Context. The interaction of solar oscillations with near surface convection is poorly understood. These interactions are likely the cause of several problems in helio- and astero-seismology, including the so-called surface effect and apparently unphysical travel time shifts as a function of center to limb distance. There is thus a clear need for further theoretical understanding and observational tests.
Aims. The aim is to determine how the observed modes are affected by the convection.
Methods. I use HMI velocity and intensity images to construct k-$ω$ diagrams showing how the oscillation amplitude and phase depend on the local granulation intensity.
Results. There is a clear and significant dependence of the observed properties of the oscillations on the local convection state.
Tens of thousands of solar-like oscillating stars have been observed by space missions. Their photometric variability in the Fourier domain can be parameterized by a sum of two super-Lorentizian functions for granulation and a Gaussian-shaped power excess for oscillation. The photometric granulation/oscillation parameters scale with stellar parameters and they can also make predictions for corresponding parameters in radial velocity measurements. Based on scaling relations, we simulate realistic radial velocity time series and examine how the root-mean-square scatter of radial velocity measure…
Context: The study of stellar structure and evolution depends crucially on accurate stellar parameters. The photometry from space telescopes has provided superb data that allowed asteroseismic characterisation of thousands of stars. However, typical targets of space telescopes are rather faint and complementary measurements are difficult to obtain. On the other hand, the brightest, otherwise well-studied stars, are lacking seismic characterization. Aims: Our goal is to use the granulation and/or oscillation time scales measured from photometric time series of bright red giants (1.6$\leq$Vmag$\…
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, and to compare them to theoretical predictions. We use a probabilistic method to compare different approaches to extracting the granulation signal.…
The Kepler mission has provided exquisite data to perform an ensemble asteroseismic analysis on evolved stars. In this work we systematically characterize solar-like oscillations and granulation for 16,094 oscillating red giants, using end-of-mission long-cadence data. We produced a homogeneous catalog of the frequency of maximum power (typical uncertainty $σ_{ν_{\rm max}}$=1.6\%), the mean large frequency separation ($σ_{Δν}$=0.6\%), oscillation amplitude ($σ_{\rm A}$=4.7\%), granulation power ($σ_{\rm gran}$=8.6\%), power excess width ($σ_{\rm width}$=8.8\%), seismically-derived stellar mass…
A poor understanding of the impact of convective turbulence in the outer layers of the Sun and Sun-like stars challenges the advance towards an improved understanding of their internal structure and dynamics. Assessing and calibrating these effects is therefore of great importance. Here we study the spatial coherence of granulation noise and oscillation modes in the Sun, with the aim of exploiting any incoherence to beat-down observed granulation noise, hence improving the detection of low-frequency p-modes. Using data from the BiSON 2B instrument, we assess the coherence between different atm…
Sensitivity of spectral lines to granulation: The Sun
Institute of Physics, University of Graz（格拉茨大学物理研究所）；Max-Planck-Institut für Sonnensystemforschung（马普太阳系研究所）；Centre for Exoplanet Science, SUPA School of Physics and Astronomy, University of St Andrews（圣安德鲁斯大学天体物理与天文学院系外行星科学中心）
The analysis of photometric time series in the context of transiting planet surveys suffers from the presence of stellar signals, often dubbed "stellar noise". These signals, caused by stellar oscillations and granulation, can usually be disregarded for main-sequence stars, as the stellar contributions average out when phase-folding the light curve. For evolved stars, however, the amplitudes of such signals are larger and the timescales similar to the transit duration of short-period planets, requiring that they be modeled alongside the transit. With the promise of TESS delivering on the order…
One of the main science motivations for the ESA PLAnetary Transit and Oscillations (PLATO) mission is to measure exoplanet transit radii with 3% precision. In addition to flares and starspots, stellar oscillations and granulation will enforce fundamental noise floors for transiting exoplanet radius measurements. We simulate light curves of Earth-sized exoplanets transiting continuum intensity images of the Sun taken by the HMI instrument aboard SDO to investigate the uncertainties introduced on the exoplanet radius measurements by stellar granulation and oscillations. After modeling the solar …
The granulation background seen in the power spectrum of a solar-like oscillator poses a serious challenge for extracting precise and detailed information about the stellar oscillations. Using a 3D hydrodynamical simulation of the Sun computed with CO$^5$BOLD, we investigate various background models to infer, using a Bayesian methodology, which one provides the best fit to the background in the simulated power spectrum. We find that the best fit is provided by an expression including the overall power level and two characteristic frequencies, one with an exponent of 2 and one with a free expo…
