1,388 research outputs found
Automated fully-stressed design with NASTRAN
An automated strength sizing capability is described. The technique determines the distribution of material among the elements of a structural model. The sizing is based on either a fully stressed design or a scaled feasible fully stressed design. Results obtained from the application of the strength sizing to the structural sizing of a composite material wing box using material strength allowables are presented. These results demonstrate the rapid convergence of the structural sizes to a usable design
A Brightening of the Symbiotic Variable SY Muscae
The symbiotic variable SY Muscae has been observed with IUE in September 1980 and June 1981 and in the photographic region in May 1981. The entire ultraviolet spectrum brightened between September and June by about a factor of 5. The spectrum shows high excitation including emission from N v and high electron density, about 10^10 cm^-3 as determined from various line ratios in the ultraviolet. The optical spectrum is dominated by permitted lines; even [0 m] is very weak again indicating high density in the ionized region. The increase in ultraviolet continuum and line emission may be due to enhanced mass transfer from the cool star whose period is 623 d and whose maximum was predicted to occur very close to the time of the June 1981 observations. Alternatively the hot star and much of the emitting gas could have been in eclipse in September 1980
The Vela Pulsar in the Ultraviolet
We describe time-tagged observations of the Vela pulsar (PSR B0833-45) with
the HST STIS MAMA-UV detectors. Using an optimal extraction technique, we
obtain a NUV light curve and crude FUV phase-resolved spectra. The pulse is
dominated by complex non-thermal emission at all phases. We obtain approximate
spectral indices for the brightest pulse components, relate these to pulsations
observed in other wavebands, and constrain the thermal surface emission from
the neutron star. An upper limit on the Rayleigh-Jeans component at UV pulse
minimum suggests a rather low mean surface temperature T_eff^\infty <4.6 x 10^5
(d_{300}/R_{14.2})^2 K. If confirmed by improved phase-resolved spectroscopy,
this observation has significant impact on our understanding of neutron star
interiors.Comment: 14 pages, 7 figures, to appear in the Astrophysical Journa
Studying the Pulsation of Mira Variables in the Ultraviolet
We present results from an empirical study of the Mg II h & k emission lines
of selected Mira variable stars, using spectra from the International
Ultraviolet Explorer (IUE). The stars all exhibit similar Mg II behavior during
the course of their pulsation cycles. The Mg II flux always peaks after optical
maximum near pulsation phase 0.2-0.5, although the Mg II flux can vary greatly
from one cycle to the next. The lines are highly blueshifted, with the
magnitude of the blueshift decreasing with phase. The widths of the Mg II lines
are also phase-dependent, decreasing from about 70 km/s to 40 km/s between
phase 0.2 and 0.6. We also study other UV emission lines apparent in the IUE
spectra, most of them Fe II lines. These lines are much narrower and not nearly
as blueshifted as the Mg II lines. They exhibit the same phase-dependent flux
behavior as Mg II, but they do not show similar velocity or width variations.Comment: 26 pages, 12 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
to appear in Ap
The Interpretation of Photoelectric Colors for Stars of Types B-F
The accumulation of photoelectric data on the Johnson-Morgan system of B - V and U - B colors makes a preliminary theoretical reconnaissance desirable The colors were predicted for atmospheres of a wide range of effective temperatures and electron pressures. The effects of the Balmer jump on the response in the U band and of the Balmer lines in the B band were included, using averages taken over spectral type and luminosity classes. Table 2 gives the predicted fluxes as compared to a black body, the corrected B - V and U - B colors, and the color temperatures The zero point is based on Code's spectral scans of two stars. The results are most useful for differential effects over small ranges of 0 and P_e; the general temperature and pressure scale derived colorimetrically seems reasonable. The large effect of lines in certain white dwarfs explains some features of the observed colors
A Differential Spectroscopic Analysis of 16 Cygni A and B
We utilize high-resolution, high signal-to-noise spectra to perform a
differential analysis of Fe abundances in the common proper-motion pair 16 Cyg
A and B. We confirm that both stars are slightly metal-rich compared to the
Sun, and we show for the first time that the primary is enhanced in Fe relative
to the secondary by a significant amount. We find Delta[Fe/H]=+0.025\pm0.009.
