Mass Transfer Between Stars: Photometric Studies

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This chapter discusses photometric methods of studying mass transfer between stars in close binary star systems, and explains how researchers use the variations in the light received from far away stars to determine the properties of the stars and the nature of mass flow between them. A new photometric method of modelling mass transfer in a certain class of binary stars is introduced. Binary systems undergoing mass exchange are said to be interacting and are referred to as interacting binary stars. The interacting binaries discussed in this chapter are Algol-type binary stars (see Section 2), or similar systems. Considerable attention in the field of interacting binary stars has recently been paid to systems containg compact objects (white dwarfs, neutron stars, black holes, etc.) and has left many systems of more normal stars, such as Algols, neglected. This circumstance actually provides opportinities for researchers to study those systems that have been studied in the past century, but have been neglected over the last couple decades. Some emphasis in this chapter is placed on a certain class of interacting binary stars that undergo strange variations in light. Basic concepts such as the light curve and ephemeris determination are introduced in section 3. Section 4 explains how a time-dependent ephemeris can be detected and used to calculate the rate at which an interacting binary’s orbital period is changing due to mass transfer between the stars, and also to calculate the rate as which the mass is transferring. The ephemeris curve is an important tool in the photometric study of interacting binary stars because it is used to verify that mass transfer is taking place. As the study of interacting binaries continues, we’re learning more about their evolution. Interacting binaries can even be used as a sort of laboratory for testing current theories of stellar evolution. Throughout our studies, we’ve learned that mass transfer between stars can be quite variable. Accretion structures have been detected around the mass-gaining component, and it has been determined that the presence and/or stability of the accretion structures depend on several factors including the size and period of the system’s orbit. The mass transfer stream in Algol systems with short orbital periods (about 3 days or less) strikes the accreting star more directly and an accretion disk is not likely to form. On the other hand, stable accretion disks do tend to form in systems with longer orbital periods (greater than about 6 days), where the accretion is not as direct. Systems with intermediate periods can exhibit wild variations, on timescales as short as one orbital cycle or less, due to a variable or transient accretion structure resulting from the system’s oscillations between states of direct 1