arXiv subjects

C. S. Luo

Publications and source records attributed to C. S. Luo.

4 recordsLinked to original sources

arXiv2026

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde VI. The photodissociation region M17SW

The kinetic temperature structure of the photodissociation region M17SW was mapped using the IRAM 30 m telescope. This mapping employed the para-H2CO triplet (J(KaKc) = 303-202, 322-221, and 321-220) near 218 GHz on a scale of ~0.2 pc. The kinetic temperatures were derived by modeling the average H2CO line ratios (322-221/303-202 + 321-220/303-202) with the RADEX non-local thermodynamic equilibrium approach. These temperatures range from 28 to 181 K with an average of 54.2 +/- 0.3 K at a spatial density of 5.5x10^5 cm^-3. Comparing with the temperature measurements obtained from multiple transitions of NH3 (1,1)-(6,6) and the far infrared (FIR) dust continuum, the H2CO lines show temperatures similar to those measured by NH3 but slightly higher than values derived from FIR observations. The high kinetic temperatures observed from H2CO are associated with the ultracompact H II region UC1, dense clumps, as well as H2O and CH3OH masers, showing a similar distribution as NH3. This indicates that dense gas in the M17SW region is heated by star formation activity. The presence of a significant gas temperature gradient across the M17SW region, as measured by H2CO and NH3, provides direct evidence for gas heated predominantly by radiation emitted from the OB star cluster NGC 6618. On a smaller scale, the dense gas surrounding the dense clumps experiences significant heating from internal protostars and/or young stellar objects. Higher temperatures traced by H2CO are linked to turbulence on a scale of ~0.2 pc. The complex temperature structure of the M17SW region is revealed by H2CO and NH3, which may be attributed to both large-scale external radiative heating and small-scale internal radiative and turbulent heating.

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arXiv2026

Molecular Clouds at the Edge of the Galaxy II. Physical properties and scaling relations

The outer Galaxy presents an optimal setting for investigating molecular clouds and star formation in environments with low metallicity. A total of 72 Galactic edge clouds were surveyed using the CO\,(2--1) line with the IRAM\,30\,m telescope, leading to the identification of 112 CO clumps within molecular clouds with linear resolutions of 0.5--0.9\,pc. Parameters such as size, mass, surface density, and velocity dispersion of these CO clumps, derived from CO\,(2--1) observations, exhibit ranges of 0.6--3.4\,pc, 34--8250\,M$_\odot$, 12--1025\,M$_{\odot}$\,pc$^{-2}$, and 0.3--1.7\,km\,s$^{-1}$, respectively. Over the Galactocentric distance range of 14--23\,kpc, no systematic variations are found in these parameters. The velocity dispersion-size relationship of the Galactic edge clumps is modeled as $σ_{\rm v}$\,=\,0.69($\pm$0.03)$R_{\rm eff}^{0.36(\pm0.10)}$, indicating that turbulence is present within the Galactic edge clumps, akin to observations in the inner Galactic disk clouds. Furthermore, the luminous mass-size relation of the Galactic edge clumps is described by $M_{\rm lum}$\,=\,196($\pm$17)$R_{\rm eff}^{\,2.18\,(\pm0.26)}$, suggesting the average column density remains almost constant for clouds of different sizes. The virial parameters range from 0.6 to 15.3, with a median value of 2.8\,$\pm$\,0.6, suggesting that most clumps are gravitationally unbound. Furthermore, the virial parameters of our Galactic edge clumps show a decreasing trend with increasing Galactocentric distances, described by an exponential relation $α_{\rm vir}$\,=\,33.0($\pm$\,10.4)\,e$^{-R_{\rm g}/6.7(\pm0.9)}$, consistent with previous results.

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arXiv2025

Molecular Clouds at the Edge of the Galaxy I. Variation of CO J=2-1/1-0 Line Ratio

The Galactic edge at Galactocentric distances of 14\,--\,22\,kpc provides an ideal laboratory to study molecular clouds in an environment that is different from the solar neighborhood, due to its lower gas density, lower metallicity, and little or no perturbation from the spiral arms. Observations of CO\,($J$\,=\,2--1) spectral lines were carried out towards 72 molecular clouds located at the Galactic edge using the IRAM\,30\,m telescope. Combined with CO\,($J$\,=\,1--0) data from the MWISP project, we investigate the variations of $R_{21}$ across these Galactic edge clouds, with $R_{21}$ representing CO(2-1)/CO(1-0) integrated intensity ratios. These are found to range from 0.3 to 3.0 with a mean of 1.0\,$\pm$\,0.1 in the Galactic edge clouds. The proportions of very low ratio gas (VLRG; $R_{21}$\,<\,0.4), low ratio gas (LRG; 0.4\,$\le$\,$R_{21}$\,<\,0.7), high ratio gas (HRG; 0.7\,$\le$\,$R_{21}$\,<\,1.0), and very high ratio gas (VHRG; $R_{21}$\,$\ge$\,1.0) are 6.9\%, 29.2\%, 26.4\%, and 37.5\%, respectively, indicating a significant presence of high $R_{21}$ ratio molecular gas within these regions. In our Galaxy, the $R_{21}$ ratio exhibits a gradient of initial radial decline followed by a high dispersion with increasing Galacticentric distance and a prevalence for high ratio gas. There is no apparent systematic variation within the Galactocentric distance range of 14 to 22\,kpc. A substantial proportion of HRG and VHRG is found to be associated with compact clouds and regions displaying star-forming activity, suggesting that the high $R_{21}$ ratios may stem from dense gas concentrations and recent episodes of star formation.

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arXiv2024

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde V. The massive filament DR21

The kinetic temperature structure of the massive filament DR21 has been mapped using the IRAM 30 m telescope. This mapping employed the para-H$_2$CO triplet ($J_{\rm K_aK_c}$ = 3$_{03}$--2$_{02}$, 3$_{22}$--2$_{21}$, and 3$_{21}$--2$_{20}$) on a scale of $\sim$0.1 pc. By modeling the averaged line ratios of para-H$_{2}$CO with RADEX under non-LTE assumptions, the kinetic temperature of the dense gas was derived at a density of $n$(H$_{2}$) = 10$^{5}$ cm$^{-3}$. The para-H$_2$CO lines reveal significantly higher temperatures than NH$_3$ (1,1)/(2,2) and FIR wavelengths. The dense clumps appear to correlate with the notable kinetic temperature. Among the four dense cores (N44, N46, N48, and N54), temperature gradients are observed on a scale of $\sim$0.1-0.3 pc. This suggests that the warm dense gas is influenced by internal star formation activity. With the exception of N54, the temperature profiles of these cores were fitted with power-law indices ranging from $-$0.3 to $-$0.5. This indicates that the warm dense gas is heated by radiation emitted from internally embedded protostar(s) and/or clusters. While there is no direct evidence supporting the idea that the dense gas is heated by shocks resulting from a past explosive event in the DR21 region, our measurements toward the DR21W1 region provide compelling evidence that the dense gas is indeed heated by shocks originating from the western DR21 flow. Higher temperatures appear to be associated with turbulence. The physical parameters of the dense gas in the DR21 filament exhibit a remarkable similarity to the results obtained in OMC-1 and N113. This may imply that the physical mechanisms governing the dynamics and thermodynamics of dense gas traced by H$_{2}$CO in diverse star formation regions may be dominated by common underlying principles despite variations in specific environmental conditions. (abbreviated)

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