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Variable Stars and Their Light Curves

This presentation by Arne Henden, director of AAVSO, provides a comprehensive overview of variable stars and their light curves. It covers fundamental concepts in photometry, including measuring brightness over time, differential photometry techniques, and the physics behind light curves. Key topics include morphological classifications, the significance of dips and bumps in light curves, and the use of multiple comparison stars for effective data analysis. The presentation emphasizes the importance of accurate measurements and understanding the underlying astrophysical processes that govern stellar variability.

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Variable Stars and Their Light Curves

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  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)

  9. 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”

  10. 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

  11. 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

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

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

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

  15. IM Aur

  16. IM Aur (EA, period=1.247296)

  17. LD355 (note temp of secondary star)

  18. 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

  19. Beta Lyr (Terrell)

  20. BV 1005

  21. FT UMa EW 0.655d

  22. BV1004

  23. DSct + EA

  24. 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)

  25. TrES-1

  26. 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

  27. V344 Lyr (Still et al. ApJ)

  28. SS Cyg, 1896-2004

  29. Z Cam

  30. 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

  31. Recent Novae

  32. V838 Mon light curve

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

  34. SN 1987A

  35. Light curve for a bright GRB afterglow, observed by amateurs

  36. 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

  37. DX Cet 0.104d

  38. GG UMa 0.135d

  39. V703 Sco 0.115 0.150

  40. AQ LeoRRd0.550 0.410

  41. W Vir phased light curve, BVRI

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

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

  44. 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"

  45. Resources • Chandra Variable Guide Star Catalog: http://cxc.harvard.edu/vguide/index.php • G. Foster, “Analyzing Light Curves”

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