40 research outputs found
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
Variable turbulent convection as the cause of the Blazhko effect - testing the Stothers model
The amplitude and phase modulation observed in a significant fraction of the
RR Lyrae variables - the Blazhko effect - represents a long-standing enigma in
stellar pulsation theory. No satisfactory explanation for the Blazhko effect
has been proposed so far. In this paper we focus on the Stothers (2006) idea,
in which modulation is caused by changes in the structure of the outer
convective zone, caused by a quasi-periodically changing magnetic field.
However, up to this date no quantitative estimates were made to investigate
whether such a mechanism can be operational and whether it is capable of
reproducing the light variation we observe in Blazhko variables. We address the
latter problem. We use a simplified model, in which the variation of turbulent
convection is introduced into the non-linear hydrodynamic models in an ad hoc
way, neglecting interaction with the magnetic field. We study the light curve
variation through the modulation cycle and properties of the resulting
frequency spectra. Our results are compared with Kepler observations of RR Lyr.
We find that reproducing the light curve variation, as is observed in RR Lyr,
requires a huge modulation of the mixing length, of the order of +/-50 per
cent, on a relatively short time-scale of less than 40 days. Even then, we are
not able to reproduce neither all the observed relations between modulation
components present in the frequency spectrum, nor the relations between Fourier
parameters describing the shape of the instantaneous light curves.Comment: 17 pages, 13 figures, accepted for publication in MNRAS; for
associated animation, see
http://homepage.univie.ac.at/radek.smolec/publications/KASC11a
Double-Mode Stellar Pulsations
The status of the hydrodynamical modelling of nonlinear multi-mode stellar
pulsations is discussed. The hydrodynamical modelling of steady double-mode
(DM) pulsations has been a long-standing quest that is finally being concluded.
Recent progress has been made thanks to the introduction of turbulent
convection in the numerical hydrodynamical codes which provide detailed results
for individual models. An overview of the modal selection problem in the HR
diagram can be obtained in the form of bifurcation diagrams with the help of
simple nonresonant amplitude equations that capture the DM phenomenon.Comment: 34 pages, to appear as a chapter in Nonlinear Stellar Pulsation in
the Astrophysics and Space Science Library (ASSL), Editors: M. Takeuti & D.
Sasselov (prints double column with pstops
'2:[email protected](22.0cm,-2cm)[email protected](22.0cm,11.0cm)' in.ps out.ps
V440 Per: the longest period overtone Cepheid
V440 Per is a Population I Cepheid with the period of 7.57 day and low
amplitude, almost sinusoidal light and radial velocity curves. With no reliable
data on the 1st harmonic, its pulsation mode identification remained
controversial. We obtained a radial velocity curve of V440 Per with our new
high precision and high throughput Poznan Spectroscopic Telescope. Our data
reach the accuracy of 130 m/s per individual measurement and yield a secure
detection of the 1st harmonic with the amplitude of A_2= 140+/- 15 m/s. The
velocity Fourier phase \phi_21 of V440 Per is inconsistent at the 7.25 \sigma
level with those of the fundamental mode Cepheids, implying that the star must
be an overtone Cepheid, as originally proposed by Kienzle et al.(1999). Thus,
V440 Per becomes the longest period Cepheid with the securely established
overtone pulsations. We show, that the convective nonlinear pulsation hydrocode
can reproduce the Fourier parameters of V440 Per very well. Requirement to
match the observed properties of V440 Per constrains free parameters of the
dynamical convection model used in the pulsation calculations, in particular
the radiative losses parameter.Comment: Submitted to MNRA
YREC: The Yale Rotating Stellar Evolution Code
The stellar evolution code YREC is outlined with emphasis on its applications
to helio- and asteroseismology. The procedure for calculating calibrated solar
and stellar models is described. Other features of the code such as a non-local
treatment of convective core overshoot, and the implementation of a
parametrized description of turbulence in stellar models, are considered in
some detail. The code has been extensively used for other astrophysical
applications, some of which are briefly mentioned at the end of the paper.Comment: 10 pages, 2 figures, ApSS accepte
Interaction Between Convection and Pulsation
This article reviews our current understanding of modelling convection
dynamics in stars. Several semi-analytical time-dependent convection models
have been proposed for pulsating one-dimensional stellar structures with
different formulations for how the convective turbulent velocity field couples
with the global stellar oscillations. In this review we put emphasis on two,
widely used, time-dependent convection formulations for estimating pulsation
properties in one-dimensional stellar models. Applications to pulsating stars
are presented with results for oscillation properties, such as the effects of
convection dynamics on the oscillation frequencies, or the stability of
pulsation modes, in classical pulsators and in stars supporting solar-type
oscillations.Comment: Invited review article for Living Reviews in Solar Physics. 88 pages,
14 figure
Amplitude Saturation in Beta Cephei Models
Although the driving mechanism acting in beta Cephei pulsators is well known
(e.g. Dziembowski & Pamyatnykh 1993), problems concerning identification of
amplitude limitation mechanism and non-uniform filling of the theoretical
instability strip, remain to be solved. In the present analysis, these problems
are addressed by non-linear modelling of radial pulsations of these stars. In
this approach radial modes are treated as representative for all acoustic
oscillations.
