Collisions versus stellar winds in the runaway merger scenario: place your bets

The runaway merger scenario is one of the most promising mechanisms to explain the formation of intermediate-mass black holes (IMBHs) in young dense star clusters (SCs). On the other hand, the massive stars that participate in the runaway merger lose mass by stellar winds. This effect is tremendously important, especially at high metallicity. We discuss N-body simulations of massive (~6x10^4 Msun) SCs, in which we added new recipes for stellar winds and supernova explosion at different metallicity. At solar metallicity, the mass of the final merger product spans from few solar masses up to ~30 Msun. At low metallicity (0.01-0.1 Zsun) the maximum remnant mass is ~250 Msun, in the range of IMBHs. A large fraction (~0.6) of the massive remnants are not ejected from the parent SC and acquire stellar or black hole companions. Finally, I discuss the importance of this result for gravitational wave detection.Comment: 4 pages, 3 figures, 1 table, to appear in Memorie della SAIt (proceedings of the Modest 16 conference, 18-22 April 2016, Bologna, Italy

Back to the green valley: how to rejuvenate an S0 galaxy through minor mergers

About half of the S0 galaxies in the nearby Universe show signatures of recent or ongoing star formation. Whether these S0 galaxies were rejuvenated by the accretion of fresh gas is still controversial. We study minor mergers of a gas-rich dwarf galaxy with an S0 galaxy, by means of N-body smoothed-particle hydrodynamics simulations. We find that minor mergers trigger episodes of star formation in the S0 galaxy, lasting for ~10 Gyr. One of the most important fingerprints of the merger is the formation of a gas ring in the S0 galaxy. The ring is reminiscent of the orbit of the satellite galaxy, and its lifetime depends on the merger properties: polar and counter-rotating satellite galaxies induce the formation of long-lived smooth gas rings.Comment: 10 pages, 4 figures, 1 table, accepted for publication in 'galaxies'. To appear in the Proceedings of the EWASS-2015 Special Session 3 "3D View on Interacting and Post-Interacting Galaxies from Clusters to Voids

The cosmic merger rate of neutron stars and black holes

Six gravitational wave detections have been reported so far, providing crucial insights on the merger rate of double compact objects. We investigate the cosmic merger rate of double neutron stars (DNSs), neutron star-black hole binaries (NSBHs) and black hole binaries (BHBs) by means of population-synthesis simulations coupled with the Illustris cosmological simulation. We have performed six different simulations, considering different assumptions for the efficiency of common envelope (CE) ejection and exploring two distributions for the supernova (SN) kicks. The current BHB merger rate derived from our simulations spans from $\sim{}150$ to $\sim{}240$ Gpc$^{-3}$ yr$^{-1}$ and is only mildly dependent on CE efficiency. In contrast, the current merger rates of DNSs (ranging from $\sim{}20$ to $\sim{}600$ Gpc$^{-3}$ yr$^{-1}$) and NSBHs (ranging from $\sim{}10$ to $\sim{}100$ Gpc$^{-3}$ yr$^{-1}$) strongly depend on the assumptions on CE and natal kicks. The merger rate of DNSs is consistent with the one inferred from the detection of GW170817 only if a high efficiency of CE ejection and low SN kicks (drawn from a Maxwellian distribution with one dimensional root mean square $\sigma{}=15$ km s$^{-1}$) are assumed.Comment: 9 pages, 6 figures, 2 tables, accepted for publication in MNRA

Signatures of planets and protoplanets in the Galactic center: a clue to understand the G2 cloud?

