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