Carbon fractionation in photon-dominated regions

Details Carbon fractionation in photon-dominated regions Carbon fractionation in photon-dominated regions

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.280o&link_type=abstract

The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Within the Herschel key projects "The Warm And Dense" ISM (WADI) and "Herschel Observations of EXtra-Ordinary Sources" (HEXOS) we observe a number of prominent photon-dominated regions (PDRs) to measure their chemical structure, determine their energy balance and dynamic state. Carbon fractionation is one of the processes that is sensitive to the conditions in the PDR front, in particular the flux of UV photons able to ionize atomic carbon and the temperature. It is mainly driven by the isotope exchange reaction of 13C+ with CO. In the transition region where ionized carbon and molecular CO coexist, we expect a noticeable reduction of the 13C+ abundance and a corresponding increase of the 13CO abundance at low temperatures. Unfortunately, due to the high optical depth of most lines of the main isotopologue, an accurate determination of the 13CO/12CO ratio is not possible. With Herschel, we had the first chance to systematically study the ratio of 12C+/13C+ in PDRs. The [13CII] lines were clearly detected in the Orion Bar, NGC3603, Mon R2, and the Carina molecular cloud. We found that for all bright PDRs, the ratio between the integrated line intensities of 12CII and 13CII is only about 30 (20 ... 50). For optically thin lines this would indicate an overabundance of 13C+ relative to the canonical isotope ratio in the elemental abundance. The low values of the [12CII]/[13CII] ratio can be explained by an optical depth of the [12CII] line around three, but require that fractionation is not efficient, i.e. the transition regions are only narrow and hot, driven by a high UV field. From the spatial intensity distribution we find that even internal surfaces in the clumpy cloud structure must face these high UV fields, i.e. that the radiation is efficiently redistributed over a large volume. The [13CII] line shows three hyperfine components allowing to test the excitation of the ion assuming that the lines are optically thin. We found systematic variations of the ratio of the hyperfine components across the PDRs which are inconsistent with normal collisional excitation or continuum pumping. The mechanism producing the observed ratios still has to be found.

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