837 research outputs found
Teff and log g dependence of velocity fields in M-stars
We present an investigation of velocity fields in early to late M-type
hydrodynamic stellar atmosphere models. These velocities will be expressed in
classical terms of micro- and macro-turbulent velocities for usage in 1D
spectral synthesis. The M-star model parameters range between log g of 3.0 -
5.0 and Teff of 2500 K - 4000 K. We characterize the Teff- and log g-dependence
of the hydrodynamical velocity fields in these models with a binning method,
and for the determination of micro-turbulent velocities, the Curve of Growth
method is used. The macro-turbulent velocities are obtained by convolutions
with Gaussian profiles. Velocity fields in M-stars strongly depend on log g and
Teff. Their velocity amplitudes increase with decreasing log g and increasing
Teff. The 3D hydrodynamical and 1D macro-turbulent velocities range from ~100
m/s for cool high gravity models to ~ 800 m/s - 1000 m/s for hot models or
models with low log g. The micro-turbulent velocities range in the order of
~100 m/s for cool models, to ~600 m/s for hot or low log g models. Our M-star
structure models are calculated with the 3D radiative-hydrodynamics (RHD) code
CO5BOLD. The spectral synthesis on these models is performed with the line
synthesis code LINFOR3D.Comment: 8 pages, 6 Figures, Proceeding fot the "Recent directions in
astrophysical quantitative spectroscopy and radiation hydrodynamics"
conferenc
Magnetic cycles of the planet-hosting star Tau Bootis: II. a second magnetic polarity reversal
In this paper, we present new spectropolarimetric observations of the
planet-hosting star Tau Bootis, using ESPaDOnS and Narval spectropolarimeters
at Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL),
respectively. We detected the magnetic field of the star at three epochs in
2008. It is a weak magnetic field of only a few Gauss, oscillating between a
predominant toroidal component in January and a dominant poloidal component in
June and July. A magnetic polarity reversal was observed relative to the
magnetic topology in June 2007. This is the second such reversal observed in
two years on this star, suggesting that Tau Boo has a magnetic cycle of about 2
years. This is the first detection of a magnetic cycle for a star other than
the Sun. The role of the close-in massive planet in the short activity cycle of
the star is questioned.
Tau Boo has strong differential rotation, a common trend for stars with
shallow convective envelope. At latitude 40 deg., the surface layer of the star
rotates in 3.31 d, equal to the orbital period. Synchronization suggests that
the tidal effects induced by the planet may be strong enough to force at least
the thin convective envelope into corotation. Tau Boo shows variability in the
Ca H & K and Halpha throughout the night and on a night to night time scale. We
do not detect enhancement in the activity of the star that may be related to
the conjunction of the planet. Further data is needed to conclude about the
activity enhancement due to the planet.Comment: 9 pages, 5 figures, 3 tables Accepted to MNRA
Evidence for Magnetic Flux Saturation in Rapidly Rotating M Stars
We present magnetic flux measurements in seven rapidly rotating M dwarfs. Our
sample stars have X-ray and H-alpha 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 HIRES 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 kilo-Gauss
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 ten 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.Comment: 10 pages, accepted for publication in Ap
Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf
[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical
observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio
emission, dominated by short-duration periodic pulses at 4.86 GHz with
P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times
demonstrates that they are due to the rotational modulation of a B~1.7 kG
magnetic field. A quiescent non-variable component is also detected, likely due
to emission from a uniform large-scale field. The H-alpha emission exhibits
identical periodicity, but unlike the radio pulses it varies sinusoidally and
is offset by exactly 1/4 of a phase. The sinusoidal variations require
chromospheric emission from a large-scale field structure, with the radio
pulses likely emanating from the magnetic poles. While both light curves can be
explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out
a symmetric dipole topology since it would result in a phase lag of 1/2
(poloidal field) or zero (toroidal field). We therefore conclude that either
(i) the field is dominated by a quadrupole configuration, which can naturally
explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions
are not trivially aligned with the field. Regardless of the field topology, we
use the measured period along with the known rotation velocity (vsini=27 km/s),
and the binary orbital inclination (i=142 deg), to derive a radius for the
primary star of 0.078+/-0.010 R_sun. This is the first measurement of the
radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides
a constraint on the mass-radius relation below 0.1 M_sun. We find that the
radius is about 30% smaller than expected from theoretical models, even for an
age of a few Gyr.Comment: Submitted to Ap
On the kinematic age of brown dwarfs: Radial velocities and space motions of 43 nearby L dwarfs
We 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.Comment: 8 pages, accepted for publication in A&
Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. III. X-ray, Radio, and H-alpha Activity Trends in M and L Dwarfs
[Abridged] As part of our on-going investigation into the magnetic field
properties of ultracool dwarfs, we present simultaneous radio, X-ray, and
H-alpha observations of three M9.5-L2.5 dwarfs (BRI0021-0214,
LSR060230.4+391059, and 2MASSJ052338.2-140302). We do not detect X-ray or radio
emission from any of the three sources, despite previous detections of radio
emission from BRI0021 and 2M0523-14. Steady and variable H-alpha emission are
detected from 2M0523-14 and BRI0021, respectively, while no H-alpha emission is
detected from LSR0602+39. Overall, our survey of nine M8-L5 dwarfs doubles the
number of ultracool dwarfs observed in X-rays, and triples the number of L
dwarfs, providing in addition the deepest limits to date, log(L_X/L_bol)<-5.
