SIMBAD references

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 Coudé 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<P<4.5 days is a predominantly single-peaked emission feature in the Hα difference profiles (Type 2). This feature is redshifted during the phase interval φ∼0.15-0.45 and blueshifted from φ∼0.55-0.85. This single-peaked emission is often composed of two closely spaced emission peaks where one peak is at the Hα rest wavelength. The other peak is blueshifted at φ∼0.65, where the line of sight is along the length of the approaching gas stream. Systems which display this morphology include RZ Cas, RW Tau, TW Dra, β Per, TX UMa, S Equ, and RS Vul. The orbital variation of the Hα observed profiles of HR 1099 was similar to that seen in the difference spectra of this group and suggests that chromospheric emission may play a significant role in these binaries. A less common morphological type in the group of short-period Algols was a widely separated double-peaked disk-like distribution (Type 1) where the gas is in a transient or classical accretion disk. All of the long-period systems (P>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<4.5 days and the longer period group 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.