arXiv subjects

M. T. Beltrán

Publications and source records attributed to M. T. Beltrán.

At least 37 records · Page 2Linked to original sources

arXiv2023

The GUAPOS project: G31.41+0.31 Unbiased ALMA sPectral Observational Survey. IV. Phosphorus-bearing molecules and their relation with shock tracers

The astrochemistry of the important biogenic element phosphorus (P) is still poorly understood, but observational evidence indicates that P-bearing molecules are likely associated with shocks. We study P-bearing molecules, as well as some shock tracers, towards one of the chemically richest hot molecular core, G31.41+0.31, in the framework of the project "G31.41+0.31 Unbiased ALMA sPectral Observational Survey" (GUAPOS), observed with the Atacama Large Millimeter Array (ALMA). We have observed the molecules PN, PO, SO, SO2, SiO, and SiS, through their rotational lines in the spectral range 84.05-115.91 GHz, covered by the GUAPOS project. PN is clearly detected while PO is tentatively detected. The PN emission arises from two regions southwest of the hot core peak, "1" and "2", and is undetected or tentatively detected towards the hot core peak. the PN and SiO lines are very similar both in spatial emission morphology and spectral shape. Region "1" is in part overlapping with the hot core and it is warmer than region "2", which is well separated from the hot core and located along the outflows identified in previous studies. The column density ratio SiO/PN remains constant in regions "1" and "2", while SO/PN, SiS/PN, and SO2/PN decrease by about an order of magnitude from region "1" to region "2", indicating that SiO and PN have a common origin even in regions with different physical conditions. Our study firmly confirms previous observational evidence that PN emission is tightly associated with SiO and it is likely a product of shock-chemistry, as the lack of a clear detection of PN towards the hot-core allows to rule out relevant formation pathways in hot gas. We propose the PN emitting region "2" as a new astrophysical laboratory for shock-chemistry studies

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arXiv2023

The GUAPOS project:III. Characterization of the O- and N-bearing complex organic molecules content and search for chemical differentiation

The G31.41+0.31 Unbiased ALMA sPectral Observational Survey (GUAPOS) project targets the hot molecular core (HMC) G31.41+0.31 (G31), to unveil the complex chemistry of one of the most chemically rich high-mass star-forming regions outside the Galactic Center (GC). In the third paper of the project, we present a study of nine O-bearing (CH$_3$OH, $^{13}$CH$_3$OH, CH$_3^{18}$OH, CH$_3$CHO, CH$_3$OCH$_3$, CH$_3$COCH$_3$ , C$_2$H$_5$OH, aGg'-(CH$_2$OH)$_2$, and gGg'-(CH$_2$OH)$_2$) and six N-bearing (CH$_3$CN, $^{13}$CH$_3$CN, CH$_3^{13}$CN, C$_2$H$_3$CN, C$_2$H$_5$CN, and C$_2$H$_5^{13}$CN) complex organic molecules toward G31. The aim of this work is to characterize the abundances in one of the most chemically-rich hot molecular cores outside the GC and to search for a possible chemical segregation between O-bearing and N-bearing species in G31, which hosts four compact sources as seen with higher angular resolution data. Observations were carried out with the interferometer ALMA and covered the entire Band 3 from 84 to 116 GHz ($\sim 32$ GHz bandwidth) with an angular resolution of $1.2''$ ($\sim4400\,\mathrm{au}$). The spectrum has been analyzed with the tool SLIM of MADCUBA to determine the physical parameters of the emitting gas. Moreover, we have analyzed the morphology of the emission of the molecular species. We have compared the abundances w.r.t methanol of COMs in G31 with other twenty-seven sources, including other hot molecular cores inside and outside the Galactic Center, hot corinos, shocked regions, envelopes around young stellar objects, and quiescent molecular clouds, and with chemical models. Different species peak at slightly different positions, and this, together with the different central velocities of the lines obtained from the spectral fitting, point to chemical differentiation of selected O-bearing species.

