Spectroscopic Mass and Host-star Metallicity Measurements for Newly Discovered Microlensing Planet OGLE-2018-BLG-0740Lb

Abstract

We report the discovery of the microlensing planet OGLE-2018-BLG-0740Lb. The planet is detected with a very strong signal of ∆χ 2 ∼ 4630, but the interpretation of the signal suffers from two types of degeneracies. One type is caused by the previously known close/wide degeneracy, and the other is caused by an ambiguity between two solutions, in which one solution requires to incorporate finite-source effects, while the other solution is consistent with a point-source interpretation. Although difficult to be firmly resolved based on only the photometric data, the degeneracy is resolved in strong favor of the point-source solution with the additional external information obtained from astrometric and spectroscopic observations. The small astrometric offset between the source and baseline object supports that the blend is the lens and this interpretation is further secured by the consistency of the spectroscopic distance estimate of the blend with the lensing parameters of the point-source solution. The estimated mass of the host is 1.0 ± 0.1 M ⊙ and the mass of the planet is 4.5 ± 0.6 M J (close solution) or 4.8 ± 0.6 M J (wide solution) and the lens is located at a distance of 3.2 ± 0.5 kpc. The bright nature of the lens, with I ∼ 17.1 (V ∼ 18.2), combined with its dominance of the observed flux suggest that radial-velocity (RV) follow-up observations of the lens can be done using high-resolution spectrometers mounted on large telescopes, e.g., VLT/ESPRESSO, and this can potentially not only measure the period and eccentricity of the planet but also probe for close-in planets. We estimate that the expected RV amplitude would be ∼ 60 sin i m s −1 .

KMT-2019-BLG-1339L: An M Dwarf with a Giant Planet or a Companion near the Planet/Brown Dwarf Boundary

“…The mode of the degeneracy between solutions A and B is similar to the degeneracy mode identified for OGLE-2018-BLG-0740 (Han et al 2019). The similarity between the two events is that the coverage of the anomaly is incomplete, and this causes the ambiguity in ρ.…”

“…This degeneracy causes an ambiguity in determining q. The third case, found from OGLE-2018-BLG-0740 with a partially covered planetary signal, was reported by Han et al (2019). For this event, the degenerate solutions yielded different values of ρ, representing the angular radius of the source θ * normalized to q E (normalized source radius), and this caused ambiguities in both s and q.…”

KMT-2019-BLG-0842Lb: A Cold Planet below the Uranus/Sun Mass Ratio

“…If the blended light proves to be aligned with the source, then it should be possible to spectroscopically classify the blend (Han et al 2019). Such an observation may also plausibly measure the radial velocity (RV) offset between the source and the blend.…”

OGLE-2018-BLG-1700L: Microlensing Planet in Binary Stellar System

“…The blend is ∼2.5 mag brighter than the source. We check the possibility of the lens being the blend itself as in the case of OGLE-2018-BLG-0740 (Han et al 2019b). For this, we measure the astrometric offset between the position of the baseline object, measured in the image obtained by combining 72 KMTC images taken before lensing magnification, and the position of the source, measured in the difference image obtained by combining 47 difference images taken during the lensing magnification.…”