359 research outputs found
Microlensing Parallax for Observers in Heliocentric Motion
Motivated by the ongoing Spitzer observational campaign, and the forecoming
K2 one, we revisit, working in an heliocentric reference frame, the geometrical
foundation for the analysis of the microlensing parallax, as measured with the
simultaneous observation of the same microlensing event from two observers with
relative distance of order AU. For the case of observers at rest we discuss the
well known fourfold microlensing parallax degeneracy and determine an equation
for the degenerate directions of the lens trajectory. For the case of observers
in motion, we write down an extension of the Gould (1994) relationship between
the microlensing parallax and the observable quantities and, at the same time,
we highlight the functional dependence of these same quantities from the
timescale of the underlying microlensing event. Furthermore, through a series
of examples, we show the importance of taking into account the motion of the
observers to correctly recover the parameters of the underlying microlensing
event. In particular we discuss the cases of the amplitude of the microlensing
parallax and that of the difference of the timescales between the observed
microlensing events, key to understand the breaking of the microlensing
parallax degeneracy. Finally, we consider the case of the simultaneous
observation of the same microlensing event from ground and two satellites, a
case relevant for the expected joint K2 and Spitzer observational programs in
2016.Comment: Accepted for publication in Ap
Newtonian Limit of Induced Gravity
We discuss the weak-field limit of induced gravity and show that results
directly depend on the coupling and self-interaction potential of the scalar
field. A static spherically symmetric exact solution is found and its conformal
properties are studied. As an application, it is shown that the light
deflection angle and the microlensing quantities can be parametrized by the
coupling of the theory.Comment: 15 pages, LATEX, to appear in Grav & Cos
Searching for variable stars in the cores of five metal-rich globular clusters using EMCCD observations
Aims. In this paper, we present the analysis of time-series observations from 2013 and 2014 of five metal-rich ([Fe/H] > −1) globular clusters: NGC 6388, NGC 6441, NGC 6528, NGC 6638, and NGC 6652. The data have been used to perform a census of the variable stars in the central parts of these clusters.
Methods. The observations were made with the electron-multiplying charge-couple device (EMCCD) camera at the Danish 1.54 m Telescope at La Silla, Chile, and they were analysed using difference image analysis to obtain high-precision light curves of the variable stars.
Results. It was possible to identify and classify all of the previously known or suspected variable stars in the central regions of the five clusters. Furthermore, we were able to identify and, in most cases, classify 48, 49, 7, 8, and 2 previously unknown variables in NGC 6388, NGC 6441, NGC 6528, NGC 6638, and NGC 6652, respectively. Especially interesting is the case of NGC 6441, for which the variable star population of about 150 stars has been thoroughly examined by previous studies, including a Hubble Space Telescope study. In this paper we are able to present 49 new variable stars for this cluster, of which one (possibly two) are RR Lyrae stars, two are W Virginis stars, and the rest are long-period semi-regular or irregular variables on the red giant branch. We have also detected the first double-mode RR Lyrae in the cluster
OGLE-2013-BLG-0911Lb: A Secondary on the Brown-dwarf Planet Boundary around an M Dwarf
We present the analysis of the binary-lens microlensing event OGLE-2013-BLG-0911. The best-fit solutions indicate the binary mass ratio of q 0.03, which differs from that reported in Shvartzvald et al. The event suffers from the well-known close/wide degeneracy, resulting in two groups of solutions for the projected separation normalized by the Einstein radius of s ~ 0.15 or s ~ 7. The finite source and the parallax observations allow us to measure the lens physical parameters. The lens system is an M dwarf orbited by a massive Jupiter companion at very close (M_(host) = 0.30^(+0.08)_(-0.06)M⊙, M_(comp) = 10.1^(+2.9)_(-2.2)M_(Jup), a_(exp) = 0.40^(+0.05)_(-0.04) au) or wide (M_(host) = 0.28^(+0.10)_(-0.08) M⊙, M_(comp) = 9.9^(+3.8)_(-3.5)M_(Jup), a(exp) = 18.0^(+3.8)_(3.5) au) separation. Although the mass ratio is slightly above the planet-brown dwarf (BD) mass-ratio boundary of q = 0.03, which is generally used, the median physical mass of the companion is slightly below the planet-BD mass boundary of 13M_(Jup). It is likely that the formation mechanisms for BDs and planets are different and the objects near the boundaries could have been formed by either mechanism. It is important to probe the distribution of such companions with masses of ~13M_(Jup) in order to statistically constrain the formation theories for both BDs and massive planets. In particular, the microlensing method is able to probe the distribution around low-mass M dwarfs and even BDs, which is challenging for other exoplanet detection methods
Microlensing Parallax for Observers in Heliocentric Motion
Motivated by the ongoing Spitzer observational campaign, and the forthcoming K2 one, we revisit, working in an heliocentric reference frame, the geometrical foundation for the analysis of the microlensing parallax, as measured with the simultaneous observation of the same microlensing event from two observers with relative distance of order au. For the case of observers at rest, we discuss the well-known fourfold microlensing parallax degeneracy and determine an equation for the degenerate directions of the lens trajectory. For the case of observers in motion, we write down an extension of the Gould relationship between the microlensing parallax and the observable quantities and, at the same time, highlight the functional dependence of these same quantities from the timescale of the underlying microlensing event. Furthermore, through a series of examples, we show the importance of taking into account themotion of the observers to correctly recover the parameters of the underlying microlensing event. In particular, we discuss the cases of the amplitude of the microlensing parallax and that of the difference of the timescales between the observed microlensing events, which are key to understand the breaking of the microlensing parallax degeneracy. Finally, we consider the case of the simultaneous observation of the same microlensing event from the ground and two satellites, a case relevant for the expected joint K2 and Spitzer observational programs in 2016
