How to Make a Singleton sdB Star via Accelerated Stellar Evolution

Many hot subdwarf B stars (sdBs) are in close binaries, and the favored formation channels for subdwarfs rely on mass transfer in a binary system to strip a core He burning star of its envelope. However, these channels cannot account for sdBs that have been observed in long period binaries nor the narrow mass distribution of isolated (or "singleton") sdBs. We propose a new formation channel involving the merger of a helium white dwarf and a low mass, hydrogen burning star, which addresses these issues. Hierarchical triples whose inner binaries merge and form sdBs by this process could explain the observed long period subdwarf+main sequence binaries. This process would also naturally explain the observed slow rotational speeds of singleton sdBs. We also briefly discuss the implications of this formation channel for extreme horizontal branch morphology in globular clusters and the UV upturn in elliptical galaxies.Comment: 5 pages, 3 figures, accepted for publication ApJ

On the Nature of Ultra-Luminous X-ray Sources from Optical/IR Measurements

We present a model for the prediction of the optical/infra-red emission from ULXs. In the model, ULXs are binary systems with accretion taking place through Roche lobe overflow. We show that irradiation effects and presence of an accretion disk significantly modify the optical/infrared flux compared to single stars, and also that the system orientation is important. We include additional constraints from the mass transfer rate to constrain the parameters of the donor star, and to a lesser extent the mass of the BH. We apply the model to fit photometric data for several ULX counterparts. We find that most donor stars are of spectral type B and are older and less massive than reported elsewhere, but that no late-type donors are admissable. The degeneracy of the acceptable parameter space will be significantly reduced with observations over a wider spectral range, and if time-resolved data become available

A Radial Velocity Study of CTCV J1300-3052

We present time-resolved spectroscopy of the eclipsing, short period cataclysmic variable CTCV J1300-3052. Using absorption features from the secondary star, we determine the radial velocity semi-amplitude of the secondary star to be K2 = 378 \pm 6 km/s, and its projected rotational velocity to be v sin i = 125 \pm 7 km/s. Using these parameters and Monte Carlo techniques, we obtain masses of M1 = 0.79 \pm 0.05 MSun for the white dwarf primary and M2 = 0.198 \pm 0.029 MSun for the M-type secondary star. These parameters are found to be in excellent agreement with previous mass determinations found via photometric fitting techniques, supporting the accuracy and validity of photometric mass determinations in short period CVs.Comment: Accepted for publication in MNRAS (24th January 2012). 10 pages, 9 figures (black and white

Spectroscopy of candidate electromagnetic counterparts to gravitational wave sources

A programme of worldwide, multi-wavelength electromagnetic follow-up of sources detected by gravitational wave detectors is in place. Following the discovery of GW150914 and GW151226, wide field imaging of their sky localisations identified a number of candidate optical counterparts which were then spectrally classified. The majority of candidates were found to be supernovae at redshift ranges similar to the GW events and were thereby ruled out as a genuine counterpart. Other candidates ruled out include AGN and solar system objects. Given the GW sources were black hole binary mergers, the lack of an identified electromagnetic counterpart is not surprising. However the observations show that is it is possible to organise and execute a campaign that can eliminate the majority of potential counterparts. Finally we note the existence of a "classification gap" with a significant fraction of candidates going unclassified

The optical emission from ultraluminous x-ray sources.

Ultraluminous X-ray sources (ULXs) are point-like, non-nuclear sources which exceed the Eddington luminosity for a stellar mass black hole (BH). The emission from these sources might be beamed or super-Eddington, but it has also been pro posed that the compact object in these sources are intermediate mass BHs (IMBHs), which fit in the mass range between the two known populations of BH in the galaxy. The existence of IMBHs is under intense debate, and study of the X-ray data has been unable to resolve this issue. This thesis describes a model I have constructed in order to examine the optical/IR emission from these sources: an alternative channel by which their nature may be understood. I assume a binary model with a black hole accreting matter from a Roche lobe filling companion star. I consider the effects of radiative transport and radiative equilibrium in the irradiated surfaces of both the star and a thin accretion disc. I use current stellar evolutionary models as an input component in this model, and hence determine the mass, radius and age of the donor stars in a range of ULX systems, and in some cases provide limits on the BH mass. In addition I determine the mass transfer rate in these systems from the X-ray luminosity and compare this to transfer rate calculations based on the stellar evolutionary models. Since this method is independent of the optical data it is a powerful additional constraint on the parameter space. For systems where optical observations are available at multiple epochs, I make further determinations of the binary parameters based on the optical variability. Where it is possible to constrain the masses of the BHs, I find them to be consistent with BHs of up to 100A . I find that in general the donor stars are older and less massive than previously thought, and are consistent with being of spectral type B. I discuss how these results affect our understanding of the evolution and history of ULXs. I discuss how future studies of ULX optical counterparts will be even more revealing, and I make predictions for these optical campaigns, estimating binary periods, variability and the results of IR observational campaigns, which my results suggest will be a important tool in future studies of the nature of this class of sources

