arXiv subjects

J. Malinen

Publications and source records attributed to J. Malinen.

At least 19 recordsLinked to original sources

arXiv2021

Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends

Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30m Large Program designed to estimate the S, C, N, and O depletions and gas ionization degree, X(e-), in a set of star-forming filaments of Taurus, Perseus and Orion. Our immediate goal is to build up a complete database of molecular abundances that can serve as an observational basis for estimating X(e-) and the C, O, N, and S depletions through chemical modeling. We observed and derived the abundances of 14 species (13CO, C18O, HCO+, H13CO+, HC18O+, HCN, H13CN, HNC, HCS+, CS, SO, 34SO, H2S, and OCS) in 244 positions, covering the AV 3 to 100 mag, n(H2) a few 10$^{3}$ to 10$^6$ cm$^{-3}$, and Tk 10 to 30 K ranges in these clouds, avoiding protostars, HII regions, and outflows. A statistical analysis is carried out to identify general trends between different species and with physical parameters. Relations between molecules reveal strong linear correlations which define three different families: (1) 13CO and C18O; (2) H13CO+, HC18O+, H13CN, and HNC; and (3) the S-bearing molecules. The abundances of the CO isotopologs increase with the gas kinetic temperature until TK 15 K. For higher temperatures, the abundance remains constant with a scatter of a factor of 3. The abundances of H13CO+, HC18O+, H13CN, and HNC are well correlated with each other, and all of them decrease with molecular hydrogen density, following the law n(H2)$^{-0.8\pm0.2}$. The abundances of S-bearing species also decrease with n(H2) at a rate of (S-bearing/H)gas n(H2)$^{-0.6\pm0.1}$. The abundances of molecules belonging to groups 2 and 3 do not present any clear trend with gas temperature. At scales of molecular clouds, the C18O abundance is the quantity that better correlates with the cloud mass. We discuss the utility of the 13CO/C18O, HCO+/H13CO+, and H13CO+/H13CN abundance ratios as chemical diagnostics of star formation in external galaxies.

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arXiv2020

Gas-phase Elemental abundances in Molecular cloudS (GEMS) III. Unlocking the CS chemistry: the CS+O reaction

CS is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. Chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. The CS+O -> CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150-400 K, but the extrapolation to lower temperatures is doubtful. Here we calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. We performed ab initio calculations to obtain the three lowest PES of the CS+O system. These PESs are used to study the reaction dynamics, using several methods to eventually calculate the CS+O thermal reaction rates. We compare the results of our theoretical calculations for 150-400 K with those obtained in the laboratory. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150-400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, which is consistent with the extrapolation of experimental data using the Arrhenius expression. We use the updated chemical network to model the sulfur chemistry in TMC1 based on molecular abundances determined from GEMS project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.

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arXiv2020

Gas phase Elemental abundances in Molecular cloudS (GEMS). II. On the quest for the sulphur reservoir in molecular clouds: the $H_{2}S$ case

Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium, and the identity of the main sulphur reservoir is still an open question. Our goal is to investigate the H$_{2}$S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir. Using millimeter observations of CS, SO, H$_{2}$S, and their isotopologues, we determine the physical conditions and H$_{2}$S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model Nautilus is then used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H$_2$S abundance. Our model shows that chemical desorption is the main source of gas-phase H$_2$S in dark cores. The measured H$_{2}$S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when $n_{\rm H}>2\times10^{4}$. This change in the desorption rate is consistent with the formation of thick H$_2$O and CO ice mantles on grain surfaces. The observed SO and H$_2$S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of $5-10$. Along the three cores, atomic S is predicted to be the main sulphur reservoir. We conclude that the gaseous H$_2$S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H$_2$S. The behavior of the observed H$_{2}$S abundance suggests a changing desorption efficiency, which would probe the snowline in these cores. Our model, however, overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.

