RR Lyrae - Theory vs Observation

The luminosities, effective temperatures and metallicities that are derived empirically by Kovacs and Jurcsik from the light curves of a large number of globular cluster and field RRab and RRc stars are compared to theoretical RR Lyrae models. The strong luminosity dependence of the empirical blue and red edges (Log L vs Log Teff diagram) is in disagreement with that of both radiative and convective models. A reexamination of the theoretical uncertainties in the modelling leads us to conclude that the disagreement is irreconcilable.Comment: 6 pages, 5 figures (revised april 2000, revisions relatively minor

Turbulent Convection in the Classical Variable Stars

We give a status report of convective Cepheid and RR Lyrae model pulsations. Some striking successes can be reported, despite the use of a rather simple treatment of turbulent convection with a 1D time-dependent diffusion equation for the turbulent energy. It is now possible to obtain stable double-mode (beat) pulsations in both Cepheid and RR Lyrae models with astrophysical parameters, i.e. periods and amplitude ratios, that are in agreement with observations. The turbulent convective models, however, have difficulties giving global agreement with the observations. In particular, the Magellanic Cloud Cepheids, that have been observed in connection with the microlensing projects have imposed novel observational constraints because of the low metallicity of the MCs.Comment: Proceedings of the 14th Florida Workshop in Nonlinear Astronomy and Physics, "Astrophysical Turbulence and Convection" Eds. J.R. Buchler and H. Kandrup to appear in the Annals of the New York Academy of Science

Hydrodynamical Survey of First Overtone Cepheids

A hydrodynamical survey of the pulsational properties of first overtone Galactic Cepheids is presented. The goal of this study is to reproduce their observed light- and radial velocity curves. The comparison between the models and the observations is made in a quantitative manner on the level of the Fourier coefficients. Purely radiative models fail to reproduce the observed features, but convective models give good agreement. It is found that the sharp features in the Fourier coefficients are indeed caused by the P1/P4 = 2 resonance, despite the very large damping of the 4th overtone. For the adopted mass-luminosity relation the resonance center lies near a period of 4.2d +/- 0.2 as indicated by the observed radial velocity data, rather than near 3.2d as the light-curves suggest.Comment: ApJ, 12 pages, (slightly) revise

