Photodynamical Modeling of the Fascinating Eclipses in the Triple-star System KOI-126

We explore the fascinating eclipses and dynamics of the compact hierarchical triple-star system KOI-126 (KIC 5897826). This system is composed of a pair of M-dwarf stars (KOI-126 B and C) in a 1.74 day orbit that revolve around an F star (KOI-126 A) every 34 days. Complex eclipse shapes are created as the M stars transit the F star, due to two effects: (1) the duration of the eclipse is a significant fraction of the M-star orbital period, so the prograde or retrograde motion of the M stars in their orbit lead to unusually short or long duration eclipses; (2) due to 3-body dynamics, the M-star orbit precesses with an astonishingly quick timescale of 1.74 yr for the periastron (apsidal) precession, and 2.73 yr for the inclination and nodal angle precession. Using the full Kepler data set, supplemented with ground-based photometry, plus 29 radial velocity measurements that span 6 yr, our photodynamical modeling yields masses of M A = 1.2713 ± 0.0047 M ⊙ (0.37%), M B = 0.23529 ± 0.00062 M ⊙ (0.26%), and M C = 0.20739 ± 0.00055 M ⊙ (0.27%) and radii of R A = 1.9984 ± 0.0027 R ⊙ (0.14%), R B = 0.25504 ± 0.00076 R ⊙ (0.3%), and R C = 0.23196 ± 0.00069 R ⊙ (0.3%). We also estimate the apsidal motion constant of the M dwarfs, a parameter that characterizes the internal mass distribution. Although it is not particularly precise, we measure a mean apsidal motion constant, k 2 ¯ , of 0.046 − 0.028 + 0.046 , which is approximately 2σ lower than the theoretical model prediction of 0.150. We explore possible causes for this discrepancy.

Photochemistry of Venus-like Planets Orbiting K- and M-dwarf Stars

Compared to the diversity seen in exoplanets, Venus is a veritable astrophysical twin of the Earth; however, its global cloud layer truncates features in transmission spectroscopy, masking its non-Earth-like nature. Observational indicators that can distinguish an exo-Venus from an exo-Earth must therefore survive above the cloud layer. The above-cloud atmosphere is dominated by photochemistry, which depends on the spectrum of the host star and therefore changes between stellar systems. We explore the systematic changes in photochemistry above the clouds of Venus-like exoplanets orbiting K-dwarf or M-dwarf host stars, using a recently validated model of the full Venus atmosphere (0–115 km) and stellar spectra from the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) Treasury survey. SO2, OCS, and H2S are key gas species in Venus-like planets that are not present in Earth-like planets, and could therefore act as observational discriminants if their atmospheric abundances are high enough to be detected. We find that SO2, OCS, and H2S all survive above the cloud layer when irradiated by the coolest K dwarf and all seven M dwarfs, whereas these species are heavily photochemically depleted above the clouds of Venus. The production of sulfuric acid molecules that form the cloud layer decreases for decreasing stellar effective temperature. Less steady-state photochemical oxygen and ozone forms with decreasing stellar effective temperature, and the effect of chlorine-catalyzed reaction cycles diminish in favor of HO x and SO x catalyzed cycles. We conclude that trace sulfur gases will be prime observational indicators of Venus-like exoplanets around M-dwarf host stars, potentially capable of distinguishing an exo-Venus from an exo-Earth.

NGTS Observations of Enhanced Nanoflare Activity in Fully Convective Stars Linked to Plasma Resistivity

Previous examination of fully-convective M-dwarf stars highlighted unexplained enhanced rates of nanoflare activity. A potential explanation was linked to the helical turbulence dynamo which operates in fully convective stars. However, recent studies have found this helical dynamo does not appear significantly different to the Solar dynamo. The specific role the convective boundary plays on observed nanoflare rates, until now, was not known. Here we find evidence that fully convective M2.5V (and later) stars display greatly enhanced nanoflare rates compared with their pre-convective boundary counterparts. Importantly, the rate of nanoflare activity increases with increasing spectral sub-type, with nanoflares exhibiting greatly enhanced flaring rates via Sweet-Parker reconnection. This occurs more favourably at increased plasma resistivities experienced in these later MV stars, suggesting a direct interplay between the rate of nanoflare occurrence and the intrinsic plasma parameters. As such, nanoflare behaviour is likely to be unrelated to the behaviour of the local dynamo.

Origins space telescope: from first light to life

The Origins Space Telescope (Origins) is one of four science and technology definition studies selected by the National Aeronautics and Space Administration (NASA) in preparation of the 2020 Astronomy and Astrophysics Decadal survey in the US. Origins will trace the history of our origins from the time dust and heavy elements permanently altered the cosmic landscape to present-day life. It is designed to answer three major science questions: How do galaxies form stars, make metals, and grow their central supermassive black holes from reionization? How do the conditions for habitability develop during the process of planet formation? Do planets orbiting M-dwarf stars support life? Origins operates at mid- to far-infrared wavelengths from ~ 2.8 μm to 588 μm, and is more than 1000 times more sensitive than prior far-IR missions due to its cold (~ 4.5 K) aperture and state-of-the-art instruments.

Exploring the possibility of Peter Pan discs across stellar mass

Recently, several accreting M dwarf stars have been discovered with ages far exceeding the typical protoplanetary disc lifetime. These ‘Peter Pan discs’ can be explained as primordial discs that evolve in a low-radiation environment. The persistently low masses of the host stars raise the question whether primordial discs can survive up to these ages around stars of higher mass. In this work we explore the way in which different mass loss processes in protoplanetary discs limit their maximum lifetimes, and how this depends on host star mass. We find that stars with masses ≲ 0.6 M⊙ can retain primordial discs for ∼50 Myr. At stellar masses ≳ 0.8 M⊙, the maximum disc lifetime decreases strongly to below 50 Myr due to relatively more efficient accretion and photoevaporation by the host star. Lifetimes up to 15 Myr are still possible for all host star masses up to ∼2 M⊙. For host star masses between 0.6 and 0.8 M⊙, accretion ceases and an inner gap forms before 50 Myr in our models. Observations suggest that such a configuration is rapidly dispersed. We conclude that Peter Pan discs can only occur around M dwarf stars.

Detection of an energetic flare from the M5V secondary star in the Polar MQ Dra

MQ Dra is a strongly magnetic Cataclysmic Variable whose white dwarf accretes material from its secondary star through a stellar wind at a low rate. TESS observations were made of MQ Dra in four sectors in Cycle 2 and show a short duration, high energy flare (∼1035 erg) which has a profile characteristic of a flare from the M5V secondary star. This is one of the few occasions where an energetic flare has been seen from a Polar. We find no evidence that the flare caused a change in the light curve following the event and consider whether a coronal mass ejection was associated with the flare. We compare the frequency of energetic flares from the secondary star in MQ Dra with M dwarf stars and discuss the overall flare rate of stars with rotation periods shorter than 0.2 d and how such fast rotators can generate magnetic fields with low differential rotation rates.