Masses of double neutron star mergers

Aims. The mass discrepancy between the observed population of double neutron star binaries by radio pulsar observations and gravitational-wave observation requires an explanation. Methods. Binary population synthesis calculations are performed, and their results are compared with the radio and the gravitational-wave observations simultaneously. Results. Simulations of binary evolution are used to link different observations of double neutron star binaries with each other. The progenitor of GW190425 is investigated in more detail. A distribution of masses and merger times of the possible progenitors is presented. Conclusions. A mass discrepancy between the radio pulsars in the Milky Way with another neutron star companion and the inferred masses from gravitational-wave observations of those kind of merging systems is naturally found in binary evolution.Comment: 5+2 pages, 3 figures, 3 tables, submitted to A&

Binary star population synthesis : Progenitors of gravitational wave driven mergers

The vast majority of all massive stars are born in binaries. The binarity of a star may influence its life drastically via a large variety of interactions with its companion star. Several astrophysical events, like the recently detected gravitational wave mergers, have a binary origin. To investigate the possible paths of binary evolution, a new stellar grid-based binary populations synthesis code ComBinE is developed. During binary evolution, many uncertain phases, including one or more mass-transfer phases, a common-envelope phase and kicks during a supernova explosion, have to be considered. The least understood among them is the common-envelope phase. A detailed analysis of the consequences of the conversion of available energy reservoirs into the ejection of the gas of the common envelope is performed at two metallicities, that of the Milky Way (Z = 0.0088) and that of the dwarf galaxy IZwicky18 (Z = 0.0002). The most crucial aspect is the bifurcation point which separates the remaining core from the lost material. ComBinE is applied to investigate a variety of parameters of the binary evolution phases and their effects on observable stages during the evolution leading to a final gravitational wave driven merger. The simulations performed with ComBinE are able to reproduce the observed Galactic double neutron star population with respect to their orbital parameters and, to some extend, their measured mass distributions. In addition, the simulations can reproduce the progenitor systems of all published merger events of double black hole mergers and the double neutron star merger, GW170817, for appropriate metallicities. Merger-rate densities in the local Universe are obtained from the simulations. Statistical uncertainties are discusses as well as predictions made for future observations and LIGO-Virgo detections. Finally, using ComBinE, the nature of the unseen companion star in the radio pulsar binary PSR J1755−2550 is probed to narrow the search window for the companion

The interaction of core-collapse supernova ejecta with a companion star

The progenitors of many CCSNe are expected to be in binary systems. After the SN explosion, the companion may suffer from mass stripping and be shock heated as a result of the impact of the SN ejecta. If the binary system is disrupted, the companion is ejected as a runaway and hypervelocity star. By performing a series of 3D hydrodynamical simulations of the collision of SN ejecta with the companion star, we investigate how CCSN explosions affect their companions. We use the BEC code to construct the detailed companion structure at the time of SN explosion. The impact of the SN blast wave on the companion is followed by means of 3D SPH simulations using the Stellar GADGET code. For main-sequence (MS) companions, we find that the amount of removed mass, impact velocity, and chemical contamination of the companion that results from the impact of the SN ejecta, strongly increases with decreasing binary separation and increasing explosion energy. Their relationship can be approximately fitted by power laws, which is consistent with the results obtained from impact simulations of SNe~Ia. However, we find that the impact velocity is sensitive to the momentum profile of the outer SN ejecta and, in fact, may decrease with increasing ejecta mass, depending on the modeling of the ejecta. Because most companions to Ib/c CCSNe are in their MS phase at the moment of the explosion, combined with the strongly decaying impact effects with increasing binary separation, we argue that the majority of these SNe lead to inefficient mass stripping and shock heating of the companion star following the impact of the ejecta. Our simulations show that the impact effects of Ib/c SN ejecta on the structure of MS companions, and thus their long-term post-explosion evolution, is in general not dramatic. We find that at most 10% of their mass is lost, and their resulting impact velocities are less than 100 km/s.Comment: Accepted for publication in Astronomy and Astrophysics, some minor typographical errors are fixed, the affiliation of second author is correcte