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arXiv2019

SCOPE: SCUBA-2 Continuum Observations of Pre-protostellar Evolution - Survey Description and Compact Source Catalogue

We present the first release of the data and compact-source catalogue for the JCMT Large Program SCUBA-2 Continuum Observations of Pre-protostellar Evolution (SCOPE). SCOPE consists of 850-um continuum observations of 1235 Planck Galactic Cold Clumps (PGCCs) made with the Submillimetre Common-User Bolometer Array 2 on the James Clerk Maxwell Telescope. These data are at an angular resolution of 14.4 arcsec, significantly improving upon the 353-GHz resolution of Planck at 5 arcmin, and allowing for a catalogue of 3528 compact sources in 558 PGCCs. We find that the detected PGCCs have significant sub-structure, with 61 per cent of detected PGCCs having 3 or more compact sources, with filamentary structure also prevalent within the sample. A detection rate of 45 per cent is found across the survey, which is 95 per cent complete to Planck column densities of $N_{H_{2}}$ $>$ 5 $\times$ 10$^{21}$ cm$^{-2}$. By positionally associating the SCOPE compact sources with YSOs, the star formation efficiency, as measured by the ratio of luminosity to mass, in nearby clouds is found to be similar to that in the more distant Galactic Plane, with the column density distributions also indistinguishable from each other.

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arXiv2018

Galactic cold cores IX. Column density structures and radiative transfer modelling

The Galactic Cold Cores (GCC) project has made Herschel observations of interstellar clouds where Planck detected compact sources of cold dust emission. Our aim is to characterise the structure of the clumps and their parent clouds. We also examine the accuracy to which the structure of dense clumps can be determined from submillimetre data. We use standard statistical methods to characterise the GCC fields. Clumps are extracted using column density thresholding and we construct for each field a three-dimensional radiative transfer (RT) model. These are used to estimate the relative radiation field intensities, clump stability, and the uncertainty of column density estimates. We examine the radial column density profiles of the clumps. In the GCC fields, the structure noise follows the relations previously established at larger scales. The fractal dimension has no significant dependence on column density and the values D = 1.25 +- 0.07 are only slightly lower than in typical molecular clouds. The column density PDFs exhibit large variations, e.g. 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~r^{-1}. In spite of a great variety of clump morphology, clumps tend to follow a similar N~r^{-1} relation below r~0.1 pc. RT calculations indicate only factor of 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. The GCC host clouds 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 statistics (e.g. D and radial profiles) are insensitive to the environment.

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arXiv2017

Correlation of gas dynamics and dust in the evolved filament G82.65-02.00

The combination of line and continuum observations can provide vital insights to the formation and fragmentation of filaments and the initial conditions for star formation. We have carried out line observations to map the kinematics of an evolved, actively star forming filament G82.65-2.00. The filament was first The combination of line and continuum observations can provide vital insights to the formation and fragmentation of filaments and the initial conditions for star formation. We have carried out line observations to map the kinematics of an evolved, actively star forming filament G82.65-2.00. The filament was first identified from the Planck data as a region of particularly cold dust emission and was mapped at 100-500 $μ$m as a part of Herschel key program Galactic Cold Cores. The observations reveal several velocity components in the field, with strongest line emission concentrated to velocity range $\sim [3,5]$ km $\rm s^{-1}$. The column density of molecular hydrogen along the filament varies from 1.0 to 2.3 $\times 10^{22}$ $\rm cm^{2}$. We have examined six cold clumps from the central part of the filament. The clumps have masses in the range $10 - 20$ $M_{\odot}$ ($\sim70M_{\odot}$ in total) and are close to or above the virial mass. Furthermore, the main filament is heavily fragmented and most of the the substructures have a mass lower than or close to the virial mass, suggesting that the filament is dispersing as a whole. Position-velocity maps of $\rm ^{12}CO$ and $\rm ^{13}CO$ lines indicate that at least one of the striations is kinematically connected to two of the clumps, potentially indicating mass accretion from the striation onto the main filament. We tentatively estimate the accretion rate to be $\dot{M}$ = $2.23 \times 10^{-6}$ $ M_{\odot} / \rm year$.

