27 research outputs found
Formation of SMBH seeds in Pop III star clusters through collisions : the importance of mass loss
Runaway collisions in dense clusters may lead to the formation of
supermassive black hole (SMBH) seeds, and this process can be further enhanced
by accretion, as recent models of SMBH seed formation in Population III star
clusters have shown. This may explain the presence of supermassive black holes
already at high redshift, . However, in this context, mass loss during
collisions was not considered and could play an important role for the
formation of the SMBH seed. Here, we study the effect of mass loss, due to
collisions of protostars, in the formation and evolution of a massive object in
a dense primordial cluster. We consider both constant mass loss fractions as
well as analytic models based on the stellar structure of the collision
components. Our calculations indicate that mass loss can significantly affect
the final mass of the possible SMBH seed. Considering a constant mass loss of
5% for every collision, we can lose between 60-80% of the total mass that is
obtained if mass loss were not considered. Using instead analytical
prescriptions for mass loss, the mass of the final object is reduced by 15-40%,
depending on the accretion model for the cluster we study. Altogether, we
obtain masses of the order of , which are still massive enough
to be SMBH seeds.Comment: 12 pages, 9 figures, accepted by MNRA
Stellar collisions in flattened and rotating Pop. III star clusters
Fragmentation often occurs in disk-like structures, both in the early
Universe and in the context of present-day star formation. Supermassive black
holes (SMBHs) are astrophysical objects whose origin is not well understood;
they weigh millions of solar masses and reside in the centers of galaxies. An
important formation scenario for SMBHs is based on collisions and mergers of
stars in a massive cluster, in which the most massive star moves to the center
of the cluster due to dynamical friction. This increases the rate of collisions
and mergers since massive stars have larger collisional cross sections. This
can lead to runaway growth of a very massive star which may collapse to become
an intermediate-mass black hole. Here we investigate the dynamical evolution of
Miyamoto-Nagai models that allow us to describe dense stellar clusters,
including flattening and different degrees of rotation. We find that the
collisions in these clusters depend mostly on the number of stars and the
initial stellar radii for a given radial size of the cluster. By comparison,
rotation seems to affect the collision rate by at most . For flatness, we
compared spherical models with systems that have a scale height of about
of their radial extent, in this case finding a change in the collision rate of
less than . Overall, we conclude that the parameters only have a minor
effect on the number of collisions. Our results also suggest that rotation
helps to retain more stars in the system, reducing the number of escapers by a
factor of depending on the model and the specific realization. After two
million years, a typical lifetime of a very massive star, we find that about
collisions occur in typical models with ,
and a half-mass radius of , leading to a mass of about
for the most massive object.Comment: 10 pages, 7 figure
C-axis electronic Raman scattering in Bi_2Sr_2CaCu_2O_{8+\delta}
We report a c-axis-polarized electronic Raman scattering study of
Bi_2Sr_2CaCu_2O_{8+\delta} single crystals. In the normal state, a resonant
electronic continuum extends to 1.5 eV and gains significant intensity as the
incoming photon energy increases. In the superconducting state, a coherence
2\Delta peak appears around 50 meV, with a suppression of the scattering
intensity at frequencies below the peak position. The peak energy, which is
higher than that seen with in-plane polarizations, signifies distinctly
different dynamics of quasiparticle excitations created with out-of-plane
polarization.Comment: 12 pages, REVTEX, 3 postscript figure
Collisions in primordial star clusters: formation pathway for intermediate mass black holes
Collisions were suggested to potentially play a role in the formation of
massive stars in present day clusters, and have likely been relevant during the
formation of massive stars and intermediate mass black holes within the first
star clusters. In the early Universe, the first stellar clusters were
particularly dense, as fragmentation typically only occurred at densities above
cm, and the radii of the protostars were enhanced due to the
larger accretion rates, suggesting a potentially more relevant role of stellar
collisions. We present here a detailed parameter study to assess how the number
of collisions as well as the mass growth of the most massive object depends on
the properties of the cluster, and we characterize the time evolution with
three effective parameters, the time when most collisions occur, the duration
of the collisions period, as well as the normalization required to obtain the
total number of collisions. We apply our results to typical Population III
(Pop.III) clusters of about M, finding that a moderate
enhancement of the mass of the most massive star by a factor of a few can be
expected. For more massive Pop.III clusters as expected in the first atomic
cooling halos, we expect a more significant enhancement by a factor of .
We therefore conclude that collisions in massive Pop.III clusters were likely
relevant to form the first intermediate mass black holes.publishe
C-axis Raman spectra of a normal plane-chain bilayer cuprate and the pseudogap
We investigate the Raman spectra in the geometry where both incident and
scattered photon polarizations are parallel to the -direction, for a
plane-chain bilayer coupled via a single-particle tunneling . The
Raman vertex is derived in the tight-binding limit and in the absence of
Coulomb screening, the Raman intensity can be separated into intraband
() and interband () transitions. In the
small- limit, the interband part dominates and a pseudogap will appear
as it does in the conductivity. Coulomb interactions bring in a two-particle
coupling and result in the breakdown of intra- and interband separation.
Nevertheless, when is small, the Coulomb screening () has little effect on the intensity to which the unscreened
interband transitions contribute most. In general, the total Raman spectra are
strongly dependent on the magnitude of .Comment: 23 pages, 6 figures, submitted to Phys. Rev.
Anomalous Self-Energy Effects of the B_1g Phonon in Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 Films
In Raman spectra of cuprate superconductors the gap shows up both directly,
via a redistribution of the electronic background, the so-called "2Delta
peaks", and indirectly, e.g. via the renormalization of phononic excitations.
