Spitzer Microlensing Program as a Probe for Globular Cluster Planets. Analysis of OGLE-2015-BLG-0448

TL;DR

This study demonstrates a methodology using Spitzer microlensing observations to investigate planetary populations in globular clusters, confirming the lens was not a cluster member and validating parallax measurements.

Contribution

We developed a method to verify globular cluster lens events and applied it to OGLE-2015-BLG-0448, establishing a framework for future planetary searches in clusters.

Findings

The lens in OGLE-2015-BLG-0448 is not a cluster member.

Microlens parallax from OGLE data matches Spitzer-based measurements.

Methodology enables verification of potential cluster lens events.

Abstract

The microlensing event OGLE-2015-BLG-0448 was observed by Spitzer and lay within the tidal radius of the globular cluster NGC 6558. The event had moderate magnification and was intensively observed, hence it had the potential to probe the distribution of planets in globular clusters. We measure the proper motion of NGC 6558 ($\mu_{\rm cl}$(N,E) = (+0.36+-0.10, +1.42+-0.10) mas/yr) as well as the source and show that the lens is not a cluster member. Even though this particular event does not probe the distribution of planets in globular clusters, other potential cluster lens events can be verified using our methodology. Additionally, we find that microlens parallax measured using OGLE photometry is consistent with the value found based on the light curve displacement between Earth and Spitzer.