Search

We developed, tested and validated a new Fast, Robust and Automated (FRA) tool to detect solar-like oscillations. FRA is based on the detection and measurement of the frequency of maximum oscillation power $\nu_{max}$, without relying on the detection of a regular frequency spacing to guide the search. We applied the FRA pipeline to 254 synthetic power spectra representative of TESS red giants, as well as 1689 red giants observed by Kepler and 2344 red giants observed by TESS. We obtain a consistency rate for $\nu_{max}$ compared with existing measurements of $\sim$ 99% for Kepler red giants and of $\sim$ 98% for TESS red giants. We find that using $\nu_{max}$ as an input parameter to guide the search for the large frequency separation $\Delta\nu$ through the existing Envelope AutoCorrelation Function (EACF) method significantly improves the consistency of the measured $\Delta\nu$ in the case of TESS stars, allowing to reach a consistency rate above 99%. Our analysis reveals that we can expect to get consistent $\nu_{max}$ and $\Delta\nu$ measurements while minimizing both the false positive measurements and the non-detections for stars with a minimum of four observed sectors and a maximum G magnitude of 9.5., Comment: 35 pages, 17 figures. Accepted for publication in Astronomische Nachrichten. This is the pre-peer reviewed (submitted) version

The Transiting Exoplanet Survey Satellite (TESS) mission searches for new exoplanets. The observing strategy of TESS results in high-precision photometry of millions of stars across the sky, allowing for detailed asteroseismic studies of individual systems. In this work, we present a detailed asteroseismic analysis of the giant star HD 76920 hosting a highly eccentric giant planet ($e = 0.878$) with an orbital period of 415 days, using 5 sectors of TESS light curve that cover around 140 days of data. Solar-like oscillations in HD 76920 are detected around $52 \, \mu$Hz by TESS for the first time. By utilizing asteroseismic modeling that takes classical observational parameters and stellar oscillation frequencies as constraints, we determine improved measurements of the stellar mass ($1.22 \pm 0.11\, M_\odot$), radius ($8.68 \pm 0.34\,R_\odot$), and age ($5.2 \pm 1.4\,$Gyr). With the updated parameters of the host star, we update the semi-major axis and mass of the planet as $a=1.165 \pm 0.035$ au and $M_{\rm p}\sin{i} = 3.57 \pm 0.22\,M_{\rm Jup}$. With an orbital pericenter of $0.142 \pm 0.005$ au, we confirm that the planet is currently far away enough from the star to experience negligible tidal decay until being engulfed in the stellar envelope. We also confirm that this event will occur within about 100\,Myr, depending on the stellar model used., Comment: 18 pages, 7 figures, 4 tables

KIC 7955301 is a hierarchical triple system with eclipse timing and depth variations discovered by the Kepler mission. It is composed of a non-eclipsing primary star at the bottom of the red giant branch on a 209-day orbit with a K/G-type main-sequence inner eclipsing binary, orbiting in 15.3 days. This system was noted for the large amplitude of its eclipse timing variations (4 hours), and the clear solar-like oscillations of the red-giant component, including p-modes of degree up to l=3 and mixed l=1 modes. The system is a single-lined spectroscopic triple. We perform a dynamical model by combining the Kepler photometric data, eclipse timing variations, and radial-velocity data obtained at Apache Point (ARCES) and Haute Provence (SOPHIE) observatories. The dynamical mass of the red-giant is determined with a 2% precision at 1.30 (+0.03,-0.02) solar mass. We perform asteroseismic modeling based on the global seismic parameters and on the individual frequencies. Both methods lead to a mass of the red giant that matches the dynamical mass within the uncertainties. Asteroseismology also reveals the rotation rate of the core (15 days), the envelope (150 days), and the inclination (75 deg) of the red giant. Three different approaches lead to an age between 3.3 and 5.8 Gyr, which highlights the difficulty of determining stellar ages despite the exceptional wealth of available information. On short timescales, the inner binary exhibits eclipses with varying depths during a 7.3-year long interval, and no eclipses during the consecutive 11.9 years. This is why Kepler could detect its eclipses, TESS will not, and the future ESA PLATO mission should. Over the long term, the system owes its evolution to the evolution of its individual components. It could end its current smooth evolution by merging by the end of the red giant or the asymptotic giant branch of the primary star., Accepted in A&A, 25 pages, 17 figures, 7 tables

