Polarimetry As a Tool to Find and Characterise Habitable Planets Orbiting White Dwarfs

Polarimetry as a tool to find and characterise habitable planets orbiting white dwarfs

Luca Fossati Argelander-Institut für Astronomie, Bonn, Germany – Open University, UK

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)

IAU Symposium 305 – 5th December 2014 Polarimetry as a tool to find and characterise habitable planets orbiting white dwarfs

Luca Fossati Argelander-Institut für Astronomie, Bonn, Germany – Open University, UK

IAU Symposium 305 – 5th December 2014 White dwarfs: formation, characteristics, evolution

Progenitor star: M < 8 Mo Ejection of the AGB WD planetary branch nebula Horizontal WD mass: 0.5 – 0.7 M branch o Giant WD radius: ~ 1 R branch ⊕ Cooling time: > 10 Gyr Main sequence > 10 Gyr Composition: CO, He, H

WD About 10% of WD are cooling magnetic with fields of sequence more than 1 MG (~700 MG)

L. Fossati – IAU Symposium 305 – 5th December 2014 White dwarfs: formation, characteristics, evolution

WD About 10% of WD are cooling magnetic with fields of sequence more than 1 MG (~700 MG)

There are lots of WD in the Galaxy!

The vast majority of stars will end up as WD! L. Fossati – IAU Symposium 305 – 5th December 2014 Planets around white dwarfs? Gänsicke+ 2006

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).

M. A. Garlick Univ. of Warwick 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

Renedo+ 2010 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).

L. Fossati – IAU Symposium 305 – 5th December 2014 The white dwarf continuous habitable zone

Agol 2011 L. Fossati – IAU Symposium 305 – 5th December 2014 Detecting planets orbiting white dwarfs: transits

A Mars-size planet would produce a transit depth of 1%

Agol 2011 L. Fossati – IAU Symposium 305 – 5th December 2014 Detecting planets orbiting white dwarfs: transits

The detection of transits requires a “lucky” system configuration, so how can we detect non-transiting planets in the CHZ?

A Mars-size planet would produce a transit depth of 1%

Agol 2011 L. Fossati – IAU Symposium 305 – 5th December 2014 Detecting planets orbiting white dwarfs: transits

The detection of transits requires a “lucky” system configuration, so how can we detect non-transiting planets in the CHZ?

A Mars-size planet would RV → no lines produce a transit depthMicrolensing of 1% → large SM axis Direct imaging → large SM axis

Agol 2011 L. Fossati – IAU Symposium 305 – 5th December 2014 Detecting planets orbiting white dwarfs: polarimetry

Stam 2008 Similar dependence to the inclination angle i, as for radial velocity planet i=0° detections.

i=30°

i=60° i=0° i≈90°

i=90°

quadrature quadrature

L. Fossati – IAU Symposium 305 – 5th December 2014 Detecting planets orbiting white dwarfs:

polarimetry total reflected polarised light reflected light Fossati+ 2012

α = 180° - i (angle between star and Earth, as seen from the planet)

r: planet radius d: star-planet distance F : stellar flux *

a1 / b1: elements of the planet scattering matrix

see Stam+ 2006 / Stam 2008 L. Fossati – IAU Symposium 305 – 5th December 2014 Detecting planets orbiting white dwarfs:

polarimetry total reflected polarised light reflected light Fossati+ 2012

α = 180° - i (angle between star and Earth, as seen from the planet)

r: planet radius d: star-planet distance F : stellar flux *

a1 / b1: elements of the planet scattering matrix

see Stam+ 2006 / Stam 2008 L. Fossati – IAU Symposium 305 – 5th December 2014 Detecting planets orbiting white dwarfs:

Fossati+ 2012 polarimetry 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.

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 CHZ Detectable 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 CHZ The 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)

Seager+ 2000 – Improved sensitivity in the blue

– Increased area because of shape distorted by Roche geometry (Fossati+ 2012)

– Add photometric analysis (intrinsic variability!) 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