High-Resolution 4.7 micron Keck/NIRSPEC Spectra of Protostars. I. Ices and Infalling Gas in the Disk of L1489 IRS

We explore the infrared M-band (4.7 μm) spectrum of the Class I protostar L1489 IRS in the Taurus molecular cloud. This is the highest-resolution wide-coverage spectrum at this wavelength of a low-mass protostar observed to date (R = 25,000; Δv = 12 km s-1). A large number of narrow absorption lines of gas-phase 12CO, 13CO, and C18O are detected, as well as a prominent band of solid 12CO. The gas-phase 12CO lines have redshifted absorption wings (up to 100 km s-1), which likely originate from warm disk material falling toward the central object. Both the isotopes and the extent of the 12CO line wings are successfully fitted with a contracting-disk model of this evolutionary transitional object. This shows that the inward motions seen in millimeter-wave emission lines continue to within ~0.1 AU of the star. The amount of high-velocity infalling gas is, however, overestimated by this model, suggesting that only part of the disk is infalling, e.g., a hot surface layer or hot gas in magnetic field tubes. The colder parts of the disk are traced by the prominent CO-ice band. The band profile results from CO in "polar" ices (CO mixed with H2O) and CO in "apolar" ices. At high spectral resolution, the "apolar" component is, for the first time, resolved into two distinct components, likely due to pure CO and CO mixed with CO2, O2, and/or N2. The ices have probably undergone thermal processing in the upper disk layer traced by our pencil absorption beam: much of the volatile "apolar" ices has evaporated, the depletion factor of CO onto grains is remarkably low (~7%), and the CO2 traced in the CO-band profile was possibly formed energetically.This study shows that high spectral resolution 4.7 μm observations provide important and unique information on the dynamics and structure of protostellar disks and on the origin and evolution of ices in these disks.

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