We use a model that allows us to study the redistribution and the related
phonon self-energy effects simultaneously. We apply this model to the B_1g
phonon of Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 films, where Pr or Ca substitution
enables us to investigate under- and overdoped samples. While various
self-energy effects can be explained by the strength and energy of the 2\Delta
peaks, anomalies remain. We discuss possible origins of these anomalies.Comment: 6 pages including 4 figure
C-axis lattice dynamics in Bi-based cuprate superconductors
We present results of a systematic study of the c axis lattice dynamics in
single layer Bi2Sr2CuO6 (Bi2201), bilayer Bi2Sr2CaCu2O8 (Bi2212) and trilayer
Bi2Sr2Ca2Cu3O10 (Bi2223) cuprate superconductors. Our study is based on both
experimental data obtained by spectral ellipsometry on single crystals and
theoretical calculations. The calculations are carried out within the framework
of a classical shell model, which includes long-range Coulomb interactions and
short-range interactions of the Buckingham form in a system of polarizable
ions. Using the same set of the shell model parameters for Bi2201, Bi2212 and
Bi2223, we calculate the frequencies of the Brillouin-zone center phonon modes
of A2u symmetry and suggest the phonon mode eigenvector patterns. We achieve
good agreement between the calculated A2u eigenfrequencies and the experimental
values of the c axis TO phonon frequencies which allows us to make a reliable
phonon mode assignment for all three Bi-based cuprate superconductors. We also
present the results of our shell model calculations for the Gamma-point A1g
symmetry modes in Bi2201, Bi2212 and Bi2223 and suggest an assignment that is
based on the published experimental Raman spectra. The
superconductivity-induced phonon anomalies recently observed in the c axis
infrared and resonant Raman scattering spectra in trilayer Bi2223 are
consistently explained with the suggested assignment.Comment: 29 pages, 13 figure
A MODEST review
We present an account of the state of the art in the fields explored by the
research community invested in 'Modeling and Observing DEnse STellar systems'.
For this purpose, we take as a basis the activities of the MODEST-17
conference, which was held at Charles University, Prague, in September 2017.
Reviewed topics include recent advances in fundamental stellar dynamics,
numerical methods for the solution of the gravitational N-body problem,
formation and evolution of young and old star clusters and galactic nuclei,
their elusive stellar populations, planetary systems, and exotic compact
objects, with timely attention to black holes of different classes of mass and
their role as sources of gravitational waves.
Such a breadth of topics reflects the growing role played by collisional
stellar dynamics in numerous areas of modern astrophysics. Indeed, in the next
decade, many revolutionary instruments will enable the derivation of positions
and velocities of individual stars in the Milky Way and its satellites and will
detect signals from a range of astrophysical sources in different portions of
the electromagnetic and gravitational spectrum, with an unprecedented
sensitivity. On the one hand, this wealth of data will allow us to address a
number of long-standing open questions in star cluster studies; on the other
hand, many unexpected properties of these systems will come to light,
stimulating further progress of our understanding of their formation and
evolution.Comment: 42 pages; accepted for publication in 'Computational Astrophysics and
Cosmology'. We are much grateful to the organisers of the MODEST-17
conference (Charles University, Prague, September 2017). We acknowledge the
input provided by all MODEST-17 participants, and, more generally, by the
members of the MODEST communit
Stellar triples on the edge
Context. Hierarchical triple stars are ideal laboratories for studying the interplay between orbital dynamics and stellar evolution. Both mass loss from stellar winds and strong gravitational perturbations between the inner and outer orbit cooperate to destabilise triple systems.
Aims. Our current understanding of the evolution of unstable triple systems is mainly built upon results from extensive binary-single scattering experiments. However, destabilised hierarchical triples cover a different region of phase space. Therefore, we aim to construct a comprehensive overview of the evolutionary pathways of destabilised triple-star systems.
Methods. Starting from generic initial conditions, we evolved an extensive set of hierarchical triples using the code TRES, combining secular dynamics and stellar evolution. We detected those triples that destabilise due to stellar winds and/or gravitational perturbations. Their evolution was continued with a direct N-body integrator coupled to stellar evolution.
Results. The majority of triples (54–69%) preserve their hierarchy throughout their evolution, which is in contradiction with the commonly adopted picture that unstable triples always experience a chaotic, democratic resonant interaction. The duration of the unstable phase was found to be longer than expected (103 − 4 crossing times, reaching up to millions), so that long-term stellar evolution effects cannot be neglected. The most probable outcome is dissolution of the triple into a single star and binary (42–45%). This occurs through the commonly known democratic channel, during which the initial hierarchy is lost and the lightest body usually escapes, but also through a hierarchical channel, during which the tertiary is ejected in a slingshot, independent of its mass. Collisions are common (13–24% of destabilised triples), and they mostly involve the two original inner binary components still on the main sequence (77–94%). This contradicts the idea that collisions with a giant during democratic encounters dominate (only 5–12%). Together with collisions in stable triples, we find that triple evolution is the dominant mechanism for stellar collisions in the Milky Way. Lastly, our simulations produce runaway and walk-away stars with speeds up to several tens of km/s, with a maximum of a few 100 km s−1. We suggest that destabilised triples can explain – or at least alleviate the tension behind – the origin of the observed (massive) runaway stars.
Conclusions. A promising indicator for distinguishing triples that will follow the democratic or hierarchical route, is the relative inclination between the inner and outer orbits. Its influence can be summed up in two rules of thumb: (1) prograde triples tend to evolve towards hierarchical collisions and ejections, and (2) retrograde triples tend to evolve towards democratic encounters and a loss of initial hierarchy, unless the system is compact, which experience collision preferentially. The trends found in this work complement those found previously from binary-single scattering experiments, and together they will help to generalise and improve our understanding on the evolution of unstable triple systems of various origins