43 research outputs found
EvryFlare II: Rotation Periods of the Cool Flare Stars in TESS Across Half the Southern Sky
We measure rotation periods and sinusoidal amplitudes in Evryscope light
curves for 122 two-minute K5-M4 TESS targets selected for strong flaring. The
Evryscope array of telescopes has observed all bright nearby stars in the
South, producing two-minute cadence light curves since 2016. Long-term,
high-cadence observations of rotating flare stars probe the complex
relationship between stellar rotation, starspots, and superflares. We detect
periods from 0.3487 to 104 d, and observe amplitudes from 0.008 to 0.216 g'
mag. We find the Evryscope amplitudes are larger than those in TESS with the
effect correlated to stellar mass (p-value=0.01). We compute the Rossby number
(Ro), and find our sample selected for flaring has twice as many intermediate
rotators (0.040.44) rotators; this may
be astrophysical or a result of period-detection sensitivity. We discover 30
fast, 59 intermediate, and 33 slow rotators. We measure a median starspot
coverage of 13% of the stellar hemisphere and constrain the minimum magnetic
field strength consistent with our flare energies and spot coverage to be 500
G, with later-type stars exhibiting lower values than earlier-types. We observe
a possible change in superflare rates at intermediate periods. However, we do
not conclusively confirm the increased activity of intermediate rotators seen
in previous studies. We split all rotators at Ro~0.2 into Prot10
d bins to confirm short-period rotators exhibit higher superflare rates, larger
flare energies, and higher starspot coverage than do long-period rotators, at
p-values of 3.2 X 10^-5, 1.0 X 10^-5, and 0.01, respectively.Comment: 16 pages, 8 figures, 3 tables. Ancillary machine-readable files
included. Accepted for publication in ApJ (proofs submitted). Includes
significant new material, including starspot color that depends on stellar
mass, more rotation periods, potential changes in activity during spin-down,
and examples of binary rotator
Variables in the Southern Polar Region Evryscope 2016 Dataset
The regions around the celestial poles offer the ability to find and
characterize long-term variables from ground-based observatories. We used
multi-year Evryscope data to search for high-amplitude (~5% or greater)
variable objects among 160,000 bright stars (Mv < 14.5) near the South
Celestial Pole. We developed a machine learning based spectral classifier to
identify eclipse and transit candidates with M-dwarf or K-dwarf host stars -
and potential low-mass secondary stars or gas giant planets. The large
amplitude transit signals from low-mass companions of smaller dwarf host stars
lessens the photometric precision and systematics removal requirements
necessary for detection, and increases the discoveries from long-term
observations with modest light curve precision. The Evryscope is a robotic
telescope array that observes the Southern sky continuously at 2-minute
cadence, searching for stellar variability, transients, transits around exotic
stars and other observationally challenging astrophysical variables. In this
study, covering all stars 9 < Mv < 14.5, in declinations -75 to -90 deg, we
recover 346 known variables and discover 303 new variables, including 168
eclipsing binaries. We characterize the discoveries and provide the amplitudes,
periods, and variability type. A 1.7 Jupiter radius planet candidate with a
late K-dwarf primary was found and the transit signal was verified with the
PROMPT telescope network. Further followup revealed this object to be a likely
grazing eclipsing binary system with nearly identical primary and secondary K5
stars. Radial velocity measurements from the Goodman Spectrograph on the 4.1
meter SOAR telescope of the likely-lowest-mass targets reveal that six of the
eclipsing binary discoveries are low-mass (.06 - .37 solar mass) secondaries
with K-dwarf primaries, strong candidates for precision mass-radius
measurements.Comment: 32 pages, 17 figures, accepted to PAS
Evryscope and K2 Constraints on TRAPPIST-1 Superflare Occurrence and Planetary Habitability
The nearby ultracool dwarf TRAPPIST-1 possesses several Earth-sized
terrestrial planets, three of which have equilibrium temperatures that may
support liquid surface water, making it a compelling target for exoplanet
characterization. TRAPPIST-1 is an active star with frequent flaring, with
implications for the habitability of its planets. Superflares (stellar flares
whose energy exceeds 10^33 erg) can completely destroy the atmospheres of a
cool star's planets, allowing ultraviolet radiation and high-energy particles
to bombard their surfaces. However, ultracool dwarfs emit little ultraviolet
flux when quiescent, raising the possibility of frequent flares being necessary
