Regions Of Molecular Clouds Research Articles

Astrochemistry and Astrobiology over the last 20 years

A review of the achievements of astrochemistry and astrobiology over the past 20 years is given. Advances in astrochemistry in understanding the processes of emergence and survival high molecular weight chemical compounds are directly related to the conditions of prebiomolecules origin, like -amino acids and complex hydrocarbons. And if on the case of amino acids and hydrocarbons synthesis , the astrophysical picture seems quite clear, then on the case of the emergence of chiral amino acids, of which the proteins of living organisms are composed, there is no generally accepted point of view. Probably they occurred in certain photochemical transformations under the influence of circularly polarized radiation in the conditions of star formation regions of molecular clouds.

The impact of shear on the rotation of Galactic plane molecular clouds

ABSTRACT Stars form in the densest regions of molecular clouds; however, there is no universal understanding of the factors that regulate cloud dynamics and their influence on the gas-to-star conversion. This study considers the impact of Galactic shear on the rotation of giant molecular clouds (GMCs) and its relation to the solenoidal modes of turbulence. We estimate the direction of rotation for a large sample of clouds in the $\mathrm{^{13}CO}$/$\mathrm{C^{18}O}$(3–2) Heterodyne Inner Milky Way Plane Survey (CHIMPS) and their corresponding sources in a new segmentation of the $\mathrm{^{12}CO}$(3–2) High-Resolution Survey. To quantify the strength of shear, we introduce a parameter that describes the shear’s ability to disrupt growing density perturbations within the cloud. Although we find no correlation between the direction of cloud rotation, the shear parameter, and the magnitude of the velocity gradient, the solenoidal fraction of the turbulence in the CHIMPS sample is positively correlated with the shear parameter and behaves similarly when plotted over Galactocentric distance. GMCs may thus not be large or long-lived enough to be affected by shear to the point of showing rotational alignment. In theory, Galactic shear can facilitate the rise of solenoidal turbulence and thus contribute to suppressing star formation. These results also suggest that the rotation of clouds is not strictly related to the overall rotation of the disc, but is more likely to be the imprint of Kelvin–Helmholtz instabilities in the colliding flows that formed the clouds.

Parsec-scale cosmic-ray ionisation rate in Orion

Cosmic rays are a key component of the interstellar medium because they regulate the dynamics and chemical processes in the densest and coldest regions of molecular clouds. Still, the cosmic-ray ionisation rate of H$_2$ (crir ) is one of the most debated parameters characterising molecular clouds because of the uncertainties in the adopted chemical networks and analysis techniques. This work aims to homogeneously estimate the crir at parsec scales towards the Orion Molecular Clouds OMC-2 and OMC-3. We explore the change in crir across a whole star-forming region by probing a range of column densities taht has never been explored before. The significant increase in statistics obtained by studying an entire region allows us to place stronger constraints on the range of crir values and exploit its connection with the physical properties of the interstellar medium. The most recent crir estimates are based on o$-$H$_2$D$^+$, which is a direct product of the interaction between cosmic rays and H$_2$ in cold clouds. Since observations of o$-$H$_2$D$^+$ are challenging, we proxy its abundance through CO depletion by employing C18O (2$-$1) observations towards OMC-2 and OMC-3, taking advantage of the existing correlation between the two parameters. Using additional observations of HCO$^+$ (1$-$0) and DCO$^+$ (3$-$2), we determine the deuteration fraction, and we finally derive the map of crir in these two regions. The C18O depletion correlates with both the total column density of H$_2$ and the N$_2$H$^+$ emission across OMC-2 and OMC-3. The obtained depletion factors and deuteration fractions are consistent with previous values obtained in low- and high-mass star-forming regions. These two parameters additionally show a positive correlation in the coldest fields of our maps. We derive cosmic-ray ionisation rates of $ $ s$^ $. These values agree well with previous estimates based on o$-$H$_2$D$^+$ observations. The crir also shows a functional dependence on the column density of H$_2$ across a full order of magnitude ($ $ cm$^ $). The estimated values of crir decrease overall for increasing $N( H_2 $), as predicted by theoretical models. The results delivered by our approach are comparable with theoretical predictions and previous independent studies. This confirms the robustness of the analytical framework and promotes CO depletion as a viable proxy of o$-$H$_2$D$^+$. We also explore the main limitations of the method by varying the physical size of the gas crossed by the cosmic rays (i.e. the path length). By employing a path length obtained from low-resolution observations, we recover values of the crir that are well below any existing theoretical and observational prediction. This discrepancy highlights the need for interferometric observations in order to reliably constrain the crir at parsec scales as well.