With this larger sample we find the first clear evidence for a substantial
reduction in X-ray activity, by about two orders of magnitude, from mid-M to
mid-L dwarfs. We find that the decline in both X-rays and H-alpha roughly
follows L_{X,Halpha}/L_bol ~ 10^[-0.4x(SP-M6)] for SP>M6. In the radio band,
however, the luminosity remains relatively unchanged from M0 to L4, leading to
a substantial increase in L_rad/L_bol. Our survey also provides the first
comprehensive set of simultaneous radio/X-ray/H-alpha observations of ultracool
dwarfs, and reveals a clear breakdown of the radio/X-ray correlation beyond
spectral type M7, evolving smoothly from L_{\nu,rad}/L_X ~ 10^-15.5 to
~10^-11.5 Hz^-1 over the narrow spectral type range M7-M9. This breakdown
reflects the substantial reduction in X-ray activity beyond M7, but its
physical origin remains unclear since, as evidenced by the uniform radio
emission, there is no drop in the field dissipation and particle acceleration
efficiency.Comment: Submitted to ApJ; 19 pages, 10 figures, 5 table
Differential rotation of main sequence F stars
The differential rotation of a 1.2 zero age MS star (spectral type
F8) is computed and the results are compared with those from a similar model of
the Sun. The rotation pattern is determined by solving the Reynolds equation
including the convective energy transport. The latter is anisotropic due to the
Coriolis force causing a horizontal temperature gradient of ~ 7 K between the
poles and the equator. Comparison of the transport mechanisms of angular
momentum (the eddy viscosity, the -effect and the meridional flow)
shows that for the F star the -effect is the most powerful transporter
for rotation periods of 7 d or less. In the limit of very fast rotation the
-effect is balanced by the meridional flow alone and the rotation is
nearly rigid. The rotation pattern found for the F star is very similar to the
solar rotation law, but the horizontal shear is about twice the solar value. As
a function of the rotation period, the total equator-pole difference of the
angular velocity has a (slight) maximum at a period of 7 d and (slowly)
vanishes in both the limiting cases of very fast and very slow rotation. A
comparison of the solar models with those for the F-type star shows a much
stronger dependence of the differential surface rotation on the stellar
luminosity rather than on the rotation rate.Comment: 7 pages, 10 figure
Rotational Velocities of Individual Components in Very Low Mass Binaries
We present rotational velocities for individual components of 11 very low mass (VLM) binaries with spectral types between M7 and L7.5. These results are based on observations taken with the near-infrared spectrograph, NIRSPEC, and the Keck II laser guide star adaptive optics system. We find that the observed sources tend to be rapid rotators (v sin i > 10 km s^(â1)), consistent with previous seeing-limited measurements of VLM objects. The two sources with the largest v sin i, LP 349â25B and HD 130948C, are rotating at ~30% of their break-up speed, and are among the most rapidly rotating VLM objects known. Furthermore, five binary systems, all with orbital semimajor axes âŸ3.5 AU, have component v sin i values that differ by greater than 3Ï. To bring the binary components with discrepant rotational velocities into agreement would require the rotational axes to be inclined with respect to each other, and that at least one component is inclined with respect to the orbital plane. Alternatively, each component could be rotating at a different rate, even though they have similar spectral types. Both differing rotational velocities and inclinations have implications for binary star formation and evolution. We also investigate possible dynamical evolution in the triple system HD 130948AâBC. The close binary brown dwarfs B and C have significantly different v sin i values. We demonstrate that components B and C could have been torqued into misalignment by the primary star, A, via orbital precession. Such a scenario can also be applied to another triple system in our sample, GJ 569AâBab. Interactions such as these may play an important role in the dynamical evolution of VLM binaries. Finally, we note that two of the binaries with large differences in component v sin i, LP 349â25AB and 2MASS 0746+20AB, are also known radio sources
Identification of the Infrared Counterpart to a Newly Discovered X-ray Source in the Galactic Center
We present first results of a campaign to find and identify new compact
objects in the Galactic Center. Selecting candidates from a combination of
Chandra and 2MASS survey data, we search for accretion disk signatures via
infrared spectroscopy. We have found the infrared counterpart to the Chandra
source CXO J174536.1-285638, the spectrum of which has strong Br-gamma and HeI
emission. The presence of CIII, NIII, and HeII indicate a binary system. We
suspect that the system is some form of high-mass binary system, either a
high-mass X-ray binary or a colliding wind binary.Comment: 11 pages, 6 figures, ApJ accepted, 200
- âŠ