Variable Stars - Eruptive
Hills Observatory: 1 January 2013 to 4 November 2018
Eruptive variables are stars varying in brightness because of violent processes and flares occurring in their chromospheres and coronae. The light changes are usually accompanied by shell events or mass outflow in the form of stellar winds of variable intensity and/or by interaction with the surrounding interstellar medium. This class includes the following types: FU   Orion variables of the FU Orionis type. Characterized by gradual increases in        brightness by about 6 mag in several months, followed by either almost complete        constancy at maximum that is sustained for long periods of time or slow decline        by 1-2 mag.  Spectral types at maximum are in the range Ae(alpha) -        Gpe(alpha).  After an outburst, a gradual development of an emission spectrum        is observed and the spectral type becomes later. These variables probably mark        one of the evolutionary stages of T Tauri-type Orion variables (INT), as evidenced        by an outburst of one member, V1057 Cyg, but its decline (2.5 mag in 11 years)        commenced immediately after maximum brightness was attained. All presently        known FU Ori variables are coupled with reflecting cometary nebulae.        GCAS   Eruptive irregular variables of the Gamma Cas type. These are rapidly rotating        B III-IVe stars with mass outflow from their equatorial zones. The formation of        equatorial rings or disks is often accompanied by temporary fading. Light        amplitudes may reach 1.5 mag in V.        I      Poorly studied irregular variables with unknown features of light variations and        spectral types. This is a very inhomogeneous group of objects.        IA    Poorly studied irregular variables of early (O-A) spectral type. IB    Poorly studied irregular variables of intermediate (F-G) to late (K-M) spectral        type. IN    Orion variables. Irregular, eruptive variables connected with bright or dark diffuse        nebulae or observed in the regions of these nebulae. Some of them may show        cyclic light variations caused by axial rotation. In the Spectrum-Luminosity        diagram, they are found in the area of the main sequence and subgiants. They        are probably young objects that, during the course of further evolution, will        become light-constant stars on the zero-age main sequence (ZAMS). The range        of brightness variations may reach several magnitudes. In the case of rapid light        variations having been observed (up to 1 mag in 1-10 days), the letter "S" is        added to the symbol for the type (INS). This type may be divided into the        following subtypes: INA  Orion variables of early spectral types (B-A or Ae). They are often characterized        by occasional abrupt Algol-like fadings (T Ori); INB  Orion variables of intermediate and late spectral types, F-M or Fe-Me (BH Cep,        AH Ori).  F-type stars may show Algol-like fadings similar to those of many INA        stars; K-M stars may produce flares along with irregular light variations;        INT  Orion variables of the T Tauri type. Stars are assigned to this type on the basis of        the following (purely spectroscopic) criteria:  spectral types are in the range        Fe-Me. The spectra of most typical stars resemble the spectrum of the solar        chromosphere. The feature specific to the type is the presence of the flourescent        emission lines Fe II 4046, 4132 A (anomalously intense in the spectra of these        stars), emission lines [Si II] and [O I], as well as the absorption line Li I 6707 A.        These variables are usually observed only in diffuse nebulae. If it is not apparent        that the star is associated with a nebula, the letter "N" in the symbol for the type        may be omitted, e.g., IT (RW AUR); IN(YY) Some Orion variables (YY Ori) show the presence of absorption components on        the redward sides of emission lines, indicating the infall of matter toward the        stars' surfaces. In such cases, the symbol for the type may be accompanied by        the symbol "YY". IS    Rapid irregular variables having no apparent connection with diffuse nebulae and        showing light changes of about 0.5 - 1.0 mag within several hours or days.        There is no strict boundary between rapid irregular and Orion variables. If a rapid        irregular star is observed in the region of a diffuse nebula, it is considered an        Orion variable and designated by the symbol INS. To attribute a variable to the IS        type, it is necessary to take much care to be certain that its light changes are        really not periodic. Quite a number of the stars assigned to this type in the third        edition of the GCVS turned out to be eclipsing binary systems, RR Lyrae        variables, and even extragalactic BL Lac objects. ISA  Rapid irregular variables of the early spectral types, B-A or Ae; ISB  Rapid irregular variables of the intermediate and late spectral types, F-M and        Fe-Me. RCB Variables of the R Coronae Borealis type. These are hydrogen-poor, carbon- and        helium-rich, high-luminosity stars belonging to the spectral types Bpe-R, which        are simultaneously eruptive and pulsating variables. They show slow nonperiodic        fadings by 1-9 mag in V lasting from a month or more to several hundred days.        These changes are superposed on cyclic pulsations with amplitudes up to several        tenths of a magnitude and periods in the range 30-100 days.        RS   Eruptive variables of the RS Canum Venaticorum type. This type is ascribed to        close binary systems with spectra showing Ca II H and K in emission, their        components having enhanced chromospheric activity that causes quasi-periodic        light variability. The period of variation is close to the orbital one, and the        variability amplitude is usually as great as 0.2 mag in V (UX Ari). They are X-ray        sources and rotating variables. RS CVn itself is also an eclipsing system (see        below). SDOR   Variables of the S Doradus type. These are eruptive, high-luminosity Bpec-        Fpec stars showing irregular (sometimes cyclic) light changes with amplitudes in        the range 1-7 mag in V.  They belong to the brightest blue stars of their parent        galaxies.  As a rule, these stars are connected with diffuse nebulae and        surrounded by expanding envelopes (P Cyg, Eta Car). UV   Eruptive variables of the UV Ceti type, these are K Ve-M Ve stars sometimes        displaying flare activity with amplitudes from several tenths of a magnitude up to        6 mag in V. The amplitude is considerably greater in the ultraviolet spectral        region.  Maximum light is attained in several seconds or dozens of seconds after        the beginning of a flare; the star returns to its normal brightness in several        minutes or dozens of minutes. UVN Flaring Orion variables of spectral types Ke-Me. These are phenomenologically        almost identical to UV Cet variables observed in the solar neighborhood. In        addition to being related to nebulae, they are normally characterized by being        of earlier spectral type and greater luminosity, with slower development of flares        (V389 Ori). They are possibly a specific subgroup of INB variables with irregular        variations superimposed by flares. WR  Eruptive Wolf-Rayet variables. Stars with broad emission features of He I and        HE II as well as C II-C IV, O II-O IV, and N III-N V.  They display irregular light        changes with amplitudes up to 0.1 mag in V, which are probably caused by        physical processes, in particular, by nonstable mass outflow from their        atmospheres.
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