Amplitudes and lifetimes of solar-like oscillations observed by CoRoT* Red-giant versus main-sequence stars

Context. The advent of space-borne missions such as CoRoT or Kepler providing photometric data has brought new possibilities for asteroseismology across the H-R diagram. Solar-like oscillations are now observed in many stars, including red giants and main- sequence stars. Aims. Based on several hundred identified pulsating red giants, we aim to characterize their oscillation amplitudes and widths. These observables are compared with those of main-sequence stars in order to test trends and scaling laws for these parameters for both main-sequence stars and red giants. Methods. An automated fitting procedure is used to analyze several hundred Fourier spectra. For each star, a modeled spectrum is fitted to the observed oscillation spectrum, and mode parameters are derived. Results. Amplitudes and widths of red-giant solar-like oscillations are estimated for several hundred modes of oscillation. Amplitudes are relatively high (several hundred ppm) and widths relatively small (very few tenths of a {\mu}Hz). Conclusions. Widths measured in main-sequence stars show a different variation with the effective temperature than red giants. A single scaling law is derived for mode amplitudes of both red giants and main-sequence stars versus their luminosity to mass ratio. However, our results suggest that two regimes may also be compatible with the observations.Comment: Accepted in A&A on 2011 February 8th, now includes corrections (results now more precise on \Gamma and A_max in Section 4.3 and 4.4, fig. 7 corrected consequently

Oscillating red giants in the CoRoT exo-field: Asteroseismic mass and radius determination

Context. Observations and analysis of solar-type oscillations in red-giant stars is an emerging aspect of asteroseismic analysis with a number of open questions yet to be explored. Although stochastic oscillations have previously been detected in red giants from both radial velocity and photometric measurements, those data were either too short or had sampling that was not complete enough to perform a detailed data analysis of the variability. The quality and quantity of photometric data as provided by the CoRoT satellite is necessary to provide a breakthrough in observing p-mode oscillations in red giants. We have analyzed continuous photometric time-series of about 11 400 relatively faint stars obtained in the exofield of CoRoT during the first 150 days long-run campaign from May to October 2007. We find several hundred stars showing a clear power excess in a frequency and amplitude range expected for red-giant pulsators. In this paper we present first results on a sub-sample of these stars. Aims. Knowing reliable fundamental parameters like mass and radius is essential for detailed asteroseismic studies of red-giant stars. As the CoRoT exofield targets are relatively faint (11-16 mag) there are no (or only weak) constraints on the star's location in the H-R diagram. We therefore aim to extract information about such fundamental parameters solely from the available time series. Methods. We model the convective background noise and the power excess hump due to pulsation with a global model fit and deduce reliable estimates for the stellar mass and radius from scaling relations for the frequency of maximum oscillation power and the characteristic frequency separation.Comment: 10 pages, 7 figures, accepted for publication in A&

Signalling mechanisms that regulate metabolic profile and autophagy of acute myeloid leukaemia cells

Acute myeloid leukaemia (AML) comprises a heterogeneous group of hematologic neoplasms characterized by diverse combinations of genetic, phenotypic and clinical features representing a major challenge for the development of targeted therapies. Metabolic reprogramming, mainly driven by deregulation of the nutrient-sensing pathways as AMPK, mTOR and PI3K/AKT, has been associated with cancer cells, including AML cells, survival and proliferation. Nevertheless, the role of these metabolic adaptations on the AML pathogenesis is still controversial. In this work, the metabolic status and the respective metabolic networks operating in different AML cells (NB-4, HL-60 and KG-1) and their impact on autophagy and survival was characterized. Data show that whereas KG-1 cells exhibited preferential mitochondrial oxidative phosphorylation metabolism with constitutive co-activation of AMPK and mTORC1 associated with increased autophagy, NB-4 and HL-60 cells displayed a dependent glycolytic profile mainly associated with AKT/mTORC1 activation and low autophagy flux. Inhibition of AKT is disclosed as a promising therapeutical target in some scenarios while inhibition of AMPK and mTORC1 has no major impact on KG-1 cells' survival. The results highlight an exclusive metabolic profile for each tested AML cells and its impact on determination of the anti-leukaemia efficacy and on personalized combinatory therapy with conventional and targeted agents.This work was developed under the scope of the project NORTE‐01‐0145‐FEDER‐000013, supported by the Northern Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (FEDER), and by FEDER, through the Competitiveness Factors Operational Programme (COMPETE), and by National funds, through the Foundation for Science and Technology (FCT), under the scope of the project POCI‐01‐0145‐FEDER‐007038. OP and BSM are supported by fellowships from the Fundação para a Ciência e Tecnologia (FCT, Portugal) (SFRH/BD/52292/2013 and SFRH/BPD/90533/2012, respectively)info:eu-repo/semantics/publishedVersio

