840 research outputs found
A phylogenetic comparative analysis on the evolution of sequential hermaphroditism in seabreams (Teleostei : Sparidae)
The Sparids are an ideal group of fish in which to study the evolution of sexual systems since they
exhibit a great sexual diversity, from gonochorism (separate sexes) to protandrous (male-first) and
protogynous (female-first) sequential hermaphroditism (sex-change). According to the size-advantage
model (SAM), selection should favour sex change when the second sex achieves greater reproductive
success at a larger body size than the first sex. Using phylogenetic comparative methods and a sample
of 68 sparid species, we show that protogyny and protandry evolve from gonochorism but evolutionary
transitions between these two forms of sequential hermaphroditism are unlikely to happen. Using male
gonadosomatic index (GSI) as a measure of investment in gametes and proxy for sperm competition,
we find that, while gonochoristic and protogynous species support the predictions of SAM, protandrous
species do not, as they exhibit higher GSI values than expected even after considering mating
systems and spawning modes. We suggest that small males of protandrous species have to invest
disproportionally more in sperm production than predicted not only when spawning in aggregations
with high levels of sperm competition, but also when spawning in pairs due to the need to fertilize
highly fecund females, much larger than themselves. We propose that this compensatory mechanism,
together with Bateman’s principles in sequential hermaphrodites, should be formally incorporated in
the SAM
The Unusual Super-Luminous Supernovae SN 2011kl and ASASSN-15lh
Two recently discovered very luminous supernovae (SNe) present stimulating
cases to explore the extents of the available theoretical models. SN 2011kl
represents the first detection of a supernova explosion associated with an
ultra-long duration gamma ray burst. ASASSN-15lh was even claimed as the most
luminous SN ever discovered, challenging the scenarios so far proposed for
stellar explosions. Here we use our radiation hydrodynamics code in order to
simulate magnetar powered SNe. To avoid explicitly assuming neutron star
properties we adopt the magnetar luminosity and spin-down timescale as free
parameters of the model. We find that the light curve (LC) of SN 2011kl is
consistent with a magnetar power source, as previously proposed, but we note
that some amount of 56^Ni (> 0.08 M_sun) is necessary to explain the low
contrast between the LC peak and tail. For the case of ASASSN-15lh we find
physically plausible magnetar parameters that reproduce the overall shape of
the LC provided the progenitor mass is relatively large (a mass of the ejecta
approx 6 M_sun). The ejecta hydrodynamics of this event is dominated by the
magnetar input, while the effect is more moderate for SN 2011kl. We conclude
that a magnetar model may be used for the interpretation of these events and
that the hydrodynamic modeling is necessary to derive the properties of
powerful magnetars and their progenitors.Comment: Accepted by Astrophysical Journal Letters, 5 pages, 5 figure
A discrepancy principle for the Landweber iteration based on risk minimization
In this paper we propose a criterion based on risk minimization to stop the Landweber algorithm for estimating the solution of a linear system with noisy data. Under the hypothesis of white Gaussian noise, we provide an unbiased estimator of the risk and we use it for defining a variant of the classical discrepancy principle. Moreover, we prove that the proposed variant satisfies the regularization property in expectation. Finally, we perform some numerical simulations when the signal formation model is given by a convolution or a Radon transform, to show that the proposed method is numerically reliable and furnishes slightly better solutions than classical estimators based on the predictive risk, namely the Unbiased Predictive Risk Estimator and the Generalized Cross Validation
Strangelet spectra from type II supernovae
We study in this work the fate of strangelets injected as a contamination in
the tail of a "strange matter-driven" supernova shock. A simple model for the
fragmentation and braking of the strangelets when they pass through the
expanding oxygen shell is presented and solved to understand the reprocessing
of this component. We find that the escaping spectrum is a scaled-down version
of the one injected at the base of the oxygen shell. The supernova source is
likely to produce low-energy particles of quite independently
of the initial conditions. However, it is difficult that ultrarrelativistic
strangelets (such as the hypothetical Centauro primaries) can have an origin in
those explosive events.Comment: RevTex file, 5 pp., no figure
Early UV/Optical Emission of The Type Ib SN 2008D
We propose an alternative explanation for the post-breakout emission of SN
2008D associated with the X-ray transient 080109. Observations of this object
show a very small contrast of 0.35 dex between the light-curve minimum
occurring soon after the breakout, and the main luminosity peak that is due to
radioactive heating of the ejecta. Hydrodynamical models show that the cooling
of a shocked Wolf-Rayet star leads to a much greater difference (> 0.9 dex).
Our proposed scenario is that of a jet produced during the explosion which
deposits 56Ni-rich material in the outer layers of the ejecta. The presence of
high-velocity radioactive material allows us to reproduce the complete
luminosity evolution of the object. Without outer 56Ni it could be possible to
reproduce the early emission purely from cooling of the shocked envelope by
assuming a larger progenitor than a Wolf-Rayet star, but that would require an
initial density structure significantly different from what is predicted by
stellar evolution models. Analytic models of the cooling phase have been
proposed reproduce the early emission of SN 2008D with an extended progenitor.
However, we found that the models are valid only until 1.5 days after the
explosion where only two data of SN 2008D are available. We also discuss the
possibility of the interaction of the ejecta with a binary companion, based on
published analytic expressions. However, the binary separation required to fit
the early emission should be < 3 Rsun which is too small for a system
containing two massive stars.Comment: 10 pages, 10 figures, Accepted for publication in Ap
The Progenitor of the Type IIb SN 2008ax Revisited
Hubble Space Telescope observations of the site of the supernova (SN) 2008ax
obtained in 2011 and 2013 reveal that the possible progenitor object detected
in pre-explosion images was in fact multiple. Four point sources are resolved
in the new, higher-resolution images. We identify one of the sources with the
fading SN. The other three objects are consistent with single supergiant stars.
