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 Non thermal emission from T Tauri stars

2010 ◽  
Vol 6 (S275) ◽  
pp. 404-405
Author(s):  
María V. del Valle ◽  
Gustavo E. Romero
Keyword(s):  
Main Sequence ◽  
Thermal Emission ◽  
High Energy ◽  
Magnetic Activity ◽  
T Tauri Stars ◽  
Main Sequence Stars ◽  
High Energy Radiation ◽  
X Ray ◽  
T Tauri ◽  
Low Mass

AbstractT Tauri stars are low mass, pre-main sequence stars. These objects are surrounded by an accretion disk and present strong magnetic activity. T Tauri stars are copious emitters of X-ray emission which belong to powerful magnetic reconnection events. Strong magnetospheric shocks are likely outcome of massive reconnection. Such shocks can accelerate particles up to relativistic energies through Fermi mechanism. We present a model for the high-energy radiation produced in the environment of T Tauri stars. We aim at determining whether this emission is detectable. If so, the T Tauri stars should be very nearby.

scholarly journals X-ray Emission from Young Stars in Suburban Orion

2004 ◽  
Vol 219 ◽  
pp. 228-232
Author(s):  
K. Briggs ◽  
M. Güdel ◽  
M. Audard ◽  
K. Smith ◽  
R. Mewe ◽  
...  
Keyword(s):  
Main Sequence ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Young Stars ◽  
Rossby Number ◽  
Low Mass Stars ◽  
X Ray ◽  
Star Forming ◽  
Low Mass ◽  
Pms Stars

X-ray emission from > 100 pre-main sequence (PMS) stars in the Orion star-forming complex is studied in a 20-ks observation by XMM-Newton. No relation between the ratio of X-ray and bolometric luminosities, LX/Lbol, and rotation period or Rossby number is exhibited, though the action of a solar-like dynamo is not excluded because all stars would appear to be in the “saturated regime” of such a dynamo. Low-mass stars showing a strong U — V excess have lower median X-ray luminosity, suggesting that accretion suppresses magnetic activity.

scholarly journals X-ray Emission from Brown Dwarfs

2004 ◽  
Vol 219 ◽  
pp. 285-295
Author(s):  
Thomas Preibisch
Keyword(s):  
Free Energy ◽  
Spectral Type ◽  
Effective Temperature ◽  
Plasma Temperature ◽  
Brown Dwarfs ◽  
Low Mass Stars ◽  
Electrical Currents ◽  
X Ray ◽  
Magnetic Free Energy ◽  
Low Mass

During the last few years, X-ray emission has been detected from numerous brown dwarfs. Most of the X-ray detected brown dwarfs are very young objects with ages of at most 107 years, and all are still relatively warm, with late M spectral types. Their typical fractional X-ray luminosities are (LX/Lbol) ∼ 10—4 — 10—3, i.e. very similar to the values observed for active very-low mass stars. Their X-ray lightcurves show low-level variability, but in most cases no large flares; this implies that the young brown dwarfs are able to produce quiescent X-ray emission, not only occasional flares. An analysis of the Chandra X-ray spectra of several brown dwarfs yields surprisingly low plasma temperatures between 3 MK and 10 MK for some of the M8-9 dwarfs and indicates a decline in plasma temperature with decreasing effective temperature (or increasing age). The lack of X-ray detections for dwarfs cooler than spectral type M9 is consistent with the strong drop of activity observed in Hα at spectral types around M9. The observed X-ray emission from the young brown dwarfs with late M spectral type can be understood as a consequence of the fact that these objects are still warm enough to maintain partially ionized atmospheres which are capable of sustaining electrical currents. In the cooler, essentially neutral atmospheres of the older L and T dwarfs such currents are probably shut off, preventing the buildup of magnetic free energy and the support for magnetically heated chromospheres and coronae.

