Diffuser-assisted Photometric Follow-up Observations of the Neptune-sized Planets K2-28b and K2-100b

Abstract

We present precision transit observations of the Neptune-sized planets K2-28b and K2-100b, using the Engineered Diffuser on the ARCTIC imager on the ARC 3.5m Telescope at Apache Point Observatory. K2-28b is a R p = 2.56R ⊕ mini-Neptune transiting a bright (J=11.7) metal-rich M4 dwarf, offering compelling prospects for future atmospheric characterization. K2-100b is a R p = 3.45R ⊕ Neptune in the Praesepe Cluster and is one of few planets known in a cluster transiting a host star bright enough (V = 10.5) for precision radial velocity observations. Using the precision photometric capabilities of the diffuser/ARCTIC system, allows us to achieve a precision of 105 +87 −37 ppm, and 38 +21 −11 ppm in 30 minute bins for K2-28b, and K2-100b, respectively. Our jointfits to the K2 and ground-based light-curves give an order of magnitude improvement in the orbital ephemeris for both planets, yielding a timing precision of 2 min in the JWST era. Although we show that the currently available broad-band measurements of K2-28b's radius are currently too imprecise to place useful constraints on K2-28b's atmosphere, we demonstrate that JWST/NIRISS will be able to discern between a cloudy/clear atmosphere in a modest number of transit observations. Our light-curve of K2-100b marks the first transit follow-up observation of this challenging-to-observe transit, where we obtain a transit depth of 819 ± 50 ppm in the SDSS i band. We conclude that diffuser-assisted photometry can play an important role in the TESS era to perform timely and precise follow-up of the expected bounty of TESS planet candidates.

A Sub-Neptune-sized Planet Transiting the M2.5 Dwarf G 9-40: Validation with the Habitable-zone Planet Finder

“…Similar and larger departures below the Gaussian expected precision have been reported by a number of groups in the literature (e.g., Blecic et al 2013;Stefansson et al 2017), and Cubillos et al (2017) show that these excursions are not statistically significant after taking into account the increasingly skewed inverse gamma distribution of the RMS values at the largest bin sizes. Following our previous work (Stefansson et al 2017(Stefansson et al , 2018a, we argue that excursions much below the Gaussian expected precision is likely an overestimate of the actual precision achieved, and we conservatively say that we achieve 138ppm precision in 30 minute bins for these transit observations.…”

“…The target rose from airmass 1.55 to airmass 1.05 during the observations. We observed the transit using the Engineered Diffuser on the ARCTIC imager, which has been described in detail in Stefansson et al (2017Stefansson et al ( , 2018a. In short, the Engineered Diffuser molds the focal-plane image of the star into a broad and stable top-hat shape, allowing us to increase our exposure times to gather more photons per exposure while minimizing correlated errors due to point spread function (PSF) variations and guiding errors.…”

Zodiacal exoplanets in time – X. The orbit and atmosphere of the young ‘neptune desert’-dwelling planet K2-100b

“…Mann et al (2017a) pointed out that the transit duration of "b" and the estimated density of the star are suggestive of an eccentric orbit (e = 0.24± 0.19 0.12 ), depending on the impact parameter. Stefansson et al (2018) also derived a transit-based density that was ∼ 3× higher than the one based on multi-band photometry and a Hipparcos parallax of the cluster (Mann et al 2017a), consistent with an eccentric orbit. However, this is based on transit photometry obtained with a 30-min cadence, only one-third of the transit duration.…”

“…The other three used the MuSCAT2 multi-bandpass photometer installed in the Carlos Sanchez Telescope (TCS) in the Teide observatory. The diffuser transit observations are described in Stefansson et al (2018), while the MuSCAT2 observations are described in Barragán et al (2019). For all data, we used the provided light curve for our MCMC analysis, only normalizing the out-of-transit baseline and updating the uncertainties based on the estimates in Barragán et al (2019).…”

“…IRD covers the Y -, J-, and Hbands simultaneously with a spectral resolution of 70,000. Using the ephemeris of Stefansson et al (2018), the transit mid-point occurred at JD = 245866.881 (UT 9:09:00) with a prediction accuracy of 50 sec. Observations commenced at JD = 2458566.81 and ended at JD = 2458566.98 and consisted of a series of 12-min integrations, the first starting 58 minutes before the predicted ingress (UT 8:21) and the last completing 93 minutes after the egress (UT 9:58).…”