Category Archives: Publications

On the inference of stellar ages and convective-core properties in main-sequence solar-like pulsators

Authors.Brandão, I. M. ; Cunha, M. S. ; Christensen-Dalsgaard, J.

Journal.Monthly Notices of the Royal Astronomical Society, Volume 438, Issue 2, p.1751-1761

Abstract.Particular diagnostic tools may isolate the signature left on the oscillation frequencies by the presence of a small convective core. Their frequency derivative is expected to provide information about convective core’s properties and stellar age. The main goal of this work is to study the potential of the diagnostic tools with regard to the inference of stellar age and stellar core’s properties. For that, we computed diagnostic tools and their frequency derivatives from the oscillation frequencies of main-sequence models with masses between 1.0 and 1.6 M☉ and with different physics. We considered the dependence of the diagnostic tools on stellar age and on the size of the relative discontinuity in the squared sound speed at the edge of the convectively unstable region. We find that the absolute value of the frequency derivatives of the diagnostic tools increases as the star evolves on the main sequence. The fraction of stellar main-sequence evolution for models with masses >1.2 M☉ may be estimated from the frequency derivatives of two of the diagnostic tools. For lower mass models, constraints on the convective core’s overshoot can potentially be derived based on the analysis of the same derivatives. For at least 35 per cent of our sample of stellar models, the frequency derivative of the diagnostic tools takes its maximum absolute value on the frequency range where observed oscillations may be expected.

Links. NASA ADS,arXiv

Interpreting the Helioseismic and Magnetic Imager (HMI) Multi-Height Velocity Measurements

Authors.Nagashima, Kaori ; Löptien, Björn ; Gizon, Laurent ; Birch, Aaron C. ; Cameron, Robert ; Couvidat, Sebastien ; Danilovic, Sanja ; Fleck, Bernhard ; Stein, Robert

Journal.Solar Physics, Volume 289, Issue 9, pp.3457-3481

Abstract.The Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI) filtergrams, taken at six wavelengths around the Fe I 6173.3 {\AA} line, contain information about the line-of-sight velocity over a range of heights in the solar atmosphere. Multi-height velocity inferences from these observations can be exploited to study wave motions and energy transport in the atmosphere. Using realistic convection simulation datasets provided by the STAGGER and MURaM codes, we generate synthetic filtergrams and explore several methods for estimating Dopplergrams. We investigate at which height each synthetic Dopplergram correlates most strongly with the vertical velocity in the model atmospheres. On the basis of the investigation, we propose two Dopplergrams other than the standard HMI-algorithm Dopplergram produced from HMI filtergrams: a line-center Dopplergram and an average-wing Dopplergram. These two Dopplergrams correlate most strongly with vertical velocities at the heights of 30 – 40 km above (line-center) and 30 – 40 km below (average-wing) the effective height of the HMI-algorithm Dopplergram. Therefore, we can obtain velocity information from two layers separated by about a half of a scale height in the atmosphere, at best. The phase shifts between these multi-height Dopplergrams from observational data as well as those from the simulated data are also consistent with the height-difference estimates in the frequency range above the photospheric acoustic cutoff frequency.

Links. NASA ADS,arXiv, Springer

Photometric magnetic-activity metrics tested with the Sun: Application to Kepler M dwarfs

Authors.Mathur, S. ; Salabert, D. ; Garcia, R. A. ; Ceillier, T.