A better understanding of the amplitudes of stellar oscillation modes and surface granulation is essential for improving theories of mode physics and the properties of the outer convection zone of solar-like stars. A proper prediction of these amplitudes is also essential for appraising the detectability of solar-like oscillations for asteroseismic analysis. Comparisons with models, or between different photometric missions, are enabled by applying a bolometric correction, which converts mission-specific amplitudes to their corresponding bolometric (full light) values. We derive the bolometric…
Context. Recently our ability to study stars using asteroseismic techniques has increased dramatically, largely through the use of space based photometric observations. Work has also been done using ground based spectroscopic observations and more is expected in the near future from the SONG network. Unfortunately, the intensity observations have an inferior signal to noise ratio and details of the observations do not agree with theory, while the data analysis used in the spectroscopic method has often been based on overly simple models of the spectra. Aims. The aim is to improve the reliabili…
Effects of Asymmetric Flows in Solar Convection on Oscillation Modes
许多日震测量都存在很大的系统误差。一个特别令人沮丧的是，时间-距离测量受到大的中心到边缘效应的影响，这看起来非常类似于有限的光旅行时间，除了幅度取决于所使用的可观测值，并且可以具有相反的符号Zhao et al.（2012）。这使得确定太阳对流区中的深纬向流的尝试受挫，Zhao等人（2012）应用了一种没有物理基础的特别校正来校正数据。在这封信中，我们提出，这种效果的一部分可以解释的高度不对称性质的太阳能颗粒，结果出现在什么样的振荡模式作为一个净径向流，从而赋予相移的模式作为一个函数的观察高度，从而日心角。
Acoustic oscillations in the field-free, gravitationally stratified Acoustic oscillations in the field-free, gravitationally stratified cavities under solar bipolar magnetic canopies
We studied the low-frequency $\lesssim 0.5\;$h$^{-1}$ (long-period $\gtrsim 2\;$h) oscillations of active regions (ARs). The investigation is based on an analysis of a time series built from Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI) photospheric magnetograms and comprises case studies of several types of AR structures. The main goals are to investigate whether ARs can be engaged in long-period oscillations as unified oscillatory entities and, if so, to determine the spectral pattern of such oscillations. Time series of characteristic parameters of the ARs, such as, …
Optimising the global detection of solar-like oscillations. Tuning the frequency range for asteroseismic detection predictions and searches
Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University（奥胡斯大学天体物理中心物理学与天文学系）；School of Physics and Astronomy, University of Birmingham（伯明翰大学物理与天文学院）
Context. The Sun and solar-like stars undergo activity cycles for which the underlying mechanisms are not well understood. The oscillations of the Sun are known to vary with its activity cycle and these changes provide diagnostics on the conditions below the photosphere. Kepler has detected oscillations in hundreds of solar-like stars but as of yet, no widespread detection of signatures of magnetic activity cycles in the oscillation parameters of these stars have been reported. Aims. We analyse the photometric short cadence Kepler time series of a set of 24 solar-like stars, which were observe…
A new method to detect solar-like oscillations at very low S/N using statistical significance testing
我们介绍了一种新的方法来检测恒星观测的频率功率谱中的类太阳振荡，在非常低的信号噪声的条件下。移动窗口功率搜索（MWPS）搜索功率谱中多余功率的特征，超过缓慢变化的（频率）恒星颗粒和射击或仪器噪声的背景贡献。我们采用假警报方法（Chaplin et al. 2011）来确定标记的过剩功率（与类太阳振荡预期的过剩功率一致）是否难以单独解释（因此是候选检测）。我们将该方法应用于太阳光度数据，其质量被系统地降级，以测试MWPS在低信噪比下的性能。我们还比较了性能的MWPS对频繁应用的功率谱的功率谱（PSxPS）检测方法。MWPS被发现优于PSxPS方法。
During the survey phase of the Kepler mission, several thousands of stars were observed in short cadence, allowing the detection of solar-like oscillations in more than 500 main-sequence and sub-giant stars. Later, the Kepler Science Office discovered an issue in the calibration that affected half of the short-cadence data, leading to a new data release (DR25) with improved corrections. We re-analyze the one-month time series of the Kepler survey phase to search for new solar-like oscillations. We study the seismic parameters of 99 stars (46 targets with new reported solar-like oscillations) i…
Detecting the presence and characteristic scale of a signal is a common problem in data analysis. We develop a fast statistical test of the null hypothesis that a Fourier-like power spectrum is consistent with noise. The null hypothesis is rejected where the local "coefficient of variation" (CV)---the ratio of the standard deviation to the mean---in a power spectrum deviates significantly from expectations for pure noise (CV~1.0 for a Chi^2 2-degrees-of-freedom distribution). This technique is of particular utility for detecting signals in power spectra with frequency-dependent noise backgroun…
The asteroseismology of red giant stars has continued to yield surprises since the onset of high-precision photometry from space-based observations. An exciting new theoretical result shows that the previously observed suppression of dipole oscillation modes in red giants can be used to detect strong magnetic fields in the stellar cores. A fundamental facet of the theory is that nearly all the mode energy leaking into the core is trapped by the magnetic greenhouse effect. This results in clear predictions for how the mode visibility changes as a star evolves up the red giant branch, and how th…
Detecting and confirming terrestrial planets is incredibly difficult due to their tiny size and mass relative to Sun-like host stars. However, recent instrumental advancements are making the detection of Earth-like exoplanets technologically feasible. For example, Kepler and TESS photometric precision means we can identify Earth-sized candidates (and PLATO in the future will add many long-period candidates to the list), while spectrographs such as ESPRESSO and EXPRES (with an aimed radial velocity precision [RV] near 10 cm/s) mean we will soon reach the instrumental precision required to confi…
