Transcript Imaging Planets around White Dwarfs - X

DEPARTMENT OF PHYSICS AND ASTRONOMY
Searching for dying solar systems:
Planets around White Dwarfs
Matt Burleigh, Leicester
Fraser Clarke, Oxford
Emma Hogan, Leicester
Simon Hodgkin, Cambridge
Improving Contrast
• To understand extrasolar planets, we need
their light!
• None of the radial velocity planets can be
imaged with current technology
• Planet is too faint and too close to the star
• Two solutions;
– Remove the starlight (technology: AO,
coronography, interferometry)
– Remove the star (stellar evolution)
Above: Gl229B – brown dwarf
companion to nearby M dwarf
Observe White Dwarfs!
Dr. Matt Burleigh
RAS November 2004
Surviving the RGB
• Red Giant expands to maximum ~5AU
• Planets inside 5AU are destroyed by
Red Giant
• Planets outside ~5AU will migrate
outward due to mass loss and survive
– Dynamical time ~10-30 yr
– mass loss time ~1000-10000yr
– planets stay bound
Dr. Matt Burleigh
RAS November 2004
The Benefits of White Dwarfs
• WD's are ~10,000 times fainter than
their progenitors
• => Huge contrast gain
• Planets orbit increases
• => Big resolution gain
• And >120 WDs within 20pc
Dr. Matt Burleigh
RAS November 2004
The Benefits of White Dwarfs
• PLUS the WD progenitors are more
massive than solar-type stars
– Mainly A and B stars
– Not generally being targeted by radial velocity
programmes
• So by targeting WDs we are probing
frequency of planets around massive stars
Dr. Matt Burleigh
RAS November 2004
Planet brightness v age
Solid lines Burrows 1997 models, dashed lines Burrows 2002 models
Models assume evolution in isolation: no addition heating source or
reflection component
Dr. Matt Burleigh
RAS November 2004
Imaging planets around white dwarfs
• Large format NIR cameras on 8m-class
telescopes
– Typically reach J~24 in 1 hour
(e.g. VLT+ISAAC, Gemini+NIRI)
– Comparable with predicted magnitudes of
planets around nearby white dwarfs
• No immediate need for AO
– Searching outside star’s PSF
Dr. Matt Burleigh
RAS November 2004
Cool companions to WDs
• Becklin & Zuckerman, Probst in the 1980s
– IR excess in WDs may indicate cool companion
– First L dwarf discovered is a companion to a WD, GD165
• But brown dwarfs are not common companions to WDs
– Only one more found so far (GD1400B, L6/7, Farihi et al. 2004)
– Confirms brown dwarf desert at wide separations
• No companions >10MJup found among Hyades WDs
– Zinnecker & Friedrich, in prep. (HST/NICMOS)
Dr. Matt Burleigh
RAS November 2004
Strategy
• Select young (<3Gyr), nearby (<20pc) white
dwarfs
– (0.2”yr < PM < few “/yr)
– Sample ~40 stars
– Progenitors mainly A/B stars (short main
sequence lifetimes)
• Obtain deep wide IR (J) images.
– Total exp time ~1hr
– Depth J~23.5 (GN) to ~24 (VLT)
– Image quality typically 0.4”-0.6”
• Wait 1—2 years…
– Obtain 2nd epoch images of all systems to
check for common proper motion companions
Dr. Matt Burleigh
RAS November 2004
Parameter space
• For comparison with
other planet search
techniques;
–
–
–
–
Planet mass; >5 Mjup
Orbit; 5—1000 AU
Age; 0.5—3 Gyr
Star mass; 2—7 Msun
(A and B stars)
• Complimentary to
other search
techniques
Dr. Matt Burleigh
RAS November 2004
Not a
discovery
Dr. Matt Burleigh
RAS November 2004
• White dwarfs
– Image depth J~24
Dr. Matt Burleigh
RAS November 2004
Two epochs for Proper Motion
• One epoch in One colour tells us nothing.
• Faint objects could be faint companions,
or they could simply be far away…
• Any object in the field could be a
companion!! (orbital expansion)
• 2nd epoch observations are needed to
confirm companions via proper motion.
Dr. Matt Burleigh
RAS November 2004
^
~90”
V
< ~120” >
• Two epochs
–
–
–
–
Dr. Matt Burleigh
June 2002 GS+Flamingos-I
October 2003 VLT+ISAAC
WD motion ~1” between images
Image depth J~23.5
RAS November 2004
Motions in field
• Arrows show direction
and degree of motion x
factor 20
• WD moved ~9 pixels in
15 months (~1”)
Dr. Matt Burleigh
RAS November 2004
A non-detection
• Circles: 1s scatter
on distribution of
proper motions of
background
objects
Dr. Matt Burleigh
RAS November 2004
A detection?
• Circles: 1s error
on PMs
• If associated,
candidates are 710MJup
• Would have
originally orbited
at ~65AU &
~75AU
• First epoch
June 2002,
second June
2003
Dr. Matt Burleigh
RAS November 2004
Add third epoch (June 2004)
• Candidates
have gone
away!
Dr. Matt Burleigh
RAS November 2004
Summary
• White dwarfs open up more parameter
space for planet surveys
– Direct imaging of planets >5MJup
– Probing frequency of planets around massive
stars (>2MSun)
• Sensitivity required is achievable with 8m
telescopes in near-IR
• Our survey is beginning to reach maturity
– 2 epochs for 12 systems
– 40 systems by 2006
Dr. Matt Burleigh
RAS November 2004
Further work….
• Is a sample of 40 enough?
– Coronographic searches of nearby young stars
indicate frequency of companions
>5MJup beyond 75AU is <3%
(McCarthy & Zuckerman 2004)
• Plus want to probe to lower masses (<5MJup)
– Spitzer mid-IR observations for photometric
excesses (several programmes in progress)
Dr. Matt Burleigh
RAS November 2004