arXiv subjects

Veli-Matti Pelkonen

Publications and source records attributed to Veli-Matti Pelkonen.

At least 19 recordsLinked to original sources

arXiv2026

Are supernovae driving turbulence in the solar neighborhood?

Turbulence plays an important role in shaping the interstellar medium, and strongly influences star formation. We aim to identify the physical processes capable of sustaining HI turbulence in the solar neighborhood. We compare recent HI line-of-sight velocity observations within a volume of radius 70-500 pc centered on the Sun with a suite of 1 kpc numerical simulations that include two distinct turbulent drivers: (i) supernova (SN) feedback and (ii) imposed large-scale turbulent forcing. For each simulation, we construct synthetic sky maps that closely mimic the observational one, allowing for a consistent comparison between the simulations and the observational data. HI observations show a median velocity dispersion of 11.1 km s-1 in the solar neighborhood. SN-driven simulations systematically underpredict this value, yielding dispersions in the range 4.9-6.7 km s-1. Simulations with strong enough large-scale forcing can reproduce not only the median observed velocity dispersion, but also the observed velocity distribution.

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arXiv2025

The Rosetta Stone Project. II. The correlation between star formation efficiency and L/M indicator for the evolutionary stages of star-forming clumps in post-processed radiative magnetohydrodynamics simulations

Context. The evolution of massive star-forming clumps that are progenitors of high-mass young stellar objects are often classified based on a variety of observational indicators ranging from near-infrared to radio wavelengths. Among them, the ratio of the bolometric luminosity to the mass of their envelope, $L/M$, has been observationally diagnosed as a good indicator for the evolutionary classification of parsec-scale star-forming clumps in the Galaxy. Aims. We developed the Rosetta Stone project$\unicode{x2013}$an end-to-end framework designed to enable an accurate comparison between simulations and observations for investigating the formation and evolution of massive clumps. In this study, we calibrate the $L/M$ indicator in relation to the star formation efficiency (SFE) and the clump age, as derived from our suite of simulations. Methods. We performed multi-wavelength radiative transfer post-processing of radiative magnetohydrodynamics (RMHD) simulations of the collapse of star-forming clumps fragmenting into protostars. We generated synthetic observations to obtain far-infrared emission from $70$ to $500\,μ$m, as was done in the Hi-GAL survey, and at $24\,μ$m in the MIPSGAL survey, which were then used to build the spectral energy distributions (SEDs) and estimate the $L/M$ parameter. An additional $1.3\,$mm wavelength in ALMA Band 6 was also produced for the comparison with observational data. We applied observational techniques$\unicode{x2013}$commonly employed by observers$\unicode{x2013}$to the synthetic data in order to derive the corresponding physical parameters. Results. We find a correlation between $L/M$ and the SFE, with a power-law form $L/M\propto {\rm SFE}^{1.20^{+0.02}_{-0.02}}$. This correlation is independent of the mass of the clumps and the choice of initial conditions of the simulations in which they formed. (Abridged)

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arXiv2025

The Rosetta Stone Project. I. A suite of radiative magnetohydrodynamics simulations of high-mass star-forming clumps

Context. Star formation and, in particular, high-mass star formation are key astrophysical processes that are far from being fully understood. Unfortunately, progress in these fields is slow because observations are hard to interpret as they cannot be directly compared to numerical simulations. Synthetic observations are therefore necessary to better constrain the models. Aims. With the Rosetta Stone project, we aim to develop an end-to-end pipeline to compare star formation simulations with observations as accurately as possible in order to study the evolution from clumps scales to stars. Methods. Using the adaptive mesh-refinement code RAMSES, we computed a first grid of model of star-forming clumps to develop our pipeline and explore the impact of the clump initial conditions on their evolution. The main purpose of this set of simulations is to be converted into synthetic observations to enable a direct comparison with real star-forming clumps observed with Herschel and ALMA. Results. The Rosetta Stone simulations presented here provide a catalog available for full post-processing and subsequent comparison with observations (RS1). Among all the parameters explored here, the strength of the magnetic field has the strongest influence on the clump evolution (fragmentation, star formation, global collapse) at both large and small scales. Numerical parameters such as the resolution per Jeans length or the threshold for accretion onto sink particles affects the formation of low-mass sinks. Finally, the widely used L/M ratio is found to be a good indicator of the clump evolutionary state regardless of its initial condition, but this could change when more feedback processes (jets, HII regions) are included. Conclusions. We now have a new suite of simulations of star-forming clumps that is available for full post-processing and subsequent comparison with the observations,

