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N.N. Samus Institute of Astronomy (Russian Acad. Sci.),

N.N. Samus Institute of Astronomy (Russian Acad. Sci.), Sternberg Astronomical Institute (Moscow University), and Eurasian Astronomical Society Modern Variable-Star Classifications

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N.N. Samus Institute of Astronomy (Russian Acad. Sci.),

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  1. N.N. Samus Institute of Astronomy (Russian Acad. Sci.), Sternberg Astronomical Institute (Moscow University), and Eurasian Astronomical Society Modern Variable-Star Classifications 7th Gamow Summer Astronomical School “Astronomy at Crossroads of Science: Astrophysics, Cosmology, Radio Astronomy, Astro-Biology”, Odessa, Ukraine, August 7–11, 2007

  2. WARNING: I am no expert in modern classification systems for variable stars, though quite an expert in traditional ones and the person responsible for collecting ideas for the new classification systems to be approved by the IAU. Classification begins as soon as people notice variable stars. Old chronicles: “a guest star” (Nova, Supernova, comet), then “Stella Nova”. 1596 – 1609: Fabricius discovers variability of Omicron Ceti (this Greek designation is from Bayer, 1603, an independent observation without noticing variations), and the star gets a new name, Mira, the name still used for a type of variable stars. Odessa, August 7–11, 2007

  3. The AAVSO light curve of Mira Ceti for the recent 60 years and the Fabricius monument in Germany Odessa, August 7–11, 2007

  4. Very many variable-star types have been introduced. The GCVS web site lists 114 types and subtypes of the classification system used in the GCVS IV (1985) and subsequent Name Lists, but some subtypes can be used additively, and the number of type and subtype combinations actually encountered in the GCVS is still larger. Conflicting signals from the community concerning what to do with the GCVS classification (“You have too many types to understand, impossible to remember; from a type, you do not get a clear idea of what the star you are dealing with can be like”; “Why don’t you introduce the new type I have invented in the GCVS? I have convincingly demonstrated it is physically meaningful”) Odessa, August 7–11, 2007

  5. The solution we propose: Let us reduce the number of types in the GCVS and make the classification simpler; let those wishing to have subtle subdivisions among the stars of their specific interest prepare specialized catalogs for corresponding variable-star classes. An additional requirement for a type to be used in the GCVS: It should be possible to attribute stars to this type on the base of observations comparatively easy to acquire in a routine study, not from very detailed studies of each particular star. Odessa, August 7–11, 2007

  6. An old example of a very detailed morphological classification for a particular type – that of H. Ludendorff (1928) for Miras: α – ascending light-curve branch considerably steeper than the descending branch subtypes α1, α2, α3, α4 (from very flat minima to narrower ones, shallower ascending branches) β – ascending branch same steep as descending branch or slightly steeper, generally symmetric curve subtypes β1, β2, β3, β4 from sharp to flat maxima γ – waves on ascending branch or double maxima γ1 – waves on ascending branch γ2 – double maxima A Mira star probably can change its type in this classification! The system has almost nothing to do with physics of Miras! Odessa, August 7–11, 2007

  7. Example of different approach – cataclysmic variables (among the best-studied groups, with physically reasonable classification) – the approach is not that different, after all! Odessa, August 7–11, 2007

  8. Example of different approach – young irregular variables (a very complex group, with old GCVS classification suggested by an expert in this particular field, wishing to introduce more types) – some simplification seems really possible! Odessa, August 7–11, 2007

  9. New automatic surveys: thousands of new variable-star discoveries, necessity of reliable automatic classification methods. Consider the associated problems using two of the best automatic surveys, ASAS-3 and NSVS, as an example. ASAS-3 (G. Pojmanski): small CCD cameras, calibrated V-band magnitudes, southern hemisphere. A catalog of variables discovered in the project: about 50000 variables, some 30000 of them new. Variability types in the ASAS-3 catalog: Ec, Esd, Ed; DSCT, RRc, RRab, DCEP, CW, Miras, ACV, BCEP; Miscellaneous. Sometimes the pulsation mode is indicated. The “type” Misc is the one most frequently used! Odessa, August 7–11, 2007

  10. Odessa, August 7–11, 2007

  11. Problems with automatic period determination and classification. One of the Galaxy’s longest-period fundamental-mode Cepheids in the ASAS-3 light-curve gallery Same star: A.V. Khruslov, PZP, 2005, 5, No. 8 Odessa, August 7–11, 2007