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arXiv2017

Galactic Cold Cores. VIII. Filament formation and evolution: Filament properties in context with evolutionary models

Filaments are key for star formation models. As part of the study carried out by the Herschel GCC Programme, here we study the filament properties presented in GCC.VII in context with theoretical models of filament formation and evolution. A conservative sample of filaments at a distance D<500pc was extracted with the Getfilaments algorithm. Their physical structure was quantified according to two main components: the central (Gaussian) region (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 its environmental column density were compared with theoretical models describing the evolution of filaments under gravity-dominated conditions. The feasibility of a transition to supercritical state by accretion is dependent on the combined effect of filament intrinsic properties and environmental conditions. Reasonably self-gravitating (high Mline-core) filaments in dense environments (av\sim3mag) can become supercritical in timescales of t\sim1Myr by accreting mass at constant or decreasing width. The trend of increasing Mline-tot (Mline-core and Mline-wing), and ridge Av with background also indicates that the precursors of star-forming filaments evolve coevally with their environment. The simultaneous increase of environment and filament Av explains the association between dense environments and high Mline-core values, and 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 collapsing clouds, presents a suitable scenario for the fulfillment of the combined filament mass-environment criterium that is in quantitative agreement with Herschel observations.

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arXiv2016

Matching dust emission structures and magnetic field in high-latitude cloud L1642: comparing Herschel and Planck maps

The nearby cloud L1642 is one of only two known very high latitude (|b| > 30 deg) clouds actively forming stars. It is a rare example of star formation in isolated conditions, and can reveal important details of star formation in general, e.g., of the effect of magnetic fields. We compare Herschel dust emission structures and magnetic field orientation revealed by Planck polarization maps in L1642. The high-resolution ($\sim20"$) Herschel data reveal a complex structure including a dense, compressed central clump, and low density striations. The Planck polarization data (at 10$'$ resolution) reveal an ordered magnetic field pervading the cloud and aligned with the surrounding striations. There is a complex interplay between the cloud structure and large scale magnetic field. This suggests that the magnetic field is closely linked to the formation and evolution of the cloud. CO rotational emission confirms that the striations are connected with the main clumps and likely to contain material either falling into or flowing out of the clumps. There is a clear transition from aligned to perpendicular structures approximately at a column density of $N_{\rm{H}} = 1.6 \times 10^{21}\, {\rm cm}^{-2}$. Comparing the Herschel maps with the Planck polarization maps shows the close connection between the magnetic field and cloud structure even in the finest details of the cloud.

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arXiv2016

Galactic Cold Cores VII: Filament Formation and Evolution - Methods & Observational Constraints

The association of filaments with protostellar objects has made these structures a priority target in star formation studies. The datasets of the Herschel Galactic Cold Cores Key Programme allow for a statistical study of filaments with a wide range of intrinsic and environmental characteristics. Characterisation of this sample can be used to identify key physical parameters and quantify the role of environment in the formation of supercritical filaments. Filaments were extracted from fields at D<500pc with the getfilaments algorithm and characterised according to their column density profiles and intrinsic properties. Each profile was fitted with a beam-convolved Plummer-like function and quantified based on the relative contributions from the filament 'core', represented by a Gaussian, and 'wing' component, dominated by the power-law of the Plummer-like function. These parameters were examined for populations associated with different background levels. We find that filaments increase their core (Mcore) and wing (Mwing) contributions while increasing their total linear mass density (Mtot). Both components appear to be linked to the local environment, with filaments in higher backgrounds having systematically more massive Mcore and Mwing. This dependence on the environment supports an accretion-based model for filament evolution in the local neighbourhood (D<500pc). Structures located in the highest backgrounds develop the highest central Av, Mcore, and Mwing as Mtot increases with time, favoured by the local availability of material and the enhanced gravitational potential. Our results indicate that filaments acquiring a significantly massive central region with Mcore>Mcrit/2 may become supercritical and form stars. This translates into a need for filaments to become at least moderately self-gravitating in order to undergo localised star formation or become star-forming filaments.

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arXiv2015

Galactic cold cores VI. Dust opacity spectral index

The Galactic Cold Cores project has made Herschel observations of 116 fields where the Planck survey has found signs of cold dust emission. The fields contain sources in different environments and different phases of star formation. The dust opacity spectral index beta and the dust colour temperature T are derived using Herschel and Planck data. The relation between beta and T is examined for the whole sample and inside individual fields. Based on IRAS and Planck data, the fields are characterised by a median colour temperature of 16.1 K and a median opacity spectral index of beta=1.84. We observe a clear T-beta anti-correlation. In Herschel observations, constrained at lower resolution by Planck data, the variations follow the column density structure and beta(FIR) can rise to ~2.2 in individual clumps. The Planck 217 GHz band shows a systematic excess that is consistent with a general flattening of the dust emission spectrum at millimetre wavelengths. When fitted separately below and above 700 um, the median spectral index values are beta(FIR) ~ 1.91 and beta(mm) ~ 1.66. The spectral index changes as a function of column density and wavelength. Beta variations are partly masked by temperature gradients and the changes in the intrinsic grain properties may be even greater.

