Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary system phi Persei

Stripped-envelope stars (SESs) form in binary systems after losing mass through Roche-lobe overflow. They bear astrophysical significance as sources of UV and ionizing radiation in older stellar populations and, if sufficiently massive, as stripped supernova progenitors. Binary evolutionary models predict them to be common, but only a handful of subdwarfs (i.e., SESs) with B-type companions are known. This could be the result of observational biases hindering detection, or an incorrect understanding of binary evolution. We reanalyze the well-studied post-interaction binary phi Persei. Recently, new data improved the orbital solution of the system, which contains a ~1.2 Msun SES and a rapidly rotating ~9.6 Msun Be star. We compare with an extensive grid of evolutionary models using a Bayesian approach and find initial masses of the progenitor of 7.2+/-0.4 Msun for the SES and 3.8+/-0.4 Msun for the Be star. The system must have evolved through near-conservative mass transfer. These findings are consistent with earlier studies. The age we obtain, 57+/-9 Myr, is in excellent agreement with the age of the alpha Persei cluster. We note that neither star was initially massive enough to produce a core-collapse supernova, but mass exchange pushed the Be star above the mass threshold. We find that the subdwarf is overluminous for its mass by almost an order of magnitude, compared to the expectations for a helium core burning star. We can only reconcile this if the subdwarf is in a late phase of helium shell burning, which lasts only 2-3% of the total lifetime as a subdwarf. This could imply that up to ~50 less evolved, dimmer subdwarfs exist for each system similar to phi Persei. Our findings can be interpreted as a strong indication that a substantial population of SESs indeed exists, but has so far evaded detection because of observational biases and lack of large-scale systematic searches.Comment: 11 pages, 5 figures, accepted for publication in A&

Constraints on the Binary Companion to the SN Ic 1994I Progenitor

Core-collapse supernovae (SNe), which mark the deaths of massive stars, are among the most powerful explosions in the universe and are responsible, e.g., for a predominant synthesis of chemical elements in their host galaxies. The majority of massive stars are thought to be born in close binary systems. To date, putative binary companions to the progenitors of SNe may have been detected in only two cases, SNe 1993J and 2011dh. We report on the search for a companion of the progenitor of the Type Ic SN 1994I, long considered to have been the result of binary interaction. Twenty years after explosion, we used the Hubble Space Telescope to observe the SN site in the ultraviolet (F275W and F336W bands), resulting in deep upper limits on the expected companion: F275W > 26.1 mag and F336W > 24.7 mag. These allow us to exclude the presence of a main sequence companion with a mass ≳ 10 M_⊙. Through comparison with theoretical simulations of possible progenitor populations, we show that the upper limits to a companion detection exclude interacting binaries with semi-conservative (late Case A or early Case B) mass transfer. These limits tend to favor systems with non-conservative, late Case B mass transfer with intermediate initial orbital periods and mass ratios. The most likely mass range for a putative main sequence companion would be ~5–12 M_⊙, the upper end of which corresponds to the inferred upper detection limit

The χeff−z\chi_\mathrm{eff}-z correlation of field binary black hole mergers and how 3G gravitational-wave detectors can constrain it

Understanding the origin of merging binary black holes is currently one of the most pressing quests in astrophysics. We show that if isolated binary evolution dominates the formation mechanism of merging binary black holes, one should expect a correlation between the effective spin parameter, $\chi_\mathrm{eff}$, and the redshift of the merger, $z$, of binary black holes. This correlation comes from tidal spin-up systems preferentially forming and merging at higher redshifts due to the combination of weaker orbital expansion from low metallicity stars given their reduced wind mass loss rate, delayed expansion and have smaller maximal radii during the supergiant phase compared to stars at higher metallicity. As a result, these tightly bound systems merge with short inspiral times. Given our fiducial model of isolated binary evolution, we show that the origin of a $\chi_\mathrm{eff}-z$ correlation in the detectable LIGO--Virgo binary black hole population is different from the intrinsic population, which will become accessible only in the future by third-generation gravitational-wave detectors such as Einstein Telescope and Cosmic Explorer. Finally, we compare our model predictions with population predictions based on the current catalog of binary black hole mergers and find that current data favor a positive correlation of $\chi_\mathrm{eff}-z$ as predicted by our model of isolated binary evolution.Comment: 14 pages, 10 figures, submitted to A&