Journal.Journal of Space Weather and Space Climate

Abstract.The Kepler mission has been providing high-quality photometric data leading to many breakthroughs in the exoplanet search and in stellar physics. Stellar magnetic activity results from the interaction between rotation, convection, and magnetic field. Constraining these processes is important if we want to better understand stellar magnetic activity. Using the Sun, we want to test a magnetic activity index based on the analysis of the photo- metric response and then apply it to a sample of M dwarfs observed by Kepler. We estimate a global stellar magnetic activity index by measuring the standard deviation of the whole time series, Sph. Because stellar variability can be related to convection, pulsations, or magnetism, we need to ensure that this index mostly takes into account magnetic effects. We define another stellar magnetic activity index as the average of the standard deviation of shorter subseries which lengths are determined by the rotation period of the star. This way we can ensure that the measured photometric variability is related to starspots crossing the visible stellar disk. This new index combined with a time-frequency analysis based on the Morlet wavelets allows us to determine the existence of magnetic activity cycles. We measure magnetic indexes for the Sun and for 34 M dwarfs observed by Kepler. As expected, we obtain that the sample of M dwarfs studied in this work is much more active than the Sun. Moreover, we find a small correlation between the rotation period and the magnetic index. Finally, by combining a time-frequency analysis with phase diagrams, we discover the presence of long-lived features suggesting the existence of active longitudes on the surface of these stars.

Links. NASA ADS,arXiv

Looking for activity cycles in late-type Kepler stars using time-frequency analysis

Authors.Vida, K. ; Oláh, K. ; Szabó, R.

Journal.Monthly Notices of the Royal Astronomical Society, Volume 441, Issue 3, p.2744-2753

Abstract.We analyse light curves covering four years of 39 fast-rotating ($P_\mathrm{rot}< 1d$) late-type active stars from the Kepler database. Using time-frequency analysis (Short-Term Fourier-Transform), we find hints for activity cycles of 300-900 days at 9 targets from the changing typical latitude of the starspots, which, with the differential rotation of the stellar surface change the observed rotation period over the activity cycle. We also give a lowest estimation for the shear parameter of the differential rotation, which is ~0.001 for the cycling targets. These results populate the less studied, short period end of the rotation-cycle length relation.

Links. NASA ADS LABS,arXiv

The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Overview and Performance

Authors. Hoeksema, J. Todd ; Liu, Yang ; Hayashi, Keiji ; Sun, Xudong ; Schou, Jesper ; Couvidat, Sebastien ; Norton, Aimee ; Bobra, Monica ; Centeno, Rebecca ; Leka, K. D. ; Barnes, Graham ; Turmon, Michael

Journal.Solar Physics, Volume 289, Issue 9, pp.3483-3530

Abstract.The Helioseismic and Magnetic Imager (HMI) began near-continuous full-disk solar measurements on 1 May 2010 from the Solar Dynamics Observatory (SDO). An automated processing pipeline keeps pace with observations to produce observable quantities, including the photospheric vector magnetic field, from sequences of filtergrams. The basic vector-field frame list cadence is 135 seconds, but to reduce noise the filtergrams are combined to derive data products every 720 seconds. The primary 720 s observables were released in mid-2010, including Stokes polarization parameters measured at six wavelengths, as well as intensity, Doppler velocity, and the line-of-sight magnetic field. More advanced products, including the full vector magnetic field, are now available. Automatically identified HMI Active Region Patches (HARPs) track the location and shape of magnetic regions throughout their lifetime. The vector field is computed using the Very Fast Inversion of the Stokes Vector (VFISV) code optimized for the HMI pipeline; the remaining 180∘ azimuth ambiguity is resolved with the Minimum Energy (ME0) code. The Milne-Eddington inversion is performed on all full-disk HMI observations. The disambiguation, until recently run only on HARP regions, is now implemented for the full disk. Vector and scalar quantities in the patches are used to derive active region indices potentially useful for forecasting; the data maps and indices are collected in the SHARP data series, hmi.sharp_720s. Definitive SHARP processing is completed only after the region rotates off the visible disk; quick-look products are produced in near real time. Patches are provided in both CCD and heliographic coordinates.HMI provides continuous coverage of the vector field, but has modest spatial, spectral, and temporal resolution. Coupled with limitations of the analysis and interpretation techniques, effects of the orbital velocity, and instrument performance, the resulting measurements have a certain dynamic range and sensitivity and are subject to systematic errors and uncertainties that are characterized in this report.

Links. Solar Physics,NASA ADS,arXiv

The eccentric massive binary V380 Cyg: revised orbital elements and interpretation of the intrinsic variability of the primary component*

Authors.Tkachenko, A. ; Degroote, P. ; Aerts, C. ; Pavlovski, K. ; Southworth, J. ; Pápics, P. I. ; Moravveji, E. ; Kolbas, V. ; Tsymbal, V. ; Debosscher, J. ; Clémer, K.

