Hydrodynamical simulations of convection-related stellar micro-variability (original) (raw)

Evidence for Granulation in Early A-Type Stars

The Astrophysical Journal, 2010

Stars with spectral types earlier than about F0 on (or close) to the main sequence have long been believed to lack observable surface convection, although evolutionary models of A-type stars do predict very thin surface convective zones. We present evidence for granulation in two δ Scuti stars of spectral type A2: HD 174936 and HD 50844. Recent analyses of space-based CoRoT (Convection, Rotation, and planetary Transits) data revealed up to some 1000 frequencies in the photometry of these stars. The frequencies were interpreted as individual pulsation modes. If true, there must be large numbers of nonradial modes of very high degree l which should suffer cancellation effects in disk-integrated photometry (even of high space-based precision). The p-mode interpretation of all the frequencies in HD 174936 and HD 50844 depends on the assumption of white (frequency independent) noise. Our independent analyses of the data provide an alternative explanation: most of the peaks in the Fourier spectra are the signature of non-white granulation background noise, and less than about 100 of the frequencies are actual stellar pmodes in each star. We find granulation time scales which are consistent with scaling relations that describe cooler stars with known surface convection. If the granulation interpretation is correct, the hundreds of low-amplitude Fourier peaks reported in recent studies are falsely interpreted as independent pulsation modes and a significantly lower number of frequencies are associated with pulsation, consistent with only modes of low degree.

Granulation in K-type dwarf stars

Astronomy & Astrophysics, 2009

Aims. To explore the impact of surface inhomogeneities on stellar spectra, granulation models need to be computed. Ideally, the most fundamental characteristics of these models should be carefully tested before applying them to the study of more practical matters, such as the derivation of photospheric abundances. Our goal is to analyze the particular case of a K-dwarf. Methods. We construct a three-dimensional radiative-hydrodynamic model atmosphere of parameters T eff = 4820 K, log g = 4.5, and solar chemical composition. Using this model and 3D spectrum synthesis, we computed a number of Fe i and Fe ii line profiles. The observations presented in the first paper of this series were used to test the model predictions. The effects of stellar rotation and instrumental imperfections are carefully taken into account in the synthesis of spectral lines. Results. The theoretical line profiles show the typical signatures of granulation: the lines are asymmetric, with their bisectors having a characteristic C-shape and their core wavelengths shifted with respect to their laboratory values. The line bisectors span from about 10 to 250 m s −1 , depending on line strength, with the stronger features showing larger span. The corresponding core wavelength shifts range from about −200 m s −1 for the weak Fe i lines to almost +100 m s −1 in the strong Fe i features. Based on observational results for the Sun, we argue that there should be no core wavelength shift for Fe i lines of EW 100 mÅ. The cores of the strongest lines show contributions from the uncertain top layers of the model, where non-LTE effects and the presence of the chromosphere, which are important in real stars, are not accounted for. The Fe ii lines suffer from stronger granulation effects due to their deeper formation depth which makes them experience stronger temperature and velocity contrasts. For example, the core wavelength shifts of the weakest Fe ii lines are about −600 m s −1 . The comparison of model predictions to observed Fe i line bisectors and core wavelength shifts for our reference star, HIP 86400, shows excellent agreement, with the exception of the core wavelength shifts of the strongest features, for which we suspect inaccurate theoretical values. Since this limitation does not affect the predicted line equivalent widths significantly, we consider our 3D model validated for photospheric abundance work.

Impact of the physical processes in the modeling of HD 49933

Astronomy and Astrophysics, 2009

Context. On its asteroseismic side, the initial run of CoRoT was partly devoted to the solar like star HD49933. The eigenmodes of this F dwarf have been observed with unprecedented accuracy. Aims. We investigate quantitatively the impact of changes in the modeling parameters like mass and composition. More importantly we investigate how a sophisticated physics affects the seismological picture of HD49933. We consider the effects of diffusion, rotation and the changes in convection efficiency. Methods. We use the CESAM stellar evolution code coupled to the ADIPLS adiabatic pulsation package to build secular models and their associated oscillation frequencies. We also exploited the hydrodynamical code STAGGER to perform surface convection calculations. The seismic variables used in this work are: the large frequency separation, the derivative of the surface phase shift, and the eigenfrequencies ν ℓ=0,n=14 and ν ℓ=0,n=27 . Results. Mass and uncertainties on the composition have much larger impacts on the seismic variables we consider than the rotation. The derivative of the surface phase shift is a promising variable for the determination of the helium content. The seismological variables of HD49933 are sensitive to the assumed solar composition and also to the presence of diffusion in the models.

