4,237 research outputs found
The UV Scattering Halo of the Central Source Associated with Eta Carinae
We have made an extensive study of the UV spectrum of Eta Carinae, and find
that we do not directly observe the star and its wind in the UV. Because of
dust along our line of sight, the UV light that we observe arises from
bound-bound scattering at large impact parameters. We obtain a reasonable fit
to the UV spectrum by using only the flux that originates outside 0.033". This
explains why we can still observe the primary star in the UV despite the large
optical extinction -- it is due to the presence of an intrinsic coronagraph in
the Eta Carinae system, and to the extension of the UV emitting region. It is
not due to peculiar dust properties alone. We have computed the spectrum of the
purported companion star, and show that it could only be directly detected in
the UV spectrum preferentially in the Far Ultraviolet Spectroscopic Explorer
(FUSE) spectral region (912-1175 Ang.). However, we find no direct evidence for
a companion star, with the properties indicated by X-ray studies and studies of
the Weigelt blobs, in UV spectra. This might be due to reprocessing of the
companion's light by the dense stellar wind of the primary. Broad FeII and
[FeII] emission lines, which form in the stellar wind, are detected in spectra
taken in the SE lobe, 0.2" from the central star. The wind spectrum shows some
similarities to the spectra of the B & D Weigelt blobs, but also shows some
marked differences in that high excitation lines, and lines pumped by Ly-alpha,
are not seen. The detection of the broad lines lends support to our
interpretation of the UV spectrum, and to our model for Eta Carinae.Comment: To appear in ApJ. 57 pages with 18 figure
Structure and superconductivity of two different phases of Re3W
Two superconducting phases of Re(3)W have been found with different physical properties. One phase crystallizes in a noncentrosymmetric cubic (alpha-Mn) structure and has a superconducting transition temperature T(c) of 7.8 K. The other phase has a hexagonal centrosymmetric structure and is superconducting with a T(c) of 9.4 K. Switching between the two phases is possible by annealing the sample or remelting it. The properties of both phases of Re(3)W have been characterized by powder neutron diffraction, magnetization, and resistivity measurements. The temperature dependences of the lower and upper critical fields have been measured for both phases. These are used to determine the penetration depths and the coherence lengths for these systems
Latitude-dependent effects in the stellar wind of Eta Carinae
The Homunculus reflection nebula around eta Carinae provides a rare
opportunity to observe the spectrum of a star from multiple latitudes. We
present STIS spectra of several positions in the Homunculus, showing directly
that eta Car has an aspherical stellar wind. P Cygni absorption in Balmer lines
depends on latitude, with high velocities and strong absorption near the poles.
Stronger absorption at high latitudes is surprising, and it suggests higher
mass flux toward the poles, perhaps resulting from radiative driving with
equatorial gravity darkening on a rotating star. Reflected profiles of He I
lines are more puzzling, offering clues to the wind geometry and ionization
structure. During eta Car's high-excitation state in March 2000, the wind was
fast and dense at the poles, with higher ionization at low latitudes.
Older STIS data obtained since 1998 reveal that this global stellar-wind
geometry changes during eta Car's 5.5 year cycle, and may suggest that this
star's spectroscopic events are shell ejections. Whether or not a companion
star triggers these outbursts remains ambiguous. The most dramatic changes in
the wind occur at low latitudes, while the dense polar wind remains relatively
undisturbed during an event. The apparent stability of the polar wind also
supports the inferred bipolar geometry. The wind geometry and its variability
have critical implications for understanding the 5.5 year cycle and long-term
variability, but do not provide a clear alternative to the binary hypothesis
for generating eta Car's X-rays.Comment: Accepted by ApJ. To appear in March 2003. Based on PhD Thesis,
Minnesota 200
A Binary Orbit for the Massive, Evolved Star HDE 326823, a WR+O System Progenitor
The hot star HDE 326823 is a candidate transition-phase object that is
evolving into a nitrogen-enriched Wolf-Rayet star. It is also a known
low-amplitude, photometric variable with a 6.123 d period. We present new, high
and moderate resolution spectroscopy of HDE 326823, and we show that the
absorption lines show coherent Doppler shifts with this period while the
emission lines display little or no velocity variation. We interpret the
absorption line shifts as the orbital motion of the apparently brighter star in
a close, interacting binary. We argue that this star is losing mass to a mass
gainer star hidden in a thick accretion torus and to a circumbinary disk that
is the source of the emission lines. HDE 326823 probably belongs to a class of
objects that produce short-period WR+O binaries.Comment: 32 pages, 10 figures, accepted to the Astronomical Journa
Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage
We report near-IR spectroscopic observations of the Eta Carinae massive
binary system during 2008-2009 using VLT/CRIRES. We detect a strong, broad
absorption wing in He I 10833 extending up to -1900 km/s across the 2009.0
spectroscopic event. Archival HST/STIS ultraviolet and optical data shows a
similar high-velocity absorption (up to -2100 km/s) in the UV resonance lines
of Si IV 1394, 1403 across the 2003.5 event. UV lines from low-ionization
species, such as Si II 1527, 1533 and C II 1334, 1335, show absorption up to
-1200 km/s, indicating that the absorption with v from -1200 to -2100 km/s
originates in a region markedly faster and more ionized than the nominal wind
of the primary star. Observations obtained at the OPD/LNA during the last 4
spectroscopic cycles (1989-2009) also display high-velocity absorption in He I
10833 during periastron. Based on the OPD/LNA dataset, we determine that
material with v < -900 km/s is present in the phase range 0.976 < phi < 1.023
of the spectroscopic cycle, but absent in spectra taken at phi < 0.947 and phi
> 1.049. Therefore, we constrain the duration of the high-velocity absorption
to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the
high-velocity absorption originates from shocked gas in the wind-wind collision
zone, at distances of 15 to 45 AU in the line-of-sight to the primary star.
