218 research outputs found
A novel eccentricity parameterization for transit-only models
We present a novel eccentricity parameterization for transit-only fits that
allows us to efficiently sample the eccentricity and argument of periastron,
while being able to generate a self-consistent model of a planet in a Keplerian
orbit around its host star. With simulated fits of 330 randomly generated
systems, we demonstrate that typical parameterizations often lead to inaccurate
and overly precise determinations of the planetary eccentricity. However, our
proposed parameterization allows us to accurately -- and often precisely --
recover the eccentricity for the simulated planetary systems with only transit
data available.Comment: 10 pages, 7 figures, submitted to PAS
First Measurement of a Rapid Increase in the AGN Fraction in High-Redshift Clusters of Galaxies
We present the first measurement of the AGN fraction in high-redshift
clusters of galaxies (z~0.6) with spectroscopy of one cluster and archival data
for three additional clusters. We identify 8 AGN in all four of these clusters
from the Chandra data, which are sensitive to AGN with hard X-ray (2-10keV)
luminosity L_{X,H} > 10^43 erg/s in host galaxies more luminous than a rest
frame M_R < -20 mag. This stands in sharp contrast to the one AGN with L_{X,H}
> 10^43 erg/s we discovered in our earlier study of eight low-redshift clusters
with z=0.06-0.31 (average z~0.2). Three of the four high-redshift cluster
datasets are sensitive to nearly L_{X,H} > 10^42 erg/s and we identify seven
AGN above this luminosity limit, compared to two in eight, low-redshift
clusters. Based on membership estimates for each cluster, we determine that the
AGN fraction at z~0.6 is f_A(L_X>10^42;M_R<-20) = 0.028 (+0.019/-0.012) and
f_A(L_X>10^43;M_R<-20) = 0.020 (+0.012/-0.008). These values are approximately
a factor of 20 greater than the AGN fractions in lower-redshift (average z~0.2)
clusters of galaxies and represent a substantial increase over the factors of
1.5 and 3.3 increase, respectively, in the measured space density evolution of
the hard X-ray luminosity function over this redshift range. Potential
systematic errors would only increase the significance of our result. The
cluster AGN fraction increases more rapidly with redshift than the field and
the increase in cluster AGN indicates the presence of an AGN Butcher-Oemler
Effect.Comment: ApJL Accepted, 5 pages, 2 figure
Photon-weighted barycentric correction and its importance for precise radial velocities
When applying the barycentric correction to a precise radial velocity
measurement, it is common practice to calculate its value only at the
photon-weighted midpoint time of the observation instead of integrating over
the entire exposure. However, since the barycentric correction does not change
linearly with time, this leads to systematic errors in the derived radial
velocities. The typical magnitude of this second-order effect is of order 10 cm
s, but it depends on several parameters, e.g. the latitude of the
observatory, the position of the target on the sky, and the exposure time. We
show that there are realistic observing scenarios, where the errors can amount
to more than 1 ms. We therefore recommend that instruments operating in
this regime always record and store the exposure meter flux curve (or a similar
measure) to be used as photon-weights for the barycentric correction. In
existing data, if the flux curve is no longer available, we argue that
second-order errors in the barycentric correction can be mitigated by adding a
correction term assuming constant flux.Comment: 9 pages, 7 figures, accepted to MNRA
KELT-4Ab: An inflated Hot Jupiter transiting the bright (V~10) component of a hierarchical triple
We report the discovery of KELT-4Ab, an inflated, transiting Hot Jupiter orbiting the brightest component of a hierarchical triple stellar system. The host star is an F star with T_(eff) = 6206 ± 75 K, log g = 4.108 ± 0.014, [Fe/H] = -0.116_(-0.069)^(+0.065, M_* = 1.201_(-0.061)^(+0.067) M_☉, and R_* = 1.603_(-0.038)^(+0.039) R_☉. The best-fit linear ephemeris is BJD_(TDB); = 2456193.29157 ± 0.00021 + E(2.9895936 ± 0.0000048). With a magnitude of V ~ 10, a planetary radius of 1.699_(-0.045)^(+0.046); R_J, and a mass of 0.902_(-0.059)^(+0.060) M_J, it is the brightest host among the population of inflated Hot Jupiters (R_P > 1.5 R_J), making it a valuable discovery for probing the nature of inflated planets. In addition, its existence within a hierarchical triple and its proximity to Earth (210 pc) provide a unique opportunity for dynamical studies with continued monitoring with high resolution imaging and precision radial velocities. The projected separation between KELT-4A and KELT-4BC is 328 ± 16 AU and the projected separation between KELT-4B and KELT-4C is 10.30 ± 0.74 AU. Assuming face-on, circular orbits, their respective periods would be 3780 ± 290 and 29.4 ± 3.6 years and the astrometric motions relative to the epoch in this work of both the binary stars around each other and of the binary around the primary star would be detectable now and may provide meaningful constraints on the dynamics of the system