Several models of different masses and metallicities were converged to limit
cycles through Stellingwerf (1974) relaxation technique. Resulting peak-to-peak
amplitudes are of order of DeltaV=0.3 mag. Such amplitudes are significantly
larger than those observed in beta Cephei pulsators. Assuming that all acoustic
modes are similar, we show that collective saturation of the driving mechanism
by several acoustic modes can easily lower predicted saturation amplitudes to
the observed level. Our calculations predict significant decrease of saturation
amplitudes as we go to high mass/high luminosity models. However, this effect
is not strong enough to explain scarcity of high mass beta Cephei variables.
We also discuss robust double-mode behaviour, encountered in our radiative
models. On a single evolutionary track we identify two double-mode domains with
two different mechanisms resposible for double-mode behaviour. The non-resonant
double-mode domain separates first overtone and fundamental mode pulsation
domains. The resonant double-mode domain appears in the middle of the first
overtone pulsation domain. Its origin can be traced to the
2omega_1=omega_0+omega_2 parametric resonance, which destabilizes the first
overtone limit cycle.Comment: 13 pages, 12 figures, accepted for publication in MNRA
Discovery of period doubling in BL Herculis stars of the OGLE survey. Observations and theoretical models
We report the discovery of a period doubling behaviour in a 2.4d BL
Herculis-type variable of the Galactic bulge. Another bulge BL Her-type star
(P=2.25d) is a strong period doubling candidate. Both objects have been
identified with the OGLE-III photometry. Possibility of period doubling in this
type of pulsators has been predicted almost twenty years ago by Buchler &
Moskalik. Our finding is the first observational confirmation of their
theoretical results.
Discovery of the first BL Herculis star showing the period doubling effect
motivates a new theoretical investigation with the state-of-the-art convective
pulsation codes. We present the results of initial model survey, specifically
aimed at studying the observed period-doubled BL Her variable. All of our
non-linear models have P=2.4d. The computations confirm that the period
doubling effect is caused by the 3:2 resonance between the fundamental mode and
the first overtone, as indicated by earlier radiative models of Buchler &
Moskalik. Comparison of the computed and the observed light curves allows to
constrain the parameters of the star, in particular its metallicity, which
appears to be high, Z is approximately 0.01. The recent evolutionary tracks put
also constraint on the mass of the star, which is close to M=0.50 solar masses.Comment: 18 pages, 18 figures, accepted for publication in MNRA
Introduction: building the history of language learning and teaching (HoLLT)
The papers presented in this issue are the result of a workshop held at the University of Nottingham in December 2012 as part of an Arts and Humanities Research Council research network Towards a History of Modern Foreign Language Teaching and Learning (2012–14) intended to stimulate historical research into language teaching and learning. This, the first workshop in the programme, focused on exchanging information on the history of language learning and teaching (HoLLT) across the different language traditions, for it had become clear to us that scholars working within their own language disciplines were often relatively unaware of work outside these. We hope that this special issue — with overview articles on the history of English, French, German, and Spanish as second/foreign languages — will help overcome that lack of awareness and facilitate further research collaboration. Charting the history of language teaching and learning will, in turn, make us all better informed in facing challenges and changes to policy and practice now and in the future. It is instructive in the current climate, for example, to realize that grave doubts were held about whether second foreign languages could survive alongside French in British schools in the early twentieth century (McLelland, forthcoming), or to look back at earlier attempts to establish foreign languages in primary schools (Bayley, 1989; Burstall et al., 1974; Hoy, 1977). As we write, language learning in England is undergoing yet more radical change. Language teaching for all children from the age of seven is being made compulsory in primary schools from 2014, while at Key Stage 3 (up to age 16), where a foreign language has not been compulsory since 2002, the most recent programme of study for England has virtually abandoned the recent focus on intercultural competence and now requires learners to ‘read great literature in the original language’,1 a radical change in emphasis compared to the previous half-century, which seems to reflect a very different view of what language learning is for. We seem to be little closer in 2014 than we were at the dawn of the twentieth century to answering with any certainty the questions that lie at the very foundations of language teaching: who should learn a foreign language, why learners learn, what they need to learn, and what we want to teach them — answers that we need before we can consider how we want to teach. The research programme begun under our research network is intended to help us to take ‘the long view’ on such questions