HST/GHRS Observations of $\beta$ Pictoris

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Session 48 -- Young Stars
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

[48.12] HST/GHRS Observations of $\beta$ Pictoris

S.R. Heap (GSFC/NASA), T. Lanz (USRA/GSFC), I. Hubeny (USRA/GSFC), D. Lindler (ACC)

In September 1994, we obtained HST/GHRS spectra and maps of $\beta$ Pic (HD 39060) and its surrounding disk. In this paper, we report our preliminary findings concerning the star and circumstellar gas disk.

Both Str\"{o}mgren photometry and Geneva photometry (as listed by SIMBAD) yield the following parameters for the star: \begin{itemize} \item T$_{\rm eff}$=8200 $\pm$ 50 K \item log g = 4.25 $\pm$ 0.05 \item $[Fe/H]$ = -0.1 $\pm$ 0.2 \end{itemize} To refine the estimated metallicity, we have compared the GHRS ultraviolet spectra with model spectra. Our model of $\beta$ Pic incorporates both a LTE stellar atmosphere plus a circumstellar gas disk, treated in a first approximation as a homogeneous, absorbing slab at T=5,000 K with a column density, N$_{\rm H}$=2x10$^{19}$ cm$^{-2}$. We confirm that the star has a near-solar metallicity.

We have compared the derived stellar parameters with evolutionary models for Z=0.017. For reference, a ZAMS star having a T$_{\rm eff}$=8,200 K has a mass, $M \approx$1.72 $M_\odot$, and a gravity, log g= 4.37. The somewhat low surface gravity of $\beta$ Pic suggests that the star has evolved slightly off the ZAMS or is still in the pre-main sequence phase. In either case, it should be somewhat more luminous than a 1.72$M_\odot$ ZAMS star. However, its absolute magnitude, M$_{\rm V}$= 2.70 (for a distance of 17 pc) is 0.21 mag fainter than a $M$=1.72 $M_\odot$ star on the ZAMS.

We therefore conclude that (1) the dust disk must diminish light from the star, and (2) it is entirely possible that $\beta$ Pic is a very young star. Both conclusions raise problems for interpreting observations of the dust disk (c.f. Paresce, 1991, A\&A, 247, L25).

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