A. Antonova

Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence

Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.

LOOKING FOR A PULSE: A SEARCH FOR ROTATIONALLY MODULATED RADIO EMISSION FROM THE HOT JUPITER, τ BOÖTIS b

Hot Jupiters have been proposed as a likely population of low-frequency radio sources due to electron cyclotron maser emission of similar nature to that detected from the auroral regions of magnetized solar system planets. Such emission will likely be confined to specific ranges of orbital/rotational phase due to a narrowly beamed radiation pattern. We report on GMRT 150 MHz radio observations of the hot Jupiter τ Bootis b, consisting of 40 hr carefully scheduled to maximize coverage of the planets 79.5 hr orbital/rotational period in an effort to detect such rotationally modulated emission. The resulting image is the deepest yet published at these frequencies and leads to a 3σ upper limit on the flux density from the planet of 1.2 mJy, two orders of magnitude lower than predictions derived from scaling laws based on solar system planetary radio emission. This represents the most stringent upper limits for both quiescent and rotationally modulated radio emission from a hot Jupiter yet achieved and suggests that either (1) the magnetic dipole moment of τ Bootis b is insufficient to generate the surface field strengths of >50 G required for detection at 150 MHz or (2) Earth lies outside the beaming pattern of the radio emission from the planet.

Volume-limited radio survey of ultracool dwarfs

Aims. We aim to increase the sample of ultracool dwarfs studied in the radio domain to allow a more statistically significant und erstanding of the physical conditions associated with these magnetically active objects. Methods. We conducted a volume-limited survey at 4.9 GHz of 32 nearby ultracool dwarfs with spectral types covering the range M7 ‐ T8. A statistical analysis was performed on the combined data from the present survey and previous radio observations of ultracool dwarfs. Results. Whilst no radio emission was detected from any of the targets, significant upper limits were placed on the radio luminosit ies that are below the luminosities of previously detected ultr acool dwarfs. Combining our results with those from the literature gives a detection rate for dwarfs in the spectral range M7 ‐ L3.5 of∼ 9%. In comparison, only one dwarf later than L3.5 is detected in 53 observations. We report the observed detection rate as a function of spectral type, and the number distribution of the dwarfs as a function of spectral type and rotation velocity. Conclusions. The radio observations to date point to a drop in the detection rate toward the ultracool dwarfs. However, the emission levels of detected ultracool dwarfs are comparable to those of earlier type active M dwarfs, which may imply that a mildly relativistic electron beam or a strong magnetic field can exist in ultracoo l dwarfs. Fast rotation may be a suffi cient condition to produce magnetic fields strengths of several hundreds Gauss to several kilo Ga uss, as suggested by the data for the active ultracool dwarfs with known rotation rates. A possible reason for the non-detection of r adio emission from some dwarfs is that maybe the centrifugal acceleration mechanism in these dwarfs is weak (due to a low rotation rate) and thus cannot provide the necessary density and/or energy of accelerated electrons. An alternative explanation could be l ong-term variability, as is the case for several ultracool d warfs whose radio emission varies considerably over long periods with emission levels dropping below the detection limit in some instances.

Phase connecting multi-epoch radio data for the ultracool dwarf TVLM 513-46546

Context. Radio data obtained for the ultracool dwarf TVLM 513-46546 has indicated a rotation period of ≈1.96 h via regular radio pulses, but how stable is this period. This has major implications regarding the stability of the magnetic field structures responsible for the radio emission from the ultracool dwarf. Aims. The aim of the present work is to investigate the stability of this rotation period using two datasets taken ≈40 days apart, some 12 months after the first report of periodical pulses in the radio data. Methods. Here we use a Bayesian analysis method which is a statistical procedure that endeavours to estimate the parameters of an underlying model probability distribution based on the observed data. Results. Periodical pulses are detected in datasets taken in April and June 2007, with the pulses being confined to a narrow range in the rotation period. This is in contradiction to a previous report of only aperiodic activity in the April 2007 dataset, while in fact both datasets have a periodic signal with a false alarm probability ≪ 10^-12. These two datasets are then used to derive a more accurate period (previously determined to be 1.96 h) of 1.96733 ± 0.00002 h. Conclusions. The similarly in the burst structure in datasets taken several weeks apart point towards the stability of an electric field structure which is somehow generated and sustained within the magnetosphere of the ultracool dwarf. The derived period of 1.96733 h is consistent with the period derived via radio and optical data taken some 12 months prior to the present observations and implies the near phase constancy of the pulsed emission. This suggest the presence of stable large-scale magnetic fields on timescales of more than 1 year. The characteristics of the pulses suggest that they are produced by the electron cyclotron maser (ECM) instability.

