1,720,996 research outputs found
A VOLUME-LIMITED SAMPLE OF 63 M7-M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO
No full textIn a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, average magnetic field strengths, and chromospheric emission from the H alpha line. We confirm earlier results that the mean level of normalized H alpha luminosity decreases with lower temperature, and we find that the scatter among H alpha luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4 kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, H alpha luminosity is related to magnetic field strength, consistent with results in earlier stars. A few very slowly rotating stars show very weak magnetic fields and H alpha emission, and all stars rotating faster than our detection limit show magnetic fields of at least a few hundred Gauss. In contrast to earlier-type stars, we observe magnetic fields weaker than 1 kG in stars rotating faster than similar to 3 km s(-1), but we find no correlation between rotation and magnetic flux generation among them. We interpret this as a fundamental change in the dynamo mechanism; in ultracool dwarfs, magnetic field generation is predominantly achieved by a turbulent dynamo, while other mechanisms can operate more efficiently at earlier spectral types.DFG [RE 1664/4-1]; NSF [AST06-06748
A VOLUME-LIMITED SAMPLE OF 63 M7-M9.5 DWARFS. I. SPACE MOTION, KINEMATIC AGE, AND LITHIUM
No full textIn a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this first paper we introduce our volume-complete sample from DENIS and 2MASS targets, and we derive radial velocities and space motion. Kinematics of our sample are consistent with the stars being predominantly members of the young disk. The kinematic age of the sample is 3.1 Gyr. We find that six of our targets show strong Li lines implying that they are brown dwarfs younger than several hundred million years. Five of the young brown dwarfs were unrecognized before. Comparing the fraction of Li detections to later spectral types, we see a hint of an unexpected local maximum of this fraction at spectral type M9. It is not yet clear whether this maximum is due to insufficient statistics, or to a combination of physical effects including spectral appearance of young brown dwarfs, Li line formation, and the star formation rate at low masses.DFG [RE1664/4-1]; NSF [AST06-06748
Chromospheric Activity, Rotation, and Rotational Braking in M and L Dwarfs
No full textWe present results from a high-resolution spectroscopic survey of 45 L dwarfs, which includes both very low mass stars and brown dwarfs. Our spectra allow us to derive a significant number of new rotational velocities, and discover a slowly rotating (in projected velocity) L dwarf that allows more accurate measurement of spectroscopic rotations for these objects. We measure chromospheric activity (and often its variability) through the H alpha emission line. Our primary new result is good evidence that magnetic braking dominates the angular momentum evolution of even brown dwarfs, although spindown times appear to increase as mass decreases. We confirm that activity decreases as effective temperature decreases, although a larger fraction of L dwarfs are active than has previously been reported. Essentially all active objects are also variable. We confirm the lack of a rotation-activity connection for L dwarfs. We find a minimum limit for rotational velocities that increases with later spectral types, rising from near zero in older mid-M stars to more than 20 km s(-1) for mid-L objects. There is strong evidence that all L dwarfs are rapid rotators. We derive a braking law that can depend on either temperature or mass which can explain all the rotational results and provides an age dependence for the angular momentum evolution. It is clear that angular momentum loss mechanisms in smaller and cooler objects become more inefficient, starting at the fully convective boundary
On the magnetic topology of partially and fully convective stars
No full textWe compare the amount of magnetic flux measured in Stokes V
and Stokes I in a sample of early- and mid-M stars around the
boundary to full convection (~M 3.5). Early-M stars possess a
radiative core, mid-M stars are fully convective. While Stokes V is
sensitive to the net polarity of magnetic flux arising mainly from
large-scale configurations, Stokes I measurements can see the total
mean flux. We find that in early-M dwarfs, only ~6% of the
total magnetic flux is detected in Stokes V. This ratio is more than
twice as large, ~14%, in fully convective mid-M dwarfs. The
bulk of the magnetic flux on M-dwarfs is not seen in Stokes V. This
is presumably because magnetic flux is mainly stored in small scale
components. There is also more to learn about the effect of the
weak-field approximation on the accuracy of strong field detections.
