Ultra-stripped supernovae and double neutron star systems

The evolution of close-orbit progenitor binaries of double neutron star (DNS) systems leads to supernova (SN) explosions of ultra-stripped stars. The amount of SN ejecta mass is very limited from such, more or less, naked metal cores with envelope masses of only 0.01-0.2 Msun. The combination of little SN ejecta mass and the associated possibility of small NS kicks is quite important for the characteristics of the resulting DNS systems left behind. Here, we discuss theoretical predictions for DNS systems, based on Case BB Roche-lobe overflow prior to ultra-stripped SNe, and briefly compare with observations.Comment: 6 pages, 3 figures, BN3 talk presented at the Marcel Grossmann Meeting (MG14), Rome, July 201

Five and a half roads to form a millisecond pulsar

In this review I discuss the characteristics and the formation of all classes of millisecond pulsars (MSPs). The main focus is on the stellar astrophysics of X-ray binaries leading to the production of fully recycled MSPs with white dwarf (WD) or substellar semi-degenerate companions. Depending on the nature of the companion star MSPs are believed to form from either low-mass X-ray binaries (LMXBs) or intermediate-mass X-ray binaries (IMXBs). For each of these two classes of X-ray binaries the evolutionary status of the donor star -- or equivalently, the orbital period -- at the onset of the Roche-lobe overflow (RLO) is the determining factor for the outcome of the mass-transfer phase and thus the nature of the MSP formed. Furthermore, the formation of binary MSPs is discussed in context of the (P,P_dot)-diagram, as well as new interpretations of the Corbet diagram. Finally, I present new models of Case A RLO of IMXBs in order to reproduce the two solar mass pulsar PSR J1614-2230.Comment: 17 pages, 9 figures (color). Invited review at ESO workshop "Evolution of Compact Binaries", March 2011, Vina del Mar (Chile), ASP Conf. Series. Fig.7 is available in high resolution on reques

Recycled Pulsars: Spins, Masses and Ages

Recycled pulsars are mainly characterized by their spin periods, B-fields and masses. All these quantities are affected by previous interactions with a companion star in a binary system. Therefore, we can use these quantities as fossil records and learn about binary evolution. Here, I briefly review the distribution of these observed quantities and summarize our current understanding of the pulsar recycling process.Comment: Brief summary of invited review talk @ MODEST-16. 4 pages, 3 figures. To appear in: "Cosmic-Lab: Star Clusters as Cosmic Laboratories for Astrophysics, Dynamics and Fundamental Physics", F.R Ferraro & B. Lanzoni eds, Mem. SAIt, Vol 87, n.

Formation of the binary pulsars PSR B2303+46 and PSR J1141-6545 - young neutron stars with old white dwarf companions

We have investigated the formation of the binary radio pulsars PSR B2303+46 and PSR J1141-6545 via Monte Carlo simulations of a large number of interacting stars in binary systems. PSR B2303+46 has recently been shown (van Kerkwijk & Kulkarni 1999) to be the first neutron star - white dwarf binary system observed, in which the neutron star was born after the formation of the white dwarf. We discuss the formation process for such a system and are able to put constraints on the parameters of the initial ZAMS binary. We present statistical evidence in favor of a white dwarf companion to the binary pulsar PSR J1141-6545, just recently discovered in the Parkes Multibeam Survey. If this is confirmed by observations this system will be the second one known in which the neutron star was born after its white dwarf companion. We also predict a minimum space velocity of 150 km/s for PSR J1141-6545, and show it must have experienced an asymmetric SN in order to explain its low eccentricity. Finally, we estimate the birthrate of these systems relative to other binary pulsar systems and present the expected distribution of their orbital periods, eccentricities and velocities.Comment: 9 pages, 4 figures, 2 tables, some revisions, accepted for publication in A&A Main Journa

Maximum speed of hypervelocity stars ejected from binaries

The recent detection of hypervelocity stars (HVSs) as late-type B-stars and HVS candidate G/K-dwarfs raises the important question of their origin. In this Letter, we investigate the maximum possible velocities of such HVSs if they are produced from binaries which are disrupted via an asymmetric supernova explosion. We find that HVSs up to ~770 and ~1280 km/s are possible in the Galactic rest frame from this scenario for these two subclasses of HVSs, respectively. We conclude that whereas a binary origin cannot easily explain all of the observed velocities of B-type HVSs (in agreement with their proposed central massive black hole origin) it can indeed account for the far majority (if not all) of the recently detected G/K-dwarf HVS candidates.Comment: 6 pages, 6 figures, including appendix, in press, MNRAS Letters (Updated and a comment added on the spin axis of SN-induced HVSs

