OGLE-2012-BLG-0563: Relative Proper Motion Constraint from Keck Follow-up finds the correct Lens

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "OGLE-2012-BLG-0563: Relative Proper Motion Constraint from Keck Follow-up finds the correct Lens".

Jocelyn: Here's the gist of the paper. The gist: Keck follow up observations of OGLE-2012-BLG-0563 yields a host mass of Mhost = 0.81 ± 0.04M⊙, a K dwarf star, orbited by a Jupiter mass planet, MPlanet = 1.12 ± 0.09MJup., which is the result of an analysis that determined the correct lens identification through image-constrained modeling and relative proper motion constraints., Discovery and Initial Findings OGLE-2012-BLG-0563 was first discovered by OGLE collaboration on May 1, 2012 using the 1.3 m Warsaw telescope, and later noticed by MOA collaboration on May 18, 2012 using the 1.8 m MOA II telescope., This event peaked around May 21 with a maximum magnification of more than 600, which showed an anomalous asymmetric feature around the peak indicating that the lens star hosted a companion., The discovery of this planet was led by Fukui et.

Vera: al.

Jocelyn: (2015), who measured the excess flux on top of the source and published the mass of the host and the planet, reporting a Saturn mass planet orbiting a 0.34 M⊙star at 1.3 kpc distance., Keck Follow-up Observations The event was observed with Keck Adaptive Optics (AO) NIRC2 during the late night of May 23, 2018, using the wide camera to detect and match bright isolated stars to the VVV catalog for calibration purposes., The Keck NIRC2 narrow camera observations were taken on May 23, 2018 in the K-band using laser guide star adaptive optics (LGSAO) with a main purpose of resolving the lens host star from the source star., The average FWHM of these narrow images is approximately 63 mas, although some images were lost due to guiding system problems or cloud coverage., For final reduction, only the best Keck images observed with LGSAO from May 23, 2018 were used for the analysis., Lens and Source Identification In Keck analysis, the two stars had similar brightness values of 17.86 ± 0.18 and 17.79 ± 0.19 in the K band, making it difficult to know which one was the source from magnitudes., The team performed two light curve fitting cases: Case A where the star in Northeast (NE) is considered the lens star and Southwest (SW) is considered the source star, and Case B where these roles are switched., The analysis showed that performing an image-constrained light curve model fit using relative proper motion direction constraints from the Keck follow-up data demonstrated that the correct solution identifies the NE star as the lens star., This outcome was driven by systematics in FTS and MOA B & C data, alongside the impact of relative proper motion direction constraints., Light Curve Modeling and Constraints The analysis involved fitting light curve data with lens flux and heliocentric relative proper motion direction constraints from Keck analysis to determine masses and distance., The team fitted several light curve models to the re-reduced MOA data combined with FTS and MOA B & C data, comparing unconstrained, magnitude-constrained, and constrained Case A and Case B models., The results showed that after removing FTS and MOA B&C data, Case A turned out to be the correct solution because it had a lower χ2 value compared to case B., The final light curve modeling parameters in Table 4 indicated that case A has a ∆χ2 ∼7 compared to the unconstrained fit, while case B has a ∆χ2 ∼18 compared to the unconstrained fit., Final Mass and Distance Determination The mass measurements were determined by solving a minimum of two of the three mass distance relations, utilizing the angular Einstein radius (θE) and microlensing parallax amplitude (πE), which can each provide mass-distance relations., The host star mass was measured to be Mhost = 0.81 ± 0.04M⊙, a K dwarf star, orbited by a Jupiter mass planet, MPlanet = 1.12 ± 0.09MJup., The lens distance was determined to be DL = 5.49+0.58−0.55 kpc., These measurements were consistent with the results obtained from HST passbands as well, confirming that the star in the south-west is the source and the star in North-East is likely to be the lens., Significance for Future Surveys The paper measures a lens-source separation of 23.28±0.48 mas in Keck narrow images, which is the smallest separation measured in all high angular resolution Keck observations., This measurement demonstrates that a lens-source separation similar to what is expected from the Roman mission can be detected clearly even with ∼20 images., The analysis shows the importance of multi-passband image analysis and obtaining independent consistent results from Keck analysis, which validates the identification of the lens star as indeed being the lens., This work developed a method that can be used to verify small-separation Roman microlensing planetary events with Keck Follow-up images.

Vera: The host mass from current analysis is 0.81M⊙ and the planet-host mass ratio is only slightly different, but this change in the host mass has resulted in a big change in planet mass., The increase in host mass would influence the big fundamental questions like, which stars host which kind of planets?, highlighting the critical importance of thoroughly checking photometric systematics and measuring the correct mass., This event is part of S16 sample (Suzuki et al.

Paper summary: Vera: So to recap, this paper argues that by using image constraints and proper motion data from Keck, they correctly identified the lens star in the OGLE-two thousand twelve-BLG-five hundred sixty-three system <ref:2610.30310#pg1>. It's about finding a way to resolve ambiguities when you have multiple stars involved in a microlensing event.

Jocelyn: The core idea is that the initial discovery papers or simple models can be misleading, but when you add the constraint of how fast those stars are actually moving across the sky, things fall into place.

Subrahmanyan: From a cosmological view, this kind of measurement helps us constrain the mass function of planets orbiting stars. If we get these masses right, we can better understand how planets form in different stellar environments across the universe.

Vera: They determined the host star mass to be zero point eight one solar masses and the planet mass to be one point one two Jupiter masses at a distance of about five hundred forty-nine kpc from us. These are their final results based on this constrained modeling approach, which they found consistent with HST results too.

Jocelyn: The separation they measure in these high-resolution Keck images is twenty-three point two eight mas, which is a very small value for a lens-source separation measured with adaptive optics <ref:2610.30310#pg1>.

Subrahmanyan: That small separation measurement means the lens and source are physically quite close to each other, which gives us tighter constraints on the Einstein radius and microlensing parallax amplitude calculations they use.

Vera: Their limitation is that they noted some systematics in the FTS and MOA B and C data, so it's important to check those across all passbands when you do this kind of analysis.

Jocelyn: They also mentioned that the method developed here, using image-constrained light curve modeling with proper motion constraints, is a new way to verify small-separation events like this one with Keck follow-up images.

Conclusion: Vera: So, the whole point of this paper is that by combining high-resolution imaging with motion data, they settled a long-standing ambiguity about which star was the lens in OGLE-two thousand twelve-BLG-five hundred sixty-three <ref:2610.30310#pg1>. They used those specific constraints to choose the correct identification.

Jocelyn: It really highlights how important it is to use all your available data sources when you're trying to pin down physical parameters like mass and distance from a microlensing event.

Subrahmanyan: For the bigger picture, this work reinforces that accurate lens identification is the first step in using these events as probes for planet populations around different types of stars. If we get the planet masses right, it helps us map out planetary demographics better.

Vera: The implication is that future surveys, like Roman Space Telescope data, can look for these small-separation events and use Keck follow-up to confirm the lens identification with a reliable method.

Jocelyn: It suggests that if you're doing time-domain astronomy, incorporating relative proper motion constraints isn't just an extra step; it’s a necessary tool to get the right picture of what you are observing.