Geary E. Albright

Morphologies of Hα Accretion Regions in Algol Binaries

We have investigated a group of 18 Algol-type binaries to determine the general morphologies and physical properties of the accretion regions in these systems. The systems studied were V505 Sgr, RZ Cas, AI Dra, TV Cas, TW Cas, δ Lib, RW Tau, TW Dra, β Per, TX UMa, U Sge, S Equ, U CrB, RS Vul, SW Cyg, CX Dra, TT Hya, and AU Mon, in order of increasing orbital period P = 1.18-11.11 days. In addition, the RS CVn-type binary HR 1099 (V711 Tau) was observed to illustrate the appearance of chromospheric Hα emission. Nearly 2200 time-resolved Hα spectra were collected from 1992 March to 1994 December with the McMath-Pierce Solar Telescope at NSO and mostly with the Coude Feed Telescope at KPNO. The spectra were obtained at phases around the entire orbit of each binary and were closely spaced to permit the detection of transitions in the profiles. Moreover, the spectra were obtained typically within three orbital cycles to reduce the influence of secular variations. Difference profiles were calculated by subtracting a composite theoretical photospheric spectrum from the observed spectrum. The analysis of the Hα difference profiles demonstrates that the accretion structures in Algol binaries have four basic morphological types: (1) double-peaked emission systems in which the accretion structure is a transient or classical accretion disk; (2) single-peaked emission systems in which the accreted gas was found along the trajectory of the gas stream and also between the two stars in an accretion annulus; (3) alternating single- and double-peaked emission systems, which can change between a single-peaked and a double-peaked type within an orbital cycle; and (4) weak spectrum systems in which there was little evidence of any accretion structure since the difference spectra are weak at all phases. The first two types are the dominant morphologies. The first type can be interpreted physically as a disklike distribution, while the second is a gas stream-like distribution. The most common type in short-period Algols with 2.7 days 6 days) were found to have slightly variable but permanent accretion disks at all epochs (i.e., CX Dra, TT Hya, and AU Mon), similar to those found in cataclysmic variables. SW Cyg (P = 4.57 days) was found to be an intermediate case between the shorter period systems with P 6 days. Two systems, U Sge and U CrB, displayed alternating single- and double-peaked emission at different epochs (Type 3), and changes from one type to another were detected within a 12 hour time interval. Observations at multiple epochs suggest that four members of the single-peaked emission group, namely RW Tau, TX UMa, S Equ, and RS Vul, may actually belong to the alternating group. So, the complete group of alternating systems covers periods of 2.7 days < P < 4.5 days. Finally, the systems V505 Sgr, δ Lib, AI Dra, TW Cas, and TV Cas had weak difference spectra or uninteresting observed spectra at most phases (Type 4). All of these systems have orbital periods P < 2.4 days and are high in the r-q diagram. In these cases, the mass-gaining star is very large relative to the binary separation, and the path of the gas stream path is very short, so there is very little room to form any substantial accretion structures as found in the other systems.

Evidence of magnetic activity in short-period algol binaries

The cool G- to K-type secondaries in short-period Algol-type binaries are expected to be magnetically active like the RS CVn binaries because of their rapid rotation and outer convective layers. There is now growing evidence of such activity, primarily from radio and X-ray observations, but there is still little or no information about most of the other indicators of magnetic activity. In this work, we have collected the evidence for magnetic activity in a selected group of 15 short-period Algols and have examined the possibility that part of the Hα emission detected in these systems may indeed arise from the chromosphere of the cool star and not exclusively from circumstellar material

Doppler Tomography of Accretion Disks in Algol Binaries

Doppler tomography of Hα spectra of four Algol-type binaries TT Hya, SW Cyg, U CrB, and U Sge has revealed quasi-stable accretion disks around the mass gaining star in each system. In the tomograms, these accretion structures are nearly symmetric distributions of gas between the stellar surface and the Roche lobe around the mass gainer. The disk is most stable in TT Hya and SW Cyg, which have orbital periods greater than 4.5 days. However, the morphology of the accretion structures in the shorter period systems, U CrB and U Sge, can change from a disklike distribution to a streamlike distribution. During a streamlike epoch of these systems, the gas stream in the Doppler tomogram closely follows the predicted free fall path from the inner Lagrangian point toward the mass gainer. However, during the disklike stage, the intensity of the disk is comparable to that of the gas stream. These tomograms display the first maps of accretion disks in the Algol-type class of interacting binaries.

Circumstellar material in TX Ursae Majoris

The analysis of the Hα difference profiles of TX UMa has revealed deep redshifted absorption during primary eclipse, weaker redshifted absorption at other phases, and strong blueshifted and redshifted emission outside primary eclipse. The absorption seen during primary eclipse is produced by circumstellar gas extending above and below the orbital plane and between us and the partially eclipsed primary star. The circumstellar material is in the form of a rotating disk because double-peaked emission features are visible at most phases outside primary eclipse, with the stronger component first redshifted near phase 0.16-0.19 but blue-shifted half an orbit away near phase 0.65-0.66

Circumstellar Matter in Direct Impact Algol Systems

Full-orbit Hα observations have been analyzed to determine the two-dimensional distribution of the circumstellar gas in the four short-period Algols, U Sge, U CrB, RS Vul, and SW Cyg. In these systems, the gas stream resulting from Roche-lobe overflow directly impacts the mass-gaining star and feeds material into a structure known as a ‘transient accretion disk’. The accretion regions observed in these systems include a transient accretion disk that at least partially surrounds the mass gainer, the gas stream, and possibly a chromospheric component associated with the magnetically active cool star.

Doppler tomography of chromospheres and accretion regions in Algol binaries

The technique of back-projection Doppler tomography has been used to produce the first set of reconstructed images of chromospheric emission sources in the class of Algol-type binaries. The Hα emission sources in the Doppler tomograms include the gas stream, a sub-Keplerian accretion annulus, a quasi-stable accretion disk, and the chromosphere of the cool secondary star. The chromospheric images of several Algol secondaries (e.g., RS Vul, β Per, TX UMa, RW Tau) are consistent with that of the RS CVn binary V711 Tau.