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Register a new userNobeyama 45m Cygnus-X CO survey I: photodissociation of molecules revealed by the unbiased large-scale CN and C$^{18}$O maps
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Mitsuyoshi Yamagishi
Publication date
2018
fields
Physics
and research's language is
English
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We present an unbiased large-scale (9 deg$^2$) CN ($N$=1-0) and C$^{18}$O ($J$=1-0) survey of Cygnus-X conducted with the Nobeyama 45m Cygnus-X CO survey. CN and C$^{18}$O are detected in various objects towards the Cygnus-X North and South (e.g., DR17, DR18, DR21, DR22, DR23, and W75N). We find that CN/C$^{18}$O integrated intensity ratios are systematically different from region to region, and are especially enhanced in DR17 and DR18 which are irradiated by the nearby OB stars. This result suggests that CN/C$^{18}$O ratios are enhanced via photodissociation reactions. We investigate the relation between the CN/C$^{18}$O ratio and strength of the UV radiation field. As a result, we find that CN/C$^{18}$O ratios correlate with the far-UV intensities, $G_0$. We also find that CN/C$^{18}$O ratios decrease inside molecular clouds, where the interstellar UV radiation is reduced due to the interstellar dust extinction. We conclude that the CN/C$^{18}$O ratio is controlled by the UV radiation, and is a good probe of photon-dominated regions.
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We report the statistical physical properties of the C$^{18}$O($J=1-0$) clumps present in a prominent cluster-forming region, Cygnus X, using the dataset obtained by the Nobeyama 45-m radio telescope. This survey covers 9 deg$^2$ of the north and south regions of Cygnus X, and totally 174 C$^{18}$O clumps are identified using the dendrogram method. Assuming a distance of 1.4 kpc, these clumps have radii of 0.2-1 pc, velocity dispersions of $<2.2~mathrm{km~s^{-1}}$, gas masses of 30-3000 $M_odot$, and H$_2$ densities of (0.2-5.5)$times10^4~mathrm{cm^{-3}}$. We confirm that the C$^{18}$O clumps in the north region have a higher H$_2$ density than those in the south region, supporting the existence of a difference in the evolution stages, consistent with the star formation activity of these regions. The difference in the clump properties of the star-forming and starless clumps is also confirmed by the radius, velocity dispersion, gas mass, and H$_2$ density. The average virial ratio of 0.3 supports that these clumps are gravitationally bound. The C$^{18}$O clump mass function shows two spectral index components, $alpha=-1.4$ in 55-140 $M_odot$ and $alpha=-2.1$ in $>140~M_odot$, which are consistent with the low- and intermediate-mass parts of the Kroupas initial mass function. The spectral index in the star-forming clumps in $>140~M_odot$ is consistent with that of the starless clumps in 55-140 $M_odot$, suggesting that the latter will evolve into star-forming clumps while retaining the gas accretion. Assuming a typical star formation efficiency of molecular clumps (10%), about ten C$^{18}$O clumps having a gas mass of $>10^3~M_odot$ will evolve into open clusters containing one or more OB stars.
We present the $^{13}$CO/C$^{18}$O (J=3-2) Heterodyne Inner Milky Way Plane Survey (CHIMPS) which has been carried out using the Heterodyne Array Receiver Program on the 15 m James Clerk Maxwell Telescope (JCMT) in Hawaii. The high-resolution spectral survey currently covers |b| < 0.5 deg and 28 < l < 46 deg, with an angular resolution of 15 arcsec in 0.5 km/s velocity channels. The spectra have a median rms of $sim$ 0.6 K at this resolution, and for optically thin gas at an excitation temperature of 10 K, this sensitivity corresponds to column densities of $N_{mathrm{H}_{2}} sim 3 times 10^{20},$cm$^{-2}$ and $N_{mathrm{H}_{2}} sim 4 times 10^{21},$cm$^{-2}$ for $^{13}$CO and C$^{18}$O, respectively. The molecular gas that CHIMPS traces is at higher column densities and is also more optically thin than in other publicly available CO surveys due to its rarer isotopologues, and thus more representative of the three-dimensional structure of the clouds. The critical density of the J=3-2 transition of CO is $gtrsim 10^{4}$ cm$^{-3}$ at temperatures of $leq 20$ K, and so the higher density gas associated with star formation is well traced. These data complement other existing Galactic plane surveys, especially the JCMT Galactic Plane Survey which has similar spatial resolution and column density sensitivity, and the Herschel infrared Galactic Plane Survey. In this paper, we discuss the observations, data reduction and characteristics of the survey, presenting integrated emission maps for the region covered. Position-velocity diagrams allow comparison with Galactic structure models of the Milky Way, and while we find good agreement with a particular four arm model, there are some significant deviations.
