Transcript here. - SUNY Oswego

Preliminary Analysis of RR Lyrae Light Curves in the
Globular Cluster M15
Timothy DeHaas • Andy Missert and Shashi Kanbur (University of Rochester and SUNY Oswego
Abstract
RR Lyraes are found mostly in globular clusters in the halo of spiral galaxies. Knowledge of their
absolute magnitude as a function of metallicity leads to constraints on the theory of galaxy formation. Thus,
it is important to study RR Lyraes in a range of environments.
We present an initial analysis of the BVI light curves of RRab stars within the metal poor Oosterhoff II
globular cluster M15 using Fourier analysis. We compare the resultant period-color relations at minimum
light with similar period-color relations from the metal rich Oosterhoff I globular cluster M3.
Using Fourier analysis to define the maximum and minimum magnitude of RRab light curves, it was
found that minimum light, the slope of period-color relation is level. At maximum light, the period-color
relation is sloped in the positive direction.
Introduction
Fourier Analysis
Globular clusters, like M15, are characterized as a spherical, density group of stars that orbit a galactic
core as a satellite. Globular clusters can be found in the halo of a galaxy and have a well defined age, older
than any object in that galaxy. For this reason, it is important to study globular clusters as it helps to
understand stellar and globular evolution as well as understand the formation history of galaxies and nearby
satellites.
Fourier analysis is an important part to analyzing
scientific data from RRab light curves. It is a
mathematical process by which a curve is fit through
data using an infinite, or partial, sum of sinusoidal
frequencies. In this scientific analysis, a Fourier
decomposition was used to define a light curve, rather
than individual point, so that maximum and minimum
magnitude can be take. A partial sum for each light
curve was taken. The order was star and color specific.
The equation used in this analysis is below.
RR Lyrae variable star, or in this case RRab stars, can be found within globular clusters. They are the
focus of this preliminary study because of their importance of defining an age and distance of a particular
cluster. They are easily identifiable, for their light curves are well define, and they are bright enough to be
observed at considerable distances.
In globular cluster M15, the RRab stars are known as being metal-poor, or an Oosterhoff type II. The
purpose of this preliminary analysis is to study the light curves of these Oosterhoff type II variables so that
the period-color (PC) relation of these stars can be compared with the period-color relation of globular
cluster M3, a metal-rich cluster, or Oosterhoff type II.
Sample Light Curves
kn
b
V  Ao  X Ak sin kwt   k
c
k1
Period Color Relation
Period
0.3677
0.5723
0.6970
0.6476
Ao
16.239
16.086
16.358
16.249
A1
0.3119
0.4506
0.2895
0.3777
Φ1
2.4693
3.7250
5.9874
2.5626
A2
0.0592
0.2033
0.1636
0.1118
Φ2
1.5333
1.6544
1.7943
0.7807
0.3697
0.6044
16.289
16.272
0.3454
0.4387
2.5072
5.3042
0.0432
0.1726
1.3908
6.1545
Conclusion
Acknowledgments
Using Fourier analysis to describe light curves of RR Lyraes in M15, maximum and minimum
magnitude of these light curves were found. .Period color relations of RR Lyraes that at minimum
light was plotted versus phase. The slope is level. The period color relations at maximum light
versus phase has a slope that is in the positive direction. Further studies will compare the slopes of
maximum and minimum period color relations of metal-poor M15 versus metal-rich M3.
The authors thank the following for support:
NSF grant OISE 0755646,
HST grant HST-AR-10673.04-A,
The 2006 Chretien Award of the American Astronomical Society to SMK,
Laboratorio Nacional de Astrofisica, Brasil,
SUNY Oswego,
The Grupo de Astrofisica, Universidade Federal de Santa Catarina.