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

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.

Tossing Black Hole Spin Axes

The detection of double black hole (BH+BH) mergers provides a unique possibility to understand their physical properties and origin. To date, the LIGO-Virgo-KAGRA network of high-frequency gravitational wave observatories have announced the detection of more than 85 BH+BH merger events (Abbott et al. 2022a). An important diagnostic feature that can be extracted from the data is the distribution of effective inspiral spins of the BHs. This distribution is in clear tension with theoretical expectations from both an isolated binary star origin, which traditionally predicts close-to aligned BH component spins (Kalogera 2000; Farr et al. 2017), and formation via dynamical interactions in dense stellar environments that predicts a symmetric distribution of effective inspiral spins (Mandel & O'Shaughnessy 2010; Rodriguez et al. 2016b). Here it is demonstrated that isolated binary evolution can convincingly explain the observed data if BHs have their spin axis tossed during their formation process in the core collapse of a massive star, similarly to the process evidently acting in newborn neutron stars. BH formation without spin-axis tossing, however, has difficulties reproducing the observed data - even if alignment of spins prior to the second core collapse is disregarded. Based on simulations with only a minimum of assumptions, constrains from empirical data can be made on the spin magnitudes of the first- and second-born BHs, thereby serving to better understand massive binary star evolution prior to the formation of BHs.Comment: 18 pages, 11 figures, 1 table, ApJ accepted version (minor edits

Comment on "A non-interacting low-mass black hole -- giant star binary system"

Thompson et al. (Reports, 1 November 2019, p. 637, Science) interpreted the unseen companion of the red giant star 2MASS J05215658+4359220 as most likely a black hole. We argue that if the red giant is about one solar mass, its companion can be a close binary consisting of two main-sequence stars. This would explain why no X-ray emission is detected from the system.Comment: 3 pages, Author version of Technical Comment published in Science on 8 May, 202

Novel modelling of ultra-compact X-ray binary evolution - stable mass transfer from white dwarfs to neutron stars

Tight binaries of helium white dwarfs (He WDs) orbiting millisecond pulsars (MSPs) will eventually "merge" due to gravitational damping of the orbit. The outcome has been predicted to be the production of long-lived ultra-compact X-ray binaries (UCXBs), in which the WD transfers material to the accreting neutron star (NS). Here we present complete numerical computations, for the first time, of such stable mass transfer from a He WD to a NS. We have calculated a number of complete binary stellar evolution tracks, starting from pre-LMXB systems, and evolved these to detached MSP+WD systems and further on to UCXBs. The minimum orbital period is found to be as short as 5.6 minutes. We followed the subsequent widening of the systems until the donor stars become planets with a mass of ~0.005 Msun after roughly a Hubble time. Our models are able to explain the properties of observed UCXBs with high helium abundances and we can identify these sources on the ascending or descending branch in a diagram displaying mass-transfer rate vs. orbital period.Comment: 6 pages, 4 figures, MNRAS Letters, in pres

Evolution towards and beyond accretion-induced collapse of massive white dwarfs and formation of millisecond pulsars

Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs), formed via type Ib/c core-collapse supernovae (SNe), which have been spun up to high rotation rates via accretion from a companion star in a low-mass X-ray binary (LMXB). In an alternative formation channel, NSs are produced via the accretion-induced collapse (AIC) of a massive white dwarf (WD) in a close binary. Here we investigate binary evolution leading to AIC and examine if NSs formed in this way can subsequently be recycled to form MSPs and, if so, how they can observationally be distinguished from pulsars formed via the standard core-collapse SN channel in terms of their masses, spins, orbital periods and space velocities. Numerical calculations with a detailed stellar evolution code were used for the first time to study the combined pre- and post-AIC evolution of close binaries. We investigated the mass transfer onto a massive WD in 240 systems with three different types of non-degenerate donor stars: main-sequence stars, red giants, and helium stars. When the WD is able to accrete sufficient mass (depending on the mass-transfer rate and the duration of the accretion phase) we assumed it collapses to form a NS and we studied the dynamical effects of this implosion on the binary orbit. Subsequently, we followed the mass-transfer epoch which resumes once the donor star refills its Roche lobe and calculated the continued LMXB evolution until the end. We demonstrate that the final properties of these MSPs are, in general, remarkably similar to those of MSPs formed via the standard core-collapse SN channel. However, the resultant MSPs created via the AIC channel preferentially form in certain orbital period intervals. Finally, we discuss the link between AIC and young NSs in globular clusters. Our calculations are also applicable to progenitor binaries of SNe Ia under certain conditions. [Abridged]Comment: 26 pages, 20 figures, 2 tables. A few references added. A&A in pres