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.