Collisions of electrons with interstellar grains

ABSTRACT We have computed cross-sections for elastic and inelastic scattering of electrons on small grains at low-collision energies. Significant differences are again found between the results obtained in the presence and the absence of a ‘permanent’ grain dipole moment. In addition to spherical grains, scattering on ellipsoidal grains is investigated. We conclude that the rate of electron attachment to interstellar grains may be substantially lower in regions of molecular clouds from which the radiation field is excluded, and where the grains are less likely to possess a dipole moment.

Reactivity of Isocyanate Radical with Formic Acid in the Tropospheric and Outer Space Regions: A DFT and MP2 Investigation in Gaseous Phase

AbstractThis work aims to investigate the nature of the structures, mechanisms and kinetics associated with the prototype reaction of formic acid (FA) with NCO radical in either the troposphere or outer space regions. The kinetic properties have been investigated to include temperature in the cold outer space molecular clouds populated regions (typically between 10 and 100 K), temperature in the warm outer space molecular cloud regions (between 90–200 K) and temperature in the troposphere region of the atmosphere (200–400 K). The study has been performed by means of selected Density Functional Theory (DFT), second order Møller–Plesset perturbation theory (MP2) and transition state theory approaches. The results of the investigation suggest that the H2NCO carbamoyl radical and carbon dioxide are the final fate for the reaction involving NCO radical and FA. HOCNH radical is considered the least formed product of the NCO+FA reaction because all pathways leading to its formation are unfavourable with regards to other competitive processes. Of all the investigated reaction channels, the thermodynamically preferred reaction pathways involve cases in which the starting isocyanate radical species has its unpaired electron on the nitrogen atom (N⋅) rather than on its oxygen atom (O⋅).

An R V Map of the Milky Way Revealed by LAMOST

The total-to-selective extinction ratio, R V, is a key parameter for tracing the properties of interstellar dust, as it directly determines the variation of the extinction curve with wavelength. By utilizing accurate color excess measurements from the optical to the mid-infrared range, we have derived R V values for approximately three million stars from LAMOST data release 7 using a forward-modeling technique. This extensive data set enables us to construct a comprehensive 2D R V map of the Milky Way within the LAMOST footprint at a spatial resolution of ∼27.′5. Based on reliable sight lines of E(B − V) > 0.1, we find that R V exhibits a Gaussian distribution centered around 3.25 with a standard deviation of 0.25. The spatial variability of R V in the Galactic disk exhibits a wide range, spanning from small scales within individual molecular clouds to large scales up to kiloparsecs. A striking correlation is observed between the distribution of R V and molecular clouds. Notably, we observe lower R V values within the regions of nearby molecular clouds compared to their surrounding areas. Furthermore, we have investigated the relationships between R V and various parameters, including dust temperature, dust emissivity spectral index, column densities and ratios of atomic and molecular hydrogen, as well as the gas-to-dust ratio. We find that these relationships vary with the level of extinction. These analyses provide new insights into the properties and evolution of dust grains in diverse interstellar environments and also hold significant importance for achieving accurate extinction corrections.

