A planet-sized transiting star around OGLE-TR-122 - Accurate mass and radius near the hydrogen-burning limit (original) (raw)

A&A 433, L21-L24 (2005)

Letter to the Editor

Accurate mass and radius near the hydrogen-burning limit

1 Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: frederic.pont@obs.unige.ch
2 European Southern Observatory, Casilla 19001, Santiago 19, Chile
3 Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France
4 Observatoire de Haute Provence, 04870 St Michel l'Observatoire, France
5 Centro de Astronomia e Astrofísica da Universidade de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal

Received: 28 January 2005
Accepted: 12 February 2005

Abstract

We report the discovery and characterisation of OGLE-TR-122b, the smallest main-sequence star to date with a direct radius determination. OGLE-TR-122b transits around its solar-type primary every 7.3-days. With $M=0.092\pm0.009 \ M_\odot$ and $R=0.120^{+0.020}_{-0.013} \ R_\odot$ , it is by far the smallest known eclipsing M-dwarf. The derived mass and radius for OGLE-TR-122b are in agreement with the theoretical expectations. OGLE-TR-122b is the first observational evidence that stars can indeed have radii comparable or even smaller than giant planets. In such cases, the photometric signal is exactly that of a transiting planet and the true nature of the companion can only be determined with high-resolution spectroscopy.

Key words: stars: low-mass, brown-dwarfs / stars: fundamental parameters / stars: binaries / binaries: eclipsing

Based on observations collected with the VLT/UT2 Kueyen telescope (Paranal Observatory, ESO, Chile) using the FLAMES+UVES spectrograph (program ID 072.C-191).