The first neutron star-neutron star (NS-NS) merger was discovered on August 17, 2017 through gravitational waves (GW170817) and followed with electromagnetic observations (Abbott et al. 2017). This merger was detected in an old elliptical galaxy with no recent star formation (Blanchard et al. 2017;Troja et al. 2017). We perform a suite of numerical calculations to understand the formation mechanism of this merger. We probe three leading formation mechanisms of double compact objects: classical isolated binary star evolution, dynamical evolution in globular clusters and nuclear cluster formation to test whether they are likely to produce NS-NS mergers in old host galaxies. Our simulations with optimistic assumptions show current NS-NS merger rates at the level of 10 −2 yr −1 from binary stars, 5 × 10 −5 yr −1 from globular clusters and 10 −5 yr −1 from nuclear clusters for all local elliptical galaxies (within 100 Mpc 3 ). These models are thus in tension with the detection of GW170817 with an observed rate 1.5 +3.2 −1.2 yr −1 (per 100 Mpc 3 ; LIGO/Virgo 90% credible limits). Our results imply that either (i) the detection of GW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is a statistical coincidence; or that (ii) physics in at least one of our three models is incomplete in the context of the evolution of stars that can form NS-NS mergers; or that (iii) another very efficient (unknown) formation channel with a long delay time between star formation and merger is at play.
We investigate here populations of cataclysmic variables (CVs) in a set of 288 globular cluster (GC) models evolved with the MOCCA code. This is by far the largest sample of GC models ever analysed with respect to CVs. Contrary to what has been argued for a long time, we found that dynamical destruction of primordial CV progenitors is much stronger in GCs than dynamical formation of CVs, and that dynamically formed CVs and CVs formed under no/weak influence of dynamics have similar white dwarf mass distributions. In addition, we found that, on average, the detectable CV population is predominantly composed of CVs formed via typical common envelope phase (CEP) ( 70 per cent), that only ≈ 2-4 per cent of all CVs in a GC is likely to be detectable, and that core-collapsed models tend to have higher fractions of bright CVs than non-core-collapsed ones. We also consistently show, for the first time, that the properties of bright and faint CVs can be understood by means of the pre-CV and CV formation rates, their properties at their formation times and cluster half-mass relaxation times. Finally, we show that models following the initial binary population proposed by Kroupa and set with low CEP efficiency better reproduce the observed amount of CVs and CV candidates in NGC 6397, NGC 6752 and 47 Tuc. To progress with comparisons, the essential next step is to properly characterize the candidates as CVs (e.g. by obtaining orbital periods and mass ratios). such as the Hubble Space Telescope (HST) and the Chandra X-ray Observatory are required to detect them. Until now the best studied GCs with respect to CV populations are NGC 6397 (Cohn et al. 2010), NGC 6752 (Lugger et al. 2017), ω Cen (Cool et al. 2013) and 47 Tuc (Rivera Sandoval et al. 2018). The identification of CVs in these GCs has been carried out by identifying HST optical counterparts to Chandra X-ray sources. Usually these counterparts show an Hα excess (suggesting the presence of an accretion disc), they are bluer than the MS stars and several also show photometric variability in different bands.In the core-collapsed 1 clusters NGC 6397 and NGC 6752, Cohn et al. (2010) and Lugger et al. (2017) found the CVs to be 1 Core collapse is a process in which the GC core evolves by releasing potential energy to the outer parts (via two-body relaxation) and thus becoming hotter and more compact, due to its negative heat capacity. The © 2018 The Authors 2 D. Belloni et al.divided into two populations, a bright and a faint one. On their optical colour-magnitude diagrams (CMDs), bright CVs lie close to the MS and faint CVs close to the WD cooling sequence, being R ≈ 21.5 mag the cut-off between both populations. Interestingly, in the non-core-collapsed clusters 47 Tuc and ω Cen, only one CV population is observed, and is mainly composed of faint CVs.Another interesting distinction between bright and faint CVs in core-collapsed GCs is related to the level of mass segregation, which is intrinsically connected with the GC relaxation time (proxy for the GC dynamical age) and ...
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