Enkelkinder und ihre Großeltern

Intergenerationelle Beziehungen und im Speziellen die Beziehung zwischen Großeltern und ihren Enkelkindern stellt ein überaus interessantes Forschungsgebiet dar, dem allerdings erst seit kurzer Zeit gebührende Aufmerksamkeit geschenkt wird. Aufgrund der steigenden Lebenserwartung einerseits, und der sinkenden Geburtenrate andererseits, erleben Jung und Alt mehr Lebenszeit miteinander, als je zuvor. Dadurch kommt bzw. kam es gerade in den letzten Jahrzehnten zu einer neuen Form der Beziehung zwischen Großeltern und ihren Enkelkindern. Ein grundlegendes Ziel dieser vorliegenden empirischen Arbeit ist es, diese besondere Beziehung, sowie ihre konkrete Ausgestaltung genauer zu analysieren. Vor allem die Verbindung zwischen Großmüttern und ihren Enkeln, sowie Unterschiede zwischen der matri- und patrilinearen Verwandtschaftslinie spielen in dieser Untersuchung eine entscheidende Rolle. Dafür wurden 228 Schüler im Alter von 12 bis 15 Jahren zu ihrer Beziehung zu ihren (leiblichen) Großmüttern mütterlicher- und väterlicherseits befragt. Konkret wurde mittels eines Enkelkind-Großmütter-Beziehungsfragebogens nach der Kontakthäufigkeit, den gemeinsamen Aktivitäten, dem wahrgenommenen Interesse von Seiten der älteren Generation, den gemeinsam diskutierten Themen, sowie nach der Bedeutsamkeit der Beziehung, separat für die Großmutter mütterlicher- und väterlicherseits, gefragt. Die Ergebnisse dieser vorliegenden Studie sprechen für sich. In allen oben genannten Bereichen nehmen Großmütter mütterlicherseits für jugendliche Enkelkinder einen signifikant höheren Stellenwert ein, im Vergleich zu Großmüttern väterlicherseits. Dies bedeutet konkret, dass mit Großmüttern mütterlicherseits die befragten Jugendlichen signifikant häufiger Kontakt haben, häufiger gemeinsame Aktivitäten unternehmen, häufiger über diverse Themen diskutieren, sowie die Beziehung als Bedeutsamer eingestuft wird, als jene zu Großmüttern väterlicherseits. Auch betonen Enkelkinder, dass sie von ihrer Großmutter mütterlicherseits signifikant mehr Interesse wahrnehmen, als von ihrer väterlichen Großmutter. Das Geschlecht des Enkelkindes spielt in den untersuchten Bereichen nur eine untergeordnete Rolle. Lediglich beim wahrgenommenen Interesse und den diskutierten Themen scheinen Mädchen ihre Großmütter höher einzustufen, als Buben. In anderen Worten, Mädchen nehmen mehr Interesse von ihren Großmüttern war und diskutieren häufiger mit ihnen über diverse Sachverhalte, als Buben. Interessante Ergebnisse liefert das Alter der Großmütter. Wird das Alter als Kovariate in den weiteren Analysen berücksichtigt, so hat dies einen entscheidenden Einfluss auf die zuvor genannten signifikanten Unterschiede zwischen Großmüttern mütterlicher- und väterlicherseits. In den Bereichen Kontakthäufigkeit, diskutierte Themen, sowie Bedeutsamkeit der Beziehung führt die Berücksichtigung des Alters der Großmütter dazu, dass die signifikanten Unterschiede verschwinden. Das Alter der Großmütter scheint demnach ein starker Einflussfaktor für die Ausgestaltung der Beziehung zu sein. Zum Schluss sei anzumerken, dass es in der vorliegenden Stichprobe offensichtlich ist, dass Jugendliche die Beziehung zu ihren beiden Großmüttern unterschiedlich erleben. Zur Großmutter mütterlicherseits pflegen Enkelkinder in vielen Bereichen eine intensivere Beziehung. Trotz der teilweise nicht signifikanten Ergebnisse ist eine gewisse Tendenz in Richtung Großmutter mütterlicherseits unübersehbar. Damit können Ergebnisse aus zahlreichen früheren Arbeiten, die diese Dominanz der matrilinearen Verwandtschaftslinie aufzeigen, bestätigt werden.Intergenerational relationships in general and the relationships between grandparents and their grandchildren in particular represent a most interesting field of research that has only very recently attracted due attention. It is due to a higher life expectancy and a lower birth rate that young and old people share a longer life span than ever before. Over the last few decades this has resulted in a new kind of relationship between grandparents and their grandchildren. It is an essential objective of this empirical study to examine their relationships and its various forms in greater detail. The contact between grandmothers and their grandchildren and the differences among matri- and patrilinear relatives are of crucial importance to this study. For this reason, 228 students at the age of 12 to 15 years have been interviewed concerning their relationships with their maternal and paternal grandmothers. A questionnaire has covered the contact frequency, the activities shared by grandchild and grandmother, the interest shown on the part of the representative of the older generation, the topics discussed by them and the importance of the relationship for the maternal and paternal grandmother individually. The results of this study demonstrate that teenage grandchildren estimate their maternal grandmothers to a considerably greater extent than their paternal grandmothers. This means concretely that with maternal grandmothers grandchildren have significantly more contact frequency, more and frequent activities shared by grandchild and grandmother and frequent discussions about miscellaneous topics. Furthermore grandchildren estimate the interest shown on the part of their maternal grandmother and the importance of the relationship significantly higher as for their paternal grandmother. The gender of the grandchild is of little significance. There are only two areas, i.e. the interest shown and the topics discussed, where girls appear to rank their grandmothers higher than boys. In other words, girls notice more interest of their maternal grandmother and discuss frequent miscellaneous topics with her than boys do. But the age of the grandmothers yields interesting results. If the age is taken into account as a co-variant in the further analyses, it has a crucial influence on the above mentioned, significant differences between maternal and paternal grandmothers. In the areas frequency of contact, topics discussed and significance of the relationship the consideration of the grandmothers` age results in a disappearance of significant differences. According to that, age seems to be a strong parameter for the definition of the relationship. In conclusion it is obvious that adolescents within the control sample experience their relationship to both of its grandmothers in a different way. Grandchildren seem to have a stronger relationship to the maternal grandmother. In spite of some non-significant results, a tendency towards the maternal grandmother is highly visible. For this reason, scores of previous analysis about this dominance of the maternal relationship are verified