NEATH – II. N2H+ as a tracer of imminent star formation in quiescent high-density gas

ABSTRACT Star formation activity in molecular clouds is often found to be correlated with the amount of material above a column density threshold of ${\sim} 10^{22} \, {\rm cm}^{-2}$. Attempts to connect this column density threshold to a volume density above which star formation can occur are limited by the fact that the volume density of gas is difficult to reliably measure from observations. We post-process hydrodynamical simulations of molecular clouds with a time-dependent chemical network, and investigate the connection between commonly observed molecular species and star formation activity. We find that many molecules widely assumed to specifically trace the dense, star-forming component of molecular clouds (e.g. HCN, HCO+, CS) actually also exist in substantial quantities in material only transiently enhanced in density, which will eventually return to a more diffuse state without forming any stars. By contrast, N2H+ only exists in detectable quantities above a volume density of $10^4 \, {\rm cm}^{-3}$, the point at which CO, which reacts destructively with N2H+, begins to deplete out of the gas phase on to grain surfaces. This density threshold for detectable quantities of N2H+ corresponds very closely to the volume density at which gas becomes irreversibly gravitationally bound in the simulations: the material traced by N2H+ never reverts to lower densities, and quiescent regions of molecular clouds with visible N2H+ emission are destined to eventually form stars. The N2H+ line intensity is likely to directly correlate with the star formation rate averaged over time-scales of around a Myr.

Parallel and perpendicular alignments of velocity gradient and magnetic field observed in the molecular clouds L1478 and L1482

ABSTRACT Star formation is a complex process that typically occurs in dense regions of molecular clouds mainly regulated by magnetic fields, magnetohydrodynamic (MHD) turbulence, and self-gravity. However, it remains a challenging endeavour to trace the magnetic field and determine regions of gravitational collapse, where the star is forming. Based on the anisotropic properties of MHD turbulence, a new technique termed velocity gradient technique (VGT) has been proposed to address these challenges. In this study, we apply the VGT to two regions of the giant California Molecular Cloud (CMC), namely, L1478 and L1482, and analyse the difference in their physical properties. We use the 12CO (J = 2–1), 13CO (J = 2–1), and C18O (J = 2–1) emission lines observed with the Heinrich Hertz Submillimeter Telescope. We compare VGT results calculated in the resolutions of 3.3 and 10 arcmin to Planck polarization at 353 GHz and 10 arcmin to determine areas of MHD turbulence dominance and self-gravity dominance. We show that the resolution difference can introduce misalignment between the two measurements. We find the VGT-measured magnetic fields globally agree with those from Planck in L1478, suggesting self-gravity’s effect is insignificant. The best agreement appears in VGT-12CO. As for L1482, the VGT measurements are statistically perpendicular to the Planck polarization indicating the dominance of self-gravity. This perpendicular alignment is more significant in VGT-13CO and VGT-C18O.

Cosmic ray-driven magnetohydrodynamic waves in magnetized self-gravitating dusty molecular clouds

ABSTRACT The impact of galactic cosmic rays (CRs) in terms of CR pressure and parallel CR diffusion has been investigated on the low-frequency magnetohydrodynamic (MHD) waves and linear gravitational instability in the typical dusty plasma environment of molecular clouds (MCs). The dusty fluid model is formulated by combining the equations of the magnetized electrons/ions and dust particles, including the CR effects. The interactions between CR fluid and gravitating magnetized dusty plasma have been studied with the help of modified dispersion properties of the MHD waves and instabilities using the hydrodynamic fluid–fluid (CR–plasma) approach. CR diffusion affects the coupling of CR pressure-driven mode with dust-Alfvén MHD mode and causes damping in the MHD waves. It persists in its effect along the direction of the magnetic field and is diminished across the magnetic field. The phase-speed diagram shows that for super-Alfvénic wave, the slow mode becomes the intermediate Alfvén mode. The fundamental Jeans instability criterion remains unaffected due to CR effects, but in the absence of CR diffusion, the effects of dust-acoustic speed and CR pressure-driven wave speed are observed in the instability criterion. It is found that CR pressure stabilizes while CR diffusion destabilizes the growth rates of Jeans instability and significantly affects the gravitational collapse of dusty MCs. The charged dust grains play a dominant role in the sub-Alfvénic and super-Alfvénic MHD waves and the collapse of MCs, triggering gravitational instability. The consequences have been discussed to understand the gravitational instability in the dense photodissociation regions of dusty MCs.

