98 research outputs found
The Rotation Period Distribution in the Young Open Cluster NGC 6709
Open clusters serve as a useful tool for calibrating models of the
relationship between mass, rotation, and age for stars with an outer convection
zone due to the homogeneity of the stars within the cluster. Cluster to cluster
comparisons are essential to determine whether the universality of spin down
relations holds. NGC 6709 is selected as a young open cluster for which no
rotation periods of members have previously been obtained. This cluster is at a
distance of over 1 kpc and has two red giant members. Isochrones place the age
of the cluster at around 150 Myr, or approximately the same age as the
Pleiades. Photometry is obtained over a multi-month observing season at the
robotic observatory STELLA. After basic processing, PSF photometry was derived
using Daophot II, and a suite of related software allowed us to create time
series of relative magnitude changes for each star. Four time series analysis
methods are then applied to these light curves to obtain rotation periods for
members stars. We obtain for the first time rotation periods for 45 FGK cluster
members of NGC 6709. We compare our rotation periods to Gaia EDR3 colors and
find a slow-rotating sequence with increasing rotation periods towards redder
stars and a smaller clump of rapid rotators that have not yet joined this
sequence. NGC 6709 has rotation periods very similar to that of another
Pleiades-age open cluster, NGC 2516.Comment: 18 pages, 14 figures, 5 table
Transmission spectroscopy of the inflated exo-Saturn HAT-P-19b
We observed the Saturn-mass and Jupiter-sized exoplanet HAT-P-19b to refine
its transit parameters and ephemeris as well as to shed first light on its
transmission spectrum. We monitored the host star over one year to quantify its
flux variability and to correct the transmission spectrum for a slope caused by
starspots. A transit of HAT-P-19b was observed spectroscopically with OSIRIS at
the Gran Telescopio Canarias in January 2012. The spectra of the target and the
comparison star covered the wavelength range from 5600 to 7600 AA. One
high-precision differential light curve was created by integrating the entire
spectral flux. This white-light curve was used to derive absolute transit
parameters. Furthermore, a set of light curves over wavelength was formed by a
flux integration in 41 wavelength channels of 50 AA width. We analyzed these
spectral light curves for chromatic variations of transit depth. The transit
fit of the combined white-light curve yields a refined value of the
planet-to-star radius ratio of 0.1390 pm 0.0012 and an inclination of 88.89 pm
0.32 degrees. After a re-analysis of published data, we refine the orbital
period to 4.0087844 pm 0.0000015 days. We obtain a flat transmission spectrum
without significant additional absorption at any wavelength or any slope.
However, our accuracy is not sufficient to significantly rule out the presence
of a pressure-broadened sodium feature. Our photometric monitoring campaign
allowed for an estimate of the stellar rotation period of 35.5 pm 2.5 days and
an improved age estimate of 5.5^+1.8_-1.3 Gyr by gyrochronology.Comment: 14 pages, 9 figures, Accepted for publication in A&
Correcting for chromatic stellar activity effects in transits with multiband photometric monitoring: Application to WASP-52
The properties of inhomogeneities on the surface of active stars (i.e. dark
spots and bright faculae) significantly influence the determination of the
parameters of an exoplanet. The chromatic effect they have on transmission
spectroscopy could affect the analysis of data from future space missions such
as JWST and Ariel.
To quantify and mitigate the effects of those surface phenomena, we developed
a modelling approach to derive the surface distribution and properties of
active regions by modelling simultaneous multi-wavelength time-series
observables. By using the StarSim code, now featuring the capability to solve
the inverse problem, we analysed 600 days of BVRI multiband photometry
from TJO and STELLA telescopes exoplanet host star WASP-52. From the results,
we simulated the chromatic contribution of surface phenomena on the observables
of its transits.
