The complete far-infrared and submillimeter spectrum of the Class 0 protostar Serpens SMM1 obtained with Herschel

Characterizing UV-irradiated shocks heating and chemistry^⋆,⋆⋆

¹ Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid Spain
e-mail: jr.goicoechea@cab.inta-csic.es
² Max-Planck-Institut für extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, HaiDian Qu, 100871 Beijing, PR China
⁵ RAL Space, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
⁶ Institute for Space Imaging Science, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta T1J 1B1, Canada
⁷ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade5-7, 1350 København K, Denmark
⁸ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
⁹ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹⁰ Department of Astronomy, University of Michigan, 500 Church St., Ann Arbor, MI 48109, USA

Received: 28 June 2012
Accepted: 17 September 2012

Abstract

We present the first complete  ~55−671 μm spectral scan of a low-mass Class 0 protostar (Serpens SMM1) taken with the PACS and SPIRE spectrometers onboard Herschel. More than 145 lines have been detected, most of them rotationally excited lines of ¹²CO (full ladder from J_u = 4−3 to 42−41 and E_u/k  =  4971 K), H₂O (up to 8₁₈−7₀₇ and E_u/k  =  1036 K), OH (up to ²Π_1/2 J = 7/2−5/2 and E_u/k  =  618 K), ¹³CO (up to J_u = 16−15), HCN and HCO⁺ (up to J_u = 12−11). Bright [O i]63, 145 μm and weaker [C ii]158 and [C i]370, 609 μm lines are also detected, but excited lines from chemically related species (NH₃, CH⁺, CO⁺, OH⁺ or H₂O⁺) are not. Mid-infrared spectra retrieved from the Spitzer archive are also first discussed here. The  ~10−37 μm spectrum has many fewer lines, but shows clear detections of [Ne ii], [Fe ii], [Si ii] and [S i] fine structure lines, as well as weaker H₂ S(1) and S(2) pure rotational lines. The observed line luminosity is dominated by CO (~54%), H₂O (~22%), [O i] (~12%) and OH (~9%) emission. A multi-component radiative transfer model allowed us to approximately quantify the contribution of the three different temperature components suggested by the ¹²CO rotational ladder (T_k^hot  ≈  800 K, T_k^warm  ≈  375 K and T_k^cool  ≈  150 K). Gas densities n(H₂)  ≳  5  ×  10⁶ cm^-3 are needed to reproduce the observed far-IR lines arising from shocks in the inner protostellar envelope (warm and hot components) for which we derive upper limit abundances of x(CO)  ≲  10^-4, x(H₂O)  ≲  0.2  ×  10^-5 and x(OH)  ≲  10^-6 withrespect to H₂. The lower energy submm ¹²CO and H₂O lines show more extended emission that we associate with the cool entrained outflow gas. Fast dissociative J-shocks (v_s  >  60 km s^-1) within an embedded atomic jet, as well as lower velocity small-scale non-dissociative shocks (v_s  ≲  20 km s^-1) are needed to explain both the atomic fine structure lines and the hot CO and H₂O lines respectively. Observations also show the signature of UV radiation (weak [C ii] and [C i] lines and high HCO⁺/HCN abundance ratios) and thus, most observed species likely arise in UV-irradiated shocks. Dissociative J-shocks produced by a jet impacting the ambient material are the most probable origin of [O i] and OH emission and of a significant fraction of the warm CO emission. In addition, H₂O photodissociation in UV-irradiated non-dissociative shocks along the outflow cavity walls can also contribute to the [O i] and OH emission.