Abstract
The Transiting Exoplanet Survey Satellite (TESS) is NASA's latest space telescope dedicated to the discovery of transiting exoplanets around nearby stars. Besides the main goal of the ...mission, asteroseismology is an important secondary goal and very relevant for the high-quality time series that TESS will make during its two-year all-sky survey. Using TESS for asteroseismology introduces strong timing requirements, especially for coherent oscillators. Although the internal clock on board TESS is precise in its own time, it might have a constant drift. Thus, it will need calibration, or else offsets might inadvertently be introduced. Here, we present simultaneous ground- and space-based observations of primary eclipses of several binary systems in the Southern ecliptic hemisphere, used to verify the reliability of the TESS timestamps. From 12 contemporaneous TESS/ground observations, we determined a time offset equal to 5.8 ± 2.5 s, in the sense that the barycentric time measured by TESS is ahead of real time. The offset is consistent with zero at the 2.3
σ
level. In addition, we used 405 individually measured mid-eclipse times of 26 eclipsing binary stars observed solely by TESS in order to test the existence of a potential drift with a monotonic growth (or decay) affecting the observations of all stars. We find a drift corresponding to
σ
drift
= 0.009 ± 0.015 s day
−1
. We find that the measured offset is of a size that will not become an issue for comparing ground-based and space data for coherent oscillations for most of the targets observed with TESS.
Context. The measurement of obliquities – the angle between the orbital and stellar rotation – in star-planet systems is of great importance for understanding planet system formation and evolution. ...The bright and well-studied HAT-P-7 (Kepler-2) system is intriguing because several Rossiter-McLaughlin (RM) measurements found a high projected obliquity in this system, but it was not possible so far to determine whether the orbit is polar and/or retrograde. Aims. The goal of this study is to measure the stellar inclination and hereby the full 3D obliquity of the HAT-P-7 system instead of only the 2D projection as measured by the RM effect. In addition, we provide an updated set of stellar parameters for the star. Methods. We used the full set of available observations from Kepler spanning Q0-Q17 to produce the power spectrum of HAT-P-7. We extracted oscillation-mode frequencies via an Markov chain Monte Carlo peak-bagging routine and used the results from this to estimate the stellar inclination angle. Combining this with the projected obliquity from RM and the inclination of the orbital plane allowed us to determine the stellar obliquity. Furthermore, we used asteroseismology to model the star from the extracted frequencies using two different approaches to the modelling, for which either the stellar evolution codes MESA or GARSTEC were adopted. Results. Our updated asteroseismic modelling shows, i.a., the following stellar parameters for HAT-P-7: M⋆ = 1.51+ 0.04-0.05 M⊙, R⋆ = 2.00+ 0.01-0.02 R⊙, and age = 2.07+ 0.28-0.23 Gyr. The modelling offers a high precision on the stellar parameters, the uncertainty on age, for instance, is of the order ∼ 11%. For the stellar inclination we estimate i⋆< 36.5°, which translates into an obliquity of 83°<ψ< 111°. The planet HAT-P-7b is likely retrograde in its orbit, and the orbit is close to being polar. The new parameters for the star give an updated planetary density of ρp = 0.65 ± 0.03 g cm-3, which is lower than previous estimates.
We have discovered a new, rare triple-mode RR Lyr star, EPIC 201585823, in the Kepler K2 mission Campaign 1 data. This star pulsates primarily in the fundamental and first-overtone radial modes, and, ...in addition, a third non-radial mode. The ratio of the period of the non-radial mode to that of the first-overtone radial mode, 0.616 285, is remarkably similar to that seen in 11 other triple-mode RR Lyr stars, and in 260 RRc stars observed in the Galactic bulge. This systematic character promises new constraints on RR Lyr star models. We detected subharmonics of the non-radial mode frequency, which are a signature of period doubling of this oscillation; we note that this phenomenon is ubiquitous in RRc and RRd stars observed from space, and from ground with sufficient precision. The non-radial mode and subharmonic frequencies are not constant in frequency or in amplitude. The amplitude spectrum of EPIC 201585823 is dominated by many combination frequencies among the three interacting pulsation mode frequencies. Inspection of the phase relationships of the combination frequencies in a phasor plot explains the ‘upward’ shape of the light curve. We also found that raw data with custom masks encompassing all pixels with significant signal for the star, but without correction for pointing changes, is best for frequency analysis of this star, and, by implication, other RR Lyr stars observed by the K2 mission. We compare several pipeline reductions of the K2 mission data for this star.
