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19 93MNRAS.2 60. .631B Mon. Not. R. Astron. Soc. 260, 631

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19 93MNRAS.2 60. .631B Mon. Not. R. Astron. Soc. 260, 631 Mon. Not. R. Astron. Soc. 260, 631-634 (1993) .631B 60. ROSAT EUV and soft X-ray studies of atmospheric composition and structure in Gl 91-B2B 93MNRAS.2 19 M. A. Barstow,1 T. A. Fleming,2 D.S. Finley,3 D. Koester4 and C. J. Diamond5 1 Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LEI 7RH 2 Max-Planck-Institutßr Extraterrestriche Physik, Giessenbachstrasse, Garching bei München, Germany 3 Center for EUV Astrophysics, University of California, Berkeley, California, USA 4 Department of Physics and Astronomy, Louisiana State University, Baton Rouge 70803-4001, Louisiana, USA 5 School of Physics and Space Research, University of Birmingham, Edgbaston, Birmingham BI5 2TT Accepted 1992 July 28. Received 1992 July 15 ABSTRACT Previous studies of the hot DA white dwarf G191-B2B have been unable to deter- mine whether the observed soft X-ray and EUV opacity arises from a stratified hydrogen and helium atmosphere or from the presence of trace metals in the photo- sphere. New EUV and soft X-ray photometry of this star, made with the ROSAT observatory, when analysed in conjunction with the earlier data, shows that the strati- fied models cannot account for the observed fluxes. Consequently, we conclude that trace metals must be a substantial source of opacity in the photosphere of G191-B2B. Key words: stars: abundances - stars: individual: G191-B2B - white dwarfs - X-rays: stars. 1 INTRODUCTION optical and ultraviolet (UV ) spectra. Furthermore, Vennes et al. (1988) have ruled out homogeneous models on theore- G191-B2B has been one the most widely studied of the tical grounds. white dwarfs since its discovery (Giclas, Burnham & Thomas We have now obtained new soft X-ray and EUV observa- 1966). Classified as a DA star from the absence of helium tions of G191-B2B using the ROSAT observatory. The features in its optical spectrum, with an effective temperature narrower bandpasses of the ROSAT instruments, compared around 60000 K it is one of the hottest members of the to EXOSAT, allow the observed count rates to place much group. However, the presence in the WE high-dispersion better limits on the allowed atmospheric parameters. We spectra of highly ionized species of C, N and Si indicates that have performed an analysis which includes our ROSAT data, the photospheric composition is not as simple as might be the long wavelength bandpass data from EXOSAT, an first supposed. Comparison of an earlier photospheric red- optical spectrum and new measurements of the H i and He i shift measurement (Trimble & Greenstein 1972) with the columns made with the Hopkins Ultraviolet Telescope velocities of these ultraviolet lines had suggested that the (Kimble et al. 1992). We are able to reject a stratified H upon material might be circumstellar (Bruhweiler & Kondo 1981), He model for the photosphere and show that additional but the velocity of the NLTE emission core in the Ha line opacity sources in the form of trace metals are required. indicates a photospheric origin for the metal features (Reid & Wegner 1988). 2 /ÏOSAJOBSERVATIONS OF G191-B2B A further clue to the composition of G191-B2B comes from the soft X-ray/EUV observations made by EXO- The ROSAT soft X-ray and EUV all-sky survey was con- SAT observatory which reveal a significant flux deficit below ducted from 1990 July to 1991 January. The data we present 300 Â compared to that expected if the star had a pure H here derive from this phase of the mission. ROSAT carries atmosphere (Paerels & Heise 1989). Several authors have on board two coaligned imaging instruments. These have analysed these observations using either uniform H/He or been well described elsewhere, but those details that are stratified (H on top of He) models (e.g. Jordan et al. 1987; relevant to this discussion are briefly reiterated here. The Koester 1989; Paerels & Heise 1989). Either interpretation X-ray telescope had a positional sensitive proportional is a reasonable explanation of the EXOSAT data, but the counter (PSPC) as a focal plane detector which covered the homogeneous model requires a He abundance that is not energy range «0.1-2.4 keV (5.2-100 Â; Pfefferman et al. consistent with the observed absence of He lines in the 1986). Although the PSPC has modest energy resolution, © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 632 M. A. Barstow et al. .631B in this study we use just the integrated count rate in the band 60. 