First ALMA Maps of Cosmic-Ray Ionization Rate in High-mass Star-forming Regions

Low-energy cosmic rays (<1 TeV) are a pivotal source of ionization of the interstellar medium, where they play a central role in determining the gas chemical composition and drastically influence the formation of stars and planets. Over the past few decades, H3 + absorption line observations in diffuse clouds have provided reliable estimates of the cosmic-ray ionization rate relative to H2 (). However, in denser clouds, where stars and planets form, this method is often inefficient due to the lack of H3 + rotational transitions. The estimates are, therefore, still provisional in this context and represent one of the least understood components when it comes to defining general models of star and planet formation. In this Letter, we present the first high-resolution maps of the in two high-mass clumps obtained with a new analytical approach recently proposed to estimate the in the densest regions of molecular clouds. We obtain that span from 3 × 10−17 to 10−16 s−1, depending on the different distribution of the main ion carriers, in excellent agreement with the most recent cosmic-ray propagation models. The cores belonging to the same parental clump show comparable , suggesting that the ionization properties of prestellar regions are determined by global rather than local effects. These results provide important information for the chemical and physical modeling of star-forming regions.

Dust extinction map of the Galactic plane based on the VVV survey data

ABSTRACT Dust extinction is one of the most reliable tracers of the gas distribution in the Milky Way. The near-infrared (NIR) Vista Variables in the Vía Láctea (VVV) survey enables extinction mapping based on stellar photometry over a large area in the Galactic plane. We devise a novel extinction mapping approach, XPNICER, by bringing together VVV photometric catalogues, stellar parameter data from StarHorse catalogues, and previously published X percentile and PNICER extinction mapping techniques. We apply the approach to the VVV survey area, resulting in an extinction map that covers the Galactic disc between 295° ≲ l ≲ 350° and −2° ≲ b ≲ 2°, and the Galactic bulge between −10° ≲ b ≲ 5°. The map has 30 arcsec spatial resolution and it traces extinctions typically up to AV ∼ 10–20 mag and maximally up to AV ∼ 30 mag. We compare our map to previous dust-based maps, concluding that it provides a high-fidelity extinction-based map, especially in its ability to recover both the diffuse dust component of the Galaxy and moderately extincted giant molecular cloud regions. The map is especially useful as independent, extinction-based data on the Galactic dust distribution and applicable for a wide range of studies from individual molecular clouds to the studies of the Galactic stellar populations.

Discovery of non-metastable ammonia masers in Sagittarius B2

We report the discovery of widespread maser emission in non-metastable inversion transitions of NH3 toward various parts of the Sagittarius B2 molecular cloud and star-forming region complex. We detect masers in the J, K = (6, 3), (7,4), (8,5), (9,6), and (10,7) transitions toward Sgr B2(M) and Sgr B2(N), an NH3 (6,3) maser in Sgr B2(NS), and NH3 (7,4), (9,6), and (10,7) masers in Sgr B2(S). With the high angular resolution data of the Karl G. Jansky Very Large Array (JVLA) in the A-configuration, we identify 18 maser spots. Nine maser spots arise from Sgr B2(N), one from Sgr B2(NS), five from Sgr B2(M), and three in Sgr B2(S). Compared to our Effelsberg single-dish data, the JVLA data indicate no missing flux. The detected maser spots are not resolved by our JVLA observations. Lower limits to the brightness temperature are &gt; 3000 K and reach up to several 105 K, manifesting the lines’ maser nature. In view of the masers’ velocity differences with respect to adjacent hot molecular cores and/or UCH II regions, it is argued that all the measured ammonia maser lines may be associated with shocks caused either by outflows or by the expansion of UCH II regions. Overall, Sgr B2 is unique in that it allows us to measure many NH3 masers simultaneously, which may be essential in order to elucidate their thus far poorly understood origin and excitation.

