Properties of protostars in the Elephant Trunk globule IC 1396A

Abstract

Extremely red objects, identified in the early Spitzer Space Telescope observations of the bright-rimmed globule IC 1396A and photometrically classified as Class I protostars Class II T Tauri stars based on their mid-infrared colors, were observed spectroscopically at 5.5 to 38 microns (IRS), at the 22 GHz water maser frequency (GBT), and in the optical (Palomar). The sources photometrically identified as Class I are confirmed as objects dominated by accretion luminosity from dense envelopes, with accretion rates 1e-5 to 1e-6 Msun/yr. The ice/silicate absorption ratio in the envelope is exceptionally low for the IC 1396A protostars, compared to those in nearby star-forming regions, suggesting the envelope chemistry is altered by the radiation field or globule pressure. Only one 22 GHz water maser was detected in IC 1396A; its infrared counterpart has luminosity <0.1 Lsun, the first H2O maser from such a low-luminosity object. The objects photometrically classified as Class II are confirmed as classical T Tauri stars with warm, luminous disks. The disk properties change significantly with source luminosity: low-mass (G-K) stars have prominent 9-11 micron emission features due to amorphous silicates while higher-mass (A-F) stars have weaker features requiring abundant crystalline silicates. The distribution of Class I sources is concentrated within the molecular globule, while the Class II sources are more widely scattered. Combined with the spectral results, this suggests two phases of star formation, the first (4 Myr ago) leading to the widespread Class II sources and the central O star of IC 1396, and the second (<1 Myr ago) occurring within the globule.