First Space-based Microlens Parallax Measurement of an Isolated Star: Spitzer Observations of OGLE-2014-BLG-0939

TL;DR

This paper reports the first space-based microlens parallax measurement of an isolated star using Spitzer, revealing the lens's properties and demonstrating methods to break degeneracies in such measurements.

Contribution

It introduces the first space-based microlens parallax measurement of an isolated star and explores new mechanisms to resolve measurement degeneracies.

Findings

Projected velocity ~ 250 km/s indicating a Galactic Disk lens

Lens mass estimated at 0.23 solar masses

Distance to lens approximately 3.1 kpc

Abstract

We present the first space-based microlens parallax measurement of an isolated star. From the striking differences in the lightcurve as seen from Earth and from Spitzer (~1 AU to the West), we infer a projected velocity v_helio,projected ~ 250 km/s, which strongly favors a lens in the Galactic Disk with mass M=0.23 +- 0.07 M_sun and distance D_L=3.1 +- 0.4 kpc. An ensemble of such measurements drawn from our ongoing program could be used to measure the single-lens mass function including dark objects, and also is necessary for measuring the Galactic distribution of planets since the ensemble reflects the underlying Galactic distribution of microlenses. We study the application of the many ideas to break the four-fold degeneracy first predicted by Refsdal 50 years ago. We find that this degeneracy is clearly broken, but by two unanticipated mechanisms.