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Properties of GX Gem
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What is GX Gem?
GX Gem is a variable star located within the
constellation Gemini. Comprised of two stars orbiting each
other nearly edge-on from an observer’s viewpoint, the
object’s variability results from the stars eclipsing each other
and blocking some of the light. GX Gem belongs to a
category of binary stars called close binaries since the two
stars cannot be individually resolved in a telescope. By
studying the variation in light in images taken from November
2001 to April 2006, the properties of the stars as well as of the
orbit have been obtained. These values were then used to
approximate the age and chemical composition of the stars.
By comparing these results with current theories of stellar
evolution, I wish to examine whether the stars behave as
predicted or, like some other stars, call for a new model of
stellar evolution.
Comparison
Laura Handzel, REU Student
Making Conclusions
Check
Figure 1: CCD image of the variable star GX Gem as well as a comparison
and check star of constant brightness with which to compare the variable.
Finding Eclipses
•Images of GX Gem were measured by using the Multi-Measure
application, including flat field and sky corrections, to measure
the magnitudes of variable, comparison, and check stars as
displayed in Figure 1. The results were plotted in both a dates
plot and phase plot to find whether each eclipse was a primary or
secondary.
•Reliable dates of minima were obtained from measurements
found with Multi-Minima 2.2 and from external papers, then
used with the Dates of Minima application to estimate the orbital
period accurately, and to learn that the orbit is probably circular.
After learning the absolute properties of the binary
system, the obtained values were then used to determine the age
and chemical composition of the two stars by interpolation in
tables from the Claret and Gimenez evolutionary model. Both
the properties and ages of the stars can be found in Figure 4.
The X value, or Hydrogen content, was found to be 0.6490, the
Y value or Helium content, was 0.321, and the Z value, amount
of heavier elements, was 0.0300. While GX Gem’s information
can neither prove or disprove current theories of stellar
evolution, it can lend credit to one side. Through the remainder
of the summer, I hope to explore its bias to one theory or another
as well as organizing my findings into a potential publication.
Figure 2: Orbital phase diagram with primary mid-eclipse set at 0
phase while mid-secondary eclipse is at 0.5 phase.
Determining Properties
•A light curve fitter (EBOPP) was used to process
the light curve data from Multi-Minima 2.2 to
find the central surface brightness of each star,
spherical radii, radii ratio, inclination, phase
correction, and magnitude outside eclipse, and
model the light curve as seen in Figure 2 (solid
curve).
Variable
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•Period and ephemeris data were combined with
radial velocity data in order to learn, using
GLSPL, the binary system radial velocity and
semi-amplitudes , resulting in the radial velocity
plot shown in Figure 3.
GX Gem Absolute Properties
Primary Uncertainty Secondary Uncertainty
Mass (Solar Units)
1.506
0.02
1.495
0.017
Radius (Solar Units)
2.271
0.05
2.237
0.05
LogG (CGS Units)
3.903
0.019
3.913
0.019
LogL (Solar Units)
0.823
0.023
0.802
0.023
•The secondary surface brightness and color index
were used to estimate the effective temperatures
of the stars (Popper 1980).
Absolute Magnitude
2.65
0.05
2.72
0.06
Syn. Rot. Vel. (km/s)
28.2
0.6
27.8
0.6
Temperature (K)
6150
50
6120
50
•Photometric and spectroscopic orbital parameters
were used to determine the absolute properties of
the stars: masses, radii, Log g, Log L, absolute
magnitudes, and synchronous rotational
velocities.
Surface Brightness
3.782
0.001
3.779
0.00275
Age (Billion yrs)
2.376
0.067
2.376
0.067
•Absolute and apparent magnitudes were used to
find the distance modulus.
Eccentricity=0
Inclination=85.8843º+-0.0878
Period=4.037934+-0.000006 days
Figure 4: Absolute properties of GX Gem found using Multi-Minima and Terminal.
Syn. Rot. Vel. stands for Synchronous Rotational Velocity. Eccentricity, inclination,
and period are for the binary system while the other values are for the individual stars.
Acknowledgements
I would like to thank:
•The orbital ephemeris was determined as HJD Min I = n P + Eo
with HJD Min I being the Julian date of the primary eclipse, n
being the cycle number, P being the period, and Eo being an
accurate date of primary eclipse, then this ephemeris was used to
refine orbital phases with the Multi-Minima 2.2 application.
•Claud H. Lacy. University of Arkansas.
Fayetteville, AR. (Mentor)
Figure 3: Radial velocity plot; black dots represent measured
velocities of primary while white dots show velocities of secondary.
•Guillermo Torres. Harvard-Smithsonian Center
for Astrophysics. Cambridge, MA. (Contributed
preliminary radial velocity measurements).