809 research outputs found
Solar-cycle variation of the sound-speed asphericity from GONG and MDI data 1995-2000
We study the variation of the frequency splitting coefficients describing the
solar asphericity in both GONG and MDI data, and use these data to investigate
temporal sound-speed variations as a function of both depth and latitude during
the period from 1995-2000 and a little beyond. The temporal variations in even
splitting coefficients are found to be correlated to the corresponding
component of magnetic flux at the solar surface. We confirm that the
sound-speed variations associated with the surface magnetic field are
superficial. Temporally averaged results show a significant excess in sound
speed around 0.92 solar radii and latitude of 60 degrees.Comment: To be published in MNRAS, accepted July 200
Deeply penetrating banded zonal flows in the solar convection zone
Helioseismic observations have detected small temporal variations of the
rotation rate below the solar surface corresponding to the so-called `torsional
oscillations' known from Doppler measurements of the surface. These appear as
bands of slower and faster than average rotation moving equatorward. Here we
establish, using complementary helioseismic observations over four years from
the GONG network and from the MDI instrument on board SOHO, that the banded
flows are not merely a near-surface phenomenon: rather they extend downward at
least 60 Mm (some 8% of the total solar radius) and thus are evident over a
significant fraction of the nearly 200 Mm depth of the solar convection zone.Comment: 4 pages, 4 figures To be published in ApJ Letters (accepted 3/3/2000
Does the Sun Shrink with Increasing Magnetic Activity?
We have analyzed the full set of SOHO/MDI f- and p-mode oscillation
frequencies from 1996 to date in a search for evidence of solar radius
evolution during the rising phase of the current activity cycle. Like Antia et
al. (2000), we find that a significant fraction of the f-mode frequency changes
scale with frequency; and that if these are interpreted in terms of a radius
change, it implies a shrinking sun. Our inferred rate of shrinkage is about 1.5
km/y, which is somewhat smaller than found by Antia et al. We argue that this
rate does not refer to the surface, but rather to a layer extending roughly
from 4 to 8 Mm beneath the visible surface. The rate of shrinking may be
accounted for by an increasing radial component of the rms random magnetic
field at a rate that depends on its radial distribution. If it were uniform,
the required field would be ~7 kG. However, if it were inwardly increasing,
then a 1 kG field at 8 Mm would suffice.
To assess contribution to the solar radius change arising above 4Mm, we
analyzed the p-mode data. The evolution of the p-mode frequencies may be
explained by a magnetic^M field growing with activity. The implications of the
near-surface magnetic field changes depend on the anisotropy of the random
magnetic field. If the field change is predominantly radial, then we infer an
additional shrinking at a rate between 1.1-1.3 km/y at the photosphere. If on
the other hand the increase is isotropic, we find a competing expansion at a
rate of 2.3 km/y. In any case, variations in the sun's radius in the activity
cycle are at the level of 10^{-5} or less, hence have a negligible contribution
to the irradiance variations.Comment: 10 pages (ApJ preprint style), 4 figures; accepted for publication in
Ap
Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits
The Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric
Observatory observed the transits of Mercury on 2003 May 7 and 2006 November 8.
Contact times between Mercury and the solar limb have been used since the 17th
century to derive the Sun's size but this is the first time that high-quality
imagery from space, above the Earth's atmosphere, has been available. Unlike
other measurements this technique is largely independent of optical distortion.
