Browsing by Subject "infrared: ISM"

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  • Juvela, Mika; Neha, Sharma; Mannfors, Emma; Saajasto, Mika; Ysard, Nathalie; Pelkonen, Veli-Matti (2020)
    Context. LDN 1642 is a rare example of a star-forming, high-latitude molecular cloud. The dust emission of LDN 1642 has already been studied extensively in the past, but its location also makes it a good target for studies of light scattering.Aims. We wish to study the near-infrared (NIR) light scattering in LDN 1642, its correlation with the cloud structure, and the ability of dust models to simultaneously explain observations of sub-millimetre dust emission, NIR extinction, and NIR scattering.Methods. We used observations made with the HAWK-I instrument to measure the NIR surface brightness and extinction in LDN 1642. These data were compared with Herschel observations of dust emission and, with the help of radiative transfer modelling, with the predictions calculated for different dust models.Results. We find, for LDN 1642, an optical depth ratio tau (250 mu m)/tau (J) approximate to 10(-3), confirming earlier findings of enhanced sub-millimetre emissivity. The relationships between the column density derived from dust emission and the NIR colour excesses are linear and consistent with the shape of the standard NIR extinction curve. The extinction peaks at A(J) = 2.6 mag, and the NIR surface brightness remains correlated with N(H-2) without saturation. Radiative transfer models are able to fit the sub-millimetre data with any of the tested dust models. However, these predict an NIR extinction that is higher and an NIR surface brightness that is lower than based on NIR observations. If the dust sub-millimetre emissivity is rescaled to the observed value of tau (250 mu m)/tau (J), dust models with high NIR albedo can reach the observed level of NIR surface brightness. The NIR extinction of the models tends to be higher than in the direct extinction measurements, which is also reflected in the shape of the NIR surface brightness spectra.Conclusions. The combination of emission, extinction, and scattering measurements provides strong constraints on dust models. The observations of LDN 1642 indicate clear dust evolution, including a strong increase in the sub-millimetre emissivity, which has not been fully explained by the current dust models yet.
  • Juvela, Mika; Guillet, Vincent; Liu, Tie; Ristorcelli, Isabelle; Pelkonen, Veli-Matti; Alina, Dana; Bronfman, Leonardo; Eden, David J.; Kim, Kee Tae; Koch, Patrick M.; Kwon, Woojin; Lee, Chang Won; Malinen, Johanna; Micelotta, Elisabetta; Montillaud, Julien; Rawlings, Mark G.; Sanhueza, Patricio; Soam, Archana; Traficante, Alessio; Ysard, Nathalie; Zhang, Chuan-Peng (2018)
    Context. The sub-millimetre polarisation of dust emission from star-forming clouds carries information on grain properties and on the effects that magnetic fields have on cloud evolution. Aims. Using observations of a dense filamentary cloud G035.39-00.33, we aim to characterise the dust emission properties and the variations of the polarisation fraction. Methods. JCMT SCUBA-2/POL-2 observations at 850 mu m were combined with Planck 850 mu m (353 GHz) data to map polarisation fraction at small and large scales. With previous total intensity SCUBA-2 observations (450 and 850 mu m) and Herschel data, the column densities were determined via modified black-body fits and via radiative transfer modelling. Models were constructed to examine how the observed polarisation angles and fractions depend on potential magnetic field geometries and grain alignment processes. Results. POL-2 data show clear changes in the magnetic field orientation. These are not in contradiction with the uniform orientation and almost constant polarisation fraction seen by Planck, because of the difference in the beam sizes and the POL-2 data being affected by spatial filtering. The filament has a peak column density of N(H-2) similar to 7 x 10(22) cm(-2), a minimum dust temperature of T similar to 12 K, and a mass of similar to 4300 M-circle dot for the area N(H-2) > 5 x 10(21) cm(-2). The estimated average value of the dust opacity spectral index is beta similar to 1.9. The ratio of sub-millimetre and J-band optical depths is tau (250 mu m)/tau(J) similar to 2.5 x 10(-3), more than four times the typical values for diffuse medium. The polarisation fraction decreases as a function of column density to p similar to 1% in the central filament. Because of noise, the observed decrease of p(N) is significant only at N(H-2) > 2 x 10(22) cm(-2). The observations suggest that the grain alignment is not constant. Although the data can be explained with a complete loss of alignment at densities above similar to 10(4) cm(-3) or using the predictions of radiative torques alignment, the uncertainty of the field geometry and the spatial filtering of the SCUBA-2 data prevent strong conclusions. Conclusions. The G035.39-00.33 filament shows strong signs of dust evolution and the low polarisation fraction is suggestive of a loss of polarised emission from its densest parts.
