1,856 research outputs found
Detecting Differential Rotation and Starspot Evolution on the M dwarf GJ 1243 with Kepler
We present an analysis of the starspots on the active M4 dwarf GJ 1243, using
four years of time series photometry from Kepler. A rapid day rotation period is measured due to the 2.2\%
starspot-induced flux modulations in the light curve. We first use a light
curve modeling approach, using a Monte Carlo Markov Chain sampler to solve for
the longitudes and radii of the two spots within 5-day windows of data. Within
each window of time the starspots are assumed to be unchanging. Only a weak
constraint on the starspot latitudes can be implied from our modeling. The
primary spot is found to be very stable over many years. A secondary spot
feature is present in three portions of the light curve, decays on 100-500 day
timescales, and moves in longitude over time. We interpret this longitude
shearing as the signature of differential rotation. Using our models we measure
an average shear between the starspots of 0.0047 rad day, which
corresponds to a differential rotation rate of
rad day. We also fit this starspot phase evolution using a series of
bivariate Gaussian functions, which provides a consistent shear measurement.
This is among the slowest differential rotation shear measurements yet measured
for a star in this temperature regime, and provides an important constraint for
dynamo models of low mass stars.Comment: 13 pages, 7 figures, ApJ Accepte
Investment intermediaries in economic development: Linking public pension funds to urban revitalization
It is difficult for large investors, such as pension funds, to make investments in EDMs because they must make very large investments. The investments in communities of need, however, are usually small. The most successful strategy to overcome these two problems is for investors to work in concert with intermediaries that can aggregate the investments and community partners that understand both the need of communities and know how to tell “the story” to investors.
Examining Values Through Aphorisms
Many psychologists agree that studying values is an important aspect of understanding human nature (Allport, Rokeach, etc.). In spite of this consensus, there is little collective research on the subject. This is due, in part, to variations in instruments. Therefore, the purpose of the present research was to (a) assess the underlying structure of responses to a set of aphorisms, (b) to examine potential reasons for differential endorsement and (c) to explore the relationship between a set of aphorisms and traditional psychosocial measure of values as well as personality.Study 1 included 928 participants, and the objective was to examine the structure underlying responses to a set of aphorisms (Laws of Life; LL). Classic psychometric tests yielded 5 factors: Direction in life, Positivity, Self-Control, Reciprocity and Cognitive Determinism.Study 2 (n =258) focused on the explanation for differential endorsement rates. Results showed that the best predictor of endorsement rate was how relevant a particular item was to the participant.Study 3 (n =193) compared the LL to other measures of values in an attempt to assess the extent of the relationship between this set of aphorisms and traditional psychological measures. Results indicated a moderate convergence between the LL factors and measures of values, with Direction in life showing to be the strongest and most extensively related factor.The purpose of Study 4 (N = 318) was to assess the relationship between the LL factors and relevant measures of personality. The five factors showed a degree of variability among the personality constructs. Again, Direction in life showed to be the factor that showed the most convergence, whereas Cognitive Determinism showed no reliable relation to any of the personality constructs.This research attempted to examine an existing set of aphorisms and use this set in order to provide insight about the nature of values. Results showed that these aphorisms are related to both measures of values as well as measures of personality
WASP-1: A lithium- and metal-rich star with an oversized planet
In this paper we present our results of a comprehensive spectroscopicanalysis
of WASP-1, the host star to the exoplanet WASP-1b. We derive T_eff = 6110 +/-
45 K, log g = 4.28 +/- 0.15, and [M/H] = 0.23 +/- 0.08, and also a high
abundance of lithium, log n(Li) = 2.91 +/- 0.05. These parameters suggests an
age for the system of 1-3 Gyr and a stellar mass of 1.25-1.35 M_sun. This means
that WASP-1 has properties very similar to those of HD 149026, the host star
for the highest density planet yet detected. Moreover, their planets orbit at
comparable distances and receive comparable irradiating fluxes from their host
stars. However, despite the similarity of WASP-1 with HD 149026, their planets
have strongly different densities. This suggests that gas-giant planet density
is not a simple function of host-star metallicity or of radiation environment
