## Stellar mergers as the origin of magnetic massive stars

Nature Springer Nature 574 (2019) 211-214

FRN Schneider, ST Ohlmann, P Podsiadlowski, FK Röpke, R Pakmor, V Springel

About ten per cent of 'massive' stars (those of more than 1.5 solar masses) have strong, large-scale surface magnetic fields1-3. It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields4,5, and the predicted fraction of merged massive stars is also about ten per cent6,7. The merger hypothesis is further supported by a lack of magnetic stars in close binaries8,9, which is as expected if mergers produce magnetic stars. Here we report three-dimensional magnetohydrodynamical simulations of the coalescence of two massive stars and follow the evolution of the merged product. Strong magnetic fields are produced in the simulations, and the merged star rejuvenates such that it appears younger and bluer than other coeval stars. This can explain the properties of the magnetic 'blue straggler' star τ Sco in the Upper Scorpius association that has an observationally inferred, apparent age of less than five million years, which is less than half the age of its birth association10. Such massive blue straggler stars seem likely to be progenitors of magnetars, perhaps giving rise to some of the enigmatic fast radio bursts observed11, and their supernovae may be affected by their strong magnetic fields12.

## Origins of Type Ibn SNe 2006jc/2015G in interacting binaries and implications for pre-SN eruptions

Monthly Notices of the Royal Astronomical Society Oxford University Press (2019)

N-C Sun, PA Crowther, P Podsiadlowski, R Hirai

Type Ibn supernovae (SNe Ibn) are intriguing stellar explosions whose spectra exhibit narrow helium lines with little hydrogen. They trace the presence of circumstellar material (CSM) formed via pre-SN eruptions of their stripped-envelope progenitors. Early work has generally assumed that SNe Ibn come from massive Wolf-Rayet (WR) stars via single star evolution. In this paper, we report ultraviolet (UV) and optical observations of two nearby Type Ibn SNe 2006jc and 2015G conducted with the Hubble Space Telescope (HST) at late times. A point source is detected at the position of SN 2006jc, and we confirm the conclusion of Maund et al. that it is the progenitor’s binary companion. Its position on the Hertzsprung-Russell (HR) diagram corresponds to a star that has evolved off the main sequence (MS); further analysis implies a low initial mass for the companion star (M2 ≤ 12.3+2.3−1.5 M⊙) and a secondary-to-primary initial mass ratio very close to unity (q = M2/M1 ∼ 1); the SN progenitor’s hydrogen envelope had been stripped through binary interaction. We do not detect the binary companion of SN 2015G. For both SNe, the surrounding stellar populations have relatively old ages and argue against any massive WR stars as their progenitors. These results suggest that SNe Ibn may have lower-mass origins in interacting binaries. As a result, they also provide evidence that the giant eruptions commonly seen in massive luminous blue variables (LBVs) can also occur in much lower-mass, stripped-envelope stars just before core collapse.

## Fast orbital shrinkage of black hole X-ray binaries driven by circumbinary disks

Astrophysical Journal Letters IOP Publishing 876 (2019) L11

Recently, the black hole X-ray binary (BHXB) Nova Muscae 1991 has been reported to be experiencing an extremely rapid orbital decay. So far, three BHXBs have anomalously high orbital period derivatives, which can not be interpreted by the standard stellar evolution theory. In this work, we investigate whether the resonant interaction between the binary and a surrounding circumbinary (CB) disk could produce the observed orbital period derivatives. Analytical calculations indicate that the observed orbital period derivatives of XTE J1118+480 and A0620-00 can originate from the tidal torque between the binary and a CB disk with a mass of $10^{-9}~\rm M_{\odot}$, which is approximately in agreement with the dust disk mass detected in these two sources. However, Nova Muscae 1991 was probably surrounded by a heavy CB disk with a mass of $10^{-7}~\rm M_{\odot}$. Based on the CB disk model and the anomalous magnetic braking theory, we simulate the evolution of the three BHXBs with intermediate-mass donor stars by using the MESA code. Our simulated results are approximately consistent with the observed donor star masses, orbital periods, and orbital-period derivatives. However, the calculated effective temperatures of the donor stars are higher than indicated by the observed spectral types of two sources.

