Abstract
We present an analysis of a volume-complete sample of 363 mid-to-late M dwarfs within 15 pc of the Sun with masses between 0.1 and 0.3
M
⊙
observed by TESS within sectors 1–42. The median ...stellar mass of the sample is 0.17
M
⊙
. We search the TESS light curves for transiting planets with orbital periods below 7 days and recover all six known planets within the sample, as well as a likely planet candidate orbiting LHS 475. Each of these planets is consistent with a terrestrial composition, with planet radii between 0.91 and 1.31
R
⊕
. We characterize the transit detection sensitivity for each star as a function of planet radius, insolation, and orbital period. We obtain a cumulative occurrence rate of
0.61
−
0.19
+
0.24
terrestrial planets per star with radii above 0.5
R
⊕
and orbital periods between 0.4 and 7 days. We find that for comparable insolations, planets larger than 1.5
R
⊕
(sub-Neptunes) are significantly less abundant around mid-to-late M dwarfs compared to earlier-type stars, while the occurrence rate of terrestrial planets is comparable to that of more massive M dwarfs. We estimate that overall, terrestrials outnumber sub-Neptunes around mid-to-late M dwarfs by 14 to 1, in contrast to GK dwarfs, where they are roughly equinumerous. We place a 1
σ
upper limit of 0.07 planets larger than 1.5
R
⊕
per star within the orbital period range of 0.5–7 days. We find evidence for a downturn in occurrence rates for planet radii below 0.9
R
⊕
, suggesting that Earth-sized and larger terrestrials may be more common around mid-to-late M dwarfs.
By targeting nearby M dwarfs, a transit search using modest equipment is capable of discovering planets as small as2 R
⊕
2
R
⊕
in the habitable zones of their host stars. The MEarth Project, a ...future transit search, aims to employ a network of ground-based robotic telescopes to monitor M dwarfs in the northern hemisphere with sufficient precision and cadence to detect such planets. Here we investigate the design requirements for the MEarth Project. We evaluate the optimal bandpass, and the necessary field of view, telescope aperture, and telescope time allocation on a star-by-star basis, as is possible for the well-characterized nearby M dwarfs. Through these considerations, 1976 late M dwarfs (
R < 0.33 R
⊙
R
<
0.33
R
⊙
) emerge as favorable targets for transit monitoring. Based on an observational cadence and on total telescope time allocation tailored to recover 90% of transit signals from planets in habitable zone orbits, we find that a network of 10 30 cm telescopes could survey these 1976 M dwarfs in less than three years. A null result from this survey would set an upper limit (at 99% confidence) of 17% for the rate of occurrence of planets larger than2 R
⊕
2
R
⊕
in the habitable zones of late M dwarfs, and even stronger constraints for planets lying closer than the habitable zone. If the true occurrence rate of habitable planets is 10%, the expected yield would be 2.6 planets.
Bacterial lipolytic enzymes were originally classified into eight different families defined by Arpigny and Jaeger (families I-VIII). Recently, the discovery of new lipolytic enzymes allowed for ...extending the original classification to fourteen families (I-XIV). We previously reported that G. thermodenitrificans EstGtA2 (access no. AEN92268) belonged to a novel group of bacterial lipolytic enzymes. Here we propose a 15.sup.th family (family XV) and suggest criteria for the assignation of protein sequences to the N' subfamily. Five selected salt bridges, hallmarks of the N' subfamily (E3/R54, E12/R37, E66/R140, D124/K178 and D205/R220) were disrupted in EstGtA2 using a combinatorial alanine-scanning approach. A set of 14 (R/Kright arrowA) mutants was produced, including five single, three double, three triple and three quadruple mutants. Despite a high tolerance to non-conservative mutations for folding, all the alanine substitutions were destabilizing (decreasing T.sub.m by 5 to 14#176;C). A particular combination of four substitutions exceeded this tolerance and prevents the correct folding of EstGtA2, leading to enzyme inactivation. Although other mutants remain active at low temperatures, the accumulation of more than two mutations had a dramatic impact on EstGtA2 activity at high temperatures suggesting an important role of these conserved salt bridge-forming residues in thermostability of lipolytic enzymes from the N' subfamily. We also identified a particular interloop salt bridge in EstGtA2 (D194/H222), located at position i -2 and i -4 residues from the catalytic Asp and His respectively which is conserved in other related bacterial lipolytic enzymes (families IV and XIII) with high tolerance to mutations and charge reversal. We investigated the role of residue identity at position 222 in controlling stability-pH dependence in EstGtA2. The introduction of a His to Arg mutation led to increase thermostability under alkaline pH. Our results suggest primary targets for optimization of EstGtA2 for specific biotechnological purposes.
