116 research outputs found
The birth properties of Galactic millisecond radio pulsars
We model the population characteristics of the sample of millisecond pulsars
within a distance of 1.5kpc.We find that for a braking index n=3, the birth
magnetic field distribution of the neutron stars as they switch on as radio
MSPs can be represented by a Gaussian with mean and
and their birth spin period by a Gaussian with mean
ms and ms. Our study, which takes into consideration
acceleration effects on the observed spin-down rate, shows that most MSPs are
born with periods that are close to the currently observed values and with
average characteristic ages typically larger by a factor 1.5 compared to the
true age. The Galactic birth rate of the MSPs is deduced to be \gsimeq 3.2
\times 10^{-6} yr near the upper end of previous estimates and larger
than the semi-empirical birth rate yr of the LMXBs. The
mean birth spin period deduced by us for the radio MSPs is a factor 2 higher
than the mean spin period observed for the accretion and nuclear powered X-ray
pulsars, although this discrepancy can be resolved if we use a braking index
, the value appropriate to spin down caused by angular momentum losses by
gravitational radiation or magnetic multipolar radiation. We discuss the
arguments for and against the hypothesis that accretion induced collapse may
constitute the main route to the formation of the MSPs, pointing out that on
the AIC scenario the low magnetic fields of the MSPs may simply reflect the
field distribution in isolated magnetic white dwarfs which has recently been
shown to be bi-modal with a dominant component that is likely to peak at fields
below G which would scale to neutron star fields below G.Comment: 8 pages, 2 figures, accepted for publication in the MNRA
Enigmas from the Sloan Digital Sky Survey DR7 Kleinman White Dwarf Catalog
We report results from a continuation of our searches for high field magnetic
white dwarfs paired in a detached binary with non degenerate companions. We
made use of the Sloan Digital Sky Survey DR7 catalog of Kleinman et al. (2013)
with 19,712 spectroscopically-identified white dwarfs. These include 1,735
white dwarf plus M dwarf detached pairs (almost 10\% of the Kleinman at al.'s
list). No new pairs were found, although we did recover the polar (AM~Herculis
system) ST\,LMi in a low state of accretion. With the larger sample the
original situation reported ten years ago remains intact now at a much higher
level of statistical significance: in the selected SDSS sample, high field
magnetic white dwarfs are not found in an apparently-detached pairing with an M
dwarf, unless they are a magnetic CV in a low state of accretion. This finding
strengthens the case that the fields in the isolated high field magnetic white
dwarfs are generated by stellar mergers but also raises questions on the nature
of the progenitors of the magnetic CVs.Comment: 12 pages, accepted for publication in the Astrophysical Journa
The Most Magnetic Stars
Observations of magnetic A, B and O stars show that the poloidal magnetic
flux per unit mass has an upper bound of 10^-6.5 G cm^2/g. A similar upper
bound is found for magnetic white dwarfs even though the highest magnetic field
strengths at their surfaces are much larger. For magnetic A and B stars there
also appears to be a well defined lower bound below which the incidence of
magnetism declines rapidly. According to recent hypotheses, both groups of
stars may result from merging stars and owe their strong magnetism to fields
generated by a dynamo mechanism as they merge. We postulate a simple dynamo
that generates magnetic field from differential rotation. The growth of
magnetic fields is limited by the requirement that the poloidal field
stabilizes the toroidal and vice versa. While magnetic torques dissipate the
differential rotation, toroidal field is generated from poloidal by an Omega
dynamo. We further suppose that mechanisms that lead to the decay of toroidal
field lead to the generation of poloidal. Both poloidal and toroidal fields
reach a stable configuration which is independent of the size of small initial
seed fields but proportional to the initial differential rotation. We pose the
hypothesis that strongly magnetic stars form from the merging of two stellar
objects. The highest fields are generated when the merge introduces
differential rotation that amounts to critical break up velocity within the
condensed object. Calibration of a simplistic dynamo model with the observed
maximum flux per unit mass for main-sequence stars and white dwarfs indicates
that about 1.5x10^-4 of the decaying toroidal flux must appear as poloidal. The
highest fields in single white dwarfs are generated when two degenerate cores
merge inside a common envelope or when two white dwarfs merge by
gravitational-radiation angular momentum loss.Comment: accepted by MNRAS 8 pages, 3 figure
The most magnetic stars
Observations of magnetic A, B and O stars show that the poloidal magnetic flux per unit mass Φp/M appears to have an upper bound of approximately 10-6.5 Gcm2 g-1. A similar upper bound to the total flux per unit mass is found for the magnetic white dwar
Formation of redbacks via accretion induced collapse
We examine the growing class of binary millisecond pulsars known as redbacks.
