Using Source Proper Motion to Validate Terrestrial Parallax: OGLE-2019-BLG-1058

Abstract

We show that because the conditions for producing terrestrial microlens parallax (TPRX; i.e., a nearby disk lens) will also tend to produce a large lens-source relative proper motion ($\hat{I}{¼}_{\mathrm{rel}}$), source proper motion (${\hat{\mathbf{I}}¼}_{\mathrm{S}}$) measurements in general provide a strong test of TPRX signals, which \citet{gould13} showed were an important probe of free-floating planet (FFP) candidates. As a case study, we report a single-lens/single-source microlensing event designated as OGLE-2019-BLG-1058. For this event, the short timescale ($\sim 2.5$ days) and very fast $\hat{I}{¼}_{\mathrm{rel}}$ ($\sim 17.6\mathrm{mas}\mathrm{y}{\mathrm{r}}^{-1}$) suggest that this isolated lens is an FFP candidate located in the disk of our Galaxy. For this event, we find a TPRX signal consistent with a disk FFP, but at low significance. A direct measurement of the ${\hat{\mathbf{I}}¼}_{\mathrm{S}}$ shows that the large $\hat{I}{¼}_{\mathrm{rel}}$ is due to an extreme ${\hat{\mathbf{I}}¼}_{\mathrm{S}}$, and thus, the lens is consistent with being a very low-mass star in the bulge and the TPRX measurement is likely spurious. By contrast, we show how a precise measurement of ${\hat{\mathbf{I}}¼}_{\mathrm{S}}$ with the mean properties of the bulge proper motion distribution would have given the opposite result; i.e., provided supporting evidence for an FFP in the disk and the TPRX measurement.

15 pages, 10 figures, 4 tables, accepted for a publication in the AJ