VELOCITY-RESOLVED [C ii] EMISSION AND [C ii]/FIR MAPPING ALONG ORION WITHHERSCHEL

Abstract

We present the first ~7.5'×11.5' velocity-resolved (~0.2 km s) map of the [C ii] 158 m line toward the Orion molecular cloud 1 (OMC 1) taken with the/HIFI instrument. In combination with far-infrared (FIR) photometric images and velocity-resolved maps of the H41 hydrogen recombination and CO =2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the [C ii] luminosity (~85 %) is from the extended, FUV-illuminated face of the cloud (>500, >5×10 cm) and from dense PDRs (≳10, ≳10 cm) at the interface between OMC 1 and the H ii region surrounding the Trapezium cluster. Around ~15 % of the [C ii] emission arises from a different gas component without CO counterpart. The [C ii] excitation, PDR gas turbulence, line opacity (from [C ii]) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the [C ii]/ and / ratios and show that [C ii]/ decreases from the extended cloud component (~10-10) to the more opaque star-forming cores (~10-10). The lowest values are reminiscent of the "[C ii] deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing [C ii]/ ratio correlates better with the column density of dust through the molecular cloud than with /. We conclude that the [C ii] emitting column relative to the total dust column along each line of sight is responsible for the observed [C ii]/ variations through the cloud.

HERSCHEL/HIFI SPECTRAL MAPPING OF C+, CH+, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH+ FORMATION

“…Outside the Hot Core region, the average column density á ñ+  N C 1 . 0 1 0 19 ( ) cm −2 agrees almost exactly with the value estimated by Ossenkopf et al (2013) for the Orion Bar where C + intensity peaks and is a factor of 3 higher than the OMC1 average quoted by Goicoechea et al (2015b), while the column densities at the positions marked in Figure 3 around BN/KL are a factor of 2-3 higher than at the Orion Bar peak.…”

“…Overall the intensitiy distribution of CH + is very similar to that of C + mapped over the same area by Goicoechea et al (2015aGoicoechea et al ( , 2015b.…”

“…By contrast, C + peaks in projection further to the NE of BN/KL than CH + , which does not match a PDR scenario for a source of UV radiation from the BN/KL complex or the Trapezium on the arcminute scale. Goicoechea et al (2015aGoicoechea et al ( , 2015b showed that the C + emitting region is widespread in OMC1, and although it is not exactly following the distribution of CO, which has its own velocity-dependent morphology, the overall ionized/PDR/molecular gas interfaces are revealed on the larger scales across OMC1, through comparison of hydrogen recombination, CO J=2−1, and C + integrated intensity distributions.…”

“…Furthermore, since CH + is produced where C + is the dominant ion, n e scales with n H 2 ; then at the H 2 density of 5×10 5 cm −3 assumed above, C/H=1.4×10 −4 , all carbon is ionized, and n e ≈140 cm −3 . To investigate the role of the background continuum in the excitation of CH + , we ran models where we ignored the background continuum measured toward Orion BN/KL (which has a mean dust temperature of ≈30 K; Goicoechea et al 2015b) and assumed only a cosmic microwave background temperature of 2.76 K. In this case, the excitation temperatures drop by factors of 1.2-2.9. The integrated intensities for the three lowest transitions increase from 1%-50%, and then decrease by 8%-21% for the J=4−3 and 5-4 transitions.…”

Gravitational collapse of the OMC-1 region

“…Figure 1b illustrates the total integrated intensity map of the N 2 H + (1−0) emission within this region. The coverage of our maps corresponds to a ∼7-pc long, north-south longitudinal cut along the entire ISF, including the ONC (e.g., O'dell 2001), the Orion BN/KL region, the Orion Bar, and the Orion HII region around the Trapezium (e.g., Goicoechea et al 2015; see also Fig.1a). Tracing the gas at densities of n(H 2 ) > 10 4.5 cm −3 (e.g., Caselli et al 2002), the N 2 H + emission reveals the intricate distribution of dense gas along the main spine of the ISF.…”