Using 3-D hydrodynamical simulations of the wind-wind collision zone, we find
that the dense high-velocity gas is in the line-of-sight to the primary star
only if the binary system is oriented in the sky so that the companion is
behind the primary star during periastron, corresponding to a longitude of
periastron of omega ~ 240 to 270 degrees. We study a possible tilt of the
orbital plane relative to the Homunculus equatorial plane and conclude that our
data are broadly consistent with orbital inclinations in the range i=40 to 60
degrees.Comment: 18 pages, 15 figures, accepted for publication in A&A;
high-resolution PDF version available also at
http://www.mpifr.de/staff/jgroh/etacar.htm
Constraining the Absolute Orientation of Eta Carinae's Binary Orbit: A 3-D Dynamical Model for the Broad [Fe III] Emission
We present a three-dimensional (3-D) dynamical model for the broad [Fe III]
emission observed in Eta Carinae using the Hubble Space Telescope/Space
Telescope Imaging Spectrograph (HST/STIS). This model is based on full 3-D
Smoothed Particle Hydrodynamics (SPH) simulations of Eta Car's binary colliding
winds. Radiative transfer codes are used to generate synthetic spectro-images
of [Fe III] emission line structures at various observed orbital phases and
STIS slit position angles (PAs). Through a parameter study that varies the
orbital inclination i, the PA {\theta} that the orbital plane projection of the
line-of-sight makes with the apastron side of the semi-major axis, and the PA
on the sky of the orbital axis, we are able, for the first time, to tightly
constrain the absolute 3-D orientation of the binary orbit. To simultaneously
reproduce the blue-shifted emission arcs observed at orbital phase 0.976, STIS
slit PA = +38 degrees, and the temporal variations in emission seen at negative
slit PAs, the binary needs to have an i \approx 130 to 145 degrees, {\theta}
\approx -15 to +30 degrees, and an orbital axis projected on the sky at a PA
\approx 302 to 327 degrees east of north. This represents a system with an
orbital axis that is closely aligned with the inferred polar axis of the
Homunculus nebula, in 3-D. The companion star, Eta B, thus orbits clockwise on
the sky and is on the observer's side of the system at apastron. This
orientation has important implications for theories for the formation of the
Homunculus and helps lay the groundwork for orbital modeling to determine the
stellar masses.Comment: 23 pages, 12 color figures, plus 2 online-only appendices (available
in the /anc folder of the Source directory). Accepted for publication in
MNRA
A spectroscopic event of eta Car viewed from different directions: The data and first results
We present spectroscopic observations with high spectral resolution of eta
Car as seen by the SE lobe of the Homunculus nebula over the 2003.5
"spectroscopic event". The observed spectra represent the stellar spectrum
emitted near the pole of the star and are much less contaminated with nebular
emission lines than direct observations of the central object. The "event" is
qualitatively similar near the pole to what is observed in direct spectra of
the star (more equator-on at 45 degree), but shows interesting differences. The
observations show that the equivalent width changes of H alpha emission and
other lines are less pronounced at the pole than in the line of sight. Also the
absorption components appear less variable. A pronounced high-velocity
absorption is present near the event in the He I lines indicating a
mass-ejection event. This feature is also seen, but less pronounced, in the
hydrogen lines. HeII4686 emission is observed for a brief period of time near
the event and appears, if corrected for light travel time, to precede similar
emission in the direct view. Our observations indicate that the event is
probably not only a change in ionization and excitation structure or a simple
eclipse-like event.Comment: 10 pages, 16 figures, accepted by A&
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