A&A 556, A64 (2013) Astronomy DOI: 10.1051/0004-6361/201321881 & c ESO 2013 Astrophysics Prospects of detecting the polarimetric signature of the Earth-mass planet α Centauri B b with SPHERE/ZIMPOL J. Milli1,2, D. Mouillet1,D.Mawet2,H.M.Schmid3, A. Bazzon3, J. H. Girard2,K.Dohlen4, and R. Roelfsema3 1 Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), University Joseph Fourier, CNRS, BP 53, 38041 Grenoble, France e-mail: [email protected] 2 European Southern Observatory, Casilla 19001, Santiago 19, Chile 3 Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland 4 Laboratoire d’Astrophysique de Marseille (LAM),13388 Marseille, France Received 12 May 2013 / Accepted 4 June 2013 ABSTRACT Context. Over the past five years, radial-velocity and transit techniques have revealed a new population of Earth-like planets with masses of a few Earth masses. Their very close orbit around their host star requires an exquisite inner working angle to be detected in direct imaging and sets a challenge for direct imagers that work in the visible range, such as SPHERE/ZIMPOL. Aims. Among all known exoplanets with less than 25 Earth masses we first predict the best candidate for direct imaging. Our primary objective is then to provide the best instrument setup and observing strategy for detecting such a peculiar object with ZIMPOL. As a second step, we aim at predicting its detectivity. Methods. Using exoplanet properties constrained by radial velocity measurements, polarimetric models and the diffraction propaga- tion code CAOS, we estimate the detection sensitivity of ZIMPOL for such a planet in different observing modes of the instrument. We show how observing strategies can be optimized to yield the best detection performance on a specific target. Results. In our current knowledge of exoplanetary systems, α Centauri B b is the most promising target with less than 25 Earth masses for ZIMPOL. With a gaseous Rayleigh-scattering atmosphere and favorable inclinations, the planet could be detected in about four hours of observing time, using the four-quadrant phase-mask coronograph in the I band. However, if α Centauri B b should display unfavorable polarimetric and reflective properties similar to that of our Moon, it is around 50 times fainter than the best sensitivity of ZIMPOL. Conclusions. α Centauri B is a primary target for SPHERE. Dedicated deep observations specifically targeting the radial velocity- detected planet can lead to a detection if the polarimetric properties of the planet are favorable. Key words. instrumentation: high angular resolution – planets and satellites: detection – instrumentation: polarimeters – polarization – planets and satellites: individual: alpha Centauri – planets and satellites: atmospheres 1. Introduction circumstellar emission down to 18 mag/arcsec2 at 1.5 on HD 169142 (Quanz et al. 2013). A dedicated instrument for exo- Imaging planets is a very attractive goal to improve our under- planet search in the visible light will now be installed at the VLT standing of planetary systems. So far, it has only been achieved 1 as part of the SPHERE instrument (Beuzit et al. 2008): ZIMPOL, in the near-infrared by detecting the thermal emission of young the Zurich IMaging POLarimeter (Schmid et al. 2006). It uses (1−100 Myr) and massive Jupiter-size planets at large distances − the SPHERE AO system and coronographic masks. ZIMPOL from their host stars (5 100 AU). Imaging planets in visible re- has demonstrated polarimetric sensitivities of 10−5 locally with flected light is also very valuable. However, while the flux re- an absolute polarimetric accuracy of 10−3. Fast polarimetric flected by the planet is highest at a very small orbit, the stellar modulation is performed using a ferroelectric liquid crystal to halo is stronger than that of the planet at such a short separation. swap two orthogonal linear polarization states at 1 kHz. A po- Moreover, the adaptive optics (AO) correction is not favorable at × −10 larization beamsplitter converts this modulation into an intensity visible wavelengths. The contrast required is around 4 10 for modulation, which is then demodulated in real-time by a special an earth at 1 AU from its host star, while the angular separation masked charge-shifting CCD detector. The same CCD pixels are is only 0.1 for a star at 10 pc. used for the detection of both polarization states to minimize dif- However, to help detection, a specific property of scat- ff ff ferential e ects. Since the modulation period is shorter than the tered light can be used: polarization. Polarimetric di erential seeing variation timescale, speckle noise is strongly reduced in imaging (PDI) is already widely used to enhance the con- the polarization