Stellar Astrophysics Center


Stellar Astrophysics Center

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Deeg H.J.,Institute of Astrophysics of Canarias | Deeg H.J.,University of La Laguna | Tingley B.,Stellar Astrophysics Center
Astronomy and Astrophysics | Year: 2017

Context. Transit or eclipse timing variations have proven to be a valuable tool in exoplanet research. However, no simple way to estimate the potential precision of such timing measures has been presented yet, nor are guidelines available regarding the relation between timing errors and sampling rate. Aims. A timing error estimator (TEE) equation is presented that requires only basic transit parameters as input. With the TEE, estimating timing precision for actual data and for future instruments, such as the TESS and PLATO space missions, is straightforward. Methods. A derivation of the timing error based on a trapezoidal transit shape is given. We also verify the TEE on realistically modelled transits using Monte Carlo simulations and determine its validity range, exploring in particular the interplay between ingress/egress times and sampling rates. Results. The simulations show that the TEE gives timing errors very close to the correct value, as long as the temporal sampling is faster than transit ingress/egress durations and transits with very low S/N are avoided. Conclusions. The TEE is a useful tool for estimating eclipse or transit timing errors in actual and future data sets. In combination with a previously published equation to estimate period-errors, predictions for the ephemeris precision of long-coverage observations are possible as well. The tests for the TEE's validity range also led to implications for instrumental design. Temporal sampling has to be faster than transit ingress or egress durations, or a loss in timing precision will occur. An application to the TESS mission shows that transits close to its detection limit will have timing uncertainties that exceed 1 h within a few months of their acquisition. Prompt follow-up observations will be needed to avoid "losing" their ephemerides. © ESO, 2017.

Lund M.N.,University of Aarhus | Lund M.N.,University of Birmingham | Lund M.N.,Stellar Astrophysics Center | Chaplin W.J.,University of Birmingham | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We introduce a new method to detect solar-like oscillations in frequency power spectra of stellar observations, under conditions of very low signal-to-noise ratio. The Moving-Windowed-Power-Search (MWPS) searches the power spectrum for signatures of excess power, over and above slowly varying (in frequency) background contributions from stellar granulation and shot or instrumental noise. We adopt a false-alarm approach to ascertain whether flagged excess power, which is consistent with the excess expected from solar-like oscillations, is hard to explain by chance alone (and hence a candidate detection). We apply the method to solar photometry data, whose quality was systematically degraded to test the performance of the MWPS at low signal-to-noise ratios. We also compare the performance of the MWPS against the frequently applied power-spectrum-of-power-spectrum (PS⊗PS) detection method. The MWPS is found to outperform the PS⊗PS method. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Mallonn M.,Leibniz Institute for Astrophysics Potsdam | Von Essen C.,Stellar Astrophysics Center | Weingrill J.,Leibniz Institute for Astrophysics Potsdam | Strassmeier K.G.,Leibniz Institute for Astrophysics Potsdam | And 6 more authors.
Astronomy and Astrophysics | Year: 2015

Context. Transiting highly inflated giant planets offer the possibility of characterizing their atmospheres. A fraction of the starlight passes through the high-altitude layers of the planetary atmosphere during transit. The resulting absorption is expected to be wavelength dependent for cloud-free atmospheres with an amplitude of up to 10-3 of the stellar flux, while a high-altitude cloud deck would cause a gray opacity. Aims. 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. Methods. 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 Å. 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 Å width. We analyzed these spectral light curves for chromatic variations of transit depth. Results. The transit fit of the combined white-light curve yields a refined value of the planet-to-star radius ratio of 0.1390 ± 0.0012 and an inclination of 88.89 ± 0.32 deg. After a re-analysis of published data, we refine the orbital period to 4.0087844 ± 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 ± 2.5 days and an improved age estimate of 5.5+ 1.8-1.3 Gyr by gyrochronology. The calculated correction of the transit depth for unocculted spots on the visible hemisphere was found to be well within the derived 1σ uncertainty of the white-light curve and the spectral data points of the transmission spectrum. © ESO, 2015.

Gilliland R.L.,Pennsylvania State University | Marcy G.W.,University of California at Berkeley | Rowe J.F.,NASA | Rogers L.,California Institute of Technology | And 31 more authors.
Astrophysical Journal | Year: 2013

NASA's Kepler Mission has revealed two transiting planets orbiting Kepler-68. Follow-up Doppler measurements have established the mass of the innermost planet and revealed a third Jovian-mass planet orbiting beyond the two transiting planets. Kepler-68b, in a 5.4 day orbit, has MP = 8.3+1.2 -2.4 M⊕, RP = 2.31 +0.06 -0.09R⊕, and ρP = 3.32 +0.86 -0.98 g cm-3, givingKepler-68b a density intermediate between that of the ice giants and Earth.Kepler-68c is Earth-sized, with a radius RP = 0.953+0.037 -0.042 R ⊕ and transits on a 9.6 day orbit; validation of Kepler-68c posed unique challenges. Kepler-68d has an orbital period of 580±15 days and a minimum mass of MP sin i = 0.947 ±0.035MJ . Power spectra of the Kepler photometry at one minute cadence exhibit a rich and strong set of asteroseismic pulsation modes enabling detailed analysis of the stellar interior. Spectroscopy of the star coupled with asteroseismic modeling of the multiple pulsation modes yield precise measurements of stellar properties, notably Teff = 5793±74 K,M* = 1.079±0.051M⊙, R* = 1.243*0.019 R ⊙, and ρ*= 0.7903±0.0054 g cm-3, all measured with fractional uncertainties of only a few percent. Models of Kepler-68b suggest that it is likely composed of rock and water, or has a H and He envelope to yield its density ∼3 g cm-3. © 2013. The American Astronomical Society. All rights reserved.

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