Kavli Affiliate: Subo Dong
| First 5 Authors: Andrew Gould, Jennifer C. Yee, Subo Dong, ,
| Summary:
Roman microlensing stands at a crossroads between its originally charted path
of cataloging a population of cool planets that has subsequently become
well-measured down to super-Earths, and the path of free-floating planets
(FFPs), which did not exist when Roman was chosen in 2010, but by now promises
revolutionary insights into planet formation and evolution via their possible
connection to a spectrum of objects spanning 18 decades in mass. Until now, it
was not even realized that the 2 paths are in conflict: Roman strategy was
optimized for bound-planet detections, and FFPs were considered only in the
context of what could be learned about them given this strategy. We derive a
simple equation that mathematically expresses this conflict and explains why
the current approach severely depresses detection of 2 of the 5 decades of
potential FFP masses, i.e., exactly the two decades, $M_{rm Pluto}< M
<2,M_{rm Mars}$, that would tie terrestrial planets to the proto-planetary
material out of which they formed. FFPs can be either truly free floating or
can be bound in "Wide", "Kuiper", and "Oort" orbits, whose separate
identification will allow further insight into planet formation. In the
(low-mass) limit that the source radius is much bigger than the Einstein
radius, $theta_*ggtheta_{rm E}$, the number of significantly magnified
points on the FFP light curve is $N=2Gammatheta_*sqrt{1-z^2}/mu$ –> 3.0,
when normalized to the adopted Roman cadence $Gamma=4/$hr, and to source
radius $theta_*=0.3,mu$as, lens-source proper motion $mu=6,$mas/yr, and
source impact parameter $z=0.5$, which are all typical values. By contrast
$N=6$ are needed for an FFP detection. Thus, unless $Gamma$ is doubled, FFP
detection will be driven into the (large-$theta_*$, small-$mu$) corner of
parameter space, reducing the detections by a net factor of 2 and cutting off
the lowest-mass FFPs.
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