1,254 research outputs found
Binary-corrected velocity dispersions from single- and multi-epoch radial velocities: massive stars in R136 as a test case
Orbital motions from binary stars can broaden the observed line-of-sight
velocity distribution of a stellar system, artificially inflating the measured
line-of-sight velocity dispersion, which can in turn lead to erroneous
conclusions about the dynamical state of the system. Cottaar et al. (2012b)
proposed a maximum likelihood procedure to recover the intrinsic velocity
dispersion of a resolved star cluster from a single epoch of radial velocity
data of individual stars, which they achieved by simultaneously fitting the
intrinsic velocity distribution of the single stars and the centres of mass of
the binaries along with the velocity shifts caused by binary orbital motions.
Assuming well-characterized binary properties, they showed that this procedure
can accurately reproduce intrinsic velocity dispersions below 1 km s for
solar-type stars. Here we investigate the systematic offsets induced in cases
where the binary properties are uncertain, and we show how two epochs of radial
velocity data with an appropriate baseline can help to mitigate these
systematic effects. We first test the method above using Monte Carlo
simulations, taking into account the large uncertainties in the binary
properties of OB stars. We then apply it to radial velocity data in the young
massive cluster R136, an example for which the intrinsic velocity dispersion of
O-type stars is known from an intensive multi-epoch approach. For typical
velocity dispersions of young massive clusters ( km s) and
with a single epoch of data, we demonstrate that the method can just about
distinguish between a cluster in virial equilibrium and an unbound cluster.
This is due to the higher spectroscopic binary fraction and more loosely
constrained distributions of orbital parameters of OB stars compared to
solar-type stars. By extending the maximum likelihood method to multi-epoch
data, Comment: Accepted by A&A; minor corrections made on November 2
On the uniqueness of kinematical signatures of intermediate-mass black holes in globular clusters
Finding an intermediate-mass black hole (IMBH) in a globular cluster (GC), or
proving its absence, is a crucial ingredient in our understanding of galaxy
formation and evolution. The challenge is to identify a unique signature of an
IMBH that cannot be accounted for by other processes. Observational claims of
IMBH detection are often based on analyses of the kinematics of stars, such as
a rise in the velocity dispersion profile towards the centre. In this
contribution we discuss the degeneracy between this IMBH signal and pressure
anisotropy in the GC. We show that that by considering anisotropic models it is
possible to partially explain the innermost shape of the projected velocity
dispersion profile, even though models that do not account for an IMBH do not
exhibit a cusp in the centre.Comment: 4 pages, 2 figures. To be published in the Proceedings IAU Symposium
No. 312, Star Clusters and Black Holes in Galaxies Across Cosmic Tim
The effect of stellar-mass black holes on the central kinematics of omega Cen: a cautionary tale for IMBH interpretations
The search for intermediate-mass black holes (IMBHs) in the centre of
globular clusters is often based on the observation of a central cusp in the
surface brightness profile and a rise towards the centre in the velocity
dispersion profiles. Similar signatures, however, could result from other
effects, that need to be taken into account in order to determine the presence
(or the absence) of an IMBH in these stellar systems. Following our previous
exploration of the role of radial anisotropy in shaping these observational
signatures, we analyse here the effects produced by the presence of a
population of centrally concentrated stellar-mass black holes. We fit dynamical
models to omega Cen data, and we show that models with ~5% of their mass in
black holes (consistent with ~100% retention fraction after natal kicks) can
reproduce the data. When simultaneously considering both radial anisotropy and
mass segregation, the best-fit model includes a smaller population of remnants,
and a less extreme degree of anisotropy with respect to the models that include
only one of these features. These results underline that before conclusions
about putative IMBHs can be made, the effects of stellar-mass black holes and
radial anisotropy need to be properly accounted for.Comment: 13 pages, 5 figures. Accepted for publication in MNRA
Testing lowered isothermal models with direct N-body simulations of globular clusters - II: Multimass models
Lowered isothermal models, such as the multimass Michie-King models, have
been successful in describing observational data of globular clusters. In this
study we assess whether such models are able to describe the phase space
properties of evolutionary -body models. We compare the multimass models as
implemented in (Gieles \& Zocchi) to -body models of star clusters with
different retention fractions for the black holes and neutron stars evolving in
a tidal field. We find that multimass models successfully reproduce the density
and velocity dispersion profiles of the different mass components in all
evolutionary phases and for different remnants retention. We further use these
results to study the evolution of global model parameters. We find that over
the lifetime of clusters, radial anisotropy gradually evolves from the low-mass
to the high-mass components and we identify features in the properties of
observable stars that are indicative of the presence of stellar-mass black
holes. We find that the model velocity scale depends on mass as ,
with for almost all models, but the dependence of central
velocity dispersion on can be shallower, depending on the dark remnant
content, and agrees well with that of the -body models. The reported model
parameters, and correlations amongst them, can be used as theoretical priors
when fitting these types of mass models to observational data.Comment: 28 pages, 22 figures, published in MNRA
Synthesis studies to single stereoisomers of the vicinal trifluoroalkane motif
This thesis focuses on the construction of individual isomers of the R-CHF-CHF-CHF-R’
motif. The multi-vicinal fluorine motif is new in organic chemistry and therefore
stereoselective methods giving rapid access to these motifs and with flexibility need to be
explored. The research in the thesis succeeded in the preparation of (2S,3R,4S)-314 and
(2S,3S,4R)-328.