On the mass of the gravitational lenses in LMC
In the self--lensing framework, we estimate the modal values of the mass of
the gravitational lenses found by the MACHO collaboration towards the Large
Magellanic Cloud (LMC). Our results suggest that only the events located near
the center can be identified as a low mass star population belonging to the LMC
disk or bar components.Comment: To appear in the proceedings of the Marcel Grossmann Meeeting
OGLE-2016-BLG-1045: A Test of Cheap Space-based Microlens Parallaxes
Microlensing is a powerful and unique technique to probe isolated objects in the Galaxy. To study the characteristics of these interesting objects based on the microlensing method, measurement of the microlens parallax is required to determine the properties of the lens. Of the various methods to measure microlens parallax, the most routine way is to make simultaneous ground- and space-based observations, i.e., by measuring the space-based microlens parallax. However, space-based campaigns usually require "expensive" resources. Gould & Yee (2012) proposed an idea called the "cheap space-based microlens parallax" that can measure the lens-parallax using only two or three space-based observations of high-magnification events (as seen from Earth). This cost-effective observation strategy to measure microlens parallaxes could be used by space-borne telescopes to build a complete sample for studying isolated objects. This would enable a direct measurement of the mass function including both extremely low-mass objects and high-mass stellar remnants. However, to adopt this idea requires a test to check how it would work in actual situations. Thus, we present the first practical test of this idea using the high-magnification microlensing event OGLE-2016-BLG-1045, for which a subset of Spitzer observations fortuitously duplicates the prescription of Gould & Yee (2012). From the test, we confirm that the measurement of the lens-parallax adopting this idea has sufficient accuracy to determine the physical properties of the isolated lens
Microlensing towards the Magellanic Clouds and M31: is the quest for MACHOs still open?
Microlensing is the tool of choice for the search and the analysis of compact
halo objects ("MACHOs"), a still viable class of dark matter candidates at the
galactic scale. Different analyses point towards an agreement in excluding dark
matter MACHOs of less than about 0.1 solar mass; it remains however an ongoing
debate for values in the mass range (0.1-1) solar mass. The more robust
constraints, though not all in agreement, come from the observational campaigns
towards the Magellanic Clouds (the LMC and the SMC). The analyses towards the
nearby galaxy of M31, in the so called "pixel lensing" regime, have expanded
the perspectives in this field of research. In this contribution first we draw
a critical view on recent results and then we focus on the pixel lensing
analysis towards M31 of the PLAN collaboration.Comment: 8 pages, no figures; to appear in the Proceedings of the 3rd
Italian-Pakistani Workshop on Relativistic Astrophysics, Lecce 20-22 June
2011, published in Journal of Physics: Conference Series (JPCS
OGLE-2013-BLG-0911Lb: A Secondary on the Brown-dwarf Planet Boundary around an M Dwarf
We present the analysis of the binary-lens microlensing event OGLE-2013-BLG-0911. The best-fit solutions indicate the binary mass ratio of q 0.03, which differs from that reported in Shvartzvald et al. The event suffers from the well-known close/wide degeneracy, resulting in two groups of solutions for the projected separation normalized by the Einstein radius of s ~ 0.15 or s ~ 7. The finite source and the parallax observations allow us to measure the lens physical parameters. The lens system is an M dwarf orbited by a massive Jupiter companion at very close (M_(host) = 0.30^(+0.08)_(-0.06)M⊙, M_(comp) = 10.1^(+2.9)_(-2.2)M_(Jup), a_(exp) = 0.40^(+0.05)_(-0.04) au) or wide (M_(host) = 0.28^(+0.10)_(-0.08) M⊙, M_(comp) = 9.9^(+3.8)_(-3.5)M_(Jup), a(exp) = 18.0^(+3.8)_(3.5) au) separation. Although the mass ratio is slightly above the planet-brown dwarf (BD) mass-ratio boundary of q = 0.03, which is generally used, the median physical mass of the companion is slightly below the planet-BD mass boundary of 13M_(Jup). It is likely that the formation mechanisms for BDs and planets are different and the objects near the boundaries could have been formed by either mechanism. It is important to probe the distribution of such companions with masses of ~13M_(Jup) in order to statistically constrain the formation theories for both BDs and massive planets. In particular, the microlensing method is able to probe the distribution around low-mass M dwarfs and even BDs, which is challenging for other exoplanet detection methods
Monte Carlo analysis of the MEGA microlensing events towards M31
We perform an analytical study and a Monte Carlo (MC) analysis of the main
features for microlensing events in pixel lensing observations towards M31. Our
main aim is to investigate the lens nature and location of the 14 candidate
events found by the MEGA collaboration. Assuming a reference model for the mass
distribution in M31 and the standard model for our galaxy, we estimate the
MACHO-to-self lensing probability and the event time duration towards M31.
Reproducing the MEGA observing conditions, as a result we get the MC event
number density distribution as a function of the event full-width half-maximum
duration and the magnitude at maximum . For a
MACHO mass of we find typical values of day
and , for both MACHO-lensing and self-lensing
events occurring beyond about 10 arcminutes from the M31 center. A comparison
of the observed features ( and ) with our MC
results shows that for a MACHO mass the four innermost MEGA
events are most likely self-lensing events, whereas the six outermost events
must be genuine MACHO-lensing events.Comment: in press on A&
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