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Register a new userThree dimensional projection effects on chemistry in a Planck galactic cold clump
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Offsets of molecular line emission peaks from continuum peaks are very common but frequently difficult to explain with a single spherical cloud chemical model. We propose that the spatial projection effects of an irregular three dimensional (3D) cloud structure can be a solution. This work shows that the idea can be successfully applied to the Planck cold clump G224.4-0.6 by approximating it with four individual spherically symmetric cloud cores whose chemical patterns overlap with each other to produce observable line maps. With the empirical physical structures inferred from the observation data of this clump and a gas-grain chemical model, the four cores can satisfactorily reproduce its 850 $mu$m continuum map and the diverse peak offsets of CCS, HC$_3$N and N$_2$H$^+$ simultaneously at chemical ages of about $8times 10^5sim 3times 10^6$ yrs. The 3D projection effects on chemistry has the potential to explain such asymmetrical distributions of chemicals in many other molecular clouds.
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We present the results from a series of ground-based radio observations toward a Planck Galactic Cold Clump (PGCC), PGCC G108.84-00.81, which is located in one curved filamentary cloud in the vicinity of an extended HII region Sh2-152 and SNR G109.1-1.0. PGCC G108.84-00.81 is mainly composed of two clumps, G108-N and G108-S. In the 850 micron dust continuum emission map, G108-N is shown as one component while G108-S is fragmented into four components. There is no infrared source associated with G108-N while there are two infrared sources (IRS 1 and IRS 2) associated with G108-S. The total mass of G108-N is larger than the jeans mass, suggesting that G108-N is gravitationally unstable and a potential place for a future star formation. The clump properties of G108-N and G108-S such as the gas temperature and the column density, are not distinctly different. However, G108-S is slightly more evolved than G108-N, in the consideration of the CO depletion factor, molecular abundances, and association with infrared sources. G108-S seems to be affected by the compression from Sh2-152, while G108-N is relatively protected from the external effect
To deepen our understanding of the chemical properties of the Planck Galactic Cold Clump (PGCC) G168.72-15.48, we performed observations of nine molecular species, namely, ce{c-C3H}, ce{H2CO}, ce{HC5N}, ce{HC7N}, ce{SO}, ce{CCH}, ce{N2H+}, ce{CH3OH}, and ce{CH3CCH}, toward two dense cores in PGCC G168.72-15.48 using the Tianma Radio Telescope and Purple Mountain Observatory Telescope. We detected ce{c-C3H}, ce{H2CO}, ce{HC5N}, ce{N2H+}, ce{CCH}, and ce{CH3OH} in both G168-H1 and G168-H2 cores, whereas ce{HC7N} and ce{CH3CCH} were detected only in G168-H1 and SO was detected only in G168-H2. Mapping observations reveal that the ce{CCH}, ce{N2H+}, ce{CH3OH}, and ce{CH3CCH} emissions are well coupled with the dust emission in G168-H1. Additionally, ce{N2H+} exhibits an exceptionally weak emission in the denser and more evolved G168-H2 core, which may be attributed to the ce{N2H+} depletion. We suggest that the ce{N2H+} depletion in G168-H2 is dominated by ce{N2} depletion, rather than the destruction by CO. The local thermodynamic equilibrium calculations indicate that the carbon-chain molecules of ce{CCH}, ce{HC5N}, ce{HC7N}, and ce{CH3CCH} are more abundant in the younger G168-H1 core. We found that starless core G168-H1 may have the properties of cold dark clouds based on its abundances of carbon-chain molecules. While, the prestellar core G168-H2 exhibits lower carbon-chain molecular abundances than the general cold dark clouds. With our gas-grain astrochemical model calculations, we attribute the observed chemical differences between G168-H1 and G168-H2 to their different gas densities and different evolutionary stages.
We present the Planck Catalogue of Galactic Cold Clumps (PGCC), an all-sky catalogue of Galactic cold clump candidates detected by Planck. This catalogue is the full version of the Early Cold Core (ECC) catalogue, which was made available in 2011 with the Early Release Compact Source Catalogue (ERCSC) and contained 915 high S/N sources. It is based on the Planck 48 months mission data that are currently being released to the astronomical community. The PGCC catalogue is an observational catalogue consisting exclusively of Galactic cold sources. The three highest Planck bands (857, 545, 353 GHz) have been combined with IRAS data at 3 THz to perform a multi-frequency detection of sources colder than their local environment. After rejection of possible extragalactic contaminants, the PGCC catalogue contains 13188 Galactic sources spread across the whole sky, i.e., from the Galactic plane to high latitudes, following the spatial distribution of the main molecular cloud complexes. The median temperature of PGCC sources lies between 13 and 14.5 K, depending on the quality of the flux density measurements, with a temperature ranging from 5.8 to 20 K after removing sources with the 1% largest temperature estimates. Using seven independent methods, reliable distance estimates have been obtained for 5574 sources, which allows us to derive their physical properties such as their mass, physical size, mean density and luminosity. The PGCC sources are located mainly in the solar neighbourhood, up to a distance of 10.5 kpc towards the Galactic centre, and range from low-mass cores to large molecular clouds. Because of this diversity and because the PGCC catalogue contains sources in very different environments, the catalogue is useful to investigate the evolution from molecular clouds to cores. Finally, the catalogue also includes 54 additional sources located in the SMC and LMC.
