variable stars and their light curves arne henden director, aavso [email protected]
TRANSCRIPT
Variable Stars and Their Light Curves
Arne Henden
Director, AAVSO
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
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
W Virginis coverage from SRO - one season
W Vir phased light curve, BVRI
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
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)
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)
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”
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
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
DU Leo 1.37d EA 13ks = 0.15d
These and similar plots are from VGUIDE
Example of total eclipse in LD 282. Note flatness. USNO 1.0m data
V477 Cyg 2.35d 14ks =0.16d
Rule: period often about 10x width of eclipse
IM Aur
IM Aur (EA, period=1.247296)
LD355 (note temp of secondary star)
Observations of HD126080 with 6cm telescope and CCDGomez-Forrellad & Garcia-Melendo 1997
3 year period; eclipse was a month longTerrell et al. 2003
Beta Lyr (Terrell)
BV 1005
FT UMa EW 0.655d
BV1004
DSct + EA
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)
TrES-1
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
V344 Lyr (Still et al. ApJ)
SS Cyg, 1896-2004
Z Cam
Obtaining light curves of microlensing candidates
Planet Mass
~13 ME
Credit: J. Skowron
Note: amateurs discovered closest microlensed star (Casseopeia) November 2006; 8th magnitude at peak
Credit: NASA
Recent Novae
V838 Mon light curve
Two type Ia light curves (Hicken 2009)SN2007af SN2006X
SN 1987A
Light curve for a bright GRB afterglow, observed by amateurs
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
DX Cet 0.104d
GG UMa 0.135d
V703 Sco 0.115 0.150
AQ Leo
RRd0.550 0.410
W Vir phased light curve, BVRI
SU Cyg
Classical Cepheid
Period 3.84 days
Note phase shift, amplitude and shape change with wavelength
(Madore & Freedman, 1991
You can observe single pulsation cycles…
…or follow decades-long trends
V Hya
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"
Resources
• Chandra Variable Guide Star Catalog: http://cxc.harvard.edu/vguide/index.php
• G. Foster, “Analyzing Light Curves”