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Astron. Astrophys. 354, 125-134 (2000)

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3. Samples of obscured AGB stars

The optical depth scales with near-IR colours: [FORMULA]. The stellar photosphere is assumed to have [FORMULA] mag. Fig. 3 in Paper IV shows that galactic M-stars have [FORMULA] mag, whilst galactic carbon stars may be slightly redder with [FORMULA] mag. The sensitivity to the choice of [FORMULA] rapidly vanishes as stars become obscured at [FORMULA] mag.

Fig. 1 shows the [FORMULA] colours of the obscured AGB stars and RSGs in the MCs (Loup et al. 1997; Zijlstra et al. 1996; van Loon et al. 1997, 1998a: Papers I to IV). These samples are based on the identification of optical and near-IR counterparts of point sources detected at 12 and/or 25 µm by IRAS. The SMC photometry is mainly from Groenewegen & Blommaert (1998). Bolometric magnitudes for the SMC stars were determined in the same way as for the LMC stars by spline fitting to the spectro-photometric energy distribution (see Whitelock et al. 1994), adopting distance moduli of 18.55 and 18.97 mag for the LMC and SMC, respectively (Walker 1999). Also included are two newly identified obscured AGB stars in the LMC (Appendix A).

[FIGURE] Fig. 1. [FORMULA] colour versus bolometric magnitude for obscured AGB stars and red supergiants in the Magellanic Clouds. Colours are bluer for the more luminous stars.

The reddest, optically thickest stars, with [FORMULA] mag, are found among AGB stars with [FORMULA] to -6 mag. No such red objects are known in the MCs among the brightest AGB stars with [FORMULA] mag, nor amongst the RSGs. This is partly because bolometrically fainter stars have smaller inner radii of the dusty CSE and smaller expansion velocities, yielding larger [FORMULA] (Eq. (1)) and redder [FORMULA] colours at a given mass-loss rate.

Whitelock et al. (1994) have searched for Long Period Variables (LPVs) in the South Galactic Cap (SGC). Their sample consists of both optically bright and obscured AGB stars. Their [FORMULA] colours are plotted versus bolometric luminosity in Fig. 2 (the Mira P-L relation was applied), where different symbols are used according to the 25 µm flux density measured if the star were at the distance of the LMC. The obscured stars that are detected by IRAS in the LMC have [FORMULA] Jy. Their SGC equivalents have very red [FORMULA] colours, though not redder than the reddest in the LMC (crosses). Optically bright AGB stars in the SGC sample have typically [FORMULA] and [FORMULA] to -5 mag. The MC samples do not contain such objects because their mid-IR emission is too faint to have been detected by IRAS at the distances of the MCs. On the other hand, the SGC sample is devoid of the brightest AGB stars with [FORMULA] mag as well as RSGs, because in the Milky Way such massive stars are preferentially found in the galactic plane.

[FIGURE] Fig. 2. [FORMULA] colour versus bolometric magnitude for AGB stars in the South Galactic Cap (Whitelock et al. 1994). Different symbols are used according to the 25 µm flux density measured if the star were at the distance of the LMC. Crosses represent obscured AGB stars in the LMC (from Fig. 1).

There is a cluster of SGC stars with [FORMULA] and [FORMULA] mag (Fig. 2). These stars clearly show circumstellar reddening, but the mid-IR emission from their CSEs is just below the detection limit of IRAS when placed at the distance of the LMC. This leaves open the possibility of the existence in the LMC of a potentially large population of AGB stars with moderate mass-loss rates and luminosities. Indeed, in Paper III several field stars were found that are not related to a nearby IRAS source but that nevertheless had near-IR colours indicative of reddening. Recent ISO observations confirm the presence of this AGB population (Loup et al. 1999).

Wood et al. (1998) find LPVs in the Galactic Centre with K-band magnitudes from 5 to 13 after correction for interstellar extinction. At the distance of the LMC this would yield K-band magnitudes from 9 to 17, i.e. within the sensitivity of the searches in Papers II & III and in Groenewegen & Blommaert (1998). The interstellar extinction corrected [FORMULA] colours of the Galactic Centre LPVs average [FORMULA] mag and are [FORMULA] mag maximum, similar to the [FORMULA] colours of the obscured AGB stars in the LMC. Wood et al. argue that their sample includes stars with initial metallicities a few times solar, and Blommaert et al. (1998) indeed find very red objects with [FORMULA] mag as inferred from their K- and L-band photometry.

Groenewegen et al. (1998) observed and modelled obscured carbon stars in the Milky Way. They used the P-L relation for carbon Miras to infer distances to the individual stars, that are found to be typically within 2 kpc from the Sun. Their two most obscured carbon stars have [FORMULA] and 8.0 mag. They also compiled a sample of oxygen-rich stars with near-IR photometry, pulsation periods and expansion velocities, without overlap with the SGC and Galactic Centre samples. The most obscured of these M stars have [FORMULA] mag.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000
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