336 research outputs found
Gravitational Settling of ^{22}Ne in Liquid White Dwarf Interiors--Cooling and Seismological Effects
We assess the impact of the trace element ^{22}Ne on the cooling and
seismology of a liquid C/O white dwarf (WD). Due to this elements' neutron
excess, it sinks towards the interior as the liquid WD cools. The subsequent
gravitational energy released slows the cooling of the WD by 0.25--1.6 Gyrs by
the time it has completely crystallized, depending on the WD mass and the
adopted sedimentation rate. The effects will make massive WDs or those in metal
rich clusters (such as NGC 6791) appear younger than their true age. Our
diffusion calculations show that the ^{22}Ne mass fraction in the crystallized
core actually increases outwards. The stability of this configuration has not
yet been determined. In the liquid state, the settled ^{22}Ne enhances the
internal buoyancy of the interior and changes the periods of the high radial
order g-modes by approximately 1%. Though a small adjustment, this level of
change far exceeds the accuracy of the period measurements. A full assessment
and comparison of mode frequencies for specific WDs should help constrain the
still uncertain ^{22}Ne diffusion coefficient for the liquid interior.Comment: 26 pages (11 text pages with 15 figures); to appear in The
Astrophysical Journa
Axion Emission from Red Giants and White Dwarfs
Using thermal field theory methods, we recalculate axion emission from dense
plasmas. We study in particular the Primakoff and the bremsstrahlung processes.
The Primakoff rate is significantly suppressed at high densities, when the
electrons become relativistic. However, the bound on the axion-photon coupling,
GeV, is unaffected, as it is constrained by the evolution of HB
stars, which have low densities. In contradistinction, the same relativistic
effects enhance the bremsstrahlung processes. From the red giants and white
dwarfs evolution, we obtain a conservative bound on the axion-electron
coupling, .Comment: 17 pp, 3 PS figures, CERN-TH-7044/9
VLT spectroscopy and non-LTE modeling of the C/O-dominated accretion disks in two ultracompact X-ray binaries
We present new medium-resolution high-S/N optical spectra of the ultracompact
low-mass X-ray binaries 4U0614+091 and 4U1626-67, taken with the ESO Very Large
Telescope. They are pure emission line spectra and the lines are identified as
due to C II-IV and O II-III Line identification is corroborated by first
results from modeling the disk spectra with detailed non-LTE radiation transfer
calculations. Hydrogen and helium lines are lacking in the observed spectra.
Our models confirm the deficiency of H and He in the disks. The lack of neon
lines suggests an Ne abundance of less than about 10 percent (by mass),
however, this result is uncertain due to possible shortcomings in the model
atom. These findings suggest that the donor stars are eroded cores of C/O white
dwarfs with no excessive neon overabundance. This would contradict earlier
claims of Ne enrichment concluded from X-ray observations of circumbinary
material, which was explained by crystallization and fractionation of the white
dwarf core.Comment: Accepted for publication in A&A. Alternative download from
http://astro.uni-tuebingen.de/publications/author_title.shtm
Double-Peaked X-Ray Lines from the Oxygen/Neon-Rich Accretion Disk in 4U1626-67
We report on a 39 ks observation of the 7.7-s low-mass X-ray binary pulsar
4U1626-67 with the High Energy Transmission Grating Spectrometer (HETGS) on the
Chandra X-Ray Observatory. This ultracompact system consists of a
disk-accreting magnetic neutron star and a very low mass, hydrogen-depleted
companion in a 42-min binary. We have resolved the previously reported Ne/O
emission line complex near 1 keV into Doppler pairs of broadened (2500 km/s
FWHM) lines from highly ionized Ne and O. In most cases, the blue and red line
components are of comparable strength, with blueshifts of 1550-2610 km/s and
redshifts of 770-1900 km/s. The lines appear to originate in hot (10^6 K),
dense material just below the X-ray-heated skin of the outer Keplerian
accretion disk, or else possibly in a disk wind driven from the pulsar's
magnetopause. The observed photoelectric absorption edges of Ne and O appear
nearly an order of magnitude stronger than expected from interstellar material
and are likely formed in cool, metal-rich material local to the source. Based
on the inferred local abundance ratios, we argue that the mass donor in this
binary is probably the 0.02 M_sun chemically fractionated core of a C-O-Ne or
O-Ne-Mg white dwarf which has previously crystallized.Comment: 9 pages. Accepted for publication in ApJ. Table 2 correcte
Prospect of Studying Hard X- and Gamma-Rays from Type Ia Supernovae
We perform multi-dimensional, time-dependent radiation transfer simulations
for hard X-ray and gamma-ray emissions, following radioactive decays of 56Ni
and 56Co, for two-dimensional delayed detonation models of Type Ia supernovae
(SNe Ia). The synthetic spectra and light curves are compared with the
sensitivities of current and future observatories for an exposure time of 10^6
seconds. The non-detection of the gamma-ray signal from SN 2011fe at 6.4 Mpc by
SPI on board INTEGRAL places an upper limit for the mass of 56Ni of \lesssim
1.0 Msun, independently from observations in any other wavelengths. Signals
from the newly formed radioactive species have not been convincingly measured
yet from any SN Ia, but the future X-ray and gamma-ray missions are expected to
deepen the observable horizon to provide the high energy emission data for a
significant SN Ia sample. We predict that the hard X-ray detectors on board
NuStar (launched in 2012) or ASTRO-H (scheduled for launch in 2014) will reach
to SNe Ia at \sim15 Mpc, i.e., one SN every few years. Furthermore, according
to the present results, the soft gamma-ray detector on board ASTRO-H will be
able to detect the 158 keV line emission up to \sim25 Mpc, i.e., a few SNe Ia
per year. Proposed next generation gamma-ray missions, e.g., GRIPS, could reach