scholarly journals X-Ray Detection of Brown Dwarfs with Chandra

2003 ◽  
Vol 211 ◽  
pp. 447-450 ◽  
Author(s):  
Scott J. Wolk
Keyword(s):  
Brown Dwarfs ◽  
Stellar Clusters ◽  
Brown Dwarf ◽  
Low Mass Stars ◽  
X Ray ◽  
Low Mass ◽  
Vast Range

I review recent observations of brown dwarfs by the Chandra X-ray Observatory. These observations fall in 2 categories, young stellar clusters which contain brown dwarfs and brown dwarf candidates and directed pointings at brown dwarfs and very low mass stars. Surprisingly, there are already over 60 published detections of brown dwarfs by Chandra. A review of the X–ray characteristics shows these objects are subject to flaring and their temperatures and luminosities have a vast range which is related to age.

scholarly journals Activity and rotation of the X-ray emitting Kepler stars

2019 ◽  
Vol 628 ◽  
pp. A41 ◽  
Author(s):  
D. Pizzocaro ◽  
B. Stelzer ◽  
E. Poretti ◽  
S. Raetz ◽  
G. Micela ◽  
...  
Keyword(s):  
White Light ◽  
Main Sequence ◽  
Magnetic Activity ◽  
Light Curves ◽  
Late Type ◽  
Main Sequence Stars ◽  
Photometric Variability ◽  
X Ray ◽  
Rotation Periods ◽  
Flare Frequency

The relation between magnetic activity and rotation in late-type stars provides fundamental information on stellar dynamos and angular momentum evolution. Rotation-activity studies found in the literature suffer from inhomogeneity in the measurement of activity indexes and rotation periods. We overcome this limitation with a study of the X-ray emitting, late-type main-sequence stars observed by XMM-Newton and Kepler. We measured rotation periods from photometric variability in Kepler light curves. As activity indicators, we adopted the X-ray luminosity, the number frequency of white-light flares, the amplitude of the rotational photometric modulation, and the standard deviation in the Kepler light curves. The search for X-ray flares in the light curves provided by the EXTraS (Exploring the X-ray Transient and variable Sky) FP-7 project allows us to identify simultaneous X-ray and white-light flares. A careful selection of the X-ray sources in the Kepler field yields 102 main-sequence stars with spectral types from A to M. We find rotation periods for 74 X-ray emitting main-sequence stars, 20 of which do not have period reported in the previous literature. In the X-ray activity-rotation relation, we see evidence for the traditional distinction of a saturated and a correlated part, the latter presenting a continuous decrease in activity towards slower rotators. For the optical activity indicators the transition is abrupt and located at a period of ~10 d but it can be probed only marginally with this sample, which is biased towards fast rotators due to the X-ray selection. We observe seven bona-fide X-ray flares with evidence for a white-light counterpart in simultaneous Kepler data. We derive an X-ray flare frequency of ~0.15 d−1, consistent with the optical flare frequency obtained from the much longer Kepler time-series.

Theoretical models of low-mass stars and brown dwarfs. I - The lower main sequence

1989 ◽  
Vol 342 ◽  
pp. 1003 ◽  
Author(s):  
Ben Dorman ◽  
Lorne A. Nelson ◽  
W. Y. Chau
Keyword(s):  
Main Sequence ◽  
Brown Dwarfs ◽  
Theoretical Models ◽  
Low Mass Stars ◽  
Low Mass

scholarly journals Properties which cannot be explained by Planetary Nebulae's progenitors

1997 ◽  
Vol 180 ◽  
pp. 367-367
Author(s):  
Noam Soker
Keyword(s):  
Main Sequence ◽  
Brown Dwarfs ◽  
Giant Planets ◽  
Main Sequence Stars ◽  
Substellar Objects ◽  
Earth Like Planets ◽  
Low Mass ◽  
Inner Region ◽  
Central Stars

