Origin of warm and hot gas emission from low-mass protostars: Herschel-HIFI observations of CO J = 16–15^⋆

I. Line profiles, physical conditions, and H₂O abundance

¹ Centre for Star and Planet Formation, Niels Bohr Institute and Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
e-mail: lars.kristensen@nbi.ku.dk
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Centre for Astronomy, Nicolaus Copernicus University, Faculty of Physics, Astronomy and Informatics, Grudziadzka 5, 87100 Torun, Poland
⁶ Jet Propulsion Laboratory, 4800 Oak Groave Drive, Pasadena, CA 91109, USA
⁷ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
⁸ LERMA, UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France
⁹ Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
¹⁰ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
¹¹ LERMA, UMR 8112 du CNRS, Observatoire de Paris, École Normale Supérieure, 61 Av. de l’Observatoire, 75014 Paris, France
¹² Heidelberg University, Center for Astronomy, Institute for Theoretical Astrophysics, Albert-Ueberle-Strasse 2, 69120 Heidelberg, Germany
¹³ Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Haidian Qu, 100871 Beijing, PR China
¹⁴ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁵ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
¹⁶ RON Netherlands Institute for Space Research, Location Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
¹⁷ Department of Astronomy & Space Science, Kyung Hee University, Gyeonggi 446-701, Korea
¹⁸ Korea School of Space Research, Kyung Hee University, Yongin-shi, Kyungki-do 449-701, Korea
¹⁹ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
²⁰ CNRS, IPAG, 38000 Grenoble, France
²¹ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII,3. 28014 Madrid, Spain
²² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
²³ Center for Space and Habitability, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

Received: 24 November 2016
Accepted: 23 May 2017

Abstract

Context. Through spectrally unresolved observations of high-J CO transitions, Herschel Photodetector Array Camera and Spectrometer (PACS) has revealed large reservoirs of warm (300 K) and hot (700 K) molecular gas around low-mass protostars. The excitation and physical origin of this gas is still not understood.

Aims. We aim to shed light on the excitation and origin of the CO ladder observed toward protostars, and on the water abundance in different physical components within protostellar systems using spectrally resolved Herschel-HIFI data.

Methods. Observations are presented of the highly excited CO line J = 16–15 (E_up/k_B = 750 K) with the Herschel Heterodyne Instrument for the Far Infrared (HIFI) toward a sample of 24 low-mass protostellar objects. The sources were selected from the Herschel “Water in Star-forming regions with Herschel” (WISH) and “Dust, Ice, and Gas in Time” (DIGIT) key programs.

Results. The spectrally resolved line profiles typically show two distinct velocity components: a broad Gaussian component with an average FWHM of 20 km s^-1 containing the bulk of the flux, and a narrower Gaussian component with a FWHM of 5 km s^-1 that is often offset from the source velocity. Some sources show other velocity components such as extremely-high-velocity features or “bullets”. All these velocity components were first detected in H₂O line profiles. The average rotational temperature over the entire profile, as measured from comparison between CO J = 16–15 and 10–9 emission, is ~300 K. A radiative-transfer analysis shows that the average H₂O/CO column-density ratio is ~0.02, suggesting a total H₂O abundance of ~2 × 10^-6, independent of velocity.

Conclusions. Two distinct velocity profiles observed in the HIFI line profiles suggest that the high-J CO ladder observed with PACS consists of two excitation components. The warm PACS component (300 K) is associated with the broad HIFI component, and the hot PACS component (700 K) is associated with the offset HIFI component. The former originates in either outflow cavity shocks or the disk wind, and the latter in irradiated shocks. The low water abundance can be explained by photodissociation. The ubiquity of the warm and hot CO components suggest that fundamental mechanisms govern the excitation of these components; we hypothesize that the warm component arises when H₂ stops being the dominant coolant. In this scenario, the hot component arises in cooling molecular H₂-poor gas just prior to the onset of H₂ formation. High spectral resolution observations of highly excited CO transitions uniquely shed light on the origin of warm and hot gas in low-mass protostellar objects.