Category Archives: Publications

Stellar modelling of Spica, a high-mass spectroscopic binary with a β Cep variable primary component

Authors. A. Tkachenko, J. M. Matthews, C. Aerts, K. Pavlovski, P. I. Papics, K. Zwintz, C. Cameron, G. A. H. Walker, R. Kuschnig, P. Degroote, J. Debosscher, E. Moravveji, V. Kolbas, D. B. Guenther, A. F. J. Moffat, J. F. Rowe, S. M. Rucinski, D. Sasselov, W. W. Weiss

Journal. Monthly Notices of the Royal Astronomical Society

Abstract. Binary stars provide a valuable test of stellar structure and evolution, because the masses of the individual stellar components can be derived with high accuracy and in a model-independent way. In this work, we study Spica, an eccentric double-lined spectroscopic binary system with a β Cep type variable primary component. We use state-of-the-art modelling tools to determine accurate orbital elements of the binary system and atmospheric parameters of both stellar components. We interpret the short-period variability intrinsic to the primary component, detected on top of the orbital motion both in the photometric and spectroscopic data. The non-LTE based spectrum analysis reveals two stars of similar atmospheric chemical composition consistent with the present day cosmic abundance standard defined by Nieva & Przybilla (2012). The masses and radii of the stars are found to be 11.43±1.15 M and 7.21±0.75 M, and 7.47±0.54 R and 3.74±0.53 R for the primary and secondary, respectively. We find the primary component to pulsate in three independent modes, of which one is identified as a radial mode, while the two others are found to be non-radial, low degree l modes. The frequency of one of these modes is an exact multiple of the orbital frequency, and the l = m = 2 mode identification suggests a tidal nature for this particular mode. We find a very good agreement between the derived dynamical and evolutionary masses for the Spica system to within the observational errors of the measured masses. The age of the system is estimated to be 12.5±1 Myr.


KIC 9246715: The Double Red Giant Eclipsing Binary with Odd Oscillations

Authors. Meredith L. Rawls, Patrick Gaulme, Jean McKeever, Jason Jackiewicz, Jerome A. Orosz, Enrico Corsaro, Paul Beck, Benoît Mosser, David W. Latham, Christian A. Latham

Journal. The Astrophysical Journal

Abstract. We combine Kepler photometry with ground-based spectra to present a comprehensive dynamical model of the double red giant eclipsing binary KIC 9246715. While the two stars are very similar in mass ({M}1={2.171}-0.008+0.006 {M}ȯ , {M}2={2.149}-0.008+0.006 {M}ȯ ) and radius ({R}1={8.37}-0.07+0.03 {R}ȯ , {R}2={8.30}-0.03+0.04{R}ȯ ), an asteroseismic analysis finds one main set of solar-like oscillations with unusually low-amplitude, wide modes. A second set of oscillations from the other star may exist, but this marginal detection is extremely faint. Because the two stars are nearly twins, KIC 9246715 is a difficult target for a precise test of the asteroseismic scaling relations, which yield M = 2.17 ± 0.14 M and R = 8.26 ± 0.18 R. Both stars are consistent with the inferred asteroseismic properties, but we suspect the main oscillator is Star 2 because it is less active than Star 1. We find evidence for stellar activity and modest tidal forces acting over the 171 day eccentric orbit, which are likely responsible for the essential lack of solar-like oscillations in one star and weak oscillations in the other. Mixed modes indicate the main oscillating star is on the secondary red clump (a core-He-burning star), and stellar evolution modeling supports this with a coeval history for a pair of red clump stars. This system is a useful case study and paves the way for a detailed analysis of more red giants in eclipsing binaries, an important benchmark for asteroseismology.

