Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths

Berger, E.

doi:10.1086/505787

Cited by 203 publications

(255 citation statements)

References 90 publications

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“…The relation between the emissions in these two wavebands has unveiled important differences from the X-ray/radio connection observed in low-mass GKM stars. Detailed discussions are found, e.g., in Berger (2006) and Berger et al (2010).…”

Section: Radio Versus X-ray Emissionmentioning

confidence: 99%

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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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Section: Radio Versus X-ray Emissionmentioning

confidence: 99%

“…A handful of M8 to early-L dwarfs in the solar neighborhood has been found to be the sources of extraordinarily strong Hα, radio, or X-ray emission (for references see Liebert et al 2003;Berger 2006;Stelzer et al 2006). In most of these cases, the emission seems to be due to flares.…”

Section: Introductionmentioning

confidence: 99%

The ultracool dwarf DENIS-P J104814.7-395606

Stelzer

Alcalá

Biazzo

et al. 2012

A&A

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“…The coronae of these objects thus must be quite Article published by EDP Sciences different from those of normal stars! Observation at 8.46 GHz with the VLA of the old BD 2MASSW J0036159+182110 (from now on called 2M0036+1821) imply a magnetic field strength of ∼175 G at about two radii above the surface of the object (Berger 2006). The field strength at the surface must be ≥1 kG.…”

Section: Introductionmentioning

confidence: 91%

“…It is the brightest, very cool dwarf at GHz-frequencies in the northern hemisphere, and it also has a strong magnetic field (Berger 2006). Similar to LP944-20, 2M0036+1821 dramatically violates the stellar relation for the flux ratio between the radio and X-ray regime as it is a factor 10 4 to 10 5 -times too bright in the radio regime.…”

Section: The Two Bdsmentioning

confidence: 99%

Short-term spectroscopic monitoring of two cool dwarfs with strong magnetic fields

Guenther¹,

Osorio

Mehner

et al. 2009

A&A

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Context. There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fields, and yet their surface temperatures are so low that the coupling is expected to be small between the matter and the magnetic field in the atmosphere. In the deeper layers, however, the coupling is expected to be much stronger. Aims. This raises the question of whether the magnetic field still leads to the formation of structures in the photosphere and of a solar-like chromosphere and corona. Methods. We carried out a spectroscopic monitoring campaign in which we observed ultracool dwarfs that have strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110. The objects were monitored over several rotation periods spectroscopically. LP944-20 was observed simultaneously in the optical and in the near infrared regime, 2MASSW J0036159+182110 only in the infrared. From the spectra, we determined the temperature of the objects in each spectrum, and measured the equivalent width in a number of diagnostically important lines. Temperature variations would indicate the presence of warm and cold regions, variations in the equivalent widths of photospheric lines are sensitive to the structure of cloud layers, and H α is a diagnostic for chromospheric structures.Results. Both dwarfs turned out to be remarkably constant. In the case of LP944-20, the T eff -variations are ≤50 K, and the rmsvariations in the equivalent widths of H α small. We also find that the equivalent widths of photospheric lines are remarkably constant. We did not find any significant variations in the case of 2MASSW J0036159+182110 either. Thus the most important result is that no significant variability was found at the time of our observations. We find that Hα-line is in emission but the equivalent width is only −4.4 ± 0.3 Å. When comparing our spectra with spectra taken over the past 11 years, we recognize significant changes during this time. Conclusions. We interpret these results as evidence that the photosphere of these objects are remarkably homogeneous, with only little structure in them, and despite the strong magnetic fields. Thus, unlike active stars, there are no prominent spots on these objects.

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“…By contrast, brown dwarfs are very bright radio emitters, but no correlation between X-ray and radio emission exists as is established for stars and solar events (Güdel-Benz relation, Benz and Güdel 1994). Since the discovery of the first radio emitting brown dwarf by Berger et al (2001), numerous surveys have detected radio emission at GHz frequencies in nearly 200 objects (Berger 2002(Berger , 2006Berger et al 2010;Hallinan et al 2006Hallinan et al , 2007Hallinan et al , 2008McLean et al 2012), including emission in the coolest brown dwarfs with spectral types as late as T6.5 (Route and Wolszczan 2012). In twelve of these objects, the radio emission is highly polarised, coherent and pulses on the rotation period of the dwarf.…”

Section: Multi-wavelength Observations Of Activity On Ultracool Dwarfsmentioning

confidence: 99%

Atmospheric Electrification in Dusty, Reactive Gases in the Solar System and Beyond

et al. 2016

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Detailed observations of the solar system planets reveal a wide variety of local atmospheric conditions. Astronomical observations have revealed a variety of extrasolar planets none of which resembles any of the solar system planets in full. Instead, the most massive amongst the extrasolar planets, the gas giants, appear very similar to the class of (young) brown dwarfs which are amongst the oldest objects in the Universe. Despite this diversity, solar system planets, extrasolar planets and brown dwarfs have broadly similar global temperatures between 300 and 2500 K. In consequence, clouds of different chemical species form in their atmospheres. While the details of these clouds differ, the fundamental physical processes are the same. Further to this, all these objects were observed to produce radio and X-ray emissions. While both kinds of radiation are well studied on Earth and to a lesser extent on the solar system planets, the occurrence of emissions that potentially originate from accelerated electrons on brown dwarfs, extrasolar planets and protoplanetary disks is not well understood yet. This paper offers an interdisciplinary view on electrification processes and their feedback on their hosting environment in meteorology, volcanology, planetology and research on extrasolar planets and planet formation.

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Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths

Cited by 203 publications

References 90 publications

The ultracool dwarf DENIS-P J104814.7-395606

The ultracool dwarf DENIS-P J104814.7-395606

Short-term spectroscopic monitoring of two cool dwarfs with strong magnetic fields

Atmospheric Electrification in Dusty, Reactive Gases in the Solar System and Beyond

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