Abstract
Doppler-based planet surveys point to an increasing occurrence rate of giant planets with stellar mass. Such surveys rely on evolved stars for a sample of intermediate-mass stars (so-called ...retired A stars), which are more amenable to Doppler observations than their main-sequence progenitors. However, it has been hypothesized that the masses of subgiant and low-luminosity red-giant stars targeted by these surveys – typically derived from a combination of spectroscopy and isochrone fitting – may be systematically overestimated. Here, we test this hypothesis for the particular case of the exoplanet-host star HD 212771 using K2 asteroseismology. The benchmark asteroseismic mass ($1.45^{+0.10}_{-0.09}\,\text{M}_{\odot }$) is significantly higher than the value reported in the discovery paper (1.15 ± 0.08 M⊙), which has been used to inform the stellar mass–planet occurrence relation. This result, therefore, does not lend support to the above hypothesis. Implications for the fates of planetary systems are sensitively dependent on stellar mass. Based on the derived asteroseismic mass, we predict the post-main-sequence evolution of the Jovian planet orbiting HD 212771 under the effects of tidal forces and stellar mass-loss.
Abstract
A poor understanding of the impact of convective turbulence in the outer layers of the Sun and Sun-like stars challenges the advance towards an improved understanding of their internal ...structure and dynamics. Assessing and calibrating these effects is therefore of great importance. Here, we study the spatial coherence of granulation noise and oscillation modes in the Sun, with the aim of exploiting any incoherence to beat down observed granulation noise, hence improving the detection of low-frequency p modes. Using data from the BiSON 2B instrument, we assess the coherence between different atmospheric heights and between different surface regions. We find that granulation noise from the different atmospheric heights probed is largely incoherent; frequency regions dominated by oscillations are almost fully coherent. We find a randomized phase difference for the granulation noise, and a near zero difference for the evanescent oscillations. A reduction of the incoherent granulation noise is shown by application of the cross-spectrum.
The Hyades open cluster was targeted during Campaign 4 (C4) of the NASA K2 mission, and short-cadence data were collected on a number of cool main-sequence stars. Here, we report results on two ...F-type stars that show detectable oscillations of a quality that allows asteroseismic analyses to be performed. These are the first ever detections of solar-like oscillations in main-sequence stars in an open cluster.
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.
In spite of the huge advances in exoplanet research provided by the NASA Kepler Mission, there remain only a small number of transit detections around evolved stars. Here, we present a reformulation ...of the noise properties of red-giant stars, where the intrinsic stellar granulation and the stellar oscillations described by asteroseismology play a key role. The new noise model is a significant improvement on the current Kepler results for evolved stars. Our noise model may be used to help understand planet detection thresholds for the ongoing K2 and upcoming TESSmissions, and serve as a predictor of stellar noise for these missions. As an application of our noise model, we explore the minimum detectable planet radii for red giant stars, and find that Neptune-sized planets should be detectable around low-luminosity red giant branch stars.
In this erratum, we provide corrected sets of r 01,10,02 difference ratio values and associated uncertainties, which were overestimated in the original paper (as noted by Roxburgh 2017) due to a ...missing trimming in the post-processing of the Markov chain Monte Carlo (MCMC) chains for these values. The typical reduction in the ratio uncertainties from performing the trimming is a factor of 10 (see Figure 3). Other parameters optimized in the peak-bagging (for instance, individual mode frequencies) are unaffected, as the trimming was performed for these in the original work (Lund et al. 2017). We also provide updated values for the n D 2 values of l=3 modes. We note that the values presented here, as with those presented in the original work, are obtained from a single peak-bagging procedure (see Lund et al. 2017 for details) and have yet to be verified by independent analyses using the same input power spectra. Examples of the updated tables from the original paper are given in Tables 1-3. We note that tables with individual mode parameters (Table 2) have been added for completeness, but the parameters in these tables are unchanged compared to the original paper. In addition to the corrected values mentioned above, we provide covariance matrices for the mode frequencies, frequency difference ratios (r 01,10,02), and second differences (n D 2) for the LEGACY sample (Lund et al. 2017), which were not published with the original work. The values provided by this erratum will be available in the online version of the paper. Figure 1. Comparison between ratio distribution of = r n 01, 25 (n » m 3090 Hz) for KIC 9139151 from the full (green) and properly thinned MCMC chains (black). The dashed red line (on top of the black curve) shows the distribution obtained by sampling from the reported frequency values and corresponding uncertainties (assuming that these are normally distributed and uncorrelated). The central peak is captured by both distributions, but the wide background signal representing the ratio prior has disappeared from the thinned chains. Dotted lines indicate the distribution medians; dashed lines bound the corresponding 68% highest probability density intervals.
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