DETERMINING THE PROPERTIES OF GX GEM. L. E. Handzel1,2 and C. H. Lacy1,3, 1Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, Arkansas 72701,2Agnes Scott College, Decatur GA 30030, , 3Department of Physics, University of Arkansas, .
Introduction: From an earth observer’s viewpoint, GX Gem appears to be a single star within the constellation Gemini that varies in brightness over time; however, it is actually a system of two stars which orbit about a common center of mass. The variation occurs when the stars eclipse each other. Since the stars’ orbit is seen nearly edge-on from the earth as shown in Figure 1, the “star” appears to dim as one star passes in front of the other thereby blocking a major fraction of its light. Examination of these variations can give observers insights to the various physical and orbital properties of the two stars.
About half of all known star systems belong to binaries, so studying these double stars is essential to understanding how stars evolve. Many theories of stellar evolution treat binary stars as if they develop independently. More recent research, though, suggests that binaries sometimes evolve differently than single stars, so the theories may require re-assessment. Information gleaned about GX Gem will give favor either for or against the current theories.
Figure 1: Representation of primary eclipse as seen from Earth. Image is not drawn to scale. The primary star is the larger, more massive of the two, so a primary eclipse occurs when the light of the primary is obscured by the secondary.
Observations: Images of GX Gem were taken by Dr. Lacy using the URSA telescope on Kimpel Hall at the University of Arkansas. The observation period began in November 2001 and ended in April 2006 using the V filter. The radial velocity data was determined by Dr. Torres of the Harvard-Smithsonian Institute for Astrophysics.
Measuring Images: In order to make use of the observational data, flat fields and sky corrections had to be included as the images were measured with the Multi-measure program written by Dr. Lacy. This program plotted the results on two different light curves. The first displayed the magnitude of the variable star as well as those of a comparison and check star of constant brightness for each HJD (Heliocentric Julian Date). This light curve revealed the times of individual eclipses. The second light curve plotted magnitude against the phase of the variable star. The 0 phase is set at the bottom of the primary eclipse while 0.5 phase is in the midst of the secondary eclipse. This curve allowed me to determine whether each minimum was a primary or secondary eclipse. The final light curve can be seen in Figure 2. From the two plots, I was able to obtain several reliable dates of eclipses.
Figure 2: Light curve of GX Gem. As shown, the binary seems to have a constant brightness outside eclipse since no light is being blocked. However, when the light from the primary is blocked by the secondary, a large dip in brightness occurs. A similar dip occurs when the primary passes in front of the secondary.
Data Analysis: Since only a limited number of minima could be accurately determined, I added dates of minima found from a variety of external papers to my own to get a better estimate of the period for GX Gem. The period was found using Dr. Lacy’s program, Dates of Minima.
After learning the period, I then used the equation HJD Min I = nP+Eo with HJD Min I being the estimate for an eclipse date, n being the cycle number since the first eclipse, P being the period, and Eo being an accurate date of minimum. This ephemeris was used to rephrase the light curve data.
Orbital Elements: Having the period, ephemeris, and light curve data, the orbital elements were then found with a collection of programs written by Dr. Lacy.
EBOPP took the variations in brightness of the stars by date, coupled with the period and ephemeris to find the photometric orbital elements: the secondary surface brightness, primary radius, ratio of radii, and orbital inclination relative to the plane of the sky.
GLSPL used the radial velocities found by Dr. Torres to create a radial velocity curve, shown in Figure 3, as well as determining the spectroscopic orbital elements which include system movement and semi amplitudes of the stars.
Figure 3: Radial velocity curve of GX Gem. The white circles display measured velocities of the primary star while the black circles show measured velocities of the secondary. Since the binary rotates nearly edge-on to Earth, the velocities are either blue shifted from movement towards the earth or red-shifted from movement away from the earth. As demonstrated in the figure, the two stars will have consistently opposite shifts.
All the orbital parameters were then combined in MRLCALC to find the absolute properties of the binary. Masses, radii, luminosities, absolute magnitudes, and synchronous rotational velocities were calculated and are displayed in Table 1.
The final step was to approximate the age and chemical composition of the stars. Over the years, several stellar models have been assembled to allow astronomers to locate such values. These values were determined by interpolation because the exact age could not be found directly.
Conclusion: As members of a binary system, the stars of GX Gem were ideal subjects to test the accuracy of current theories of stellar evolution. Though unable to disprove the theories by themselves, GX Gem does give evidence against the theories. Being each about one and a half times more massive than the sun, the stars of GX Gem should have a shorter main sequence lifetime than the sun since more massive stars consume their hydrogen supply sooner. However, these stars are less than half the age of the sun, yet they have already begun hydrogen shell burning while the sun has not. In addition, 3% of GX Gem’s composition is heavier elements, so the stars have a higher metallicity than the sunwhich is only 1% heavier elements. Like many binary stars studied before, GX Gem calls for a re-evaluation of the current theories of evolution.
References: [1]Torres G. (2006) Harvard-SmithsonianCenter for Astrophysics, Cambridge, MA 02138. [2]Claret A. (1997) Astron. Astrophys. Suppl. Ser., 125, 439-443. [3]Claret A. and Gimenez A. (1992) Astron. Astrophys. Suppl. Ser., 96, 255-267. [4]Hubscher J. (2005) IAU Inform. Bull. Var. Stars, 5643, 1. [5]Sanchez-Bajo F. et. al. (2003) Astron. Nachr., 324, 511-515. [6]Agerer F. and Hubscher J. (2003) IAU Inform. Bull. Var. Stars, 5484, 1. [7]Lacy C. H. (2002) IAU Inform. Bull. Var. Stars, 5357, 1. [4]Agerer F. and Hubscher J. (1999) IAU Inform. Bull. Var. Stars, 4711, 1-4.
GX Gem Properties / Primary / Uncertainty / Secondary / UncertaintyMass (Solar Units) / 1.515 / 0.014 / 1.499 / 0.013
Radius (Solar Units) / 2.274 / 0.05 / 2.24 / 0.05
LogG (CGS Units) / 3.904 / 0.019 / 3.913 / 0.019
LogL (Solar Units) / 0.825 / 0.023 / 0.803 / 0.023
Absolute Magnitude / 2.65 / 0.06 / 2.71 / 0.07
Synchronous Rotational Velocity (km/s) / 28.2 / 0.6 / 27.8 / 0.6
Temperature (K) / 6150 / 50 / 6120 / 50
Surface Brightness / 3.782 / 0.001 / 3.779 / 0.00275
Age (Billion years) / 2.376 / 0.067 / 2.376 / 0.067
Eccentricity=0 / Inclination=85.8843º+-0.0878
Period=4.037934+-0.000006 days
Table 1: Table displaying the properties of GX Gem. Properties of each star in the binary are given first with their uncertainties followed by the properties of the binary.