7 research outputs found
LMC self lensing for OGLE-II microlensing observations
In the framework of microlensing searches towards the Large Magellanic Cloud
(LMC), we discuss the results presented by the OGLE collaboration for their
OGLE-II campaign \citep{lukas09}. We evaluate the optical depth, the duration
and the expected rate of events for the different possible lens populations:
both luminous, dominated by the LMC self lensing, and "dark", the would be
compact halo objects (MACHOs) belonging to either the Galactic or to the LMC
halo. The OGLE-II observational results, 2 microlensing candidate events
located in the LMC bar region with duration of 24.2 and 57.2 days, compare well
with the expected signal from the luminous lens populations:
, with typical duration, for LMC self lensing, of about 50
days. Because of the small statistics at disposal, however, the conclusions
that can be drawn as for the halo mass fraction, , in form of compact halo
objects are not too severe. By means of a likelihood analysis we find an
\emph{upper} limit for , at 95% confidence level, of about 15% in the mass
range and 26% for .Comment: accepted for publication in MNRA
Detecting Variability in Massive Astronomical Time-Series Data I: application of an infinite Gaussian mixture model
We present a new framework to detect various types of variable objects within
massive astronomical time-series data. Assuming that the dominant population of
objects is non-variable, we find outliers from this population by using a
non-parametric Bayesian clustering algorithm based on an infinite
GaussianMixtureModel (GMM) and the Dirichlet Process. The algorithm extracts
information from a given dataset, which is described by six variability
indices. The GMM uses those variability indices to recover clusters that are
described by six-dimensional multivariate Gaussian distributions, allowing our
approach to consider the sampling pattern of time-series data, systematic
biases, the number of data points for each light curve, and photometric
quality. Using the Northern Sky Variability Survey data, we test our approach
and prove that the infinite GMM is useful at detecting variable objects, while
providing statistical inference estimation that suppresses false detection. The
proposed approach will be effective in the exploration of future surveys such
as GAIA, Pan-Starrs, and LSST, which will produce massive time-series data.Comment: accepted for publication in MNRA
Global dynamics of advection-dominated accretion flows with magnetically driven outflow
We study the global dynamics of advection-dominated accretion flows (ADAFs)
with magnetically driven outflows. A fraction of gases in the accretion flow is
accelerated into the outflows, which leads to decreasing of the mass accretion
rate in the accretion flow towards the black hole. We find that the r-dependent
mass accretion rate is close to a power-law one, m_dot r^s, as assumed in the
advection-dominated inflow-outflow solution (ADIOS), in the outer region of the
ADAF, while it deviates significantly from the power-law r-dependent accretion
rate in the inner region of the ADAF. It is found that the structure of the
ADAF is significantly changed in the presence of the outflows. The temperatures
of the ions and electrons in the ADAF decreases in the presence of outflows, as
a fraction of gravitational power released in the ADAF is tapped to accelerate
the outflows.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Swift observations of the X-ray and UV evolution of V2491 Cyg (Nova Cyg 2008 No. 2)
We present extensive, high-density Swift observations of V2491 Cyg (Nova Cyg
2008 No. 2). Observing the X-ray emission from only one day after the nova
discovery, the source is followed through the initial brightening, the
Super-Soft Source phase and back to the pre-outburst flux level. The evolution
of the spectrum throughout the outburst is demonstrated. The UV and X-ray
light-curves follow very different paths, although changes occur in them around
the same times, indicating a link between the bands. Flickering in the
late-time X-ray data indicates the resumption of accretion.
We show that if the white dwarf is magnetic, it would be among the most
magnetic known; the lack of a periodic signal in our later data argues against
a magnetic white dwarf, however. We also discuss the possibility that V2491 Cyg
is a recurrent nova, providing recurrence timescale estimates.Comment: 12 pages, 8 figure (2 in colour), accepted for publication in MNRA
Long-lived planetesimal discs
We investigate the survival of planetesimal discs over Gyr timescales, using
a unified approach that is applicable to all Keplerian discs of solid bodies --
dust grains, asteroids, planets, etc. Planetesimal discs can be characterized
locally by four parameters: surface density, semi-major axis, planetesimal size
and planetesimal radial velocity dispersion. Any planetesimal disc must have
survived all dynamical processes, including gravitational instability,
dynamical chaos, gravitational scattering, physical collisions, and radiation
forces, that would lead to significant evolution over its lifetime. These
processes lead to a rich set of constraints that strongly restrict the possible
properties of long-lived discs. Within this framework, we also discuss the
detection of planetesimal discs using radial velocity measurements, transits,
microlensing, and the infrared emission from the planetesimals themselves or
from dust generated by planetesimal collisions.Comment: 31 pages (single column, font size 10), 10 figures, 2 tables.
Accepted by MNRAS. This amended version corrects minor errors in Figures 3, 4
and 5 (erratum submitted to MNRAS). Text and conclusions unchanged
The chemical composition of donors in AM CVn stars and ultra-compact X-ray binaries: observational tests of their formation
We study the formation of ultra-compact binaries (AM CVn stars and
ultra-compact X-ray binaries) with emphasis on the surface chemical abundances
of the donors in these systems. Hydrogen is not convincingly detected in the
spectra of these systems. Three different proposed formation scenarios involve
different donor stars, white dwarfs, helium stars or evolved main-sequence
stars. Using detailed evolutionary calculations we show that the abundances of
helium WD donors and evolved main-sequence stars are close to equilibrium
CNO-processed material, and the detailed abundances correlate with the core
temperature and thus mass of the MS progenitors. Evolved MS donors typically
have traces of H left. For hybrid or CO white dwarf donors, the carbon and
oxygen abundances depend on the temperature of the helium burning and thus on
the helium core mass of the progenitors. For helium star donors in addition to
their mass, the abundances depend strongly on the amount of helium burnt before
mass transfer starts and can range from unprocessed and thus almost equal to
CNO-processed matter, to strongly processed and thus C/O rich and N-deficient.
We briefly discuss the relative frequency of these cases for helium star
donors, based on population synthesis results. Finally we give diagnostics for
applying our results to observed systems and find that the most important test
is the N/C ratio, which can indicate the formation scenario as well as, in some
cases, the mass of the progenitor of the donor. In addition, if observed, the
N/O, O/He and O/C ratios can distinguish between helium star and WD donors.
Applied to the known systems we find evidence for WD donors in the AM CVn
systems GP Com, CE 315 and SDSS J0804+16 and evidence for hybrid WD or very
evolved helium star donors in the UCXBs 4U 1626-67 and 4U 0614+09. [Abridged]Comment: Accepted for publication in MNRA