Colette Salyk¹, John Lacy², Matt Richter³, Ke Zhang⁴, Klaus Pontoppidan⁵, John S. Carr⁶, Joan R. Najita⁷, and Geoffrey A. Blake⁸
Astrophysical Journal 874, 24 Link to Article [DOI: 10.3847/1538-4357/ab05c3 ]
¹Department of Physics and Astronomy, Vassar College, 124 Raymond Avenue, Poughkeepsie, NY 12604, USA
²Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA
³Physics Department, University of California at Davis, Davis, CA 95616, USA
⁴Department of Astronomy, University of Michigan, 311 West Hall, 1085 South University Avenue, Ann Arbor, MI 48109, USA
⁵Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶Naval Research Laboratory, Code 7213, Washington, DC 20375, USA
⁷National Optical Astronomy Observatory, 950 N. Cherry Avenue, Tucson, AZ 85719, USA
⁸Division of Geological and Planetary Sciences, California Institute of Technology, MC 150-21, 1200 E California Boulevard, Pasadena, CA 91125, USA

We present the largest survey of spectrally resolved mid-infrared water emission to date, with spectra for 11 disks obtained with the Michelle and TEXES spectrographs on Gemini North. Water emission is detected in six of eight disks around classical T Tauri stars. Water emission is not detected in the transitional disks SR 24 N and SR 24 S, in spite of SR 24 S having pretransitional disk properties like DoAr 44, which does show water emission. With R ~ 100,000, the TEXES water spectra have the highest spectral resolution possible at this time, and allow for detailed line shape analysis. We find that the mid-IR water emission lines are similar to the “narrow component” in CO rovibrational emission, consistent with disk radii of a few astronomical units. The emission lines are either single peaked, or consistent with a double peak. Single-peaked emission lines cannot be produced with a Keplerian disk model, and may suggest that water participates in the disk winds proposed to explain single-peaked CO emission lines. Double-peaked emission lines can be used to determine the radius at which the line emission luminosity drops off. For HL Tau, the lower limit on this measured dropoff radius is consistent with the 13 au dark ring. We also report variable line/continuum ratios from the disks around DR Tau and RW Aur, which we attribute to continuum changes and line flux changes, respectively. The reduction in RW Aur line flux corresponds with an observed dimming at visible wavelengths.

Debanjan Sengupta^1,2, Sarah E. Dodson-Robinson^1,3, Yasuhiro Hasegawa², and Neal J. Turner²
Astrophysical Journal 874, 26 Link to Article [DOI: 10.3847/1538-4357/aafc36 ]
¹Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA
²Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³Bartol Research Institute, Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA

Despite making a small contribution to total protoplanetary disk mass, dust affects the disk temperature by controlling the absorption of starlight. As grains grow from their initial interstellar-medium-like size distribution, settling depletes the disk’s upper layers of dust and decreases the optical depth, cooling the interior. Here we investigate the effect of collisional growth of dust grains and their dynamics on the thermal and optical profile of the disk, and explore the possibility that cooling induced by grain growth and settling could lead to gravitational instability. We develop a Monte Carlo dust collision model with a weighting technique and allow particles to collisionally evolve through sticking and fragmentation, along with vertical settling and turbulent mixing. We explore three disk models and perform simulations for both constant and spatially variable turbulence profile. We then calculate mean wavelength-dependent opacities for the evolving disks and perform radiative transfer to calculate the temperature profile. Finally, we calculate the Toomre Q parameter, a measure of the disk’s stability against self-gravity, after it reaches a steady-state dust-size distribution. We find that even weak turbulence can keep submicrometer-sized particles stirred in the disk’s upper layer, affecting its optical and thermal profiles, and the growth of large particles in the midplane can make a massive disk optically thick at millimeter wavelengths, making it difficult to calculate the surface density of dust available for planet formation in the inner disk. Also, for all our initially marginally stable annuli, we find a small but noticeable reduction in Q.

Yanqin Wu
Astrophysical Journal 874, 91 Link to Article [DOI: 10.3847/1538-4357/ab06f8 ]
Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada

The majority of the transiting planets discovered by the Kepler mission (called super-Earths here, includes the so-called “sub-Neptunes”) orbit close to their stars. As such, photoevaporation of their hydrogen envelopes etches sharp features in an otherwise bland space spanned by planet radius and orbital period. This, in turn, can be exploited to reveal the mass of these planets, in addition to techniques such as radial velocity and transit-timing-variation. Here, using updated radii for Keplerplanet hosts from Gaia DR2, I show that the photoevaporation features shift systematically to larger radii for planets around more massive stars (ranging from M-dwarfs to F-dwarfs), corresponding to a nearly linear scaling between planet mass and its host mass. By modeling planet evolution under photoevaporation, one further deduces that the masses of super-Earths peak narrowly around 8 M_⊕(M _*/M _⊙). When such a stellar mass dependence is scaled out, Kepler planets appear to be a homogeneous population surprisingly uniform in mass, in core composition (likely terrestrial), and in initial mass fraction of their H/He envelope (a couple percent). The masses of these planets do not appear to depend on the metallicity values of their host stars, while they may weakly depend on the orbital separation. Taken together, the simplest interpretation of our results is that super-Earths are at the so-called “thermal mass”, where the planet’s Hill radius is equal to the vertical scale height of the gas disk.

¹C.J.Manser et al. (>10)
Science 364, 66-69 Link to Article [DOI: 10.1126/science.aat5330]
¹Department of Physics, University of Warwick, Coventry CV4 7AL, UK. Reprinted with permission from AAAS

Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4-minute periodic variation in the strength and shape of the Ca ii emission line profiles originating from the debris disc around the white dwarf SDSS J122859.93+104032.9. We interpret this short-period signal as the signature of a solid-body planetesimal held together by its internal strength.

¹D.S.Lauretta et al. (>10)
Nature 568, 55-60 Link to Article [https://doi.org/10.1038/s41586-019-1033-6]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

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