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 $\log B(G)= 8.1$ and $\sigma_{\log B}=0.4$ and their birth spin period by a Gaussian with mean $P_0=4$ ms and $\sigma_{P_0}=1.3$ 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$^{-1}$ near the upper end of previous estimates and larger than the semi-empirical birth rate $\sim 10^{-7}$ yr$^{-1}$ 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 $n=5$, 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 $10^3$ G which would scale to neutron star fields below $10^9$ 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 $^{56}$Ni produced. In ELDs there are two sites of $^{56}$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 $^{44}$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