Information on Service 'Candidates for astrometric microlensing'

The Idea behind this Catalog

We constructed the candidate list to identify stars with large proper motions (>0.15 arcsec/year) that are potential candidates for the astrometric microlensing effect during the Gaia mission [PDW]. With the Gaia mission it will be possible to measure this effect with the required accuracy at best 30 micro-arcsec for a single measurement. The satellite, which is observing since July 2014, surveys the whole sky and has an expected lifetime of five years. It is doing astrometry, photometry and spectroscopy of approximately one billion stars in our galaxy brighter than ~20 mag in visual band [GAIA].

The astrometric microlensing effect allows a precise measurement of the mass of a star that is acting as a lens [PAC]. Ideally, the microlensing event is observed both astrometrically and photometrically. When the unaffected source position is not known or the lens is not visible both measurements are necessary. Photometric microlensing events (with small angular distances of lens and source) are about ten times shorter than the corresponding astrometric event. Hence Gaia will do no or only few photometric measurements. Therefore, an accompanying ground based photometric observations of the events would be advantageous. For this purpose it is very helpful to predict microlensing events.

To predict microlensing events it is essential to have positions and proper motions with a high accuracy (maximal 100 mas in position and 10 mas/year in proper motion). Hence it is important to choose suitable catalogs for the lenses and sources. For the sources, we chose PPMXL [PPMXL], for the lenses the LSPM-NORTH [LSPM] on the northern sky and PPMX [PPMX] and UCAC3 [UCAC3].

Problems

After manual inspection of about one hundred lens proper motions we found out that the majority of the high proper motions in PPMX an UCAC3 are erroneous. While nearly all proper motions from LSPM are correct, only a small fraction of high proper motions (<1%) from PPMX and UCAC3 are physical. This means that a high number of the predicted events from our candidate list is wrong.

Confirmed Microlensing Candidates (2012-2019)

In our previous work the assembled microlensing catalog comprised 910 candidates for the years 2012 to 2017. Their proper motion analysis yielded that 96% of the (high) proper motions of the lensing stars were erroneous. We were thus left with only 43 confirmed microlensing candidates. Due to an improvement of the search width around potential lensing stars, we can predict 50 additional astrometric microlensing events with correct proper motion now between 2014 and 2019. All confirmed astrometric microlensing candidates are marked with true in the "Confirmed?"-column.

References