Modelling the environment around five ultracool dwarfs via the radio domain

Abstract

We present the results of a series of short radio observations of six ultracool dwarfs made using the upgraded VLA in S (2-4GHz) and C (4-7GHz) bands. LSR J1835+3259 exhibits a 100 percent right-hand circularly polarised burst which shows intense narrowband features with a fast negative frequency drift of about −30 MHz s −1 . They are superimposed on a fainter broadband emission feature with a total duration of about 20 minutes, bandwidth of about 1 GHz, centred at about 3.5 GHz, and a slow positive frequency drift of about 1 MHz s −1 . This makes it the first such event detected below 4 GHz and the first one exhibiting both positive and negative frequency drifts. Polarised radio emission is also seen in 2MASS J00361617+1821104 and NLTT 33370, while LP 349-25 and TVLM 513-46546 have unpolarised emission and BRI B0021-0214 was not detected. We can reproduce the main characteristics of the burst from LSR J1835+3259 using a model describing the magnetic field of the dwarf as a tilted dipole. We also analyse the origins of the quiescent radio emission and estimate the required parameters of the magnetic field and energetic electrons. Although our results are non-unique, we find a set of models which agree well with the observations.

Low-frequency GMRT observations of ultra-cool dwarfs

“…2MASS J00361617+1821104 (hereafter 2M 0036+18): this is an L3.5 dwarf at a distance of 8.74 ± 0.01 pc (Gaia Collaboration et al 2018). It is a well-studied object at radio frequencies, having been detected, for example, by Berger (2002); Berger et al (2005), and Metodieva et al (2017).…”

“…We found that the modelled flux densities depend weakly on the viewing angle θ between the line of sight and the magnetic field, for values between 10 − 80 • . Therefore, to simplify the modelling procedure, we chose to keep θ fixed at 25 • , within the range of acceptable values reported by Metodieva et al (2017), ignoring the sense of polarization. We empirically determined the spectral turnover frequency by running a Markov Chain Monte Carlo sampler, from the emcee package (Foreman-Mackey et al 2013), to fit a broken power law of the form…”

“…On the other hand, the persistent, non-flaring emission from UCDs has been argued to originate from gyrosynchrotron (e.g. Metodieva et al 2017;Lynch et al 2016;Williams et al 2015b;Berger et al 2005) or ECMI (e.g. Llama et al 2018;Schlieder et al 2014;Hallinan et al 2008) processes.…”

“…Using an input magnetospheric structure, low-frequency observations can therefore be used to constrain the coronal volume where conditions are favourable for the ECMI to take place (Llama et al 2018;Jaeger et al 2011). For gyrosynchrotron emission, the high-frequency regime is typically characterised by a power-law SED, where it is difficult but not impossible to uniquely constrain coronal magnetic field strength, electron density, and other coronal parameters (Metodieva et al 2017;Lynch et al 2016Lynch et al , 2015Ravi et al 2011;Burgasser & Putman 2005). However, at low frequencies the SED transitions to the optically-thick regime, which, when coupled with high-frequency measurements, allows coronal properties to be determined.…”

Probing the magnetosphere of the M8.5 dwarf TVLM 513−46546 by modelling its auroral radio emission. Hint of star exoplanet interaction?

“…The auroral radio emission gives a snapshot of the stellar magnetic field topology at the epoch of its detection, and its modelling has been already used as a method to describe the field topology of UCD magnetospheres (Metodieva et al 2017). …”