255 research outputs found
The embedded clusters DBS 77, 78, 102, and 160-161 and their link with the interstellar medium
Aims. We report a study of the global properties of some embedded clusters
placed in the fourth quadrant of the Milky Way to clarify some issues related
with their location into the Galaxy and their stellar formation processes.
Methods. We performed BVI photometric observations in the region of DBS 77, 78,
102, 160, and 161 clusters and infrared spectroscopy in DBS 77 region. They
were complemented with JHK data from VVV survey combined with 2MASS catalogue,
and used mid-infrared information from GLIMPSE catalogue. We also searched for
HI data from SGPS and PMN radio surveys, and previous spectroscopic stellar
classification. The spectroscopic and photometric information allowed us to
estimate the spectral classification of the brightest stars of each studied
region. On the other hand, we used the radio data to investigate the
interstellar material parameters and the continuum sources probably associated
with the respective stellar components. Results. We estimated the basic
physical parameters of the clusters (reddening, distance, age, and initial mass
function). We searched for HII regions located near to the studied clusters and
we analyzed the possible link between them. In the particular case of DBS
160-161 clusters, we identified the HI bubble B332.5-0.1-42 located around
them. We found that the mechanical energy injected to the interstellar medium
by the more massive stars of this couple of clusters was enough to generate the
bubble.Comment: 15 pages, 14 figures, 6 tables, accepted for publication in A&
New Metallicities of RR Lyrae Stars in omega Centauri: Evidence for a Non He-Enhanced Metal-Intermediate Population
We present new spectroscopic metal abundances for 74 RR Lyrae stars in omega
Cen obtained with FLAMES. The well-known metallicity spread is visible among
the RR Lyrae variables. The metal-intermediate (MInt) RR Lyrae stars ([Fe/H] ~
-1.2) are fainter than the bulk of the dominant metal-poor population ([Fe/H] ~
-1.7), in good agreement with the corresponding zero-age horizontal branch
models with cosmological helium abundance Y = 0.246. This result conflicts with
the hypothesis that the progenitors of the MInt RR Lyrae stars correspond to
the anomalous blue main-sequence stars, which share a similar metallicity but
whose properties are currently explained by assuming for them a large helium
enhancement. Therefore, in this scenario, the coexistence within the cluster of
two different populations with similar metallicities ([Fe/H] ~ -1.2) and
different helium abundances has to be considered.Comment: 9 pages, 4 figures, accepted for publication by ApJ
Molecular Gas in the Low Metallicity, Star Forming Dwarf IC 10
We present a complete survey of CO 1->0 emission in the Local Group dwarf
irregular IC 10. The survey, conducted with the BIMA interferometer, covers the
stellar disk and a large fraction of the extended HI envelope with the
sensitivity and resolution necessary to detect individual giant molecular
clouds (GMCs) at the distance of IC 10 (950 kpc). We find 16 clouds with a
total CO luminosity of 1 x 10^6 K km s^-1 pc^2, equivalent to 4 x 10^6 Msun of
molecular gas using the Galactic CO-to-H2 conversion factor. Observations with
the ARO 12m find that BIMA may resolve out as much as 50% of the CO emission,
and we estimate the total CO luminosity as 2.2 x 10^6 K km s^-1 pc^2. We
measure the properties of 14 GMCs from high resolution OVRO data. These clouds
are very similar to Galactic GMCs in their sizes, line widths, luminosities,
and CO-to-H2 conversion factors despite the low metallicity of IC 10 (Z ~ 1/5
Zsun). Comparing the BIMA survey to the atomic gas and stellar content of IC 10
we find that most of the CO emission is coincident with high surface density
HI. IC 10 displays a much higher star formation rate per unit molecular (H2) or
total (HI+H2) gas than most galaxies. This could be a real difference or may be
an evolutionary effect - the star formation rate may have been higher in the
recent past.Comment: 21 pages, 14 figures, Accepted to Ap
Massive open star clusters using the VVV survey II. Discovery of six clusters with Wolf-Rayet stars
Context: The ESO Public Survey "VISTA Variables in the V\'ia L\'actea" (VVV)
provides deep multi-epoch infrared observations for an unprecedented 562 sq.
degrees of the Galactic bulge, and adjacent regions of the disk. In this survey
nearly 150 new open clusters and cluster candidates have been discovered. Aims:
This is the second in a series of papers about young, massive open clusters
observed using the VVV survey. We present the first study of six recently
discovered clusters. These clusters contain at least one newly discovered
Wolf-Rayet (WR) star. Methods: Following the methodology presented in the first
paper of the series, wide-field, deep JHKs VVV observations, combined with new
infrared spectroscopy, are employed to constrain fundamental parameters for a
subset of clusters. Results: We affirm that the six studied stellar groups are
real young (2-7 Myr) and massive (between 0.8 and 2.2 10^3 Msol) clusters. They
are highly obscured (Av ~ 5-24 mag) and compact (1-2 pc). In addition to WR
stars, two of the six clusters also contain at least one red supergiant star.
