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Register a new userCensus of High- and Medium-mass Protostars V. CO Abundance and the Galactic $X_{text{CO}}$ Factor
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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.
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We report the second complete molecular line data release from the {em Census of High- and Medium-mass Protostars} (CHaMP), a large-scale, unbiased, uniform mapping survey at sub-parsec resolution, of mm-wave line emission from 303 massive, dense molecular clumps in the Milky Way. This release is for all $^{12}$CO $J$=1$rightarrow$0 emission associated with the dense gas, the first from Phase II of the survey, which includes $^{12}$CO, $^{13}$CO, and C$^{18}$O. The observed clump emission traced by both $^{12}$CO and HCO$^+$ (from Phase I) shows very similar morphology, indicating that, for dense molecular clouds and complexes of all sizes, parsec-scale clumps contain $Xi$ ~ 75% of the mass, while only 25% of the mass lies in extended (>~ 10 pc) or low density components in these same areas. The mass fraction of all gas above a density 10$^9$ m$^{-3}$ is $xi_9$ >~ 50%. This suggests that parsec-scale clumps may be the basic building blocks of the molecular ISM, rather than the standard GMC concept. Using $^{12}$CO emission, we derive physical properties of these clumps in their entirety, and compare them to properties from HCO$^+$, tracing their denser interiors. We compare the standard X-factor converting $I_{CO}$ to $N_{H_2}$ with alternative
The Census of High- and Medium-mass Protostars (CHaMP) is the first large-scale, unbiased, uniform mapping survey at sub-parsec scale resolution of 90 GHz line emission from massive molecular clumps in the Milky Way. We present the first Mopra (ATNF) maps of the CHaMP survey region (300{deg}>l>280{deg}) in the HCO+ J=1-0 line, which is usually thought to trace gas at densities up to 10^11 m-3. In this paper we introduce the survey and its strategy, describe the observational and data reduction procedures, and give a complete catalogue of moment maps of the HCO+ J=1-0 emission from the ensemble of 301 massive molecular clumps. From these maps we also derive the physical parameters of the clumps, using standard molecular spectral-line analysis techniques. This analysis yields the following range of properties: integrated line intensity 1-30 K km s-1, peak line brightness 1-7 K, linewidth 1-10 km s-1, integrated line luminosity 0.5-200 K km s-1 pc^2, FWHM size 0.2-2.5 pc, mean projected axial ratio 2, optical depth 0.08-2, total surface density 30-3000 M{sun} pc-2, number density 0.2-30 x 10^9 m-3, mass 15-8000 M{sun}, virial parameter 1-55, and total gas pressure 0.3-700 pPa. We find that the CHaMP clumps do not obey a Larson-type size-linewidth relation. Among the clumps, there exists a large population of subthermally excited, weakly-emitting (but easily detectable) dense molecular clumps, confirming the prediction of Narayanan et al. (2008). These weakly-emitting clumps comprise 95% of all massive clumps by number, and 87% of the molecular mass, in this portion of the Galaxy; their properties are distinct from the brighter massive star-forming regions that are more typically studied. If the clumps evolve by slow contraction, the 95% of fainter clumps may represent a long-lived stage of pressure-confined, gravitationally stable massive clump evolution, while the CHaMP ... (abridged)
We present $^{12}$CO, $^{13}$CO, and C$^{18}$O data as the next major release for the CHaMP project, an unbiased sample of Galactic molecular clouds in $l$ = 280$^{circ}$-300$^{circ}$. From a radiative transfer analysis, we self-consistently compute 3D cubes of optical depth, excitation temperature, and column density for $sim$300 massive clumps, and update the $I_{rm CO}$-dependent CO$rightarrow$H$_2$ conversion law of Barnes et al (2015). For $N$ $propto$ $I^p$, we find $p$ = 1.92$pm$0.05 for the velocity-resolved conversion law aggregated over all clumps. A practical, integrated conversion law is $N_{rm CO}$ = (4.0$pm$0.3)$times$10$^{19}$m$^{-2}$ $I_{rm CO}^{1.27pm0.02}$, confirming an overall 2$times$ higher total molecular mass for Milky Way clouds, compared to the standard $X$ factor. We use these laws to compare the kinematics of clump interiors with their foreground $^{12}$CO envelopes, and find evidence that most clumps are not dynamically uniform: irregular portions seem to be either slowly accreting onto the interiors, or dispersing from them. We compute the spatially-resolved mass accretion/dispersal rate across all clumps, and map the local flow timescale. While these flows are not clearly correlated with clump structures, the inferred accretion rate is a statistically strong function of the local mass surface density $Sigma$, suggesting near-exponential growth or loss of mass over effective timescales $sim$30-50 Myr. At high enough $Sigma$, accretion dominates, suggesting gravity plays an important role in both processes. If confirmed by numerical simulations, this sedimentation picture would support arguments for long clump lifetimes mediated by pressure confinement, with a terminal crescendo of star formation, suggesting a resolution to the 40-yr-old puzzle of the dynamical state of molecular clouds and their low star formation efficiency.
