A&A 556, A89 (2013) DOI: 10.1051/0004-6361/201220849 c  ESO 2013 Astronomy & Astrophysics High-J CO survey of low-mass protostars observed with Herschel -HIFI ⋆,⋆⋆ U. A. Yıldız 1 , L. E. Kristensen 1,2 , E. F. van Dishoeck 1,3 , I. San José-García 1 , A. Karska 3 , D. Harsono 1 , M. Tafalla 4 , A. Fuente 5 , R. Visser 6 , J. K. Jørgensen 7,8 , and M. R. Hogerheijde 1 1 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: yildiz@strw.leidenuniv.nl 2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 3 Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany 4 Observatorio Astronómico Nacional (IGN), Calle Alfonso XII, 3, 28014 Madrid, Spain 5 Observatorio Astronómico Nacional, Apartado 112, 28803 Alcalá de Henares, Spain 6 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA 7 Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø., Denmark 8 Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K., Denmark Received 4 December 2012 / Accepted 14 June 2013 ABSTRACT Context. In the deeply embedded stage of star formation, protostars start to heat and disperse their surrounding cloud cores. The evolution of these sources has traditionally been traced through dust continuum spectral energy distributions (SEDs), but the use of CO excitation as an evolutionary probe has not yet been explored due to the lack of high- J CO observations. Aims. The aim is to constrain the physical characteristics (excitation, kinematics, column density) of the warm gas in low-mass protostellar envelopes using spectrally resolved Herschel data of CO and compare those with the colder gas traced by lower excitation lines. Methods. Herschel-HIFI observations of high- J lines of 12 CO, 13 CO, and C 18 O (up to J u = 10, E u up to 300 K) are presented toward 26 deeply embedded low-mass Class 0 and Class I young stellar objects, obtained as part of the Water In Star-forming regions with Herschel (WISH) key program. This is the ﬁrst large spectrally resolved high- J CO survey conducted for these types of sources. Complementary lower J CO maps were observed using ground-based telescopes, such as the JCMT and APEX and convolved to matching beam sizes. Results. The 12 CO 10–9 line is detected for all objects and can generally be decomposed into a narrow and a broad component owing to the quiescent envelope and entrained outﬂow material, respectively. The 12 CO excitation temperature increases with velocity from ∼60 K up to ∼130 K. The median excitation temperatures for 12 CO, 13 CO, and C 18 O derived from single-temperature ﬁts to the J u = 2–10 integrated intensities are ∼70 K, 48 K and 37 K, respectively, with no signiﬁcant diﬀerence between Class 0 and Class I sources and no trend with M env or L bol . Thus, in contrast to the continuum SEDs, the spectral line energy distributions (SLEDs) do not show any evolution during the embedded stage. In contrast, the integrated line intensities of all CO isotopologs show a clear decrease with evolutionary stage as the envelope is dispersed. Models of the collapse and evolution of protostellar envelopes reproduce the C 18 O results well, but underproduce the 13 CO and 12 CO excitation temperatures, due to lack of UV heating and outﬂow components in those models. The H 2 O1 10 - 1 01 /CO 10–9 intensity ratio does not change signiﬁcantly with velocity, in contrast to the H 2 O/CO 3–2 ratio, indicating that CO 10–9 is the lowest transition for which the line wings probe the same warm shocked gas as H 2 O. Modeling of the full suite of C 18 O lines indicates an abundance proﬁle for Class 0 sources that is consistent with a freeze-out zone below 25 K and evaporation at higher temperatures, but with some fraction of the CO transformed into other species in the cold phase. In contrast, the observations for two Class I sources in Ophiuchus are consistent with a constant high CO abundance proﬁle. Conclusions. The velocity resolved line proﬁles trace the evolution from the Class 0 to the Class I phase through decreasing line intensities, less prominent outﬂow wings, and increasing average CO abundances. However, the CO excitation temperature stays nearly constant. The multiple components found here indicate that the analysis of spectrally unresolved data, such as provided by SPIRE and PACS, must be done with caution. Key words. astrochemistry – stars: formation – stars: protostars – ISM: molecules – techniques: spectroscopic 1. Introduction Low-mass stars like our Sun form deep inside collapsing molec- ular clouds by accreting material onto a central dense source. As the source evolves, gas and dust move from the envelope to ⋆ Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with im- portant participation from NASA. ⋆⋆ Appendices C and D are available in electronic form at http://www.aanda.org the disk and onto the star, resulting in a decrease in the enve- lope mass and a shift in the peak of the continuum spectral en- ergy distribution to shorter wavelengths (e.g., Lada 1999; André et al. 2000; Young & Evans 2005). At the same time, jets and winds from the protostar entrain material and disperse the enve- lope. Spectral lines at submillimeter wavelengths trace this dense molecular gas and reveal both the kinematic signature of col- lapse (Gregersen et al. 1997; Myers et al. 2000; Kristensen et al. 2012) as well as the high velocity gas in the outﬂows (Arce et al. 2007). Article published by EDP Sciences A89, page 1 of 46