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Article overview
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Low-frequency GMRT observations of ultra-cool dwarfs | | Andrew Zic
; Christene Lynch
; Tara Murphy
; David L. Kaplan
; Poonam Chandra
; | | Date: |
22 Nov 2018 | | Abstract: | Observations of radio emission in about 10 per cent of ultra-cool dwarfs
(UCDs) indicate the presence of strong, persistent magnetic fields in these
stars. These results are in contrast to early theoretical expectations on
fully-convective dynamos, and to other tracers of magnetic activity, such as H
{alpha} and X-ray luminosity. Radio-frequency observations have been key to
physically characterising UCD magnetospheres, although explaining the diverse
behaviour within them remains challenging. Most radio-frequency studies of UCDs
have been conducted in the 4-8 GHz band, where traditional radio
interferometers are typically most sensitive. Hence, the nature of UCD radio
emission at low frequencies ($lesssim 1.4,mathrm{GHz}$) remains relatively
unexplored, but can probe optically thick emission, and regions of lower
magnetic field strengths -- regimes not accessible to higher-frequency
observations. In this work, we present the results from Giant Metrewave Radio
Telescope observations of nine UCDs taken at $sim 610$ and
$1300,mathrm{MHz}$. These are the first observations of UCDs in this
frequency range to be published in the literature. Using these observations, we
are able to constrain the coronal magnetic field strength and electron number
density of one of the targets to $1 lesssim B lesssim 90,mathrm{G}$ and $4
lesssim log(N_e) lesssim 10$, respectively. We do not detect the flaring
emission observed at higher frequencies, to a limit of a few millijanskys.
These results show that some UCDs can produce low-frequency radio emission, and
highlights the need for simultaneous multi-wavelength radio observations to
tightly constrain the coronal and magnetospheric properties of these stars. | | Source: | arXiv, 1811.9294 | | Services: | Forum | Review | PDF | Favorites | |
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