The ultracool dwarf DENIS-P J104814.7-395606

Abstract

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.

HADES RV Programme with HARPS-N at TNG

“…which is consistent within 3σ with similar previous studies performed on samples of M dwarfs (Martínez-Arnáiz et al 2011;Stelzer et al 2012). From Eq.…”

Trends in Ultracool Dwarf Magnetism. I. X-Ray Suppression and Radio Enhancement

“…Slow rotators ( 20 km s −1 ) have strong-field dynamos and thus stay close to the GBR, as found by Stelzer et al (2012). Rapid rotators may have dynamos in either mode, so some are strong GBR violators while others are not, explaining the results of McLean et al (2012) and this work.…”

“…With the addition of our new measurements, there is strong evidence that UCDs display a wide range of behavior with regard to the GBR: some are strongly radio overluminous, while others could be consistent with it. This range can be, and has been, interpreted as a bimodality in the UCD population (McLean et al 2012;Stelzer et al 2012), which has support from both ZDI observations and geodynamo simulations (Morin et al 2010;Gastine et al 2013). We have argued that one group of sources can maintain a population of gyrosynchrotron-emitting particles in the corona, setting an effective floor on L ν,R , while the other group is less active than earlier-type stars but emits following the standard chromospheric evaporation model.…”

“…The strong-field sources are likely also able to maintain the requisite converging fields but, lacking the corona-filling supply of particles, do not display the periodic bursts. As pointed out by Stelzer et al (2012), however, the strong-field sources are slower rotators, and so the lack of observed bursts may be a selection effect because UCD radio observations are generally shorter in duration than the rotational period. Hallinan et al (2008) identify another possible selection effect, providing evidence that the sources with ECMI bursts are observed at high inclinations.…”

“…Despite these similarities, our new data show that while LP 851-346 is at least an order of magnitude brighter than LSR J1835+3259 in the X-ray, it is also at least an order of magnitude fainter in the radio. Stelzer et al (2012) considered the GBR in the UCD regime and proposed the existence of two populations: one comprising rapidly rotating, radio-bright, X-ray-dim UCDs; the other comprising slower-rotating, radio-dim, X-ray-bright objects. They additionally noted that only the radio-bright objects display bright, highly polarized radio pulses, while only the radio-dim objects produce X-ray flares, with the exception of LP 944-20, which seems to flare in both bands.…”