Search

Ultra-precise photometric data gathered by recent space missions allow to improve our knowledge of stars significantly but challenges to find models that fit the observations with the state-of-the-art theory are raised. In addition, the huge amount of data requires the application of innovative data analysis techniques to exploit the information. Machine learning could help us both to deal with the large amount of data and to explore new ways to process the lightcurves of pulsating stars. Of special interest for studies of intermediate mass pulsating stars is the decades-old unsolved non-linear phenomenon. The aim of this project is to use machine learning tools in order to characterize a sample of pulsating stars through clustering analysis to shed some light into the nature of the nonlinear phenomenon. Specifically, we focus here on the classification of Delta Scuti stars as High Amplitude (HADS) and Low Amplitude (LADS) through the analysis of the nonlinear parameters characterizing the lightcurves (e.g. harmonics and combinations). The input parameters are extracted from the lightcurves of a sample of stars observed by TESS space satellite. The application of ML for understanding nonlinearities have a great potential use for the Complementary Science Program of PLATO.

In this poster, we present two contrasting studies: First, we want to investigate the hypothesis that non-linear interactions in the power spectra of δ Scuti stars are related to their evolutionary state. For this purpose, we examine a set of light curves belonging to a sample of young δ Scuti stars observed by the TESS satellite. We expect an absence of sum combinations in them, and the results agree with such an assumption. The second study outlines the procedure of numerically solving the non-linear differential equations of oscillations for High Amplitude Delta Scuti (HADS) stars. The equations are described in terms of the so-called Volterra expansion, and these coefficients can be estimated observationally through amplitude and phase relations between parents and children frequencies (see Lares-Martiz et al. 2020). After obtaining these coefficients, the equations can be numerically solved.

delta Scuti stars are intermediate-mass stars (i.e. between 1.5 and 3 solar masses) pulsators located in the classical Cepheids instability strip, either in their pre-main sequence stage, or already in the main sequence or also moving from the main sequence to the giant branch. Their spectral types range from A to F. Before the era of space telescopes, these stars were thought to have a small number of pulsating frequencies, but with the data coming from space missions like CoRoT, Kepler, TESS, etc., it has been found that there are some $\delta$ Scuti stars with hundreds of pulsation frequencies. In the last years, some authors found an amplitude modulation in a few $\delta$ Scuti stars but a time-frequency analysis has not yet been performed up to date. In our work we have chosen the wavelet transform as the more appropriate tool to perform such analysis. Unlike the short time Fourier transform, that uses a fixed windows size, the wavelet transform uses short windows at high frequencies and long windows at low frequencies, making it like an adaptive analysis tool. The wavelet analysis relies on the use of a mother wavelet, i.e. a wave-like function of finite energy that can be scaled and shifted in a way that conserves the energy, so it can correlate with the variations on the signal, thus providing a measure of the instantaneous frequencies of the signal. Our results show that some frequencies are stable, at least within the observation run time, although some others show some kind of variation.

delta Scuti stars are far from being understood as much as solar-like pulsators. One of the open questions on these stars started with the first long-run observations of CoRoT from space. To observe the so-called grass (i.e. the low-amplitude signature of their power spectra), long duration, high photometric precision and high duty cycle light curves are needed which are impossible to gather from ground-based observatories. Its origin is still a long-standing debate but the presence of a flat plateau may be related with extrinsic or intrinsic processes of this kind of stars such as fast rotation. Here we deeply study the grass not only analysing the reliable harmonic variations but using the Coarse Grain Spectral Analysis (CGSA) to characterize its fractal signature. We applied this methodology to a sample of well-known delta Scuti stars observed by TESS space telescope approximately for a year. Our results point to a dependence of the grass with rotation and also with time.

