Astron. Astrophys. 354, 125-134 (2000)
3. Samples of obscured AGB stars
The optical depth scales with near-IR colours:
. The stellar photosphere is assumed
to have mag. Fig. 3 in Paper IV
shows that galactic M-stars have
mag, whilst galactic carbon stars may be slightly redder with
mag. The sensitivity to the choice
of rapidly vanishes as stars become
obscured at mag.
Fig. 1 shows the 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]](img39.gif) |
Fig. 1. 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
mag, are found among AGB stars with
to -6 mag. No such red objects are
known in the MCs among the brightest AGB stars with
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
(Eq. (1)) and redder
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
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 Jy. Their SGC equivalents have
very red colours, though not redder
than the reddest in the LMC (crosses). Optically bright AGB stars in
the SGC sample have typically and
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
mag as well as RSGs, because in the
Milky Way such massive stars are preferentially found in the galactic
plane.
![[FIGURE]](img49.gif) |
Fig. 2. 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
and
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 colours of the
Galactic Centre LPVs average mag and
are mag maximum, similar to the
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
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 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 mag.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000
helpdesk.link@springer.de  |