91 research outputs found
Land subsidence, seismicity and pore pressure monitoring: the new requirements for the future development of oil and gas fields in Italy
The Emilia earthquake of 2012 (Italy) stimulated a controversial debate
concerning the possibility that the event could have been induced or
triggered by underground fluids production. The public discussion led the
Italian Government to issue a protocol of guidelines for the monitoring of
microseismic activity, ground deformation and reservoir pore pressure. The
guidelines will be put into operation as soon as practicable when licensing
is being considered, and all data provided by mining operators must be made
available to the relevant Authorities. The implementation of an outreach and
communication program to local residents and administrative authorities is
prescribed, so that the civil and scientific community at large can gain
confidence that operations are being managed optimally
An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate
There is wide agreement that Type Ia supernovae (used as standard candles for
cosmology) are associated with the thermonuclear explosions of white dwarf
stars. The nuclear runaway that leads to the explosion could start in a white
dwarf gradually accumulating matter from a companion star until it reaches the
Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in
a compact binary system. The X-ray signatures of these two possible paths are
very different. Whereas no strong electromagnetic emission is expected in the
merger scenario until shortly before the supernova, the white dwarf accreting
material from the normal star becomes a source of copious X-rays for ~1e7 yr
before the explosion. This offers a means of determining which path dominates.
Here we report that the observed X-ray flux from six nearby elliptical galaxies
and galaxy bulges is a factor of ~30-50 less than predicted in the accretion
scenario, based upon an estimate of the supernova rate from their K-band
luminosities. We conclude that no more than ~5 per cent of Type Ia supernovae
in early type galaxies can be produced by white dwarfs in accreting binary
systems, unless their progenitors are much younger than the bulk of the stellar
population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs
make a significant contribution to the supernova rate.Comment: 10 pages, 1 tabl
Theoretical UBVRI colors of iron core white dwarfs
We explore photometric properties of hypothetical iron core white dwarfs and
compute their expected colors in UBVRI Johnson broadband system. Atmospheres of
iron core WDs in this paper consist of pure iron covered by a pure hydrogen
layer of an arbitrary column mass. LTE model atmospheres and theoretical
spectra are calculated on the basis of Los Alamos TOPS opacities and the
equation of state from the OPAL project, suitable for nonideal Fe and H gases.
We have also computed UBVRI colors of the models and determined an area on the
B-V vs. U-B and U-B vs. V-I planes, occupied by both pure Fe, and pure H model
atmospheres of WD stars. Finally, we search for iron core white dwarf
candidates in the available literature.Comment: 13 pages, 12 figures, Astronomy & Astrophysics (2003) in prin
Very low mass white dwarfs with a C-O core
The lower limit for the mass of white dwarfs (WDs) with C-O core is commonly
assumed to be roughly 0.5 Msun. As a consequence, WDs of lower masses are
usually identified as He-core remnants. However, when the initial mass of the
progenitor star is in between 1.8 and 3 Msun, which corresponds to the so
called red giant (RGB) phase transition, the mass of the H-exhausted core at
the tip of the RGB is 0.3 < M_H/Msun < 0.5. Prompted by this well known result
of stellar evolution theory, we investigate the possibility to form C-O WDs
with mass M < 0.5 Msun. The pre-WD evolution of stars with initial mass of
about 2.3 Msun, undergoing anomalous mass-loss episodes during the RGB phase
and leading to the formation of WDs with He-rich or CO-rich cores have been
computed. The cooling sequences of the resulting WDs are also described. We
show that the minimum mass for a C-O WD is about 0.33 Msun, so that both He and
C-O core WDs can exist in the mass range 0.33-0.5 Msun. The models computed for
the present paper provide the theoretical tools to indentify the observational
counterpart of very low mass remnants with a C-O core among those commonly
ascribed to the He-core WD population in the progressively growing sample of
observed WDs of low mass. Moreover, we show that the central He-burning phase
of the stripped progeny of the 2.3 Msun star lasts longer and longer as the
total mass decreases. In particular, the M= 0.33 Msun model takes about 800 Myr
to exhausts its central helium, which is more than three time longer than the
value of the standard 2.3 Msun star: it is, by far, the longest core-He burning
lifetime. Finally, we find the occurrence of gravonuclear instabilities during
the He-burning shell phase.Comment: Accepted for publication in Astronomy and Astrophysics, 9 page
GALEX and Optical Light Curves of WX LMi, SDSSJ103100.5+202832.2 and SDSSJ121209.31+013627.7
{\it GALEX} near ultraviolet (NUV) and far-ultraviolet (FUV) light curves of
three extremely low accretion rate polars show distinct modulations in their UV
light curves. While these three systems have a range of magnetic fields from 13
to 70 MG, and of late type secondaries (including a likely brown dwarf in
SDSSJ121209.31+013627.7), the accretion rates are similar, and the UV
observations imply some mechanism is operating to create enhanced emission
zones on the white dwarf. The UV variations match in phase to the two magnetic
poles viewed in the optical in WX LMi and to the single poles evident in the
optical in SDSSJ1212109.31+013627.7 and SDSSJ103100.55+202832.2. Simple spot
models of the UV light curves show that if hot spots are responsible for the UV
variations, the temperatures are on the order of 10,000-14,000K. For the single
pole systems, the size of the FUV spot must be smaller than the NUV and in all
cases, the geometry is likely more complicated than a simple circular spot.Comment: 29 pages, 4 tables, 10 figures, Astrophysical Journal, accepte
