The atmosphere of Sirius B. II - Extreme-ultraviolet observations

Details The atmosphere of Sirius B. II - Extreme-ultraviolet observations The atmosphere of Sirius B. II - Extreme-ultraviolet observations

Apr 1977

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977apj...213l..25s&link_type=abstract

Astrophysical Journal, Part 2 - Letters to the Editor, vol. 213, Apr. 1, 1977, p. L25-L28. Research supported by the University

Astronomy and Astrophysics

Astrophysics

19

Far Ultraviolet Radiation, Spaceborne Telescopes, Stellar Atmospheres, White Dwarf Stars, X Ray Sources, Apollo Soyuz Test Project, Atmospheric Models, Grazing Incidence Telescopes, Spectrum Analysis, Stellar Models, Stellar Temperature

Scientific paper

The letter reports an observation of Sirius in the extreme-ultraviolet (100-1000 A) band, using a grazing-incidence telescope flown aboard the Apollo-Soyuz mission. No positive flux is detected; under the arbitrary assumption of flat incident spectrum, an upper limit to the flux in the 170-620 A band is 5 billion erg/sq cm per sec. A detailed model-atmosphere analysis, when combined with the EUV limits, places severe constraints on models which attribute the previously reported soft X-ray (44-60 A) flux to thermal radiation from deep layers of the atmosphere of the white dwarf Sirius B. EUV radiation should be detectable from Sirius B just below the sensitivity threshold of the current data, or a thermal origin for the X-ray flux is untenable. If the X-ray flux is thermal, the present results provide extremely sensitive constraints on the temperature and helium abundance of Sirius B: the white dwarf has an effective temperature of 32,000 K to 32,500 K and a helium abundance (relative to hydrogen) of 0.0001 to 0.0002.

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