229 research outputs found
The Sisyphus particle detector
The particle measurement subsystem planned for the MJS 77 mission is described. Scientific objectives with respect to Saturn's rings are as follows: (1) measure particles outside the visible rings, including particulates orbiting in more distant rings and particles scattered out of visible rings, (2) measure meteoroid environment in vicinity of Saturn, and (3) develop an understanding of the dynamics of the rings with respect to their collisional interaction with the environment
Meteoroid hazards in deep space Final report
Design and development of Sisyphus meteoroid detection system for interplanetary spacecraf
The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems
With a view to understanding the formation of double neutron-stars (DNS), we
investigate the late stages of evolution of helium stars with masses of 2.8 -
6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found
that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in
very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and
spiral-in phase due to the development of a convective helium envelope. If the
neutron star has sufficient time to complete the spiraling-in process before
the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d)
with a merger timescale of the order of 1 Myr or less. These systems would have
important consequences for the detection rate of GWR and for the understanding
of GRB progenitors. On the other hand, if the time left until the explosion is
shorter than the orbital-decay timescale, the system will undergo a SN
explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider
orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through
CE evolution. The remnants of these massive helium stars are DNSs with periods
in the range of 0.1 - 1 d. This suggests that this range of mass includes the
progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A
minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12,
respectively) must have been imparted at the birth of the pulsar's companion.
The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced
from helium star-neutron star binaries which avoid RLOF, with the helium star
more massive than 2.5 Msun. For these systems the minimum kick velocities are
50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).Comment: 16 pages, latex, 12 figures, accepted for publication in MNRA
The Role of Helium Stars in the Formation of Double Neutron Stars
We have calculated the evolution of 60 model binary systems consisting of
helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star
companion to investigate the formation of double neutron star systems.Orbital
periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe
overflow starting from the helium main sequence to after the ignition of carbon
burning in the core. We have also examined the evolution into a common envelope
phase via secular instability, delayed dynamical instability, and the
consequence of matter filling the neutron star's Roche lobe. The survival of
some close He-star neutron-star binaries through the last mass transfer episode
(either dynamically stable or unstable mass transfer phase) leads to the
formation of extremely short-period double neutron star systems (with
P<~0.1days). In addition, we find that systems throughout the entire calculated
mass range can evolve into a common envelope phase, depending on the orbital
period at the onset of mass transfer. The critical orbital period below which
common envelope evolution occurs generally increases with M_He. In addition, a
common envelope phase may occur during a short time for systems characterized
by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun).
The existence of a short-period population of double neutron stars increases
the predicted detection rate of inspiral events by ground-based
gravitational-wave detectors and impacts their merger location in host galaxies
and their possible role as gamma-ray burst progenitors. We use a set of
population synthesis calculations and investigate the implications of the
mass-transfer results for the orbital properties of DNS populations.Comment: 30 pages, Latex (AASTeX), 1 table, 8 figures. To appear in ApJ, v592
n1 July 20, 200
Numerical Simulations of the Onset and Stability of Dynamical Mass Transfer in Binaries
Hydrodynamical simulations of semi-detached, polytropic binary stars are
presented in an effort to study the onset and stability of dynamical mass
transfer events. Initial, synchronously rotating equilibrium models are
constructed using a self-consistent-field technique and then evolved with an
Eulerian hydrodynamics code in a fully self-consistent manner. We describe code
improvements introduced over the past few years that permit us to follow
dynamical mass-transfer events through more than 30 orbits. Mass-transfer
evolutions are presented for two different initial configurations: A
dynamically unstable binary with initial mass ratio (donor/accretor) that leads to a complete merger in orbits; and a
double-degenerate binary with initial mass ratio that, after some
initial unstable growth of mass transfer, tends to separate as the
mass-transfer rate levels off.Comment: 47 pages, 11 figures, submitted to the Astrophysical Journal. See
http://www.phys.lsu.edu/faculty/tohline/astroph/dmtf05 for high resolution
figures and mpeg animation
Type Ia Supernovae: An Examination of Potential Progenitors and the Redshift Distribution
We examine the possibility that supernovae type Ia (SN Ia) are produced by
white dwarfs accreting from Roche-lobe filling evolved companions, under the
assumption that a strong optically thick stellar wind from accretor is able to
stabilize the mass transfer. We show that if a mass transfer phase on a thermal
timescale precedes a nuclear burning driven phase, then such systems (of which
the supersoft X-ray sources are a subgroup) can account for about 10% of the
inferred SN Ia rate.
