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Imaging Accretion the Coude Way (1Jun95) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 42, June 1995) Mercedes Richards and Geary Albright (Virginia) used the KPNO 0.9-m Coude‚ Feed telescope to obtain images of the gas streaming between the two stars in Algol-type interacting binary systems. Their technique was to was obtain spectra of Ha emission over the orbital periods of the systems, and then use tomography to construct maps of the accreting gas. Their images suggest that the disks in Algol systems are similar in some respects to those found in cataclysmic variables. Interacting binary star systems typically contain at least one star that fills or nearly fills its Roche lobe. In the Roche lobe-filling binaries, mass transfer feeds gas along a stream from the Inner Lagrangian point of the binary towards the mass gainer. Eclipsing Algol-type binaries provide an excellent laboratory for the study of mass transfer, containing an evolved Roche-lobe filling G to K giant or subgiant star and a luminous B to A main sequence companion (the primary). In these systems, the mass gainer (the primary) is large relative to the binary separation. In this situation, the morphology of accretion structures is determined by the radius of the mass gainer relative to the distance between the stars. In the long period Algols, there is enough room for an accretion disk to form around the mass gainer. However, for the shorter period systems (Porb < 6 days), the size of the mass gainer prevents the formation of a classic accretion disk. Instead, the gas stream strikes the surface of the star and the resulting accretion regions have been referred to as a transient accretion disk. [Figure not included] Figure 1: Ha difference of U CrB plotted in the rest frame of the primary star to illustrate the relative motions of the emission components. Richards and Albright used the Coude‚ Feed to obtain roughly 1000 Ha spectra around the entire orbit of over 10 Algol binaries. The observed Ha line profiles typically showed only weak emission peaks in the wings of the observed line profiles. This emission was enhanced by subtracting the combined theoretical (ATLAS9) photospheric absorption line profiles of the stars from the observed line profile. The resulting Hadifference profiles represent the non-photospheric emission and excess absorption in the binary. A representative set of difference profiles are shown in Figure 1 for U CrB. [Figure not included] Figure 2: Doppler Tomogram of U CrB. The solid trajectory in the Doppler image represents the path of the gas stream. The outermost solid circle represents the Keplerian velocity at the surface of the primary and the inner circle around the binary represents the Keplerian velocity at the Roche surface. A standard accretion disk would lie between these two circles. The U CrB tomogram shows that the Ha emission arises primarily along the predicted free fall trajectory. Richards and Albright, with the assistance of undergraduate summer student Larissa Bowles, used the Ha difference profiles and the technique of Doppler Tomography to reconstruct images of the accretion regions in Algol binaries. In brief, the spectra serve as 1D "slices" through a 2D Doppler image that contain both velocity and spatial information as we view the binary at different positions in its orbit. The spatial information is provided by the Doppler radial velocities of the emission features in the spectra. The quality of the reconstruction improves with orbital resolution. With the advent of large, efficient CCDs, fast computers, and multi-night observing shifts, it is now possible to reconstruct observational images of the accretion regions. Images of the accretion disks in cataclysmic variables have been successfully reconstructed using this technique, for example. The Doppler images of the Ha emitting regions of the short-period Algols show a very distinct flow of gas along the predicted gravitational free fall path of the gas stream in the rotating reference frame. The most prominent gas stream was found in U CrB (Figure 2). This contour map was obtained from the difference profiles shown in Figure 1. These images of gas streams in the Algols are the first such images for the entire class of interacting binaries. Initial results also show that systems with Porb > 4.5 days display a near-Keplerian accretion disk around the mass gainer with little evidence of any emission from a gas stream. These results suggest that the disks seen in the Algols are similar in some respects to those found in the cataclysmic variables (see Richards, Albright, Bowles 1995 ApJ, 438, L103).
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