Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243

The recently reported observation of VFTS 243 is the first example of a massive black-hole binary system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M_{⊙}) and near-circular orbit (e≈0.02) of VFTS 243 suggest that the progenitor star experienced complete collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence level, the natal kick velocity (mass decrement) is ≲10  km/s (≲1.0M_{⊙}), with a full probability distribution that peaks when ≈0.3M_{⊙} were ejected, presumably in neutrinos, and the black hole experienced a natal kick of 4  km/s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0-0.2%. Such a small neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.

A metal-poor star with abundances from a pair-instability supernova

The most massive and shortest-lived stars dominate the chemical evolution of the pre-galactic era. On the basis of numerical simulations, it has long been speculated that the mass of such first-generation stars was up to several hundred solar masses^1-4. The very massive first-generation stars with a mass range from 140 to 260 solar masses are predicted to enrich the early interstellar medium through pair-instability supernovae (PISNe)⁵. Decades of observational efforts, however, have not been able to uniquely identify the imprints of such very massive stars on the most metal-poor stars in the Milky Way^6,7. Here we report the chemical composition of a very metal-poor (VMP) star with extremely low sodium and cobalt abundances. The sodium with respect to iron in this star is more than two orders of magnitude lower than that of the Sun. This star exhibits very large abundance variance between the odd- and even-charge-number elements, such as sodium/magnesium and cobalt/nickel. Such peculiar odd-even effect, along with deficiencies of sodium and α elements, are consistent with the prediction of primordial pair-instability supernova (PISN) from stars more massive than 140 solar masses. This provides a clear chemical signature indicating the existence of very massive stars in the early universe.

Compact Binary Coalescences: Astrophysical Processes and Lessons Learned

On 11 February 2016, the LIGO and Virgo scientific collaborations announced the first direct detection of gravitational waves, a signal caught by the LIGO interferometers on 14 September 2015, and produced by the coalescence of two stellar-mass black holes. The discovery represented the beginning of an entirely new way to investigate the Universe. The latest gravitational-wave catalog by LIGO, Virgo and KAGRA brings the total number of gravitational-wave events to 90, and the count is expected to significantly increase in the next years, when additional ground-based and space-born interferometers will be operational. From the theoretical point of view, we have only fuzzy ideas about where the detected events came from, and the answers to most of the five Ws and How for the astrophysics of compact binary coalescences are still unknown. In this work, we review our current knowledge and uncertainties on the astrophysical processes behind merging compact-object binaries. Furthermore, we discuss the astrophysical lessons learned through the latest gravitational-wave detections, paying specific attention to the theoretical challenges coming from exceptional events (e.g., GW190521 and GW190814).

Impact of AGB Stars on the Chemical Evolution of Neutron-Capture Elements

In this review, we discuss the impact of s-process nucleosynthesis in asymptotic giant branch stars on the enrichment of heavy elements. We review the main steps made on this subject in the last 40 years and discuss the importance of modelling the evolution of the abundances of such elements in our Milky Way. From the comparison between model results and observations, we can impose strong constraints on stellar nucleosynthesis, as well as on the evolution of the Milky Way.

Star and Black Hole Formation at High Redshift

Evidence for dark matter (DM) was originally discovered in 1933 by Zwicky (Zwicky 1933, 1937), and has defied all explanations since then. The original discovery was based on the motions of galaxies in clusters of galaxies. The MicroWave Back Ground (MWBG) observations by the Planck mission and other satellites give definitive numbers. Galaxy correlations give results down to small galaxies, which match theoretical expectations. Here we focus on a few interesting aspects, that may allow to determine the nature of dark matter: (1) Ultra Faint Dwarf (UFD) galaxies, that represent the oldest galaxies known. UFDs are almost devoid of baryonic matter. (2) Calculations show that there can be super-sonic flow of baryonic matter. It follows that there are ubiquitous shockwaves; commonly oblique they generate vorticity. (3) Early virialized clumps, mini-halos, have a density that is consistent with the density implied by Super Massive Black Holes (SMBHs) today, if we assume that SMBHs grow by merging, akin to the Press & Schechter (1974) picture for galaxies. This implies that the oldest SMBHs observed today give powerful constraints on the very early phases.

The RADIOSTAR Project

Radioactive nuclei are the key to understanding the circumstances of the birth of our Sun because meteoritic analysis has proven that many of them were present at that time. Their origin, however, has been so far elusive. The ERC-CoG-2016 RADIOSTAR project is dedicated to investigating the production of radioactive nuclei by nuclear reactions inside stars, their evolution in the Milky Way Galaxy, and their presence in molecular clouds. So far, we have discovered that: (i) radioactive nuclei produced by slow (107Pd and 182Hf) and rapid (129I and 247Cm) neutron captures originated from stellar sources —asymptotic giant branch (AGB) stars and compact binary mergers, respectively—within the galactic environment that predated the formation of the molecular cloud where the Sun was born; (ii) the time that elapsed from the birth of the cloud to the birth of the Sun was of the order of 107 years, and (iii) the abundances of the very short-lived nuclei 26Al, 36Cl, and 41Ca can be explained by massive star winds in single or binary systems, if these winds directly polluted the early Solar System. Our current and future work, as required to finalise the picture of the origin of radioactive nuclei in the Solar System, involves studying the possible origin of radioactive nuclei in the early Solar System from core-collapse supernovae, investigating the production of 107Pd in massive star winds, modelling the transport and mixing of radioactive nuclei in the galactic and molecular cloud medium, and calculating the galactic chemical evolution of 53Mn and 60Fe and of the p-process isotopes 92Nb and 146Sm.