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arXiv2015

The physical state of selected cold clumps

Context. he study of prestellar cores is essential to understanding the initial stages of star formation. With $Herschel$ more cold clumps have been detected than ever before. For this study we have selected 21 cold clumps from 20 $Herschel$ fields observed as a follow-up on original $Planck$ detections. We have observed these clumps in $^{13}$CO (1-0), C$^{18}$O (1-0), and N$_2$H$^+$ (1-0) lines. Aims. Our aim is to find out if these cold clumps are prestellar. We have examined to what extent independent analysis of the dust and the molecular lines lead to similar conclusions about the masses of these objects. Methods. We calculate the clump masses and densities from the dust continuum and molecular line observations and compare these to each other and to the virial and Bonnor-Ebert masses calculated for each clump. Finally we examine two of the fields with radiative transfer models to estimate CO abundances. Results. When excitation temperatures could be estimated, the column densities derived from molecular line observations were comparable to those from dust continuum data. The median column density estimates are 4.2$\times 10^{21}$cm$^{-2}$ and 5.5$\times 10^{21}$cm$^{-2}$ for the line and dust emission data, respectively. The calculated abundances, column densities, volume densities, and masses all have large uncertainties and one must be careful when drawing conclusions. Abundance of $^{13}$CO was found in modeling the two clumps in the field G131.65$+$9.75 to be close to the usual value of 10$^{-6}$. The abundance ratio of $^{13}$CO and C$^{18}$O was $\sim$10. Molecular abundances could only be estimated with modeling, relying on dust column density data. Conclusions. The results indicate that most cold clumps, even those with dust color temperatures close to 11 K, are not necessarily prestellar.

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arXiv2015

Galactic cold cores V. Dust opacity

The project Galactic Cold Cores has made Herschel observations of interstellar clouds where the Planck satellite survey has located cold and compact clumps. The sources range from starless clumps to protostellar cores. We examine 116 Herschel fields to estimate the submillimetre dust opacity and its variations. The submillimetre dust opacity was derived from Herschel data, and near-infrared observations of the reddening of background stars are converted into near-infrared optical depth. We studied the systematic errors affecting these parameters and used modelling to correct for the expected biases. The ratio of 250um and J band opacities is correlated with the cloud location and star formation activity. We find a median ratio of tau(250um)/tau(J)= (1.6+-0.2)*10^-3, which is more than three times the mean value in diffuse medium. Assuming a spectral index beta=1.8 instead of beta=2.0, the value would be lower by ~30%. No significant systematic variation is detected with Galactocentric distance or with Galactic height. The tau(250um)/tau(J) maps reveal six fields with clear increase of submillimetre opacity of up to tau(250um)/tau(J) ~ 4*10^-3. These are all nearby fields with spatially resolved clumps of high column density. We interpret the increase in the far-infrared opacity as a sign of grain growth in the densest and coldest regions of interstellar clouds.

