Browsing by Subject "DARK CLOUDS"

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  • Tatematsu, Ken'ichi; Liu, Tie; Kim, Gwanjeong; Yi, Hee-Weon; Lee, Jeong-Eun; Hirano, Naomi; Liu, Sheng-Yuan; Ohashi, Satoshi; Sanhueza, Patricio; Di Francesco, James; Evans, Neal J.; Fuller, Gary A.; Kandori, Ryo; Choi, Minho; Kang, Miju; Feng, Siyi; Hirota, Tomoya; Sakai, Takeshi; Lu, Xing; Lu'o'ng, Quang Nguyen; Thompson, Mark A.; Wu, Yuefang; Li, Di; Kim, Kee-Tae; Wang, Ke; Ristorcelli, Isabelle; Juvela, Mika; Toth, L. Viktor (2020)
    We mapped two molecular cloud cores in the Orion A cloud with the 7 m Array of the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeterArray (ALMA) and with the Nobeyama 45 m radio telescope. These cores have bright N2D+ emission in single-pointing observations with the Nobeyama 45 m radio telescope, have a relatively high deuterium fraction, and are thought to be close to the onset of star formation. One is a star-forming core, and the other is starless. These cores are located along filaments observed in N2H+ and show narrow line widths of 0.41 km s(-1) and 0.45 km s(-1) in N2D+, respectively, with the Nobeyama 45 m telescope. Both cores were detected with the ALMA ACA 7 m Array in the continuum and molecular lines at Band 6. The starless core G211 shows a clumpy structure with several sub-cores, which in turn show chemical differences. Also, the sub-cores in G211 have internal motions that are almost purely thermal. The starless sub-core G211D, in particular, shows a hint of the inverse P Cygni profile, suggesting infall motion. The star-forming core G210 shows an interesting spatial feature of two N2D+ peaks of similar intensity and radial velocity located symmetrically with respect to the single dust continuum peak. One interpretation is that the two N2D+ peaks represent an edge-on pseudo-disk. The CO outflow lobes, however, are not directed perpendicular to the line connecting both N2D+ peaks.
  • Harju, J.; Daniel, F.; Sipilä, O.; Caselli, P.; Pineda, J. E.; Friesen, R. K.; Punanova, A.; Guesten, R.; Wiesenfeld, L.; Myers, P. C.; Faure, A.; Hily-Blant, P.; Rist, C.; Rosolowsky, E.; Schlemmer, S.; Shirley, Y. L. (2017)
    Context. Ammonia and its deuterated isotopologues probe physical conditions in dense molecular cloud cores. The time-dependence of deuterium fractionation and the relative abundances of different nuclear spin modifications are supposed to provide a means of determining the evolutionary stages of these objects. Aims. We aim to test the current understanding of spin-state chemistry of deuterated species by determining the abundances and spin ratios of NH2D, NHD2 and ND3 in a quiescent, dense cloud. Methods. Spectral lines of NH3, NH2D, NHD2, ND3 and N2D+ were observed towards a dense, starless core in Ophiuchus with the APEX, GBT and IRAM 30-m telescopes. The observations were interpreted using a gas-grain chemistry model combined with radiative transfer calculations. The chemistry model distinguishes between the different nuclear spin states of light hydrogen molecules, ammonia and their deuterated forms. Different desorption schemes can be considered. Results. High deuterium fractionation ratios with NH2D = NH3 similar to 0 : 4, NHD2 = NH2D similar to 0 : 2 and ND3 = NHD2 similar to 0 : 06 are found in the core. The observed ortho/para ratios of NH2D and NHD2 are close to the corresponding nuclear spin statistical weights. The chemistry model can approximately reproduce the observed abundances, but consistently predicts too low ortho/para-NH2D, and too large ortho/para-NHD2 ratios. The longevity of N2H+ and NH3 in dense gas, which is prerequisite to their strong deuteration, can be attributed to the chemical inertia of N-2 on grain surfaces. Conclusions. The discrepancies between the chemistry model and the observations are likely to be caused by the fact that the model assumes complete scrambling in principal gas-phase deuteration reactions of ammonia, which means that all the nuclei are mixed in reactive collisions. If, instead, these reactions occur through proton hop/hydrogen abstraction processes, statistical spin ratios are to be expected. The present results suggest that while the deuteration of ammonia changes with physical conditions and time, the nuclear spin ratios of ammonia isotopologues do not probe the evolutionary stage of a cloud.
