arXiv subjects

A. Miglio

Publications and source records attributed to A. Miglio.

At least 145 records · Page 8Linked to original sources

arXiv2009

Theoretical amplitudes and lifetimes of non-radial solar-like oscillations in red giants

Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars? We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. Very distinct results are found depending on the evolutionary status of the star.

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arXiv2009

Solar-like oscillations in a massive star

Seismology of stars provides insight into the physical mechanisms taking place in their interior, with modes of oscillation probing different layers. Low-amplitude acoustic oscillations excited by turbulent convection were detected four decades ago in the Sun and more recently in low-mass main-sequence stars. Using data gathered by the Convection Rotation and Planetary Transits mission, we report here on the detection of solar-like oscillations in a massive star, V1449 Aql, which is a known large-amplitude (b Cephei) pulsator.

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arXiv2009

Ground-based observations of the beta Cephei CoRoT main target HD 180642: abundance analysis and mode identification

The known beta Cephei star HD 180642 was observed by the CoRoT satellite in 2007. From the very high-precision light curve, its pulsation frequency spectrum could be derived for the first time (Degroote and collaborators). In this paper, we obtain additional constraints for forthcoming asteroseismic modeling of the target. Our results are based on both extensive ground-based multicolour photometry and high-resolution spectroscopy. We determine T_eff = 24 500+-1000 K and log g = 3.45+-0.15 dex from spectroscopy. The derived chemical abundances are consistent with those for B stars in the solar neighbourhood, except for a mild nitrogen excess. A metallicity Z = 0.0099+-0.0016 is obtained. Three modes are detected in photometry. The degree l is unambiguously identified for two of them: l = 0 and l = 3 for the frequencies 5.48694 1/d and 0.30818 1/d, respectively. The radial mode is non-linear and highly dominant with an amplitude in the U-filter about 15 times larger than the strongest of the other modes. For the third frequency of 7.36673 1/d found in photometry, two possibilities remain: l = 0 or 3. In the radial velocities, the dominant radial mode presents a so-called stillstand but no clear evidence of the existence of shocks is observed. Four low-amplitude modes are found in spectroscopy and one of them, with frequency 8.4079 1/d, is identified as (l,m)=(3,2). Based on this mode identification, we finally deduce an equatorial rotational velocity of 38+-15 km/s.

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arXiv2009

Discriminating between overshooting and rotational mixing in massive stars: any help from asteroseismology?

Chemical turbulent mixing induced by rotation can affect the internal distribution of mu near the energy-generating core of main-sequence stars, having an effect on the evolutionary tracks similar to that of overshooting. However, this mixing also leads to a smoother chemical composition profile near the edge of the convective core, which is reflected in the behaviour of the buoyancy frequency and, therefore, in the frequencies of gravity modes. We show that for rotational velocities typical of main-sequence B-type pulsating stars, the signature of a rotationally induced mixing significantly perturbs the spectrum of gravity modes and mixed modes, and can be distinguished from that of overshooting. The cases of high-order gravity modes in Slowly Pulsating B stars and of low-order g modes and mixed modes in beta Cephei stars are discussed.

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arXiv2009

Testing the forward approach in modelling beta Cephei pulsators: setting the stage

The information on stellar parameters and on the stellar interior we can get by studying pulsating stars depends crucially on the available observational constraints: both seismic constraints precision and number of detected modes, identification, nature of the modes) and "classical" observations (photospheric abundances, effective temperature, luminosity, surface gravity). We consider the case of beta Cephei pulsators and, with the aim of estimating quantitatively how the available observational constraints determine the type and precision of our inferences, we set the stage for Hare&Hound exercises. In this contribution we present preliminary results for one simple case, where we assume as "observed" frequencies a subset of frequencies of a model and then evaluate a seismic merit function on a dense and extensive grid of models of B-type stars. We also compare the behaviour of chi^2 surfaces obtained with and without mode identification.

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arXiv2008

Close-up of primary and secondary asteroseismic CoRoT targets and the ground-based follow-up observations

To optimise the science results of the asteroseismic part of the CoRoT satellite mission a complementary simultaneous ground-based observational campaign is organised for selected CoRoT targets. The observations include both high-resolution spectroscopic and multi-colour photometric data. We present the preliminary results of the analysis of the ground-based observations of three targets. A line-profile analysis of 216 high-resolution FEROS spectra of the delta Sct star HD 50844 reveals more than ten pulsation frequencies in the frequency range 5-18 c/d, including possibly one radial fundamental mode (6.92 c/d). Based on more than 600 multi-colour photometric datapoints of the beta Cep star HD180642, spanning about three years and obtained with different telescopes and different instruments, we confirm the presence of a dominant radial mode nu1=5.48695 c/d, and detect also its first two harmonics. We find evidence for a second mode nu2=0.3017 c/d, possibly a g-mode, and indications for two more frequencies in the 7-8 c/d domain. From Stromgren photometry we find evidence for the hybrid delta Sct/gamma Dor character of the F0 star HD 44195, as frequencies near 3 c/d and 21 c/d are detected simultaneously in the different filters.

