480 research outputs found
Age, Metallicity, and the Distance to the Magellanic Clouds From Red Clump Stars
We show that the luminosity dependence of the red clump stars on age and
metallicity can cause a difference of up to < ~0.6 mag in the mean absolute I
magnitude of the red clump between different stellar populations. We show that
this effect may resolve the apparent ~0.4 mag discrepancy between red
clump-derived distance moduli to the Magellanic Clouds and those from, e.g.,
Cepheid variables. Taking into account the population effects on red clump
luminosity, we determine a distance modulus to the LMC of 18.36 +/- 0.17 mag,
and to the SMC of 18.82 +/- 0.20 mag. Our alternate red clump LMC distance is
consistent with the value (m-M){LMC} = 18.50 +/- 0.10 adopted by the HST
Cepheid Key Project. We briefly examine model predictions of red clump
luminosity, and find that variations in helium abundance and core mass could
bring the Clouds closer by some 0.10--0.15 mag, but not by the ~0.4 mag that
would result from setting the mean absolute I-magnitude of the Cloud red clumps
equal to the that of the Solar neighborhood red clump.Comment: Accepted for publication in The Astrophysical Journal Letters, AASTeX
4.0, 10 pages, 1 postscript figur
Imaging the Radio Photospheres of Mira Variables
We have used the VLA at 43 GHz to image the radio continuum emission from o
Ceti, R Leo, and W Hya and to precisely locate their SiO maser emission with
respect to the star. The radio continuum emission region for all three stars
has a diameter close to 5.6 AU. These diameters are similar to those measured
at infrared wavelengths in bands containing strong molecular opacity and about
twice those measured in line-free regions of the infrared spectrum. Thus, the
radio photosphere and the infrared molecular layer appear to be coextensive.
The 43 GHz continuum emission is consistent with temperatures near 1600 K and
opacity from H-minus free-free interactions. While the continuum image of o
Ceti appears nearly circular, both R Leo and W Hya display significant
elongations. The SiO masers for all three stars show partial rings with
diameters close to 8 AU.Comment: 14 pages; 3 figure
Hipparcos period-luminosity relations for Miras and semiregular variables
We present period-luminosity diagrams for nearby Miras and semiregulars,
selecting stars with parallaxes better than 20 per cent and well-determined
periods. Using K-band magnitudes, we find two well-defined P-L sequences, one
corresponding to the standard Mira P-L relation and the second shifted to
shorter periods by a factor of about 1.9. The second sequence only contains
semiregular variables, while the Mira sequence contains both Miras and
semiregulars. Several semiregular stars show double periods in agreement with
both relations. The Whitelock evolutionary track is shown to fit the data,
indicating that the semiregulars are Mira progenitors. The transition between
the two sequences may correspond to a change in pulsation mode or to a change
in the stellar structure. Large amplitude pulsations leading to classical Mira
classification occur mainly near the tip of the local AGB luminosity function.Comment: 10 pages with figures, accepted by ApJ Letter
Long Period Variables in Globular Clusters and the Galactic Bulge: Their Dependence on Metallicity
We derive the frequency of occurrence of luminous long period variables
(LLPVs) in globular clusters and in the Baade's Window field of the Galactic
bulge. LLPVs occur only in clusters with [Fe/H]>/= -1.0. In these clusters
their frequency of occurrence relative to the number of giant stars appears to
be independent of metallicity. Integrated over all metallicities, Baade's
Window appears to be deficient in LLPVs. We estimate [Fe/H] values for Baade's
Window LLPVs from their period and a log P vs. [Fe/H] relation derived from
cluster variables and find that LLPVs with [Fe/H]>/= 0.0 are absent from
Baade's Window. We propose that this is because of enhanced mass loss rates in
these LLPVs with a consequently abbreviated lifetime compared to lower
metallicity LLPVs. A typical lifetime for cluster LLPVs is about 3 x10^5 yrs.
Finally, we call attention to the need for a much more complete survey for
LLPVs in globular clusters.Comment: 20 pages of text plus tables in PS format created by MS WORD97; 9
figures in PS format generated by SigmaPlot; all in one gzipped tar file;
originally submitted to ApJ but has now been accepted and is in press at the
AJ with minor revisions and some amplifications from previous versio
The Infrared Surface Brightness Fluctuation Distances to the Hydra and Coma Clusters
We present IR surface brightness fluctuation (SBF) distance measurements to
NGC 4889 in the Coma cluster and to NGC 3309 and NGC 3311 in the Hydra cluster.
We explicitly corrected for the contributions to the fluctuations from globular
clusters, background galaxies, and residual background variance. We measured a
distance of 85 +/- 10 Mpc to NGC 4889 and a distance of 46 +/- 5 Mpc to the
Hydra cluster. Adopting recession velocities of 7186 +/- 428 km/s for Coma and
4054 +/- 296 km/s for Hydra gives a mean Hubble constant of H_0 = 87 +/- 11
km/s/Mpc. Corrections for residual variances were a significant fraction of the
SBF signal measured, and, if underestimated, would bias our measurement towards
smaller distances and larger values of H_0. Both NICMOS on the Hubble Space
Telescope and large-aperture ground-based telescopes with new IR detectors will
make accurate SBF distance measurements possible to 100 Mpc and beyond.Comment: 24 pages, 4 PostScript figures, 2 JPEG images; accepted for
publication in Ap
The Age and Structure of the Galactic Bulge from Mira Variables
We report periods and JHKL observations for 648 oxygen-rich Mira variables
found in two outer bulge fields at b=-7 degrees and l=+/-8 degrees and combine
these with data on 8057 inner bulge Miras from the OGLE, Macho and 2MASS
surveys, which are concentrated closer to the Galactic centre. Distance moduli
are estimated for all these stars. Evidence is given showing that the bulge
structure is a function of age. The longer period Miras (log P > 2.6, age about
5 Gyr and younger) show clear evidence of a bar structure inclined to the line
of sight in both the inner and outer regions. The distribution of the shorter
period (metal-rich globular cluster age) Miras, appears spheroidal in the outer
bulge. In the inner region these old stars are also distributed differently
from the younger ones and possibly suggest a more complex structure. These data
suggest a distance to the Galactic centre, R0, of 8.9 kpc with an estimated
uncertainty of 0.4 kpc. The possible effect of helium enrichment on our
conclusions is discussed.Comment: Accepted for MNRAS, 12 pages, 12 figure
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