Day 1468

Tuesday.

1809.08243
First resolution of micro lensed images
Dong, et al

Employ VLTI GRAVITY to resolve, for the first time, the two images generated by a gravitational microlens.  The measurements of the image separation theta_-,+=3.78±0.05 mas, and hence the Einstein radius theta_E=1.87±0.03 mas, are precise.  This demonstrates the robustness of the method, provided that the source is bright enough for GRAVITY (K <~ 10.5) and the image separation is of order or larger than the fringe spacing.  When theta_E is combined with a measurement of the "microlens parallax" pi_E, the two will together yield the lens mass and lens-source relative parallax and proper motion.  Because the source parallax and proper motion are well measured by Gaia, this means that the lens characteristics will be fully determined, whether or not it proves to be luminous.  This method can be a powerful probe of dark, isolated objects, which are otherwise quite difficult to identify, much less characterize.  The measurement contradicts Einstein's (1936) prediction that "the luminous circle [i.e., microlnsed image] cannot be distinguished" from a star.


1809.08344
The Elephant in the room: the importance of where and when massive stars form in molecular clouds
Grudic, Hopkins

Most simulations of galaxies and massive giant molecular clouds (GMCs) cannot explicitly resolve the formation (or predict the MS masses) of individual stars.  So they must use some prescription for the amount of feedback from an assumed population of massive stars (e.g. sampling the IMF).  Perform a methods study of simulations of a star-forming GMC with stellar feedback from UV radiation, varying only the prescription for determining the luminosity of each stellar mass element formed (according to different IMF sampling schemes).  Show that different prescriptions can lead to widely varying (factor of ~3) SF efficiencies (on GMC scales) even though the average mass-to-light ratios agree.  Discreteness of sources is important: radiative feedback from fewer, more-luminous sources has a greater effect for a given total luminosity.  These differences can dominate over other, more widely-recognized differences between similar literature GMC-scale studies (e.g. numerical methods, cloud initial conditions, presence of magnetic fields).  Moreover the differences in these methods are not purely numerical: some make different implicit assumptions about where and how massive stars form, and this remains deeply uncertain in SF theory.