arXiv subjects

L. Velilla-Prieto

Publications and source records attributed to L. Velilla-Prieto.

18 recordsLinked to original sources

arXiv2026

The formation radius of HCN in O-rich asymptotic giant branch stars

Molecules detected in circumstellar envelopes around asymptotic giant branch (AGB) stars are generally well explained in terms of formation under chemical equilibrium in the stellar atmosphere or photochemistry in the outer expanding layers. However, several molecules are detected in the inner circumstellar regions with abundances orders of magnitude above the predictions of chemical equilibrium. Anomalously abundant molecules comprise H2O, NH3, SiH4, and PH3 in C-rich envelopes and HCN, CS, and NH3 in O-rich sources. These molecules are formed by some yet unknown nonequilibrium processes. Aims: We aim to shed light on the nonequilibrium process that enhances the abundance of anomalously abundant molecules in the inner regions of circumstellar envelopes. Here, we focus on HCN in O-rich stars to constrain its formation radius. Method: We observed the O-rich AGB stars RCrt and IKTau with ALMA at high angular resolution (HPBW=~0.05") in the J=4-3 line of HCN in both the ground and the \nu_2=1 vibrational states. The radial distribution of the emission was modeled using a large velocity gradient radiative transfer code in which we considered an abundance profile with a central hole. Results: The models with a hole in the abundance distribution of HCN reproduce the specific shape of the radial emission distribution around the two studied O-rich stars. The best models locate the formation radius of HCN at 4-6 Rstar in RCrt and 3-5 Rstar in IKTau. Conclusion: The formation radius of HCN inferred from observations is consistent with models that invoke shocked-induced chemistry but also with models where photochemistry acts as the solely disequilibrium process. High angular resolution observations of more O-rich stars and tailored shocked-induced and/or photochemical models are needed to unambiguously unveil the underlying mechanism behind anomalously abundant molecules in AGB envelopes.

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arXiv2026

Chemistry in the High expansion-velocity C-rich evolved star AFGL2233. Isotopic ratios, peculiarities and evolutionary status

High expansion velocity carbon stars (HVCs) are a rare class of evolved stars whose circumstellar envelopes (CSEs) combine C-rich chemistry with unusually high expansion velocities typical of O-rich massive evolved stars. AFGL2233 has been proposed as a high-mass evolved object that exhausted hot-bottom burning. Studying its chemistry is essential to understand the nature and evolution of these objects. We characterize the chemical composition and isotopic ratios of the CSE of AFGL2233 and investigate chemical peculiarities, including the presence of N- and O-bearing species in a C-rich environment. We carried out a complete line survey at 3 mm and 1 mm using the IRAM 30m telescope, complemented by Herschel/HIFI FIR observations and interferometric maps of SiO, C2H, and HCN. Molecular emission was analyzed using rotational diagrams and radiative transfer modeling under the LVG approximation. Column densities and fractional abundances were derived for more than 30 molecular species, including isotopologues, and compared with other evolved stars. The Gaia DR3 distance of 1.236 kpc implies a luminosity of ~2 Lsun, consistent with an initial mass of 4.5-9 Msun. The molecular inventory confirms C-rich chemistry but reveals unusually high abundances of NH3, H2O, and SiN. The isotopic ratios vary among species, with 12C/13C ranging from 7 to 55. The C2H/C4H ratio is abnormally high compared with C-rich AGB stars. The presence of SiN and high NH3 may indicate N-enrichment or the influence of a companion. AFGL2233 is likely a high-mass AGB or super-AGB star with a complex evolutionary history involving nucleosynthesis, shocks, and possible binary interaction.

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arXiv2025

X TrA through the eyes of MATISSE: More evidence of clumpy molecular layers around C-type asymptotic giant branch stars

Aims. The goal of this study is to further the understanding of the wind formation mechanism in asymptotic giant branch (AGB) stars through the analysis of the close environment (within a few stellar radii) of the carbon star X TrA. Methods. X TrA was observed for the first time with the Mid-Infrared SpectroScopic Experiment instrument (MATISSE) in the L and N bands in low spectral resolution mode (R=30), and its close surroundings were mapped in specific wavelength ranges corresponding to specific molecules ($C_2H_2$ and HCN, at 3.1 and 3.8 $\mu$m) and dust (amorphous carbon and, for example, Sic at 11.3 $\mu$m), via image reconstruction techniques. Results. The angular diameter of the star ranges from 10 mas in the L band pseudo-continuum (3.5 $\mu$m) to 20 mas at 3.1 and 11.3 $\mu$m. The reconstructed images show some mild elongated features (along the east-west direction) and asymmetric protrusions, which are most evident around 3.1 $\mu$m. Imaging results highlight the clumpy nature of the circumstellar environment, starting from the photospheric region up to more distant layers. Conclusions. The angular diameters found for X TrA in the image data are in agreement with previous photospheric diameter estimates (following VLTI/MIDI 8-13 $\mu$m observations), and their wavelength dependence is similar to values found for other carbon stars observed with MATISSE (R Scl and V Hya). The 3.1 $\mu$m images presented here show highly asymmetric features, another case of a C-rich star with irregular morphologies close to the stellar disk; this supports the notion that the $C_2H_2+HCN$ abundance distribution usually originates from a clumpy layer around carbon stars.

