arXiv subjects

J. C. Suarez

Publications and source records attributed to J. C. Suarez.

At least 37 records · Page 2Linked to original sources

arXiv2009

HD 50844: the new look of Delta Sct stars from CoRoT space photometry

It has also been suggested that the detection of a wealth of very low amplitude modes in Delta Sct stars was only a matter of signal--to--noise ratio. Access to this treasure, impossible from the ground, is one of the scientific aims of the space mission CoRoT, a space mission developed and operated by CNES. This work presents the results obtained on HD 50844: the 140,016 datapoints were analysed using independent approaches and several checks performed. A level of 10^{-5} mag was reached in the amplitude spectra of the CoRoT timeseries. The frequency analysis of the CoRoT timeseries revealed hundreds of terms in the frequency range 0--30 d^{-1}. All the cross--checks confirmed this new result. The initial guess that Delta Sct stars have a very rich frequency content is confirmed. The spectroscopic mode identification gives theoretical support since very high--degree modes (up to ell=14) are identified. We also prove that cancellation effects are not sufficient in removing the flux variations associated to these modes at the noise level of the CoRoT measurements. The ground--based observations indicate that HD 50844 is an evolved star that is slightly underabundant in heavy elements, located on the Terminal Age Main Sequence. Probably due to this unfavourable evolutionary status, no clear regular distribution is observed in the frequency set. The predominant term (f_1=6.92 d^{-1}) has been identified as the fundamental radial mode combining ground-based photometric and spectroscopic data. This work is also based on observations made with ESO telescopes under the ESO Large Programme LP178.D-0361 and on data collected at the Observatorio de Sierra Nevada, at the Observatorio Astronomico Nacional San Pedro Martir, and at the Piszkesteto Mountain Station of Konkoly Observatory.

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arXiv2008

Seismology of beta Cephei stars: differentially-rotating models for interpreting the oscillation spectrum of nu-Eridani

A method for the asteroseismic analysis of beta Cephei stars is presented and applied to the star nu Eridani. The method is based on the analysis of rotational splittings, and their asymmetries using differentially-rotating asteroseismic models. Models with masses around 7.13 M_sun, and ages around 14.9 Myr, were found to fit better 10 of the 14 observed frequencies, which were identified as the fundamental radial mode and the three L=1 triplets g, p, and p. The splittings and aymmetries found for these modes recover those provided in the literature, except for p. For this last mode, all its non-axysimmetric components are predicted by the models. Moreover, opposite signs of the observed and predicted splitting asymmetries are found. If identification is confirmed, this can be a very interesting source of information about the internal rotation profile, in particular in the outer regions of the star. In general, the seismic models which include a description for shellular rotation yield slightly better results as compared with those given by uniformly-rotating models. Furthermore, we show that asymmetries are quite dependent on the overshooting of the convective core, which make the present technique suitable for testing the theories describing the angular momentum redistribution and chemical mixing due to rotationally-induced turbulence.

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arXiv2008

The gamma Dor CoRoT target HD49434. I-Results from the ground-based campaign

Context: We present the results of an extensive ground-based photometric and spectroscopic campaign on the gamma Dor CoRoT target HD49434. This campaign was preparatory to the CoRoT satellite observations, which took place from October 2007 to March 2008. Results: The frequency analysis clearly shows the presence of four frequencies in the 0.2-1.7 c/d interval, as well as six frequencies in the 5-12 c/d domain. The low frequencies are typical for gamma Dor variables while the high frequencies are common for delta Sct pulsators. We propose the frequency 2.666 c/d as a possible rotational frequency. All modes, for which an identification was possible, seem to be high-degree modes (3 <= l <= 8). We did not find evidence for a possible binary nature of HD49434. The element abundances we derived are consistent with the values obtained in previous analyses. Conclusions: We classify the gamma Dor star HD49434 as a hybrid pulsator, which pulsates simultaneously in p- and g-modes. This finding makes HD49434 an extremely interesting target for asteroseismic modelling. Observations: Based on observations made with the 2.2m ESO/MPI telescope at the La Silla Observatory under the ESO Large Programme: LP178.D-0361. Also based on observations obtained at Observatorio de Sierra Nevada (Spain), at the Centro Astronomico Hispano Aleman at Calar Alto (Spain), at Observatorio Astronomico Nacional San Pedro Martir (Mexico), at the Piszkesteto Mountain Station of Konkoly Observatory (Hungary), at Observatoire de Haute Provence (France) and at Mount John University Observatory (New Zealand).