image. trast between a star and circumstellar material, e.g., to reveal The large majority of low-mass exoplanets (Mpl ≤ 25 MEarth) protoplanetary disks. Currently, two 8-m class telescopes pro- detected in transit or radial velocity (RV) have a projected an- vide subarcsec-resolved imaging with a dual-beam polarimeter: / / gular separation at quadrature smaller than the ZIMPOL in- Subaru HiCIAO and VLT NaCo. The latter revealed polarized ner working angle however, 2λ/d at 600 nm or 0.03. Those 1 Except for Fomalhaut b detected by Kalas et al. (2008) with with a preliminary intensity contrast higher than one part per −9 HST/ACS and confirmed by Galicher et al. (2013)andCurrie et al. billion (10 ) and a projected separation larger than 0.03 are (2012), but this is a controversial case because the nature of the object named in Fig. 1 and constitute our sample selection. The pre- 2 has yet to be revealed. liminary intensity contrast is given by f · (Rpl/a) assuming the Article published by EDP Sciences A64, page 1 of 5 A&A 556, A64 (2013) 10−6 circularized given the low value of a. Four discrete in- clinations 10◦,30◦,60◦, and 90◦ are used in our simula- . α Cen B b tions, corresponding to a planet true mass of 6 6, 2 2, 1 3, and 1.1 Earth masses. α, λ 10−7 – The disk integrated reflectance f ( ) and polarization frac- tion p(α, λ) as a function of α. The product f × p is called polarized reflectance hereafter. Gl 581 d – The planet radius R estimated from the mass-radius relation Gl 785 b Gliese 876 e pl HD 20794 c 61 Vir d derived for terrestrial planets by Sotin et al. (2007), with Rpl −8 HD 102365 b . Contrast to the star 10 0 274 ◦ HD 20794 d proportional to Mpl . It ranges between 1.7 REarth for a 10 HD 69830 d inclination and 1.0 REarth for an edge-on system. HD 40307 g Rpl 2 HD 192310 c φ, ,λ φ, ,λ All in all, the polarimetric contrast is p( i ) f ( i ) a .We HD 10180 g 10−9 investigate two polarization models for the planet: 0.01 0.10 1.00 Angular separation (arcsec) Moon-like planet. This model corresponds to a rocky planet with polarimetric properties like the Moon. Given the small Fig. 1. Preliminary intensity contrast of known exoplanets of less mass and orbital distance of α Cen B b, a tiny atmosphere or than 25 Earth masses confirmed before May 2013 (from exoplanets.eu). even no atmosphere at all is a realistic assumption. The light The size and color (from blue to red) of the dots are proportional to the planet mass. is reflected from the solid surface and Moon- or Mercury-like properties are therefore plausible. From a detection point of same reflectance f = 0.2 for all planets (corresponding to the view this would represent a worst case scenario. We used Rayleigh-scattering atmosphere described below and a scatter- reflectance and polarization fraction measurements of the ff ing angle α = 87◦). The radius R is computed from the Moon derived from Coyne & Pellicori (1970)andKie er & pl Stone (2005) for this scenario. They are displayed in Fig. 2. RV mass Mpl sin i assuming an Earth bulk density. The ver- The polarized reflectance reaches a maximum of 0.13% for a tical dotted line shows the ZIMPOL inner working angle. Of ◦ the 11 targets, α Cen B b is by far the most promising with its scattering angle of 60 and there is little wavelength depen- − intensity contrast of more than 2 × 10−7. It has a semi-major dence in the 600 900 nm range. axis a = 0.04 AU (Dumusque et al. 2012). Because its parent Planet with a Rayleigh-scattering atmosphere. In more favor- star is the second-closest star after Proxima Centauri at a dis- able conditions, the planet is assumed to have a rocky core tance of 1.34 pc, the projected separation is enhanced but re- and to retain a Rayleigh-scattering atmosphere that reflects mains small: 0.03 at quadrature. and polarizes much more incident starlight. We used the po- Polarimetric differential imaging is complementary to larization model presented in Buenzli & Schmid (2009)for RV techniques, which have a projection ambiguity because the that purpose. It assumes a multiple-scattering atmosphere system inclination i is unknown and only the projected mass above a Lambertian surface. It is described by three pa- M × sin i can be determined. This degeneracy can be broken rameters: the surface albedo As, the atmosphere total opti- τ ω with multi-epoch direct images. α Cen B b could then become cal depth , and the single scattering albedo . We chose the first exoplanet to be unambiguously detected both in RV the most favorable parameter set corresponding to a deep τ = ω = and direct imaging. The only other planet directly imaged with ( 30) conservative ( 1) Rayleigh-scattering atmo- = tight RV constraints is β Pic b (Lagrange et al.