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arXiv2014

Mapping of interstellar clouds with infrared light scattered from dust: TMC-1N

Mapping of near-infrared (NIR) scattered light is a recent method for the study of interstellar clouds, complementing other, more commonly used methods, like dust emission and extinction. Our goal is to study the usability of this method on larger scale, and compare the properties of a filament using NIR scattering and other methods. We also study the radiation field and differences in grain emissivity between diffuse and dense areas. We have used scattered J, H, and K band surface brightness WFCAM-observations to map filament TMC-1N in Taurus, covering an area of 1dx1d corresponding to ~(2.44 pc)^2. We have converted the data into optical depth and compared the results with NIR extinction and Herschel observations of submm dust emission. We see the filament in scattered light in all three NIR bands. We note that our WFCAM observations in TMC-1N show notably lower intensity than previous results in Corona Australis using the same method. We show that 3D radiative transfer simulations predict similar scattered surface brightness levels as seen in the observations. However, changing the assumptions about the background can change the results of simulations notably. We derive emissivity by using optical depth in the J band as an independent tracer of column density. We obtain opacity sigma(250um) values 1.7-2.4x10^-25 cm^2/H, depending on assumptions of the extinction curve, which can change the results by over 40%. These values are twice as high as obtained for diffuse areas, at the lower limit of earlier results for denser areas. We show that NIR scattering can be a valuable tool in making high resolution maps. We conclude, however, that NIR scattering observations can be complicated, as the data can show relatively low-level artefacts. This suggests caution when planning and interpreting the observations.

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arXiv2014

Multiwavelength study of the high-latitude cloud L1642: chain of star formation

L1642 is one of the two high galactic latitude (|b| > 30deg) clouds confirmed to have active star formation. We examine the properties of this cloud, especially the large-scale structure, dust properties, and compact sources in different stages of star formation. We present high-resolution far-infrared and submm observations with the Herschel and AKARI satellites and mm observations with the AzTEC/ASTE telescope, which we combined with archive data from near- and mid-infrared (2MASS, WISE) to mm observations (Planck). The Herschel observations, combined with other data, show a sequence of objects from a cold clump to young stellar objects at different evolutionary stages. Source B-3 (2MASS J04351455-1414468) appears to be a YSO forming inside the L1642 cloud, instead of a foreground brown dwarf, as previously classified. Herschel data reveal striation in the diffuse dust emission around L1642. The western region shows striation towards NE and has a steeper column density gradient on its southern side. The densest central region has a bow-shock like structure showing compression from the west and a filamentary tail extending towards east. The differences suggest that these may be spatially distinct structures, aligned only in projection. We derive values of the dust emission cross-section per H nucleon for different regions of the cloud. Modified black-body fits to the spectral energy distribution of Herschel and Planck data give emissivity spectral index beta values 1.8-2.0 for the different regions. The compact sources have lower beta values and show an anticorrelation between T and beta. Markov chain Monte Carlo calculations demonstrate the strong anticorrelation between beta and T errors and the importance of mm Planck data in constraining the estimates. L1642 reveals a more complex structure and sequence of star formation than previously known.

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arXiv2013

Estimation of high-resolution dust column density maps. Comparison of modified black-body fits and radiative transfer modelling

Sub-millimetre dust emission is often used to derive the column density N of dense interstellar clouds. The observations consist of data at several wavelengths but of variable resolution. We examine two procedures that been proposed for the estimation of high resolution N maps. Method A uses a low-resolution temperature map combined with higher resolution intensity data while Method B combines N estimates from different wavelength ranges. Our aim is to determine the accuracy of the methods relative to the true column densities and the estimates obtainable with radiative transfer modelling. We use magnetohydrodynamical (MHD) simulations and radiative transfer calculations to simulate sub-millimetre observations at the wavelengths of the Herschel Space Observatory. The observations are analysed with the methods and the results compared to the true values and to the results from radiative transfer modelling of observations. Both methods A and B give relatively reliable column density estimates at the resolution of 250um data while also making use of the longer wavelengths. For high signal-to-noise data, the results of Method B are better correlated with the true column density, while Method A is less sensitive to noise. When the cloud has internal heating, results of Method B are consistent with those that would be obtained with high-resolution data. Because of line-of-sight temperature variations, these underestimate the true column density and, because of a favourable cancellation of errors, Method A can sometimes give more correct values. Radiative transfer modelling, even with very simple 3D cloud models, can provide better results. However, the complexity of the models required for improvements increases rapidly with the complexity and opacity of the clouds.

