Non-adiabatic Pulsation Models Research Articles

KIC 5950759: a high-amplitude δ Sct star with amplitude and frequency modulation near the terminal age main sequence

ABSTRACT Among the intermediate mass pulsating stars known as δ Sct stars is a subset of high-amplitude and predominantly radial-mode pulsators known as high-amplitude δ Sct (HADS) stars. From more than 2000 δ Sct stars observed by the Kepler space mission, only two HADS stars were detected. We investigate the more perplexing of these two HADS stars, KIC 5950759. We study its variability using ground- and space-based photometry, determine its atmospheric parameters from spectroscopy and perform asteroseismic modelling to constrain its mass and evolutionary stage. From spectroscopy, we find that KIC 5950759 is a metal-poor star, which is in agreement with the inferred metallicity needed to reproduce its pulsation mode frequencies from non-adiabatic pulsation models. Furthermore, we combine ground-based WASP and Kepler space photometry, and measure a linear change in period of order $\dot{P}/P \simeq 10^{-6}$ yr−1 for both the fundamental and first overtone radial modes across a time base of several years, which is at least two orders of magnitude larger than predicted by evolution models, and is the largest measured period change in a δ Sct star to date. Our analysis indicates that KIC 5950759 is a metal-poor HADS star near the short-lived contraction phase and the terminal-age main sequence, with its sub-solar metallicity making it a candidate SX Phe star. KIC 5950759 is a unique object among the thousands of known δ Sct stars and warrants further study to ascertain why its pulsation modes are evolving remarkably faster than predicted by stellar evolution.

A New Interpretation of the Period–Luminosity Sequences of Long-period Variables

Abstract Period–luminosity (PL) sequences of long-period variables (LPVs) are commonly interpreted as different pulsation modes, but there is disagreement on the modal assignment. Here, we re-examine the observed PL sequences in the Large Magellanic Cloud, including the sequence of long secondary periods (LSPs), and their associated pulsation modes. First, we theoretically model the sequences using linear, radial, nonadiabatic pulsation models and a population synthesis model of the LMC red giants. Then, we use a semi-empirical approach to assign modes to the pulsation sequences by exploiting observed multi-mode pulsators. As a result of the combined approaches, we consistently find that sequences B and C′ both correspond to first overtone pulsation, although there are some fundamental mode pulsators at low luminosities on both sequences. The masses of these fundamental mode pulsators are larger at a given luminosity than the mass of the first overtone pulsators. These two sequences, B and C′, are separated by a small period interval in which large amplitude pulsation in a long secondary period (sequence D variability) occurs, meaning that the first overtone pulsation is not seen as the primary mode of pulsation. Observationally, this leads to the splitting of the first overtone pulsation sequence into the two observed sequences, B and C′. Our two independent examinations also show that sequences A′, A, and C correspond to third overtone, second overtone, and fundamental mode pulsation, respectively.

Linear nonadiabatic pulsation models of the double-mode Cepheid V371 Per

A recently found double-mode Cepheid with the shortest known period in our Galaxy and abnormal period ratio, V371 Per, is investigated by linear nonadiabatic pulsation models. V371 Per is likely to be crossing the instability strip for the first time, because the mass derived from pulsation models is larger than the evolution mass for the second or higher crossing objects. This result seems to support the conclusion obtained from spectroscopic observation. We also found that models with observed period and period ratio of V371 Per need to have mass and Teff in a narrow range which shifts as heavy element abundance Z changes. We have checked the agreement between Teff ranges estimated observationally and derived from pulsation models using observational Z. We found that those ranges overlap marginally. We need more spectroscopic estimations of Teff and [Fe/H], and more photometric monitoring to estimate the evolutional period change for confirmation of our result.