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arXiv2008

FILOU oscillation code

The present paper provides a description of the oscillation code FILOU, its main features, type of applications it can be used for, and some representative solutions. The code is actively involved in CoRoT/ESTA exercises (this volume) for the preparation for the proper interpretation of space data from the CoRoT mission. Although CoRoT/ESTA exercises have been limited to the oscillations computations for non-rotating models, the main characteristic of FILOU is, however, the computation of radial and non-radial oscillation frequencies in presence of rotation. In particular, FILOU calculates (in a perturbative approach) adiabatic oscillation frequencies corrected for the effects of rotation (up to the second order in the rotation rate) including near degeneracy effects. Furthermore, FILOU works with either a uniform rotation or a radial differential rotation profile (shellular rotation), feature which makes the code singular in the field.

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arXiv2007

Inter-comparison of the g-, f- and p-modes calculated using different oscillation codes for a given stellar model

In order to make astroseismology a powerful tool to explore stellar interiors, different numerical codes should give the same oscillation frequencies for the same input physics. This work is devoted to test, compare and, if needed, optimize the seismic codes used to calculate the eigenfrequencies to be finally compared with observations. The oscillation codes of nine research groups in the field have been used in this study. The same physics has been imposed for all the codes in order to isolate the non-physical dependence of any possible difference. Two equilibrium models with different grids, 2172 and 4042 mesh points, have been used, and the latter model includes an explicit modelling of semiconvection just outside the convective core. Comparing the results for these two models illustrates the effect of the number of mesh points and their distribution in particularly critical parts of the model, such as the steep composition gradient outside the convective core. A comprehensive study of the frequency differences found for the different codes is given as well. These differences are mainly due to the use of different numerical integration schemes. The use of a second-order integration scheme plus a Richardson extrapolation provides similar results to a fourth-order integration scheme. The proper numerical description of the Brunt-Vaisala frequency in the equilibrium model is also critical for some modes. An unexpected result of this study is the high sensitivity of the frequency differences to the inconsistent use of values of the gravitational constant (G) in the oscillation codes, within the range of the experimentally determined ones, which differ from the value used to compute the equilibrium model.

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arXiv2007

The role of rotation on Petersen Diagrams. II The influence of near-degeneracy

In the present work, the effect of near-degeneracy on rotational Petersen diagrams (RPD) is analysed. Seismic models are computed considering rotation effects on both equilibrium models and adiabatic oscillation frequencies (including second-order near-degeneracy effects). Contamination of coupled modes and coupling strength on the first radial modes are studied in detail. Analysis of relative intrinsic amplitudes of near-degenerate modes reveals that the identity of the fundamental radial mode and its coupled quadrupole pair are almost unaltered once near-degeneracy effects are considered. However, for the first overtone, a mixed radial/quadrupole identity is always predicted. The effect of near-degeneracy on the oscillation frequencies becomes critical for rotational velocities larger than 15-20 km/s, for which large wriggles in the evolution of the period ratios are obtained (up $10^{-2}$). Such wriggles imply uncertainties, in terms of metallicity determinations using RPD, reaching up to 0.50 dex, which can be critical for Pop. I HADS (High Amplitude \dss). In terms of mass determinations, uncertainties reaching up to 0.5 M_sun are predicted. The location of such wriggles is found to be independent of metallicity and rotational velocity, and governed mainly by the avoided-crossing phenomenon.

