T. R. Marsh

The origin of subdwarf B stars – I. The formation channels

Subdwarf B (sdB) stars (and related sdO/sdOB stars) are believed to be helium-core-burning objects with very thin hydrogen-rich envelopes. In recent years it has become increasingly clear from observational surveys that a large fraction of these objects are members of binary systems. To understand their formation better, we present the results of a detailed investigation of the three main binary evolution channels that can lead to the formation of sdB stars: the common-envelope (CE) ejection channel, the stable Roche lobe overflow (RLOF) channel, and the double helium white dwarfs (WDs) merger channel. The CE ejection channel leads to the formation of sdB stars in short-period binaries with typical orbital periods between 0.1 and 10 d, very thin hydrogen-rich envelopes and a mass distribution sharply peaked around similar to0.46 M-.. On the other hand, under the assumption that all mass transferred is soon lost, the stable RLOF channel produces sdB stars with similar masses but long orbital periods (400-1500 d) and with rather thick hydrogen-rich envelopes. The merger channel gives rise to single sdB stars whose hydrogen-rich envelopes are extremely thin but which have a fairly wide distribution of masses (0.4-0.65 M-.). We obtained the conditions for the formation of sdB stars from each of these channels using detailed stellar and binary evolution calculations where we modelled the detailed evolution of sdB stars and carried out simplified binary population synthesis simulations. The observed period distribution of sdB stars in compact binaries strongly constrains the CE ejection parameters. The best fits to the observations are obtained for very efficient CE ejection where the envelope ionization energy is included, consistent with previous results. We also present the distribution of sdB stars in the T (eff) -log g diagram, the Hertzsprung-Russell diagram and the distribution of mass functions.

The origin of subdwarf B stars – II

We have carried out a detailed binary population synthesis (BPS) study of the formation of subdwarf B (sdB) stars and related objects (sdO, sdOB stars) using the latest version of the BPS code developed by Han and co-workers. We systematically investigate the importance of the five main evolutionary channels in which the sdB stars form after one or two common-envelope (CE) phases, one or two phases of stable Roche lobe overflow (RLOF) or as the result of the merger of two helium white dwarfs (WDs). Our best BPS model can satisfactorily explain the main observational characteristics of sdB stars, in particular their distributions in the orbital period-minimum companion mass (log P-M-comp) diagram and in the effective temperature-surface gravity (T-eff- log g) diagram, their distributions of orbital period, log (gtheta(4))(theta = 5040 K / T-eff) and mass function, their binary fraction and the fraction of sdB binaries with WD companions, their birth rates and their space density. We obtain a Galactic formation rate for sdB stars of 0.014- 0.063 yr(-1) with a best estimate of similar to0.05 yr(-1) and a total number in the Galaxy of 2.4-9.5 x 10(6) with a best estimate of similar to6 x 10(6); half of these may be missing in observational surveys owing to selection effects. The intrinsic binary fraction is 76-89 per cent, although the observed frequency may be substantially lower owing to the selection effects. The first CE ejection channel, the first stable RLOF channel and the merger channel are intrinsically the most important channels, although observational selection effects tend to increase the relative importance of the second CE ejection and merger channels. We also predict a distribution of masses for sdB stars that is wider than is commonly assumed and that some sdB stars have companions of spectral type as early as B. The percentage of A-type stars with sdB companions can in principle be used to constrain some of the important parameters in the binary evolution model. We conclude that (i) the first RLOF phase needs to be more stable than is commonly assumed, either because the critical mass ratio q(crit) for dynamical mass transfer is higher or because of tidally enhanced stellar wind mass loss; (ii) mass transfer in the first stable RLOF phase is non-conservative, and the mass lost from the system takes away a specific angular momentum similar to that of the system; and (iii) common-envelope ejection is very efficient.

ULTRACAM: an ultrafast, triple‐beam CCD camera for high‐speed astrophysics

Charge-Coupled Devices (CCD) have revolutionised observational astronomy since they were introduced in the 1970s. Their only limitation is their inability to operate at high frame rates. This has meant that faint objects that vary on timescales of less than of order seconds cannot be temporally resolved, ruling out, for example, the study of variability in compact objects, such as white dwarfs, neutron stars and black holes. ULTRACAM is a triple-beam camera designed to overcome this limitation and enable the study of astrophysics on fast timescales using CCDs. The project was awarded £292,034 in July 1999 by the Particle Physics and Astronomy Research Council and the instrument saw first light on 16 May 2002 on the William Herschel Telescope, on budget and three months ahead of schedule. This thesis describes my contribution to the ULTRACAM project. It begins with a description of the motivation for building ULTRACAM and lists its functional and performance requirements. An instrument design which meets these requirements is then presented, followed by a description of the manufacture and integration phase of the project. A whole chapter is then devoted to an in-depth analysis of the commissioning data obtained on the WHT, which verifies that ULTRACAM performs to specification. The thesis concludes with some suggestions for enhancements and future work.

