51 Matching Results

Search Results

Advanced search parameters have been applied.

Delta Scuti stars: Theory

Description: The purpose of asteroseismology is not only to derive the internal structure of individual stars from their observed oscillation frequencies, but also to test and extend one`s understanding of the physics of matter under the extremes of temperature, density, and pressure found in stellar interiors. In this review, the author hopes to point out what one can learn about the Sun by studying {delta} Scuti stars, as well as what one can learn about stars more massive or evolved than the Sun. He discusses some of the difficulties in theoretical approaches to asteroseismology for {delta} Scuti stars, using FG Vir, {delta} Scuti, and CD-24{degree} 7599 as examples.
Date: March 1, 1998
Creator: Guzik, J.A.
Partner: UNT Libraries Government Documents Department

Pulsating variable stars in the MACHO bulge database: the semiregular variables

Description: We review the pulsating stars contained in the top 24 fields of the MACHO bulge database, with special emphasis on the red semireg-ular stars. Based on period, amplitude and color cuts, we have selected a sample of 2000 semireguku variables with 15 < P < 100 days. Their period-luminosity relation is studied, as well ss their spatial distribution. We find that they follow the bar, unlike the RR Lyrae in these fields.
Date: November 1, 1997
Creator: Minniti, D.; Alcock, C. & Allsman, R.A.
Partner: UNT Libraries Government Documents Department

Linear pulsations of strange modes in LBVs

Description: Outbursts of the luminous blue variables have been studied for a long time, but a detailed understanding of the mechanism has eluded astronomers. In the last few years it has been recognized that the dramatic increase in outburst brightness is due almost entirely to the luminosity being shifted into the visual band, rather than a true luminosity increase. Some ideas about how these very massive and very luminous stars might display their dramatic increase of visual brightness have been given by many. A sampling is given here. The strange modes we consider in this paper have been studied by the G{umlt o}ttingen group under Fricke and Glatzel. We are interested in strange modes because some are very rapidly growing when conditions are right, and amplitudes reach large radial velocity (200 km/s) and luminosity (0.1 mag). Then the radiative luminosity in deep layers can surpass the Eddington limit during each pulsation cycle, and outbursts occur.
Date: December 31, 1996
Creator: Cox, A.N.; Guzik, J.A. & Soukup, M.S.
Partner: UNT Libraries Government Documents Department

The effects of metallicity on {delta} Scuti star asteroseismology

Description: In {delta} Scuti star seismology, most researchers use evolution and pulsation models assuming a solar element mixture and Z = 0.02 for preliminary determinations of stellar masses or evolutionary state from observed frequencies. Here the authors investigate the consequences of this assumption by considering the effects of metallicity changes in the models on their inferences of the internal structure of {delta} Scuti stars. They use the main-sequence {delta} Scuti star FG Vir, and the more evolved shell hydrogen burning star {delta} Scuti to illustrate their results.
Date: March 1, 1998
Creator: Guzik, J.A.; Bradley, P.A. & Templeton, M.R.
Partner: UNT Libraries Government Documents Department

A nonlinear study of luminous blue variables and possible outbursts

Description: Linear pulsation analysis of luminous blue variable models shows instability to pulsations in multiple radial and nonradial strange modes (see Glatzel, these proceedings). These modes have large linear growth rates, sometimes exceeding several hundred percent per period, which prompted us to investigate the nonlinear behavior of envelope models. While the nonradial modes are predicted in the linear analysis to have higher growth rates than the radial modes, nonlinear nonradial pulsations are beyond the capabilities of pulsation hydrodynamics codes developed to date. As for relevant radial nonlinear calculations, Stothers & Chin (1993) report briefly on nonlinear hydrodynamic calculations of one dynamically unstable massive star envelope model. Aikawa & Sreenivasan (1996) have done nonlinear oscillation modeling of strange modes in low-mass AGB stars. Kiriakidis et al. (these proceedings) present nonlinear models (not including convection) of two types of strange-mode pulsators, massive stars and Wolf-Rayet stars. They find periodic or irregular pulsations, and suggest that pulsation drives mass loss. Here we present new nonlinear hydrodynamic calculations to explore the link between strange-mode pulsations and LBV outbursts.
Date: December 31, 1996
Creator: Guzik, J.A.; Cox, A.N.; Despain, K.M. & Soukup, M.S.
Partner: UNT Libraries Government Documents Department

