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Searching for Planets
Around White Dwarf Stars
Fergal Mullally
Princeton
- or -
I still haven’t found what I’m looking for
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The Big Goal Direct Detection of Extra-solar planets
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The Big Challenge: Need high contrast observations:
HR 8799 (Marois et al 2008)
~30AU
Formalhaut (Kalas et al. 2008)
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White Dwarf Stars mitigate our chief constraint
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Other advantages
• High proper motions
• Improved contrasts for high mass progenitors
• Different age range (108--109 yr), compared to young MS stars
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Finding planets around pulsating white dwarf stars
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White Dwarf Stars A brief review
• All stars with M < 8-10 M will eventually become white dwarf stars
• That’s ~ 98 % of all stars
• Progenitor systems live for 108-1010 years, and age can be estimated
• Faint: L* ~ 10-3--10-4L
• WD planets at larger orbital separations than progenitor star
• Few, very broad absorption lines
• Unsuitable for precision radial velocity measurements
• Instrumentation coming on-line now that can detect Jovian analogues around WDs (GPI, HiCIAO).
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http://whitedwarf.org
l =1 m =0
l =2 m =1
l =3 m =0
DAVs are multi-periodic, non-radial pulsating white dwarf stars
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DAVs divide into two sub-types Br
ight
ness
Time
hDAVs
• Small number of modes
• Symmetric pulse shapes
• Periods of 100-400s
• Low amplitude
cDAVs
• Often many modes
• Sharply peaked pulse shapes
• 500-1000s
• Larger amplitude
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Some DAVs show extreme stability in phase of pulsation
(dP/dt ~ 10-15)
25 years of data Kepler et al. (2005)
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Cooling of the star causes a monotonic increase in period
...which causes a parabolic trend in arrival time
Kepler (2005), Kawalar (1986)
...which can be used to test models of stellar evolution
rate of period change
...which can be used to measure core composition...
Avg. atomic mass of nucleons in core
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Distance from Earth
• Change in arrival time (O-C) greater for planets with: – Large Mass – Large orbital
Separation – Orbits along our line
of sight.
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A planet around an sdB star
Silvotti et al. (2007) O-C values after effect of secular cooling removed
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The McDonald Observatory WD Planet Search Program Mullally et al. (2008)
Sample of 15 DAVs with the 2.1m Otto Struve telescope at McDonald Observatory
Searching for planets through variations in the pulsation arrival time (same as Silvotti 2008)
Data spans 2003 to 2009 with timing uncertainties of 0.5--2 seconds
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Results WD1354+0108 Mullally 2008
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Results WD1355+5454
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Results WD0214-0823
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Results WD2214-0025
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Results R548
1970
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Results G117-B15A
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A Promising Candidate GD66 as of July 2007
Pulsations later than expected
Pulsations earlier than expected
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Cooling of the star produces a parabola in the O-C diagram
Effect of cooling predicted, and observed to be ~10-15 in another DAV, G117-B15A, but 10-12 in GD66
Cooling rate is dependent on average atomic weight of core
Average atomic weight of core must be ≈ 5900 to explain such a high
What else could it be? Cooling of the star?
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d
x √d2 + x2
Proper motion produces a parabolic change in distance to the star and therefore a parabola in the O-C diagram
What else could it be? Proper motion?
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Proper motion is a smaller effect even than cooling
Pajdosz (1995)
• µ= 133 mas/yr
•
• P=302s d≈ 50pc • pm= 6.5 x 10-16
• obs ~ 10-12
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GD66
No known effect can explain observed O-C diagram for GD66
Of course, there’s always the unknown...
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G29-38: A DAV that seemed to have a companion...
(but doesn’t)
Winget (1990)
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12,000 K DA White Dwarf model spectrum
10MJ 1 Gyr old model planet spectrum
Seeing is believing Confirmation through direct detection with Spitzer
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10MJ 1 Gyr old model planet spectrum
Combined Spectrum
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10MJ 1 Gyr old model planet spectrum
Combined Spectrum
J H K Spitzer IRAC bands
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Credit: NASA/JPL-Caltech
The Plan
If the planet is bright enough, we can detect it as an excess flux at 4.5µm relative to 3.8µm using IRAC on Spitzer.
We observed GD66 for 1600s at 3.8µm and 6400s at 4.5µm
We observed ZZ Ceti and L19-2 as calibration stars to determine the expected flux ratio
Limit on the least massive planet we can detect is set by the systematic uncertainty in IRAC.
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Detection Limit is set by our systematic uncertainty
BD +60 1753
Photometric Error Repeat observations of the same star produces a spread in values greater than the photometric error.
This places a detection limit on any excess of 0.54%
Flux Ratio (Ch2/Ch1)
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Result
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Result
There does seem to be an slight excess, but it’s only 1.7σ signifcant
GD66
ZZ Ceti L19-2
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Teff = 11,989 K Bergeron (2004) log(g)=8.05
⇒ Mass = 0.64 M Wood (1992) " Cooling Time = 500 Myr
⇒ Main-sequence Mass = 2.26--2.64 M Kalirai (2007), Dobbie (2006), Meng (2007)
Main-sequence Age = 700--1100 Myr Pols (1998)
Total Age: 1.2--1.6 Gyr
Upper limit on companion mass requires an estimate of age
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ref
Isolated WD
Planet models from Burrows (2003)
Upper limit on planet mass is 5-7MJ
Mullally (2009)
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So, Spitzer observations didn’t see anything conclusive
But additional ground based observations will soon cover an entire orbit, right?
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More ground based observations GD66 as of July 2007 2008
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More ground based observations GD66 as of July 2008
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Most recent observations
Circular Fit
Eccentric Fit
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More ground based observations
Amplitude 5.42(33) sec
Period 6.50(28) years
Orbital Separation 3.003(99) AU
Mass >2.34(15) MJ
Current best fit parameters
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More ground based observations
Amplitude 5.953(26) sec
Period 6.78(01) years
Eccentricity 0.237(24) AU
Mass >2.34(15) MJ
Current best fit parameters to eccentric fit
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Future work Confirmation from an independent pulsation period
Brig
htne
ss
Time
302s
271s
197s
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The End.
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