In these systems the pulsar's companion has a mass between 0.1 and about 0.5
solar masses in an orbital period of less than 1.5 days. All show extended
radio eclipses associated with circumbinary material. They do not lie on the
period-companion mass relation expected from the canonical intermediate-mass
X-ray binary evolution in which the companion filled its Roche lobe as a red
giant and has now lost its envelope and cooled as a white dwarf. The redbacks
lie closer to, but usually at higher period than, the period-companion mass
relation followed by cataclysmic variables and low-mass X-ray binaries. In
order to turn on as a pulsar mass accretion on to a neutron star must be
sufficiently weak, considerably weaker than expected in systems with low-mass
main-sequence companions driven together by magnetic braking or gravitational
radiation. If a neutron star is formed by accretion induced collapse of a white
dwarf as it approaches the Chandrasekhar limit some baryonic mass is abruptly
lost to its binding energy so that its effective gravitational mass falls. We
propose that redbacks form when accretion induced collapse of a white dwarf
takes place during cataclysmic variable binary evolution because the loss of
gravitational mass makes the orbit expand suddenly so that the companion no
longer fills its Roche lobe. Once activated, the pulsar can ablate its
companion and so further expand the orbit and also account for the extended
eclipses in the radio emission of the pulsar that are characteristic of these
systems. The whole period-companion mass space occupied by the redbacks can be
populated in this way.Comment: 12 pages, 7 figure
Could Edge-Lit Type Ia Supernovae be Standard Candles
The progenitors of Type Ia supernovae (SNe Ia) have not been identified.
Though they are no longer fashionable we investigate the consequences if a
significant number of SNe Ia were edge-lit detonations (ELDs) of carbon/oxygen
white dwarfs that have accreted a critical mass of helium. Our best
understanding of the Phillips relation between light curve speed and peak
luminosity assigns both these phenomena to the amount of Ni produced. In
ELDs there are two sites of Ni synthesis. If the peak luminosity is
determined primarily by the C/O ratio in the core it is primarily a function of
its progenitor's initial mass. If the light curve decay speed is determined by
the total mass of iron group elements ejected this is a function of the total
mass of the ELD at the time of explosion. In general these two masses are
correlated and an empirical relation between peak luminosity and light curve
shape can be expected. However when we perform population synthesis for
progenitors of different metallicities we find a systematic shift in this
relation that would make distant SNe Ia fainter than those nearby. The
abundances of alpha-rich isotopes, such as Ca, in the solar system
indicate that only about 40 per cent of SNe Ia are edge-lit so any systematic
effect that could be present would be correspondingly diluted. If we examine
only the small subset of ELDs that accrete from a naked helium star, rather
than a He white dwarf, the systematic effect disappears.Comment: 14 pages, 3figure
On the Orbital Period of the New Cataclysmic Variable EUVE J2115-586
We have obtained phase-resolved spectroscopy (3660-6040 Å) of the recently discovered cataclysmic variable EUVE J21 15-586 using the 74-inch telescope at Mount Stromlo Observatory. The radial velocity is modulated over a period of 110.8 min with a possible one-cycle-per-day alias of 102.8 min, and a semiamplitude of ≍270 km s-1 at Hβ and ≍390 km s-1 at He II λ4686. The spectroscopic appearance (H I Balmer, Ca II, He I, He II emission lines), the orbital period, and the velocity amplitude indicate that this cataclysmic variable is probably an AM Her type; the absence of cyclotron humps indicates a low intensity magnetic field (B\u3c20 MG). Extreme ultraviolet emission phased at the orbital period shows evidence of variability, but additional EUV/soft x-ray observations are recommended
