5,102 research outputs found
A possible black hole in the gamma-ray microquasar LS 5039
The population of high energy and very high energy gamma-ray sources,
detected with EGRET and the new generation of ground-based Cherenkov
telescopes, conforms a reduced but physically important sample. Most of these
sources are extragalactic (e.g., blazars), while among the galactic ones there
are pulsars and SN remnants. The microquasar LS 5039, previously proposed to be
associated with an EGRET source by Paredes et al. (2000), has recently been
detected at TeV energies, confirming that microquasars should be regarded as a
class of high energy gamma-ray sources. To model and understand how the
energetic photons are produced and escape from LS 5039 it is crucial to unveil
the nature of the compact object, which remains unknown. Here we present new
intermediate-dispersion spectroscopy of this source which, combined with values
reported in the literature, provides an orbital period of 3.90603+/-0.00017 d,
a mass function f(M)=0.0053+/-0.0009 M_sun, and an eccentricity e=0.35+/-0.04.
Atmosphere model fitting to the spectrum of the optical companion, together
with our new distance estimate of d=2.5+/-0.1 kpc, yields R_opt=9.3+0.7-0.6
R_sun, log (L_opt/L_sun)=5.26+/-0.06, and M_opt=22.9+3.4-2.9 M_sun. These,
combined with our dynamical solution and the assumption of
pseudo-synchronization, yield an inclination i=24.9+/-2.8 degree and a compact
object mass M_X=3.7+1.3-1.0 M_sun. This is above neutron star masses for most
of the standard equations of state and, therefore, we propose that the compact
object in LS 5039 is a black hole. We finally discuss about the implications of
our orbital solution and new parameters of the binary system on the CNO
products, the accretion/ejection energetic balance, the SN explosion scenario,
and the behaviour of the TeV emission with the new orbital period.Comment: 10 pages, 8 figures. Accepted for publication in MNRAS. Minor changes
according to referee repor
The Best Brown Dwarf Yet?: A Companion to the Hyades Eclipsing Binary V471 Tau
We have carried out an analysis of about 160 eclipse timings spanning over 30
years of the Hyades eclipsing binary V471 Tauri that shows a long-term
quasi-sinusoidal modulation of its observed eclipse arrival times. The O-Cs
have been analyzed for the ``light-time'' effect that arises from the
gravitational influence of a tertiary companion. The presence of a third body
causes the relative distance of the eclipsing pair to the Earth to change as it
orbits the barycenter of the triple system. The result of the analysis of the
eclipse times yields a light-time semi-amplitude of 137.2+/-12.0 s, an orbital
period of P_3 = 30.5+/-1.6 yr and an eccentricity of e_3 = 0.31+/-0.04. The
mass of the tertiary component is M_3 sin i_3 = 0.0393+/-0.0038 Mo when a total
mass of 1.61+/-0.06 Mo for V471 Tau is adopted. For orbital inclinations i_3 >
35 deg, the mass of the third body would be below the stable hydrogen burning
limit of M = 0.07 Mo and it thus would be a brown dwarf. In the next several
years (near maximum elongation), it should be feasible to obtain IR images and
spectra of V471 Tau C that, when combined with the known mass, age, distance,
and [Fe/H], will serve as a benchmark for understanding the physical properties
and evolution of brown dwarfs.Comment: 9 pages, 3 figures, accepted for publication in ApJ Letter
On the formation and evolution of the first Be star in a black hole binary MWC 656
We find that the formation of MWC 656 (the first Be binary containing a black
hole) involves a common envelope phase and a supernova explosion. This result
supports the idea that a rapidly rotating Be star can emerge out of a common
envelope phase, which is very intriguing because this evolutionary stage is
thought to be too fast to lead to significant accretion and spin up of the B
star.
We predict of B BH binaries to currently reside in the Galactic
disk, among which around contain a Be star, but there is only a small
chance to observe a system with parameters resembling MWC 656. If MWC 656 is
representative of intrinsic Galactic Be BH binary population, it may indicate
that standard evolutionary theory needs to be revised. This would pose another
evolutionary problem in understanding BH binaries, with BH X-ray Novae
formation issue being the prime example.
