----- Submission 1 (Fri Nov 14 20:16:17 GMT 2003) ----- Verifying the White Dwarf Mass-Radius Relation with Sirius B We propose timely STIS observations of Sirius B, that will provide fundamental astrophysical data on this very important, but difficult to study, white dwarf. High quality spectra of the Balmer line region will provide accurate, modern measurements of its gravitational redshift, effective temperature, surface gravity and absolute flux. From these measurements, precise determinations of the spectroscopic mass and radius can be made, that will approach a 1% uncertainty. At this level, it will be possible to: 1) directly compare the spectroscopic and astrometric masses at a comparable level of accuracy, 2) precisely test the theoretical mass-radius relation for a 1 solar mass white dwarf, 3) distinguish between thick and thin H layer masses and 4) confirm the widely used method of estimating white dwarf masses from the Balmer lines. The only existing Balmer line observations of Sirius B are those of Greenstein et al. (1971) from the days of photographic plates. The increasing separation of the components now permits STIS observations at a time when new astrometric and far UV observations are available.
We require STIS observations of Sirius B to obtain a complete Balmer series spectrum, including Ha. The spectra will be obtained using the combined G430L and G750M gratings with the CCD detector. The G430L grating provides complete coverage of the Balmer line series from H?, yielding a resolution of 5.4?, well-matched to the requirement for determination of accurate Teff and log g (see Bergeron et al. 1992). The current estimate of the gravitational redshift is ~90km s-1, hence the G430L grating does not yield adequate resolution (170km s-1 per pixel) for a sufficiently accurate radial velocity measurement. We will obtain a single G750M exposure, centered on Ha for this purpose. The 0.56?/pixel spectral resolution of the grating translates into 30km/s in velocity space and cross-correlation techniques can be used to improve the accuracy beyond this by a factor 10. Since Sirius has a very close, bright main sequence companion (!), our observing strategy for STIS needs to both minimise the possible scattered light from the primary while evaluating the actual level of contamination. We have evaluated the likely level of the scattered light component and find that the white dwarf is ~2 magnitudes brighter than the PSF wing from the primary star. We will place a long slit perpendicular to the line joining the two stars, so that the scattered light component can be determined and subtracted directly. HOT STARS 12 false false false false SUPPORTED STAR DA Position and proper motion have been determined from WFPC2 images of Sirius B taken in October 2001, 2002, and 2003, and have been corrected for annual parallax. Note that this results in an "effective" p.m. for Sirius B that includes its orbital motion around Sirius A, and this p.m. will not be the same as the p.m. of the center of mass of the binary. Alpha-CMa-B J2000 06 45 8.8 -16 42 52.1 J2000 STIS/CCD ACCUM Spectroscopic No STIS/CCD ACCUM /var/tmp/OPVisit16212.xml The ORIENTs are used to place Sirius B halfway between the diffraction spikes from Sirius A. (Tic-Visit :proposal-id 9762 :visit-id "01" :proposal-version -1 :trans-version "20030713" :date "14-Nov-2003 14:43:16.0" :trans-errors '(:WARNINGS 1 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 294880 :tdrss-contact-duration 0 ) (Tic-Obset :proposal-id 9762 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 9762 :visit-id "01" :orbit-number 1 :pcs-time 365 :si-vis-utilization 2829 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3207 :pcs-gap 0 ) true /klingsor/data1/bond/hstcycle12/siriusb/OP-cache/9762-v01.xml