A Gaseous Metal Disk Around a White Dwarf

The destiny of planetary systems through the late evolution of their host stars is very uncertain. We report a metal-rich gas disk around a moderately hot and young white dwarf. A dynamical model of the double-peaked emission lines constrains the outer disk radius to just 1.2 solar radii. The likely origin of the disk is a tidally disrupted asteroid, which has been destabilized from its initial orbit at a distance of more than 1000 solar radii by the interaction with a relatively massive planetesimal object or a planet. The white dwarf mass of 0.77 solar mass implies that planetary systems may form around high-mass stars.

Mass transfer between double white dwarfs

Three periodically variable stars have recently been discovered (V407 Vul, P=9.5 min; ES Cet, P=10.3 min; RX J0806.3+1527, P=5.3 min) with properties that suggest that their photometric periods are also their orbital periods, making them the most compact binary stars known. If true, this might indicate that close, detached, double white dwarfs are able to survive the onset of mass transfer caused by gravitational wave radiation and emerge as the semi-detached, hydrogen-deficient stars known as the AM CVn stars. The accreting white dwarfs in such systems are large compared to the orbital separations. This has two effects. First, it makes it likely that the mass-transfer stream can hit the accretor directly. Secondly, it causes a loss of angular momentum from the orbit which can destabilize the mass transfer unless the angular momentum lost to the accretor can be transferred back to the orbit. The effect of the destabilization is to reduce the number of systems which survive mass transfer by as much as one hundredfold. In this paper we analyse this destabilization and the stabilizing effect of a dissipative torque between the accretor and the binary orbit. We obtain analytical criteria for the stability of both disc-fed and direct impact accretion, and we carry out numerical integrations to assess the importance of secondary effects, the chief one being that otherwise stable systems can exceed the Eddington accretion rate. We show that to have any effect upon survival rates, the synchronizing torque must act on a time-scale of the order of 1000 yr or less. If synchronization torques are this strong, then they will play a significant role in the spin rates of white dwarfs in cataclysmic variable stars as well.

Orbital periods of 22 subdwarf B stars

Subdwarf B (sdB) stars are thought to be core helium burning stars with low mass hydrogen envelopes. In recent years it has become clear that many sdB stars lose their hydrogen through interaction with a binary companion and continue to reside in binary systems today. In this paper we present the results of a programme to measure orbital parameters of binary sdB stars. We determine the orbits of 22 binary sdB stars from 424 radial velocity measurements, raising the sample of sdBs with known orbital parameters to 38. We calculate lower limits for the masses of the companions of the sdB stars which, when combined with the orbital periods of the systems, allow us to discuss approximate evolutionary constraints. We find that a formation path for sdB stars consisting of mass transfer at the tip of the red giant branch (RGB) followed by a common envelope phase explains most, but not all of the observed systems. It is particularly difficult to explain both long period systems and short period, massive systems. We present new measurements of the effective temperature, surface density and surface helium abundance for some of the sdB stars by fitting their blue spectra. We find that two of them (PG 0839 + 399 and KPD 1946 + 4340) do not lie in the extreme horizontal branch (EHB) band, indicating that they are post-EHB stars.

Two planets orbiting the recently formed post-common envelope binary NN Serpentis

Planets orbiting post-common envelope binaries provide fundamental information on planet formation and evolution. We searched for such planets in NN Ser ab, an eclipsing short-period binary that shows long-term eclipse time variations. Using published, reanalysed, and new mid-eclipse times of NN Ser ab obtained between 1988 and 2010, we find excellent agreement with the light-travel-time effect produced by two additional bodies superposed on the linear ephemeris of the binary. Our multi-parameter fits accompanied by N-body simulations yield a best fit for the objects NN Ser (ab)c and d locked in the 2:1 mean motion resonance, with orbital periods P-c similar or equal to 15.5 yrs and P-d similar or equal to 7.7 yrs, masses M-c sin i(c) similar or equal to 6.9 M-Jup and M-d sin i(d) similar or equal to 2.2 M-Jup, and eccentricities e(c) similar or equal to 0 and e(d) similar or equal to 0.20. A secondary chi(2) minimum corresponds to an alternative solution with a period ratio of 5:2. We estimate that the progenitor binary consisted of an A star with similar or equal to 2 M-circle dot and the present M dwarf secondary at an orbital separation of similar to 1.5 AU. The survival of two planets through the common-envelope phase that created the present white dwarf requires fine tuning between the gravitational force and the drag force experienced by them in the expanding envelope. The alternative is a second-generation origin in a circumbinary disk created at the end of this phase. In that case, the planets would be extremely young with ages not exceeding the cooling age of the white dwarf of 10(6) yrs.

