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Alphabet Soup

... or Defining Ourselves by What We Observe.

by Kate Hutton

The GCVS can be quite an alphabet soup, so I think it is appropriate to extract that part of it which will be under the purview of the LPV (long-period variable) Section.

I am not going to reproduce the entire list of GCVS types here. If you are interested, you can see it at the source: http://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt. A brief summary would help answer some questions that have come up on AAVSO-DIS, however. Basically, the GCVS or General Catalog of Variable Stars divides the subject matter into the usual broad classes of eruptive, pulsating, rotating, etc. variable stars.
[edit] GCVS Variability Types.

An improved system of variability classification is used in the fourth edition of the GCVS, based on recent developments in classification principles and taking into account the suggestions of a number of specialists. Variability types are grouped according to the major astrophysical reasons for variability, viz.,

  1. eruptive (FU, GCAS, I, IA, IB, IN, INA, INB, INT, IT, IN(YY), IS, ISA, ISB, RCB, RS, SDOR, UV, UVN, WR),
  3. rotating (ACV, ACVO, BY, ELL, FKCOM, PSR, SXARI),
  4. cataclysmic (explosive and novalike) variables (N, NA, NB, NC, NL, NR, SN, SNI, SNII, UG, UGSS, UGSU, UGZ, ZAND),
  5. eclipsing binary systems (E, EA, EB, EW, GS, PN, RS, WD, WR, AR, D, DM, DS, DW, K, KE, KW, SD),
  6. intense variable X-ray sources (X, XB, XF, XI, XJ, XND, XNG, XP, XPR, XPRM, XM),
  7. other symbols (BLLAC, CST, GAL, L:, QSO, S, *, +, :).
  8. the new variability types (ZZO, AM, R, BE, LBV, BLBOO, EP, SRS, LPB)
Notice that there is no classification called LPV. The LPV section will be concerned with any of those pulsating variables that take a relatively long time to go through their cycles. These include M (Mira), SR (semiregular), SRA, SRB, SRC & SRD (sub-classifications of SR), and L, LB & LC (poorly studied LPV's that have not been otherwise classified).

If we want to be one step more general, we might include any star that is appropriate to observe once per week to ten days. Then we could add RCB (R Corona Borealis stars) & possibly SDOR (S Doradus types) such as P Cygni. The RV Tauri types (RV, RVA & RVB) might also be included, although they do require somewhat more frequent observations.

Below, I am reproducing the relevant portions of the GCVS descriptions. I will leave discussion of pulsation modes & the relationships between these types to future posts by the scientific advisors.

Here goes:
  • M Mira (Omicron) Ceti-type variables. These are long-period variable giants with characteristic late-type emission spectra (Me, Ce, Se) and light amplitudes from 2.5 to 11 mag in V. Their periodicity is well pronounced, and the periods lie in the range between 80 and 1000 days. Infrared amplitudes are usually less than in the visible and may be <2.5 mag. For example, in the K band they usually do not exceed 0.9 mag. If the amplitudes exceed 1 - 1.5 mag , but it is not certain that the true light amplitude exceeds 2.5 mag, the symbol "M" is followed by a colon, or the star is attributed to the semiregular class with a colon following the symbol for that type (SR).
  • SR Semiregular variables, which are giants or supergiants of intermediate and late spectral types showing noticeable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to >2000 days, while the shapes of the light curves are rather different and variable, and the amplitudes may be from several hundredths to several magnitudes (usually 1-2 mag in V).
  • SRA Semiregular variables, late-type (M, C, S or Me, Ce, Se) giants displaying persistent periodicity and usually small (<2.5 mag in V) light amplitudes (Z Aqr). Amplitudes and light-curve shapes generally vary and periods are in the range of 35-1200 days. Many of these stars differ from Miras only by showing smaller light amplitudes;
  • SRB Semiregular variables, late-type (M, C, S or Me, Ce, Se) giants with poorly defined periodicity (mean cycles in the range of 20 to 2300 days) or with alternating intervals of periodic and slow irregular changes, and even with light constancy intervals (RR CrB, AF Cyg). Every star of this type may usually be assigned a certain mean period (cycle), which is the value given in the Catalogue. In a number of cases, the simultaneous presence of two or more periods of light variation is observed;
  • SRC Semiregular variables, late-type (M, C, S or Me, Ce, Se) supergiants (Mu Cep) with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days;
  • SRD Semiregular variable giants and supergiants of F, G, or K spectral types, sometimes with emission lines in their spectra. Amplitudes of light variation are in the range from 0.1 to 4 mag, and the range of periods is from 30 to 1100 days (SX Her, SV UMa).
  • L Slow irregular variables. The light variations of these stars show no evidence of periodicity, or any periodicity present is very poorly defined and appears only occasionally. Like for the type I, stars are often attributed to this type because of being insufficiently studied. Many type L variables are really semiregulars or belong to other types.
  • LB Slow irregular variables of late spectral types (K, M, C, S); as a rule, they are giants (CO Cyg). This type is also ascribed, in the GCVS, to slow red irregular variables in the case of unknown spectral types and luminosities.
  • LC Irregular variable supergiants of late spectral types having amplitudes of about 1 mag in V (TZ Cas).
  • RV Variables of the RV Tauri type. These are radially pulsating supergiants having spectral types F-G at maximum light and K-M at minimum. The light curves are characterized by the presence of double waves with alternating primary and secondary minima that can vary in depth so that primary minima may become secondary and vice versa. The complete light amplitude may reach 3-4 mag in V. Periods between two adjacent primary minima (usually called formal periods) lie in the range 30-150 days (these are the periods appearing in the Catalogue). Two subtypes, RVA and RVB, are recognized:
  • RVA RV Tauri variables that do not vary in mean magnitude (AC Her);
  • RVB RV Tauri variables that periodically (with periods from 600 to 1500 days and amplitudes up to 2 mag in V) vary in mean magnitude (DF Cyg, RV Tau).
  • 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).
  • 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.