1,460 research outputs found
Progenitors of supernova Ibc: a single Wolf-Rayet star as the possible progenitor of the SN Ib iPTF13bvn
Core-collapse supernova (SN) explosions mark the end of the tumultuous life
of massive stars. Determining the nature of their progenitors is a crucial step
towards understanding the properties of SNe. Until recently, no progenitor has
been directly detected for SN of type Ibc, which are believed to come from
massive stars that lose their hydrogen envelope through stellar winds and from
binary systems where the companion has stripped the H envelope from the
primary. Here we analyze recently reported observations of iPTF13bvn, which
could possibly be the first detection of a SN Ib progenitor based on
pre-explosion images. Very interestingly, the recently published Geneva models
of single stars can reproduce the observed photometry of the progenitor
candidate and its mass-loss rate, confirming a recently proposed scenario. We
find that a single WR star with initial mass in the range 31-35 Msun fits the
observed photometry of the progenitor of iPTF13bvn. The progenitor likely has a
luminosity of log (L/Lsun)~5.55, surface temperature ~45000 K, and mass of
~10.9 Msun at the time of explosion. Our non-rotating 32 Msun model
overestimates the derived radius of the progenitor, although this could likely
be reconciled with a fine-tuned model of a more massive (between 40 and 50
Msun), hotter, and luminous progenitor. Our models indicate a very uncertain
ejecta mass of ~8 Msun, which is higher than the average of the SN Ib ejecta
mass that is derived from the lightcurve (2-4 Msun). This possibly high ejecta
mass could produce detectable effects in the iPTF13bvn lightcurve and spectrum.
If the candidate is indeed confirmed to be the progenitor, our results suggest
that stars with relatively high initial masses (>30 Msun) can produce visible
SN explosions at their deaths and do not collapse directly to a black hole.Comment: 4 pages, 2 figures, accepted for publication in A&
Massive star evolution in close binaries:conditions for homogeneous chemical evolution
We investigate the impact of tidal interactions, before any mass transfer, on
various properties of the stellar models. We study the conditions for obtaining
homogeneous evolution triggered by tidal interactions, and for avoiding any
Roche lobe overflow during the Main-Sequence phase. We consider the case of
rotating stars computed with a strong coupling mediated by an interior magnetic
field. In models without any tidal interaction (single stars and wide
binaries), homogeneous evolution in solid body rotating models is obtained when
two conditions are realized: the initial rotation must be high enough, the loss
of angular momentum by stellar winds should be modest. This last point favors
metal-poor fast rotating stars. In models with tidal interactions, homogeneous
evolution is obtained when rotation imposed by synchronization is high enough
(typically a time-averaged surface velocities during the Main-Sequence phase
above 250 km s), whatever the mass losses. In close binaries, mixing is
stronger at higher than at lower metallicities. Homogeneous evolution is thus
favored at higher metallicities. Roche lobe overflow avoidance is favored at
lower metallicities due to the fact that stars with less metals remain more
compact. We study also the impact of different processes for the angular
momentum transport on the surface abundances and velocities in single and close
binaries. In models where strong internal coupling is assumed, strong surface
enrichments are always associated to high surface velocities in binary or
single star models. In contrast, models computed with mild coupling may produce
strong surface enrichments associated to low surface velocities. Close binary
models may be of interest for explaining homogeneous massive stars, fast
rotating Wolf-Rayet stars, and progenitors of long soft gamma ray bursts, even
at high metallicities.Comment: 21 pages, 13 figures, 3 tables, accepted for publication in Astronomy
and Astrophysic
Taking the self out of self-rule
Many philosophers believe that agents are self-ruled only when ruled by their (authentic) selves. Though this view is rarely argued for explicitly, one tempting line of thought suggests that self-rule is just obviously equivalent to rule by the self. However, the plausibility of this thought evaporates upon close examination of the logic of ‘self-rule’ and similar reflexives. Moreover, attempts to rescue the account by recasting it in negative terms are unpromising. In light of these problems, this paper instead proposes that agents are self-ruled only when not ruled by others. One reason for favouring this negative social view is its ability to yield plausible conclusions concerning various manipulation cases that are notoriously problematic for nonsocial accounts of self-rule. A second reason is that the account conforms with ordinary usage. It is concluded that self-rule may be best thought of as an essentially social concept
Close binary evolution. III. Impact of tides, wind magnetic braking, and internal angular momentum transport
