ABSTRACT
We determine rotation periods for 127 stars in the ∼115-Myr-old Blanco 1 open cluster using ∼200 d of photometric monitoring with the Next Generation Transit Survey. These stars span F5–M3 ...spectral types (1.2 M⊙ ≳ M ≳ 0.3 M⊙) and increase the number of known rotation periods in Blanco 1 by a factor of four. We determine rotation periods using three methods: Gaussian process (GP) regression, generalized autocorrelation function (G-ACF), and Lomb–Scargle (LS) periodogram, and find that the GP and G-ACF methods are more applicable to evolving spot modulation patterns. Between mid-F and mid-K spectral types, single stars follow a well-defined rotation sequence from ∼2 to 10 d, whereas stars in photometric multiple systems typically rotate faster. This may suggest that the presence of a moderate-to-high mass ratio companion inhibits angular momentum loss mechanisms during the early pre-main sequence, and this signature has not been erased at ∼100 Myr. The majority of mid-F to mid-K stars display evolving modulation patterns, whereas most M stars show stable modulation signals. This morphological change coincides with the shift from a well-defined rotation sequence (mid-F to mid-K stars) to a broad rotation period distribution (late-K and M stars). Finally, we compare our rotation results for Blanco 1 to the similarly aged Pleiades: the single-star populations in both clusters possess consistent rotation period distributions, which suggests that the angular momentum evolution of stars follows a well-defined pathway that is, at least for mid-F to mid-K stars, strongly imprinted by ∼100 Myr.
ABSTRACT
We present the results of a coordinated campaign to simultaneously observe the M star binary Ross 733 in the optical and near-ultraviolet (NUV) with TESS and Swift, respectively. We observed ...two flares in the Swift NUV light curve. One of these was decay phase of a flare that was also detected with TESS and the other was only detected in the NUV. We used the TESS light curve to measure the white-light flare rate of Ross 733, and calculate that the system flares with an energy of 1033 erg once every 1.5 d. We used our simultaneous observations to measure a pseudo-continuum temperature of $7340^{+810}_{-900}$ K during the flare decay. We also used our observations to test the NUV predictions of the 9000 K blackbody flare model, and find that it underestimates number of flares we detect in our Swift NUV light curve. We discuss the reasons for this and attribute it to the unaccounted contributions from emission lines and continuum temperatures above 9000 K. We discuss how additional observations are required to break the degeneracy between the two in future multiwavelength flare campaigns.
Abstract
Stellar variability is a limiting factor for planet detection and characterization, particularly around active M-type stars. Here we revisit one of the most active stars from the Kepler ...mission, the M4 star GJ 1243, and use a sample of 414 flare events from 11 months of 1-minute cadence light curves to study the empirical morphology of white-light stellar flares. We use a Gaussian process detrending technique to account for the underlying starspots. We present an improved analytic, continuous flare template that is generated by stacking the flares onto a scaled time and amplitude and uses a Markov Chain Monte Carlo analysis to fit the model. Our model is defined using classical flare events but can also be used to model complex, multipeaked flare events. We demonstrate the utility of our model using TESS data at the 10-minute, 2-minute, and 20 s cadence modes. Our new flare model code is made publicly available on GitHub.
5
5
https://github.com/lupitatovar/Llamaradas-Estelares
ABSTRACT
We present the results of a search for stellar flares from stars neighbouring the target sources in the Kepler short cadence data. These flares have been discarded as contaminants in ...previous surveys and therefore provide an unexplored resource of flare events, in particular high-energy events from faint stars. We have measured M dwarf flare energies up to 1.5 × 1035 erg, pushing the limit for flare energies measured using Kepler data. We have used our sample to study the flaring activity of wide binaries, finding that the lower mass counterpart in a wide binary flares more often at a given energy. Of the 4430 flares detected in our original search, 298 came from a neighbouring star, a rate of 6.7 ± 0.4 per cent for the Kepler short cadence light curves. We have used our sample to estimate a 5.8 ± 0.1 per cent rate of false positive flare events in studies using Transiting Exoplanet Survey Satellite short cadence data.
ABSTRACT
We present the results of a search for stellar flares in the first data release from the Next Generation Transit Survey (NGTS). We have found 610 flares from 339 stars, with spectral types ...between F8 and M6, the majority of which belong to the Galactic thin disc. We have used the 13-s cadence NGTS light curves to measure flare properties such as the flare amplitude, duration, and bolometric energy. We have measured the average flare occurrence rates of K and early to mid-M stars and present a generalized method to measure these rates while accounting for changing detection sensitivities. We find that field age K and early M stars show similar flare behaviour, while fully convective M stars exhibit increased white-light flaring activity, which we attribute to their increased spin-down time. We have also studied the average flare rates of pre-main-sequence K and M stars, showing they exhibit increased flare activity relative to their main-sequence counterparts.
