171 research outputs found
Space suit
A pressure suit for high altitude flights, particularly space missions is reported. The suit is designed for astronauts in the Apollo space program and may be worn both inside and outside a space vehicle, as well as on the lunar surface. It comprises an integrated assembly of inner comfort liner, intermediate pressure garment, and outer thermal protective garment with removable helmet, and gloves. The pressure garment comprises an inner convoluted sealing bladder and outer fabric restraint to which are attached a plurality of cable restraint assemblies. It provides versitility in combination with improved sealing and increased mobility for internal pressures suitable for life support in the near vacuum of outer space
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
First-light LBT nulling interferometric observations: warm exozodiacal dust resolved within a few AU of eta Corvi
We report on the first nulling interferometric observations with the Large
Binocular Telescope Interferometer (LBTI), resolving the N' band (9.81 - 12.41
um) emission around the nearby main-sequence star eta Crv (F2V, 1-2 Gyr). The
measured source null depth amounts to 4.40% +/- 0.35% over a field-of-view of
140 mas in radius (~2.6\,AU at the distance of eta Corvi) and shows no
significant variation over 35{\deg} of sky rotation. This relatively low null
is unexpected given the total disk to star flux ratio measured by Spitzer/IRS
(~23% across the N' band), suggesting that a significant fraction of the dust
lies within the central nulled response of the LBTI (79 mas or 1.4 AU).
Modeling of the warm disk shows that it cannot resemble a scaled version of the
Solar zodiacal cloud, unless it is almost perpendicular to the outer disk
imaged by Herschel. It is more likely that the inner and outer disks are
coplanar and the warm dust is located at a distance of 0.5-1.0 AU,
significantly closer than previously predicted by models of the IRS spectrum
(~3 AU). The predicted disk sizes can be reconciled if the warm disk is not
centrosymmetric, or if the dust particles are dominated by very small grains.
Both possibilities hint that a recent collision has produced much of the dust.
Finally, we discuss the implications for the presence of dust at the distance
where the insolation is the same as Earth's (2.3 AU).Comment: 9 pages, 6 figures, accepted for publication in Ap
The HOSTS Survey for Exozodiacal Dust: Preliminary results and future prospects
[abridged] The presence of large amounts of dust in the habitable zones of
nearby stars is a significant obstacle for future exo-Earth imaging missions.
We executed an N band nulling interferometric survey to determine the typical
amount of such exozodiacal dust around a sample of nearby main sequence stars.
The majority of our data have been analyzed and we present here an update of
our ongoing work. We find seven new N band excesses in addition to the high
confidence confirmation of three that were previously known. We find the first
detections around Sun-like stars and around stars without previously known
circumstellar dust. Our overall detection rate is 23%. The inferred occurrence
rate is comparable for early type and Sun-like stars, but decreases from 71%
[+11%/-20%] for stars with previously detected mid- to far-infrared excess to
11% [+9%/-4%] for stars without such excess, confirming earlier results at high
confidence. For completed observations on individual stars, our sensitivity is
five to ten times better than previous results. Assuming a lognormal luminosity
function of the dust, we find upper limits on the median dust level around all
stars without previously known mid to far infrared excess of 11.5 zodis at 95%
confidence level. The corresponding upper limit for Sun-like stars is 16 zodis.
An LBTI vetted target list of Sun-like stars for exo-Earth imaging would have a
corresponding limit of 7.5 zodis. We provide important new insights into the
occurrence rate and typical levels of habitable zone dust around main sequence
stars. Exploiting the full range of capabilities of the LBTI provides a
critical opportunity for the detailed characterization of a sample of
exozodiacal dust disks to understand the origin, distribution, and properties
of the dust.Comment: To appear in SPIE Astronomical Telescopes + Instrumentation 2018
proceedings. Some typos fixed, one reference adde
The discontinuous nature of chromospheric activity evolution
Chromospheric activity has been thought to decay smoothly with time and,
hence, to be a viable age indicator. Measurements in solar type stars in open
clusters seem to point to a different conclusion: chromospheric activity
undergoes a fast transition from Hyades level to that of the Sun after about 1
Gyr of main--sequence lifetime and any decaying trend before or after this
transition must be much less significant than the short term variations.Comment: 6 pages, 1 figure, to be published in Astrophysics and Space Scienc
Cyclic competition of four species: domains and interfaces
We study numerically domain growth and interface fluctuations in one- and
two-dimensional lattice systems composed of four species that interact in a
cyclic way. Particle mobility is implemented through exchanges of particles
located on neighboring lattice sites. For the chain we find that domain growth
strongly depends on the mobility, with a higher mobility yielding a larger
domain growth exponent. In two space dimensions, when also exchanges between
mutually neutral particles are possible, both domain growth and interface
fluctuations display universal regimes that are independent of the predation
and exchange rates.Comment: 14 pages, 7 figures, version accepted for publication in J. Stat.
