Stephen Justham

Detection of Circumstellar Material in a Normal Type Ia Supernova

Type Ia supernovae are important cosmological distance indicators. Each of these bright supernovae supposedly results from the thermonuclear explosion of a white dwarf star that, after accreting material from a companion star, exceeds some mass limit, but the true nature of the progenitor star system remains controversial. Here we report the spectroscopic detection of circumstellar material in a normal type Ia supernova explosion. The expansion velocities, densities, and dimensions of the circumstellar envelope indicate that this material was ejected from the progenitor system. In particular, the relatively low expansion velocities suggest that the white dwarf was accreting material from a companion star that was in the red-giant phase at the time of the explosion.

Supernova Shock Breakout from a Red Supergiant

Massive stars undergo a violent death when the supply of nuclear fuel in their cores is exhausted, resulting in a catastrophic “core-collapse” supernova. Such events are usually only detected at least a few days after the star has exploded. Observations of the supernova SNLS-04D2dc with the Galaxy Evolution Explorer space telescope reveal a radiative precursor from the supernova shock before the shock reached the surface of the star and show the initial expansion of the star at the beginning of the explosion. Theoretical models of the ultraviolet light curve confirm that the progenitor was a red supergiant, as expected for this type of supernova. These observations provide a way to probe the physics of core-collapse supernovae and the internal structures of their progenitor stars.

Explosive common-envelope ejection: implications for gamma-ray bursts and low-mass black-hole binaries

We present a new mechanism for the ejection of a common envelope in a massive binary, where the energy source is nuclear energy rather than orbital energy. This can occur during the slow merger of a massive primary with a secondary of 1–3xa0 M⊙ when the primary has already completed helium core burning. We show that in the final merging phase, hydrogen-rich material from the secondary can be injected into the helium-burning shell of the primary. This leads to a nuclear runaway and the explosive ejection of both the hydrogen and the helium layers, producing a close binary containing a CO star and a low-mass companion. We argue that this presents a viable scenario to produce short-period black-hole binaries and long-duration gamma-ray bursts (LGRBs). We estimate an LGRB rate of ∼ 10−6xa0yr−1 at solar metallicity, which implies that this may account for a significant fraction of all LGRBs and that this rate should be higher at lower metallicity.

Magnetic braking of Ap/Bp stars: application to compact black-hole X-ray binaries

We examine the proposal that the subset of neutron-star and black-hole X-ray binaries that form with Ap or Bp star companions will experience systemic angular-momentum losses due to magnetic braking, not otherwise operative with intermediate-mass companion stars. We suggest that for donor stars possessing the anomalously high magnetic fields associated with Ap and Bp stars, a magnetically coupled, irradiation-driven stellar wind can lead to substantial systemic loss of angular momentum. Hence, these systems, which would otherwise not be expected to experience ‘magnetic braking’, evolve to shorter orbital periods during mass transfer. In this paper, we detail how such a magnetic braking scenario operates. We apply it to a specific astrophysics problem involving the formation of compact black-hole binaries with low-mass donor stars. At present, it is not understood how these systems form, given that lowmass companion stars are not likely to provide sufficient gravitational potential to unbind the envelope of the massive progenitor of the black hole during a prior ‘common-envelope’ phase. On the other hand, intermediate-mass companions, such as Ap and Bp stars, could more readily eject the common envelope. However, in the absence of magnetic braking, such systems tend to evolve to long orbital periods. We show that, with the proposed magnetic braking properties afforded by Ap and Bp companions, such a scenario can lead to the formation of compact black-hole binaries with orbital periods, donor masses, lifetimes and production rates that are in accord with the observations. In spite of these successes, our models reveal a significant discrepancy between the calculated effective temperatures and the observed spectral types of the donor stars. Finally, we show that this temperature discrepancy would still exist for other scenarios invoking initially intermediate-mass donor stars, and this presents a substantial unresolved mystery.

