502.01 — Are Exoplanets Orbiting M Dwarfs Ex- treme?
Philip Steven Muirhead1; Aurora Kesseli1; Eunkyu Han1; Mark Veyette2
1 Astronomy, Boston University (Boston, Massachusetts, United States)
2 Lockheed Martin (Denver, Colorado, United States)
M dwarf stars have long spin-down timescales, long activity lifetimes and persistent magnetic activity, all of which have implications for the potential habit- ability of orbiting planets. I will present results from several research programs investigating M dwarf ro- tation, activity and evolution. I will discuss a new technique to measure chemical-kinematic ages of main-sequence M dwarf stars. We applied that tech- nique to a variety of nearby M dwarfs, both planet hosts and non-planet hosts, and rapid (young) and slow (old) rotators. We find that relatively slow ro- tators (P∼100 days) do not appear to be α enriched, indicating that they are not over 10 Gyrs old. Sec- ond, for the rapid rotators, we see clear evidence of Zeeman enhancement of Y-band Ti I lines as a func- tion of Rossby number. While other activity indi- cators, such as H-α and X-ray emission, appear to saturate with low Rossby number, Zeeman enhance- ment does not, indicating that the saturation mech- anism is confined to the chromosphere and corona. Finally, I will present new results on the M dwarf radius problem. Using spectral synthesis methods, we find that large magnetic star spot fractions are primarily responsible for observed discrepancies be- tween model and measured stellar radii in fully con- vective M dwarf stars. As most M dwarfs appear dis- crepant, our results suggest the vast majority of M dwarfs have large spot fractions and correspondingly high localization of magnetic fields.
502.02 — Flare Statistics and High Resolution Spec- troscopy of a Volume Complete Sample of Mid-to- Late M dwarfs within 15 Parsecs
Amber Medina1; David Charbonneau1; Jennifer
Winters1; Jonathan Irwin1
1 Astronomy, Center for Astrophysics | Harvard and Smithsonian (Cambridge, Massachusetts, United States)
Main-sequence stars with masses less than 30% that of the Sun are fully convective and are the most abun- dant stars in the galaxy. The question of how fully convective stars generate their magnetic field is of in- trinsic interest and also bears upon the habitability of their orbiting planets. These stars currently provide the best opportunities to study planets in the habit- able zone, so it is essential we characterize their mag- netic activity. We are currently undertaking a multi- epoch high-resolution spectroscopic survey in addi- tion to obtaining (through a TESS GI program) two- minute cadence data of a volume-complete sample of stars with masses between 0.1-0.3 the solar value and within 15 parsecs. The stars in the sample are well-characterized with accurate masses and radii, and photometric rotation periods from the MEarth project. We determined the statistics of flares on all mid-to-late M dwarfs within 15 parsecs observed by TESS to-date. We use our complementary high- resolution spectroscopic measurements of rotational velocities, H-α equivalent widths, along with our galactic space motions (calculated from our mea- sured radial velocities) to correlate the ages and ac- tivity levels of this population to the flare rates, lu- minosities, and durations.
This work is supported by grants from the John Templeton Foundation, the David and Lucile Packard Foundation, and the US National Science Foundation.
502.03 — Volatile- and Water-rich Planetary Mate- rial Accreting onto a White Dwarf
Matthew Hoskin1; Odette Toloza1; Boris Gaensicke1;
Roberto Raddi3; Detlev Koester2; Jay Farihi4
1 Physics, University of Warwick (Coventry, WEST MIDLANDS, United Kingdom)
2 Institut fur Theoretische Physik und Astrophysik, Christian- Albrechts-Universität zu Kiel (Kiel, Germany)
3 Dr. Karl Remeis-Sternwarte, Friedrich–Alexander Universität Erlangen-Nürnberg (Bamberg, Germany)
4 Physics & Astronomy, University College London (London, United Kingdom)
We report the discovery of a white dwarf that has an unusually large amount of hydrogen,∼5% by mass, within its helium atmosphere. Such a mixture cannot result from the past evolution of this star alone (Rol- land+, 2018). The only plausible explanation is that this white dwarf has recently accreted∼1022g of wa-
ter — as much as 1% of the Earth’s oceans. Absorp- tion lines of the major mineral-forming elements (O, Mg, Si, Ca, Fe, Ni) and of volatile elements (C, S, P) detected in ourVLTandHubble Space Telescopespec- troscopy unambiguously demonstrate that the star
is currently accreting planetary debris. Our abun- dance analysis indicates a comet-like nature of the disrupted planetesimal, carrying the material nec- essary for seeding terrestrial exo-planets with the building-blocks of life. Small traces of hydrogen are common in helium atmosphere white dwarfs, and are often found alongside pollution by planetary de- bris, providing clear statistical evidence that water- rich rocky bodies prevale into the final stages of stel- lar and planetary evolution (Gentile Fusillo+, 2017). This connection is corroborated by this spectacularly polluted white dwarf, which has accreted a sufficient amount of water to change its past and future spectra evolution.
