MOA-2019-BLG-008Lb: A New Microlensing Detection of an Object at the Planet/Brown Dwarf Boundary

Abstract

We report on the observations, analysis and interpretation of the microlensing event MOA-2019-BLG-008. The observed anomaly in the photometric light curve is best described through a binary lens model. In this model, the source did not cross caustics and no finite-source effects were observed. Therefore, the angular Einstein ring radius θ E cannot be measured from the light curve alone. However, the large event duration, t E ∼ 80 days, allows a precise measurement of the microlensing parallax π E. In addition to the constraints on the angular radius θ * and the apparent brightness I s of the source, we employ the Besançon and GalMod galactic models to estimate the physical properties of the lens. We find excellent agreement between the predictions of the two galactic models: the companion is likely a resident of the brown dwarf desert with a mass M p ∼ 30 M Jup, and the host is a main-sequence dwarf star. The lens lies along the line of sight to the Galactic bulge, at a distance of ≤4 kpc. We estimate that in about 10 yr the lens and source will be separated by ∼55 mas, and it will be possible to confirm the exact nature of the lensing system by using high-resolution imaging from ground- or space-based observatories.

Estimating Microlensing Parameters from Observables and Stellar Isochrones with pyLIMASS

“…The downside is that the results are inevitably biased because of the different models assumptions. For example, Bachelet et al (2022b) found a good agreement with the prediction from the Besançon (Robin et al 2003), GalMod (Pasetto et al 2018), Dominik (2006), andKoshimoto et al (2021) models, but small discrepancies exist.…”

“…These measurements will be based on the lightcurve modeling as well as the modeling of the Roman images near the event location. A double point-spread function fit provides strong constraints on the source and lens magnitudes as well as the relative proper motion μ rel (Bachelet et al 2022a), which is included as the last observable. The typical astrometric precision can be approximated by (Lindegren 1978;Sozzetti 2005), where FWHM is the full width at half-maximum of the optical system and S/N is the photometric signal-to-noise ratio.…”