Mec
Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars
A knowledge of stellar ages is crucial for our understanding of many
astrophysical phenomena, and yet ages can be difficult to determine. As they
become older, stars lose mass and angular momentum, resulting in an observed
slowdown in surface rotation. The technique of 'gyrochronology' uses the
rotation period of a star to calculate its age. However, stars of known age
must be used for calibration, and, until recently, the approach was untested
for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for
stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for
old field stars whose ages have been determined with asteroseismology. The data
for the cluster agree with previous period-age relations, but these relations
fail to describe the asteroseismic sample. Here we report stellar evolutionary
modelling, and confirm the presence of unexpectedly rapid rotation in stars
that are more evolved than the Sun. We demonstrate that models that incorporate
dramatically weakened magnetic braking for old stars can---unlike existing
models---reproduce both the asteroseismic and the cluster data. Our findings
might suggest a fundamental change in the nature of ageing stellar dynamos,
with the Sun being close to the critical transition to much weaker magnetized
winds. This weakened braking limits the diagnostic power of gyrochronology for
those stars that are more than halfway through their main-sequence lifetimes.Comment: 25 pages, 3 figures in main paper, 6 extended data figures, 1 table.
Published in Nature, January 2016. Please see https://youtu.be/O6HzYgP5uyc
for a video description of the resul
ARGOS: the laser guide star system for the LBT
ARGOS is the Laser Guide Star adaptive optics system for the Large Binocular Telescope. Aiming for a wide field adaptive optics correction, ARGOS will equip both sides of LBT with a multi laser beacon system and corresponding wavefront sensors, driving LBT's adaptive secondary mirrors. Utilizing high power pulsed green lasers the artificial beacons are generated via Rayleigh scattering in earth's atmosphere. ARGOS will project a set of three guide stars above each of LBT's mirrors in a wide constellation. The returning scattered light, sensitive particular to the turbulence close to ground, is detected in a gated wavefront sensor system. Measuring and correcting the ground layers of the optical distortions enables ARGOS to achieve a correction over a very wide field of view. Taking advantage of this wide field correction, the science that can be done with the multi object spectrographs LUCIFER will be boosted by higher spatial resolution and strongly enhanced flux for spectroscopy. Apart from the wide field correction ARGOS delivers in its ground layer mode, we foresee a diffraction limited operation with a hybrid Sodium laser Rayleigh beacon combination.12 page(s
An investigation of chromospheric activity spanning the Vaughan--Preston gap: impact on stellar ages
Chromospheric activity is widely used as an age indicator for solar-type
stars based on the early evidence that there is a smooth evolution from young
and active to old and inactive stars. We analysed chromospheric activity in
five solar-type stars in two open clusters, in order to study how chromospheric
activity evolves with time. We took UVES high-resolution, high S/N ratio
spectra of 3 stars in IC 4756 and 2 in NGC 5822, which were combined with a
previously studied data-set and reanalysed here. The emission core of the deep,
photospheric Ca II K line was used as a probe of the chromospheric activity.
All of the 5 stars in the new sample, including those in the 1.2 Gyr-old NGC
5822, have activity levels comparable to those of Hyades and Praesepe. A likely
interpretation of our data is that solar-type-star chromospheric activity, from
the age of the Hyades until that of the Sun, does not evolve smoothly. Stars
change from active to inactive on a short timescale. Evolution before and after
such a transition is much less significant than cyclical and long-term
variations. We show that data presented in the literature to support a
correlation between age and activity could be also interpreted differently in
the light of our results.Comment: 5 pages, 2 Figures, 1 Table, published in A&A
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