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Register a new userThe changing face of AU Mic b: stellar spots, spin-orbit commensurability, and Transit Timing Variations as seen by CHEOPS and TESS
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AU Mic is a young planetary system with a resolved debris disc showing signs of planet formation and two transiting warm Neptunes near mean-motion resonances. Here we analyse three transits of AU Mic b observed with the CHaracterising ExOPlanet Satellite (CHEOPS), supplemented with sector 1 and 27 Transiting Exoplanet Survey Satellite (TESS) photometry, and the All-Sky Automated Survey (ASAS) from the ground. The refined orbital period of AU Mic b is 8.462995 pm 0.000003 d, whereas the stellar rotational period is P_{rot}=4.8367 pm 0.0006 d. The two periods indicate a 7:4 spin--orbit commensurability at a precision of 0.1%. Therefore, all transits are observed in front of one of the four possible stellar central longitudes. This is strongly supported by the observation that the same complex star-spot pattern is seen in the second and third CHEOPS visits that were separated by four orbits (and seven stellar rotations). Using a bootstrap analysis we find that flares and star spots reduce the accuracy of transit parameters by up to 10% in the planet-to-star radius ratio and the accuracy on transit time by 3-4 minutes. Nevertheless, occulted stellar spot features independently confirm the presence of transit timing variations (TTVs) with an amplitude of at least 4 minutes. We find that the outer companion, AU Mic c may cause the observed TTVs.
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KELT-9 b is an ultra hot Jupiter transiting a rapidly rotating, oblate early-A-type star in a polar orbit. We model the effect of rapid stellar rotation on KELT-9 bs transit light curve using photometry from the Transiting Exoplanet Survey Satellite (tess) to constrain the planets true spin-orbit angle and to explore how KELT-9 b may be influenced by stellar gravity darkening. We constrain the host stars equatorial radius to be $1.089pm0.017$ times as large as its polar radius and its local surface brightness to vary by $sim38$% between its hot poles and cooler equator. We model the stellar oblateness and surface brightness gradient and find that it causes the transit light curve to lack the usual symmetry around the time of minimum light. We take advantage of the light curve asymmetry to constrain KELT-9 bs true spin orbit angle (${87^circ}^{+10^circ}_{-11^circ}$), agreeing with citet{gaudi2017giant} that KELT-9 b is in a nearly polar orbit. We also apply a gravity darkening correction to the spectral energy distribution model from citet{gaudi2017giant} and find that accounting for rapid rotation gives a better fit to available spectroscopy and yields a more reliable estimate for the stars polar effective temperature.
We report measurements of the sky-projected spin-orbit angle for AU,Mic,b, a Neptune-size planet orbiting a very young ($sim20$,Myr) nearby pre-main sequence M dwarf star which also hosts a bright, edge-on, debris disk. The planet was recently discovered from preliminary analysis of radial velocity observations and confirmed to be transiting its host star from photometric data from the NASAs textit{TESS} mission. We obtained radial velocity measurements of AU,Mic over the course of two partially observable transits and one full transit of planet b from high-resolution spectroscopic observations made with the {textsc{Minerva}}-Australis telescope array. Only a marginal detection of the Rossiter--McLaughlin effect signal was obtained from the radial velocities, in part due to AU Mic being an extremely active star and the lack of full transit coverage plus sufficient out-of-transit baseline. As such, a precise determination of the obliquity for AU,Mic,b is not possible in this study and we find a sky-projected spin-orbit angle of $lambda = 47{^{+26}_{-54}}^{circ}$. This result is consistent with both the planets orbit being aligned or highly misaligned with the spin-axis of its host star. Our measurement independently agrees with, but is far less precise than observations carried out on other instruments around the same time that measure a low obliquity orbit for the planet. AU,Mic is the youngest exoplanetary system for which the projected spin-orbit angle has been measured, making it a key data point in the study of the formation and migration of exoplanets -- particularly given that the system is also host to a bright debris disk.
