A multi‐transition CO and 13CO survey of the Galactic Plane

A survey of the Galactic Plane in the (2→1) and (3→2) rotational transitions of CO and 13CO has been carried out using the 10 m telescope of the Caltech Submillimeter Observatory (CSO). These data, plus previous observations of (1→0) emission from CO and 13CO obtained with at the Five College Radio Astronomy Observatory (FCRAO), have been used to determine the excitation properties of the molecular gas [Tk, n(H2)] throughout the disk of the Milky Way. The bulk of the molecular gas outside the galactic center (R/R⊙≳0.05 kpc) appears to be slightly colder, Tk=10–15 °K, and denser, n(H2)=103–104 cm−3, than previously assumed. There are no obvious gradients in temperature and cloud H2 density with galactocentric radius, although the molecular clouds in the galactic center and near the peak of the galactic ring at R/R⊙=0.4–0.6 are on average warmer (Tk∼15–20 °K) than the molecular material at other radii. The large H2 volume density derived from the CO data implies that a general internal property of molecular...