Type Ia supernovae and the formation of single low-mass white dwarfs

Context. There is still considerable debate over the progenitors of type Ia supernovae (SNe Ia). Likewise, it is not agreed how single white dwarfs with masses less than or similar to 0.5 M(circle dot) can be formed in the field, even though they are known to exist. Aims. We consider whether single low-mass white dwarfs (LMWDs) could have been formed in binary systems where their companions have exploded as an SN Ia. In this model, the observed single LMWDs are the remnants of giant-branch donor stars whose envelopes have been stripped off by the supernova explosion. Methods. We investigate the likely remnants of SNe Ia, including the effects of the explosion on the envelope of the donor star. We also use evolutionary arguments to examine alternative formation channels for single LMWDs. In addition, we calculate the expected kinematics of the potential remnants of SNe Ia. Results. SN Ia in systems with giant-branch donor stars can naturally explain the production of single LMWDs. It seems difficult for any other formation mechanism to account for the observations, especially for those single LMWDs with masses <= 0.4 M(circle dot). Independent of those results, we find that the kinematics of one potentially useful population containing single LMWDs is consistent with our model. Studying remnant white-dwarf kinematics seems to be a promising way to investigate SN Ia progenitors. Conclusions. The existence of single LMWDs appears to constitute evidence for the production of SNe Ia in binary systems with a red-giant donor star. Other single white dwarfs with higher space velocities support a second, probably dominant, population of SN Ia progenitors which contained main-sequence or subgiant donor stars at the time of explosion. The runaway stars LP 400-22 and US 708 suggest the possibility of a third formation channnel for some SNe Ia in systems where the donor stars are hot subdwarfs.

A Multiphysics and Multiscale Software Environment for Modeling Astrophysical Systems

We present MUSE, a software framework for tying together existing computational tools for different astrophysical domains into a single multiphysics, multiscale workload. MUSE facilitates the coupling of existing codes written in different languages by providing inter-language tools and by specifying an interface between each module and the framework that represents a balance between generality and computational efficiency. This approach allows scientists to use combinations of codes to solve highly-coupled problems without the need to write new codes for other domains or significantly alter their existing codes. MUSE currently incorporates the domains of stellar dynamics, stellar evolution and stellar hydrodynamics for a generalized stellar systems workload. MUSE has now reached a Noahs Ark milestone, with two available numerical solvers for each domain. MUSE can treat small stellar associations, galaxies and everything in between, including planetary systems, dense stellar clusters and galactic nuclei. Here we demonstrate an examples calculated with MUSE: the merger of two galaxies. In addition we demonstrate the working of MUSE on a distributed computer. The current MUSE code base is publicly available as open source at http://muse.li.

Models of Ultraluminous X-Ray Sources with Intermediate-Mass Black Holes

We have computed models for ultraluminous X-ray sources (ULXs) consisting of a black hole accretor of intermediate mass (IMBH; e.g., ~1000 M☉) and a captured donor star. For each of four different sets of initial donor masses and orbital separations we computed 30,000 binary evolution models using a full Henyey stellar evolution code. To our knowledge, this is the first time that a population of X-ray binaries this large has been carried out with other than approximation methods, and it serves to demonstrate the feasibility of this approach to large-scale population studies of mass transfer binaries. In the present study, we find that in order to have a plausible efficiency for producing active ULX systems with IMBHs having luminosities 1040 ergs s-1, there are two basic requirements for the capture of companion/donor stars. First, the donor stars should be massive, i.e., 8 M☉. Second, the initial orbital separations after circularization should be close, i.e., 6-30 times the radius of the donor star when on the main sequence. Even under these optimistic conditions, we show that the production rate of IMBH-ULX systems may fall short of the observed values by factors of 10-100.

