14,154 research outputs found
New ultracool subdwarfs identified in large-scale surveys using Virtual Observatory tools: II. SDSS DR7 vs UKIDSS LAS DR6, SDSS DR7 vs UKIDSS LAS DR8, SDSS DR9 vs UKIDSS LAS DR10, and SDSS DR7 vs 2MASS
We aim at developing an efficient method to search for late-type subdwarfs
(metal-depleted dwarfs with spectral types >M5) to improve the current
statistics. Our objectives are: improve our knowledge of metal-poor low-mass
dwarfs, bridge the gap between the late-M and L types, determine their surface
density, and understand the impact of metallicity on the stellar and substellar
mass function.
We carried out a search cross-matching the SDSS, 2MASS, and UKIDSS using
STILTS, Aladin, and Topcat. We considered different photometric and proper
motion criteria for our selection. We identified 29 and 71 late-type subdwarf
candidates in each cross-correlation over 8826 and 3679 square degrees,
respectively. We obtained low-resolution optical spectra for 71 of our
candidates with GTC, NOT, and VLT and retrieved spectra for 30 candidates from
the SDSS spectroscopic database. We classified 92 candidates based on 101
optical spectra using two methods: spectral indices and comparison with
templates of known subdwarfs.
We confirmed 86% and 94% of the candidates as late-type subdwarfs from the
SDSS vs 2MASS and SDSS vs UKIDSS cross-matches, respectively. These subdwarfs
have spectral types ranging between M5 and L0.5 and SDSS magnitudes in the
r=19.4-23.3 mag range. Our new late-type M discoveries include 49 subdwarfs, 25
extreme subdwarfs, six ultrasubdwarfs, one subdwarf/extreme subdwarf, and two
dwarfs/subdwarfs. We derived a surface density of late-type subdwarfs of
0.040 per square degree in the SDSS DR7 vs UKIDSS LAS DR10
cross-match. We also checked the AllWISE photometry of known and new subdwarfs
and found that mid-infrared colours of M subdwarfs do not appear to differ from
their solar-metallicity counterparts of similar spectral types. However, the
J-W2 and J-W1 colours are bluer for lower metallicity dwarfs. (abstract
strongly abridged)Comment: 28 pages, 4 Tables, 10 figures, 1 appendix. Accepted to A&A.
Photometry and spectra available in a dedicated archive on late-type
subdwarfs at http://svo2.cab.inta-csic.es/vocats/ltsa
Spatial dispersion in Casimir forces: A brief review
We present the basic principles of non-local optics in connection with the
calculation of the Casimir force between half-spaces and thin films.
At currently accessible distances , non-local corrections amount to about
half a percent, but they increase roughly as 1/L at smaller separations. Self
consistent models lead to corrections with the opposite sign as models with
abrupt surfaces.Comment: Proceedings of QFEXT05, Barcelona, Sept. 5-9, 200
Radiative non-isothermal Bondi accretion onto a massive black hole
In this paper, we present the classical Bondi accretion theory for the case
of non-isothermal accretion processes onto a supermassive black hole (SMBH),
including the effects of X-ray heating and the radiation force due to electron
scattering and spectral lines. The radiation field is calculated by considering
an optically thick, geometrically thin, standard accretion disk as the emitter
of UV photons and a spherical central object as a source of X-ray emission. In
the present analysis, the UV emission from the accretion disk is assumed to
have an angular dependence, while the X-ray/central object radiation is assumed
to be isotropic. This allows us to build streamlines in any angular direction
we need to. The influence of both types of radiation is evaluated for different
flux fractions of the X-ray and UV emissions with and without the effects of
spectral line driving. We find that the radiation emitted near the SMBH
interacts with the infalling matter and modifies the accretion dynamics. In the
presence of line driving, a transition resembles from pure type 1 & 2 to type 5
solutions (see Fig2.1 of Frank etal. 2002), which takes place regardless of
whether or not the UV emission dominates over the X-ray emission. We compute
the radiative factors at which this transition occurs, and discard type 5
solution from all our models. Estimated values of the accretion radius and
accretion rate in terms of the classical Bondi values are also given. The
results are useful for the construction of proper initial conditions for
time-dependent hydrodynamical simulations of accretion flows onto SMBH at the
centre of galaxies.Comment: 10 pages, 10 figures, Accepted to be published in A&
The polarized expression of Na+,K+-ATPase in epithelia depends on the association between beta-subunits located in neighboring cells