SCATTER: A New Common Envelope Formalism

One of the most mysterious astrophysical states is the common envelope (CE) phase of binary evolution, in which two stars are enshrouded by the envelope shed by one of them. Interactions between the stars and the envelope shrinks the orbit. The CE can lead to mergers or to a subsequent phase of interactions. Mergers may involve any combination of two compact objects and/or stars. Some involving white dwarfs, may produce Type Ia supernovae, while merging neutron stars may yield gamma-ray bursts, and merging compact objects of all kinds produce gravitational radiation. Since CEs can arise from a variety of different initial conditions, and due to the complexity of the processes involved, it is difficult to predict their end states. When many systems are being considered, as in population synthesis calculations, conservation principles are generally employed. Here we use angular momentum in a new way to derive a simple expression for the final orbital separation. This method provides advantages for the study of binaries and is particularly well-suited to higher order multiples, now considered to be important in the genesis of potential mergers. Here we focus on CEs in binaries, and the follow-up paper extends our formalism to multiple star systems within which a CE occurs.Comment: Accepted for publication in the Astrophysical Journal, 23 pages, 13 figure

Progenitors of gravitational wave mergers: Binary evolution with the stellar grid-based code ComBinE

The first gravitational wave detections of mergers between black holes and neutron stars represent a remarkable new regime of high-energy transient astrophysics. The signals observed with LIGO-Virgo detectors come from mergers of extreme physical objects which are the end products of stellar evolution in close binary systems. To better understand their origin and merger rates, we have performed binary population syntheses at different metallicities using the new grid-based binary population synthesis code ComBinE. Starting from newborn pairs of stars, we follow their evolution including mass loss, mass transfer and accretion, common envelopes and supernova explosions. We apply the binding energies of common envelopes based on dense grids of detailed stellar structure models, make use of improved investigations of the subsequent Case BB Roche-lobe overflow and scale supernova kicks according to the stripping of the exploding stars. We demonstrate that all the double black hole mergers, GW150914, LVT151012, GW151226, GW170104, GW170608 and GW170814, as well as the double neutron star merger GW170817, are accounted for in our models in the appropriate metallicity regime. Our binary interaction parameters are calibrated to match the accurately determined properties of Galactic double neutron star systems, and we discuss their masses and types of supernova origin. Using our default values for the input physics parameters, we find a double neutron star merger rate of about 3.0 Myr^-1 for Milky-Way equivalent galaxies. Our upper limit to the merger-rate density of double neutron stars is R=400 yr^-1 Gpc^-3 in the local Universe (z=0).Comment: 36 pages, 26 figures, 8 tables, plus 9 pages appendix. Accepted 2018 August 6 by MNRAS after revision according to referee report (in particular, including further discussions on the progenitor binary of GW170817

Progenitors of gravitational wave mergers: Binary evolution with the stellar grid-based code ComBinE

The first gravitational wave detections of mergers between black holes and neutron stars represent a remarkable new regime of high-energy transient astrophysics. The signals observed with LIGO-Virgo detectors come from mergers of extreme physical objects which are the end products of stellar evolution in close binary systems. To better understand their origin and merger rates, we have performed binary population syntheses at different metallicities using the new grid-based binary population synthesis code ComBinE. Starting from newborn pairs of stars, we follow their evolution including mass loss, mass transfer and accretion, common envelopes and supernova explosions. We apply the binding energies of common envelopes based on dense grids of detailed stellar structure models, make use of improved investigations of the subsequent Case BB Roche-lobe overflow and scale supernova kicks according to the stripping of the exploding stars. We demonstrate that all the double black hole mergers, GW150914, LVT151012, GW151226, GW170104, GW170608 and GW170814, as well as the double neutron star merger GW170817, are accounted for in our models in the appropriate metallicity regime. Our binary interaction parameters are calibrated to match the accurately determined properties of Galactic double neutron star systems, and we discuss their masses and types of supernova origin. Using our default values for the input physics parameters, we find a double neutron star merger rate of about 3.0 Myr-1 for Milky-Way equivalent galaxies. Our upper limit to the merger-rate density of double neutron stars is R≃400 yr-1 Gpc-3 in the local Universe (z=0)

