Detection of a dense clump in a filament interacting with W51e2

Abstract

In the framework of the Herschel/PRISMAS guaranteed time key program, the line of sight to the distant ultracompact H ii region W51e2 has been observed using several selected molecular species. Most of the detected absorption features are not associated with the background high-mass star-forming region and probe the diffuse matter along the line of sight. We present here the detection of an additional narrow absorption feature at ∼70 km s −1 in the observed spectra of HDO, NH 3 and C 3 . The 70 km s −1 feature is not uniquely identifiable with the dynamic components (the main cloud and the large-scale foreground filament) so-far identified toward this region. The narrow absorption feature is similar to the one found toward low-mass protostars, which is characteristic of the presence of a cold external envelope. The far-infrared spectroscopic data were combined with existing ground-based observations of 12 CO, 13 CO, CCH, CN, and C 3 H 2 to characterize the 70 km s −1 component. Using a non-LTE analysis of multiple transitions of NH 3 and CN, we estimated the density (n(H 2 ) ∼ (1-5) × 10 5 cm −3 ) and temperature (10-30 K) for this narrow feature. We used a gasgrain warm-up based chemical model with physical parameters derived from the NH 3 data to explain the observed abundances of the different chemical species. We propose that the 70 km s −1 narrow feature arises in a dense and cold clump that probably undergoes collapse to form a low-mass protostar, formed on the trailing side of the high-velocity filament, which is thought to be interacting with the W51 main cloud. While the fortuitous coincidence of the dense clump along the line of sight with the continuum-bright W51e2 compact H ii region has contributed to its nondetection in the continuum images, this same attribute makes it an appropriate source for absorption studies and in particular for ice studies of star-forming regions.

Ortho-to-para ratio of NH2

“…chemical models for the nitrogen hydrides in different physical conditions in Persson et al 2014). We did, however, find redshifted absorption in W51, at V LSR ∼ 68 km s −1 , tracing a dense clump in a filament interacting with W51, also detected in C 3 , HDO and NH 3 (Mookerjea et al 2014). The absence of emission from any of the rotational NH 3 transitions in this component suggests a cold core.…”

“…( f ) Derived from CN and NH 3 rotational transitions (Mookerjea et al 2014). be considered as more robust than the results in the molecular envelopes.…”

Tracing the cold and warm physico-chemical structure of deeply embedded protostars: IRAS 16293−2422 vs. VLA 1623−2417

“…In high-mass star-forming regions, c−C 3 H 2 and C 2 H tend to be strongly correlated, with both lines presenting similar spatial distributions (Pilleri et al 2013;Mookerjea et al 2012Mookerjea et al , 2014. In contrast, toward IRAS 20343+4129, c−C 3 H 2 and C 2 H show an anti-correlation around the outflow cavity walls of IRS 1 (Fontani et al 2012) but not around the UC HII-region of IRS 3.…”

First detection of the carbon chain molecules13CCC and C13CC towards SgrB2(M)

“…However, this chemical pathway can not be used to explain the abundance of C 3 in the warm envelopes of hot cores. Post warm-up gas phase chemistry of CH 4 applicable at moderate temperatures of 30 K is required to explain the abundance of C 3 and associated hydrocarbons satisfactorily for a few sources (Mookerjea et al 2010(Mookerjea et al , 2012(Mookerjea et al , 2014. Further observations of high density warm regions are needed to clearly establish this hypothesis of carbon chain formation in these regions.…”