Variable Stars and Their Light Curves

1. Variable Stars and Their Light Curves Arne Henden Director, AAVSO arne@aavso.org

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 man’s 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. 15:02:01.3 +83:03:49 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 http://www.binarymaker.com Windows only • PHOEBE/Wilson-DeVinney, primarily Linux http://phoebe.fiz.uni-lj.si/ • 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=1.247296)

18. LD355 (note temp of secondary star)

19. Observations of HD126080 with 6cm telescope and CCD Gomez-Forrellad & Garcia-Melendo 1997 3 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, 1896-2004

30. Z Cam

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

32. Recent Novae

33. V838 Mon light curve

34. SN2007af SN2006X Two type Ia light curves (Hicken 2009)

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: don’t trust catalog periods • Rule: primary classification by period

38. DX Cet 0.104d

39. GG UMa 0.135d

40. V703 Sco 0.115 0.150

41. AQ LeoRRd0.550 0.410

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… V Hya …or follow decades-long trends

46. Resources • AAVSO Variable Star of the Season archive: http://www.aavso.org/vsots_archive • GCVS web site: http://www.sai.msu.su/gcvs/gcvs/index.htm • 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: http://cxc.harvard.edu/vguide/index.php • G. Foster, “Analyzing Light Curves”