scholarly journals Nonlinear Models of Miras, Including Time-Dependent Convection

1993 ◽  
Vol 139 ◽  
pp. 208-208
Author(s):  
Dale A. Ostlie ◽  
Arthur N. Cox
Keyword(s):  
Effective Temperature ◽  
Fundamental Mode ◽  
Chaotic Behavior ◽  
Nonlinear Models ◽  
Variable Stars ◽  
Time Dependent ◽  
Phase Lag ◽  
Spatial Averaging ◽  
Mira Variable ◽  
Turbulent Pressure

AbstractNonlinear calculations of Mira variable stars of Population I are presented. Each model is 1 M⊙, with a luminosity of 5000 L⊙ and an effective temperature near 3000 K. These models incorporate our theory of time-dependent convection, which is based on a convective phase lag formalism and includes spatial averaging of convective eddies from adjacent zonal interfaces. The theory also includes turbulent pressure, energy, and viscosity terms and allows for negative convective luminosities in subadiabatic regions where overshooting occurs.Results of the present study suggest that based upon the dynamic behavior of the models, fundamental mode pulsations are the preferred mode of oscillation. In particular, we do not obtain the chaotic behavior that has been noted in previous nonlinear studies of the fundamental mode oscillations of Miras.

scholarly journals Spectroscopic properties of a two-dimensional time-dependent Cepheid model

2018 ◽  
Vol 611 ◽  
pp. A19 ◽  
Author(s):  
V. Vasilyev ◽  
H.-G. Ludwig ◽  
B. Freytag ◽  
B. Lemasle ◽  
M. Marconi
Keyword(s):  
Effective Temperature ◽  
Spectroscopic Properties ◽  
Variable Stars ◽  
Time Dependent ◽  
Surface Gravity ◽  
Current Standard ◽  
Static Approximation ◽  
Cepheid Variables ◽  
Microturbulent Velocity ◽  
1D Model

Context. Standard spectroscopic analyses of variable stars are based on hydrostatic 1D model atmospheres. This quasi-static approach has not been theoretically validated. Aim. We aim at investigating the validity of the quasi-static approximation for Cepheid variables. We focus on the spectroscopic determination of the effective temperature Teff, surface gravity log g, microturbulent velocity ξt, and a generic metal abundance log A, here taken as iron.Methods. We calculated a grid of 1D hydrostatic plane-parallel models covering the ranges in effective temperature and gravity that are encountered during the evolution of a 2D time-dependent envelope model of a Cepheid computed with the radiation-hydrodynamics code CO5BOLD. We performed 1D spectral syntheses for artificial iron lines in local thermodynamic equilibrium by varying the microturbulent velocity and abundance. We fit the resulting equivalent widths to corresponding values obtained from our dynamical model for 150 instances in time, covering six pulsational cycles. In addition, we considered 99 instances during the initial non-pulsating stage of the temporal evolution of the 2D model. In the most general case, we treated Teff, log g, ξt, and log A as free parameters, and in two more limited cases, we fixed Teff and log g by independent constraints. We argue analytically that our approach of fitting equivalent widths is closely related to current standard procedures focusing on line-by-line abundances.Results. For the four-parametric case, the stellar parameters are typically underestimated and exhibit a bias in the iron abundance of ≈−0.2 dex. To avoid biases of this type, it is favorable to restrict the spectroscopic analysis to photometric phases ϕph ≈ 0.3…0.65 using additional information to fix the effective temperature and surface gravity.Conclusions. Hydrostatic 1D model atmospheres can provide unbiased estimates of stellar parameters and abundances of Cepheid variables for particular phases of their pulsations. We identified convective inhomogeneities as the main driver behind potential biases. To obtain a complete view on the effects when determining stellar parameters with 1D models, multidimensional Cepheid atmosphere models are necessary for variables of longer period than investigated here.

scholarly journals Irregularity Interpreted as Low Dimension Chaos for Convective Models of W VIR Variables

1989 ◽  
Vol 111 ◽  
pp. 267-267
Author(s):  
S. Ami Glasner ◽  
J. Robert Buchler
Keyword(s):  
Effective Temperature ◽  
Control Parameter ◽  
Chaotic Behavior ◽  
Time Dependent ◽  
Special Cases ◽  
Low Dimension ◽  
Rv Tau Stars ◽  
Linear And Nonlinear ◽  
Stellar Envelopes ◽  
Irregular Behavior

