11,867 research outputs found
MIL-H-8501B: Application to shipboard terminal operations
The philosophy and structure of the proposed U.S. Military Specification for Handling Qualities Requirements for Military Rotorcraft, MIL-H-8501B, are presented with emphasis on shipboard terminal operations. The impact of current and future naval operational requirements on the selection of appropriate combinations of basic vehicle dynamics and usable cue environments are identified. An example 'walk through' of MIL-H-8501B is conducted from task identification to determination of stability and control requirements. For selected basic vehicle dynamics, criteria as a function of input/response magnitude are presented. Additionally, rotorcraft design development implications are discussed
Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry
We have simultaneously monitored the photometric and polarimetric variations
of the Classical T Tauri star AA Tau during the fall of 2002. We combine these
data with previously published polarimetric data covering two earlier epochs.
The phase coverage is complete, although not contiguous. AA Tau clearly shows
cyclic variations coupled with the rotation of the system. The star-disk system
produces a repeatable polarisation curve where the polarisation increases with
decreasing brightness. The data fit well with the model put forward by Bouvier
et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively
dumping material onto the central star via magnetospheric accretion. The inner
edge of the disk is deformed by its interaction with the tilted magnetosphere,
producing eclipses as it rotates and occults the photosphere periodically. From
the shape of the polarisation curve in the QU-Plane we confirm that the
accretion disk is seen at a large inclination, almost edge-on, and predict that
its position angle is PA~90 deg., i.e., that the disk's major axis is oriented
in the East-West direction.Comment: Astron. Astrophys., in pres
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Astrocytes expressing GFP in 3D collagen gels provide an effective model for screening the glial response to potential CNS cell therapies
Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 I. Spectropolarimetric observations in all four Stokes parameters
High-resolution spectropolarimetric observations provide simultaneous
information about stellar magnetic field topologies and three-dimensional
distributions of chemical elements. Here we present analysis of a unique full
Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap
star HD 24712. The goal of our work is to examine circular and linear
polarization signatures inside spectral lines and to study variation of the
stellar spectrum and magnetic observables as a function of rotational phase. HD
24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over
a period of 2010-2011. The resulting spectra have S/N ratio of 300-600 and
resolving power exceeding 100000. The multiline technique of least-squares
deconvolution (LSD) was applied to combine information from the spectral lines
of Fe-peak and rare-earth elements. We used the HARPSPol spectra of HD 24712 to
study the morphology of the Stokes profile shapes in individual spectral lines
and in LSD Stokes profiles corresponding to different line masks. From the LSD
Stokes V profiles we measured the longitudinal component of the magnetic field,
, with an accuracy of 5-10 G. We also determined the net linear
polarization from the LSD Stokes Q and U profiles. We determined an improved
rotational period of the star, P_rot = 12.45812 +/- 0.00019d. We measured
from the cores of Halpha and Hbeta lines. The analysis of measurements
showed no evidence for a significant radial magnetic field gradient in the
atmosphere of HD 24712. We used our and net linear polarization
measurements to determine parameters of the dipolar magnetic field topology. We
found that magnetic observables can be reasonably well reproduced by the
dipolar model. We discovered rotational modulation of the Halpha core and
related it a non-uniform surface distribution of rare-earth elements.Comment: Accepted for publication in A&
Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM513-46546
[Abridged] We present the first simultaneous radio, X-ray, ultraviolet, and
optical spectroscopic observations of the M8.5 dwarf TVLM513-46546, with a
duration of 9 hours. These observations are part of a program to study the
origin of magnetic activity in ultracool dwarfs, and its impact on
chromospheric and coronal emission. Here we detect steady quiescent radio
emission superposed with multiple short-duration, highly polarized flares;
there is no evidence for periodic bursts previously reported for this object,
indicating their transient nature. We also detect soft X-ray emission, with
L_X/L_bol~10^-4.9, the faintest to date for any object later than M5, and a
possible weak X-ray flare. TVLM513-46546 continues the trend of severe
violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4
orders of magnitude. From the optical spectroscopy we find that the Balmer line