Journal.Monthly Notices of the Royal Astronomical Society, Volume 438, Issue 4, p.3093-3110

Abstract.We present a detailed analysis and interpretation of the high-mass binary V380 Cyg, based on high-precision space photometry gathered with the Kepler space mission as well as high-resolution ground-based spectroscopy obtained with the HERMES spectrograph attached to the 1.2 m Mercator telescope. We derive a precise orbital solution and the full physical properties of the system, including dynamical component mass estimates of 11.43 ± 0.19 and 7.00 ± 0.14 M⊙ for the primary and secondary, respectively. Our frequency analysis reveals the rotation frequency of the primary in both the photometric and spectroscopic data and additional low-amplitude stochastic variability at low frequency in the space photometry with characteristics that are compatible with recent theoretical predictions for gravity-mode oscillations excited either by the convective core or by sub-surface convective layers. Doppler imaging analysis of the silicon lines of the primary suggests the presence of two high-contrast stellar surface abundance spots which are located either at the same latitude or longitude. Comparison of the observed properties of the binary with present-day single-star evolutionary models shows that the latter are inadequate and lack a serious amount of near-core mixing.


Towards asteroseismically calibrated age-rotation-activity relations for Kepler solar-like stars

Authors.Garcia, R. A. ; Ceillier, T. ; Salabert, D. ; Mathur, S. ; van Saders, J. L. ; Pinsonneault, M. ; Ballot, J. ; Beck, P. G. ; Bloemen, S. ; Campante, T. L. ; Davies, G. R. ; do Nascimento, J. -D., Jr. ; Mathis, S. ; Metcalfe, T. S. ; Nielsen, M. B. ; Suarez, J. C. ; Chaplin, W. J. ; Jimenez, A. ; Karoff, C.

Journal.Submited to Astronomy & Astrophysics

Abstract.Kepler ultra-high precision photometry of long and continuous observations provide a unique dataset in which surface rotation and variability can be studied for thousands of stars. Because many of these old field stars also have independently measured asteroseismic ages, measurements of rotation and activity are particularly interesting in the context of age-rotation-activity relations. These relations generally lack good calibrators at old ages, a problem that this Kepler sample of stars is uniquely suited to address. We study the surface rotation and the photometric magnetic activity of a subset of 540 solar-like stars on the main-sequence and the subgiant branch for which stellar pulsations have been measured. The rotation period is determined by comparing the results from two different sets of calibrated data and from two complementary analyses. Global photometric levels of magnetic activity in this sample of stars are also extracted by using a photometric activity index, which takes into account the rotation period of the stars. Out of the 540 solar-like pulsating stars in our sample, we successfully measured the rotation period of 310 stars (excluding known binaries and candidate planet host stars). The rotation periods lay between 1 and 100 days. The remaining stars are classified into two categories: those not showing any surface rotation (6 stars), and those in which the four analyses did not converge to a single and robust rotation period (213). The photometric magnetic activity levels were computed and for 61.5% of the dwarfs, its value is comparable to the solar one. We then extract an age-rotation relation only for the dwarfs with very precise asteroseismic age estimations, highlighting the necessity of excluding the hot stars and the subgiants when inferring such relations. We also studied age-activity-rotation relations with a hint of correlation for the subgiants.

Links. NASA ADS,arXiv

Prospects for detecting asteroseismic binaries in Kepler data

Authors. Miglio, A.; Chaplin, W. J.; Farmer, R.; Kolb, U.; Girardi, L.; Elsworth, Y.; Appourchaux, T.; Handberg, R.