Several hundred young stars lie in the innermost parsec of our Galaxy. The super-massive black hole (SMBH) might capture planets orbiting these stars, and bring them onto nearly radial orbits. The same fate might occur to planetary embryos (PEs), i.e. protoplanets born from gravitational instabilities in protoplanetary disks. In this paper, we investigate the emission properties of rogue planets and PEs in the Galactic center. In particular, we study the effects of photoevaporation, caused by the ultraviolet background. Rogue planets can hardly be detected by current or forthcoming facilities, unless they are tidally disrupted and accrete onto the SMBH. In contrast, photoevaporation of PEs (especially if the PE is being tidally stripped) might lead to a recombination rate as high as ~10^45 s^-1, corresponding to a Brackett-gamma luminosity ~10^31 erg s^-1, very similar to the observed luminosity of the dusty object G2. We critically discuss the possibility that G2 is a rogue PE, and the major uncertainties of this model.Comment: 10 pages, 5 figures, 1 table, ApJ, accepte

Roche-lobe overflow systems powered by black holes in young star clusters: the importance of dynamical exchanges

We have run 600 N-body simulations of intermediate-mass (~3500 Msun) young star clusters (SCs) with three different metallicities (Z=0.01, 0.1 and 1 Zsun). The simulations include the dependence of stellar properties and stellar winds on metallicity. Massive stellar black holes (MSBHs) with mass >25 Msun are allowed to form through direct collapse of very massive metal-poor stars (Z<0.3 Zsun). We focus on the demographics of black hole (BH) binaries that undergo mass transfer via Roche lobe overflow (RLO). We find that 44 per cent of all binaries that undergo an RLO phase (RLO binaries) formed through dynamical exchange. RLO binaries that formed via exchange (RLO-EBs) are powered by more massive BHs than RLO primordial binaries (RLO-PBs). Furthermore, the RLO-EBs tend to start the RLO phase later than the RLO-PBs. In metal-poor SCs (0.01-0.1 Zsun), >20 per cent of all RLO binaries are powered by MSBHs. The vast majority of RLO binaries powered by MSBHs are RLO-EBs. We have produced optical color-magnitude diagrams of the simulated RLO binaries, accounting for the emission of both the donor star and the irradiated accretion disk. We find that RLO-PBs are generally associated with bluer counterparts than RLO-EBs. We compare the simulated counterparts with the observed counterparts of nine ultraluminous X-ray sources. We discuss the possibility that IC 342 X-1, Ho IX X-1, NGC 1313 X-2 and NGC 5204 X-1 are powered by a MSBH.Comment: 17 pages, 10 figures, 9 tables, ApJ, accepte

Is NGC 6752 Hosting a Single or a Binary Black Hole?

The five millisecond pulsars that inhabit NGC 6752 display locations or accelerations remarkably different with respect to all other pulsars known in globular clusters. This may reflect the occurrence of an uncommon dynamics in the cluster core that could be attributed to the presence of a massive perturber. We here investigate whether a single intermediate-mass black hole, lying on the extrapolation of the mass versus sigma relation observed in galaxy spheroids, or, a less massive black hole binary could play the requested role.Comment: To appear in "Carnegie Observatories Astrophysics Series, Vol. 1: Coevolution of Black Holes and Galaxies," ed. L. C. Ho (Pasadena: Carnegie Observatories, http://www.ociw.edu/ociw/symposia/series/symposium1/proceedings.html

Perturbations induced by a molecular cloud on the young stellar disc in the Galactic Centre

The Galactic centre (GC) is a crowded environment: observations have revealed the presence of (molecular, atomic and ionized) gas, of a cusp of late-type stars, and of ~100 early-type stars, about half of which lying in one or possibly two discs. In this paper, we study the perturbations exerted on a thin stellar disc (with outer radius ~0.4 pc) by a molecular cloud that falls towards the GC and is disrupted by the supermassive black hole (SMBH). The initial conditions for the stellar disc were drawn from the results of previous simulations of molecular cloud infall and disruption in the SMBH potential. We find that most of the gas from the disrupted molecular cloud settles into a dense and irregular disc surrounding the SMBH. If the gas disc and the stellar disc are slightly misaligned (~5-20 deg), the precession of the stellar orbits induced by the gas disc significantly increases the inclinations of the stellar orbits (by a factor of ~3-5 in 1.5 Myr) with respect to the normal vector to the disc. Furthermore, the distribution of orbit inclinations becomes significantly broader. These results might be the clue to explain the broad distribution of observed inclinations of the early-type stars with respect to the normal vector of the main disc. We discuss the implications for the possibility that fresh gas was accreted by the GC after the formation of the disc(s) of early-type stars.Comment: 12 pages, 12 figures, 2 tables, accepted for publication in MNRA