White Light Flare Continuum Observations with ULTRACAM

We present sub-second, continuous-coverage photometry of three flares on the dM3.5e star, EQ Peg A, using custom continuum filters with WHT/ULTRACAM. These data provide a new view of flare continuum emission, with each flare exhibiting a very distinct light curve morphology. The spectral shape of flare emission for the two large-amplitude flares is compared with synthetic ULTRACAM measurements taken from the spectra during the large 'megaflare' event on a similar type flare star. The white light shape during the impulsive phase of the EQ Peg flares is consistent with the range of colors derived from the megaflare continuum, which is known to contain a Hydrogen recombination component and compact, blackbody-like components. Tentative evidence in the ULTRACAM photometry is found for an anti-correlation between the emission of these components.Comment: 8 pages, 3 figures. Proceedings of the 16th Workshop on Cool Stars, Stellar Systems, and the Sun (PASP conference series, in press

The evolutionary state of short-period magnetic white dwarf binaries

We present phase-resolved spectroscopy of two new short-period low accretion rate magnetic binaries, SDSS J125044.42+154957.3 (Porb= 86 min) and SDSS J151415.65+074446.5 (Porb= 89 min). Both systems were previously identified as magnetic white dwarfs from the Zeeman splitting of the Balmer absorption lines in their optical spectra. Their spectral energy distributions exhibit a large near-infrared excess, which we interpret as a combination of cyclotron emission and possibly a late-type companion star. No absorption features from the companion are seen in our optical spectra. We derive the orbital periods from a narrow, variable Hα emission line which we show to originate on the companion star. The high radial velocity amplitude measured in both systems suggests a high orbital inclination, but we find no evidence for eclipses in our data. The two new systems resemble the polar EF Eri in its prolonged low state and also SDSS J121209.31+013627.7, a known magnetic white dwarf plus possible brown dwarf binary, which was also recovered by our method

The fast rotating, low gravity subdwarf B star EC 22081-1916 - Remnant of a common envelope merger event

Hot subdwarf B stars (sdBs) are evolved core helium-burning stars with very thin hydrogen envelopes. In order to form an sdB, the progenitor has to lose almost all of its hydrogen envelope right at the tip of the red giant branch. In binary systems, mass transfer to the companion provides the extraordinary mass loss required for their formation. However, apparently single sdBs exist as well and their formation is unclear since decades. The merger of helium white dwarfs leading to an ignition of core helium-burning or the merger of a helium core and a low mass star during the common envelope phase have been proposed. Here we report the discovery of EC 22081-1916 as a fast rotating, single sdB star of low gravity. Its atmospheric parameters indicate, that the hydrogen envelope must be unusually thick, which is at variance with the He-WD merger scenario, but consistent with a common envelope merger of a low-mass, possibly substellar object with a red-giant core.Comment: 16 pages, 4 figures, ApJL, accepte

Accurate positions for the ULXs NGC 7319-X4 and NGC 5474-X1 and limiting magnitudes for their optical counterparts

In this paper we report accurate Chandra positions for two ultraluminous X-ray sources: NGC 7319-X4 at Right Ascension (RA) = 339.02917(2) deg, Declination (Dec) = 33.97476(2) deg and NGC 5474-X1 at RA = 211.24859(3) deg, Dec = 53.63584(3) deg. We perform bore-sight corrections on the Chandra X-ray Satellite observations of these sources to get to these accurate positions of the X-ray sources and match these positions with archival optical data from the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. We do not find the optical counterparts: the limiting absolute magnitudes of the observations in the WFPC2 standard magnitude system are B = -7.9, V = -8.7 and I = -9.3 for NGC 7319-X4 and U = -6.4 for NGC 5474-X1. We report on the X-ray spectral properties and we find evidence for X-ray variability in NGC 5474-X1. Finally, we briefly discuss several options for the nature of these ULXs.Comment: 8 pages, 4 figures, accepted for publication in MNRA