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Register a new userAU Mic b is the Youngest Planet to have a Spin-Orbit Alignment Measurement
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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.
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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.
Here, we study the dichotomy of the escaping atmosphere of the newly discovered close-in exoplanet AU Mic b. On one hand, the high EUV stellar flux is expected to cause a strong atmospheric escape in AU Mic b. On the other hand, the wind of this young star is believed to be very strong, which could reduce or even inhibit the planets atmospheric escape. AU Mic is thought to have a wind mass-loss rate that is up to $1000$ times larger than the solar wind mass-loss rate ($dot{M}_odot$). To investigate this dichotomy, we perform 3D hydrodynamics simulations of the stellar wind--planetary atmosphere interactions in the AU Mic system and predict the synthetic Ly-$alpha$ transits of AU Mic b. We systematically vary the stellar wind mass-loss rate from a `no wind scenario to up to a stellar wind with a mass-loss rate of $1000~dot{M}_odot$. We find that, as the stellar wind becomes stronger, the planetary evaporation rate decreases from $6.5times 10^{10}$ g/s to half this value. With a stronger stellar wind, the atmosphere is forced to occupy a smaller volume, affecting transit signatures. Our predicted Ly-$alpha$ absorption drops from $sim 20%$, in the case of `no wind to barely any Ly-$alpha$ absorption in the extreme stellar wind scenario. Future Ly-$alpha$ transits could therefore place constraints not only on the evaporation rate of AU Mic b, but also on the mass-loss rate of its host star.
We present high resolution near-infrared spectropolarimetric observations using the SPIRou instrument at CFHT during a transit of the recently detected young planet AU Mic b, with supporting spectroscopic data from iSHELL at IRTF. We detect Zeeman signatures in the Stokes V profiles, and measure a mean longitudinal magnetic field of $overline{B}_ell=46.3pm0.7$~G. Rotationally modulated magnetic spots likely cause long-term variations of the field with a slope of $d{B_ell}/dt=-108.7pm7.7$~G/d. We apply the cross-correlation technique to measure line profiles and obtain radial velocities through CCF template matching. We find an empirical linear relationship between radial velocity and $B_ell$, which allows us to estimate the radial velocity variations which stellar activity induces through rotational modulation of spots for the five hours of continuous monitoring of AU Mic with SPIRou. We model the corrected radial velocities for the classical Rossiter-McLaughlin effect, using MCMC to sample the posterior distribution of the model parameters. This analysis shows that the orbit of AU Mic b is prograde and aligned with the stellar rotation axis with a sky-projected spin-orbit obliquity of $lambda=0^{+18}_{-15}$ degrees. The aligned orbit of AU Mic b indicates that it formed in the protoplanetary disk that evolved to the current debris disk around AU Mic.
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.
We present a velocimetric and spectropolarimetric analysis of 27 observations of the 22-Myr M1 star AU Microscopii (Au Mic) collected with the high-resolution $YJHK$ (0.98-2.35 $mu$m) spectropolarimeter SPIRou from 2019 September 18 to November 14. Our radial velocity (RV) time-series exhibits activity-induced fluctuations of 45 m/s RMS, about three times smaller than those measured in the optical domain, that we filter using Gaussian Process Regression. We report a 3.9$sigma$-detection of the recently-discovered 8.46-d transiting planet AU Mic b, with an estimated mass of $17.1^{+4.7}_{-4.5}$ M$_{odot}$ and a bulk density of $1.3 pm 0.4$ g/cm$^{-3}$, inducing a RV signature of semi-amplitude $K=8.5^{+2.3}_{-2.2}$ m/s in the spectrum of its host star. A consistent detection is independently obtained when we simultaneously image stellar surface inhomogeneities and estimate the planet parameters with Zeeman-Doppler Imaging (ZDI). Using ZDI, we invert the time series of unpolarized and circularly-polarized spectra into surface brightness and large-scale magnetic maps. We find a mainly poloidal and axisymmetric field of 475 G, featuring, in particular, a dipole of 450 G tilted at 19{deg} to the rotation axis. Moreover, we detect a strong differential rotation of d$Omega = 0.167 pm 0.009$ rad/d shearing the large-scale field, about twice stronger than that shearing the brightness distribution, suggesting that both observables probe different layers of the convective zone. Even though we caution that more RV measurements are needed to accurately pin down the planet mass, AU Mic b already appears as a prime target for constraining planet formation models, studying the interactions with the surrounding debris disk, and characterizing its atmosphere with upcoming space- and ground-based missions.
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