polarimetry as a tool to find and characterise habitable ... · white dwarfs: cooling sequence...

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


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


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


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


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%



Agol 2011


The detection of transits requires a “lucky” system configuration, so how can we

detect non-transiting planets in the CHZ?



Agol 2011


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


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.


i=0° i≈90°


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



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.


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


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


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


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?


polarimetry as a tool to find and characterise habitable ... · white dwarfs: cooling sequence...

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