The detection of gravity modes is expected to give us unprecedented insights into the inner dynamics of the Sun. Within this framework, predicting their amplitudes is essential to guide future observational strategies and seismic studies. In this wor
k, we predict the amplitude of low-frequency asymptotic gravity modes generated by penetrative convection at the top of the radiative zone. The result is found to depend critically on the time evolution of the plumes inside the generation region. Using a solar model, we compute the GOLF apparent surface radial velocity of low-degree gravity modes in the frequency range $10~mu H_zle u le 100~mu H_z$. In case of a Gaussian plume time evolution, gravity modes turn out to be undetectable because of too small surface amplitudes. This holds true despite a wide range of values considered for the parameters of the model. In the other limiting case of an exponential time evolution, plumes are expected to drive gravity modes in a much more efficient way because of a much higher temporal coupling between the plumes and the modes than in the Gaussian case. Using reasonable values for the plume parameters based on semi-analytical models, the apparent surface velocities in this case turn out to be one order of magnitude smaller than the 22-years GOLF detection threshold and than the previous estimates considering turbulent pressure as the driving mechanism, with a maximum value of $0.05$ cm s${}^{-1}$ for $ell =1$ and $ uapprox 100~mu H_z$. When accounting for uncertainties on the plume parameters, the apparent surface velocities in the most favorable plausible case become comparable to those predicted with turbulent pressure, and the GOLF observation time required for a detection at $ u approx100~mu H_z$ and $ell=1$ is reduced to about 50 yrs.
The existence of mixed modes in stars is a marker of stellar evolution. Their detection serves for a better determination of stellar age. The goal of this paper is to identify the dipole modes in an automatic manner without human intervention. I use
the power spectra obtained by the Kepler mission for the application of the method. I compute asymptotic dipole mode frequencies as a function of coupling factor and dipole period spacing, and other parameters. For each star, I collapse the power in an echelle diagramme aligned onto the monopole and dipole mixed modes. The power at the null frequency is used as a figure of merit. Using a genetic algorithm, I then optimise the figure of merit by adjusting the location of the dipole frequencies in the power spectrum}. Using published frequencies, I compare the asymptotic dipole mode frequencies with published frequencies. I also used published frequencies for deriving coupling factor and dipole period spacing using a non-linear least squares fit. I use Monte-Carlo simulations of the non-linear least square fit for deriving error bars for each parameters. From the 44 subgiants studied, the automatic identification allows to retrieve within 3 $mu$Hz at least 80% of the modes for 32 stars, and within 6 $mu$Hz at least 90% of the modes for 37 stars. The optimised and fitted gravity-mode period spacing and coupling factor agree with previous measurements. Random errors for the mixed-mode parameters deduced from Monte-Carlo simulation are about 30-50 times smaller than previously determined errors, which are in fact systematic errors. The period spacing and coupling factors of mixed modes in subgiants are confirmed. The current automated procedure will need to be improved using a more accurate asymptotic model and/or proper statistical tests.
The Spectral Imaging of the Coronal Environment (SPICE) instrument is a high-resolution imaging spectrometer operating at extreme ultraviolet (EUV) wavelengths. In this paper, we present the concept, design, and pre-launch performance of this facilit
y instrument on the ESA/NASA Solar Orbiter mission. The goal of this paper is to give prospective users a better understanding of the possible types of observations, the data acquisition, and the sources that contribute to the instruments signal. The paper discusses the science objectives, with a focus on the SPICE-specific aspects, before presenting the instruments design, including optical, mechanical, thermal, and electronics aspects. This is followed by a characterisation and calibration of the instruments performance. The paper concludes with descriptions of the operations concept and data processing. The performance measurements of the various instrument parameters meet the requirements derived from the missions science objectives. The SPICE instrument is ready to perform measurements that will provide vital contributions to the scientific success of the Solar Orbiter mission.
The NASA Kepler space telescope has detected solar-like oscillations in several hundreds of single stars, thereby providing a way to determine precise stellar parameters using asteroseismology. In this work, we aim to derive the fundamental parameter
s of a close triple star system, HD 188753, for which asteroseismic and astrometric observations allow independent measurements of stellar masses. We used six months of Kepler photometry available for HD 188753 to detect the oscillation envelopes of the two brightest stars. For each star, we extracted the individual mode frequencies by fitting the power spectrum using a maximum likelihood estimation approach. We then derived initial guesses of the stellar masses and ages based on two seismic parameters and on a characteristic frequency ratio, and modelled the two components independently with the stellar evolution code CESTAM. In addition, we derived the masses of the three stars by applying a Bayesian analysis to the position and radial-velocity measurements of the system. Based on stellar modelling, the mean common age of the system is $10.8 pm 0.2,$Gyr and the masses of the two seismic components are $M_A =$ $0.99 pm 0.01,M_odot$ and $M_{Ba} =$ $0.86 pm 0.01,M_odot$. From the mass ratio of the close pair, $M_{Bb}/M_{Ba} = 0.767 pm 0.006$, the mass of the faintest star is $M_{Bb} =$ $0.66 pm 0.01,M_odot$ and the total seismic mass of the system is then $M_{syst} =$ $2.51 pm 0.02,M_odot$. This value agrees perfectly with the total mass derived from our orbital analysis, $M_{syst} =$ $2.51^{+0.20}_{-0.18},M_odot$, and leads to the best current estimate of the parallax for the system, $pi = 21.9 pm 0.2,$mas. In addition, the minimal relative inclination between the inner and outer orbits is $10.9^circ pm 1.5^circ$, implying that the system does not have a coplanar configuration.
