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Stellar Variability:
A Broad and Narrow Perspective
Monnier et al. 2007
J. Robert Parks
Georgia State University
Advisor: Russel White
Committee: Peter Plavchan (IPAC) John Monnier (Umich) Fabien Baron Doug Gies Gary Hastings Hal McAlister
Credit: NASA/JPL-Caltech/T. Pyle
Here's hoping nobody gets beheaded during or after this talk...
Types of Stellar Variability
Era of High Precision Astronomy
Photometry to parts per million, RV to m/s
Complicates detection and characterization of exoplanets
Complicates determination of fundamental parameters of stars
Explores stellar interiors
Explores stellar environments
Dumusque et al. 2012
http://astro.phys.au.dk/KASC/seismology/measurements.html
www.crh.noaa.gov
The Broad
Long temporal baseline, daily cadence photometric survey of a group of coeval stars.
The Narrow
High spatial resolution of a starspots on a stellar surface.
Young stellar objects often found in chaotic environments
Many different variability mechanism can be working concurrently
Temporal behavior along with changes in stellar brightness and color point to variability mechanism
Photometric Monitoring of Young Stars
Isella (2006)
Ophiuchi
Well studied low mass star forming region
D ~ 125 pc, Age ~ 1 Myr
Observations2MASS Cal-PSWDB, 8 x 1 FOV
1678 star, 1582 obs. per star (JHKs)~ 8 million data points
B ~ 2.5 yrs, cadence of ~ 1 day
Heavy visual extinction (Av ~ 5 to 25 mag)
Cool Starspots
Magnetic in origin
Stellar photosphere
Stellar colors not correlated with single band variability
PeriodicP < 14 days (Rebull 2001)
Hot Starspots
Accretion shock or magnetic flare
Stellar photosphere
Star becomes bluer as star brightens
Periodic or aperiodicP < 14 days (Rebull 2001)
TS ~ days
Variable Extinction
Circumstellar environment or intra-cloud material
Inner disk (~1 3 AU) or beyond
Star becomes redder as star dims
Periodic or aperiodicP < 250 days
TS > 50 days
Variable Mass Accretion
Circumstellar disk heats via mass transfer
Inner Disk (~1 3 AU)
Star becomes bluer as star dims
AperiodicTS ~ days to years
Variability Mechanisms
Cool Starspots
Hot Starspots (Accretion)
Extinction
Accretion
7 point variability test
101 (6%) variable stars
72 known YSOs, 79% variableClass I: 92% (12 of 13)
Class II: 72% (34 of 47)
Class III: 92% (11 of 12)
22 new candidate members
Variable sub-categoriesPeriodicSinusoidal-like
Eclipse-like
Long time-scale
Irregular
25 stars
Periods: 0.49 to 25.55 days
Ks 0.06 to 1.64 mag, median 0.29 mag
3 Class I, 8 Class II, & 8 Class III
Dominant mechansim -> cool starspots
Sinusoidal-like Periodic Variables
6 stars; all Class II
Periods: 2.95 to 8.00 days
Ks 0.21 to 0.51 mag, median 0.31 mag
Dominant mechansim -> extinction
Eclipse-like Periodic Variables
31 stars
7 Class I, 15 Class II, & 3 Class III
Time-scales: 64 to 790 days
Ks 0.05 to 2.31 mag, median 0.29 mag
Dominant mechansim -> extinction/accretion
Long Time-scale Variables
Irregular Variables
40 stars
1 Class I, 7 Class II, & 1 Class III
Ks 0.04 to 1.11 mag, median 0.14 mag
Dominant mechansim -> extinction/accretion
ISO-Oph 126
Ps = 9.114 0.090 daysKs = 0.06 mag(H-Ks) = 0.10 magCool Starspots
TS = 349 daysKs = 0.1 mag(H-Ks) = 0.37 magExtinction
WL 15
Ps = 19.412 0.085 daysKs = 0.90 mag(H-Ks) = 0.40 magCool Starspots (?)
TS 47 daysKs ~ 1 mag(H-Ks) = 0.50 magExtinction
Ps = 5.7752 0.0085 days
Pe = 5.9514 0.0014 days
Phased to Sinusoidal Period
Phased to Eclipse-like Period
P ~ 4 hours
Distinct to 20 levelYLW 1C
Potential hot proto-Jupiter
AA Tau-like systems found in other surveys [citations]
Driven occultations over millions of dynamical timescales
Fundamental to understanding planet formation
Sinusoidal-like Variability: Cool Starspots
Eclipse-like Variability: Extinction from occulting body
Huelamo et al. 2008
Starspots are common
Inhibit determining stellar properties, finding planets
Effects can be better understood if starspots could bespatially resolved, and
temporally monitored
Transition Slide
Previous Starspot Studies
Light Curve Inversion
Doppler Imaging
Indirect methods
Rely heavily on initial assumptions
Direct method Long Baseline Interferometric Imaging!
