469 research outputs found
Direct evidence for an early reionization of the Universe?
We examine the possible reionization of the intergalactic medium (IGM) by the
source UDF033238.7-274839.8 (hereafter HUDF-JD2), which was discovered in deep
{\it HST}/VLT/{\it Spitzer} images obtained as part of the Great Observatory
Origins Deep Survey and {\it Hubble} Ultra-Deep Field projects. Mobasher et al
(2005) have identified HUDF-JD2 as a massive ()
post-starburst galaxy at redshift z. We find that HUDF-JD2 may be
capable of reionizing its surrounding region of the Universe, starting the
process at a redshift as high as z.Comment: 6 pages, 2 figures. Accepted for publication in ApJ Letter
Mass Determination of Black Holes in LMC X-1 and Nova Muscae 1991 from their High-Energy Spectra
We offer a brief description of the bulk-motion Comptonization (BMC) model
for accretion onto black holes, illustrated by its application to observational
data for LMC X-1, and Nova Muscae 1991. We then extract some physical
parameters of these systems from observables (within the context of the BMC
model}, drawing from results on GRO J1655-40, for which we presented extensive
analysis previously. We derive estimates of the mass, (16 +/- 1) solar masses
and mass accretion rate in the disk in Eddington units around 2 for LMC X-1,
and (24 +/- 1)d_{5.5} and the disk mass acretion rate around 3 for Nova Muscae
1991 [where d_{5.5} stands for the distance in 5.5 kpc units]. Differences
between these estimates and previous estimates based on dynamical studies are
discussed. It is further shown that the disk inner radius increases with the
high-to-low state transition in Nova Muscae 1991. Specifically, our analysis
suggests that the inner-disk radius increases to 17 Scwarzschild radii as the
transition to the low-hard state occurs.Comment: 14 pages, 3 figures, The paper is accepted for publication in the
Astrophysical Journal Letter
VLA Detection of the Ionized Stellar Winds Arising from Massive Stars in the Galactic Center Arches Cluster
The Galactic center Arches stellar cluster, detected and studied until now
only in the near-infrared, is comprised of at least one hundred massive (M>20
Msun) stars. Here we report the detection at centimeter wavelengths of radio
continuum emission from eight radio sources associated with the cluster. Seven
of these radio sources have rising spectral indices between 4.9 and 8.5 GHz and
coincide spatially with the brightest stars in the cluster, as determine from
JHK photometry and Brackett alpha and Brackett Gamma spectroscopy. Our results
confirm the presence of powerful ionized winds in these stars. The eighth radio
source has a nonthermal spectrum and its nature is yet unclear, but it could be
associated with a lower mass young star in the cluster.Comment: 6 pages, 2 embedded figures, accepted to ApJLetter
Chandra X-Ray Point Sources, including Supernova 1979C, in the Spiral Galaxy M100
Six x-ray point sources, with luminosities of in the 0.4--7 keV band, were detected in Chandra
observations of the spiral galaxy M100. One source is identified with supernova
SN 1979C and appears to have roughly constant x-ray flux for the period 16--20
years after the outburst. The x-ray spectrum is soft, as would be expected if
the x-ray emission is due to the interaction of supernova ejecta with
circumstellar matter. Most of the other sources are variable either within the
Chandra observation or when compared to archival data. None are coincident with
the peak of the radio emission at the nucleus. These sources have harder
spectra than the supernova and are likely x-ray binaries. M100 has more bright
x-ray sources than typical for spiral galaxies of its size. This is likely
related to active star formation occurring in the galaxy.Comment: accepted by the Astrophysical Journal, 7 page
Mass accretion rates from multi-band photometry in the Carina Nebula: the case of Trumpler 14
We present a study of the mass accretion rates of pre-Main Sequence (PMS)
stars in the cluster Trumpler 14 (Tr14) in the Carina Nebula. Using optical
multi-band photometry we were able to identify 356 PMS stars showing H-alpha
excess emission with equivalent width EW(H-alpha)>20\AA. We interpret this
observational feature as indication that these objects are still actively
accreting gas from their circumstellar medium. From a comparison of the HR
diagram with PMS evolutionary models we derive ages and masses of the PMS
stars. We find that most of the PMS objects are younger than 10 Myr with a
median age of ~3 Myr. Surprisingly, we also find that ~20% of the mass
accreting objects are older than 10 Myr. For each PMS star in Trumpler 14 we
determine the mass accretion rate () and discuss its dependence
on mass and age. We finally combine the optical photometry with near-IR
observations to build the spectral energy distribution (SED) for each PMS star
in Tr14. The analysis of the SEDs suggests the presence of transitional discs
in which a large amount of gas is still present and sustains accretion onto the
PMS object at ages older than 10 Myr. Our results, discussed in light of recent
recent discoveries with Herschel of transitional discs containing a massive gas
component around the relatively old PSM stars TW Hydrae, 49 Ceti, and HD 95086,
support a new scenario in which old and evolved debris discs still host a
significant amount of gas.Comment: 12 pages, 13 figures, accepted for publication on A&
Mass accretion rates from multiband photometry in the Carina Nebula: The case of Trumpler 14
open8siWe present a study of the mass accretion rates of pre-main sequence (PMS) stars in the cluster Trumpler 14 (Tr 14) in the Carina Nebula. Using optical multiband photometry we were able to identify 356 PMS stars showing Hα excess emission with equivalent width EW(Hα) > 20 Ă
. We interpret this observational feature as an indication that these objects are still actively accreting gas from their circumstellar medium. From a comparison of the HR diagram with PMS evolutionary models we derive ages and masses of the PMS stars. We find that most of the PMS objects are younger than 10 Myr with a median age of ~3 Myr. Surprisingly, we also find that ~20% of the mass accreting objects are older than 10 Myr. For each PMS star in Trumpler 14 we determine the mass accretion rate (áčacc) and discuss its dependence on mass and age. We finally combine the optical photometry with near-IR observations to build the spectral energy distribution (SED) for each PMS star in Tr 14. The analysis of the SEDs suggests the presence of transitional discs in which a large amount of gas is still present and sustains accretion onto the PMS object at ages older than 10 Myr. Our results, discussed in light of recent recent discoveries with Herschel of transitional discs containing a massive gas component around the relatively old PMS stars TW Hydrae, 49 Ceti, and HD 95086, support a new scenario n which old and evolved debris discs still host a significant amount of gas.openBeccari, G.; De Marchi, G.; Panagia, N.; Valenti, E.; Carraro, G.; Romaniello, M.; Zoccali, M.; Weidner, C.Beccari, G.; De Marchi, G.; Panagia, N.; Valenti, E.; Carraro, Giovanni; Romaniello, M.; Zoccali, M.; Weidner, C
WFPC2 Observations of NGC 454: an Interacting Pair of Galaxies
We present WFPC2 images in the F450W, F606W and F814W filters of the
interacting pair of galaxies NGC 454. Our data indicate that the system is in
the early stages of interaction. A population of young star-clusters has formed
around the late component, and substantial amounts of gas have sunk into the
center of the earlier component, where it has not yet produced significant
visible star formation or nuclear activity. We have photometric evidence that
the star-clusters have strong line emission, which indicate the presence of a
substantial component of hot, massive stars which formed less than 5-10 Myrs
ago.Comment: 14 pages, 4 figures, Latex (AAS macros), ApJL in pres
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