ABSTRACT
The ultraviolet (UV) emission of stellar flares may have a pivotal role in the habitability of rocky exoplanets around low-mass stars. Previous studies have used white-light observations to ...calibrate empirical models which describe the optical and UV flare emission. However, the accuracy of the UV predictions of models has previously not been tested. We combined TESS optical and GALEX UV observations to test the UV predictions of empirical flare models calibrated using optical flare rates of M stars. We find that the canonical 9000-K black-body model used by flare studies underestimates the GALEX near-ultraviolet (NUV) energies of field age M stars by up to a factor of 6.5 ± 0.7 and the GALEX far-ultraviolet energies of fully convective field age M stars by 30.6 ± 10.0. We calculated energy correction factors that can be used to bring the UV predictions of flare models closer in line with observations. We calculated pseudo-continuum flare temperatures that describe both the white-light and GALEX NUV emission. We measured a temperature of 10 700 K for flares from fully convective M stars after accounting for the contribution from UV line emission. We also applied our correction factors to the results of previous studies of the role of flares in abiogenesis. Our results show that M stars do not need to be as active as previously thought in order to provide the NUV flux required for prebiotic chemistry, however, we note that flares will also provide more FUV flux than previously modelled.
ABSTRACT
High-cadence observations of high-energy stellar flares from cool and ultracool dwarfs are often limited by the faint nature of their host stars. Many low-mass sources cannot be detected in ...quiescence by photometric surveys, meaning they are not targeted for high-cadence observations. This reduces the chances of detecting the rarest high-energy flare events. We used the 13-s cadence full-frame images of Next-Generation Transit Survey (NGTS) to search for flares from M and L dwarfs. This included stars that were too faint to detect in quiescence. We detect 160 flares from 135 stars, with spectral types ranging from M3 to L2.5. We use our sample to study the energies, amplitudes and durations of flares from M and L dwarfs. We measure bolometric flare energies up to 4.5 × 1034 erg for ultracool dwarfs, but conclude that we have not reached a maximum limit to the energy released during white-light flares. We use our results to study the incidence rate of flares of mid- and late-M stars, not accounting for age or binarity, and find that 1.4 ± 0.4 and $9^{+16}_{-3}$ per cent of mid- and late-M stars, respectively, exhibit flares with amplitudes above 1 mag in the NGTS bandpass. Future studies with greater numbers of NGTS fields will expand upon this work.
Abstract
Variability in the far-ultraviolet (FUV) emission produced by stellar activity affects photochemistry and heating in orbiting planetary atmospheres. We present a comprehensive analysis of ...the FUV variability of GJ 436, a field-age M2.5V star (
P
rot
≈ 44 days) that is orbited by a warm Neptune-sized planet (
M
≈ 25
M
⊕
,
R
≈ 4.1
M
⊕
,
P
orb
≈ 2.6 days). Observations at three epochs from 2012 to 2018 span nearly a full activity cycle, sample two rotations of the star and two orbital periods of the planet, and reveal a multitude of brief flares. From 2012 to 2018, the star’s 7.75 ± 0.10 yr activity cycle produced the largest observed variations, 38% ± 3% in the summed flux of the major FUV emission lines. In 2018, the variability due to rotation was 8% ± 2%. An additional 11% ± 1% scatter at a cadence of 10 minutes, which is treated as white noise in the fits, likely has both instrumental and astrophysical origins. Flares increased time-averaged emission by 15% over the 0.88 days of cumulative exposure, peaking as high as 25× quiescence. We interpret these flare values as lower limits given that flares too weak or too infrequent to have been observed likely exist. GJ 436’s flare frequency distribution at FUV wavelengths is unusual compared to other field-age M dwarfs, exhibiting a statistically significant dearth of high-energy (>4 × 10
28
erg) events, which we hypothesize to be the result of a magnetic star–planet interaction (SPI) triggering premature flares. If an SPI is present, GJ 436 b’s magnetic field strength must be ≲100 G to explain the statistically insignificant increase in the orbit-phased FUV emission.
Abstract
Low-mass stars (≤1
M
⊙
) are some of the best candidates for hosting planets with detectable life because of these stars’ long lifetimes and relative ratios of planet to star mass and ...radius. An important aspect of these stars to consider is the amount of ultraviolet (UV) and X-ray radiation incident on planets in the habitable zones due to the ability of UV and X-ray radiation to alter the chemistry and evolution of planetary atmospheres. In this work, we build on the results of the HAZMAT I and HAZMAT III M-star studies to determine the intrinsic UV and X-ray flux evolution with age for M stars using Gaia parallactic distances. We then compare these results to the intrinsic fluxes of K stars adapted from HAZMAT V. We find that although the intrinsic M-star UV flux is 10–100 times lower than that of K stars, the UV fluxes in their respective habitable zone are similar. However, the habitable zone X-ray flux evolutions are slightly more distinguishable with a factor of 3–15 times larger X-ray flux for late M stars than for K stars. These results suggest that there may not be a K-dwarf advantage compared to M stars in the UV, but one may still exist in the X-ray.
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