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arXiv2025

Exploring circumstellar chemistry in X-ray emitting AGB stars

Aims. Our goal is to characterize the chemistry and physical conditions of the circumstellar envelopes (CSEs) of Asymptotic Giant Branch (AGB) binary candidate stars with UV-excess and X-ray emission, in particular, to identify the effects of the internal X-ray emission in the abundance of certain key molecules. Methods. We observed the 86.0-94.0 and 260.0-272.5 GHz spectral ranges searching for rotational transitions of the X-ray sensitive molecule $HCO^{+}$ in four AGB stars, two of them detected in both UV and X-ray emission and the other two detected only in UV. We derived the CSEs's physical parameters from previous CO observations and determined the molecular abundances of the detected species using radiative transfer models. We developed chemical kinetics models that account for the effects of internal X-ray emission (as well as UV radiation) and compared our predictions with observations. Results. We report the detection of $HCO^{+}$ in the X-ray emitting C-rich AGB T\,Dra, while it remains undetected in the spectra of the other three sources. In T\,Dra we also detect SiO, HCN, HNC, $HC_{3}N$, $SiC_{2}$, $C_{2}H$ and SiS. For the other targets only HCN and SiO are detected. The high fractional abundance of $HCO^{+}$ derived for T\,Dra ($[1.5-3.0]\times 10^{-8}$) is in good agreement with the predictions from our chemical kinetics models including the effects of internal X-ray emission, and one order of magnitude higher than the values expected for C-rich AGB stars. Additionally, we identify abundance enhancements for HNC, and $HC_{3}N$ alongside a depletion of CO in the innermost regions of T\,Dra's envelope. Conclusions. An internal X-ray source can significantly alter molecular abundances in AGB CSEs, enhancing $HCO^{+}$, $N_{2}H^{+}$, HNC, and $HC_{3}N$ while depleting parent species like CO. UV radiation has a weaker effect unless the envelope is optically thin or porous.

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arXiv2025

Reevaluation of ALMA detection of circumstellar PH3 in the AGB envelope IRC+10216: evidence for misidentification with HCN

The article "Confirmation of interstellar phosphine towards asymptotic giant branch star IRC+10216" by A. Manna and S. Pal uses ALMA data of the C-star envelope IRC+10216 to claim a confirmation of the detection of PH3 in this source. The article however incorrectly assign an emission feature observed in the ALMA spectrum of IRC+10216 to PH3, while we find that it arises from a highly vibrationally excited state of HCN. Concretely the feature can be confidently assigned to the J=3-2 l=0 transition of HCN in the v1+4v2 vibrational state based on the observation of the l=+2 and l=-2 components of the same rotational transition, J=3-2, with the observed relative intensities in agreement with the relative line strengths. The detection of PH3 in IRC+10216 remains confirmed based on the observation of the J=1-0 and J=2-1 lines with the single-dish telescopes IRAM-30m, ARO SMT-10m, and Herschel (Agundez et al. 2008, 2014; Tenenbaum et al. 2008).

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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

Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars

During their thermally pulsing phase, Asymptotic Giant Branch (AGB) stars eject material that forms extended dusty envelopes. Visible polarimetric imaging found clumpy dust clouds within two stellar radii of several oxygen-rich stars. Inhomogeneous molecular gas has also been observed in multiple emission lines within several stellar radii of different oxygen rich stars, including W Hya and Mira. At the stellar surface level, infrared images have revealed intricate structures around the carbon semi-regular variable R Scl and in the S-type star $\pi^{\mathrm{1}}$ Gru. Infrared images have also shown clumpy dust structures within a few stellar radii of the prototypical carbon AGB star IRC+10216, and studies of the molecular gas distribution beyond the dust formation zone have also shown complex circumstellar structures. Because of the lack of sufficient spatial resolution, however, the distribution of molecular gas in the stellar atmosphere and the dust formation zone of AGB carbon stars is not known, nor is how it is subsequently expelled. Here we report observations with a resolution of one stellar radius of the recently formed dust and molecular gas in the atmosphere of IRC+10216. Lines of HCN, SiS, and SiC$_2$ appear at different radii and in different clumps, which we interpret as large convective cells in the photosphere, as seen in Betelgeuse. The convective cells coalesce with pulsation causing anisotropies that, together with companions, shape its circumstellar envelope.