Ultra-precise observations from space missions such as CoRoT, Kepler/K2 or TESS can be contrasted with data from ground-based surveys such as CARMENES or HARPS to validate extra-solar planets detection through spectroscopy or using different photometric bands to discard false positives. Though several thousands of exoplanets have already been confirmed the number of candidates is orders of magnitude higher. Since it is not possible to allocate time from ground-based surveys to perform the follow-up of all the candidates, an automatic validation procedure is required which is robust enough to confirm the detections. I introduce here a new validation method for exoplanet transit candidates based on modeling the data as auto-regressive processes. These can fit either stellar pulsations, stellar activity or any intrinsic variability so they are very suitable to model out of transit light variations. This procedure does not require any a priori from the star or planet such as period, depth, duration or shape of the signal. It neither does require any colors, spectroscopic observations or any informed assumptions, only one light curve with several transits is required. Here I show that transits can be validated with no necessity of follow-up observations in many cases and false positives can be reliably identified with auto-regressive models., https://github.com/javier-iaa/MIARMA, {"references":["Pascual-Granado, J., Garrido R., and Suárez, J.C. 2015, A&A, 575, A78","Pascual-Granado, J., Suárez, J.C., Garrido, R., et al. 2018, A&A, 614, A40","Holdsworth et al. 2021, MNRAS, 506, 1073","Tarnopolski, M. et al. 2020, ApJS, 250, 1"]}

Asteroseismology has been able to provide some information on stellar rotation for the Sun, solar-like stars, and compact objects like white dwarfs. However, this study is still rather arduous for intermediate-mass stars, which are moderate-to-rapid rotators. This is so because rotation causes splittings and shiftings in the oscillation modes, thus increasing the complexity of the oscillation spectrum and making it harder to decipher. We present a study of the oscillation patterns of a sample of benchmark $\delta$~Scuti stars that belong to eclipsing binary systems. Our objective was finding the frequency spacing related to the rotational splitting. For this task, we combined three complementary techniques: the Fourier transform, the autocorrelation function, and the histogram of frequency differences. We were able to find the rotational splittings for the majority of the stars, especially when using the last two methods (with both of them showing a similar behaviour). Hence, this is the first time we may clearly state that one of the periodicities present in the p~modes oscillation spectra of $\delta$~Scuti stars corresponds to the rotational splitting. Furthermore, we found that this is true independently of the stellar rotation rate. Additionally, for most of the stars, it was necessary to determine the large separation prior to spot the rotational splitting. These promising results pave the way to find a robust methodology to determine rotational splittings from the oscillation spectra of $\delta$~Scuti stars and, thus, to the understanding of the rotational profile of intermediate-mass pulsating stars.

It is clear that for the identification of pulsation modes a necessary first step is the unambiguous detection and extraction of frequencies from observations. It is no less clear that this task is arduous and full of obstacles. To name just a few: irregular sampling introduce correlation between frequency bins in the periodogram; noise level cannot be estimated properly when the statistical properties deviate from a uncorrelated stochastic process with Gaussian distribution; the false alarm test, which is based on a proper estimation of the noise level, also suffer from biases due to assumptions like an exponentially distributed noise; different fitting methods produce different results, etc. Here I will discuss some of these issues and introduce an ambitious project aimed to formalize a set of consistent criteria for a reliable frequency extraction in pulsating stars observations. This will produce a living recipe with the purpose of serving as a reference for the present and future data analysts of the ultra-precise observations gathered by asteroseismic space missions. That is, the necessary criteria to be a connoisseur of stellar light curves and identify pulsation modes with seismic models.

We present the results of the Fermi-Large Area Telescope (LAT) light curve (LC) modelling of selected blazars: six flat spectrum radio quasars (FSRQs) and five BL Lacertae (BL Lacs). All objects have densely sampled and long-term LCs, over 10 years. For each blazar we generated three LCs with 7, 10, and 14 days binning, using the latest LAT 8-year source catalog and binned analysis provided within the fermipy package. The LCs were modelled with several tools: the Fourier transform, the Lomb-Scargle periodogram (LSP), the autoregressive moving average (ARMA), the fractional autoregressive integrated moving average, the continuous-time autoregressive moving average (CARMA) processes, the Hurst exponents ($H$), the $\mathcal{A}-\mathcal{T}$ plane, and the wavelet scalogram. The power law indices $��$ calculated from the Fourier and LSP modelling are consistent with each other. Many objects yield $��\simeq 1$, with PKS 2155$-$304 even flatter, but some are significantly steeper, e.g. Mrk 501 and B2 1520+31. The power law PSD is indicative of a self-affine stochastic process characterised by $H$, underlying the observed variability. Several algorithms for the $H$ estimations are employed. For some objects we found $H>0.5$, indicating long-term memory. We confirm a quasi-periodic oscillation (QPO) in the PKS 2155$-$304 data with the period of $612\pm 42$ days at a $3��$ significance level, but do not detect any QPOs in other objects. The ARMA results give in general higher orders for 7 days binned LCs and lower orders for 10 and 14 days binned LCs, implying temporal variations in the LCs are consistently captured by the fitted models. CARMA modelling leads to featureless PSDs. The recently introduced $\mathcal{A}-\mathcal{T}$ plane allows us to successfully classify the PSDs based on the LCs alone and clearly separates the FSRQ and BL Lac types of blazars., To appear in the proceedings of the High Energy Astrophysics in Southern Africa 2021 (HEASA2021) in Proceedings of Science (PoS)