Gas at high Galactic latitude is a relatively little-noticed component of the interstellar medium. In an effort to address this, forty-one Planck Galactic Cold Clumps at high Galactic latitude (HGal; $|b|>25^{circ}$) were observed in $^{12}$CO, $^{13}$CO and C$^{18}$O J=1-0 lines, using the Purple Mountain Observatory 13.7-m telescope. $^{12}$CO (1-0) and $^{13}$CO (1-0) emission was detected in all clumps while C$^{18}$O (1-0) emission was only seen in sixteen clumps. The highest and average latitudes are $71.4^{circ}$ and $37.8^{circ}$, respectively. Fifty-one velocity components were obtained and then each was identified as a single clump. Thirty-three clumps were further mapped at 1$^prime$ resolution and 54 dense cores were extracted. Among dense cores, the average excitation temperature $T_{mathrm{ex}}$ of $^{12}$CO is 10.3 K. The average line widths of thermal and non-thermal velocity dispersions are $0.19$ km s$^{-1}$ and $0.46$ km s$^{-1}$ respectively, suggesting that these cores are dominated by turbulence. Distances of the HGal clumps given by Gaia dust reddening are about $120-360$ pc. The ratio of $X_{13}$/$X_{18}$ is significantly higher than that in the solar neighbourhood, implying that HGal gas has a different star formation history compared to the gas in the Galactic disk. HGal cores with sizes from $0.01-0.1$ pc show no notable Larsons relation and the turbulence remains supersonic down to a scale of slightly below $0.1$ pc. None of the HGal cores which bear masses from 0.01-1 $M_{odot}$ are gravitationally bound and all appear to be confined by outer pressure.
Based on the 850 $mu$m dust continuum data from SCUBA-2 at James Clerk Maxwell Telescope (JCMT), we compare overall properties of Planck Galactic Cold Clumps (PGCCs) in the $lambda$ Orionis cloud to those of PGCCs in the Orion A and B clouds. The Orion A and B clouds are well known active star-forming regions, while the $lambda$ Orionis cloud has a different environment as a consequence of the interaction with a prominent OB association and a giant Hii region. PGCCs in the $lambda$ Orionis cloud have higher dust temperatures ($Td=16.13pm0.15$ K) and lower values of dust emissivity spectral index ($ beta=1.65pm0.02$) than PGCCs in the Orion A (Td=13.79$pm 0.21$K, $beta=2.07pm0.03$) and Orion B ($Td=13.82pm0.19$K, $beta=1.96pm0.02$) clouds. We find 119 sub-structures within the 40 detected PGCCs and identify them as cores. Of total 119 cores, 15 cores are discovered in the $lambda$ Orionis cloud, while 74 and 30 cores are found in the Orion A and B clouds, respectively. The cores in the $lambda$ Orionis cloud show much lower mean values of size R=0.08 pc, column density N(H2)=$(9.5pm1.2) times 10^{22}$ cm$^{-2}$, number density n(H2)=$(2.9 pm 0.4)times10^{5}$ cm$^{-3}$, and mass $M_{core}$=$1.0pm0.3$ M$_{odot}$ compared to the cores in the Orion A (R=0.11pc, $N(H2)=(2.3pm0.3) times 10^{23}$ cm$^{-2}$, n(H2)=$(3.8pm0.5) times 10^{5}$cm$^{-3}$, and $M_{core}$=$2.4 pm 0.3$ M$_{odot}$) and Orion B (R=0.16pc, N(H2)=$(3.8 pm 0.4) times 10^{23}$cm$^{-2}$, n(H2)=$(15.6pm1.8)times10^{5}$ cm$^{-3}$, and $M_{core}$= $2.7pm0.3$ M$_{odot}$) clouds. These core properties in the $lambda$ Orionis cloud can be attributed to the photodissociation and external heating by the nearby Hii region, which may prevent the PGCCs from forming gravitationally bound structures and eventually disperse them. These results support the idea of negative stellar feedback on core formation.
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