Stellar binary companions account for bipolar PNe (∼ 11% of all PNe1), and some ellipticalls (22%2). The rest of axisymmetrical PNe (40% to 60% of all PNe) are due to substellar objects (planets and brown dwarfs)3. This classification of axi symmetrical PNe suggests that substellar objects are commonly present within several × AU around main sequence stars, and that several substellar objects must be present around most main sequence stars3. Some substellar and low mass stellar companions will enter the primary envelope only as the primary reaches the upper AGB. Thus, the early mass loss episode will be spherical, leading to the formation of a spherical halo around an elliptical inner region. Gas giant planets and brown dwarfs close to the primary, will not allow Earth-like planets to have stable orbits. Systems with no Jupiter-like planets will allow Earth-like planets to be present. These stars will form spherical PNe (10%-20% of all PNe, including spherically ejected PNe that have been deformed by the ISM through which they move4). Systems with substellar objects at large separation, as Jupiter in the solar system, will also allow Earth-like planets to be present. These systems will form PNe with spherical halo. Therefore, life may have been present in planets around the central stars of round PNe and elliptical PNe with round halos.

scholarly journals Measurements of the Ca ii infrared triplet emission lines of pre-main-sequence stars

2020 ◽  
Vol 72 (5) ◽  
Author(s):  
Mai Yamashita ◽  
Yoichi Itoh ◽  
Yuhei Takagi
Keyword(s):  
Main Sequence ◽  
Open Clusters ◽  
Emission Lines ◽  
Main Sequence Stars ◽  
Low Mass Stars ◽  
Chromospheric Activity ◽  
Low Mass ◽  
Pms Stars ◽  
Infrared Triplet ◽  
Narrow Emission

Abstract We investigated the chromospheric activity of 60 pre-main-sequence (PMS) stars in four molecular clouds and five moving groups. It is considered that strong chromospheric activity is driven by the dynamo processes generated by stellar rotation. In contrast, several researchers have pointed out that the chromospheres of PMS stars are activated by mass accretion from their protoplanetary disks. In this study, the Ca ii infrared triplet (IRT) emission lines were investigated utilizing medium- and high-resolution spectroscopy. The observations were conducted with Nayuta/MALLS and Subaru/HDS. Additionally, archive data obtained by Keck/HIRES, VLT/UVES, and VLT/X-Shooter were used. The small ratios of the equivalent widths indicate that Ca ii IRT emission lines arise primarily in dense chromospheric regions. Seven PMS stars show broad emission lines. Among them, four PMS stars have more than one order of magnitude brighter emission line fluxes compared to the low-mass stars in young open clusters. The four PMS stars have a high mass accretion rate, which indicates that the broad and strong emission results from a large mass accretion. However, most PMS stars exhibit narrow emission lines. No significant correlation was found between the accretion rate and flux of the emission line. The ratios of the surface flux of the Ca ii IRT lines to the stellar bolometric luminosity, $R^{\prime }_{\rm IRT}$, of the PMS stars with narrow emission lines are as large as the largest $R^{\prime }_{\rm IRT}$ of the low-mass stars in the young open clusters. This result indicates that most PMS stars, even in the classical T Tauri star stage, have chromospheric activity similar to zero-age main-sequence stars.

scholarly journals The rotational evolution of young low mass stars

2007 ◽  
Vol 3 (S243) ◽  
pp. 231-240 ◽  
Author(s):  
Jérôme Bouvier
Keyword(s):  
Angular Momentum ◽  
Accretion Disk ◽  
Main Sequence ◽  
Young Stars ◽  
Sequence Evolution ◽  
Main Sequence Stars ◽  
Low Mass Stars ◽  
Rotational Evolution ◽  
Low Mass

AbstractStar-disk interaction is thought to drive the angular momentum evolution of young stars. In this review, I present the latest results obtained on the rotational properties of low mass and very low mass pre-main sequence stars. I discuss the evidence for extremely efficient angular momentum removal over the first few Myr of pre-main sequence evolution and describe recent results that support an accretion-driven braking mechanism. Angular momentum evolution models are presented and their implication for accretion disk lifetimes discussed.

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/).

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