Draft version May 6, 2013 Preprint typeset using LATEX style emulateapj v. 11/10/09 EMBEDDED PROTOSTARS IN THE DUST, ICE, AND GAS IN TIME (DIGIT) HERSCHEL KEY PROGRAM: CONTINUUM SEDS, AND AN INVENTORY OF CHARACTERISTIC FAR-INFRARED LINES FROM PACS SPECTROSCOPY Joel D. Green 1, Neal J. Evans II1, Jes K. Jørgensen2,3, Gregory J. Herczeg4,5, Lars E. Kristensen6,7, Jeong-Eun Lee8, Odysseas Dionatos3,2,9, Umut A. Yildiz6, Colette Salyk10, Gwendolyn Meeus11, Jeroen Bouwman12, Ruud Visser13, Edwin A. Bergin13, Ewine F. van Dishoeck6,5, Michelle R. Rascati1, Agata Karska5, Tim A. van Kempen6,14, Michael M. Dunham15, Johan E. Lindberg3,2, Davide Fedele5, & the DIGIT Team 1. The University of Texas at Austin, Department of Astronomy, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA; [email protected] 2. Niels Bohr Institute, University of Copenhagen. Denmark 3. Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Denmark 4. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, 100871, PR China 5. Max-Planck Institute for Extraterrestrial Physics, Postfach 1312, 85741, Garching, Germany 6. Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands 7. Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, 02183, USA 8. Department of Astronomy & Space Science, Kyung Hee University, Gyeonggi, 446-701, Korea 9. University of Vienna, Department of Astronomy, T¨urkenschanzstrasse 17, 1180 Vienna, Austria 10. National Optical Astronomy Observatory, 950 N Cherry Ave Tucson, AZ 85719, USA 11. Universidad Autonoma de Madrid, Dpt. Fisica Teorica, Campus Cantoblanco, Spain 12. Max Planck Institute for Astronomy, Heidelberg, Germany 13. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA 14. Joint ALMA offices, Av. Alonso de Cordova, Santiago, Chile 15. Dept. of Astronomy, Yale University, New Haven, CT, USA Draft version May 6, 2013 ABSTRACT We present 50-210 µm spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel- PACS, and 0.5-1000 µm SEDs, as part of the Dust, Ice, and Gas in Time (DIGIT) Key Program. Some sources exhibit up to 75 H2O lines ranging in excitation energy from 100-2000 K, 12 transitions of OH, and CO rotational lines ranging from J = 14 → 13 up to J = 40 → 39. [O I] is detected in all but one source in the entire sample; among the sources with detectable [O I] are two Very Low Luminosity Objects (VeLLOs). The mean 63/145 µm [O I] flux ratio is 17.2 ± 9.2. The [O I] 63 µm line correlates with Lbol, but not with the time-averaged outflow rate derived from low-J CO maps. [C II] emission is in general not local to the source. The sample Lbol increased by 1.25 (1.06) and Tbol decreased to 0.96 (0.96) of mean (median) values with the inclusion of the Herschel data. Most CO rotational diagrams are characterized by two optically thin components (N = (0.70 ± 1.12) × 1049 total particles). N CO correlates strongly with Lbol, but neither Trot nor N CO(warm)/N CO(hot) cor- relates with Lbol, suggesting that the total excited gas is related to the current source luminosity, but that the excitation is primarily determined by the physics of the interaction (e.g., UV-heating/shocks). Rotational temperatures for H2O (Trot = 194 ± 85 K) and OH (Trot = 183 ± 117 K) are generally lower than for CO, and much of the scatter in the observations about the best fit is attributed to differences in excitation conditions and optical depths amongst the detected lines. 1. INTRODUCTION Large samples of protostars in relatively nearby (d ≤ Embedded protostars represent a transition from the 300 pc) clouds have been developed through recent sur- core collapse within molecular clouds to the eventual veys with the Spitzer Space Telescope (e.g, Evans et al. young star and protoplanetary disk system, revealed as 2009, Allen et al. in prep.), along with ground-based sur- the surrounding envelope is accreted and/or removed. veys (e.g., Jørgensen et al. 2009, Dunham et al. 2013). Processes occurring in the protostellar stage of evolu- We have selected a sample of well-studied protostars from tion determine the conditions in the disks, including the these studies, including both Class 0 and Class I objects. amount and nature of material available for planet forma- Class 0 and Class I sources are characterized observa- tion in later stages of evolution. In a picture of episodic tionally by rising spectral energy distributions (SEDs) accretion (e.g., Dunham et al. 2010), the disk mass at the between near-infrared and mid-infrared wavelengths. end of the protostellar phase may be determined by the In addition to the continuum emission, the far- phasing of the last burst of accretion onto the star and infrared/submillimeter bands contain numerous pure ro- the end of infall. Tracing the physical processes