University of Hertfordshrie

Hatfield, United Kingdom

University of Hertfordshrie

Hatfield, United Kingdom

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Yu S.,Armagh Observatory | Hallinan G.,National University of Ireland | Hallinan G.,University of California at Berkeley | Doyle J.G.,Armagh Observatory | And 6 more authors.
Astronomy and Astrophysics | Year: 2011

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 × 105-5 × 105 cm-3 and temperature between 107 and 5 × 107 K. The active region has a size <1 R Jup, 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. © 2010 ESO.


Zhang Z.H.,University of Hertfordshrie | Pinfield D.J.,University of Hertfordshrie | Day-Jones A.C.,University of Chile | Burningham B.,University of Hertfordshrie | And 11 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

We identify 806 ultracool dwarfs (of which 34 are newly discovered L dwarfs) from their Sloan Digital Sky Survey (SDSS) riz photometry and obtain proper motions through cross-matching with the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS) and Two-Micron All-Sky Survey (2MASS). Proper-motion and distance constraints show that nine of our ultracool dwarfs are members of widely separated binary systems: SDSS 0101 (K5V+M9.5V), SDSS 0207 (M1.5V+L3V), SDSS 0832 (K3III+L3.5V), SDSS 0858 (M4V+L0V), SDSS 0953 (M4V+M9.5V), SDSS 0956 (M2V+M9V), SDSS 1304 (M4.5V+L0V), SDSS 1631 (M5.5V+M8V) and SDSS 1638 (M4V+L0V). One of these (SDSS 0832) is shown to be a companion to the bright K3 giant η Cancri. Such primaries can provide age and metallicity constraints for any companion objects, yielding excellent benchmark objects. η Cancri AB is the first wide ultracool dwarf + giant binary system identified. We present new observations and analysis that constrain the metallicity of η Cancri A to be near-solar, and use recent evolutionary models to constrain the age of the giant to be 2.2-6.1 Gyr. If η Cancri B is a single object, we estimate its physical attributes to be mass = 63-82 MJup, Teff = 1800 ± 150 K, log g = 5.3-5.5, [M/H] = 0.0 ± 0.1. Its colours are non-typical when compared with other ultracool dwarfs, and we also assess the possibility that η Cancri B is itself an unresolved binary, showing that the combined light of an L4 + T4 system could provide a reasonable explanation for its colours. © 2010 The Authors. Journal compilation © 2010 RAS.


Zhang Z.H.,University of Hertfordshrie | Pinfield D.J.,University of Hertfordshrie | Day-Jones A.C.,University of Chile | Burningham B.,University of Hertfordshrie | Jones H.R.A.,University of Hertfordshrie
EPJ Web of Conferences | Year: 2011

Ultra-cool dwarfs as wide companions to subgiants, giants, white dwarfs and main sequence stars can be very good benchmark objects, for which we can infer physical properties with minimal reference to theoretical models, through association with the primary stars. We have searched for benchmark ultra-cool dwarfs in widely separated binary systems using SDSS, UKIDSS, and 2MASS. We then estimate spectral types using SDSS spectroscopy and multi-band colors, place constraints on distance, and perform proper motions calculations for all candidates which have sufficient epoch baseline coverage. Analysis of the proper motion and distance constraints show that eight of our ultra-cool dwarfs are members of widely separated binary systems. Another L3.5 dwarf, SDSS 0832, is shown to be a companion to the bright K3 giant η Cancri. Such primaries can provide age and metallicity constraints for any companion objects, yielding excellent benchmark objects. This is the first wide ultra-cool dwarf + giant binary system identified. © Owned by the authors, published by EDP Sciences, 2011.

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