ModellingHerschelobservations of hot molecular gas emission from embedded low-mass protostars

Abstract

Aims. Young stars interact vigorously with their surroundings, as evident from the highly rotationally excited CO (up to E u /k = 4000 K) and H 2 O emission (up to 600 K) detected by the Herschel Space Observatory in embedded low-mass protostars. Our aim is to construct a model that reproduces the observations quantitatively, to investigate the origin of the emission, and to use the lines as probes of the various heating mechanisms. Methods. The model consists of a spherical envelope with a power-law density structure and a bipolar outflow cavity. Three heating mechanisms are considered: passive heating by the protostellar luminosity, ultraviolet irradiation of the outflow cavity walls, and small-scale C-type shocks along the cavity walls. Most of the model parameters are constrained from independent observations; the two remaining free parameters considered here are the protostellar UV luminosity and the shock velocity. Line fluxes are calculated for CO and H 2 O and compared to Herschel data and complementary ground-based data for the protostars NGC 1333 IRAS2A, HH 46 and DK Cha. The three sources are selected to span a range of evolutionary phases (early Stage 0 to late Stage I) and physical characteristics such as luminosity and envelope mass. Results. The bulk of the gas in the envelope, heated by the protostellar luminosity, accounts for 3-10% of the CO luminosity summed over all rotational lines up to J = 40-39; it is best probed by low-J CO isotopologue lines such as C 18 O 2-1 and 3-2. The UV-heated gas and the C-type shocks, probed by 12 CO 10-9 and higher-J lines, contribute 20-80% each. The model fits show a tentative evolutionary trend: the CO emission is dominated by shocks in the youngest source and by UV-heated gas in the oldest one. This trend is mainly driven by the lower envelope density in more evolved sources. The total H 2 O line luminosity in all cases is dominated by shocks (>99%). The exact percentages for both species are uncertain by at least a factor of 2 due to uncertainties in the gas temperature as function of the incident UV flux. However, on a qualitative level and within the context of our model, both UV-heated gas and C-type shocks are needed to reproduce the emission in far-infrared rotational lines of CO and H 2 O.

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

“…Around SMM3 and SMM4, the bulk of [CII] emission is associated with SMM6; however, some [CII] emission is also observed along the outflows. A protostellar source with UV luminosity of ∼0.1 L , as indicated by best-fit model-sources of Visser et al (2012), would give an unattenuated UV flux of ∼10 G 0 at a distance of 4000 AU from the source (e.g., the distance of the outflow positions from SMM3). Adopting an envelope density of ∼10 4 cm −3 as found in Visser et al (2012) and using the PDR model of Kaufman et al (1999), we find [CII] flux levels of ∼10 −14 erg cm −2 s −1 within the area of a PACS spaxel.…”

“…Spectro-imaging is a powerful tool for the study of extended emission structures, permitting us to simultaneously retrieve the spatial distribution and spectral information of an excited region. This allows us to disentangle the contribution of UV-heated outflow cavities from emission due to shocks along the outflow propagation axis (e.g., Visser et al 2012;Herczeg et al 2012;van Kempen et al 2010). The spatial distribution of isolated spectral features provides morphological evidence of the origin of the excited gas, whereas the study of the spectral information provides information on the underlying physical conditions (e.g., Kristensen et al 2010b).…”

High-JCO emission in the Cepheus E protostellar outflow observed with SOFIA/GREAT

“…Eventually, the combination of a maximum number of CO lines will provide the optimal way to shed light on the physical processes responsible for the existence of these components (e.g. Gusdorf et al 2012;Visser et al 2012). …”

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

“…Detailed models of the emission from the extended and passively heated envelope (having most of the mass) predict 12 CO rotational temperatures around 30−60 K (Visser et al 2012;Harsono et al, in prep. ).…”

“…Herczeg et al (2012) line is not even detected) led these authors to conclude that the hot gas where H 2 O dominates the gas cooling is heated by nondissociative C-shocks shielded from UV radiation. Passive heating by the protostellar luminosity is also thought to contribute to the mid-J 12 CO and 13 CO emission (Visser et al 2012;Yıldız et al 2012). In NGC 1333 IRAS 4A/4B, however, the 12 CO intensities and broad line-profiles of lower-J transitions (J = 1−0 up to 10−9) probe swept-up or entrained shocked gas along the outflow (Yıldız et al 2012).…”