90 research outputs found
Period-colour and amplitude-colour relations in classical Cepheid variables IV: The multi-phase relations
The superb phase resolution and quality of the OGLE data on LMC and SMC
Cepheids, together with existing data on Galactic Cepheids, are combined to
study the period-colour (PC) and amplitude-colour (AC) relations as a function
of pulsation phase. Our results confirm earlier work that the LMC PC relation
(at mean light) is more consistent with two lines of differing slopes,
separated at a period of 10 days. However, our multi-phase PC relations reveal
much new structure which can potentially increase our understanding of Cepheid
variables. These multi-phase PC relations provide insight into why the Galactic
PC relation is linear but the LMC PC relation is non-linear. This is because
the LMC PC relation is shallower for short (log P < 1) and steeper for long
(log P > 1) period Cepheids than the corresponding Galactic PC relation. Both
of the short and long period Cepheids in all three galaxies exhibit the
steepest and shallowest slopes at phases around 0.75-0.85, respectively. A
consequence is that the PC relation at phase ~0.8 is highly non-linear.
Further, the Galactic and LMC Cepheids with log P > 1 display a flat slope in
the PC plane at phases close to the maximum light. When the LMC
period-luminosity (PL) relation is studied as a function of phase, we confirm
that it changes with the PC relation. The LMC PL relation in V- and I-band near
the phase of 0.8 provides compelling evidence that this relation is also
consistent with two lines of differing slopes joined at a period close to 10
days.Comment: 12 pages, 1 table and 13 figures, MNRAS accepte
On the Variation of Fourier Parameters for Galactic and LMC Cepheids at Optical, Near-Infrared and Mid-Infrared Wavelengths
We present a light curve analysis of fundamental-mode Galactic and Large
Magellanic Cloud (LMC) Cepheids based on the Fourier decomposition technique.
We have compiled light curve data for Galactic and LMC Cepheids in optical
({\it VI}), near-infrared ({\it JHK}) and mid-infrared (3.6
4.5-m) bands from the literature and determined the variation of their
Fourier parameters as a function of period and wavelength. We observed a
decrease in Fourier amplitude parameters and an increase in Fourier phase
parameters with increasing wavelengths at a given period. We also found a
decrease in the skewness and acuteness parameters as a function of wavelength
at a fixed period. We applied a binning method to analyze the progression of
the mean Fourier parameters with period and wavelength. We found that for
periods longer than about 20 days, the values of the Fourier amplitude
parameters increase sharply for shorter wavelengths as compared to wavelengths
longer than the -band. We observed the variation of the Hertzsprung
progression with wavelength. The central period of the Hertzsprung progression
was found to increase with wavelength in the case of the Fourier amplitude
parameters and decrease with increasing wavelength in the case of phase
parameters. We also observed a small variation of the central period of the
progression between the Galaxy and LMC, presumably related to metallicity
effects. These results will provide useful constraints for stellar pulsation
codes that incorporate stellar atmosphere models to produce Cepheid light
curves in various bands.Comment: 22 pages, 19 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
Period-color and amplitude-color relations in classical Cepheid variables V: The Small Magellanic Cloud Cepheid models
Period-colour (PC) and amplitude-colour (AC) relations at maximum, mean and
minimum light are constructed from a large grid of full amplitude hydrodynamic
models of Cepheids with a composition appropriate for the SMC (Small Magellanic
Cloud). We compare these theoretical relations with those from observations.
The theoretical relations are in general good agreement with their
observational counterparts though there exist some discrepancy for short period
(log [P] < 1) Cepheids. We outline a physical mechanism which can, in
principle, be one factor to explain the observed PC/AC relations for the long
and short period Cepheids in the Galaxy, LMC and SMC. Our explanation relies on
the hydrogen ionization front-photosphere interaction and the way this
interaction changes with pulsation period, pulsation phase and metallicity.
Since the PC relation is connected with the period-luminosity (PL) relation, it
is postulated that such a mechanism can also explain the observed properties of
the PL relation in these three galaxies.Comment: 10 pages, 6 figures and 6 tables, MNRAS accepte
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