An official website of the United States government
Abstract M dwarf flares observed by the Transiting Exoplanet Survey Satellite (TESS) sometimes exhibit apeak-bumplight-curve morphology, characterized by a secondary, gradual peak well after the main, impulsive peak. A similarlate phaseis frequently detected in solar flares observed in the extreme ultraviolet from longer hot coronal loops distinct from the impulsive flare structures. White-light emission has also been observed in off-limb solar flare loops. Here, we perform a suite of one-dimensional hydrodynamic loop simulations for M dwarf flares inspired by these solar examples. Our results suggest that coronal plasma condensation following impulsive flare heating can yield high electron number density in the loop, allowing it to contribute significantly to the optical light curves via free-bound and free–free emission mechanisms. Our simulation results qualitatively agree with TESS observations: the longer evolutionary timescale of coronal loops produces a distinct, secondary emission peak; its intensity increases with the injected flare energy. We argue that coronal plasma condensation is a possible mechanism for the TESS late-phase flares.
more »
« less
This content will become publicly available on May 9, 2026
Analyzing the Morphology of Late-phase Stellar Flares from G-, K-, and M-type Stars
Abstract Stellar flares occasionally present apeak-bumplight-curve morphology, consisting of an initial impulsive phase followed by a gradual late phase. Analyzing this specific morphology can uncover the underlying physics of stellar flare dynamics, particularly the plasma heating–evaporation–condensation process. While previous studies have mainly examined peak-bump occurrences on M dwarfs, this report extends the investigation to G-, K-, and M-type stars. We utilize the flare catalog published by J. Crowley et al., encompassing 12,597 flares, detected by using Transiting Exoplanet Survey Satellite (TESS) observations. Our analysis identifies 10,142 flares with discernible classical and complex morphology, of which 197 (∼1.9%) exhibit the peak-bump feature. We delve into the statistical properties of these TESS late-phase flares, noting that both the amplitude and FWHM durations of both the peaks and bumps show positive correlations across all source-star spectral types, following a power law with indices 0.69 ± 0.09 and 1.0 ± 0.15, respectively. Additionally, a negative correlation between the flare amplitude and the effective temperature of their host stars is observed. Compared to the other flares in our sample, peak-bump flares tend to have larger and longer initial peak amplitudes and FWHM durations and possess energies ranging from 1031to 1036erg.
more »
« less
- Award ID(s):
- 1848250
- PAR ID:
- 10610534
- Publisher / Repository:
- American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 984
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 186
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.55https://github.com/lupitatovar/Llamaradas-Estelaresmore » « less
-
Abstract Nuclear star clusters (NSCs), made up of a dense concentration of stars and the compact objects they leave behind, are ubiquitous in the central regions of galaxies surrounding the central supermassive black hole (SMBH). Close interactions between stars and stellar-mass black holes (sBHs) lead to tidal disruption events (TDEs). We uncover an interesting new phenomenon: for a subset of these, the unbound debris (to the sBH) remains bound to the SMBH, accreting at a later time, thus giving rise to a second flare. We compute the rate of such events and find them ranging within 10−6–10−3yr−1gal−1for SMBH mass ≃106–109M⊙. Time delays between the two flares spread over a wide range, from less than a year to hundreds of years. The temporal evolution of the light curves of the second flare can vary between the standardt−5/3power law to much steeper decays, providing a natural explanation for observed light curves in tension with the classical TDE model. Our predictions have implications for learning about NSC properties and calibrating its sBH population. Some double flares may be electromagnetic counterparts to LISA extreme-mass-ratio inspiral sources. Another important implication is the possible existence of TDE-like events in very massive SMBHs, where TDEs are not expected. Such flares can affect spin measurements relying on TDEs in the upper SMBH range.more » « less
-
Abstract “Quasiperiodic eruptions” (QPE) are recurrent nuclear transients with periods of several hours to almost a day, which thus far have been detected exclusively in the X-ray band. We have shown that many of the key properties of QPE flares (period, luminosity, duration, emission temperature, alternating long-short recurrence time behavior, and source rates) are naturally reproduced by a scenario involving twice-per-orbit collisions between a solar-type star on a mildly eccentric orbit, likely brought into the nucleus as an extreme mass-ratio inspiral (EMRI), and the gaseous accretion disk of a supermassive black hole (SMBH). The flare is generated by the hot shocked debris expanding outwards from either side of the disk midplane, akin to dual miniature supernovae. Here, we consider the conditions necessary for disk–star collisions to generate lower-temperature flares that peak in the ultraviolet (UV) instead of the X-ray band. We identify a region of parameter space at low SMBH massM•∼ 105.5M⊙and QPE periodsP≳ 10 hr for which the predicted flares are sufficiently luminousLUV∼ 1041erg s−1to outshine the quiescent disk emission at these wavelengths. The prospects to discover such “UV QPEs” with future satellite missions such as ULTRASAT and Ultraviolet Explorer depend on the prevalence of very low-mass SMBHs and the occurrence rate of stellar EMRIs onto them. For gaseous disks produced by the tidal disruption of stars, we predict that X-ray QPEs will eventually shut off, only to later reappear as UV QPEs as the accretion rate continues to drop.more » « less
-
The high-energy radiative output, from the X-ray to the ultraviolet, of exoplanet host stars drives photochemical reactions and mass loss in the upper regions of planetary atmospheres. In order to place constraints on the atmospheric properties of the three closest terrestrial exoplanets transiting M dwarfs, we observe the high-energy spectra of the host stars LTT 1445A and GJ 486 in the X-ray withXMM-NewtonandChandraand in the ultraviolet with HST/COS and STIS. We combine these observations with estimates of extreme-ultraviolet flux, reconstructions of the Lyαlines, and stellar models at optical and infrared wavelengths to produce panchromatic spectra from 1 Å to 20 µm for each star. While LTT 1445Ab, LTT 1445Ac, and GJ 486b do not possess primordial hydrogen-dominated atmospheres, we calculate that they are able to retain pure CO2atmospheres if starting with 10, 15, and 50% of Earth’s total CO2budget, respectively, in the presence of their host stars’ stellar wind. We use age-activity relationships to place lower limits of 2.2 and 6.6 Gyr on the ages of the host stars LTT 1445A and GJ 486. Despite both LTT 1445A and GJ 486 appearing inactive at optical wavelengths, we detect flares at ultraviolet and X-ray wavelengths for both stars. In particular, GJ 486 exhibits two far-ultraviolet flares with absolute energies of 1029.5and 1030.1erg (equivalent durations of 4357 ± 96 and 19 724 ± 169 s) occurring 3 h apart. Based on the timing of the observations, we suggest that these high-energy flares are related and indicative of heightened flaring activity that lasts for a period of days, but our interpretations are limited by sparse time-sampling. Consistent high-energy monitoring is needed to determine the duration and extent of high-energy activity on individual M dwarfs and the population as a whole.more » « less
