Visibilities and bolometric corrections for stellar oscillation modes observed by<i>Kepler</i>

Ballot, J.; Barban, C.; Veer-Menneret, C. van 't

doi:10.1051/0004-6361/201016230

Cited by 82 publications

(127 citation statements)

References 24 publications

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“…Brown et al (1991) and Kjeldsen & Bedding (1995) have conjectured that ν max scales as the star cutoff frequency, ν c ∝ g/ √ T eff . This relation was shown to work for a larger variety of stars (Bedding & Kjeldsen 2003;Stello et al 2009;Huber et al 2009;Mosser et al 2010) and its underlying physical origin has been explained recently by Belkacem et al (2011). For the rms brightness fluctuation σ, the observations show that σ roughly scales as (ν max ) −1/2 (Mathur et al 2011;Chaplin et al 2011b).…”

Section: Theoretical Scaling Relationsmentioning

confidence: 80%

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Stellar granulation as seen in disk-integrated intensity

Samadi

Belkacem

Ludwig

et al. 2013

A&A

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Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τ eff ) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (ν max ) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-integrated intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τ eff ) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τ eff and σ, which show the same dependence on ν max as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (M a ) estimated at the photosphere. The theoretical scaling relations for τ eff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τ eff with ν max . It also highlights the important role of M a in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on M a . Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with M a .

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Section: Theoretical Scaling Relationsmentioning

confidence: 80%

“…(21) was obtained according to the relation σ 2 = C 2 bol H g /τ eff /4, where C bol is a bolometric correction that scales for the Kepler bandpass as C bol = (T eff /T 0 ) α , where T 0 = 5934 K and α = 0.8 (Ballot et al 2011, see also Michel et al 2009).…”

Section: Observationsmentioning

confidence: 99%

Stellar granulation as seen in disk-integrated intensity

Samadi

Belkacem

Ludwig

et al. 2013

A&A

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“…For this we use a power law B a (ν) = P a ν e a . To obtain the best fit we have followed the same procedure as in Ballot et al (2011b). The results of the fit are τ g (s) = 1112 ± 21, σ g = 102.2 ± 0.5 ppm, and α g = 2.41 ± 0.02.…”

Section: Background Parametersmentioning

confidence: 99%

Study of KIC 8561221 observed byKepler: an early red giant showing depressed dipolar modes

García

Hernández

Benomar

et al. 2014

A&A

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Context. The continuous high-precision photometric observations provided by the CoRoT and Kepler space missions have allowed us to understand the structure and dynamics of red giants better using asteroseismic techniques. A small fraction of these stars show dipole modes with unexpectedly low amplitudes. The reduction in amplitude is more pronounced for stars with a higher frequency of maximum power, ν max . Aims. In this work we want to characterise KIC 8561221 in order to confirm that it is currently the least evolved star among this peculiar subset and to discuss several hypotheses that could help explain the reduction of the dipole mode amplitudes. Methods. We used Kepler short-and long-cadence data combined with spectroscopic observations to infer the stellar structure and dynamics of KIC 8561221. We then discussed different scenarios that could contribute to reducing the dipole amplitudes, such as a fast-rotating interior or the effect of a magnetic field on the properties of the modes. We also performed a detailed study of the inertia and damping of the modes. Results. We have been able to characterise 36 oscillations modes, in particular, a few dipole modes above ν max that exhibit nearly normal amplitudes. The frequencies of all the measured modes were used to determine the overall properties and the internal structure of the star. We have inferred a surface rotation period of ∼91 days and uncovered a variation in the surface magnetic activity during the last 4 years. The analysis of the convective background did not reveal any difference compared to "normal" red giants. As expected, the internal regions of the star probed by the = 2 and 3 modes spin 4 to 8 times faster than the surface. Conclusions. With our grid of standard models we are able to properly fit the observed frequencies. Our model calculation of mode inertia and damping give no explanation for the depressed dipole modes. A fast-rotating core is also ruled out as a possible explanation. Finally, we do not have any observational evidence of a strong deep magnetic field inside the star.

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“…We are more interested in the total background signal (or so to say a proxy for the total energy) locally around ν max and therefore have to merge the two individual amplitudes to a single value that can be compared to other observations or model predictions. This is straightforward and we define the total bolometric intensity fluctuation due to granulation as A 2 gran = C 2 bol (a 2 1 + a 2 2 ), where C bol is a bolometric correction that scales for the Kepler bandpass as C bol = (T eff /T 0 ) α , where T 0 = 5934 K and α = 0.8 (Ballot et al 2011;Michel et al 2009). The estimated uncertainties of ±250 K in T eff typically add about 1-2% uncertainty to A gran , which are then about 7% on average.…”

Section: Intensity Fluctuation a Granmentioning

confidence: 99%

The connection between stellar granulation and oscillation as seen by theKeplermission

Kallinger

Ridder

Hekker

et al. 2014

A&A

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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. Even though they are consistent on a global scale, large systematic differences of an unknown origin remain between different methods, as well as between the observations and simulations. 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 (i.e., inverse timescale) and amplitude of both background components to tightly scale with ν max for a wide variety of stars (about 2-2000 μHz in ν max ), and quantify a mass dependency of the latter. To enable comparison with theoretical predictions (which do not include the observed power depression), 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 . We have also checked our scaling relations with solar reference values and find them in good agreement. 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. The resulting scaling relations allow a simple estimate of the overall spectral shape of any solar-ty...

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Visibilities and bolometric corrections for stellar oscillation modes observed byKepler

Cited by 82 publications

References 24 publications

Stellar granulation as seen in disk-integrated intensity

Stellar granulation as seen in disk-integrated intensity

Study of KIC 8561221 observed byKepler: an early red giant showing depressed dipolar modes

The connection between stellar granulation and oscillation as seen by theKeplermission

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