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Astron. Astrophys. 354, 125-134 (2000)
4. Relative mass-loss rates and dust-to-gas ratios
In determining mass-loss rates and dust-to-gas ratios in the MCs
one can rely on the use of luminosities and eliminate
from Eq. (1). Luminosities are
difficult to measure for stars in the Milky Way, however, due to
unknown distances and severe interstellar extinction. Often the Mira
P-L relation is applied to infer distances to individual
stars, but this relation possibly breaks down for stars whose stellar
mantles have been significantly reduced due to mass loss (e.g.
Blommaert et al. 1998; Wood et al. 1998; Wood 1998), or obscured AGB
stars may fall on other, parallel sequences (Bedding & Zijlstra
1998; Wood 1999). For the Milky Way stars, L is eliminated from
Eq. (1), leaving to be measured.
Because has been measured for only a
few LMC stars, Eq. (3) will be used for the remaining LMC stars to
estimate from L (see Appendix
B). Although these estimates are likely to be accurate within a few km
s-1, the contribution of the uncertainty in
is enlarged by a factor three in Eq.
(5).
Eqs. (4) & (5) are used here to compare the combination of
mass-loss rate and dust-to-gas ratio between the MCs and between the
LMC and the Milky Way, respectively. If the mass-loss rate depends on
luminosity but not on initial metallicity and if the luminosity
distributions of the stars in these samples are identical - implying
identical star formation histories - then the distributions over the
combination of and
are identical except for possible
offsets due to different mean values for
among the different samples. The
stars within each of the samples are likely to cover a range in
initial metallicities due to their different progenitor masses and
hence different formation epochs. Similar star formation histories,
however, are anticipated to result in similar distributions over
initial metallicity, and it is therefore meaningful to assign a mean
initial metallicity and a mean value for
to each of the samples.
Eq. (4) is used to calculate the combination of mass-loss rate and
dust-to-gas ratio from the optical depths and luminosities for the
magellanic stars (Fig. 3). The stars with
and
mag are considered to be RSGs and
obscured AGB stars, respectively (see van Loon et al. 1999b), except
for the well-studied luminous obscured AGB star IRAS05298-6957
( mag) and the weakly mass-losing RSG
HV2700 ( mag). IRAS04498-6842
( mag) was classified as an AGB star
(Paper II), but here it is re-classified as a RSG (see also
Paper IV). Fig. 4 shows the cumulative distributions (normalised
to unity) of the obscured AGB stars in the LMC (solid) and SMC
(dotted) over the value of . These do
not include the few stars that have lower limits to their
colours. The distributions for the
subsamples of spectroscopically confirmed carbon stars and oxygen-rich
(M-type) stars (van Loon et al. 1999b, and references therein;
Groenewegen & Blommaert 1998) are boldfaced in Figs. 4a and b,
respectively. Despite the small numbers for some of the
sub-distributions, especially the M-type stars in the SMC, the shapes
of the cumulative distributions are very similar. Both in the LMC and
SMC the obscured M-type AGB stars and the obscured carbon stars have
distributions that are coincident to a very high degree
(Table 1). The only difference in the distributions is the
systematic offset between those in the LMC compared to those in the
SMC (Table 1). This offset is a
result for the obscured AGB stars as a whole, a
result for the obscured carbon
stars, and consistent for the obscured M-type AGB stars (though not
significant by itself).
![[FIGURE]](img75.gif) |
Fig. 3. Mass-loss rate and dust-to-gas ratio calculated from Eq. (4), versus . There is a maximum to , increasing with higher luminosity. Mass-loss rates may be the same in SMC and LMC if is sufficiently lower in the SMC.
|
![[FIGURE]](img83.gif) |
Fig. 4a and b. Normalised distribution of obscured AGB stars ( mag) over mass-loss rate and dust-to-gas ratio . Boldfaced are the sub-distributions of confirmed M-type stars a and confirmed carbon stars b amongst the obscured AGB stars.
|
![[TABLE]](img87.gif)
Table 1. Mean and error in the mean of the values of for the distributions of obscured AGB stars in the LMC and SMC. Also the differences between the various (sub-)distributions are listed.
Eq. (5) is used to calculate the combination of mass-loss rate and
dust-to-gas ratio from the optical depths and expansion velocities for
the obscured AGB stars in the LMC and the Milky Way (Fig. 5). Only LMC
stars for which periods are known from the literature (Wood et al.
1992; Wood 1998) are plotted. Stars in the different samples are not
similarly distributed over P, with the LMC sample having an
average pulsation period longer than that of the Milky Way sample,
possibly due to selection effects or differences in the star formation
histories. Hence the populations of stars in these samples may not be
directly compared.
Two regimes of pulsation period will be focussed on: one for
d, and another for
d. The first group consists of stars
that evolve beyond the optically bright Mira phase with moderate
and into the obscured AGB phase (Jura
1986). The distribution of these stars in Fig. 5 suggests that they
still increase in mass-loss rate and/or that they represent a large
range in stellar masses. The second group consists of more evolved (or
more massive) stars for which has
become so high that they shed their mantles on a timescale much
shorter than the nuclear burning timescale. Hence these stars stay at
constant luminosity while their pulsation periods keep increasing as
their mantles are steadily diminished. The lack of any clear
correlation between and P in
this part of Fig. 5 suggests that thereby the mass-loss rate remains
essentially constant. Stars in the LMC sample are predominantly found
in the second class of objects, whilst these kind of stars are very
rare in the Galactic Centre sample due to their faintness in the
K-band. M-type stars in the "Solar neighbourhood" sample of
Groenewegen et al. (1998) populate both regimes rather evenly.
The relative distributions of mass-loss rates and dust-to-gas
ratios are derived from Fig. 6 for the LMC (dotted), "Solar
neighbourhood" (M-type; solid) and Galactic Centre (dashed), in the
same way as before by estimating the relative offsets in
of the normalised cumulative
distributions. The sample of obscured AGB stars with
d in the "Solar neighbourhood"
(Groenewegen et al. 1998) is clearly inhomogeneous: (i) the
distribution of obscured M-type AGB stars is much flatter than all
other distributions, indicating a wide range of stellar parameters,
and (ii) the distribution of obscured carbon stars is offset with
respect to the distribution of obscured M-type AGB stars - a
result (Table 2). This may
reflect differences in age (initial stellar mass) and/or initial
metallicity, as the sub-samples of obscured M-type AGB stars and
obscured carbon stars in the Groenewegen et al. sample have been
selected in a very different way. The distributions of the obscured
M-type AGB stars and obscured carbon stars in the LMC sample are,
again, indistinguishable (Table 2). The Galactic Centre sample is
positively offset with respect to both the Groenewegen et al. sample
( result for
d) and the LMC sample
( result).
![[FIGURE]](img113.gif) |
Fig. 6a and b Normalised distribution of obscured AGB stars in the LMC and in the Milky Way over mass-loss rate and dust-to-gas ratio for pulsation periods d a and d b .
|
![[TABLE]](img119.gif)
Table 2. Mean and error in the mean of the values of for the distributions of obscured AGB stars in the LMC, in the Galactic Centre (GC), and in the "Solar neighbourhood" (Groen; excluding the point at d and a value of -2.7). Also the differences between the various (sub-)distributions are listed.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000
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