The formation of low-mass helium white dwarfs orbiting pulsars: Evolution of low-mass X-ray binaries below the bifurcation period

Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs) which have been spun up to high rotation rates via accretion of matter from a companion star in a low-mass X-ray binary (LMXB). However, many details of this recycling scenario remain to be understood. Here we investigate binary evolution in close LMXBs to study the formation of radio MSPs with low-mass helium white dwarf companions (He WDs) in tight binaries with orbital periods P_orb = 2-9 hr. In particular, we examine: i) if such observed systems can be reproduced from theoretical modelling using standard prescriptions of orbital angular momentum losses (i.e. with respect to the nature and the strength of magnetic braking), ii) if our computations of the Roche-lobe detachments can match the observed orbital periods, and iii) if the correlation between WD mass and orbital period (M_WD, P_orb) is valid for systems with P_orb < 2 days. Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in ~ 400 close LMXB systems with different initial values of donor star mass, NS mass, orbital period and the so-called gamma-index of magnetic braking. Subsequently, we followed the orbital and the interior evolution of the detached low-mass (proto) He WDs, including stages with residual shell hydrogen burning. We find that a severe fine-tuning is necessary to reproduce the observed MSPs in tight binaries with He WD companions of mass < 0.20 M_sun, which suggests that something needs to be modified or is missing in the standard input physics of LMXB modelling. We demonstrate that the theoretically calculated (M_WD, P_orb)-relation is in general also valid for systems with P_orb < 2 days, although with a large scatter in He WD masses between 0.15-0.20 M_sun. The results of the thermal evolution of the (proto) He WDs are reported in a follow-up paper (Paper II).Comment: 14 pages, 13 figures, 1 table, A&A, accepte

The case of PSR J1911-5958A in the outskirts of NGC 6752: signature of a black hole binary in the cluster core?

We have investigated different scenarios for the origin of the binary millisecond pulsar PSR J1911-5958A in NGC 6752, the most distant pulsar discovered from the core of a globular cluster to date. The hypothesis that it results from a truly primordial binary born in the halo calls for accretion-induced collapse and negligible recoil speed at the moment of neutron star formation. Scattering or exchange interactions off cluster stars are not consistent with both the observed orbital period and its offset position. We show that a binary system of two black holes with (unequal) masses in the range of 3-100 solar masses can live in NGC 6752 until present time and can have propelled PSR J1911-5958A into an eccentric peripheral orbit during the last ~1 Gyr.Comment: Accepted by ApJ Letter. 5 pages, 1 figure, 1 tabl

Formation of the Galactic Millisecond Pulsar Triple System PSR J0337+1715 - a Neutron Star with Two Orbiting White Dwarfs

The millisecond pulsar in a triple system (PSR J0337+1715, recently discovered by Ransom et al.) is an unusual neutron star with two orbiting white dwarfs. The existence of such a system in the Galactic field poses new challenges to stellar astrophysics for understanding evolution, interactions and mass-transfer in close multiple stellar systems. In addition, this system provides the first precise confirmation for a very wide-orbit system of the white dwarf mass-orbital period relation. Here we present a self-consistent, semi-analytical solution to the formation of PSR J0337+1715. Our model constrains the peculiar velocity of the system to be less than 160 km/s and brings novel insight to, for example, common envelope evolution in a triple system, for which we find evidence for in-spiral of both outer stars. Finally, we briefly discuss our scenario in relation to alternative models.Comment: ApJ Letters, in press (6 pages, 3 figures, 1 table

Progenitors of ultra-stripped supernovae

The explosion of ultra-stripped stars in close binaries may explain new discoveries of weak and fast optical transients. We have demonstrated that helium star companions to neutron stars (NSs) may evolve into naked metal cores as low as ~1.5 Msun, barely above the Chandrasekhar mass limit, by the time they explode. Here we present a new systematic investigation of the progenitor evolution leading to such ultra-stripped supernovae (SNe), in some cases yielding pre-SN envelopes of less than 0.01 Msun. We discuss the nature of these SNe (electron-capture vs iron core-collapse) and their observational light-curve properties. Ultra-stripped SNe are highly relevant for binary pulsars, as well as gravitational wave detection of merging NSs by LIGO/VIRGO, since these events are expected to produce mainly low-kick NSs in the mass range 1.10-1.80 Msun.Comment: 7 pages, 5 figures, NS4 talk presented at the Marcel Grossmann Meeting (MG14), Rome, July 201