The fundamental parameters of the Ap star 78 Virginis

Context. Determining the effective temperature of Ap stars, including the roAp stellar pulsators, is a difficult task owing to their strong magnetic field and their related spotted surfaces. It is, however, an important step towards constraining models of their complex atmosphere and testing proposed pulsation excitation mechanisms. Aims. Using the unique angular resolution provided by long-baseline visible interferometry, we aim at deriving accurate angular diameters of a number of Ap targets, so as to determine their unbiased effective temperature (Teff) and their accurate position in the Hertzsprung-Russell diagram, to estimate their mass and age, and to test non-adiabatic pulsation models. Interferometric results on four Ap stars have been published in earlier works. Here we report the results on a fifth, significantly hotter star. Methods. We observed 78 Vir with the visible spectrograph VEGA installed at the combined focus of the CHARA long-baseline optical array. We derived the limb-darkened diameter of this Ap star from our interferometric measurements. Based on photometric and spectroscopic data available in the literature, we estimated the star’s bolometric flux and used it, in combination with its parallax and angular diameter, to determine the star’s luminosity and effective temperature. We then used the derived fundamental parameters to perform a non-adiabatic pulsation analysis. Results. We determined a limb-darkened angular diameter of 0.346 ± 0.006 mas and deduced a linear radius of R = 2.11 ± 0.04 R� . Considering a bolometric flux of 2.73 ± 0.20 10 −7 erg/cm 2 /s we obtained a luminosity of L/L� = 27 ± 2a nd an effective temperature of Teff = 9100 ± 190 K. The non-adiabatic pulsation modeling allows us to predict that high overtone pulsations could be excited in 78 Vir at frequencies ranging from 1.2 to 1.9 mHz, provided that the magnetic field is capable of suppressing envelope convection in the polar regions. Conclusions. Visible long-baseline interferometry is a unique means of deriving accurate fundamental parameters of Ap stars. The Ap star 78 Vir is found to be a promising roAp-star candidate and one that would allow us to extend recent tests on the roAp stars’ excitation mechanism towards the blue edge of the instability strip. Asteroseismic data of this star would, therefore, be of strong interest.

Multisite photometric campaign on the high-amplitude δ Scuti star KIC 6382916

We present results of a multi-site photometric campaign on the high-amplitude $\delta$\,Scuti star KIC\,6382916 in the {\it Kepler} field. The star was observed over a 85-d interval at five different sites in North America and Europe during 2011. {\it Kepler} photometry and ground-based multicolour light curves of KIC\,6382916 are used to investigate the pulsational content and to identify the principal modes. High-dispersion spectroscopy was also obtained in order to derive the stellar parameters and projected rotational velocity. From an analysis of the {\it Kepler} time series, three independent frequencies and a few hundred combination frequencies are found. The light curve is dominated by two modes with frequencies $f_{1}$= 4.9107 and $f_{2}$= 6.4314\,d$^{-1}$. The third mode with $f_{3}$= 8.0350\,d$^{-1}$ has a much lower amplitude. We attempt mode identification by examining the amplitude ratios and phase differences in different wavebands from multicolour photometry and comparing them to calculations for different spherical harmonic degree, $l$. We find that the theoretical models for $f_1$ and $f_2$ are in a best agreement with the observations and lead to value of l = 1 modes, but the mode identification of $f_3$ is uncertain due to its low amplitude. Non-adiabatic pulsation models show that frequencies below 6\,d$^{-1}$ are stable, which means that the low frequency of $f_1$ cannot be reproduced. This is further confirmation that current models predict a narrower pulsation frequency range than actually observed.

An asteroseismic study of the Southern [formula omitted] Cephei star ALS 3721

We present the results of a new investigation aimed to identify the pulsational characteristics of the Southern β Cephei star ALS 3721. Spectroscopic and multicolour photometric data were acquired at the South African Astronomical Observatory (SAAO), South Africa in 2011. Frequency analysis showed that the oscillations of ALS 3721 could be attributed to the two main frequencies with higher significance. Stellar parameters and projected rotational velocity obtained by the spectra were also used to perform photometric mode identification. In order to determine spherical harmonic degrees (l), a principal method was followed by comparing the observed light amplitude ratios in different passbands with those computed from non-adiabatic pulsation models. In general case, therefore, the spherical harmonic degrees corresponding of the frequencies were found in the expected β Cephei range.

Mode identification in the high-amplitude δ Scuti star V2367 Cyg

We report on a multi-site photometric campaign on the high-amplitude $\delta$ Scuti star V2367 Cyg in order to determine the pulsation modes. We also used high-dispersion spectroscopy to estimate the stellar parameters and projected rotational velocity. Time series multicolour photometry was obtained during a 98-d interval from five different sites. These data were used together with model atmospheres and non-adiabatic pulsation models to identify the spherical harmonic degree of the three independent frequencies of highest amplitude as well as the first two harmonics of the dominant mode. This was accomplished by matching the observed relative light amplitudes and phases in different wavebands with those computed by the models. In general, our results support the assumed mode identifications in a previous analysis of Kepler data.

