scholarly journals Looking for Very Low-Mass Pre-Main Sequence Objects with SDSS

2003 ◽  
Vol 211 ◽  
pp. 525-526
Author(s):  
Peregrine M. McGehee ◽  
Suzanne L. Hawley ◽  
Kevin R. Covey
Keyword(s):  
Molecular Cloud ◽  
Brown Dwarfs ◽  
Circumstellar Disks ◽  
Mass Function ◽  
Magnetic Activity ◽  
Ultraviolet Excess ◽  
Coronal Emission ◽  
Intrinsic Nature ◽  
Primordial Magnetic Fields ◽  
Low Mass

Determining the process(es) by which brown dwarfs form is key to understanding their intrinsic nature. If their origins are within circumstellar disks they are akin to giant planets. If, on the other hand, they coalesce from molecular cloud cores, then they share a common lineage with low mass stars. These two mechanisms can be distinguished by investigation of young (< 10 Myr) substellar objects. If brown dwarfs are small failed stars, we expect to find very low mass analogs of the Classical T Tauris, with primordial magnetic fields from the molecular cloud collapse. Accretion onto these objects leads to characteristic magnetic activity signatures such as chromospheric and coronal emission resulting in an ultraviolet excess continuum.The Orion OB1b association (m-M = 7.9, 2 Myr) provides a laboratory for following the strength and occurrence of the accretion process as a function of mass. Studies of the substellar mass function within the sigma Orionis cluster at the southern end of the association indicate that brown dwarfs are common. Based on model isochrones and the SDSS M dwarf sequence we expect the 95% completeness limit of the “Orion” scans to correspond to 0.1 and 0.03 solar masses for the u and g bands.

scholarly journals The Formation Mechanism and Resulting Properties of Brown Dwarfs

2003 ◽  
Vol 211 ◽  
pp. 27-30 ◽  
Author(s):  
Matthew R. Bate ◽  
Ian A. Bonnell ◽  
Volker Bromm
Keyword(s):  
Star Formation ◽  
Formation Mechanism ◽  
Binary Stars ◽  
Molecular Cloud ◽  
Large Scale ◽  
Brown Dwarfs ◽  
Circumstellar Disks ◽  
Mass Function ◽  
Initial Mass Function ◽  
Initial Mass

We present results from the most complex hydrodynamical star formation calculation performed to date. It follows the collapse and fragmentation of a large-scale turbulent molecular cloud to form dozens of stars and brown dwarfs. It resolves all fragmentation down to the opacity limit, binary stars with separations as small as 1 AU, and circumstellar disks with radii down to ≈ 10 AU. In this proceedings, we examine the formation mechanism of the brown dwarfs and compare the initial mass function and the properties of the brown dwarfs with observations.

scholarly journals Magnetic Activity of Pre-main Sequence Stars near the Stellar-Substellar Boundary

2015 ◽  
Vol 10 (S314) ◽  
pp. 126-127
Author(s):  
David Principe ◽  
Joel. H. Kastner ◽  
David Rodriguez
Keyword(s):  
Spectral Type ◽  
Main Sequence ◽  
Brown Dwarfs ◽  
Magnetic Activity ◽  
Main Sequence Stars ◽  
Coronal Emission ◽  
Low Mass Stars ◽  
X Ray ◽  
Low Mass ◽  
Age Range

AbstractX-ray observations of pre-main sequence (pre-MS) stars of M-type probe coronal emission and offer a means to investigate magnetic activity at the stellar-substellar boundary. Recent observations of main sequence (MS) stars at this boundary display a decrease in fractional X-ray luminosity (LX/Lbol) by almost two orders of magnitude for spectral types M7 and later. We investigate magnetic activity and search for a decrease in X-ray emission in the pre-MS progenitors of these MS stars. We present XMM-Newton X-ray observations and preliminary results for ~10 nearby (30-70 pc), very low mass pre-MS stars in the relatively unexplored age range of 10-30 Myr. We compare the fractional X-ray luminosities of these 10-30 Myr old stars to younger (1-3 Myr) pre-MS brown dwarfs and find no dependence on spectral type or age suggesting that X-ray activity declines at an age later than ~30 Myr in these very low-mass stars.

