Dense and warm molecular gas in the envelopes and outflows of southern low-mass protostars

Abstract

Context. Observations of dense molecular gas lie at the basis of our understanding of the density and temperature structure of protostellar envelopes and molecular outflows. The Atacama Pathfinder EXperiment (APEX) opens up the study of southern (Dec < −35 • ) protostars. Aims. We aim to characterize the properties of the protostellar envelope, molecular outflow and surrounding cloud, through observations of high excitation molecular lines within a sample of 16 southern sources presumed to be embedded YSOs, including the most luminous Class I objects in Corona Australis and Chamaeleon. Methods. Observations of submillimeter lines of CO, HCO + and their isotopologues, both single spectra and small maps (up to 80 × 80 ), were taken with the FLASH and APEX-2a instruments mounted on APEX to trace the gas around the sources. The HARP-B instrument on the JCMT was used to map IRAS 15398-3359 in these lines. HCO + mapping probes the presence of dense centrally condensed gas, a characteristic of protostellar envelopes. The rare isotopologues C 18 O and H 13 CO + are also included to determine the optical depth, column density, and source velocity. The combination of multiple CO transitions, such as 3-2, 4-3 and 7-6, allows to constrain outflow properties, in particular the temperature. Archival submillimeter continuum data are used to determine envelope masses. Results. Eleven of the sixteen sources have associated warm and/or dense (≥10 6 cm −3 ) quiescent gas characteristic of protostellar envelopes, or an associated outflow. Using the strength and degree of concentration of the HCO + 4-3 and CO 4-3 lines as a diagnostic, five sources classified as Class I based on their spectral energy distributions are found not to be embedded YSOs. The C 18 O 3-2 lines show that for none of the sources, foreground cloud layers are present. Strong molecular outflows are found around six sources, with outflow forces an order of magnitude higher than for previously studied Class I sources of similar luminosity. Conclusions. This study provides a starting point for future ALMA and Herschel surveys by identifying truly embedded southern YSOs and determining their larger scale envelope and outflow characteristics.

Herschelobservations of the hydroxyl radical (OH) in young stellar objects

“…Combined with the flux derived from the unresolved line observed with PACS, this constrains the line width to FWHM > ∼ 11 km s −1 . This width is much broader than expected for the quiescent envelope from ground-based observations (van Kempen et al 2009a) and indicates that the observed OH emission most likely stems from shocked gas in HH 46.…”

“…For comparison, the FWHM of the H 2 O 1 10 −1 01 transition derived from recent unpublished HIFI observations is about 16 km s −1 . The CO(6−5) and CO(7−6) lines observed with APEX by van Kempen et al (2009a) are narrower. In a similar HIFI observation, OH emission was tentatively detected below the 5σ level for the strongest triplet component in NGC 1333 IRAS 2A, in agreement with the upper limits on OH emission in the PACS observation of the same source.…”

Star formation in Cometary Globule 1: the second generation

“…Even though the position where van Kempen et al (2009) observed the C 18 O (3-2) and the HCO + (4-3) lines, the YSO, lies between the mapping positions in this paper, their measurement can be used to evaluate the nature of the C 18 O_maximum. The APEX beam size at 319 GHz is similar to the SEST beam size at 220 GHz, which makes comparing the line intensities observed with APEX and SEST realistic.…”

“…Even though the emission from high density tracers is weak in the CG 12 C 18 O hotspot, the C 18 O (3-2) T MB of 10 K is high compared to the T MB temperatures in the (2-1) and (1-0) transitions (5 K and 2.2 K, respectively). Observations of the high density tracers (van Kempen et al 2009) and C 18 O thus indicate that C 18 O_maximum is a moderately dense elongated structure west of the YSO position.…”

“…The HCO + (4-3) line observed by van Kempen et al (2009) in the direction of the YSO is weak (T MB 0.6 K) and H 13 CO + (4-3) was a upper limit of 0.08 K. This rules out a dense core in this direction (van Kempen et al 2009). Even though the position where van Kempen et al (2009) observed the C 18 O (3-2) and the HCO + (4-3) lines, the YSO, lies between the mapping positions in this paper, their measurement can be used to evaluate the nature of the C 18 O_maximum.…”