Spectroscopic characterisation of microlensing events

Towards a new interpretation of OGLE-2011-BLG-0417^⋆,⋆⋆

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
² Aix-Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13388 Marseille, France
e-mail: alexandre.santerne@lam.fr
³ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁴ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
⁵ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁶ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble, UMR 5274, 38041 Grenoble, France
⁷ Laboratory for Exoplanets and Stellar Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556, USA
⁹ Observatoire Astronomique de l’Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
¹⁰ Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
¹¹ Astronomy Department, University of Washington, PO Box 351580, U.W., Seattle, WA 98195-1580, USA
¹² Astronomy Department, Cornell University, Ithaca, NY 14853, USA
¹³ European Southern Observatory (ESO), Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
¹⁴ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal

Received: 6 November 2015
Accepted: 12 October 2016

Abstract

The microlensing event OGLE-2011-BLG-0417 is an exceptionally bright lens binary that was predicted to present radial velocity variation at the level of several km s^-1. Pioneer radial velocity follow-up observations with the UVES spectrograph at the ESO−VLT of this system clearly ruled out the large radial velocity variation, leaving a discrepancy between the observation and the prediction. In this paper, we further characterise the microlensing system by analysing its spectral energy distribution (SED) derived using the UVES spectrum and new observations with the ARCoIRIS (CTIO) near-infrared spectrograph and the Keck adaptive optics instrument NIRC2 in the J, H, and Ks-bands. We determine the mass and distance of the stars independently from the microlensing modelling. We find that the SED is compatible with a giant star in the Galactic bulge and a foreground star with a mass of 0.94 ± 0.09 M_⊙  at a distance of 1.07 ± 0.24 kpc. We find that this foreground star is likely the lens. Its parameters are not compatible with the ones previously reported in the literature (0.52 ± 0.04 M_⊙ at 0.95 ± 0.06 kpc), based on the microlensing light curve. A thoughtful re-analysis of the microlensing event is mandatory to fully understand the reason of this new discrepancy. More importantly, this paper demonstrates that spectroscopic follow-up observations of microlensing events are possible and provide independent constraints on the parameters of the lens and source stars, hence breaking some degeneracies in the analysis. UV-to-NIR low-resolution spectrographs like X-shooter (ESO−VLT) could substantially contribute to this follow-up efforts, with magnitude limits above all microlensing events detected so far.