  12. An example of modern approach to ASAS-(1,2) variable-star classification: L. Eyer, C. Blake, 2005, MNRAS, 358, 30. A completely automatic algorithm applied. Six major types: eclipsing binaries, “sinusoidal curves”, Cepheids, small-amplitude red variables, SR, Mira stars. Twelve types is the more detailed version: ~EA, EB, ~EW, eclipsing (more marginal cases), Cepheids, Cepheids (with more marginal cases), LPVs, RR Lyrae candidates, various case class, small amplitude variables, SARVs, SRs, Miras. The authors estimate the type classification error level to be about 7 percent. In my opinion, first, this level is too high, and second, the authors underestimate it. Odessa, August 7–11, 2007

  13. Here it seems to be relevant to make an important note concerning all compact automatic classification systems. Looking for variable stars: what for? If we are not mainly after applied-science goals (like: We want to use this star for spacecraft orientation, what can we expect from it?) and are thinking about astrophysics, then we need 10000 new variable stars to find 100 really interesting objects among them. It is these 100 stars that will never be classified correctly – and not isolated as interesting cases – by existing automatic classification systems. Odessa, August 7–11, 2007

  14. The NSVS project: a part of the ROTSE-I project, small CCD cameras, northern hemisphere, instrumental R magnitudes. Though potentially as rich in new variables as ASAS-3, the project has announced 10 times fewer new variables so far. It has been demonstrated by T.I. Ignatieva (MSci Thesis, Moscow University, 2005) that some 18% of all stars announced by the NSVS team as new Cepheids were actually non-eclipsing RS CVn binaries with chromospheric activity. Odessa, August 7–11, 2007

  15. It is easy to recognize an eclipsing RS CVn binary: A new RS CVn Algol discovered by A. Khruslov (Tula) in the NSVS data Odessa, August 7–11, 2007

  16. RS CVn binaries (eclipsing or not) with periods in the Cepheid range are mostly quite detectable X-ray sources. Only the very brightest Cepheids ( Cep,  Aql) are known X-ray sources. This is the criterion used by Ignatieva but not taken into account by the NSVS team. Khruslov’s RS CVn star is the X-ray source 1RXS J060751.0+724636. Odessa, August 7–11, 2007

  17. A TYPICAL PROBLEM OF AUTOMATIC SKY SURVEYS No. 1 varies! Odessa, August 7–11, 2007

  18. The general structure of the GCVS IV classification scheme • Large classes (groups of types): • Eruptive variable stars • Pulsating variable stars • Rotating variable stars • Cataclysmic (explosive and nova-like) variables • Close binary eclipsing systems • Optically variable close-binary sources of strong variable X-ray radiation • These classes are just a way to group types; in our opinion, they can be kept if slightly better described (e.g., removeclose from item 5, simplify item 6). Moreover, users ask for more hierarchy in the classification! Odessa, August 7–11, 2007

  19. Our trial revision of the GCVS data for several southern constellations show that type changes are not needed for most stars. In the existing scheme, there can be combined types (like UV+BY), meaning the same star simultaneously having properties of two or more types. We suggest to add one more symbol combining types, in the cases when two or more classification options remain for the same star, for example, EC|Ell, EC|RR (here EC means a contact eclipsing binary, this is one of our suggestions from next slides). This symbol will permit us to have more information in the main GCVS Table, people do not read remarks very attentively. Moreover, it will simplify moving stars from the suspected-variable catalogue to the GCVS. Odessa, August 7–11, 2007

  20. For those knowing the current GCVS system and wishing to know our plans of its modification: PULSATING VARIABLES Present classification:Suggested: ACYG ACyg BCEP, BCEPS BCep (BCEPS seem not justified) CEP Cep (users suggest to have all Cepheids, Pop I and II, combined at a higher level) DCEP, DCEPS, CEP(B) DCep, DCep0, DCep1, DCepB01, DCepB12 Remark: 0 – fundamental, 1 – 1st overtone, 01 – fundamental plus overtone, 12 – 1st and 2nd overtones. For non-beat Cepheids, it is safer not to indicate the mode yet Odessa, August 7–11, 2007

