860 research outputs found
Ultra-low Amplitude Variables in the LMC -- Classical Cepheids, Pop. II Cepheids, RV Tau Stars and Binary Variables
A search for variable stars with ultra-low amplitudes (ULA), in the millimag
range, has been made in the combined MACHO and OGLE data bases in the broad
vicinity of the Cepheid instability strip in the HR diagram. A total of 25
singly periodic and 4 multiply periodic ULA objects has been uncovered. Our
analysis does not allow us to distinguish between pulsational and ellipsoidal
(binary) variability, nor between LMC and foreground objects. However, the
objects are strongly clustered and appear to be associated with the pulsational
instability strips of LMC Pop. I and II variables. When combined with the ULA
variables of Buchler et al (2005) a total of 20 objects fall close to the
classical Cepheid instability strip. However, they appear to fall on parallel
period-magnitude relations that are shifted to slightly higher magnitude which
would confer them a different evolutionary status. Low amplitude RV Tauri and
Pop. II Cepheids have been uncovered that do not appear in the MACHO or OGLE
catalogs. Interestingly, a set of binaries seem to lie on a PM relation that is
essentially parallel to that of the RV Tauri/Pop. II Cepheids.Comment: 13 pages, 13 (color) figures. Astrophysical Journal (accepted for
publlication
Thermal flickers: A semianalytical approach
In order to enhance physical insight into the nature of thermal oscillations arising from a thin helium burning shell, the behavior in its phase plane of a simple two zone model which, however, contains all the relevant physics is analyzed. This simple model very naturally reproduces thermal flickers and is relatively insensitive to all but two parameters
Modulations in Multi-Periodic Blue Variables in the LMC
As shown by Mennickent, et al(2003), a subset of the blue variable stars in
the Large Magellanic Cloud exhibit brightness variability of small amplitude in
the period range 2.4 to 16 days as well as larger amplitude variability with
periods of 140 to 600 days, with a remarkably tight relation between the long
and the short periods. Our re-examination of these objects has led to the
discovery of additional variability. The Fourier spectra of 11 of their 30
objects have 3 or 4 peaks above the noise level and a linear relation of the
form f_a = 2(f_b - f_L) among three of the frequencies. An explanation of this
relation requires an interplay between the binary motion and that of a third
object. The two frequency relations together with the Fourier amplitude ratios
pose a challenging modeling problem.Comment: 4 pages, 3 figures, Astrophysical Journal (in press
Strange Cepheids and RR Lyrae
Strange modes can occur in radiative classical Cepheids and RR Lyrae models.
These are vibrational modes that are trapped near the surface as a result of a
'potential barrier' caused by the sharp hydrogen partial ionization region.
Typically the modal number of the strange mode falls between the 7th and 12th
overtone, depending on the astrophysical parameters of the equilibrium stellar
models (L, M, \Teff, X, Z). Interestingly these modes can be linearly unstable
outside the usual instability strip, in which case they should be observable as
new kinds of variable stars, 'strange Cepheids' or 'strange RR Lyrae' stars.
