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Register a new userThe dichotomy of atmospheric escape in AU Mic b
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S. Carolan
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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.
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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.
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.
(Abridged) Short-period gas giant exoplanets are susceptible to intense atmospheric escape due to their large scale heights and strong high-energy irradiation. This process is thought to occur ubiquitously, but to date we have only detected direct evidence of atmospheric escape in hot Jupiters and warm Neptunes. The paucity of cases for intermediate, Saturn-sized exoplanets at varying levels of irradiation precludes a detailed understanding of the underlying physics in atmospheric escape of hot gas giants. Our objectives here are to assess the high-energy environment of the warm ($T_mathrm{eq} = 970$ K) Saturn WASP-29 b and search for signatures of atmospheric escape. We used far-ultraviolet (FUV) observations from the Hubble Space Telescope to analyze the flux time series of H I, C II, Si III, Si IV, and N V during the transit of WASP-29 b. At 3$sigma$ confidence, we rule out any in-transit absorption of H Ilarger than 92% in the Lyman-$alpha$ blue wing and 19% in the red wing. We found an in-transit flux decrease of $39%^{+12%}_{-11%}$ in the ground-state C II emission line at 133.45 nm. But due to this signal being significantly present in only one visit, it is difficult to attribute a planetary or stellar origin for the ground-state C II signal. We place 3$sigma$ absorption upper limits of 40%, 49% and 24% for Si III, Si IV, and for excited-state C II at 133.57 nm, respectively. Low activity levels and the faint X-ray luminosity suggest that WASP-29 is an old, inactive star. An energy-limited approximation combined with the reconstructed EUV spectrum of the host suggests that the planet is losing its atmosphere at a rate of $4 times 10^9$ g s$^{-1}$. The non-detection at Lyman-$alpha$ could be partly explained by a low fraction of escaping neutral hydrogen, or by the state of fast radiative blow-out we infer from the reconstructed stellar Lyman-$alpha$ line.
We present 1.3 millimeter ALMA Cycle 0 observations of the edge-on debris disk around the nearby, ~10 Myr-old, M-type star AU Mic. These observations obtain 0.6 arcsec (6 AU) resolution and reveal two distinct emission components: (1) the previously known dust belt that extends to a radius of 40 AU, and (2) a newly recognized central peak that remains unresolved. The cold dust belt of mass about 1 lunar mass is resolved in the radial direction with a rising emission profile that peaks sharply at the location of the outer edge of the birth ring of planetesimals hypothesized to explain the midplane scattered light gradients. No significant asymmetries are discerned in the structure or position of this dust belt. The central peak identified in the ALMA image is ~6 times brighter than the stellar photosphere, which indicates an additional emission process in the inner regions of the system. Emission from a stellar corona or activity may contribute, but the observations show no signs of temporal variations characteristic of radio-wave flares. We suggest that this central component may be dominated by dust emission from an inner planetesimal belt of mass about 0.01 lunar mass, consistent with a lack of emission shortward of 25 microns and a location <3 AU from the star. Future millimeter observations can test this assertion, as an inner dust belt should be readily separated from the central star at higher angular resolution.
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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