SCATTER: A New Common Envelope Formalism

© 2023. The Author(s). Published by the American Astronomical Society. https://creativecommons.org/licenses/by/4.0/One of the most mysterious astrophysical states is the common envelope (CE) phase of binary evolution, in which two stars are enshrouded by the envelope shed by one of them. Interactions between the stars and the envelope shrinks the orbit. The CE can lead to mergers or to a subsequent phase of interactions. Mergers may involve any combination of two compact objects and/or stars. Some involving white dwarfs may produce Type Ia supernovae, while merging neutron stars may yield gamma-ray bursts, and merging compact objects of all kinds produce gravitational radiation. Since CEs can arise from a variety of different initial conditions, and due to the complexity of the processes involved, it is difficult to predict their end states. When many systems are being considered, as in population synthesis calculations, conservation principles are generally employed. Here we use angular momentum in a new way to derive a simple expression for the final orbital separation. This method provides advantages for the study of binaries and is particularly well suited to higher-order multiples, now considered to be important in the genesis of potential mergers. Here we focus on CEs in binaries, and the follow-up paper extends our formalism to multiple-star systems within which a CE occurs.Peer reviewe

Can Neutron Star Mergers Alone Explain the r-process Enrichment of the Milky Way?

© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article under the terms of the Creative Commons Attribution License, https://creativecommons.org/licenses/by/4.0/Comparing Galactic chemical evolution models to the observed elemental abundances in the Milky Way, we show that neutron star mergers can be a leading r-process site only if at low metallicities such mergers have very short delay times and significant ejecta masses that are facilitated by the masses of the compact objects. Namely, black hole–neutron star mergers, depending on the black hole spins, can play an important role in the early chemical enrichment of the Milky Way. We also show that none of the binary population synthesis models used in this Letter, i.e., COMPAS, StarTrack, Brussels, ComBinE, and BPASS, can currently reproduce the elemental abundance observations. The predictions are problematic not only for neutron star mergers, but also for Type Ia supernovae, which may point to shortcomings in binary evolution models.Peer reviewe

"What's (the) Matter?", A Show on Elementary Particle Physics with 28 Demonstration Experiments

We present the screenplay of a physics show on particle physics, by the Physikshow of Bonn University. The show is addressed at non-physicists aged 14+ and communicates basic concepts of elementary particle physics including the discovery of the Higgs boson in an entertaining fashion. It is also demonstrates a successful outreach activity heavily relying on the university physics students. This paper is addressed at anybody interested in particle physics and/or show physics. This paper is also addressed at fellow physicists working in outreach, maybe the experiments and our choice of simple explanations will be helpful. Furthermore, we are very interested in related activities elsewhere, in particular also demonstration experiments relevant to particle physics, as often little of this work is published. Our show involves 28 live demonstration experiments. These are presented in an extensive appendix, including photos and technical details. The show is set up as a quest, where 2 students from Bonn with the aid of a caretaker travel back in time to understand the fundamental nature of matter. They visit Rutherford and Geiger in Manchester around 1911, who recount their famous experiment on the nucleus and show how particle detectors work. They travel forward in time to meet Lawrence at Berkeley around 1950, teaching them about the how and why of accelerators. Next, they visit Wu at DESY, Hamburg, around 1980, who explains the strong force. They end up in the LHC tunnel at CERN, Geneva, Switzerland in 2012. Two experimentalists tell them about colliders and our heroes watch live as the Higgs boson is produced and decays. The show was presented in English at Oxford University and University College London, as well as Padua University and ICTP Trieste. It was 1st performed in German at the Deutsche Museum, Bonn (5/'14). The show has eleven speaking parts and involves in total 20 people.Comment: 113 pages, 88 figures. An up to date version of the paper with high resolution pictures can be found at http://www.th.physik.uni-bonn.de/People/dreiner/Downloads/. In v2 the acknowledgements and a citation are correcte