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arXiv2025

The Rosetta Stone project. III. ALMA synthetic observations of fragmentation in high-mass star-forming clumps

The physical mechanisms that regulate the collapse of high-mass parsec-scale clumps and allow them to form clusters of new stars represent a crucial aspect of star formation. To investigate these mechanisms, we developed the Rosetta Stone project: an end-to-end (simulations-observations) framework that is based on the systematic production of realistic synthetic observations of clump fragmentation and their comparison with real data. In this work, we compare ALMA 1.3mm continuum dust emission observations from the SQUALO survey with a new set of 24 radiative magnetohydrodynamical simulations of high-mass clump fragmentation, post-processed using the CASA software to mimic the observing strategy of SQUALO. The simulations were initialized combining typical values of clump mass (500,1000 solar masses) and radius (~0.4pc) with two levels of turbulence (Mach number of 7,10) and three levels of magnetization (mass-to-flux ratio of ~3,10,100). Following the clump evolution over time with two random seeds projected along three orthogonal directions, we produced a collection of 732 synthetic fields. The synthetic observations of clump fragmentation at ~7000AU revealed between 2 and 14 fragments per field. Among the initial conditions of the simulations, magnetic fields have the largest impact on the fragment multiplicity at these scales. In advanced stages of clump evolution, a lower number of fragments is preferentially associated with magnetized clumps. Fragments identified at ~7000AU correspond to individual or multiple sink particles in ~75% of the cases, suggesting that not all fragments are actively forming stars. Both sinks and fragments accrete mass throughout the whole clump evolution, favoring a scenario in which fragments are not isolated from the environment. Our study demonstrates the importance of synthetic observations in interpreting results from interferometric observations.

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arXiv2025

The Formation of Protoplanetary Disks through Pre-Main Sequence Bondi-Hoyle Accretion

Protoplanetary disks are traditionally described as finite mass reservoirs left over by the gravitational collapse of the protostellar core, a view that strongly constrains both disk evolution and planet formation models. We propose a different scenario where protoplanetary disks of pre-main sequence stars are primarily assembled by Bondi-Hoyle accretion from the parent gas cloud. We demonstrate that Bondi-Hoyle accretion can supply not only the mass, but also the angular momentum necessary to explain the observed size of protoplanetary disks. Additionally, we predict how the specific angular momentum of protoplanetary disks scales with stellar mass. Our conclusions are based on a new analytical derivation of the scaling of the angular momentum in turbulent flows, which we confirm with a numerical simulation of supersonic turbulence. A key outcome of our analysis is the recognition that density fluctuations in supersonic turbulence--previously overlooked in studies of cloud and core rotation--lead to a significant increase in angular momentum at disk-forming scales. This revised understanding of disk formation and evolution alleviates several longstanding observational discrepancies and compels substantial revisions to current models of disk and planet formation.

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arXiv2025

Origin and Evolution of Angular Momentum of Class II Disks

Context: While class II pre-main-sequence (PMS) stars have already accreted most of their mass, the continued inflow of fresh material via Bondi-Hoyle accretion acts as an additional mass reservoir for their circumstellar disks. This may explain the observed accretion rates of PMS stars, as well as observational inconsistencies in the mass and angular momentum balance of their disks. Aims: Using a new simulation that reproduces the stellar initial mass function (IMF), we want to quantify the role of Bondi-Hoyle accretion in the formation of class II disks, as well as address the prospect of its observational detection with the James Webb Space Telescope (JWST). Methods: We studied the mass and angular momentum of the accreting gas using passively advected tracer particles in the simulation, and we carried out radiative transfer calculations of near-infrared scattering to generate synthetic JWST observations of Bondi-Hoyle trails of PMS stars. Results: Gas accreting on class II PMS stars approximately 1 Myr after their formation has enough mass and angular momentum to strongly affect the evolution of the preexisting disks. The accreted angular momentum is large enough to also explain the observed size of class II disks. The orientation of the angular momentum vector can differ significantly from that of the previously accreted gas, which may result in a significant disk warping or misalignment. We also predict that JWST observations of class II stars will be able to detect Bondi-Hoyle trails with a 80%-100% success rate with only a 2 min exposure time, depending on the filter, if stars with both an accretion rate \dot{M} > 5e-10 Msol/yr and a luminosity L > 0.5 Lsol are selected.