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arXiv2008

A seismic approach to testing different formation channels of subdwarf B stars

There are many unknowns in the formation of subdwarf B stars. Different formation channels are considered to be possible and to lead to a variety of helium-burning subdwarfs. All seismic models to date, however, assume that a subdwarf B star is a post-helium-flash-core surrounded by a thin inert layer of hydrogen. We examine an alternative formation channel, in which the subdwarf B star originates from a massive (>~2 Msun) red giant with a non-degenerate helium-core. Although these subdwarfs may evolve through the same region of the log g-Teff diagram as the canonical post-flash subdwarfs, their interior structure is rather different. We examine how this difference affects their pulsation modes and whether it can be observed. Using detailed stellar evolution calculations we construct subdwarf B models from both formation channels. The iron accumulation in the driving region due to diffusion, which causes the excitation of the modes, is approximated by a Gaussian function. The pulsation modes and frequencies are calculated with a non-adiabatic pulsation code. A detailed comparison of two subdwarf B models from different channels, but with the same log g and Teff, shows that their mode excitation is different. The excited frequencies are lower for the post-flash than for the post-non-degenerate subdwarf B star. This is mainly due to the differing chemical composition of the stellar envelope. A more general comparison between two grids of models shows that the excited frequencies of most post-non-degenerate subdwarfs cannot be well-matched with the frequencies of post-flash subdwarfs. In the rare event that an acceptable seismic match is found, additional information, such as mode identification and log g and Teff determinations, allows us to distinguish between the two formation channels.

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arXiv2008

Asteroseismology and Interferometry

Asteroseismology provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Recent developments, including the first systematic studies of solar-like pulsators, have boosted the impact of this field of research within Astrophysics and have led to a significant increase in the size of the research community. In the present paper we start by reviewing the basic observational and theoretical properties of classical and solar-like pulsators and present results from some of the most recent and outstanding studies of these stars. We centre our review on those classes of pulsators for which interferometric studies are expected to provide a significant input. We discuss current limitations to asteroseismic studies, including difficulties in mode identification and in the accurate determination of global parameters of pulsating stars, and, after a brief review of those aspects of interferometry that are most relevant in this context, anticipate how interferometric observations may contribute to overcome these limitations. Moreover, we present results of recent pilot studies of pulsating stars involving both asteroseismic and interferometric constraints and look into the future, summarizing ongoing efforts concerning the development of future instruments and satellite missions which are expected to have an impact in this field of research.

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arXiv2008

Analysis of 70 Ophiuchi AB including seismic constraints

The analysis of solar-like oscillations for stars belonging to a binary system provides a unique opportunity to probe the internal stellar structure and to test our knowledge of stellar physics. Such oscillations have been recently observed and characterized for the A component of the 70 Ophiuchi system. A model of 70 Ophiuchi AB that correctly reproduces all observational constraints available for both stars is determined. An age of 6.2 +- 1.0 Gyr is found with an initial helium mass fraction Y_i=0.266 +- 0.015 and an initial metallicity (Z/X)_i=0.0300 +- 0.0025 when atomic diffusion is included and a solar value of the mixing-length parameter assumed. A precise and independent determination of the value of the mixing-length parameter needed to model 70 Oph A requires accurate measurement of the mean small separation, which is not available yet. Current asteroseismic observations, however, suggest that the value of the mixing-length parameter of 70 Oph A is lower or equal to the solar calibrated value. The effects of atomic diffusion and of the choice of the adopted solar mixture were also studied. We also tested and compared the theoretical tools used for the modeling of stars for which p-modes frequencies are detected by performing this analysis with three different stellar evolution codes and two different calibration methods. We found that the different evolution codes and calibration methods we used led to perfectly coherent results.

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arXiv2008

Grids of Stellar Models and Frequencies with CLES + LOSC

We present a grid of stellar models, obtained with the CLES evolution code, following the specification of ESTA-Task1, and the corresponfing seismic properties, computed with the LOSC code. We provide a complete description of the corresponding files that will be available on the ESTA web-pages.

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arXiv2008

Probing the properties of convective cores through g modes: high-order g modes in SPB and gamma Doradus stars

In main sequence stars the periods of high-order gravity modes are sensitive probes of stellar cores and, in particular, of the chemical composition gradient that develops near the outer edge of the convective core. We present an analytical approximation of high-order g modes that takes into account the effect of the mu gradient near the core. We show that in main-sequence models, similarly to the case of white dwarfs, the periods of high-order gravity modes are accurately described by a uniform period spacing superposed to an oscillatory component. The periodicity and amplitude of such component are related, respectively, to the location and sharpness of the mu gradient. We investigate the properties of high-order gravity modes for stellar models in a mass domain between 1 and 10 Msun, and the effects of the stellar mass, evolutionary state, and extra-mixing processes on period spacing features. In particular, we show that for models of a typical SPB star, a chemical mixing that could likely be induced by the slow rotation observed in these stars, is able to significantly change the g-mode spectra of the equilibrium model. Prospects and challenges for the asteroseismology of gamma Doradus and SPB stars are also discussed.