at ages of ~2 Gyr.Comment: Accepted for publication in MNRAS. 6 pages, 4 figure
Detection limits for close eclipsing and transiting sub-stellar and planetary companions to white dwarfs in the WASP survey
We used photometric data from the WASP (Wide-Angle Search for Planets) survey
to explore the possibility of detecting eclipses and transit signals of brown
dwarfs, gas giants and terrestrial companions in close orbit around white
dwarfs. We performed extensive Monte Carlo simulations and we found that for
Gaussian random noise WASP is sensitive to companions as small as the Moon
orbiting a 12 white dwarf. For fainter stars WASP is sensitive to
increasingly larger bodies. Our sensitivity drops in the presence of co-variant
noise structure in the data, nevertheless Earth-size bodies remain readily
detectable in relatively low S/N data. We searched for eclipses and transit
signals in a sample of 194 white dwarfs in the WASP archive however, no
evidence for companions was found. We used our results to place tentative upper
limits to the frequency of such systems. While we can only place weak limits on
the likely frequency of Earth-sized or smaller companions; brown dwarfs and gas
giants (radius R) with periods 0.2 days must certainly be
rare (). More stringent constraints requires significantly larger white
dwarf samples, higher observing cadence and continuous coverage. The short
duration of eclipses and transits of white dwarfs compared to the cadence of
WASP observations appears to be one of the main factors limiting the detection
rate in a survey optimised for planetary transits of main sequence stars.Comment: 8 pages, 3 figure
An Improved Method for Estimating the Masses of Stars with Transiting Planets
To determine the physical parameters of a transiting planet and its host star
from photometric and spectroscopic analysis, it is essential to independently
measure the stellar mass. This is often achieved by the use of evolutionary
tracks and isochrones, but the mass result is only as reliable as the models
used. The recent paper by Torres et al (2009) showed that accurate values for
stellar masses and radii could be obtained from a calibration using T_eff, log
g and [Fe/H]. We investigate whether a similarly good calibration can be
obtained by substituting log rho - the fundamental parameter measured for the
host star of a transiting planet - for log g, and apply this to star-exoplanet
systems. We perform a polynomial fit to stellar binary data provided in Torres
et al (2009) to obtain the stellar mass and radius as functions of T_eff, log
rho and [Fe/H], with uncertainties on the fit produced from a Monte Carlo
analysis. We apply the resulting equations to measurements for seventeen
SuperWASP host stars, and also demonstrate the application of the calibration
in a Markov Chain Monte Carlo analysis to obtain accurate system parameters
where spectroscopic estimates of effective stellar temperature and metallicity
are available. We show that the calibration using log rho produces accurate
values for the stellar masses and radii; we obtain masses and radii of the
SuperWASP stars in good agreement with isochrone analysis results. We ascertain
that the mass calibration is robust against uncertainties resulting from poor
photometry, although a good estimate of stellar radius requires good-quality
transit light curve to determine the duration of ingress and egress.Comment: 5 pages, 2 figures, accepted for publication in A&
Rotation of Late-Type Stars in Praesepe with K2
We have Fourier analyzed 941 K2 light curves of likely members of Praesepe,
measuring periods for 86% and increasing the number of rotation periods (P) by
nearly a factor of four. The distribution of P vs. (V-K), a mass proxy, has
three different regimes: (V-K)<1.3, where the rotation rate rapidly slows as
mass decreases; 1.3<(V-K)<4.5, where the rotation rate slows more gradually as
mass decreases; and (V-K)>4.5, where the rotation rate rapidly increases as
mass decreases. In this last regime, there is a bimodal distribution of
periods, with few between 2 and 10 days. We interpret this to mean
that once M stars start to slow down, they do so rapidly. The K2 period-color
distribution in Praesepe (790 Myr) is much different than in the Pleiades
(125 Myr) for late F, G, K, and early-M stars; the overall distribution
moves to longer periods, and is better described by 2 line segments. For mid-M
stars, the relationship has similarly broad scatter, and is steeper in
Praesepe. The diversity of lightcurves and of periodogram types is similar in
the two clusters; about a quarter of the periodic stars in both clusters have
multiple significant periods. Multi-periodic stars dominate among the higher
masses, starting at a bluer color in Praesepe ((V-K)1.5) than in the
Pleiades ((V-K)2.6). In Praesepe, there are relatively more light curves
that have two widely separated periods, 6 days. Some of these could
be examples of M star binaries where one star has spun down but the other has
not.Comment: Accepted by Ap
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