I spy a planet: the impact of stellar variability on microlensing

PI: Newton, Elisabeth, Dartmouth College
Wide-Field Science – Regular

M dwarfs are the most common type of star in our Galaxy, with the stellar mass function peaking around 0.23M☉ or T* = 3200 K (M4V). Given Roman’s deep sensitivity, these stars will dominate both the sources and lenses of exoplanet microlensing events observed in Roman’s Galactic Bulge Time Domain Survey (GBTDS). M dwarfs are highly magnetically active, resulting in photometric variability from both stellar rotation and flares. Those cooler than M4 retain rapid rotation and high levels of magnetic activity for several Gyr, and both signatures persist even for the old, slowly rotating subset. In this proposal, we demonstrate the likelihood of intrinsic variability of M dwarf photometric variability to masquerade as, or hinder the detection of, microlensing events. We show that that even though flares are typically asymmetric and have greater fluxes at bluer wavelengths, their morphologies have the potential to closely resemble those of microlensing signals. This resemblance necessitates the consideration of M dwarf flares in the ongoing work to develop algorithms for microlensing event detection, because such algorithms will need to be trained to distinguish true planetary signals from those arising from stellar activity.

We will answer the following key science questions:

• How does stellar variability manifest in Roman observations? Through simulations using existing data from TESS and other sources, we will characterize the expected signatures of stellar variability in the context of Roman’s observational parameters.

•  How can we distinguish flares from free-floating planets? We will develop a probabilistic framework similar to existing transit detection models, which will assess the likelihood that observed events are due to stellar flares as opposed to genuine microlensing events.

•  How can we support community efforts in data analysis? We will create an open-source package, ispy, designed to simulate realistic stellar variability for use in the community’s microlensing detection pipelines.

We aim to provide the knowledge and resources necessary for the community to meet the challenge of stellar variability head-on. Addressing this issue in advance will facilitate more robust detection of planetary candidates and rejection of false positives from the outset of the Roman mission.