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arXiv2025

Comparison of Herschel and ArTéMiS observations of massive filaments

Context: OMC-3 in the Orion A Cloud is a nearby, high-mass star-forming region, and therefore ideal to study massive filaments in detail. Aims: We analyze how the inclusion of higher-resolution data changes the estimates of the filament properties and test the robustness of filament fitting routines. Methods. ArTéMiS and Herschel data are combined to create high-resolution images. Column densities and temperatures are estimated with modified blackbody fitting. The nearby OMC-3 cloud is compared to the more distant G202 and G17 clouds. We compare the OMC-3 cloud as it appears at Herschel and ArTéMiS resolution. Results. Column densities of dense clumps in OMC-3 are higher in combined ArTéMiS and Herschel data (FWHM 8.5"), when compared to Herschel-only data (FWHM 20"). Estimated filament widths are smaller in the combined maps, and also show signs of further fragmentation when observed with the ArTéMiS resolution. In the analysis of Herschel data the estimated filament widths are correlated with the distance of the field. Conclusions. Median filament FWHM in OMC-3 at higher resolution is 0.05 pc, but 0.1 pc with the Herschel resolution, 0.3 pc in G202 and 1.0 pc in G17, also at the Herschel resolution. It is unclear what causes the steep relation between distance and filament FWHM, but likely reasons include the effect of the limited telescope resolution combined with existing hierarchical structure, and convolution of large-scale background structures within the ISM. Estimates of the asymptotic power-law index of the filament profile function p is high. When fit with the Plummer function, the individual parameters of the profile function are degenerate, while the FWHM is better constrained. OMC-3 shows negative kurtosis, and all but OMC-3 at the Herschel resolution some asymmetry.

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arXiv2024

Updated kinematics of the Radcliffe Wave: non-synchronous, dipole-like vertical oscillations

The kinematic information of the Radcliffe Wave (RW) is essential for determining its existence and gaining insights into its origin and evolution. In this work, we present an accurate measurement of the vertical velocity ($V_Z$) of RW by incorporating the radial velocity (RV) measures through two methods, which is crucial but was neglected previously. First, the velocities are measured towards young stars, using their RV measurements from APOGEE-2 and proper motion measurements from Gaia DR3. Second, we combine RV measurements toward clouds with proper motion measurements of associated Young Stellar Objects (YSOs) to determine the vertical velocities of the clouds. The results reveal that the oscillations in $V_Z$ are not synchronous with the vertical coordinate $Z$, which differs from the conclusions of previous studies. Instead, we find a 5 km$\cdot$s$^{-1}$$\cdot$kpc$^{-1}$ gradient in $V_Z$ along the RW, exhibiting a dipole-like pattern. Consequently, the kinematic arrangement does not show a corresponding coherence with the spatial arrangement, bringing the Radcliffe Wave model into question.

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arXiv2023

Will ALMA Reveal the True Core Mass Function of Protoclusters?

Characterizing prestellar cores in star-forming regions is an important step towards the validation of theoretical models of star formation. Thanks to their sub-arcsecond resolution, ALMA observations can potentially provide samples of prestellar cores up to distances of a few kpc, where regions of massive star formation can be targeted. However, the extraction of real cores from dust-continuum observations of turbulent star-forming clouds is affected by complex projection effects. In this work, we study the problem of core extraction both in the idealized case of column-density maps and in the more realistic case of synthetic 1.3\,mm ALMA observations. The analysis is carried out on 12 regions of high column density from our 250 pc simulation. We find that derived core masses are highly unreliable, with only {\em a weak correlation between the masses of cores selected in the synthetic ALMA maps and those of the corresponding three-dimensional cores}. The fraction of real three-dimensional cores detected in the synthetic maps increases monotonically with mass and remains always below 50\%. Above $\sim 1\,M_{\odot}$, the core mass function derived from the column-density maps is steeper than that of the three-dimensional cores, while the core mass function from the synthetic ALMA maps has a slope closer to that of the real three-dimensional cores. Because of the mass uncertainties, proper guidance from realistic simulations is essential if ALMA observations of protoclusters at kpc distances are to be used to test star-formation models.