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arXiv2007

Gravity modes and mixed modes as probes of stellar cores in main-sequence stars: from solar-like to beta Cep stars

We investigate how the frequencies of gravity modes depend on the detailed properties of the chemical composition gradient that develops near the core of main-sequence stars and, therefore, on the transport processes that are able to modify the μprofile in the central regions. We show that in main-sequence models, similarly to the case of white dwarfs, the periods of high-order gravity modes are accurately described by a uniform period spacing superposed to an oscillatory component. The periodicity and amplitude of such component are related, respectively, to the location and sharpness of the μgradient. We briefly discuss and interpret, by means of this simple approximation, the effect of turbulent mixing near the core on the periods of both high-order and low-order g modes, as well as of modes of mixed pressure-gravity character.

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arXiv2007

The effect of turbulent mixing on the g-mode spectrum of MS stars

The understanding of transport processes inside the stars is one of the main goals of asteroseismology. Chemical turbulent mixing can affect the internal distribution of μnear the energy generating core, having an effect on the evolutionary tracks similar to that of overshooting. This mixing leads to a smoother chemical composition profile near the edge of the convective core, which is reflected in the behavior of the buoyancy frequency and, therefore, in the frequencies of gravity modes. We describe the effects of convective overshooting and turbulent mixing on the frequencies of gravity modes in B-type main sequence stars. In particular, the cases of p-g mixed modes in beta Cep stars and high-order modes in SPBs are considered.

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arXiv2007

The Liege Oscillation Code

The Liege Oscillation code can be used as a stand-alone program or as a library of subroutines that the user calls from a Fortran main program of his own to compute radial and non-radial adiabatic oscillations of stellar models. We describe the variables and the equations used by the program and the methods used to solve them. A brief account is given of the use and the output of the program.

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arXiv2007

CLES, Code Liegeois d'Evolution Stellaire

Cles is an evolution code recently developed to produce stellar models meeting the specific requirements of studies in asteroseismology. It offers the users a lot of choices in the input physics they want in their models and its versatility allows them to tailor the code to their needs and implement easily new features. We describe the features implemented in the current version of the code and the techniques used to solve the equations of stellar structure and evolution. A brief account is given of the use of the program and of a solar calibration realized with it.

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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

An asteroseismic study of the beta Cephei star theta Ophiuchi: constraints on global stellar parameters and core overshooting

We present a seismic study of the beta Cephei star theta Ophiuchi. Our analysis is based on the observation of one radial mode, one rotationally split l = 1 triplet and three components of a rotationally split l = 2 quintuplet for which the m-values were well identified by spectroscopy. We identify the radial mode as fundamental, the triplet as p_1 and the quintuplet as g_1. Our NLTE abundance analysis results in a metallicity and CNO abundances in full agreement with the most recent updated solar values. With X \in [0.71,0.7211] and Z \in [0.009,0.015], and using the Asplund et al. (2005) mixture but with a Ne abundance about 0.3 dex larger (Cunha et al. 2006), the matching of the three independent modes, enables us to deduce constrained ranges for the mass (M = 8.2 +/- 0.3 Msun) and central hydrogen abundance (X_c = 0.38 +/- 0.02) of theta Oph and to prove the occurrence of core overshooting (alpha_ov = 0.44 +/- 0.07). We also derive an equatorial rotation velocity of 29 +/- 7 km/s. Moreover, we show that the observed non-equidistance of the l=1 triplet can be reproduced by second order effects of rotation. Finally, we show that the observed rotational splitting of two modes cannot rule out a rigid rotation model.

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arXiv2007

Multiperiodicity in the newly discovered mid-late Be star V2104 Cygni

We obtained the first long, homogenous time-series of V2104Cyg, consisting of 679 datapoints, with the uvbybeta photometers of Sierra Nevada and San Pedro Martir Observatories with the aim to detect and subsequently interpret the intrinsic frequencies of this previously unstudied variable star, which turned out to be a Be star. We try to figure out its place among the variable B stars on the upper Main Sequence. In order to obtain additional information on physical parameters we collected a few spectra with the ELODIE and FIES instruments. We searched for frequencies in the uvby passbands using 2 different frequency analysis methods and used the S/N>4 criterion to select the significant periodicities. We obtained an estimate of the physical parameters of the underlying B star of spectral type between B5 and B7, by correcting for the presence of a circumstellar disk, using a formalism based on the strenght of the Halpha line emission. We detected 3 independent frequencies with amplitudes below 0.01mag, f1 = 4.7126 c/d, f2 = 2.2342 c/d and f3 = 4.671 c/d, and discovered that V2104Cyg is a Be star. The fast rotation (vsini=290+/-10 km/s, and 27<i<45) hampered the investigation of the associated pulsational parameters l. Nevertheless, the most plausible explanation for the observed variability of this mid-late type Be star is a non-radial pulsation model. This paper is based on observations obtained at the Observatorio Astronomico Nacional San Pedro Martir (Mexico), Observatorio de Sierra Nevada (Spain), Observatoire de Haute Provence (France), and on observations made with the Nordic Optical Telescope, Observatorio Roque de los Muchachos, La Palma, Spain.

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