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1988Apj. . .335. .953R the Astrophysical Journal, 335:953-961 .953R The Astrophysical Journal, 335:953-961,1988 December 15 © 1988. The American Astronomical Society. All rights reserved. Printed in U.S.A. .335. 1988ApJ. DETECTION OF H-ALPHA EMISSION IN THE HOT WHITE DWARF G191-B2B1 Neill Reid Palomar Observatory, California Institute of Technology AND Gary Wegner Dartmouth College Received 1988 March 16; accepted 1988 May 27 ABSTRACT We have obtained high-resolution spectra of G191-B2B, the hottest known DA white dwarf, which reveal emission in the core of the Ha line. Our observations show little variation in the line profile over a period of four days, ruling out line-doubling in a close binary as an explanation. The observed emission cannot be due to a nearby red dwarf companion, while the absence of any spatially extended emission argues against either a planetary nebula remnant or local ionization of the interstellar medium. Our determination of the systemic velocity, using the companion red dwarf G191-B2A, is 5 ± 2 km s-1 and shows that both the Ha emission and the high-excitation species observed in the ultraviolet are redshifted by 19 ± 3 km s_1, suggesting a photospheric origin. The low redshift implies a mass of 0.45 M0 for this hot white dwarf, although the uncer- tainties in the effective temperature and parallax permit masses in the (1 o) range 0.29 to 0.60 M0. Subject headings: stars: emission-line — stars: individual (G191-B2B) — stars: white dwarfs I. INTRODUCTION 5 ± 5 km s_ 1 radial velocity of the interstellar lower ionization -1 G191-B2B (or EG 247) is one of the many stars discovered lines and with the 34 km s velocity ascribed to G191-B2B during the Lowell proper motion survey (Giclas, Burnham, (Trimble and Greenstein 1972). and Thomas 1972) which, although having a proper motion Bruhweiler and Kondo (1981, 1982, 1983) have proposed below the main survey limit, was deemed sufficiently unusual that radiative acceleration in the white dwarf photosphere to be cataloged, in this case as a blue common proper motion leads to transport of metals to the surface where they are companion of a main-sequence K dwarf. Subsequent observa- expelled as a low-velocity wind and that the observed absorp- tions by Eggen & Greenstein (1967) and Greenstein (1969) tion lines have their origin in an expanding shell. Theoretical showed that this star is among the hottest DA white dwarfs analysis by Fontaine and Michaud (1979) and by Yauclair, known. Analyzing broadband photometry, Koester, Schulz Vauclair and Greenstein (1979) suggests that the radiative and Weidemann (1979) determined the effective temperature as force is sufficient to support traces of C, N, and O in hydrogen- 56,800 ± 3300 K and calculated log g as 5.95, although they rich envelopes, while higher abundances could be expected in note that this result is based on extrapolating from their helium-rich atmospheres. Expelling the matter from the star in models to higher temperatures. The more recent acquisition of a stellar wind is a more difficult problem. However, as is dis- cussed further in § III, we have redetermined the heliocentric observations at shorter wavelengths from satellite observa- -1 tories leads to an upward revision in the temperature estimate, velocity of G191-B2A as 5 ± 2 km s and find that the highly ionized lines in the ultraviolet spectrum of G191-B2B are red- with analysis of the Lya profile giving Teff ~ 62,250 ± 3520 K (Holberg, Wesemael, and Basile 1986), and EX OS AT EUV shifted with respect to the systemic velocity, and probably have their origin in the stellar photosphere. Dupree and Raymond observations indicating Teff ~ 65,000 K (Jordan et al 1987). Holberg et al. calculate the gravity as log g = 7.55 ± 0.35, (1983) suggested that in the case of Feige 24, where there are indentifying the star as a white dwarf rather than a hot sub- high-velocity lines well separated from the interstellar lines, the 6 metals could be accreted from material expelled from the M dwarf. As such the cooling time is ic < 5 x 10 yr (Koester and Schoenberner 1986), and thus the star has only recently dwarf companion. Sion and Starrfield (1984) have developed evolved through the planetary nebula stage. this hypothesis suggesting that the lines originate in a stellar With a relatively bright apparent magnitude, G191-B2B is wind from the M dwarf. G191-B2B, however, lacks a close accessible to high-resolution observations and these have companion. Finally, Dupree and Raymond (1983) have sug- revealed several peculiarities apart from those discussed in this gested that the high-excitation lines in G191-B2B are due to paper. High-resolution IUE data show