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
Artikel i vetenskaplig tidskrift, 2016

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 × 105 L?. Results. The targeted lines of CH+, OH+, H2O+, C+, and CH are detected mostly in blue-shifted absorption. H3O+ 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 H2 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 H3O+ 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.

Ultraviolet: ISM

Stars: formation

Astrochemistry

Stars: low-mass

Stars: massive

ISM: molecules

Författare

A. O. Benz

Eidgenössische Technische Hochschule Zürich (ETH)

S. Bruderer

Max Planck-institutet

Eidgenössische Technische Hochschule Zürich (ETH)

E. F. van Dishoeck

Universiteit Leiden

Max Planck-institutet

M. Melchior

University of Applied Sciences Northwestern Switzerland, School of Enginnering

Eidgenössische Technische Hochschule Zürich (ETH)

S. F. Wampfler

Niels Bohr Institute

Eidgenössische Technische Hochschule Zürich (ETH)

F. F. S. van der Tak

Rijksuniversiteit Groningen

Netherlands Institute for Space Research (SRON)

J. R. Goicoechea

Consejo Superior de Investigaciones Científicas

N. Indriolo

University of Michigan

L. Kristensen

Harvard-Smithsonian Center for Astrophysics

D. C. Lis

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

CalTech

J. C. Mottram

Max Planck-institutet

Universiteit Leiden

E. A. Bergin

University of Michigan

P. Caselli

Max Planck-institutet

F. Herpin

Laboratoire d'Astrophysique de Bordeaux

M. R. Hogerheijde

Universiteit Leiden

D. Johnstone

National Research Council Canada

University of Victoria

René Liseau

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

B. Nisini

Osservatorio Astronomico di Roma

M. Tafalla

Observatorio Astronomico Nacional

R. Visser

European Southern Observatory (ESO)

F. Wyrowski

Max Planck-institutet

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 590 Art. no. A105-

Ämneskategorier

Astronomi, astrofysik och kosmologi

Fundament

Grundläggande vetenskaper

DOI

10.1051/0004-6361/201525835