in these tational transitions of the CO ladder, as well as OH and systems will lead to a better understanding of the con- H2O, and several fine structure lines, all useful tracers straints on planet formation. Physical processes can be of gas content and properties. The transitions and colli- traced through a wealth of molecular, atomic, and ionic sional rates of these simple molecules are well-understood tracers available to optical, infrared, and millimeter-wave (see, e.g., Yang et al. 2010; Neufeld 2012, for a recent telescopes. update on CO). Thus these lines make excellent diagnos- 2 Green et al. tics of opacity, density, temperature, and shock velocities the sample in the context of other surveys, and describe (e.g., Kaufman & Neufeld 1996; Flower & Pineau Des the observations. In §3 we present our detailed data re- Forˆets 2010) of the gas surrounding these systems. duction method for acquiring accurate continuum SEDs ISO/LWS covered the 20-200 µm spectral region and and linefluxes. In §4 we present results and statistics for was well-suited to study the warm (T > 100 K) region the sample. In §5 we analyze rotational diagrams for all of protostellar envelopes, distinguishable from the am- detected molecular species. In §6 we present correlations bient cloud typically probed in ground-based millimeter amongst the detected and derived parameters. In §7 we studies. ISO-LWS detected gas phase H2O, high-J CO consider the origin of the line emission. We conclude in rotational transitions, and fine structure emission lines §8. toward protostars and related sources (e.g., Lorenzetti 2. et al. 1999; Giannini et al. 1999; Ceccarelli et al. 1999; OBSERVATIONS Lorenzetti et al. 2000; Giannini et al. 2001; Nisini et al. The DIGIT program uses Herschel to achieve far- 2002). These lines are indicative of the innermost re- infrared spectral coverage of protostars and more evolved gions of the protostellar envelope, exposed to heating by young stellar objects in order to make a full inventory the central object, and the outflow cavity region, where of their chemical tracers. The full DIGIT sample con- winds and jets may interact with the envelope and the sists of 94 sources: 24 Herbig Ae/Be stars, 9 T Tauri surrounding cloud. stars, 30 protostars observed with PACS and HIFI spec- The Herschel Space Observatory (Pilbratt et al. 2010) troscopy, and an additional 31 weak-line T Tauri stars provides greater spatial and spectral resolution, as well as observed with PACS imaging. Cieza et al. (2013) pre- sensitivity, allowing study of sources with weaker contin- sented the results for the weak-line T Tauri stars; vari- uum and line emission. Herschel is an ESA space-based ous papers are summarizing the results on disks in the submillimeter telescope with a 3.5-meter primary mir- Herbig and T Tauri star spectroscopic survey (Sturm et ror. The Herschel-PACS spectrograph (Photodetector al., A&A, in press., Meeus et al., Fedele et al., subm.) In Array Camera and Spectrometer, Poglitsch et al. 2010) this paper we focus on the embedded protostar sample. provides a generational improvement over the spectral We presented the first DIGIT observations of a proto- range covered by ISO-LWS, and is complementary with star, DK Cha, in van Kempen et al. (2010a). The 30 the Infrared Spectrograph (Houck et al. 2004) on the object sample is drawn from previous studies (summa- Spitzer Space Telescope (Werner et al. 2004). Spitzer- rized by Lahuis et al. 2006), focusing on protostars with IRS revealed a wealth of diagnostics in the mid-infrared, high-quality Spitzer-IRS 5-40 µm and UV, optical, in- at relatively low spectral (R ∼ 60-600) and spatial (2– frared, and submillimeter complementary data. Thus all ′′ 5 ) resolution. For example, H2O and OH were detected of the sources in the DIGIT sample are well-studied at in some protostars (Watson et al. 2007) but in transi- other wavelengths, and many are part of ongoing studies tions with typically greater critical densities than those with Herschel in other programs, especially in the Water detectable by Herschel. Combined with enhanced spa- in Star-Forming Regions with Herschel (WISH) program tial resolution, this difference in excitation requirements (van Dishoeck et al. 2011; see also Nisini et al. (2010); can lead to significantly different conclusions on the ori- Kristensen et al. (2012); Karska et al. (2013); Wampfler gin of the emission without the context of the Herschel et al. (2013)). Specific outflow positions that are well lines (Herczeg et al. 2012). Similarly while pure rota- separated from the central source positions are observed tional lines of H2 were detected with Spitzer in many re- by, e.g., Lefloch et al. (2010); Vasta et al.