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arXiv2024

Multi-line study of the radial extent of SiO, CS, and SiS in AGB envelopes

The spatial distribution of molecules in AGB circumstellar envelopes is regulated by different processes. In the outer layers all molecules are destroyed due to the interaction with interstellar ultraviolet photons. Here we aim to characterize in a coherent and uniform way the radial extent of three molecules (SiO, CS, and SiS) in envelopes around AGB stars of O- and C-rich character, and to study their dependence with mass loss rate. To that purpose, we used the Yebes 40m and IRAM 30m telescopes to observe 7 M-type and 7 C-type AGB envelopes covering a wide range of mass loss rates (1e-7 - 1e-5 Msun/yr) in lines of SiO, CS, and SiS spanning a range of upper level energies of 2-130 K. We carried out excitation and radiative transfer calculations over a wide parameter space to characterize the molecular abundance and radial extent. A chi2 analysis indicates that the abundance is well constrained while the radial extent is more difficult to constrain. The radial extent increases with increasing envelope density, in agreement with previous observational findings. At high envelope densities, Mdot/vexp > 1e-6 (Msun/yr)/(km/s), the radial extent of SiO, CS, and SiS are similar, while at low envelope densities, Mdot/vexp < 1e-7 (Msun/yr)/(km/s), the radial extent differ among the three molecules, in agreement with theoretical expectations based on destruction due to photodissociation. At low envelope densities we find a sequence of increasing radial extent, SiS -> CS -> SiO. We also find a tentative dependence of the radial extent with the chemical type (O- or C-rich) of the star for SiO and CS. Interferometric observations and further investigation of the photodissociation of SiO, CS, and SiS should allow to clarify the situation on the relative photodissociation radius of SiO, CS, and SiS in AGB envelopes and the dependence with envelope density and C/O ratio.

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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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arXiv2022

History of two mass loss processes in VY CMa. Fast outflows carving older ejecta

Red supergiant stars (RSGs, Minit = 10-40Msun) are known to eject large amounts of material, as much as half of their initial mass during this evolutionary phase. However, the processes powering the mass ejection in low- and intermediate-mass stars do not work for RSGs and the mechanism that drives the ejection remains unknown. Different mechanisms have been proposed as responsible for this mass ejection but so far little is known about the actual processes taking place in these objects. Here we present high angular resolution interferometric ALMA maps of VY CMa continuum and molecular emission, which resolve the structure of the ejecta with unprecedented detail. The study of the molecular emission from the ejecta around evolved stars has been shown to be an essential tool in determining the characteristics of the mass loss ejections. Our aim is thus to use the information provided by these observations to understand the ejections undergone by VY CMa and to determine their possible origins. We inspected the kinematics of molecular emission observed. We obtained position-velocity diagrams and reconstructed the 3D structure of the gas traced by the different species. It allowed us to study the morphology and kinematics of the gas traced by the different species surrounding VY CMa. Two types of ejecta are clearly observed: extended, irregular, and vast ejecta surrounding the star that are carved by localized fast outflows. The structure of the outflows is found to be particularly flat. We present a 3D reconstruction of these outflows and proof of the carving. This indicates that two different mass loss processes take place in this massive star. We tentatively propose the physical cause for the formation of both types of structures. These results provide essential information on the mass loss processes of RSGs and thus of their further evolution.

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arXiv2022

Dissecting the central regions of OH 231.8 + 4.2 with ALMA: a salty rotating disk at the base of a young bipolar outflow