  21. PULSATING VARIABLES (CONTINUED) Present classification:Suggested: CW, CWA, CWB CW (period will also be given) DSCT, DSCTC, SXPHE DSct (amplitude will also be given, population membership is not always obvious) L, LB, LC L, Lb (for red stars) M M (alternative idea: merge with former SRA) PVTEL PVTel RR, RRAB, RRC, RR(B) RR, RR0, RR1, RRB01 RV, RVA, RVB RV, RVb (if the secondary wave’s amplitude is at least equal to that of the main oscillation) Odessa, August 7–11, 2007

  22. PULSATING VARIABLES (CONTINUED) Present classification:Suggested: SR, SRA, SRB, SRC, SRS SR (the regularity level should be indicated by the presentation of the light elements; if a star is a supergiant, it will be obvious from its spectral type) (alternative: merge former SRA with Miras or combine all red stars at a higher level) SRD SRd (we feel not inclined to introduce the UU Her type) ZZ, ZZA, ZZB, ZZO ZZ (subtype is contained in the spectral type) BLBOO BLBoo (alternative: merge with Cepheids) GDOR GDor Odessa, August 7–11, 2007

  23. PULSATING VARIABLES (CONTINUED) Present classification:Suggested: RPHS RPHS (rapidly pulsating hot subdwards; we repeat our call for suggestions of a better designation) LPB or LBV SPB Odessa, August 7–11, 2007

  24. ERUPTIVE VARIABLES Present classification:Suggested: FU, I, IA, IB, INA, INB, I, TT, TTc, TTw, HAeBe INT, IT, IN(YY), IS, ISA, Remark: I is for irregulars with ISB doubts about their being young, TTc for classical and TTw, for weak-lined T Tauri stars, HAeBe for Herbig Ae/Be stars. Alternative: keep FU? GCAS, BE Be RCB RCB RS Move to rotating variables SDOR SDor UV, UVN UV Odessa, August 7–11, 2007

  25. ERUPTIVE VARIABLES (CONTINUED) Present classification:Suggested: WR WR New type: PP (a proto-planetary object) New type: dL (L dwarf), the causes of variability not quite clear yet Odessa, August 7–11, 2007

  26. ROTATING VARIABLES Present classification:Suggested: ACV, SXARI ACV ACVO roAp, ACV+roAp BY BY; also should include chromospherically active giants ELL Ell FKCOM FKCom PSR Psr R R RS (earlier among eruptive) Odessa, August 7–11, 2007

  27. CATACLYSMIC VARIABLES AND OTHER STARS WITH OUTBURSTS Present classification:Suggested: No subtype: CV N, NA, NB N, Nhf (final helium flash; suggestions of a different designation are welcome) NR Nr, NSym (symbiotic Nova), UGWZ (see below) NC NSym NL CV, UXUMa (“permanent outburst”), VY (VY Scl type, fadings from time to time) Odessa, August 7–11, 2007

  28. CATACLYSMIC VARIABLES AND OTHER STARS WITH OUTBURSTS (CONTINUED) Present classification:Suggested: SN, SNI, SNIISN (to avoid introducing the current complex classification) AM AM (including XM from X-ray variables), DQ (intermediate polars) UG, UGSS, UGZ UG, UGSS, UGZ UGSU UGSU, UGWZ (WZ Sge type, extremely long supercycles, large amplitudes), UG_ER (ER UMa type, short supercycles and normal cycles, small amplitudes; ideas of a better abbreviation are welcome) Odessa, August 7–11, 2007

  29. CATACLYSMIC VARIABLES AND OTHER STARS WITH OUTBURSTS (CONTINUED) Present classification:Suggested: ZAND Sym, NSym Odessa, August 7–11, 2007

  30. ECLIPSING VARIABLES (the description of the class should not mention close binarity) Present classification:Suggested: E, EA, EB, EW, EP E, EA, EC – contact (merging EB and EW), EP PN, WD, WR Remove from the classification, indicate through the spectral type or Remarks RS If such a star has eclipses, give notation like EA+RS, RS+EA AR, D, DM, DS, DW, K, Remove from the classification; KE, KW, SD as a rule, these types in the GCVS are based on indirect evidence Odessa, August 7–11, 2007

  31. X-RAY VARIABLES Present classification:Suggested: X, XB, XF, XI, XJ, XND, AM (move to cataclysmics), X, XN, XNB (approximately corresponds to XNG, a B star orbiting a compact object), Xlm (low mass), Xhm (high mass), Xmq (microquasars). In this class, advice from experts is especially needed Odessa, August 7–11, 2007

  32. THANK YOU! СПАСИБО! ДЯКУЮ! Odessa, August 7–11, 2007

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