830,931 research outputs found
A Tidally-Disrupted Asteroid Around the White Dwarf G29-38
The infrared excess around the white dwarf G29-38 can be explained by
emission from an opaque flat ring of dust with an inner radius 0.14 of the
radius of the Sun and an outer radius approximately equal to the Sun's. This
ring lies within the Roche region of the white dwarf where an asteroid could
have been tidally destroyed, producing a system reminiscent of Saturn's rings.
Accretion onto the white dwarf from this circumstellar dust can explain the
observed calcium abundance in the atmosphere of G29-38. Either as a bombardment
by a series of asteroids or because of one large disruption, the total amount
of matter accreted onto the white dwarf may have been comparable to the total
mass of asteroids in the Solar System, or, equivalently, about 1% of the mass
in the asteroid belt around the main sequence star zeta Lep.Comment: ApJ Letters, in pres
A Shapiro delay detection in the binary system hosting the millisecond pulsar PSR J1910-5959A
PSR J1910-5959A is a binary pulsar with a helium white dwarf companion
located about 6 arcmin from the center of the globular cluster NGC6752. Based
on 12 years of observations at the Parkes radio telescope, the relativistic
Shapiro delay has been detected in this system. We obtain a companion mass Mc =
0.180+/-0.018Msun (1sigma) implying that the pulsar mass lies in the range
1.1Msun <= Mp <= 1.5Msun. We compare our results with previous optical
determinations of the companion mass, and examine prospects for using this new
measurement for calibrating the mass-radius relation for helium white dwarfs
and for investigating their evolution in a pulsar binary system. Finally we
examine the set of binary systems hosting a millisecond pulsar and a low mass
helium white dwarf for which the mass of both stars has been measured. We
confirm that the correlation between the companion mass and the orbital period
predicted by Tauris & Savonije reproduces the observed values but find that the
predicted Mp - Pb correlation over-estimates the neutron star mass by about
0.5Msun in the orbital period range covered by the observations. Moreover, a
few systems do not obey the observed Mp - Pb correlation. We discuss these
results in the framework of the mechanisms that inhibit the accretion of matter
by a neutron star during its evolution in a low-mass X-ray binary.Comment: 4 figures, 2 tables, accepted for publication in the Astrophysical
Journa
White Lies Matter: The Evolution, Persistence, and Impact of Scientific Racism
A dissection of the origins, history, persistence, and impacts of various threads of scientific racism. This paper traces scientific racism from its roots in evolutionary biology through Social Darwinism and eugenics. By exploring the historical connection between scientific racism and white supremacy, this research aims to reveal some of the ways in which contemporary science, racism, and society at large have been shaped by the past. Tracing these threads will follow scientific racism from its origins to the beginnings of Social Darwinism and eugenics, through American cultivation of eugenics and its connections to Nazi Germany, to ongoing eugenicist policies and white supremacist terrorism in our modern world
Delay time distribution of type Ia supernovae: theory vs. observation
Two formation scenarios are investigated for type Ia supernovae in elliptical
galaxies: the single degenerate scenario (a white dwarf reaching the
Chandrasekhar limit through accretion of matter transferred from its companion
star in a binary) and the double degenerate scenario (the inspiraling and
merging of two white dwarfs in a binary as a result of the emission of
gravitational wave radiation). A population number synthesis code is used,
which includes the latest physical results in binary evolution and allows to
differentiate between certain physical scenarios (such as the description of
common envelope evolution) and evolutionary parameters (such as the mass
transfer efficiency during Roche lobe overflow). The thus obtained theoretical
distributions of type Ia supernova delay times are compared to those that are
observed, both in morphological shape and absolute number of events. The
critical influence of certain parameters on these distributions is used to
constrain their values. The single degenerate scenario alone is found to be
unable in reproducing the morphological shape of the observational delay time
distribution, while use of the double degenerate one (or a combination of both)
does result in fair agreement. Most double degenerate type Ia supernovae are
formed through a normal, quasi-conservative Roche lobe overflow followed by a
common envelope phase, not through two successive common envelope phases as is
