2,596 research outputs found
The open cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution
Recent studies of white dwarfs in open clusters have provided new constraints
on the initial - final mass relationship (IFMR) for main sequence stars with
masses in the range 2.5 - 6.5 Mo. We re-evaluate the ensemble of data that
determines the IFMR and argue that the IFMR can be characterised by a mean
initial-final mass relationship about which there is an intrinsic scatter. We
investigate the consequences of the IFMR for the observed mass distribution of
field white dwarfs using population synthesis calculations. We show that while
a linear IFMR predicts a mass distribution that is in reasonable agreement with
the recent results from the PG survey, the data are better fitted by an IFMR
with some curvature. Our calculations indicate that a significant (~28%)
percentage of white dwarfs originating from single star evolution have masses
in excess of ~0.8 Mo, obviating the necessity for postulating the existence of
a dominant population of high-mass white dwarfs that arise from binary star
mergers.Comment: 5 pages, 2 color Postscript figures. Accepted for publication in
MNRA
Galactic Escape Speeds in Mirror and Cold Dark Matter Models
The mirror dark matter (MDM) model of Berezhiani et al. has been shown to
reproduce observed galactic rotational curves for a variety of spiral galaxies,
and has been presented as an alternative to cold dark matter (CDM) models. We
investigate possible additional tests involving the properties of stellar
orbits, which may be used to discriminate between the two models. We
demonstrate that in MDM and CDM models fitted equally well to a galactic
rotational curve, one generally expects predictable differences in escape
speeds from the disc. The recent radial velocity (RAVE) survey of the Milky Way
has pinned down the escape speed from the solar neighbourhood to
km s, placing an additional constraint on dark
matter models. We have constructed an MDM model for the Milky Way based on its
rotational curve, and find an escape speed that is just consistent with the
observed value given the current errors, which lends credence to the viability
of the MDM model. The Gaia-ESO spectroscopic survey is expected to lead to an
even more precise estimate of the escape speed that will further constrain dark
matter models. However, the largest differences in stellar escape speeds
between both models are predicted for dark matter dominated dwarf galaxies such
as DDO 154, and kinematical studies of such galaxies could prove key in
establishing, or abolishing, the validity of the MDM model.Comment: Accepted for publication in the European Physical Journal
The effects of tidally induced disc structure on white dwarf accretion in intermediate polars
We investigate the effects of tidally induced asymmetric disc structure on
accretion onto the white dwarf in intermediate polars. Using numerical
simulation, we show that it is possible for tidally induced spiral waves to
propagate sufficiently far into the disc of an intermediate polar that
accretion onto the central white dwarf could be modulated as a result. We
suggest that accretion from the resulting asymmetric inner disc may contribute
to the observed X-ray and optical periodicities in the light curves of these
systems. In contrast to the stream-fed accretion model for these periodicities,
the tidal picture predicts that modulation can exist even for systems with
weaker magnetic fields where the magnetospheric radius is smaller than the
radius of periastron of the mass transfer stream. We also predict that
additional periodic components should exist in the emission from low mass ratio
intermediate polars displaying superhumps.Comment: 9 pages, 5 figures, accepted for publication in MNRA
WD1953-011 - a magnetic white dwarf with peculiar field structure
We present H-alpha spectra of the magnetic white dwarf star WD1953-011 which
confirm the presence of the broad Zeeman components corresponding to a field
strength of about 500kG found by Maxted & Marsh (1999). We also find that the
line profile is variable over a timescale of a day or less. The core of the
H-alpha line also shows a narrow Zeeman triplet corresponding to a field
strength of of about 100kG which appears to be almost constant in shape. These
observations suggest that the magnetic field on WD1953-011 has a complex
structure and that the star has a rotational period of hours or days which
causes the observed variability of the spectra. We argue that neither an offset
dipole model nor a double-dipole model are sufficient to explain our
observations. Instead, we propose a two component model consisting of a high
field region of magnetic field strength of about 500kG covering about 10% of
the surface area of the star superimposed on an underlying dipolar field of
mean field strength of about 70kG. Radial velocity measurements of the narrow
Zeeman triplet show that the radial velocity is constant to within a few km/s
