Searching for the near-infrared counterpart of Proxima c using multi-epoch high-contrast SPHERE data at VLT^★,★★

¹ INAF – Osservatorio Astronomico di Padova, Italy
e-mail: raffaele.gratton@inaf.it
² Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
³ Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
⁴ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁵ INAF – Osservatorio Astrofisico di Torino, Italy
⁶ Institute of Astrophysics, FORTH, GR-71110 Heraklion, Greece
⁷ Department of Physics, University of Crete, 70013 Heraklion, Greece
⁸ CRAL, UMR 5574, CNRS, Université Lyon 1, ENS, 9 avenue Charles André, 69561 Saint Genis Laval Cedex, France
⁹ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
¹⁰ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹¹ Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹² Observatoire de Genève, Université de Genève, 51 Chemin des Mailletes, 1290 Sauverny, Switzerland
¹³ Université Côte d’Azur, OCA, CNRS, Lagrange, France
¹⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ Department of Astronomy, Stockholm University, Stockholm, Sweden
¹⁷ INAF – Osservatorio Astronomico di Brera, Milano, Italy
¹⁸ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹⁹ Department of Astronomy, University of Michigan, 1085 S. Univer-sity Ave, Ann Arbor, MI 48109-1107, USA
²⁰ ONERA (Office National d’Etudes et de Recherches Aérospatiales), BP72, 92322 Chatillon, France
²¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²² Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zürich, Zürich, Switzerland
²³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
²⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received: 28 January 2020
Accepted: 14 April 2020

Abstract

Context. Proxima Centauri is the closest star to the Sun and it is known to host an Earth-like planet in its habitable zone; very recently a second candidate planet was proposed based on radial velocities. At quadrature, the expected projected separation of this new candidate is larger than 1 arcsec, making it a potentially interesting target for direct imaging.

Aims. While identification of the optical counterpart of this planet is expected to be very difficult, successful identification would allow for a detailed characterization of the closest planetary system.

Methods. We searched for a counterpart in SPHERE images acquired over four years through the SHINE survey. In order to account for the expected large orbital motion of the planet, we used a method that assumes the circular orbit obtained from radial velocities and exploits the sequence of observations acquired close to quadrature in the orbit. We checked this with a more general approach that considers Keplerian motion, called K-stacker.

Results. We did not obtain a clear detection. The best candidate has signal-to-noise ratio (S∕N) = 6.1 in the combined image. A statistical test suggests that the probability that this detection is due to random fluctuation of noise is <1%, but this result depends on the assumption that the distribution of noise is uniform over the image, a fact that is likely not true. The position of this candidate and the orientation of its orbital plane fit well with observations in the ALMA 12 m array image. However, the astrometric signal expected from the orbit of the candidate we detected is 3σ away from the astrometric motion of Proxima as measured from early Gaia data. This, together with the unexpectedly high flux associated with our direct imaging detection, means we cannot confirm that our candidate is indeed Proxima c.

Conclusions. On the other hand, if confirmed, this would be the first observation in imaging of a planet discovered from radial velocities and the second planet (after Fomalhaut b) of reflecting circumplanetary material. Further confirmation observations should be done as soon as possible.