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Polarimetry as a tool to find and characterise habitable planets orbiting white dwarfs
S. Bagnulo (Armagh Observatory, UK)
C. Haswell, M. Patel, R. Busuttil (Open University, UK)
P. Kowalski (Potsdam, Germany)
D. Shulyak (Goettingen, Germany)
M. Sterzik (ESO Garching, Germany)
G. Valyavin (SAO, Russia)
Luca FossatiArgelander-Institut für Astronomie,
Bonn, Germany – Open University, UK
IAU Symposium 305 – 5th December 2014
Polarimetry as a tool to find and characterise habitable planets orbiting white dwarfs
S. Bagnulo (Armagh Observatory, UK)
C. Haswell, M. Patel, R. Busuttil (Open University, UK)
P. Kowalski (Potsdam, Germany)
D. Shulyak (Goettingen, Germany)
M. Sterzik (ESO Garching, Germany)
G. Valyavin (SAO, Russia)
Luca FossatiArgelander-Institut für Astronomie,
Bonn, Germany – Open University, UK
IAU Symposium 305 – 5th December 2014
L. Fossati – IAU Symposium 305 – 5th December 2014
WD cooling sequence
Main sequence
Giant branch
Horizontal branch
AGB branch
Ejection of the planetary nebula
WD
> 10 Gyr
Progenitor star: M < 8 Mo
WD mass: 0.5 – 0.7 Mo
WD radius: ~ 1 R⊕
Cooling time: > 10 Gyr
Composition: CO, He, H
About 10% of WD are magnetic with fields of more than 1 MG (~700 MG)
White dwarfs: formation, characteristics, evolution
WD cooling sequence
Main sequence
Giant branch
Horizontal branch
AGB branch
Ejection of the planetary nebula
WD
> 10 Gyr
Progenitor star: M < 8 Mo
WD mass: 0.5 – 0.7 Mo
WD radius: ~ 1 R⊕
Cooling time: > 10 Gyr
Composition: CO, He, H
About 10% of WD are magnetic with fields of more than 1 MG (~700 MG)
There are lots of WD in the Galaxy!
The vast majority of stars will end up as WD!
White dwarfs: formation, characteristics, evolution
L. Fossati – IAU Symposium 305 – 5th December 2014
Planets around white dwarfs?
M. A. Garlick Univ. of Warwick
Metals in the atmosphere of WD, indicative of the continuous pollution of the surface from falling Earth-like debris (e.g. Farihi+ 2010; Melis+ 2011; Klein+ 2011; Gänsicke+ 2012).
Gänsicke+ 2006
L. Fossati – IAU Symposium 305 – 5th December 2014
Planets around white dwarfs?– Planets have been found around sub-dwarf (close-in Earth-mass – e.g. Charpinet+ 2011; Silvotti+ 2014) and horizontal branch (Jupiter mass – Silvotti+ 2007) stars.
– Gaseous planet (~2.5 MJ) has been found orbiting a binary system composed of a pulsar and a WD with a semi-major axis of ~23 AU and an orbital period of ~2 years (Ford+ 2000; Sigurdsson+ 2003).
– Mullally+ 2008 reported the detection of a ~2MJ planet in a 4.5 years orbit around
a pulsating WD.
– Finally, planets orbiting red giants have been found as well (Frink+ 2002, Sato+ 2003, Hatzes+ 2005).
Theoretical models predict the possibility also of the presence of short-period planets orbiting WDs through a common envelope phase or orbit migration (Faedi+ 2011, Veras & Gaensike 2014)
Planets might indeed be able to survive the death of their parent star
L. Fossati – IAU Symposium 305 – 5th December 2014
White dwarfs: cooling sequence
Crystallization of the core slows down the cooling process
WDs provide a stable luminosity source for several Gyr, without the damaging radiation produced by stellar activity on main sequence stars (M-dwarf, in particular).
Renedo+ 2010
L. Fossati – IAU Symposium 305 – 5th December 2014
The white dwarf continuous habitable zone
Agol 2011Agol 2011L. Fossati – IAU Symposium 305 – 5th December 2014
Detecting planets orbiting white dwarfs:transits
Agol 2011
A Mars-size planet would produce a transit depth of 1%
L. Fossati – IAU Symposium 305 – 5th December 2014
Detecting planets orbiting white dwarfs:transits
Agol 2011
A Mars-size planet would produce a transit depth of 1%
The detection of transits requires a “lucky” system configuration, so how can we
detect non-transiting planets in the CHZ?