so this star is unlikely to be a close binary.Comment: Accpeted for publication in MNRAS. 4 pages, 2 figure
Superconducting Superstructure for the TESLA Collider
We discuss the new layout of a cavity chain (superstructure) allowing, we
hope, significant cost reduction of the RF system of both linacs of the TESLA
linear collider. The proposed scheme increases the fill factor and thus makes
an effective gradient of an accelerator higher. We present mainly computations
we have performed up to now and which encouraged us to order the copper model
of the scheme, still keeping in mind that experiments with a beam will be
necessary to prove if the proposed solution can be used for the acceleration.Comment: 11 page
Modelling of isolated radio pulsars and magnetars on the fossil field hypothesis
We explore the hypothesis that the magnetic fields of neutron stars are of
fossil origin. For parametrised models of the distribution of magnetic flux on
the Main Sequence and of the birth spin period of the neutron stars, we
calculate the expected properties of isolated radio pulsars in the Galaxy using
as our starting point the initial mass function and star formation rate as a
function of galacto-centric radius. We then use the 1374 MHz Parkes Multi-Beam
Survey of isolated radio pulsars to constrain the parameters in our model and
to deduce the required distribution of magnetic fields on the main sequence. We
find agreement with observations for a model with a star formation rate that
corresponds to a supernova rate of 2 per century in the Galaxy from stars with
masses in the range 8 - 45 solar masses and predict 447,000 active pulsars in
the Galaxy with luminosities greater than 0.19 mJy kpc^2. The progenitor OB
stars have a field distribution which peaks near 46 Gauss with about 8 percent
of stars having fields in excess of 1,000 Gauss. The higher field progenitors
yield a population of 24 neutron stars with fields in excess of 10^14 Gauss,
periods ranging from 5 to 12 seconds, and ages of up to 100,000 years, which we
identify as the dominant component of the magnetars. We also predict that high
field neutron stars (log B>13.5) originate preferentially from higher mass
progenitors and have a mean mass of 1.6 solar masses, which is significantly
above the mean mass of 1.4 solar masses calculated for the overall population
of radio pulsars.Comment: 6 pages, 4 figure
Role of olmesartan in combination therapy in blood pressure control and vascular function
Angiotensin receptor blockers have emerged as a first-line therapy in the management of hypertension and hypertension-related comorbidities. Since national and international guidelines have stressed the need to control blood pressure to <140/90 mmHg in uncomplicated hypertension and <130/80 mmHg in those with associated comorbidities such as diabetes or chronic kidney disease, these goal blood pressures can only be achieved through combination therapy. Of several drugs that can be effectively combined to attain the recommended blood pressure goals, fixed-dose combinations of angiotensin receptor blockers and the calcium channel blocker amlodipine provide additive antihypertensive effects associated with a safe profile and increased adherence to therapy. In this article, we review the evidence regarding the beneficial effects of renin–angiotensin system blockade with olmesartan medoxomil and amlodipine in terms of blood pressure control and improvement of vascular function and target organ damage
V405 Aurigae: A High Magnetic Field Intermediate Polar
Our simultaneous multicolor (UBVRI) circular polarimetry has revealed nearly
sinusoidal variation over the WD spin cycle, and almost symmetric positive and
negative polarization excursions. Maximum amplitudes are observed in the B and
V bands (+-3 %). This is the first time that polarization peaking in the blue
has been discovered in an IP, and suggests that V405 Aur is the highest
magnetic field IP found so far. The polarized flux spectrum is similar to those
found in polars with magnetic fields in the range B ~ 25-50 MG. Our low
resolution circular spectropolarimetry has given evidence of transient features
which can be fitted by cyclotron harmonics n = 6, 7, and 8, at a field of B =
31.5 +- 0.8 MG, consistent with the broad-band polarized flux spectrum. Timings
of the circular polarization zero crossovers put strict upper limits on WD spin
period changes and indicate that the WD in V405 Aur is currently accreting
closely at the spin equilibrium rate, with very long synchronization
timescales, T_s > 10^9 yr. For the observed spin to orbital period ratio,
P_{spin}/P_{orb} = 0.0365, and P_{orb} ~ 4.15 hr, existing numerical accretion
models predict spin equilibrium condition with B ~ 30 MG if the mass ratio of
the binary components is q_1 ~ 0.4. The high magnetic field makes V405 Aur a
likely candidate as a progenitor of a polar.Comment: To appear in The Astrophysical Journal, September 1 Issue (2008), 9
pages, 10 figure
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