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arXiv2023

Kinematics and stability of high-mass protostellar disk candidates at sub-arcsecond resolution -- Insights from the IRAM NOEMA large program CORE

The fragmentation mode of high-mass molecular clumps and the accretion processes that form the most massive stars ($M\gtrsim 8M_\odot$) are still not well understood. To this end, we have undertaken a large observational program (CORE) making use of interferometric observations from the Northern Extended Millimetre Array (NOEMA) for a sample of 20 luminous ($L>10^4L_\odot$) protostellar objects in the 1.37 mm wavelength regime in both continuum and line emission, reaching $\sim$0.4" resolution (800 au at 2 kpc). Using the dense gas tracer CH$_3$CN, we find velocity gradients across 13 cores perpendicular to the directions of bipolar molecular outflows, making them excellent disk candidates. Specific angular momentum ($j$) radial profiles are on average $\sim10^{-3}$ km /s pc and follow $j \propto r^{1.7}$, consistent with a poorly resolved rotating and infalling envelope/disk model. Fitting the velocity profiles with a Keplerian model, we find protostellar masses in the range of $\sim 10-25$ $M_\odot$. Modelling the level population of CH$_3$CN lines, we present temperature maps and find median gas temperatures in the range $70-210$ K. We create Toomre $Q$ maps to study the stability of the disks and find almost all (11 of 13) disk candidates to be prone to fragmentation due to gravitational instabilities at the scales probed by our observations. In particular, disks with masses greater than $\sim10-20\%$ of the mass of their host (proto)stars are Toomre unstable, and more luminous protostellar objects tend to have disks that are more massive and hence more prone to fragmentation. Our finings show that most disks around high-mass protostars are prone to disk fragmentation early in their formation due to their high disk to stellar mass ratio. This impacts the accretion evolution of high-mass protostars which will have significant implications for the formation of the most massive stars.

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arXiv2023

The SOUL view of IRAS20126+4104. Kinematics and variability of the H$_2$ jet from a massive protostar

We exploit the increased sensitivity of the recently installed AO SOUL at the LBT to obtain new high-spatial-resolution NIR images of the massive young stellar object IRAS20126+4104 and its outflow. We aim to derive the jet proper motions and kinematics, as well as to study its photometric variability by combining the novel performances of SOUL together with previous NIR images. We used both broad-band ($K_{s}$, $K'$) and narrow-band (Br$γ$, H2) observations from a number of NIR cameras (UKIRT/UFTI,SUBARU/CIAO,TNG/NICS,LBT/PISCES,and LBT/LUCI1) to derive maps of the continuum and the H$_2$ emission in the 2.12 $μ$m line. Three sets of images, obtained with AO systems (CIAO,2003; FLAO,2012; SOUL,2020), allowed us to derive the proper motions of a large number of H$_2$ knots along the jet. Photometry from all images was used to study the jet variability. We derived knot proper motions in the range of 1.7-20.3 mas yr$^{-1}$ (i.e. 13-158 km s$^{-1}$ at 1.64 kpc, avg. outflow tangential velocity $\sim$ 80 km s$^{-1}$). The derived knot dynamical age spans a $\sim$ 200-4000 yr interval. A ring-like H$_2$ feature near the protostar location exhibits peculiar kinematics and may represent the outcome of a wide-angle wind impinging on the outflow cavity. Both H$_2$ geometry and velocities agree with those inferred from proper motions of the H$_2$O masers, located at a smaller distance from the protostar. Although the total H$_2$ line emission from the knots does not exhibit time variations at a $\widetilde{>}$ 0.3 mag level, we have found a clear continuum flux variation (radiation scattered by the dust in the cavity opened by the jet) which is anti-correlated between the blue-shifted and red-shifted lobes and may be periodic (with a period of $\sim$ 12-18 yr). We suggest that the continuum variability might be related to inner-disc oscillations which have also caused the jet precession.