The origin of high magnetic fields in white dwarfs
The lack of evidence for Zeeman splitting of the hydrogen lines in the spectra of the 1,253 close but detached binary systems consisting of a white dwarf and a nondegenerate star, a sample that includes the pre-Cataclysmic Variables, identified in the Sloan Digital Sky Survey indicates that there are no identifiable progenitors for the Magnetic Cataclysmic Variables (MCVs), even though these comprise some 25per cent of all Cataclysmic Variables (CVs). Indeed, all high-field white dwarfs appear to be either single stars or components of AM Her systems. This suggests that all such white dwarfs have a binary origin. We resolve this dilemma by postulating that the 106-108G magnetic fields that are observed in the white dwarfs in the MCVs are generated in the common envelope phase of pre-CV evolution in systems which almost merge. Systems that merge in the common envelope phase yield a population of isolated magnetic white dwarfs with fields of 106-109G that make up the entire single magnetic white dwarf population
Cosmic biology in perspective
Abstract: A series of astronomical observations obtained over the period 1986 to 2018 supports the idea that life is a cosmic rather than a purely terrestrial or planetary phenomenon. These include (1) the detection of biologically relevant molecules in interstellar clouds and in comets, (2) mid-infrared spectra of interstellar grains and the dust from comets, (3) a diverse set of data from comets including the Rosetta mission showing consistency with biology and (4) the frequency of Earth-like or habitable planets in the Galaxy. We argue that the conjunction of all the available data suggests the operation of cometary biology and interstellar panspermia rather than the much weaker hypothesis of comets being only the source of the chemical building blocks of life. We conclude with specific predictions on the properties expected of extra-terrestrial life if it is discovered on Enceladus, Europa or beyond. A radically different biochemistry elsewhere can be considered as a falsification of the theory of interstellar panspermia
Hubble Space Telescope FUV Spectra of the Post-Common-Envelope Hyades Binary V471 Tauri
We have carried out an analysis of the HST STIS archival spectra of the
magnetic white dwarf in the Hyades eclipsing-spectroscopic, post-common
envelope binary V471 Tauri, time resolved on the orbit and on the X-ray
rotational phase of the magnetic white dwarf. An HST STIS spectrum obtained
during primary eclipse reveals a host of transition region/chromospheric
emission features including N V (1238, 1242), Si IV (1393, 1402), C IV (1548,
1550) and He II (1640). The spectroscopic characteristics and emission line
fluxes of the transition region/chromosphere of the very active, rapidly
rotating, K2V component of V471 Tauri, are compared with the emission
characteristics of fast rotating K dwarfs in young open clusters. We have
detected a number of absorption features associated with metals accreted onto
the photosphere of the magnetic white dwarf from which we derive radial
velocities. All of the absorption features are modulated on the 555s rotation
period of the white dwarf with maximum line strength at rotational phase 0.0
when the primary magnetic accretion region is facing the observer. The
photospheric absorption features show no clear evidence of Zeeman splitting and
no evidence of a correlation between their variations in strength and orbital
phase. We report clear evidence of a secondary accretion pole. We derive C and
Si abundances from the Si IV and C III features. All other absorption lines are
either interstellar or associated with a region above the white dwarf and/or
with coronal mass ejection events illuminated as they pass in front of the
white dwarf.Comment: The Astrophysical Journal, May 10, 2012 issue - 16 figure
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