RR Lyrae stars in the MACHO database

Description: The MACHO Project has catalogued {approximately} 8000 RR Lyrae stars in the Large Magellanic Cloud, {approximately} 180 in the Galactic bulge, and {approximately} 50 in the Sgr dwarf galaxy. These variables are excellent distance indicators, and are used as tools to study the structure of the Large Magellanic Cloud and the bulge. The large datasets also probe uncommon pulsation modes. A number of double-mode RR Lyrae stars (RRd) are found in the Large Magellanic Cloud sample. These stars provide important clues for understanding the formation and evolution of the inner Galaxy, the Large Magellanic Cloud and the Sgr dwarf galaxy. A large number of second overtone pulsators (RRe) are found in the LMC and bulge. Finally, the RR Lyrae belonging to the Sgr dwarf yield an accurate distance to this galaxy. Their presence also alerts us of the very interesting possibility of distant sources for bulge microlensing effects. 49 refs., 7 figs., 1 tab.
Date: October 1, 1996
Creator: Minniti, D.; Alcock, C; Alves, D. R.; Axelrod, T. S.; Bennett, D. P.; Cook, K. H. et al.
Partner: UNT Libraries Government Documents Department

Pulsations and outbursts of luminous blue variables

Description: We propose an outburst mechanism for the most luminous stars in our and other galaxies. These million solar luminosity stars, with masses (after earlier mass loss) of between 20 and maybe 70 solar masses, are pulsationally unstable for both radial and low-degree nonradial modes. Some of these modes are ``strange,`` meaning mostly that the pulsations are concentrated near the stellar surface and have very rapid growth rates in linear theory. The pulsation driving is by both the high iron line opacity (near 150,000 K) and the helium opacity (near 30,000 K) kappa effects. Periods range from 5 to 40 days. Depending on the composition, pulsations periodically produce luminosities above the Eddington limit for deep layers. The radiative luminosity creates an outward push that readily eases the very low gamma envelope to very large outburst radii. A key point is that a super-Eddington luminosity cannot be taken up by the sluggish convection rapidly enough to prevent an outward acceleration of much of the envelope. As the helium abundance in the envelope stellar material increases by ordinary wind mass loss and the luminous blue variable outbursts, the opacity in the deep pulsation driving layers decreases. This makes the current Eddington luminosity even higher so that pulsations can then no longer give radiative luminosities exceeding the limit. For the lower mass and luminosity luminous blue variables there is considerably less iron line opacity driving, and pulsations are almost all caused by the helium ionization kappa effect.
Date: June 1, 1997
Creator: Cox, A.N.; Guzik, J.A.; Soukup, M.S. & Despain, K.M.
Partner: UNT Libraries Government Documents Department

Radial and nonradial periods and growth rates of an AI Velorum model

Description: Walraven, Walraven, and Balona recently discovered several new periodicities in addition to the well-known fundamental and first overtone periods of the high-amplitude {delta} Scuti star AI Velorum. Linear nonadiabatic pulsation calculations were performed for an AI Velorum model of mass 1.96 M{sub {circle dot}}, 24.05 L{sub {circle dot}}, and T{sub eff}7566 K for the radial and low-degree nonradial modes to help verify the tentative identifications made by Walraven, et al. Comparison of the calculated periods with the observations suggests some alternatives to the identifications proposed by Walraven, et al.
Date: January 1, 1992
Creator: Guzik, J.A.
Partner: UNT Libraries Government Documents Department

Nonlinear Pulsation Modeling of Luminous Blue Variables

Description: Using an updated version of the Ostlie and Cox (1993) nonlinear hydrodynamics code, we show the results of Luminous Blue Variable (LBV) envelope models based on evolution models of initial mass 50-80 M solar. including mass loss. The models use OPAL opacities, contain 60-120 Lagrangian zones, include time dependent convection, and are given an initial photospheric radial velocity amplitude of 1 km/sec. Our goal is to explain the reason for the LBV instability strip and suggest a cause for LBV outbursts observed in massive stars in our Galaxy as well as the LMC and SMC.
Date: December 31, 1997
Creator: Despain, Kate M.; Guzik, Joyce A. & Cox, Arthur N.
Partner: UNT Libraries Government Documents Department

