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Abstract The optical and near-ultraviolet (NUV) continuum radiation in M-dwarf flares is thought to be the impulsive response of the lower stellar atmosphere to magnetic energy release and electron acceleration at coronal altitudes. This radiation is sometimes interpreted as evidence of a thermal photospheric spectrum withT≈ 10⁴K. However, calculations show that standard solar flare coronal electron beams lose their energy in a thick target of gas in the upper and middle chromosphere (log₁₀column mass/[g cm⁻²] ≲ −3). At larger beam injection fluxes, electric fields and instabilities are expected to further inhibit propagation to low altitudes. We show that recent numerical solutions of the time-dependent equations governing the power-law electrons and background coronal plasma (Langmuir and ion-acoustic) waves from Kontar et al. produce order-of-magnitude larger heating rates than those that occur in the deep chromosphere through standard solar flare electron beam power-law distributions. We demonstrate that the redistribution of beam energy aboveE≳ 100 keV in this theory results in a local heating maximum that is similar to a radiative-hydrodynamic model with a large, low-energy cutoff and a hard power-law index. We use this semiempirical forward-modeling approach to produce opaque NUV and optical continua at gas temperaturesT≳ 12,000 K over the deep chromosphere with log₁₀column mass/[g cm⁻²] of −1.2 to −2.3. These models explain the color temperatures and Balmer jump strengths in high-cadence M-dwarf flare observations, and they clarify the relation among atmospheric, radiation, and optical color temperatures in stellar flares. 

Abstract Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) observations reveal a class of solar flares with substantial energy and momentum impacts in the photosphere, concurrent with white-light emission and helioseismic responses, known as sunquakes. Previous radiative hydrodynamic modeling has demonstrated the challenges of explaining sunquakes in the framework of the standard flare model of “electron beam” heating. One of the possibilities to explain the sunquakes and other signatures of the photospheric impact is to consider additional heating mechanisms involved in solar flares, for example via flare-accelerated protons. In this work, we analyze a set of single-loop Fokker–Planck and radiative hydrodynamics RADYN+FP simulations where the atmosphere is heated by nonthermal power-law-distributed proton beams which can penetrate deeper than the electron beams into the low atmospheric layers. Using the output of the RADYN models, we calculate synthetic Fei6173 Å line Stokes profiles and from those the line-of-sight observables of the SDO/HMI instrument, as well as the 3D helioseismic response, and compare them with the corresponding observational characteristics. These initial results show that the models with proton beam heating can produce the enhancement of the HMI continuum observable and explain qualitatively the generation of sunquakes. The continuum observable enhancement is evident in all models but is more prominent in ones withE_c≥ 500 keV. In contrast, the models withE_c≤ 100 keV provide a stronger sunquake-like helioseismic impact according to the 3D acoustic modeling, suggesting that low-energy (deka- and hecto-keV) protons have an important role in the generation of sunquakes. 

Abstract Redshifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resolution of the Interface Region Imaging Spectrograph. Radiative-hydrodynamic flare models show that these redshifts are generally reproduced by electron-beam-generated chromospheric condensations. The models produce large ambient electron densities, and the pressure broadening of the hydrogen Balmer series should be readily detected in observations. To accurately interpret the upcoming spectral data of flares with the DKIST, we incorporate nonideal, nonadiabatic line-broadening profiles of hydrogen into the RADYN code. These improvements allow time-dependent predictions for the extreme Balmer line wing enhancements in solar flares. We study two chromospheric condensation models, which cover a range of electron-beam fluxes (1 − 5 × 10 11 erg s −1 cm −2 ) and ambient electron densities (1 − 60 × 10 13 cm −3 ) in the flare chromosphere. Both models produce broadening and redshift variations within 10 s of the onset of beam heating. In the chromospheric condensations, there is enhanced spectral broadening due to large optical depths at H α , H β , and H γ , while the much lower optical depth of the Balmer series H12−H16 provides a translucent window into the smaller electron densities in the beam-heated layers below the condensation. The wavelength ranges of typical DKIST/ViSP spectra of solar flares will be sufficient to test the predictions of extreme hydrogen wing broadening and accurately constrain large densities in chromospheric condensations. 

