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Astron. Astrophys. 354, 125-134 (2000)
Appendix A: new near-IR counterparts of IRAS sources in the LMC
Periods of weather conditions that were too poor for long-term
photometric monitoring at the South African Astronomical Observatory
(SAAO) at Sutherland, South Africa, in December 1997 were used to
search for near-IR counterparts of IRAS point sources in the direction
of the LMC. This was done on the 1.9 m telescope with the
Mk III scanning photometer in the K-band. An
aperture of was used, chopping and
nodding with a throw of . The search
was limited to objects brighter than
mag. The areas around five IRAS
point sources suspected to be obscured AGB stars (Paper I) were
searched. The candidate near-IR counterparts found are listed in
Table A1, where the photometry is in the SAAO system (Carter
1990), i.e. the J-band magnitude is transformed to the 0.75 m
telescope system. One object was re-observed under good photometric
conditions (LI-LMC1284), together with the star HR2015
( Dor) for photometric calibration.
Positions have been estimated by comparing the position of the
diaphragm in the (red) acquisition video images with the second
generation Digital Sky Survey, and are accurate to
.
![[TABLE]](img153.gif)
Table A1. Near-IR stars near IRAS point sources in the direction of the LMC (LI=LI-LMC: Schwering & Israel 1990) that are candidate obscured AGB stars. Listed are IRAS flux densities (in Jy), near-IR position, distance to the IRAS source (in arcsec), near-IR magnitudes, and bolometric magnitude assuming association (see text). Values between parentheses are 1- errors.
I retrieved 12, 25 and 60 µm data from the IRAS data
base server in
Groningen 1
(Assendorp et al. 1995). Point sources were recovered by means of
square degree maps with
pixels. The flux density was
measured from a trace through the position of the source using the
command SCANAID in the Groningen GIPSY
data analysis software. LI-LMC203 shows a hint of duplicity: two
similarly bright sources separated by one arcminute. LI-LMC987 looks
slightly extended, and LI-LMC1284 is on top of brighter emission.
LI-LMC1522 and especially LI-LMC1795 are isolated. Assuming
identification of near-IR and IRAS sources, the spectral energy
distribution were integrated graphically to yield bolometric
magnitudes.
LI-LMC203 is not identified with certainty. The best spatial
coincidence is for the first listed in Table A1, that has blue
near-IR colours incompatible with mass-losing AGB stars. There are two
much brighter near-IR sources with moderately red
nearby, of which the third listed
in Table A1 is a cluster of
stars within a diameter of . The
proposed near-IR counterparts of LI-LMC987 and 1795 have near-IR
colours consistent with red giants without mass loss and are probably
not associated with the IRAS sources. The near-IR counterpart of
LI-LMC1284 is a heavily obscured AGB star, with IR colours compatible
with either carbon or oxygen-rich dust. LI-LMC1522 is also identified
as a dust-enshrouded star, with IR colours suggesting oxygen-rich
dust.
Appendix B: expansion velocities
Here the expansion velocities of AGB stars are briefly discussed in
order to arrive at a justified calibration of the
scaling relation (Eq. (3)) for LMC
stars. I consider derived from the
separation of the peaks of OH maser line profiles, and from the width
of CO(1-0) emission for Milky Way stars without OH measurements. The
latter are divided by 1.12, following Groenewegen et al. (1998; see
also Lewis 1991).
The is plotted against P
in Fig. B1. Periods for the LMC stars are from Wood et al. (1992) and
Wood (1998), and their are from Wood
et al. (1992) and van Loon et al. (1998b). Carbon stars (filled
symbols) are distinguished from oxygen-rich, M-type stars (open
symbols). As an AGB star evolves, P increases. Mass loss
becomes important for d and reaches
a maximum for d (cf. Jura 1986).
This evolution is also reflected in ,
which increases when d and levels
off at a value km s-1
for d (dotted line in Fig. B1, see
also Lewis 1991). The individual data sets (and Lewis 1991) suggest
that AGB stars with d evolve at
constant
typically 3 to 4 km s-1.
This is also seen in (intrinsic) S-type stars with semi-regular
variability (Fig. 18 in Jorissen & Knapp 1998). This may be a slow
wind supported by another mechanism, such as radiation pressure on
molecules (cf. Sahai & Liechti 1995; Steffen et al. 1997, 1998).
It agrees with the observation that SiO masers, that depend on shocks
by stellar pulsation (Alcolea et al. 1990), may be present for
d and become ubiquitous when
d (Izumiura et al. 1994).
![[FIGURE]](img179.gif) |
Fig. B1. Expansion velocity versus pulsation period, for samples in the LMC (Wood et al. 1992; Wood 1998; van Loon et al. 1998b), South Galactic Cap (Whitelock et al. 1994), Galactic Centre (Wood et al. 1998) and "Solar neighbourhood" (Groenewegen et al. 1998). Carbon stars are represented by filled symbols. The dotted line is drawn to guide the eye: expansion velocities are low for , increase for longer periods, and reach a more constant level for . Low-metallicity stars in the LMC and SGC samples (at more than 1 kpc from the galactic plane) have lower than stars with solar metallicity.
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Carbon stars appear to have somewhat larger
than M-type stars at the same
P, although the (few) SGC carbon stars do not obey this trend.
The data is also consistent with smaller P for carbon stars at
the same .
SGC stars within 1 kpc from the galactic plane (squares) are
distinguished from SGC stars beyond that (large circles). The latter
are presumably of sub-solar initial metallicity and have smaller
than the former.
Wood et al. (1992) showed that is
smaller at LMC metallicity than at solar metallicity, providing
supportive evidence for Eq. (3). The LMC star with
km s-1 is
IRAS04553-6825, a very luminous RSG (van Loon et al. 1998b, and
references therein). The other LMC stars are AGB stars with
km s-1. The OH/IR stars
in the Groenewegen sample with have
to 20 km s-1, suggesting
that initial metallicities of these Milky Way stars are higher than
the LMC stars. The expansion velocities of the obscured AGB stars in
the Galactic Centre range up to km
s-1, and initial metallicities of two to three times solar
have been suggested by Wood et al. (1998). Considering all this, Eq.
(3) is calibrated by demanding a star with LMC metallicity and
( mag) to have
km s-1.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000
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