High-Resolution 4.7 micron Keck/NIRSPEC Spectra of Protostars. I. Ices and Infalling Gas in the Disk of L1489 IRS
Abstract
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 ^(12)CO, ^(13)CO, and C^(18)O are detected, as well as a prominent band of solid ^(12)CO. The gas-phase ^(12)CO 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 ^(12)CO 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 H_2O) 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 CO_2, O_2, and/or N_2. 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 CO_2 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.
Additional Information
© 2002 American Astronomical Society. Received 2001 October 5; accepted 2001 December 4. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The UCLA NIRSPEC instrument team is acknowledged for constructing an excellent instrument. We thank M. Brown (Caltech, USA) for providing a procedure to rectify two-dimensional spectra in IDL, T. Teixeira (University of Aarhus, Denmark) for generously providing her compilation of laboratory CO spectra to us, and the anonymous referee for careful reading of the manuscript, which helped to clarify the description of several aspects of the analysis. The research of A. C. A. B. at the Caltech Submillimeter Observatory is funded by the NSF through contract AST 99- 80846. The research of M. R. H. is supported by the Miller Institute for Basic Research in Science. NASA support to G. A. B. is gratefully acknowledged. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible.Attached Files
Published - 0004-637X_568_2_761.pdf
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Additional details
- Eprint ID
- 34176
- Resolver ID
- CaltechAUTHORS:20120918-112454667
- AST 99-80846
- NSF
- Miller Institute for Basic Research in Science
- NASA
- Created
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2012-09-18Created from EPrint's datestamp field
- Updated
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2021-11-09Created from EPrint's last_modified field
- Caltech groups
- Division of Geological and Planetary Sciences