130 research outputs found
Eclipsing binaries observed with the WIRE satellite. II, ÎČ Aurigae and non-linear limb darkening in light curves
Aims. We present the most precise light curve ever obtained of a detached eclipsing binary star and use it investigate the inclusion of non-linear limb darkening laws in light curve models of eclipsing binaries. This light curve, of the bright eclipsing system ÎČ Aurigae, was obtained using the star tracker aboard the wire satellite and contains 30 000 datapoints with a point-to-point scatter of 0.3mmag.
Methods. We analyse the wire light curve using a version of the ebop code modified to include non-linear limb darkening laws and to directly incorporate observed times of minimum light and spectroscopic light ratios into the photometric solution as individual observations. We also analyse the dataset with the Wilson-Devinney code to ensure that the two models give consistent results.
Results. ebop is able to provide an excellent fit to the high-precision wire data.Whilst the fractional radii of the stars are only defined to a precision of 5% by this light curve, including an accurate published spectroscopic light ratio improves this dramatically to 0.5%. Using non-linear limb darkening improves the quality of the fit significantly compared to the linear law and causes the measured radii to increase by 0.4%. It is possible to derive all of the limb darkening coefficients from the light curve, although they are strongly correlated with each other. The fitted coefficients agree with theoretical predictions to within their fairly large error estimates. We
were able to obtain a reasonably good fit to the data using the Wilson-Devinney code, but only using the highest available integration accuracy and by iterating for a long time. Bolometric albedos of 0.6 were found, which are appropriate to convective rather than radiative envelopes.
Conclusions. The radii and masses of the components of ÎČ Aur are RA = 2.762 ± 0.017 R, RB = 2.568 ± 0.017 R, MA = 2.376 ±0.027 M and MB = 2.291 ± 0.027 M, where A and B denote the primary and secondary star, respectively. Theoretical stellar evolutionary models can match these parameters for a solarmetal abundance and an age of 450â500 Myr. The Hipparcos trigonometric parallax and an interferometrically-derived orbital parallax give distances to ÎČ Aur which are in excellent agreement with each other
and with distances derived using surface brightness relations and several sets of empirical and theoretical bolometric corrections
Oscillations in Arcturus from WIRE photometry
Observations of the red giant Arcturus (Alpha Boo) obtained with the star
tracker on the Wide Field Infrared Explorer (WIRE) satellite during a baseline
of 19 successive days in 2000 July-August are analysed. The amplitude spectrum
has a significant excess of power at low-frequencies. The highest peak is at
about 4.1 micro-Hz (2.8 d), which is in agreement with previous ground-based
radial velocity studies. The variability of Arcturus can be explained by sound
waves, but it is not clear whether these are coherent p-mode oscillations or a
single mode with a short life-time.Comment: 6 pages, 1 Latex file, 4 .eps figures, 2 .sty files, ApJL, 591, L151
See erratum (astro-ph/0308424
Evidence for Granulation and Oscillations in Procyon from Photometry with the WIRE satellite
We report evidence for the granulation signal in the star Procyon A, based on
two photometric time series from the star tracker on the WIRE satellite. The
power spectra show evidence of excess power around 1 milliHz, consistent with
the detection of p-modes reported from radial velocity measurements. We see a
significant increase in the noise level below 3 milliHz, which we interpret as
the granulation signal. We have made a large set of numerical simulations to
constrain the amplitude and timescale of the granulation signal and the
amplitude of the oscillations. We find that the timescale for granulation is
T(gran) = 750(200) s, the granulation amplitude is 1.8(0.3) times solar, and
the amplitude of the p-modes is 8(3) ppm. We found the distribution of peak
heights in the observed power spectra to be consistent with that expected from
p-mode oscillations. However, the quality of the data is not sufficient to
measure the large separation or detect a comb-like structure, as seen in the
p-modes of the Sun. Comparison with the recent negative result from the MOST
satellite reveal that the MOST data must have an additional noise source that
prevented the detection of oscillations.Comment: 23 pages, 12 figures, submitted to ApJ; v2 revisions: one reference