The polarized distribution of Na+,K+-ATPase plays a paramount physiological role, because either directly or through coupling with co- and countertransporters, it is responsible for the net movement of, for example, glucose, amino acids, Ca2+, K+, Cl-, and CO3H- across the whole epithelium. We report here that the beta-subunit is a key factor in the polarized distribution of this enzyme. 1) Madin-Darby canine kidney (MDCK) cells (epithelial from dog kidney) express the Na+,K+-ATPase over the lateral side, but not on the basal and apical domains, as if the contact with a neighboring cell were crucial for the specific membrane location of this enzyme. 2) MDCK cells cocultured with other epithelial types (derived from human, cat, dog, pig, monkey, rabbit, mouse, hamster, and rat) express the enzyme in all (100%) homotypic MDCK/MDCK borders but rarely in heterotypic ones. 3) Although MDCK cells never express Na+,K+-ATPase at contacts with Chinese hamster ovary (CHO) cells, they do when CHO cells are transfected with beta(1)-subunit from the dog kidney (CHO-beta). 4) This may be attributed to the adhesive property of the beta(1)-subunit, because an aggregation assay using CHO (mock-transfected) and CHO-beta cells shows that the expression of dog beta(1)-subunit in the plasma membrane does increase adhesiveness. 5) This adhesiveness does not involve adherens or tight junctions. 6) Transfection of beta(1)-subunit forces CHO-beta cells to coexpress endogenous a-subunit. Together, our results indicate that MDCK cells express Na+,K+-ATPase at a given border provided the contacting cell expresses the dog P,-subunit. The cell-cell interaction thus established would suffice to account for the polarized expression and positioning of Na+,K+-ATPase in epithelial cells
MWL observations of VHE blazars in 2006
In 2006 the MAGIC telescope observed the well known very high energy (VHE, >
80 GeV) blazars Mrk 421 and Mrk 501 in the course of multi-wavelength
campaigns, comprising measurements in the optical, X-ray and VHE regime. MAGIC
performed additional snapshot observations on Mrk 421 around the MWL campaigns
and detected the source each night with high significance, establishing once
more flux variability on nightly scales for this object. For certain nights,
the integral flux exceeded the one of Crab significantly, whereas the truly
simultaneous observations have been conducted in a rather low flux state. The
MAGIC observations contemporaneous to XMM-Newton revealed clear intra-night
variability. No significant correlation between the spectral index and the flux
could be found for the nine days of observations. The VHE observations of Mrk
501 have been conducted during one of the lowest flux states ever measured by
MAGIC for this object. The VHE and optical light curves do not show significant
variability, whereas the flux in X-rays increased by about 50 %. In this
contribution, the results of the MAGIC observations will be presented in
detail.Comment: Contribution to the 31st ICRC, Lodz, Poland, July 200
Tracing the Conversion of Gas into Stars in Young Massive Cluster Progenitors
Whilst young massive clusters (YMCs; 10 M, age
100 Myr) have been identified in significant numbers, their
progenitor gas clouds have eluded detection. Recently, four extreme molecular
clouds residing within 200 pc of the Galactic centre have been identified as
having the properties thought necessary to form YMCs. Here we utilise far-IR
continuum data from the Herschel Infrared Galactic Plane Survey (HiGAL) and
millimetre spectral line data from the Millimetre Astronomy Legacy Team 90 GHz
Survey (MALT90) to determine their global physical and kinematic structure. We
derive their masses, dust temperatures and radii and use virial analysis to
conclude that they are all likely gravitationally bound -- confirming that they
are likely YMC progenitors. We then compare the density profiles of these
clouds to those of the gas and stellar components of the Sagittarius B2 Main
and North proto-clusters and the stellar distribution of the Arches YMC. We
find that even in these clouds -- the most massive and dense quiescent clouds
in the Galaxy -- the gas is not compact enough to form an Arches-like ( =
2x10 M, R = 0.4 pc) stellar distribution. Further
dynamical processes would be required to condense the resultant population,
indicating that the mass becomes more centrally concentrated as the
(proto)-cluster evolves. These results suggest that YMC formation may proceed
hierarchically rather than through monolithic collapse.Comment: 12 pages, 8 figures, 1 table. Accepted by MNRA
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