ALMA OBSERVATIONS OF THE MOLECULAR GAS IN THE DEBRIS DISK OF THE 30 Myr OLD STAR HD 21997
- European Space Agency (ESA-ESTEC, SRE-SA), P.O. Box 299, 2200AG, Noordwijk (Netherlands)
- Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, P.O. Box 67, 1525 Budapest (Hungary)
- Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333 CA Leiden (Netherlands)
- Department of Astronomy and Department of Planetary Sciences, The University of Arizona, Tucson, AZ 85721 (United States)
- Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn (Germany)
- NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771 (United States)
- Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg (Germany)
- Astronomy Department, Wesleyan University, Middletown, CT 06459 (United States)
The 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 {sup 12}CO and {sup 13}CO in the J = 2-1 and J = 3-2 transitions and C{sup 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, M{sub *}=1.8{sup +0.5}{sub -0.2} M{sub ☉}, and i = 32.°6 ± 3.°1. The total CO mass, as calculated from the optically thin C{sup 18}O line, is about (4-8) × 10{sup –2} M{sub ⊕}, 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 Moór 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.
- OSTI ID:
- 22270761
- Journal Information:
- Astrophysical Journal, Vol. 776, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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