1,614 research outputs found
Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-star
We have derived the mean broadening profile of the star V102 in the region of
the open cluster IC4665 from high resolution spectroscopy. At a projected
equatorial rotation velocity of vsini = (105 +- 12)km/s we find strong
deviation from classical rotation. We discuss several scenarios, the most
plausible being strong differential rotation in latitudinal direction. For this
scenario we find a difference in angular velocity of DeltaOmega = 3.6 +- 0.8
rad/d (DeltaOmega/Omega = 0.42 +- 0.09). From the Halpha line we derive a
spectral type of A9 and support photometric measurements classifying IC4665
V102 as a non-member of IC4665. At such early spectral type this is the
strongest case of differential rotation observed so far. Together with three
similar stars, IC4665 V102 seems to form a new class of objects that exhibit
extreme latitudinal shear in a very shallow convective envelope.Comment: accepted for A&A Letter
From solar-like to anti-solar differential rotation in cool stars
Stellar differential rotation can be separated into two main regimes:
solar-like when the equator rotates faster than the poles and anti-solar when
the polar regions rotate faster than the equator. We investigate the transition
between these two regimes with 3-D numerical simulations of rotating spherical
shells. We conduct a systematic parameter study which also includes models from
different research groups. We find that the direction of the differential
rotation is governed by the contribution of the Coriolis force in the force
balance, independently of the model setup (presence of a magnetic field,
thickness of the convective layer, density stratification). Rapidly-rotating
cases with a small Rossby number yield solar-like differential rotation, while
weakly-rotating models sustain anti-solar differential rotation. Close to the
transition, the two kinds of differential rotation are two possible bistable
states. This study provides theoretical support for the existence of anti-solar
differential rotation in cool stars with large Rossby numbers.Comment: 5 pages, 6 figures, accepted for publication in MNRA
Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method
The detection of planets around very low-mass stars with the radial velocity
method is hampered by the fact that these stars are very faint at optical
wavelengths where the most high-precision spectrometers operate. We investigate
the precision that can be achieved in radial velocity measurements of low mass
stars in the near infrared (nIR) Y-, J-, and H-bands, and we compare it to the
precision achievable in the optical. For early-M stars, radial velocity
measurements in the nIR offer no or only marginal advantage in comparison to
optical measurements. Although they emit more flux in the nIR, the richness of
spectral features in the optical outweighs the flux difference. We find that
nIR measurement can be as precise than optical measurements in stars of
spectral type ~M4, and from there the nIR gains in precision towards cooler
objects. We studied potential calibration strategies in the nIR finding that a
stable spectrograph with a ThAr calibration can offer enough wavelength
stability for m/s precision. Furthermore, we simulate the wavelength-dependent
influence of activity (cool spots) on radial velocity measurements from optical
to nIR wavelengths. Our spot simulations reveal that the radial velocity jitter
does not decrease as dramatically towards longer wavelengths as often thought.
The jitter strongly depends on the details of the spots, i.e., on spot
temperature and the spectral appearance of the spot. Forthcoming nIR
spectrographs will allow the search for planets with a particular advantage in
mid- and late-M stars. Activity will remain an issue, but simultaneous
observations at optical and nIR wavelengths can provide strong constraints on
spot properties in active stars.Comment: accepted by ApJ, v2 accepted revision with new precision
calculations, abstract abride
What controls the large-scale magnetic fields of M dwarfs?
Observations of active M dwarfs show a broad variety of large-scale magnetic
fields encompassing dipole-dominated and multipolar geometries. We detail the
analogy between some anelastic dynamo simulations and spectropolarimetric
observations of 23 M stars. In numerical models, the relative contribution of
inertia and Coriolis force in the global force balance -estimated by the
so-called local Rossby number- is known to have a strong impact on the magnetic
field geometry. We discuss the relevance of this parameter in setting the
large-scale magnetic field of M dwarfs.Comment: 4 pages, 3 figures, conference proceeding, IAUS 302 'Magnetic Fields
Throughout the Stellar Evolution', (26-30 Aug 2013, Biarritz, France
Differential rotation in rapidly rotating F-stars
We obtained high quality spectra of 135 stars of spectral types F and later
and derived ``overall'' broadening functions in selected wavelength regions
utilizing a Least Squares Deconvolution (LSD) procedure. Precision values of
the projected rotational velocity were derived from the first zero
of the Fourier transformed profiles and the shapes of the profiles were
analyzed for effects of differential rotation. The broadening profiles of 70
stars rotating faster than km s show no indications of multiplicity nor of spottedness. In
those profiles we used the ratio of the first two zeros of the Fourier
transform to search for deviations from rigid rotation. In the vast
majority the profiles were found to be consistent with rigid rotation. Five
stars were found to have flat profiles probably due to cool polar caps, in
three stars cuspy profiles were found. Two out of those three cases may be due
to extremely rapid rotation seen pole on, only in one case ( km
s) solar-like differential rotation is the most plausible explanation
for the observed profile. These results indicate that the strength of
differential rotation diminishes in stars rotating as rapidly as v \sin{i} \ga
50 km s.Comment: 10 pages, accepted for publication in A&
What controls the magnetic geometry of M dwarfs?
