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

Abstract

Context. Mid-and far-infrared observations of the environment around embedded protostars reveal a plethora of high excitation molecular and atomic emission lines. Different mechanisms for the origin of these lines have been proposed, including shocks induced by protostellar jets and radiation or heating by the embedded protostar of its immediate surroundings. Aims. By studying of the most important molecular and atomic coolants, we aim at constraining the physical conditions around the embedded protostars SMM3 and SMM4 in the Serpens molecular cloud core and measuring the CO/H 2 ratio in warm gas. The excitation conditions for molecular species are assessed through rotational diagrams. Emission lines from major coolants are compared to the results of steady-state C-and J-type shock models. Results. Line emission tends to peak at distances of ∼10-20 from the protostellar sources with all but [CII] peaking at the positions of outflow shocks seen in near-IR and sub-millimeter interferometric observations. The [CII] emission pattern suggests that it is most likely excited from energetic UV radiation originating from the nearby flat-spectrum source SMM6. Excitation analysis indicates that H 2 and CO originate in gas at two distinct rotational temperatures of ∼300 K and 1000 K, while the excitation temperature for H 2 O and OH is ∼100-200 K. The morphological and physical association between CO and H 2 suggests a common excitation mechanism, which allows direct comparisons between the two molecules. The CO/H 2 abundance ratio varies from ∼10 −5 in the warmer gas up to ∼10 −4 in the hotter regions. Shock models indicate that C-shocks can account for the observed line intensities if a beam filling factor and a temperature stratification in the shock front are considered. C-type shocks can best explain the emission from H 2 O. The existence of J-shocks is suggested by the strong atomic/ionic (except for [CII]) emission and a number of line ratio diagnostics. Dissociative shocks can account for the CO and H 2 emission in a single excitation temperature structure. Conclusions. The bulk of cooling from molecular and atomic lines is associated with gas excited in outflow shocks. The strong association between H 2 and CO constrain their abundance ratio in warm gas. Both C-and J-type shocks can account for the observed molecular emission; however, J-shocks are strongly suggested by the atomic emission and provide simpler and more homogeneous solutions for CO and H 2 . The variations in the CO/H 2 abundance ratio for gas at different temperatures can be interpreted by their reformation rates in dissociative J-type shocks, or the influence of both C and J shocks.

Herschel GASPS spectral observations of T Tauri stars in Taurus. Unraveling far-infrared line emission from jets and discs

“…The agreement between observations and shock model predictions depends on the specific line ratio (see Karska et al 2014b) and the evolutionary stage of each source. Similar conditions are found in the few cases where individual sources have been compared to shock models (Lee et al 2013;Dionatos et al 2013). However, we must stress that C-type shocks are probably the main driver of the molecular emission (see below).…”

Warm gas towards young stellar objects in Corona Australis

“…When comparing the non-deconvolved and deconvolved spectra, we find that the spectral lines are stronger than the continuum in the deconvolved data (especially in the IRS7 sources), indicating that the spectral line emission is less extended than the continuum emission, a pattern which is also seen in the larger DIGIT sample of embedded objects (hereby referred to as the DIGIT sample; Green et al 2013), although data of more crowded regions (e.g. Serpens; Dionatos et al 2013) or strong outflow sources (e.g. L1448-MM; Lee et al 2013) show spatial extent.…”

The Evolution of Far-Infrared Co Emission From Protostars

“…Herschel/ PACS spectra for eightPBRs were later obtained by the HOPS team as part of the Herschel open time (OT) program (Tobin et al 2016). The DIGIT program surveyed 94 young stellar objects in different evolutionary stages with Herschel and obtained far-IR spectra of 24 Herbig Ae/Be stars, 40 T Tauri stars,and 30 protostars in nearby (d  400 pc) star-forming regions (van Kempen et al 2010;Cieza et al 2013;Dionatos et al 2013;Fedele et al 2013;Green et al 2013bGreen et al , 2013aGreen et al , 2016Meeus et al 2013;Sturm et al 2013;Lee et al 2014bLee et al , 2014a. The Herschel/PACS spectra of 30 protostars in the DIGIT sample were presented in Green et al (2013b).…”