The origin of pulsating ultra-luminous X-ray sources: Low- and intermediate-mass X-ray binaries containing neutron star accretors

Ultra-luminous X-ray sources (ULXs) are those X-ray sources located away from the centre of their host galaxy with luminosities exceeding the Eddington limit of a stellar-mass black hole ($L_X>10^{39}\;{\rm erg\,s}^{-1}$). The discovery of X-ray pulsations in some of these objects (e.g. M82~X-2) suggests that a certain fraction of the ULX population may have a neutron star accretor. We present systematic modelling of low- and intermediate-mass X-ray binaries (LMXBs and IMXBs; donor-star mass range $0.92$--$8.0$~M$_{\odot}$ and neutron-star accretors) to explain the formation of this sub-population of ULXs. Using MESA, we explored the allowed initial parameter space of binary systems consisting of a neutron star and a low- or intermediate-mass donor star that could explain the observed properties of ULXs. Our simulations take into account beaming effects, stellar rotation, general angular momentum losses, and a detailed and self-consistent calculation of the mass-transfer rate. We study the conditions that lead to dynamical stability of these systems, which depends strongly on the response of the donor star to mass loss. Using two values for the initial neutron star mass ($1.3$~M$_{\odot}$ and $2.0$~M$_{\odot}$), we present two sets of mass-transfer calculation grids. We find that LMXBs/IMXBs can produce NS-ULXs with typical time-averaged isotropic-equivalent X-ray luminosities of $10^{39}$--$10^{41}\;{\rm erg\,s}^{-1}$ on a timescale up to $\sim\!1.0\;{\rm Myr}$ for the lower luminosities. We also estimate their likelihood of detection, the types of white-dwarf remnants left behind by the donors, and the total amount of mass accreted by the neutron stars. We also compare our results to the observed pulsating ULXs. Our results suggest that a large subset of the observed pulsating ULX population can be explained by LMXBs/IMXBs undergoing a super-Eddington mass-transfer phase.Comment: 19 pages, 13 figures, Accepted by A&A. Parameter space was increased to include low-mass XRBs and corresponding changes made to the text (including the title) and figures 4, 6-11. Changed axes for figures 1 and 2. Fixed typos and updated references. Added arguments about why spin period is not an accurate reflection of mass accretion rate in the introductio

Ultraviolet Detection of the Binary Companion to the Type IIb SN 2001ig

We present HST/WFC3 ultraviolet imaging in the F275W and F336W bands of the Type IIb SN 2001ig at an age of more than 14 years. A clear point source is detected at the site of the explosion having $m_{\rm F275W}=25.39 \pm 0.10$ and $m_{\rm F336W}=25.88 \pm 0.13$ mag. Despite weak constraints on both the distance to the host galaxy NGC 7424 and the line-of-sight reddening to the supernova, this source matches the characteristics of an early B-type main sequence star having $19,000 < T_{\rm eff} < 22,000$ K and $\log (L_{\rm bol}/L_{\odot})=3.92 \pm 0.14$. A BPASS v2.1 binary evolution model, with primary and secondary masses of 13 M$_{\odot}$ and 9 M$_{\odot}$ respectively, is found to resemble simultaneously in the Hertzsprung-Russell diagram both the observed location of this surviving companion, and the primary star evolutionary endpoints for other Type IIb supernovae. This same model exhibits highly variable late-stage mass loss, as expected from the behavior of the radio light curves. A Gemini/GMOS optical spectrum at an age of 6 years reveals a narrow He II emission line, indicative of continuing interaction with a dense circumstellar medium at large radii from the progenitor. We review our findings on SN 2001ig in the context of binary evolution channels for stripped-envelope supernovae. Owing to the uncrowded nature of its environment in the ultraviolet, this study of SN 2001ig represents one of the cleanest detections to date of a surviving binary companion to a Type IIb supernova.Comment: 8 pages, 3 figures. Resubmitted to ApJ after minor changes requested by refere