In Chapter 1, an overview of the impact of fluorine in organic molecules is given. Recent
developments in asymmetric electrophilic and nucleophilic fluorination are described, as well
as the preparation of multivicinal fluoroalkane motifs.
Aldol reactions of either (R)- or (S)-N-(α-fluoropropyl)-2-oxazolidinones, mediated by TiCl 4
are reported in Chapter 2. Such aldol reactions gave rise to identical α-fluoro-β-hydroxy-
aldol products with high diastereoselectivities (95% dr). After removal from the auxiliary α-
fluoro-β-hydroxy- products were converted to the corresponding α,β-difluoro products.
The synthesis of non symmetric vicinal trifluoro motifs (2S,3R,4S)-314 and (2S,3S,4R)-328 is
described in Chapter 3. They were prepared by direct fluorination in three steps of the
corresponding (2R,3R,4R)-erythro and (2R,3S,4S)-threo enantio-enriched epoxy-alcohols. The
two erythro and threo epoxy-alcohol isomers behave very differently during the first
fluorination step and then an attempt to study and rationalise this difference in behaviour is
made
Multiple populations in globular clusters: the distinct kinematic imprints of different formation scenarios
Several scenarios have been proposed to explain the presence of multiple
stellar populations in globular clusters. Many of them invoke multiple
generations of stars to explain the observed chemical abundance anomalies, but
it has also been suggested that self-enrichment could occur via accretion of
ejecta from massive stars onto the circumstellar disc of low-mass pre-main
sequence stars. These scenarios imply different initial conditions for the
kinematics of the various stellar populations. Given some net angular momentum
initially, models for which a second generation forms from gas that collects in
a cooling flow into the core of the cluster predict an initially larger
rotational amplitude for the polluted stars compared to the pristine stars.
This is opposite to what is expected from the accretion model, where the
polluted stars are the ones crossing the core and are on preferentially radial
(low-angular momentum) orbits, such that their rotational amplitude is lower.
Here we present the results of a suite of -body simulations with initial
conditions chosen to capture the distinct kinematic properties of these
pollution scenarios. We show that initial differences in the kinematics of
polluted and pristine stars can survive to the present epoch in the outer parts
of a large fraction of Galactic globular clusters. The differential rotation of
pristine and polluted stars is identified as a unique kinematic signature that
could allow us to distinguish between various scenarios, while other kinematic
imprints are generally very similar from one scenario to the other.Comment: 22 pages, 16 figures + appendix. Accepted for publication in MNRA
A stellar-mass black hole population in the globular cluster NGC 6101?
Dalessandro et al. observed a similar distribution for blue straggler stars
and main-sequence turn-off stars in the Galactic globular cluster NGC 6101, and
interpreted this feature as an indication that this cluster is not
mass-segregated. Using direct N-body simulations, we find that a significant
amount of mass segregation is expected for a cluster with the mass, radius and
age of NGC 6101. Therefore, the absence of mass segregation cannot be explained
by the argument that the cluster is not yet dynamically evolved. By varying the
retention fraction of stellar-mass black holes, we show that segregation is not
observable in clusters with a high black hole retention fraction (>50% after
supernova kicks and >50% after dynamical evolution). Yet all model clusters
have the same amount of mass segregation in terms of the decline of the mean
mass of stars and remnants with distance to the centre. We also discuss how
kinematics can be used to further constrain the presence of a stellar-mass
black hole population and distinguish it from the effect of an
intermediate-mass black hole. Our results imply that the kick velocities of
black holes are lower than those of neutron stars. The large retention fraction
during its dynamical evolution can be explained if NGC 6101 formed with a large
initial radius in a Milky Way satellite.Comment: 10 pages, 7 figure
Massive binary stars and the kinematics of Young Massive Clusters
Located in the Large Magellanic Cloud, R136 is a rare example of a nearby young and
dense massive star cluster in which individual stars can be resolved. Often suggested
as a globular cluster in formation, its study is of great interest and promises to provide
insights into the early dynamical evolution of massive star clusters. This is crucial
to understand more extreme and distant starburst clusters, which contribute to a
significant fraction of all current star formation in the Local Universe, in particular
in interacting galaxies.