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arXiv2022

CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy III. Nitrogen isotopic ratios in the outer Galaxy

Nitrogen isotopic ratios are a key tool for tracing Galactic stellar nucleosynthesis. We present the first study of the $^{14}$N/$^{15}$N abundance ratio in the outer regions of the Milky Way (namely, for galactocentric distances, $R_{\rm GC}$, from 12 kpc up to 19 kpc), with the aim to study the stellar nucleosynthesis effects in the global Galactic trend. We analysed IRAM 30m observations towards a sample of 35 sources in the context of the CHEMical complexity in star-forming regions of the OUTer Galaxy (CHEMOUT) project. We derived the $^{14}$N/$^{15}$N ratios from HCN and HNC for 14 and 3 sources, respectively, using the $J$ = 1-0 rotational transition of HN$^{13}$C, H$^{15}$NC, H$^{13}$CN, and HC$^{15}$N. The results found in the outer Galaxy have been combined with previous measurements obtained in the inner Galaxy. We find an overall linear decreasing H$^{13}$CN/HC$^{15}$N ratio with increasing $R_{\rm GC}$. This translates to a parabolic $^{14}$N/$^{15}$N ratio with a peak at 11 kpc. Updated Galactic chemical evolution models have been taken into account and compared with the observations. The parabolic trend of the $^{14}$N/$^{15}$N ratio with $R_{\rm GC}$ can be naturally explained (i) by a model that assumes novae as the main $^{15}$N producers on long timescales ($\ge$1 Gyr) and (ii) by updated stellar yields for low- and intermediate-mass stars.

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arXiv2022

N-bearing complex organics toward high-mass protostars: Constant ratios pointing to formation in similar pre-stellar conditions across a large mass range

No statistical study of COMs toward a large sample of high-mass protostars with ALMA has been carried out so far. We aim to study six N-bearing species: CH$_3$CN, HNCO, NH$_2$CHO, C$_2$H$_5$CN, C$_2$H$_3$CN and CH$_3$NH$_2$ in a large sample of high-mass protostars. From the ALMAGAL survey, 37 of the most line-rich hot molecular cores are selected. Next, we fit their spectra and find column densities and excitation temperatures of the above N-bearing species, in addition to CH$_3$OH. We (tentatively) detect CH$_3$NH$_2$ in $\sim32%$ of the sources. We find three groups of species when comparing their excitation temperatures: hot (NH$_2$CHO; Tex > 250 K), warm (C$_2$H$_3$CN, HN$^{13}$CO and CH$_{3}^{13}$CN; 100 K < Tex < 250 K) and cold species (CH$_3$OH and CH$_3$NH$_2$; Tex < 100 K). This temperature segregation reflects the trend seen in their sublimation temperatures and validates the idea of onion-like structure of COMs around protostars. Moreover, the molecules studied here show constant column density ratios across low- and high-mass protostars with scatter less than a factor $\sim3$ around the mean. The constant column density ratios point to a common formation environment of COMs or their precursors, most likely in the pre-stellar ices. The scatter around the mean of the ratios, although small, varies depending on the species considered. This spread can either have a physical origin (source structure, line or dust optical depth) or a chemical one. Formamide is most prone to the physical effects as it is tracing the closest regions to the protostars, whereas such effects are small for other species. Assuming that all molecules form in the pre-stellar ices, the scatter variations could be explained by differences in lifetimes or physical conditions of the pre-stellar clouds. If the pre-stellar lifetimes are the main factor, they should be similar for low- and high-mass protostars.

