182 research outputs found
CHORIZOS: a CHi-square cOde for parameteRized modelIng and characteriZation of phOtometry and Spectrophotometry
We have developed a CHi-square cOde for parameteRized modelIng and
characteriZation of phOtometry and Spectrophotometry (CHORIZOS). CHORIZOS can
use up to two intrinsic free parameters (e.g. temperature and gravity for
stars; type and redshift for galaxies; or age and metallicity for stellar
clusters) and two extrinsic ones (amount and type of extinction). The code uses
chi-square minimization to find all models compatible with the observed data in
the model N-dimensional (N=1,2,3,4) parameter space. CHORIZOS can use either
correlated or uncorrelated colors as input and is especially designed to
identify possible parameter degeneracies and multiple solutions. The code is
written in IDL and is available to the astronomical community. Here we present
the techniques used, test the code, apply it to a few well-known astronomical
problems, and suggest possible applications. As a first scientific result from
CHORIZOS, we confirm from photometry the need for a revised
temperature-spectral type scale for OB stars previously derived from
spectroscopy.Comment: 32 pages, 13 figures. To appear in the September 2004 issue of PAS
The young stellar population of NGC 4214 as observed with HST. I. Data and methods
We present the data and methods that we have used to perform a detailed
UV-optical study of the nearby dwarf starburst galaxy NGC 4214 using
multifilter HST/WFPC2+STIS photometry. We explain the process followed to
obtain high-quality photometry and astrometry of the stellar and cluster
populations of this galaxy. We describe the procedure used to transform
magnitudes and colors into physical parameters using spectral energy
distributions. The data show the existence of both young and old stellar
populations that can be resolved at the distance of NGC 4214 (2.94 Mpc) and we
perform a general description of those populations.Comment: 33 pages, 9 figures, and 8 table
The spatial distribution of O-B5 stars in the solar neighborhood as measured by Hipparcos
We have developed a method to calculate the fundamental parameters of the
vertical structure of the Galaxy in the solar neighborhood from trigonometric
parallaxes alone. The method takes into account Lutz-Kelker-type biases in a
self-consistent way and has been applied to a sample of O-B5 stars obtained
from the Hipparcos catalog. We find that the Sun is located 24.2 +/- 1.7
(random) +/- 0.4 (systematic) pc above the galactic plane and that the disk
O-B5 stellar population is distributed with a scale height of 34.2 +/- 0.8
(random) +/- 2.5 (systematic) pc and an integrated surface density of (1.62 +/-
0.04 (random) +/- 0.14 (systematic)) 10^{-3} stars pc^{-2}. A halo component is
also detected in the distribution and constitutes at least ~5% of the total
O-B5 population. The O-B5 stellar population within ~100 pc of the Sun has an
anomalous spatial distribution, with a less-than-average number density. This
local disturbance is probably associated with the expansion of Gould's belt.Comment: 14 pages, 3 figures, to appear in the May 2001 issue of the
Astronomical Journa
The dust SED of dwarf galaxies
Context. High-resolution data from Spitzer, Herschel, and Planck allow us to probe the entire spectral energy distribution (SED) of morphologically separated components of the dust emission from nearby galaxies and allow a more detailed comparison between data and models. Aims. We wish to establish the physical origin of dust heating and emission based on radiation transfer models, that self-consistently connect the emission components from diffuse dust and the dust in massive star forming regions. Methods. NGC 4214 is a nearby dwarf galaxy with a large set of ancillary data, ranging from the ultraviolet (UV) to radio, including maps from Spitzer and Herschel and detections from Planck. We mapped this galaxy with MAMBO at 1.2mm at the IRAM 30m telescope. We extracted separate dust emission components for the HII regions (plus their associated PDRs on pc scales) and for the diffuse dust (on kpc scales). We analysed the full UV to FIR/submm SED of the galaxy using a radiation transfer model that self-consistently treats the dust emission from diffuse and star forming (SF) complexes components, considering the illumination of diffuse dust both by the distributed stellar populations and by escaping light from the HII regions. While maintaining consistency within the framework of this model, we additionally used a model that provides a detailed description of the dust emission from the HII regions and their surrounding PDRs on pc scales. Thanks to the large amount of available data and many previous studies for NGC 4214, very few free parameters remained in the model fitting process. Results. We achieve a satisfactory fit for the emission from HII + PDR regions on pc scales, with the exception of the emission at 8 μm, which is underpredicted by the model. For the diffuse emission we achieve a good fit if we assume that about 40-65% of the emission escaping the HII + PDR regions is able to leave the galaxy without passing through a diffuse ISM, which is not an unlikely scenario for a dwarf galaxy that has recently undergone a nuclear starburst. We determine a dust-to-gas mass ratio of 350-470, which is close to the expected value based on the metallicity. © 2012 ESO