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arXiv2023

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): Detection of extremely high density compact structure of prestellar cores and multiple substructures within

Prestellar cores are self-gravitating dense and cold structures within molecular clouds where future stars are born. They are expected, at the stage of transitioning to the protostellar phase, to harbor centrally concentrated dense (sub)structures that will seed the formation of a new star or the binary/multiple stellar systems. Characterizing this critical stage of evolution is key to our understanding of star formation. In this work, we report the detection of high density (sub)structures on the thousand-au scale in a sample of dense prestellar cores. Through our recent ALMA observations towards the Orion molecular cloud, we have found five extremely dense prestellar cores, which have centrally concentrated regions $\sim$ 2000 au in size, and several $10^7$ $cm^{-3}$ in average density. Masses of these centrally dense regions are in the range of 0.30 to 6.89 M$_\odot$. {\it For the first time}, our higher resolution observations (0.8$'' \sim $ 320 au) further reveal that one of the cores shows clear signatures of fragmentation; such individual substructures/fragments have sizes of 800 -1700 au, masses of 0.08 to 0.84 M$_\odot$, densities of $2 - 8\times 10^7$ $cm^{-3}$ and separations of $\sim 1200$ au. The substructures are massive enough ($\gtrsim 0.1~M_\odot$) to form young stellar objects and are likely examples of the earliest stage of stellar embryos which can lead to widely ($\sim$ 1200 au) separated multiple systems.

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arXiv2022

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): Evidence for a Molecular Jet Launched at an Unprecedented Early Phase of Protostellar evolution

Protostellar outflows and jets play a vital role in star formation as they carry away excess angular momentum from the inner disk surface, allowing the material to be transferred toward the central protostar. Theoretically, low velocity and poorly collimated outflows appear from the beginning of the collapse, at the first hydrostatic core (FHSC) stage. With growing protostellar core mass, high-density jets are launched which entrain an outflow from the infalling envelope. Until now, molecular jets have been observed at high velocity ($\gtrsim$ 100 km/s) in early Class\,0 protostars. We, for the first time, detect a dense molecular jet in SiO emission with small-velocity ($\sim$ 4.2 km\,s$^{-1}$, deprojected $\sim$ 24 km\,s$^{-1}$) from source G208.89-20.04Walma (hereafter, G208Walma) using ALMA Band\,6 observations. This object has some characteristics of FHSCs, such as a small outflow/jet velocity, extended 1.3\,mm continuum emission, and N$_2$D$^+$ line emission. Additional characteristics, however, are typical of early protostars: collimated outflow and SiO jet. The full extent of the outflow corresponds to a dynamical time scale of $\sim$ 930$^{+200}_{-100}$ years. The spectral energy distribution also suggests a very young source having an upper limit of T$_{bol}$ $\sim$ 31 K and L$_{bol}$ $\sim$ 0.8 L$_\sun$. We conclude that G208Walma is likely in the transition phase from FHSC to protostar, and the molecular jet has been launched within a few hundred years of initial collapse. Therefore, G208Walma may be the earliest object discovered in the protostellar phase with a molecular jet.

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arXiv2021

The Dynamical State of Massive Clumps

The dynamical state of massive clumps is key to our understanding of the formation of massive stars. In this work, we study the kinematic properties of massive clumps using synthetic observations. We have previously compiled a very large catalog of synthetic dust-continuum compact sources from our 250 pc, SN-driven, star formation simulation. Here, we compute synthetic $\rm N_{2}H^{+}$ line profiles for a subsample of those sources and compare their properties with the observations and with those of the corresponding three-dimensional (3D) clumps in the simulation. We find that the velocity dispersion of the sources estimated from the $\rm N_{2}H^{+}$ line is a good estimate of that of the 3D clumps, although its correlation with the source size is weaker than the velocity-size correlation of the 3D clumps. The relation between the mass of the 3D clumps, $M_{\rm main}$, and that of the corresponding synthetic sources, $M_{\rm SED}$, has a large scatter and a slope of 0.5, $M_{\rm main} \propto M_{\rm SED}^{0.5}$, due to uncertainties arising from the observational band-merging procedure and from projection effects along the line of sight. As a result, the virial parameters of the 3D clumps are not correlated with the clump masses, even if a negative correlation is found for the compact sources, and the virial parameter of the most massive sources may significantly underestimate that of the associated clumps.

