Space-based asteroseismology has been playing an important role in the characterization of exoplanet-host stars and their planetary systems. The future looks even brighter, with space missions such ...as NASA's TESS and ESA's PLATO ready to take on this legacy. In this contribution, we provide an outlook on the synergy between asteroseismology and exoplanet science, namely, on the prospect of conducting a populational study of giant planets around oscillating evolved stars with the TESS mission.
Transition from spot to faculae domination Reinhold, Timo; Bell, Keaton J.; Kuszlewicz, James ...
Astronomy and astrophysics (Berlin),
01/2019, Letnik:
621
Journal Article
Recenzirano
Odprti dostop
Context.
The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes magnetic activity signatures such as dark spots and bright faculae. Having knowledge ...about the dominant source of surface activity might allow us to draw conclusions about the stellar age and magnetic field topology, and to put the solar cycle in context.
Aims.
We investigate the underlying process that causes magnetic activity by studying the appearance of activity signatures in contemporaneous photometric and chromospheric time series.
Methods.
Lomb-Scargle periodograms are used to search for cycle periods present in the photometric and chromospheric time series. To emphasize the signature of the activity cycle we account for rotation-induced scatter in both data sets by fitting a quasi-periodic Gaussian process model to each observing season. After subtracting the rotational variability, cycle amplitudes and the phase difference between the two time series are obtained by fitting both time series simultaneously using the same cycle period.
Results.
We find cycle periods in 27 of the 30 stars in our sample. The phase difference between the two time series reveals that the variability in fast-rotating active stars is usually in anti-phase, while the variability of slowly rotating inactive stars is in phase. The photometric cycle amplitudes are on average six times larger for the active stars. The phase and amplitude information demonstrates that active stars are dominated by dark spots, whereas less-active stars are dominated by bright faculae. We find the transition from spot to faculae domination to be at the Vaughan–Preston gap, and around a Rossby number equal to one.
Conclusions.
We conclude that faculae are the dominant ingredient of stellar activity cycles at ages ≳2.55 Gyr. The data further suggest that the Vaughan–Preston gap cannot explain the previously detected dearth of
Kepler
rotation periods between 15 and 25 days. Nevertheless, our results led us to propose an explanation for the lack of rotation periods to be due to the non-detection of periodicity caused by the cancelation of dark spots and bright faculae at ∼800 Myr.
We report the discovery of a warm sub-Saturn, TOI-257b (HD 19916b), based on data from NASA's Transiting Exoplanet Survey Satellite (TESS). The transit signal was detected by TESS and confirmed to be ...of planetary origin based on radial velocity observations. An analysis of the TESS photometry, the Minerva-Australis, FEROS, and HARPS radial velocities, and the asteroseismic data of the stellar oscillations reveals that TOI-257b has a mass of \(M_P=0.138\pm0.023\)\,\(\rm{M_J}\) (\(43.9\pm7.3\)\,\(M_{\rm \oplus}\)), a radius of \(R_P=0.639\pm0.013\)\,\(\rm{R_J}\) (\(7.16\pm0.15\)\,\(R_{\rm \oplus}\)), bulk density of \(0.65^{+0.12}_{-0.11}\) (cgs), and period \(18.38818^{+0.00085}_{-0.00084}\)\,\(\rm{days}\). TOI-257b orbits a bright (\(\mathrm{V}=7.612\)\,mag) somewhat evolved late F-type star with \(M_*=1.390\pm0.046\)\,\(\rm{M_{\odot}}\), \(R_*=1.888\pm0.033\)\,\(\rm{R_{\odot}}\), \(T_{\rm eff}=6075\pm90\)\,\(\rm{K}\), and \(v\sin{i}=11.3\pm0.5\)\,km\,s\(^{-1}\). Additionally, we find hints for a second non-transiting sub-Saturn mass planet on a \(\sim71\)\,day orbit using the radial velocity data. This system joins the ranks of a small number of exoplanet host stars (\(\sim100\)) that have been characterized with asteroseismology. Warm sub-Saturns are rare in the known sample of exoplanets, and thus the discovery of TOI-257b is important in the context of future work studying the formation and migration history of similar planetary systems.