Nonradial instability strips for post-AGB stars

Description: We test several pre-degenerate (PNN and DO) and degenerate (DB) models for stability against nonradial oscillations. These models lie on the 0.6 M{sub {circle dot}} evolutionary track calculated by Iben. The post-AGB stars have a residual CO core with only a little surface hydrogen and helium. In order to match all the observed pulsators. We use three different surface compositions for the DO stars, and a pure helium surface for the DB white dwarfs. We find 3 DO and 1 DB instability strips that we compare to the available observations. 16 refs., 1 fig.
Date: January 1, 1990
Creator: Stanghellini, L. (Osservatorio Astronomico di Bologna (Italy)); Cox, A.N. (Los Alamos National Lab., NM (USA)) & Starrfield, S.G. (Arizona State Univ., Tempe, AZ (USA). Dept. of Physics and Astronomy Los Alamos National Lab., NM (USA))
Partner: UNT Libraries Government Documents Department

Nonlinear pulsation masses

Description: The advent of nonlinear pulsation theory really coincides with the development of the large computers after the second world war. Christy and Stobbie were the first to make use of finite difference techniques on computers to model the bumps'' observed in the classical Cepheid light and velocity curves, the so-called Hertzsprung'' sequence. Following this work a more sophisticated analysis of the light and velocity curves from the models was made by Simon and Davis using Fourier techniques. Recently a simpler amplitude equation formalism has been developed that helps explain this resonance mechanism. The determination of Population I Cepheid masses by nonlinear methods will be discussed. For the lower mass objects, such as RR Lyrae and BL Her. stars, we find general agreement using evolutionary masses and nonlinear pulsation theory. An apparent difficulty of nonlinear pulsation theory occurs in the understanding of double'' mode pulsation, which will also be discussed. Recent studies in nonlinear pulsation theory have dealt with the question of mode selection, period doubling and the trends towards chaotic behavior such as is observed in the transition from W Virginis to RV Tauri-like stars. 10 refs., 1 fig., 2 tabs.
Date: January 1, 1990
Creator: Davis, C.G.
Partner: UNT Libraries Government Documents Department

Light curves for ''bump Cepheids'' computed with a dynamically zoned pulsation code

Description: The dynamically zoned pulsation code developed by Castor, Davis, and Davison has been used to recalculate the Goddard model and to calculate three other Cepheid models with the same period (9.8 days). This family of models shows how the bumps and other features of the light and velocity curves change as the mass is varied at constant period. This study, with a code that is capable of producing reliable light curves, shows again that the light and velocity curves for 9.8-day Cepheid models with standard homogeneous compositions do not show bumps like those that are observed unless the mass is significantly lower than the ''evolutionary mass.'' The light and velocity curves for the Goddard model presented here are similar to those computed independently by Fischel, Sparks, and Karp. They should be useful as standards for future investigators.
Date: January 1, 1978
Creator: Adams, T.F.; Castor, J.E. & Davis, C.G.
Partner: UNT Libraries Government Documents Department

Nonradial pulsations of hot evolved stars

Description: There are three classes of faint blue variable stars: the ZZ Ceti variables (DAV degenerate dwarfs), the DBV variables (DB degenerate dwarfs), and the GW Vir variables (DOV degenerate dwarfs). None of these classes of variable stars were known at the time of the last blue star meeting. Observational and theoretical studies of the ZZ Ceti variables, the DBV variables, and the GW Vir variables have shown them to be pulsating in nonradial g-modes. The cause of the pulsation has been determined for each class of variable star and, in all cases, also involves predictions of the stars envelope composition. The predictions are that the ZZ Ceti variables must have pure hydrogen surface layers, the DBV stars must have pure helium surface layers, and the GW Vir stars must have carbon and oxygen rich surface layers with less than 30% (by mass) of helium. Given these compositions, it is found that pulsation driving occurs as a result of the kappa and gamma effects operating in the partial ionization zones of either hydrogen or helium. In addition, a new driving mechanism, called convection blocking, also occurs in these variables. For the GW Vir variables, it is the kappa and gamma effects in the partial ionization regions of carbon and oxygen. 45 refs.
Date: January 1, 1987
Creator: Starrfield, S.G.
Partner: UNT Libraries Government Documents Department