The present paper reexamines the linear stability properties of the strange
modes by taking into account the effects of an isothermal atmosphere, and of
turbulent convection. It is found that the linear vibrational instability of
the strange modes is resistant to both of these effects. Nonlinear hydrodynamic
calculations indicate that the pulsation amplitude of these modes is likely to
saturate at the millimagnitude level. These modes should therefore be
detectable albeit not without effort.Comment: 6 pages, 7 figures, submitted to Ap
Ultra-Low Amplitude Cepheids in the Large Magellanic Cloud
The MACHO variables of LMC Field 77 that lie in the vicinity of the Cepheid
instability strip are reexamined. Among the 144 variables that we identify as
Cepheids we find 14 that have Fourier amplitudes <0.05 mag in the MACHO red
band, of which 7 have an amplitude <0.006 mag : we dub the latter group of
stars ultra-low amplitude (ULA) Cepheids. The variability of these objects is
verified by a comparison of the MACHO red with the MACHO blue lightcurves and
with those of the corresponding OGLE LMC stars. The occurrence of ULA Cepheids
is in agreement with theory. We have also discovered 2 low amplitude variables
whose periods are about a factor of 5--6 smaller than those of F Cepheids of
equal apparent magnitude. We suggest that these objects are Cepheids undergoing
pulsations in a surface mode and that they belong to a novel class of Strange
Cepheids (or Surface Mode Cepheids) whose existence was predicted by Buchler et
al. (1997).Comment: 4 pages, 5 figures, slightly revised, to appear in ApJ Letter
Evidence for Low-Dimensional Chaos in Semiregular Variable Stars
An analysis of the photometric observations of the light curves of the five
large amplitude, irregularly pulsating stars R UMi, RS Cyg, V CVn, UX Dra and
SX Her is presented. First, multi-periodicity is eliminated for these
pulsations, i.e. they are not caused by the excitation of a small number of
pulsation modes with constant amplitudes. Next, on the basis of energetics we
also eliminate stochasticity as a cause, leaving low dimensional chaos as the
only alternative. We then use a global flow reconstruction technique in an
attempt to extract quantitative information from the light curves, and to
uncover common physical features in this class of irregular variable stars that
straddle the RV Tau to the Mira variables. Evidence is presented that the
pulsational behavior of R UMi, RS Cyg, V CVn and UX Dra takes place in a
4-dimensional dynamical phase space, suggesting that two vibrational modes are
involved in the pulsation. A linear stability analysis of the fixed points of
the maps further indicates the existence of a two-mode resonance, similar to
the one we had uncovered earlier in R Sct: The irregular pulsations are the
result of a continual energy exchange between two strongly nonadiabatic modes,
a lower frequency pulsation mode and an overtone that are in a close 2:1
resonance. The evidence is particularly convincing for R UMi, RS Cyg and V CVn,
but much weaker for UX Dra. In contrast, the pulsations of SX Her appear to be
more complex and may require a 6D space.Comment: 20 pages, 14 figures, accepted in ApJ - paper with clearer figures is
available at http://www.phys.ufl.edu/~buchler/publications/u12.ps.gz (1Mb
Automated Nonlinear Stellar Pulsation Calculations: Applications to RR Lyrae stars. The Slope of the Fundamental Blue Edge and the First RRd Model Survey
We describe a methodology that allows us to follow the pulsational behavior
of an RR Lyrae model consistently and automatically along its evolutionary
track throughout the whole instability strip. It is based on the powerful
amplitude equation formalism, and resorts to a judicious combination of
numerical hydrodynamical simulations, the analytical signal time-series
analysis, and amplitude equations. A large-scale survey of the nonlinear
pulsations in RR Lyr instability strip is then presented, and the mode
selection mechanism is delineated throughout the relevant regions of parameter
space. We obtain and examine two regions with hysteresis, where the pulsational
state depends on the direction of the evolutionary tracks, namely a region with
either fundamental (RRab) or first overtone (RRc) pulsations and a region with
either fundamental (RRab) or double-mode (RRd) pulsations. The regions where
stable double-mode (DM, or RRd) pulsations occur are very narrow and hard to
find in astrophysical parameter (L, M, T_eff, X, Z) space with hydrodynamic
simulations, but our systematic and efficient methodology allows us to
investigate them with unprecedented detail. It is shown that by simultaneously
considering the effects of mode selection and of horizontal branch evolution we
can naturally solve one of the extant puzzles involving the topologies of the
theoretical and observed instability strips, namely the slope of the
fundamental blue edge. The importance of the interplay between mode selection
and stellar evolutionary effects is also demonstrated for the properties of
double-mode RR Lyr. Finally, the Petersen diagram of double-mode RR Lyr models
is discussed for the first time.Comment: 13 pages, 6 figures, accepted to be published in A&
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