408 research outputs found
VLA 3.5 cm continuum sources in the Serpens cloud core
We present VLA 3.5 cm continuum observations of the Serpens cloud core. 22
radio continuum sources are detected. 16 out of the 22 cm sources are suggested
to be associated with young stellar objects (Class 0, Class I, flat-spectrum,
and Class II) of the young Serpens cluster. The rest of the VLA sources
plausibly are background objects. Most of the Serpens cm sources likely
represent thermal radio jets; on the other hand, the radio continuum emission
of some sources could be due to a gyrosynchroton mechanism arising from
coronally active young stars. The Serpens VLA sources are spatially distributed
into two groups; one of them located towards the NW clump of the Serpens core,
where only Class 0 and Class I protostars are found to present cm emission, and
a second group located towards the SE clump, where radio continuum sources are
associated with objects in evolutionary classes from Class 0 to Class II. This
subgrouping is similar to that found in the near IR, mid-IR and mm wavelength
regimes.Comment: 2 figures, accepted by Astronomical journa
Thin-shell wormholes from charged black holes in generalized dilaton-axion gravity
This paper discusses a new type of thin-shell wormhole constructed by
applying the cut-and-paste technique to two copies of a charged black hole in
generalized dilaton-axion gravity, which was inspired by low-energy string
theory. After analyzing various aspects of this thin-shell wormhole, we discuss
its stability to linearized spherically symmetric perturbations.Comment: Minor changes, 6 pages, 4 figures. Accepted for publication in Gen.
Rel. Gra
d-dimensional non-asymptotically flat thin-shell wormholes in Einstein-Yang-Mills-Dilaton gravity
Thin-shell wormholes in Einstein-Yang-Mills-dilaton (EYMD) gravity are
considered. We show that a non-asymptotically flat (NAF) black hole solution of
the d-dimensional EYMD theory provides stable thin-shell wormholes which are
supported entirely by exotic matter. The presence of dilaton makes the
spacetime naturally NAF, and with our conclusion it remains still open to
construct wormholes supported by normal matter between two such spacetimes.Comment: 6 pages, 5 figures. Title changed, the first version has been
generalized to d-dimensions. The EYMGB part was removed and was considered in
a more general form separately in arXiv:1007.462
A Theoretical Construction of Thin Shell Wormhole from Tidal Charged Black hole
Recently, Dadhich et al [ Phys.Lett.B 487, 1 (2000)] have discovered a black
hole solution localized on a three brane in five dimensional gravity in the
Randall-Sundrum scenario. In this article, we develop a new class of thin shell
wormhole by surgically grafting above two black hole spacetimes. Various
aspects of this thin wormhole are also analyzed.Comment: 14 pages, 6 figures, Accepted in Gen.Rel.Gra
Shadow of a rotating braneworld black hole
We investigate the shadow cast by a rotating braneworld black hole, in the
Randall-Sundrum scenario. In addition to the angular momentum, the tidal charge
term deforms the shape of the shadow. For a given value of the rotation
parameter, the presence of a negative tidal charge enlarges the shadow and
reduces its deformation with respect to Kerr spacetime, while for a positive
charge, the opposite effect is obtained. We also analyze the case in which the
combination of the rotation parameter and the tidal charge results in a naked
singularity. We discuss the observational prospects corresponding to the
supermassive black hole at the Galactic center.Comment: 11 pages, 6 figures. v2: improved version (includes 2 new figures
Collisional modelling of the debris disc around HIP 17439
We present an analysis of the debris disc around the nearby K2 V star HIP
17439. In the context of the Herschel DUNES key programme the disc was observed
and spatially resolved in the far-IR with the Herschel PACS and SPIRE
instruments. In a first model, Ertel et al. (2014) assumed the size and radial
distribution of the circumstellar dust to be independent power laws. There, by
exploring a very broad range of possible model parameters several scenarios
capable of explaining the observations were suggested. In this paper, we
perform a follow-up in-depth collisional modelling of these scenarios trying to
further distinguish between them. In our models we consider collisions, direct
radiation pressure, and drag forces, i.e. the actual physical processes
operating in debris discs. We find that all scenarios discussed in Ertel et al.
are physically sensible and can reproduce the observed SED along with the PACS
surface brightness profiles reasonably well. In one model, the dust is produced
beyond 120au in a narrow planetesimal belt and is transported inwards by
Poynting-Robertson and stellar wind drag. A good agreement with the observed
radial profiles would require stellar winds by about an order of magnitude
stronger than the solar value, which is not supported, although not ruled out,
by observations. Another model consists of two spatially separated planetesimal
belts, a warm inner and a cold outer one. This scenario would probably imply
the presence of planets clearing the gap between the two components. Finally,
we show qualitatively that the observations can be explained by assuming the
dust is produced in a single, but broad planetesimal disc with a surface
density of solids rising outwards, as expected for an extended disc that
experiences a natural inside-out collisional depletion. Prospects of
discriminating between the competing scenarios by future observations are
discussed.Comment: Astronomy and Astrophysics (accepted for publication). 11 pages, 8
figure
Thin-shell wormholes with a generalized Chaplygin gas in Einstein-Born-Infeld theory
We construct spherically symmetric thin-shell wormholes supported by a
generalized Chaplygin gas in Born-Infeld electrodynamics coupled to Einstein
gravity, and we analyze their stability under radial perturbations. For
different values of the Born-Infeld parameter and the charge, we compare the
results with those obtained in a previous work for Maxwell electrodynamics. The
stability region in the parameter space reduces and then disappears as the
value of the Born-Infeld parameter is modified in the sense of a larger
departure from Maxwell theory.Comment: 9 pages, 6 figures; v2: improved versio
Strong field limit analysis of gravitational retro-lensing
We present a complete treatment in the strong field limit of gravitational
retro-lensing by a static spherically symmetric compact object having a photon
sphere. The results are compared with those corresponding to ordinary lensing
in similar strong field situations. As examples of application of the
formalism, a supermassive black hole at the galactic center and a stellar mass
black hole in the galactic halo are studied as retro-lenses, in both cases
using the Schwarzschild and Reissner-Nordstrom geometries.Comment: 11 pages, 1 figure; v2: minor changes. Accepted for publication in
Physical Review
The onset of X-ray emission in young stellar objects: a Chandra observation of the Serpens star-forming region
AIMS: To study the properties of X-ray emissions from young stellar objects
(YSOs), through their evolution from Class I to Class III and determine whether
Class 0 protostars emit in X-rays. METHODS: A deep Chandra X-ray observation of
the Serpens star-forming region was obtained. The Serpens Cloud Core is ideally
suited for this type of investigation, being populated by a dense and extremely
young cluster whose members are found in all different evolutionary stages,
including six well studied Class 0 sources. RESULTS: None of the six Class 0
protostars is detected in our observations, excluding the presence of sources
with X-ray luminosities > 0.4 10^30 erg/s (for column densities of the order of
4 10^{23} cm^-2, or A_V ~ 200). A total of 85 X-ray sources are detected and
the light curves and spectra of 35 YSOs are derived. There is a clear trend of
decreasing absorbing column densities as one moves from Class I to Class III
sources, and, possibly, evidence of decreasing plasma temperatures, too. We
observe a strong, long-duration, flare from a Class II low-mass star, for which
we derive a flaring loop length of the order of 20 stellar radii. We interpret
the flaring event as originating from a magnetic flux tube connecting the star
to its circumstellar disk. The presence of such a disk is supported by the
detection, in the spectrum of this star, of 6.4 keV Fe fluorescent emission.Comment: Accepted for publication in A&
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