58,947 research outputs found
Effects of galaxy-halo alignment and adiabatic contraction on gravitational lens statistics
We study the strong gravitational lens statistics of triaxial cold dark
matter (CDM) halos occupied by central early-type galaxies. We calculate the
image separation distribution for double, cusp and quad configurations. The
ratios of image multiplicities at large separations are consistent with the
triaxial NFW model, and at small separations are consistent with the singular
isothermal ellipsoid (SIE) model. At all separations, the total lensing
probability is enhanced by adiabatic contraction. If no adiabatic contraction
is assumed, naked cusp configurations become dominant at approximately 2.5'',
which is inconsistent with the data. We also show that at small-to-moderate
separations, the image multiplicities depend sensitively on the alignment of
the shapes of the luminous and dark matter projected density profiles. In
constrast to other properties that affect these ratios, the degree of alignment
does not have a significant effect on the total lensing probability. These
correlations may therefore be constrained by comparing the theoretical image
separation distribution to a sufficiently large lens sample from future wide
and deep sky surveys such as Pan-Starrs, LSST and JDEM. Understanding the
correlations in the shapes of galaxies and their dark matter halo is important
for future weak lensing surveys.Comment: 10 pages, 7 figure
Detecting Unresolved Binaries in TESS Data with Speckle Imaging
The Transiting Exoplanet Survey Satellite (TESS) is conducting a two-year
wide-field survey searching for transiting exoplanets around nearby bright
stars that will be ideal for follow-up characterization. To facilitate studies
of planet compositions and atmospheric properties, accurate and precise
planetary radii need to be derived from the transit light curves. Since 40 -
50% of exoplanet host stars are in multiple star systems, however, the observed
transit depth may be diluted by the flux of a companion star, causing the
radius of the planet to be underestimated. High angular resolution imaging can
detect companion stars that are not resolved in the TESS Input Catalog, or by
seeing-limited photometry, to validate exoplanet candidates and derive accurate
planetary radii. We examine the population of stellar companions that will be
detectable around TESS planet candidate host stars, and those that will remain
undetected, by applying the detection limits of speckle imaging to the
simulated host star populations of Sullivan et al. (2015) and Barclay et al.
(2018). By detecting companions with contrasts of delta m < 7 - 9 and
separations of ~0.02 - 1.2'', speckle imaging can detect companion stars as
faint as early M stars around A - F stars and stars as faint as mid-M around G
- M stars, as well as up to 99% of the expected binary star distribution for
systems located within a few hundred parsecs.Comment: Accepted for publication in The Astronomical Journal; 16 pages, 8
figures, 2 table
Long-term stability of the HR 8799 planetary system without resonant lock
HR 8799 is a star accompanied by four massive planets on wide orbits. The
observed planetary configuration has been shown to be unstable on a timescale
much shorter than the estimated age of the system (~ 30 Myr) unless the planets
are locked into mean motion resonances. This condition is characterised by
small-amplitude libration of one or more resonant angles that stabilise the
system by preventing close encounters. We simulate planetary systems similar to
the HR 8799 planetary system, exploring the parameter space in separation
between the orbits, planetary masses and distance from the Sun to the star. We
find systems that look like HR 8799 and remain stable for longer than the
estimated age of HR 8799. None of our systems are forced into resonances. We
find, with nominal masses and in a narrow range of orbit separations, that 5 of
100 systems match the observations and lifetime. Considering a broad range of
orbit separations, we find 12 of 900 similar systems. The systems survive
significantly longer because of their slightly increased initial orbit
separations compared to assuming circular orbits from the observed positions. A
small increase in separation leads to a significant increase in survival time.
The low eccentricity the orbits develop from gravitational interaction is
enough for the planets to match the observations. With lower masses, but still
comfortably within the estimated planet mass uncertainty, we find 18 of 100
matching and long-lived systems in a narrow orbital separation range. In the
broad separation range, we find 82 of 900 matching systems. Our results imply
that the planets in the HR 8799 system do not have to be in strong mean motion
resonances.Comment: Accepted for publication in A&
Charge Transfer from Regularized Symmetry-Adapted Perturbation Theory
16 pages, 16 figure
The Effects of Close Companions (and Rotation) on the Magnetic Activity of M Dwarfs
We present a study of close white dwarf and M dwarf (WD+dM) binary systems
and examine the effect that a close companion has on the magnetic field
generation in M dwarfs. We use a base sample of 1602 white dwarf -- main
sequence binaries from Rebassa et al. to develop a set of color cuts in GALEX,
SDSS, UKIDSS, and 2MASS color space to construct a sample of 1756 WD+dM
high-quality pairs from the SDSS DR8 spectroscopic database. We separate the
individual WD and dM from each spectrum using an iterative technique that
compares the WD and dM components to best-fit templates. Using the absolute
height above the Galactic plane as a proxy for age, and the H{\alpha} emission
line as an indicator for magnetic activity, we investigate the age-activity
relation for our sample for spectral types \leqM7. Our results show that
early-type M dwarfs (\leqM4) in close binary systems are more likely to be
active and have longer activity lifetimes compared to their field counterparts.
However, at a spectral type of M5 (just past the onset of full convection in M
dwarfs), the activity fraction and lifetimes of WD+dM binary systems becomes
more comparable to that of the field M dwarfs. One of the implications of
having a close binary companion is presumed to be increased stellar rotation
through disk-disruption, tidal effects, or angular momentum exchange. Thus, we
interpret the similarity in activity behavior between late-type dMs in WD+dM
pairs and late-type field dMs to be due to a decrease in sensitivity in close
binary companions (or stellar rotation), which has implications for the nature
of magnetic activity in fully-convective stars. (Abridged)Comment: 21 pages, 19 figures, emulateapj style, accepted to Astronomical
Journal June 28, 201
gravity theories in the Palatini Formalism constrained from strong lensing
gravity, capable of driving the late-time acceleration of the
universe, is emerging as a promising alternative to dark energy. Various
gravity models have been intensively tested against probes of the expansion
history, including type Ia supernovae (SNIa), the cosmic microwave background
(CMB) and baryon acoustic oscillations (BAO). In this paper we propose to use
the statistical lens sample from Sloan Digital Sky Survey Quasar Lens Search
Data Release 3 (SQLS DR3) to constrain gravity models. This sample can
probe the expansion history up to , higher than what probed by
current SNIa and BAO data. We adopt a typical parameterization of the form
with and
constants. For (CDM), we obtain the best-fit value of the
parameter , for which the 95% confidence interval that is
[-4.633, -3.754]. This best-fit value of corresponds to the matter
density parameter , consistent with constraints from other
probes. Allowing to be free, the best-fit parameters are . Consequently, we give and the
deceleration parameter . At the 95% confidence level, and
are constrained to [-4.67, -2.89] and [-0.078, 0.202] respectively.
Clearly, given the currently limited sample size, we can only constrain
within the accuracy of and thus can not distinguish
between CDM and gravity with high significance, and actually,
the former lies in the 68% confidence contour. We expect that the extension of
the SQLS DR3 lens sample to the SDSS DR5 and SDSS-II will make constraints on
the model more stringent.Comment: 10 pages, 7 figures. Accepted for publication in MNRA
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