  • Juvela, M.; Malinen, J.; Montillaud, J.; Pelkonen, V.-M.; Ristorcelli, I.; Tóth, L. V. (2018)
    Context. The Galactic Cold Cores (GCC) project has made Herschel photometric observations of interstellar clouds where Planck detected compact sources of cold dust emission. The fields are in different environments and stages of star formation. Aims. Our aim is to characterise the structure of the clumps and their parent clouds, and to study the connections between the environment and the formation of gravitationally bound objects. We also examine the accuracy to which the structure of dense clumps can be determined from sub-millimetre data. Methods. We use standard statistical methods to characterise the GCC fields. Individual clumps are extracted using column density thresholding. Based on sub-millimetre measurements, we construct a three-dimensional radiative transfer (RT) model for each field. These are used to estimate the relative radiation field intensities, to probe the clump stability, and to examine the uncertainty of column density estimates. We examine the structural parameters of the clumps, including their radial column density profiles. Results. In the GCC fields, the structure noise follows the relations previously established at larger scales and in lower-density clouds. The fractal dimension has no significant dependence on column density and the values D-p = 1.25 +/- 0.07 are only slightly lower than in typical molecular clouds. The column density probability density functions (PDFs) exhibit large variations, for example, in the case of externally compressed clouds. At scales r > 0.1 pc, the radial column density distributions of the clouds follow an average relation of N similar to r(-1). In spite of a great variety of clump morphologies (and a typical aspect ratio of 1.5), clumps tend to follow a similar N similar to r(-1) relation below r similar to 0.1 pc. RT calculations indicate only factor 2.5 variation in the local radiation field intensity. The fraction of gravitationally bound clumps increases significantly in regions with A v > 5 mag but most bound objects appear to be pressure-confined. Conclusions. The host clouds of the cold clumps in the GCC sample have statistical properties similar to general molecular clouds. The gravitational stability, peak column density, and clump orientation are connected to the cloud background while most other statistical clump properties (e.g. D-p and radial profiles) are insensitive to the environment. The study of clump morphology should be continued with a comparison with numerical simulations.
  • Juvela, Mika; He, Jinhua; Pattle, Katherine; Liu, Tie; Bendo, George; Eden, David J.; Feher, Orsolya; Fich, Michel; Fuller, Gary; Hirano, Naomi; Kim, Kee-Tae; Li, Di; Liu, Sheng-Yuan; Malinen, Johanna; Marshall, Douglas J.; Paradis, Deborah; Parsons, Harriet; Pelkonen, Veli-Matti; Rawlings, Mark G.; Ristorcelli, Isabelle; Samal, Manash R.; Tatematsu, Ken'ichi; Thompson, Mark; Traficante, Alessio; Wang, Ke; Ward-Thompson, Derek; Wu, Yuefang; Yi, Hee-Weon; Yoo, Hyunju (2018)
    Context. Analysis of all-sky Planck submillimetre observations and the IRAS 100 mu m data has led to the detection of a population of Galactic cold clumps. The clumps can be used to study star formation and dust properties in a wide range of Galactic environments. Aims. Our aim is to measure dust spectral energy distribution ( SED) variations as a function of the spatial scale and the wavelength. Methods. We examined the SEDs at large scales using IRAS, Planck, and Herschel data. At smaller scales, we compared JCMT/SCUBA-2 850 mu m maps with Herschel data that were filtered using the SCUBA-2 pipeline. Clumps were extracted using the Fellwalker method, and their spectra were modelled as modified blackbody functions. Results. According to IRAS and Planck data, most fields have dust colour temperatures T-C similar to 14-18K and opacity spectral index values of beta = 1.5-1.9. The clumps and cores identified in SCUBA-2 maps have T similar to 13K and similar beta values. There are some indications of the dust emission spectrum becoming flatter at wavelengths longer than 500 mu m. In fits involving Planck data, the significance is limited by the uncertainty of the corrections for CO line contamination. The fits to the SPIRE data give a median beta value that is slightly above 1.8. In the joint SPIRE and SCUBA-2 850 mu m fits, the value decreases to beta similar to 1.6. Most of the observed T-beta anticorrelation can be explained by noise. Conclusions. The typical submillimetre opacity spectral index fi of cold clumps is found to be similar to 1.7. This is above the values of diffuse clouds, but lower than in some previous studies of dense clumps. There is only tentative evidence of a T-beta anticorrelation and beta decreasing at millimetre wavelengths.
