arXiv subjects

A. A. Pamyatnykh

Publications and source records attributed to A. A. Pamyatnykh.

At least 37 records · Page 2Linked to original sources

arXiv2008

Toward mode selection in Delta Scuti stars: Regularities in observed and theoretical frequency spectra

Only a fraction of the theoretically predicted nonradial pulsation modes have so far been observed in Delta Scuti stars. Nevertheless, the large number of frequencies detected in recent photometric studies of selected Delta Scuti stars allow us to look for regularities in the frequency spacing of modes. Mode identifications are used to interpret these results. Statistical analyses of several Delta Scuti stars (FG Vir, 44 Tau, BL Cam and others) show that the photometrically observed frequencies are not distributed at random, but that the excited nonradial modes cluster around the frequencies of the radial modes over many radial orders. The observed regularities can be partly explained by modes trapped in the stellar envelope. This mode selection mechanism was proposed by Dziembowski & Krolikowska (1990) and shown to be efficient for l = 1 modes. New pulsation model calculations confirm the observed regularities. We present the s-f diagram, which compares the average separation of the radial frequencies (s) with the frequency of the lowest-frequency unstable radial mode (f). This provides an estimate for the log g value of the observed star, if we assume that the centers of the observed frequency clusters correspond to the radial mode frequencies. This assumption is confirmed by examples of well-studied Delta Scuti variables in which radial modes were definitely identified.

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arXiv2008

Regularities in the frequency spacings of Delta Scuti stars and the s-f Diagram

Statistical analyses of several Delta Scuti stars (FG Vir, 44 Tau, BL Cam and others) show that the photometrically observed frequencies cluster around the frequencies of the radial modes over many radial orders. The observed regularities can be partly explained by modes trapped in the stellar envelope. This mode selection mechanism was already proposed by Dziembowski & Krolikowska (1990) and was shown to be efficient for l=1 modes. New pulsation model calculations confirm the observed regularities. We present the s-f diagram, which compares the average separation of the radial frequencies (s) with the frequency of the lowest unstable radial mode (f). The diagram provides an estimate for the log g value of the observed star, if we assume that the centers of the observed frequency clusters correspond to the radial mode frequencies. This assumption is confirmed by examples of well-studied Delta Scuti variables in which radial modes were definitely identified.

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arXiv2008

Examples of seismic modelling

Findings of a few recent asteroseismic studies of the main-sequence pulsating stars, as performed in Wojciech Dziembowski's group in Warsaw and in Michel Breger's group in Vienna, are briefly presented and discussed. The selected objects are three hybrid pulsators Nu Eridani, 12 Lacertae and Gamma Pegasi, which show both Beta Cephei and SPB type modes, and the Delta Scuti type star 44 Tauri.

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arXiv2008

The two hybrid B-type pulsators: Nu Eridani and 12 Lacertae

The rich oscillation spectra determined for the two stars, Nu Eridani and 12 Lacertae, present an interesting challenge to stellar modelling. The stars are hybrid objects showing a number of modes at frequencies typical for Beta Cep stars but also one mode at frequency typical for SPB stars. We construct seismic models of these stars considering uncertainties in opacity and element distribution. We also present estimate of the interior rotation rate and address the matter of mode excitation. We use both the OP and OPAL opacity data and find significant difference in the results. Uncertainty in these data remains a major obstacle in precise modelling of the objects and, in particular, in estimating the overshooting distance. We find evidence for significant rotation rate increase between envelope and core in the two stars. Instability of low-frequency g-modes was found in seismic models of Nu Eri built with the OP data, but at frequencies higher than those measured in the star. No such instability was found in models of 12 Lac. We do not have yet a satisfactory explanation for low frequency modes. Some enhancement of opacity in the driving zone is required but we argue that it cannot be achieved by the iron accumulation, as it has been proposed.