luminosity exceeds the X-ray luminosity by a factor of a few, suggesting that,
unlike in early M dwarfs, chromospheric heating may not be due to coronal X-ray
emission. More importantly, we detect a sinusoidal H-alpha light curve with a
period of 2 hr, matching the rotation period of TVLM513-46546. This is the
first known example of such Balmer line behavior, which points to a co-rotating
chromospheric hot spot or an extended magnetic structure, with a covering
fraction of about 50%. This feature may be transitory based on the apparent
decline in light curve peak during the four observed maxima. From the radio
data we infer a large scale steady magnetic field of ~100 G, in good agreement
with the value required for confinement of the X-ray emitting plasma. The radio
flares, on the other hand, are produced in a component of the field with a
strength of ~3 kG and a likely multi-polar configuration.Comment: 13 pages, 4 figure
Gas Absorption in the KH 15D System: Further Evidence for Dust Settling in the Circumbinary Disk
Na I D lines in the spectrum of the young binary KH 15D have been analyzed in
detail. We find an excess absorption component that may be attributed to
foreground interstellar absorption, and to gas possibly associated with the
solids in the circumbinary disk. The derived column density is log N_NaI = 12.5
cm^-2, centered on a radial velocity that is consistent with the systemic
velocity. Subtracting the likely contribution of the ISM leaves log N_NaI ~
12.3 cm^-2. There is no detectable change in the gas column density across the
"knife edge" formed by the opaque grain disk, indicating that the gas and
solids have very different scale heights, with the solids being highly settled.
Our data support a picture of this circumbinary disk as being composed of a
very thin particulate grain layer composed of millimeter-sized or larger
objects that are settled within whatever remaining gas may be present. This
phase of disk evolution has been hypothesized to exist as a prelude to the
formation of planetesimals through gravitational fragmentation, and is expected
to be short-lived if much gas were still present in such a disk. Our analysis
also reveals the presence of excess Na I emission relative to the comparison
spectrum at the radial velocity of the currently visible star that plausibly
arises within the magnetosphere of this still-accreting young star.Comment: Accepted for publication in ApJ, 23 pages, 6 figure
Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani
According to our current understanding, a subclass of the upper main sequence
chemically peculiar stars, called mercury-manganese (HgMn), is non-magnetic.
Nevertheless, chemical inhomogeneities were recently discovered on their
surfaces. At the same time, no global magnetic fields stronger than 1-100 G are
detected by modern studies. The goals of our study are to search for magnetic
field in the HgMn binary system 66 Eri and to investigate chemical spots on the
stellar surfaces of both components. Our analysis is based on high quality
spectropolarimetric time-series observations obtained during 10 consecutive
nights with the HARPSpol instrument at the ESO 3.6-m telescope. To increase the
sensitivity of the magnetic field search we employed a least-squares
deconvolution (LSD). We used spectral disentangling to measure radial
velocities and study line profile variability. Chemical spot geometry was
reconstructed using multi-line Doppler imaging. We report a non-detection of
magnetic field in 66 Eri, with error bars 10-24 G for the longitudinal field.
Circular polarization profiles also do not indicate any signatures of complex
surface magnetic fields. For a simple dipolar field configuration we estimated
an upper limit of the polar field strength to be 60-70 G. For the HgMn
component we found variability in spectral lines of Ti, Ba, Y, and Sr with the
rotational period equal to the orbital one. The surface maps of these elements
reconstructed with the Doppler imaging technique, show relative underabundance
on the hemisphere facing the secondary component. The contrast of chemical
inhomogeneities ranges from 0.4 for Ti to 0.8 for Ba.Comment: 13 pages, 14 figure
The Transit Light Curve Project. IX. Evidence for a Smaller Radius of the Exoplanet XO-3b
We present photometry of 13 transits of XO-3b, a massive transiting planet on
an eccentric orbit. Previous data led to two inconsistent estimates of the
planetary radius. Our data strongly favor the smaller radius, with increased
precision: R_p = 1.217 +/- 0.073 R_Jup. A conflict remains between the mean
stellar density determined from the light curve, and the stellar surface
gravity determined from the shapes of spectral lines. We argue the light curve
should take precedence, and revise the system parameters accordingly. The
planetary radius is about 1 sigma larger than the theoretical radius for a
hydrogen-helium planet of the given mass and insolation. To help in planning
future observations, we provide refined transit and occultation ephemerides.Comment: To appear in ApJ [22 pages
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