On the evolutionary status of short-period cataclysmic variables

We present high-speed, three-colour photometry of seven short-period (P-orb <= 95 min) eclipsing cataclysmic variables (CVs) from the Sloan Digital Sky Survey. We determine the system parameters via a parametrized model of the eclipse fitted to the observed light curve by chi(2) minimization. Three out of seven of the systems possess brown dwarf donor stars and are believed to have evolved past the orbital period minimum. This is in line with the predictions that 40-70 per cent of CVs should have evolved past the orbital period minimum. Therefore, the main result of our study is that the missing population of post-period minimum CVs has finally been identified. The donor star masses and radii are, however, inconsistent with model predictions; the donor stars are approximately 10 per cent larger than expected across the mass range studied here. One explanation for the discrepancy is the enhanced angular momentum loss (e.g. from circumbinary discs); however, the mass-transfer rates, as deduced from white dwarf effective temperatures, are not consistent with enhanced angular momentum loss. We show that it is possible to explain the large donor radii without invoking enhanced angular momentum loss by a combination of geometrical deformation and the effects of starspots due to strong rotation and expected magnetic activity. Choosing unambiguously between these different solutions will require independent estimates of the mass-transfer rates in short-period CVs. The white dwarfs in our sample show a strong tendency towards high masses. We show that this is unlikely to be due to selection effects. The dominance of high-mass white dwarfs in our sample implies that erosion of the white dwarf during nova outbursts must be negligible, or even that white dwarfs grow in mass through the nova cycle. Amongst our sample, there are no helium-core white dwarfs, despite predictions that 30-80 per cent of short-period CVs should contain helium-core white dwarfs. We are unable to rule out selection effects as the cause of this discrepancy.

First Kepler results on compact pulsators – I. Survey target selection and the first pulsators

We present results from the first two quarters of a survey to search for pulsations in compact stellar objects with the Kepler spacecraft. The survey sample and the various methods applied in its compilation are described, and spectroscopic observations are presented to separate the objects into accurate classes. From the Kepler photometry we clearly identify nine compact pulsators and a number of interesting binary stars. Of the pulsators, one shows the strong, rapid pulsations typical of a V361 Hya-type sdB variable (sdBV); seven show long-period pulsation characteristics of V1093 Her-type sdBVs; and one shows low-amplitude pulsations with both short and long periods. We derive effective temperatures and surface gravities for all the subdwarf B stars in the sample and demonstrate that below the boundary region where hybrid sdB pulsators are found, all our targets are pulsating. For the stars hotter than this boundary temperature a low fraction of strong pulsators (<10 per cent) is confirmed. Interestingly, the short-period pulsator also shows a low-amplitude mode in the long-period region, and several of the V1093 Her pulsators show low-amplitude modes in the short-period region, indicating that hybrid behaviour may be common in these stars, also outside the boundary temperature region where hybrid pulsators have hitherto been found.

Kepler observations of the beaming binary KPD 1946+4340

The Kepler Mission has acquired 33.5 d of continuous 1-min photometry of KPD 1946+4340, a short-period binary system that consists of a subdwarf B star (sdB) and a white dwarf. In the light curve, eclipses are clearly seen, with the deepest occurring when the compact white dwarf crosses the disc of the sdB (0.4 per cent) and the more shallow ones (0.1 per cent) when the sdB eclipses the white dwarf. As expected, the sdB is deformed by the gravitational field of the white dwarf, which produces an ellipsoidal modulation of the light curve. Spectacularly, a very strong Doppler beaming (also known as Doppler boosting) effect is also clearly evident at the 0.1 per cent level. This originates from the sdB’s orbital velocity, which we measure to be 164.0 ± 1. 9k m s −1 from supporting spectroscopy. We present light-curve models that account for all these effects, as well as gravitational lensing, which decreases the apparent radius of the white dwarf by about 6 per cent, when it eclipses the sdB. We derive system parameters and uncertainties from the light curve using Markov chain Monte Carlo simulations. Adopting a theoretical white dwarf mass–radius relation, the mass of the subdwarf is found ,