AbstractLinear and nonlinear pulsational properties of convective stellar envelopes relevant for W Vir and RV Tau stars are surveyed. All models show the same trend to pass from regular to irregular behavior when a control parameter is changed (the effective temperature). The transition to irregular pulsation follows well known systematic routes to chaos (as in the radiative case). Some rich structures were found in special cases; they deserve further research. We show that the chaotic behavior is sustained even when convection is taken into account. The effect of the inclusion of time dependent convection shows up mostly as a shift of Kovacs and Buchler (Ap.J 1988) results in the parameters plane (L,Teff) towards more realistic models.

scholarly journals Detecting shock waves in non-fundamental mode RR Lyrae using large sample of spectra in SDSS and LAMOST

2020 ◽  
pp. 181-186
Author(s):  
Xiao-Wei Duan ◽  
Xiao-Dian Chen ◽  
Li-Cai Deng ◽  
Fan Yang ◽  
Chao Liu ◽  
...  
Keyword(s):  
Shock Waves ◽  
Fundamental Mode ◽  
Variable Stars ◽  
Ionization Zone ◽  
Rr Lyrae Stars ◽  
Hydrogen Emission ◽  
Emission Signal ◽  
Rr Lyrae ◽  
Hydrogen Emission Lines ◽  
Blazhko Effect

Steps toward the nature inside RR Lyrae variables can not only improve our understanding of variable stars but also innovate the precision when we use them as tracers to map the structure of the universe. In this work, we develop a hand-crafted one-dimensional pattern recognition pipeline to fetch out the "first apparitions", the most prominent observational characteristic of shock. We report the first detection of hydrogen emission lines in the first-overtone and multi-mode RR Lyrae variables. We find that there is an anti-correlation between the intensity and the radial velocity of the emission signal, which is possibly caused by opacity changing in the helium ionization zone. Moreover, we find one RRd star with hydrogen emission that possibly shows Blazhko-type modulations. According to our discoveries, with an enormous volume of upcoming data releases of variable stars and spectra, it may become possible to build up the bridge between shock waves and big problems like the Blazhko effect in non-fundamental mode RR Lyrae stars.

scholarly journals About the existence of warm H-rich pulsating white dwarfs

2019 ◽  
Vol 633 ◽  
pp. A20 ◽  
Author(s):  
Leandro G. Althaus ◽  
Alejandro H. Córsico ◽  
Murat Uzundag ◽  
Maja Vučković ◽  
Andrzej S. Baran ◽  
...  
Keyword(s):  
Transition Zone ◽  
Effective Temperature ◽  
Observational Evidence ◽  
Time Dependent ◽  
Theoretical Studies ◽  
Ionization Zone ◽  
Transition Zones ◽  
Element Diffusion ◽  
Effective Temperatures ◽  
Thermal Pulses

Context. The possible existence of warm (Teff ∼ 19 000 K) pulsating DA white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical studies more than 30 yr ago. These studies reported the occurrence of g-mode pulsational instabilities due to the κ mechanism acting in the partial ionization zone of He below the H envelope in models of DA WDs with very thin H envelopes (MH/M⋆ ≲ 10−10). However, to date, no pulsating warm DA WD has been discovered, despite the varied theoretical and observational evidence suggesting that a fraction of WDs should be formed with a range of very low H content. Aims. We re-examine the pulsational predictions for such WDs on the basis of new full evolutionary sequences. We analyze all the warm DAs observed by the TESS satellite up to Sector 9 in order to search for the possible pulsational signal. Methods. We computed WD evolutionary sequences of masses 0.58 and 0.80 M⊙ with H content in the range −14.5 ≲ log(MH/M⋆)≲ − 10, appropriate for the study of pulsational instability of warm DA WDs. Initial models were extracted from progenitors that were evolved through very late thermal pulses on the early cooling branch. We use LPCODE stellar code into which we have incorporated a new full-implicit treatment of time-dependent element diffusion to precisely model the H–He transition zone in evolving WD models with very low H content. The nonadiabatic pulsations of our warm DA WD models were computed in the effective temperature range of 30 000 − 10 000 K, focusing on ℓ = 1 g modes with periods in the range 50 − 1500 s. Results. We find that traces of H surviving the very late thermal pulse float to the surface, eventually forming thin, growing pure H envelopes and rather extended H–He transition zones. We find that such extended transition zones inhibit the excitation of g modes due to partial ionization of He below the H envelope. Only in the cases where the H–He transition is assumed much more abrupt than predicted by diffusion do models exhibit pulsational instability. In this case, instabilities are found only in WD models with H envelopes in the range of −14.5 ≲ log(MH/M⋆)≲ − 10 and at effective temperatures higher than those typical for ZZ Ceti stars, in agreement with previous studies. None of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Conclusions. Our study suggests that the nondetection of pulsating warm DAs, if WDs with very thin H envelopes do exist, could be attributed to the presence of a smooth and extended H–He transition zone. This could be considered as indirect proof that element diffusion indeed operates in the interior of WDs.