Upper limit for circumstellar gas around the type Ia SN 2000cx

Context. The nature of the companion stars in type Ia Supernova (SNe) progenitor systems remains unclear. One possible way to discriminate between different scenarios is the presence (or absence) of circumstellar material, the left overs from the progenitor evolution that may be revealed by their interaction with the SN. nAims. A new method to probe the circumstellar environment has been exploited for the normal type Ia SN 2006X, leading for the first time to the direct detection of material which escaped the progenitor system. In this paper we apply the same analysis to the peculiar type Ia SN 2000cx, with the aim of constraining the properties of its progenitor system. nMethods. Using multi-epoch, high-resolution spectroscopy we have studied the spectral region where narrow, time-variable Na ID absorption features are expected in case circumstellar material is present along the line of sight. nResults. No Na ID absorption is detected in the rest-frame of the host galaxy to a level of a few mA, setting a stringent upper limit to the column density of the absorbing material (N(NaI) 2 ≤ 10^(10) cm^(-2)). nConclusions. In this respect the peculiar type Ia SN 2000cx is different from the normal Ia SN 2006X. Whether this is to be attributed to a different progenitor system, to viewing-angle effects or to a low metallicity remains to be clarified.

On the formation of single and binary helium-rich subdwarf O stars

We propose a formation channel for the previously unexplained helium-rich subdwarf O (He-rich sdO) stars in which post-subdwarf B (sdB) stars (i.e. hybrid COHe white dwarfs) reignite helium burning in a shell after gaining matter from their helium white-dwarf (WD) companions. Such short-period binaries containing post-sdB WDs and helium WDs are predicted by one of the major binary formation channels for sdB stars. In the majority of cases, mass transfer is expected to lead to a dynamically unstable merger event, leaving a single-star remnant. Calculations of the evolution of these stars show that their properties are consistent with the observed He-rich sdO stars. The luminosity of these stars is about an order of magnitude higher than that of canonical sdB stars. We also suggest that binary systems such as PG 1544+488 (Ahmad et al. 2004) and HE 0301-3039 (Lisker et al. 2004), which each contain two hot subdwarfs, could be the outcome of a double-core common-envelope phase. Since this favours intermediate-mass progenitors, this may also explain why the subdwarfs in these systems are He-rich.

Eclipsing binaries in extrasolar planet transit surveys: the case of SuperWASP

Extrasolar planet transit surveys will also detect eclipsing binaries. Using a comprehensive binary population synthesis scheme, we investigate the statistical properties of a sample of eclipsing binaries that is detectable by an idealized extrasolar planet transit survey with specifications broadly similar to those of the Wide Angle Search for Planets (SuperWASP) project. nIn this idealized survey, the total number of detectable single stars in the Galactic disc is of the order of 106-107, while, for a flat initial mass ratio distribution, the total number of detectable eclipsing binaries is of the order of 104-105. The majority of the population of detectable single stars is made up of main-sequence stars (~60 per cent), horizontal-branch stars (~20 per cent), and giant-branch stars (~10 per cent). The largest contributions to the population of detectable eclipsing binaries stem from detached double main-sequence star binaries (~60 per cent), detached giant-branch main-sequence star binaries (~20 per cent), and detached horizontal-branch main-sequence star binaries (~10 per cent). White dwarf main-sequence star binaries make up approximately 0.3 per cent of the sample. nThe ratio of the number of eclipsing binaries to the number of single stars detectable by the idealized SuperWASP survey varies by less than a factor of 2.5 across the sky, and decreases with increasing Galactic latitude. It is found to be largest in the direction of the Galactic longitude l=-7 b 5 and the Galactic latitude b=-22 b 5. nWe also show that the fractions of systems in different subgroups of eclipsing binaries are sensitive to the adopted initial mass ratio or initial secondary mass distribution, which is one of the poorest constrained input parameters in present-day binary population synthesis calculations. This suggests that once statistically meaningful results from transit surveys are available, they will be able to significantly improve the predictive power of population synthesis studies of interacting binaries and related objects.