High-J CO emission spatial distribution and excitation in the Orion Bar

Abstract

Context. With Herschel, we can for the first time observe a wealth of high-J CO lines in the interstellar medium with a high angular resolution. These lines are specifically useful for tracing the warm and dense gas and are therefore very appropriate for a study of strongly irradiated dense photodissocation regions (PDRs). Aims. We characterize the morphology of CO J = 19-18 emission and study the high-J CO excitation in a highly UV-irradiated prototypical PDR, the Orion Bar. Methods. We used fully sampled maps of CO J = 19-18 emission with the Photoconductor Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory over an area of ∼110 × 110 with an angular resolution of 9 . We studied the morphology of this high-J CO line in the Orion Bar and in the region in front and behind the Bar, and compared it with lower-J lines of CO from J = 5-4 to J = 13-12 and 13 CO from J = 5-4 to J = 11-10 emission observed with the Herschel Spectral and Photometric Imaging Receiver (SPIRE). In addition, we compared the high-J CO to polycyclic aromatic hydrocarbon (PAH) emission and vibrationally excited H 2 . We used the CO and 13 CO observations and the RADEX model to derive the physical conditions in the warm molecular gas layers. Results. The CO J = 19-18 line is detected unambiguously everywhere in the observed region, in the Bar, and in front and behind of it. In the Bar, the most striking features are several knots of enhanced emission that probably result from column and/or volume density enhancements. The corresponding structures are most likely even smaller than what PACS is able to resolve. The high-J CO line mostly arises from the warm edge of the Orion Bar PDR, while the lower-J lines arise from a colder region farther inside the molecular cloud. Even if it is slightly shifted farther into the PDR, the high-J CO emission peaks are very close to the H/H 2 dissociation front, as traced by the peaks of H 2 vibrational emission. Our results also suggest that the high-J CO emitting gas is mainly excited by photoelectric heating. The CO J = 19-18/J = 12-11 line intensity ratio peaks in front of the CO J = 19-18 emission between the dissociation and ionization fronts, where the PAH emission also peak. A warm or hot molecular gas could thus be present in the atomic region where the intense UV radiation is mostly unshielded. In agreement with recent ALMA detections, low column densities of hot molecular gas seem to exist between the ionization and dissociation fronts. As found in other studies, the best fit with RADEX modeling for beam-averaged physical conditions is for a density of 10 6 cm −3 and a high thermal pressure (P/k = n H × T ) of ∼1-2 × 10 8 K cm −3 . Conclusions. The high-J CO emission is concentrated close to the dissociation front in the Orion Bar. Hot CO may also lie in the atomic PDR between the ionization and dissociation fronts, which is consistent with the dynamical and photoevaporation effects.

Photodissociation and X-Ray Dominated Regions

“…Molecular emission in the form of globules or plumes extends into the atomic gas, indicating advection of the molecular gas through the DF (Goicoechea et al 2016). In addition, the HCO + (J = 4 − 3), high-J CO (Parikka et al 2018), and H * 2 (v = 1 − 0) are nearly coincident, suggesting a merging of the H2 and CO dissociation layers (Goicoechea et al 2016, Kirsanova & Wiebe 2019. However, we note that this may also be due to high densities and resulting small scale sizes, with vibrationally excited H2 driving the carbon chemistry to produce both CO and HCO + .…”

“…Other fields that are in use are the Mathis field (U ; Mathis et al 1983, Weingartner & Draine 2001c) and the Draine field (χ; Draine 1978). In terms of the Habing field, the median local Galactic interstellar radiation field is estimated to be G0∼1.6 (Parravano et al 2003), comparable to the Draine field, while G0∼10 5 for the PDR behind the Trapezium cluster.…”

“…The layered structure in the Orion Bar is apparent in many atomic and molecular tracers (e.g., Tielens et al 1993, Walmsley et al 2000, Lis & Schilke 2003, van der Wiel et al 2009, Bernard-Salas et al 2012, Joblin et al 2018, Parikka et al 2018) demonstrating an edge-on geometry. Nevertheless, on small scales a more complex structure is seen at the DF.…”

Merged H/H2 and C+/C/CO transitions in the Orion Bar

“…In this sense, these two models compliment each other. A more advanced modelling that includes both dynamics and chemistry, considers the ionized, atomic and molecular gas, and takes into account more molecular line data (e.g., Parikka et al 2018;Goicoechea et al 2019), will definitely help to resolve the issue. Fig.…”

Herschel survey and modelling of externally-illuminated photoevaporating protoplanetary disks

“…For the proplyd 244-440, we also included the OH 84 μm line detected in a PACS survey of the Orion Bar and described in Parikka et al (2016). HST10 has been observed in the H 2 1-0 S(1) line by Chen et al (1998), and we use their value for the intensity of this line.…”