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 outflowComment: Accepted to Astrophysical Journal Letters, 2011 April 19: 6 pages, 3
figures, 2 table
