The first detection of ionized helium in the local ISM - EUVE and IUE spectroscopy of the hot DA white dwarf GD 246

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We present an analysis of the extreme ultraviolet (EUV) spectroscopy of a sample of 10 DA white dwarfs observed by the Extreme Ultraviolet Explorer (EUVE). We have selected white dwarfs cooler than about 50,000 K and with presumably low heavy element abundances. The goal of this study is to determine the fundamental atmospheric parameters, namely the effective temperature and chemical composition, of these stars by fitting their continua with synthetic spectra computed from pure hydrogen LTE/line-blanketed model atmospheres. The question of the presence (or absence) of trace elements is explored by comparing EUV-determined effective temperatures to the one obtained from a fit of hydrogen balmer lines. It is found that the majority of the DA in the sample are consistent with having a pure hydrogen atmosphere. One of the star, MCT0027-634, is another possible example of a HZ 43-type white dwarf, having an effective temperature above 50000 K and a low heavy element abundance, i.e., much lower than predicted by diffusion theory.

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confidence: 75%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A Spectroscopic Survey in the EUV of the “Coolest” Hot DA Stars

Dupuis

Vennes

1996

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“…For the high-Z (above Ne) Li-like lines, the emission in our model comes primarily from 10 6 -8 K. by Linsky et al (1993). Vennes et al (1993) show that the fraction of ionized He might be as high as 25%. The abundances in Table 1 derived from lines between 227 and 504 A would then be somewhat reduced.…”

Section: Elemental Abundances At High Temperaturementioning

confidence: 97%

Dissecting the EUV Spectrum of Capella

Brickhouse

1996

Extreme ultraviolet spectra of Capella, obtained at various orbital phases over the past two years by the EUVE satellite, show strong emission lines from a continuous distribution of temperatures (~ 10 5 -10 7 ' 3 K). In addition to the strong He II A303.8, the spectra are dominated by emission lines of highly ionized iron. Strong lines of Fe IX, XV, XVI, and XVIII-XXIV are used to construct emission measure distributions for the individual pointings, which show several striking features, including a minimum near 10 6 K and a local maximum at 10 6 ' 8 K. Furthermore, intensities of the highest temperature lines (T" > 10 7 K) show variations (factors of 2-3) at different orbital phases, while the lower temperature Fe lines show variations of about 30% or less. The low variability of most of the strong low temperature features motivates a detailed analysis of the summed spectrum. With ~ 280 ks of total exposure time, we have measured over 200 emission features with S/N > 3.0 in the summed spectrum. We report here initial results from the analysis of this spectrum. We can now identify lines of Fe VIII and X-XIV, as well as a number of electron density and abundance diagnostic lines.We also report here the first direct measurement of the continuum flux around ~ 100 A in a cool star atmosphere with EUVE. The continuum flux can be predicted from the emission measure model based on Fe line emission, and demonstrates that the Fe/H abundance ratio is close to the solar photospheric value.

“…In both density cases the charge exchange contribution to the hydrogen ionization balance is still less than 1% of the photoionization caused by e CMa. We can compare this range of helium ionization fraction to the only directly measured ionization fraction of helium observed toward the hot white dwarf GD246 (Vennes et al 1993). Using the EUVE derived spectrum, Vennes et al were able to detect the interstellar He II edge at 228 A as well as the autoionization feature of He I at 206 A giving both the neutral and singly ionized column densities of helium.…”

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confidence: 97%

ϵ Canis Majoris and the Ionization of the Local Cloud

Vallerga

Welsh

1996

Using the EUV (70 -730 A) spectrum of the brightest EUV source, e CMa taken with the Extreme Ultraviolet Explorer Satellite (EUVE) and simple models that extrapolate this spectrum to the Lyman edge at 912 A, we have determined the local interstellar hydrogen photoionization parameter, T, solely from this B2 II star to be 1.1 x 10 -15 s -1 . This figure is a factor of 7 greater than previous estimates of T calculated for all nearby stars combined (Bruhweiler & Cheng 1988). Using measured values of the density and temperature of neutral interstellar hydrogen gas in the Local Cloud, we derive a particle density of ionized hydrogen, n(H + ), and electrons, n e , of 0.015 -0.019 cm -3 assuming ionization equilibrium and a helium ionization fraction of less than 20%. These values correspond to a hydrogen ionization fraction, Xs from 19% to 15%, respectively.