Giant stars as known exoplanet hosts are relatively rare due to the potential challenges in acquiring precision radial velocities and the small predicted transit depths. However, these giant host ...stars are also some of the brightest in the sky and so enable high signal-to-noise follow-up measurements. Here we report on new observations of the bright (V ~ 3.3) giant star \(\iota\) Draconis (\(\iota\) Dra), known to host a planet in a highly eccentric ~511 day period orbit. TESS observations of the star over 137 days reveal asteroseismic signatures, allowing us to constrain the stellar radius, mass, and age to ~2%, ~6%, and ~28%, respectively. We present the results of continued radial velocity monitoring of the star using the Automated Planet Finder over several orbits of the planet. We provide more precise planet parameters of the known planet and, through the combination of our radial velocity measurements with Hipparcos and Gaia astrometry, we discover an additional long-period companion with an orbital period of ~\(68^{+60}_{-36}\) years. Mass predictions from our analysis place this sub-stellar companion on the border of the planet and brown dwarf regimes. The bright nature of the star combined with the revised orbital architecture of the system provides an opportunity to study planetary orbital dynamics that evolve as the star moves into the giant phase of its evolution.
The analysis of photometric time series in the context of transiting planet surveys suffers from the presence of stellar signals, often dubbed "stellar noise". These signals, caused by stellar ...oscillations and granulation, can usually be disregarded for main-sequence stars, as the stellar contributions average out when phase-folding the light curve. For evolved stars, however, the amplitudes of such signals are larger and the timescales similar to the transit duration of short-period planets, requiring that they be modeled alongside the transit. With the promise of TESS delivering on the order of \(\sim\!10^5\) light curves for stars along the red-giant branch, there is a need for a method capable of describing the "stellar noise" while simultaneously modelling an exoplanet's transit. In this work, a Gaussian Process regression framework is used to model stellar light curves and the method validated by applying it to TESS-like artificial data. Furthermore, the method is used to characterize the stellar oscillations and granulation of a sample of well-studied \textit{Kepler} low-luminosity red-giant branch stars. The parameters determined are compared to equivalent ones obtained by modelling the power spectrum of the light curve. Results show that the method presented is capable of describing the stellar signals in the time domain and can also return an accurate and precise measurement of \(\nu_\text{max}\), i.e., the frequency of maximum oscillation amplitude. Preliminary results show that using the method in transit modelling improves the precision and accuracy of the ratio between the planetary and stellar radius, \(R_p/R_\star\). The method's implementation is publicly available.
KOI-3890 is a highly eccentric, 153-day period eclipsing, single-lined spectroscopic binary system containing a red-giant star showing solar-like oscillations alongside tidal interactions. The ...combination of transit photometry, radial velocity observations, and asteroseismology have enabled the detailed characterisation of both the red-giant primary and the M-dwarf companion, along with the tidal interaction and the geometry of the system. The stellar parameters of the red-giant primary are determined through the use of asteroseismology and grid-based modelling to give a mass and radius of \(M_{\star}=1.04\pm0.06\;\textrm{M}_{\odot}\) and \(R_{\star}=5.8\pm0.2\;\textrm{R}_{\odot}\) respectively. When combined with transit photometry the M-dwarf companion is found to have a mass and radius of \(M_{\mathrm{c}}=0.23\pm0.01\;\textrm{M}_{\odot}\) and \(R_{\mathrm{c}}=0.256\pm0.007\;\textrm{R}_{\odot}\). Moreover, through asteroseismology we constrain the age of the system through the red-giant primary to be \(9.1^{+2.4}_{-1.7}\;\mathrm{Gyr}\). This provides a constraint on the age of the M-dwarf secondary, which is difficult to do for other M-dwarf binary systems. In addition, the asteroseismic analysis yields an estimate of the inclination angle of the rotation axis of the red-giant star of \(i=87.6^{+2.4}_{-1.2}\) degrees. The obliquity of the system\textemdash the angle between the stellar rotation axis and the angle normal to the orbital plane\textemdash is also derived to give \(\psi=4.2^{+2.1}_{-4.2}\) degrees showing that the system is consistent with alignment. We observe no radius inflation in the M-dwarf companion when compared to current low-mass stellar models.
