Properties of GX Gem

1. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. ` ` What is GX Gem? Properties of GX Gem Laura Handzel, REU Student 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. Making Conclusions 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). • 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. • The secondary surface brightness and color index were used to estimate the effective temperatures of the stars (Popper 1980). • 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. • Absolute and apparent magnitudes were used to find the distance modulus. Variable Comparison 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. • 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. 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: • Claud H. Lacy. University of Arkansas. Fayetteville, AR. (Mentor) • Guillermo Torres. Harvard-Smithsonian Center for Astrophysics. Cambridge, MA. (Contributed preliminary radial velocity measurements). Figure 3: Radial velocity plot; black dots represent measured velocities of primary while white dots show velocities of secondary.