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arXiv2007

Mode stability in delta Scuti stars: linear analysis versus observations in open clusters

A comparison between linear stability analysis and observations of pulsation modes in five delta Scuti stars, belonging to the same cluster, is presented. The study is based on the work by Michel et al. (1999), in which such a comparison was performed for a representative set of model solutions obtained independently for each individual star considered. In this paper we revisit the work by Michel et al. (1999) following, however, a new approach which consists in the search for a single, complete, and coherent solution for all the selected stars, in order to constrain and test the assumed physics describing these objects. To do so, refined descriptions for the effects of rotation on the determination of the global stellar parameters and on the adiabatic oscillation frequency computations are used. In addition, a crude attempt is made to study the role of rotation on the prediction of mode instabilities.The present results are found to be comparable with those reported by Michel et al. (1999). Within the temperature range log T_eff = 3.87-3.88 agreement between observations and model computations of unstable modes is restricted to values for the mixing-length parameter alpha_nl less or equal to 1.50. This indicates that for these stars a smaller value for alpha_nl is required than suggested from a calibrated solar model. We stress the point that the linear stability analysis used in this work still assumes stellar models without rotation and that further developments are required for a proper description of the interaction between rotation and pulsation dynamics.

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arXiv2007

Multisite campaign on the open cluster M67. III. Delta Scuti pulsations in the blue stragglers

We have made an asteroseismic analysis of the variable blue stragglers in the open cluster M67. The data set consists of photometric time series from eight sites using nine 0.6-2.1 meter telescopes with a time baseline of 43 days. In two stars, EW Cnc and EX Cnc, we detect the highest number of frequencies (41 and 26) detected in delta Scuti stars belonging to a stellar cluster, and EW Cnc has the second highest number of frequencies detected in any delta Scuti star. We have computed a grid of pulsation models that take the effects of rotation into account. The distribution of observed and theoretical frequencies show that in a wide frequency range a significant fraction of the radial and non-radial low-degree modes are excited to detectable amplitudes. Despite the large number of observed frequencies we cannot constrain the fundamental parameters of the stars. To make progress we need to identify the degrees of some of the modes either from multi-colour photometry or spectroscopy.

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arXiv2006

Asteroseismology with the WIRE satellite. I. Combining Ground- and Space-based Photometry of the Delta Scuti Star Epsilon Cephei

We have analysed ground-based multi-colour Stromgren photometry and single-filter photometry from the star tracker on the WIRE satellite of the delta scuti star Epsilon Cephei. The ground-based data set consists of 16 nights of data collected over 164 days, while the satellite data are nearly continuous coverage of the star during 14 days. The spectral window and noise level of the satellite data are superior to the ground-based data and this data set is used to locate the frequencies. However, we can use the ground-based data to improve the accuracy of the frequencies due to the much longer time baseline. We detect 26 oscillation frequencies in the WIRE data set, but only some of these can be seen clearly in the ground-based data. We have used the multi-colour ground-based photometry to determine amplitude and phase differences in the Stromgren b-y colour and the y filter in an attempt to identify the radial degree of the oscillation frequencies. We conclude that the accuracies of the amplitudes and phases are not sufficient to constrain theoretical models of Epsilon Cephei. We find no evidence for rotational splitting or the large separation among the frequencies detected in the WIRE data set. To be able to identify oscillation frequencies in delta scuti stars with the method we have applied, it is crucial to obtain more complete coverage from multi-site campaigns with a long time baseline and in multiple filters. This is important when planning photometric and spectroscopic ground-based support for future satellite missions like COROT and KEPLER.

