3,234 research outputs found
Nuclear star cluster formation in energy-space
In a virialized stellar system, the mean-square velocity is a direct tracer
of the energy per unit mass of the system. Here, we exploit this to estimate
and compare root-mean-square velocities for a large sample of nuclear star
clusters and their host (late- or early-type) galaxies. Traditional
observables, such as the radial surface brightness and second-order velocity
moment profiles, are subject to short-term variations due to individual
episodes of matter infall and/or star formation. The total mass, energy and
angular momentum, on the other hand, are approximately conserved. Thus, the
total energy and angular momentum more directly probe the formation of galaxies
and their nuclear star clusters, by offering access to more fundamental
properties of the nuclear cluster-galaxy system than traditional observables.
We find that there is a strong correlation, in fact a near equality, between
the root-mean-square velocity of a nuclear star cluster and that of its host.
Thus, the energy per unit mass of a nuclear star cluster is always comparable
to that of its host galaxy. We interpret this as evidence that nuclear star
clusters do not form independently of their host galaxies, but rather that
their formation and subsequent evolution are coupled. We discuss how our
results can potentially be used to offer a clear and observationally testable
prediction to distinguish between the different nuclear star cluster formation
scenarios, and/or quantify their relative contributions.Comment: 12 pages, 3 figures, 4 tables; accepted for publication in MNRA
Ongoing astrometric microlensing events from VVV and Gaia
6 pages, 2 figures, accepted MNRAS LettersWe extend predictive microlensing event searches using the Vista Variables in the Via Lactea survey and the second Gaia data release. We identify two events with maxima in 2019 that require urgent follow-up. First, we predict that the nearby M2 dwarf L 338-152 will align with a background source with a closest approach of mas on 2019 November d. This will cause a peak astrometric shift and photometric amplification of the background source of mas and mmag respectively. This event should be astrometrically detectable by both the Hubble Space Telescope (HST) and the Spectro-Polarimetric High-contrast Exoplanet Research instrument on the Very Large Telescope. Secondly, we predict the likely K dwarf NLTT 45128 will lens a background source with a closest approach of mas on 2019 September d. This will produce a peak astrometric shift of mas. NLTT 45128 is only 3.6 magnitudes brighter than the background source which makes it an excellent candidate for follow-up with HST. Characterisation of these signals will allow direct gravitational masses to be inferred for both L 338-152 and NLTT 45128 with an estimated precision of and per cent respectively.Peer reviewedFinal Published versio
The effects of a background potential in star cluster evolution: a delay in the relaxation time-scale and runaway collision processes
Runaway stellar collisions in dense star clusters are invoked to explain the
presence of very massive stars or blue stragglers in the center of those
systems. This process has also been explored for the first star clusters in the
Universe and shown to yield stars that may collapse at some points into an
intermediate mass black hole. Although the early evolution of star clusters
requires the explicit modeling of the gas out of which the stars form, these
calculations would be extremely time-consuming and often the effects of the gas
can be accurately treated by including a background potential to account for
the extra gravitational force. We apply this approximation to model the early
evolution of the first dense star clusters formed in the Universe by performing
-body simulations, our goal is to understand how the additional
gravitational force affects the growth of a very massive star through stellar
mergers in the central parts of the star cluster. Our results show that the
background potential increases the velocities of the stars, causing an overall
delay in the evolution of the clusters and in the runaway growth of a massive
star at the center. The population of binary stars is lower due to the
increased kinetic energy of the stars, initially reducing the number of stellar
collisions, and we show that relaxation processes are also affected. Despite
these effects, the external potential enhances the mass of the merger product
by a factor 2 if the collisions are maintained for long times.Comment: 16 pages. Accepted for publication in Astronomy and Astrophysic
Fronto-striatal cognitive deficits at different stages of Parkinson's disease
