Sharif Digital Repository

We present an analysis of potential follow-up polarimetric microlensing observations to study the stellar atmospheres of distant stars. First, we produce synthetic microlensing events using the Galactic model, stellar population and interstellar dust toward the Galactic bulge. We simulate the polarization microlensing light curves and pass them through the instrument specifications of the FOcal Reducer and low dispersion Spectrograph (FORS2) polarimeter at the Very Large Telescope (VLT), and then analyse them.We find that the accuracy of the VLT allows us to constrain the atmospheres of cool red giant branch (RGB) stars. Assuming that about 3000 microlensing events are detected per year by... 

The analysis of gravitational wave (GW) data from advanced LIGO provides the mass of each companion of binary black holes as the source of GWs. The mass of events corresponding to the binary black holes from GW is above 20 M Δ which is much larger than the mass of astrophysical black holes detected by x-ray observations. In this work, we examine primordial black holes (PBHs) as the source of LIGO events. Assuming that 100% of the dark matter is made of PBHs, we estimate the rate at which these objects make binaries, merge, and produce GWs as a function of the redshift. The gravitational lensing of GWs by PBHs can also enhance the amplitude of the strain. We simulate GWs sourced by binary... 

HATS-18 b is a transiting planet with a large mass and a short orbital period, and is one of the best candidates for the detection of orbital decay induced by tidal effects. We present extensive photometry of HATS-18 from which we measure 27 times of mid-transit. Two further transit times were measured from data from the Transiting Exoplanet Survey Satellite (TESS) and three more taken from the literature. The transit timings were fitted with linear and quadratic ephemerides and an upper limit on orbital decay was determined. This corresponds to a lower limit on the modified stellar tidal quality factor of $Q_star ^{, prime } gt 10^{5.11 pm 0.04}$. This is at the cusp of constraining the... 

Transits in the planetary system WASP-4 were recently found to occur 80 s earlier than expected in observations from the TESS satellite. We present 22 new times of mid-transit that confirm the existence of transit timing variations, and are well fitted by a quadratic ephemeris with period decay dP/dt = −9.2 ± 1.1 ms yr−1. We rule out instrumental issues, stellar activity, and the Applegate mechanism as possible causes. The light-time effect is also not favoured due to the non-detection of changes in the systemic velocity. Orbital decay and apsidal precession are plausible but unproven. WASP-4 b is only the third hot Jupiter known to show transit timing variations to high confidence. We... 

Cometary activity is a manifestation of sublimation-driven processes at the surface of nuclei. However, cometary outbursts may arise from other processes that are not necessarily driven by volatiles. In order to fully understand nuclear surfaces and their evolution, we must identify the causes of cometary outbursts. In that context, we present a study of mini-outbursts of comet 46P/Wirtanen. Six events are found in our long-term lightcurve of the comet around its perihelion passage in 2018. The apparent strengths range from -0.2 to -1.6 mag in a 5″ radius aperture and correspond to dust masses between ∼104 and 106 kg, but with large uncertainties due to the unknown grain size distributions.... 

Context. Brown dwarfs are transition objects between stars and planets that are still poorly understood, for which several competing mechanisms have been proposed to describe their formation. Mass measurements are generally difficult to carry out for isolated objects as well as for brown dwarfs orbiting low-mass stars, which are often too faint for a spectroscopic follow-up. Aims. Microlensing provides an alternative tool for the discovery and investigation of such faint systems. Here, we present an analysis of the microlensing event OGLE-2019-BLG-0033/MOA-2019-BLG-035, which is caused by a binary system composed of a brown dwarf orbiting a red dwarf. Methods. Thanks to extensive ground... 

We present an analysis of microlensing event OGLE-2019-BLG-0825. This event was identified as a planetary candidate by preliminary modeling. We find that significant residuals from the best-fit static binary-lens model exist and a xallarap effect can fit the residuals very well and significantly improves χ 2 values. On the other hand, by including the xallarap effect in our models, we find that binary-lens parameters such as mass ratio, q, and separation, s, cannot be constrained well. However, we also find that the parameters for the source system such as the orbital period and semimajor axis are consistent between all the models we analyzed. We therefore constrain the properties of the... 

We report an analysis of the planetary microlensing event OGLE-2018-BLG-1185, which was observed by a large number of ground-based telescopes and by the Spitzer Space Telescope. The ground-based light curve indicates a low planet-host star mass ratio of q = (6.9 0.2) 10-5, which is near the peak of the wide-orbit exoplanet mass-ratio distribution. We estimate the host star and planet masses with a Bayesian analysis using the measured angular Einstein radius under the assumption that stars of all masses have an equal probability of hosting the planet. The flux variation observed by Spitzer is marginal, but still places a constraint on the microlens parallax. Imposing a conservative constraint...