A Systematic TMRT Observational Study of Galactic ¹²C/¹³C Ratios from Formaldehyde

Abstract

We present observations of the C-band 1 10 − 1 11 (4.8 GHz) and Ku-band 2 11 − 2 12 (14.5 GHz) K-doublet lines of H 2 CO and the C-band 1 10 − 1 11 (4.6 GHz) line of H 2 13 CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65-m radio telescope (TMRT). Our sample with 112 sources includes strong H 2 CO sources from the TMRT molecular line survey at C-band and other known H 2 CO sources. All three lines are detected toward 38 objects (43 radial velocity components) yielding a detection rate of 34%. Complementary observations of their continuum emission at both C-and Ku-bands were performed. Combining spectral line parameters and continuum data, we calculate the column densities, the optical depths and the isotope ratio H 2 12 CO/H 2 13 CO for each source. To evaluate photon trapping caused by sometimes significant opacities in the main isotopologue's rotational mm-wave lines connecting our measured K-doublets, and to obtain 12 C/ 13 C abundance ratios, we used the RADEX non-LTE model accounting for radiative transfer effects. This implied the use of the new collision rates from Wiesenfeld & Faure (2013). Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of 12 C/ 13 C = (5.08±1.10)D GC + (11.86±6.60), with a correlation coefficient of 0.58. D GC refers to Galactocentric distances. Our 12 C/ 13 C ratios agree very well with the ones deduced from CN and C 18 O but are lower than those previously reported on the basis of H 2 CO, tending to suggest that the bulk of the H 2 CO in our sources was formed on dust grain mantles and not in the gas phase.

ALMA view of the 12C/13C isotopic ratio in starburst galaxies

“…Previous observations of CN in the Milky Way indicate a 12 C/ 13 C isotope ratio gradient with Galactocentric distance (Savage et al 2002;Milam et al 2005), which agrees rather well with the gradient derived from measurements of CO and H 2 CO (e.g., Henkel et al 1980Henkel et al , 1982Henkel et al , 1983Henkel et al , 1985Henkel et al , 1994bLanger & Penzias 1990Wilson & Rood 1994;Giannetti et al 2014;Yan et al 2019). This suggests that the 12 C/ 13 C isotope ratios obtained from CN are an excellent indicator of Galactic chemical evolution (Milam et al 2005).…”

First detection of 13CH in the interstellar medium

“…4), we derive a revised 12 C/ 13 C Galactic gradient of 12 C/ 13 C = 5.87(0.45) R GC + 13.25(2.94), where the values in parentheses represent 1σ uncertainties. The addition of our CH data points plus those from Wouterloot & Brand (1996), Ritchey et al (2011), Giannetti et al (2014) and Yan et al (2019) result in values for the best-fit slope and intercept that are within the combined uncertainties consistent with the values derived by Halfen et al (2017). The small uncertainties of the CH data result in somewhat smaller formal uncertainties of the fitted values.…”

“…Plot of 12 C/ 13 C isotope ratios as a function of galactocentric distance, R GC (kpc). The filled red circles represent the 12 C/ 13 C ratio obtained using CH (this paper), while the empty black and purple triangles, grey pentagons, yellow circles, dark blue squares, and green, light blue diamonds and tan stars are those obtained using isotopologues of C 18 O ( J = 1−0 and J = 2−1 transitions) (Wouterloot & Brand 1996;Giannetti et al 2014), CN (Savage et al 2002;Milam et al 2005), CO (Langer & Penzias 1990), CH + (Ritchey et al 2011), H 2 CO (Henkel et al 1985;Yan et al 2019), and complex organic molecules (Halfen et al 2017), respectively. The black solid line represents the weighted fit to the data, and the grey shaded region demarcates the 1σ interval of this fit.…”

“…spread is indeed due to opacity effects, then the ground-state rotational transitions of CH studied in this work, which are free from such effects, should be well suited to quantitatively constrain the 12 C/ 13 C ratio. By combining the 12 C/ 13 C ratio values derived using CH with those derived by Henkel et al (1985), Langer & Penzias (1990), Wouterloot & Brand (1996) 4 , Milam et al (2005, Giannetti et al (2014), Ritchey et al (2011), Halfen et al (2017), and Yan et al (2019) and carrying out a weighted least-squares fit (see Fig. 4), we derive a revised 12 C/ 13 C Galactic gradient of 12 C/ 13 C = 5.87(0.45) R GC + 13.25(2.94), where the values in parentheses represent 1σ uncertainties.…”

Sulphur and carbon isotopes towards Galactic centre clouds

“…Therefore, it is important to also cover the inner region of the Milky Way to find out whether the trend proposed by Chin et al (1996) is also valid for the inner Galaxy as has been reported for the 12 C/ 13 C (see e.g. Yan et al 2019), 14 N/ 15 N (Adande & Ziurys 2012) and 18 O/ 17 O (Wouterloot et al 2008;Zhang et al 2015) isotopic ratios.…”