Context: observations of rapidly rotating M dwarfs show a broad variety of
large-scale magnetic fields encompassing dipole-dominated and multipolar
geometries. In dynamo models, the relative importance of inertia in the force
balance -- quantified by the local Rossby number -- is known to have a strong
impact on the magnetic field geometry. Aims: we aim to assess the relevance of
the local Rossby number in controlling the large-scale magnetic field geometry
of M dwarfs. Methods: we explore the similarities between anelastic dynamo
models in spherical shells and observations of active M-dwarfs, focusing on
field geometries derived from spectropolarimetric studies. To do so, we
construct observation-based quantities aimed to reflect the diagnostic
parameters employed in numerical models. Results: the transition between
dipole-dominated and multipolar large-scale fields in early to mid M dwarfs is
tentatively attributed to a Rossby number threshold. We interpret late M dwarfs
magnetism to result from a dynamo bistability occurring at low Rossby number.
By analogy with numerical models, we expect different amplitudes of
differential rotation on the two dynamo branches.Comment: 4 pages, 4 figures, accepted for publication in A&
Multi-wavelength observations of Proxima Centauri
We report simultaneous observations of the nearby flare star Proxima Centauri
with VLT/UVES and XMM-Newton over three nights in March 2009. Our optical and
X-ray observations cover the star's quiescent state, as well as its flaring
activity and allow us to probe the stellar atmospheric conditions from the
photosphere into the chromosphere, and then the corona during its different
activity stages. Using the X-ray data, we investigate variations in coronal
densities and abundances and infer loop properties for an intermediate-sized
flare. The optical data are used to investigate the magnetic field and its
possible variability, to construct an emission line list for the chromosphere,
and use certain emission lines to construct physical models of Proxima
Centauri's chromosphere.
We report the discovery of a weak optical forbidden Fe xiii line at 3388 AA
during the more active states of Proxima Centauri. For the intermediate flare,
we find two secondary flare events that may originate in neighbouring loops,
and discuss the line asymmetries observed during this flare in H i, He i, and
Ca ii lines. The high time-resolution in the H alpha line highlights strong
temporal variations in the observed line asymmetries, which re-appear during a
secondary flare event. We also present theoretical modelling with the stellar
atmosphere code PHOENIX to construct flaring chromospheric models.Comment: 19 pages, 22 figures, accepted by A&
On a transition from solar-like coronae to rotation-dominated jovian-like magnetospheres in ultracool main-sequence stars
For main-sequence stars beyond spectral type M5 the characteristics of
magnetic activity common to warmer solar-like stars change into the brown-dwarf
domain: the surface magnetic field becomes more dipolar and the evolution of
the field patterns slows, the photospheric plasma is increasingly neutral and
decoupled from the magnetic field, chromospheric and coronal emissions weaken
markedly, and the efficiency of rotational braking rapidly decreases. Yet,
radio emission persists, and has been argued to be dominated by
electron-cyclotron maser emission instead of the gyrosynchrotron emission from
warmer stars. These properties may signal a transition in the stellar extended
atmosphere. Stars warmer than about M5 have a solar-like corona and
wind-sustained heliosphere in which the atmospheric activity is powered by
convective motions that move the magnetic field. Stars cooler than early-L, in
contrast, may have a jovian-like rotation-dominated magnetosphere powered by
the star's rotation in a scaled-up analog of the magnetospheres of Jupiter and
Saturn. A dimensional scaling relationship for rotation-dominated
magnetospheres by Fan et al. (1982) is consistent with this hypothesis
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
Geometry Technology Module (GTM). Volume 1: Engineering description and utilization manual
The geometry technology module (GTM) is described as a system of computerized elements residing in the engineering design integration system library developed for the generation, manipulation, display, computation of mass properties, and data base management of panelled geometry. The GTM is composed of computer programs and associated data for performing configuration analysis on geometric shapes. The program can be operated in batch or demand mode and is designed for interactive use
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