The CHEOPS space mission dedicated to exoplanet follow-up was launched in December 2019, equipped with the capacity to perform photometric measurements at the 20 ppm level. As CHEOPS carries out its observations in a broad optical passband, it can provide insights into the reflected light from exoplanets and constrain the short-wavelength thermal emission for the hottest of planets by observing occultations and phase curves. Here, we report the first CHEOPS observation of an occultation, namely, that of the hot Jupiter WASP-189b, a $M_P approx 2 M_J$ planet orbiting an A-type star. We detected the occultation of WASP-189 b at high significance in individual measurements and derived an occultation depth of $dF = 87.9 pm 4.3$ppm based on four occultations. We compared these measurements to model predictions and we find that they are consistent with an unreflective atmosphere heated to a temperature of $3435 pm 27$K, when assuming inefficient heat redistribution. Furthermore, we present two transits of WASP-189b observed by CHEOPS. These transits have an asymmetric shape that we attribute to gravity darkening of the host star caused by its high rotation rate. We used these measurements to refine the planetary parameters, finding a $sim25%$ deeper transit compared to the discovery paper and updating the radius of WASP-189b to $1.619pm0.021 R_J$. We further measured the projected orbital obliquity to be $lambda = 86.4^{+2.9}_{-4.4}$deg, a value that is in good agreement with a previous measurement from spectroscopic observations, and derived a true obliquity of $Psi = 85.4pm4.3$deg. Finally, we provide reference values for the photometric precision attained by the CHEOPS satellite: for the V=6.6 mag star, and using a one-hour binning, we obtain a residual RMS between 10 and 17ppm on the individual light curves, and 5.7ppm when combining the four visits.
We obtained spectra of the pre-main sequence star AU Microscopii during a transit of its Neptune-sized planet to investigate its orbit and atmosphere. We used the high-dispersion near-infrared spectrograph IRD on the Subaru telescope to detect the Doppler shadow from the planet and constrain the projected stellar obliquity. Modeling of the observed planetary Doppler shadow suggests a spin-orbit alignment of the system ($lambda=-4.7_{-6.4}^{+6.8}$ degrees), but additional observations are needed to confirm this finding. We use both the IRD data and spectra obtained with NIRSPEC on Keck-II to search for absorption in the 1083 nm line of metastable triplet He I by the planets atmosphere and place an upper limit for the equivalent width of 3.7 mAA at 99 $%$ confidence. With this limit and a Parker wind model we constrain the escape rate from the atmosphere to $<0.15-0.45, M_{oplus}$ Gyr$^{-1}$, comparable to the rates predicted by an XUV energy-limited escape calculation and hydrodynamic models, but refinement of the planet mass is needed for rigorous tests.
AU Mic is a young ($sim$24 Myr), pre-Main Sequence M~dwarf star that was observed in the first month of science observations of the Transiting Exoplanet Survey Satellite (TESS) and re-observed two years later. This target has photometric variability from a variety of sources that is readily apparent in the TESS light curves; spots induce modulation in the light curve, flares are present throughout (manifesting as sharp rises with slow exponential decay phases), and transits of AU Mic b may be seen by eye as dips in the light curve. We present a combined analysis of both TESS Sector 1 and Sector 27 AU Mic light curves including the new 20-second cadence data from TESS Year 3. We compare flare rates between both observations and analyze the spot evolution, showing that the activity levels increase slightly from Sector 1 to Sector 27. Furthermore, the 20-second data collection allows us to detect more flares, smaller flares, and better resolve flare morphology in white light as compared to the 2-minute data collection mode. We also refine the parameters for AU Mic b by fitting three additional transits of AU Mic b from Sector 27 using a model that includes stellar activity. We show that the transits exhibit clear transit timing variations (TTVs) with an amplitude of $sim$80 seconds. We also detect three transits of a 2.8 $R_oplus$ planet, AU Mic c, which has a period of 18.86 days.
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