4,315 research outputs found
White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy
The nature of the several microlensing events observed by the MACHO team
towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass
substellar objects and stars with masses larger than ~M_{sun} have been ruled
out as major components of a Massive Astrophysical Compact Halo Object (MACHO)
Galactic halo, while stars of half a solar mass seem to be viable candidates.
Main sequence stars have been already discarded, and there are tight
restrictions on the role played by white dwarfs with hydrogen-dominated
atmospheres. In this paper we evaluate the contribution to the dark matter
content of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. For
this purpose we use a Monte Carlo simulator which incorporates up-to-date
evolutionary sequences of white dwarfs with hydrogen-rich and
hydrogen-deficient atmospheres. We also take into account detailed descriptions
of the thick disk and the halo of our Galaxy as well as of a reliable model of
the LMC. We find that the contribution of white dwarfs with hydrogen-deficient
atmospheres moderately increases the theoretical estimate of the optical depth
with respect to the value obtained when only hydrogen-rich white dwarfs are
considered. We also find that the contribuiton of the thick disk population of
white dwarfs is comparable to the halo contribution. However, the contributions
of both the halo and the thick disk white-dwarf populations are still
insufficient to explain the number of events observed by the MACHO team.
Finally, we find that the contribution to the halo dark matter of the entire
population under study is less than 10% at the 95% conficence level.Comment: 12 pages, 5 figures, accepted for publication in A&
Confirmation Via the Continuum-Fitting Method that the Spin of the Black Hole in Cygnus X-1 is Extreme
In Gou et al. (2011), we reported that the black hole primary in the X-ray
binary Cygnus X-1 is a near-extreme Kerr black hole with a spin parameter
a*>0.95(3{\sigma}). We confirm this result while setting a new and more
stringent limit: a*>0.983 at the 3{\sigma}(99.7%) level of confidence. The
earlier work, which was based on an analysis of all three useful spectra that
were then available, was possibly biased by the presence in these spectra of a
relatively strong Compton power-law component: The fraction of the thermal seed
photons scattered into the power law was f_s=23-31%, while the upper limit for
reliable application of the continuum-fitting method is f_s<25%. We have
subsequently obtained six additional spectra of Cygnus X-1 suitable for the
measurement of spin. Five of these spectra are of high quality with f_s in the
range 10% to 19%, a regime where the continuum-fitting method has been shown to
deliver reliable results. Individually, the six spectra give lower limits on
the spin parameter that range from a*>0.95 to a*>0.98, allowing us to
conservatively conclude that the spin of the black hole is a*>0.983
(3{\sigma}).Comment: 14 pages in emulated ApJ format, including 6 figures and 4 tables,
ApJ in press. Discussion on the pileup effect to our spin measurement is
added, including a subsection and a new figure, to reflect the referee's
comments; the conclusions are unchange
Water maser detections in southern candidates to post-AGB stars and Planetary Nebulae
We intended to study the incidence and characteristics of water masers in the
envelopes of stars in the post-AGB and PN evolutionary stages.
We have used the 64-m antenna in Parkes (Australia) to search for water maser
emission at 22 GHz, towards a sample of 74 sources with IRAS colours
characteristic of post-AGB stars and PNe, at declination . In our
sample, 39% of the sources are PNe or PNe candidates, and 50% are post-AGB
stars or post-AGB candidates.
We have detected four new water masers, all of them in optically obscured
sources: three in PNe candidates (IRAS 12405-6219, IRAS 15103-5754, and IRAS
16333-4807); and one in a post-AGB candidate (IRAS 13500-6106). The PN
candidate IRAS 15103-5754 has water fountain characteristics, and it could be
the first PN of this class found.
We confirm the tendency suggested in Paper I that the presence of water
masers in the post-AGB phase is favoured in obscured sources with massive
envelopes. We propose an evolutionary scenario for water masers in the post-AGB
and PNe stages, in which ``water fountain'' masers could develop during
post-AGB and early PN stages. Later PNe would show lower velocity maser
emission, both along jets and close to the central objects, with only the
central masers remaining in more evolved PNe.Comment: 11 pages, 5 figures. Accepted by Astronomy and Astrophysic
The Evolution of the Kinematics of Nebular Shells in Planetary Nebulae in the Milky Way Bulge
We study the line widths in the [\ion{O}{3}]5007 and H lines
for two groups of planetary nebulae in the Milky Way bulge based upon
spectroscopy obtained at the Observatorio Astron\'omico Nacional in the Sierra
San Pedro M\'artir (OAN-SPM) using the Manchester Echelle Spectrograph. The
first sample includes objects early in their evolution, having high H
luminosities, but [\ion{O}{3}]. The second
sample comprises objects late in their evolution, with \ion{He}{2} . These planetary nebulae represent evolutionary
phases preceeding and following those of the objects studied by Richer et al.
(2008). Our sample of planetary nebulae with weak [\ion{O}{3}]5007 has
a line width distribution similar to that of the expansion velocities of the
envelopes of AGB stars, and shifted to systematically lower values as compared
to the less evolved objects studied by Richer et al. (2008). The sample with
strong \ion{He}{2} has a line width distribution
indistinguishable from that of the more evolved objects from Richer et al.
