Frequency Separation Δν Research Articles

The Asteroseismology Analysis of 14 Solar-like Stars Based on TESS and LAMOST Observations

The Transiting Exoplanet Survey Satellite (TESS) is performing a near all-sky survey for planets that transit bright stars. It provides excellent photometric data for asteroseismology of solar-like stars that exhibit oscillation by a convection-driven mechanism. In this paper, we analyze the light curves of 14 stars that are observed by TESS. These stars mainly focus on Sector 21 and the other Sectors intersecting with Sector 21. Through crossmatching with the LAMOST surveys, 14 stars with signals of solar-like oscillations are selected, and asteroseismic analysis is present. The frequency of maximum power and the large frequency separation Δν are extracted from its frequency power spectrum using the pySYD pipeline. Scaling relations are an attractive method that can easily and immediately be applied to observations to derive the stellar properties. To validate the different calibrated scaling relations for 14 TESS stars, the fundamental stellar properties are estimated by the relations and compared with grid-based modeling results, which use the constraints of global asteroseismic parameters and nonasteroseismic parameters. The new red-giant branch (RGB) scaling relations are found to overestimate the radius and mass when compared to the grid results, while the f Δν scaling relations lead to overall lower radius and mass estimates. The average uncertainties of these stars from grid modeling are within 2% in the radius and 3% in the mass. Regardless of the method, we can get almost equally good results of log g for all the stars. Besides the grid method, the new RGB scaling relations for ages provide a simple and accurate method to determine the stellar ages.

Seismic signature of electron degeneracy in the core of red giants: Hints for mass transfer between close red-giant companions

The detection of mixed modes in red giants with space missions COROT and Kepler has revealed their deep internal structure. These modes allow us to characterize the pattern of pressure modes (through the measurement of their asymptotic frequency separation Δν) and the pattern of gravity modes (through the determination of their asymptotic period spacing ΔΠ1). It has been shown that red giant branch (RGB) stars regroup on a well-defined sequence in the Δν − ΔΠ1 plane. Our first goal is to theoretically explain the features of this sequence and understand how it can be used to probe the interiors of red giants. Using a grid of red giant models computed with MESA, we demonstrate that red giants join the Δν − ΔΠ1 sequence whenever electron degeneracy becomes strong in the core. We argue that this can be used to estimate the central densities of these stars, and potentially to measure the amount of core overshooting during the main sequence part of the evolution. We also investigate a puzzling subsample of red giants that are located below the RGB sequence, in contradiction with stellar evolution models. After checking the measurements of the asymptotic period spacing for these stars, we show that they are mainly intermediate-mass red giants. This is doubly peculiar because these stars should have nondegenerate cores and they are expected to be located well above the RGB sequence. We show that these peculiarities are well accounted for if these stars result from the interaction between two low-mass (M ≲ 2 M⊙) close companions during the red giant branch phase. If the secondary component has already developed a degenerate core before mass transfer begins, it becomes an intermediate-mass giant with a degenerate core. The secondary star is then located below the degenerate sequence, which is in agreement with the observations.

Asteroseismology of evolved stars with EGGMiMoSA

Context.In the context of an ever increasing amount of highly precise data, thanks to the numerous space-borne missions, came a revolution in stellar physics. This data allowed asteroseismology to thrive and improve our general knowledge of stars. Important results were obtained about giant stars owing to the presence of ‘mixed modes’ in their oscillation spectra. These modes carry information about the whole stellar interior, enabling the comprehensive characterisation of their structure.Aims.The current study is part of a series of papers that provide a technique to coherently and robustly analyse the mixed-modes frequency spectra and characterise the stellar structure of stars on both the subgiant branch and red-giant branch (RGB). In this paper we aim at defining seismic indicators, relevant of the stellar structure, as well as studying their evolution along a grid of models.Methods.The proposed method, EGGMiMoSA, relies on the asymptotic description of mixed modes. It defines appropriate initial guesses for the parameters of the asymptotic formulation and uses a Levenberg-Marquardt minimisation scheme in order to adjust the complex mixed-modes pattern in a fast and robust way.Results.We are able to follow the evolution of the mixed-modes parameters along a grid of models from the subgiant phase to the RGB bump, therefore extending previous works. We show the impact of the stellar mass and composition on the evolution of these parameters. We observe that the evolution of the period spacing Δπ1, pressure offsetϵp, gravity offsetϵg, and coupling factorqas a function of the large frequency separation Δνis little affected by the chemical composition and that it follows two different regimes depending on the evolutionary stage. On the subgiant branch, the stellar models display a moderate core-envelope density contrast. Therefore, the evolution of Δπ1,ϵp,ϵg, andqsignificantly changes with the stellar mass. Furthermore, we demonstrate that, for a given metallicity and with proper measurements of the period spacing Δπ1and large frequency separation Δν, we may unambiguously constrain the stellar mass, radius and age of a subgiant star. Conversely, as the star reaches the red-giant branch, the core-envelope density contrast becomes very large. Consequently, the evolution ofϵp,ϵgandqas a function of Δνbecomes independent of the stellar mass. This is also true for Δπ1in stars with masses ≲1.8 M⊙because of core electron degeneracy. This degeneracy in Δπ1is lifted for higher masses, again allowing for a precise measurement of the stellar age. Overall, our computations qualitatively agree with previous observed and theoretical studies.Conclusions.The method provides automated measurements of the adjusted parameters along a grid of models and opens the way to the precise seismic characterisation of both subgiants and red giants. In the following papers of the series, we will explore further refinements to the technique as well as its application to observed stars.