corrected and a comment in Figure 7 correcte
Properties of 42 Solar-type Kepler Targets from the Asteroseismic Modeling Portal
Recently the number of main-sequence and subgiant stars exhibiting solar-like oscillations that are resolved
into individual mode frequencies has increased dramatically. While only a few such data sets were available
for detailed modeling just a decade ago, the Kepler mission has produced suitable observations for hundreds
of new targets. This rapid expansion in observational capacity has been accompanied by a shift in analysis
and modeling strategies to yield uniform sets of derived stellar properties more quickly and easily. We use
previously published asteroseismic and spectroscopic data sets to provide a uniform analysis of 42 solar-type
Kepler targets from the Asteroseismic Modeling Portal (AMP). We find that fitting the individual frequencies
typically doubles the precision of the asteroseismic radius, mass and age compared to grid-based modeling of
the global oscillation properties, and improves the precision of the radius and mass by about a factor of three
over empirical scaling relations. We demonstrate the utility of the derived properties with several applications
A Sensitive Search for Variability in Late L Dwarfs: The Quest for Weather
We have conducted a photometric monitoring program of three field late L brown dwarfs (DENIS-P J0255-4700, 2MASS J0908+5032, and 2MASS J2244+2043) looking for evidence of nonaxisymmetric structure or temporal variability in their photospheres. The observations were performed using Spitzer IRAC 4.5 and 8 ÎŒm bandpasses and were designed to cover at least one rotational period of each object; 1 Ï rms uncertainties of less than 3 mmag at 4.5 ÎŒm and around 9 mmag at 8 ÎŒm were achieved. Two out of the three objects studied exhibit some modulation in their light curves at 4.5 ÎŒmâbut not 8 ÎŒmâwith periods of 7.4 hr (DENIS 0255) and 4.6 hr (2MA 2244) and peak-to-peak amplitudes of 10 and 8 mmag. Although the lack of detectable 8 ÎŒm variation suggests an instrumental origin for the detected variations, the data may nevertheless still be consistent with intrinsic variability, since the shorter wavelength IRAC bandpasses probe more deeply into late L dwarf atmospheres than the longer wavelengths. A cloud feature occupying a small percentage (1%-2%) of the visible hemisphere could account for the observed amplitude of variation. If, instead, the variability is indeed instrumental in origin, then our nonvariable L dwarfs could be either completely covered with clouds or objects whose clouds are smaller and uniformly distributed. Such scenarios would lead to very small photometric variations. Follow-up IRAC photometry at 3.6 and 5.8 ÎŒm bandpasses should distinguish between the two cases. In any event, the present observations provide the most sensitive search to date for structure in the photospheres of late L dwarfs at mid-IR wavelengths, and our photometry provides stringent upper limits to the extent to which the photospheres of these transition L dwarfs are structured
Asteroseismology with the WIRE satellite. I. Combining Ground- and Space-based Photometry of the Delta Scuti Star Epsilon Cephei
We have analysed ground-based multi-colour Stromgren photometry and
single-filter photometry from the star tracker on the WIRE satellite of the
delta scuti star Epsilon Cephei. The ground-based data set consists of 16
nights of data collected over 164 days, while the satellite data are nearly
continuous coverage of the star during 14 days. The spectral window and noise
level of the satellite data are superior to the ground-based data and this data
set is used to locate the frequencies. However, we can use the ground-based
data to improve the accuracy of the frequencies due to the much longer time
baseline. We detect 26 oscillation frequencies in the WIRE data set, but only
some of these can be seen clearly in the ground-based data. We have used the
multi-colour ground-based photometry to determine amplitude and phase
differences in the Stromgren b-y colour and the y filter in an attempt to
identify the radial degree of the oscillation frequencies. We conclude that the
accuracies of the amplitudes and phases are not sufficient to constrain
theoretical models of Epsilon Cephei. We find no evidence for rotational
splitting or the large separation among the frequencies detected in the WIRE
data set. To be able to identify oscillation frequencies in delta scuti stars
with the method we have applied, it is crucial to obtain more complete coverage
from multi-site campaigns with a long time baseline and in multiple filters.