The future evolution of MWC 656 with a M black hole and
with a M main sequence companion on a day orbit
may lead to the formation of a coalescing BH-NS system. The estimated Advanced
LIGO/Virgo detection rate of such systems is up to yr. This
empirical estimate is a lower limit as it is obtained with only one particular
evolutionary scenario, the MWC 656 binary. This is only a third such estimate
available (after Cyg X-1 and Cyg X-3), and it lends additional support to the
existence of so far undetected BH--NS binaries.Comment: revised and extended version after MNRAS review 17 pages, 10 figure
Transmission spectroscopy of the inflated exo-Saturn HAT-P-19b
We observed the Saturn-mass and Jupiter-sized exoplanet HAT-P-19b to refine
its transit parameters and ephemeris as well as to shed first light on its
transmission spectrum. We monitored the host star over one year to quantify its
flux variability and to correct the transmission spectrum for a slope caused by
starspots. A transit of HAT-P-19b was observed spectroscopically with OSIRIS at
the Gran Telescopio Canarias in January 2012. The spectra of the target and the
comparison star covered the wavelength range from 5600 to 7600 AA. One
high-precision differential light curve was created by integrating the entire
spectral flux. This white-light curve was used to derive absolute transit
parameters. Furthermore, a set of light curves over wavelength was formed by a
flux integration in 41 wavelength channels of 50 AA width. We analyzed these
spectral light curves for chromatic variations of transit depth. The transit
fit of the combined white-light curve yields a refined value of the
planet-to-star radius ratio of 0.1390 pm 0.0012 and an inclination of 88.89 pm
0.32 degrees. After a re-analysis of published data, we refine the orbital
period to 4.0087844 pm 0.0000015 days. We obtain a flat transmission spectrum
without significant additional absorption at any wavelength or any slope.
However, our accuracy is not sufficient to significantly rule out the presence
of a pressure-broadened sodium feature. Our photometric monitoring campaign
allowed for an estimate of the stellar rotation period of 35.5 pm 2.5 days and
an improved age estimate of 5.5^+1.8_-1.3 Gyr by gyrochronology.Comment: 14 pages, 9 figures, Accepted for publication in A&
Biogeography And Diversification Of Rhegmatorhina (Aves: Thamnophilidae): Implications For The Evolution Of Amazonian Landscapes During The Quaternary
Aim: To test the importance of alternative diversification drivers and biogeographical processes for the evolution of Amazonian upland forest birds through a densely sampled analysis of diversification of the endemic Amazonian genus Rhegmatorhina at multiple taxonomic and temporal scales. Location: Amazonia. Taxon: Antbirds (Thamnophilidae). Methods: We sequenced four mtDNA and nuclear gene regions of 120 individuals from 50 localities representing all recognized species and subspecies of the genus. We performed molecular phylogenetic analyses using both gene tree and species tree methods, molecular dating analysis and estimated population demographic history and gene flow. Results: Dense sampling throughout the distribution of Rhegmatorhina revealed that the main Amazonian rivers delimit the geographic distribution of taxa as inferred from mtDNA lineages. Molecular phylogenetic analyses resulted in a strongly supported phylogenetic hypothesis for the genus, with two main clades currently separated by the Madeira River. Molecular dating analysis indicated diversification during the Quaternary. Reconstruction of recent demographic history of populations revealed a trend for population expansion in eastern Amazonia and stability in the west. Estimates of gene flow corroborate the possibility that migration after divergence had some influence on the current patterns of diversity. Main Conclusions: Based on broad-scale sampling, a clarification of taxonomic boundaries, and strongly supported phylogenetic relationships, we confirm that, first, mitochondrial lineages within this upland forest Amazonian bird genus agree with spatial patterns known for decades based on phenotypes, and second, that most lineages are geographically delimited by the large Amazonian rivers. The association between past demographic changes related to palaeoclimatic cycles and the historically varying strength and size of rivers as barriers to dispersal may be the path to the answer to the long-standing question of identifying the main drivers of Amazonian diversification
First detection of thermal radio jets in a sample of proto-brown dwarf candidates
We observed with the JVLA at 3.6 and 1.3 cm a sample of 11 proto-brown dwarf
candidates in Taurus in a search for thermal radio jets driven by the most
embedded brown dwarfs. We detected for the first time four thermal radio jets
in proto-brown dwarf candidates. We compiled data from UKIDSS, 2MASS, Spitzer,
WISE and Herschel to build the Spectral Energy Distribution (SED) of the
objects in our sample, which are similar to typical Class~I SEDs of Young
Stellar Objects (YSOs). The four proto-brown dwarf candidates driving thermal
radio jets also roughly follow the well-known trend of centimeter luminosity
against bolometric luminosity determined for YSOs, assuming they belong to
Taurus, although they present some excess of radio emission compared to the
known relation for YSOs. Nonetheless, we are able to reproduce the flux
densities of the radio jets modeling the centimeter emission of the thermal
radio jets using the same type of models applied to YSOs, but with
corresponding smaller stellar wind velocities and mass-loss rates, and
exploring different possible geometries of the wind or outflow from the star.
Moreover, we also find that the modeled mass outflow rates for the bolometric
luminosities of our objects agree reasonably well with the trends found between
the mass outflow rates and bolometric luminosities of YSOs, which indicates
that, despite the "excess" centimeter emission, the intrinsic properties of
proto-brown dwarfs are consistent with a continuation of those of very low mass
stars to a lower mass range. Overall, our study favors the formation of brown
dwarfs as a scaled-down version of low-mass stars.Comment: 18 pages, 8 figures, 14 tables, accepted by the Astrophysical Journa
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