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1. Impact of Electron Precipitation on Brown Dwarf Atmospheres and the Missing Auroral H3+ Emission

   https://doi.org/10.3847/1538-4357/ad2f9e  

   Pineda, J. Sebastian ; Hallinan, Gregg ; Desert, Jean-Michel ; Harding, Leon K. ( April 2024 , The Astrophysical Journal)

   Recent observations have demonstrated that very low-mass stars and brown dwarfs are capable of sustaining strong magnetic fields despite their cool and neutral atmospheres. These kilogauss field strengths are inferred based on strong, highly circularly polarized gigahertz radio emission, a consequence of the electron cyclotron maser instability. Crucially, these observations imply the existence of energetic nonthermal electron populations, associated with strong current systems, as are found in the auroral regions of the magnetized planets of the solar system. Intense auroral electron precipitation will lead to electron collisions with the H₂gas that should generate the ion${\mathrm{H}}_3^{+}$. With this motivation, we targeted a sample of ultracool dwarfs, known to exhibit signatures associated with aurorae, in search of theK-band emission features of${\mathrm{H}}_3^{+}$using the Keck telescopes on Maunakea. From our sample of nine objects, we found no clear indication of${\mathrm{H}}_3^{+}$emission features in our low-to-medium-resolution spectra (R∼ 3600). We also modeled the impact of an auroral electron beam on a brown dwarf atmosphere, determining the depth at which energetic beams deposit their energy and drive particle impact ionization. We find that the${\mathrm{H}}_3^{+}$nondetections can be explained by electron beams of typical energies ≳2–10 keV, which penetrate deeply enough that any${\mathrm{H}}_3^{+}$produced is chemically destroyed before radiating energy through its infrared transitions. Strong electron beams could further explain the lack of UV auroral detections and suggest that most or nearly all of the precipitating auroral energy must ultimately emerge as thermal emissions deep in brown dwarf atmospheres.

    

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2. SPT-CL J2215−3537: A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster

   https://doi.org/10.3847/1538-4357/acc6c2  

   Calzadilla, Michael S. ; Bleem, Lindsey E. ; McDonald, Michael ; Gladders, Michael D. ; Mantz, Adam B. ; Allen, Steven W. ; Bayliss, Matthew B. ; Eilers, Anna-Christina ; Floyd, Benjamin ; Hlavacek-Larrondo, Julie ; et al ( April 2023 , The Astrophysical Journal)

   We present the discovery of the most distant, dynamically relaxed cool core cluster, SPT-CL J2215−3537 (SPT2215), and its central brightest cluster galaxy (BCG) atz= 1.16. Using new X-ray observations, we demonstrate that SPT2215 harbors a strong cool core with a central cooling time of 200 Myr (at 10 kpc) and a maximal intracluster medium cooling rate of 1900 ± 400M_⊙yr⁻¹. This prodigious cooling may be responsible for fueling the extended, star-forming filaments observed in Hubble Space Telescope imaging. Based on new spectrophotometric data, we detect bright [Oii] emission in the BCG, implying an unobscured star formation rate (SFR) of${320}_{-140}^{+230}$M_⊙yr⁻¹. The detection of a weak radio source (2.0 ± 0.8 mJy at 0.8 GHz) suggests ongoing feedback from an active galactic nucleus (AGN), though the implied jet power is less than half the cooling luminosity of the hot gas, consistent with cooling overpowering heating. The extreme cooling and SFR of SPT2215 are rare among known cool core clusters, and it is even more remarkable that we observe these at such high redshift, when most clusters are still dynamically disturbed. The high mass of this cluster, coupled with the fact that it is dynamically relaxed with a highly isolated BCG, suggests that it is an exceptionally rare system that must have formed very rapidly in the early universe. Combined with the high SFR, SPT2215 may be a high-zanalog of the Phoenix cluster, potentially providing insight into the limits of AGN feedback and star formation in the most massive galaxies.

    

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3. Searching for stellar flares from low-mass stars using ASKAP and TESS

   https://doi.org/10.1093/mnras/stac2143  

   Rigney, Jeremy ; Ramsay, Gavin ; Carley, Eoin P. ; Doyle, J. Gerry ; Gallagher, Peter T. ; Wang, Yuanming ; Pritchard, Joshua ; Murphy, Tara ; Lenc, Emil ; Kaplan, David L. ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   Solar radio emission at low frequencies (<1 GHz) can provide valuable information on processes driving flares and coronal mass ejections (CMEs). Radio emission has been detected from active M dwarf stars, suggestive of much higher levels of activity than previously thought. Observations of active M dwarfs at low frequencies can provide information on the emission mechanism for high energy flares and possible stellar CMEs. Here, we conducted two observations with the Australian Square Kilometre Array Pathfinder Telescope totalling 26 h and scheduled to overlap with the Transiting Exoplanet Survey Satellite Sector 36 field, utilizing the wide fields of view of both telescopes to search for multiple M dwarfs. We detected variable radio emission in Stokes I centred at 888 MHz from four known active M dwarfs. Two of these sources were also detected with Stokes V circular polarization. When examining the detected radio emission characteristics, we were not able to distinguish between the models for either electron cyclotron maser or gyrosynchrotron emission. These detections add to the growing number of M dwarfs observed with variable low-frequency emission.

    

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4. CONSTRAINTS ON AURORAL RADIO EMISSION FROM Y DWARFS

   Melodie M. Kao, Gregg Hallinan ( April 2018 , Astrophysical journal)

   As an initial pilot study of magnetism in Y dwarfs, we have observed the three known IR variable Y dwarfs WISE J085510.83-071442.5, WISE J140518.40+553421.4, and WISEP J173835.53+273258.9 with the Karl G. Jansky Very Large Array (VLA) from 4-8 GHz to investigate the presence of quiescent radio emission as a proxy for highly circularly polarized radio emission associated with large-scale auroral currents. Measurements of magnetic fields on Y dwarfs, currently only possible by observing auroral radio emission, are essential for constraining fully convective magnetic dynamo models. We do not detect any pulsed or quiescent radio emission, down to rms noise levels of 7.2 uJy for WISE J085510.83-071442.5, 2.2 uJy for WISE J140518.40+553421.4, and 3.2 uJy for WISEP J173835.53+273258.9. The fractional detection rate of radio emission from T dwarfs is <10% and suggests that a much larger sample of deep observations of Y dwarfs is needed to rule out radio emission in the Y dwarf population. The significance of a single detection provides strong motivation for such a search. 

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5. A scaling relationship for non-thermal radio emission from ordered magnetospheres: from the top of the main sequence to planets

   https://doi.org/10.1093/mnras/stab2168  

   Leto, P ; Trigilio, C ; Krtička, J ; Fossati, L ; Ignace, R ; Shultz, M E ; Buemi, C S ; Cerrigone, L ; Umana, G ; Ingallinera, A ; et al ( August 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed, where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broad-band radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broad-band radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere’s co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultracool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons. 

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