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HST Absolute Spectrophotometry of Vega from the Far-UV to the IR
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HST ABSOLUTE SPECTROPHOTOMETRY OF VEGA FROM
THE FAR-UV TO THE IR
R. C. Bohlin R. L. Gilliland
Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD 21218
bohlin@, gillil@
To be published in the 4 June 2004 issue of the Astronomical Journal
Received: 2 February 2004; Accepted: 19 February 2004
ABSTRACT
The Space Telescope Imaging Spectrograph has measured the absolute flux for Vega from
0.17–1.01 μm on the HST White Dwarf flux scale. These data are saturated by up to a
factor of 80 overexposure but retain linearity to a precision of 0.2%, because the charge bleeds
along the columns and is recovered during readout of the CCD. The S/N per pixel exceeds
1000, and the resolution R is about 500. A V magnitude of 0.026±0.008 is established for
Vega; and the absolute flux level is 3.46 × 10?9 erg cm?2 s?1 at 5556 A?. In the regions of
Balmer and Paschen lines, the STIS equivalent widths differ from the pioneering work of
Hayes in 1985 but do agree with predictions of a Kurucz model atmosphere, so that the
STIS flux distribution is preferred to that of Hayes. Over the full wavelength range, the
model atmosphere calculation shows excellent agreement with the STIS flux distribution
and is used to extrapolate predicted fluxes into the IR region. However, the IR fluxes are
2% low with respect to the standard Vega model of Cohen. IUE data provide the extension
of the measured STIS flux distribution from 0.17 down to 0.12 μm. The STIS relative flux
calibration is based on model atmosphere calculations of pure hydrogen WDs, while the
Hayes flux calibration is based on the physics of laboratory lamps and black body ovens.
The agreement to 1% of these two independent methods for determining the relative stellar
flux distributions suggests that both methods may be correct from 0.5–0.8 μm and adds
confidence to claims that the fluxes relative to 5500 A? are determined to better than 4% by
the pure h
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