Turbulent convection: comparing the moment equations to numerical simulations

The non-local hydrodynamic moment equations for compressible convection are compared to numerical simulations. Convective and radiative flux typically deviate less than 20% from the 3D simulations, while mean thermodynamic quantities are accurate to at least 2% for the cases we have investigated. The moment equations are solved in minutes rather than days on standard workstations. We conclude that this convection model has the potential to considerably improve the modelling of convection zones in stellar envelopes and cores, in particular of A and F stars.Comment: 10 pages (6 pages of text including figure captions + 4 figures), Latex 2e with AAS Latex 5.0 macros, accepted for publication in ApJ

Mode Switching Time Scales in the Classical Variable Stars

Near the edges of the instability strip the rate of stellar evolution is larger than the growth-rate of the pulsation amplitude, and the same holds whenever the star is engaged in pulsational mode switching. Stellar evolution therefore controls both the onset of pulsation at the edges of the instability strip and of mode switching inside it. Two types of switchings (bifurcations) occur. In a soft bifurcation the switching time scale is the inverse harmonic mean of the pulsational modal growth-rate and of the stellar evolution rate. In a hard bifurcation the switching times can be substantially longer than the thermal time scale which is typically of the order of a hundred periods for Cepheids and RR Lyrae. We discuss some of the observational consequences, in particular the paucity of low amplitude pulsators at the edges of the instability strip.Comment: 5 pages, 3 figures, ApJ (in press

Nonlinear Beat Cepheid and RR Lyrae Models

The numerical hydrodynamic modelling of beat Cepheid behavior has been a long standing quest in which purely radiative models had failed consistently. We find that beat pulsations occur naturally when {\it turbulent convection} is accounted for in our hydrodynamics code. The development of a relaxation code and of a Floquet stability analysis greatly facilitates the search for and the analysis of beat Cepheid models. The conditions for the occurrence of beat behavior can be understood easily and at a fundamental level with the help of amplitude equations.Comment: 8 pages, 5 figures, Astronomy and Astrophysics, submitte

A test of time-dependent theories of stellar convection

Context: In Cepheids close to the red edge of the classical instability strip, a coupling occurs between the acoustic oscillations and the convective motions close to the surface.The best topical models that account for this coupling rely on 1-D time-dependent convection (TDC) formulations. However, their intrinsic weakness comes from the large number of unconstrained free parameters entering in the description of turbulent convection. Aims: We compare two widely used TDC models with the first two-dimensional nonlinear direct numerical simulations (DNS) of the convection-pulsation coupling in which the acoustic oscillations are self-sustained by the kappa-mechanism. Methods: The free parameters appearing in the Stellingwerf and Kuhfuss TDC recipes are constrained using a chi2-test with the time-dependent convective flux that evolves in nonlinear simulations of highly-compressible convection with kappa-mechanism. Results: This work emphasises some inherent limits of TDC models, that is, the temporal variability and non-universality of their free parameters. More importantly, within these limits, Stellingwerf's formalism is found to give better spatial and temporal agreements with the nonlinear simulation than Kuhfuss's one. It may therefore be preferred in 1-D TDC hydrocodes or stellar evolution codes.Comment: 7 pages, 5 figures, 2 tables, accepted for publication in A&

Fourier decomposition and frequency analysis of the pulsating stars with P<1 d in the OGLE database. I. Monoperiodic Delta Scuti, RRc and RRab variables. Separation criteria and particularities

The OGLE database is revisited to investigate in more detail the properties of the Fourier parameters. Methodological improvements led us to identify a clear separation among High-Amplitude Delta Scuti (HADS), RRc and RRab stars. The bimodal distribution of the R21 parameter in HADS stars is explained as a contamination effect from RRc stars: there is evidence that all stars with 0.20<P<0.25 d are RRc variables. The previously claimed existence of a subclass of unusual HADS is demonstrated to be a spurious result. Candidate overtone pulsators are found among HADS and RRc variables. The properties of the Fourier parameters are discussed as a function of the physical conditions in the stars involved. Among the field RRab stars we detected different light-curve groups producing distinct "tails" in the Fourier plots for P>0.55 d; evolutionary phases or the combination of different physical conditions (not only metallicity) are suggested to explain this separation, observed also in the cluster RRab stars. The stellar parameters of RRc stars in a given globular cluster show different tendencies than those of RRc stars from different clusters.Comment: 12 pages (in A&A style), 14 eps figures. Accepted for A&A Main Journal. Table 3, 4 and 5 are also included as ascii files. The atlas of the light curves and least-squares fits can be requested from the autho