On the formation of neon-enriched donor stars in ultracompact X-ray binaries
We study the formation of neon-enriched donor stars in ultracompact X-ray
binaries (orbital periods P<80 min) and show that their progenitors have to be
low-mass (0.3 - 0.4 solar mass) ``hybrid'' white dwarfs (with CO cores and
thick helium mantles). Stable mass transfer is possible if in the initial
stages of mass exchange mass is lost from the system, taking away the specific
orbital angular momentum of the accretor (``isotropic re-emission''). The
excess of neon in the transferred matter is due to chemical fractionation of
the white dwarf which has to occur prior to the Roche lobe overflow by the
donor. The estimated lower limit of the orbital periods of the systems with
neon-enriched donors is close to 10 min. We show that the X-ray pulsar 4U
1626-67, which likely also has a neon-enriched companion, may have been formed
via accretion induced collapse of an oxygen-neon white dwarf accretor if the
donor was a hybrid white dwarf.Comment: 6 pages, 3 figures, uses aa.cls 5.1 version class file, accepted for
publication in Astronomy and Astrophysic
The system parameters of DW Ursae Majoris
We present new constraints on the system parameters of the SW Sextantis star
DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the
Hubble Space Telescope. Our data were obtained during a low state of the
system, in which the UV light was dominated by the hot white dwarf (WD)
primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set
firm limits on the masses and radii of the system components and the distance
between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014,
M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5
the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's
mass-period relation for CV secondaries, our estimates for the system
parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001,
M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82
\pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral
type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160
pc, from time-resolved I- and K-band photometry. Finally, we have repeated
Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa
in order to account for the higher surface gravity indicated by our eclipse
analysis. In this way we obtained a second estimate for the distance, d = 590
\pm 100 pc, which allows us to obtain a second estimate for the spectral type
of the secondary, M7 \pm 2.0. We conclude that the true value for the distance
and spectral type will probably be in between the values obtained by the two
methods.Comment: 23 pages including 5 figures and 3 tables. Accepted for publication
in Ap
Spread of vaccinia virus to cattle herds, Argentina, 2011
To the Editor: Since 1999, several zoonotic outbreaks of vaccinia virus (VACV) infection have been reported in cattle and humans in rural areas of Brazil. The infections have caused exanthematous lesions on cows and persons who milk them, and thus are detrimental to the milk industry and public health services (1,2). In Brazil during the last decade, VACV outbreaks have been detected from the north to the extreme south of the country (1–4). Because Brazil shares extensive boundaries with other South American countries, humans and cattle on dairy and beef-producing farms in those countries may be at risk of exposure to VACV. To determine if VACV has spread from Brazil to Argentina, we investigated the presence of VACV in serum samples from cattle in Argentina.Facultad de Ciencias VeterinariasComisión de Investigaciones Científicas de la provincia de Buenos Aire
Spread of vaccinia virus to cattle herds, Argentina, 2011
To the Editor: Since 1999, several zoonotic outbreaks of vaccinia virus (VACV) infection have been reported in cattle and humans in rural areas of Brazil. The infections have caused exanthematous lesions on cows and persons who milk them, and thus are detrimental to the milk industry and public health services (1,2). In Brazil during the last decade, VACV outbreaks have been detected from the north to the extreme south of the country (1–4). Because Brazil shares extensive boundaries with other South American countries, humans and cattle on dairy and beef-producing farms in those countries may be at risk of exposure to VACV. To determine if VACV has spread from Brazil to Argentina, we investigated the presence of VACV in serum samples from cattle in Argentina.Facultad de Ciencias VeterinariasComisión de Investigaciones Científicas de la provincia de Buenos Aire
The Hot Components of AM CVn Helium Cataclysmics
We present the results of a multi-component synthetic spectral analysis of
the archival far ultraviolet spectra of the hot components of several AM CVn
double degenerate interacting binaries with known distances from trigonometric
parallaxes. Our analysis was carried out using the code BINSYN (Linnell &
Hubeny 1996) which takes into account the donor companion star, the shock front
which forms at the disk edge and the FUV and NUV energy distribution. We fixed
the distance of each system at its parallax-derived value and adopted
appropriate values of orbital inclination and white dwarf mass. We find that
the accretion-heated "DO/DB" WDs are contributing significantly to the FUV flux
in four of the systems (ES Ceti, CR Boo, V803 Cen, HP Lib, GP Com). In two of
the systems, GP Com and ES Ceti, the WD dominates the FUV/NUV flux. We present
model-derived accretion rates which agree with the low end of the range of
accretion rates derived earlier from black body fits over the entire spectral
energy distribution. We find that the WD in ES Ceti is very likely not a direct
impact accretor but has a small disk. The WD in ES Ceti has K. This is far cooler than the previous estimate of Espaillat
et al.(2005). We find that the WD in GP Com has K,
which is hotter than the previously estimated temperature of 11,000K. We
present a comparison between our empirical results and current theoretical
predictions for these systems.Comment: Accepted for publication in the Astrophysical Journal (Sept.20, 2011
issue, in press
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