  • Auddy, Sayantan; Myers, Philip C.; Basu, Shantanu; Harju, Jorma; Pineda, Jaime E.; Friesen, Rachel K. (2019)
    We develop a new "core field structure" (CFS) model to predict the magnetic field strength and magnetic field fluctuation profile of dense cores using gas kinematics. We use spatially resolved observations of the nonthermal velocity dispersion from the Green Bank Ammonia survey along with column density maps from SCUBA-2 to estimate the magnetic field strength across seven dense cores located in the L1688 region of Ophiuchus. The CFS model predicts the profile of the relative field fluctuation, which is related to the observable dispersion in the direction of the polarization vectors. Within the context of our model, we find that all of the cores have a transcritical mass-to-flux ratio.
  • Kim, Gwanjeong; Tatematsu, Ken'ichi; Liu, Tie; Yi, Hee-Weon; He, Jinhua; Hirano, Naomi; Liu, Sheng-Yuan; Choi, Minho; Sanhueza, Patricio; Toth, L. Viktor; Evans, Neal J.; Feng, Siyi; Juvela, Mika; Kim, Kee-Tae; Vastel, Charlotte; Lee, Jeong-Eun; Lu'o'ng, Quang Nguyen; Kang, Miju; Ristorcelli, Isabelle; Feher, Orsolya; Wu, Yuefang; Ohashi, Satoshi; Wang, Ke; Kandori, Ryo; Hirota, Tomoya; Sakai, Takeshi; Lu, Xing; Thompson, Mark A.; Fuller, Gary A.; Li, Di; Shinnaga, Hiroko; Kim, Jungha (2020)
    We present the results of a single-pointing survey of 207 dense cores embedded in Planck Galactic Cold Clumps distributed in five different environments (lambda Orionis, Orion A, Orion B, the Galactic plane, and high latitudes) to identify dense cores on the verge of star formation for the study of the initial conditions of star formation. We observed these cores in eight molecular lines at 76-94 GHz using the Nobeyama 45 m telescope. We find that early-type molecules (e.g., CCS) have low detection rates and that late-type molecules (e.g., N(2)H(+)and c-C3H2) and deuterated molecules (e.g., N(2)D(+)and DNC) have high detection rates, suggesting that most of the cores are chemically evolved. The deuterium fraction (D/H) is found to decrease with increasing distance, indicating that it suffers from differential beam dilution between the D/H pair of lines for distant cores (>1 kpc). For lambda Orionis, Orion A, and Orion B located at similar distances, D/H is not significantly different, suggesting that there is no systematic difference in the observed chemical properties among these three regions. We identify at least eight high-D/H cores in the Orion region and two at high latitudes, which are most likely to be close to the onset of star formation. There is no clear evidence of the evolutionary change in turbulence during the starless phase, suggesting that the dissipation of turbulence is not a major mechanism for the beginning of star formation as judged from observations with a beam size of 0.04 pc.