We present an analysis of the first 20 second cadence light curves obtained by the TESS space telescope during its extended mission. We find improved precision of 20 second data compared to 2 minute data for bright stars when binned to the same cadence (˜10%-25% better for T ? 8 mag, reaching equal precision at T ˜ 13 mag), consistent with pre-flight expectations based on differences in cosmic-ray mitigation algorithms. We present two results enabled by this improvement. First, we use 20 second data to detect oscillations in three solar analogs (? Pav, ? Tuc, and p Men) and use asteroseismology to measure their radii, masses, densities, and ages to ˜1%, ˜3%, ˜1%, and ˜20% respectively, including systematic errors. Combining our asteroseismic ages with chromospheric activity measurements, we find evidence that the spread in the activity-age relation is linked to stellar mass and thus the depth of the convection zone. Second, we combine 20 second data and published radial velocities to recharacterize p Men c, which is now the closest transiting exoplanet for which detailed asteroseismology of the host star is possible. We show that p Men c is located at the upper edge of the planet radius valley for its orbital period, confirming that it has likely retained a volatile atmosphere and that the "asteroseismic radius valley"remains devoid of planets. Our analysis favors a low eccentricity for p Men c (, D.H. acknowledges support from the Alfred P. Sloan Foundation, the National Aeronautics and Space Administration (80NSSC19K0379, 80NSSC21K0652), and the National Science Foundation (AST-1717000). T.S.M. acknowledges support from NASA grant 80NSSC20K0458. Computational time at the Texas Advanced Computing Center was provided through XSEDE allocation TG-AST090107. A.C. acknowledges support from the National Science Foundation through the Graduate Research Fellowship Program (DGE 1842402). W.H.B. performed computations using the University of Birmingham's BlueBEAR High Performance Computing service. T.R.B. acknowledges support from the Australian Research Council through Discovery Project DP210103119. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (Grant DNRF106). M.S.C. and M.D. acknowledge the support by FCT/MCTES through the research grants UIDB/04434/2020, UIDP/04434/2020 and PTDC/FIS-AST/30389/2017, and by FEDER—Fundo Europeu de Desenvolvimento Regional through COMPETE2020—Programa Operacional Competitividade e Internacionalização (grant: POCI-01-0145-FEDER-030389). T.L.C. is supported by Fundação para a Ciência e a Tecnologia (FCT) in the form of a work contract (CEECIND/00476/2018). M.S.C. is supported by national funds through FCT in the form of a work contract. H.K. and E.P. acknowledge the grant from the European Social Fund via the Lithuanian Science Council (LMTLT) grant No. 09.3.3-LMT-K-712-01-0103. R.A.G. and S.N.B. acknowledge the support received from the CNES with the PLATO and GOLF grants. B.N. acknowledges postdoctoral funding from the Alexander von Humboldt Foundation and "Branco Weiss fellowship Science in Society" through the SEISMIC stellar interior physics group. S.M. acknowledges support by the Spanish Ministry of Science and Innovation with the Ramon y Cajal fellowship number RYC-2015-17697 and the grant number PID2019-107187GB-I00. T.W. acknowledges support from the B-type Strategic Priority Program of the Chinese Academy of Sciences (grant No. XDB41000000) from the NSFC of China (grant Nos. 11773064, 11873084, and 11521303), from the Youth Innovation Promotion Association of Chinese Academy of Sciences, and from the Ten Thousand Talents Program of Yunnan for Top-notch Young Talents. T.W. also gratefully acknowledges the computing time granted by the Yunnan Observatories and provided by the facilities at the Yunnan Observatories Supercomputing Platform. T.D. acknowledges support from MIT's Kavli Institute as a Kavli postdoctoral fellow.

According to dynamo theory, stars with convective envelopes efficiently generate surface magnetic fields, which manifest as magnetic activity in the form of starspots, faculae, flares, when their rotation period is shorter than their convective turnover time. Most red giants, having undergone significant spin down while expanding, have slow rotation and no spots. However, based on a sample of about 4500 red giants observed by the NASA Kepler mission, a previous study showed that about 8 % display spots, including about 15 % that belong to close binary systems. Here, we shed light on a puzzling fact: for rotation periods less than 80 days, a red giant that belongs to a close binary system displays a photometric modulation about an order of magnitude larger than that of a single red giant with similar rotational period and physical properties. We investigate whether binarity leads to larger magnetic fields when tides lock systems, or if a different spot distribution on single versus close binary stars can explain this fact. For this, we measure the chromospheric emission in the CaII H & K lines of 3130 of the 4465 stars studied in a previous work thanks to the LAMOST survey. We show that red giants in a close-binary configuration with spin-orbit resonance display significantly larger chromospheric emission than single stars, suggesting that tidal locking leads to larger magnetic fields at a fixed rotational period. Beyond bringing interesting new observables to study the evolution of binary systems, this result could be used to distinguish single versus binary red giants in automatic pipelines based on machine learning., 10 pages, 8 Figures, accepted for publication in A&A

