Multi-season optical modulation phased with the orbit of the super-Earth 55 Cancri e

Abstract

Context. 55 Cnc e is a transiting super-Earth orbiting a solar-like star with an orbital period of ∼ 17.7 hours. In 2011, using the Microvariability and Oscillations in Stars (MOST) space telescope, a quasi-sinusoidal modulation in flux was detected with the same period as the planetary orbit. The amplitude of this modulation was too large to be explained as the change in light reflected or emitted by the planet. Aims. The MOST telescope continued to observe 55 Cnc e for a few weeks per year over five years (from 2011 to 2015), covering 143 individual transits. This paper presents the analysis of the observed phase modulation throughout these observations and a search for the secondary eclipse of the planet. Methods. The most important source of systematic noise in MOST data is due to stray-light reflected from the Earth, which is modulated with both the orbital period of the satellite (101.4 minutes) and the Earth's rotation period. We present a new technique to deal with this source of noise, which we combined with standard detrending procedures for MOST data. We then performed Markov Chain Monte Carlo analyses of the detrended light curves, modeling the planetary transit and phase modulation. Results. We find phase modulations similar to those seen in 2011 in most of the subsequent years; however, the amplitude and phase of maximum light are seen to vary, from year to year, from 113 to 28 ppm and from 0.1 to 3.8 rad. The secondary eclipse is not detected, but we constrain the geometric albedo of the planet to less than 0.47 (2σ). Conclusions. While we cannot identify a single origin of the observed optical modulation, we propose a few possible scenarios. Those include star-planet interaction, such as coronal rains and spots rotating with the motion of the planet along its orbit, or the presence of a transiting circumstellar torus of dust. However, a detailed interpretation of these observations is limited by their photometric precision. Additional observations at optical wavelengths could measure the variations at higher precision, contribute to uncovering the underlying physical processes, and measure or improve the upper limit on the albedo of the planet.

Demonstrating High-precision Photometry with a CubeSat: ASTERIA Observations of 55 Cancri e

“…We show the probability distribution function of the retrieved transit depth in Figure 8. This is in agreement with the values published in the literature at similar wavelengths : Winn et al (2011) found 380±52 ppm with MOST, De Mooij et al (2014) 361±49 ppm between 457 and 671nm, 331±36 ppm with HST STIS/G750L (Bourrier et al 2018) and 346±15 ppm in the global anaylsis of MOST data (Sulis et al 2019). Table 3 lists all the retrieved parameters for the fit with a prior on T 0 .…”

“…The deviations in these two parameters are allowed, because the fit prefers parameters such that there is no coverage of the ingress and egress phases of the transit, leaving the transit parameters less constrained. The resulting best-fit value for the T 0 is off by 63 min compared to the most recent published ephemeris of 55 Cnc e (Sulis et al 2019).…”

“…Both fits used the priors on b and P listed in Table 3. When using the published T 0 as a prior (Sulis et al 2019), we find a transit depth median value of 374±170 ppm (Figure 9, top). We show the probability distribution function of the retrieved transit depth in Figure 8.…”

Circumstellar environment of 55 Cancri

“…Using these simulations, we estimated the possible position of chromospheric hotspots due to this interaction, and found they would be offset in phase from the planet's orbital position and that apparent activity due to SPI may be modulated with a period close to twice the orbital or synodic period. Recently, Sulis et al (2019) detected a stellar flux modulation in phase with the orbital period of planet e. Furthermore, the amplitude of this modulation appears to be directly related to stellar activity, indicating that this could be caused by magnetic interaction occurring as a consequence of the planet lying entirely within the Alfvén surface of the star. Assuming the interpretation of the observation by Sulis et al (2019) in terms of SPI is correct, the possible apparent discrepancy in the timing of the modulation between the observation and the model prediction may be caused by the fact that the photometric and spectropolarimetric observations have been obtained more than two years apart: within this time, the geometry and strength of the stellar magnetic field may have changed.…”

Atmospheric compositions and observability of nitrogen-dominated ultra-short-period super-Earths

“…For this comparison we use the available measured spectra of 55 Cancri from Crossfield (2012), which has higher flux at 5 µm compared to our generated PHOENIX model. The variability of 55 Cnc e is a known issue, and several possible explanations have been published in the last years, ranging from large scale volcanic plumes, a gas-dust torus, reflective grains or magnetic interaction, to other effects (Demory et al 2016a,b;Bourrier et al 2018;Tamburo et al 2018;Sulis et al 2019;Folsom et al 2020). More observations are needed to decipher the issue.…”