A Spitzer-IRS Detection of Crystalline Silicates in a Protostellar Envelope

TL;DR

This study reports the first clear detection of crystalline silicates in a protostellar envelope using Spitzer-IRS data, indicating in-situ formation or processing of silicates in early star formation stages.

Contribution

It provides the first unambiguous evidence of crystalline silicate absorption in a cold protostellar envelope, expanding understanding of dust processing in early stellar environments.

Findings

Crystalline silicates detected at multiple infrared wavelengths.

Crystalline mass fraction estimated at 14-17%.

Crystalline silicates are more abundant than in the interstellar medium.

Abstract

We present the Spitzer Space Telescope Infrared Spectrograph spectrum of the Orion A protostar HOPS-68. The mid-infrared spectrum reveals crystalline substructure at 11.1, 16.1, 18.8, 23.6, 27.9, and 33.6 microns superimposed on the broad 9.7 and 18 micron amorphous silicate features; the substructure is well matched by the presence of the olivine end-member forsterite. Crystalline silicates are often observed as infrared emission features around the circumstellar disks of Herbig Ae/Be stars and T Tauri stars. However, this is the first unambiguous detection of crystalline silicate absorption in a cold, infalling, protostellar envelope. We estimate the crystalline mass fraction along the line-of-sight by first assuming that the crystalline silicates are located in a cold absorbing screen and secondly by utilizing radiative transfer models. The resulting crystalline mass fractions of 0.14 and 0.17, respectively, are significantly greater than the upper limit found in the interstellar medium (< 0.02-0.05). We propose that the amorphous silicates were annealed within the hot inner disk and/or envelope regions and subsequently transported outward into the envelope by entrainment in a protostellar outflow