Kepler observations of δ Scuti stars

We analyse observations of 1568 δ Scuti stars in the public archive of the Kepler satellite. We compare the location of these stars in the Hertzsprung–Russell (HR) diagram with that calculated by linear, non-adiabatic pulsation models. There is generally a good agreement and it appears that many of the hotter δ Scuti stars are pulsating in overtones as high as radial order n= 8. Models predict unstable modes of intermediate and high spherical harmonic, l, which are trapped in the envelope. The instability strip for these modes extends well beyond the blue edge for l≤ 4 and should be visible in the Kepler data. However, stars with these predicted properties are not seen. Moreover, we find that the observed frequency range does not agree with the models. Another challenge is to understand why more than half of the stars in the δ Sct instability strip are not pulsating. The distribution of amplitudes argues strongly against the notion that the pulsation amplitudes are below the Kepler detection limit. The mode density of δ Scuti stars is surprisingly low and suggests that modes of a high degree are not common. We do not find any stars with mode densities as high as that found in the CoRoT observations of HD 50844. The periodograms of stars in the same part of the HR diagram vary widely. However, we have identified a group of δ Scuti stars characterized by a single dominant frequency in which a period–luminosity law is present. In many cases the dominant frequency is accompanied by a close frequency of lower amplitude. The problem of low frequencies in hot δ Scuti stars remains unresolved. We confirm that these frequencies have long lifetimes and are not likely to be of stochastic origin. By carefully examining the periodograms of δ Sct stars in the low-frequency regime, we were able to identify a group of δ Sct stars with low-frequency peaks closely resembling those attributed to pure γ Dor stars. These stars lie in the region of the HR diagram where ground-based γ Dor stars are located. None is found hotter than the blue edge of γ Dor pulsation. We suspect that these are genuine γ Dor/δ Sct hybrids.

Oscillations in Young Stellar Objects

AbstractWe report the discovery of periodicities in the light curve of the Herbig Ae star V346 Ori. We interpret these variations as the superposition of at least two signals with periods P1=42±6 min and P2=68±12 min resulting from stellar oscillations. The computation of linear non-adiabatic pulsation models for Pre-Main Sequence (PMS) stars reproduces these periods for a 1.5 M⊙ star with Teff= 7300 K and log L/L⊙= 0.74, pulsating in the fundamental and second overtones.

Non-Adiabatic Quantities in δ Scuti and γ Dor Stars: a Comparison between Observations and Theoretical Non-Adiabatic Models

AbstractPhase lag, ψT and R, a parameter measuring the departure from adiabaticity, can be derived directly from multicolor photometry. The same quantities can also be calculated from theoretical non-adiabatic pulsation models. We present here some preliminary theoretical results which indicate that these quantities depend on the mixing length parameter used to treat convection in the standard ML theory. The comparison favours models with α values between 1 and 1.5Hp. Non-adiabatic models also predict the observed phase lags for the newly discovered α Dor g-mode pulsating stars.

Beat cepheid period ratios from opal opacities.

view Abstract Citations (19) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Beat cepheid period ratios from opal opacities. Morgan, Siobahn M. ; Welch, Douglas L. Abstract The discovery of a large number of beat Cepheids in the Large Magellanic Cloud in the MACHO survey, provides an opportunity to compare the characteristics of such Cepheids over a range of metallicities. We produced a large grid of linear nonadiabatic pulsation models using the OPAL opacity tables and with compositions corresponding to those of the Milky Way, and the Large and Small Magellanic Clouds. Using the relationship between the period ratio and the main pulsation period, we are able to define a range of models which correspond to the observed beat Cepheids, and thereby constrain the physical characteristics of the LMC beat Cepheids. We are also able to make some predictions about the nature of the yet-to-be-discovered SMC beat Cepheids. Publication: The Astronomical Journal Pub Date: September 1997 DOI: 10.1086/118550 arXiv: arXiv:astro-ph/9607068 Bibcode: 1997AJ....114.1183M Keywords: CEPHEIDS; STARS: OSCILLATIONS; Astrophysics E-Print: 20 pages, 6 embedded Postscript figures, uses aasms4.sty. Accepted for publication in the 1997 September AJ full text sources arXiv | ADS | data products SIMBAD (2) NED (2)

Theoretical growth rates, periods, and pulsation constants for long-period variables

view Abstract Citations (195) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Theoretical growth rates, periods, and pulsation constants for long-period variables. Fox, M. W. ; Wood, P. R. Abstract An extensive set of linear, nonadiabatic pulsation models for red giant and supergiant stars is computed, in order that the dependence of pulsation periods (P), pulsation constants (Q), and growth rate on physical input parameters can be determined from the systematic behavior seen in the models. Also investigated is the extent of the dependence of P, Q, and growth rate on uncertain quantities such as atmospheric molecular opacity, surface boundary conditions, and effective temperature. The growth rate for the fundamental mode is found to increase with luminosity on the giant branch while the growth rate for the first overtone decreases. Dynamical instabilities found in previous adiabatic models of extreme red giants do not occur when nonadiabatic effects are included in the models. Publication: The Astrophysical Journal Pub Date: August 1982 DOI: 10.1086/160160 Bibcode: 1982ApJ...259..198F Keywords: Long Term Effects; Pulse Duration; Red Giant Stars; Stellar Oscillations; Supergiant Stars; Variable Stars; Linear Equations; Magellanic Clouds; Mass To Light Ratios; Propagation Modes; Stellar Models; Astrophysics full text sources ADS | data products SIMBAD (2)