This is important when planning photometric and spectroscopic ground-based
support for future satellite missions like COROT and KEPLER.Comment: 13 pages, 12 figures, 4 tables. Fig. 4 reduced in quality. Accepted
by A&
A Tight Upper Limit on Oscillations in the Ap star Epsilon Ursae Majoris from WIRE Photometry
Observations of Epsilon UMa obtained with the star tracker on the Wide Field
Infrared Explorer (WIRE) satellite during a month in mid-2000 are analyzed.
This is one of the most precise photometry of an Ap star. The amplitude
spectrum is used to set an upper limit of 75 parts per million for the
amplitude of stellar pulsations in this star unless it accidentally oscillates
with a single mode at the satellite orbit, its harmonics or their one day
aliases. This is the tightest limit put on the amplitude of oscillations in an
Ap star. As the rotation period of Epsilon UMa is relatively short (5.1 d), it
cannot be argued that the observations were made at a wrong rotational phase.
Our results thus support the idea that some Ap stars do not pulsate at all.Comment: 4 pages, 4 figures, 2 style files, accepted for publication in ApJ
Asteroseismology of red giants from the first four months of Kepler data: Global oscillation parameters for 800 stars
We have studied solar-like oscillations in ~800 red-giant stars using Kepler
long-cadence photometry. The sample includes stars ranging in evolution from
the lower part of the red-giant branch to the Helium main sequence. We
investigate the relation between the large frequency separation (Delta nu) and
the frequency of maximum power (nu_max) and show that it is different for red
giants than for main-sequence stars, which is consistent with evolutionary
models and scaling relations. The distributions of nu_max and Delta nu are in
qualitative agreement with a simple stellar population model of the Kepler
field, including the first evidence for a secondary clump population
characterized by M ~> 2 M_sun and nu_max ~ 40-110 muHz. We measured the small
frequency separations delta nu_02 and delta nu_01 in over 400 stars and delta
nu_03 in over 40. We present C-D diagrams for l=1, 2 and 3 and show that the
frequency separation ratios delta nu_02/Delta nu and delta nu_01/Delta nu have
opposite trends as a function of Delta nu. The data show a narrowing of the l=1
ridge towards lower nu_max, in agreement with models predicting more efficient
mode trapping in stars with higher luminosity. We investigate the offset
epsilon in the asymptotic relation and find a clear correlation with Delta nu,
demonstrating that it is related to fundamental stellar parameters. Finally, we
present the first amplitude-nu_max relation for Kepler red giants. We observe a
lack of low-amplitude stars for nu_max ~> 110 muHz and find that, for a given
nu_max between 40-110 muHz, stars with lower Delta nu (and consequently higher
mass) tend to show lower amplitudes than stars with higher Delta nu.Comment: 12 pages, 12 figures, accepted for publication in Ap
Sounding stellar cycles with Kepler - II. Ground-based observations
We have monitored 20 Sun-like stars in the Kepler field-of-view for excess
flux with the FIES spectrograph on the Nordic Optical Telescope since the
launch of Kepler spacecraft in 2009. These 20 stars were selected based on
their asteroseismic properties to sample the parameter space (effective
temperature, surface gravity, activity level etc.) around the Sun. Though the
ultimate goal is to improve stellar dynamo models, we focus the present paper
on the combination of space-based and ground-based observations can be used to
test the age-rotation-activity relations.
In this paper we describe the considerations behind the selection of these 20
Sun-like stars and present an initial asteroseismic analysis, which includes
stellar age estimates. We also describe the observations from the Nordic
Optical Telescope and present mean values of measured excess fluxes. These
measurements are combined with estimates of the rotation periods obtained from
a simple analysis of the modulation in photometric observations from Kepler
caused by starspots, and asteroseismic determinations of stellar ages, to test
relations between between age, rotation and activity.Comment: Accepted for publication in MNRA
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