502.04 — M-dwarf Activity Driven 3D Climate and Photochemistry of Inner Habitable Zone Tidally- Locked Planets
Howard Chen1,2; Eric Wolf4; Zhuchang Zhan3; Daniel
Horton1,2
1 Department of Earth and Planetary Sciences, Northwestern Uni- versity (Evanston, Illinois, United States)
2 Center for Interdisciplinary Exploration and Research in Astro- physics (CIERA), Northwestern University (Evanston, Illinois, United States)
3 Department of Earth, Atmospheric and Planetary Sciences, Mas- sachusetts Institute of Technology (Cambridge, Massachusetts, United States)
4 Laboratory for Atmospheric and Space Physics, University of Colorado Boulder (Boulder, Colorado, United States)
Planets residing in circumstellar habitable zones (CHZs) offer our best opportunities to test hypothe- ses of life’s potential pervasiveness and complexity. Constraining the precise boundaries of habitability and its observational discriminants is thus critical to maximizing our chances at remote life detection for future instruments. Conventionally, calculations of the inner edge of the habitable zone (IHZ) have been performed using both 1D climate models and 3D general circulation models. However, these models lack interactive three-dimensional chemistry and do not resolve the observationally-critical mesosphere and lower thermosphere (MLT). Here we employ a 3D chemistry-climate model (CCM) to simulate the atmospheres of synchronously- rotating planets or- biting at the inner edge of habitable zones of K- and M-dwarf stars (between Teff = 4000 K and 2600 K)
with N2-O2-H2O-CO2 atmospheres. With the in- clusion of interactive chemistry, we find that sim- ulated runaway and moist greenhouse thresholds are in good agreement with previous GCM studies. However, around quiescent stars, our prognostic hy- drogen mixing ratios are orders of magnitude lower
than previous diagnostic estimates, suggesting that planets in these systems are less vulnerable to desic- cation via water escape. Additionally, we find that around active M-dwarfs, increases in upper atmo- spheric moisture and photodissociation rates allow hydrogen mixing ratios to approach that of water vapor, leading to elevated water loss efficiency via diffusion-limited escape. Using our CCM results as inputs, translated transmission and emission spectra show that both water vapor and ozone features could be detectable by future missions such as the James Webb Space Telescope.
502.05 — Dark water oceans on exoplanets orbiting cool stars
Lisa Kaltenegger1
1 Astronomy, Carl Sagan Institute Cornell University (Ithaca, New York, United States)
Several thousand extrasolar planets orbiting other stars provide a first glimpse into the diversity of other worlds. We show that oceans on worlds or- biting different alien Suns will differ from Earth’s oceans because the penetration depth of light can be very different, altering ocean dynamics signifi- cantly. While Sunlight can penetrate up to 250m in a water ocean, light may penetrate as little as 2m for oceans illuminated by cool red stars. Dynamics and photosynthesis in water oceans on exoplanets and exomoons orbiting other Suns can be very dif- ferent from Earth’s. We introduce a new paradigm for the hydrosphere-atmosphere interaction in plan- etary models. The idea is fundamental – when you convolute the absorption of water with wavelength with the irradiation exoplanets receive from different host stars it fundamentally changes how deep that light can penetrate water, especially for cool host star planets like the detected, potentially habitable plan- ets around our neighboring stars Proxima-b and the planets in the Trappist-1 system. Our paper shows the huge impact the host star irradiation has on the thermal structure and resulting dynamics on water oceans on extrasolar planets. Additionally, the depth below which there is generally insufficient light for photoactive organisms in oceans on a red star will be much shallower than on Earth. Thus, photosynthetic ocean life, if it exists, will be much closer to the ocean surface, and can be more readily detected on planets and moons orbiting red stars.
502.06 — Prebiotic Planets: Evaluating Planetary Conditions for Origins of Life
Dimitar Sasselov1
1 Astronomy, Harvard University (Cambridge, Massachusetts, United States)
We often discuss exoplanet habitability, but rarely fo- cus on the prebiotic planets – the ones with geochem- ical conditions conducive to the emergence of life. How could prebiotic exoplanets, if we could identify them, help us solve life’s origins? In this talk I will focus on the prebiotic synthesis of the nucleotides, amino acids and lipids needed for life as we know it
and the planetary environmental context that makes that synthesis possible. I will argue that, as we still struggle to understand life’s origins on Earth, there are general predictions about the global planetary conditions that are testable with upcoming spectro- scopic observations of the atmospheres of rocky ex- oplanets.