Links. ApJNASA ADS, arXiv

Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars

Authors. Jennifer L. van Saders, Tugdual Ceillier, Travis S. Metcalfe, Victor Silva Aguirre, Marc H. Pinsonneault, Rafael A. García, Savita Mathur, Guy R. Davies

Journal. Nature

Abstract. A knowledge of stellar ages is crucial for our understanding of many astrophysical phenomena, and yet ages can be difficult to determine. As they become older, stars lose mass and angular momentum, resulting in an observed slowdown in surface rotation. The technique of ‘gyrochronology’ uses the rotation period of a star to calculate its age. However, stars of known age must be used for calibration, and, until recently, the approach was untested for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for old field stars whose ages have been determined with asteroseismology. The data for the cluster agree with previous period─age relations, but these relations fail to describe the asteroseismic sample. Here we report stellar evolutionary modelling, and confirm the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun. We demonstrate that models that incorporate dramatically weakened magnetic braking for old stars can—unlike existing models—reproduce both the asteroseismic and the cluster data. Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes.

Links. NatureNASA ADS, arXiv

The Kepler-454 System: A Small, Not-rocky Inner Planet, a Jovian World, and a Distant Companion

Authors. Sara Gettel, David Charbonneau, Courtney D. Dressing, Lars A. Buchhave, Xavier Dumusque, Andrew Vanderburg, Aldo S. Bonomo, Luca Malavolta, Francesco Pepe, Andrew Collier Cameron, David W. Latham, Stephane Udry, Geoffrey W. Marcy, Howard Isaacson, Andrew W. Howard, Guy R. Davies, Victor Silva Aguirre, Hans Kjeldsen, Timothy R. Bedding, Eric Lopez, Laura Affer, Rosario Cosentino, Pedro Figueira, Aldo F. M. Fiorenzano, Avet Harutyunyan, John Asher Johnson, Mercedes Lopez-Morales, Christophe Lovis, Michel Mayor, Giusi Micela, Emilio Molinari, Fatemeh Motalebi, David F. Phillips, Giampaolo Piotto, Didier Queloz, Ken Rice, Dimitar Sasselov, Damien Segransan, Alessandro Sozzetti, Chris Watson, Sarbani Basu, Tiago L. Campante, Jorgen Christensen-Dalsgaard, Steven D. Kawaler, Travis S. Metcalfe, Rasmus Handberg, Mikkel N. Lund, Mia S. Lundkvist, Daniel Huber, William J. Chaplin

Journal. The Astrophysical Journal

Abstract. Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like star that hosts a transiting planet candidate in a 10.6 day orbit. From spectroscopy, we estimate the stellar temperature to be 5687 ± 50 K, its metallicity to be [m/H] = 0.32 ± 0.08, and the projected rotational velocity to be v sin i 〈 2.4 km s-1. We combine these values with a study of the asteroseismic frequencies from short cadence Kepler data to estimate the stellar mass to be {1.028}-0.03+0.04{M}, the radius to be 1.066 ± 0.012 R, and the age to be {5.25}-1.39+1.41 Gyr. We estimate the radius of the 10.6 day planet as 2.37 ± 0.13 R. Using 63 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 36 observations made with the HIRES spectrograph at the Keck Observatory, we measure the mass of this planet to be 6.8 ± 1.4 M. We also detect two additional non-transiting companions, a planet with a minimum mass of 4.46 ± 0.12 MJ in a nearly circular 524 day orbit and a massive companion with a period 〉10 years and mass 〉12.1 MJ. The 12 exoplanets with radii 〈2.7 R and precise mass measurements appear to fall into two populations, with those 〈1.6 R following an Earth-like composition curve and larger planets requiring a significant fraction of volatiles. With a density of 2.76 ± 0.73 g cm-3, Kepler-454b lies near the mass transition between these two populations and requires the presence of volatiles and/or H/He gas.