Asteroseismology is playing an increasingly important role in the characterization of red-giant host stars and their planetary systems. Here, we conduct detailed asteroseismic modeling of the evolved ...red-giant branch (RGB) hosts KOI-3886 and \(\iota\) Draconis, making use of end-of-mission Kepler (KOI-3886) and multi-sector TESS (\(\iota\) Draconis) time-series photometry. We also model the benchmark star KIC 8410637, a member of an eclipsing binary, thus providing a direct test to the seismic determination. We test the impact of adopting different sets of observed modes as seismic constraints. Inclusion of \(\ell=1\) and 2 modes improves the precision on the stellar parameters, albeit marginally, compared to adopting radial modes alone, with \(1.9\)-\(3.0\%\) (radius), \(5\)-\(9\%\) (mass), and \(19\)-\(25\%\) (age) reached when using all p-dominated modes as constraints. Given the very small spacing of adjacent dipole mixed modes in evolved RGB stars, the sparse set of observed g-dominated modes is not able to provide extra constraints, further leading to highly multimodal posteriors. Access to multi-year time-series photometry does not improve matters, with detailed modeling of evolved RGB stars based on (lower-resolution) TESS data sets attaining a precision commensurate with that based on end-of-mission Kepler data. Furthermore, we test the impact of varying the atmospheric boundary condition in our stellar models. We find mass and radius estimates to be insensitive to the description of the near-surface layers, at the expense of substantially changing both the near-surface structure of the best-fitting models and the values of associated parameters like the initial helium abundance, \(Y_{\rm i}\). Attempts to measure \(Y_{\rm i}\) from seismic modeling of red giants may thus be systematically dependent on the choice of atmospheric physics.
We investigate the masses of "retired A stars" using asteroseismic detections on seven low-luminosity red-giant and sub-giant stars observed by the NASA Kepler and K2 Missions. Our aim is to explore ...whether masses derived from spectroscopy and isochrone fitting may have been systematically overestimated. Our targets have all previously been subject to long term radial velocity observations to detect orbiting bodies, and satisfy the criteria used by Johnson et al. (2006) to select survey stars that may have had A-type (or early F-type) main-sequence progenitors. The sample actually spans a somewhat wider range in mass, from \(\approx 1\,\rm M_{\odot}\) up to \(\approx 1.7\,\rm M_{\odot}\). Whilst for five of the seven stars the reported discovery mass from spectroscopy exceeds the mass estimated using asteroseismology, there is no strong evidence for a significant, systematic bias across the sample. Moreover, comparisons with other masses from the literature show that the absolute scale of any differences is highly sensitive to the chosen reference literature mass, with the scatter between different literature masses significantly larger than reported error bars. We find that any mass difference can be explained through use of differing constraints during the recovery process. We also conclude that underestimated uncertainties on the input parameters can significantly bias the recovered stellar masses, which may have contributed to the controversy on the mass scale for retired A stars.
Since the onset of the `space revolution' of high-precision high-cadence photometry, asteroseismology has been demonstrated as a powerful tool for informing Galactic archaeology investigations. The ...launch of the NASA TESS mission has enabled seismic-based inferences to go full sky -- providing a clear advantage for large ensemble studies of the different Milky Way components. Here we demonstrate its potential for investigating the Galaxy by carrying out the first asteroseismic ensemble study of red giant stars observed by TESS. We use a sample of 25 stars for which we measure their global asteroseimic observables and estimate their fundamental stellar properties, such as radius, mass, and age. Significant improvements are seen in the uncertainties of our estimates when combining seismic observables from TESS with astrometric measurements from the Gaia mission compared to when the seismology and astrometry are applied separately. Specifically, when combined we show that stellar radii can be determined to a precision of a few percent, masses to 5-10% and ages to the 20% level. This is comparable to the precision typically obtained using end-of-mission Kepler data
Over the course of its history, the Milky Way has ingested multiple smaller
satellite galaxies. While these accreted stellar populations can be
forensically identified as kinematically distinct ...structures within the Galaxy,
it is difficult in general to precisely date the age at which any one merger
occurred. Recent results have revealed a population of stars that were accreted
via the collision of a dwarf galaxy, called \textit{Gaia}-Enceladus, leading to
a substantial pollution of the chemical and dynamical properties of the Milky
Way. Here, we identify the very bright, naked-eye star $\nu$\,Indi as a probe
of the age of the early in situ population of the Galaxy. We combine
asteroseismic, spectroscopic, astrometric, and kinematic observations to show
that this metal-poor, alpha-element-rich star was an indigenous member of the
halo, and we measure its age to be $11.0 \pm 0.7$ (stat) $\pm 0.8$ (sys)$\,\rm
Gyr$. The star bears hallmarks consistent with it having been kinematically
heated by the \textit{Gaia}-Enceladus collision. Its age implies that the
earliest the merger could have begun was 11.6 and 13.2 Gyr ago at 68 and 95%
confidence, respectively. Input from computations based on hierarchical
cosmological models tightens (i.e. reduces) slightly the above limits.
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