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arXiv2022

Methanol deuteration in high-mass protostars

The deuteration of molecules forming in the ices such as methanol (CH$_3$OH) is sensitive to the physical conditions during their formation in dense cold clouds and can be probed through observations of deuterated methanol in hot cores. Observations with ALMA containing transitions of CH$_3$OH, CH$_2$DOH, CHD$_2$OH, $^{13}$CH$_3$OH, and CH$_3^{18}$OH are investigated. The column densities of CH$_2$DOH, CHD$_2$OH, and CH$_3$OH are determined for all sources, where the column density of CH$_3$OH is derived from optically thin $^{13}$C and $^{18}$O isotopologues. Consequently, the D/H ratio of methanol is derived taking statistical effects into account. Singly deuterated methanol (CH$_2$DOH) is detected toward 25 of the 99 sources in our sample of the high-mass protostars. Including upper limits, the $\rm (D/H)_{CH_3OH}$ ratio inferred from $N_\mathrm{CH_2DOH}/N_\mathrm{CH_3OH}$ was derived for 38 of the 99 sources and varies between $\sim10^{-3}-10^{-2}$. Including other high-mass hot cores from the literature, the mean methanol D/H ratio is $1.1\pm0.7\times10^{-3}$. This is more than one order of magnitude lower than what is seen for low-mass protostellar systems ($2.2\pm1.2\times10^{-2}$). Doubly deuterated methanol (CHD$_2$OH) is detected toward 11 of the 99 sources. Including upper limits for 15 sources, the $\rm (D/H)_{CH_2DOH}$ ratios derived from $N_\mathrm{CHD_2OH}/N_\mathrm{CH_2DOH}$ are more than two orders of magnitude higher than $\rm (D/H)_{CH_3OH}$ with an average of $2.0\pm0.8\times10^{-1}$ which is similar to what is found for low-mass sources. Comparison with literature GRAINOBLE models suggests that the high-mass prestellar phases are either warm ($>20$ K) or live shorter than the free-fall timescale. In contrast, for low-mass protostars, both a low temperature of $<15$ K and a prestellar phase timescale longer than the free-fall timescale are necessary.

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arXiv2022

Cloud-cloud collision as origin of the G31.41+0.31 massive protocluster

The G31.41+0.31 (G31) hot molecular core (HMC) is a high-mass protocluster showing accelerated infall and rotational spin-up that is well studied at high-angular resolution. To complement the accurate view of the small scale in G31, we have traced the kinematics of the large-scale material by carrying out N$_2$H$^+$\,(1--0) observations with the IRAM 30m telescope of an area of $\sim$6$\times6$\,arcmin$^2$ around the HMC. The N$_2$H$^+$ observations have revealed a large-scale (5\,pc) hub-filament system (HFS) composed by at least four filamentary arms and a NNE--SSW velocity gradient ($\sim$0.4\,km/s/pc) between the northern and southern filaments. The linewidth increases towards the hub at the center of the HFS reaching values of 2.5--3\,km\,s$^{-1}$ in the central 1\,pc. The origin of the large-scale velocity gradient is likely cloud-cloud collision. In this scenario, the filaments in G31 would have formed by compression resulting from the collision and the rotation of the HMC observed at scales of 1000\,au would have been induced by shear caused by the cloud-cloud collision at scales of a few pc. We conclude that G31 represents a HFS in a compressed layer with an orthogonal orientation to the plane of the sky, and represents a benchmark for the filaments-to-clusters paradigm of star formation.

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arXiv2022

CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy. I. Organic molecules and tracers of star-formation activity

The outer Galaxy is an environment with metallicity lower than the Solar one. Because of this, the formation and survival of molecules in star-forming regions located in the inner and outer Galaxy is expected to be different. To gain understanding on how chemistry changes throughout the Milky Way, it is crucial to observe outer Galaxy star-forming regions to constrain models adapted for lower metallicity environments. In this paper we present a new observational project: chemical complexity in star-forming regions of the outer Galaxy (CHEMOUT). The goal is to unveil the chemical composition in 35 dense molecular clouds associated with star-forming regions of the outer Galaxy through observations obtained with the IRAM 30m telescope. In this first paper, we present the sample, and report the detection at 3~mm of simple organic species HCO+, H13CO+, HCN, c-C3H2, HCO, C4H, and HCS+, of the complex hydrocarbon CH3CCH, and of SiO, CCS and SO. From c-C3H2, we estimate new kinematic heliocentric and Galactocentric distances based on an updated rotation curve of the Galaxy. The detection of the molecular tracers does not seem to have a clear dependence on the Galactocentric distance. We also analyse the HCO+ line profiles. We find high velocity wings in ~71% of the targets, and their occurrence does not depend on the Galactocentric distance. Our results, confirmed by a statistical analysis, show that the presence of organic molecules and tracers of protostellar activity is ubiquitous in the low-metallicity environment of the outer Galaxy. Based on this, and on the additional evidence that small, terrestrial planets are omnipresent in the Galaxy, we support previous claims that the definition of Galactic Habitable Zone should be rediscussed in view of the ubiquitous capacity of the interstellar medium to form organic molecules.

