723 research outputs found
A Determination of the Hubble Constant from Cepheid Distances and a Model of the Local Peculiar Velocity Field
We present a measurement of the Hubble Constant based on Cepheid distances to
27 galaxies within 20 Mpc. We take the Cepheid data from published measurements
by the Hubble Telescope Key Project on the Distance Scale (H0KP). We calibrate
the Cepheid Period-Luminosity (PL) relation with data from over 700 Cepheids in
the LMC obtained by the OGLE collaboration; we assume an LMC distance modulus
of 18.50 mag (d=50.1 kpc). Using this PL calibration we obtain new distances to
the H0KP galaxies. We correct the redshifts of these galaxies for peculiar
velocities using two distinct velocity field models: the phenomenological model
of Tonry et al. and a model based on the IRAS density field and linear
gravitational instability theory. We combine the Cepheid distances with the
corrected redshifts for the 27 galaxies to derive H_0, the Hubble constant. The
results are H_0 = 85 +/- 5 km/s/Mpc (random error) at 95% confidence when the
IRAS model is used, and 92 +/- 5 km/s/Mpc when the phenomenological model is
used. The IRAS model is a better fit to the data and the Hubble constant it
returns is more reliable. Systematic error stems mainly from LMC distance
uncertainty which is not directly addressed by this paper. Our value of H_0 is
significantly larger than that quoted by the H0KP, H_0 = 71 +/- 6 km/s/Mpc.
Cepheid recalibration explains ~30% of this difference, velocity field analysis
accounts for ~70%. We discuss in detail possible reasons for this discrepancy
and future study needed to resolve it.Comment: 33 pages, 8 embedded figures. New table, 5 new references, text
revision
The AGB population of NGC 6822: distribution and the C/M ratio from JHK photometry
NGC 6822 is an irregular dwarf galaxy and part of the Local Group. Its close
proximity and apparent isolation provide a unique opportunity to study galactic
evolution without any obvious strong external influences. This paper aims to
study the spatial distribution of the asymptotic giant branch (AGB) population
and metallicity in NGC 6822. Using deep, high quality JHK photometry, taken
with WFCAM on UKIRT, carbon- and oxygen-rich AGB stars have been isolated. The
ratio between their number, the C/M ratio, has then been used to derive the
[Fe/H] abundance across the galaxy. The tip of the red giant branch is located
at K0 = 17.41 \pm 0.11 mag and the colour separation between carbon- and
oxygen-rich AGB stars is at (J - K)0 = 1.20 \pm 0.03 mag (i.e. (J - K)2MAS S
{\guillemotright} 1.28 mag). A C/M ratio of 0.62 \pm 0.03 has been derived in
the inner 4 kpc of the galaxy, which translates into an iron abundance of
[Fe/H] = -1.29\pm0.07 dex. Variations of these parameters were investigated as
a function of distance from the galaxy centre and azimuthal angle. The AGB
population of NGC 6822 has been detected out to a radius of 4 kpc giving a
diameter of 56 arcmin. It is metal-poor, but there is no obvious gradient in
metallicity with either radial distance from the centre or azimuthal angle. The
detected spread in the TRGB magnitude is consistent with that of a galaxy
surrounded by a halo of old stars. The C/M ratio has the potential to be a very
useful tool for the determination of metallicity in resolved galaxies but a
better calibration of the C/M vs. [Fe/H] relation and a better understanding of
the sensitivities of the C/M ratio to stellar selection criteria is first
required
Imaging of the Stellar Population of IC10 with Laser Guide Star Adaptive Optics and the Hubble Space Telescope
We present adaptive optics (AO) images of the central starburst region of the
dwarf irregular galaxy IC10. The Keck 2 telescope laser guide star was used to
achieve near diffraction-limited performance at H and K' (Strehls of 18% and
32%, respectively). The images are centered on the putative Wolf-Rayet (W-R)
object [MAC92]24. We combine our AO images with F814W data from HST. By
comparing the K' vs. [F814W]-K' color-magnitude diagram (CMD) with theoretical
isochrones, we find that the stellar population is best represented by at least
two bursts of star formation, one ~ 10 Myr ago and one much older (150-500
Myr). Young, blue stars are concentrated in the vicinity of [MAC92]24. This
population represents an OB association with a half-light radius of about 3 pc.