## Comprehensive Study of Ejecta-companion Interaction for Core-collapse Supernovae in Massive Binaries

ASTROPHYSICAL JOURNAL 864 (2018) ARTN 119

## An excess of massive stars in the local 30 Doradus starburst

Science American Association for the Advancement of Science 359 (2018) 69-71

JM Bestenlehner, S Simón-Díaz, F Tramper, PA Crowther, A de Koter, DM de Mink, PL Dufton, M Garcia, M Gieles, V Kalari, DJ Lennon, J Maíz Apellániz, N Markova, F Schneider, H Sana, CJ Evans, N Castro, L Fossati, V Hénault-Brunet, A Herrero, RG Izzard, F Najarro, P Podsiadlowski, J Puls, WD Taylor

The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analog of large star-formation events in the distant universe. We determined the recent formation history and the initial mass function (IMF) of massive stars in 30 Doradus on the basis of spectroscopic observations of 247 stars more massive than 15 solar masses ([Formula: see text]). The main episode of massive star formation began about 8 million years (My) ago, and the star-formation rate seems to have declined in the last 1 My. The IMF is densely sampled up to 200 [Formula: see text] and contains 32 ± 12% more stars above 30 [Formula: see text] than predicted by a standard Salpeter IMF. In the mass range of 15 to 200 [Formula: see text], the IMF power-law exponent is [Formula: see text], shallower than the Salpeter value of 2.35.

## Do SN 2002cx-like and SN Ia-CSM Objects Share the Same Origin?

ASTROPHYSICAL JOURNAL 861 (2018) ARTN 127

## On the formation history of Galactic double neutron stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 481 (2018) 4009-4029

A Vigna-Gomez, CJ Neijssel, S Stevenson, JW Barrett, K Belczynski, S Justham, SE de Mink, B Mueller, P Podsiadlowski, M Renzo, D Szecsi, I Mandel

## Response to Comment on “An excess of massive stars in the local 30 Doradus starburst”

Science American Association for the Advancement of Science 361 (2018) eaat7032

F Schneider, H Sana, CJ Evans, JM Bestenlehner, N Castro, L Fossati, G Gräfener, N Langer, OH Ramírez-Agudelo, C Sabín-Sanjulián, S Simón-Díaz, F Tramper, PA Crowther, A De Koter, PL Dufton, M Garcia, M Gieles, V Hénault-Brunet, A Herrero, RG Izzard, DJ Lennon, V Kalari, J Maíz Apellániz, N Markova

Farr and Mandel reanalyze our data, finding initial mass function slopes for high-mass stars in 30 Doradus that agree with our results. However, their reanalysis appears to underpredict the observed number of massive stars. Their technique results in more precise slopes than in our work, strengthening our conclusion that there is an excess of massive stars (&gt;30 solar masses) in 30 Doradus.

## A kilonova as the electromagnetic counterpart to a gravitational-wave source.

Nature Nature 551 (2017) 75-79

SJ Smartt, T-W Chen, A Jerkstrand, M Coughlin, E Kankare, C Inserra, M Fraser, K Maguire, KC Chambers, ME Huber, T Krühler, G Leloudas, M Magee, LJ Shingles, KW Smith, J Tonry, A Gal-Yam, R Kotak, C Agliozzo, JP Anderson, JD Lyman, DS Homan, C Barbarino

<p>Gravitational waves were discovered with the detection of binary black hole mergers and they should also be detectable from lower mass neutron star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal called a kilonova. The gravitational wave source GW170817 arose from a binary neutron star merger in the nearby Universe with a relatively well confined sky position and distance estimate6. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC4993, which is spatially coincident with GW170817 and a weak short gamma-ray burst. The transient has physical parameters broadly matching the theoretical predictions of blue kilonovae from neutron star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01M⊙ with an opacity of 𝓀 ≤ 0.5 cm2 g^-1 at a velocity of 0:2 ± 0:1c. The power source is constrained to have a power law slope of β = -1.2+0:3-0:3, consistent with radioactive powering from r-process nuclides. We identify line features in the spectra that are consistent with light r-process elements (90 &lt; A &lt; 140). As it fades, the transient rapidly becomes red, and emission may have contribution by a higher opacity, lanthanide-rich ejecta component. This indicates that neutron star mergers produce gravitational waves, radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.</p>

## Episodic mass ejections from common-envelope objects

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 470 (2017) 1788-1808

M Clayton, P Podsiadlowski, N Ivanova, S Justham

## Multi-messenger observations of a binary neutron star merger

Astrophysical Journal Letters Institute of Physics 848 (2017) L12-

BP Abbott, TD Abbott, R Abbott, RP Fender, KP Mooley, S Sarkar, P Podsiadlowski, AJ Stewart

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼1.7s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg(2) at a luminosity distance of 40+8−8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M⊙. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼40Mpc) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼9 and ∼16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

## He-accreting carbon-oxygen white dwarfs and Type Ia supernovae

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 472 (2017) 1593-1599

B Wang, P Podsiadlowski, Z Han

## Formation of Double Neutron Star Systems

ASTROPHYSICAL JOURNAL 846 (2017) ARTN 170

TM Tauris, M Kramer, PCC Freire, N Wex, H-T Janka, N Langer, P Podsiadlowski, E Bozzo, S Chaty, MU Kruckow, EPJ van den Heuvel, J Antoniadis, RP Breton, DJ Champion