We are able to determine the relevant activity parameters of WASP-52 and
reconstruct the time-evolving longitudinal map of active regions. The star
shows a heterogeneous surface composed of dark spots with a mean temperature
contrast of K with filling factors ranging from 3 to 14 %. We
studied the chromatic effects on the depths of transits obtained at different
epochs with different stellar spot distributions. For WASP-52, with
peak-to-peak photometric variations of 7 % in the visible, the residual
effects of dark spots on the measured transit depth, after applying the
calculated corrections, are about at 550 nm and at
6m.
We demonstrate that by using contemporaneous ground-based multiband
photometry of an active star, it is possible to reconstruct the parameters and
distribution of active regions over time, and thus, quantify the chromatic
effects on the planetary radii measured with transit spectroscopy and mitigate
them by about an order of magnitude.Comment: 17 pages, 12 figure
Broad-band spectrophotometry of the hot Jupiter HAT-P-12b from the near-UV to the near-IR
The detection of trends or gradients in the transmission spectrum of
extrasolar planets is possible with observations at very low spectral
resolution. Transit measurements of sufficient accuracy using selected
broad-band filters allow for an initial characterization of the atmosphere of
the planet. We obtained time series photometry of 20 transit events and
analyzed them homogeneously, along with eight light curves obtained from the
literature. In total, the light curves span a range from 0.35 to 1.25 microns.
During two observing seasons over four months each, we monitored the host star
to constrain the potential influence of starspots on the derived transit
parameters. We rule out the presence of a Rayleigh slope extending over the
entire optical wavelength range, a flat spectrum is favored for HAT-P-12b with
respect to a cloud-free atmosphere model spectrum. A potential cause of such
gray absorption is the presence of a cloud layer at the probed latitudes.
Furthermore, in this work we refine the transit parameters, the ephemeris and
perform a TTV analysis in which we found no indication for an unseen companion.
The host star showed a mild non-periodic variability of up to 1%. However, no
stellar rotation period could be detected to high confidence.Comment: 13 pages, 6 figures, Accepted for publication in A&
Absolute dimensions and apsidal motion of the eclipsing binaries V889 Aquilae and V402 Lacertae
Context. Double-lined eclipsing binaries allow the direct determination of masses and radii, which are key for testing stellar models. With the launch of the TESS mission, many well-known eclipsing binaries have been observed at higher photometric precision, permitting the improvement of the absolute dimension determinations. Aims. Using TESS data and newly obtained spectroscopic observations, we aim to determine the masses and radii of the eccentric eclipsing binary systems V889 Aql and V402 Lac, together with their apsidal motion parameters. Methods. We simultaneously modelled radial velocity curves and times of eclipse for each target to precisely determine the orbital parameters of the systems, which we used to analyse the light curves and then obtain their absolute dimensions. We compared the obtained values with those predicted by theoretical models. Results. We determined masses and radii of the components of both systems with relative uncertainties lower than 2%. V889 Aql is composed of two stars with masses 2:17±0:02 M⊙ and 2:13±0:01 M⊙ and radii 1:87±0:04 R⊙ and 1:85±0:04 R⊙.We find conclusive evidence of the presence of a third body orbiting V889 Aql with a period of 67 yr. Based on the detected third light and the absence of signal in the spectra, we suggest that this third body could in turn be a binary composed of two ±1.4 M⊙ stars. V402 Lac is composed of two stars with masses 2:80 ± 0:05 M⊙ and 2:78 ± 0:05 M⊙ and radii 2:38 ± 0:03 R⊙ and 2:36 ± 0:03 R⊙. The times of minimum light are compatible with the presence of a third body for this system too, although its period is not yet fully sampled. In both cases we have found a good agreement between the observed apsidal motion rates and the model predictions
GU Boo: A New 0.6 Msun Detached Eclipsing Binary
We have found a new low-mass, double-lined, detached eclipsing binary, GU
Boo, among a sample of new variables from the ROTSE-I database. The binary has
an orbital period of 0.488728 +/- 0.000002 days, and estimated apparent
magnitudes Vrotse = 13.7 and I = 11.8. Our analysis of the light and radial