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arXiv2022

Importance of source structure on complex organics emission. I. Observations of CH$_3$OH from low-mass to high-mass protostars

Complex organic molecules (COMs) are often observed toward embedded Class 0 and I protostars. However, not all Class 0 and I protostars exhibit COMs emission. In this work, variations in methanol (CH$_3$OH) emission are studied to test if absence of CH$_3$OH emission can be linked to source properties. Combining both new and archival observations with ALMA and sources from the literature, a sample of 184 low-mass and high-mass protostars is investigated. The warm (T > 100 K) gaseous CH$_3$OH mass, $M_{\rm CH_3OH}$, is determined for each source using primarily optically thin isotopologues. On average, Class I protostellar systems seem to have less warm $M_{\rm CH_3OH}$ ($<10^{-10}$ M$_\odot$) than younger Class 0 sources ($\sim10^{-7}$ M$_\odot$). High-mass sources in our sample show higher warm $M_{\rm CH_3OH}$ up to $10^{-7}-10^{-3}$ M$_\odot$. To take into account the effect of the source's overall mass on $M_{\rm CH_3OH}$, a normalized CH$_3$OH mass is defined as $M_{\rm CH_3OH}/M_{\rm dust,0}$, where $M_{\rm dust,0}$ is the cold + warm dust mass within a fixed radius. Excluding upper limits, a simple power-law fit to the normalized warm CH$_3$OH masses results in $M_{\rm CH_3OH}/M_{\rm dust,0}\propto L_{\rm bol}^{0.70\pm0.05}$. This is in good agreement with a simple hot core toy model which predicts that the normalized $M_{\rm CH_3OH}$ increases with $L_{\rm bol}^{0.75}$ due to the snowline moving outward. Sources for which the size of the disk is equivalent or smaller than the estimated 100 K radius agree well with the best-fit power-law model, whereas sources with significantly larger disks show up to two orders of magnitude lower normalized warm CH$_3$OH masses. Based on the latter results, we suggest that source structure such as a disk can result in colder gas and thus fewer COMs in the gas phase. Additionally, optically thick dust can hide the emission of COMs.

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arXiv2022

The sharp ALMA view of infall and outflow in the massive protocluster G31.41+0.31

Context. To better understand the formation of high-mass stars, it is fundamental to investigate how matter accretes onto young massive stars, how it is ejected, and how all this differs from the low-mass case. The massive protocluster G31.41+0.31 is the ideal target to study all these processes because observations at millimeter and centimeter wavelengths have resolved the emission of the Main core into at least four massive dust continuum sources, named A, B, C, and D, within 1" or 0.018 pc, and have identified signatures of infall and several outflows associated with the core. Aims. We study the interplay between infall and outflow in G31.41+0.31 by investigating at a spatial resolution of a few 100 au their properties and their possible impact on the core. Methods. We carried out molecular line observations of typical high-density tracers, such as CH3CN or H2CO, and shock and outflow tracers, such as SiO, with ALMA at 1.4 mm that achieved an angular resolution of 0.09" (340 au). Results. The observations have revealed inverse P-Cygni profiles in CH3CN and H2CO toward the four sources embedded in the Main core, suggesting that all of them are undergoing collapse. The infall rates, estimated from the red-shifted absorption are on the order of 1E-2 Msun/yr. The individual infall rates imply that the accretion timescale of the Main core is an order of magnitude smaller than its rotation timescale. This confirms that rotating toroids such as the G31 Main core are non-equilibrium, transient collapsing structures that need to be constantly replenished with fresh material from a large-scale reservoir. For sources B, C, and D, the infall could be accelerating inside the sources, while for source A, the presence of a second emission component complicates the interpretation. The SiO observations have revealed the presence of at least six outflows in the G31.41+0.31 star-forming region, ...