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arXiv2007

An asteroseismic study of the Delta Scuti star 44 Tau

In this paper we investigate theoretical pulsation models for the delta Scuti star 44 Tau. The star was monitored during several multisite campaigns which confirmed the presence of radial and nonradial oscillations. Moreover, its exceptionally low rotational velocity makes 44 Tau particulary interesting for an asteroseismic study. Due to the measured log g value of 3.6 +/- 0.1, main sequence and post-main sequence models have to be considered. We perform mode identification based on photometric and spectroscopic data. A nonadiabatic pulsation code is used to compute models that fit the identified modes. The influence of different opacity tables and element mixtures on the results is tested. The observed frequencies of 44 Tau can be fitted in both the main sequence and the post-main sequence evolutionary stage. Post-main sequence models are preferable as they fulfill almost all observational constraints (fit of observed frequencies, position in the HRD and instability range). These models can be obtained with normal chemical composition which is in agreement with recent spectroscopic measurements. The efficiency of envelope convection (in the framework of the mixing-length theory) is predicted to be very low in 44 Tau. We show that the results are sensitive to the choice between the OPAL and OP opacities. While the pulsation models of 44 Tau computed with OP opacities are considerably too cool and too faint, the use of OPAL opacities results in models within the expected temperature and luminosity range.

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arXiv2007

The effect of different opacity data and chemical element mixture on the Petersen diagram

The Petersen diagram is a frequently used tool to constrain model parameters such as metallicity of radial double-mode pulsators. In this diagram the period ratio of the radial first overtone to the fundamental mode, P_1/P_0, is plotted against the period of the fundamental mode. The period ratio is sensitive to the chemical composition as well as to the rotational velocity of a star. In the present study we compute stellar pulsation models to demonstrate the sensitivity of the radial period ratio to the opacity data (OPAL and OP tables) and we also examine the effect of different relative abundances of heavy elements. We conclude that the comparison with observed period ratios could be used successfully to test the opacity data.

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arXiv2007

On the Prospects for Detection and Identification of Low-Frequency Oscillation Modes in Rotating B Type Stars

We study how rotation affects observable amplitudes of high-order g- and mixed r/g-modes and examine prospects for their detection and identification. Our formalism, which is described in some detail, relies on a nonadiabatic generalization of the traditional approximation. Numerical results are presented for a number of unstable modes in a model of SPB star, at rotation rates up to 250 km/s. It is shown that rotation has a large effect on mode visibility in light and in mean radial velocity variations. In most cases, fast rotation impairs mode detectability of g-modes in light variation, as Townsend (2003b) has already noted, but it helps detection in radial velocity variation. The mixed modes, which exist only at sufficiently fast rotation, are also more easily seen in radial velocity. The amplitude ratios and phase differences are strongly dependent on the aspect, the rotational velocity and on the mode. The latter dependence is essential for mode identification.

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arXiv2007

The Beta Cephei instability domain for the new solar composition and with new OP opacities

The recent revision of the solar chemical composition (Asplund, Grevesse and Sauval 2005)is characterized by about 40 per cent decrease of C, N, O, Ne, Ar abundances and by 20 percent decrease of Fe and some other metal abundances. We tested the effect of these modifications on the instability of Beta Cephei models. For the opacities, the newest OP data from the Opacity Project (Seaton 2005) were used. We show that the Beta Cephei instability domain in the Hertzsprung-Russel diagram, when computed with new data for Z=0.012 (revised solar value), is very similar to the instability domain computed earlier using the OPAL opacities for the older solar composition with Z=0.02. Almost all observed Beta Cephei variables are located within the instability domain. Two effects are responsible for stronger instability when using the new data: (i) Metal opacity bump in the OP case is located slightly deeper in the star than that in the OPAL case, which results in more effective driving; (ii) at a fixed Z value, the new Fe-group abundances are higher than the older ones because the Z value is determined mainly by the abundances of C, N, 0, and Ne.

