D. T. Wickramasinghe

Asteroseismology of the DOV star PG 1159 - 035 with the Whole Earth Telescope

Results are reported from 264.1 hr of nearly continuous time-series photometry on the pulsating prewhite dwarf star (DPV) PG 1159 - 035. The power spectrum of the data set is completely resolved into 125 individual frequencies; 101 of them are identified with specific quantized pulsation modes, and the rest are completely consistent with such modal assignment. It is argued that the luminosity variations are certainly the result of g-mode pulsations. Although the amplitudes of some of the peaks exhibit significant variations on the time scales of a year or so, the underlying frequency structure of the pulsations is stable over much longer intervals. The existing linear theory is invoked to determine, or strongly constrain, many of the fundamental physical parameters describing this star. Its mass is found to be 0.586 solar mass, is rotation period 1.38 days, its magnetic field less than 6000 G, its pulsation and rotation axes to be aligned, and its outer layers to be compositionally stratified.

Binary star origin of high field magnetic white dwarfs

White dwarfs with surface magnetic fields in excess of 1MG are found as isolated single stars and relatively more often in magnetic cataclysmic variables. Some 1,253 white dwarfs with a detached low-mass main-sequence companion are identified in the Sloan Digital Sky Survey but none of these is observed to show evidence for Zeeman splitting of hydrogen lines associated with a magnetic field in excess of 1MG. If such high magnetic fields on white dwarfs result from the isolated evolution of a single star then there should be the same fraction of high field white dwarfs among this SDSS binary sample as among single stars. Thus we deduce that the origin of such high magnetic fields must be intimately tied to the formation of cataclysmic variables. The formation of a CV must involve orbital shrinkage from giant star to main-sequence star dimensions. It is believed that this shrinkage occurs as the lowmass companion and the white dwarf spiral together inside a common envelope. CVs emerge as very close but detached binary stars that are then brought together by magnetic braking or gravitational radiation. We propose that the smaller the orbital separation at the end of the common envelope phase, the stronger the magnetic field. The magnetic cataclysmic variables originate from those common envelope systems that almost merge. We propose further that those common envelope systems that do merge are the progenitors of the single high field white dwarfs. Thus all highly magnetic white dwarfs, be they single stars or the components of MCVs, have a binary origin. This hypothesis also accounts for the relative dearth of single white dwarfs with fields of 10 4 10 6 G. Such intermediate-field white dwarfs are found preferentially in cataclysmic variables. In addition the bias towards higher masses for highly magnetic white dwarfs is expected if a fraction of these form when two degenerate cores merge in a common envelope. Similar scenarios may account for very high field neutron stars. From the space density of single highly magnetic white dwarfs we estimate that about three times as many common envelope events lead to a merged core as to a cataclysmic variable.

Spectropolarimetric Survey of Hydrogen-rich White Dwarf Stars

We have conducted a survey of 61 southern white dwarfs searching for magnetic fields using Zeeman spectropolarimetry. Our objective is to obtain a magnetic field distribution for these objects and, in particular, to find white dwarfs with weak fields. We found one possible candidate (WD 0310-688) that may have a weak magnetic field of -6.1 ±2.2 kG. Next, we determine the fraction and distribution of magnetic white dwarfs in the solar neighborhood and investigate the probability of finding more of these objects based on the current incidence of magnetism in white dwarfs within 20 pc of the Sun. We have also analyzed the spectra of the white dwarfs to obtain effective temperatures and surface gravities.

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 M ○. . We re-evaluate the ensemble of data that determines the IFMR and argue that the IFMR can be characterized by a mean IFMR 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 Palomar-Green 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 a single star evolution has masses in excess of ∼0.8 M ○. , obviating the necessity for postulating the existence of a dominant population of high-mass white dwarfs that arise from binary star mergers.

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 parametrized 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 Galactocentric 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 M ⊙ 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 at ∼46 G with ∼8 per cent of stars having fields in excess of 1000 G. The higher-field progenitors yield a population of 24 neutron stars with fields in excess of 10 14 G, periods ranging from 5 to 12 s, and ages of up to 100 000 yr, 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 M ⊙ , which is significantly above the mean mass of 1.4 M ⊙ calculated for the overall population of radio pulsars.

Understanding the Cool DA White Dwarf Pulsator, G29-38

The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been explored through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modelling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modelling, thereby joining the rest of the known white dwarf pulsators.

Planets around White Dwarfs

The fate of a planetary system like our own, as the parent star expands through the red giant phase and becomes a white dwarf (WD), has been a topic of some discussion. For an Earth-like inner planet, the conducting core may remain intact, even though severe ablation occurs of the outer layers. We argue that a planetary core in orbit around a WD may reveal its presence through its interaction with the magnetosphere of the WD. As the planet moves through the magnetosphere, electrical currents will be generated, which will heat the atmosphere of the WD near its magnetic poles. The results of such a heating may be detected in the optical as Hα emission. Ohmic dissipation will result in the slow decay of the planetary orbit, and such a planet will merge with the WD in less than a Hubble time, unless the initial orbital separation is greater than about 10 solar radii. We propose that the peculiar emission-line WD GD 356 may be a system in the process of such a merger.

The birth properties of Galactic millisecond radio pulsars

We model the population characteristics of the sample of millisecond pulsars (MSPs) within a distance of 1.5 kpc. We find that for a braking index n = 3, the birth magnetic field distribution of the neutron stars as they switch on as radio-emitting MSPs can be represented by a Gaussian in the logarithm with mean log B(G) = 8.1 and σ log B = 0.4 and their birth spin period by a Gaussian with mean P 0 = 4 ms and σ P 0 = 1.3 ms. We assume no field decay during the lifetime of MSPs. 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 that are typically larger by a factor of ∼1.5 compared to the true age. The Galactic birth rate of the MSPs is deduced to be ≥3.2 x 10 -6 yr -1 near the upper end of previous estimates and larger than the semi-empirical birth rate ∼10 -7 yr -1 of the low-mass X-ray binaries (LMXBs), the currently favoured progenitors. The mean birth spin period deduced by us for the radio MSPs is a factor of ∼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 (AIC) 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, under magnetic flux conservation.

Infrared spectroscopy over the 2.9?3.9 ?m waveband in biochemistry and astronomy

The infrared spectrum of the galactic centre source IRS 7 over the 2.9–3.9 μm waveband is interpreted as strong evidence for bacterial grains.

The most magnetic stars

Observations of magnetic A, B and O stars show that the poloidal mag- netic flux per unit massp/M appears to have an upper bound of approximately 10 6.5 Gcm 2 g 1 . A similar upper bound to the total flux per unit mass is found for the 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. Accord- ing 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. We limit the growth of magnetic fields 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 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. Cali- bration of a simplistic dynamo model with the observed maximum flux per unit mass for main-sequence stars and white dwarfs indicates that about 1.5×10 4 of the decay- ing 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. Magnetars are the most magnetic neutron stars. Though these are expected to form directly from single stars, their magnetic flux to mass ratio indicates that a similar dynamo, driven by differential rotation acquired at their birth, may also be the source of their strong magnetism.