We claim the discovery of 8 new WR stars, and 3 stars showing WR-like emission
lines which could be classified WR or OIf. Preliminary analysis provides
initial masses of ~30-50 Msol for the WR stars. Finally,we discuss the spiral
structure of the Galaxy using as tracers the six new clusters together with the
previously studied VVV clusters.Comment: 17 pages, 8 figurs, accepted in A&
On the distance and reddening of the starburst galaxy IC10
We present deep and accurate optical photometry of the Local Group starburst
galaxy IC10. The photometry is based on two sets of images collected with the
Advanced Camera for Surveys and with the Wide Field Planetary Camera 2 on board
the Hubble Space Telescope. We provide new estimates of the Red Giant Branch
tip (TRGB) magnitude, m_{F814W}^{TRGB}=21.90+-0.03, and of the reddening,
E(B-V)=0.78+-0.06, using field stars in the Small Magellanic Cloud (SMC) as a
reference. Adopting the SMC and two globulars, Omega Centauri and 47 Tucanae,
as references we estimate the distance modulus to IC10: independent
calibrations give weighted average distances of mu=24.51+-0.08 (TRGB) and
mu=24.56+-0.08 (RR Lyrae). We also provide a new theoretical calibration for
the TRGB luminosity, and using these predictions we find a very similar
distance to IC10 (mu~24.60+-0.15). These results suggest that IC10 is a likely
member of the M31 subgroup.Comment: 4 pages, 4 figures, ApJ Letters accepte
HST Snaphot Study of Variable Stars in Globular Clusters: Inner Region of NGC 6441
[Abridged] We present the results of a Hubble Space Telescope snapshot
program to survey the inner region of the globular cluster NGC 6441 for its
variable stars. A total of 57 variable stars was found including 38 RR Lyrae
stars, 6 Population II Cepheids, and 12 long period variables. Of the RR Lyrae
stars observed in this survey, 26 are pulsating in the fundamental mode with a
mean period of 0.753d and 12 are first-overtone mode pulsators with a mean
period of 0.365d. These values match up very well with those found in
ground-based surveys. Combining all the available data for NGC 6441, we find
mean periods of 0.759d and 0.375d for the RRab and RRc stars, respectively. We
also find that the RR Lyrae in this survey are located in the same regions of a
period-amplitude diagram as those found in ground-based surveys. Although NGC
6441 is a metal-rich globular cluster, its RR Lyrae more closely resemble those
in Oosterhoff type II globular clusters. However, even compared to typical
Oosterhoff type II systems, the mean period of its RRab stars is unusually
long. We also derived I-band period-luminosity relations for the RR Lyrae
stars. Of the six Population II Cepheids, five are of W Virginis type and one
is a BL Herculis variable stars. This makes NGC 6441, along with NGC 6388, the
most metal-rich globular cluster known to contain these types of variable
stars. Another variable, V118, may also be a Population II Cepheid given its
long period and its separation in magnitude from the RR Lyrae stars. We argue
that there does not appear to be a change in the period-luminosity relation
slope between the BL Herculis and W Virginis stars, but that a change of slope
does occur when the RV Tauri stars are added to the period-luminosity relation.Comment: 28 pages, including 9 figures and 8 tables, emulateapj5/apjfonts
style. Accepted by the Astronomical Journal. Approximate publication date
September 2003. We recommend the interested reader to download the preprint
with full-resolution figures, which can be found at
http://www.astro.puc.cl/~mcatelan/Pritzl.zi
Deep Hubble Space Telescope Imaging of IC 1613 II. The Star Formation History
We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxy
IC 1613. The photometry is the deepest to date for an isolated dwarf irregular
galaxy. The resulting color-magnitude diagram (CMD) is analyzed using three
different methods to derive a star formation history (SFH). All three find an
enhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similar
age-metallicity relationships (AMR). A comparison of the newly observed outer
field with an earlier studied central field of IC 1613 shows that the SFR in
the outer field has been significantly depressed during the last Gyr. This
implies that the optical scale length of the galaxy has been decreasing with
time and that comparison of galaxies at intermediate redshift with present day
galaxies should take this effect into account. We find strong similarities
between IC 1613 and the more distant Milky Way dSph companions in that all are
dominated by star formation at intermediate ages. In particular, the SFH and
AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr
galaxies cannot be distinguished from dSphs by their intermediate age stellar
populations. This type of a SFH may also be evidence for slower or suppressed
early star formation in dwarf galaxies due to photoionization after the
reionization of the universe by background radiation. Assuming that IC 1613 is
typical of a dIrr evolving in isolation, since most of the star formation
occurs at intermediate ages, these dwarf systems cannot be responsible for the
fast chemical enrichment of the IGM which is seen at high redshift. There is no
evidence for any large amplitude bursts of star formation in IC 1613, and we
find it highly unlikely that analogs of IC 1613 have contributed to the excess
of faint blue galaxies in existing galaxy redshift surveys.Comment: 32 pages, including 1 table and 17 figures, accepted for publication
in the Astrophysical Journal October 10, 2003 issu
Milky Way Demographics with the VVV Survey II. Color Transformations and Near-Infrared Photometry for 136 Million Stars in the Southern Galactic Disk
The new multi-epoch near-infrared VVV survey (VISTA Variables in the Via
Lactea) is sampling 562 sq. deg of the Galactic bulge and adjacent regions of
the disk. Accurate astrometry established for the region surveyed allows the
VVV data to be merged with overlapping surveys (e.g., GLIMPSE, WISE, 2MASS,
etc.), thereby enabling the construction of longer baseline spectral energy
distributions for astronomical targets. However, in order to maximize use of
the VVV data, a set of transformation equations are required to place the VVV
JHKs photometry onto the 2MASS system. The impetus for this work is to develop
those transformations via a comparison of 2MASS targets in 152 VVV fields
sampling the Galactic disk. The transformation coefficients derived exhibit a
reliance on variables such as extinction. The transformed data were
subsequently employed to establish a mean reddening law of
E_{J-H}/E_{H-Ks}=2.13 +/- 0.04, which is the most precise determination to date
and merely emphasizes the pertinence of the VVV data for determining such
important parameters.Comment: 24 pages, 12 figures, published in A&
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