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arXiv2007

44 Tau: Discrimination between MS and post-MS models

For the Delta Scuti star 44 Tau 13 independent frequencies have been detected in previous studies. This star is unusual among the Delta Scuti stars because of its very low v sin i value of 2 +/- 1 km/s, indicating that it is either an extremely slow rotator or seen pole-on. 44 Tau shows unusually high amplitudes for a typical nonradially pulsating Delta Scuti star and is believed to be a connecting link between the high-amplitude Delta Scuti stars (HADS) and the low-amplitude Delta Scuti stars. We performed an asteroseimic study to find the appropriate theoretical model for this star. The fact that the radial fundamental and first overtone frequencies have been identified allows for reducing the number of possible solutions significantly. Standard post-main sequence models with inefficient convection in the envelope seem to be preferable in comparison with various models on the main sequence. They fit both observed global parameters of 44 Tau and most of identified radial and nonradial frequencies.

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arXiv2006

Excitation and visibility of slow modes in rotating B-type stars

We use the traditional approximation to describe oscillations with frequencies comparable to the angular rotation rate. Validity of this approximation in application to main-sequence B stars is discussed. Numerical results regarding mode stability and visibility are presented for a model of the Be star HD 163868. For this object, Walker et al.(2005) detected a record number of mode frequencies using data from the small space telescope MOST. Our interpretation of these data differs from that of Walker et al. In particular, we interpret peaks in the lowest frequency range as retrograde g modes. We find instability in a large number of modes that remain undetectable because of unfavourable aspect and/or effect of cancellation. There is no clear preference to excitation of prograde modes.

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arXiv2006

Amplitude variability or close frequencies in pulsating stars? The Delta Scuti star FG Vir

The nature of the observed amplitude variability of several modes in the Delta Scuti star FG Vir is examined. In this star, three frequencies show strong amplitude and phase variations. In the power spectrum, these frequencies also show up as frequency doublets. However, since true amplitude variability of a single frequency can also lead to (false) frequency doublets in the power spectrum, a specific test examining in detail the observed amplitude and phase variations of an assumed single frequency is applied. For the frequencies at 12.15 and 23.40 cycle/day it is shown that amplitude variability of a single mode can be ruled out. In particular, an important property of beating between two modes is fulfilled: the amplitude and phase vary synchronously with a phase shift close to 90 degrees. The origin of the amplitude variability of a third mode, viz. near 19.86 c/d, is not clear due to the long beat period of 20+ years, for which the amplitude/phase test suffers from gaps in the coverage. However, even for this frequency the amplitude variations can be expressed well by a mathematical two-mode model. If we examine these three close frequency pairs together with other (usually more widely separated) close frequencies in FG Vir, 18 pairs of frequencies with separations closer than 0.10 c/d have been detected. It is shown that the majority of the pairs occur near the theoretically expected frequencies of radial modes. Mode identifications are available for only a few modes: the only detected radial mode at 12.15 c/d is part of a close pair. It is shown that accidental agreements between the frequencies of excited modes can be ruled out because of the large number of detected close frequency doublets.

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arXiv2005

Close frequencies in pulsating stars: common and mysterious!

Amplitude and phase variability are commonly found in many different types of pulsating stars. This suggests a common, presently unknown physical origin. We have examined the phenomenon in several Delta Scuti stars with extensive data and find the beating of close frequencies to be responsible. This is demonstrated for the star FG Vir by testing the relationship between the observed amplitude and phase variations. Most close frequency pairs are situated near the observed or theoretically predicted frequencies of radial modes. The large number of detected close frequencies excludes the possibility of accidental frequency agreements.

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arXiv2005

On the nature of small amplitude peaks in $δ$ Scuti oscillation spectra

The standard assumption in interpretation of stellar oscillation spectra from photometry is that the excited mode have low angular degrees, typically $\ell< 3$. Considering the case of FG Vir, the $δ$ Scuti star with the richest known oscillation spectrum, we show that this assumption is not justified for low amplitude peaks. The $\ell<3$ identifications have been found for 12 dominant peaks from pulsation amplitudes and phases. However, we show that for the rest of the peaks (55), whose amplitudes are typically below 1 mmag, much higher $\ell$'s are most likely. We argue that improving amplitude resolution to the micromagnitude level, as expected from space observations, is not likely to be rewarded with a credible mode identifications because the spectra will be dominated by high-$\ell$ modes of unknown azimuthal order, $m$.

