Oxygen isotopic ratios toward molecular clouds in the Galactic disk

Abstract

We present our observations of the J = 1 − 0 rotation transitions in molecular isotopes C 18 O and C 17 O toward a sample of molecular clouds with different galactocentric distances, using the Delingha 13.7 m (DLH 13.7 m) telescope, administered by Purple Mountain Observatory, and its 9-beam SIS receiver. Complementary observations toward several sources with large galactocentric distance are obtained with the IRAM 30m and Mopra 22m telescopes. C 18 O/C 17 O abundance ratios reflecting the 18 O/ 17 O isotope ratios are obtained from integrated intensity ratios of C 18 O and C 17 O. We derived the ratio value for 13 sources covering a galactocentric distance range of 3 kpc to 16 kpc. In combination with our mapping results that provide a ratio value of 3.01±0.14 in the Galactic center region, it shows that the abundance ratio tends to increase with galactocentric distance, i.e., it supports a radial gradient along the Galactic disk for the abundance ratio. This is consistent with the inside-out formation scenario of our Galaxy. However, our results may suffer from small samples with large galactocentric distance. Combining our data with multitransition lines of C 18 O and C 17 O will be helpful for constraining opacities and abundances and further confirming the Galactic radial gradient shown by the isotope ratio 18 O/ 17 O.

The evolution of CNO isotopes: a new window on cosmic star formation history and the stellar IMF in the age of ALMA

“…A radial 18 O/ 17 O gradient is suggested by Wouterloot et al (2008), who combine observations of different CO transitions in Galactic sources covering the Galactocentric distance range 0 < R GC /kpc < 17. Their finding is further supported by recent independent analysis and observations by Li et al (2016). Ratios of 16 O to 18 O across the disc are taken from the compilation of Wilson & Rood (1994), from OH data of Polehampton et al (2005), and derived by combining the CO data from Wouterloot et al (2008) with the 12 C/ 13 C gradient from Milam et al (2005) following Young et al (2011).…”

The evolution of CNO isotopes: the impact of massive stellar rotators

“…3, middle and lower righthand panels; the original data have been combined with the 12 C/ 13 C gradient by Milam et al 2005 to trace the 16 O/ 18 O gradient) and b) the 18 O/ 17 O gradient, from J = 1−0, 2−1 and 3−2 transitions of C 18 O and C 17 O (small light-grey circles in Fig. 3); (v) Li et al (2016), which derived C 18 O/C 17 O abundance ratios for 13 sources spanning the Galactocentric distance range 3-16 kpc (small filled diamonds in Fig. 3); and (vi) Colzi et al (2018), which targeted the J = 1−0 rotational transitions of hydrogen isocyanide and hydrogen cyanide and derived the 14 N/ 15 N ratios for both molecules (Fig.…”

A Systematic Observational Study on Galactic Interstellar Ratio 18O/17O. II. C18O and C17O J = 2–1 Data Analysis

“…Therefore, we started a systematic observational study based on the 18 O/ 17 O isotope ratio, measuring rotational transitions of C 18 O and C 17 O. Our earlier work indicated lower abundance ratios in the GC region compared to molecular clouds in the Galactic disk, based on mapping toward GC molecular clouds and a single-pointing pilot survey of Galactic disk molecular clouds (Zhang et al 2015;Li et al 2016). The first article in our series of 18 O/ 17 O studies involving large samples of sources was based on measurements of the J = 1-0 lines of C 18 O and C 17 O (Zhang et al 2020, hereafter Paper I).…”