The corona of a fully convective star with a near-polar flare
Astronomy & Astrophysics(2024)
摘要
In 2020, the Transiting Exoplanet Survey Satellite (TESS) observed a rapidly
rotating M7 dwarf, TIC 277539431, produce a flare at 81 latitude, the
highest latitude flare located to date. This is in stark contrast to solar
flares that occur much closer to the equator, typically below 30. The
mechanisms that allow flares at high latitudes to occur are poorly understood.
We studied five Sectors of TESS monitoring, and obtained 36 ks of XMM-Newton
observations to investigate the coronal and flaring activity of TIC 277539431.
From the observations, we infer the optical flare frequency distribution, flare
loop sizes and magnetic field strengths, the soft X-ray flux, luminosity and
coronal temperatures, as well as the energy, loop size and field strength of a
large flare in the XMM-Newton observations. We find that TIC 277539431's corona
does not differ significantly from other low mass stars on the canonical
saturated activity branch with respect to coronal temperatures and flaring
activity, but shows lower luminosity in soft X-ray emission by about an order
of magnitude, consistent with other late M dwarfs. The lack of X-ray flux, the
high latitude flare, the star's viewing geometry, and the otherwise typical
stellar corona taken together can be explained by the migration of flux
emergence to the poles in rapid rotators like TIC 277539431 that drain the
star's equatorial regions of magnetic flux, but preserve its ability to produce
powerful flares.
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