the presence of weak, the hot white dwarf ionizing the local interstellar medium. narrow absorption lines due to N v, C iv, and Si iv (Bruhweiler It is possible that these metals also play a role in explaining and Kondo 1981). Similar high-excitation lines subsequently the soft X-ray spectrum of G191-B2B. Petre, Shipman, and were found in several other DA white dwarfs, notably Feige 24 Cañizares (1986) have shown that pure hydrogen DA white (Dupree and Raymond 1982). In G191-B2B these features have dwarfs are strong X-ray emitters to the extent that all such a radial velocity of 18 ± 4.1 km s-1 as compared with the stars with Teff > 30,000 K and d < 100 pc were detectable by the Einstein Observatory HRI instrument. In fact few stars 1 Based on observations obtained with the 60 inch (1.5 m) telescope at among the known hot white dwarfs were detected, implying Palomar Observatory which is jointly owned by the California Institue of the presence of absorption at 2 < 200 Â. The absorber is Technology and the Carnegie Institution of Washington. usually assumed to be helium but, as Jordan et al. (1987) point 953 © American Astronomical Society • Provided by the NASA Astrophysics Data System .953R 954 REID AND WEGNER Vol. 335 .335. out, Si iv, C iv, and N v could also partially contribute. Feige The echelle spectrograph mounted on the 60 inch telescope 24, which also exhibits high-excitation lines in the ultraviolet, is described in detail by McCarthy (1985). The use of a prism has a soft EUV spectrum resembling that of G191-B2B. The cross disperser gives a possible wavelength coverage of from helium abundance in these stars can be derived from model ~ 3200 to ~ 8000 Â, but the response of the 800 x 800 format 1988ApJ. atmosphere analysis of EUV and X-ray data, and Jordan et al TI CCD detector leads to an effective short wavelength limit of derive a value of He/H ~ 5 x 10-3. Although more than a ~4000 Â. The instrumental resolution is ~ 20,000 (2 pixels factor of 10 higher than HZ 43 (Teff - 57,000 K), this abun- FWHM), but our observations were obtained using a slit of dance is marginally consistent with the absence of He n lines in width 2", giving a resolution, measured from He-Ar wave- the optical spectrum. In addition, Vennes et al (1988) have length calibration spectra, of 3.5 pixels, or ~0.55 Â, at Ha. The derived a similar temperature and interpret G191-B2B as echelle format obviously does not permit long-slit observa- having a thin outer layer of hydrogen. tions. However, the order separation is sufficient to allow the We have recently obtained high-resolution optical spectros- use of slit lengths up to 6" and the 2" x 5" slit used does copy of G191-B2B which show that this star exhibits narrow provide some spatial information. emission in the core of the Ha line. We have also observed the Our data reduction was carried out using the FIGARO companion red dwarf and derive a radial velocity of ~5 km package on DEIMOS, the astronomy department VAX 11/780 s“1, rather than the 34 km s~1 published by Trimble and at Caltech, and on the microvax computer at the Palomar 60 Greenstein (1972). The following section presents our observa- inch telescope. The echelle optical design leads to curved spec- tions and we discuss their interpretation and the implications tral orders on the detector. Thus the first stage in the reduction in §111. involves straightening the orders to permit their easy extrac- II. OBSERVATIONS tion. These correction were made using the s-distortion correc- tion routines in FIGARO, with a bright star (in this case the B2 a) Data Reduction subgiant BS 8733) being used to define a template, a two- Our initial observations of G191-B2B (and of the compan- dimensional polynomial calculated and the data rebinned. The ion K dwarf) were obtained on the Hale 200 inch (5.1 m) tele- calibrating spectrum was obtained while BS 8733 was close to scope as part of a larger survey of red dwarf-white dwarf pairs. the zenith to match the G191-B2 observations. Our spectra, with a resolution of ~1.3 Â, appeared to show Once the spectra were straightened, we reduced the CCD structure in the core of the Ha line. We therefore reobserved data in the standard manner, subtracting the bias level using this star at higher resolution using the echelle spectrograph on the “ erase line ” overscan obtained with each data frame. The the Palomar 60 inch (1.5 m) telescope. The latter spectra con- order separation decreases with increasing wavelength, but the sisted of 2400 s exposures obtained on 1987 November 27 and individual orders are still well separated at Ha, as Figure 1 28 and December 1. On each occasion G191-B2B was at an shows.
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