9,740 research outputs found
Temporal fluctuations in the differential rotation of cool active stars
This paper reports positive detections of surface differential rotation on
two rapidly rotating cool stars at several epochs, by using stellar surface
features (both cool spots and magnetic regions) as tracers of the large scale
latitudinal shear that distorts the convective envelope in this type of stars.
We also report definite evidence that this differential rotation is different
when estimated from cool spots or magnetic regions, and that it undergoes
temporal fluctuations of potentially large amplitude on a time scale of a few
years. We consider these results as further evidence that the dynamo processes
operating in these stars are distributed throughout the convective zone rather
than being confined at its base as in the Sun. By comparing our observations
with two very simple models of the differential rotation within the convective
zone, we obtain evidence that the internal rotation velocity field of the stars
we investigated is not like that of the Sun, and may resemble that we expect
for rapid rotators. We speculate that the changes in differential rotation
result from the dynamo processes (and from the underlying magnetic cycle) that
periodically converts magnetic energy into kinetic energy and vice versa. We
emphasise that the technique outlined in this paper corresponds to the first
practical method for investigating the large scale rotation velocity field
within convective zones of cool active stars, and offers several advantages
over asteroseismology for this particular purpose and this specific stellar
class.Comment: 14 pages, 4 figure
The evolution of surface magnetic fields in young solar-type stars
The surface rotation rates of young solar-type stars decrease rapidly with
age from the end of the pre-main sequence though the early main sequence. This
suggests that there is also an important change in the dynamos operating in
these stars, which should be observable in their surface magnetic fields. Here
we present early results in a study aimed at observing the evolution of these
magnetic fields through this critical time period. We are observing stars in
open clusters and stellar associations to provide precise ages, and using
Zeeman Doppler Imaging to characterize the complex magnetic fields. Presented
here are results for six stars, three in the in the beta Pic association (~10
Myr old) and three in the AB Dor association (~100 Myr old).Comment: To appear in the proceedings of IAU symposium 302: Magnetic fields
throughout stellar evolution. 2 pages, 3 figure
Transient behavior of surface plasmon polaritons scattered at a subwavelength groove
We present a numerical study and analytical model of the optical near-field
diffracted in the vicinity of subwavelength grooves milled in silver surfaces.
The Green's tensor approach permits computation of the phase and amplitude
dependence of the diffracted wave as a function of the groove geometry. It is
shown that the field diffracted along the interface by the groove is equivalent
to replacing the groove by an oscillating dipolar line source. An analytic
expression is derived from the Green's function formalism, that reproduces well
the asymptotic surface plasmon polariton (SPP) wave as well as the transient
surface wave in the near-zone close to the groove. The agreement between this
model and the full simulation is very good, showing that the transient
"near-zone" regime does not depend on the precise shape of the groove. Finally,
it is shown that a composite diffractive evanescent wave model that includes
the asymptotic SPP can describe the wavelength evolution in this transient
near-zone. Such a semi-analytical model may be useful for the design and
optimization of more elaborate photonic circuits whose behavior in large part
will be controlled by surface waves.Comment: 12 pages, 10 figure
Termination of planetary accretion due to gap formation
The process of gap formation by a growing planetary embryo embedded in a
planetesimal disk is considered. It is shown that there exists a single
parameter characterizing this process, which represents the competition between
the gravitational influence of the embryo and planetesimal-planetesimal
scattering. For realistic assumptions about the properties of the planetesimal
disk and the planetary embryo, a gap is opened long before the embryo can
accrete all the bodies within its region of influence. The implication of this
result is that the embryo stops growing and, thus, large bodies formed during
the coagulation stage should be less massive than is usually assumed. For
conditions expected at 1 AU in the solar protoplanetary disk, gap formation is
expected to occur around bodies of mass < 10^24 g. The effect of protoplanetary
radial migration is also discussed.Comment: 21 pages, 3 figures, submitted to A
Simulated CII observations for SPICA/SAFARI
We investigate the case of CII 158 micron observations for SPICA/SAFARI using
a three-dimensional magnetohydrodynamical (MHD) simulation of the diffuse
interstellar medium (ISM) and the Meudon PDR code. The MHD simulation consists
of two converging flows of warm gas (10,000 K) within a cubic box 50 pc in
length. The interplay of thermal instability, magnetic field and self-gravity
leads to the formation of cold, dense clumps within a warm, turbulent
interclump medium. We sample several clumps along a line of sight through the
simulated cube and use them as input density profiles in the Meudon PDR code.
This allows us to derive intensity predictions for the CII 158 micron line and
provide time estimates for the mapping of a given sky area.Comment: 4 pages, 5 figures, to appear in the proceedings of the workshop "The
Space Infrared Telescope for Cosmology & Astrophysics: Revealing the Origins
of Planets and Galaxies" (July 2009, Oxford, United Kingdom
A MiMeS analysis of the magnetic field and circumstellar environment of the weak-wind O9 sub-giant star HD 57682
I will review our recent analysis of the magnetic properties of the O9IV star
HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the
Canada-France-Hawaii telescope within the context of the Magnetism in Massive
Stars (MiMeS) Large Program. I discuss our most recent determination of the
rotational period from longitudinal magnetic field measurements and Halpha
variability - the latter obtained from over a decade's worth of professional
and amateur spectroscopic observations. Lastly, I will report on our
investigation of the magnetic field geometry and the effects of the field on
the circumstellar environment.Comment: 2 pages, 2 figures, IAUS272 - Active OB Stars: Structure, Evolution,
Mass Loss and Critical Limit
SPIRE-FTS observations of RCW 120
The expansion of Galactic HII regions can trigger the formation of a new
generation of stars. However, little is know about the physical conditions that
prevail in these regions. We study the physical conditions that prevail in
specific zones towards expanding HII regions that trace representative media
such as the photodissociation region, the ionized region, and condensations
with and without ongoing star formation. We use the SPIRE Fourier Transform
Spectrometer (FTS) on board to observe the HII region RCW 120.
Continuum and lines are observed in the m range. Line intensities
and line ratios are obtained and used as physical diagnostics of the gas. We
used the Meudon PDR code and the RADEX code to derive the gas density and the
radiation field at nine distinct positions including the PDR surface and
regions with and without star-formation activity. For the different regions we
detect the atomic lines [NII] at m and [CI] at and m,
the ladder between the and levels and the
ladder between the and levels, as well as CH in absorption. We find gas temperatures in the range K for
densities of , and a high column density on the order
of that is in agreement with dust
analysis. The ubiquitousness of the atomic and CH emission suggests the
presence of a low-density PDR throughout RCW 120. High-excitation lines of CO
indicate the presence of irradiated dense structures or small dense clumps
containing young stellar objects, while we also find a less dense medium
() with high temperatures (K).Comment: 11 pages, 11 figures, accepted by A&
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