The great success of Helioseismology resides in the remarkable progress achieved in the understanding of the structure and dynamics of the solar interior. This success mainly relies on the ability to conceive, implement, and operate specific instrume
ntation with enough sensitivity to detect and measure small fluctuations (in velocity and/or intensity) on the solar surface that are well below one meter per second or a few parts per million. Furthermore the limitation of the ground observations imposing the day-night cycle (thus a periodic discontinuity in the observations) was overcome with the deployment of ground-based networks --properly placed at different longitudes all over the Earth-- allowing longer and continuous observations of the Sun and consequently increasing their duty cycles. In this chapter, we start by a short historical overview of helioseismology. Then we describe the different techniques used to do helioseismic analyses along with a description of the main instrumental concepts. We in particular focus on the instruments that have been operating long enough to study the solar magnetic activity. Finally, we give a highlight of the main results obtained with such high-duty cycle observations (>80%) lasting over the last few decades.
The evolved solar-type stars 16 Cyg A & B have long been studied as solar analogs, yielding a glimpse into the future of our own Sun. The orbital period of the binary system is too long to provide meaningful dynamical constraints on the stellar prope
rties, but asteroseismology can help because the stars are among the brightest in the Kepler field. We present an analysis of three months of nearly uninterrupted photometry of 16 Cyg A & B from the Kepler space telescope. We extract a total of 46 and 41 oscillation frequencies for the two components respectively, including a clear detection of octupole (l=3) modes in both stars. We derive the properties of each star independently using the Asteroseismic Modeling Portal, fitting the individual oscillation frequencies and other observational constraints simultaneously. We evaluate the systematic uncertainties from an ensemble of results generated by a variety of stellar evolution codes and fitting methods. The optimal models derived by fitting each component individually yield a common age (t=6.8+/-0.4 Gyr) and initial composition (Z_i=0.024+/-0.002, Y_i=0.25+/-0.01) within the uncertainties, as expected for the components of a binary system, bolstering our confidence in the reliability of asteroseismic techniques. The longer data sets that will ultimately become available will allow future studies of differential rotation, convection zone depths, and long-term changes due to stellar activity cycles.
Solar-like oscillations have been observed by {{it Kepler}} and CoRoT in several solar-type stars. We study the variations of stellar p-mode linewidth as a function of effective temperature. Time series of 9 months of Kepler data have been used.
The power spectra of 42 cool main-sequence stars and subgiants have been analysed using both Maximum Likelihood Estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths and mode heights. Here we report on the mode linewidth at maximum power and at maximum mode height for these 42 stars as a function of effective temperature. We show that the mode linewidth at either maximum mode height or maximum amplitude follows a scaling relation with effective temperature, which is a combination of a power law plus a lower bound. The typical power law index is about 13 for the linewidth derived from the maximum mode height, and about 16 for the linewidth derived from the maximum amplitude while the lower bound is about 0.3 microHz and 0.7 microHz, respectively. We stress that this scaling relation is only valid for the cool main-sequence stars and subgiants, and does not have predictive power outside the temperature range of these stars.
We report on the first asteroseismic analysis of solar-type stars observed by Kepler. Observations of three G-type stars, made at one-minute cadence during the first 33.5d of science operations, reveal high signal-to-noise solar-like oscillation spec
tra in all three stars: About 20 modes of oscillation can clearly be distinguished in each star. We discuss the appearance of the oscillation spectra, including the presence of a possible signature of faculae, and the presence of mixed modes in one of the three stars.
We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5d of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars:
About 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies and frequency separations to provide first results on the radii, masses and ages of the stars, and comment in the light of these results on prospects for inference on other solar-type stars that Kepler will observe.
The POLAR Investigation of the Sun (POLARIS) mission uses a combination of a gravity assist and solar sail propulsion to place a spacecraft in a 0.48 AU circular orbit around the Sun with an inclination of 75 degrees with respect to solar equator. Th
is challenging orbit is made possible by the challenging development of solar sail propulsion. This first extended view of the high-latitude regions of the Sun will enable crucial observations not possible from the ecliptic viewpoint or from Solar Orbiter. While Solar Orbiter would give the first glimpse of the high latitude magnetic field and flows to probe the solar dynamo, it does not have sufficient viewing of the polar regions to achieve POLARIS primary objective : determining the relation between the magnetism and dynamics of the Suns polar regions and the solar cycle.
