arXiv subjects

W. H. T. Vlemmings

Publications and source records attributed to W. H. T. Vlemmings.

At least 19 recordsLinked to original sources

arXiv2026

CO rotational line emission in very red carbon stars in the Magellanic Clouds

Stars of low and intermediate initial mass lose most of their stellar mass at the end of their lives during the asymptotic giant branch (AGB) phase. Determining their gas and dust mass-loss rates (MLRs) is crucial for quantifying the contribution of evolved stars to the life cycle of dust and gas in the Universe. The Atacama Large Millimeter/submillimeter Array was used to observe 38 carbon stars (C stars) in the large Magellanic cloud (LMC) and three C stars in the small Magellanic cloud (SMC) in the CO J= 2-1 line. Line profiles were fitted to derive stellar velocities and wind-expansion velocities (Vexp). CO emission is detected in two C stars in the SMC and 33 C stars in the LMC. This is the first detection of carbon monoxide around an AGB star in the SMC. One object in the LMC shows emission in $^{13}$CO. The wind-expansion velocity ranges from \sim7.5 to \sim30 km/s. Archival data were used to determine the pulsation periods as well as construct and model the spectral energy distributions using two dust radiative transfer codes. Mass-loss rates were independently derived from these two codes as well as from the intensity of the CO line, using a simple formula. On average, the dust-based MLRs higher than the MLRs based on the CO line by a factor of 1.6. Additional CO data in other transitions, combined with proper modelling, is required to further investigate this possible discrepancy. Mass-loss rates, pulsation periods, and expansion velocities were compared to a sample of Galactic C stars. There is a strong bias, as the Magellanic Cloud targets sample the highest MLRs and luminosities, yet they represent only a minority of stars in a Galactic sample. Comparing this sample with a similarly extreme set of Galactic stars with periods longer than 500 days, we identify no correlation between metallicity and either the MLR or Vexp.

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arXiv2026

OH/IR stars in the inner Galactic bulge: I. Colour, CO line, and stellar light curve characteristics

Stars on the asymptotic giant branch (AGB) play important roles in a number of astronomical contexts. To quantify these roles, it is necessary to establish the mass-loss characteristics of stars as they evolve up and beyond the AGB. We used an equidistant sample of 77 AGB stars in the inner Galactic bulge, selected on the existence and strength of OH1612\,MHz maser emission, to obtain information on the mass loss of O-rich AGB stars, and on its transformations in geometry and kinematics that occur at the tip of the AGB. Observations of circumstellar lines from several rotational transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O were performed with the Atacama Large Millimeter/submillimeter Array (ALMA), and, for a sub-sample, with the Atacama Pathfinder EXperiment telescope (APEX). The ALMA observations also provide continuum data. Existing infrared photometry was used to estimate colours and produce stellar light curves. Based on mid-infrared colour, CO line, and near-infrared variability characteristics, the objects were divided into four categories of distinct stellar and/or circumstellar properties. Various circumstellar CO line characteristics are presented and compared between the categories, such as morphologies and extents of brightness distributions (BDs), line profiles (LPs), line intensity ratios, and kinematics. A majority of the objects form a homogenous group with sharply, centrally peaked BDs and LPs of the soft-parabola type, while the rest show extended and complex BDs and/or LPs with high-velocity wings. The C$^{18}$O line and ALMA continuum detection rates vary significantly between the categories. CO line emission is also detected from interaction zones where the expanding circumstellar gas meets the interstellar medium.

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arXiv2026

Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. III. SiO and SiS abundances

