Gravitationalwave (GW) detections of merging neutron star–black hole (NSBH) systems probe astrophysical neutron star (NS) and black hole (BH) mass distributions, especially at the transition between NS and BH masses. Of particular interest are the maximum NS mass, minimum BH mass, and potential mass gap between them. While previous GW population analyses assumed all NSs obey the same maximum mass, if rapidly spinning NSs exist, they can extend to larger maximum masses than nonspinning NSs. In fact, several authors have proposed that the ∼2.6
Gravitationalwave (GW) detections are starting to reveal features in the mass distribution of double compact objects. The lower end of the black hole (BH) mass distribution is especially interesting as few formation channels contribute here and because it is more robust against variations in the cosmic star formation than the highmass end. In this work we explore the stable mass transfer channel for the formation of GW sources with a focus on the lowmass end of the mass distribution. We conduct an extensive exploration of the uncertain physical processes that impact this channel. We note that, for fiducial assumptions, this channel reproduces the peak at ∼9
 Award ID(s):
 2009131
 NSFPAR ID:
 10486235
 Publisher / Repository:
 DOI PREFIX: 10.3847
 Date Published:
 Journal Name:
 The Astrophysical Journal
 Volume:
 940
 Issue:
 2
 ISSN:
 0004637X
 Format(s):
 Medium: X Size: Article No. 184
 Size(s):
 Article No. 184
 Sponsoring Org:
 National Science Foundation
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Observations of Xray binaries indicate a dearth of compact objects in the mass range from ∼2 − 5 M ⊙ . The existence of this (first mass) gap has been used to discriminate between proposed engines behind corecollapse supernovae. From LIGO/Virgo observations of binary compact remnant masses, several candidate first mass gap objects, either neutron stars (NSs) or black holes (BHs), were identified during the O3 science run. Motivated by these new observations, we study the formation of BHNS mergers in the framework of isolated classical binary evolution, using population synthesis methods to evolve large populations of binary stars (Population I and II) across cosmic time. We present results on the NS to BH mass ratios ( q = M NS / M BH ) in merging systems, showing that although systems with a mass ratio as low as q = 0.02 can exist, typically BHNS systems form with moderate mass ratios q = 0.1 − 0.2. If we adopt a delayed supernova engine, we conclude that ∼30% of BHNS mergers may host at least one compact object in the first mass gap (FMG). Even allowing for uncertainties in the processes behind compact object formation, we expect the fraction of BHNS systems ejecting mass during the merger to be small (from ∼0.6 − 9%). In our reference model, we assume: (i) the formation of compact objects within the FMG, (ii) natal NS/BH kicks decreased by fallback, (iii) low BH spins due to TaylerSpruit angular momentum transport in massive stars. We find that ≲1% of BHNS mergers will have any mass ejection and about the same percentage will produce kilonova bright enough to have a chance of being detected with a large (Subaruclass) 8 m telescope. Interestingly, all these mergers will have both a BH and an NS in the FMG.more » « less

Abstract Gravitationalwave (GW) detections of binary black hole (BH) mergers have begun to sample the cosmic BH mass distribution. The evolution of single stellar cores predicts a gap in the BH mass distribution due to pairinstability supernovae (PISNe). Determining the upper and lower edges of the BH mass gap can be useful for interpreting GW detections of merging BHs. We use
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