3 OPTICAL OBSERVATIONS « 0.1-0.4 keV (25-100 À). The EUV waveband is spanned by the Wide Field Camera (Wells et al. 1990). For survey We include in our analysis a very high quality optical spec- observations two broadband filters were employed - SI trum of G191-B2B, with a signal-to-noise ratio of 300 and a covering the range 90-200 eV (60-140 À) and S2 the range resolution of 8 À obtained by DSF using the 3-m telescope at 93MNRAS.2 60-110 eV ( 112-200 Ä). Two additional filters are available the Lick Observatory. The gravity of the star was initially 19 for the post-survey part of the mission, extending the determined by fitting the hydrogen Balmer ß, ô and y lines coverage to lower energies - PI spans 56-83 eV (150-220 with pure-H model atmospheres (e.g. Kidder et al. 1992), Ä) and P2 spans 17-24 eV (500-730 Â). Fig. 1 shows the yielding log g= 7.5 ±0.07 (la error). A second fit was per- effective areas of all spectral bands except P2. For com- formed for temperature T and gravity using a stratified 15 parison the responses of the prime thin lexan (3Lx) and model with an assumed H layer mass (raH) of 10" M0 also aluminium/parylene (AIP) EXOSAT filters are also shown. It giving a value 7.5 for logg. Consequently, in subsequent can be seen that the ROSAT instruments yield about three analyses logg was held fixed at 7.5 with T and mH treated as times better spectral resolution than that available with free parameters. EXOSAT. To determine the ranges of T and raH allowed by the opti- Although observations with the PI and P2 filters have cal spectrum we simultaneously fitted H/?, Hó and Hy as been scheduled in the ROSAT pointed observation pro- well as the region around the (undetected) He n 4686-Â line. gramme, they have yet to be executed successfully. In Fig. 1 a The 99 per cent confidence interval obtained for T and raH comparison of the EXOSAT A1P and ROSAT WFC PI is shown in Fig. 2. The result is consistent with a pure H filter responses shows that they have similar bandpasses. atmosphere at «60 000 K, or with a range of T down to Therefore, we include the observed A1P count rate in the somewhat lower than 56 000 K and log mH as low as - 14.2. data used here, published in the analysis of Paerels & Heise Note that as raH is decreased, a lower temperature is (1989). Table 1 summarizes the observed count rates in each required to achieve a good fit to the Balmer lines. The band. The errors quoted are purely statistical. A small drift stringent lower limit placed on the allowed H layer mass is a in the WFC response was observed throughout the sky result of including the 4684-Â line in the analysis. survey. At the date when G191-B2B was observed (MJD 48 125), this amounted to a decrease in efficiency of 5 and 4 EUV/X-RAY DATA ANALYSIS 3 per cent in SI and S2 filters respectively and has been taken into account in the subsequent analysis. Our EUV/X-ray analysis was performed using both homo- geneous and stratified H +He atmospheres. The homo- geneous models were calculated by one of us (MAB) using the NTLE code tlusty of Hubeny (1988). They do not include full line blanketing. A detailed study, which we will report elsewhere, shows that at temperatures above 30 000 K the emergent EUV and X-ray fluxes are not signifi- cantly affected by the level of blanketing included in the model calculations. The stratified models were computed by DK and are fully blanketed (Jordan & Koester 1986; Koe- sterl991). Fits to these models were performed using a suite of idl routines written by DSF to fold model atmospheres through each instrument response and interstellar absorption. Pre- dicted count rates are compared with the observed values in each filter and instrument. Ax2 statistic is then calculated which determines the quality of agreement between data and predictions, taking into account the uncertainties in the measured count rates. The program searches through the range of all free parameters to determine the best fit and associated errors. A valid analysis requires that the number Figure 1. Effective area as a function of photon energy for the of degrees of freedom v, that is the number of data points ROSAT PSPC, the WFC SI, S2 and PI filters and the EXOSAT minus the number of free parameters, must be greater than thin lexan and aluminium/parylene filters. or equal to one. There are five variables that determine the emergent EUV and X-ray fluxes for each of our model struc- tures - effective temperature, interstellar column, surface Table 1. Observed G191-B2B count rates. gravity, helium opacity and a distance/radius-related normal- Instrument -1- filter Count rate (s 1) ization constant. In the homogeneous models the He opacity EXOSAT A1P 23.1 ±0.1 is defined by fractional abundance with respect to H and in the stratified case the appropriate free parameter is the H WFC S2 2.77 ±0.04 layer mass.
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