Relevance of the small frequency separation for asteroseismic stellar age, mass, and radius

Aims. We performed a theoretical analysis aimed at quantifying the relevance of the small frequency separation δν in determining stellar ages, masses, and radii. We aimed to establish a minimum uncertainty on these quantities for low-mass stars across different evolutionary stages of the main sequence and to evaluate the biases that come from some systematic differences between the stellar model grid adopted for the recovery and the observed stars. Methods. We adopted the Stellar CharactEristics Pisa Estimation gRid (SCEPtER) pipeline for low-mass stars, [0.7, 1.05] M⊙, from the zero-age main sequence (ZAMS) to the central hydrogen depletion. For each model in the grid, we computed oscillation frequencies. Synthetic stars were generated and reconstructed based on different assumptions about the relative precision in the δν parameter (namely 5% and 2%). The quantification of the systematic errors arising from a possible mismatch between synthetic stars and the recovery grid was performed by generating stars from synthetic grids of stellar models with different initial helium abundance and microscopic diffusion efficiency. The results obtained without δν as an observable are included for comparison. Results. The investigation highlighted and confirmed the improvement in the age estimates when δν is available, which has already been reported in the literature. While the biases were negligible, the statistical error affecting age estimates was strongly dependent on the stellar evolutionary phase. The error is at its maximum at ZAMS and it decreases to about 11% and 6% (δν known at 5% and 2% level, respectively) when stars reach the 30% of their evolutionary MS lifetime. The usefulness of small frequency separation in improving age estimates vanishes in the last 20% of the MS. The availability of δν in the fit for mass and radius estimates provided an effect that was nearly identical to its effect on age, assuming an observational uncertainty of 5%. As a departure, with respect to age estimates, no benefit was detected for mass and radius determinations from a reduction of the observational error in δν to 2%. The age variability attributed to differences in the initial helium abundance resulted in negligible results owing to compensation effects that have already been discussed in previous works. On the other hand, the current uncertainty in the initial helium abundance leads to a greater bias (2% and 1% level) in mass and radius estimates whenever δν is in the observational pool. This result, together with the presence of further unexplored uncertainty sources, suggest that precision in the derived stellar quantities below these thresholds may possibly be overoptimistic. The impact of microscopic diffusion was investigated by adopting a grid of models for the recovery which totally neglected the process. The availability of the small frequency separation resulted in biases lower than 5% and 2% for observational errors of 5% and 2%, respectively. The estimates of mass and radius showed again a greater distortion when δν is included among the observables. These biases are at the level of 1%, confirming that threshold as a minimum realistic uncertainty on the derived stellar quantities. Finally, we compared the estimates by the SCEPtER pipeline for 13 Kepler asteroseismic LEGACY sample stars with those given by six different pipelines from literature. This procedure demonstrated a fair agreement for the results. The comparison suggests that a realistic approach to the determination of the error on the estimated parameters consists of approximately doubling the error in the recovered stellar characteristics from a single pipeline. Overall, on the LEGACY sample data, we obtained a multi-pipeline precision of about 4.4%, 1.7%, and 11% on the estimated masses, radii, and ages, respectively.

Asteroseismology of the Hyades red giant and planet host ϵ Tauri

Context. Asteroseismic analysis of solar-like stars allows us to determine physical parameters such as stellar mass, with a higher precision compared to most other methods. Even in a well-studied cluster such as the Hyades, the masses of the red giant stars are not well known, and previous mass estimates are based on model calculations (isochrones). The four known red giants in the Hyades are assumed to be clump (core-helium-burning) stars based on their positions in colour-magnitude diagrams, however asteroseismology offers an opportunity to test this assumption. Aims. Using asteroseismic techniques combined with other methods, we aim to derive physical parameters and the evolutionary stage for the planet hosting star ϵ Tau, which is one of the four red giants located in the Hyades. Methods. We analysed time-series data from both ground and space to perform the asteroseismic analysis. By combining high signal-to-noise radial-velocity data from the ground-based SONG network with continuous space-based data from the revised Kepler mission K2, we derive and characterize 27 individual oscillation modes for ϵ Tau, along with global oscillation parameters such as the large frequency separation Δν and the ratio between the amplitude of the oscillations measured in radial velocity and intensity as a function of frequency. The latter has been measured previously for only two stars, the Sun and Procyon. Combining the seismic analysis with interferometric and spectroscopic measurements, we derive physical parameters for ϵ Tau, and discuss its evolutionary status. Results. Along with other physical parameters, we derive an asteroseismic mass for ϵ Tau of M = 2.458 ± 0.073 M⊙, which is slightly lower than previous estimates, and which leads to a revised minimum mass of the planetary companion. Noting that the SONG and K2 data are non-simultaneous, we estimate the amplitude ratio between intensity and radial velocity to be 42.2 ± 2.3 ppm m−1 s, which is higher than expected from scaling relations.

