N95- 27078 13 DISCUSSION ON SELECTED SYMBIOTIC STARS R. Viotti and M. Hack I. INTRODUCTION made of its variations" (Mayall, 1969). This pessimistic remark should be consid- Because of its large variety of aspects, the ered as a note of caution for those involved in symbiotic phenomenon is not very suitable for the interpretation of the observations. In the a statistical treatment. It is also not clear following, we shall discuss a number of indi- whether symbiotic stars really represent a vidual symbiotic stars for which the amount of homogeneous group of astrophysical objects or observational data is large enough to draw a a collection of objects of different natures but rather complete picture of their general behav- showing similar phenomena. However, as al- ior and to make consistent models. We shall ready discussed in the introduction to the sym- especially illustrate the necessary steps toward biotic stars, in this monograph we are espe- an empirical model and take the discussion of cially interested in the symbiotic phenomenon, the individual objects as a useful occasion to i.e., in those physical processes occurring in describe different techniques of diagnosis. the atmosphere of each individual object and in their time dependence. Such a research can be I1. Z ANDROMEDAE AND THE DIAGNOS- performed through the detailed analysis of TICS OF THE SYMBIOTIC STARS individual objects. This study should be done for a time long enough to cover all the different II.A. INTRODUCTION phases of their activity, in all the spectral ranges. Since the typical time scale of the symbiotic phenomena is up to several years and Z And has been considered as the prototype decades, this represents a problem since, for of the symbiotic stars, from its light history and instance, making astronomy outside the visual the spectral variation during outburst. For a region is a quite new field of research. It was a long time, its light curve has been the basis for fortunate case that a few symbiotic stars (Z theoretical studies of the symbiotic stars. The And, AG Dra, CH Cyg, AX Per, and PU Vul) optical spectrum of the star has been studied had undergone remarkable light variations (or extensively since the pioneer work of Plaskett "outbursts") in recent years, which could have (1928). Of particular interest is the work of been followed in the space ultraviolet with Swings and Struve (1941; see also Swings, IUE, and simultaneously in the optical and IR 1970) describing the spectral evolution during with ground-based telescopes. But, in general, and after 1939 outburst, and of Boyarchuck the time coverage of most of the symbiotic (1968), who discussed the 1960 outburst and objects is too short to have a complete picture the following decline phase. Ultraviolet obser- of their behavior. In this regard, one should vations started after the launch of the Interna- recall Mayall's remark about the light curve of tional Ultraviolet Explorer (Altamore et al., Z And: "Z Andromedae is another variable that 1981) and continued to the time of writing the shows it will requires several hundred years of present review. In particular, this allowed the observations before a good analysis can be study in the UV of the two minor outbursts that 663 occurredin 1984and 1985.Theseobserva- emissionlines of several different atomic spe- tions,togetherwith theextensivemonitoring cies and with a large range of ionization en- of theIR andof theemissionlineprofiles,still ergy. A compendium of the ions whose transi- makeZ Andoneof thebest-studiedsymbiotic tions have been identified in the optical spec- stars,andanidealtargetfor investigationthe trum of Z And was given in Table 11-2. Both symbioticphenomenon. permitted and forbidden transitions are pres- ent, as well as lines typical of stellar chromo- Thevisualluminosityof Z Andis variable spheres, Be, Of, and WR stars, planetary nebu- betweenV = 8to 11ontimescalesfromafew lae, coronal regions, etc. These features are daysto severalmonths,withoutclearevidence also seen in the ultraviolet. Altamore et al. of anyperiodicity.Thefull lightcurveisrepro- (1981) identified low-excitation lines of OI, ducedin Figure11-2in Chapter11.Figure13- MglI, and Fell, and high-ionization lines of 1 is a condensedplot of IO0-daymeansof Hell, OV, and MgV]. But the main result is that photographicand visual observationsfrom the intensity of the emission lines is largely 1887to 1969(Mayall,1969). variable on long time scale. Variation of H(x was reported by Altamore et al. (1979), while Altamore et al. (1982) found that in November 1982, the UV continuum and emission lines Thelightcurveis characterizedby (1)four mainphasesof higherluminositystartingin were about 40 percent stronger than in August 1895,1914,1939,and1959,andby(2)periods 1980. They also noted that the IR spectrum did of lowluminositylastingroughlyonedecadein not show a significant change since 1981. From between.In thefollowing,weshalldiscussthe a more thorough analysis of the UV observa- tions collected since 1978 with the IUE satel- behaviorofZ Andduringquiescenceandactiv- ity. lite, Fernandez-Castro