824 research outputs found
On identifying the neutron star that was born in the supernova that placed 60Fe onto the Earth
Recently, 60Fe was found in the Earth crust formed in a nearby recent
supernova (SN). If the distance to the SN and mass of the progenitor of that SN
was known, then one could constrain SN models. Knowing the positions, proper
motions, and distances of dozens of young nearby neutron stars, we can
determine their past flight paths and possible kinematic origin. Once the birth
place of a neutron star in a SN is found, we would have determined the distance
of the SN and the mass of the SN progenitor star.Comment: refereed NPA5 conference proceedings, in pres
Binary populations and stellar dynamics in young clusters
We first summarize work that has been done on the effects of binaries on
theoretical population synthesis of stars and stellar phenomena. Next, we
highlight the influence of stellar dynamics in young clusters by discussing a
few candidate UFOs (unconventionally formed objects) like intermediate mass
black holes, Eta Carinae, Zeta Puppis, Gamma Velorum and WR 140.Comment: Contributed paper IAU 250: Massive Stars as Cosmic Engine
The effects of supernovae on the dynamical evolution of binary stars and star clusters
In this chapter I review the effects of supernovae explosions on the
dynamical evolution of (1) binary stars and (2) star clusters.
(1) Supernovae in binaries can drastically alter the orbit of the system,
sometimes disrupting it entirely, and are thought to be partially responsible
for `runaway' massive stars - stars in the Galaxy with large peculiar
velocities. The ejection of the lower-mass secondary component of a binary
occurs often in the event of the more massive primary star exploding as a
supernova. The orbital properties of binaries that contain massive stars mean
that the observed velocities of runaway stars (10s - 100s km s) are
consistent with this scenario.
(2) Star formation is an inherently inefficient process, and much of the
potential in young star clusters remains in the form of gas. Supernovae can in
principle expel this gas, which would drastically alter the dynamics of the
cluster by unbinding the stars from the potential. However, recent numerical
simulations, and observational evidence that gas-free clusters are observed to
be bound, suggest that the effects of supernova explosions on the dynamics of
star clusters are likely to be minimal.Comment: 16 pages, to appear in the 'Handbook of Supernovae', eds. Paul Murdin
and Athem Alsabti. This version replaces an earlier version that contained
several typo
Local Pulsars; A note on the Birth-Velocity Distribution
We explore a simple model for the representation of the observed
distributions of the motions, and the characteristic ages of the local
population of pulsars. The principal difference from earlier models is the
introduction of a unique value, S, for the kick velocity with which pulsars are
born. We consider separately the proper motion components in galactic longitude
and latitude, and find that the distributions of the velocity components
parallel and perpendicular to the galactic plane are represented satisfactorily
by S=200 km/sec, and leave no room for a significant fraction of much higher
velocities. The successful proposition of a unique value for the kick velocity
may provide an interesting tool in attempts to understand the physical process
leading to the expulsion of the neutron star.Comment: To be published in JAA, 14 pages, 7 figure
OB Stars in the Solar Neighborhood I: Analysis of their Spatial Distribution
We present a newly-developed, three-dimensional spatial classification
method, designed to analyze the spatial distribution of early type stars within
the 1 kpc sphere around the Sun. We propose a distribution model formed by two
intersecting disks -the Gould Belt (GB) and the Local Galactic Disk (LGD)-
defined by their fundamental geometric parameters. Then, using a sample of
about 550 stars of spectral types earlier than B6 and luminosity classes
between III and V, with precise photometric distances of less than 1 kpc, we
estimate for some spectral groups the parameters of our model, as well as
single membership probabilities of GB and LGD stars, thus drawing a picture of
the spatial distribution of young stars in the vicinity of the Sun.Comment: 28 pages including 9 Postscript figures, one of them in color.