The majority of this thesis is based on multi-epoch spectroscopic observations in
and around R136 obtained as part of the VLT-FLAMES Tarantula Survey (VFTS),
an ambitious programme which targeted nearly 1 000 massive stars in the intricate
30 Doradus star-forming region. The motivations and observing strategy of this survey,
designed to address key questions about the evolution of massive stars and clusters, are
first introduced. The data reduction procedures applied to VFTS data are described,
with an emphasis on the tasks accomplished in the context of this thesis.
The VFTS data are first used to perform a detailed kinematic study of R136, determine
its dynamical state, and evaluate the importance of gas expulsion in the early evolution
of massive star clusters. Orbital motions of binary stars are found to dominate the line-
of-sight velocity dispersion of the cluster, illustrating the risk of interpreting velocity
dispersion measurements for unresolved extragalactic young massive clusters. However,
once the detected binaries are rejected and the contribution of undetected binaries is
accounted for through Monte Carlo simulations, the true velocity dispersion of the
cluster is found to be low and consistent with it being in virial equilibrium. This
suggests that gas expulsion has not had a dramatic effect on the early dynamical
evolution of R136.
Using the velocity measurements of R136 as a test case, a maximum likelihood method
that fits the velocity dispersion of a cluster from a single epoch of radial velocity data
is then tested. The method must be applied with care given the high binary fraction of
massive stars and the large uncertainties in their binary orbital parameter distributions,
but for typical velocity dispersions of young massive clusters (& 4 kms−1), it is shown
that the velocity dispersion can be measured with an accuracy of 40% or better. This
offers an efficient way of constraining the dynamics of these systems.
The radial velocity measurements of apparently single stars in R136 are also used to
investigate the internal rotation of the cluster, a potentially important but largely
unexplored characteristic of young clusters. Evidence is found, at the 95% confidence
level, for rotation of the cluster as a whole. A simple maximum likelihood method is
presented to fit rotation curves to the data, from which a typical rotational velocity
of 3 kms−1 is found. When compared to the low velocity dispersion of R136, this
suggests that star clusters may form with as much as 20% of their kinetic energy in
rotation.
Finally, a smaller-scale survey of massive stars in the Wing of the Small Magellanic
Cloud is introduced. As an example of the particularly interesting massive binaries
that can be revealed by the synergy between large optical spectroscopic surveys of
young clusters and observations at other wavelengths, the discovery of a new Be/X-ray
pulsar binary and associated supernova remnant is reported. With a long spin period
of over 1 000 seconds and a young age of 104 years constrained by its association with
the supernova remnant, the pulsar in this system is quickly emerging as a unique object
that challenges our understanding of the spin evolution of accreting neutron stars
Mass models of NGC 6624 without an intermediate-mass black hole
An intermediate-mass black hole (IMBH) was recently reported to reside in the
centre of the Galactic globular cluster (GC) NGC 6624, based on timing
observations of a millisecond pulsar (MSP) located near the cluster centre in
projection. We present dynamical models with multiple mass components of NGC
6624 - without an IMBH - which successfully describe the surface brightness
profile and proper motion kinematics from the Hubble Space Telescope (HST) and
the stellar mass function at different distances from the cluster centre. The
maximum line-of-sight acceleration at the position of the MSP accommodates the
inferred acceleration of the MSP, as derived from its first period derivative.
With discrete realizations of the models we show that the higher-order period
derivatives - which were previously used to derive the IMBH mass - are due to
passing stars and stellar remnants, as previously shown analytically in
literature. We conclude that there is no need for an IMBH to explain the timing
observations of this MSP.Comment: 8 pages, 7 figures, MNRAS. Updated to match final journal styl
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