Ph. Podsiadlowski

Presupernova evolution in massive interacting binaries

The way in which binary interaction affects the presupernova evolution of massive close binaries and the resulting supernova explosions is investigated systematically by means of a Henyey-type stellar evolution code that was modified to allow its application to binary stellar evolution calculations. The code makes it possible to trace the effects of mass and angular momentum loss from the binary, as well as mass transfer within the binary system. It is found that a large number of binary scenarios can be distinguished, depending on the type of binary interaction and the evolutionary stage of the supernova progenitor at the time of the interaction. Monte Carlo simulations are performed to estimate the frequencies of the occurrence of various scenarios. It is found that, because of a previous binary interaction, 15-30 percent of all massive stars (with initial masses greater than about 8 solar masses) become helium stars, and another 5 percent of all massive stars end their lives as blue supergiants rather than as red supergiants.

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.

Common envelope evolution: where we stand and how we can move forward

This work aims to present our current best physical understanding of common-envelope evolution (CEE). We highlight areas of consensus and disagreement, and stress ideas which should point the way forward for progress in this important but long-standing and largely unconquered problem. Unusually for CEE-related work, we mostly try to avoid relying on results from population synthesis or observations, in order to avoid potentially being misled by previous misunderstandings. As far as possible we debate all the relevant issues starting from physics alone, all the way from the evolution of the binary system immediately before CEE begins to the processes which might occur just after the ejection of the envelope. In particular, we include extensive discussion about the energy sources and sinks operating in CEE, and hence examine the foundations of the standard energy formalism. Special attention is also given to comparing the results of hydrodynamic simulations from different groups and to discussing the potential effect of initial conditions on the differences in the outcomes. We compare current numerical techniques for the problem of CEE and also whether more appropriate tools could and should be produced (including new formulations of computational hydrodynamics, and attempts to include 3D processes within 1D codes). Finally we explore new ways to link CEE with observations. We compare previous simulations of CEE to the recent outburst from V1309 Sco, and discuss to what extent post-common-envelope binaries and nebulae can provide information, e.g. from binary eccentricities, which is not currently being fully exploited.

A PHOTOMETRIC REDSHIFT OF z ∼ 9.4 FOR GRB 090429B

Gamma-ray bursts (GRBs) serve as powerful probes of the early universe, with their luminous afterglows revealing the locations and physical properties of star-forming galaxies at the highest redshifts, and potentially locating first-generation (Population III) stars. Since GRB afterglows have intrinsically very simple spectra, they allow robust redshifts from low signal-to-noise spectroscopy, or photometry. Here we present a photometric redshift of z ~ 9.4 for the Swift detected GRB 090429B based on deep observations with Gemini-North, the Very Large Telescope, and the GRB Optical and Near-infrared Detector. Assuming a Small Magellanic Cloud dust law (which has been found in a majority of GRB sight lines), the 90% likelihood range for the redshift is 9.06 7. The non-detection of the host galaxy to deep limits (Y(AB) ~ 28, which would correspond roughly to 0.001L* at z = 1) in our late-time optical and infrared observations with the Hubble Space Telescope strongly supports the extreme-redshift origin of GRB 090429B, since we would expect to have detected any low-z galaxy, even if it were highly dusty. Finally, the energetics of GRB 090429B are comparable to those of other GRBs and suggest that its progenitor is not greatly different from those of lower redshift bursts.

The Rates of Hypernovae and Gamma-Ray Bursts: Implications for Their Progenitors

A critical comparison of estimates for the rates of hypernovae (HNe) and gamma-ray bursts (GRBs) is presented. Within the substantial uncertainties, the estimates are shown to be quite comparable and give a galactic rate of 10-6 to 10-5 yr-1 for both events. These rates are several orders of magnitude lower than the rate of core-collapse supernovae, suggesting that the evolution leading to an HN/GRB requires special circumstances, very likely due to binary interactions. Various possible binary channels are discussed, and it is shown that these are generally compatible with the inferred rates.

A New Population of High-Redshift Short-Duration Gamma-Ray Bursts

The redshift distribution of the short-duration gamma-ray bursts (GRBs) is a crucial, but currently fragmentary, cluetothenatureoftheirprogenitors.HerewepresentopticalobservationsofnineshortGRBsobtainedwithGemini, Magellan, and the Hubble Space Telescope. We detect the afterglows and host galaxies of two short bursts, and host galaxiesfortwoadditionalburstswithknownopticalafterglowpositions,andfivewithX-raypositions(P6 00 radius).In eightoftheninecaseswefindthatthemostprobablehostgalaxiesarefaint,R � 23 26:5mag,andarethereforestarkly different from the first few short GRB hosts with R � 17 22 mag and z P0:5. Indeed, we measure spectroscopic redshifts ofz � 0:4 1:1for the four brightest hosts. A comparison to largefield galaxy samples, as well as the hosts of longGRBsandpreviousshortGRBs,indicatesthatthefainterhostslikelyresideatz k1. Ourmostconservativelimit is that at least half of the five hosts without a known redshift reside at z > 0:7 (97% confidence level), suggesting that about 1 to 2 of all short GRBs originate at higher redshifts than previously determined. This has two important implications: (1) we constrain the acceptable age distributions to a wide lognormal (� k1) with � � � 4 8G yr, or to a powerlaw,P(� ) / � n ,with � 1Pn P0;and(2)theinferredisotropicenergies,E�; iso � 1050 10 52 ergs,aresignificantly larger than � 10 48 ‐10 49 ergs for the low-redshift, short GRBs, indicating a large spread in energy release or jet opening angles. Finally, we reiterate the importance of short GRBs as potential gravitational-wave sources and find a conservative detection rate with the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) of � 2‐6 yr � 1 .

