The VMC survey

Abstract

Context. Asymptotic giant branch (AGB) stars are major contributors to both the chemical enrichment of the interstellar medium and the integrated light of galaxies. Despite its importance, the AGB is one of the least understood phases of stellar evolution. The main difficulties associated with detailed modelling of the AGB are related to the mass-loss process and the 3rd dredge-up efficiency Aims. We provide direct measures of mass-loss rates and luminosities for a complete sample of AGB stars in the Large Magellanic Cloud, disentangling the C-and O-rich stellar populations. Methods. Dust radiative transfer models are presented for all 374 AGB stars candidates in one of the fields observed by the new VISTA survey of the Magellanic Clouds (VMC). Mass-loss rates, luminosities and a classification of C-and O-rich stars are derived by fitting the models to the spectral energy distribution (SED) obtained by combining VMC data with existing optical, near-, and mid-infrared photometry. Results. The classification technique is reliable at a level of -at worst -75% and significantly better for the reddest dusty stars. We classified none of the stars with a relevant mass-loss rate as O-rich, and we can exclude the presence of more than one dusty O-rich star at a ∼94% level. The bolometric luminosity function we obtained is fully consistent with most of the literature data on the LMC and with the prediction of theoretical models, with a peak of the C-star distribution at M bol −4.8 mag and no stars brighter than the classical AGB tip, at M bol = −7.1 mag.Conclusions. This exploratory study shows that our method provides reliable mass-loss rates, luminosities and chemical classifications for all AGB stars. These results offer already important constraints to AGB evolutionary models. Most of our conclusions, especially for the rarer dust-enshrouded extreme AGB stars, are however strongly limited by the relatively small area covered by our study. Forthcoming VMC observations will easily remove this limitation.

Synthetic photometry for carbon-rich giants

“…7, where the models were contrasted with the objects of the W06 sample in thė M-(J -K)-plane. Also shown in the figure is the LMC relation derived by Gullieuszik et al (2012), which fits both kinds of data quite well. The second case concerns the bolometric corrections compiled in Fig.…”

“…It is interesting that the overplotted empirical relation found for field carbon stars observed by Le Bertre (1997; dotted line) lies below the individual data over the whole range. On the other hand, the more recent relation for C-rich LMC stars adopted from Gullieuszik et al (2012;dashed line) fits the galactic W06 sample better, although these are two independent studies. Anyway, the different dynamic model atmospheres, also shown in Fig.…”

“…In addition, the empirical relation found by Le Bertre (1997) for galactic mass-losing AGB stars of spectral type C is drawn (dotted line). Overplotted is also the more recent fit derived by Gullieuszik et al (2012) for C-rich giants in the LMC (dashed line).…”

“…7 is given in the ESO photometric system and was transformed to the Johnson-Glass system according to the equations given in Bessell & Brett (1988). The other relation given in Gullieuszik et al (2012;their Eq. (2)) was transformed from the 2MASS system by applying the conversion of Carpenter (2001).…”

The VMC survey

“…The saturation limit is shown in magenta. Sources are extracted by the aperture-photometry pipeline up to 2 mag above this limit; see (Gullieuszik et al 2012, hereafter Paper III, for details). The patterns are very similar in each band; when sources are fainter the photometric error becomes larger and a similar change occurs in sharpness.…”