The present understanding of C-rich AGB chemistry largely relies on observations of the archetypal carbon star IRC+10216. Current molecular abundance estimates for carbon stars are based either on single-dish spectra sampling a range of excitation conditions, or on interferometric mapping of a few lines. We aim to estimate the circumstellar abundances of SiO, SiS, and their most abundant isotopologues ($^{29}$SiO, $^{30}$SiO, $^{29}$SiS, $^{30}$SiS, and Si$^{34}$S) for a sample of five carbon stars. We derived molecular abundances using detailed 1D non-local thermodynamic equilibrium (non-LTE) radiative transfer (RT) modelling, constrained by both morphological and excitation information obtained from spatially resolved ALMA maps and single-dish observations. We further compared the derived abundances to chemical modelling results. We obtain good fits to the SiO and SiS line profiles, and derived well-constrained abundance profiles and reliable isotopic ratios for all sources except AFGL 3068. While the SiS peak abundances are very similar across the sample (2.0$\times$10$^{-6}-4.7\times$10$^{-6}$), we find that the SiO peak abundances of the rest of the stars are a factor of $\sim$5 larger than that of IRC+10216. The $e$-folding radii ($R_\mathrm{e}$) are in the range 1.3$\times$10$^{16}-7.0\times$10$^{16}$ cm for SiO and 6.0$\times$10$^{15}-1.0\times$10$^{17}$ cm for SiS. The $R_\mathrm{e}$ increases with gas density for both SiO and SiS. Chemical models reproduce the derived SiO abundance profiles well, while over-predicting the SiS $R_\mathrm{e}$ values. Our models highlight the necessity of having spatially resolved observations across a broad range of excitation conditions, while also making evident the limitations inherent in 1D RT modelling using simplified (circum)stellar models. We find that the currently assumed SiS photodissociation rate in chemical models is underestimated.

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arXiv2026

A surprisingly large asymmetric ejection from Mira A

Stars with masses between roughly 1 and 8~$M_\odot$ end their lives on the asymptotic giant branch (AGB), when intense mass loss takes place. The outflows are generally accepted to be driven by radiation pressure acting on dust grains that form in the dense extended atmospheres created by the action of convection and stellar pulsations. The complex physics underlying convection, stellar pulsations, and dust nucleation precludes predicting AGB mass loss from first principles. We investigated the evolution of two lobes observed to be expanding away from the AGB star Mira~A using images of polarized light obtained at six epochs using SPHERE on the VLT and of molecular emission at two epochs obtained with ALMA. While dust seems confined to the edges of the lobes, gas fills the lobes and displays higher densities than expected at the observed radii based on the large-scale mass-loss rate of Mira~A, with a total gas mass in the lobes of $\sim 2 \times 10^{-5}~M_\odot$. We find the expansion of the lobes to be consistent with both a constant velocity (ejection time in 2010 or 2011) or a decelerating expansion (ejection time in 2012). If ejection events with a similar magnitude happen periodically, we derive periods between 50 and 200~years to account for the mass-loss rate of Mira~A. This periodicity is uncertain because the average mass-loss rate of Mira A on larger scales is uncertain. We find abundances in the lobes of $\sim 1.5 \times 10^{-6}$ and $\sim 2.5 \times 10^{-6}$ for SO and SO$_2$, respectively, and of $2\times10^{-10}$, $6.5\times10^{-10}$, and $4\times10^{-7}$ for AlO, AlF, and PO. The strong variation in brightness of the different features identified in the polarized-light images is puzzling. We suggest that an asymmetric stellar radiation field preferentially illuminates specific regions of the circumstellar envelope at a given time, producing a lighthouse-like effect.

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arXiv2025

A compact Keplerian-like disc and H30$α$ emission towards the radio jet in the massive protostar W75N(B)-VLA3

Studying young, high-mass stellar objects is challenging for testing models of massive star formation due to their great distances, often kiloparsecs away. This requires extremely high-angular resolution to resolve features like accretion discs around massive protostars. Consequently, while powerful, collimated outflows are evident in massive protostars, the compact accretion discs anticipated at their centres are still proving difficult to pinpoint. This study presents ALMA continuum and molecular line observations at 1.3 mm of the massive protostar W75N(B)-VLA3. The observations achieve an angular resolution of $\sim$0.12$^{\prime\prime}$ ($\sim$156 au). Dense gas tracers reveal a circumstellar disc of $\sim$450 au in radius surrounding VLA3, with an orientation perpendicular to its associated thermal radio jet. From the millimetre continuum, a total mass of $\approx$0.43-1.74 $M_\odot$ is estimated for the disc. The disc's velocity profile is consistent with Keplerian rotation around a protostar of $\approx$16 $M_\odot$. This adds VLA3 to the small number of massive disc-protostar-jet systems documented in the literature with a centrifugally supported disc with a radius less than 500 au. Additionally, we detected H30$α$ recombination line emission towards the radio jet powered by VLA3. Despite limitations in the spatial and spectral resolution, our data reveal a very broad line, indicative of high-velocity motions, as well as a tentative velocity gradient in the jet's direction, thus favouring the H30$α$ emission to originate from the radio jet. Should this interpretation be confirmed with new observations, W75N(B)-VLA3 could represent the first protostellar radio jet for which a thermal radio recombination line has been detected.

