Issue |
A&A
Volume 443, Number 1, November III 2005
|
|
---|---|---|
Page(s) | 143 - 156 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20053131 | |
Published online | 21 October 2005 |
Mira variables in the OGLE bulge fields
1
Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: groen@ster.kuleuven.ac.be
2
Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
Received:
25
March
2005
Accepted:
9
June
2005
The 222 000 I-band light curves of variable stars detected by the
ogle-ii survey in the direction of the Galactic Bulge have been
fitted and also correlated with the denis and 2mass all-sky release databases and with lists of known
objects. Lightcurves and the results of the lightcurve fitting
(periods and amplitudes) and denis and 2mass data are presented for 2691
objects with I-band semi-amplitude larger than 0.45 mag,
corresponding to classical Mira variables.
That the Mira period distribution of 6 fields at similar longitude but
spanning latitudes from -1.2 to -5.8 are statistically
indistinguishable indicates similar populations with initial masses of
1.5-2 , corresponding to ages of 1-3 Gyr. A field at similar
longitude at
from Glass et al. (2001, MNRAS, 321, 77; erratum: 2002, MNRAS, 336, 1390) does show a
significantly different period distribution, indicating the presence
of a younger population of 2.5-3
and ages below 1 Gyr.
The K-band period-luminosity relation is presented for the whole
sample and for sub-fields. The zero point depends on Galactic
longitude. Simulations are carried out to show that the observed
dependence of the zero point with l, and the number of stars per
field are naturally explained using the model of disk and bulge stars
of Binney et al. (1997, MNRAS, 288, 365), for a viewing angle (major-axis Bar – axis
perpendicular to the line-of-sight to the Galactic Centre) of
degrees. The simulations also show that biases in the
observed zero point are small, <0.02 mag.
A comparison is made with similar objects in the Magellanic
Clouds. The slope of the PL-relation in the Bulge and the MCs agree
within the errorbars. Assuming the zero point does not depend on
metallicity, a distance modulus difference of 3.72 between Bulge and
LMC is derived. This implies a LMC DM of 18.21 for an assumed distance
to the Galactic Centre (GC) of 7.9 kpc, or, assuming a LMC DM of
18.50, a distance to the GC of 9.0 kpc.
From the results in Groenewegen (2004, A&A, 425, 595) it is found for carbon-rich
Miras that the PL-relation implies a relative SMC-LMC DM of 0.38,
assuming no metallicity dependence. This is somewhat smaller than the
often quoted value near 0.50. Following theoretical work by Wood
(1990, in From Miras to Planetary Nebulae,
ed. M. O. Mennessier, & A. Omont (Gif-sur-Yvette: Éditions Frontières), 67) a metallicity term of the form
is
introduced. If a relative SMC-LMC DM of 0.50 is imposed,
is required, and for that value the distance to the GC becomes
kpc (for a LMC DM of 18.50), within the errorbar of the
geometric determination of
kpc. An independent estimate
leads to a distance estimate to the GC of
kpc.
Key words: stars: AGB and post-AGB / Galaxy: bulge / Galaxy: center
© ESO, 2005
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