Links. ApJNASA ADS, arXiv

Data compression for local correlation tracking of solar granulation

Authors. Björn Löptien, Aaron C. Birch, Tom L. Duvall Jr., Laurent Gizon, Jesper Schou

Journal. Astronomy & Astrophysics

Abstract. Context. Several upcoming and proposed space missions, such as Solar Orbiter, will be limited in telemetry and thus require data compression. Aims. We test the impact of data compression on local correlation tracking (LCT) of time-series of continuum intensity images. We evaluate the effect of several lossy compression methods (quantization, JPEG compression, and a reduced number of continuum images) on measurements of solar differential rotation with LCT. Methods. We apply the different compression methods to tracked and remapped continuum intensity maps obtained by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory. We derive 2D vector velocities using the local correlation tracking code FLCT and determine the additional bias and noise introduced by compression to differential rotation. Results. We find that probing differential rotation with LCT is very robust to lossy data compression when using quantization. Our results are severely affected by systematic errors of the LCT method and the HMI instrument. The sensitivity of LCT to systematic errors is a concern for Solar Orbiter.

Links. A&ANASA ADS, arXiv

CoRoT space photometry of seven Cepheids

Authors. Ennio Poretti, Jean-Francois Le Borgne, Monica Rainer, Annie Baglin, Jozsef Benko, Jonas Debosscher, Werner W. Weiss

Journal. Monthly Notices of the Royal Astronomical Society

Abstract. A few Galactic classical Cepheids were observed in the programmes of space missions as Coriolis, MOST, and Kepler. An appealing opportunity was to detect additional non-radial modes, thus opening the possibility to perform asteroseismic studies and making the pulsational content of Galactic Cepheids more similar to that of Magellanic Clouds ones. However, only hints of cycle-to-cycle variations were found, without any strict periodicity. In this context the potential of the CoRoT exoplanetary data base was not fully exploited despite the wide area covered on the Galactic plane. Therefore, we investigated all the candidate Cepheids pointed out by the automatic classification of the CoRoT curves. At the end, we could identify seven bona fide Cepheids. The light curves were investigated to remove some instrumental effects. The frequency analysis was particularly delicate since these small effects can be enhanced by the large amplitude, resulting in the presence of significant, but spurious, peaks in the power spectrum. Indeed, the careful evaluation of a very attracting peak in the spectra of CoRoT 0102618121 allowed us to certify its spurious origin. Once that the instrumental effects were properly removed, no additional mode was detected. On the other hand, cycle-to-cycle variations of the Fourier parameters were observed, but very small and always within ±3σ. Among the seven Cepheids, there are two Pop. I first-overtone pulsators, four Pop. I fundamental mode pulsators, and one Pop. II star. The CoRoT colours allowed us to measure that times of maximum brightness occur a little earlier (about 0.01 period) at short wavelengths than at long ones.


Uncertainties on near-core mixing in red-clump stars: effects on the period spacing and on the luminosity of the AGB bump

Authors. Diego Bossini, Andrea Miglio, Maurizio Salaris, Adriano Pietrinferni, Josefina Montalbán, Alessandro Bressan, Arlette Noels, Santi Cassisi, Léo Girardi, Paola Marigo

Journal. Monthly Notices of the Royal Astronomical Society

Abstract. Low-mass stars in the He-core-burning (HeCB) phase play a major role in stellar, galactic, and extragalactic astrophysics. The ability to predict accurately the properties of these stars, however, depends on our understanding of convection, which remains one of the key open questions in stellar modelling. We argue that the combination of the luminosity of the AGB bump (AGBb) and the period spacing of gravity modes (ΔΠ1) during the HeCB phase provides us with a decisive test to discriminate between competing models of these stars. We use the Modules for Experiments in Stellar Astrophysics (MESA), a Bag of Stellar Tracks and Isochrones (BaSTI), and PAdova & TRieste Stellar Evolution Code (PARSEC) stellar evolution codes to model a typical giant star observed by Kepler. We explore how various near-core-mixing scenarios affect the predictions of the above-mentioned constraints, and we find that ΔΠ1 depends strongly on the prescription adopted. Moreover we show that the detailed behaviour of ΔΠ1 shows the signature of sharp variations in the Brunt-Väisälä frequency, which could potentially give additional information about near-core features. We find evidence for the AGBb among Kepler targets, and a first comparison with observations shows that, even if standard models are able to reproduce the luminosity distribution, no standard model can account for satisfactorily the period spacing of HeCB stars. Our analysis allows us to outline a candidate model to describe simultaneously the two observed distributions: a model with a moderate overshooting region characterized by an adiabatic thermal stratification. This prescription will be tested in the future on cluster stars, to limit possible observational biases.


