2,513 research outputs found
A compilation and analysis of typical approach and landing data for a simulator study of an externally blown flap STOL aircraft
A piloted simulation study has been made of typical landing approaches with an externally blown flap STOL aircraft to ascertain a realistic dispersion of parameter values at both the flare window and touchdown. The study was performed on a fixed-base simulator using standard cockpit instrumentation. Six levels of stability and control augmentation were tested during a total of 60 approaches (10 at each level). A detached supplement containing computer printouts of the flare-window and touchdown conditions for all 60 runs has been prepared
Parameter-free Stark Broadening of Hydrogen Lines in DA White Dwarfs
We present new calculations for the Stark broadening of the hydrogen line
profiles in the dense atmospheres of white dwarf stars. Our improved model is
based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but
it also includes non-ideal gas effects from the Hummer & Mihalas occupation
probability formalism directly inside the line profile calculations. This
approach improves upon previous calculations that relied on the use of an
ad-hoc free parameter to describe the dissolution of the line wing opacity in
the presence of high electric microfields in the plasma. We present here the
first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has
no free parameters. The atmospheric parameters obtained from optical and UV
spectroscopic observations using these improved models are shown to differ
substantially from those published in previous studies.Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample
We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs
in the UV spectra of quasars selected from the SDSS Early Data Release. These
quasars have strong intervening MgII-FeII systems which are known signatures of
high column density neutral gas. In total, UV observations of Ly-alpha
absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW)
W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of
systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases
to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The
mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36
lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit
redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M =
0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is
constant for 0.5<z<5.0 to within the uncertainties. This is larger than
Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution
does not change significantly with redshift. However, the low redshift
distribution is marginally flatter due to the higher fraction of high N(HI)
systems in our sample. (6) Finally, using the precision of MgII survey
statistics, we find that there may be evidence of a decreasing Omega_{DLA} from
z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very
high columns of neutral gas might be missed by DLA surveys because of their
very small cross sections, and therefore, that Omega_{DLA} might not include
the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure
Optical properties and spatial distribution of MgII absorbers from SDSS image stacking
We present a statistical analysis of the photometric properties and spatial
distribution of more than 2,800 MgII absorbers with 0.37<z<1 and rest
equivalent width W_0(\lambda2796)>0.8\AA detected in SDSS quasar spectra. Using
an improved image stacking technique, we measure the cross-correlation between
MgII gas and light (in the g, r, i and z-bands) from 10 to 200 kpc and infer
the light-weighted impact parameter distribution of MgII absorbers. Such a
quantity is well described by a power-law with an index that strongly depends
on W_0, ranging from ~-1 for W_0~ 1.5\AA. At redshift
0.37<z<0.55, we find the average luminosity enclosed within 100 kpc around MgII
absorbers to be M_g=-20.65+-0.11 mag, which is ~0.5 L_g*. The global
luminosity-weighted colors are typical of present-day intermediate type
galaxies. However, while the light of weaker absorbers originates mostly from
red passive galaxies, stronger systems display the colors of blue star-forming
galaxies. Based on these observations, we argue that the origin of strong MgII
absorber systems might be better explained by models of metal-enriched gas
outflows from star-forming/bursting galaxies. Our analysis does not show any
redshift dependence for both impact parameter and rest-frame colors up to z=1.
However, we do observe a brightening of the absorbers related light at high
redshift (~50% from z~0.4 to 1). We argue that MgII absorbers are a phenomenon
typical of a given evolutionary phase that more massive galaxies experience
earlier than less massive ones, in a downsizing fashion. (abridged)Comment: ApJ in press, 28 pages, 16 figures, using emulateapj. Only typo
corrections wrt the original submission (v1
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Numerical simulations of the convective dilution process in helium-rich white dwarfs
The 21st European Workshop on White Dwarfs was held in Austin, TX from July 23rd to 27th of 2018DB and DBA white dwarfs are generally believed to
be the result of a process by which a thin radiative hydrogen
atmosphere floating in diffusive equilibrium
on top of a helium envelope is eventually completely
diluted in the underlying more massive helium convection
zone that develops with cooling. However,
the observed hydrogen abundances in these objects
exceed by several orders of magnitude the predictions
obtained from such a scenario invoking diffusive
equilibrium, thus currently leaving the very existence
of DB and DBA white dwarfs unaccounted
for in any satisfactory way. We present here the results
of new numerical simulations aimed at improving
the modeling of this convective dilution process.
In particular, we show how DA white dwarfs can be
transformed into DB stars below 20,000 K, and more
importantly, we propose a model that predicts the
correct amount of hydrogen observed in DBA stars
without invoking any accretion mechanism, an alternative
model that has been proposed over the years
to account qualitatively for the presence of hydrogen
in the atmospheres of DBA stars.Astronom
Combinatorial Hopf algebra of superclass functions of type
We provide a Hopf algebra structure on the space of superclass functions on
the unipotent upper triangular group of type D over a finite field based on a
supercharacter theory constructed by Andr\'e and Neto. Also, we make further
comments with respect to types B and C. Type A was explores by M. Aguiar et. al
(2010), thus this paper is a contribution to understand combinatorially the
supercharacter theory of the other classical Lie types.Comment: Last section modified. Recent development added and correction with
respect to previous version state
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