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Variable Stars and Their Light Curves Arne Henden Director, AAVSO

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1 Variable Stars and Their Light Curves Arne Henden Director, AAVSO

2 Photometry Basics Brightness as a function of time Possible color information if you use more than one filter (visual is always one filter, no color vision at night). Wide-band photometry is poor mans spectroscopy. Almost always differential; comparing brightness of target vs. constant stars If combining estimates from more than one person/system, need to use same comparison stars and/or possibly transform data to same system

3 Light Curve Basics Morphology: shape and structure General Catalog of Variable Stars (GCVS) classifications almost always morphology- based Rule: plotting light curves is easy; understanding the underlying physics is hard Rule: light curves only tell part of the story; use ancillary information whenever possible Time-series light curves Phased light curves

4 W Virginis coverage from SRO - one season

5 W Vir phased light curve, BVRI

6 Dips, Bumps and Wiggles General morphological classifications Stars can exhibit one or more of these features Features can be transitory No two stars are alike Rule: to determine if a feature is periodic, you need to see it replicated at least twice, and preferably 3-5 times Rule: use two or more comparison stars, as any dip/bump might be in the comp rather than the target

7 Dips Momentary decrease in brightness of star Can be caused by extrinsic obscuration by another object (eclipsing binary, exoplanet transit) Can be intrinsic decrease (R CrB, VY Scl)

8 More dips Rule: Never trust an individual measurement (lots of equipment/sky problems can make one point bright or faint) Rule: Never trust a dip that occurs at the beginning or end of a time series (airmass changes cause systematic changes; twilight does the same) Rule: Eclipsing systems often have periods twice what you think (equal-depth minima)


10 Z UMi - a circumpolar RCB Note near complete BVRI coverage (dropouts due to summer monsoon) of this circumpolar object at SRO. 15:02: :03:49 Nearly grey

11 Eclipsing binaries Contact (K) - the two stars are in contact, usually no clean start/stop of eclipse Semidetached (SD) - the two stars are near one another, often ellipsoidal in shape, with perhaps Roche lobe overflow Detached (D) - no influence by one star on the other, usually flat between eclipses Period is helpful in determining category, but primary classifier is light curve shape

12 Modelling binaries Binary Maker 3 Windows only PHOEBE/Wilson-DeVinney, primarily Linux Usually require standardized filter photometry Multiple filters improve results as it gives temperature as well as geometry Gives orbit size, inclination, relative sizes of two stars

13 DU Leo 1.37d EA 13ks = 0.15d These and similar plots are from VGUIDE

14 Example of total eclipse in LD 282. Note flatness. USNO 1.0m data

15 V477 Cyg 2.35d 14ks =0.16d Rule: period often about 10x width of eclipse

16 IM Aur

17 IM Aur (EA, period= )

18 LD355 (note temp of secondary star)

19 Observations of HD with 6cm telescope and CCD Gomez-Forrellad & Garcia-Melendo year period; eclipse was a month long Terrell et al. 2003

20 Beta Lyr (Terrell)

21 BV 1005

22 FT UMa EW 0.655d

23 BV1004

24 DSct + EA

25 Transiting exoplanets Similar to detached light curves, with very small dip (planet is small compared to star) With high precision, eclipse has D-shape Eclipse gives size of planet; radial velocity wobble gives mass Excellent probe of stellar surface (limb darkening, star spots)

26 TrES-1

27 Bumps Momentary increase in brightness of star Almost always intrinsic (star gets brighter) Wide range of physics, from flare (M dwarf) to stellar disruption (SNe) Differentiate by luminosity, as outbursts look very similar

28 V344 Lyr (Still et al. ApJ)

29 SS Cyg,

30 Z Cam

31 Obtaining light curves of microlensing candidates Planet Mass ~13 M E Credit: J. Skowron Note: amateurs discovered closest microlensed star (Casseopeia) November 2006; 8th magnitude at peak Credit: NASA

32 Recent Novae

33 V838 Mon light curve

34 Two type Ia light curves (Hicken 2009) SN2007afSN2006X

35 SN 1987A

36 Light curve for a bright GRB afterglow, observed by amateurs

37 Wiggles Light curve that contains both bumps and dips Irregular when no obvious period can be determined (semiregular variables) Periodic include most pulsating stars, such as RR Lyr, Cepheid, Mira Periodic wiggles give information about stellar structure. Multiple periods probe the interior of the star. Can be radial or non-radial pulsation. Rule: dont trust catalog periods Rule: primary classification by period

38 DX Cet 0.104d

39 GG UMa 0.135d

40 V703 Sco

41 AQ Leo RRd


43 W Vir phased light curve, BVRI

44 SU Cyg Classical Cepheid Period 3.84 days Note phase shift, amplitude and shape change with wavelength (Madore & Freedman, 1991

45 You can observe single pulsation cycles… …or follow decades-long trends V Hya

46 Resources AAVSO Variable Star of the Season archive: GCVS web site: J.R. Percy, "Understanding Variable Stars D. Terrell, J.D. Mukherjee & R.E. Wilson, "Binary Stars: A Pictorial Atlas C. Sterken & C. Jaschek, "Light Curves of Variable Stars: A Pictorial Atlas J. Kallrath & E.F. Milone, "Eclipsing Binary Stars: Modeling and Analysis"

47 Resources Chandra Variable Guide Star Catalog: G. Foster, Analyzing Light Curves

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