A Direct Measurement of Hydrogen Column Density towards Sirius B by EUVE photometry at 60 nm

The hydrogen column density to Sirius B has yet to be measured directly even though it is the nearest and brightest white dwarf. The fundamental parameters of temperature, radius, and mass of this important white dwarf have remained uncertain, largely due to the difficulty of observing a much fainter white dwarf in close proximity to the brightest star in the sky, Sirius A. Recent spectral measurements of Sirius B in the extreme ultraviolet (EUV), which are free from contamination from Sirius A, have allowed accurate determinations of the temperature and gravity and therefore the radius of the white dwarf (Holberg et al. 1998). However, these results depend critically on the assumed H and He column density towards the source. Holberg et al. rederived the H column using the UV absorption lines of SiII, OI and CII measured with IUE and scaled the results to hydrogen by adopting solar abundances and characteristic depletion factors. Other estimates of the H column have used the measured deuterium column scaled by the D/H ratio determined towards other nearby stars (Bertin et al, 1995), or by using other proxy elements such as MgII (Frisch, 1994). We will present an attempt to derive this neutral H column directly, by measuring the Lyman continuum flux from Sirius B above 50.4 nm using the long wavelength photometer (Scanner C) on the Extreme Ultraviolet Explorer satellite (EUVE). Neutral H is the only element in the ISM that can significantly absorb this continuum at these low column densities. During a five orbit observation on November 30, 1998, Sirius B was detected at a level of 0.022 counts per second in the 60 nm (Tin) bandpass of EUVE. The H column densities derived depend on the source models assumed. We will discuss these dependencies as well as possible systematic experimental errors. Holberg J.B., et al. (1998), ApJ. 497,935 Bertin P., et al. (1995), A&A,302,889 Frisch, P.C. (1994), Science, 265,1423.