Groups of patients with idiopathic Parkinson's disease, either medicated or unmedicated, were compared with matched groups of normal controls on a computerized battery previously shown to be sensitive to frontal lobe dysfunction, including tests of planning, spatial working memory and attentional set-shifting. In a series of problems based on the 'Tower of London' test, medicated patients with Parkinson's disease were shown to be impaired in the amount of time spent thinking about (planning) the solution to each problem. Additionally, an impairment in terms of the accuracy of the solution produced on this test was only evident in those patients with more severe clinical symptoms and was accompanied by deficits in an associated test of spatial short-term memory. Medicated patients with both mild and severe clinical symptoms were also impaired on a related test of spatial working memory. In contrast, a group of patients who were unmedicated and 'early in the course' of the disease were unimpaired in all three of these tests. However, all three Parkinson's disease groups were impaired in the test of attentional set-shifting ability, although unimpaired in a test of pattern recognition which is insensitive to frontal lobe damage. These data are compared with those previously published from a group of young neurosurgical patients with localized excisions of the frontal lobes and are discussed in terms of the specific nature of the cognitive deficit at different stages of Parkinson's disease
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
New candidate hypervelocity red clump stars in the inner Galactic bulge
We search for high-velocity stars in the inner region of the Galactic bulge
using a selected sample of red clump stars. Some of those stars might be
considered hypervelocity stars (HVSs). Even though the HVSs ejection relies on
an interaction with the supermassive black hole (SMBH) at the centre of the
Galaxy, there are no confirmed detections of HVSs in the inner region of our
Galaxy. With the detection of HVSs, ejection mechanism models can be
constrained by exploring the stellar dynamics in the Galactic centre through a
recent stellar interaction with the SMBH. Based on a previously developed
methodology by our group, we searched with a sample of preliminary data from
version 2 of the Vista Variables in the Via Lactea (VVV) Infrared Astrometric
Catalogue (VIRAC2) and Gaia DR3 data, including accurate optical and NIR proper
motions. This search resulted in a sample of 46 stars with transverse
velocities larger than the local escape velocity within the Galactic bulge, of
which 4 are prime candidate HVSs with high-proper motions consistent with being
ejections from the Galactic centre. Adding to that, we studied a sample of
reddened stars without a Gaia DR3 counterpart and found 481 stars with
transverse velocities larger than the local escape velocity, from which 65
stars have proper motions pointing out of the Galactic centre and are candidate
HVSs. In total, we found 69 candidate HVSs pointing away from the Galactic
centre with transverse velocities larger than the local escape velocity.Comment: Accepted for publication in MNRAS. 18 pages (5 of Appendix), 15
figure
Gravity in the 3+1-Split Formalism I: Holography as an Initial Value Problem
We present a detailed analysis of the 3+1-split formalism of gravity in the
presence of a cosmological constant. The formalism helps revealing the intimate
connection between holography and the initial value formulation of gravity. We
show that the various methods of holographic subtraction of divergences
correspond just to different transformations of the canonical variables, such
that the initial value problem is properly set up at the boundary. The
renormalized boundary energy momentum tensor is a component of the Weyl tensor.Comment: 28 pages; v2: minor improvements, references adde
Large angular momentum closed strings colliding with D-branes
We investigate colliding processes of closed strings with large angular
momenta with D-branes. We give explicit CFT calculations for closed string
states with an arbitrary number of bosonic excitations and no or one fermion
excitation. The results reproduce the correspondence between closed string
states and single trace operators in the boundary gauge theory recently
suggested by Berenstein, Maldacena and Nastase.Comment: LaTeX, 18pages, explanation about the two conditions in section 2
added, some statements about closed string vertex operators correcte
On the Baryonic Branch Root of N=2 MQCD
We investigate the brane exchange in the framework of N=2 MQCD by using a
specific family of M fivebrane configurations relevant to describe the baryonic
branch root. An exchange of M fivebranes is realized in the Taub-NUT geometry
and controlled by the moduli parameter of the configurations. This family also
provides two different descriptions of the root. These descriptions are
examined carefully using the Taub-NUT geometry. It is shown that they have the
same baryonic branch and are shifted each other by the brane exchange.Comment: LaTeX, 25 pages, 7 figures, references adde
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