et al. (1988) found that the UV continuum and the emission line fluxes vary quasi-periodically on a time scale of about 760 days. The amplitude is larger for the ll.B. THEBEHAVIOROFZ ANDDURING Balmer near-UV continuum and for most of the QUIESCENCE emission lines, and lower for the far-UV con- tinuum and the CIII] line. The time variation of the SilII]/CIII] ratio suggests a variable mean Themostrecentquiescentphase(1972to electron density of the emitting region from 0.6 about1984)of Z Andwasalsothelongest,and to 2.2 x 10 "_ cm -3 in phase with the UV light thisfact hasalloweda detailedstudyof the curve. At maximum, the density and the emis- behaviorof a symbioticstarduringminimum sion measure are larger, while the effective light. The optical-ultravioletspectrumof Z emitting volume is smaller. Thus at maximum, Andat minimumisrich in strongandnarrow the emission mostly comes from a compact 1_ 1_ 20000 2_ 3_ 3_ 40000 61 I I I I I I - 6 10- -10 8- _ -8 12 - -12 I I I I I I I I I 1890 1900 1910 1920 1930 1940 1950 1960 1970 232848 Z ANDROMEDAE, 1887-1969 Figure 13-1. Plot of the lO0-day average magnitudes of Z And during 1887 to 1969 (from Mayall, 1969). 664 region,andatminimum,theemittingvolumeis lengths, but this needs to be confirmed by fu- morediluted.Theseobservationsareconsis- ture observations. tentwith a modelof highly ionizeddiffuse regionthat is periodicallyocculted,leaving visibleonlytheouterlow-densityregions. II.C. THE ACTIVE PHASES OF Z AND Muchaboutthenatureof Z And canbe learnedfromthesimultaneousstudyin differ- The active phase is characterized mostly by entwavelengthregions.Figure13-2illustrates an increase of one to two magnitudes of the thevariationsof Z AndfromUVto IRduring visual magnitude after a long-lasting quiescent thelastquiescentphase. phase. The brightening is rather slow, the rise time being about 100 days/mag, even com- ThelargeUVvariabilityisclearlypresentin pared with the slow novae. This "outburst" is theU-bandandin theHcf-variationandcanbe generally followed by a sequence of minima tracedbackto beforethefirstultravioletobser- and maxima resembling a damped oscillator. vation.Thevariationis lessevidentin thevis- The time interval between two successive ual.Duringminimum,V variedbetween10.2 maxima (or minima) is not constant, but varies and11.0in anapparentlyirregularmode.This from 31(1 to 79(1 days (Mallei 19781 in an ir- variationduringminimumhasbeennotedby regular way and is not in phase with the UV- severalauthors,(see Kenyon,1986),and variability during quiescence discussed above. KenyonandWebbink(19841,fromtheanalysis of all theminimaduringquiescence,foundthat During the rise to outburst and the subse- theyareclearlyperiodicwithameanperiodof quent oscillations, the optical spectrum under- 756.85daysin agreementwiththeUVandHa goes large changes, which have been exten- results.The radialvelocityof the M-giant sively described in a number of papers. As the componentwas measuredby Garcia and stellar luminosity increases, the high-ioniza- Kenyon(19881overabout6years.Theyfound tion lines fade, and at maximum light, the spec- a smoothedsinusoidalcurvewith a periodof trum displays a strong blue continuum with 750+ 8 days, and a semiamplitude of K = 8.1 prominent hydrogen emission. These lines + 0.5 km s t. This fact provides further evidence have absorption cores that dominate the emis- of binarity, but the radial velocity and photom- sion at the higher members of the Balmer se- etric curves do not have the relative phasing (a ries. In some cases, P Cygni profiles are present quarter of phase) expected from eclipse or re- in the H and Hei lines. At maximum, the ab- flection effects. This might suggest a more sorption bands of the M spectrum are hardly complex geometry of the system, as also sug- visible. However, the weakening of the cool gested by the ultraviolet observations as dis- spectrum is only apparent since, as it has been cussed below. Finally, concerning the infrared, found, for instance Boyarchuck (1968), the Taranova and Yudin (1981) and others reported TiO bands are only veiled by the enhanced blue small fluctuations, clearly not in phase with the continuum, while the luminosity of the cool UV variations. These can be attributed to the component has not changed within the errors. irregular behavior of the late-type component The high-ionization lines and the TiO absorp- that is a common feature of the normal (normal tion bands strengthen again as the blue contin- = not a symbiotic or peculiar system) late-type uum decreases during the light fading. giant and supergiant stars. The little variability of the cool component was also noted by Alta- Recently, two minor outbursts of Z And more et al. (1979, 1981) who, from their analy- were reported. After nearly 12 years of quies- sis of a collection of blue-infrared objective cence, the star brightened to V = 9.6 in March prism plates taken during October 1977 to June 1984 and again to V = 10.1 in September 1985 1979, found that the near-lR continuum re- (Mattei, 1984, 1985). The 1984 outburst dis- mained constant within +0.1 mag.