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 um) spectrum of the class I protostarL1489 IRS in the Taurus Molecular Cloud. This is the highest resolution widecoverage spectrum at this wavelength of a low mass protostar observed to date(R=25,000; Dv=12 km/s). Many narrow absorption lines of gas phase 12CO, 13CO,and C18O are detected, as well as a prominent band of solid 12CO. The gas phase12CO lines have red shifted absorption wings (up to 100 km/s), likelyoriginating from warm disk material falling toward the central object. Theisotopes and the 12CO line wings are successfully fitted with a contractingdisk model of this evolutionary transitional object (Hogerheijde 2001). Thisshows that the inward motions seen in millimeter wave emission lines continueto within ~0.1 AU from the star. The colder parts of the disk are traced by theprominent CO ice band. The band profile results from CO in 'polar' ices (COmixed with H2O), and CO in 'apolar' ices. At the high spectral resolution, the'apolar' component is, for the first time, resolved into two distinctcomponents, likely due to pure CO and CO mixed with CO2, O2 and/or N2. The iceshave probably experienced thermal processing in the upper disk layer traced byour pencil absorption beam: much of the volatile 'apolar' ices has evaporatedand the depletion factor of CO onto grains is remarkably low (~7%). This studyshows that high spectral resolution 4.7 um observations provide important andunique information on the dynamics and structure of protostellar disks and theevolution of ices in these disks.Comment: 11 pages, 6 figures Scheduled to appear in ApJ 568 n2, 1 April 200