Double-mode pulsation

Description: Double mode pulsation is a very pervasive phenomenon in stars all over the Hertzsprung-Russell diagram. In order of increasing radius, examples are: ZZ Ceti stars, the sun, the delta Scuti stars, RR Lyrae variables, the ..beta.. Cephei variables and those related to them, Cepheids, and maybe even the Mira stars. These many modes have been interpreted as both radial and nonradial modes, but in many cases the actual mode has not been clearly identified. Yellow giants seem to be the most simple pulsators with a large majority of the RR Lyrae variables and Cepheids showing only one pulsation period. We limit this review to those very few cases for classical Cepheids and RR Lyrae variables which display two modes. For these we know many facts about these stars, but the actual cause of the pulsation in two modes simultaneously remains unknown.
Date: July 30, 1982
Creator: Cox, A.N.
Partner: UNT Libraries Government Documents Department

Application of two-time methods in stellar pulsations

Description: The method of two-time expansions is extended to include the effects of the outer, non-adiabatic layers in stellar pulsation. The evolution of pulsating stellar models can be examined, including the approach to limit cycle behavior. The method is demonstrated by a calculation of the eigenvectors for a ..beta.. Cepheid model. It is argued that the method is promising as a practical tool for treating the approach to limiting oscillations, as well as resonant and multimode behavior.
Date: July 28, 1982
Creator: Pesnell, W.D.; Regev, O. & Buchler, J.R.
Partner: UNT Libraries Government Documents Department

Using nonradial pulsations to determine the envelope composition of very evolved stars

Description: Recent observational and theoretical studies of the ZZ Ceti variables (DA degenerate dwarfs), the DBV variables (DB degenerate dwarfs), and the GW Vir variables (DO degenerate dwarfs) have shown them to be pulsating in nonradial g/sup +/-modes. The pulsation mechanism has been identified for each class of variable star and, in all cases, involves predictions of the stars envelope composition. The ZZ Ceti variables must have pure hydrogen surface layers, the DBV stars must have pure helium surface layers, and the GW Vir stars must have carbon and oxygen rich surface layers. 44 refs.
Date: July 7, 1986
Creator: Starrfield, S.
Partner: UNT Libraries Government Documents Department

Some masses for population I and II Cepheids

Description: The masses of Cepheids can be obtained in several ways. If a Cepheid luminosity is known from membership in a galactic cluster, the mass-luminosity relation obtained from stellar evolution theory gives its mass. This evolution mass depends slightly on the composition, that is, the mass fraction of helium, Y, and on the mass fraction of all the heavier elements, Z, but the composition dependence is small.
Date: January 1, 1984
Creator: Kidman, R.B. & Cox, A.N.
Partner: UNT Libraries Government Documents Department

Nonlinear RR Lyrae models with new Livermore opacities

Description: A.N. Cox recently showed that a 20% opacity decrease in the 20,000--30,000 K region as indicated by the new Livermore OPAL opacities reconciles the discrepancy between pulsation and evolution masses of double-mode RR Lyrae variables. Nonlinear hydrodynamic calculations were performed for RR Lyrae models of mass 0l75 M{circle dot}, 51 L{circle dot}, and Z=0.0001 including this opacity decrease. The Stellingwerf periodic relaxation method was used to converge the models to a limit cycle, and the Floquet matrix eigenvalues calculated to search for a tendency of the fundamental mode to grow from the full-amplitude overtone solution, and the overtone mode to grow from the full-amplitude fundamental solution, thereby predicting double-mode behavior. Models of T{sup eff} < 7000 K with the opacity decrease have positive fundamental-mode growth rates in the overtone solution, in contrast to earlier results by Hodson and Cox, and models with T{sub eff} < 7000 have positive 1st overtone growth rates in the fundamental-mode behavior was not found.
Date: January 1, 1992
Creator: Guzik, J.A. & Cox, A.N.
Partner: UNT Libraries Government Documents Department