ALMA-IMF

We present the first data release of the ALMA-IMF Large Program, which covers the 12m-array continuum calibration and imaging. The ALMA-IMF Large Program is a survey of fifteen dense molecular cloud regions spanning a range of evolutionary stages that aims to measure the core mass function. We describe the data acquisition and calibration done by the Atacama Large Millimeter/submillimeter Array (ALMA) observatory and the subsequent calibration and imaging we performed. The image products are combinations of multiple 12 m array configurations created from a selection of the observed bandwidth using multi-term, multi-frequency synthesis imaging and deconvolution. The data products are self-calibrated and exhibit substantial noise improvements over the images produced from the delivered data. We compare different choices of continuum selection, calibration parameters, and image weighting parameters, demonstrating the utility and necessity of our additional processing work. Two variants of continuum selection are used and will be distributed: the “best-sensitivity” (bsens) data, which include the full bandwidth, including bright emission lines that contaminate the continuum, and “cleanest” (cleanest), which select portions of the spectrum that are unaffected by line emission. We present a preliminary analysis of the spectral indices of the continuum data, showing that the ALMA products are able to clearly distinguish free-free emission from dust emission, and that in some cases we are able to identify optically thick emission sources. The data products are made public with this release.

Characterization of the Coriolis Coupled Far-Infrared Bands of syn-Vinyl Alcohol.

Rotational emission from vibrationally excited molecules are responsible for a large fraction of lines in the spectra of interstellar molecular clouds. Vinyl alcohol (VA) has two rotamers that differ in energy by 6.4 kJ/mol, both of which have been observed toward the molecular cloud, Sagittarius B2(N) [Turner and Apponi, Astrophys. J. 2001, 561, 207]. Previously, we reported an analysis of the far-infrared spectrum of the higher energy rotamer, anti-VA [Bunn et al. Astrophys. J. 2017, 847, 67], yielding rotational and higher order distortion constants in the first excited vibrational state, and here, we report an analysis of the far-infrared spectrum of the lower energy rotamer, syn-VA, whose spectrum is significantly more complicated on account of Coriolis interactions that result in perturbations to the rovibrational spectrum. We account for those perturbations with the inclusion of Coriolis coupling constants in the fit, which couples the first excited OH torsional (ν15) and CCO bending (ν11) states. Inclusion of them resulted in more physically meaningful rotational and centrifugal distortion constants, and allows for accurate pure rotational line predictions to be made up to high energies. These will be particularly useful in searches for vibrationally excited syn-VA toward warm regions of interstellar molecular clouds, where we predict that it may be significantly abundant.

Gravitational instability in radiative molecular clouds including cosmic ray diffusion and ion Larmor radius corrections

ABSTRACT The effects of cosmic ray (CR) diffusion and finite Larmor radius (FLR) corrections have been studied on the linear gravitational instability of thermally conducting plasmas typically in the H ii regions of molecular clouds. The hydrodynamic fluid–fluid approach is considered for interacting CRs with gravitating, magnetized, and thermally conducting gas in molecular clouds. The magnetohydrodynamic fluid model is formulated considering CR pressure gradients, CR diffusion, and radiative and FLR effects in terms of particle Larmor radius. The dispersion relation of the gravitational instability is analytically derived using the normal mode analysis, and the effects of CRs and FLR corrections have been discussed in longitudinal and transverse modes. It is observed that in the absence of CRs, the FLR effects (magnetic viscosity) reduce the growth rate for wavenumber smaller than a critical value, and above it gets increased. However, the growth rate is strongly suppressed in the presence of combined CRs and FLR effects. The individual behaviour of FLR effects is observed to destabilize the growth rate of the gravitational instability in the presence of CR effects. The CR pressure decreases the growth rates of the gravitational and thermal instabilities, whereas parallel CR diffusion enhances the growth rate of the gravitational instability. The Jeans length of the gravitating gas cloud gets increased due to an increase in the CR-to-gas pressure ratio. It is found that the gravitational collapse of the system is supported by high-energy (above knee) CR particles with the Larmor radii comparable to the cloud size. The present results have been applied to understand the role of CRs and FLR corrections on the gravitational collapse in the H ii regions of molecular clouds.