Attempt to detect the granulation signature in the HgMn star HD 175640 from CoRoT light curves

Monthly Notices of the Royal Astronomical Society, 2013

HgMn stars are affected by atomic diffusion processes. Because theoretical models based on atomic diffusion require that the atmospheres of ApBp stars have to be much more stable than those of normal stars, the detection or the absence of detection of granulation in them should be an interesting information for the modelling. We would like to address the question of the stability of the atmospheres of HgMn stars by analysing the light curves provided by the Convection Rotation and planetary Transits (CoRoT) satellite. We used the CoRoT light curves of the bright HgMn star HD 175640 observed through asteroseismology channel, to look for granulation signature. We have developed a new quasi-automatic IDL procedure, which allows us to correct abnormal jumps and variations which exist in CoRoT N2 data. The earlier results show that this procedure is an appropriate tool for the analysis of data obtained through asteroseismology and exoplanet channels. Calculations we carried out for this target show that granulation signature in this HgMn star is not detected from CoRoT light curves. On the other hand, we do not detect clear pulsation signal for this star. That shows that even if some HgMn stars are pulsating, this is not a general property of the group.

The connection between stellar granulation and oscillation as seen by the Kepler mission

Astronomy & Astrophysics, 2014

Context. The long and almost continuous observations by Kepler show clear evidence of a granulation background signal in a large sample of stars, which is interpreted as the surface manifestation of convection. It has been shown that its characteristic timescale and rms intensity fluctuation scale with the peak frequency (ν max ) of the solar-like oscillations. Various attempts have been made to quantify the observed signal, to determine scaling relations for its characteristic parameters, and to compare them to theoretical predictions. Aims. We aim to study different approaches to quantifying the signature of stellar granulation and to search for a unified model that reproduces the observed signal best in a wide variety of stars. We then aim to define empirical scaling relations between the granulation properties and ν max and various other stellar parameters. Methods. We use a probabilistic method to compare different approaches to extracting the granulation signal. We fit the power density spectra of a large set of Kepler targets, determine the granulation and global oscillation parameter, and quantify scaling relations between them. Results. We establish that a depression in power at about ν max /2, known from the Sun and a few other main-sequence stars, is also statistically significant in red giants and that a super-Lorentzian function with two components is best suited to reproducing the granulation signal in the broader vicinity of the pulsation power excess. We also establish that the specific choice of the background model can affect the determination of ν max , introducing systematic uncertainties that can significantly exceed the random uncertainties. We find the characteristic frequency and amplitude of both background components to tightly scale with ν max for a wide variety of stars, and quantify a mass dependency of the latter. To enable comparison with theoretical predictions, we computed effective timescales and bolometric intensity fluctuations and found them to approximately scale as τ eff ∝ g −0.85 T −0.4 and A gran ∝ (g 2 M) −1/4 (or more conveniently R/M 3/4 ), respectively. Similarly, the bolometric pulsation amplitude scales approximately as A puls ∝ (g 2 M) −1/3 (or R 4/3 /M), which implicitly verifies a separate mass and luminosity dependence of A puls . Conclusions. We provide a thorough analysis of the granulation background signal in a large sample of stars, from which we establish a unified model that allows us to accurately extract the granulation and global oscillation parameter.

Changes in granulation scales over the solar cycle seen with SDO/HMI and Hinode/SOT

Astronomy & Astrophysics, 2021

Context. The Sun is the only star where the superficial turbulent convection can be observed at very high spatial resolution. The Solar Dynamics Observatory (SDO) has continuously observed the full Sun from space with multi-wavelength filters since July 2010. In particular, the Helioseismic and Magnetic Imager (HMI) instrument takes high-cadence frames (45 s) of continuum intensity in which solar granulation is visible. Aims. We aimed to follow the evolution of the solar granules over an activity cycle and look for changes in their spatial properties. Methods. We investigated the density of granules and their mean area derived directly from the segmentation of deconvolved images from SDO/HMI. To perform the segmentation, we define granules as convex elements of images. Results. We measured an approximately 2% variation in the density and the mean area of granules over the cycle, the density of granules being greater at solar maximum with a smaller granule mean area. The maximum dens...