The X-Ray Luminosity Function of Ultra Luminous X-Ray Sources in Collisional Ring Galaxies

Ring galaxies are fascinating laboratories: a catastrophic impact between two galaxies (one not much smaller than the other) has produced fireworks especially in the larger one, when hit roughly perpendicularly to the plane. We analyze the point sources, produced by the starburst episode following the impact, in the rings of seven galaxies and determine their X-ray luminosity function (XLF). In total we detect 63 sources, of which 50 have luminosity L$_X \geq 10^{39}$ erg s$^{-1}$, classifying them as ultra luminous X-ray sources (ULXs). We find that the total XLF is not significantly different from XLFs derived for other kinds of galaxies, with a tendency of having a larger fraction of high X-ray luminosity objects. Both the total number of ULXs and the number of ULXs per unit star formation rate are found in the upper envelope of the more normal galaxies distribution. Further analysis would be needed to address the issue of the nature of the compact component in the binary system.Comment: To appear in The Astrophysical Journa

Relic Signatures of Reionization Sources

We have exploited the recent determination of the radial distribution of BSS in four GCs, in order to investigate which mechanism of BSS formation prevails in these stellar systems. Our conclusion is that the two main formation paths proposed so far, i.e. masstransfer in PBs and merging of MS stars due to collisions in the cluster core, must coexist and have similar efficiency both in a low density cluster (M3) and in much denser clusters, like 47 Tuc and NGC 6752. In particular, in M3, 47 Tuc, and NGC 6752 the COL-BSS sum to around 50 - 60% of the total and mostly reside in the central region of the cluster. The MT-BSS are slightly less abundant than the COL-BSS, but populate all the GC. The density of BSS reaches a minimum in a so-called zone of avoidance, which separates the portion of the GC mostly occupied by COL-BSS from the cluster outskirts, where the MT-BSS dominate. The location of the zone of avoidance is explained by accounting for the effects of the dynamical friction on the PBs which were massive enough for generating the observed BSS. The picture described above can also be applied to ! Cen; but in this case the lack of a central peak in the BSS radial distribution requires that the large majority of the BSS derive from PBs. The very low rate of production of COL-BSS could be in turn attributed to the fact that mass segregation has not yet driven a sizeable number of PBs to the central region of the cluster to produce BSS. A very interesting further development of this research will be to perform a comparison between the location of a significant sample of BSS in a GC and their spectroscopic properties. According to the findings of this work, the position in the GC might represent a strong dynamical clue for the formation mechanism of a given BSS. If it is located outside the zone of avoidance, the BSS almost certainly results from evolution of a PB; if it is harbored in the cluster core, the BSS has most likely a collisional origin. On the other hand, indication about the origin of the same BSS can be independently obtained from high resolution spectroscopy. Indeed the chemical signature of the MT-BSS formation process has been recently discovered in 47 Tuc (Ferraro et al. 2006b). The acquisition of similar sets of data in clusters with different structural parameters and/or in different regions of the same cluster will provide an unprecedented tool for conforming the scenario presented here and to finally address the BSS formation processes and their complex interplay with the dynamical evolution of the cluster

The Maxwell's demon of star clusters

Stellar binaries are the most important energy reservoir of star clusters. Via three-body encounters, binaries can reverse the core collapse and prevent a star cluster from reaching equipartition. Moreover, binaries are essential for the formation of stellar exotica, such as blue straggler stars, intermediate-mass black holes and massive ($\gtrsim{}30$ M$_\odot$) black hole binaries.Comment: 17 pages, 8 figures, Submitted Manuscript Under Review: To appear in The Impact of Binaries on Stellar Evolution, Beccari G. & Boffin H.M.J. (Eds.).\copyright\ 2018 Cambridge University Pres