for prebiotic chemistry that requires ultraviolet light. We combine Evryscope
and Kepler observations to characterize the high-energy flare rate of
TRAPPIST-1. The Evryscope is an array of 22 small telescopes imaging the entire
Southern sky in g' every two minutes. Evryscope observations, spanning 170
nights over 2 years, complement the 80-day continuous short-cadence K2
observations by sampling TRAPPIST-1's long-term flare activity. We update
TRAPPIST-1's superflare rate, finding a cumulative rate of 4.2 (+1.9 -0.2)
superflares per year. We calculate the flare rate necessary to deplete ozone in
the habitable-zone planets' atmospheres, and find that TRAPPIST-1's flare rate
is insufficient to deplete ozone if present on its planets. In addition, we
calculate the flare rate needed to provide enough ultraviolet flux to power
prebiotic chemistry. We find TRAPPIST-1's flare rate is likely insufficient to
catalyze some of the Earthlike chemical pathways thought to lead to RNA
synthesis, and flux due to flares in the biologically relevant UV-B band is
orders of magnitude less for any TRAPPIST-1 planet than has been experienced by
Earth at any time in its history.Comment: 12 pages, 9 figures. Accepted to The Astrophysical Journal, in pres
Germinal Center B Cells Regulate Their Capability to Present Antigen by Modulation of HLA-DO
Peptide acquisition by MHC class II molecules is catalyzed by HLA-DM (DM). In B cells, HLA-DO (DO) inhibits or modifies the peptide exchange activity of DM. We show here that DO protein levels are modulated during B cell differentiation. Remarkably, germinal center (GC) B cells, which have low levels of DO relative to naive and memory B cells, are shown to have enhanced antigen presentation capabilities. DM protein levels also were somewhat reduced in GC B cells; however, the ratio of DM to DO in GC B cells was substantially increased, resulting in more free DM in GC B cells. We conclude that modulation of DM and DO in distinct stages of B cell differentiation represents a mechanism by which B cells regulate their capacity to function as antigen-presenting cells. Efficient antigen presentation in GC B cells would promote GC B cell–T cell interactions that are essential for B cells to survive positive selection in the GC
Evryscope and K2 Constraints on TRAPPIST-1 Superflare Occurrence and Planetary Habitability
The nearby ultracool dwarf TRAPPIST-1 possesses several Earth-sized terrestrial planets, three of which have equilibrium temperatures that may support liquid surface water, making it a compelling target for exoplanet characterization. TRAPPIST-1 is an active star with frequent flaring, with implications for the habitability of its planets. Superflares (stellar flares whose energy exceeds 1033 erg) can completely destroy the atmospheres of a cool star's planets, allowing ultraviolet radiation and high-energy particles to bombard their surfaces. However, ultracool dwarfs emit little ultraviolet flux when quiescent, raising the possibility of frequent flares being necessary for prebiotic chemistry that requires ultraviolet light. We combine Evryscope and Kepler observations to characterize the high-energy flare rate of TRAPPIST-1. The Evryscope is an array of 22 small telescopes imaging the entire Southern sky in g' every two minutes. Evryscope observations, spanning 170 nights over 2 yr, complement the 80 day continuous short-cadence K2 observations by sampling TRAPPIST-1's long-term flare activity. We update TRAPPIST-1's superflare rate, finding a cumulative rate of 4.2−0.2+1.9 superflares per year. We calculate the flare rate necessary to deplete ozone in the habitable-zone planets' atmospheres, and find that TRAPPIST-1's flare rate is insufficient to deplete ozone if present on its planets. In addition, we calculate the flare rate needed to provide enough ultraviolet flux to power prebiotic chemistry. We find TRAPPIST-1's flare rate is likely insufficient to catalyze some of the Earthlike chemical pathways thought to lead to ribonucleic acid synthesis, and flux due to flares in the biologically relevant UV-B band is orders of magnitude less for any TRAPPIST-1 planet than has been experienced by Earth at any time in its history
EvryFlare. I. Long-term Evryscope Monitoring of Flares from the Cool Stars across Half the Southern Sky
