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 Patrick W. Morris
Carbon Chemistry in Orion BN/KL:  C+, CH+, and CH Distributions, Excitation, and Abundances

The Puzzle of CH+
The simplest C-bearing molecules CH, CH+, and CN were the first molecules of any kind discovered in the ISM, over 75 years ago, through their absorption lines at optical wavelengths.  With a strong source of continuum emission in the background (a star-forming region with thermal emission from heated dust, for example), and today's instrumentation, the electronic transitions are readily observed in the optical and UV.

They each also have rotational states associated with the ground electronic states --- in fact it was such a rotational transition between j = 1 and j = 2 electronic states of CN that lead astronomers to suspect some background radiation which is populating the  j = 1 level, with a rotational energy of 2.3 K from the j = 0 level.  This was the first indication of a widespread background radiation field, although they didn't know its cosmic origin... it would be another 25 years before the 2.7 K CMB was discovered by Penzias & Wilson.

The CH+ ion also caused head scratching up until recently, since it is observed in much higher abundances than expected for an equilibrium chemistry that should apply to the ISM.  It is very reactive, so it is not enough to simply ionize CH, which is usually observed in normal (steady-state) abundances in dense (star-forming) regions, while CH+ is at much lower densities.  Also, combining C+ with neutral H has much too low a reaction rate at normal ISM and molecular cloud temperatures.

The most efficient way to produce CH+ is to get C+ to react with H2:

Shocks and/or PDRs?

The above reaction is highly endothermic, so catching it in the act requires an energetic environment, with an energy input of ~4300 K (~0.3 eV), i.e., a reservoir of vibrationally-excited H2.  While quite atypical of normal ISM temperatures, excited H2 is not hard to find, e.g., in gas exposed to shocked outflows found in dense star forming regions, in the vortices of intermittent turbulence dissipation regions in the ISM, and possibly in large scale shocks from SNe or galactic shear.  H2 can also be excited by UV irradiation in dense PDRs, i.e., near OB-type stars with strong UV radiation fields.   These scenarios are all developed observationally and theoretically within the last ~decade in order to solve basic problems in interstellar carbon chemistry, giving some insight into the prevailing physical conditions in diverse regions (including other galaxies where CH+ is observed).

CH+ has a ground-state dipole moment, so the environments where it is formed can be explored with observations of pure rotational lines at far IR wavelengths.

The Astrophysical Laboratory of Orion BN/KL
 Orion BN/KL has something for everyone --- YSOs, outflows, shocked gas, young OB stars emitting strong FUV radiation, dense and diluted atomic and molecular gas, dust... and it's all close enough (~2.4 kpc) and extended on angular scales that are well-suited to map with HIFI.  The beam size for HIFI observations of C+ is just about the same as shown by the 10" fiduciary in the schematic.

Using the Herschel/HIFI instrument, our team mapped CH+ low-J rotational transitions, the C+ 1901 GHz line, CH 537, 1471/1477 and 1657/1661 GHz Lambda-doubling transitions, and a dozen methanol CH3OH 835 GHz K=5-4 torsional transitions.   The maps cover the areas shown in the boxes centered on BN/KL.  The JHK image on the left is from VIRCAM on  ESO's VISTA telescope.   The image on the right was obtained with Herschel/SPIRE 350 microns (red), HIFI CH J=1-0 (green), and HIFI 1900 GHz continuum (blue, looks violet).

Examples of the very nice C
+, CH+, CH, and methanol data are shown below.

 C+ integrated intensities.  Contours indicate the 1900 GHz continuum. Velocity channel maps of C+.  A snappy video clip that rolls through the velocities at 0.2 km/s intervals ca be viewed here.

 CH+ J=1-0 integrated intensities and total column densities.   Contours indicate the C+ integrated intensities. CH+ J=2-1 integrated intensities.  Negative values around the Hot Core are due to strong absorption, detected only in this CH+ line.
 The distributions and kinematic characteristics of our data show that all of the emission we observed is produced in the extended PDR surrounding the explosive BN/KL outflow --- but (surprisingly) none in the shocked outflow itself.  Our non-LTE radiative transfer and PDR models can reproduce the CH+ and CH line intensities, supporting the prevailing role of UV-irradiation in CH+ formation in this environment.  The PDR characteristics are similar to those of the Orion Bar. Surprisingly, no C+, CH+, or CH appears to be emitted in the shock-heated gas associated with the bipolar outflow.

The formation of CH
+ where C+ and excited H2 are abundant supports the role of UV-driven chemistry, in dense PDRs.  The observed distributions and kinematics of C+, CH+, and CH pose a couple of problems:
1.  C+ and CH+ do not correlate with the H2 S(1) 2.12 micron emission tracing the bipolar eruption $\to$  the reaction is absent from shock-heated gas.

Rather than C+ reacting with H2 to form CH+ in the outflow, we suggest that it's going entirely into the production of CO, which is very abundant throughout OMC1.  The standard gas phase chemistry can follow a couple of different paths after C+ reacts with OH:

There is good evidence for either one (or both) of these paths occurring in the outflow, from observations of fast (80-100 km/s) CO and OH that correlate well with the bipolar geometry:

 The energetic BN/KL outflow imaged in  H2 S(1) 2.12 microns (false color) compared with fast CO J=10-9 (blue and red) and OH 1837 GHz triplet transitions (white). Thick contours represent the upper 60% of radiated power, thin lines correspond to the 30% to 60% power range, and all are on intensity scales normalized to peak integrated emission on intervals of 5% relative flux. Fast blue-shifted OH was not measured due to line confusion.

2.  Where is the reservoir of vibrationally-excited H2 located, for the reaction with C+ to form CH+?

Excited H2 S(1) is emitting in the shock-heated gas of the eruptive outflow, but that's not where we observe CH
+ and C+.  The answer might be in the energetics of H2, in the PDR vs shocked gas.  H2 is thermalized at the nu=1 level in shocks, at an energy that is sufficient to overcome the CH+ formation barrier, leaving ~1500 K of excitation energy.  In the PDR, fluorescence and thermal heating can excite H2 up to high vibrational and rotational levels, providing plenty of energy to excite CH+, resulting in the observed emission that can be matched to PDR models.  CH+ might be forming in the shocked gas, but emitting too weakly to be detected with HIFI.

What's next?
Our team is studying ground-based CO+ observations from APEX, and HIFI and other ground-based data of C+, CO, HCO+, HOC+, and OH to test the hypothesis that CO formation is favored over CH+ in the shocked gas of the outflow.  Why?  Because it puts to test the utility of CH+ and other species with high formation enthalpies as probes of processes that regulate the thermal balance of the interstellar gas and influence star formation in molecular clouds.

Some reading:

Morris, Patrick W.; Gupta, Harshal; Nagy, Zsofia; Pearson, John C.; Ossenkopf-Okada, Volker; et al.  ApJ 2016, 829, 15, "Herschel/HIFI Spectral Mapping of C+, CH+, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH+ Formation"

Nagy, Z.; Van der Tak, F. F. S.; Ossenkopf, V.; Gerin, M.; Le Petit, F., et al. 2013, A&A, 550, 96, "The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+. The effect of high electron density and vibrationally excited H2 in a warm PDR surface"

Faure, A.; Halvick, P.; Stoecklin, T.; Honvault, P.; Epee Epee, M. D., et al.  2017, Astro-ph , "State-to-state chemistry and rotational excitation of CH+ in photon-dominated regions"

Orion KL BN/KL Molecular Cloud CH CH+ CH3OH C+ methanol shock PDR outflow molecule formation ISM stellar radiation OB O-type star

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 Last Updated 11/5/2011