This tends to support the ``self-pollution'' scenario proposed by Gonzalez
(1998), though lack of a complete understanding of small primordial metallicity
variations among binaries and open cluster members prevents a definitive
conclusion
The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables
Abundance analyses from high-resolution optical spectra are presented for 19
Type II Cepheids in the Galactic field. The sample includes both short-period
(BL Her) and long-period (W Vir) stars. This is the first extensive abundance
analysis of these variables. The C, N, and O abundances with similar spreads
for the BL Her and W Vir show evidence for an atmosphere contaminated with
-process and CN-cycling products. A notable anomaly of the BL Her
stars is an overabundance of Na by a factor of about five relative to their
presumed initial abundances. This overabundance is not seen in the W Vir stars.
The abundance anomalies running from mild to extreme in W Vir stars but not
seen in the BL Her stars are attributed to dust-gas separation that provides an
atmosphere deficient in elements of high condensation temperature, notably Al,
Ca, Sc, Ti, and -process elements. Such anomalies have previously been seen
among RV Tau stars which represent a long-period extension of the variability
enjoyed by the Type II Cepheids. Comments are offered on how the contrasting
abundance anomalies of BL Her and W Vir stars may be explained in terms of the
stars' evolution from the blue horizontal branch.Comment: 41 pages including 11 figures and 4 tables; Accepted for publication
in Ap
Modeling RR Tel through the Evolution of the Spectra
We investigate the evolution of RR Tel after the outburst by fitting the
emission spectra in two epochs. The first one (1978) is characterized by large
fluctuations in the light curve and the second one (1993) by the slow fading
trend. In the frame of a colliding wind model two shocks are present: the
reverse shock propagates in the direction of the white dwarf and the other one
expands towards or beyond the giant. The results of our modeling show that in
1993 the expanding shock has overcome the system and is propagating in the
nearby ISM. The large fluctuations observed in the 1978 light curve result from
line intensity rather than from continuum variation. These variations are
explained by fragmentation of matter at the time of head-on collision of the
winds from the two stars. A high velocity (500 km/s) wind component is revealed
from the fit of the SED of the continuum in the X-ray range in 1978, but is
quite unobservable in the line profiles. The geometrical thickness of the
emitting clumps is the critical parameter which can explain the short time
scale variabilities of the spectrum and the trend of slow line intensity
decrease.Comment: 26 pages, LaTeX (including 5 Tables) + 6 PostScript figures. To
appear in "The Astrophysical Journal
The evolutionary status of the semiregular variable QYSge
Repeated spectroscopic observations made with the 6m telescope of yielded new
data on the radial-velocity variability of the anomalous yellow supergiant
QYSge. The strongest and most peculiar feature in its spectrum is the complex
profile of NaI D lines, which contains a narrow and a very wide emission
components. The wide emission component can be seen to extend from -170 to +120
km/s, and at its central part it is cut by an absorption feature, which, in
turn, is split into two subcomponents by a narrow (16km/s at r=2.5) emission
peak. An analysis of all the Vr values leads us to adopt for the star a
systemic velocity of Vr=-21.1 km/s, which corresponds to the position of the
narrow emission component of NaI. The locations of emission-line features of
NaI D lines are invariable, which point to their formation in regions that are
external to the supergiant's photosphere. Differential line shifts of about
10km/s are revealed. The absorption lines in the spectrum of QYSge have a
substantial width of FWHM~45 km/s. The method of model atmospheres is used to
determine the following parameters: Teff=6250K, lg g=2.0, and microturbulence
Vt=4.5km/s. The metallicity of the star is found to be somewhat higher than the
solar one with an average overabundance of iron-peak elements of [Met/H]=+0.20.
The star is found to be slightly overabundant in carbon and nitrogen,
[C/Fe]=+0.25, [N/Fe]=+0.27. The alpha-process elements Mg, Si, and Ca are
slightly overabundant [alpha/H]=+0.12. The strong sodium excess, [Na/Fe]=+0.75,
is likely to be due to the dredge-up of the matter processed in the NeNa cycle.
Heavy elements of the s-process are underabundant relative to the Sun. On the
whole, the observed properties of QYSge do not give grounds for including this
star into the group of RCrB or RVTau-type type objects.Comment: 29 pages, 8 figures, 4 tables; accepted by Astrophys. Bulleti
alpha-nucleus potentials for the neutron-deficient p nuclei
alpha-nucleus potentials are one important ingredient for the understanding
of the nucleosynthesis of heavy neutron-deficient p nuclei in the astrophysical
gamma-process where these p nuclei are produced by a series of (gamma,n),
(gamma,p), and (gamma,alpha) reactions. I present an improved alpha-nucleus
potential at the astrophysically relevant sub-Coulomb energies which is derived
from the analysis of alpha decay data and from a previously established
systematic behavior of double-folding potentials.Comment: 6 pages, 3 figures, accepted for publication in Phys. Rev.
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