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arXiv2006

A comprehensive asteroseismic modelling of the high-amplitude delta Scuti star RV Arietis

We present a comprehensive asteroseismic study of the double-mode high-amplitude delta Scuti star HD 187642 (RV Arietis). The modelling includes some of the most recent techniques: 1) effects of rotation on both equilibrium models and adiabatic oscillation spectrum, 2) non-adiabatic study of radial and non-radial modes, 3) relationship between the fundamental radial mode and the first overtone in the framework of Petersen diagrams. The analysis reveals that two of the observed frequencies are very probably identified as the fundamental and first overtone radial modes. Analysis of the colour index variations, together with theoretical non-adiabatic calculations, points to models in the range of [7065,7245] K in effective temperature and of [1190, 1270] Myr in stellar age. These values were found to be compatible with those obtained using the three other asteroseismic techniques.

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arXiv2006

Discovery and application of Frequency Ratio Method to the new multiperiodic gamma Dor star HD 218427

gamma Dor-type oscillations have been discovered in the star HD218427 through simultaneous uvby photometric observations carried out in the year 2003. A few Hbeta-Crawford measurements were also collected for calibration purposes which place this star well-located inside the gamma Dor instability region. Deficiency in metal content, similar to other well-defined gamma Dor stars, is found in HD218427 and the possibility of a lambda Boo nature is discussed. Frequency analysis was carried out for different filters, the combined "vby" filter was also used and five frequencies were found as significant with periods ranging between 0.3 and 0.8 days. The recently developed Frequency Ratio Method is used in order to perform an identification of the excited modes. The results are consistent with an l=2 identification for all the modes and high radial quantum numbers (n~40) for the three main observed periodicities. The possibility of multiplet structures is also discussed. However, no consistency is found when the Time-Dependent Convection treatment is used for modes discrimination. The disagreement can be due to the large rotation velocity taking place in HD218427 and, consequently, the significant coupling between the modes.

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arXiv2005

The effects of moderately fast shellular rotation on adiabatic oscillations

We investigate adiabatic oscillations for delta Scuti star models, taking into account a moderate rotation velocity ~100 \km/s. The resulting oscillation frequencies include corrections for rotation up to second order in the rotation rate including those of near degeneracy. Effects of either a uniform rotation or a rotation profile assuming local angular momentum conservation of the form Omega=Omega(r) on oscillation frequencies are compared. As expected, important differences (around 3 microHz) are obtained in the $g$ and mixed mode regions. For higher frequency p modes, differences range between 1 microHz and 3 microHz. Such differences are likely to be detectable with future space missions such as COROT, where precisions in frequency around 0.5 microHz will be reachable.

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arXiv2005

A seismological analysis of Delta Scuti stars in the Pleiades cluster

A comparison between the oscillation frequencies of six multi-periodic Delta Scuti stars of the Pleiades cluster and the eigenfrequencies of rotating stellar models that match the corresponding stellar parameters has been carried out. The assumption that all the stars considered have some common parameters, such as metallicity, distance or age, is imposed as a constraint. As a result, we have a best fit solution associated with a cluster metallicity of [Fe/H] ~ 0.067, an age between 70x10^6 and 100x10^6 yr and a distance modulus of mv-Mv=5.60-5.70 mag. All the stars were found to oscillate mainly in non-radial, low degree, low order p modes. Estimates of mass and rotation rates for each star are also obtained

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arXiv2005

The role of rotation on Petersen Diagrams. The $ Pi_{1/0}(Omega)$ period ratios

The present work explores the theoretical effects of rotation in calculating the period ratios of double-mode radial pulsating stars with special emphasis on high-amplitude delta Scuti stars (HADS). Diagrams showing these period ratios vs. periods of the fundamental radial mode have been employed as a good tracer of non-solar metallicities and are known as Petersen diagrams (PD).In this paper we consider the effect of moderate rotation on both evolutionary models and oscillation frequencies and we show that such effects cannot be completely neglected as it has been done until now. In particular it is found that even for low-to-moderate rotational velocities (15-50 km/s), differences in period ratios of some hundredths can be found. The main consequence is therefore the confusion scenario generated when trying to fit the metallicity of a given star using this diagram without a previous knowledge of its rotational velocity.

