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Session 10 - Interstellar Medium and Star Formation.
Display session, Monday, June 08
Atlas Ballroom,
We report both ROSAT observations and ground-based 10.8 \mum imaging of the wide binary HR 4796, which consists of a main sequence A-type star with a large amount of circumstellar dust, HR 4796A, and, at a separation of 7\farcs7, a pre-main sequence M-type companion, HR 4796B. From the ROSAT data, we find that the X-ray emission is centered on HR 4796B, with L_X/L_Bol \approx 3 \times 10^-4. The 10.8 \mum flux, which arises from HR 4796A, displays an excess over the photospheric emission of 0.08 \pm 0.02 Jy, a result consistent with the previous characterization of the emission from the circumstellar dust in the wavelength range 12 \mum \leq \lambda \leq 100 \mum as a 110 K black body.
The Hipparcos data can be used to argue that the three main sequence A type stars in the Bright Star Catalog with L_IR/L_Bol > 10^-3 (HR 4796A, \beta Pic and 49 Cet) all have low luminosities for their colors. We suggest that very approximately 20% of all A-type stars pass through an early phase where they possess an amount of circumstellar dust comparable to that found around HR 4796A or \beta Pic.
In order to explain the result that the grain emission can be approximated by a 110 K black body, we propose that the circumstellar grains are largely composed of ice particles with a typical radius near 100 \mum and that they sublimate rapidly when they are closer than \sim35 AU to HR 4769A. This swarm of ice particles may be a proto-cometary cloud.