Journal. The Astrophysical Journal Letters, Volume 784, Issue 1, article id. L3, 6 pp. (2014)

Abstract. Asteroseismology may in principle be used to detect unresolved stellar binary systems comprised of solar-type stars and/or red giants. This novel method relies on the detection of the presence of two solar-like oscillation spectra in the frequency spectrum of a single lightcurve. Here, we make predictions of the numbers of systems that may be detectable in data already collected by the NASA Kepler Mission. Our predictions, which are based upon TRILEGAL and BiSEPS simulations of the Kepler field of view, indicate that as many as 200 or more “asteroseismic binaries” may be detectable in this manner. Most of these binaries should be comprised of two He-core-burning red giants. Owing largely to the limited numbers of targets with the requisite short-cadence Kepler data, we expect only a small number of detected binaries containing solar-type stars. The predicted yield of detections is sensitive to the assumed initial mass ratio distribution of the binary components and therefore represents a sensitive calibration of the much debated initial mass ratio distribution near mass ratio unity.

Links. NASA ADS, arXiv

Measurement of Acoustic Glitches in Solar-type Stars from Oscillation Frequencies Observed by Kepler

Authors. Mazumdar, A.; Monteiro, M. J. P. F. G.; Ballot, J.; Antia, H. M.; Basu, S.; Houdek, G.; Mathur, S.; Cunha, M. S.; Silva Aguirre, V.; García, R. A.; Salabert, D.; Verner, G. A.; Christensen-Dalsgaard, J.; Metcalfe, T. S.; Sanderfer, D. T.; Seader, S. E.; Smith, J. C.; Chaplin, W. J.

Journal. The Astrophysical Journal, Volume 782, Number 1

Abstract. For the very best and brightest asteroseismic solar-type targets observed by Kepler, the frequency precision is sufficient to determine the acoustic depths of the surface convective layer and the helium ionization zone. Such sharp features inside the acoustic cavity of the star, which we call acoustic glitches, create small oscillatory deviations from the uniform spacing of frequencies in a sequence of oscillation modes with the same spherical harmonic degree. We use these oscillatory signals to determine the acoustic locations of such features in 19 solar-type stars observed by the Kepler mission. Four independent groups of researchers utilized the oscillation frequencies themselves, the second differences of the frequencies and the ratio of the small and large separation to locate the base of the convection zone and the second helium ionization zone. Despite the significantly different methods of analysis, good agreement was found between the results of these four groups, barring a few cases. These results also agree reasonably well with the locations of these layers in representative models of the stars. These results firmly establish the presence of the oscillatory signals in the asteroseismic data and the viability of several techniques to determine the location of acoustic glitches inside stars.

Links. IOP Science, NASA ADS, arXiv

Understanding the dynamical structure of pulsating stars. HARPS spectroscopy of the δ Scuti stars ρ Puppis and DX Ceti

Authors. Nardetto, N.; Poretti, E.; Rainer, M.; Guiglion, G.; Scardia, M.; Schmid, V. S.; Mathias, P.

Journal. A&A Volume 561, January 2014

Abstract. Context. High-resolution spectroscopy is a powerful tool to study the dynamical structure of a pulsating star’s atmosphere.
Aims. We aim at comparing the line asymmetry and velocity of the two δ Sct stars ρ Pup and DX Cet with previous spectroscopic data obtained on classical Cepheids and β Cep stars.
Methods. We obtained, analysed and discuss HARPS high-resolution spectra of ρ Pup and DX Cet. We derived the same physical quantities as used in previous studies, which are the first-moment radial velocities and the bi-Gaussian spectral line asymmetries.
Results. The identification of f = 7.098 d-1 as a fundamental radial mode and the very accurate Hipparcos parallax promote ρ Pup as the best standard candle to test the period–luminosity relations of δ Sct stars. The action of small-amplitude nonradial modes can be seen as well-defined cycle-to-cycle variations in the radial velocity measurements of ρ Pup. Using the spectral-line asymmetry method, we also found the centre-of-mass velocities of ρ Pup and DX Cet, Vγ = 47.49 ± 0.07 km s-1 and Vγ = 25.75 ± 0.06 km s-1, respectively. By comparing our results with previous HARPS observations of classical Cepheids and β Cep stars, we confirm the linear relation between the atmospheric velocity gradient and the amplitude of the radial velocity curve, but only for amplitudes larger than 22.5 km s-1. For lower values of the velocity amplitude (i.e.,

Links. A&A, NASA ADS, arXiv