We present ALMA continuum and molecular line emission maps at $\sim$1 mm of OH 231.8, a well studied bipolar nebula around an AGB star. The excellent angular resolution of our maps ($\sim$20 mas) allows us to scrutinise the central nebular regions of OH 231.8, which hold the clues to unravel how this iconic object assembled its complex nebular architecture. We report, for the first time in this object and others of its kind, the discovery of a rotating circumbinary disk selectively traced by NaCl, KCl, and H$_2$O emission lines. This represents the first detection of KCl in an oxygen-rich AGB circumstellar envelope. The rotating disk, of radius $\sim$30 au, lies at the base of a young bipolar wind traced by SiO and SiS emission, which also presents signs of rotation at its base. The NaCl equatorial structure is characterised by a mean rotation velocity of $\sim$4 km s$^{-1}$ and extremely low expansion speeds, $\sim$3 km s$^{-1}$. The outflow has a predominantly expansive kinematics characterised by a constant radial velocity gradient of $\sim$65 km s$^{-1}$ arcsec$^{-1}$ at its base. Beyond $r$$\sim$350 au, the gas in the outflow continues radially flowing at a constant terminal speed of $\sim$16 km s$^{-1}$. Our continuum maps reveal a spatially resolved dust disk-like structure perpendicular to the outflow, with the NaCl, KCl and H$_2$O emission arising from the disk's surface layers. Within the disk, we also identify an unresolved point continuum source, which likely represents the central Mira-type star QX Pup enshrouded by a $\sim$3 $R_{\star}$ component of hot ($\sim$1400 K) freshly formed dust. The point source is slightly off-centered from the disk centroid, enabling us for the first time to place constraints to the orbital separation and period of the central binary system, $a$$\sim$20 au and $P_{\rm orb}$$\sim$55 yr, respectively. (abridged).

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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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arXiv2020

The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars

Aims: We aim to determine the abundances of SiO, CS, SiS, SO, and SO$_2$ in a large sample of oxygen-rich AGB envelopes covering a wide range of mass loss rates to investigate the potential role that these molecules could play in the formation of dust in these environments. Methods:We surveyed a sample of 30 oxygen-rich AGB stars in the $λ$ 2 mm band using the IRAM 30m telescope. We performed excitation and radiative transfer calculations based on the LVG method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. Results:We detected SiO in all 30 targeted envelopes, as well as CS, SiS, SO, and SO$_2$ in 18, 13, 26, and 19 sources, respectively. Remarkably, SiS is not detected in any envelope with a mass loss rate below $10^{-6}$ M$_{\odot}$ yr$^{-1}$, whereas it is detected in all envelopes with mass loss rates above that threshold. From a comparison with a previous, similar study on C-rich sources, it becomes evident that the fractional abundances of CS and SiS show a marked differentiation between C-rich and O-rich sources, being two orders of magnitude and one order of magnitude more abundant in C-rich sources, respectively, while the fractional abundance of SiO turns out to be insensitive to the C/O ratio. The abundance of SiO in O-rich envelopes behaves similarly to C-rich sources, that is, the denser the envelope the lower its abundance. A similar trend, albeit less clear than for SiO, is observed for SO in O-rich sources. Conclusions: The marked dependence of CS and SiS abundances on the C/O ratio indicates that these two molecules form more efficiently in C- than O-rich envelopes. The decline in the abundance of SiO with increasing envelope density and the tentative one for SO indicate that SiO and possibly SO act as gas-phase precursors of dust in circumstellar envelopes around O-rich AGB stars.

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arXiv2019

IRC+10216 mass loss properties through the study of $λ$3mm emission: Large spatial scale distribution of SiO, SiS, and CS

The study of the gas in the envelopes surrounding asymptotic giant branch (AGB) stars through observations in the millimetre wavelength range provides information about the history and nature of these molecular factories. Here we present ALMA observations at subarsecond resolution, complemented with IRAM-30m data, of several lines of SiO, SiS, and CS towards the best-studied AGB circumstellar envelope, IRC+10216. We aim to characterise their spatial distribution and determine their fractional abundances mainly through radiative transfer and chemical modelling. The three species display extended emission with several enhanced emission shells. CS displays the most extended distribution reaching distances up to approximately 20''. SiS and SiO emission have similar sizes of approximately 11'', but SiS emission is slightly more compact. We have estimated fractional abundances relative to H$_2$, which on average are equal to f(SiO)$\sim$10$^{-7}$, f(SiS)$\sim$10$^{-6}$, and f(CS)$\sim$10$^{-6}$ up to the photo-dissociation region. The observations and analysis presented here show evidence that the circumstellar material displays clear deviations from an homogeneous spherical wind, with clumps and low density shells that may allow UV photons from the interstellar medium (ISM) to penetrate deep into the envelope, shifting the photo-dissociation radius inwards. Our chemical model predicts photo-dissociation radii compatible with those derived from the observations, although it is unable to predict abundance variations from the starting radius of the calculations ($\sim$10$R_{*}$), which may reflect the simplicity of the model. We conclude that the spatial distribution of the gas proves the episodic and variable nature of the mass loss mechanism of IRC+10216, on timescales of hundreds of years.

