Microlens Terrestrial Parallax Mass Measurements: A Rare Probe of Isolated Brown Dwarfs and Free-Floating Planets

TL;DR

This paper discusses a method to measure the mass and density of isolated dark low-mass objects like free-floating planets and brown dwarfs using terrestrial microlens parallax, highlighting its potential for rare event detection.

Contribution

The authors derive an analytic expression for the rate of microlensing events suitable for mass measurement and propose strategies to significantly increase detection sensitivity.

Findings

Rate of events is proportional to the population's number density.

Rate increases sharply with smaller source stars.

Proposed modifications could boost sensitivity by a factor of 100.

Abstract

Terrestrial microlens parallax is one of the very few methods that can measure the mass and number density of isolated dark low-mass objects, such as old free-floating planets and brown dwarfs. Terrestrial microlens parallax can be measured whenever a microlensing event differs substantially as observed from two or more well-separated sites. If the lens also transits the source during the event, then its mass can be measured. We derive an analytic expression for the expected rate of such events and then use this to derive two important conclusions. First the rate is directly proportional to the number density of a given population, greatly favoring low-mass populations relative to their contribution to the general microlensing rate, which further scales as sqrt{M} where M is the lens mass. Second, the rate rises sharply as one probes smaller source stars, despite the fact that the probability of transit falls directly with source size. We propose modifications to current observing strategies that could yield a factor 100 increase in sensitivity to these rare events.