to SNe Ia at \sim20 - 35 Mpc by MeV observations. Those would provide new
diagnostics and strong constraints on explosion models, detecting rather
directly the main energy source of supernova light.Comment: 14 pages, 7 figures, 1 table, accepted for publication in Ap
Webifying the computerized execution of Clinical Practice Guidelines
The means through which Clinical Practice Guidelines are dissemi-nated and become accessible are a crucial factor in their later adoption by health care professionals. Making these guidelines available in Clinical Decision Sup-port Systems renders their application more personal and thus acceptable at the moment of care. Web technologies may play an important role in increasing the reach and dissemination of guidelines, but this promise remains largely unful-filled. There is a need for a guideline computer model that can accommodate a wide variety of medical knowledge along with a platform for its execution that can be easily used in mobile devices. This work presents the CompGuide frame-work, a web-based and service-oriented platform for the execution of Computer-Interpretable Guidelines. Its architecture comprises different modules whose in-teraction enables the interpretation of clinical tasks and the verification of clinical constraints and temporal restrictions of guidelines represented in OWL. It allows remote guideline execution with data centralization, more suitable for a work en-vironment where physicians are mobile and not bound to a machine. The solution presented in this paper encompasses a computer-interpretable guideline model, a web-based framework for guideline execution and an Application Programming Interface for the development of other guideline execution systems.This work is part-funded by ERDF - European Regional Development Fund through the COMPETE Programme (operational programme for competitiveness) and by National Funds through the FCT â Fundação para a CiĂȘncia e a Tecnologia (Portuguese Foundation for Science and Technology) within project FCOMP-01-0124-FEDER-028980 (PTDC/EEI-SII/1386/2012). The work of Tiago Oliveira is supported by doctoral grant by FCT (SFRH/BD/85291/2012)
Synthesis of radioactive elements in novae and supernovae and their use as a diagnostic tool
Novae and supernovae play a key role in many fields of Astrophysics and
Cosmology. Despite their importance, an accurate description of which objects
explode and why and how they explode is still lacking. One of the main
characteristics of such explosions is that they are the main suppliers of newly
synthesized chemical elements in the Galaxy. Since some of these isotopes are
radioactive, it is possible to use the corresponding gamma-rays as a diagnostic
tool of the explosion thanks to their independence on the thermal state of the
debris. The drawback is the poor sensitivity of detectors in the MeV energy
domain. As a consequence, the radioactive lines have only been detected in one
core collapse supernova (SN 1987A), one Type Ia supernova (SN 2014J), and one
supernova remnant (Cas A). Nevertheless these observations have provided and
are providing important information about the explosion mechanisms.
Unfortunately, novae are still eluding detection. These results emphasize the
necessity to place as soon as possible a new instrument in orbit with enough
sensitivity to noticeably enlarge the sample of detected eventsComment: Contribution to Review Book "15 years of INTEGRAL". Accepted for
publication in New Astronomy Review
AT 2017be - a new member of the class of Intermediate-Luminosity Red Transients
We report the results of our spectrophotometric monitoring campaign for
AT~2017be in NGC~2537. Its lightcurve reveals a fast rise to an optical
maximum, followed by a plateau lasting about 30 days, and finally a fast
decline. Its absolute peak magnitude ( 12 ) is
fainter than that of core-collapse supernovae, and is consistent with those of
supernova impostors and other Intermediate-Luminosity Optical Transients. The
quasi-bolometric lightcurve peaks at 2 10 erg s,
and the late-time photometry allows us to constrain an ejected Ni mass
of 8 10\msun. The spectra of AT~2017be show minor
evolution over the observational period, a relatively blue continuum showing at
early phases, which becomes redder with time. A prominent H emission
line always dominates over other Balmer lines. Weak Fe {\sc ii} features,
Ca~{\sc ii} HK and the Ca {\sc ii} NIR triplet are also visible, while
P-Cygni absorption troughs are found in a high resolution spectrum. In
addition, the [Ca~{\sc ii}] 7291,7324 doublet is visible in all
spectra. This feature is typical of Intermediate-Luminosity Red Transients
(ILRTs), similar to SN~2008S. The relatively shallow archival Spitzer data are
not particularly constraining. On the other hand, a non-detection in deeper
near-infrared HST images disfavours a massive Luminous Blue Variable eruption
as the origin for AT~2017be. As has been suggested for other ILRTs, we propose
that AT~2017be is a candidate for a weak electron-capture supernova explosion
of a super-asymptotic giant branch star, still embedded in a thick dusty
envelope.Comment: 21 pages, 15 figures, accepted by MNRA
The Electric Charge of Neutrinos and Plasmon Decay
By using both thermal field theory and a somewhat more intuitive method, we
define the electric charge as well as the charge radius of neutrinos
propagating inside a plasma. We show that electron neutrinos acquire a charge
radius of order cm, regardless of the properties of
the medium. Then, we compute the rate of plasmon decay which such an electric
charge or a charge radius implies. Taking into account the relativistic effects
of the degenerate electron gas, we compare our results to various
approximations as well as to recent calculations and determine the regimes
where the electric charge or the charge radius does mediate the decay of
plasmons. Finally, we discuss the stellar limits on any anomalous charge radius
of neutrinos.Comment: 19pp, 4 figures (available upon request), CERN-TH-7076/9
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