velocity curves of the system yields individual masses and radii of M1= 0.610
+/- 0.007 Msun, M2 = 0.599 +/- 0.006 Msun, R1= 0.623 +/- 0.016 Rsun, R2= 0.620
+/- 0.020 Rsun. The stars in GU Boo are therefore very similar to the
components of the eclipsing binary YY Gem. For this study we have adopted a
mean effective temperature for the binary of Teff = 3870 +/- 130 K. Based on
its space velocities we suggest that GU Boo is a main sequence binary, possibly
with an age of several Gyr. The metallicity of the binary is not well
constrained at this point but we speculate that it should not be very different
from solar. We have compared the physical parameters of GU Boo with current
low-mass stellar models, where we accounted for uncertainties in age and
metallicity by considering a wide range of values for those parameters. Our
comparisons reveal that all the models underestimate the radii of the
components of GU Boo by at least 10-15%. This result is in agreement with the
recent studies of YY Gem and CU Cnc.Comment: 41 pages, 10 figures, 11 tables; accepted by Ap
GJ1214: Rotation period, starspots, and uncertainty on the optical slope of the transmission spectrum
Brightness inhomogeneities in the stellar photosphere (dark spots or bright
regions) affect the measurements of the planetary transmission spectrum. To
investigate the star spots of the M dwarf GJ 1214, we conducted a multicolor
photometric monitoring from 2012 to 2016. The measured variability shows a
periodicity of 125 +- 5 days, which we interpret as the signature of the
stellar rotation period. This value overrules previous suggestions of a
significantly shorter stellar rotation period. A light curve inversion of the
monitoring data yields an estimation of the flux dimming of a permanent spot
filling factor not contributing to the photometric variability, a temperature
contrast of the spots of about 370 K and persistent active longitudes. The
derived surface maps over all five seasons were used to estimate the influence
of the star spots on the transmission spectrum of the planet from 400 nm to
2000 nm. The monitoring data presented here do not support a recent
interpretation of a measured transmission spectrum of GJ 1214b as to be caused
by bright regions in the stellar photosphere. Instead, we list arguments as to
why the effect of dark spots likely dominated over bright regions in the period
of our monitoring. Furthermore, our photometry proves an increase in
variability over at least four years, indicative for a cyclic activity
behavior. The age of GJ 1214 is likely between 6 and 10 Gyr. The long-term
photometry allows for a correction of unocculted spots. For an active star such
as GJ 1214, there remains a degeneracy between occulted spots and the transit
parameters used to build the transmission spectrum. This degeneracy can only be
broken by high-precision transit photometry resolving the spot crossing
signature in the transit light curve.Comment: 11 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Magnetic fields and differential rotation on the pre-main sequence I: The early-G star HD 141943 - brightness and magnetic topologies
Spectroscopic and spectropolarimetric observations of the pre-main sequence
early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007,
2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian
Telescope using the UCLES echelle spectrograph and the SEMPOL
spectropolarimeter visitor instrument. Brightness and surface magnetic field
topologies were reconstructed for the star using the technique of least-squares
deconvolution to increase the signal-to-noise of the data.
The reconstructed brightness maps show that HD 141943 had a weak polar spot
and a significant amount of low latitude features, with little change in the
latitude distribution of the spots over the 4 years of observations. The
surface magnetic field was reconstructed at three of the epochs from a high
order (l <= 30) spherical harmonic expansion of the spectropolarimetric
observations. The reconstructed magnetic topologies show that in 2007 and 2010
the surface magnetic field was reasonably balanced between poloidal and
toroidal components. However we find tentative evidence of a change in the
poloidal/toroidal ratio in 2009 with the poloidal component becoming more
dominant. At all epochs the radial magnetic field is predominantly
non-axisymmetric while the azimuthal field is predominantly axisymmetric with a
ring of positive azimuthal field around the pole similar to that seen on other
active stars.Comment: 18 pages, 17 figures, accepted by MNRA
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