often assumed. This may cast doubt on the determination of delay times by using
analytical formalisms, as is sometimes done in other studies. The theoretical
absolute number of events in old elliptical galaxies lies a factor of at least
three below the rates that are observed. While this may simply be the result of
observational uncertainties, a better treatment of the effects of rotation on
stellar structure could mitigate the discrepancy.Comment: 5 pages, 4 figures, to appear in proceedings of "Binary Star
Evolution: Mass Loss, Accretion, and Mergers
Evolution of Hot Gas and Dark Halos in Group-Dominant Elliptical Galaxies: Influence of Cosmic Inflow
We study the complete dynamical evolution of hot interstellar gas in massive
elliptical galaxies born into a simple flat universe beginning with an
overdense perturbation. Within the turn-around radius dark matter flows in a
self-similar fashion into a stationary Navarro-Frenk-White halo and the
baryonic gas shocks. After a few gigayears, when enough gas accumulates within
the accretion shock, the de Vaucouleurs stellar system is constructed and the
energy from Type II supernovae is released. The stars and dark halo are matched
to NGC 4472. Gas continues to enter the galaxy by secondary infall and by
stellar mass loss based on a Salpeter IMF. After about 13 Gyrs the temperature
and density distribution in the hot gas agree quite well with the hot
interstellar gas observed in NGC 4472. As a result of supernova-driven outflow,
the present day baryonic fraction has a deep minimum in the outer galactic
halo. When relatively gas-rich, X-ray luminous models are spatially truncated
at early times, simulating tidal events that may have occurred during galaxy
group dynamics, the current locus of truncated models lies just along the
, X-ray size correlation among well-observed ellipticals, providing
another striking confirmation of our simple model of elliptical evolution.Comment: 16 pages in AASTEX LaTeX with 14 figures; accepted by Astrophysical
Journa
The Progenitors of Type Ia Supernovae: II. Are they Double-Degenerate Binaries? The Symbiotic Channel
In order for a white dwarf (WD) to achieve the Chandrasekhar mass, M_C, and
explode as a Type Ia supernova (SNIa), it must interact with another star,
either accreting matter from or merging with it. The failure to identify the
types of binaries which produce SNeIa is the "progenitor problem". Its solution
is required if we are to utilize the full potential of SNeIa to elucidate basic
cosmological and physical principles. In single-degenerate models, a WD
accretes and burns matter at high rates. Nuclear-burning WDs (NBWDs) with mass
close to M_C are hot and luminous, potentially detectable as supersoft x-ray
sources (SSSs). In previous work we showed that > 90-99% of the required number
of progenitors do not appear as SSSs during most of the crucial phase of mass
increase. The obvious implication is that double-degenerate (DD) binaries form
the main class of progenitors. We show in this paper, however, that many
binaries that later become DDs must pass through a long-lived NBWD phase during
which they are potentially detectable as SSSs. The paucity of SSSs is therefore
not a strong argument in favor of DD models. Those NBWDs that are the
progenitors of DD binaries are likely to appear as symbiotic binaries for
intervals > 10^6 years. In fact, symbiotic pre-DDs should be common, whether or
not the WDs eventually produce SNeIa. The key to solving the progenitor problem
lies in understanding the appearance of NBWDs. Most do not appear as SSSs most
of the time. We therefore consider the evolution of NBWDs to address the
question of what their appearance may be and how we can hope to detect them.Comment: 24 pages; 5 figures; submitted to Ap
From Diffusion MRI to Brain Connectomics
International audienceDiffusion MRI (dMRI) is a unique modality of MRI which allows one to indirectly examine the microstructure and integrity of the cerebral white matter in vivo and non-invasively. Its success lies in its capacity to reconstruct the axonal connectivity of the neurons, albeit at a coarser resolution, without having to operate on the patient, which can cause radical alterations to the patient's cognition. Thus dMRI is beginning to assume a central role in studying and diagnosing important pathologies of the cerebral white matter, such as Alzheimer's and Parkinson's diseases, as well as in studying its physical structure in vivo. In this chapter we present an overview of the mathematical tools that form the framework of dMRI - from modelling the MRI signal and measuring diffusion properties, to reconstructing the axonal connectivity of the cerebral white matter, i.e., from Diffusion Weighted Images (DWIs) to the human connectome
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