Mercedes T. Richards

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.

Circumstellar material in Algol - A study of the Balmer line profiles

The kinematics and spatial distribution of the circumstellar H II structures in Algol (β Persei) have been found from a study of the Balmer Hα, Hβ, and Hγ line profiles. The main focus of the study was the Hα line profile which contained contributions from the circumstellar material. Since these contributions were weak, they were enhanced by removing the composite theoretical photospheric line profiles of the three stars in Algol from the observed line profile. These residual or difference profiles were studied to provide a model for the circumstellar gas in Algol. The 98 IIIa-F (4700-6700 A) photographic spectra which were used in the analysis were obtained over 15 months during 1976 and 1977 and cover the entire orbit of the Algol binary, with a resolution of approximately 0.02 in phase

Hydrodynamic Simulations of Hα Emission in Algol-Type Binaries

Two-dimensional hydrodynamic simulations of mass transfer in short-period Algol-type binaries were performed using the numerical code Virginia Hydrodynamics 1. This code uses the piecewise parabolic method with a Lagrangian remap. Our version of the code also accounts for radiative cooling and collisional ionization and excitation processes. The purpose of performing the simulations was to study the Hα emission from circumstellar gas in the Algols. Using observational evidence from the literature to constrain the gas stream properties, hydrodynamic maps of the Hα emissivity in the two systems β Per (P = 2.87 days) and TT Hya (P = 6.95 days) were made in both Cartesian and velocity coordinates from the simulation data. The velocity maps were then compared to Doppler tomograms constructed from observed Hα line emission in these systems. Since the tomograms cannot be directly transformed to maps of emission in spatial coordinates, the simulated Cartesian maps enabled us to interpret the dynamical processes that produce the features observed in the Doppler tomograms. We find that the simulations produce asymmetric accretion structures with many features similar to those found in the Doppler tomograms of Algol 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.

Consequences of the star-stream interaction in Algol

The Hα line profile of Algol was studied to determine the nature of the interaction between the gas stream and the primary, and evidence was found of an asymmetric transient accretion disk with an average radius of (1.8 ± 0.2)R A , an electron density N e ≥ 10 10 cm -3 , and mass ≥ 10 -12 M ⊙ . The trailing side of the disk is wider and denser than the leading side. Algol is the first partially eclipsing short-period Algol-type system in which a transient disk has been found

A Multiwavelength Study of Spectral Variations in the CX Draconis Binary

An extensive collection of spectroscopic observations of the interacting binary CX Dra spanning a 23 year interval have been analyzed. This study includes a re—nement of the orbital solution of CX Dra ; equivalent width measurements that show short-, medium-, and long-term behavior of the diUerence pro- —les ; a calculation of the Balmer decrement ; velocity maps based on the velocity curves of the Ha and He I diUerence emission peaks ; trailed spectrograms of the Ha ,H b ,H e I ,and Si II lines ; and Doppler tomograms at these four wavelengths. The main conclusions are : 1. The circumstellar environment in the system changes in cycles of hundreds of days. The length of the cycles is variable. These cycles may be part of a ii super ˇˇ 4000 day cycle. 2. The equivalent widths of the diUerence Ha and He I j6678 lines are modulated with the orbital period of 6.696 days. The corresponding phase diagrams indicate a great deal of scatter, but the modula- tion is quite conspicuous when long data strings are used. The pattern of the phase diagram suggests a permanent presence of the emission in Ha ,H e I j6678, and Si II j6371 lines. 3. The radial velocities of the Ha emission peak follow an S-wave. The resulting velocity map shows that the source of the single-peaked emission lies close to the L1 point, roughly between the primary and L1 point. 4. Doppler tomograms constructed for Ha observed and diUerence pro—les show that the emission comes from a region of low velocity, a gas stream, and an accretion disk. The Hb emission arises from a region that is cospatial with the Ha source. The Doppler tomograms for He I j6678 and Si II j6371 lines suggest that emission in this gas also originates from a locus of a disk around the primary star. 5. The model based on the equivalent widths of the diUerence pro—les, the trailed spectrograms, and Doppler tomograms of the Ha ,H e I j6678, Si II j6371, and Hb lines suggests that the main source of the Ha emission is about halfway between the stars at a distance of 0.49a from the primary star, and that the He I and Si II emission sources arise from an accretion disk centered on the primary star. Subject headings : accretion, accretion disksbinaries : closecircumstellar matter ¨ stars : emission-line, Bestars : individual (CX Dra)

ANALYSIS OF THE Si IV ULTRAVIOLET SPECTRA OF U SAGITTAE

Archival IUE spectra of U Sge have been used to study hot (D105 K) circumstellar gas in this system. The observed spectra contain several UV resonance lines, of which the Si IV lines (jj1394 and 1405) are the strongest. These lines are observed in absorption, so we must remove the photospheric absorption line contribution to the spectra in order to see the emission produced by the circumstellar gas. We have developed a robust method for creating such diUerence spectra from IUE data. In the observed spectra we see a variation in the line pro—les and line centers with an orbital phase suggesting the presence of circumstellar gas. The residual emission seen in the diUerence spectra is strongest between phases / 0.3 and / 0.7, with a strength of up to 0.2 of the continuum —ux. This is con—rmed by the presence of an emission region visible in the Doppler tomogram of the j1394 line. This is consistent with the location of an Ha ii absorption zone ˇˇ seen in tomograms of U Sge and U CrB, and with theoretical predictions of a high temperature accretion region. Subject headings : accretion, accretion disksbinaries : eclipsingcircumstellar matter ¨ stars : individual (U Sagittae) ¨ ultraviolet : stars

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

The RS Canum Venaticorum characteristics of the infrared light curves of Algol

The 1.2-micron infrared light curve of Algol has been analyzed to determine the nature of the large scatter in the observations. This scatter is the result of time-dependent changes in the depths and phases of both eclipses, and in the asymmetric shape of secondary minimum. These changes can be explained by cyclic variations in the mean temperature of the secondary and by a photometric distortion of the light curve. This type of variability is similar to that seen in the optical light curves of RS Canum Venaticorum (or RS CVn) binaries which are distorted by the motions of starspots on the chromospherically active K-type secondaries in these systems. The discovery of similar photometric distortions in the 1.2-micron light curve of Algol provides evidence of starspot activity. In particular, this activity is associated with the K2 subgiant secondary in Algol. 31 refs.