Carbon fractionation in photo-dissociation regions

Abstract

We upgraded the chemical network from the UMIST Database for Astrochemistry 2006 to include isotopes such as 13 C and 18 O. This includes all corresponding isotopologues, their chemical reactions and the properly scaled reaction rate coefficients. We study the fractionation behavior of astrochemically relevant species over a wide range of model parameters, relevant for modelling of photo-dissociation regions (PDRs). We separately analyze the fractionation of the local abundances, fractionation of the total column densities, and fractionation visible in the emission line ratios. We find that strong C + fractionation is possible in cool C + gas. Optical thickness as well as excitation effects produce intensity ratios between 40 and 400. The fractionation of CO in PDRs is significantly different from the diffuse interstellar medium. PDR model results never show a fractionation ratio of the CO column density larger than the elemental ratio. Isotope-selective photo-dissociation is always dominated by the isotope-selective chemistry in dense PDR gas. The fractionation of C, CH, CH + and HCO + is studied in detail, showing that the fractionation of C, CH and CH + is dominated by the fractionation of their parental species. The light hydrides chemically derive from C + , and, consequently, their fractionation state is coupled to that of C + . The fractionation of C is a mixed case depending on whether formation from CO or HCO + dominates. Ratios of the emission lines of [C ii], [C i], 13 CO, and H 13 CO + provide individual diagnostics to the fractionation status of C + , C, and CO.

Formation and Fractionation of CO (Carbon Monoxide) in Diffuse Clouds Observed at Optical and Radio Wavelengths

“…Unlike the current work, Röllig & Ossenkopf (2013) and Szucs et al (2014) Szucs et al (2014) in regard to their Figure 14, equivalent to our Figure 3, their results are much better suited to comparison with our mm-wave absorption results and the UV absorption results must reflect different conditions than those that were considered. In the present work the difference is attributed to higher density and weaker UV illumination along sightlines removed from early-type stars.…”

“…In doing so they followed a remark by Sheffer et al (2007) to the effect that the older result provided a better fit to the UV absorption line observations, a few of which have high N( 12 CO)/N( 13 CO) ratios. We, along with Röllig & Ossenkopf (2013) (also see Roueff et al (2015)) approximated the temperature-dependent rate coefficients for C + exchange measured by Smith & Adams (1980). These are higher than the value k f = 2 × 10 −10 cm 3 s −1 of Watson et al (1976) by factors of 2, 3 and 4 at at kinetic temperatures 160 K, 80 K, and 30 K, respectively.…”

“…Röllig & Ossenkopf (2013) discuss carbon fractionation chemistry in PDR with n(H) = 10 3 − 10 7 cm −3 while Szucs et al (2014) discuss carbon monoxide chemistry in turbulent clouds of typical mass 10 4 M Sun and mean density n(H) = 300 cm −3 , equivalent in some rough sense to a uniform sphere of radius R=7 pc and central column density 2n(H)R = 1.27 × 10 22 cm −2 or A V = 7 mag, well beyond the range of column density considered here.…”

The Intrinsic Abundance Ratio and X-Factor of Co Isotopologues in L 1551 Shielded From Fuv Photodissociation

“…In L 1551, we do not find the decreasing trend even at the low A V range, which indicates the FUV strength in L 1551 is less than that in Perseus. Under the mean ISRF, 12 CO is selfshielded atà 0.5 V mag (Röllig & Ossenkopf 2013;Szűcs et al 2014), which may be the case in L 1551. Figure 8 shows the correlation between the X-factor and the T ex value, and the points are color-coded by A V .…”

“…It is possible that besides the interstellar FUV radiation, embedded OB stars provide strong FUV radiation so that the distance of the penetration will increase. Another possibility is that the higher temperature in the massive cores will also change the fractionation of 12 C and 13 C (Röllig & Ossenkopf 2013). In the intermediate-mass star-forming regions, Kong et al (2015) showed that the intensity ratio of 13 CO ( J = 2-1) to C 18 O ( J = 2-1), which is equivalent to X X CO C O 13 18 for the optically thin case, rises to a peak of up to ∼40 atà 5 V mag, then decreases to ∼4.5 with increasing A V in the southeast of the California molecular cloud.…”