  • 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.
  • Rivera-Ingraham, A.; Ristorcelli, I.; Juvela, M.; Montillaud, J.; Men'shchikov, A.; Malinen, J.; Pelkonen, V. -M.; Marston, A.; Martin, P. G.; Pagani, L.; Paladini, R.; Paradis, D.; Ysard, N.; Ward-Thompson, D.; Bernard, J. -P.; Marshall, D. J.; Montier, L.; Toth, L. V. (2017)
    Context. The onset of star formation is intimately linked with the presence of massive unstable filamentary structures. These filaments are therefore key for theoretical models that aim to reproduce the observed characteristics of the star formation process in the Galaxy. Aims. As part of the filament study carried out by the Herschel Galactic Cold Cores Key Programme, here we study and discuss the filament properties presented in GCC VII (Paper I) in context with theoretical models of filament formation and evolution. Methods. A conservatively selected sample of filaments located at a distance D <500 pc was extracted from the GCC fields with the getfilaments algorithm. The physical structure of the filaments was quantified according to two main components: the central (Gaussian) region of the filament (core component), and the power-law-like region dominating the filament column density profile at larger radii (wing component). The properties and behaviour of these components relative to the total linear mass density of the filament and the column density of its environment were compared with the predictions from theoretical models describing the evolution of filaments under gravity-dominated conditions. Results. The feasibility of a transition from a subcritical to supercritical state by accretion at any given time is dependent on the combined effect of filament intrinsic properties and environmental conditions. Reasonably self-gravitating (high M-line,M-core) filaments in dense environments (Av greater than or similar to 3 mag) can become supercritical on timescales of t similar to 1 Myr by accreting mass at constant or decreasing width. The trend of increasing M-line,M-tot (M-line,M-core and M-line,M-wing) and ridge A(v) with background for the filament population also indicates that the precursors of star-forming filaments evolve coevally with their environment. The simultaneous increase of environment and filament Av explains the observed association between dense environments and high Mlille,co values, and it argues against filaments remaining in constant single-pressure equilibrium states. The simultaneous growth of filament and background in locations with efficient mass assembly, predicted in numerical models of filaments in collapsing clouds, presents a suitable scenario for the fulfillment of the combined filament mass-environment criterium that is in quantitative agreement with Herschel observations.
  • Abergel, A.; Ade, P. A. R.; Aghanim, N.; Alves, M. I. R.; Aniano, G.; Armitage-Caplan, C.; Arnaud, M.; Ashdown, M.; Atrio-Barandela, F.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Bartlett, J. G.; Battaner, E.; Benabed, K.; Benoit, A.; Benoit-Levy, A.; Bernard, J. -P.; Bersanelli, M.; Bielewicz, P.; Bobin, J.; Bock, J. J.; Bonaldi, A.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bridges, M.; Bucher, M.; Burigana, C.; Butler, R. C.; Cardoso, J. -F.; Catalano, A.; Chamballu, A.; Chary, R. -R.; Chiang, H. C.; Chiang, L. -Y; Christensen, P. R.; Church, S.; Clemens, M.; Juvela, M.; Keihänen, Elina; Keskitalo, R.; Kurki-Suonio, H.; Poutanen, T.; Suur-Uski, A. -S.; Tuovinen, J.; Valiviita, J.; Ysard, N. (2014)
  • Ade, P. A. R.; Aghanim, N.; Alina, D.; Alves, M. I. R.; Aniano, G.; Annitage-Caplan, C.; Arnaud, M.; Arzoumanian, D.; Ashdown, M.; Atrio-Barandela, F.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Battaner, E.; Benabed, K.; Benoit-Levy, A.; Bernard, J. -P.; Bersanelli, M.; Bielewicz, P.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bracco, A.; Burigana, C.; Cardoso, J. -F.; Catalano, A.; Chamballu, A.; Chiang, H. C.; Christensen, P. R.; Colombi, S.; Colombo, L. P. E.; Combet, C.; Couchot, F.; Coulais, A.; Crill, B. P.; Curto, A.; Cuttaia, F.; Danese, L.; Davies, R. D.; Davis, R. J.; de Bernardis, P.; de Rosa, A.; Juvela, M.; Keihanen, E.; Kurki-Suonio, H.; Pelkonen, V. -M.; Suur-Uski, A. -S.; Valiviita, J. (2015)
