Variable Stars - Cataclysmic
Hills Observatory: 1 January 2013 to 14 August 2018
Cataclysmic (Explosive and Novalike) variables are stars showing outbursts caused by thermonuclear burst processes in their surface layers (novae) or deep in their interiors (supernovae). We use the term "novalike" for variables that show novalike outbursts caused by rapid energy release in the surrounding space (UG-type stars - see below) and also for objects not displaying outbursts but resembling explosive variables at minimum light by their spectral (or other) characteristics. The majority of explosive and novalike variables are close binary systems, their components having strong mutual influence on the evolution of each star. It is often observed that the hot dwarf component of the system is surrounded by an accretion disk formed by matter lost by the other, cooler, and more extended component. This category is subdivided into the following types: N     Novae. Close binary systems with orbital periods from 0.05 to 230 days. One of        the components of these systems is a hot dwarf star that suddenly, during a time        interval from one to several dozen or several hundred days, increases its        brightness by 7-19 mag in V, then returns gradually to its former brightness over        several months, years, or decades. Small changes at minimum light may be        present. Cool components may be giants, subgiants, or dwarfs of K-M type. The        spectra of novae near maximum light resemble A-F absorption spectra of        luminous stars at first. Then broad emission lines (bands) of hydrogen, helium        and other elements with absorption components indicating the presence of a        rapidly expanding envelope appear in the spectrum. As the light decreases, the        composite spectrum begins to show forbidden lines characteristic of the spectra        of gas nebulae excited by hot stars. At minimum light, the spectra of novae are        generally continuous or resemble the spectra of Wolf-Rayet stars.  Only spectra        of the most massive systems show traces of cool components.               Some novae reveal pulsations of hot components with periods of approximately        100 s and amplitudes of about 0.05 mag in V after an outburst. Some novae        eventually turn out to be eclipsing systems. According to the features of their        light variations, novae are subdivided into fast (NA), slow (NB), very slow (NC)        and recurrent (NR) categories. NA   Fast novae displaying rapid light increases and then, having achieved maximum        light, fading by 3 mag in 100 or fewer days (GK Per); NB   Slow novae that fade after maximum light by 3 mag in >= 150 days (RR Pic).        Here the presence of the well-known "dip" in the light curves of novae similar to        T Aur and DQ Her is not taken into account:  The rate of fading is estimated on        the basis of a smooth curve, its parts before and after the "dip" being a direct        continuation of one another; NC   Novae with a very slow development and remaining at maximum light for more        than a decade, then fading very slowly. Before an outburst these objects may        show long-period light changes with amplitudes of 1-2 mag in V (RR Tel);  cool        components of these systems are probably giants or supergiants, sometimes        semiregular variables, and even Mira variables. Outburst amplitudes may reach        10 mag.  High excitation emission spectra resemble those of planetary nebulae,        Wolf-Rayet stars, and symbiotic variables. The possibility that these objects are        planetary nebulae in the process of formation is not excluded; NL   Novalike variables, which are insufficiently studied objects resembling novae by        the characteristics of their light changes or by spectral features. This type        includes, in addition to variables showing novalike outbursts, objects with no        bursts ever observed; the spectra of novalike variables resemble those of old        novae, and small light changes resemble those typical for old novae at minimum        light. However, quite often a detailed investigation makes it possible to reclassify        some representatives of this highly inhomogeneous group of objects into other        types; NR   Recurrent novae, which differ from typical novae by the fact that two or more        outbursts (instead of a single one) separated by 10-80 years have been observed        (T CrB). SN   Supernovae (B Cas, CM Tau). Stars that increase, as a result of an outburst, their        brightnesses by 20 mag and more, then fade slowly.  The spectrum during an        outburst is characterized by the presence of very broad emission bands, their        widths being several times greater than those of the bright bands observed in the        spectra of novae. The expansion velocities of SN envelopes are in the thousands        of km/s. The structure of a star after outburst alters completely. An expanding        emission nebula results and a (not always observable) pulsar remains at the        position of the original star. According to the light curve shape and the spectral        features, supernovae are subdivided into types I and II. SNI  Type I supernovae. Absorption lines of Ca II, Si, etc., but no hydrogen lines are        present in the spectra. The expanding envelope almost lacks hydrogen. During        20-30 days following maximum light, the brightness decreases by approximately        0.1 mag per day, then the rate of fading slows and reaches a constant value of        0.014/day; SNII  Type II supernovae. Lines of hydrogen and other elements are apparent in their        spectra. The expanding envelope consists mainly of H and He. Light curves show        greater diversity than those of type I supernovae. Usually after 40-100 days since        maximum light, the rate of fading is 0.1 mag per day. UG   U Geminorum-type variables, quite often called dwarf novae. They are close        binary systems consisting of a dwarf or subgiant K-M star that fills the volume of        its inner Roche lobe and a white dwarf surrounded by an accretion disk. Orbital        periods are in the range 0.05-0.5 days. Usually only small, in some cases rapid,        light fluctuations are observed, but from time to time the brightness of a system        increases rapidly by several magnitudes and, after an interval of from several        days to a month or more, returns to the original state. Intervals between two        consecutive outbursts for a given star may vary greatly, but every star is        characterized by a certain mean value of these intervals, i.e., a mean cycle that        corresponds to the mean light amplitude. The longer the cycle, the greater the        amplitude. These systems are frequently sources of X-ray emission. The        spectrum of a system at minimum is continuous, with broad H and He emission        lines. At maximum these lines almost disappear or become shallow absorption        lines. Some of these systems are eclipsing, possibly indicating that the primary        minimum is caused by the eclipse of a hot spot that originates in the accretion        disk from the infall of a gaseous stream from the K-M star. According to the        characteristics of the light changes, U Gem variables may be subdivided into        three types: SS Cyg, SU UMa and Z Cam. UGSS  SS Cygni-type variables (SS Cyg, U Gem). They increase in brightness by 2-6        mag in V in 1-2 days and in several subsequent days return to their original        brightnesses. The values of the ycle are in the range 10 days to several        thousand; UGSU  SU Ursae Majoris-type variables. These are characterized by the presence of        two types of outbursts called "normal" and "supermaxima". Normal, short        outbursts are similar to those of UGSS stars, while supermaxima are brighter by        2 mag, are more than five times longer (wider), and occur several times less        frequently.  During supermaxima the light curves show superposed periodic        oscillations (superhumps), their periods being close to the orbital ones and        amplitudes being about 0.2-0.3 mag in V. Orbital periods are shorter than 0.1        days; companions are of dM spectral type; UGZ  Z Camelopardalis-type stars. These also show cyclic outbursts, differing from        UGSS variables by the fact that sometimes after an outburst they do not return        to the original brightness, but during several cycles retain a magnitude between        maximum and minimum.  The values of cycles are from 10 to 40 days, while        light amplitudes are from 2 to 5 mag in V.        ZAND  Symbiotic variables of the Z Andromedae type. They are close binaries        consisting of a hot star, a star of late type, and an extended envelope excited by        the hot star's radiation. The combined brightness displays irregular variations        with amplitudes up to 4 mag in V. A very inhomogeneous group of objects.              
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