The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs

Luhman, K. L.

doi:10.1146/annurev-astro-081811-125528

Cited by 207 publications

(168 citation statements)

References 380 publications

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“…BDs are now routinely observed in star forming regions and therefore it is important to understand how they form and evolve (Luhman 2012). A key way to do this is to investigate their accretion and outflow properties and compare them to YSOs.…”

Section: Introductionmentioning

confidence: 99%

Accretion-ejection connection in the young brown dwarf candidate ISO-ChaI 217

Whelan

Alcalá

Bacciotti

et al. 2014

A&A

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As the number of observed brown dwarf outflows is growing it is important to investigate how these outflows compare to the wellstudied jets from young stellar objects. A key point of comparison is the relationship between outflow and accretion activity and in particular the ratio between the mass outflow and accretion rates (Ṁ out /Ṁ acc ). The brown dwarf candidate ISO-ChaI 217 was discovered by our group, as part of a spectro-astrometric study of brown dwarfs, to be driving an asymmetric outflow with the blueshifted lobe having a position angle of ∼20 • . The aim here is to further investigate the properties of ISO-ChaI 217, the morphology and kinematics of its outflow, and to better constrainṀ out /Ṁ acc . The outflow is spatially resolved in the [S ] λλ6716, 6731 lines and is detected out to ∼1. 6 in the blue-shifted lobe and 1 in the red-shifted lobe. The asymmetry between the two lobes is confirmed although the velocity asymmetry is less pronounced with respect to our previous study. Using thirteen different accretion tracers we measure log (Ṁ acc ) [M /yr] = −10.6 ± 0.4. As it was not possible to measure the effect of extinction on the ISO-ChaI 217 outflowṀ out was derived for a range of values of A v , up to a value of A v = 2.5 mag estimated for the source extinction. The logarithm of the mass outflow (Ṁ out ) was estimated in the range −11.7 to −11.1 for both jets combined. ThusṀ out /Ṁ acc [M /yr] lies below the maximum value predicted by magneto-centrifugal jet launching models. Finally, both model fitting of the Balmer decrements and spectro-astrometric analysis of the Hα line show that the bulk of the H I emission comes from the accretion flow.

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Section: Introductionmentioning

confidence: 99%

Accretion-ejection connection in the young brown dwarf candidate ISO-ChaI 217

Whelan

Alcalá

Bacciotti

et al. 2014

A&A

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“…While it seems clear that these objects are too massive to generally form by core-accretion in a circumstellar disk (Fig. 3 of Mordasini et al 2009), a number of plausible formation theories have been advanced (see reviews by Whitworth et al 2007;Luhman 2012), but the role each of them plays in the actual formation process is not clear. The small number of BD companions to solar-type stars prevents reliable statistical studies from being made, and although radial-velocity surveys have detected a few tens of objects with minimum masses in the BD regime (e.g.…”

Section: Introductionmentioning

confidence: 99%

SOPHIE velocimetry ofKeplertransit candidates

Díaz

Damiani

Deleuil

et al. 2013

A&A

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We report the discovery of a transiting brown dwarf companion to KOI-205, a K0 main-sequence star, in a 11.720125-day period orbit. The transits were detected by the Kepler space telescope, and the reflex motion of the star was measured using radial velocity observations obtained with the SOPHIE spectrograph. The atmospheric parameters of the host stars were determined from the analysis of high-resolution, high signal-to-noise ratio ESPaDOns spectra obtained for this purpose. Together with spectrophotometric measurements recovered from the literature, these spectra indicate that the star is a mildly metallic K0 dwarf with T eff 5237 ± 60 K. The mass of the companion is 39.9 ± 1.0 M Jup and its radius is 0.81 ± 0.02 R Jup , in agreement with current theoretical predictions. This is the first time a bona fide brown dwarf companion is detected in orbit around a star of this type. The formation and orbital evolution of brown dwarf companions is briefly discussed in the light of this new discovery.

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“…The lower bound mass for the definition of a brown dwarf has often been taken to be the deuterium-burning limit, but such a distinction is currently debated, since deuterium-burning has a negligible impact on stellar structure and evolution (Chabrier & Baraffe 2000). Based on observations, it may be more relevant to distinguish brown dwarfs and giant planets based on their dominant formation mechanisms, which allows a mass overlap between these two populations (Luhman 2012;Chabrier et al 2014). Of course, such a definition is not without its challenges, since it is not straightforward to infer the formation of a particular object based on its observable properties.…”

Section: Introductionmentioning

confidence: 99%

Can brown dwarfs survive on close orbits around convective stars?

Damiani

Díaz

2016

A&A

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Context. The mass range of brown dwarfs extends across the planetary domain to stellar objects. There is a relative paucity of brown dwarfs companions around FGKM-type stars compared to exoplanets for orbital periods of less than a few years, but most of the short-period brown dwarf companions that are fully characterised by transits and radial velocities are found around F-type stars. Aims. We examine the hypothesis that brown dwarf companions could not survive on close orbit around stars with important convective envelopes because the tides and angular momentum loss, the result of magnetic braking, would lead to a rapid orbital decay with the companion being quickly engulfed. Methods. We use a classical Skumanich-type braking law and constant time-lag tidal theory to assess the characteristic timescale for orbital decay for the brown dwarf mass range as a function of the host properties. Results. We find that F-type stars may host massive companions for a significantly longer time than G-type stars for a given orbital period, which may explain the paucity of G-type hosts for brown dwarfs with an orbital period less than five days. On the other hand, we show that the small radius of early M-type stars contributes to orbital decay timescales that are only half those of F-type stars, despite their more efficient tidal dissipation and magnetic braking. For fully convective later type M-dwarfs, orbital decay timescales could be orders of magnitude greater than for F-type stars. Moreover, we find that, for a wide range of values of tidal dissipation efficiency and magnetic braking, it is safe to assume that orbital decay for massive companions can be neglected for orbital periods greater than ten days. Conclusions. For orbital periods greater than ten days, brown dwarf occurrence should largely be unaffected by tidal decay, whatever the mass of the host. On closer orbital periods, the rapid engulfment of massive companions could explain the lack of G and K-type hosts in the sample of known systems with transiting brown dwarfs. However, the paucity of M-type hosts cannot be an effect of tidal decay alone, but may be the result of a selection effect in the sample and/or the formation mechanism.

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The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs

Cited by 207 publications

References 380 publications

Accretion-ejection connection in the young brown dwarf candidate ISO-ChaI 217

Accretion-ejection connection in the young brown dwarf candidate ISO-ChaI 217

SOPHIE velocimetry ofKeplertransit candidates

Can brown dwarfs survive on close orbits around convective stars?

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