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arXiv2025

A search for the favored hyperfine transition of a 6.7 GHz methanol maser line

The polarized emission of astrophysical masers, especially OH and methanol lines, is an effective tool to study the magnetic field in high-mass star-forming regions. The magnetic field strength measurement via the Zeeman effect of OH maser emission is well established, but that of the methanol maser emission is still under debate because of its complex hyperfine structure. We aim to identify the dominating hyperfine transition of the Class II methanol maser emission by comparing the magnetic field strength measured with the excited OH maser emission and the Zeeman splitting of the methanol maser emission. We used quasi-simultaneous EVN observations of the two maser emissions at 6.035 GHz ex-OH and 6.668 GHz methanol toward two well-known high-mass young stellar objects: ON 1 and W75N. The observations were performed in full polarimetric mode and in phase-referencing mode to couple the maser features of the two maser emissions in each source. We detected linearly and circularly polarized emission in both maser transitions and HMYSOs. Specifically, we measured the magnetic field strength in twelve and five ex-OH maser features toward ON1 and W75N, respectively, and the Zeeman splitting of the methanol maser spectra in one and three maser features toward ON 1 and W75N, respectively. We determined that the two maser emissions likely probe the same magnetic field but at different densities. Indeed, a direct comparison of the magnetic field strength and the Zeeman splitting as measured with the ex-OH and methanol maser spots, respectively, provided values of the Zeeman splitting coefficient for the 6.7 GHz maser that do not match any of the table values present in the literature. We are not able to uniquely identify the dominating hyperfine transition; however, through density considerations, we can narrow the choice down to three hyperfine transitions: 3->4, 6->7A, and 7->8.

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arXiv2025

Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. II. Abundances and spatial distributions of CS

The circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars harbour a rich variety of molecules and are sites of complex chemistry. Our current understanding of the circumstellar chemical processes of carbon-rich AGB stars is predominantly based on observations of a single star, IRC+10216, often regarded as an archetypical carbon star. We aim to estimate stellar and circumstellar properties for five carbon stars, and constrain their circumstellar CS abundances. This study compares the CS abundances among the sources, informs circumstellar chemical models, and helps to assess if IRC+10216 is a good representative of the physics and chemistry of carbon star CSEs. We modelled the spectral energy distributions (SEDs) and CO line emission to derive the stellar and outflow properties. Using these, we then retrieved CS abundance profiles with detailed radiative transfer modelling, imposing spatial and excitation constraints from ALMA and single-dish observations. We obtain good fits to the SEDs and CO lines for all sources and reproduce the CS line emission across various transitions and apertures, yielding robust estimates of the CS abundance profiles. Peak CS fractional abundances range from 1$\times$10$^{-6}$ - 4$\times$10$^{-6}$, with e-folding radii of 1.8$\times$10$^{16}$ - 6.8$\times$10$^{16}$ cm. We also derive reliable $^{12}$C/$^{13}$C and $^{32}$S/$^{34}$S ratios from CS isotopologue modelling. Our results refine previous single-dish CS abundance estimates and improve the relative uncertainty on the CS e-folding radius for IRAS 07454$-$7112 by a factor of $\sim$2.5. Chemical models reproduce our estimates of the CS radial extent, corroborating the CS photodissociation framework used therein. We find no significant differences between the derived CS abundance profiles for IRC+10216 and the rest of the sample, apart from the expected density-driven variations.

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arXiv2025

The circumstellar environment around the extreme Galactic red supergiant NML Cygni: Dense, dusty, and asymmetric

Supernova observations imply the presence of a dense and asymmetric circumstellar environment around SN Type II progenitors, whereas the mass loss from these progenitors, namely, red supergiants, is still poorly constrained. We aim to characterise the dust and gas in the circumstellar environment of the extreme Galactic red supergiant \nmlcyg in terms of mass, morphology, and kinematics. Based on interferometric observations with NOEMA at 230 GHz we estimated dust masses and temperatures, and measured the extent and morphological complexity of the circumstellar environment. We detected two strong continuum components, amounting to an estimated total dust mass of $\sim2\times10^{-3}M_{\odot}$ located out to ~2000 AU from the star, largely beyond the dust detected at optical/infrared wavelengths. The extent of the detected CO emission supports the notion that the outflow is formed by a mass-loss rate of several $10^{-4}\,M_{\odot}$/year and that it is not primarily shaped by extreme irradiation from the Cyg OB2 cluster it has been associated with. We have detected, but not resolved, previously unseen high-velocity components close to the star. The observations reveal a very complex circumstellar morphology and we propose that some of the detected components could be the imprint of a hitherto unknown binary companion.