New Acetylenic Amine Derivatives of 5,8-Quinolinediones: Synthesis, Crystal Structure and Antiproliferative Activity

Acetylenic amine derivatives of the 5,8-quinolinedione were synthesized and characterized by the 1H and 13C NMR, IR spectroscopy and MS spectra. Additionally, the 6- and 7-substituted allylamine-5,8-quinolinediones were synthesized for comparison purposes. The crystal structure was determined for the 6-chloro-7-propargylamine-5,8-quinolinedione and 7-chloro-6-propargylamine-5,8-quinolinedione. Additionally, the IR spectral analysis supplemented by the density functional theory (DFT) calculations were carried out. It was found that different positions of the propargylamine side chain had a distinct influence on crystal structure, formation of H-bonds and the carbonyl stretching IR bands. Correlation between the frequency separation Δν of the carbonyl IR bands and the position of the 6- and 7-substituents was found. The 7-substituted derivatives exhibited a higher frequency separation Δν. The observed correlation could provide an opportunity to use the IR spectroscopy to study substitution reactions. Cytotoxic activities against three human cancer cell lines for the 5,8-quinolinedione derivatives with different amine substituents, i.e., propargylamine, N-methylpropargylamine, 1,1-dimethylpropargylamine, allylamine and propylamine were also analysed with respect to their molecular structure.

Oscillation parameters of red giants observed in the CoRoT exofield

AbstractThe high-precision data obtained by the CoRoT (Convection, Rotation and planetary Transits) space mission allows firm detections of solar-like oscillations in a great variety of different stars. We derived reliable estimates of the frequency of maximum oscillation power νmax and the large frequency separation Δν of more than 300 cool red giants, that were observed by CoRoT. A detailed study of their seismic parameters provides an exclusive view of the population of the observed giants.

Discovery of Kilohertz Quasi‐periodic Oscillations and Shifted Frequency Correlations in the Accreting Millisecond Pulsar XTE J1807−294

We report the discovery of twin kilohertz quasi-periodic oscillations (kHz QPOs) in the X-ray flux of XTE J1807-294, the fourth accreting millisecond pulsar (AMP). This is the second AMP exhibiting twin kHz QPOs. In contrast to the first case, SAX J1808.4-3658, the frequency separation Δν between these kHz QPOs is consistent with the pulse frequency (190.6 Hz), not with half that value, confirming for the first time from pulsation measurements the inference, based on burst oscillations, that slow rotators (spin frequency less than 400 Hz) have Δν approximately equal to the spin frequency. While the QPOs move in frequency together over a range of more than 200 Hz, Δν remains constant with an average value of 205 ± 6 Hz. Variability components were found in the 5-130 Hz range, similar to those seen in other LMXBs. The correlations between the QPO and noise frequencies are also similar to those in other sources, but shifted by a factor of 1.59 in kHz QPO frequencies, similar to the factor of 1.45 shift found for SAX J1808.4-3658. Our results argue in favor of a spin-related formation mechanism for twin kHz QPOs and against a spin-related cause of the shift in the frequency correlations.

Electric Field Gradient and Magnetic Hyperfine Interaction at Eu Nuclei in Ferromagnetic EuS

The NMR spectra of 151 Eu and 153 Eu nuclei with the nuclear spin I =5/2 were observed in ferromagnetic EuS using a single-crystal sphere by the cw method in fields up to 21 kOe below 4.2 K. In fields above magnetic saturation the 153 Eu spectrum consists of quadrupole-split five lines whose relative intensities depend on field direction. The central-line frequency which gives a hyperfine field at nuclei exhibits a small anisotropy in good agreement with the previous results, but the frequency separation Δν q between the quadrupole multiplets is isotropic, which is determined to be 0.358±0.005 MHz at 4.2 K. The temperature dependence of Δν q is rapid compared with that of the Eu magnetic moment. A linear extrapolation to absolute zero gives a value of Δν q =0.422±0.005 MHz which yields a quadrupole coupling of e 2 q Q /h=2.81±0.03 MHz. The 151 Eu spectrum is a single line even in magnetic fields.