  • JCMT Large Program SCOPE Collabora; TRAO Key Sci Program TOP Collabora; Yi, Hee-Weon; Lee, Jeong-Eun; Liu, Tie; Kim, Kee-Tae; Choi, Minho; Eden, David; Evans, Neal J.; Di Francesco, James; Fuller, Gary; Hirano, N.; Juvela, Mika; Kang, Sung-ju; Kim, Gwanjeong; Koch, Patrick M.; Lee, Chang Won; Li, Di; Liu, H-Y B.; Liu, Hong-Li; Liu, Sheng-Yuan; Rawlings, Mark G.; Ristorcelli, I.; Sanhueza, Patrico; Soam, Archana; Tatematsu, Ken'ichi; Thompson, Mark; Toth, L.; Wang, Ke; White, Glenn J.; Wu, Yuefang; Yang, Yao-Lun (2018)
    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 A Orionis cloud has a different environment as a consequence of the interaction with a prominent OB association and a giant H-II region. PGCCs in the lambda Orionis cloud have higher dust temperatures (T-d = 16.13 +/- 0.15 K) and lower values of dust emissivity spectral index (beta = 1.65 +/- 0.02) than PGCCs in the Orion A (T-d = 13.79 +/- 0.21 K, beta = 2.07 +/- 0.03) and Orion B (T-d = 13.82 +/- 0.19 K, beta =1.96 +/- 0.02) clouds. We find 119 substructures within the 40 detected PGCCs and identify them as cores. Out of a total of 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(H-2) (9.5 +/- 1.2) x 10(22)cm(-2) , number density n(H-2) - (2.9 +/- 0.4) x 10 5 CM -3 , and mass M-core = 1.0 +/- 0.3 M(circle dot)compared to the cores in the Orion A [R = 0.11 pc, N(H-2) = (2.3 +/- 0.3) x 10(23) cm(-2), n(H-2) = (3.8 +/- 0.5) x 10(5)cm(-3) , and M-core = 2.4 +/- 0.3 M-circle dot] and Orion B [R = 0.16 pc, N(H-2) (3.8 +/- 0.4) x 10(23) cm(-2), n(H-2) = (15.6 +/- 1.8) x 10(5) cm(-3) , and M-core = 2.7 +/- 0.3 M-circle dot] clouds. These core properties in the A Orionis cloud can be attributed to the photodissociation and external heating by the nearby H rr 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.
  • Juvela, Mika; Padoan, Paolo; Ristorcelli, Isabelle; Pelkonen, Veli-Matti (2019)
    Context. The Planck Catalogue of Galactic Cold Clumps (PGCC) contains over 13 000 sources that are detected based on their cold dust signature. They are believed to consist of a mixture of quiescent, pre-stellar, and already star-forming objects within molecular clouds. Aims. We extracted PGCC-type objects from cloud simulations and examined their physical and polarisation properties. The comparison with the PGCC catalogue helps to characterise the properties of this large sample of Galactic objects and, conversely, provides valuable tests for numerical simulations of large volumes of the interstellar medium and the evolution towards pre-stellar cores. Methods. We used several magnetohydrodynamical simulation snapshots to define the density field of our model clouds. Sub-millimetre images of the surface brightness and polarised signal were obtained with radiative transfer calculations. We examined the statistics of synthetic cold clump catalogues extracted with methods similar to the PGCC. We also examined the variations of the polarisation fraction p in the clumps. Results. The clump sizes, aspect ratios, and temperatures in the synthetic catalogue are similar to the PGCC. The fluxes and column densities of synthetic clumps are smaller by a factor of a few. Rather than with an increased dust opacity, this could be explained by increasing the average column density of the model by a factor of two to three, close to N(H-2) = 10(22) cm(-2). When the line of sight is parallel to the mean magnetic field, the polarisation fraction tends to increase towards the clump centres, which is contrary to observations. When the field is perpendicular, the polarisation fraction tends to decrease towards the clumps, but the drop in p is small (e.g. from p similar to 8% to p similar to 7%). Conclusions. Magnetic field geometry reduces the polarisation fraction in the simulated clumps by only Delta p similar to 1% on average. The larger drop seen towards the actual PGCC clumps therefore suggests some loss of grain alignment in the dense medium, such as predicted by the radiative torque mechanism. The statistical study is not able to quantify dust opacity changes at the scale of the PGCC clumps.