# Summary of K2 Program GO16104

Title: ASASSN-14mz: An Extreme flare on an ultracool dwarf

PI: Shappee, Benjamin J (Carnegie Observatories)
CoIs: van Saders, Jennifer Lynn; Schmidt, Sarah Jane

Stellar flares are dramatic multi-wavelength emission events that occur in the surface magnetic fields of stars across the main sequence. Flares are particularly common on early- to mid-M dwarfs, where decades of observations have indicated that these flares are analogs of weaker events on our own Sun. Beyond their intrinsic value as tests of stellar astrophysics, M dwarfs are important as exoplanet host stars: they are the most abundant stars in the galaxy, and, because Earth-size planets in the stellar habitable zone are far easier to detect around M dwarfs, they are primary targets of many upcoming and ongoing planet-hunting missions and surveys (K2, MEarth, TESS, MINERVA- Red, etc.). However, because they are so magnetically active, M dwarfs pose unique challenges to our understanding of planetary habitability. Several authors have discussed the effects of flaring M dwarfs on nearby planets (Heath et al. 1999; Tarter et al. 2007; Segura et al. 2010), but studies thus far are restricted to scenarios in which planets experience flares fluxes a factor of more than 100 below that predicted for planets in the habitable zones of the strongest discovered flares. Furthermore, because these large flares are rare, the flaring rate at the highest energies is poorly constrained.

The All-Sky Automated Survey for Supernovae (ASAS-SN) is a long-term project, designed to monitor the entire visible sky every 2-3 days, whose primary goal is to find all of the closest supernovae (SNe) with an unbiased search. This systematic all-sky technique has enabled ASAS- SN to discover 30+ extreme flares (V e 4 mag) from low-mass stars, including the four largest M-dwarf flares ever detected (Stanek et al. 2013; Schmidt et al. 2014; Simonian et al. 2014, 2016) and the second and largest (V e 11 mag) L-dwarf flare (Schmidt et al. 2016). The key advantage of ASAS-SN is that our surveyed volume is nearby, by the virtue of our shallow scanning strategy across the entire sky. Thus, when we detect a flare, they are typically V < 16 mag or brighter, making it possible to characterize the star in quiescence.

On 2014 Dec. 24.4 UT the ASAS-SN transient pipeline triggered on a 13.21 mag transient at RA = 08:51:13.9 Dec = +19:12:21. 5 J2000. The transient then faded 0.23 mag in 110 seconds between our two dithered images. Archival SDSS and 2MASS data show a faint source at this location (r = 18.73 and V < 19.5) whose broad-band photometry is consistent with a mid-M dwarf (M6). Other than this one dramatic event, no other flares have been seen in the 462 times ASAS-SN has observed this location over the last 5 years.

Out of the unique sample of M-dwarf flares that ASAS-SN has found, only ASASSN-14mz will be in an upcoming K2 field. This provides a unique opportunity to constrain the rate of smaller flares ( < 0.5  1 mag) on a star that we known has undergone a flare 100 times larger. These smaller flares are expected from an average field M6 roughly once an hour. While large flares can last an hour, smaller flares last minutes, so short cadence observations are required. Such observations will also yield a rotation period, the critical second dimension in understanding the flaring behavior of low-mass stars as a function of stellar type and age. ASASSN-14mz expected to be on silicon (neither on a dead module nor chip gap) during in Campaign 16. Measuring rotation and the rates of smaller flares on ASASSN-14mz will provide a new insights into the magnetic fields of these stars, regardless of whether we find that ASASSN-14mzs flare rate is below-normal, average, or active.

# Targets requested by this program that have been observed (1)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
211932063, 132.808479, 19.2062, 17.807, GO16104_LC|GO16104_SC