Herschel/HIFI spectral line survey of the Orion Bar

Abstract

Context. Photon dominated regions (PDRs) are interfaces between the mainly ionized and mainly molecular material around young massive stars. Analysis of the physical and chemical structure of such regions traces the impact of far-ultraviolet radiation of young massive stars on their environment. Aims. We present results on the physical and chemical structure of the prototypical high UV-illumination edge-on Orion Bar PDR from an unbiased spectral line survey with a wide spectral coverage which includes lines of many important gas coolants such as [Cii], [Ci], and CO and other key molecules such as H 2 CO, H 2 O, HCN, HCO + , and SO. Methods. A spectral scan from 480-1250 GHz and 1410-1910 GHz at 1.1 MHz resolution was obtained by the HIFI instrument on board the Herschel Space Observatory. We obtained physical parameters for the observed molecules. For molecules with multiple transitions we used rotational diagrams to obtain excitation temperatures and column densities. For species with a single detected transition we used an optically thin LTE approximation. In the case of species with available collisional rates, we also performed a non-LTE analysis to obtain kinetic temperatures, H 2 volume densities, and column densities. Results. About 120 lines corresponding to 29 molecules (including isotopologues) have been detected in the Herschel/HIFI line survey, including 11 transitions of CO, 7 transitions of 13 CO, 6 transitions of C 18 O, 10 transitions of H 2 CO, and 6 transitions of H 2 O. The rotational temperatures are in the range between ∼22 and ∼146 K and the column densities are in the range between 1.8 × 10 12 cm −2 and 4.5 × 10 17 cm −2. For species with at least three detected transitions and available collisional excitation rates we derived a best fit kinetic temperature and H 2 volume density. Most species trace kinetic temperatures in the range between 100 and 150 K and H 2 volume densities in the range between 10 5 and 10 6 cm −3. The species with temperatures and/or densities outside this range include the H 2 CO transitions tracing a very high temperature (315 K) and density (1.4 × 10 6 cm −3) component and SO corresponding to the lowest temperature (56 K) measured as a part of this line survey. Conclusions. The observed lines/species reveal a range of physical conditions (gas density/temperature) involving structures at high density/high pressure, making the traditional clump/interclump picture of the Orion Bar obsolete.

Spatially resolved images of reactive ions in the Orion Bar

“…They also reproduce (±3 K) the T rot inferred from the rotational population diagrams for a 10 -wide filament (see Table B.1). For H 13 CO + , HOC + , and SO + , we obtain n(H 2 ) 10 5 cm −3 , n(e − ) = 10 cm −3 , and T k 200 K. These conditions agree with those inferred by Nagy et al (2017) for the H 12 CO + rotationally excited emission (J = 6−5 to 11−10) observed by Herschel/HIFI.…”

“…Herschel allowed the detection of OH, CH + , and SH + emission toward dense PDRs (Goicoechea et al 2011;Nagy et al 2013Nagy et al , 2017Parikka et al 2017). Unfortunately, the limited size of the space telescope did not permit us to resolve the ∆A V 1 mag extent of the DF (a few arcsec for the closest PDRs).…”

CO Spectral Line Energy Distributions in Galactic Sources: Empirical Interpretation of Extragalactic Observations∗

“…Individual analyses of the full spectra have been reported for OrionKL (Crockett et al 2014), OrionS (Tahani 2013;Tahani et al 2016), the Orion Bar (Nagy et al 2017), and SgrB2(N) + at 692GHz, J=7-6 by OT1_mgerin_4 in observations targeting CI at 809GHz, and J=8-7 by OT1_cvastel_2 in observations targeting HDO at 919GHz. Observations of these three transitions utilized the WBS, and were made in DBS mode.…”

STATCONT: A statistical continuum level determination method for line-rich sources

“…Similarly, a complex chemistry is also found towards disks (e.g., Öberg et al 2015;Guzmán et al 2015), shocked gas (e.g., Codella et al 2010;Sugimura et al 2011) and photon-dominated regions (e.g., Treviño-Morales et al 2014Nagy et al 2017). In extragalactic astronomy, recent observations reach good enough sensitivities to detect an increasing number of spectral line features in the central regions of many galaxies (e.g., González-Alfonso et al 2004;Martín et al 2006Martín et al , 2011Aladro et al 2011;Rangwala et al 2011;Muller et al 2011Muller et al , 2014Meier et al 2015), and it is expected that individual hot molecular cores will soon be detected in other galaxies (see e.g., Shimonishi et al 2016).…”