We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS 2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these, and the singly-periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor 6 smaller than that in the B filter which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 micromag, respectively, in two of the best characterised non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars., We thank the referee for very detailed and useful comments to the original manuscript. This work was supported by FCT - Fundação para a Ciência e a Tecnologia through national funds and by FEDER through COMPETE2020 - Programa Operacional Competitividade e Internacionalização by these grants: UID/FIS/04434/2019, PTDC/FIS-AST/30389/2017 & POCI-01- 0145-FEDER-030389. MC is supported in the form of work contract funded by national funds through FCT. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (Grant agreement no.: DNRF106). DLH and DWK acknowledge financial support from the Science and Technology Facilities Council (STFC) via grant ST/M000877/1. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. LFM acknowledges support from the UNAM by the way of DGAPA project PAPIIT IN100918. The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement N 670519: MAMSIE) and from the Fonds Wetenschappelijk Onderzoek - Vlaanderen (FWO) under the grant agreement G0H5416N (ERC Opvangproject). MS acknowledges the financial support of Postdoc@MUNI project CZ.02.2.69/0.0/0.0/16 027/0008360. EN acknowledges the Polish National Science Center grants no.2014/13/B/ST9/00902. JCS acknowledges funding support from Spanish public funds for research under projects ESP2017-87676-2-2 and ESP2015- 65712-C5-5-R, and from project RYC-2012-09913 under the ‘Ramón y Cajal’ program of the Spanish Ministry of Science and Education. AGH acknowledges funding support from Spanish public funds for research under projects ESP2017-87676-2-2 and ESP2015-65712-C5-5-R of the Spanish Ministry of Science and Education. A´ S, ZsB, and RSz acknowledge the financial support of the GINOP-2.3.2-15-2016- 00003, K-115709, K-113117, K-119517 and PD-123910 grants of the Hungarian National Research, Development and Innovation Office (NKFIH), and the Lendület Program of the Hungarian Academy of Sciences, project No. LP2018-7/2018. GH has been supported by the Polish NCN grant 2015/18/A/ST9/00578. MLM acknowledges funding support from Spanish public funds for research under project ESP2015- 65712-C5-3-R. JPG acknowledges funding support from Spanish public funds for research under project ESP2017-87676-C5-5-R. MLM and JPG also acknowledges funding support from the State Agency for Research of the Spanish MCIU through the ”Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). IS acknowledges funding support of NSF under projects DN 08-1/2016 and DN 18/13-12.12.2017. P. Kołaczek-Szymański acknowledges support from the NCN grant no. 2016/21/B/ST9/01126. This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program.

International audience; In this work, we present a new method to determine the surface gravity of delta Sct stars. We used a refined Delta nu-rho relation and the stellar parallaxes or luminosities to determine their masses and radii. A comparison with the data obtained from the binary analysis, has shown that the values found by both methods are equivalent, within the uncertainties. Moreover, thanks to the refined relation, the uncertainties in log g are of the order of those usually estimated with high-resolution spectroscopy. Because of that, this new method to determine the surface gravity is an important step-forward to break the degeneracy problem in the spectroscopic analysis.