    Polarized emission observed by Planck HFI at 353GHz towards a sample of nearby fields is presented, focusing on the statistics of polarization fractions p and angles psi. The polarization fractions and column densities in these nearby fields are representative of the range of values obtained over the whole sky. We find that: (i) the largest polarization fractions are reached in the most diffuse fields; (ii) the maximum polarization fraction p(max) decreases with column density N-H in the more opaque fields with N-H > 10(21) cm(-2); and (iii) the polarization fraction along a given line of sight is correlated with the local spatial coherence of the polarization angle. These observations are compared to polarized emission maps computed in simulations of anisotropic magnetohydrodynamical turbulence in which we assume a uniform intrinsic polarization fraction of the dust grains. We find that an estimate of this parameter may be recovered from the maximum polarization fraction p(max) in diffuse regions where the magnetic field is ordered on large scales and perpendicular to the line of sight. This emphasizes the impact of anisotropies of the magnetic field on the emerging polarization signal. The decrease of the maximum polarization fraction with column density in nearby molecular clouds is well reproduced in the simulations, indicating that it is essentially due to the turbulent structure of the magnetic field: an accumulation of variously polarized structures along the line of sight leads to such an anti-correlation. In the simulations, polarization fractions are also found to anti-correlate with the angle dispersion function S. However, the dispersion of the polarization angle for a given polarization fraction is found to be larger in the simulations than in the observations, suggesting a shortcoming in the physical content of these numerical models. In summary, we find that the turbulent structure of the magnetic field is able to reproduce the main statistical properties of the dust polarization as observed in a variety of nearby clouds, dense cores excluded, and that the large-scale field orientation with respect to the line of sight plays a major role in the quantitative analysis of these statistical properties.
  • Ade, P. A. R.; Keihanen, E.; Kurki-Suonio, H.; Suur-Uski, A. -S.; Valiviita, J.; Planck Collaboration (2015)
    Planck has mapped the intensity and polarization of the sky at microwave frequencies with unprecedented sensitivity. We use these data to characterize the frequency dependence of dust emission. We make use of the Planck 353 GHz I, Q, and U Stokes maps as dust templates, and cross-correlate them with the Planck and WMAP data at 12 frequencies from 23 to 353 GHz, over circular patches with 10 degrees radius. The cross-correlation analysis is performed for both intensity and polarization data in a consistent manner. The results are corrected for the chance correlation between the templates and the anisotropies of the cosmic microwave background. We use a mask that focuses our analysis on the diffuse interstellar medium at intermediate Galactic latitudes. We determine the spectral indices of dust emission in intensity and polarization between 100 and 353 GHz, for each sky patch. Both indices are found to be remarkably constant over the sky. The mean values, 1.59 +/- 0.02 for polarization and 1.51 +/- 0.01 for intensity, for a mean dust temperature of 19.6 K, are close, but significantly different (3.6 sigma). We determine the mean spectral energy distribution (SED) of the microwave emission, correlated with the 353 GHz dust templates, by averaging the results of the correlation over all sky patches. We find that the mean SED increases for decreasing frequencies at v <60 GHz for both intensity and polarization. The rise of the polarization SED towards low frequencies may be accounted for by a synchrotron component correlated with dust, with no need for any polarization of the anomalous microwave emission. We use a spectral model to separate the synchrotron and dust polarization and to characterize the spectral dependence of the dust polarization fraction. The polarization fraction (p) of the dust emission decreases by (21 +/- 6)% from 353 to 70 GHz. We discuss this result within the context of existing dust models. The decrease in p could indicate differences in polarization efficiency among components of interstellar dust (e.g., carbon versus silicate grains). Our observational results provide inputs to quantify and optimize the separation between Galactic and cosmological polarization.