scholarly journals Very Low-Luminosity Objects in Star-Forming Regions

1998 ◽  
Vol 11 (1) ◽  
pp. 423-424
Author(s):  
Motohide Tamura ◽  
Yoichi Itoh ◽  
Yumiko Oasa ◽  
Alan Tokunaga ◽  
Koji Sugitani
Keyword(s):  
Brown Dwarfs ◽  
Mass Function ◽  
T Tauri Stars ◽  
Initial Mass Function ◽  
Formation Mechanisms ◽  
Star Forming ◽  
Star Forming Regions ◽  
T Tauri ◽  
Low Mass ◽  
Stellar Sources

Abstract In order to tackle the problems of low-mass end of the initial mass function (IMF) in star-forming regions and the formation mechanisms of brown dwarfs, we have conducted deep infrared surveys of nearby molecular clouds. We have found a significant population of very low-luminosity sources with IR excesses in the Taurus cloud and the Chamaeleon cloud core regions whose extinction corrected J magnitudes are 3 to 8 mag fainter than those of typical T Tauri stars in the same cloud. Some of them are associated with even fainter companions. Follow-up IR spectroscopy has confirmed for the selected sources that their photospheric temperature is around 2000 to 3000 K. Thus, these very low-luminosity young stellar sources are most likely very low-mass T Tauri stars, and some of them might even be young brown dwarfs.

scholarly journals Brown Dwarf Model Atmospheres Based on Multi-Dimensional Radiation Hydrodynamics

2009 ◽  
Vol 5 (H15) ◽  
pp. 756-756 ◽  
Author(s):  
France Allard ◽  
Bernd Freytag
Keyword(s):  
Cloud Model ◽  
Molecular Transport ◽  
Brown Dwarfs ◽  
Model Atmosphere ◽  
Magnetic Activity ◽  
Cloud Formation ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
The Status ◽  
Low Mass

AbstractThe atmospheres of Brown Dwarfs (BDs) are the site of molecular opacities and cloud formation, and control their cooling rate, radius and brightness evolution. Brown dwarfs evolve from stellar-like properties (magnetic activity, spots, flares, mass loss) to planet-like properties (electron degeneracy of the interior, cloud formation, dynamical molecular transport) while retaining, due to their fully convective interior, larger rotational velocities (≤ 30 km/s i.e. P < 4 hrs versus 11 hrs for Jupiter). Model atmospheres treating all this complexity are therefore essential to understand the evolution properties, and to interpret the observations of these objects. While the pure gas-phase based NextGen model atmospheres (Allard et al. 1997, Hauschildt et al. 1999) have allowed the understanding of the several populations of Very Low Mass Stars (VLMs), the AMES-Dusty models (Allard et al. 2001) based on equilibrium chemistry have reproduced some near-IR photometric properties of M and L-type brown dwarfs, and played a key role in the determination of the mass of brown dwarfs and Planetary Mass Objects (PMOs) in the eld and in young stellar clusters. In this paper, we present a new model atmosphere grid for VLMs, BDs, PMOs named BT-Settl, which includes a cloud model and dynamical molecular transport based on mixing information from 2D Radiation Hydrodynamic (RHD) simulations (Freytag et al. 2009). We also present the status of our 3D RHD simulations including rotation (Coriolis forces) of a cube on the surface of a brown dwarf. The BT-Settl model atmosphere grid will be available shortly via the Phoenix web simulator (http://phoenix.ens-lyon.fr/simulator/).

scholarly journals Low‐Mass Stars and Brown Dwarfs in NGC 2024: Constraints on the Substellar Mass Function

2006 ◽  
Vol 646 (2) ◽  
pp. 1215-1229 ◽  
Author(s):  
Joanna L. Levine ◽  
Aaron Steinhauer ◽  
Richard J. Elston ◽  
Elizabeth A. Lada
Keyword(s):  
Brown Dwarfs ◽  
Mass Function ◽  
Low Mass Stars ◽  
Low Mass

scholarly journals The Initial Mass Function of Low‐Mass Stars and Brown Dwarfs in Young Clusters

