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ALMA Observations of the Molecular Gas in the Debris Disk of the 30 Myr Old Star HD 21997The 30 Myr old A3-type star HD 21997 is one of the two known debris dust disks having a measurable amount of cold molecular gas. With the goal of understanding the physical state, origin, and evolution of the gas in young debris disks, we obtained CO line observations with the Atacama Large Millimeter/submillimeter Array (ALMA). Here, we report on the detection of (12)CO and (13)CO in the J = 2-1 and J = 3-2 transitions and C(18)O in the J = 2-1 line. The gas exhibits a Keplerian velocity curve, one of the few direct measurements of Keplerian rotation in young debris disks. The measured CO brightness distribution could be reproduced by a simple star+disk system, whose parameters are r(sub in) < 26 AU, r(sub out) = 138 +/- 20 AU, Stellar M = 1.8 +0.5/−0.2 Solar M, and i = 32. Deg. 6 +/- 3 deg..1. The total CO mass, as calculated from the optically thin C(18)O line, is about (4-8) ×10(exp −2 ) Solar M, while the CO line ratios suggest a radiation temperature on the order of 6-9 K. Comparing our results with those obtained for the dust component of the HD 21997 disk from ALMA continuum observations by Moor et al., we conclude that comparable amounts of CO gas and dust are present in the disk. Interestingly, the gas and dust in the HD 21997 system are not colocated, indicating a dust-free inner gas disk within 55 AU of the star. We explore two possible scenarios for the origin of the gas. A secondary origin, which involves gas production from colliding or active planetesimals, would require unreasonably high gas production rates and would not explain why the gas and dust are not colocated. We propose that HD 21997 is a hybrid system where secondary debris dust and primordial gas coexist. HD 21997, whose age exceeds both the model predictions for disk clearing and the ages of the oldest T Tauri-like or transitional gas disks in the literature, may be a key object linking the primordial and the debris phases of disk evolution.
Document ID
20140013343
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
External Source(s)
doi:10.1088/0004-637X/776/2/77
Authors
Kospal, A. (European Space Agency. European Space Research and Technology Center, ESTEC Noordwijk, Netherlands)
Moor, A. (Konkoly Observatory Budapest, Hungary)
Juhasz, A. (Leiden Univ. Netherlands)
Abraham, P. (Konkoly Observatory Budapest, Hungary)
Apai, D. (Arizona Univ. Tucson, AZ, United States)
Csengeri, T. (Max-Planck-Inst. fuer Radioastronomie Bonn, Germany)
Grady, C. A. (Eureka Scientific, Inc. Oakland, CA, United States)
Henning, Th. (Max-Planck-Inst. fuer Astronomie Heidelberg, Germany)
Hughes, A. M. (Wesleyan Univ. Middletown, CT, United States)
Kiss, Cs. (Konkoly Observatory Budapest, Hungary)
Pascucci, I. (Arizona Univ. Tucson, AZ, United States)
Schmalzl, M. (Leiden Univ. Netherlands)
Date Acquired
November 5, 2014
Publication Date
September 30, 2013
Publication Information
Publication: The Astrophysical Journal
Publisher: American Astronomical Journal
Volume: 776
Issue: 2
ISSN: 0004-637X
Subject Category
Astrophysics
Report/Patent Number
GSFC-E-DAA-TN12445
Funding Number(s)
CONTRACT_GRANT: NNG12PP44P
Distribution Limits
Public
Copyright
Public Use Permitted.
Keywords
molecular gas
primordial
ALMA continuum observations

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