Massive stars with solar metallicity lose important amounts of rotational
angular momentum through their winds. When a magnetic field is present at the
surface of a star, efficient angular momentum losses can still be achieved even
when the mass-loss rate is very modest, at lower metallicities, or for
lower-initial-mass stars. In a close binary system, the effect of wind magnetic
braking also interacts with the influence of tides, resulting in a complex
evolution of rotation. We study the interactions between the process of wind
magnetic braking and tides in close binary systems. We discuss the evolution of
a 10 M star in a close binary system with a 7 M companion using
the Geneva stellar evolution code. The initial orbital period is 1.2 days. The
10 M star has a surface magnetic field of 1 kG. Various initial
rotations are considered. We use two different approaches for the internal
angular momentum transport. In one of them, angular momentum is transported by
shear and meridional currents. In the other, a strong internal magnetic field
imposes nearly perfect solid-body rotation. The evolution of the primary is
computed until the first mass-transfer episode occurs. The cases of different
values for the magnetic fields and for various orbital periods and mass ratios
are briefly discussed. We show that, independently of the initial rotation rate
of the primary and the efficiency of the internal angular momentum transport,
the surface rotation of the primary will converge, in a time that is short with
respect to the main-sequence lifetime, towards a slowly evolving velocity that
is different from the synchronization velocity. (abridged).Comment: 11 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Children's construction task performance and spatial ability: controlling task complexity and predicting mathematics performance.
This paper presents a methodology to control construction task complexity and examined the relationships between construction performance and spatial and mathematical abilities in children. The study included three groups of children (N = 96); ages 7-8, 10-11, and 13-14 years. Each group constructed seven pre-specified objects. The study replicated and extended previous findings that indicated that the extent of component symmetry and variety, and the number of components for each object and available for selection, significantly predicted construction task difficulty. Results showed that this methodology is a valid and reliable technique for assessing and predicting construction play task difficulty. Furthermore, construction play performance predicted mathematical attainment independently of spatial ability
Gauging Agency Involvement in Environmental Management Using Text Analysis of Laws and Regulations
Resolving Vega and the inclination controversy with CHARA/MIRC
Optical and infrared interferometers definitively established that the
photometric standard Vega (alpha Lyrae) is a rapidly rotating star viewed
nearly pole-on. Recent independent spectroscopic analyses could not reconcile
the inferred inclination angle with the observed line profiles, preferring a
larger inclination. In order to resolve this controversy, we observed Vega
using the six-beam Michigan Infrared Combiner on the Center for High Angular
Resolution Astronomy Array. With our greater angular resolution and dense
(u,v)-coverage, we find Vega is rotating less rapidly and with a smaller
gravity darkening coefficient than previous interferometric results. Our models
are compatible with low photospheric macroturbulence and also consistent with
the possible rotational period of ~0.71 days recently reported based on
magnetic field observations. Our updated evolutionary analysis explicitly
incorporates rapid rotation, finding Vega to have a mass of 2.15+0.10_-0.15
Msun and an age 700-75+150 Myrs, substantially older than previous estimates
with errors dominated by lingering metallicity uncertainties
(Z=0.006+0.003-0.002).Comment: Accepted for publication in ApJ Letter
Momentum transfer using chirped standing wave fields: Bragg scattering
We consider momentum transfer using frequency-chirped standing wave fields.
Novel atom-beam splitter and mirror schemes based on Bragg scattering are
presented. It is shown that a predetermined number of photon momenta can be
transferred to the atoms in a single interaction zone.Comment: 4 pages, 3 figure
High-precision calculations of dispersion coefficients, static dipole polarizabilities, and atom-wall interaction constants for alkali-metal atoms
The van der Waals coefficients for the alkali-metal atoms from Na to Fr
interacting in their ground states, are calculated using relativistic ab initio
methods. The accuracy of the calculations is estimated by also evaluating
atomic static electric dipole polarizabilities and coefficients for the
interaction of the atoms with a perfectly conducting wall. The results are in
excellent agreement with the latest data from ultra-cold collisions and from
studies of magnetic field induced Feshbach resonances in Na and Rb. For Cs we
provide critically needed data for ultra-cold collision studies
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