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arXiv2025

ALMA observations of CO isotopologues towards six obscured post-AGB stars

Low- and intermediate-mass stars evolve through the asymptotic giant branch (AGB), when an efficient mass-loss process removes a significant fraction of their initial mass. A substantial increase in the mass-loss rate at the end of the AGB is observed for at least some stars for unknown reasons. This creates post-AGB objects that are completely enshrouded in thick dusty envelopes and might be associated with binary interactions. We observed the $J=2-1$ line of $^{13}$CO, C$^{17}$O, and C$^{18}$O with the Atacama Large Millimeter / submillimeter Array (ALMA) towards six obscured post-AGB stars (four C-rich and two O-rich sources) to constrain the properties of their circumstellar envelopes, recent mass-loss histories, and initial mass of the central stars. Based on the inferred $^{17}$O/$^{18}$O isotopic ratios, we find all stars to have relatively low initial masses ($< 2~M_\odot$) contrary to suggestions in the literature of higher masses for some sources. We infer a mass for HD~187885 $\sim 1.15~M_\odot$, which is relatively low for a carbon star. For all but one source (GLMP~950), we observe kinematic components with velocities $\gtrsim 30$~km~s$^{-1}$, which are faster than typical AGB wind expansion velocities. For most sources, these higher-velocity outflows display point-symmetric morphologies. The case of Hen~3-1475 is particularly spectacular, with the high-velocity molecular outflow interleaved with the high-velocity outflow of ionised gas observed at optical wavelengths. Based on the size of the emission regions of the slow components of the outflows, we derive typical kinematic ages associated with the C$^{18}$O~$J=2-1$ emission $\lesssim 1500$~years and obtain relatively high associated mass-loss rates ($\gtrsim10^{-4}~M_\odot~{\rm yr}^{-1}$). The sources with known spectral types are found to have evolved faster than expected based on stellar evolutionary models.

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arXiv2024

An impressionist view of V Hydrae. When MATISSE paints Asymmetric Giant Blobs

Our purpose is to study the effect of binary companions located within the first 10 stellar radii from the primary AGB star. In this work, we target the mass-losing carbon star V Hydrae (V Hya), looking for signatures of its companion in the dust forming region of the atmosphere. The star was observed in the L- and N-bands with the VLTI/MATISSE instrument at low spectral resolution. We reconstructed images of V Hya's photosphere and surroundings using the two bands and compared our interferometric observables with VLTI/MIDI and VISIR archival data. To constrain the dust properties, we used DUSTY to model the spectral energy distribution. The star is dominated by dust emission in the L- and N- bands. The VISIR image confirms the presence of a large-scale dusty circumstellar envelope surrounding V Hya. The MATISSE reconstructed images show asymmetric and elongated structures in both infrared bands. In the L-band, we detected an elongated shape of approximately 15 mas, likely to be of photospheric origin. In the N-band, we found a 20 mas extension North-East from the star, and perpendicular to the L-band elongated axis. The position angle and the size of the N-band extension match the prediction of the companion position at MATISSE epoch. By comparing MATISSE N-band with MIDI data, we deduce that the elongation axis in the N-band has rotated since the previous interferometric measurements 13 years ago, supporting the idea that the particle enhancement is related to the dusty clump moving along with the companion. The MATISSE images unveil the presence of a dust enhancement at the companion position, opening new doors for further analysis on the binary interaction with an AGB component.

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arXiv2024

Probing the dynamical and kinematical structures of detached shells around AGB stars