(2008), but a distribution in angular size that is systematically larger and so
they are clearly more evolved. These data and those of Richer et al. (2008)
form a homogeneous sample from a single Galactic population of planetary
nebulae, from the earliest evolutionary stages until the cessation of nuclear
burning in the central star. They confirm the long-standing predictions of
hydrodynamical models of planetary nebulae, where the kinematics of the nebular
shell are driven by the evolution of the central star.Comment: accepted for publication in the Astrophysical Journa
The mediterranean overflow in the Gulf of Cadiz: a rugged journey
The pathways and transformations of dense water overflows, which depend on small-scale interactions between flow dynamics and erosional-depositional processes, are a central piece in the ocean's large-scale circulation. A novel, high-resolution current and hydrographic data set highlights the intricate pathway travelled by the saline Mediterranean Overflow as it enters the Atlantic. Interaction with the topography constraints its spreading. Over the initial 200 km west of the Gibraltar gateway, distinct channels separate the initial gravity current into several plunging branches depth-sorted by density. Shallow branches follow the upper slope and eventually detach as buoyant plumes. Deeper branches occupy mid slope channels and coalesce upon reaching a diapiric ridge. A still deeper branch, guided by a lower channel wall marked by transverse furrows, experiences small-scale overflows which travel downslope to settle at mid-depths. The Mediterranean salt flux into the Atlantic has implications for the buoyancy balance in the North Atlantic. Observations on how this flux enters at different depth levels are key to accurately measuring and understanding the role of Mediterranean Outflow in future climate scenarios.project INGRES3 [CTM2010-21229]; project STOCA (IEO); project PESCADIZ (IEO); project INDEMARES [LIFE07 NAT/E/000732+]; project MOC2 [CTM2008-06438-C02-01]; project MED-OUTFLOW [CTM2008-03422-E/MAR, CTM2010-11488-E]; project PELCOSAT (IEO); project SEMANE; project DILEMA [CTM2014-59244-C3-2-R]; project INPULSE [CTM2016-75129-C3-1-R]; SISMER data center; PANGEA data center; IEO data center; ICES data center; BODC data center; NOAA data center; CONTOURIBER project [CTM2008-06399-C04-01/MAR]info:eu-repo/semantics/publishedVersio
Extended parametric resonances in nonlinear Schrodinger systems
We study an example of exact parametric resonance in a extended system ruled
by nonlinear partial differential equations of nonlinear Schr\"odinger type. It
is also conjectured how related models not exactly solvable should behave in
the same way. The results have applicability in recent experiments in
Bose-Einstein condensation and to classical problems in Nonlinear Optics.Comment: 1 figur
The white dwarf luminosity function. I. Statistical errors and alternatives
Over the years, several methods have been proposed to compute galaxy
luminosity functions, from the most simple ones -counting sample objects inside
a given volume- to very sophisticated ones -like the C- method, the STY method
or the Choloniewski method, among others. However, only the V/Vmax method is
usually employed in computing the white dwarf luminosity function and other
methods have not been applied so far to the observational sample of
spectroscopically identified white dwarfs. Moreover, the statistical
significance of the white dwarf luminosity function has also received little
attention and a thorough study still remains to be done. In this paper we
study, using a controlled synthetic sample of white dwarfs generated using a
Monte Carlo simulator, which is the statistical significance of the white dwarf
luminosity function and which are the expected biases. We also present a
comparison between different estimators for computing the white dwarf
luminosity function. We find that for sample sizes large enough the V/Vmax
method provides a reliable characterization of the white dwarf luminosity
function, provided that the input sample is selected carefully. Particularly,
the V/Vmax method recovers well the position of the cut-off of the white dwarf
luminosity function. However, this method turns out to be less robust than the
Choloniewski method when the possible incompletenesses of the sample are taken
into account. We also find that the Choloniewski method performs better than
the V/Vmax method in estimating the overall density of white dwarfs, but misses
the exact location of the cut-off of the white dwarf luminosity function.Comment: 14 pages, 12 figures, accepted for publication in MNRA
Are beryllium abundances anomalous in stars with giant planets?
In this paper we present beryllium (Be) abundances in a large sample of 41
extra-solar planet host stars, and for 29 stars without any known
planetary-mass companion, spanning a large range of effective temperatures. The
Be abundances were derived through spectral synthesis done in standard Local
Thermodynamic Equilibrium, using spectra obtained with various instruments. The
results seem to confirm that overall, planet-host stars have ``normal'' Be
abundances, although a small, but not significant, difference might be present.
This result is discussed, and we show that this difference is probably not due
to any stellar ``pollution'' events. In other words, our results support the
idea that the high-metal content of planet-host stars has, overall, a
``primordial'' origin. However, we also find a small subset of planet-host
late-F and early-G dwarfs that might have higher than average Be abundances.
The reason for the offset is not clear, and might be related either to the
engulfment of planetary material, to galactic chemical evolution effects, or to
stellar-mass differences for stars of similar temperature.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Vortices and Jacobian varieties
We investigate the geometry of the moduli space of N-vortices on line bundles
over a closed Riemann surface of genus g > 1, in the little explored situation
where 1 =< N < g. In the regime where the area of the surface is just large
enough to accommodate N vortices (which we call the dissolving limit), we
describe the relation between the geometry of the moduli space and the complex
geometry of the Jacobian variety of the surface. For N = 1, we show that the
metric on the moduli space converges to a natural Bergman metric on the Riemann
surface. When N > 1, the vortex metric typically degenerates as the dissolving
limit is approached, the degeneration occurring precisely on the critical locus
of the Abel-Jacobi map at degree N. We describe consequences of this phenomenon
from the point of view of multivortex dynamics.Comment: 36 pages, 2 figure
Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: implications for its evolution and recent reactivation
We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on 'growing' the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years
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