621 research outputs found
Many new variable stars discovered in the core of the globular cluster NGC 6715 (M 54) with EMCCD observations
Context. We show the benefits of using electron-multiplying CCDs and the shift-and-add technique as a tool to minimise the effects of atmospheric turbulence, such as blending between stars in crowded fields, and to avoid saturated stars in the fields observed. We intend to complete, or improve on, the census of the variable star population in globular cluster NGC 6715.
Aims. Our aim is to obtain high-precision time-series photometry of the very crowded central region of this stellar system via the collection of better angular resolution images than has been previously achieved with conventional CCDs on ground-based telescopes.
Methods. Observations were carried out using the Danish 1.54-m telescope at the ESO La Silla observatory in Chile. The telescope is equipped with an electron-multiplying CCD that enables short-exposure-time images to be obtained (ten images per second) that were stacked using the shift-and-add technique to produce the normal-exposure-time images (minutes). The high precision photometry was performed via difference image analysis employing the DanDIA pipeline. We attempted automatic detection of variable stars in the field.
Results. We statistically analysed the light curves of 1405 stars in the crowded central region of NGC 6715 to automatically identify the variable stars present in this cluster. We found light curves for 17 previously known variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67 new variables (30 RR Lyrae, 21 irregular (long-period type), 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11 unclassified). Photometric measurements for these stars are available in electronic form through the Strasbourg Astronomical Data Centre
OGLE-2011-BLG-0265Lb: A Jovian Microlensing Planet Orbiting an M Dwarf
We report the discovery of a Jupiter-mass planet orbiting an M-dwarf star that gave rise to the microlensing event OGLE-2011-BLG-0265. Such a system is very rare among known planetary systems and thus the discovery is important for theoretical studies of planetary formation and evolution. High-cadence temporal coverage of the planetary signal, combined with extended observations throughout the event, allows us to accurately model the observed light curve. However, the final microlensing solution remains degenerate, yielding two possible configurations of the planet and the host star. In the case of the preferred solution, the mass of the planet is M_p = 0.9 ± 0.3 M_J, and the planet is orbiting a star with a mass M = 0.22 ± 0.06 M_☉. The second possible configuration (2σ away) consists of a planet with M_p = 0.6 ± 0.3 M_J and host star with M = 0.14 ± 0.06 M_☉. The system is located in the Galactic disk 3–4 kpc toward the Galactic bulge. In both cases, with an orbit size of 1.5–2.0 AU, the planet is a "cold Jupiter"—located well beyond the "snow line" of the host star. Currently available data make the secure selection of the correct solution difficult, but there are prospects for lifting the degeneracy with additional follow-up observations in the future, when the lens and source star separate
MOA-2016-BLG-227Lb: A Massive Planet Characterized by Combining Light-curve Analysis and Keck AO Imaging
We report the discovery of a microlensing planet—MOA-2016-BLG-227Lb—with a large planet/host mass ratio of q ≃ 9 × 10^(−3). This event was located near the K2 Campaign 9 field that was observed by a large number of telescopes. As a result, the event was in the microlensing survey area of a number of these telescopes, and this enabled good coverage of the planetary light-curve signal. High angular resolution adaptive optics images from the Keck telescope reveal excess flux at the position of the source above the flux of the source star, as indicated by the light-curve model. This excess flux could be due to the lens star, but it could also be due to a companion to the source or lens star, or even an unrelated star. We consider all these possibilities in a Bayesian analysis in the context of a standard Galactic model. Our analysis indicates that it is unlikely that a large fraction of the excess flux comes from the lens, unless solar-type stars are much more likely to host planets of this mass ratio than lower mass stars. We recommend that a method similar to the one developed in this paper be used for other events with high angular resolution follow-up observations when the follow-up observations are insufficient to measure the lens–source relative proper motion
First microlensing candidate towards M31 from the Nainital Microlensing Survey
