912 research outputs found
PSF and field of view characteristics of imaging and nulling interferometers
In this communication are presented some complements to a recent paper
entitled "Simple Fourier optics formalism for high angular resolution systems
and nulling interferometry", dealing with imaging and nulling capacities of a
few types of multi-aperture optical systems. Herein the characteristics of such
systems in terms of Point Spread Function (PSF) and Field of View (FoV) are
derived from simple analytical expressions that are further evaluated
numerically for various configurations. We consider successively the general
cases of Fizeau and Michelson interferometers, and those of a monolithic pupil,
nulling telescope, of a nulling, Sheared-Pupil Telescope (SPT), and of a sparse
aperture, Axially Combined Interferometer (ACI). The analytical formalism also
allows establishing the exact Object-Image relationships applicable to nulling
PSTs or ACIs that are planned for future space missions searching for habitable
extra-solar planets
Multi-spectral piston sensor for co-phasing giant segmented mirrors and multi-aperture interferometric arrays
This paper presents the optical design of a multi-spectral piston sensor
suitable to co-phasing giant segmented mirrors equipping the Future Extremely
Large Telescopes (ELTs). The general theory of the sensor is described in
detail and numerical simulations have been carried out, demonstrating that
direct piston and tip-tilt measurements are feasible within accuracies
respectively close to 20 nm and 10 nano-radians. Those values are compatible
with the co-phasing requirements, although the method seems to be perturbed by
uncorrected atmospheric seein
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
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