arXiv subjects

M. Andriantsaralaza

Publications and source records attributed to M. Andriantsaralaza.

6 recordsLinked to original sources

arXiv2026

A giant magnetic eruption from the asymptotic giant branch star Mira A

Mass loss during the advanced evolutionary stages of low- and intermediate-mass stars is traditionally attributed to dust-driven winds, whereas the influence of magnetic fields remains poorly constrained. However, recent observations show massive directional mass ejections from the nearby asymptotic giant branch (AGB) star Mira~A that appear not to be driven by radiation pressure on the dust. We observationally investigate whether magnetic fields are responsible for the mass loss ejections from Mira A. We obtained full-polarisation ALMA observations of the SiO $v=1$, $J=5\rightarrow4$ maser transition at 215 GHz with angular resolutions of 1.5-2.6 au. The observations reveal a highly magnetized loop-like mass ejection traced by strongly polarised SiO masers extending to approximately eight stellar radii. The morphology, polarisation pattern, energetics, and kinematics indicate a magnetically driven eruption analogous to a stellar coronal mass ejection. The observations demonstrate that magnetic eruptions can dominate mass loss from evolved stars and imply that magnetic fields must become an integral component of future models of AGB, and potentially other cool giant, winds.

astro-ph.SR↗

arXiv2026

Periodic activity on the (sub)mm surface of the AGB star Mira A

Evolved stars on the asymptotic giant branch (AGB) lose mass through stellar winds launched from their dynamic extended atmospheres. Recent high-angular-resolution observations of the archetypal AGB star Mira A (o Ceti) have revealed compact hotspots on its submillimetre (submm) surface, whose origin remains unclear. We aim to characterise the submm photosphere of Mira A and investigate the properties and temporal behaviour of the compact hotspots detected on its surface. We analysed 19 epochs of high-angular-resolution observations obtained with the Atacama Large Millimetre/submillimetre Array in Bands 4, 6, 7, and 9, ranging from 134.34 to 669.25 GHz. The complex visibilities were fitted with parametric models to derive the flux density, size, morphology, and brightness temperature of the stellar disc and an additional more compact component across multiple pulsation phases and observing frequencies.Our results show that the flux density of the stellar disc increases with frequency while its apparent size decreases, in agreement with the expected stratified radio photosphere. In Band 6, both the flux density and the size of the stellar disc vary with pulsation phase, reaching maximum values close to optical maximum light and decreasing toward minimum light. Compact hotspots are detected in Bands~6 and 7 near maximum light, with their brightness temperatures exceeding those of the stellar disc by several thousand kelvins, reaching T>8600 K in Band 6. They are highly energetic and persist on timescales of weeks but evolve over longer timescales, showing changes in both position and intensity. Hotspot activity appears to occur repeatedly across multiple pulsation cycles and may be linked to strong shocks, magnetic activity, and/or episodic mass ejections, and could therefore play an important role in the mass-loss process in AGB stars.

astro-ph.SR↗

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.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2022

Distance estimates for AGB stars from parallax measurements

Estimating the distances to asymptotic giant branch (AGB) stars using optical measurements of their parallaxes is not straightforward because of the large uncertainties introduced by their dusty envelopes, their large angular sizes, and their surface brightness variability. We compared the parallaxes from Gaia DR3 with parallaxes measured with maser observations with very long baseline interferometry (VLBI) to determine a statistical correction factor for the DR3 parallaxes using a sub-sample of 33 maser-emitting oxygen-rich nearby AGB stars. We found that the nominal errors on the Gaia DR3 parallaxes are underestimated by a factor of 5.44 for the brightest sources ($G<8$ mag). Fainter sources ($8\leq G<12$) require a lower parallax error inflation factor of 2.74. We obtain a Gaia DR3 parallax zero-point offset of -0.077 mas for bright AGB stars. The offset becomes more negative for fainter AGB stars. We calculated the distances of about 200 AGB stars in the DEATHSTAR project using a Bayesian statistical approach on the corrected DR3 parallaxes and a prior based on the previously determined Galactic distribution of AGB stars. We found that the derived Gaia distances are associated with significant, asymmetrical errors for more than 40% of the sources in our sample. We performed radiative transfer modelling of the stellar and dust emission to determine the luminosity of the sources in the VLBI sub-sample based on the distances derived from maser parallaxes, and derived a new bolometric period-luminosity relation for Galactic oxygen-rich Mira variables. A new distance catalogue based on these results is provided for the sources in the DEATHSTAR sample. We found that a RUWE (re-normalised unit weight error) below 1.4 does not guarantee reliable distance estimates and we advise against the use of only the RUWE to measure the quality of Gaia DR3 astrometric data for individual AGB stars.

astro-ph.SR↗

arXiv2021

DEATHSTAR: Nearby AGB stars with the Atacama Compact Array II. CO envelope sizes and asymmetries: The S-type stars

We aim to constrain the sizes of the CO circumstellar envelopes (CSEs) of 16 S-type stars, along with an additional 7 and 4 CSEs of C-type and M-type AGB stars, respectively. We map the emission from the CO J=2-1 and 3-2 lines observed with the Atacama Compact Array (ACA) and its total power (TP) antennas, and fit with a Gaussian distribution in the uv- and image planes for ACA-only and TP observations, respectively. The major axis of the fitted Gaussian for the CO(2-1) line data gives a first estimate of the size of the CO-line-emitting CSE. We investigate possible signs of deviation from spherical symmetry by analysing the line profiles, the results from visibility fitting, and by investigating the deconvolved images. The sizes of the CO-line-emitting CSEs of low-mass-loss-rate (low-MLR) S-stars fall between the sizes of the CSEs of C-stars, which are larger, and those of M-stars, which are smaller, as expected because of the differences in their respective CO abundances. The sizes of the low-MLR S-type stars show no dependence on circumstellar density, while a steeper density dependence is observed at high MLR. Furthermore, our results show that the CO CSEs of most of the S-stars in our sample are consistent with a spherically symmetric and smooth outflow. The CO envelope sizes obtained in this paper will be used to constrain detailed radiative transfer modelling to directly determine more accurate MLR estimates for the stars in our sample. For several of our sources that present signs of deviation from spherical symmetry, further high-resolution observations would be necessary to investigate the nature of, and the physical processes behind, these asymmetrical structures. This will provide further insight into the mass-loss process and its related chemistry in S-type AGB stars.

astro-ph.SR↗