Aims. We aim to resolve the spatial and kinematic sub-structures in five detached-shell sources to provide detailed constraints for hydrodynamic models that describe the formation and evolution of the shells. Methods. We use observations of the 12 CO (1-0) emission towards five carbon-AGB stars with ALMA. The data have angular resolutions of 0.3 arcsec to 1arcsec and a velocity resolution of 0.3 km/s . This enables us to quantify spatial and kinematic structures in the shells. Results. The observed emission is separated into two distinct components: a more coherent, bright outer shell and a more filamentary, fainter inner shell. The kinematic information shows that the inner sub-shells move at a higher velocity relative to the outer sub-shells. The observed sub-structures confirm the predictions from hydrodynamical models. However, the models do not predict a double-shell structure, and the CO emission likely only traces the inner and outer edges of the shell, implying a lack of CO in the middle layers of the detached shell. Previous estimates of the masses and temperatures are consistent with originating mainly from the brighter subshell, but the total shell masses are likely lower limits. Conclusions. The observed spatial and kinematical splittings of the shells appear consistent with results from hydrodynamical models, provided the CO emission does not trace the H2 density distribution in the shell but rather traces the edges of the shells. It is therefore not possible to constrain the total shell mass based on the CO observations alone. Complementary observations of, e.g., CI as a dissociation product of CO would be necessary to understand the distribution of CO compared to H2.

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arXiv2024

Molecular line polarisation from the circumstellar envelopes of Asymptotic Giant Branch stars

Polarisation observations of masers in the circumstellar envelopes (CSEs) around Asymptotic Giant Branch (AGB) stars have revealed strong magnetic fields. However, masers probe only specific lines-of-sight through the CSE. Non-masing molecular line polarisation observation can more directly reveal the large scale magnetic field morphology and hence probe the effect of the magnetic field on AGB mass-loss and the shaping of the AGB wind. Observations and models of CSE molecular line polarisation can now be used to describe the magnetic field morphology and estimate its strength throughout the entire CSE. We use observations taken with ALMA of molecular line polarisation in the envelope of two AGB stars (CW~Leo and R~Leo). We model the observations using the multi-dimensional polarised radiative transfer tool PORTAL. We find linearly polarised emission, with maximum fractional polarisation on the order of a few percent, in several molecular lines towards both stars. We can explain the observed differences in polarisation structure between the different molecular lines by alignment of the molecules by a combination of the Goldreich-Kylafis effect and radiative alignment effects. We specifically show that the polarisation of CO traces the morphology of the magnetic field. Competition between the alignment mechanisms allows us to describe the behaviour of the magnetic field strength with radius throughout the circumstellar envelope of CW~Leo. The magnetic field strength derived using this method is inconsistent with the magnetic field strength derived using a structure function analysis of the CO polarisation and the strength previously derived using CN Zeeman observations. In contrast with CW~Leo, the magnetic field in the outer envelope of R~Leo appears to be advected outwards by the stellar wind. {abridged abstract}

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arXiv2024

An empirical view of the extended atmosphere and inner envelope of the AGB star R Doradus I. Physical model based on CO lines

The mass loss experienced on the asymptotic giant branch (AGB) at the end of the lives of low- and intermediate-mass stars is widely accepted to rely on radiation pressure acting on dust grains formed in the extended AGB atmospheres. The interaction of convection, stellar pulsation, and heating and cooling processes cause the density, velocity and temperature distributions in the inner regions of the envelope to be complex, making the dust-formation process difficult to calculate. Hence, characterising the extended atmospheres and inner outflow empirically is paramount to advance our understanding of the dust-formation and wind-driving processes. To this end, we observe the AGB star R Dor using ALMA and modelled the $^{12}$CO $v=0, J=2-1$, $v=1, J=2-1$ and $3-2$ and $^{13}$CO $v=0, J=3-2$ lines using the 3D radiative transfer code LIME up to a distance of $\sim 4$ times the radius of the star at sub-mm wavelengths. We find a complex velocity field with structure down to scales at least equal to the resolution of the observations. The observed maps are well reproduced assuming spherical symmetry for the gas temperature and density distributions. We find the radial profiles of these two quantities to be very steep close to the star and shallower for radii larger than $\sim 1.6$ times the stellar sub-mm radius. This change is consistent with the transition between extended atmosphere and outflow. We constrain the standard deviation of the stochastic velocity distribution in the large-scale outflow to be $\lesssim 0.4$ km/s. We observe two emission blobs in the CO $v=0, J=2-1$ line and find their gas densities and radial velocities to be substantially larger than those of the surrounding gas. Monitoring the evolution of these blobs will lead to a better understanding of the role of these structures in the mass-loss process of R Dor.

