Rising Near-ultraviolet Spectra in Stellar Megaflares
Flares from M dwarf stars can attain energies up to 10 ^4 times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and particle acceleration. Larger heating rates in the low atmosphere are needed to reproduce the shape and strength of the o...
Saved in:
| Main Authors: | , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2024-01-01
|
| Series: | The Astrophysical Journal |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/1538-4357/ad9395 |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
| _version_ | 1850035414689120256 |
|---|---|
| author | Adam F. Kowalski Rachel A. Osten Yuta Notsu Isaiah I. Tristan Antigona Segura Hiroyuki Maehara Kosuke Namekata Shun Inoue |
| author_facet | Adam F. Kowalski Rachel A. Osten Yuta Notsu Isaiah I. Tristan Antigona Segura Hiroyuki Maehara Kosuke Namekata Shun Inoue |
| author_sort | Adam F. Kowalski |
| collection | DOAJ |
| description | Flares from M dwarf stars can attain energies up to 10 ^4 times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and particle acceleration. Larger heating rates in the low atmosphere are needed to reproduce the shape and strength of the observed continua in stellar flares, which are often simplified to a blackbody model from the optical to the far-ultraviolet (FUV). The near-ultraviolet (NUV) has been woefully undersampled in spectral observations despite this being where the blackbody radiation should peak. We present Hubble Space Telescope NUV spectra in the impulsive phase of a flare with E _TESS ≈ 7.5 × 10 ^33 erg and a flare with E _TESS ≈ 10 ^35 erg and the largest NUV flare luminosity observed to date from an M star. The composite NUV spectra are not well represented by a single blackbody that is commonly assumed in the literature. Rather, continuum flux rises toward shorter wavelengths into the FUV, and we calculate that an optical T = 10 ^4 K blackbody underestimates the short-wavelength NUV flux by a factor of ≈6. We show that rising NUV continuum spectra can be reproduced by collisionally heating the lower atmosphere with beams of E ≳ 10 MeV protons or E ≳ 500 keV electrons and flux densities of 10 ^13 erg cm ^−2 s ^−1 . These are much larger than the canonical values describing accelerated particles in solar flares. |
| format | Article |
| id | doaj-art-b186be18d51d40ffbd1c541336efb5ff |
| institution | DOAJ |
| issn | 1538-4357 |
| language | English |
| publishDate | 2024-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj-art-b186be18d51d40ffbd1c541336efb5ff2025-08-20T02:57:29ZengIOP PublishingThe Astrophysical Journal1538-43572024-01-0197818110.3847/1538-4357/ad9395Rising Near-ultraviolet Spectra in Stellar MegaflaresAdam F. Kowalski0https://orcid.org/0000-0001-7458-1176Rachel A. Osten1https://orcid.org/0000-0001-5643-8421Yuta Notsu2https://orcid.org/0000-0002-0412-0849Isaiah I. Tristan3https://orcid.org/0000-0001-5974-4758Antigona Segura4https://orcid.org/0000-0002-2240-2452Hiroyuki Maehara5https://orcid.org/0000-0003-0332-0811Kosuke Namekata6https://orcid.org/0000-0002-1297-9485Shun Inoue7https://orcid.org/0000-0003-3085-304XDepartment of Astrophysical and Planetary Sciences, University of Colorado Boulder , CO 80305, USA; National Solar Observatory , Boulder, CO 80303, USA; Laboratory for Atmospheric and Space Physics , Boulder, CO 80303, USASpace Telescope Science Institute , Baltimore, MD 21218, USANational Solar Observatory , Boulder, CO 80303, USA; Laboratory for Atmospheric and Space Physics , Boulder, CO 80303, USADepartment of Astrophysical and Planetary Sciences, University of Colorado Boulder , CO 80305, USA; National Solar Observatory , Boulder, CO 80303, USA; Laboratory for Atmospheric and Space Physics , Boulder, CO 80303, USAInstituto de Ciencias Nucleares, Universidad Nacional Autónoma de México , Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México, M é xicoSubaru Telescope Okayama Branch Office, National Astronomical Observatory of Japan, National Institutes of Natural Sciences , 3037-5 Honjo, Kamogata, Asakuchi, Okayama 719-0232, JapanThe Hakubi Center for Advanced Research, Kyoto University , Kyoto 606-8302, Japan; Department of Physics, Kyoto University , Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan; Division of Science, National Astronomical Observatory of Japan , NINS, Osawa, Mitaka, Tokyo, 181-8588, JapanDepartment of Physics, Kyoto University , Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, JapanFlares from M dwarf stars can attain energies up to 10 ^4 times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and particle acceleration. Larger heating rates in the low atmosphere are needed to reproduce the shape and strength of the observed continua in stellar flares, which are often simplified to a blackbody model from the optical to the far-ultraviolet (FUV). The near-ultraviolet (NUV) has been woefully undersampled in spectral observations despite this being where the blackbody radiation should peak. We present Hubble Space Telescope NUV spectra in the impulsive phase of a flare with E _TESS ≈ 7.5 × 10 ^33 erg and a flare with E _TESS ≈ 10 ^35 erg and the largest NUV flare luminosity observed to date from an M star. The composite NUV spectra are not well represented by a single blackbody that is commonly assumed in the literature. Rather, continuum flux rises toward shorter wavelengths into the FUV, and we calculate that an optical T = 10 ^4 K blackbody underestimates the short-wavelength NUV flux by a factor of ≈6. We show that rising NUV continuum spectra can be reproduced by collisionally heating the lower atmosphere with beams of E ≳ 10 MeV protons or E ≳ 500 keV electrons and flux densities of 10 ^13 erg cm ^−2 s ^−1 . These are much larger than the canonical values describing accelerated particles in solar flares.https://doi.org/10.3847/1538-4357/ad9395Red dwarf flare starsNear ultraviolet astronomy |
| spellingShingle | Adam F. Kowalski Rachel A. Osten Yuta Notsu Isaiah I. Tristan Antigona Segura Hiroyuki Maehara Kosuke Namekata Shun Inoue Rising Near-ultraviolet Spectra in Stellar Megaflares The Astrophysical Journal Red dwarf flare stars Near ultraviolet astronomy |
| title | Rising Near-ultraviolet Spectra in Stellar Megaflares |
| title_full | Rising Near-ultraviolet Spectra in Stellar Megaflares |
| title_fullStr | Rising Near-ultraviolet Spectra in Stellar Megaflares |
| title_full_unstemmed | Rising Near-ultraviolet Spectra in Stellar Megaflares |
| title_short | Rising Near-ultraviolet Spectra in Stellar Megaflares |
| title_sort | rising near ultraviolet spectra in stellar megaflares |
| topic | Red dwarf flare stars Near ultraviolet astronomy |
| url | https://doi.org/10.3847/1538-4357/ad9395 |
| work_keys_str_mv | AT adamfkowalski risingnearultravioletspectrainstellarmegaflares AT rachelaosten risingnearultravioletspectrainstellarmegaflares AT yutanotsu risingnearultravioletspectrainstellarmegaflares AT isaiahitristan risingnearultravioletspectrainstellarmegaflares AT antigonasegura risingnearultravioletspectrainstellarmegaflares AT hiroyukimaehara risingnearultravioletspectrainstellarmegaflares AT kosukenamekata risingnearultravioletspectrainstellarmegaflares AT shuninoue risingnearultravioletspectrainstellarmegaflares |