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Period-Color and Amplitude-Color Relations in OGLE III RR Lyraes
Joyce
1
Wu ,
Dilyana
2
Mihaylova ,
Shashi
3
Kanbur ,
Chow-Choong
4
Ngeow
(1) Ithaca College Physics Dept. (2) University of Rochester Physics and Astronomy Dept. (3) SUNY Oswego Physics Dept. (4) Graduate Institute of Astronomy, National Central
University, Taiwan
The Theory
RR Lyraes
Our results for the amplitude color relations do support
the theory, as seen in the figures below. The slopes of
the fit on the maximum light is .380 while the slope for
the minimum light is .109 in the LMC and respectively,
.372 and .129 in the minimum suggesting at the very
least that the amplitude is much more reliant on the
temperature at maximum.
RR Lyraes are variable stars, which means
their apparent brightness as seen from Earth
varies. Their intrinsic brightness is around 0.75
and their period is usually less than a day.
They’re commonly found in globular clusters,
though the stars involved in out research were
field stars, and are used as standard candles
which are astronomical objects with known
luminosities to measure galactic distances.
OGLE-III Survey
The Optical Gravitation Lensing Experiment
(OGLE-III) is a Polish astronomical survey
primarily intended to detect gravitational
microlensing events and transiting planets. Due
to constant monitoring of the night sky, they
were able to construct one of the largest
catalogues of variable stars. This catalogue
provided information of periods and V and I
bands intensities for over 17,000 RR Lyraes in
the Large Magellanic Cloud (LMC) and the
Small Magellanic Cloud (SMC) that we used in
our project.
Amplitude-Color Relations
The theory begins with the Stefan Boltzmann Law which states that the
luminosity is proportionate to the surface area of a sphere and the
temperature. We looked at the ratio between the maximum and
minimum, and assuming the radius is fairly similar at maximum and
minimum light, we are left with an equation that states the amplitude
should be directly related to the temperature at maximum and minimum
temperature.
Period-Color Relations
As seen in the figures below and to
the right, is that the period color for
minimum is relatively flat, suggesting
the amplitude of the star is reliant
solely on the temperature of maximum
light.
Evidence of Evolved Stars in Data
Fourier Decomposition and Light
Curves
We ran the OGLE-III data through a Fourier
decomposition code which provided us
theoretical light curves to fit to curves of our
actual data. The figure above displays what a
typical light curve from our data of RR Lyraes
should look like.
Acknowledgements
The authors thank SUNY Oswego, National
Central University and the Graduate Institute for
Astronomy and the National Science Foundation's
Office of International Science and Engineering's
award 1065093.
In the period-amplitude relations there is evidence in a curve
parallel to the bulk of the data that may suggest there are
evolved stars in the data moving off the horizontal branch that
RR Lyraes occupy. Below are period amplitude relations in the
SMC and LMC that are fitted with a curve of data from M3 RR
Lyraes from a previous project. A paper by Smith et al. also
theorizes the presence of evolved stars in the data.