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arXiv2023

Charting Circumstellar Chemistry of Carbon-rich AGB Stars: I. ALMA 3 mm spectral surveys

AGB stars are major contributors to the chemical enrichment of the ISM through nucleosynthesis and extensive mass loss. Most of our current knowledge of AGB atmospheric and circumstellar chemistry, in particular in a C-rich environment, is based on observations of the carbon star IRC+10216. We aim to obtain a more generalised understanding of the chemistry in C-rich AGB CSEs by studying a sample of three carbon stars, IRAS15194-5115, IRAS15082-4808, and IRAS07454-7112, and test the archetypal status often attributed to IRC+10216. We performed spatially resolved, unbiased spectral surveys in ALMA Band 3. We identify a total of 132 rotational transitions from 49 molecular species. There are two main morphologies of the brightness distributions: centrally-peaked (e.g. HCN) and shell-like (e.g. C$_2$H). We estimated the sizes of the molecular emitting regions using azimuthally-averaged radial profiles of the line brightness distributions, and derived abundance estimates. Of the shell distributions, the cyanopolyynes peak at slightly smaller radii than the hydrocarbons, and CN and HNC show the most extended emission. The emitting regions for each species are the smallest for IRAS07454-7112. We find that, within the uncertainties of the analysis, the three stars present similar abundances for most species, also compared to IRC+10216. We find that SiO is more abundant in our three stars compared to IRC+10216. Our estimated isotopic ratios match well the literature values for the sources. The observed circumstellar chemistry appears very similar across our sample and compared to that of IRC+10216, both in terms of the relative location of the emitting regions and molecular abundances. This implies that, to a first approximation, the chemical models tailored to IRC+10216 are able to reproduce the observed chemistry in C-rich envelopes across roughly an order of magnitude in wind density.

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arXiv2023

Monitoring of the polarized $H_{2}O$ maser emission around the massive protostars W75N(B)-VLA1 and W75N(B)-VLA2

Several radio sources have been detected in the HMSFR W75N(B), among them the massive YSOs VLA1 and VLA2 are of great interest. These are thought to be in different evolutionary stages. In particular, VLA1 is at the early stage of the photoionization and it is driving a thermal radio jet, while VLA2 is a thermal, collimated ionized wind surrounded by a dusty disk or envelope. In both sources 22 GHz water masers have been detected in the past. Those around VLA1 show a persistent distribution along the radio jet and those around VLA2 have instead traced the evolution from a non-collimated to a collimated outflow over a period of 20 years. By monitoring the polarized emission of the water masers around both VLA1 and VLA2 over a period of 6 years, we aim to determine whether the maser distributions show any variation over time and whether the magnetic field behaves accordingly. The EVN was used in full polarization and phase-reference mode to measure the absolute positions of the masers and to determine both the orientation and the strength of the magnetic field. We observed four epochs separated by two years from 2014 to 2020. We detected polarized emission from the water masers around both the YSOs in all the epochs. We find that the masers around VLA1 are tracing a nondissociative shock originating from the expansion of the thermal radio jet, while the masers around VLA2 are tracing an asymmetric expansion of the gas that is halted in the northeast where the gas likely encounters a very dense medium. We also found that the magnetic field inferred from the water masers in each epoch can be considered as a portion of a quasi-static magnetic field estimated in that location rather than in that time. This allowed us to study locally the morphology of the magnetic field around both YSOs in a larger area by considering the vectors estimated in all the epochs as a whole.

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arXiv2023

Polarisation of molecular lines in the circumstellar envelope of the post-Asymptotic Giant Branch star OH 17.7-2.0

(abridged) The role of magnetic field in the shaping of Planetary Nebulae (PNe), either directly or indirectly after being enhanced by binary interaction, has long been a topic of debate. Large scale magnetic fields around pre-PNe have been inferred from polarisation observations of masers. However, because masers probe very specific regions, it is still unclear if the maser results are representative of the intrinsic magnetic field in the circumstellar envelope (CSE). Molecular line polarisation can provide important information about the magnetic field. A comparison between the field morphology determined from maser observations and that observed in the more diffuse CO gas, can reveal if the two tracers probe the same magnetic field. We compare observations taken with ALMA of molecular line polarisation around the post-Asymptotic Giant Branch)/pre-PNe star OH~17.7-2.0 with previous observations of polarisation in the 1612~MHz OH maser region. We detect CO~$J=2-1$ molecular line polarisation at a level of $\sim4\%$ that displays an ordered linear polarisation structure. We find that, correcting for Faraday rotation of the OH~maser linear polarisation vectors, the OH and CO linearly polarised emission trace the same large scale magnetic field. A structure function analysis of the CO linear polarisation reveals a plane-of-the-sky magnetic field strength of $B_\perp\sim1$~mG in the CO region, consistent with previous OH Zeeman observations. The consistency of the ALMA CO molecular line polarisation with maser observations indicate that both can be used to determine the magnetic field in CSEs. The existence of a strong, ordered, magnetic-field around OH 17.7-2.0 indicates that magnetic fields are likely involved in the formation of this bipolar pre-PNe.