Testing the predicted mass-loss bi-stability jump at radio wavelengths
Context. Massive stars play a dominant role in the Universe, but one of the main drivers for their evolution, their mass loss, remains poorly understood. Aims. In this study, we test the theoretically predicted mass-loss behaviour as a function of stellar effective temperature across the so-called "bi-stability" jump. Methods. We observe OB supergiants in the spectral range 08-B3 at radio wavelengths to measure their thermal radio flux densities, and complement these measurements with data from the literature. We derive the radio mass-loss rates and wind efficiencies, and compare our results with Hα- mass-loss rates and predictions based on radiation-driven wind models. Results. The wind efficiency shows the possible presence of a local maximum around an effective temperature of 21 000 K - in qualitative agreement with predictions. Furthermore, we find that the absolute values of the radio mass-loss rates show good agreement with empirical Hα rates derived assuming homogeneous winds - for the spectral range under consideration. However, the empirical mass-loss rates are larger (by a factor of a few) than the predicted rates from radiation-driven wind theory for objects above the bi-stability jump (BSJ) temperature, whilst they are smaller (by a factor of a few) for stars below the BSJ temperature. The reason for these discrepancies remains as yel unresolved. A new wind momenta-luminosity relation (WLR) for 08-B0 stars has been derived using the radio observations. The validity of the WLR as a function of the fitting parameter related to the force multiplier αeff (Kudritzki & Puls 2000) is discussed. Conclusions. Our most interesting finding is that the qualitative behaviour of the empirical wind efficiencies with effective temperature is in line with the predicted behaviour, and this presents the first hint of empirical evidence for the predicted mass-loss bi-stability jump. However, a larger sample of stars around the BSJ needs to be observed to confirm this finding.Facultad de Ciencias Astronómicas y Geofísica
Testing the predicted mass-loss bi-stability jump at radio wavelengths
In this study, we test the theoretically predicted mass-loss behaviour as a
function of stellar effective temperature across the so-called `bi-stability'
jump (BSJ). We gathered radio observations of 30 OB supergiants (O8-B3). We
derived the radio mass-loss rates and wind efficiencies, and compared our
results with Halpha mass-loss rates and predictions based on radiation-driven
wind models. he wind efficiency shows the possible presence of a local maximum
around an effective temperature of 21~000 K -- in qualitative agreement with
predictions. We also find that the radio mass-loss rates show good agreement
with empirical Halpha rates. However, the empirical mass-loss rates are larger
than the predicted rates from radiation-driven wind theory for objects above
the BSJ temperature, whilst they are smaller for the rest. A new wind
momenta-luminosity relation for O8-B0 stars has been derived.Comment: 13 pages, 13 figures, A&
The young stellar population of NGC 4214 as observed with HST. II. Results
We present the results of a detailed UV-optical study of the nearby dwarf
starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. The
stellar extinction is found to be quite patchy, with some areas having values
of E(4405-5495)< 0.1 mag and others, associated with star forming regions, much
more heavily obscured, a result which is consistent with previous studies of
the nebular extinction. We determined the ratio of blue-to-red supergiants and
found it to be consistent with theoretical models for the metallicity of the
SMC. The stellar IMF of the field in the range 20-100 solar masses is found to
be steeper than Salpeter. A number of massive clusters and associations with
ages between a few and 200 million years are detected and their properties are
discussed.Comment: 49 pages, 12 figures, 6 table
The type IIb SN 2008ax: the nature of the progenitor
A source coincident with the position of the type IIb supernova (SN) 2008ax
is identified in pre-explosion Hubble Space Telescope (HST) Wide Field
Planetary Camera 2 observations in three optical filters. We identify and
constrain two possible progenitor systems: (i) a single massive star that lost
most of its hydrogen envelope through radiatively driven mass loss processes,
prior to exploding as a helium-rich Wolf-Rayet star with a residual hydrogen
envelope, and (ii) an interacting binary in a low mass cluster producing a
stripped progenitor. Late time, high resolution observations along with
detailed modelling of the SN will be required to reveal the true nature of this
progenitor star.Comment: 5 pages, 2 figures, resolution of figure 1 reduced, figure 2 revised,
some revision following referee's comments, accepted for publication in MNRAS
letter
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