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Water in star-forming regions with Herschel (WISH). VI. Constraints on UV and X-ray irradiation from a survey of hydrides in low- to high-mass young stellar objects

Benz, A. O. and Bruderer, S. and van Dishoeck, E. F. and Melchior, M. and Wampfler, S. F. and van der Tak, F. and Goicoechea, J. R. and Indriolo, N. and Kristensen, L. E. and Lis, D. C. and Mottram, J. C. and Bergin, E. A. and Caselli, P. and Herpin, F. and Hogerheijde, M. R. and Johnstone, D. and Liseau, R. and Nisini, B. and Tafalla, M. and Vissers, R. and Wyrowski, F. (2016) Water in star-forming regions with Herschel (WISH). VI. Constraints on UV and X-ray irradiation from a survey of hydrides in low- to high-mass young stellar objects. Astronomy and Astrophysics, 590 . Art. No. A105. ISSN 0004-6361. http://resolver.caltech.edu/CaltechAUTHORS:20160719-083324611

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Abstract

Context. Hydrides are simple compounds containing one or a few hydrogen atoms bonded to a heavier atom. They are fundamental precursor molecules in cosmic chemistry and many hydride ions have become observable in high quality for the first time thanks to the Herschel Space Observatory. Ionized hydrides such as CH^+ and OH^+ (and also HCO^+), which affect the chemistry of molecules such as water, provide complementary information on irradiation by far-UV (FUV) or X-rays and gas temperature. Aims. We explore hydrides of the most abundant heavier elements in an observational survey covering young stellar objects (YSOs) with different mass and evolutionary state. The focus is on hydrides associated with the dense protostellar envelope and outflows, contrary to previous work that focused on hydrides in diffuse foreground clouds. Methods. Twelve YSOs were observed with HIFI on Herschel in six spectral settings providing fully velocity-resolved line profiles as part of the Water in star-forming regions with Herschel (WISH) program. The YSOs include objects of low (Class 0 and I), intermediate, and high mass, with luminosities ranging from 4 L_⊙ to 2 × 10^5 L_⊙. Results. The targeted lines of CH^+, OH^+, H_2O^+, C^+, and CH are detected mostly in blue-shifted absorption. H_3O^+ and SH^+ are detected in emission and only toward some high-mass objects. The observed line parameters and correlations suggest two different origins related to gas entrained by the outflows and to the circumstellar envelope. The derived column densities correlate with bolometric luminosity and envelope mass for all molecules, best for CH, CH^+, and HCO^+. The column density ratios of CH^+/OH^+ are estimated from chemical slab models, assuming that the H_2 density is given by the specific density model of each object at the beam radius. For the low-mass YSOs the observed ratio can be reproduced for an FUV flux of 2–400 times the interstellar radiation field (ISRF) at the location of the molecules. In two high-mass objects, the UV flux is 20–200 times the ISRF derived from absorption lines, and 300–600 ISRF using emission lines. Upper limits for the X-ray luminosity can be derived from H_3O^+ observations for some low-mass objects. Conclusions. If the FUV flux required for low-mass objects originates at the central protostar, a substantial FUV luminosity, up to 1.5 L_⊙, is required. There is no molecular evidence for X-ray induced chemistry in the low-mass objects on the observed scales of a few 1000 AU. For high-mass regions, the FUV flux required to produce the observed molecular ratios is smaller than the unattenuated flux expected from the central object(s) at the Herschel beam radius. This is consistent with an FUV flux reduced by circumstellar extinction or by bloating of the protostar.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1051/0004-6361/201525835DOIArticle
http://www.aanda.org/articles/aa/abs/2016/06/aa25835-15/aa25835-15.htmlPublisherArticle
http://arxiv.org/abs/1603.08721arXivDiscussion Paper
ORCID:
AuthorORCID
van Dishoeck, E. F.0000-0001-7591-1907
Lis, D. C.0000-0002-0500-4700
Alternate Title:WISH VI. Constraints on UV and X-ray irradiation from a survey of hydrides in low- to high-mass YSOs
Additional Information:© 2016 ESO. Received 6 February 2015. Accepted 28 March 2016. Published online 24 May 2016. Herschel is an ESA space observatory with science instruments provided by a European-led Principal Investigator consortia and with important participation from NASA. We thank the WISH team for inspiring discussions and support. We acknowledge the referee for helpful criticism and Göran Sandell for the confirmation of the NGC6334 I(N) object parameters. This program is made possible thanks to the Swiss Herschel guaranteed time program. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research Groningen, The Netherlands and with major contributions from Germany, France, and the US. Consortium members are: Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland: NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronomico Nacional (IGN), Centro de Astrobiologia (CSIC-INT); Sweden: Chalmers University of Technology, Onsala Space Observatory, Stockholm University; Switzerland: ETH Zurich, FHNW Windisch; USA: Caltech, JPL, NHSC. The work on star formation at ETH Zurich was partially funded by the Swiss National Science Foundation (grant No. 200020-113556). Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA), by a Royal Netherlands Academy of Arts and Sciences (KNAW) professor prize, and by the European Union A-ERC grant 291141 CHEMPLAN. Support for this work was also provide by NASA (Herschel OT funding) through an award issued by JPL/ Caltech.
Funders:
Funding AgencyGrant Number
Swiss National Science Foundation (SNSF)200020-113556
Nederlandse Onderzoekschool voor de Astronomie (NOVA)UNSPECIFIED
Royal Netherlands Academy of Arts and Sciences (KNAW)UNSPECIFIED
European Research Council (ERC)291141 CHEMPLAN
NASA/JPL/CaltechUNSPECIFIED
Subject Keywords:stars: formation stars: low-mass stars: massive ISM: molecules ultraviolet: ISM astrochemistry
Record Number:CaltechAUTHORS:20160719-083324611
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20160719-083324611
Official Citation:Water in star-forming regions with Herschel (WISH) - VI. Constraints on UV and X-ray irradiation from a survey of hydrides in low- to high-mass young stellar objects A. O. Benz, S. Bruderer, E. F. van Dishoeck, M. Melchior, S. F. Wampfler, F. van der Tak, J. R. Goicoechea, N. Indriolo, L. E. Kristensen, D. C. Lis, J. C. Mottram, E. A. Bergin, P. Caselli, F. Herpin, M. R. Hogerheijde, D. Johnstone, R. Liseau, B. Nisini, M. Tafalla, R. Visser and F. Wyrowski A&A, 590 (2016) A105 DOI: http://dx.doi.org/10.1051/0004-6361/201525835
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:69106
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:19 Jul 2016 17:41
Last Modified:30 Aug 2017 21:05

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