An Exploration of Systematic Errors in Transiting Planets and Their Host Stars
Transiting planet systems offer the best opportunity to measure the masses
and radii of a large sample of planets and their host stars. However, relative
photometry and radial velocity measurements alone only constrain the density of
the host star. Thus, there is a one-parameter degeneracy in the mass and radius
of the host star, and by extension the planet. Several theoretical,
semi-empirical, and nearly empirical methods have been used to break this
degeneracy and independently measure the mass and radius of the host star and
planets(s). As we approach an era of few percent precisions on some of these
properties, it is critical to assess whether these different methods are
providing accuracies that are of the same order, or better than, the stated
statistical precisions. We investigate the differences in the planet parameter
estimates inferred when using the Torres empirical relations, YY isochrones,
MIST isochrones, and a nearly-direct empirical measurement of the radius of the
host star using its spectral energy distribution, effective temperature, and
\textit{Gaia} parallax. We focus our analysis on modelling KELT-15b, a fairly
typical hot Jupiter, using each of these methods. We globally model TESS
photometry, optical-to-NIR flux densities of the host star, and \textit{Gaia}
parallaxes, in conjunction with extant KELT ground-based follow-up photometric
and radial velocity measurements. We find systematic differences in several of
the inferred parameters of the KELT-15 system when using different methods,
including a () difference in the inferred stellar and
planetary radii between the MIST isochrones and SED fitting.Comment: 12 Figures, 20 Tables, Submitted to Ap
Seeing-limited Coupling of Starlight into Single-mode Fiber with a Small Telescope
An optical fiber link to a telescope provides many advantages for
spectrometers designed to detect and characterize extrasolar planets through
precise radial velocity (PRV) measurements. In the seeing-limited regime, a
multi-mode fiber is typically used so that a significant amount of starlight
may be captured. In the near-diffraction-limited case, either with an adaptive
optics system or with a small telescope at an excellent site, efficiently
coupling starlight into a much smaller, single-mode fiber may be possible. In
general, a spectrometer designed for single-mode fiber input will be
substantially less costly than one designed for multi-mode fiber input. We
describe the results of tests coupling starlight from a 70 cm telescope at Mt.
Hopkins, Arizona into the single-mode fiber of the MINERVA-Red spectrometer at
a wavelength of ~850 nm using a low-speed tip/tilt image stabilization system
comprising all commercial, off-the-shelf components. We find that approximately
0.5% of the available starlight is coupled into the single-mode fiber under
seeing conditions typical for observatories hosting small telescopes, which is
close to the theoretical expectation. We discuss scientific opportunities for
small telescopes paired with inexpensive, high-resolution spectrometers, as
well as upgrade paths that should significantly increase the coupling
efficiency for the MINERVA-Red system.Comment: 8 pages, 4 figures. Accepted for publication in Astronomische
Nachrichte
HST/WFC3 Light Curve Confirms the Closest Exoplanet to Transit an M Dwarf is Terrestrial
Previous studies of the exoplanet LTT 1445Ac concluded that the light curve
from the Transiting Exoplanet Survey Satellite (TESS) was consistent with both
grazing and non-grazing geometries. As a result, the radius and hence density
of the planet remained unknown. To resolve this ambiguity, we observed the LTT
1445 system for six spacecraft orbits of the Hubble Space Telescope (HST) using
WFC3/UVIS imaging in spatial scan mode, including one partial transit of LTT
1445Ac. This imaging produces resolved light curves of each of the three stars
in the LTT 1445 system. We confirm that the planet transits LTT 1445A and that
LTT 1445C is the source of the rotational modulation seen in the TESS light
curve, and we refine the estimate of the dilution factor for the TESS data. We
perform a joint fit to the TESS and HST observations, finding that the transit
of LTT 1445Ac is not grazing with 97% confidence. We measure a planetary radius
of 1.10 R. Combined with previous radial velocity
observations, our analysis yields a planetary mass of M
and a planetary density of 5.6 g cm. LTT 1445Ac is an
Earth analog with respect to its mass and radius, albeit with a higher
instellation, and is therefore an exciting target for future atmospheric
studies.Comment: Submitted to AJ. 9 pages, 7 figures, 3 table
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