2000 ◽  
Vol 540 (2) ◽  
pp. 1016-1040 ◽  
Author(s):  
K. L. Luhman ◽  
G. H. Rieke ◽  
Erick T. Young ◽  
Angela S. Cotera ◽  
H. Chen ◽  
...  
Keyword(s):  
Brown Dwarfs ◽  
Mass Function ◽  
Initial Mass Function ◽  
Initial Mass ◽  
Low Mass Stars ◽  
Low Mass ◽  
Young Clusters

scholarly journals Theory of Young Clusters

2004 ◽  
Vol 221 ◽  
pp. 257-264
Author(s):  
Matthew R. Bate ◽  
Ian A. Bonnell
Keyword(s):  
Star Formation ◽  
Molecular Cloud ◽  
Initial Conditions ◽  
Star Clusters ◽  
Brown Dwarfs ◽  
Mass Function ◽  
Large Cluster ◽  
Close Binaries ◽  
Stellar Clusters ◽  
Small Clusters

We review recent results from hydrodynamical calculations of the formation of young stellar clusters. The calculations present a highly dynamical picture of star formation where the mass function of stars originates from competitive accretion between protostars and dynamical ejections which halt accretion. Large star clusters form hierarchically; a molecular cloud forms many small clusters that later merge into one large cluster, erasing the initial substructure. Using calculations that resolve fragmentation down to the opacity limit, we examine the implications of dynamical star formation for the IMF, the formation of brown dwarfs and close binaries, and the sizes of protoplanetary discs. Finally, we discuss the dependence of the results on the initial conditions.

scholarly journals Disks around Young Brown Dwarfs

2003 ◽  
Vol 211 ◽  
pp. 133-136
Author(s):  
Michael C. Liu
Keyword(s):  
Brown Dwarfs ◽  
Circumstellar Disks ◽  
Disk Accretion ◽  
Low Mass Stars ◽  
Star Forming ◽  
Star Forming Regions ◽  
Planetary Mass ◽  
T Tauri ◽  
Hα Emission ◽  
Low Mass

We present some results from a systematic survey for disks around spectroscopically identified young brown dwarfs and very low mass stars. We find that ≈75% of our sample show intrinsic IR excesses, indicative of circum(sub)stellar disks. The observed excesses are well-correlated with Hα emission, consistent with a common disk accretion origin. Because the excesses are modest, conventional analyses using only IR colors would have missed most of the sources with disks. In the same star-forming regions, we find that disks around brown dwarfs and T Tauri stars are contemporaneous; assuming coevality, this demonstrates that substellar disks are at least as long-lived as stellar disks. Altogether, the frequency and properties of circumstellar disks are similar from the stellar regime down to the substellar and planetary-mass regime. This offers compelling evidence of a common origin for most stars and brown dwarfs.

Circumstellar disk fragmentation and the origin of massive planetary companions, brown dwarfs, and very low-mass stars

2018 ◽  
Vol 14 (S345) ◽  
pp. 239-240 ◽  
Author(s):  
M. B. N. Kouwenhoven ◽  
Yun Li ◽  
D. Stamatellos ◽  
S. P. Goodwin
Keyword(s):  
Binary Systems ◽  
Brown Dwarfs ◽  
Mass Range ◽  
Mass Function ◽  
Initial Mass Function ◽  
Low Mass Stars ◽  
Triple Systems ◽  
Low Mass ◽  
Viable Mechanism

AbstractThe low-mass end of the initial mass function remains poorly understood. In this mass range, very low-mass stars, brown dwarfs, and massive planets are able to form through a variety of physical processes. Here, we study the long-term evolution of disk-fragmented systems around low-mass stars, for the epoch up to 10 Myr (the typical lifetime of an embedded cluster) and up to 10 Gyr (the age of the Milky Way). We carry out N-body simulations to study the decay of disk-fragmented systems and the resulting end products. Our simulations indicate rapid decay and frequent physical collisions during the first 10 Myr. We find that disk fragmentation provides a viable mechanism for explaining hierarchical triple systems, the brown dwarf desert, single and binary brown dwarfs, and very low-mass binary systems in the solar neighbourhood.

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