The Complex Behaviour of s-Process Element Abundances at Young Ages

Open clusters appear as simple objects in many respects, with a high degree of homogeneity in their (initial) chemical composition, and the typical solar-scaled abundance pattern that they exhibit for the majority of the chemical species. The striking singularity is represented by heavy elements produced from the slow process of the neutron-capture reactions. In particular, young open clusters (ages less than a few hundred Myr) give rise to the so-called barium puzzle: that is an extreme enhancement in their [Be/Fe] ratios, up to a factor of four of the solar value, which is not followed by other nearby s-process elements (e.g., lanthanum and cerium). The definite explanation for such a peculiar trend is still wanting, as many different solutions have been envisaged. We review the status of this field and present our new results on young open clusters and the pre-main sequence star RZ Piscium.

Mixing and Magnetic Fields in Asymptotic Giant Branch Stars in the Framework of FRUITY Models

In the last few years, the modeling of asymptotic giant branch (AGB) stars has been much investigated, both focusing on nucleosynthesis and stellar evolution aspects. Recent advances in the input physics required for stellar computations made it possible to construct more accurate evolutionary models, which are an essential tool to interpret the wealth of available observational and nucleosynthetic data. Motivated by such improvements, the FUNS stellar evolutionary code has been updated. Nonetheless, mixing processes occurring in AGB stars’ interiors are currently not well-understood. This is especially true for the physical mechanism leading to the formation of the 13C pocket, the major neutron source in low-mass AGB stars. In this regard, post-processing s-process models assuming that partial mixing of protons is induced by magneto-hydrodynamics processes were shown to reproduce many observations. Such mixing prescriptions have now been implemented in the FUNS code to compute stellar models with fully coupled nucleosynthesis. Here, we review the new generation of FRUITY models that include the effects of mixing triggered by magnetic fields by comparing theoretical findings with observational constraints available either from the isotopic analysis of trace-heavy elements in presolar grains or from carbon AGB stars and Galactic open clusters.

GW190521 from the Merger of Ultradwarf Galaxies

We present an alternative formation scenario for the gravitational wave event GW190521 that can be explained as the merger of central black holes (BHs) from two ultradwarf galaxies of stellar mass ∼10^{5}-10^{6}  M_{⊙}, which had themselves previously undergone a merger. The GW190521 components' masses of 85_{-14}^{+21}  M_{⊙} and 66_{-18}^{+17}  M_{⊙} challenge standard stellar evolution models, as they fall in the so-called mass gap. We demonstrate that the merger history of ultradwarf galaxies at high redshifts (1≲z≲2) matches well the LIGO-Virgo inferred merger rate for BHs within the mass range of the GW190521 components, resulting in a likely time delay of ≲4  Gyr considering the redshift of this event. We further demonstrate that the predicted timescales are consistent with expectations for central BH mergers, although with large uncertainties due to the lack of high-resolution simulations in low-mass dwarf galaxies. Our findings show that this BH production and merging channel is viable and extremely interesting as a new way to explore galaxies' BH seeds and galaxy formation. We recommend this scenario be investigated in detail with simulations and observations.

The origin of the elements: a century of progress

This review assesses the current state of knowledge of how the elements were produced in the Big Bang, in stellar lives and deaths, and by interactions in interstellar gas. We begin with statements of fact and discuss the evidence that convinced astronomers that the Sun is fusing hydrogen, that low-mass stars produce heavy elements through neutron capture, that massive stars can explode as supernovae and that supernovae of all types produce new elements. Nucleosynthesis in the Big Bang, through cosmic ray spallation, and in exploding white dwarfs is only ranked below the above facts in certainty because the evidence, while overwhelming, is so far circumstantial. Next, we highlight the flaws in our current understanding of the predictions for lithium production in the Big Bang and/or its destruction in stars and for the production of the elements with atomic number [Formula: see text]. While the theory that neutron star mergers produce elements through neutron-capture has powerful circumstantial evidence, we are unconvinced that they produce all of the elements past nickel. Also in dispute is the exact mechanism or mechanisms that cause the white dwarfs to explode. It is difficult to determine the origin of rare isotopes because signatures of their production are weak. We are uncertain about the production sites of some lithium and nitrogen isotopes and proton-rich heavy nuclei. Finally, Betelgeuse is probably not the next star to become a supernovae in the Milky Way, in part because Betelgeuse may collapse directly to a black hole instead. The accumulated evidence in this review shows that we understand the major production sites for the elements, but islands of uncertainty in the periodic table exist. Resolving these uncertainties requires in particular understanding explosive events with compact objects and understanding the nature of the first stars and is therefore primed for new discoveries in the next decades. This article is part of the theme issue 'Mendeleev and the periodic table'.

Phosphorus-rich stars with unusual abundances are challenging theoretical predictions

Almost all chemical elements have been made by nucleosynthetic reactions in various kind of stars and have been accumulated along our cosmic history. Among those elements, the origin of phosphorus is of extreme interest because it is known to be essential for life such as we know on Earth. However, current models of (Galactic) chemical evolution under-predict the phosphorus we observe in our Solar System. Here we report the discovery of 15 phosphorus-rich stars with unusual overabundances of O, Mg, Si, Al, and Ce. Phosphorus-rich stars likely inherit their peculiar chemistry from another nearby stellar source but their intriguing chemical abundance pattern challenge the present stellar nucleosynthesis theoretical predictions. Specific effects such as rotation or advanced nucleosynthesis in convective-reactive regions in massive stars represent the most promising alternatives to explain the existence of phosphorus-rich stars. The phosphorus-rich stars progenitors may significantly contribute to the phosphorus present on Earth today.

Current models of Galactic chemical evolution under predict the phosphorus we observe in our Solar System. Here, the authors show the discovery of 15 phosphorus-rich stars with a peculiar abundance pattern that challenges the present stellar nucleosynthesis theoretical predictions, but which could explain the missing source of phosphorus in the Galaxy.