Metallicity effect on stellar granulation detected from oscillating red giants in open clusters

Astronomy & Astrophysics

Context. The effect of metallicity on the granulation activity in stars, and hence on the convective motions in general, is still poorly understood. Available spectroscopic parameters from the updated APOGEE-Kepler catalog, coupled with high-precision photometric observations from NASA's Kepler mission spanning more than four years of observation, make oscillating red giant stars in open clusters crucial testbeds. Aims. We aim to determine the role of metallicity on the stellar granulation activity by discriminating its effect from that of different stellar properties such as surface gravity, mass, and temperature. We analyze 60 known red giant stars belonging to the open clusters NGC 6791, NGC 6819, and NGC 6811, spanning a metallicity range from [Fe/H] −0.09 to 0.32. The parameters describing the granulation activity of these stars and their frequency of maximum oscillation power, ν max , are studied while taking into account different masses, metallicities, and stellar evolutionary stages. We derive new scaling relations for the granulation activity, re-calibrate existing ones, and identify the best scaling relations from the available set of observations. Methods. We adopted the Bayesian code Diamonds for the analysis of the background signal in the Fourier spectra of the stars. We performed a Bayesian parameter estimation and model comparison to test the different model hypotheses proposed in this work and in the literature. Results. Metallicity causes a statistically significant change in the amplitude of the granulation activity, with a dependency stronger than that induced by both stellar mass and surface gravity. We also find that the metallicity has a significant impact on the corresponding time scales of the phenomenon. The effect of metallicity on the time scale is stronger than that of mass. Conclusions. A higher metallicity increases the amplitude of granulation and meso-granulation signals and slows down their characteristic time scales toward longer periods. The trend in amplitude is in qualitative agreement with predictions from existing 3D hydrodynamical simulations of stellar atmospheres from main sequence to red giant stars. We confirm that the granulation activity is not sensitive to changes in the stellar core and that it only depends on the atmospheric parameters of stars.

Stellar Granulation as the Source of High-Frequency Flicker in Kepler Light Curves

The Astrophysical Journal

A large fraction of cool, low-mass stars exhibit brightness fluctuations that arise from a combination of convective granulation, acoustic oscillations, magnetic activity, and stellar rotation. Much of the short-timescale variability takes the form of stochastic noise, whose presence may limit the progress of extrasolar planet detection and characterization. In order to lay the groundwork for extracting useful information from these quasi-random signals, we focus on the origin of the granulation-driven component of the variability. We apply existing theoretical scaling relations to predict the star-integrated variability amplitudes for 508 stars with photometric light curves measured by the Kepler mission. We also derive an empirical correction factor that aims to account for the suppression of convection in F-dwarf stars with magnetic activity and shallow convection zones. So that we can make predictions of specific observational quantities, we performed Monte Carlo simulations of ...

Abundance and stratification analysis of the chemically peculiar star HD 103498

Monthly Notices of the Royal Astronomical Society, 2011

The slow rotation and the absence of strong mixing processes in the atmospheres of chemically peculiar stars develop the ideal conditions for the appearance of abundance anomalies through the mechanism of microscopic particle diffusion. This makes these objects look spectroscopically and photometrically different from their 'normal' analogues. As a result, it is often difficult to accurately determine the atmospheric parameters of these stars, and special methods are needed for a consistent analysis of their atmospheres. The main aim of the present paper is to analyse atmospheric abundance and stratification of chemical elements in the atmosphere of the chemically peculiar star HD 103498. We find that there are two model atmospheres, computed with individual and stratified abundances, that provide a reasonable fit to the observed spectroscopic and photometric indicators: T eff = 9300 K, log g = 3.5 and T eff = 9500 K, log g = 3.6. It is shown that Mg has a large abundance gradient in the star's atmosphere with accumulation of Mg ions in the uppermost atmospheric layers, whereas Si demonstrates the opposite behaviour with accumulation in deep layers. In addition, a detailed non-local thermodynamic equilibrium (non-LTE) analysis showed that none of the Mg transitions under consideration is a subject of noticeable non-LTE effects. By comparing the photometry observations after transforming them to physical units, we estimated the radius of HD 103498 to be between R = (4.56 ± 0.77) R for T eff = 9300 K, log g = 3.5, and R = (4.39 ± 0.75) R for T eff = 9500 K, log g = 3.6 models, respectively. We note that the lack of suitable observations in absolute units prevents us from uniquely determining the T eff of the star at the current stage of analysis.