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arXiv2021

Physical Properties and Real Nature of Massive Clumps in the Galaxy

Systematic surveys of massive clumps have been carried out to study the conditions leading to the formation of massive stars. These clumps are typically at large distances and unresolved, so their physical properties cannot be reliably derived from the observations alone. Numerical simulations are needed to interpret the observations. To this end, we generate synthetic Herschel observations using our large-scale star-formation simulation, where massive stars explode as supernovae driving the interstellar-medium turbulence. From the synthetic observations, we compile a catalog of compact sources following the exact same procedure as for the Hi-GAL compact source catalog. We show that the sources from the simulation have observational properties with statistical distributions consistent with the observations. By relating the compact sources from the synthetic observations to their three-dimensional counterparts in the simulation, we find that the synthetic observations overestimate the clump masses by about an order of magnitude on average due to line-of-sight projection, and projection effects are likely to be even worse for Hi-GAL Inner Galaxy sources. We also find that a large fraction of sources classified as protostellar are likely to be starless, and propose a new method to partially discriminate between true and false protostellar sources.

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arXiv2020

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP) II. Survey overview: a first look at 1.3 mm continuum maps and molecular outflows

Planck Galactic Cold Clumps (PGCCs) are contemplated to be the ideal targets to probe the early phases of star formation. We have conducted a survey of 72 young dense cores inside PGCCs in the Orion complex with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3\,mm (band 6) using three different configurations (resolutions $\sim$ 0$\farcs$35, 1$\farcs$0, and 7$\farcs$0) to statistically investigate their evolutionary stages and sub-structures. We have obtained images of the 1.3\,mm continuum and molecular line emission ($^{12}$CO, and SiO) at an angular resolution of $\sim$ 0$\farcs$35 ($\sim$ 140\,au) with the combined arrays. We find 70 substructures within 48 detected dense cores with median dust-mass $\sim$ 0.093\,M$_{\sun}$ and deconvolved size $\sim$ 0$\farcs$27. Dense substructures are clearly detected within the central 1000\,au of four candidate prestellar cores. The sizes and masses of the substructures in continuum emission are found to be significantly reduced with protostellar evolution from Class\,0 to Class\,I. We also study the evolutionary change in the outflow characteristics through the course of protostellar mass accretion. A total of 37 sources exhibit CO outflows, and 20 ($>$50\%) show high-velocity jets in SiO. The CO velocity-extents ($Δ$Vs) span from 4 to 110 km/s with outflow cavity opening angle width at 400\,au ranging from $[Θ_{obs}]_{400}$ $\sim$ 0$\farcs$6 to 3$\farcs$9, which corresponds to 33$\fdg$4$-$125$\fdg$7. For the majority of the outflow sources, the $Δ$Vs show a positive correlation with $[Θ_{obs}]_{400}$, suggesting that as protostars undergo gravitational collapse, the cavity opening of a protostellar outflow widens and the protostars possibly generate more energetic outflows.

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arXiv2020

Dust emission, extinction, and scattering in LDN 1642

We study the near-infrared (NIR) scattering in LDN 1642, its correlation with the cloud structure, and the ability of dust models to simultaneously explain sub-millimetre emission, NIR extinction, and NIR scattering. We use observations from the HAWK-I instrument to measure the NIR surface brightness and extinction. These are compared with Herschel data on dust emission and, with radiative transfer modelling, with predictions calculated for different dust models. We find an optical depth ratio $τ(250\,μ{\rm m})/τ(J)\approx 10^{-3}$, confirming earlier findings of high sub-millimetre emissivity. The relationships between the column density derived from dust emission and the NIR colour excesses is linear and consistent with the standard NIR extinction curve. The extinction peaks at $A_J=2.6\,$mag, the NIR surface brightness remaining correlated with $N({\rm H}_2)$ without saturation. Radiative transfer models can fit the sub-millimetre data with any of the tested dust models. However, these predict a NIR extinction that is higher and a NIR surface brightness that is lower than in observations. If the dust sub-millimetre emissivity is rescaled to the observed value of $τ(250\,μ{\rm m})/τ(J)$, 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 directly measured, which is reflected in the shape of the NIR surface brightness spectra. 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, not yet fully explained by the current dust models.

