ALBERT

Description: Context. Multiplanet systems are excellent laboratories to test planet formation models as all planets are formed under the same initial conditions. In this context, systems transiting bright stars can play a key role, since planetary masses, radii, and bulk densities can be measured. Aims. GJ 9827 (K2-135) has recently been found to host a tightly packed system consisting of three transiting small planets whose orbital periods of 1.2, 3.6, and 6.2 days are near the 1:3:5 ratio. GJ 9827 hosts the nearest planetary system (~30 pc) detected by NASA’s Kepler or K2 space mission. Its brightness (V = 10.35 mag) makes the star an ideal target for detailed studies of the properties of its planets. Methods. Combining the K2 photometry with high-precision radial-velocity measurements gathered with the FIES, HARPS, and HARPS-N spectrographs we revised the system parameters and derive the masses of the three planets. Results. We find that GJ 9827 b has a mass of Mb = 3.69−0.46+0.48 M⊕ and a radius of Rb = 1.58−0.13+0.14 R⊕, yielding a mean density of ρb = 5.11−1.27+1.74 g cm−3. GJ 9827 c has a mass of Mc = 1.45−0.57+0.58 M⊕, radius of Rc = 1.24−0.11+0.11 R⊕, and a mean density of ρc = 4.13−1.77+2.31 g cm−3. For GJ 9827 d, we derive Md = 1.45−0.57+0.58 M⊕, Rd = 1.24−0.11+0.11 R⊕, and ρd = 1.51−0.53+0.71 g cm−3. Conclusions. GJ 9827 is one of the few known transiting planetary systems for which the masses of all planets have been determined with a precision better than 30%. This system is particularly interesting because all three planets are close to the limit between super-Earths and sub-Neptunes. The planetary bulk compositions are compatible with a scenario where all three planets formed with similar core and atmosphere compositions, and we speculate that while GJ 9827 b and GJ 9827 c lost their atmospheric envelopes, GJ 9827 d maintained its primordial atmosphere, owing to the much lower stellarirradiation. This makes GJ 9827 one of the very few systems where the dynamical evolution and the atmosphericescape can be studied in detail for all planets, helping us to understand how compact systems form and evolve.

Description: Protodunes emerge from a flat sand bed at the upwind margin of White Sands Dune Field, and, over several hundred meters, transition into fully developed dunes. Here, we investigate spatial and temporal changes in topography across this transition from 2007 to 2016 using lidar-derived topography, structure-from-motion-derived topography, and RTK GPS. We characterize the deposits present in 2015 using ground penetrating radar. Symmetric protodunes give way downwind to an asymmetric protodune at the transition to slipface development. Between 2007 and 2016, protodune amplitude increased from 0.2 m to 4.0 m, migration rate increased from 3.2 m/yr to 6.1 m/yr, and wavelength increased from 76 m to 122 m. Ground-penetrating radar surveys show strata between flat and 15° make up the stratigraphic architecture of the protodunes. Strata increase in steepness commensurate with an increase in amplitude. Decimeter accumulations of low-angle strata associated with initial protodune stages give way to 4 m of accumulation composed of sets up to 1 m thick prior to slipface development. Topsets present in the thickest sets indicate near critical angles of bedform climb. Growth and slipface development occur by aerodynamic sand trapping and protodune merging. Changes in asymmetry erase initial slipfaces prior to permanent slipface development, after which efficient sand trapping and scour promotes the transition to a dune across 20 m in 5 years. Protodune stratification has hallmarks of sandsheet stratification and can be appreciated within the greater suite of processes that create low-angle eolian stratification found in modern and ancient environments.