Abstract We confirm the planetary nature of two gas giants discovered by TESS to transit M dwarfs with stellar companions at wide separations. TOI-3984 A ( J = 11.93) is an M4 dwarf hosting a short-period (4.353326 ± 0.000005 days) gas giant ( M p = 0.14 ± 0.03 M J and R p = 0.71 ± 0.02 R J ) with a wide-separation white dwarf companion. TOI-5293 A ( J = 12.47) is an M3 dwarf hosting a short-period (2.930289 ± 0.000004 days) gas giant ( M p = 0.54 ± 0.07 M J and R p = 1.06 ± 0.04 R J ) with a wide-separation M dwarf companion. We characterize both systems using a combination of ground- and space-based photometry, speckle imaging, and high-precision radial velocities from the Habitable-zone Planet Finder and NEID spectrographs. TOI-3984 A b ( T eq = 563 ± 15 K and TSM = 138 − 27 + 29 ) and TOI-5293 A b ( T eq = 675 − 30 + 42 K and TSM = 92 ± 14) are two of the coolest gas giants among the population of hot Jupiter–sized gas planets orbiting M dwarfs and are favorable targets for atmospheric characterization of temperate gas giants and 3D obliquity measurements to probe system architecture and migration scenarios. 

Abstract Early in their lives, planets endure extreme amounts of ionizing radiation from their host stars. For planets with primordial hydrogen and helium-rich envelopes, this can lead to substantial mass loss. Direct observations of atmospheric escape in young planetary systems can help elucidate this critical stage of planetary evolution. In this work, we search for metastable helium absorption—a tracer of tenuous gas in escaping atmospheres—during transits of three planets orbiting the young solar analog V1298 Tau. We characterize the stellar helium line using HET/HPF, and find that it evolves substantially on timescales of days to months. The line is stable on hour-long timescales except for one set of spectra taken during the decay phase of a stellar flare, where absoprtion increased with time. Utilizing a beam-shaping diffuser and a narrowband filter centered on the helium feature, we observe four transits with Palomar/WIRC: two partial transits of planet d ( P = 12.4 days), one partial transit of planet b ( P = 24.1 days), and one full transit of planet c ( P = 8.2 days). We do not detect the transit of planet c, and we find no evidence of excess absorption for planet b, with Δ R b / R ⋆ < 0.019 in our bandpass. We find a tentative absorption signal for planet d with Δ R d / R ⋆ = 0.0205 ± 0.054, but the best-fit model requires a substantial (−100 ± 14 minutes) transit-timing offset on a two-month timescale. Nevertheless, our data suggest that V1298 Tau d may have a high present-day mass-loss rate, making it a priority target for follow-up observations. 

Abstract We present high-resolution observations of a flaring event in the M8 dwarf vB 10 using the near-infrared Habitable-zone Planet Finder (HPF) spectrograph on the Hobby-Eberly Telescope. The high stability of HPF enables us to accurately subtract a vB 10 quiescent spectrum from the flare spectrum to isolate the flare contributions and study the changes in the relative energy of the Caiiinfrared triplet, several Paschen lines, the Heλ10830 triplet lines, and to select iron and magnesium lines in HPF's bandpass. Our analysis reveals the presence of a red asymmetry in the Heλ10830 triplet, which is similar to signatures of coronal rain in the Sun. Photometry of the flare derived from an acquisition camera before spectroscopic observations and the ability to extract spectra from up-the-ramp observations with the HPF infrared detector enable us to perform time-series analysis of part of the flare and provide coarse constraints on the energy and frequency of such flares. We compare this flare with historical observations of flares around vB 10 and other ultracool M dwarfs and attempt to place limits on flare-induced atmospheric mass loss for hypothetical planets around vB 10.