Stellar rotation periods are valuable both for constraining models of angular momentum loss and for understanding how magnetic features impact inferences of exoplanet parameters. Building on our ...previous work in the northern hemisphere, we have used long-term, ground-based photometric monitoring from the MEarth Observatory to measure 234 rotation periods for nearby, southern hemisphere M dwarfs. Notable examples include the exoplanet hosts GJ 1132, LHS 1140, and Proxima Centauri. We find excellent agreement between our data and K2 photometry for the overlapping subset. Among the sample of stars with the highest quality data sets, we recover periods in 66%; as the length of the data set increases, our recovery rate approaches 100%. The longest rotation periods we detect are around 140 days, which we suggest represent the periods that are reached when M dwarfs are as old as the local thick disk (about 9 Gyr).
Most known terrestrial planets orbit small stars with radii less than 60 per cent of that of the Sun
. Theoretical models predict that these planets are more vulnerable to atmospheric loss than their ...counterparts orbiting Sun-like stars
. To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve
. Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point (latitude and longitude of 0 degrees)-possibly indicative of atmospheric circulation
. Here we report a phase curve measurement for the smaller, cooler exoplanet LHS 3844b, a 1.3-Earth-radii world in an 11-hour orbit around the small nearby star LHS 3844. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1,040 ± 40 kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are susceptible to erosion by stellar wind. The data are well fitted by a bare-rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.
Most stars become white dwarfs after they have exhausted their nuclear fuel (the Sun will be one such). Between one-quarter and one-half of white dwarfs have elements heavier than helium in their ...atmospheres, even though these elements ought to sink rapidly into the stellar interiors (unless they are occasionally replenished). The abundance ratios of heavy elements in the atmospheres of white dwarfs are similar to the ratios in rocky bodies in the Solar System. This fact, together with the existence of warm, dusty debris disks surrounding about four per cent of white dwarfs, suggests that rocky debris from the planetary systems of white-dwarf progenitors occasionally pollutes the atmospheres of the stars. The total accreted mass of this debris is sometimes comparable to the mass of large asteroids in the Solar System. However, rocky, disintegrating bodies around a white dwarf have not yet been observed. Here we report observations of a white dwarf--WD 1145+017--being transited by at least one, and probably several, disintegrating planetesimals, with periods ranging from 4.5 hours to 4.9 hours. The strongest transit signals occur every 4.5 hours and exhibit varying depths (blocking up to 40 per cent of the star's brightness) and asymmetric profiles, indicative of a small object with a cometary tail of dusty effluent material. The star has a dusty debris disk, and the star's spectrum shows prominent lines from heavy elements such as magnesium, aluminium, silicon, calcium, iron, and nickel. This system provides further evidence that the pollution of white dwarfs by heavy elements might originate from disrupted rocky bodies such as asteroids and minor planets.
ABSTRACT Rotation is a directly observable stellar property, and it drives magnetic field generation and activity through a magnetic dynamo. Main-sequence stars with masses below approximately 0.35 M ...(mid-to-late M dwarfs) are fully convective, and are expected to have a different type of dynamo mechanism than solar-type stars. Measurements of their rotation rates provide insight into these mechanisms, but few rotation periods are available for these stars at field ages. Using photometry from the MEarth Project, we measure rotation periods for 387 nearby, mid-to-late M dwarfs in the northern hemisphere, finding periods from 0.1 to 140 days. The typical rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5%-1%. We find no period-amplitude relation for stars below 0.25 M and an anticorrelation between period and amplitude for higher-mass M dwarfs. We highlight the existence of older, slowly rotating stars without H emission that nevertheless have strong photometric variability. We use parallaxes, proper motions, radial velocities, photometry, and near-infrared metallicity estimates to further characterize the population of rotators. The Galactic kinematics of our sample is consistent with the local population of G and K dwarfs, and rotators have metallicities characteristic of the solar neighborhood. We use the W space velocities and established age-velocity relations to estimate that stars with P < 10 days have ages of on average <2 Gyr, and that those with P > 70 days have ages of about 5 Gyr. The period distribution is dependent on mass: as the mass decreases, the slowest rotators at a given mass have longer periods, and the fastest rotators have shorter periods. We find a lack of stars with intermediate rotation periods, and the gap between the fast and slow rotators is larger for lower masses. Our data are consistent with a scenario in which these stars maintain rapid rotation for several gigayears, then spin down quickly, reaching periods of around 100 days by a typical age of 5 Gyr.