  • Paradis, D.; Meny, C.; Juvela, M.; Noriega-Crespo, A.; Ristorcelli, I. (2019)
    Context. Some Galactic molecular clouds show signs of dust evolution as compared to the diffuse interstellar medium, most of the time through indirect evidence such as color ratios, increased dust emissivity, or scattering (coreshine). These signs are not a feature of all Galactic clouds. Moreover, molecular clouds in the Large Magellanic Cloud (LMC) have been analyzed in a previous study based on Spitzer and IRIS data, at 4' angular resolution, with the use of one single dust model, and did not show any signs of dust evolution. Aims. In this present analysis we investigate the dust properties associated with the different gas phases (including the ionized phase this time) of the LMC molecular clouds at 1' angular resolution (four times greater than the previous analysis) and with a larger spectral coverage range thanks to Herschel data. We also ensure the robustness of our results in the framework of various dust models. Methods. We performed a decomposition of the dust emission in the infrared (from 3.6 to 500 mu m) associated with the atomic, molecular, and ionized gas phases in the molecular clouds of the LMC. The resulting spectral energy distributions were fitted with four distinct dust models. We then analyzed the model parameters such as the intensity of the radiation field and the relative dust abundances, as well as the slope of the emission spectra at long wavelengths. Results. This work allows dust models to be compared with infrared data in various environments for the first time, which reveals important differences between the models at short wavelengths in terms of data fitting (mainly in the polycyclic aromatic hydrocarbon bands). In addition, this analysis points out distinct results according to the gas phases, such as dust composition directly affecting the dust temperature and the dust emissivity in the submillimeter and different dust emission in the near-infrared (NIR). Conclusions. We observe direct evidence of dust property evolution from the diffuse to the dense medium in a large sample of molecular clouds in the LMC. In addition, the differences in the dust component abundances between the gas phases could indicate different origins of grain formation. We also point out the presence of a NIR-continuum in all gas phases, with an enhancement in the ionized gas. We favor the hypothesis of an additional dust component as the carrier of this continuum.
  • 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.
  • Miettinen, O. (2018)
    Context. Filamentary molecular clouds, such as many of the infrared dark clouds (IRDCs), can undergo hierarchical fragmentation into substructures (clumps and cores) that can eventually collapse to form stars. Aims. We aim to determine the occurrence of fragmentation into cores in the clumps of the filamentary IRDC G304.74 + 01.32 (hereafter, G304.74). We also aim to determine the basic physical characteristics (e.g. mass, density, and young stellar object (YSO) content) of the clumps and cores in G304.74. Methods. We mapped the G304.74 filament at 350 mu m using the Submillimetre APEX Bolometer Camera (SABOCA) bolometer. The new SABOCA data have a factor of 2.2 times higher resolution than our previous Large APEX BOlometer CAmera (LABOCA) 870 mu m map of the cloud (9 '' vs. 19 ''.86). We also employed the Herschel far-infrared (IR) and submillimetre, andWide-field Infrared Survey Explorer (WISE) IR imaging data available for G304.74. The WISE data allowed us to trace the IR emission of the YSOs associated with the cloud. Results. The SABOCA 350 mu m data show that G304.74 is composed of a dense filamentary structure with a mean width of only 0.18 +/- 0.05 pc. The percentage of LABOCA clumps that are found to be fragmented into SABOCA cores is 36% +/- 16%, but the irregular morphology of some of the cores suggests that this multiplicity fraction could be higher. The WISE data suggest that 65% +/- 18% of the SABOCA cores host YSOs. The mean dust temperature of the clumps, derived by comparing the Herschel 250, 350, and 500 mu m flux densities, was found to be 15.0 +/- 0.8 K. The mean mass, beam-averaged H-2 column density, and H2 number density of the LABOCA clumps are estimated to be 55 +/- 10 M-circle dot, (2.0 +/- 0.2) x 10(22) cm(-2), and (3.1 +/- 0.2) x 10(4) cm(-3). The corresponding values for the SABOCA cores are 29 +/- 3 M-circle dot, (2.9 +/- 0.3) x 10(22) cm(-2), and (7.9 +/- 1.2) x 10(4) cm(-3). The G304.74 filament is estimated to be thermally supercritical by a factor of greater than or similar to 3.5 on the scale probed by LABOCA, and by a factor of greater than or similar to 1.5 for the SABOCA filament. Conclusions. Our data strongly suggest that the IRDC G304.74 has undergone hierarchical fragmentation. On the scale where the clumps have fragmented into cores, the process can be explained in terms of gravitational Jeans instability. Besides the filament being fragmented, the finding of embedded YSOs in G304.74 indicates its thermally supercritical state, although the potential non-thermal (turbulent) motions can render the cloud a virial equilibrium system on scale traced by LABOCA. The IRDC G304.74 has a seahorse-like morphology in the Herschel images, and the filament appears to be attached by elongated, perpendicular striations. This is potentially evidence that G304.74 is still accreting mass from the surrounding medium, and the accretion process can contribute to the dynamical evolution of the main filament. One of the clumps in G304.74, IRAS 13039-6108, is already known to be associated with high-mass star formation, but the remaining clumps and cores in this filament might preferentially form low and intermediate-mass stars owing to their mass reservoirs and sizes. Besides the presence of perpendicularly oriented, dusty striations and potential embedded intermediate-mass YSOs, G304.74 is a relatively nearby (d similar to 2.5 kpc) IRDC, which makes it a useful target for future star formation studies. Owing to its observed morphology, we propose that G304.74 could be nicknamed the Seahorse Nebula.