VY Canis Majoris. II. Interpretation of the Energy Distribution
Abstract
IuIticolor photometric observations of VY C a between 0.36 and 22 have been interpreted in terms of a model consisting of a dust disk surrounding and hiding a star with an energy distribution like that of a1 Her. The emitted continuous spectrum was separated into two components: one scattered in the disk, and another due to thermal emission by solid particles. The radial T(r) distribution in the disk was obtained by a method, due to Huang, for solving the transfer equation in spherical coordinates under the simplification of gray particles and the Eddington approximation. The surface flux was then found at each wavelength byusingaproceduredue to Larson of integrationover angle and depth. The resulting nonPlanckian spectrum represents the multicolor observations satisfactorily; the temperature range is from 3500 K at the outer surface of the disk to 15150 Qr 18500 K at the inner surface, depending upon which of two alternative models for the radial density distribution is chosen. The absolute values of the inner and outer rad depend on the disk thickness that is assumed; for a case where the dust is confined between latitudes :1:200, these radii are about 60-90 and 1000-1500 a.u., respectively. The fit requires that, within the framework of this theory, the particles have a high value of the single-scattering albedo, about 0.9. The total mass of the disk (gas plus dust) is about 0.1 M0. The time required, only a few hundred years, for a particle to move the radial thickness of the disk, traveling at the mean expansion velocity inferred from the duplicity of the microwave lines of OH and H20, suggests that the disk is a short-lived phenomenon. These calculations all assume that VY CMa is a member of the young association associated with r CMa, NGC 2362, and the His region Sharpless 310, at a distance of 1.5 kpc. The overall picture is that of formation of dust and molecules in this circumstellar ring or "solar nebula" about NN CMa, the disk now being in tho process of radial dissipation by particle or radiation pressure from the young M-type central star.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1970
- DOI:
- 10.1086/150688
- Bibcode:
- 1970ApJ...162..557H