—> Astronomers Discovered the Smallest Microlensing planet OGLE-2019-BLG-0960 Lb.
—> OGLE-2019-BLG-0960Lb is the 19th microlensing planet with a mass-ratio below the fiducial power-law break in the mass-ratio distribution.
—> Astronomers claimed that such planets primarily found in moderate magnification and “Hollywood” events.
A team of international astronomers reported on the analysis of OGLE-2019-BLG-0960, which contains the smallest mass-ratio microlensing planet, “OGLE-2019-BLG-0960 Lb”, found to date. It has a mass ratio of q ∼ 1.27 ± 0.07 or ∼ 1.45 ± 0.15 × 10^-5.
The measurement of the annual parallax effect combined with the finite source effect allowed them to determine the mass of the host (M-dwarf) star (ML = 0.3–0.6 M), the mass of its planet (mp =1.4–3.1 M), the projected separation between the host and planet (a⊥ = 1.2–2.3 au), and the distance to the lens system (DL = 0.6–1.2 kpc).
OGLE-2019-BLG-0960Lb is the 19th microlensing planet with a mass-ratio below the fiducial power-law break in the mass-ratio distribution, qbr = 1.7 × 10-⁴, posited by Suzuki et al. (2016). It is the fourth planet below the revised break of qbr = 0.55 × 10-⁴ from Jung et al. (2019).
The three smallest planets (including this one) have all been discovered since the advent of continuous survey observations from KMTNet. This indicates that the current generation of microlensing experiments is now capable of measuring both the precise location of qbr and the power-law slope, p, of the mass-ratio distribution below qbr.
By comparing OGLE-2019-BLG-0960 with other published planets below qbr, they found that they (such planets) are primarily found in moderate magnification and “Hollywood” events. Moderate magnification events are the primary source of small planets because the cross-section for a planetary perturbation is largest when the planetary caustics are near resonance. In fact, the planet sensitivity is maximized for planets just outside resonance (| log s| > | log s_resonant|) because significant perturbations to the magnification field extend well beyond the caustic structures. However, the planet sensitivity decreases rapidly for |log s| & few| log s_resonant|.
They found three of the nineteen planets with mass ratios smaller than q = 1.7 × 10-⁴ in “Hollywood” events, for which the cross-section for light-curve anomalies is set by the source size rather than the caustic size. For these planets s >> s_resonant. The expected yield for such events is likely to be much lower than for moderate magnification events. Typical source sizes are ρ ∼ 0.01, and thus, the cross-section is a factor of ∼ 10 smaller than for moderate magnification events. At the same time, events with giant sources represent only a fraction of all microlensing events, making intensive work on such events tractable.
Hence, the focus for discovering planets with log q ≲ – 4 should be on moderate magnification events and events with giant sources. This search could be conducted within existing survey data or be supplemented by a followup campaign. Even though OGLE-2019-BLG-0960Lb could be recovered from survey data alone (Fig 3), this recovery was aided by the special geometry of the light curve (Jung et al. 2020) and suggests that similar, but shorter and potentially more numerous, perturbations would be missed in the low-cadence survey fields. For followup observations, Abe et al. (2013) previously suggested that the focus for small planets should be events with 50 < Amax < 200. This current investigation showed that events should be monitored for the full time that they have A > 5, which suggests this focus should be extended to events with peak magnification Amax > 10 or even smaller. If additional resources are available, followup observations could be further extended to include events with giant sources. This strategy will maximize the number of small planets found and enable a robust measurement of the mass-ratio distribution of microlensing planets with q < qbr.
Finally, they demonstrated with an empirical investigation that most planets showing a degeneracy between (s > 1) and (s < 1) solutions are not in the regime (|log s|>> 0) for which the “close”/“wide” degeneracy was derived. This investigation suggested a link between the “close”/“wide” and “inner/outer” degeneracies and also that the symmetry in the lens equation goes much deeper than symmetries uncovered for the limiting cases.
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