## Subdwarf B stars from the common envelope ejection channel

Astronomy & Astrophysics EDP Sciences 54 (2017) A54-

H Xiong, X Chen, P Podsiadlowski, Z Han

<h4>Context</h4> <p>Subdwarf B stars (sdB) are important to stellar evolutionary theory and asteroseismology, and are crucial to our understanding of the structure and evolution of the Galaxy. From the canonical binary scenario, the majority of sdBs are produced from low-mass stars with degenerate cores where helium is ignited in a way of flashes. Due to numerical difficulties, the models of produced sdBs are generally constructed from more massive stars with non-degenerate cores. This leaves several uncertainties on the exact characteristics of sdB stars.</p> <h4>Aims</h4> <p>The purpose of this paper is to study the characteristics of sdBs produced from the common envelope (CE) ejection channel.</p> <h4>Methods</h4> <p>We use the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), which can resolve flashes during stellar evolution. To mimic the CE ejection process, we firstly evolve a single star to a position near the tip of red giant branch, then artificially remove its envelope with a very high mass loss rate until the star begins to collapse. Finally, we followed the evolution of the remnant until it becomes a helium or a carbon-oxygen white dwarf.</p> <h4>Results</h4> <p>The sdB stars produced from the CE ejection channel appear to form two distinct groups on the effective temperature-gravity diagram. One group, referred as the flash-mixing sdBs, almost has no H-rich envelope and crows at the hottest temperature end of the extremely horizontal branch (EHB), while the other group, called as the canonical sdBs, has significant H-rich envelope and spreads over the whole canonical EHB region. The key factor for the dichotomy of the sdB properties is the development of convection during the first helium flash, i.e. the convection region penetrates into the H-rich envelope for the flash-mixing sdBs but doesnot for the canonical sdBs.</p> <h4>Conclusions</h4> <p>The dichotomy of the sdB properties from the CE ejection channel is intrinsic and caused by the interior structure of the star after the CE ejection. The modelling of the CE ejection process will change the parameter spaces much for the two typical groups of sdB stars. For a given initial stellar mass and a given core mass at the onset of the CE, if the CE ejection stops early, the star has a relatively massive H-rich envelope, resulting in a canonical sdB generally. The fact of only a few short-orbital-period sdB binaries being in the flash-mixing sdB region and the lack of He-rich sdBs in short-orbital-period binaries indicate that the flash mixing is not very often in the products of the CE ejection. A falling back process after the CE ejection, similar to that happened in nova, is an appropriate way of increasing the envelope mass, then prevents the flash mixing.</p>

## Light-curve and spectral properties of ultrastripped core-collapse supernovae leading to binary neutron stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 466 (2017) 2085-2098

TJ Moriya, PA Mazzali, N Tominaga, S Hachinger, SI Blinnikov, TM Tauris, K Takahashi, M Tanaka, N Langer, P Podsiadlowski

## A common-envelope wind model for Type Ia supernovae - I. Binary evolution and birth rate

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 469 (2017) 4763-4787

## Ultra-luminous X-ray sources and neutron-star-black-hole mergers from very massive close binaries at low metallicity

ASTRONOMY & ASTROPHYSICS 604 (2017) ARTN A55

P Marchant, N Langer, P Podsiadlowski, TM Tauris, S de Mink, I Mandel, TJ Moriya

## Binary Evolution and the Progenitor of SN 1987A

ArXiv ArXiv (2017) 1-12

Since the majority of massive stars are members of binary systems, an understanding of the intricacies of binary interactions is essential for understanding the large variety of supernova types and sub-types. I therefore briefly review the basic elements of binary evolution theory and discuss how binary interactions affect the presupernova structure of massive stars and the resulting supernovae. SN 1987A was a highly anomalous supernova, almost certainly because of a previous binary interaction. The most likely scenario at present is that the progenitor was a member of a massive close binary that experienced dynamical mass transfer during its second red-supergiant phase and merged completely with its companion as a consequence. This can naturally explain the three main anomalies of SN 1987A: the blue color of the progenitor, the chemical anomalies and the complex triple-ring nebula.

## Rapid orbital decay in detached binaries: Evidence for circumbinary disks

Astrophysical Journal Letters Institute of Physics 837 (2017) L19-

Some short-period, detached binary systems have recently been reported to experience very rapid orbital decay, much faster than is expected from the angular-momentum loss caused by gravitational radiation alone. As these systems contain fully convective stars, magnetic braking is not believed to be operative, making the large orbital-period derivative puzzling. Here we explore whether a resonant interaction between the binary and a surrounding circumbinary (CB) disk could account for the observed orbital decay. Our calculations indicate that the observed orbital-period derivatives in seven detached binaries can be produced by the resonant interaction between the binary and a CB disk if the latter has a mass in the range of $10^{-4}-10^{-2}~ M_{\odot}$, which is of the same order as the inferred disk mass ($\sim2.4\times 10^{-4}~ M_{\odot}$) in the post-common-envelope binary NN Ser.