The CoRoT target HD175726: an active star with weak solar-like oscillations

Context. The CoRoT short runs give us the opportunity to observe a large variety of late-type stars through their solar-like oscillations. We report observations of the star HD175726 that lasted for 27 days during the first short run of the mission. The time series reveals a high-activity signal and the power spectrum presents an excess due to solar-like oscillations with a low signal-to-noise ratio. Aims. Our aim is to identify the most efficient tools to extract as much information as possible from the power density spectrum. Methods. The most productive method appears to be the autocorrelation of the time series, calculated as the spectrum of the filtered spectrum. This method is efficient, very rapid computationally, and will be useful for the analysis of other targets, observed with CoRoT or with forthcoming missions such as Kepler and Plato. Results. The mean large separation has been measured to be 97.2+-0.5 microHz, slightly below the expected value determined from solar scaling laws.We also show strong evidence for variation of the large separation with frequency. The bolometric mode amplitude is only 1.7+-0.25 ppm for radial modes, which is 1.7 times less than expected. Due to the low signal-to-noise ratio, mode identification is not possible for the available data set of HD175726. Conclusions. This study shows the possibility of extracting a seismic signal despite a signal-to-noise ratio of only 0.37. The observation of such a target shows the efficiency of the CoRoT data, and the potential benefit of longer observing runs.Comment: 8 pages. Accepted in A&

The universal red-giant oscillation pattern; an automated determination with CoRoT data

The CoRoT and Kepler satellites have provided thousands of red-giant oscillation spectra. The analysis of these spectra requires efficient methods for identifying all eigenmode parameters. The assumption of new scaling laws allows us to construct a theoretical oscillation pattern. We then obtain a highly precise determination of the large separation by correlating the observed patterns with this reference. We demonstrate that this pattern is universal and are able to unambiguously assign the eigenmode radial orders and angular degrees. This solves one of the current outstanding problems of asteroseismology hence allowing precise theoretical investigation of red-giant interiors.Comment: Accepted in A&A letter

Line-profile variations of stochastically excited oscillations in four evolved stars

Since solar-like oscillations were first detected in red-giant stars, the presence of non-radial oscillation modes has been debated. Spectroscopic line-profile analysis was used in the first attempt to perform mode identification, which revealed that non-radial modes are observable. Despite the fact that the presence of non-radial modes could be confirmed, the degree or azimuthal order could not be uniquely identified. Here we present an improvement to this first spectroscopic line-profile analysis. Aims: We aim to study line-profile variations of stochastically excited solar-like oscillations in four evolved stars to derive the azimuthal order of the observed mode and the surface rotational frequency. Methods: Spectroscopic line-profile analysis is applied to cross-correlation functions, using the Fourier Parameter Fit method on the amplitude and phase distributions across the profiles. Results: For four evolved stars, beta Hydri (G2IV), epsilon Ophiuchi (G9.5III), eta Serpentis (K0III) and delta Eridani (K0IV) the line-profile variations reveal the azimuthal order of the oscillations with an accuracy of ~1. Furthermore, our analysis reveals the projected rotational velocity and the inclination angle. From these parameters we obtain the surface rotational frequency. Conclusions: We conclude that line-profile variations of cross-correlation functions behave differently for different frequencies and that they provide additional information in terms of the surface rotational frequency and azimuthal order.Comment: Accepted for publication in Astronomy and Astrophysics, 9 pages, 10 figures and 3 tables. A version with figure 1 in full resolution can be obtained upon request from first autho

Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints

Context. The availability of asteroseismic constraints for a large sample of red giant stars from the CoRoT and Kepler missions paves the way for various statistical studies of the seismic properties of stellar populations. Aims. We use the first detailed spectroscopic study of 19 CoRoT red-giant stars (Morel et al 2014) to compare theoretical stellar evolution models to observations of the open cluster NGC 6633 and field stars. Methods. In order to explore the effects of rotation-induced mixing and thermohaline instability, we compare surface abundances of carbon isotopic ratio and lithium with stellar evolution predictions. These chemicals are sensitive to extra-mixing on the red-giant branch. Results. We estimate mass, radius, and distance for each star using the seismic constraints. We note that the Hipparcos and seismic distances are different. However, the uncertainties are such that this may not be significant. Although the seismic distances for the cluster members are self consistent they are somewhat larger than the Hipparcos distance. This is an issue that should be considered elsewhere. Models including thermohaline instability and rotation-induced mixing, together with the seismically determined masses can explain the chemical properties of red-giants targets. However, with this sample of stars we cannot perform stringent tests of the current stellar models. Tighter constraints on the physics of the models would require the measurement of the core and surface rotation rates, and of the period spacing of gravity-dominated mixed modes. A larger number of stars with longer times series, as provided by Kepler or expected with Plato, would help for ensemble asteroseismology.Comment: Accepted 03/05/201

Non-radial oscillations in the red giant HR7349 measured by CoRoT

Convection in red giant stars excites resonant acoustic waves whose frequencies depend on the sound speed inside the star, which in turn depends on the properties of the stellar interior. Therefore, asteroseismology is the most robust available method for probing the internal structure of red giant stars. Solar-like oscillations in the red giant HR7349 are investigated. Our study is based on a time series of 380760 photometric measurements spread over 5 months obtained with the CoRoT satellite. Mode parameters were estimated using maximum likelihood estimation of the power spectrum. The power spectrum of the high-precision time series clearly exhibits several identifiable peaks between 19 and 40 uHz showing regularity with a mean large and small spacing of Dnu = 3.47+-0.12 uHz and dnu_02 = 0.65+-0.10 uHz. Nineteen individual modes are identified with amplitudes in the range from 35 to 115 ppm. The mode damping time is estimated to be 14.7+4.7-2.9 days.Comment: 8 pages, A&A accepte

Characteristics of solar-like oscillations in red giants observed in the CoRoT exoplanet field

Observations during the first long run (~150 days) in the exo-planet field of CoRoT increase the number of G-K giant stars for which solar-like oscillations are observed by a factor of 100. This opens the possibility to study the characteristics of their oscillations in a statistical sense. We aim to understand the statistical distribution of the frequencies of maximum oscillation power (nu_max) in red giants and to search for a possible correlation between nu_max and the large separation (delta_nu). The nu_max distribution shows a pronounced peak between 20 - 40 microHz. For about half of the stars we obtain delta_nu with at least two methods. The correlation between nu_max and delta_nu follows the same scaling relation as inferred for solar-like stars. The shape of the nu_max distribution can partly be explained by granulation at low frequencies and by white noise at high frequencies, but the population density of the observed stars turns out to be also an important factor. From the fact that the correlation between delta_nu and nu_max for red giants follows the same scaling relation as obtained for sun-like stars, we conclude that the sound travel time over the pressure scale height of the atmosphere scales with the sound travel time through the whole star irrespective of evolution.Comment: Accepted for publication in Astronomy and Astrophysics (CoRoT special issue), 5 pages, 7 figures and 1 tabl

Spin down of the core rotation in red giants

The space mission Kepler provides us with long and uninterrupted photometric time series of red giants. We are now able to probe the rotational behaviour in their deep interiors using the observations of mixed modes. We aim to measure the rotational splittings in red giants and to derive scaling relations for rotation related to seismic and fundamental stellar parameters. We have developed a dedicated method for automated measurements of the rotational splittings in a large number of red giants. Ensemble asteroseismology, namely the examination of a large number of red giants at different stages of their evolution, allows us to derive global information on stellar evolution. We have measured rotational splittings in a sample of about 300 red giants. We have also shown that these splittings are dominated by the core rotation. Under the assumption that a linear analysis can provide the rotational splitting, we observe a small increase of the core rotation of stars ascending the red giant branch. Alternatively, an important slow down is observed for red-clump stars compared to the red giant branch. We also show that, at fixed stellar radius, the specific angular momentum increases with increasing stellar mass. Ensemble asteroseismology indicates what has been indirectly suspected for a while: our interpretation of the observed rotational splittings leads to the conclusion that the mean core rotation significantly slows down during the red giant phase. The slow-down occurs in the last stages of the red giant branch. This spinning down explains, for instance, the long rotation periods measured in white dwarfsComment: Accepted in A&