We present full spectral scans from 200-670$mu$m of 26 Class 0+I protostellar sources, obtained with $Herschel$-SPIRE, as part of the COPS-SPIRE Open Time program, complementary to the DIGIT and WISH Key programs. Based on our nearly continuous, line-free spectra from 200-670 $mu$m, the calculated bolometric luminosities ($L_{rm bol}$) increase by 50% on average, and the bolometric temperatures ($T_{rm bol}$) decrease by 10% on average, in comparison with the measurements without Herschel. Fifteen protostars have the same Class using $T_{rm bol}$ and $L_{rm bol}$/$L_{rm submm}$. We identify rotational transitions of CO lines from J=4-3 to J=13-12, along with emission lines of $^{13}$CO, HCO$^+$, H$_{2}$O, and [CI]. The ratios of $^{12}$CO to $^{13}$CO indicate that $^{12}$CO emission remains optically thick for $J_{rm up}$ < 13. We fit up to four components of temperature from the rotational diagram with flexible break points to separate the components. The distribution of rotational temperatures shows a primary population around 100 K with a secondary population at $sim$350 K. We quantify the correlations of each line pair found in our dataset, and find the strength of correlation of CO lines decreases as the difference between $J$-level between two CO lines increases. The multiple origins of CO emission previously revealed by velocity-resolved profiles are consistent with this smooth distribution if each physical component contributes to a wide range of CO lines with significant overlap in the CO ladder. We investigate the spatial extent of CO emission and find that the morphology is more centrally peaked and less bipolar at high-$J$ lines. We find the CO emission observed with SPIRE related to outflows, which consists two components, the entrained gas and shocked gas, as revealed by our rotational diagram analysis as well as the studies with velocity-resolved CO emission.
We investigate the evolution of far-IR CO emission from protostars observed with Herschel/PACS for 50 sources from the combined sample of HOPS and DIGIT Herschel key programs. From the uniformly sampled spectral energy distributions, we computed $L_{rm{bol}}$, $T_{rm{bol}}$ and $L_{rm {bol}}/L_{rm {smm}}$ for these sources to search for correlations between far-IR CO emission and protostellar properties. We find a strong and tight correlation between far-IR CO luminosity ($L^{rm fir}_{rm CO}$) and the bolometric luminosity ($L_{rm{bol}}$) of the protostars with $L^{rm fir}_{rm CO}$ $propto$ $L_{rm{bol}}^{0.7}$. We, however, do not find a strong correlation between $L^{rm fir}_{rm CO}$ and protostellar evolutionary indicators, $T_{rm{bol}}$ and $L_{rm {bol}}/L_{rm {smm}}$. FIR CO emission from protostars traces the currently shocked gas by jets/outflows, and $L^{rm fir}_{rm CO}$ is proportional to the instantaneous mass loss rate, $dot{M}_{rm{out}}$. The correlation between $L^{rm fir}_{rm CO}$ and $L_{rm{bol}}$ is indicative of instantaneous $dot{M}_{rm{out}}$ tracking instantaneous $dot{M}_{rm{acc}}$. The lack of correlation between $L^{rm fir}_{rm CO}$ and evolutionary indicators $T_{rm{bol}}$ and $L_{rm {bol}}/L_{rm {smm}}$ suggests that $dot{M}_{rm{out}}$ and, therefore, $dot{M}_{rm{acc}}$ do not show any clear evolutionary trend. These results are consistent with mass accretion/ejection in protostars being episodic. Taken together with the previous finding that the time-averaged mass ejection/accretion rate declines during the protostellar phase (e.g., Bontemps et al. 1996), our results suggest that the instantaneous accretion/ejection rate of protostars is highly time variable and episodic, but the amplitude and/or frequency of this variability decreases with time such that the time averaged accretion/ejection rate declines with system age.
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