L. Fossati – IAU Symposium 305 – 5th December 2014
Detecting planets orbiting white dwarfs:transits
Agol 2011
A Mars-size planet would produce a transit depth of 1%
The detection of transits requires a “lucky” system configuration, so how can we
detect non-transiting planets in the CHZ?
RV → no linesMicrolensing → large SM axis
Direct imaging → large SM axis
L. Fossati – IAU Symposium 305 – 5th December 2014
Detecting planets orbiting white dwarfs:polarimetry
Stam 2008
quadrature quadrature
i=0°
i=60°
i=30°
i=90°
Similar dependence to the inclination angle i, as for radial velocity planet detections.
L. Fossati – IAU Symposium 305 – 5th December 2014
i=0° i≈90°
Detecting planets orbiting white dwarfs:polarimetry
Fossati+ 2012
α = 180° - i (angle between star and Earth, as seen from the planet)
r: planet radiusd: star-planet distance
F*: stellar flux
a1 / b
1: elements of the planet
scattering matrix
see Stam+ 2006 / Stam 2008
polarised reflected light
total reflected light
L. Fossati – IAU Symposium 305 – 5th December 2014
Detecting planets orbiting white dwarfs:polarimetry
Fossati+ 2012
α = 180° - i (angle between star and Earth, as seen from the planet)
r: planet radiusd: star-planet distance
F*: stellar flux
a1 / b
1: elements of the planet
scattering matrix
see Stam+ 2006 / Stam 2008
polarised reflected light
total reflected light
L. Fossati – IAU Symposium 305 – 5th December 2014
Detecting planets orbiting white dwarfs:polarimetry
Fossati+ 2012
Cloud-free Lambertian planet surface with wavelength independent albedo of 1.
Note: Earth's albedo ~0.3 and a snowball Earth's albedo ~0.85.
HZ
The polarisation signal of any planet in the WD CHZ is larger than that of a comparable planet in the HZ of any other type of star.
L. Fossati – IAU Symposium 305 – 5th December 2014
Polarimetry of planets orbiting WD in the CHZDetectable polarisation (at 3σ) as a function of stellar magnitude, telescope size, and wavelength band.
FORS-like instrument @ VLT and 2.5 hours of exposure time (~ 12 hours orbital period)
Brightest cool WD:WD 0046+051 (Teff ~ 6000 K)V ~ 12.4 mag
1) B-band polarisation easiest to detect, despite lower fluxes and CCD sensitivity (see also Berdyugina+ 2011)
2) Currently detectable only Jupiter-size planets
L. Fossati – IAU Symposium 305 – 5th December 2014
Polarimetry of planets orbiting WD in the CHZThe detection threshold, hence size of the detectable planet, can be improved, up to the detection of a SuperEarth orbiting the brightest cool WD.
– Broad-band polarimetry
– Atmosphereless planets provide a color independent polarimetric signal (e.g., Bagnulo+ 2012)
– Improved sensitivity in the blue
– Increased area because of shape distorted by Roche geometry(Fossati+ 2012)
– Add photometric analysis (intrinsic variability!)
Seager+ 2000
L. Fossati – IAU Symposium 305 – 5th December 2014
Conclusions– Presence of planets orbiting giants and sdBs; solid bodies orbiting WD
– CWDs provide a stable source of light for several Gyr
– CHZ around 0.01 AU
– In addition to transits, polarimetry is the only other (secure) way to detect planets in the CHZ; it would provide simultaneous characterisation as well
– The polarimetric signal of a terrestrial planet in the CHZ of a CWD is 102 (104) larger than that of the same planet in the HZ of an M-dwarf (Sun-like star)
– Atmosphereless super-Earth orbiting the brightest CWD detectable with VLT
L. Fossati – IAU Symposium 305 – 5th December 2014
Conclusions– Presence of planets orbiting giants and sdBs; solid bodies orbiting WD
– CWDs provide a stable source of light for several Gyr
– CHZ around 0.01 AU
– In addition to transits, polarimetry is the only other (secure) way to detect planets in the CHZ; it would provide simultaneous characterisation as well
– The polarimetric signal of a terrestrial planet in the CHZ of a CWD is 102 (104) larger than that of the same planet in the HZ of an M-dwarf (Sun-like star)
– Atmosphereless super-Earth orbiting the brightest CWD detectable with VLT
– Earth-like planet orbiting in the CHZ of a CWD would be habitable in terms of UV radiation and photosynthesis (Fossati+ 2012)
– For a magnetic WD the cooling rate decreases (Valyavin+ 2014)→ planet still habitable?→ stable orbit?→ polarimetry still useful?
L. Fossati – IAU Symposium 305 – 5th December 2014