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arXiv2020

The Effect of Supernovae on the Turbulence and Dispersal of Molecular Clouds

While the importance of supernova feedback in galaxies is well established, its role on the scale of molecular clouds is still debated. In this work, we focus on the impact of supernovae on individual clouds, using a high-resolution magneto-hydrodynamic simulation of a region of 250 pc where we resolve the formation of individual massive stars. The supernova feedback is implemented with real supernovae that are the natural evolution of the resolved massive stars, so their position and timing are self-consistent. We select a large sample of molecular clouds from the simulation to investigate the supernova energy injection and the resulting properties of molecular clouds. We find that molecular clouds have a lifetime of a few dynamical times, less then half of them contract to the point of becoming gravitationally bound, and the dispersal time of bound clouds, of order one dynamical time, is a factor of two shorter than that of unbound clouds. We stress the importance of internal supernovae, that is massive stars that explode inside their parent cloud, in setting the cloud dispersal time, and their huge overdensity compared to models where the supernovae are randomly distributed. We also quantify the energy injection efficiency of supernovae as a function of supernova distance to the clouds. We conclude that intermittent driving by supernovae can maintain molecular-cloud turbulence and may be the main process of cloud dispersal. The role of supernovae in the evolution of molecular clouds cannot be fully accounted for without a self-consistent implementation of their feedback.

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arXiv2019

Synthetic observations of dust emission and polarisation of Galactic cold clumps

The Planck Catalogue of Galactic Cold Clumps (PGCC) contains over 13000 sources detected based on their cold dust signature. They are believed to consist of a mixture of quiescent, pre-stellar, and already star-forming objects. 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 PGCC sample and provides valuable tests for numerical simulations of interstellar medium. We used several MHD snapshots to define the density field of our models. Sub-millimetre images of the surface brightness and polarisation were obtained with radiative transfer calculations. We examined the statistics of synthetic cold clump catalogues and examined the variations of the clump polarisation fraction p. The clump sizes, aspect ratios, and temperatures in the synthetic catalogue are similar to the PGCC. The fluxes and column densities 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 models by a factor of two to three, close to N(H2)= 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, 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~8% to p~7%). Magnetic field geometry reduces the polarisation fraction in the simulated clumps by only Δp~1% on average. The larger drop seen towards the actual PGCC clumps 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.

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arXiv2018

Dust spectrum and polarisation at 850 um in the massive IRDC G035.39-00.33

The dust sub-millimetre polarisation of star-forming clouds carries information on dust and the role of magnetic fields in cloud evolution. With 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. JCMT SCUBA-2/POL-2 data at 850um are combined with Planck 850um (353GHz) data to map polarisation fractions. With previous SCUBA-2 observations (450um and 850um) and Herschel data, the column densities are determined via modified blackbody fits and via radiative transfer modelling. Models are constructed to examine how the polarisation angles and fractions depend on potential magnetic field geometries and grain alignment. POL-2 data show clear changes in the magnetic field orientation. The filament has a peak column density of N(H2)~7 10^22 cm-2, a minimum dust temperature of T~12 K, and a mass of some 4300Msun for the area N(H2)> 5 10^21 cm-2. The estimated average value of the dust opacity spectral index is beta ~ 1.9. The ratio of sub-millimetre and J band optical depths is tau(250 um)/tau(J) ~ 2.5 10^-3, more than four times the typical values for diffuse medium. The polarisation fraction decreases as a function of column density to p ~ 1% in the central filament. Because of noise, the observed decrease of p(N) is significant only at N(H2)>2 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 ~ 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. G035.39-00.33 shows strong signs of dust evolution and the low polarisation fraction is suggestive of a loss of polarised emission from its densest parts.

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