Primer I: UV & Aperture Synthesis
van Cittert-Zernike
Primer II: Visibility and Closure Phase
Visibility Source Distributionranges from 0 to 1
Closure Phase Source AsymmetryRanges from -180 to 180
Asymmetry if non-zero or 180
Monnier 2003
Primer III: Effects of Starspots on Vis & CP
Starspots warp diffraction pattern
180 visibility ambiguity; CP necessary to resolve
Andromeda
G8 III-IV
SB1
= 38.74 0.68 mas
D ~ 25 pcvsini = 6.5 km/sPphot ~ 54 days
H = 1.501 mag V = 0.22 mag
~ 2.75 mas
Andromeda is a bright, large, essentially single star with high amplitude photometric variability attributed to starspots.
Fairborn Observatory
V 0.22 mag
Cool starspot driven variability with P ~ 54 days
11 years
Long Term Photometric Monitoring
6 x 1-m telescopes
Dense [u,v] coverage
MIRC combiner in H band8 spectral channels
2010 photometric channels
2011 6 telescope combination
~0.4 mas maximum resolution
27 epochs from 2007 to 2011
S1
S2
E1
E2
W2
W1
Outer Array, S1-E1-W1-W2
Inner Array, S2-E2-W1-W2
CHARA/MIRC
Periodic Photometric Observations
P1 = 26.978 0.032 daysP2 = 54.25 0.91 daysP3 = 55.0 1.1 daysP4 = 54.8 1.9 daysPavg = 54.5 2.4 days
2007
2008
2009
2010
Parametric Model
SQUEEZE Reconstruction
GA and AMOEBA minimization
Free parameters:Stellar angular diameter
Limb-darkening coefficient
Starspot size, latitude, longitude, and flux ratio
Baron et. al 2010
Monte-Carlo Markov Chain minimization
Minimizes flux gradients
Starspots
Artifact
Angular Resolution
2007 & 2008 Observing Strategy
Nov 17th, 2007
1 Inner Array snapshot
Aug 17th through 21st, 2008 &Sep 20th and Sep 27th,2008
1 to 3 Outer Array snapshots
[u,v] Coverage
48 to 144 points
Each Data Block
6 visibilities
4 closure phase & triple amplitudes
Inconclusive results based on limited [u,v] coverage
2007 & 2008 Data Sets
2009 & 2010 Observing Strategy
Aug 24th & Aug 25th, 2009
Bracketed obs. with Outer Array during night's first half
Bracketed obs. with Inner Array during night's second half
Combine both sets of data
Repeat on consecutive night
Combine nights together
Aug 2nd through Sep 10th, 2010
Same as 2009 except Sep 10th
[u,v] Coverage
624 to 1128 points (336 on Sep 10th)
Each Data Block
11 visibilities
8 closure phase & triple amplitudes
Strategy provides nearly 8x improvement in [u,v] coverage and 2 additional closure phases
Indicates presence of starspots even at maximum brightness
2009 Data Set
ParameterModelRecon
14.1 4.13.6
b1-2.3 1.9-3.4
l1-13.4 5.3-26.8
Tr10.931 0.0240.922
22.2 2.22.0
b2-0.8 1.4-2.3
l222.9 2.021.1
Tr20.979 0.0100.924
2010 Data Set
2010 Light Curve
Points extracted from model images
Scaled to match level and amplitude of photometric light curve
2011 Observing Strategy
Sep 2nd & Sep 24th, 2011
Bracketed obs. with all 6 telescopes simultaneously
[u,v] Coverage
200 to 864 points
Each Data Block
11 visibilities
20 closure phase & triple amplitudes
Reduction in [u,v] coverage, however gains 12 additional closure phases
2011 Data Set
2011 Light Curve
2010 Andromeda Rotation
Pspots = 60 13 days
Pphot = 54.8 1.9 days
2011 Andromeda Rotation
Pspots = 54.0 7.6 days
Pphot = 54.5 2.4 days
Still Don't Believe Me?
Andromeda Rotation Axis
2010
2011
=20 6.8i =78 1.5
=23 6.4i =77.98 0.18
Conducted a high cadence, long baseline variability of the molecular cloud Oph
Characterized the amplitude and timescales of variability
Discovered candidate Oph members
Discovered a number of stars with obvious evidence of multiple variability mechanisms
Summary of Results I
Established that cool spots can be resolved on stars besides the Sun.
Tentative evidence for motion of cool starspots across stellar disc.
Performed ~2.5 year variability survey with a cadence of ~1 day.
Identified 101 variables from 1678 target stars
Identified 32 periodic variable stars
Determined variability time-scales for 31 stars
Tentatively identified dominant variability mechanism for most variable stars
Summary of Results II
Acknowledgements
Bonus Slides
Plavchan-Parks Algorithm
Determination of Time-scales
Parameter Extraction from Reconstructions
Simulated Images for Artifact Identification
Ph.D. Dissertation Defense
July 14th, 2014
7/11/14