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

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 ($\mu_{\rm rel}$), source proper motion (${\boldsymbol\mu}_{\rm S}$) 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 $\mu_{\rm rel}$ ($\sim 17.6\, {\rm mas\, yr^{-1}}$) 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 ${\boldsymbol\mu}_{\rm S}$ shows that the large $\mu_{\rm rel}$ is due to an extreme ${\boldsymbol\mu}_{\rm S}$, 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 ${\boldsymbol\mu}_{\rm S}$ 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.