The importance of radiative transfer in stellar pulsation models

Description: With the advent of the new astrophysical opacities it seems appropriate to discuss the need for a full radiative transfer (RT) theory instead of the usual equilibrium diffusion theory used in most nonlinear pulsation codes. Early studies on the importance of RT in the calculation of light curves for Cepheid models showed little effect over diffusion theory. The new opacities though may help to explain the bump'' mass discrepancy problem. For RR Lyrae models the use of RT theory causes some effects both in the color differences (U-B) as well as the light curves. New opacities help to explain the period ratios for double mode RR Lyrae and beat Cepheids. A new area of research is in the modeling of stars with high luminosity to mass ratios that show tendencies for doubling and transitions to chaos, such as W Virginis and RV Tauri stars. For these stars it has been shown the RT is necessary in calculating their light curves and that the understanding of the shock dynamics depends on the transfer of lines in the pulsating RT dependent atmospheres.
Date: January 1, 1992
Creator: Davis, C.G.
Partner: UNT Libraries Government Documents Department

Pulsations of B star models by an opacity mechanism

Description: The pulsation mechanism for B stars has been sought for 30 years. No proposed radial or nonradial mechanism, either deeply seated or in the surface layers, has been successful in explaining all the observational details. Perhaps the missing piece in the puzzle is the opacity of the stellar material. Many times the first author has tried to make unconventional surface compositions give instability, but none were ever found. We now propose that the sudden appearance of a tremendous number of iron lines, as the temperature rises above about 150,000 K, gives a high sensitivity of the opacity to temperature at the very low densities found in these blue giants. Opacities need to increase quickly to a factor of three or more above the Cox-Tabor (1976) values in the range around 200,000 K. These increases are the same needed to decrease theoretical period ratios of double-mode Cepheids and {delta} Scuti variables to agree better with observations for conventional yellow giant masses. The reason why not all B stars pulsate is that a slight primordial deficit in the iron abundance in the surface layer (1 {times} 10{sup {minus}6} of the mass) can reduce the opacity and its sensitivity to temperature. A slight amount of iron concentration by radiative levitation could make a star pulsate even if it did not originally have enough primordial iron to cause this opacity mechanism to operate. Then any slow slight mixing caused by the unstable nonradial pulsations could restabilize the pulsations as actually observed in {alpha} Vir and {beta} CMa. Rapid levitation and mixing for the very luminous B stars with their very low density envelopes could even explain the puzzling luminous blue variables with this standard {kappa} mechanism. Large amplitude pulsations like those seen in BW Vul would indicate a somewhat larger iron abundance compared to ...
Date: January 1, 1990
Creator: Cox, A.N. (Los Alamos National Lab., NM (USA)) & Morgan, S.M. (Washington Univ., Seattle, WA (USA). Dept. of Astronomy)
Partner: UNT Libraries Government Documents Department

Cepheid masses for models with enhanced opacities

Description: Cepheid models with enhanced opacity are constructed and used to determine pulsation constants, Q{sub 0}. These are compared to models without an enhanced opacity. Methods of determining Cepheid masses are investigated using the different models and are compared to results obtained by Gieren. The methods to determine the pulsation and Wesselink masses by ourselves and Gieren differ significantly, due to the use of a Q{sub 0} that varies with mass, radius and luminosity. 14 refs., 1 fig., 1 tab.
Date: January 1, 1990
Creator: Morgan, S.M. (Washington Univ., Seattle, WA (USA). Dept. of Astronomy) & Cox, A.N. (Los Alamos National Lab., NM (USA))
Partner: UNT Libraries Government Documents Department

Reminiscence of a stellar pulsation theorist

Description: The author started working in stellar pulsation theory in 1965. Since this time, we have studied the effects of improved radiative transfer and dynamic zoning on models of Cepheids, RR Lyrae, and W Virginis stars. In this paper we discuss the relevant equations and some results in comparisons to observations of W Virginis and long-period Cepheids. Some suggestions for the next generation of stellar pulsation codes is given. 3 refs., 7 figs.
Date: January 1, 1989
Creator: Davis, C.G.
Partner: UNT Libraries Government Documents Department