There are several lipid binding sites on serum albumins. The aim of this study was to examine the binding of bovine serum albumin (BSA) to cholesterol (Chol), ...1,2-dioleoyl-3-(trimethylammonium)propane (DOTAP), (dioctadecyldimethyl)ammonium bromide (DDAB), and dioleoylphosphatidylethanolamine (DOPE), at physiological conditions, using constant protein concentration and various lipid contents. Fourier transform infrared (FTIR), circular dichroism (CD) and fluorescence spectroscopic methods were used to analyze the lipid binding mode, the binding constant, and the effects of lipid complexation on BSA stability and conformation. Structural analysis showed that lipids bind BSA via both hydrophilic and hydrophobic contacts with overall binding constants of K Chol = (1.12 ± 0.40) × 103 M−1, K DDAB = (1.50 ± 0.50) × 103 M−1, K DOTAP = (2.45 ± 0.80) × 103 M−1, and K DOPE = (1.35 ± 0.60) × 103 M−1. The numbers of bound lipid (n) were 1.1 (cholesterol), 1.28 (DDAB), 1.02 (DOPE), and 1.21 (DOTAP) in these lipid−BSA complexes. DDAB and DOTAP induced major alterations of BSA conformation, causing a partial protein unfolding, while cholesterol and DOPE stabilized protein secondary structure.
ABSTRACT The Transiting Exoplanet Survey Satellite (TESS) is a NASA-sponsored Explorer mission that will perform a wide-field survey for planets that transit bright host stars. Here, we predict the ...properties of the transiting planets that TESS will detect along with the EB stars that produce false-positive photometric signals. The predictions are based on Monte Carlo simulations of the nearby population of stars, occurrence rates of planets derived from Kepler, and models for the photometric performance and sky coverage of the TESS cameras. We expect that TESS will find approximately 1700 transiting planets from pre-selected target stars. This includes 556 planets smaller than twice the size of Earth, of which 419 are hosted by M dwarf stars and 137 are hosted by FGK dwarfs. Approximately 130 of the planets will have host stars brighter than . Approximately 48 of the planets with lie within or near the habitable zone ( ); between 2 and 7 such planets have host stars brighter than . We also expect approximately 1100 detections of planets with radii 2-4 , and 67 planets larger than 4 . Additional planets larger than 2 can be detected around stars that are not among the pre-selected target stars, because TESS will also deliver full-frame images at a 30-minute cadence. The planet detections are accompanied by over 1000 astrophysical false positives. We discuss how TESS data and ground-based observations can be used to distinguish the false positives from genuine planets. We also discuss the prospects for follow-up observations to measure the masses and atmospheres of the TESS planets.
We present a study of flare rates, rotation periods, and spectroscopic activity indicators of 125 single stars within 15 parsecs and with masses between 0.1 and 0.3 M observed during the first year ...of the TESS mission, with the goal of elucidating the relationship between these various magnetically connected phenomena. We gathered multiepoch high-resolution spectra of each target, and we measured equivalent widths of the activity indicators helium I D3, H , and the calcium infrared triplet line at 8542.09 . We present 18 new rotation periods from MEarth photometry and 19 new rotation periods from TESS photometry. We present a catalog of 1392 flares. After correcting for sensitivity, we find the slope of the flare frequency distribution for all stars to have a standard value of = 1.98 0.02. We determine R31.5, the rate of flares per day with energies above E = 3.16 × 1031 ergs in the TESS bandpass. We find that below a critical value of H EW = −0.71 , log R31.5 increases linearly with increasing H emission; above this value, log R31.5 declines rapidly. The stars divide into two groups: 26% have H in emission, high flare rates with typical values of log R31.5 = −1.30 0.08, and have Rossby numbers <0.50. The remaining 74% show little to no H in emission and exhibit log R31.5 < −3.86, with the majority of these stars not showing a single flare during the TESS observations.
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