Hubble Space Telescope spectroscopy of the Balmer lines in Sirius B

Sirius B is the nearest and brightest of all white dwarfs, but it is very difficult to observe at visible wavelengths due to the overwhelming scattered light contribution from Sirius A. However, from space we can take advantage of the superb spatial resolution of the Hubble Space Telescope (HST) to resolve the A and B components. Since the closest approach in 1993, the separation between the two stars has become increasingly favourable and we have recently been able to obtain a spectrum of the complete Balmer line series for Sirius B using the HST Space Telescope Imaging Spectrograph (STIS). The quality of the STIS spectra greatly exceeds that of previous ground-based spectra, and can be used to provide an important determination of the stellar temperature (T_eff = 25 193 K) and gravity (log g = 8.556). In addition, we have obtained a new, more accurate, gravitational redshift of 80.42 ± 4.83 km s^-1 for Sirius B. Combining these results with the photometric data and the Hipparcos parallax, we obtain new determinations of the stellar mass for comparison with the theoretical mass-radius relation. However, there are some disparities between the results obtained independently from log g and the gravitational redshift which may arise from flux losses in the narrow 50 × 0.2 arcsec² slit. Combining our measurements of T_eff and log g with the Wood evolutionary mass-radius relation, we obtain a best estimate for the white dwarf mass of 0.978 M_⊙. Within the overall uncertainties, this is in agreement with a mass of 1.02 M_⊙ obtained by matching our new gravitational redshift to the theoretical mass-radius relation.