In addition, we examine the cosmic history of the supernova rate, and we show
that the ratio of the rate of SN Ia to the rate of supernovae produced by
massive stars (supernovae of types II, Ib, Ic) should increase from about z = 1
towards lower redshifts.Comment: 29 pages, Latex, 6 figures, aasms4.sty, psfig.sty, to appear in The
Astrophysical Journa
An RGS4-Mediated Phenotypic Switch of Bronchial Smooth Muscle Cells Promotes Fixed Airway Obstruction in Asthma
Abstract In severe asthma, bronchodilator-and steroid-insensitive airflow obstruction develops through unknown mechanisms characterized by increased lung airway smooth muscle (ASM) mass and stiffness. We explored the role of a Regulator of Gprotein Signaling protein (RGS4) in the ASM hyperplasia and reduced contractile capacity characteristic of advanced asthma. Using immunocytochemical staining, ASM expression of RGS4 was determined in endobronchial biopsies from healthy subjects and those from subjects with mild, moderate and severe asthma. Cell proliferation assays, agonist-induced calcium mobilization and bronchoconstriction were determined in cultured human ASM cells and in human precision cut lung slices. Using gain-and loss-of-function approaches, the precise role of RGS proteins was determined in stimulating human ASM proliferation and inhibiting bronchoconstriction. RGS4 expression was restricted to a subpopulation of ASM and was specifically upregulated by mitogens, which induced a hyperproliferative and hypocontractile ASM phenotype similar to that observed in recalcitrant asthma. RGS4 expression was markedly increased in bronchial smooth muscle of patients with severe asthma, and expression correlated significantly with reduced pulmonary function. Whereas RGS4 inhibited G proteincoupled receptor (GPCR)-mediated bronchoconstriction, unexpectedly RGS4 was required for PDGF-induced proliferation and sustained activation of PI3K, a mitogenic signaling molecule that regulates ASM proliferation. These studies indicate that increased RGS4 expression promotes a phenotypic switch of ASM, evoking irreversible airway obstruction in subjects with severe asthma
Modelling the formation of double white dwarfs
We investigate the formation of the ten double-lined double white dwarfs that
have been observed so far. A detailed stellar evolution code is used to
calculate grids of single-star and binary models and we use these to
reconstruct possible evolutionary scenarios. We apply various criteria to
select the acceptable solutions from these scenarios. We confirm the conclusion
of Nelemans et al. (2000) that formation via conservative mass transfer and a
common envelope with spiral-in based on energy balance or via two such
spiralins cannot explain the formation of all observed systems. We investigate
three different prescriptions of envelope ejection due to dynamical mass loss
with angular-momentum balance and show that they can explain the observed
masses and orbital periods well. Next, we demand that the age difference of our
model is comparable to the observed cooling-age difference and show that this
puts a strong constraint on the model solutions. However, the scenario in which
the primary loses its envelope in an isotropic wind and the secondary transfers
its envelope, which is then re-emitted isotropically, can explain the observed
age differences as well. One of these solutions explains the DB-nature of the
oldest white dwarf in PG1115+116 along the evolutionary scenario proposed by
Maxted et al. (2002a), in which the helium core of the primary becomes exposed
due to envelope ejection, evolves into a giant phase and loses its
hydrogen-rich outer layers.Comment: 20 pages, 17 figures, 6 tables, accepted for publication in Astronomy
and Astrophysics. See http://www.astro.uu.nl/~sluys/publications/ for
high-resolution versions of Figs. 15 and 1
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
Recommended from our members
20-HETE Mediates Ozone-Induced, Neutrophil-Independent Airway Hyper-Responsiveness in Mice
Background Ozone, a pollutant known to induce airway hyper-responsiveness (AHR), increases morbidity and mortality in patients with obstructive airway diseases and asthma. We postulate oxidized lipids mediate in vivo ozone-induced AHR in murine airways.Methodology/Principal Findings Male BALB/c mice were exposed to ozone (3 or 6 ppm) or filtered air (controls) for 2 h. Precision cut lung slices (PCLS; 250 µm thickness) containing an intrapulmonary airway (∼0.01 mm lumen area) were prepared immediately after exposure or 16 h later. After 24 h, airways were contracted to carbachol (CCh). Log EC and E values were then calculated by measuring the airway lumen area with respect to baseline. In parallel studies, dexamethasone (2.5 mg/kg), or 1-aminobenzotriazol (ABT) (50 mg/kg) were given intraperitoneal injection to naïve mice 18 h prior to ozone exposure. Indomethacin (10 mg/kg) was administered 2 h prior. Cell counts, cytokine levels and liquid chromatography-mass spectrometry (LC-MS) for lipid analysis were assessed in bronchoalveolar lavage (BAL) fluid from ozone exposed and control mice. Ozone acutely induced AHR to CCh. Dexamethasone or indomethacin had little effect on the ozone-induced AHR; while, ABT, a cytochrome P450 inhibitor, markedly attenuated airway sensitivity. BAL fluid from ozone exposed animals, which did not contain an increase in neutrophils or interleukin (IL)-6 levels, increased airway sensitivity following in vitro incubation with a naïve PCLS. In parallel, significant increases in oxidized lipids were also identified using LC-MS with increases of 20-HETE that were decreased following ABT treatment.Conclusions/Significance These data show that ozone acutely induces AHR to CCh independent of inflammation and is insensitive to steroid treatment or cyclooxygenase (COX) inhibition. BAL fluid from ozone exposed mice mimicked the effects of in vivo ozone exposure that were associated with marked increases in oxidized lipids. 20-HETE plays a pivotal role in mediating acute ozone-induced AHR
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