The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X‐Ray, and Hα Observations

Berger, E.; Rutledge, Robert E.; Reid, I. Neill; Bildsten, Lars; Gizis, John E.; Liebert, James; Martín, E. L.; Basri, Gibor; Jayawardhana, Ray; Brandeker, Alexis; Fleming, T. A.; Johns-Krull, C. M.; Giampapa, M. S.; Hawley, Suzanne L.; Schmitt, J. H. M. M.

doi:10.1086/430343

Cited by 98 publications

(176 citation statements)

References 68 publications

(107 reference statements)

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“…-2MASS J0036 has been detected as variable by Berger et al (2005) in its radio emission, by Maiti (2007) in the R and I bands (not simultaneously), not by Gelino et al (2002) and Caballero et al (2003), and we do not find it to be polarised above the 0.13% level (M02 and ZO05, in R only, do). -2MASS J1507 has shown some limited variability, over a few hours (Goldman et al, in prep.…”

Section: Comparison With Atmospheric Variability Studiescontrasting

confidence: 57%

Polarisation of very-low-mass stars and brown dwarfs

Goldman¹,

Pitann²,

Osorio

et al. 2009

A&A

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Context. Ultra-cool dwarfs of the L spectral type (T eff = 1400-2200 K) are known to have dusty atmospheres. Asymmetries of the dwarf surface may arise from rotationally-induced flattening and dust-cloud coverage, and may result in non-zero linear polarisation through dust scattering. Aims. We aim to study the heterogeneity of ultra-cool dwarfs' atmospheres and the grain-size effects on the polarisation degree in a sample of nine late M, L and early T dwarfs. Methods. We obtain linear polarimetric imaging measurements using FORS1 at the Very Large Telescope, in the Bessel I filter, and for a subset in the Bessel R and the Gunn z filters. Results. We measure a polarisation degree of (0.31 ± 0.06)% for LHS102BC. We fail to detect linear polarisation in the rest of our sample, with upper-limits on the polarisation degree of each object of 0.09% to 0.76% (95% of confidence level), depending on the targets and the bands. For those targets we do not find evidence of large-scale cloud horizontal structure in our data. Together with previous surveys, our results set the fraction of ultra-cool dwarfs with detected linear polarisation to 30 +10 −6 % (1-σ errors). From the whole sample of well-measured objects with errors smaller than 0.1%, the fraction of ultra-cool dwarfs with polarisation degree larger than 0.3% is smaller than 16% (95% confidence level). Conclusions. For three brown dwarfs, our observations indicate polarisation degrees different (at the 3-σ level) than previously reported, giving hints of possible variations. Our results fail to correlate with the current model predictions for ultra-cool dwarf polarisation for a flattening-induced polarisation, or with the variability studies for a polarisation induced by an heterogeneous cloud cover. This stresses the intricacy of each of those tasks, but may arise as well from complex and dynamic atmospheric processes.

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Section: Comparison With Atmospheric Variability Studiescontrasting

confidence: 57%

Polarisation of very-low-mass stars and brown dwarfs

Goldman¹,

Pitann²,

Osorio

et al. 2009

A&A

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“…Direct magnetic field measurements derived from near-infrared (NIR) spectroscopy have demonstrated the presence of kilo-Gauss fields on UCDs but it is unclear at present whether these fields have a structure that is adequate for storing energy. However, UCDs seem to be overluminous in radio by factors of >1000 with respect to the L R versus (vs.) L X relation observed for earlier-type active stars (Berger et al 2005). The detection of radio emission indicates a high-density electron population and the presence of magnetic fields.…”

Section: Introductionmentioning

confidence: 80%

The ultracool dwarf DENIS-P J104814.7-395606

Stelzer

Alcalá

Biazzo

et al. 2012

A&A

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Context. Several diagnostics ranging from the radio to the X-ray band are suitable for investigating the magnetic activity of late-type stars. Empirical connections between the emission at different wavelengths place constraints on the nature and efficiency of the emission mechanism and the physical conditions in different atmospheric layers. The activity of ultracool dwarfs, at the low-mass end of the main-sequence, is poorly understood. Aims. We perform a multi-wavelength study of one of the nearest M9 dwarfs, DENIS-P J104814.7-395606 (4 pc), to examine its position within the group of magnetically active ultracool dwarfs, and, in general, advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Methods. We obtained an XMM-Newton observation of DENIS-P J104814.7-395606 and a broad-band spectrum from the ultraviolet to the near-infrared with X-Shooter. From this dataset, we derive the X-ray properties, stellar parameters, kinematics, and the emission-line spectrum tracing chromospheric activity. We integrate these data by compiling the activity parameters of ultracool dwarfs from the literature. Results. Our deep XMM-Newton observation provides the first X-ray detection of DENIS-P J104814.7-395606 (log L x = 25.1), as well as the first measurement of its V band brightness (V = 17.35 mag). The flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreement of DENIS-P J104814.7-395606 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker as judged from their lower fluxes. The observed Balmer decrements of DENIS 1048-3956 are compatible with optically thick plasma in local thermal equilibrium (LTE) at low, nearly photospheric temperature or optically thin LTE plasma at 20 000 K. Describing the decrements with case B recombination requires different emitting regions for Hα and the higher Balmer lines. The high observed Hα/Hβ flux ratio is also poorly fitted by the optically thin models. We derive a similarly high value for the Hα/Hβ ratio of vB 10 and LHS 2065 and conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P J104814.7-395606 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Güdel relation between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the presence of two types of ultracool dwarfs with distinct radio and X-ray behaviors.

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“…There are now a number of confirmed radio detections at and below the substellar boundary in recent years (Berger et al 2001;Berger 2002;Burgasser & Putman 2005;Berger et al 2005;Hallinan et al 2006;Phan-Bao et al 2007;Antonova et al 2007) violating the Güdel & Benz relation by up to four orders of magnitude.…”

Section: Introductionmentioning

confidence: 99%

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

Doyle¹,

Antonova

Marsh

et al. 2010

A&A

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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.

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The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X‐Ray, and Hα Observations

Cited by 98 publications

References 68 publications

Polarisation of very-low-mass stars and brown dwarfs

Polarisation of very-low-mass stars and brown dwarfs

The ultracool dwarf DENIS-P J104814.7-395606

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

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