Kepler’s first view of O-star variability: K2 data of five O stars in Campaign 0 as a proof of concept for O-star asteroseismology

Authors. B. Buysschaert, C. Aerts, S. Bloemen, J. Debosscher, C. Neiner, M. Briquet, J. Vos, P. Papics, R. Manick, V. Schmid, H. Van Winkel, A. Tkachenko

Journal. Monthly Notices of the Royal Astronomical Society

Abstract. We present high-precision photometric light curves of five O-type stars observed with the refurbished Kepler satellite during its Campaign 0. For one of the stars, we also assembled high-resolution ground-based spectroscopy with the HERMES spectrograph attached to the 1.2 m Mercator telescope. The stars EPIC 202060097 (O9.5V) and EPIC 202060098 (O7V) exhibit monoperiodic variability due to rotational modulation with an amplitude of 5.6 and 9.3 mmag and a rotation period of 2.63 and 5.03 d, respectively. EPIC 202060091 (O9V) and EPIC 202060093 (O9V:pe) reveal variability at low frequency but the cause is unclear. EPIC 202060092 (O9V:p) is discovered to be a spectroscopic binary with at least one multiperiodic β Cep-type pulsator whose detected mode frequencies occur in the range [0.11, 6.99] d-1 and have amplitudes between 0.8 and 2.0 mmag. Its pulsation spectrum is shown to be fully compatible with the ones predicted by core-hydrogen burning O-star models. Despite the short duration of some 33 d and the limited data quality with a precision near 100 μmag of these first K2 data, the diversity of possible causes for O-star variability already revealed from campaigns of similar duration by the MOST and CoRoT satellites is confirmed with Kepler. We provide an overview of O-star space photometry and give arguments why future K2 monitoring during Campaigns 11 and 13 at short cadence, accompanied by time-resolved high-precision high-resolution spectroscopy, opens up the possibility of in-depth O-star seismology.


Horizontal Flows in Active Regions from Ring-diagram and Local Correlation Tracking Methods

Authors. Jain, Kiran; Tripathy, S. C.; Ravindra, B.; Komm, R.; Hill, F.

Journal.  The Astrophysical Journal

Abstract. Continuous high-cadence and high-spatial resolution Dopplergrams allow us to study sub-surface dynamics that may be further extended to explore precursors of visible solar activity on the surface. Since the p-mode power is absorbed in the regions of high magnetic field, the inferences in these regions are often presumed to have large uncertainties. In this paper, using the Dopplergrams from space-borne Helioseismic Magnetic Imager (HMI), we compare horizontal flows in a shear layer below the surface and the photospheric layer in and around active regions. The photospheric flows are calculated using local correlation tracking (LCT) method while the ring-diagram (RD) technique of helioseismology is used to infer flows in the sub-photospheric shear layer. We find a strong positive correlation between flows from both methods near the surface. This implies that despite the absorption of acoustic power in the regions of strong magnetic field, the flows inferred from the helioseismology are comparable to those from the surface measurements. However, the magnitudes are significantly different; the flows from the LCT method are smaller by a factor of 2 than the helioseismic measurements. Also, the median difference between direction of corresponding vectors is 49 degree.

Links. ApJNASA ADS, arXiv

Determining the evolutionary stage of HD163899 on the basis of its oscillation spectrum

Authors. Ostrowski, Jakub; Daszyńska-Daszkiewicz, Jadwiga; Cugier, Henryk

Journal.  Astronomy in Focus, Volume 1

Abstract. We present the new interpretation of the oscillation spectrum of HD 163899 based on the new determinations of the effective temperature, mass-luminosity ratio and rotational velocity. These new parameters strongly prefer the more massive models than previously considered. Now it is also possible that the star could be in the main sequence stage. Using the oscillation spectrum as a gauge, we intend to establish which stage of evolution corresponds better to HD 163899.

Links. NASA ADS, arXiv