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Probing the Gas Content of Late-stage Protoplanetary Disks with N_2H^+

Anderson, Dana E. and Blake, Geoffrey A. and Bergin, Edwin A. and Zhang, Ke and Carpenter, John M. and Schwarz, Kamber R. and Huang, Jane and Öberg, Karin I. (2019) Probing the Gas Content of Late-stage Protoplanetary Disks with N_2H^+. Astrophysical Journal, 881 (2). Art. No. 127. ISSN 1538-4357. https://resolver.caltech.edu/CaltechAUTHORS:20190821-142017134

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Abstract

The lifetime of gas in circumstellar disks is a fundamental quantity that informs our understanding of planet formation. Studying disk gas evolution requires measurements of disk masses around stars of various ages. Because H_2 gas is unobservable under most disk conditions, total disk masses are based on indirect tracers such as sub-mm dust and CO emission. The uncertainty in the relation between these tracers and the disk mass increases as the disk evolves. In a few well-studied disks, CO exhibits depletions of up to 100× below the assumed interstellar value. Thus, additional tracers are required to accurately determine the total gas mass. The relative lack of nitrogen found in solid solar system bodies may indicate that it persists in volatile form, making nitrogen-bearing species more robust tracers of gas in more evolved disks. Here we present Atacama Large Millimeter/submillimeter Array detections of N_2H^+ in two mature, ~5–11 Myr old disks in the Upper Scorpius OB Association. Such detections imply the presence of H_2-rich gas and sources of ionization, both required for N_2H^+ formation. The Upper Sco disks also show elevated N_2H^+/CO flux ratios when compared to previously observed disks with ≳10× higher CO fluxes. Based on line ratio predictions from a grid of thermochemical disk models, a significantly reduced CO/H_2 abundance of <10^(−6) for a gas-to-dust ratio of ≳100 is required to produce the observed N_2H^+ fluxes. These systems appear to maintain H_2 gas reservoirs and indicate that carbon- and nitrogen-bearing species follow distinct physical or chemical pathways as disks evolve.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.3847/1538-4357/ab2cb5DOIArticle
ORCID:
AuthorORCID
Anderson, Dana E.0000-0002-8310-0554
Blake, Geoffrey A.0000-0003-0787-1610
Bergin, Edwin A.0000-0003-4179-6394
Zhang, Ke0000-0002-0661-7517
Carpenter, John M.0000-0003-2251-0602
Schwarz, Kamber R.0000-0002-6429-9457
Huang, Jane0000-0001-6947-6072
Öberg, Karin I.0000-0001-8798-1347
Alternate Title:Probing the Gas Content of Late-stage Protoplanetary Disks with N2H+
Additional Information:© 2019 The American Astronomical Society. Received 2018 December 14; revised 2019 June 19; accepted 2019 June 23; published 2019 August 21. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant No. DGE-1144469 and funding from NSF grants AST-1514670 and AST-1344133 (INSPIRE) as well as NASA NNX16AB48G. J.C. acknowledges support from the National Aeronautics and Space Administration under grant No. 15XRP15_20140 issued through the Exoplanets Research Program. K.Z. acknowledges the support of NASA through Hubble Fellowship grant HST-HF2-51401.001-A. J.H. acknowledges support from the National Science Foundation Graduate Research Fellowship under grant No. DGE-1144152. This work makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.01199.S, ADS/JAO.ALMA#2011.0.00526.S, ADS/JAO.ALMA#2015.1.00964.S, and ADS/JAO.ALMA#2013.1.00226.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We thank NRAO for assistance with data reduction and the anonymous reviewer for critically reading the manuscript and providing insightful feedback. Software: APLpy (Robitaille & Bressert 2012), Astropy (Astropy Collaboration 2013, 2018), CASA (McMullin et al. 2007), Matplotlib (Hunter 2007).
Group:Astronomy Department
Funders:
Funding AgencyGrant Number
NSF Graduate Research FellowshipDGE-1144469
NSFAST-1514670
NSFAST-1344133
NASANNX16AB48G
NASA15XRP15_20140
NASA Hubble FellowshipHST-HF2-51401.001-A
NSF Graduate Research FellowshipDGE-1144152
Subject Keywords:astrochemistry – molecular data – protoplanetary disks
Issue or Number:2
Record Number:CaltechAUTHORS:20190821-142017134
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20190821-142017134
Official Citation:Dana E. Anderson et al 2019 ApJ 881 127
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98083
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:21 Aug 2019 21:30
Last Modified:20 Apr 2020 08:47

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