In our limited sample, we see evidence for a change in magnetic
topology at the boundary to full convection. Fully convective stars
store a 2–3 times higher fraction of their flux in fields visible
to Stokes V. We estimate the total magnetic energy detected in
Stokes I and compare it to results from Stokes V. We find that in
early-M dwarfs only ~0.5% of the total magnetic energy is
detected in Stokes V while this fraction is ~2.5% in mid-M
dwarfs
The moderate magnetic field of the flare star Proxima Centauri
No full textWe report moderate magnetic flux of 450 G < Bf <
750 G (3σ) on the nearby M 5.5 flare star Proxima Centauri.
A high resolution UVES spectrum was used to measure magnetic flux
from Zeeman broadening in absorption lines of molecular FeH around
1 μm. The magnetic flux we find is relatively weak compared
with classical strong flare stars, but so are Proxima's flaring
rates and actual emission levels. We compare what is known about the
rotation rate, Rossby number, and activity levels in this star to
relations between these quantities that are being developed more
generally for M dwarfs. We conclude that the magnetic flux is higher
than the best estimates of the Rossby number from period
measurements. On the other hand, the activity levels of Proxima
Centauri are at the high end of what could be expected based on the
measured field, but not so high as to exceed the natural scatter in
these relations (other stars lie along this high envelope as well)
COMPARISON OF KEPLER PHOTOMETRIC VARIABILITY WITH THE SUN ON DIFFERENT TIMESCALES
No full textWe utilize Kepler data to study the precision differential photometric variability of solar-type and cooler stars at different timescales, ranging from half an hour to three months. We define a diagnostic that characterizes the median differential intensity change between data bins of a given timescale. We apply the same diagnostics to Solar and Heliospheric Observatory data that has been rendered comparable to Kepler. The Sun exhibits similar photometric variability on all timescales as comparable solar-type stars in the Kepler field. The previously defined photometric "range" serves as our activity proxy (driven by starspot coverage). We revisit the fraction of comparable stars in the Kepler field that are more active than the Sun. The exact active fraction depends on what is meant by "more active than the Sun" and on the magnitude limit of the sample of stars considered. This active fraction is between a quarter and a third (depending on the timescale). We argue that a reliable result requires timescales of half a day or longer and stars brighter than M-Kep of 14, otherwise non-stellar noise distorts it. We also analyze main sequence stars grouped by temperature from 6500 to 3500 K. As one moves to cooler stars, the active fraction of stars becomes steadily larger (greater than 90% for early M dwarfs). The Sun is a good photometric model at all timescales for those cooler stars that have long-term variability within the span of solar variability.NSF [AST-0606748]; NASA Kepler fellowshi
Evidence for Magnetic Flux Saturation in Rapidly Rotating M Stars
No full textCopyright © 2009 American Astronomical Society / IOP PublishingWe present magnetic flux measurements in seven rapidly rotating M dwarfs. Our sample stars have X-ray and Hα emission indicative of saturated emission, i.e., emission at a high level, independent of rotation rate. Our measurements are made using near-infrared FeH molecular spectra observed with the High Resolution Echelle Spectrometer at Keck. Because of their large convective overturn times, the rotation velocity of M stars with small Rossby numbers is relatively slow and does not hamper the measurement of Zeeman splitting. The Rossby numbers of our sample stars are as small as 0.01. All our sample stars exhibit magnetic flux of kG strength. We find that the magnetic flux saturates in the same regime as saturation of coronal and chromospheric emission, at a critical Rossby number of around 0.1. The filling factors of both field and emission are near unity by then. We conclude that the strength of surface magnetic fields remains independent of rotation rate below that; making the Rossby number yet smaller by a factor of 10 has little effect. These saturated M-star dynamos generate an integrated magnetic flux of roughly 3 kG, with a scatter of about 1 kG. The relation between emission and flux also has substantial scatter
Surprisingly Weak Magnetism on Young Accreting Brown Dwarfs
No full textWe have measured the surface magnetic flux on four accreting young brown dwarfs and one nonaccreting young very low mass (VLM) star utilizing high-resolution spectra of absorption lines of the FeH molecule. A magnetic field of 1-2 kG had been proposed for one of the brown dwarfs, Two Micron All Sky Survey (2MASS) J1207334-393254, because of its similarities to higher mass T Tauri stars as manifested in accretion and the presence of a jet. We do not find clear evidence for a kilogauss field in any of our young brown dwarfs but do find a 2 kG field on the young VLM star. Our 3 sigma upper limit for the magnetic flux in 2MASS J1207334-393254 just reaches 1 kG. We estimate the magnetic field required for accretion in young brown dwarfs given the observed rotations, and find that fields of only a few hundred gauss are sufficient for magnetospheric accretion. This predicted value is less than our observed upper limit. We conclude that magnetic fields in young brown dwarfs are a factor of 5 or more lower than in young stars of about one solar mass, and in older stars with spectral types similar to our young brown dwarfs. It is interesting that, during the first few million years, the fields scale down with mass in line with what is needed for magnetospheric accretion, yet no such scaling is observed at later ages within the same effective temperature range. This scaling is opposite to the trend in rotation, with shorter rotation periods for very young accreting brown dwarfs compared with accreting solar-mass objects (and very low Rossby numbers in all cases). We speculate that in young objects a deeper intrinsic connection may exist between magnetospheric accretion and magnetic field strength, or that magnetic field generation in brown dwarfs may be less efficient than in stars. Neither of these currently has an easy physical explanation
On the kinematic age of brown dwarfs: radial velocities and space motions of 43 nearby L dwarfs
No full textWe present radial velocity measurements of a sample of L0–L8 dwarfs
observed with VLT/UVES and Keck/HIRES. We combine these measurements
with distance and proper motion from the literature to determine
space motions for 43 of our targets. We identify nine candidate
members of young moving groups, which have ages of 50–600 Myr according to their
space motion. From the total velocity dispersion of the 43 L dwarfs,
we calculate a kinematic age of ~5 Gyr for our sample. This age
is significantly higher than the ~3 Gyr age known for late M
dwarfs in the solar neighbourhood. We find that the distributions of
the U and V velocity components of our sample are clearly
non-Gaussian, placing the age estimate inferred from the full space motion
vector into question. The W-component exhibits a distribution more
consistent with a normal distribution, and from W alone we derive
an age of ~3 Gyr, which is the same age found for late-M dwarf
samples. Our brightness-limited sample is probably contaminated by a
number of outliers that predominantly bias the U and V velocity
components. The origin of the outliers remain unclear, but we
suggest that these brown dwarfs may have gained their high velocities
by means of ejection from multiple systems during their formation
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