Delta Scuti ($��$ Sct) stars are intermediate-mass pulsators, whose intrinsic oscillations have been studied for decades. However, modelling their pulsations remains a real theoretical challenge, thereby even hampering the precise determination of global stellar parameters. In this work, we used space photometry observations of eclipsing binaries with a $��$ Sct component to obtain reliable physical parameters and oscillation frequencies. Using that information, we derived an observational scaling relation between the stellar mean density and a frequency pattern in the oscillation spectrum. This pattern is analogous to the solar-like large separation but in the low order regime. We also show that this relation is independent of the rotation rate. These findings open the possibility of accurately characterizing this type of pulsator and validate the frequency pattern as a new observable for $��$ Sct stars., 11 pages, including 2 pages of appendix, 2 figures, 2 tables, accepted for publication in ApJL

Tesis Univ. Granada. Departamento de Física Estelar

Esta investigación fue financiada por la Agencia Estatal de Investigación y el Fondo Europeo de Desarrollo Regional bajo el proyecto con referencia ESP2017-87676-C5-5-R, titulado "CONTRIBUCIÓN DEL IAA-CSIC A LA MISIÓN ESPACIAL PLATO2.0: FASES B2/C/D. OPERACION DE NOMAD-EXOMARS" y fue realizada en las instalaciones del Instituto de Astrofísica de Andalucía, a cuyo personal extiendo mis agradecimientos por mantener un centro de investigación donde el ambiente de trabajo fue en todo momento más que agradable., Los modelos de pulsación estelar disponibles hasta la fecha no explican en su totalidad los espectros de potencias observados para las estrellas del tipo Scuti. Dichos espectros presentan una distribución en frecuencias muy diferentes, incluso para estrellas que poseen parámetros estelares muy similares. Este es el caso, por ejemplo, de las estrellas llamadas híbridas, que pulsan en un régimen tanto de modos p como de modos g, o la existencia de estrellas sin oscilaciones dentro de las bandas de inestabilidad de pulsación. Estos hechos sugieren que se han de revisar los mecanismos de excitación y de amortiguamiento que operan en estas estrellas, o introducir otros mecanismos causantes de la aparición de frecuencias en los espectros de potencias que los modelos actuales no son capaces de reproducir. Uno de los mecanismos físicos que dan origen a máximos en los espectros de potencia y que los modelos actuales no tienen en cuenta, son los procesos no-lineales. Una respuesta no-lineal del medio estelar a la oscilación, debido a cambios de grosor de la capa convectiva, entre otros procesos, pueden causar distorsión en la forma sinusoidal de las curvas de luz. Como consecuencia, aparecen en los espectros de potencia frecuencias de combinaciones entre modos de oscilación propios de la estrella, llamadas términos no-lineales o no-linealidades. En esta tesis, se estudian los términos no-lineales presentes en las estrellas Scuti con el objetivo de identificarlos inequívocamente. La motivación radica tanto en conseguir extraerlos correctamente de los espectros de potencia (para así explicar la distribución en frecuencia con los modelos de pulsación lineal disponibles), como en caracterizarlos, para en un futuro construir modelos no-lineales de pulsación. Como resultado del trabajo de investigación, en esta memoria se presenta un método autoconsistente para extraer no-linealidades que garantiza residuos no correlados con dichos términos. El método permite identificar frecuencias con precisiones muy altas, del orden de las alcanzadas por el método O-C, en el caso de estrellas mono-periódicas. También permite, en el caso de estrellas de doble-modo o multi-periódicas, identificar estructuras de frecuencias ocultas en lo que, antes de la extracción, se consideraba ruido. Además, se muestran resultados de una posible caracterización de los términos no-lineales en estrellas High Amplitude Scuti. Se presenta un método basado en dicha caracterización de no-linealidades que permitiría discriminar modos radiales en estrellas Low Amplitude Scuti, proporcionando así una restricción adicional que posibilitaría la identificación modal. Esta sería la primera vez que los términos no-lineales son útiles, aunque indirectamente, para llevar a cabo un análisis astrosismológico en estrellas pulsantes de masa intermedia. Finalmente, se presentan también resultados para algunas estrellas variable del tipo Doradus, constituyendo el inicio de la caracterización de términos no-lineales producto de la interacción de modos no-radiales., Tesis Univ. Granada., Agencia Estatal de Investigación y el Fondo Europeo de Desarrollo Regional (ESP2017-87676-C5-5-R)