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Register a new userDEATHSTAR: Nearby AGB stars with the Atacama Compact Array II. CO envelope sizes and asymmetries: The S-type stars
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Added by
Miora Andriantsaralaza
Publication date
2021
fields
Physics
and research's language is
English
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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.
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This is the first publication of the DEATHSTAR project. The goal of the project is to reduce the uncertainties of observational estimates of mass-loss rates from Asymptotic Giant Branch (AGB) stars. Line emission from 12CO J=2-1 and 3-2 were mapped using the ACA. In this initial analysis, the emission distribution was fit to a Gaussian distribution in the uv-plane. Detailed radiative transfer analysis will be presented in the future. The axes of the best-fit Gaussian at the line center of the 12CO J=2-1 emission gives a first indication of the size of the emitting region. Furthermore, the fitting results, such as the major and minor axis, center position, and the goodness of fit across both lines, constrain the symmetry of the emission distribution. We find that the CO envelope sizes are, in general, larger for C-type than for M-type AGB stars, which is expected if the CO/H2 ratio is larger in C-type stars. Furthermore, a relation between the 12CO J=2-1 size and circumstellar density is shown that, while in broad agreement with photodissociation calculations, reveals large scatter and systematic differences between the stellar types. The majority of the sources have CO envelopes that are consistent with a spherically symmetric, smooth outflow. For about a third of the sources, indications of strong asymmetries are found. This is consistent with previous interferometric investigations of northern sources. Smaller scale asymmetries are found in a larger fraction of sources. These results for CO envelope radii and shapes can be used to constrain detailed radiative transfer modeling of the same stars so as to determine mass-loss rates that are independent of photodissociation models. For a large fraction of the sources, observations at higher spatial resolution will be necessary to further investigate the complex circumstellar dynamics revealed by our ACA observations.
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We study the evolved stellar population of the Local Group galaxy IC10, with the aim of characterizing the individual sources observed and to derive global information on the galaxy, primarily the star formation history and the dust production rate. To this aim, we use evolutionary sequences of low- and intermediate-mass ($M < 8~M_{odot}$) stars, evolved through the asymptotic giant branch phase, with the inclusion of the description of dust formation. We also use models of higher mass stars. From the analysis of the distribution of stars in the observational planes obtained with IR bands, we find that the reddening and distance of IC10 are $E(B-V)=1.85$ mag and $d=0.77$ Mpc, respectively. The evolved stellar population is dominated by carbon stars, that account for $40%$ of the sources brighter than the tip of the red giant branch. Most of these stars descend from $sim 1.1-1.3~M_{odot}$ progenitors, formed during the major epoch of star formation, which occurred $sim 2.5$ Gyr ago. The presence of a significant number of bright stars indicates that IC10 has been site of significant star formation in recent epochs and currently hosts a group of massive stars in the core helium-burning phase. Dust production in this galaxy is largely dominated by carbon stars; the overall dust production rate estimated is $7times 10^{-6}~M_{odot}$/yr.
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