The space-borne missions CoRoT and Kepler opened up a new opportunity for better understanding stellar evolution by probing stellar interiors with unrivalled high-precision photometric data. Kepler has observed stellar oscillation for four years, which gave access to excellent frequency resolution that enables deciphering the oscillation spectrum of evolved red giant branch and asymptotic giant branch stars. The internal structure of stars in the upper parts of the red and asymptotic giant branches is poorly constrained, which makes the distinction between red and asymptotic giants difficult. We perform a thorough seismic analysis to address the physical conditions inside these stars and to distinguish them. We studied the oscillation mode properties of about 2.000 evolved giants in a model described by the asymptotic pressure-mode pattern of red giants, which includes the signature of the helium second-ionisation zone. We extracted the mode properties up to the degree l = 3 and investigated their dependence on stellar mass, metallicity, and evolutionary status. We identify a clear difference in the signature of the helium second-ionisation zone between red and asymptotic giants. We also detect a clear shortage of the energy of l = 1 modes after the core-He-burning phase. Furthermore, we note that the mode damping observed on the asymptotic giant branch is similar to that observed on the red giant branch. We highlight that the signature of the helium second-ionisation zone varies with stellar evolution. This provides us with a physical basis for distinguishing red giant branch stars from asymptotic giants. Here, our investigation of stellar oscillations allows us to constrain the physical processes and the key events that occur during the advanced stages of stellar evolution, with emphasis on the ascent along the asymptotic giant branch, including the asymptotic giant branch bump., Comment: 20 pages, 12 figures, accepted in A&A

Red giant stars present mixed modes, which behave as pressure modes in the convective envelope and as gravity modes in the radiative interior. This mixed character allows to probe the physical conditions in their core. With the advent of long-duration time series from space-borne missions such as CoRoT and Kepler, it becomes possible to study the red giant core rotation. As more than 15 000 red giant light curves have been recorded, it is crucial to develop a robust and efficient method to measure this rotation. Such measurements of thousands of mean core rotation would open the way to a deeper understanding of the physical mechanisms that are able to transport angular momentum from the core to the envelope in red giants. In this work, we detail the principle of the method we developed to obtain automatic measurements of the red giant mean core rotation. This method is based on the stretching of the oscillation spectra and on the use of the so-called Hough transform. We finally validate this method for stars on the red giant branch, where overlapping rotational splittings and mixed-mode spacings produce complicated frequency spectra., Comment: 8 pages, 3 figures, 1 table

Asteroseismology allows us to probe stellar interiors. Mixed modes can be used to probe the physical conditions in red giant cores. However, we still need to identify the physical mechanisms that transport angular momentum inside red giants, leading to the slow-down observed for the red giant core rotation. Thus large-scale measurements of the red giant core rotation are of prime importance to obtain tighter constraints on the efficiency of the internal angular momentum transport, and to study how this efficiency changes with stellar parameters. This work aims at identifying the components of the rotational multiplets for dipole mixed modes in a large number of red giant oscillation spectra observed by Kepler. Such identification provides us with a direct measurement of the red giant mean core rotation. We compute stretched spectra that mimic the regular pattern of pure dipole gravity modes. Mixed modes with same azimuthal order are expected to be almost equally spaced in stretched period. The departure from this regular pattern allows us to disentangle the various rotational components and therefore to determine the mean core rotation rates of red giants. We obtained mean core rotation measurements for 875 red giant branch stars. This large sample includes stars with a mass as large as 2.5 $M_{\odot}$, allowing us to test the dependence of the core slow-down rate on the stellar mass. This work on a large sample allows us to refine previous measurements of the evolution of the mean core rotation on the red giant branch. Rather than a slight slow down, our results suggest rotation to be constant along the red giant branch, with values independent on the mass., Comment: 14 pages, 14 figures, 4 tables, submitted to A&A

