The origin of the [C II] emission in the S140 photon-dominated regions. New insights from HIFI

Abstract

Using Herschel's HIFI instrument, we observe [C ii] along a cut through S140, as well as high-J transitions of CO and HCO + at two positions on the cut, corresponding to the externally irradiated ionization front and the embedded massive star-forming core IRS1. The HIFI data were combined with available ground-based observations and modeled using the KOSMA-τ model for photon-dominated regions (PDRs). We derive the physical conditions in S140 and in particular the origin of [C ii] emission around IRS1. We identify three distinct regions of [C ii] emission from the cut, one close to the embedded source IRS1, one associated with the ionization front, and one further into the cloud. The line emission can be understood in terms of a clumpy model of PDRs. At the position of IRS1, we identify at least two distinct components contributing to the [C ii] emission, one of them a small, hot component, which can possibly be identified with the irradiated outflow walls. This is consistent with the [C ii] peak at IRS1 coinciding with shocked H 2 emission at the edges of the outflow cavity. We note that previously available observations of IRS1 can be reproduced well by a single-component KOSMA-τ model. Thus, it is HIFI's unprecedented spatial and spectral resolution, as well as its sensitivity that has allowed us to uncover an additional hot gas component in the S140 region.

Velocity profiles of [CII], [CI], CO, and [OI] and physical conditions in four star-forming regions in the Large Magellanic Cloud

“…The significant difference in the velocity profile of the [C ii] emission observed in many sources is interpreted as an indication of a substantial fraction of the [C ii] emitting material being accelerated relative to the quiescent material, e.g. it undergoes ablation (Dedes et al 2010;Mookerjea et al 2012;Okada et al 2012;Schneider et al 2012;Simon et al 2012;Pilleri et al 2012), or photoevaporated (Sandell et al 2015). Although an order of a few km s −1 displacement is consistent with modeled photoevaporation in globules (Lefloch & Lazareff 1994), only a few studies to compare the observed line profiles of different emission lines with simulation incorporating full PDR chemistry have been conducted (Bisbas et al 2018).…”

Resolving the Far-Ir Line Deficit: Photoelectric Heating and Far-Ir Line Cooling in NGC 1097 and NGC 4559

“…The deconvolved [C ii] line width in most regions is narrower than the PACS instrumental profile at 157 μm (FWHM = 240 km s −1 ); thus, line widths in our data are dominated by the instrumental profile. Observations of [C ii] emission in a galactic star-forming region and its associated PDR show widths of 5-10 km s −1 using the Heterodyne Instrument for the Far Infrared (HIFI) on Herschel's (Dedes et al 2010), which has a velocity resolution of 0.7 km s −1 . For narrow [C ii] emission, i.e., 4-10 km s −1 , the [C ii] line becomes optically thick at N(H) = 3.65-6.25 × 10 21 cm −2 (Russell et al 1980), assuming N (C + )/N(H) = 1.6 × 10 −4 .…”

PDR MODEL MAPPING OF PHYSICAL CONDITIONS VIASPITZER/IRS SPECTROSCOPY OF H2: THEORETICAL SUCCESS TOWARD NGC 2023-SOUTH

“…Habart et al (2011) found order-of-magnitude discrepancies between their PDR model results and Spitzer H 2 data for rotational levels J 3 toward mainly low-excitation PDRs. Dedes et al (2010) were able to fit observations of high-J CO lines by using spherical PDR models for an ensemble of clumps distributed in size and mass. Weingartner & Draine (1999) have suggested that radiation forces on grains increase the dust/gas ratio in PDRs, thus leading to enhanced heating rates.…”