211 research outputs found
Transition from a supersonic to a subsonic solar wind
The transition from a supersonic to a subsonic corona was investigated by increasing the density N sub O at the base of the corona while keeping the temperature T sub O constant. For the initial values of N sub O and T sub O, the solution of the inviscid solar wind equations is of the Parker type. As N sub O is increased, Parker type supersonic solutions cease to exist. No subsonic solutions exist
Equilibrium configurations between density and topographic surface irregularities in a purely elastic earth model technical report no. 2
Equilibrium configurations between density and topographic surface irregularities in two- dimensional purely elastic earth mode
Large scale flows in the solar interior: Effect of asymmetry in peak profiles
Ring diagram analysis can be used to study large scale velocity fields in the
outer part of the solar convection zone. All previous works assume that the
peak profiles in the solar oscillation power spectrum are symmetric. However,
it has now been demonstrated that the peaks are not symmetric. In this work we
study how the explicit use of asymmetric peak profiles in ring-diagram analysis
influences the estimated velocity fields. We find that the use of asymmetric
profiles leads to significant improvement in the fits, but the estimated
velocity fields are not substantially different from those obtained using a
symmetric profile to fit the peaks. The resulting velocity fields are compared
with those obtained by other investigators.Comment: To appear in Ap
X-ray detection of the substellar twin 2MASS J11011926-7732383 AB
2MASS J11011926-7732383 AB (hereafter 2M1101AB), located in the Cha I star
forming region, is a rare wide-separation brown dwarf binary. XMM-Newton and
Chandra observations of 2M1101AB have allowed us to examine the influence of
physical parameters (mass, bolometric luminosity and effective temperature) on
X-ray emission from a coeval pair of substellar objects. The spatial resolution
of XMM-Newton is not sufficient to separate contributions from the two
components in the binary. The X-ray source detected with XMM-Newton has a
column density compatible with the infrared extinction of component A. On the
other hand, the binary is resolved with Chandra, and the bulk of the X-ray
emission is clearly associated with the photospherically cooler component B.
These apparently contradictory results point at strong variability of 2M1101's
X-ray emission. Combined with previous sensitive X-ray observations from
low-mass members of ChaI, we find a decline of X-ray luminosity with decreasing
(sub)stellar mass that is typical for star forming regions. 2M1101B is the
coolest (spectral type M8.25) and least massive brown dwarf of ChaI detected in
X-rays so far. It is also among the youngest (~1 Myr) substellar ChaI members,
and therefore relatively luminous. Most bona fide brown dwarfs of Cha I have
remained below the sensitivity limits of available X-ray observations, because
of their low luminosity associated with higher age.Comment: accepted for publication in Astronomy & Astrophysic
Simultaneous Water Vapor and Dry Air Optical Path Length Measurements and Compensation with the Large Binocular Telescope Interferometer
The Large Binocular Telescope Interferometer uses a near-infrared camera to
measure the optical path length variations between the two AO-corrected
apertures and provide high-angular resolution observations for all its science
channels (1.5-13 m). There is however a wavelength dependent component to
the atmospheric turbulence, which can introduce optical path length errors when
observing at a wavelength different from that of the fringe sensing camera.
Water vapor in particular is highly dispersive and its effect must be taken
into account for high-precision infrared interferometric observations as
described previously for VLTI/MIDI or the Keck Interferometer Nuller. In this
paper, we describe the new sensing approach that has been developed at the LBT
to measure and monitor the optical path length fluctuations due to dry air and
water vapor separately. After reviewing the current performance of the system
for dry air seeing compensation, we present simultaneous H-, K-, and N-band
observations that illustrate the feasibility of our feedforward approach to
stabilize the path length fluctuations seen by the LBTI nuller.Comment: SPIE conference proceeding
Exoplanet science with the LBTI: instrument status and plans
The Large Binocular Telescope Interferometer (LBTI) is a strategic instrument
of the LBT designed for high-sensitivity, high-contrast, and high-resolution
infrared (1.5-13 m) imaging of nearby planetary systems. To carry out a
wide range of high-spatial resolution observations, it can combine the two
AO-corrected 8.4-m apertures of the LBT in various ways including direct
(non-interferometric) imaging, coronagraphy (APP and AGPM), Fizeau imaging,
non-redundant aperture masking, and nulling interferometry. It also has
broadband, narrowband, and spectrally dispersed capabilities. In this paper, we
review the performance of these modes in terms of exoplanet science
capabilities and describe recent instrumental milestones such as first-light
Fizeau images (with the angular resolution of an equivalent 22.8-m telescope)
and deep interferometric nulling observations.Comment: 12 pages, 6 figures, Proc. SPI
L'-band AGPM vector vortex coronagraph's first light on LBTI/LMIRCam
We present the first observations obtained with the L'-band AGPM vortex
coronagraph recently installed on LBTI/LMIRCam. The AGPM (Annular Groove Phase
Mask) is a vector vortex coronagraph made from diamond subwavelength gratings.
It is designed to improve the sensitivity and dynamic range of high-resolution
imaging at very small inner working angles, down to 0.09 arcseconds in the case
of LBTI/LMIRCam in the L' band. During the first hours on sky, we observed the
young A5V star HR\,8799 with the goal to demonstrate the AGPM performance and
assess its relevance for the ongoing LBTI planet survey (LEECH). Preliminary
analyses of the data reveal the four known planets clearly at high SNR and
provide unprecedented sensitivity limits in the inner planetary system (down to
the diffraction limit of 0.09 arcseconds).Comment: 9 pages, 4 figures, SPIE proceeding
Solar dynamo model with nonlocal alpha-effect
The first results of the solar dynamo model that allows for the diamagnetic
effect of inhomogeneous turbulence and the nonlocal alpha-effect due to the
rise of magnetic loops are discussed. The nonlocal alpha-effect is not subject
to the catastrophic quenching related to the conservation of magnetic helicity.
Given the diamagnetic pumping, the magnetic fields are concentrated near the
base of the convection zone, although the distributed-type model covers the
entire thickness of the convection zone. The magnetic cycle period, the
equatorial symmetry of the field, its meridional drift, and the
polar-to-toroidal field ratio obtained in the model are in agreement with
observations. There is also some disagreement with observations pointing the
ways of improving the model.Comment: To appear in Astronomy Letters, 10 pages, 5 figure
EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars
We observe the slowly-rotating, active, single giant, EK Eri, to study and
infer the nature of its magnetic field directly. We used the spectropolarimeter
NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least
Square Deconvolution method to create high signal-to-noise ratio Stokes V
profiles. We fitted the Stokes V profiles with a model of the large-scale
magnetic field. We studied the classical activity indicators, the CaII H and K
lines, the CaII infrared triplet, and H\alpha line. We detected the Stokes V
signal of EK Eri securely and measured the longitudinal magnetic field Bl for
seven individual dates spanning 60% of the rotational period. The measured
longitudinal magnetic field of EK Eri reached about 100 G and was as strong as
fields observed in RSCVn or FK Com type stars: this was found to be
extraordinary when compared with the weak fields observed at the surfaces of
slowly-rotating MS stars or any single red giant previously observed with
NARVAL. From our modeling, we infer that the mean surface magnetic field is
about 270 G, and that the large scale magnetic field is dominated by a poloidal
component. This is compatible with expectations for the descendant of a
strongly magnetic Ap star.Comment: 8 pages, 6 figures. Accepted for publication in A&
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