Oscillation modes with a mixed character, as observed in evolved low-mass stars, are highly sensitive to the physical properties of the innermost regions. Measuring their properties is therefore extremely important to probe the core, but requires some care, due to the complexity of the mixed-mode pattern. This work aims at providing a consistent description of the mixed-mode pattern of low-mass stars, based on the asymptotic expansion. We also aim at studying the variation of the gravity offset $\varepsilon_{g}$ with stellar evolution. We revisit previous work about mixed modes in red giants and empirically test how period spacings, rotational splittings, mixed-mode widths and heights can be estimated in a consistent view, based on the properties of the mode inertia ratios. From the asymptotic fit of the mixed-mode pattern of a large set of red giants at various evolutionary stages, we derive unbiased and precise asymptotic parameters. As the asymptotic expansion of gravity modes is verified with a precision close to the frequency resolution for stars on the red giant branch (10$^{-4}$ in relative values), we can derive accurate values of the asymptotic parameters. We decipher the complex pattern in a rapidly rotating star, and explain how asymmetrical splittings can be inferred, as well as the stellar inclinations. This allows us to revisit the stellar inclinations in two open clusters, NGC 6819 and NGC 6791: our results show that the stellar inclinations in these clusters do not have privileged orientation in the sky. The variation of the asymptotic gravity offset along with stellar evolution is investigated in detail. We also derive generic properties that explain under which conditions mixed modes can be observed., Comment: Accepted in A&A, AA/2018/32777

Stellar oscillations give seismic information on the internal properties of stars. Red giants are targets of interest since they present mixed modes, which behave as pressure modes in the convective envelope and as gravity modes in the radiative core. Mixed modes thus directly probe red giant cores, and allow in particular the study of their mean core rotation. The high-quality data obtained by CoRoT and Kepler satellites represent an unprecedented perspective to obtain thousands of measurements of red giant core rotation, in order to improve our understanding of stellar physics in deep stellar interiors. We developed an automated method to obtain such core rotation measurements and validated it for stars on the red giant branch. In this work, we particularly focus on the specific application of this method to red giants having a rapid core rotation. They show complex spectra where it is tricky to disentangle rotational splittings from mixed-mode period spacings. We demonstrate that the method based on the identification of mode crossings is precise and efficient. The determination of the mean core rotation directly derives from the precise measurement of the asymptotic period spacing {\Delta}{\Pi}1 and of the frequency at which the crossing of the rotational components is observed., Comment: 4 pages, 2 figures, 2 tables, to be published in the Astro Fluid 2016 Conference Proceedings, editor EAS Publications Series

Asteroseismology consists in probing stellar interiors through the detection of seismic waves. Red giants are evolved low-mass stars that haveexhausted hydrogen in their core. These stars are solar-type pulsators presenting mixed modes that allow us to have a direct access to the physical properties of their core. The available seismic measurements indicate that one or several mechanisms that remain poorly understood counterbalance the acceleration of the core rotation, resulting from its contraction, by transporting angular momentum. The greatest part of this PhD thesis was devoted to the development of a method allowing a measurement as automated as possible of the mean core rotation of stars on the red giant branch that were observed by the Kepler satellite (NASA). The measurements that were derived for almost 900 stars highlight that the core rotation is almost constant along the red giant branch, with values largely independent of the stellar mass. The second part of this PhD thesis isdevoted to the interpretation of these results based on stellar modelling. The challenge consists in using the large-scale measurements obtained in the first part to characterise the quantity of angular momentum that has to be extracted from each layer of the core, at different timesteps on the red giant branch, for different stellar masses.; L’astérosismologie consiste à sonder les intérieurs stellaires en détectant les ondes sismiques qui s’y propagent. Les géantes rouges, des étoiles évoluées peu massives dont l’hydrogène du cœur est épuisé, sont des pulsateurs de type solaire présentant des modes mixtes qui nous permettent d’accéder directement aux propriétés physiques de leur cœur. Les mesures sismiques disponibles indiquent qu’un ou plusieurs mécanismes physiques encore mal compris contrebalancent l’accélération de la rotation du cœur sous l’effet de sa contraction, en transportant du moment cinétique. La majeure partie de cette thèse a été consacrée au développement d’une méthode permettant une mesure aussi automatisée quepossible de la rotation moyenne du cœur des étoiles de la branche des géantes rouges observées par le satellite Kepler (NASA). Les mesures obtenues pour environ 900 étoiles mettent en évidence que la rotation du cœur est à peu près constante le long de la branche des géantes rouges, avec des valeurs indépendantes de la masse des étoiles. Le deuxième volet de cette thèse est consacré à l’interprétation de ces résultats basée sur la modélisation stellaire. L’enjeu consiste à utiliser les mesures à grande échelle obtenues durant la première partie pour caractériser la quantitéde moment cinétique qui doit être extraite localement de chaque région du cœur, à différents instants sur la branche des géantes rouges, pourdifférentes masses stellaires.