. Bruhweiler, Absorption line studies and the distribution of neutral gas in the local interstellar medium, in: Local Interstellar Medium, ed. F. Bruhweiler, Y. Kondo, and B. D. Savage, NASA CP 2345, p. 39 (1984)

3.

A. K. Dupree, S. L. Baliunas, and H. L. Shipman, Deuterium and Hydrogen in the local interstellar medium, Astrophys. J. 218, 316 (1977)

4.

W. McClintock, R. C. Henry, J. L. Linsky, and H. W. Moos, Ultraviolet observations of cool stars. VII. Local interstellar hydrogen and deuterium Lyman alpha, Astrophy. J. 225, 465 (1978)

5.

R. C. Anderson, R. C. Henry, H. W. Moos, and J. L. Linsky, Ultraviolet observations of cool stars VIII. Interstellar Matter toward Procyon, Astrophys. J. 226, 883 (1978)

6.

R. C. Anderson and E. J. Weiler, Copernicus observations of neutral hydrogen and deutenium in the diraction of HR1099, Pub. Astr. Soc. Pac. 91, 431 (1979)

7.

W. B. Landsman, R. C. Henry, H. W. Moos, and J. L. Linsky, Observation of Interstellar Hydrogen and Deutenium toward Alpha Centauri A, Astrophys. J. 285, 801 (1984)

90

W. B. Landsman et at.

8.

W. B. Landsman, R. C. Henry, H. W. Moos, and J. L. Linsky, Observations of interstellar HI toward nearby late-type stars, in: Local Interstellar Medium, ed. F. Bruhweller, Y. Kondo, and B. D. Savage, NASA CP 2345, p. 60 (1984)

9.

W. B. Landsman, J. Murthy, R. C. Henry, H. W. Moos, J. L. Linsky, and J. L. Russell, TUE observations of interstellar hydrogen and deuterium toward Alpha Centauni B, Astrophys. J. 303, 791 (1986)

10.

J. Murthy, R. C. Henry, H. W. Moos, W. Landsman, J. L. Linsky, A. Vidal-Madjar, and C. Gry, TUE observations of hydrogen and deutenium in the local interstellar medium, in preparation.

11.

R. Ferlet, R. Lallement, and A. Vidal-Madjar, The local interstellar medium over 5 Pc the direction of Aiph Aql, Astron. Ap., in press (1986)

12.

A. Vidal-Madjar and C. Gry, Deutenium and the local interstellar cloud(s?), Astron. Ap. 155, 407 (1986)

13.

R. Lallement, A. Vidal-Madjar, and R. Ferlet, Multi-component velocity structure of the local interstellar medium, Astron. Ap., in press (1986)

14.

J. L. Linsky, W. B. Landsman, B. D. Savage, S. R. Heap, A. M. Smith, and J. C. Brandt, this issue.

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