Understanding the evolution and dust formation of carbon stars in the Large Magellanic Cloud via the JWST

Abstract

Context. Carbon stars have been, and still are, extensively studied. Given their complex internal structure and their peculiar chemical composition, they are living laboratories in which we can test stellar structure and evolution theories of evolved stars. Furthermore, they are the most relevant dust manufacturers, thus playing a crucial role in the evolution of galaxies. Aims. We aim to study the dust mineralogy of the circumstellar envelope of carbon stars in the Large Magellanic Cloud (LMC) to achieve a better understanding of the dust formation process in the outflow of these objects. We intend to investigate the expected distribution of carbon stars in the observational planes built with the filters of the Mid-Infrared Instrument (MIRI) mounted onboard the James Webb Space Telescope (JWST) to select the best planes allowing an exhaustive characterisation of the stars. Methods. We compared the synthetic spectral energy distributions, obtained by modelling asymptotic giant branch stars and the dust formation process in the wind, with the spectra of carbon stars in the LMC, taken with the Infrared Spectrograph onboard the Spitzer Space Telescope. From the detailed comparison between synthetic modelling and observation we characterise the individual sources and derive the detailed mineralogy of the dust in the circumstellar envelope. Results. The sample of stars considered here is composed of stars of diverse mass, formation epoch, degree of obscuration, and metallicity. We find that precipitation of MgS on SiC seeds is common to all non-metal-poor carbon stars. Solid carbon is the dominant dust component, with percentages above 80% in all cases; a percentage between 10% and 20% of carbon dust is under the form of graphite, the remaining being amorphous carbon. Regarding the observational planes based on the MIRI filters, the colour-magnitude ([F770W]–[F1800W], [F1800W]) plane allows the best understanding of the degree of obscuration of the stars, while the ([F1800W]–[F2550W], [F1800W]) diagram allows better discrimination among stars of different metallicities.

Understanding dust production and mass loss in the AGB phase using post-AGB stars in the Magellanic Clouds

“…In addition, the comprehension of the TAGB turns crucial to interpret the distribution of stars of galaxies in the observational colour-colour and colour-magnitude planes, because it is while evolving thorugh the TAGB that the stars are expected to exhibit the largest infrared excesses, thus the reddest IR colours (Marini et al 2021;Dell'Agli et al 2021). Fig.…”

“…The AGB evolution of these stars was studied in details by Dell'Agli et al (2015a) and more recently by Marini et al (2021), who outlined the large quantities of carbon accumulated in the surface regions and the intense dust production taking place, particularly during the very final AGB phases. Results from stellar evolution and dust formation modelling suggest that at the tip of AGB this class of stars experiences mass loss rates slightly in excess of 10 −4 M /yr and efficient dust production, with rates of the order of a few 10 −6 M /yr; a thick dust shell composed mainly of solid carbon is expected to surround the star while evolving at the TAGB, with optical depths τ 10 ∼ 5 (Dell'Agli et al 2021).…”

“…The stars are interpreted and characterized under the light of updated AGB evolutionary sequences, extended on purpose until and past the post-AGB phase. These tracks also include the description of the dust formation process, following the schematization proposed by the Heidelberg group (Ferrarotti & Gail 2006), on the wake of previous studies on dust production by evolved stars proposed by our team (Ventura et al 2012(Ventura et al , 2014, which have been used to interpret the evolved stellar populations of the Magellanic Clouds (Dell'Agli et al 2014a,b, 2015a and of Local Group galaxies (Dell'Agli et al 2016, and to explore the potentialities in this field offered by the recent launch of the James Webb Space Telescope (Marini et al 2020(Marini et al , 2021.…”

AGB Winds with Gas-Dust Drift in Stellar Evolution Codes

“…Beyond this radius, the pressure term is insignificant and the conventional way of taking the pressure effects into account is to assume a non-zero gas velocity at the inner boundary (r in = r c ) and that dust nucleated at this location will have the same velocity. Second, one has to assume strict stationarity of the considered mean-flow, such that the equation of continuity (1) integrates to a constant mass flux at all r. This should apply to the dust component as well, in the present context. Third, the temperature structure (needed for the dust modelling) has to follow some approximate model; either a radiative equilibrium model or the Lucy approximation [29,30].…”

“…Knowledge about how asymptotic giant branch (AGB) stars enrich the interstellar medium (ISM) with metals (i.e., elements heavier than helium) requires understanding of the mechanisms behind their mass loss and how the formation of dust changes throughout their evolution on the AGB. Observational constraints provide important clues, such as the composition of dust produced and abundances of important molecular species for dust growth [1][2][3][4], which allow for testing and calibration of dust-formation models used in stellar evolution codes.…”