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arXiv2021

The trans/cis ratio of formic, HCOOH, and thioformic, HC(O)SH, acids in the Interstellar Medium

We studied theoretically the cis-trans isomerization reactions of two astrophysically relevant acids, formic acid (HCOOH) and thioformic acid (HC(O)SH), where the latter has recently been discovered in space. We also searched for these molecules towards the hot core G31.41+0.31 to compare their abundances with the expected theoretical isomerization results. Our results demonstrate that these isomerizations are viable in the conditions of the ISM due to ground-state tunneling effects, which allow the system to reach the thermodynamic equilibrium at moderately low temperatures. At very low temperatures (Tkin 10 K), the reaction rate constants for the cis to trans isomerizations are very small, which implies that the cis isomers should not be formed under cold ISM conditions. This is in disagreement with observations of the cis/trans isomers of HCOOH in cold cores where the cis isomer is found to be 5-6% the trans isomer. At high temperatures (150-300 K), our theoretical data not only match the observed behaviour of the trans/cis abundance ratios for HCOOH (the cis form is undetected), but they support our tentative detection of the trans and, for the first time in the insterstellar medium, of the cis isomer of HC(O)SH towards the hot molecular core G31.41+0.31 (with a measured trans/cis abundance ratio of 3.7). While the trans/cis ratio for HC(O)SH in the ISM depends on the relative stability of the isomers, the trans/cis ratio for HCOOH cannot be explained by isomerization, and it is determined by other competitive chemical processes

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arXiv2021

Clustered star formation at early evolutionary stages. Physical and chemical analysis of the young star-forming regions ISOSS J22478+6357 and ISOSS J23053+5953

We aim to characterize the physical and chemical properties of fragmented cores during the earliest evolutionary stages in the very young star-forming regions ISOSS J22478+6357 and ISOSS J23053+5953. NOEMA 1.3 mm data are used in combination with archival mid- and far-infrared observations to construct and fit the SEDs of individual fragmented cores. The radial density profiles are inferred from the 1.3 mm continuum visibility profiles and the radial temperature profiles are estimated from H2CO rotation temperature maps. Molecular column densities are derived with the line fitting tool XCLASS. The physical and chemical properties are combined by applying the physical-chemical model MUSCLE in order to constrain the chemical timescales of a few line-rich cores. The morphology and spatial correlations of the molecular emission are analyzed using the HOG method. The mid-infrared data show that both regions contain a cluster of young stellar objects. Bipolar molecular outflows are observed in the CO 2-1 transition toward the strong mm cores indicating protostellar activity. We find strong molecular emission of SO, SiO, H2CO, and CH3OH in locations which are not associated with the mm cores. These shocked knots can be either associated with the bipolar outflows or, in the case of ISOSS J23053+5953, with a colliding flow that creates a large shocked region between the mm cores. The mean chemical timescale of the cores is lower (20 000 yr) compared to that of the sources of the more evolved CORE sample (60 000 yr). With the HOG method, we find that the spatial emission of species tracing the extended emission and of shock-tracing molecules are well correlated within transitions of these groups.

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arXiv2021

Disk fragmentation in high-mass star formation. High-resolution observations towards AFGL 2591-VLA 3

Increasing evidence suggests that, similar to their low-mass counterparts, high-mass stars form through a disk-mediated accretion process. At the same time, formation of high-mass stars still necessitates high accretion rates, and hence, high gas densities, which in turn can cause disks to become unstable against gravitational fragmentation. We study the kinematics and fragmentation of the disk around the high-mass star forming region AFGL 2591-VLA 3 which was hypothesized to be fragmenting based on the observations that show multiple outflow directions. We use a new set of high-resolution (0.19 arcsec) IRAM/NOEMA observations at 843 micron towards VLA 3 which allow us to resolve its disk, characterize the fragmentation, and study its kinematics. In addition to the 843 micron continuum emission, our spectral setup targets warm dense gas and outflow tracers such as HCN, HC$_3$N and SO$_2$, as well as vibrationally excited HCN lines. The high resolution continuum and line emission maps reveal multiple fragments with subsolar masses within the inner 1000 AU of VLA 3. Furthermore, the velocity field of the inner disk observed at 843 micron shows a similar behavior to that of the larger scale velocity field studied in the CORE project at 1.37 mm. We present the first observational evidence for disk fragmentation towards AFGL 2591-VLA 3, a source that was thought to be a single high-mass core. While the fragments themselves are low-mass, the rotation of the disk is dominated by the protostar with a mass of 10.3$\pm 1.8~M_{\odot}$. These data also show that NOEMA Band 4 can obtain the highest currently achievable spatial resolution at (sub-)mm wavelengths in observations of strong northern sources.

