6,512 research outputs found
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
X-ray binaries powered by massive stellar black holes
The mass of stellar black holes (BHs) is currently thought to be in the 3-20
Msun range, but is highly uncertain: recent observations indicate the existence
of at least one BH with mass >20 Msun. The metallicity of the progenitor star
strongly influences the mass of the remnant, as only metal-poor stars can have
a final mass higher than ~40 Msun, and are expected to directly collapse into
BHs with mass >25 Msun. By means of N-body simulations, we investigate the
formation and evolution of massive stellar BHs (MSBHs, with mass >25 Msun) in
young dense star clusters. We study the effects of MSBHs on the population of
X-ray sources.Comment: 4 pages, 4 figures, 1 table, to appear in X-ray Astronomy: towards
the next 50 years!, Proceedings of the conference held October 1-5, 2012 in
Milano, Italy. To be published on Mem. SAI
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 to Gpc
yr and is only mildly dependent on CE efficiency. In contrast, the
current merger rates of DNSs (ranging from to Gpc
yr) and NSBHs (ranging from to Gpc
yr) 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 km
s) 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
Primordial gas heating by dark matter and structure formation
Dark matter (DM) decays and annihilations might heat and partially reionize
the Universe at high redshift. Although this effect is not important for the
cosmic reionization, the gas heating due to DM particles might affect the
structure formation. In particular, the critical halo mass for collapse is
increased up to a factor of ~2. Also the fraction of gas which collapses inside
the smallest halos is substantially reduced with respect to the cosmological
value. These effects imply that DM decays and annihilations might delay the
formation of the first structures and reduce the total star mass in the
smallest halos.Comment: 4 pages, 4 figures, to appear in the "Memorie della SAIt",
proceedings of the "LI congresso della Societa' Astronomica Italiana",
Firenze, April 17-20 200
Broad [OIII] in the globular cluster RZ 2109: X-ray ionized nova ejecta?
We study the possibility that the very broad (~1500 km/s) and luminous
(L_5007 ~ 1.4e37 erg/s) [OIII] line emission observed in the globular cluster
RZ 2109 might be explained with the photoionization of nova ejecta by the
bright (L_X ~ 4e39 erg/s) X-ray source hosted in the same globular cluster. We
find that such scenario is plausible and explains most of the features of the
RZ 2109 spectrum (line luminosity, absence of H emission lines, peculiar
asymmetry of the line profile); on the other hand, it requires the nova ejecta
to be relatively massive (>~ 0.5e-3 Msun}), and the nova to be located at a
distance <~ 0.1 pc from the X-ray source. We also predict the time evolution of
the RZ 2109 line emission, so that future observations can be used to test this
scenario.Comment: 11 pages, 2 figures, 6 tables; accepted for publication on MNRA
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