DISK EVOLUTION IN W5: INTERMEDIATE-MASS STARS AT 2-5 Myr

Abstract

We present the results of a survey of young intermediate mass stars (age $<$~5 Myr, 1.5 $<M_{\star} \leq $ 15 $M_{\odot}$) in the W5 massive star forming region. We use combined optical, near-infrared and {\it Spitzer} Space Telescope photometry and optical spectroscopy to define a sample of stars of spectral type A and B and examine their infrared excess properties. We find objects with infrared excesses characteristic of optically thick disks, i.e. Herbig AeBe stars. These stars are rare: $<$1.5% of the entire spectroscopic sample of A and B stars, and absent among stars more massive than 2.4 $M_\odot$. 7.5% of the A and B stars possess infrared excesses in a variety of morphologies that suggest their disks are in some transitional phase between an initial, optically thick accretion state and later evolutionary states. We identify four morphological classes based on the wavelength dependence of the observed excess emission above theoretical photospheric levels: (a) the optically thick disks; (b) disks with an optically thin excess over the wavelength range 2 to 24 $\micron$, similar to that shown by Classical Be stars; (c) disks that are optically thin in their inner regions based on their infrared excess at 2-8 $\micron$ and optically thick in their outer regions based on the magnitude of the observed excess emission at 24 $\micron$; (d) disks that exhibit empty inner regions (no excess emission at $\lambda$ $\leq$ 8 $\micron$) and some measurable excess emission at 24 $\micron$. A sub-class of disks exhibit no significant excess emission at $\lambda \leq$ 5.8 $\micron$, have excess emission only in the {\it Spitzer} 8 $\micron$ band and no detection at 24 $\micron$. We discuss these spectral energy distribution (SED) types, suggest physical models for disks exhibiting these emission patterns and additional observations to test these theories.