We resolve the W-R object [MAC92]24 into at least six blue stars. Four of these
components have near-IR colors and luminosities that make them robust WN star
candidates. By matching the location of C-stars in the CMD with those in the
SMC we derive a distance modulus for IC10 of about 24.5 mag. and a foreground
reddening of E(B-V) = 0.95. We find a more precise distance by locating the tip
of the giant branch in the F814W, H, and K' luminosity functions. We find a
weighted mean distance modulus of 24.48 +/- 0.08. The systematic error in this
measurement, due to a possible difference in the properties of the RGB
populations in IC10 and the SMC, is +/- 0.16 mag.Comment: 13 pages, 13 figures, ApJ in pres
The VMC survey III : Mass-loss rates and luminosities of LMC AGB stars
Context. Asymptotic giant branch (AGB) stars are major contributors to both the chemical enrichment of the interstellar medium and the integrated light of galaxies. Despite its importance, the AGB is one of the least understood phases of stellar evolution. The main difficulties associated with detailed modelling of the AGB are related to the mass-loss process and the 3rd dredge-up efficiency Aims. We provide direct measures of mass-loss rates and luminosities for a complete sample of AGB stars in the Large Magellanic Cloud, disentangling the C- and O-rich stellar populations. Methods. Dust radiative transfer models are presented for all 374 AGB stars candidates in one of the fields observed by the new VISTA survey of the Magellanic Clouds (VMC). Mass-loss rates, luminosities and a classification of C-and O-rich stars are derived by fitting the models to the spectral energy distribution (SED) obtained by combining VMC data with existing optical, near-, and mid-infrared photometry. Results. The classification technique is reliable at a level of - at worst -75% and significantly better for the reddest dusty stars. We classified none of the stars with a relevant mass-loss rate as O-rich, and we can exclude the presence of more than one dusty O-rich star at a similar to 94% level. The bolometric luminosity function we obtained is fully consistent with most of the literature data on the LMC and with the prediction of theoretical models, with a peak of the C-star distribution at M-bol similar or equal to -4.8 mag and no stars brighter than the classical AGB tip, at M-bol = -7.1 mag. Conclusions. This exploratory study shows that our method provides reliable mass-loss rates, luminosities and chemical classifications for all AGB stars. These results offer already important constraints to AGB evolutionary models. Most of our conclusions, especially for the rarer dust-enshrouded extreme AGB stars, are however strongly limited by the relatively small area covered by our study. Forthcoming VMC observations will easily remove this limitation.Peer reviewe
Distances to Populous Clusters in the LMC via the K-Band Luminosity of the Red Clump
We present results from a study of the distances and distribution of a sample
of intermediate-age clusters in the Large Magellanic Cloud. Using deep
near-infrared photometry obtained with ISPI on the CTIO 4m, we have measured
the apparent K-band magnitude of the core helium burning red clump stars in 17
LMC clusters. We combine cluster ages and metallicities with the work of
Grocholski & Sarajedini to predict each cluster's absolute K-band red clump
magnitude, and thereby calculate absolute cluster distances. An analysis of
these data shows that the cluster distribution is in good agreement with the
thick, inclined disk geometry of the LMC, as defined by its field stars. We
also find that the old globular clusters follow the same distribution,
suggesting that the LMC's disk formed at about the same time as the globular
clusters, ~ 13 Gyr ago. Finally, we have used our cluster distances in
conjunction with the disk geometry to calculate the distance to the LMC center,
for which we find (m-M)o = 18.40 +/- 0.04_{ran} +/- 0.08_{sys}, or Do = 47.9
+/- 0.9 +/- 1.8 kpc.Comment: 31 pages including 5 figures and 7 tables. Accepted for publication
in the August 2007 issue of A
- …