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arXiv2012

Profiles of interstellar cloud filaments. Observational effects in synthetic sub-millimetre observations

Sub-millimetre observations suggest that the filaments of interstellar clouds have rather uniform widths and can be described with the so-called Plummer profiles. The shapes of the filament profiles are linked to their physical state. Before drawing conclusions on the observed column density profiles, we must evaluate the observational uncertainties. We want to estimate the bias that could result from radiative transfer effects or from variations of submm dust emissivity. We use cloud models obtained with magnetohydrodynamic simulations and carry out radiative transfer calculations to produce maps of sub-millimetre emission. Column densities are estimated based on the synthetic observations. For selected filaments, the estimated profiles are compared to those derived from the original column density. Possible effects from spatial variations of dust properties are examined. With instrumental noise typical of the Herschel observations, the parameters derived for nearby clouds are correct to within a few percent. The radiative transfer effects have only a minor effect on the results. If the signal-to-noise ratio is degraded by a factor of four, the errors become significant and for half of the examined filaments the values cannot be constrained. The errors increase in proportion to the cloud distance. Assuming the resolution of Herschel instruments, the model filaments are barely resolved at a distance of ~400 pc and the errors in the parameters of the Plummer function are several tens of per cent. The Plummer parameters, in particular the power-law exponent p, are sensitive to noise but can be determined with good accuracy using Herschel data. One must be cautious about possible line-of-sight confusion. In our models, a large fraction of the filaments seen in the column density maps are not continuous structures in three dimensions.

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arXiv2012

Profiling filaments: comparing near-infrared extinction and submillimetre data in TMC-1

Interstellar filaments are an important part of star formation. To understand the structure of filaments, cross-section profiles are often fitted with Plummer profiles. This profiling is often done with submm studies, such as Herschel. It would be convenient if filament properties could also be studied using groundbased NIR data. We compare the filament profiles obtained by NIR extinction and submm observations to find out if reliable profiles can be derived using NIR data. We use J-, H-, and K-band data of a filament north of TMC-1 to derive an extinction map from colour excesses of background stars. We compare the Plummer profiles obtained from extinction maps with Herschel dust emission maps. We present 2 methods to estimate profiles from NIR: Plummer profile fits to median Av of stars or directly to the Av of individual stars. We compare the methods by simulations. In simulations extinction maps and the new methods give correct results to within ~10-20 for modest densities. Direct fit to data on individual stars gives more accurate results than extinction map, and can work in higher density. In profile fits to real observations, values of Plummer parameters are generally similar to within a factor of ~2. Although parameter values can vary significantly, estimates of filament mass usually remain accurate to within some tens of per cent. Our results for TMC-1 are in agreement with earlier results. High resolution NIR data give more details, but 2MASS data can be used to estimate profiles. NIR extinction can be used as an alternative to submm observations to profile filaments. Direct fits of stars can also be a valuable tool. Plummer profile parameters are not always well constrained, and caution should be taken when making fits. In the evaluation of Plummer parameters, one can use the independence of dust emission and NIR data and the difference in the shapes of the confidence regions.

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arXiv2012

Modelling the dust emission from dense interstellar clouds: disentangling the effects of radiative transfer and dust properties

With Planck and Herschel, we now have the spectral coverage and angular resolution required to observe dense and cold molecular clouds. As these clouds are optically thick at short wavelength but optically thin at long wavelength, it is tricky to conclude anything about dust properties without a proper treatment of the radiative transfer (RT). Our aim is to disentangle the effects of RT and of dust properties on the variations in the dust emission to provide observers with keys to analyse the emission arising from dense clouds. We model cylindrical clouds, illuminated by the ISRF, and carry out full RT calculations. Dust temperatures are solved using DustEM for amorphous carbons and silicates, grains coated with carbon mantles, and mixed aggregates of carbon and silicate. We allow variations of the grain optical properties with wavelength and temperature. We determine observed colour temperatures, T, and emissivity spectral indices, beta, by fitting the dust emission with modified blackbodies, to compare our models with observations. RT effects can neither explain the low T nor the increased submm emissivity measured at the centre of dense clouds, nor the observed beta-T anti-correlation. Adding noise to the modelled data, we show that it is not likely to be the unique explanation for the beta-T anti-correlation observed in starless clouds. It may be explained by intrinsic variations in the grain optical properties with temperature. As for the increased submm emissivity and the low T, they have to originate in variations in the grain optical properties, probably caused by their growth to form porous aggregates. We find it difficult to track back the nature of grains from the spectral variations in their emission. Finally, the column density is underestimated when determined with blackbody fitting because of the discrepancy between T and the true dust temperature at the cloud centre.

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