Accepted for publication in The Astronomical Journal, 30 January 200
MRF4 negatively regulates adult skeletal muscle growth by repressing MEF2 activity
The myogenic regulatory factor MRF4 is highly expressed in adult skeletal muscle but its function is unknown. Here we show that Mrf4 knockdown in adult muscle induces hypertrophy and prevents denervation-induced atrophy. This effect is accompanied by increased protein synthesis and widespread activation of muscle-specific genes, many of which are targets of MEF2 transcription factors. MEF2-dependent genes represent the top-ranking gene set enriched after Mrf4 RNAi and a MEF2 reporter is inhibited by co-transfected MRF4 and activated by Mrf4 RNAi. The Mrf4 RNAi-dependent increase in fibre size is prevented by dominant negative MEF2, while constitutively active MEF2 is able to induce myofibre hypertrophy. The nuclear localization of the MEF2 corepressor HDAC4 is impaired by Mrf4 knockdown, suggesting that MRF4 acts by stabilizing a repressor complex that controls MEF2 activity. These findings open new perspectives in the search for therapeutic targets to prevent muscle wasting, in particular sarcopenia and cachexia
Hyperfast pulsars as the remnants of massive stars ejected from young star clusters
Recent proper motion and parallax measurements for the pulsar PSR B1508+55
indicate a transverse velocity of ~1100 km/s, which exceeds earlier
measurements for any neutron star. The spin-down characteristics of PSR
B1508+55 are typical for a non-recycled pulsar, which implies that the velocity
of the pulsar cannot have originated from the second supernova disruption of a
massive binary system. The high velocity of PSR B1508+55 can be accounted for
by assuming that it received a kick at birth or that the neutron star was
accelerated after its formation in the supernova explosion. We propose an
explanation for the origin of hyperfast neutron stars based on the hypothesis
that they could be the remnants of a symmetric supernova explosion of a
high-velocity massive star which attained its peculiar velocity (similar to
that of the pulsar) in the course of a strong dynamical three- or four-body
encounter in the core of dense young star cluster. To check this hypothesis we
investigated three dynamical processes involving close encounters between: (i)
two hard massive binaries, (ii) a hard binary and an intermediate-mass black
hole, and (iii) a single star and a hard binary intermediate-mass black hole.
We find that main-sequence O-type stars cannot be ejected from young massive
star clusters with peculiar velocities high enough to explain the origin of
hyperfast neutron stars, but lower mass main-sequence stars or the stripped
helium cores of massive stars could be accelerated to hypervelocities. Our
explanation for the origin of hyperfast pulsars requires a very dense stellar
environment of the order of 10^6 -10^7 stars pc^{-3}. Although such high
densities may exist during the core collapse of young massive star clusters, we
caution that they have never been observed.Comment: 11 pages, 6 figures, 1 table, accepted to MNRA
Spectroscopic study of the O-type runaway supergiant HD 195592
The scope of this paper is to perform a detailed spectroscopic study of the
northern O-type supergiant HD 195592. We use a large sample of high quality
spectra in order to investigate its multiplicity, and to probe the line profile
variability. Our analysis reveals a clear spectroscopic binary signature in the
profile of the He {\sc i} 6678 line, pointing to a probable O + B
system. We report on low amplitude radial velocity variations in every strong
absorption line in the blue spectrum of HD 195592. These variations are ruled
by two time-scales respectively of 5.063 and about 20 days. The former is
firmly established, whilst the latter is poorly constrained. We report also on
a very significant line profile variability of the H line, with time
scales strongly related to those of the radial velocities. Our results provide
significant evidence that HD 195592 is a binary system, with a period that
might be the variability time-scale of about 5 days. The second time scale may
be the signature of an additional star moving along a wider orbit provided its
mass is low enough, even though direct evidence for the presence of a third
star is still lacking. Alternatively, the second time-scale may be the
signature of a variability intrinsic to the stellar wind of the primary,
potentially related to the stellar rotation.Comment: 9 pages, 5 postscript figures, accepted for publication in New
Astronom
Nova Sco and coalescing low mass black hole binaries as LIGO sources
Double neutron star binaries, analogous to the well known Hulse--Taylor
pulsar PSR 1913+16, are guaranteed-to-exist sources of high frequency
gravitational radiation detectable by LIGO. There is considerable uncertainty
in the estimated rate of coalescence of such systems, with conservative
estimates of ~1 per million years per galaxy, and optimistic theoretical
estimates one or more magnitude larger. Formation rates of low-mass black
hole-neutron star binaries may be higher than those of NS-NS binaries, and may
dominate the detectable LIGO signal rate.
We estimate the enhanced coalescence rate for BH-BH binaries due to weak
asymmetric kicks during the formation of low mass black holes like Nova Sco,
and find they may contribute significantly to the LIGO signal rate, possibly
dominating the phase I detectable signals if the range of BH masses for which
there is significant kick is broad enough. For a standard Salpeter IMF,
assuming mild natal kicks, we project that the R6 merger rate of BH-BH systems
is ~0.5, smaller than that of NS-NS systems. However, the higher chirp mass of
these systems produces a signal nearly four times greater, on average, with a
commensurate increase in search volume.
The BH-BH coalescence channel considered here also predicts that a
substantial fraction of BH-BH systems should have at least one component with
near-maximal spin (a/M ~ 1).The waveforms produced by the coalescence of such a
system should produce a clear spin signature, so this hypothesis could be
directly tested by LIGO.Comment: 16 pages, LaTeX/AASTeX, 5 figure
Sexually transmitted infections - factors associated with quality of care among private general practitioners
- …