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arXiv2005

The Frequency Ratio Method for the seismic modelling of gamma Doradus stars. II The role of rotation

The effect of rotation on the Frequency Ratio Method (Moya et al. 2005) is examined. Its applicability to observed frequencies of rotating gamma Doradus stars is discussed taking into account the following aspects: the use of a perturbative approach to compute adiabatic oscillation frequencies; the effect of rotation on the observational Brunt-Vaisala integral determination and finally, the problem of disentangling multiplet-like structures from frequency patterns due to the period spacing expected for high-order gravity modes in asymptotic regime. This analysis reveals that the FRM produces reliable results for objects with rotational velocities up to 70 kms/s, for which the FRM intrinsic error increases one order of magnitude with respect to the typical FRM errors given in Moya et al. (2005). Our computations suggest that, given the spherical degree "l" identification, the FRM may be discriminating for m = 0 modes, in the sense that the method avoids any misinterpretation induced by the presence of rotationally split multiplet-like structures, which reinforces the robustness of the method. However, if "l" is unknown, such discrimination is not ensured. In order to check the FRM in presence of slow-moderate rotation, we have applied it to the three observed frequencies of the slowly rotating (vsini = 29 km/s) gamma Doradus star HD48501.

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arXiv2005

The double-mode nature of the HADS star GSC 00144-03031 and the Petersen diagram of the class

The double--mode pulsation of GSC 00144-03031 has been detected when searching for COROT targets. A very large dataset composed of 4722 photometric measurements was collected at six observatories in Europe and America. There is no hint of the excitation of additional modes (down to 0.6 mmag) and therefore GSC 00144-03031 seems to be a pure double--mode pulsator, with a very short fundamental radial mode (P=84 min). From Stromgren photometry and evolutionary tracks it appears to be a Pop. I star with M=1.75 solar masses, located in the middle of the instability strip, close to the Zero--Age Main Sequence. We also discovered other new double--mode pulsators in the databases of large--scale projects: OGLE BW2_V142, OGLE BW1_V207, ASAS3 094303-1707.3, ASAS3 000116-6037.0, NSVS 3234596 and NSVS 3324715. An observational Petersen diagram is presented and explained by means of new models. A common sequence connecting Pop. I stars from the shortest to the longest periods is proposed and the spreads in the period ratios are ascribed to different metallicities (at the shortest periods) and to different masses (at the longest ones). The paper is based on data collected at S. Pedro Martir and Sierra Nevada Observatories and on the contributions from several amateur astronomers.

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arXiv2005

Modelling of the fast rotating delta Scuti star Altair

We present an asteroseismic study of the fast rotating star HD187642 (Altair), recently discovered to be a delta Scuti pulsator. We have computed models taking into account rotation for increasing rotational velocities. We investigate the relation between the fundamental radial mode and the first overtone in the framework of Petersen diagrams. The effects of rotation on such diagrams, which become important at rotational velocities above 150 km/s, as well as the domain of validity of our seismic tools are discussed. We also investigate the radial and non-radial modes in order to constrain models fitting the five most dominant observed oscillation modes.

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arXiv2005

Preparing the COROT space mission: new variable stars in the galactic Anticenter direction

The activities related to the preparation of the asteroseismic, photometric space mission COROT are described. Photoelectric observations, wide--field CCD photometry, uvbyB calibrations and further time--series have been obtained at different observatories and telescopes. They have been planned to complete the COROT programme in the direction of the galactic Anticenter. In addition to suitable asteroseismic targets covering the different evolutionary stages between ZAMS and TAMS, we discovered several other variable stars, both pulsating and geometrical. We compared results on the incidence of variability in the galactic Center and Anticenter directions. Physical parameters have been obtained and evolutionary tracks fitting them have been calculated. The peculiarities of some individual stars alre pointed out. Paper based on observations collected at the San Pedro Martir, Sierra Nevada, Teide, La Silla, Haute-Provence and Roque de Los Muchachos (Telescopio Nazionale Galileo and Mercator telescopes) observatories.

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