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arXiv2022

First detection of AlF line emission towards M-type AGB stars

The nucleosynthesis production of fluorine (F) is still a matter of debate. Asymptotic giant branch (AGB) stars are one of the main candidates for F production. However, their contribution to the total F budget is not fully known due to the lack of observations. In this paper, we report the detection of AlF line emission, one of the two main carriers of F in the gas-phase in the outflow of evolved stars, towards five nearby oxygen-rich AGB stars, $o$ Ceti, R Leo, IK Tau, R Dor, and W Hya. From spatially resolved observations, we estimated the AlF emitting region with a radius $\sim11R_{\star}$ for $o$ Ceti and $\sim9R_{\star}$ for R Leo. From population diagram analysis, we report the AlF column densities of $\sim 5.8\times10^{15}$ cm$^{-2}$ and $\sim 3\times10^{15}$ cm$^{-2}$ for $o$ Ceti and R Leo, respectively, within these regions. For $o$ Ceti, we used the C$^{18}$O ($v=0$, $J=3-2$) observations to estimate the H$_2$ column density of the emitting region. We found a fractional abundance of $f_{\rm AlF/H_2}\sim(2.5\pm1.7)\times10^{-8}$. This gives a lower limit on the F budget in $o$ Ceti and is compatible with the solar F budget $f_{\rm F/H_2}=(5\pm2)\times10^{-8}$. For R Leo, a fractional abundance $f_{\rm AlF/H_2}=(1.2\pm0.5)\times10^{-8}$ is estimated. For other sources, we cannot precisely determine the emitting region based on the available data. Assuming an emitting region with a radius of $\sim 11R_{\star}$ and the rotational temperatures derived for $o$ Ceti and R Leo, we crudely approximated the AlF column density to be $\sim(1.2-1.5)\times10^{15}$ cm$^{-2}$ in W Hya, $\sim(2.5-3.0)\times10^{14}$ cm$^{-2}$ in R Dor, and $\sim(0.6-1.0)\times10^{16}$ cm$^{-2}$ in IK Tau. These result in fractional abundances within a range of $f_{\rm AlF/H_2}\sim(0.1-4)\times10^{-8}$ in W Hya, R Dor, and IK Tau.

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arXiv2021

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions. V. Completion of the flux-limited sample

Although the role of magnetic fields in launching molecular outflows in massive YSOs has been convincingly demonstrated by theoretical arguments, observationally, the alignment of the magnetic field lines with the molecular outflows is still under debate. We aim to complete the measurements of the direction of the magnetic fields at mas resolution around a sample of massive star-forming regions (MSFRs) to determine whether the magnetic field and outflows are aligned. In 2012, we started a large VLBI campaign with the EVN to measure the magnetic field orientation and strength toward a sample of 31 MSFRs (the flux-limited sample) by analyzing the polarized emission of 6.7GHz CH3OH masers. In the previous papers of the series, we have presented 80% of the sample. Here, we report the linearly and circularly polarized emission of 6.7GHz CH3OH masers toward the last five MSFRs of the flux-limited sample. The sources are G30.70-0.07, G30.76-0.05, G31.28+0.06, G32.03+0.06, and G69.52-0.97. We detected a total of 209 masers, 15% of which show linearly polarized emission (0.07%-16.7%), and 2% of which show circularly polarized emission (0.2%-4.2%). Zeeman splitting was measured toward G30.70-0.07, G32.03+0.06, and G69.52-0.97. The statistical analysis of the entire flux-limited sample shows that the observations are consistent with a bimodal distribution in the difference between the 3D magnetic field direction and the outflow axis, with half the magnetic field directions being perpendicular and the other half being parallel to the outflow. In addition, we determined that typical values of the linear and circular polarization fractions for 6.7 GHz CH3OH masers are Pl=1.0%-2.5% and Pv=0.5%-0.75%, respectively. We found that a typical Zeeman splitting is in the range between 0.5 m/s and 2.0 m/s. This would correspond to 9 mG<$|B_{||}|$<40 mG if F=3->4 is the most favored hyperfine transition.

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