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arXiv2005

Constraints on parameters of B-type pulsators from combined multicolour photometry and radial velocity data. I. $β$ Cephei stars

We analyze data on pulsation amplitudes and phases for two $β$ Cephei stars, $δ$ Cet and $ν$ Eri. Strömgren photometry and radial velocity measurements are used simultaneously to obtain constraints on mean parameters of the stars and identification of the excited modes. The inference about the radial mode order and mean star parameters is based on comparison of certain complex parameter, $f$, determined from data, with its counterpart derived from linear nonadiabatic modelling of stellar oscillations. The theoretical $f$ values are very sensitive to the adopted opacity data. In our modelling we rely on the data from OPAL and OP projects. Significant differences were found. New seismic models of $ν$ Eri were constructed with both the OPAL and OP opacities.

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arXiv2005

Cluster AgeS Experiment (CASE): SX Phe stars from the globular cluster Omega Centauri

We present an analysis and interpretation of oscillation spectra for all 69 SX Phoenicis stars discovered in the field of the cluster. For most of the stars we have reliable absolute magnitudes and colors. Except of one, or perhaps two, objects, the stars are cluster members. Their pulsational behaviour is very diversified. Multiperiodic variability with at least part of the excited modes being nonradial is most common but there are also many cases of high amplitude, presumably radial mode, pulsators. In a number of such cases we have evidence for two radial modes being excited. Parameters of radial mode pulsators are in most cases consistent with standard evolutionary models for stars in the mass range 0.9-1.15 Mo. However, in four cases we have evidence that the masses are significantly lower than expected. Three objects show frequency triplets that may be interpreted in terms of rotational frequency splitting of l=1 modes. Implied equatorial velocities of rotation are from 10 to over 100 km/s. Nearly all measured frequencies fall in the ranges predicted for unstable modes. Two cases of low frequency variability are interpreted as being caused by tidal distortion induced by close companions.

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arXiv2005

Inferences from pulsational amplitudes and phases for multimode $δ$ Sct star FG Vir

We combine photometric and spectroscopic data on twelve modes excited in FG Vir to determine their spherical harmonic degrees, $\ell$, and to obtain constraints on the star model. The effective temperature consistent with the mean colours and the pulsation data is about 7200K. In six cases, the $\ell$ identification is unique with above 80 % probability. Two modes are identified as radial. Simultaneously with $\ell$, we determine a complex parameter $f$ which probes subphotospheric stellar layers. Comparing its values with those derived from models assuming different treatment of convection, we find evidence that convective transport in the envelope of this star is inefficient.

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arXiv2004

Various applications of multicolour photometry and radial velocity data for multimode $δ$ Scuti stars

In addition to revealing spherical harmonic degrees, $\ell$, of excited modes, pulsational amplitudes and phases from multicolour photometry and radial velocity data yield a valuable constraints on stellar atmospheric parameters and on subphotospheric convection. Multiperiodic pulsators are of particular interest because each mode yields independent constraints. We present an analysis of data on twelve modes observed in FG Vir star.

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arXiv2004

Asteroseismology of the Beta Cephei star Nu Eridani: Interpretation and applications of the oscillation spectrum

The oscillation spectrum of Nu Eri is the richest known for any variable of the Beta Cephei type. We interpret the spectrum in terms of normal mode excitation and construct seismic models of the star. The frequency data combined with data on mean colours sets the upper limit on the extent of overshooting from the convective core. We use data on rotational splitting of two dipole (l=1) modes (g_1 and p_1) to infer properties of the internal rotation rate. Adopting a plausible hypothesis of nearly uniform rotation in the envelope and increasing rotation rate in the mu-gradient zone, we find that the mean rotation rate in this zone is about three times faster than in the envelope. In our standard model only the modes in the middle part of the oscillation spectrum are unstable. To account for excitation of a possible high-order g-mode at nu = 0.43 c/d as well as p-modes at nu > 6 c/d we have to invoke an overabundance of Fe in the driving zone.

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