Planck Galactic Cold Clumps at High Galactic Latitude-A Study with CO Lines

Abstract

Gas at high Galactic latitude is a relatively little-noticed component of the interstellar medium. In an effort to address this, forty-one Planck Galactic Cold Clumps at high Galactic latitude (HGal; $|b|>25^{\circ }$) were observed in ${}^{12}$CO, ${}^{13}$CO and C${}^{18}$O J=1-0 lines, using the Purple Mountain Observatory 13.7-m telescope. ${}^{12}$CO (1-0) and ${}^{13}$CO (1-0) emission was detected in all clumps while C${}^{18}$O (1-0) emission was only seen in sixteen clumps. The highest and average latitudes are $71.4^{\circ }$ and $37.8^{\circ }$, respectively. Fifty-one velocity components were obtained and then each was identified as a single clump. Thirty-three clumps were further mapped at 1${}^{\prime }$ resolution and 54 dense cores were extracted. Among dense cores, the average excitation temperature $T_{\mathrm{ex}}$ of ${}^{12}$CO is 10.3 K. The average line widths of thermal and non-thermal velocity dispersions are $0.19$ km s${}^{-1}$ and $0.46$ km s${}^{-1}$ respectively, suggesting that these cores are dominated by turbulence. Distances of the HGal clumps given by Gaia dust reddening are about $120-360$ pc. The ratio of $X_{13}$/$X_{18}$ is significantly higher than that in the solar neighbourhood, implying that HGal gas has a different star formation history compared to the gas in the Galactic disk. HGal cores with sizes from $0.01-0.1$ pc show no notable Larson's relation and the turbulence remains supersonic down to a scale of slightly below $0.1$ pc. None of the HGal cores which bear masses from 0.01-1 $M_{\odot }$ are gravitationally bound and all appear to be confined by outer pressure.

35 pages, 13 figures