We report our first microlensing candidate NMS-E1 towards M31 from the data
accumulated during the four years of Nainital Microlensing Survey. Cousin R and
I band observations of ~13'x13' field in the direction of M31 have been carried
out since 1998 and data is analysed using the pixel technique proposed by the
AGAPE collaboration. NMS-E1 lies in the disk of M31 at \alpha = 0:43:33.3 and
\delta = +41:06:44, about 15.5 arcmin to the South-East direction of the center
of M31. The degenerate Paczy\'{n}ski fit gives a half intensity duration of ~59
days. The photometric analysis of the candidate shows that it reached R~20.1
mag at the time of maximum brightness and the colour of the source star was
estimated to be (R-I)_0 ~ 1.1 mag. The microlensing candidate is blended by red
variable stars; consequently the light curves do not strictly follow the
characteristic Paczy\'{n}ski shape and achromatic nature. However its long
period monitoring and similar behaviour in R and I bands supports its
microlensing nature.Comment: no changes except typos corrected, to appear in A&
The evolution of tensor perturbations in scalar-tensor theories of gravity
The evolution equations for tensor perturbations in a generic scalar tensor
theory of gravity are presented. Exact solution are given for a specific class
of theories and Friedmann-Lema\^{i}tre-Robertson-Walker backgrounds. In these
cases it is shown that, although the evolution of tensor models depends on the
choice of parameters of the theory, no amplification is possible if the
gravitational interaction is attractive.Comment: 11 pages, 2 figures, submitted to Physical Review
Deformation of the Planetary Orbits Caused by the Time Dependent Gravitational Potential in the Universe
In the paper are studied the deformations of the planetary orbits caused by
the time dependent gravitational potential in the universe. It is shown that
the orbits are not axially symmetric and the time dependent potential does not
cause perihelion precession. It is found a simple formula for the change of the
orbit period caused by the time dependent gravitational potential and it is
tested for two binary pulsars.Comment: 7 page
Mirror dark matter and the new DAMA/LIBRA results: A simple explanation for a beautiful experiment
Recently, the DAMA/LIBRA experiment has convincingly confirmed the DAMA/NaI
annual modulation signal, experimentally demonstrating the existence of
non-baryonic dark matter in the halo of our galaxy. Meanwhile, in another part
of town, other experiments such as CDMS and XENON10 have not detected any
evidence for dark matter. One promising dark matter candidate which can
reconcile the positive DAMA annual modulation signal with the null results from
the other experiments, is mirror dark matter. We re-analyse the mirror matter
interpretation of the DAMA annual modulation signal utilizing a) the new data
from DAMA/LIBRA, including the measured energy dependence of the annual
modulation signal b) an updated quenching factor which takes into account the
channeling effect in crystals and c) the latest constraints from CDMS/Ge,
CDMS/Si and XENON10 experiments. We show that the simplest possibility of a
(and/or ) dominated halo with a small component is sufficient to
fully explain all of the dark matter experiments. We also point out that a
certain class of hidden sector dark matter models, although theoretically less
appealing and less constrained, can mimic the success of the mirror dark matter
model and hence are also viable.Comment: about 20 pages, 6 figure
First Space-based Microlens Parallax Measurement of an Isolated Star: Spitzer Observations of OGLE-2014-BLG-0939
We present the first space-based microlens parallax measurement of an
isolated star. From the striking differences in the lightcurve as seen from
Earth and from Spitzer (~1 AU to the West), we infer a projected velocity
v_helio,projected ~ 250 km/s, which strongly favors a lens in the Galactic Disk
with mass M=0.23 +- 0.07 M_sun and distance D_L=3.1 +- 0.4 kpc. An ensemble of
such measurements drawn from our ongoing program could be used to measure the
single-lens mass function including dark objects, and also is necessary for
measuring the Galactic distribution of planets since the ensemble reflects the
underlying Galactic distribution of microlenses. We study the application of
the many ideas to break the four-fold degeneracy first predicted by Refsdal 50
years ago. We find that this degeneracy is clearly broken, but by two
unanticipated mechanisms.Comment: 26 pages, 3 figures, submitted to Ap
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