Understanding EROS2 observations toward the spiral arms within a classical Galactic model framework^⋆

¹ Laboratoire de l’Accélérateur Linéaire, IN2P3-CNRS, Université de Paris-Sud, BP 34, 91898 Orsay Cedex, France
e-mail: moniez@lal.in2p3.fr
² Department of Physics, Isfahan University of Technology, 84156-83111 Isfahan, Iran
³ Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, N2L 2Y5 Ontario, Canada
⁴ Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada
⁵ Department of Physics, Sharif University of Technology, PO Box 11155-9161, Tehran, Iran

Received: 23 January 2017
Accepted: 10 April 2017

Abstract

Aims. EROS (Expérience de Recherche d’Objets Sombres) has searched for microlensing toward four directions in the Galactic plane away from the Galactic center. The interpretation of the catalog optical depth is complicated by the spread of the source distance distribution. We compare the EROS microlensing observations with Galactic models (including the Besançon model), tuned to fit the EROS source catalogs, and take into account all observational data such as the microlensing optical depth, the Einstein crossing durations, and the color and magnitude distributions of the catalogued stars.

Methods. We simulated EROS-like source catalogs using the HIgh-Precision PARallax COllecting Satellite (Hipparcos) database, the Galactic mass distribution, and an interstellar extinction table. Taking into account the EROS star detection efficiency, we were able to produce simulated color–magnitude diagrams that fit the observed diagrams. This allows us to estimate average microlensing optical depths and event durations that are directly comparable with the measured values.

Results. Both the Besançon model and our Galactic model allow us to fully understand the EROS color–magnitude data. The average optical depths and mean event durations calculated from these models are in reasonable agreement with the observations. Varying the Galactic structure parameters through simulation, we were also able to deduce contraints on the kinematics of the disk, the disk stellar mass function (at a few kpc distance from the Sun), and the maximum contribution of a thick disk of compact objects in the Galactic plane (M_thick< 5 − 7 × 10¹⁰M_⊙ at 95%, depending on the model). We also show that the microlensing data toward one of our monitored directions are significantly sensitive to the Galactic bar parameters, although much larger statistics are needed to provide competitive constraints.

Conclusions. Our simulation gives a better understanding of the lens and source spatial distributions in the microlensing events. The goodness of a global fit taking into account all the observables (from the color-magnitude diagrams and microlensing observations) shows the validity of the Galactic models. Our tests with the parameters excursions show the unique sensitivity of the microlensing data to the kinematical parameters and stellar initial mass function.