Census of High- and Medium-mass Protostars. V. CO Abundance and the Galactic X _CO Factor

We present the second dust continuum data release in Census of High- and Medium-mass Protostars (CHaMP), expanding methodology trialed Pitts et al. 2019 to entire CHaMP survey area ($280^{\circ}<l<300^{\circ}$, $-4^{\circ}<b<+2^{\circ}$). This includes maps temperature ($T_d$), H$_2$ column density ($N_{H_2}$), gas-phase CO abundance, temperature-density plots for every prestellar clump with Herschel coverage, showing no evidence internal heating most clumps our sample. show that abundance is a strong function $T_d$, can be fit second-order polynomial log-space, typical dispersion factor 2--3. The peaks at $20.0^{+0.4}_{-1.0}$ K value $7.4^{+0.2}_{-0.3}\times10^{-5}$ per H$_2$; low $T_d$ which this maximal occurs relative laboratory results likely due interstellar UV bombardment largest fields. Finally, we that, as predicted by theoretical literature hinted previous studies individual clouds, conversion from integrated $^{12}$CO line intensity ($I_{^{12}CO}$) $N_{H_2}$, $X_{CO}$-factor, varies broken power-law $I_{^{12}CO}$ transition zone between 70 90 km$^{-1}$. $X_{CO}$-function propose has $N_{H_2}\propto I_{^{12}CO}^{0.51}$ $I_{^{12}CO}\lesssim70$ km$^{-1}$ I_{^{12}CO}^{2.3}$ $I_{^{12}CO}\gtrsim90$ high-$I_{^{12}CO}$ side should generalizable known adjustments metallicity, but influence fields on low-$I_{^{12}CO}$ may sample specific. discuss how these expand upon works series, help tie together observational, theoretical, over past decade.