Modelling the radio pulses of an ultracool dwarf

Context. Recently, unanticipated magnetic activity in ultracool dwarfs (UCDs, spectral classes later than M7) has emerged from a number of radio observations. The highly (up to 100%) circularly polarized nature and high brightness temperature of the emission have been interpreted as requiring an effective amplification mechanism of the high-frequency electromagnetic waves − the electron cyclotron maser instability (ECMI). Aims. We aim to understand the magnetic topology and the properties of the radio emitting region and associated plasmas in these ultracool dwarfs, interpreting the origin of radio pulses and their radiation mechanism. Methods. An active region model was built, based on the rotation of the UCD and the ECMI mechanism. Results. The high degree of variability in the brightness and the diverse profile of pulses can be interpreted in terms of a large-scale hot active region with extended magnetic structure existing in the magnetosphere of TVLM 513-46546. We suggest the time profile of the radio light curve is in the form of power law in the model. Combining the analysis of the data and our simulation, we can determine the loss-cone electrons have a density in the range of 1.25 ×10 5 −5 ×10 5 cm −3 and temperature between 10 7 and 5 ×10 7 K. The active region has a size <1 RJup, while the pulses produced by the ECMI mechanism are from a much more compact region (e.g. ∼0.007 RJup). A surface magnetic field strength of ≈7000 G is predicted. Conclusions. The active region model is applied to the radio emission from TVLM 513-46546, in which the ECMI mechanism is responsible for the radio bursts from the magnetic tubes and the rotation of the dwarf can modulate the integral of flux with respect to time. The radio emitting region consists of complicated substructures. With this model, we can determine the nature (e.g. size, temperature, density) of the radio emitting region and plasma. The magnetic topology can also be constrained. We compare our predicted X-ray flux with Chandra X-ray observation of TVLM 513-46546. Although the X-ray detection is only marginally significant, our predicted flux is significantly lower than the observed flux. Further multi-wavelength observations will help us better understand the magnetic field structure and plasma behavior on the ultracool dwarf.

ELECTRON-BEAM-INDUCED RADIO EMISSION FROM ULTRACOOL DWARFS

We present the numerical simulations for an electron-beam-driven and loss-cone-driven electron-cyclotron maser (ECM) with different plasma parameters and different magnetic field strengths for a relatively small region and short timescale in an attempt to interpret the recent discovered intense radio emission from ultracool dwarfs. We find that a large amount of electromagnetic (EM) field energy can be effectively released from the beam-driven ECM, which rapidly heats the surrounding plasma. A rapidly developed high-energy tail of electrons in velocity space (resulting from the heating process of the ECM) may produce the radio continuum depending on the initial strength of the external magnetic field and the electron beam current. Both significant linear polarization and circular polarization of EM waves can be obtained from the simulations. The spectral energy distributions of the simulated radio waves show that harmonics may appear from 10 to 70ν_(pe) (ν_(pe) is the electron plasma frequency) in the non-relativistic case and from 10 to 600ν_(pe) in the relativistic case, which makes it difficult to find the fundamental cyclotron frequency in the observed radio frequencies. A wide frequency band should therefore be covered by future radio observations.

VVV high proper motion stars - I. The catalogue of bright KS ≤ 13.5 stars

The final, definitive version of this paper has been published in Monthly Notices of the Royal Astronomical Society, Vol. 464(1): 1247-1258, January 2017, DOI: 10.1093/mnras/stw2357, first published on line September 16, 2016, published by Oxford University Press on behalf of MNRAS.

Modelling the environment around five ultracool dwarfs via the radio domain

We present the results of a series of short radio observations of six ultracool dwarfs made using the upgraded Very Large Array in S (2–4GHz) and C (4–7GHz) bands. LSR J1835+3259 exhibits a 100 per cent right-hand circularly polarized burst that shows intense narrow-band features with a fast negative frequency drift of about −30 MHz s−1. They are superimposed on a fainter broad-band emission feature with a total duration of about 20 min, 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. Polarized radio emission is also seen in 2MASS J00361617+1821104 and NLTT 33370, while LP 349-25 and TVLM 513-46546 have unpolarized 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 that agree well with the observations.

Low-resolution optical spectra of ultracool dwarfs with OSIRIS/GTC

We present the results of low-resolution optical spectrosc opy with OSIRIS/GTC (Optical System for Imaging and Low Resolution Integrated Spectroscopy/ Gran Telescopio Canarias) for a sample of ultracool dwarfs. For a subsample of seven objects, based on 2MASS NIR photometric colours, a ’photometric’ spectral type is determine d and compared to the results of the optical spectroscopy. For the stars, showing Hα line in emission, equivalent widths were measured, and the ratio of Hα to bolometric luminosity were calculated. We find that two dw arfs show the presence of magnetic activity over long periods, LP 326-21 ‐ quasi-constant-like, and 2MASS J17071830+6439331 ‐ variable.

Mapping the Radio Coronae of Cool Stars and Brown Dwarfs

The pulsing radio emission detected from ultracool dwarfs can be used as a powerful diagnostic of magnetic field strengths and topologies at and below the substellar boundary. Studies thus far have confirmed magnetic field strengths of 3 kG for two late M dwarfs and 1.7 kG for an L3.5 dwarf, the latter being the first confirmation of kG magnetic fields for an L dwarf. Ongoing long term monitoring of the radio pulses will also investigate the stability of the associated large‐scale magnetic fields over timescales > 1  year. We also present the preliminary results of a lengthy radio monitoring campaign of the rapidly rotating M4 star V374 Peg, with the resulting light curves phased with magnetic maps previously obtained through Zeeman Doppler Imaging. The radio emission from V374 Peg is strongly modulated by the large scale dipolar magnetic field, with two clear peaks in the radio light curve per period of rotation, occurring when the dipolar field lies in the plane of the sky. These results provide strong evidence that the electron cyclotron maser instability plays a pivotal role in the production of quiescent radio emission from V374 Peg, representing a significant departure from the accepted model of gyrosynchrotron emission as the dominant source of quiescent radio emission from active M dwarfs.