Tests of Extreme Physics in Very Cool White Dwarfs
A program of low resolution Near UV STIS spectroscopy is proposed that will critically test new improved models of photospheric opacity in cool (Teff < 6000 K) DA and DC white dwarf stars. Such stars possess either weak or no Balmer lines from which spectroscopic information such as temperature, surface gravity and photospheric composition can be determined. These critical parameters must be estimated from fits to the overall Spectral Energy Distribution (SED), a method that is inherently less sensitive than spectroscopic methods. Our new models implement a detailed treatment of collisional perturbations to H atoms in cool dense WD photospheres that naturally account for an acknowledged missing opacity, due the red wing of the H I Lyman alpha transition. This in turn modifies the SED in the wavelength region from 2500 A to 4000 A. A full validation of these models over a significant range of temperature and gravity will greatly improve the analysis of the cool white dwarfs and provide new insights into atmospheric compositions, cooling ages, and luminosities of this important component of the white dwarf population.
This proposal will obtain single STIS observations of seven of our targets and two observations of the eighth and weakest target. All observations will use the STIS NUV MAMA with the G230L grating and the 52x2 slit since our observations are primarily spectrophotometric. All targets are cool white dwarf stars with spectral energy distributions that rapidly fall towards short wavelengths. We have simulated expected count rates and S/N for each target, using an appropriate model atmosphere and the STIS Spectroscopic ETC. Maximum signal levels are expected between 2800 A and 3000 A, with useful signals extending as short as 2400 A for the strongest targets. Our observations will be most sensitive to the NUV MAMA dark count.
COOL STARS
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This object was generated by the targetselector and retrieved from the SIMBAD database.
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