We conclude that their light contaminated the previously identified progenitor
candidate. After subtraction of these stars, the progenitor appears to be
significantly fainter and bluer than previously measured. Post-explosion
photometry at the SN location indicates that the progenitor object has
disappeared. If single, the progenitor is compatible with a supergiant star of
B to mid-A spectral type, while a Wolf-Rayet (WR) star would be too luminous in
the ultraviolet to account for the observations. Moreover, our hydrodynamical
modelling shows the pre-explosion mass was and the radius was
, which is incompatible with a WR progenitor. We present a
possible interacting binary progenitor computed with our evolutionary models
that reproduces all the observational evidence. A companion star as luminous as
an O9-B0 main-sequence star may have remained after the explosion.Comment: ApJ accepted, 14 pages, 7 figure
Quantifying pigment cover to assess variation in animal colouration
The study of animal colouration addresses fundamental and applied aspects relevant to a wide range
of fields, including behavioural ecology, environmental adaptation and visual ecology. Although a
variety of methods are available to measure animal colours, only few focus on chromatophores
(specialized cells containing pigments) and pigment migration. Here, we illustrate a freely available
and user friendly method to quantify pigment cover (PiC) with high precision and low effort using
digital images, where the foreground (i.e., pigments in chromatophores) can be detected and
separated from the background. Images of the brown shrimp, Crangon crangon were used to
compare PiC with the traditional Chromatophore Index (CI). Results indicate that PiC outcompetes CI
for pigment detection and transparency measures in terms of speed, accuracy and precision. The
proposed methodology provides researchers with a useful tool to answer essential physiological,
behavioural and evolutionary questions on animal colouration in a wide range of species
iPTF13bvn: The First Evidence of a Binary Progenitor for a Type Ib Supernova
The recent detection in archival HST images of an object at the the location
of supernova (SN) iPTF13bvn may represent the first direct evidence of the
progenitor of a Type Ib SN. The object's photometry was found to be compatible
with a Wolf-Rayet pre-SN star mass of ~11 Msun. However, based on
hydrodynamical models we show that the progenitor had a pre-SN mass of ~3.5
Msun and that it could not be larger than ~8 Msun. We propose an interacting
binary system as the SN progenitor and perform evolutionary calculations that
are able to self-consistently explain the light-curve shape, the absence of
hydrogen, and the pre-SN photometry. We further discuss the range of allowed
binary systems and predict that the remaining companion is a luminous O-type
star of significantly lower flux in the optical than the pre-SN object. A
future detection of such star may be possible and would provide the first
robust identification of a progenitor system for a Type Ib SN.Comment: Accepted to AJ on July 26. Slight changes from original, however
delayed by slow refereeing proces
Curvas de luz de supernovas superluminosas: modelos hidrodinámicos
Fil: Orellana, Mariana. Universidad Nacional de RÃo Negro. RÃo Negro, Argentina.Fil: Orellana, Mariana. Consejo Nacional de Investigaciones CientÃficas y Tecnológicas (CONICET), Argentina.Fil: Bersten, Melina. Instituto de AstrofÃsica de La Plata, CONICET-UNLP; Argentina.Fil: Benvenuto, Omar G. Instituto de AstrofÃsica de La Plata, CONICET-UNLP; Argentina.Fil: Benvenuto, Omar G. Comisión de Investigaciones CientÃficas de la Provincia de Buenos Aires, Argentina.Superluminous supernovae (SLSNe) have only recently been detected. The physical origins of their extreme luminosity, a factor 10 to 100 times brighter than normal SNe, remains speculative. One popular mechanism invoked to explain SLSNe is that a magnetar is formed by the collapse of a massive star. The magnetar is a strongly-magnetized, rapidly-rotating neutron star that loses rotational energy via magnetic dipole radiation. That energy provides the extra kick and luminosity for the SLSN. We study the effect of the shock wave propagation through the envelope with the spindown of the magnetar as the central engine. First we reproduce the analytic simplified treatment that is popular in the literature and then compare the results from our radiative hydrodynamic code. The magnetar properties can be constrained through fitting of the observed light curve. As an illustration, we apply this method to SN 2011kg.Las supernovas superluminosas (SLSNe) se han detectado recientemente. Los orÃgenes fÃsicos de su extrema luminosidad, un factor de 10 a 100 veces más brillante que el SNe normal, siguen siendo especulaciones. Un mecanismo popular invocado para explicar SLSNe es que una magnetar se forma por el colapso de una estrella masiva. El magnetar es una estrella de neutrones fuertemente magnetizada y de rotación rápida que pierde energÃa de rotación a través de la radiación del dipolo magnético. Esa energÃa proporciona la patada extra y la luminosidad para el SLSN. Estudiamos el efecto de la propagación de la onda de choque a través de la envolvente con el spindown de la magnetar como motor central. Primero reproducimos el tratamiento analÃtico simplificado que es popular en la literatura y luego comparamos los resultados de nuestro código hidrodinámico radiativo. Las propiedades del magnetar se pueden restringir mediante el ajuste de la curva de luz observada. Como ilustración, aplicamos este método a SN 2011kg
Snapping shrimp and their crustaceous cacophony
We humans are a noisy bunch. Our sounds fill the land and air around us, and even the oceans and seas. But we are not the only ones filling the sea with sound. Tiny snapping shrimp, also known as pistol shrimp, are some of the loudest animals in the ocean! They capture their prey by blasting it with a powerful shockwave from an enlarged claw. While the sound from each individual shrimp is small, the noise they make as a group has been known to mask the presence of submarines! How does something so small make such a loud noise?How can scientists use this noise to better understand the health of the seabed
- …