scholarly journals The ASAS-SN catalogue of variable stars – VII. Contact binaries are different above and below the Kraft break

2020 ◽  
Vol 493 (3) ◽  
pp. 4045-4057 ◽  
Author(s):  
T Jayasinghe ◽  
K Z Stanek ◽  
C S Kochanek ◽  
B J Shappee ◽  
M H Pinsonneault ◽  
...  
Keyword(s):  
Effective Temperature ◽  
Type Systems ◽  
Variable Stars ◽  
Log P ◽  
Early Type ◽  
Late Type ◽  
V Band ◽  
Contact Binaries ◽  
Clean Break ◽  
Two Populations

ABSTRACT We characterize ${\sim } 71\, 200$ W Ursae Majoris (UMa) type (EW) contact binaries, including ${\sim } 12\, 600$ new discoveries, using All-Sky Automated Survey for SuperNovae (ASAN-SN)V-band all-sky light curves along with archival data from Gaia, 2MASS, AllWISE, LAMOST, GALAH, RAVE, and APOGEE. There is a clean break in the EW period–luminosity relation at $\rm \log (\it P/{\rm d})\,{\simeq }\,{\rm -0.30}$, separating the longer period, early-type EW binaries from the shorter period, late-type systems. The two populations are even more cleanly separated in the space of period and effective temperature, by $T_{\rm eff}=6710\,{\rm K}-1760\,{\rm K}\, \log (P/0.5\,{\rm d})$. Early-type and late-type EW binaries follow opposite trends in Teff with orbital period. For longer periods, early-type EW binaries are cooler, while late-type systems are hotter. We derive period–luminosity relationships in the WJK, V, Gaia DR2 G, J, H, Ks, and W1 bands for the late-type and early-type EW binaries separated by both period and effective temperature, and by period alone. The dichotomy of contact binaries is almost certainly related to the Kraft break and the related changes in envelope structure, winds, and angular momentum loss.

scholarly journals The period–luminosity relation of Mira variable stars

2012 ◽  
Vol 8 (S289) ◽  
pp. 217-217
Author(s):  
Akiharu Nakagawa ◽  
Tomoharu Kurayama ◽  
Toshihiro Omodaka ◽  
Tatsuya Kamezaki ◽  
Yoshiro Nishida ◽  
...  
Keyword(s):  
Very Long Baseline Interferometry ◽  
Variable Stars ◽  
Distance Measurement ◽  
Current Status ◽  
Long Baseline ◽  
Mira Variables ◽  
Mira Variable ◽  
Interferometry Method

AbstractWe use astrometry to measure the distances to Galactic Mira variable stars. Our purpose is to determine a precise period–luminosity relation (PLR). At present, we do not have a precise PLR for Galactic Mira stars because of the large uncertainties affecting their distance estimates. To reduce the uncertainties, we adopted the Very Long Baseline Interferometry method and measured annual parallaxes of Mira variable stars with VERA. In addition to our previous results, we obtained three new distances for Mira variable stars. Based on our observations, the typical uncertainty in a given distance measurement is reduced to below 10%. At this conference, we present the current status of our project. To establish a precise Galactic PLR, we continue to observe more Mira variables. In addition, the apparent magnitudes of the target stars should be studied carefully.

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