The true solar radius is still a matter of debate in the literature as measured
differences of several tenths of an arcsecond (i.e., about 500 km) are
apparent. This is due mainly to systematic errors from different instruments
and observers since the claimed uncertainties for a single instrument are
typically an order of magnitude smaller. From the MDI transit data we find the
solar radius to be 960".12 +/- 0".09 (696,342 +/- 65 km). This value is
consistent between the transits and consistent between different MDI focus
settings after accounting for systematic effects.Comment: Accepted for publication in The Astrophysical Journal (2012 March 5
A note on the torsional oscillation at solar minimim
We examine the evolution of the zonal flow pattern in the upper solar convection zone during the current extended solar minimum, and compare it with that during the previous minimum. The results suggest that a configuration matching that at the previous minimum was reached during 2008, but that the flow band corresponding to the new cycle has been moving more slowly toward the equator than was observed in the previous cycle, resulting in a gradual increase in the apparent length of the cycle during the 2007-2008 period. The current position of the lower-latitude fast-rotating belt corresponds to that seen around the onset of activity in the previous cycle
TWIST1 Is Expressed in Colorectal Carcinomas and Predicts Patient Survival
TWIST1 is a transcription factor that belongs to the family of basic helix-loop-helix proteins involved in epithelial-to-mesenchymal transition and invasion processes. The TWIST1 protein possesses oncogenic, drug-resistant, angiogenic and invasive properties, and has been related with several human tumors and other pathologies. Colorectal cancer is one of the tumors in which TWIST1 is over-expressed, but its involvement in the clinical outcome of the disease is still unclear. We tested, by RT-PCR, the expression levels of TWIST1 in normal and tumor paired-sample tissues from a series of 151 colorectal cancer patients, in order to investigate its prognostic value as a tumor marker. TWIST1 expression was restricted to tumor tissues (86.1%) and correlated with lymph node metastasis (LNM). Adjusted analysis showed that the expression levels of TWIST1 correlated with overall survival (OS) and disease-free survival (DFS). Importantly, TWIST1 expression levels predicted OS specifically at stages I and II. Moreover, patients with stage II tumors and high TWIST1 levels showed even shorter survival than patients with stage III tumors. These results suggest that TWIST1 expression levels could be a tumor indicator in stage II patients and help select patients at greater risk of poor prognosis who might benefit from adjuvant chemotherapy
Studying asphericity in the solar sound speed from MDI and GONG data
We study the variation of the frequency splitting coefficients describing the solar asphericity in both GONG and MDI data, and use these data to investigate temporal sound-speed variations as a function of both depth and latitude during the period 1995-2000. The temporal variations in even splitting coefficients are found to be correlated with the corresponding component of magnetic flux at the solar surface. The sound-speed variations associated with the surface magnetic field appear to be superficial. Temporally averaged results show a significant excess in sound speed around r=0.92 R⊙ and latitude of 60°
Rotation profiles of solar-like stars with magnetic fields
The aim of this work is to investigate rotation profile of solar-like stars
with magnetic fields. A diffusion coefficient of magnetic angular momentum
transport is deduced. Rotating stellar models with different mass are computed
under the effect of the coefficient. Then rotation profiles are obtained from
the theoretical stellar models. The total angular momentum of solar model with
only hydrodynamic instabilities is about 13 times larger than that of the Sun
at the age of the Sun, and this model can not reproduce quasi-solid rotation in
the radiative region. However, not only can the solar model with magnetic
fields reproduce an almost uniform rotation in the radiative region, but its
total angular momentum is consistent with helioseismic result at the level of 3
at the age of the Sun. The rotation of solar-like stars with magnetic
fields is almost uniform in the radiative region. But there is an obvious
transition region of angular velocity between the convective core and the
radiative region of models with 1.2 - 1.5 , where angular velocity
has a sharp radial change, which is different from the rotation profile of the
Sun and massive stars with magnetic fields. Moreover the changes of the angular
velocity in the transition region increase with the increasing in the age and
mass.Comment: Accepted for publication in ChjA
Seismic evidence for a rapidly rotating core in a lower-giant-branch star observed with Kepler
Rotation is expected to have an important influence on the structure and the
evolution of stars. However, the mechanisms of angular momentum transport in
stars remain theoretically uncertain and very complex to take into account in
stellar models. To achieve a better understanding of these processes, we
desperately need observational constraints on the internal rotation of stars,
which until very recently were restricted to the Sun. In this paper, we report
the detection of mixed modes - i.e. modes that behave both as g modes in the
core and as p modes in the envelope - in the spectrum of the early red giant
KIC7341231, which was observed during one year with the Kepler spacecraft. By
performing an analysis of the oscillation spectrum of the star, we show that
its non-radial modes are clearly split by stellar rotation and we are able to
determine precisely the rotational splittings of 18 modes. We then find a
stellar model that reproduces very well the observed atmospheric and seismic
properties of the star. We use this model to perform inversions of the internal
rotation profile of the star, which enables us to show that the core of the
star is rotating at least five times faster than the envelope. This will shed
new light on the processes of transport of angular momentum in stars. In
particular, this result can be used to place constraints on the angular
momentum coupling between the core and the envelope of early red giants, which
could help us discriminate between the theories that have been proposed over
the last decades.Comment: Accepted in ApJ, 39 pages, 16 figure
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