A Protocometary Cloud around HR 4796A?

Abstract

We report both ROSAT observations and ground-based 10.8 μm 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 77, 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 ≈ 3 × 10^-4. The 10.8 μm flux, which arises from HR 4796A, displays an excess over the photospheric emission of 0.08 ± 0.02 Jy, a result consistent with the previous characterization of the emission from the circumstellar dust in the wavelength range 12 μm ≤ λ ≤ 100 μm as a 110 K blackbody. The Hipparcos data can be used to argue that the three main-sequence A-type stars in the Bright Star Catalogue with L_IR/L_bol > 10^-3 (HR 4796A, β Pic, and 49 Cet) all have low luminosities for their colors. We argue that 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 β Pic. In order to explain the result that the grain emission can be approximated by a 110 K blackbody, we propose that the circumstellar grains are largely composed of ice particles with a typical radius near 100 μm, and that they sublimate rapidly when they are closer than ~35 AU to HR 4796A. This swarm of ice particles might be a protocometary cloud.