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arXiv2021

The GUAPOS project II. A comprehensive study of peptide-like bond molecules

Peptide-like bond molecules, which can take part to the formation of proteins in a primitive Earth environment, have been detected up to now only towards a few sources. We present a study of HNCO, HC(O)NH$_{2}$, CH$_{3}$NCO, CH$_{3}$C(O)NH$_{2}$, CH$_{3}$NHCHO, CH$_{3}$CH$_{2}$NCO, NH$_{2}$C(O)NH$_{2}$, NH$_{2}$C(O)CN, and HOCH$_{2}$C(O)NH$_{2}$ towards the hot core G31.41+0.31. We have used the spectrum obtained from the ALMA 3mm spectral survey GUAPOS, with an angular resolution of 1.2"$\times$1.2" ($\sim$4500 au), to derive column densities of all the molecular species, together with other 0.2"$\times$0.2" ($\sim$750 au) ALMA observations to study the morphology of HNCO, HC(O)NH$_{2}$ and CH$_{3}$C(O)NH$_{2}$. We have detected HNCO, HC(O)NH$_{2}$, CH$_{3}$NCO, CH$_{3}$C(O)NH$_{2}$, and CH$_{3}$NHCHO, for the first time all together outside the Galactic center. We have obtained molecular fractional abundances with respect to H$_{2}$ from 10$^{-7}$ down to a few 10$^{-9}$ and with respect to CH$_{3}$OH from 10$^{-3}$ to $\sim$4$\times$10$^{-2}$. From the comparison with other sources, we find that regions in an earlier stage of evolution, such as pre-stellar cores, show abundances at least two orders of magnitude lower than those in hot cores, hot corinos or shocked regions. Moreover, molecular abundance ratios towards different sources are found to be consistent between them within $\sim$1 order of magnitude, regardless of the physical properties (e.g. different masses and luminosities), or the source position throughout the Galaxy. New correlations between pairs of molecular abundances have also been found. These results suggest that all these species are formed on grain surfaces in early evolutionary stages of molecular clouds, and that they are subsequently released back to the gas-phase through thermal desorption or shock-triggered desorption.

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arXiv2021

The TOPGöt high-mass star-forming sample. I. Methyl cyanide emission as tracer of early phases of star formation

The TOPGöt project studies a sample of 86 high-mass star-forming regions in different evolutionary stages from starless cores to ultra compact HII regions. The aim of the survey is to analyze different molecular species in a statistically significant sample to study the chemical evolution in high-mass star-forming regions, and identify chemical tracers of the different phases. The sources have been observed with the IRAM 30m telescope in different spectral windows at 1, 2, and 3 mm. In this first paper, we present the sample and analyze the spectral energy distributions (SEDs) of the TOPGöt sources to derive physical parameters. We use the MADCUBA software to analyze the emission of methyl cyanide (CH$_3$CN), a well-known tracer of high-mass star formation. The emission of the $\rm{CH_3CN(5_{K}-4_{K})}$ K-transitions has been detected towards 73 sources (85% of the sample), with 12 non-detections and one source not observed in the frequency range of $\rm{CH_3CN(5_{K}-4_{K})}$. The emission of CH$_3$CN has been detected towards all evolutionary stages, with the mean abundances showing a clear increase of an order of magnitude from high-mass starless-cores to later evolutionary stages. We found a conservative abundance upper limit for high-mass starless cores of $X_{\rm CH_3CN}<4.0\times10^{-11}$, and a range in abundance of $4.0\times10^{-11}<X_{\rm CH_3CN}<7.0\times10^{-11}$ for those sources that are likely high-mass starless cores or very early high-mass protostellar objects. In fact, in this range of abundance we have identified five sources previously not classified as being in a very early evolutionary stage. The abundance of $\rm{CH_3CN}$ can thus be used to identify high-mass star-forming regions in early phases of star-formation.