We use the IRAM Large Program EMPIRE and new high-resolution ALMA data to measure 13CO(1-0)/C18O(1-0) intensity ratios across nine nearby spiral galaxies. These isotopologues of CO are typically optically thin across most of the area in galaxy disks, and this ratio allows us to gauge their relative abundance due to chemistry or stellar nucleosynthesis effects. Resolved 13CO/C18O gradients across normal galaxies have been rare due to the faintness of these lines. We find a mean 13CO/C18O ratio of 6.0$pm$0.9 for the central regions of our galaxies. This agrees well with results in the Milky Way, but differs from results for starburst galaxies (3.4$pm$0.9) and ultraluminous infrared galaxies (1.1$pm$0.4). In our sample, the 13CO/C18O ratio consistently increases with increasing galactocentric radius and decreases with increasing star formation rate surface density. These trends qualitatively agree with expectations for carbon and oxygen isotopic abundance variations due to stellar nucleosynthesis, with a possible effect of fractionation.
Fully sampled degree-scale maps of the 13CO 2-1 and CO 4-3 transitions toward three members of the Lupus Molecular Cloud Complex - Lupus I, III, and IV - trace the column density and temperature of the molecular gas. Comparison with IR extinction maps from the c2d project requires most of the gas to have a temperature of 8-10 K. Estimates of the cloud mass from 13CO emission are roughly consistent with most previous estimates, while the line widths are higher, around 2 km/s. CO 4-3 emission is found throughout Lupus I, indicating widespread dense gas, and toward Lupus III and IV. Enhanced line widths at the NW end and along the edge of the B228 ridge in Lupus I, and a coherent velocity gradient across the ridge, are consistent with interaction between the molecular cloud and an expanding HI shell from the Upper-Scorpius subgroup of the Sco-Cen OB Association. Lupus III is dominated by the effects of two HAe/Be stars, and shows no sign of external influence. Slightly warmer gas around the core of Lupus IV and a low line width suggest heating by the Upper-Centaurus-Lupus subgroup of Sco-Cen, without the effects of an HI shell.
The FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) project is one of the legacy projects using the new multi-beam FOREST receiver installed on the Nobeyama 45-m telescope. This project aims to investigate the distribution, kinematics, and physical properties of both diffuse and dense molecular gas in the Galaxy at once by observing 12CO, 13CO, and C18O J=1-0 lines simultaneously. The mapping regions are a part of the 1st quadrant (10d < l < 50d, |b| < 1d) and the 3rd quadrant (198d < l <236d, |b| < 1d) of the Galaxy, where spiral arms, bar structure, and the molecular gas ring are included. This survey achieves the highest angular resolution to date (~20) for the Galactic plane survey in the CO J=1-0 lines, which makes it possible to find dense clumps located farther away than the previous surveys. FUGIN will provide us with an invaluable dataset for investigating the physics of the galactic interstellar medium (ISM), particularly the evolution of interstellar gas covering galactic scale structures to the internal structures of giant molecular clouds, such as small filament/clump/core. We present an overview of the FUGIN project, observation plan, and initial results, which reveal wide-field and detailed structures of molecular clouds, such as entangled filaments that have not been obvious in previous surveys, and large-scale kinematics of molecular gas such as spiral arms.
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