Interstellar Objects Follow the Collapse of Molecular Clouds

AbstractInterstellar objects (ISOs), the parent population of 1i/‘Oumuamua and 2i/Borisov, are abundant in the interstellar medium of the Milky Way. This means that the interstellar medium, including molecular-cloud regions, has three components: gas, dust, and ISOs. From observational constraints of the field density of ISOs drifting in the solar neighborhood, we infer that a typical molecular cloud of 10 pc diameter contains some 1018ISOs. At typical sizes ranging from hundreds of meters to tens of kilometers, ISOs are entirely decoupled from the gas dynamics in these molecular clouds. Here we address the question of whether ISOs can follow the collapse of molecular clouds. We perform low-resolution simulations of the collapse of molecular clouds containing initially static ISO populations toward the point where stars form. In this proof-of-principle study, we find that the interstellar objects definitely follow the collapse of the gas—and many become bound to the new-forming numerical approximations to future stars (sinks). At minimum, 40% of all sinks have one or more ISO test particles gravitationally bound to them for the initial ISO distributions tested here. This value corresponds to at least 1010actual ISOs being bound after three initial freefall times. Thus, ISOs are a relevant component of star formation. We find that more massive sinks bind disproportionately large fractions of the initial ISO population, implying competitive capture of ISOs. Sinks can also be solitary, as their ISOs can become unbound again—particularly if sinks are ejected from the system. Emerging planetary systems will thus develop in remarkably varied environments, ranging from solitary to richly populated with bound ISOs.

Ice mantles on dust grains: dramatic variation of thickness with grain size

ABSTRACT We compute the desorption rate of icy mantles on dust grains as a function of the size and composition of both the grain and the mantle. We combine existing models of cosmic ray (CR)-related desorption phenomena with a model of CR transport to accurately calculate the desorption rates in dark regions of molecular clouds. We show that different desorption mechanisms dominate for grains of different sizes and in different regions of the cloud. We then use these calculations to investigate a simple model of the growth of mantles, given a distribution of grain sizes. We find that modest variations of the desorption rate with grain size lead to a strong dependence of mantle thickness on grain size. Furthermore, we show that freeze-out is almost complete in the absence of an external ultraviolet (UV) field, even when photodesorption from CR-produced UV is taken into consideration. Even at gas densities of $10^4\, {\rm cm^{-3}}$, less than 30 per cent of the CO remain in the gas phase after 3 × 105 yr for standard values of the CR ionization rate.

Modeling chemistry during star formation: water deuteration in dynamic star-forming regions

Context. Recent observations of the HDO/H2O ratio toward protostars in isolated and clustered environments show an apparent dichotomy, where isolated sources show higher D/H ratios than clustered counterparts. Establishing which physical and chemical processes create this differentiation can provide new insights into the chemical evolution of water during star formation and the chemical diversity during the star formation process and in young planetary systems. Aims. We seek to determine to what degree the local cloud environment influences the D/H ratio of water in the hot corinos toward low-mass protostars and establish which physical and chemical conditions can reproduce the observed HDO/H2O and D2O/HDO ratios in hot corinos. Methods. The evolution of water during star formation is modeled using 3D physicochemical models of a dynamic star-forming environment. The physical evolution during the protostellar collapse is described by tracer particles from a 3D MHD simulation of a molecular cloud region. Each particle trajectory is post-processed using RADMC-3D to calculate the temperature and radiation field. The chemical evolution is simulated using a three-phase grain-surface chemistry model and the results are compared with interferometric observations of H2O, HDO, and D2O in hot corinos toward low-mass protostars. Results. The physicochemical model reproduces the observed HDO/H2O and D2O/HDO ratios in hot corinos, but shows no correlation with cloud environment when similar initial conditions are tested. The observed dichotomy in water D/H ratios requires variation in the initial conditions, for example the duration and temperature of the prestellar phase. Reproducing the observed D/H ratios in hot corinos requires a prestellar phase duration t ~ 1−3 Myr and temperatures in the range T ~ 10−20 K prior to collapse. Furthermore, high cosmic-ray ionization rates (ξH2 ~ 10−15 s−1) appear to be incompatible with the observed D/H ratios toward low-mass protostars. Conclusions. This work demonstrates that the observed differentiation between clustered and isolated protostars stems from differences in the molecular cloud or prestellar core conditions and does not arise during the protostellar collapse itself. The observed D/H ratios for water in hot corinos are consistent with chemical inheritance of water, and no resetting during the protostellar collapse, providing a direct link between the prestellar chemistry and the hot corino.