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arXiv2021

Fragmentation in the massive G31.41+0.31 protocluster

Context. ALMA observations at 1.4 mm and 0.2'' (750au) angular resolution of the Main core in the high-mass star forming region G31.41+0.31 have revealed a puzzling scenario: on the one hand, the continuum emission looks very homogeneous and the core appears to undergo solid-body rotation, suggesting a monolithic core stabilized by the magnetic field; on the other hand, rotation and infall speed up toward the core center, where two massive embedded free-free continuum sources have been detected, pointing to an unstable core having undergone fragmentation. Aims. To establish whether the Main core is indeed monolithic or its homogeneous appearance is due to a combination of large dust opacity and low angular resolution, we carried out millimeter observations at higher angular resolution and different wavelengths. Methods. We carried out ALMA observations at 1.4 mm and 3.5 mm that achieved angular resolutions of 0.1''(375 au) and 0.075'' (280 au), respectively. VLA observations at 7 mm and 1.3 cm at even higher angular resolution, 0.05'' (190 au) and 0.07'' (260 au), respectively, were also carried out to better study the nature of the free-free continuum sources detected in the core. Results. The millimeter continuum emission of the Main core has been clearly resolved into at least four sources, A, B, C, and D, within 1'', indicating that the core is not monolithic. The deconvolved radii of the dust emission of the sources, estimated at 3.5 mm, are 400-500au, their masses range from 15 to 26 Msun, and their number densities are several 1E9 cm-3. Sources A and B, located closer to the center of the core and separated by 750 au, are clearly associated with two free-free continuum sources, likely thermal radio jets, and are the brightest in the core. The spectral energy distribution of these two sources and their masses and sizes are similar and suggest a common origin.

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arXiv2021

Multi-scale view of star formation in IRAS 21078+5211: From clump fragmentation to disk wind

In the massive star-forming region IRAS 21078+5211, a highly fragmented cluster (0.1~pc in size) of molecular cores is observed, located at the density peak of an elongated (1~pc in size) molecular cloud. A small (1~km/s per 0.1~pc) LSR velocity (Vlsr) gradient is detected across the axis of the molecular cloud. Assuming we are observing a mass flow from the harboring cloud to the cluster, we derive a mass infall rate of about 10^{-4}~M_{sun}~yr^{-1}. The most massive cores (labeled 1, 2, and 3) are found at the center of the cluster, and these are the only ones that present a signature of protostellar activity in terms of emission from high-excitation molecular lines or a molecular outflow. We reveal an extended (size about 0.1~pc), bipolar collimated molecular outflow emerging from core 1. We believe this is powered by a (previously discovered) compact (size <= 1000~au) radio jet, ejected by a YSO embedded in core 1 (named YSO-1), since the molecular outflow and the radio jet are almost parallel and have a comparable momentum rate. By means of high-excitation lines, we find a large (14~km/s over 500~au) Vlsr gradient at the position of YSO-1, oriented approximately perpendicular to the radio jet. Assuming this is an edge-on, rotating disk and fitting a Keplerian rotation pattern, we determine the YSO-1 mass to be 5.6+/-2.0~M_{sun}. The water masers (previously observed with VLBI) emerge within 100-300~au from YSO-1 and are unique tracers of the jet kinematics. Their three-dimensional (3D) velocity pattern reveals that the gas flows along, and rotates about, the jet axis. We show that the 3D maser velocities are fully consistent with the magneto-centrifugal disk-wind models predicting a cylindrical rotating jet. Under this hypothesis, we determine the jet radius to be about 16~au and the corresponding launching radius and terminal velocity to be about 2.2~au and 200~km/s, respectively.

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