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arXiv2019

Discovery of the first Ca-bearing molecule in space: CaNC

We report on the detection of calcium isocyanide, CaNC, in the carbon-rich evolved star IRC+10216. We derived a column density for this species of (2.0$\pm$0.5)$\times$10$^{11}$ cm$^{-2}$. Based on the observed line profiles and the modelling of its emission through the envelope, the molecule has to be produced in the intermediate and outer layers of the circumstellar envelope where other metal-isocyanides have previously been found in this source. The abundance ratio of CaNC relative to MgNC and FeCN is $\simeq$1/60 and $\simeq$1, respectively. We searched for the species CaF, CaCl, CaC, CaCCH, and CaCH$_3$ for which accurate frequency predictions are available. Only upper limits have been obtained for these molecules.

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arXiv2019

Detection of highly excited OH towards AGB stars: A new probe of shocked gas in the extended atmospheres

We report the detection and investigate the properties of high-excitation lambda-doubling line emission of hydroxyl (OH) detected towards three asymptotic giant branch (AGB) stars (W Hya, R Dor, and IK Tau) using ALMA. The OH lines are produced very close to the central stars and seem optically thin and with no maser effect. We analyse the molecular excitation using population diagrams and find rotational temperatures of $\sim 2500$ K and column densities of $\sim 10^{19}$ cm$^{-2}$ for both W Hya and R Dor. For W Hya, we observe emission from vibrationally excited H2O arising from the same region as the OH emission. Moreover, CO $v = 1; J = 3 - 2$ emission also shows a brightness peak in the same region. Considering optically thin emission and the rotational temperature derived for OH, we find a CO column density $\sim 15$ times higher than that of OH, within an area of ($92 \times 84$) mas$^2$ centred on the OH emission peak. These results should be considered tentative because of the simple methods employed. The observed OH line frequencies differ significantly from the predicted transition frequencies in the literature, and provide the possibility of using OH lines observed in AGB stars to improve the accuracy of the Hamiltonian used for the OH molecule. We predict stronger OH lambda-doubling lines at millimetre wavelengths than those we detected. These lines will be a good probe of shocked gas in the extended atmosphere and are possibly even suitable as probes of the magnetic field in the atmospheres of close-by AGB stars through the Zeeman effect.

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arXiv2019

Gas infall and possible circumstellar rotation in R Leo

We present new interferometer molecular observations of R Leo taken at 1.2 mm with the Atacama Large Millimeter Array with an angular resolution up to ~0.026 arcsec. These observations permit us to resolve the innermost envelope of this star revealing the existence of a complex structure that involves extended continuum emission and molecular emission showing a non-radial gas velocity distribution. This molecular emission displays prominent red-shifted absorptions located right in front to the star typical of material infall and lateral gas motions compatible with the presence of a torus-like structure.

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arXiv2018

Through the magnifying glass: ALMA acute viewing of the intricate nebular architecture of OH231.8+4.2

We present continuum and molecular line emission ALMA observations of OH 231.8+4.2, a well studied bipolar nebula around an asymptotic giant branch (AGB) star. The high angular resolution (~0.2-0.3 arcsec) and sensitivity of our ALMA maps provide the most detailed and accurate description of the overall nebular structure and kinematics of this object to date. We have identified a number of outflow components previously unknown. Species studied in this work include 12CO, 13CO, CS, SO, SO2, OCS, SiO, SiS, H3O+, Na37Cl, and CH3OH. The molecules Na37Cl and CH3OH are first detections in OH 231.8+4.2, with CH3OH being also a first detection in an AGB star. Our ALMA maps bring to light the totally unexpected position of the mass-losing AGB star (QX Pup) relative to the large-scale outflow. QX Pup is enshrouded within a compact (<60 AU) parcel of dust and gas (clump S) in expansion (V~5-7 km/s) that is displaced by 0.6arcsec to the south of the dense equatorial region (or waist) where the bipolar lobes join. Our SiO maps disclose a compact bipolar outflow that emerges from QX Pup's vicinity. This outflow is oriented similarly to the large-scale nebula but the expansion velocities are about ten times lower (~35 km/s). We deduce short kinematical ages for the SiO outflow, ranging from ~50-80 yr, in regions within ~150 AU, to ~400-500 yr at the lobe tips (~3500 AU). Adjacent to the SiO outflow, we identify a small-scale hourglass-shaped structure (mini-hourglass) that is probably made of compressed ambient material formed as the SiO outflow penetrates the dense, central regions of the nebula. The lobes and the equatorial waist of the mini-hourglass are both radially expanding with a constant velocity gradient. The mini-waist is characterized by extremely low velocities, down to ~1 km/s at ~150 AU, which tentatively suggest the presence of a stable structure. (abridged)

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