Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

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Boron in the Very Metal-Poor Star BD-13^o 3442

L.M. Rebull, D.K. Duncan
Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637 USA

A.M. Boesgaard
Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 USA

Constantine P. Deliyannis
Department of Astronomy, Yale University, New Haven, CT 06520 USA

L.M. Hobbs
Yerkes Observatory, University of Chicago, Williams Bay, WI 53191 USA

J.R. King
Department of Astronomy, University of Texas, Austin, TX 78712 USA

S. Ryan
Anglo-Australian Observatory, Sydney, Australia

Abstract:

The Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) has been used to obtain a spectrum of the boron 2500Å region of BD-13^o 3442. At a metallicity of [Fe/H], this is the most metal-poor star ever observed for B, and 30 hrs of spacecraft time (8.25 hours exposure time) resulted in a definite detection. Spectrum synthesis using the latest Kurucz model atmospheres yields an abundance of (B)0.25. This value fits a linear extrapolation of the B vs. [Fe/H] relation found for 11 halo and disk stars of less extreme metallicity (Duncan et al. 1996b), arguing against an inhomogeneous Big Bang or direct production in supernovae as origins of B. The slope of the B vs. [Fe/H] fit is almost exactly 1, which could be evidence of light element production by cosmic ray spallation of C,N,O nuclei onto protons, rather than the reverse, an alteration of the theory of light element production which has been accepted for many years.

Introduction

The evolution of Li, Be, and B is a powerful discriminant between different models of the chemical and dynamical evolution of the Galaxy. The theory of light element production which has been accepted for many years (e.g., Reeves & Meyer 1978) suggests production by cosmic ray (CR) spallation of protons onto CNO nuclei in the interstellar medium. Thus, the light element abundances should depend on the intensity and shape of the cosmic ray spectrum, which in turn depends on the supernova and massive star formation rates. They also could depend on the rise of the (progenitor) CNO abundances and the decline of the gas mass fraction, rates of infall and outflow, etc. The present observations were sought to constrain these models.

Data

The Goddard High-Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) was used with the G270M grating to obtain a spectrum in the B I region (2500Å). Data was collected for a total of 8.25 hours exposure time (30 hours spacecraft time) in six separate ``visits'' over several months from December 1994 to June 1995. The S/N of the final spectrum was 100, limited only by photon statistics; resolution was 24,000 (0.025Å/pixel).

Analysis

Boron abundances were determined via spectrum synthesis using the Synthe program distributed by Kurucz (1993) on CD-ROM, modified to run on Unix SPARCstations by Steve Allen (UC, Santa Cruz). We used the line list of Duncan et al. (1996a), which consists almost entirely of laboratory-measured lines, and which fits both the Hyades giants and metal-poor stars.

Considerable care was spent in determining the error for each of the B determinations. Sources of random error we considered included temperature, metallicity, continuum placement, and photon statistics in the points defining the line itself. In more metal-poor stars, the continuum is easier to define, but the B line is weaker. At disk metallicity, continuum errors are larger but make less difference since the line is deep. Suggestions of NLTE effects which would increase B abundances will be tested observationally in 1995 by HST observations of BI and BII.

Figure 1 shows a comparison of BD-13^o 3442, BD3^o 740, and HD 140283, the three most metal-poor stars observed in the B region. It is clear that BD-13^o 3442 is the most metal-poor star of the three. (BD3^o 740 was observed by Duncan et al. 1996b; HD 140283 was observed by Duncan, Lambert, & Lemke 1992.)

 
Figure: Comparison of BD-13 ([Fe/H]=), BD-3 ([Fe/H]=), and HD 140283 ([Fe/H]=) in the boron region. BD-13 is clearly the most metal-poor and HD 140283 is clearly the most metal-rich.

Published values of stellar parameters for BD-13^o 3442 are [Fe/H] = -2.9 to -3.14, = 3.6 to 3.8, and = 6100 to 6300. Our final best-fit was achieved with a model of [Fe/H] = -2.96, = 3.0, and = 6100.

Considerable time was spent in determining errors; the results of this analysis appear in the table. The spectrum synthesis fit is shown in Figure 2.

 
Figure: The best-fit value for [B/H] was ; boron abundances also shown here are [B/H] and [B/H].

Discussion

When the data of BD-13^o 3442 is combined with that of Duncan et al. (1996; this volume, p.), a straight line of slope 0.98 provides an excellent fit to the data. This may indicate a mechanism in which B and Be are produced by spallation of C,N, and O onto protons either near supernovae, as suggested by Duncan, Lambert, & Lemke (1992), or in winds from massive stars in star-forming regions (Casse, Lehoucq, & Vangioni-Flam, 1995), rather than primarily from CR proton spallation onto C,N, and O nuclei in the general interstellar medium. More detailed comparison with such models is being carried out.

Acknowledgments:

This research was based on observations obtained with the NASA/ESA Hubble Space Telescope through the Space Telescope Science Institute, which is operated by the AURA, Inc., under NASA contract NAS5-26555.

References:

Casse, M., Lehoucq, R., & Vangioni-Flam, E. 1995, Nature, 373, 318

Duncan, D.K., Lambert, D.L., & Lemke, M. 1992, ApJ, 401, 584

Duncan, D., Peterson, R., Thorburn, J., Pinsonneault, M., & Deliyannis, C. 1996a, ApJ, in press

Duncan, D., Primas, F., Coble, K.A., Rebull, L.M., Boesgaard, A.M., Deliyannis, C.P., Hobbs, L.M., King, J.R., & Ryan, S. 1996b, in Science with the Hubble Space Telescope - II, eds. Benvenuti, Macchetto, & Schreier, this volume, p.

Kurucz, R.L. 1993, CDROM # 18

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