We search for superflares from 4068 cool stars in 2+ yr of Evryscope photometry, focusing on those with high-cadence data from both Evryscope and the Transiting Exoplanet Survey Satellite (TESS). The Evryscope array of small telescopes observed 575 flares from 284 stars, with a median energy of 1034.0 erg. Since 2016, Evryscope has enabled the detection of rare events from all stars observed by TESS through multi-year, high-cadence continuous observing. We report around twice the previous largest number of 1034 erg high-cadence flares from nearby cool stars. We find eight flares with amplitudes of 3+ g' magnitudes, with the largest reaching 5.6 mag and releasing 1036.2 erg. We observe a 1034 erg superflare from TOI-455 (LTT 1445), a mid-M with a rocky planet candidate. We measure the superflare rate per flare-star and quantify the average flaring of active stars as a function of spectral type, including superflare rates, flare frequency distributions, and typical flare amplitudes in g'. We confirm superflare morphology is broadly consistent with magnetic reconnection. We estimate starspot coverage necessary to produce superflares, and hypothesize maximum allowed superflare energies and waiting times between flares corresponding to 100% coverage of the stellar hemisphere. We observe decreased flaring at high Galactic latitudes. We explore the effects of superflares on ozone loss to planetary atmospheres: we observe one superflare with sufficient energy to photodissociate all ozone in an Earth-like atmosphere in one event. We find 17 stars that may deplete an Earth-like atmosphere via repeated flaring. Of the 1822 stars around which TESS may discover temperate rocky planets, we observe 14.6% ± 2% emit large flares
Orbital Foregrounds for Ultra-Short Duration Transients
Reflections from objects in Earth orbit can produce sub-second, star-like
optical flashes similar to astrophysical transients. Reflections have
historically caused false alarms for transient surveys, but the population has
not been systematically studied. We report event rates for these orbital
flashes using the Evryscope Fast Transient Engine, a low-latency transient
detection pipeline for the Evryscopes. We select single-epoch detections likely
caused by Earth satellites and model the event rate as a function of both
magnitude and sky position. We measure a rate of
sky hour, peaking at , for flashes morphologically
degenerate with real astrophysical signals in surveys like the Evryscopes. Of
these, sky hour are bright enough to be
visible to the naked eye in typical suburban skies with a visual limiting
magnitude of . These measurements place the event rate of orbital
flashes orders of magnitude higher than the combined rate of public alerts from
all active all-sky fast-timescale transient searches, including neutrino,
gravitational-wave, gamma-ray, and radio observatories. Short-timescale orbital
flashes form a dominating foreground for un-triggered searches for fast
transients in low-resolution, wide-angle surveys. However, events like fast
radio bursts (FRBs) with arcminute-scale localization have a low probability
() of coincidence with an orbital flash, allowing optical surveys
to place constraints on their potential optical counterparts in single images.
Upcoming satellite internet constellations, like SpaceX Starlink, are unlikely
to contribute significantly to the population of orbital flashes in normal
operations.Comment: 8 pages, 4 figure
EvryFlare. II. Rotation Periods of the Cool Flare Stars in TESS across Half the Southern Sky
We measure rotation periods and sinusoidal amplitudes in Evryscope light curves for 122 two-minute K5–M4 TESS targets selected for strong flaring. The Evryscope array of telescopes has observed all bright nearby stars in the south, producing 2-minute cadence light curves since 2016. Long-term, high-cadence observations of rotating flare stars probe the complex relationship between stellar rotation, starspots, and superflares. We detect periods from 0.3487 to 104 days and observe amplitudes from 0.008 to 0.216 g' mag. We find that the Evryscope amplitudes are larger than those in TESS with the effect correlated to stellar mass (p-value = 0.01). We compute the Rossby number (Ro) and find that our sample selected for flaring has twice as many intermediate rotators (0.04 Ro Ro Ro > 0.44) rotators; this may be astrophysical or a result of period detection sensitivity. We discover 30 fast, 59 intermediate, and 33 slow rotators. We measure a median starspot coverage of 13% of the stellar hemisphere and constrain the minimum magnetic field strength consistent with our flare energies and spot coverage to be 500 G, with later-type stars exhibiting lower values than earlier-type stars. We observe a possible change in superflare rates at intermediate periods. However, we do not conclusively confirm the increased activity of intermediate rotators seen in previous studies. We split all rotators at Ro ∼ 0.2 into bins of PRot PRot > 10 days to confirm that short-period rotators exhibit higher superflare rates, larger flare energies, and higher starspot coverage than do long-period rotators, at p-values of 3.2 × 10−5, 1.0 × 10−5, and 0.01, respectively