Organic residues in astrophysical ice analogues: Thermal processing of hydrogenated glyoxal ices under interstellar conditions

ABSTRACT Organic residues are considered as part of the chemical composition of the interstellar dust grains. They are formed under the extreme conditions of the interstellar medium and play an important role in exobiology. They may contain pre-biotic organic species such as amino acids, constituents of proteins and building blocks of DNA and RNA, key elements of life. By investigating the formation of organic residues in an astrophysical context, many groups have been focusing in the UV irradiation and subsequent warm-up of astrophysical ice analogues. This aims to suggest that organic residues are mainly formed in regions of molecular clouds exposed to UV light or cosmic rays. This study shows an organic residue formation involving glyoxal ice and H atoms. While the hydrogenation of glyoxal at 10 K leads mainly to small molecules such as CO and H2CO and CH3OH, we show that the heating of the hydrogenated ice in the 10–300 K temperature range leads to solid residues whose structure is similar to that of glycolaldehyde but they remain stable in solid phase at 300 K and atmospheric pressure. The analysis of the IR data shows that the organic residues formed through the thermal processing of CHOCHO + H reaction would be a mixture of hydroxypyruvaldehyde and methyl glyoxylate, two solid organics whose formation starts with an H-abstraction from glyoxal to form CHOCO• radical which recombines to •CH2OH and •OCH3 radicals. These latter may be formed and trapped in glyoxal ice as secondary products from H2CO + H secondary reaction.

Dust polarization studies on MHD simulations of molecular clouds: comparison of methods for the relative-orientation analysis

Context. The all-sky survey from the Planck space telescope has revealed that thermal emission from Galactic dust is polarized on scales ranging from the whole sky down to the inner regions of molecular clouds. Polarized dust emission can therefore be used as a probe for magnetic fields on different scales. In particular, the analysis of the relative orientation between the density structures and the magnetic field projected on the plane of the sky can provide information on the role of magnetic fields in shaping the structure of molecular clouds where star formation takes place. Aims. The orientation of the magnetic field with respect to the density structures has been investigated using different methods. The goal of this paper is to explicitly compare two of these: the Rolling Hough Transform (RHT) and the gradient technique (GRAD). Methods. We generated synthetic surface brightness maps at 353 GHz (850 μm) via magnetohydrodynamic simulations. We applied RHT and GRAD to two morphologically different regions identified in our maps. Region 1 is dominated by a dense and thick filamentary structure with some branches, while Region 2 includes a thinner filament with denser knots immersed in a more tenuous medium. Both methods derive the relative orientation between the magnetic field and the density structures, to which we applied two statistics, the histogram of relative orientation and the projected Rayleigh statistic, to quantify the variations of the relative orientation as a function of column density. Results. Both methods find areas with significant signal, and these areas are substantially different. In terms of relative orientations, in all our considered cases the predominant orientation of the density structures is perpendicular to the direction of the magnetic field. When the methods are applied to the same selected areas the results are consistent with each other in Region 2 but show some noticeable differences in Region 1. In Region 1, RHT globally finds the relative orientation becoming more perpendicular for increasing column density, while GRAD, applied at the same resolution as RHT, gives the opposite trend. These disparities are caused by the intrinsic differences in the methods and in the structures that they select. Conclusions. Our results indicate that the interpretation of the relative orientation between the magnetic field and density structures should take into account the specificity of the methods used to determine such orientation. The combined use of complementary techniques such as RHT and GRAD provides more complete information, which can be advantageously used to better understand the physical mechanisms operating in magnetized molecular clouds.