Unlocking the Origins of Ultracool Dwarf Radio Emission

Empirical trends in stellar X-ray and radio luminosities suggest that low mass ultracool dwarfs (UCDs) should not produce significant radio emission. Defying these expectations, strong non-thermal emission has been observed in a few UCDs in the 1-10 GHz range, with a variable component often attributed to global aurorae and a steady component attributed to other processes such as gyrosynchrotron emission. While both auroral and gyrosynchrotron emission peak near the critical frequency, only the latter radiation is expected to extend into millimeter wavelengths. We present ALMA 97.5 GHz and VLA 33 GHz observations of a small survey of 5 UCDs. LP 349-25, LSR J1835+3259, and NLTT 33370 were detected at 97.5 GHz, while LP 423-31 and LP 415-20 resulted in non-detections at 33 GHz. A significant flare was observed in NLTT 33370 that reached a peak flux of 4880 +/- 360 microJy, exceeding the quiescent flux by nearly an order of magnitude, and lasting 20 seconds. These ALMA observations show bright 97.5 GHz emission with spectral indices ranging from alpha = -0.76 to alpha = -0.29, suggestive of optically thin gyrosynchrotron emission. If such emission traces magnetic reconnection events, then this could have consequences for both UCD magnetic models and the atmospheric stability of planets in orbit around them. Overall, our results provide confirmation that gyrosynchrotron radiation in radio loud UCDs can remain detectable into the millimeter regime.