Download Planet Mass ~13 M E

Variable Stars and Their Light
Curves
Arne Henden
Director, AAVSO
[email protected]
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 morphologybased
• 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 CCD
Gomez-Forrellad & Garcia-Melendo 1997
3 year
period;
eclipse was
a month long
Terrell 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
Credit: J. Skowron
Planet Mass
~13 ME
Credit: NASA
Note: amateurs
discovered closest
microlensed star
(Casseopeia)
November 2006; 8th
magnitude at peak
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
RRd
0.550 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…
V Hya
…or follow decades-long
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”