High-Velocity Features: A Ubiquitous Property of Type Ia Supernovae

Evidence of high-velocity features (HVFs) such as those seen in the near-maximum spectra of some Type Ia supernovae (SNe Ia; e.g., SN 2000cx) has been searched for in the available SN Ia spectra observed earlier than 1 week before B maximum. Recent observational efforts have doubled the number of SNe Ia with very early spectra. Remarkably, all SNe Ia with early data (seven in our Research Training Network sample and 10 from other programs) show signs of such features, to a greater or lesser degree, in Ca II IR and some also in the Si II λ6355 line. HVFs may be interpreted as abundance or density enhancements. Abundance enhancements would imply an outer region dominated by Si and Ca. Density enhancements may result from the sweeping up of circumstellar material (CSM) by the highest velocity SN ejecta. In this scenario, the high incidence of HVFs suggests that a thick disk and/or a high-density companion wind surrounds the exploding white dwarf, as may be the case in single degenerate systems. Large-scale angular fluctuations in the radial density and abundance distribution may also be responsible: this could originate in the explosion and would suggest a deflagration as the more likely explosion mechanism. CSM interaction and surface fluctuations may coexist, possibly leaving different signatures on the spectrum. In some SNe, the HVFs are narrowly confined in velocity, suggesting the ejection of blobs of burned material.

SUBARU HIGH-RESOLUTION SPECTROSCOPY OF STAR G IN THE TYCHO SUPERNOVA REMNANT*

It is widely believed that Type Ia supernovae (SN Ia) originate in binary systems where a white dwarf accretes material from a companion star until its mass approaches the Chandrasekhar mass and carbon is ignited in the white dwarf’s core. This scenario predicts that the donor star should survive the supernova explosion, providing an opportunity to understand the progenitors of Type Ia supernovae.In this paper we argue that rotationis a generic signature expected of most non-giant donor stars that is easily measurable. (Ruiz-Lapuente et al. 2004) examined stars in the center of the remnant of SN 1572 (Tycho’s SN) and showed evidence that a subgiant star (Star-G by their naming convention) near the remnant’s centre was the system’s donor star. We present high-resolution (R ≃ 40000) spectra taken with the High Dispersion Spectrograph on Subaru of this candidate donor star and measure the star’s radial velocity as 79 ± 2 kms −1 with respect to the LSR and put an upper limit on the star’s rotation of 7.5 kms −1 . In addition, by comparing images that were taken in 1970 and 2004, we measure the proper motion of Star-G to be µl = −1.6 ± 2.1masyr −1 and µb = −2.7 ± 1.6masyr −1 . We demonstrate that all of the measured properties of Star-G presented in this paper are consistent with those of a star in the direction of Tycho’s SN that is not associated with the supernova event. However, we discuss an unlikely, but still viable scenario for Star-G to be the donor star, and suggest further observations that might be able to confirm or refute it. Subject headings: astrometry – techniques: spectroscopic – binaries: close – supernovae: general – supernova remnants

Stellar-mass black hole binaries as ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) with Lx>10^{39} ergs/s have been discovered in great numbers in external galaxies with ROSAT, Chandra, and XMM. The central question regarding this important class of sources is whether they represent an extension in the luminosity function of binary X-ray sources containing neutron stars and stellar-mass black holes (BHs), or a new class of objects, e.g., systems containing intermediate-mass black holes (100-1000 Msun). We have carried out a theoretical study to test whether a large fraction of the ULXs, especially those in galaxies with recent star formation activity, can be explained with binary systems containing stellar-mass black holes. To this end, we have applied a unique set of binary evolution models for black-hole X-ray binaries, coupled to a binary population synthesis code, to model the ULXs observed in external galaxies. We find that for donor stars with initial masses>10 Msun the mass transfer driven by the normal nuclear evolution of the donor star is sufficient to potentially power most ULXs. This is the case during core hydrogen burning and, to an even more pronounced degree, while the donor star ascends the giant branch, though the latter phases lasts only ~5% of the main sequence phase. We show that with only a modest violation of the Eddington limit, e.g., a factor of ~10, both the numbers and properties of the majority of the ULXs can be reproduced. One of our conclusions is that if stellar-mass black-hole binaries account for a significant fraction of ULXs in star-forming galaxies, then the rate of formation of such systems is ~3 x 10^{-7} per year normalized to a core-collapse supernova rate of 0.01 per year.

The double pulsar J0737–3039: testing the neutron star equation of state

The double pulsar J0737--3039 has become an important astrophysical laboratory for testing fundamental physics. Here we demonstrate that the low measured mass of Pulsar B can be used to constrain the equation of state of neutron star matter {\em under the assumption} that it formed in an electron-capture supernova. We show that the observed orbital parameters as well as the likely evolutionary history of the system support such a hypothesis and discuss future refinements that will improve the constraints this test may provide.