37 research outputs found
A Lighthouse Effect in Eta Carinae
We present a new model for the behavior of scattered time-dependent,
asymmetric near-UV emission from the nearby ejecta of {\eta} Car. Using a 3-D
hydrodynamical simulation of {\eta} Car's binary colliding winds, we show that
the 3-D binary orientation derived by Madura et al. (2012) is capable of
explaining the asymmetric near-UV variability observed in the Hubble Space
Telescope Advanced Camera for Surveys/High Resolution Camera (HST ACS/HRC)
F220W images of Smith et al. (2004b). Models assuming a binary orientation with
i ~ 130 to 145 degrees, {\omega} ~ 230 to 315 degrees, PAz ~ 302 to 327 degrees
are consistent with the observed F220W near-UV images. We find that the hot
binary companion does not significantly contribute to the near-UV excess
observed in the F220W images. Rather, we suggest that a bore-hole effect and
the reduction of Fe II optical depths inside the wind-wind collision cavity
carved in the extended photosphere of the primary star lead to the
time-dependent directional illumination of circum-binary material as the
companion moves about in its highly elliptical orbit.Comment: 14 pages, 4 figures, 1 table. Accepted for publication in ApJ
X-ray Modeling of \eta\ Carinae and WR140 from SPH Simulations
The colliding wind binary (CWB) systems \eta\ Carinae and WR140 provide
unique laboratories for X-ray astrophysics. Their wind-wind collisions produce
hard X-rays that have been monitored extensively by several X-ray telescopes,
including RXTE. To interpret these RXTE X-ray light curves, we model the
wind-wind collision using 3D smoothed particle hydrodynamics (SPH) simulations.
Adiabatic simulations that account for the absorption of X-rays from an assumed
point source at the apex of the wind-collision shock cone by the distorted
winds can closely match the observed 2-10keV RXTE light curves of both \eta\
Car and WR140. This point-source model can also explain the early recovery of
\eta\ Car's X-ray light curve from the 2009.0 minimum by a factor of 2-4
reduction in the mass loss rate of \eta\ Car. Our more recent models relax the
point-source approximation and account for the spatially extended emission
along the wind-wind interaction shock front. For WR140, the computed X-ray
light curve again matches the RXTE observations quite well. But for \eta\ Car,
a hot, post-periastron bubble leads to an emission level that does not match
the extended X-ray minimum observed by RXTE. Initial results from incorporating
radiative cooling and radiatively-driven wind acceleration via a new
anti-gravity approach into the SPH code are also discussed.Comment: 5 pages, 3 figures, Proceedings of the 39th Li\'ege Astrophysical
Colloquium, held in Li\`ege 12-16 July 2010, edited by G. Rauw, M. De Becker,
Y. Naz\'e, J.-M. Vreux, P. William
Multi-Wavelength Implications of the Companion Star in Eta Carinae
Eta Carinae is considered to be a massive colliding wind binary system with a
highly eccentric (e \sim 0.9), 5.54-yr orbit. However, the companion star
continues to evade direct detection as the primary dwarfs its emission at most
wavelengths. Using three-dimensional (3-D) SPH simulations of Eta Car's
colliding winds and radiative transfer codes, we are able to compute synthetic
observables across multiple wavebands for comparison to the observations. The
models show that the presence of a companion star has a profound influence on
the observed HST/STIS UV spectrum and H-alpha line profiles, as well as the
ground-based photometric monitoring. Here, we focus on the Bore Hole effect,
wherein the fast wind from the hot secondary star carves a cavity in the dense
primary wind, allowing increased escape of radiation from the hotter/deeper
layers of the primary's extended wind photosphere. The results have important
implications for interpretations of Eta Car's observables at multiple
wavelengths.Comment: 5 pages, 4 figures, To be published in the proceedings of the meeting
'Four Decades of Research on Massive Stars' in honor of Tony Moffat, 11-15
July 2011, Saint-Michel-des-Saints, Quebe
Constraining the Properties of the Eta Carinae System via 3-D SPH Models of Space-Based Observations: The Absolute Orientation of the Binary Orbit
The extremely massive (> 90 Solar Mass) and luminous (= 5 x 10(exp 6) Solar Luminosity) star Eta Carinae, with its spectacular bipolar "Homunculus" nebula, comprises one of the most remarkable and intensely observed stellar systems in the galaxy. However, many of its underlying physical parameters remain a mystery. Multiwavelength variations observed to occur every 5.54 years are interpreted as being due to the collision of a massive wind from the primary star with the fast, less dense wind of a hot companion star in a highly elliptical (e approx. 0.9) orbit. Using three-dimensional (3-D) Smoothed Particle Hydrodynamics (SPH) simulations of the binary wind-wind collision in Eta Car, together with radiative transfer codes, we compute synthetic spectral images of [Fe III] emission line structures and compare them to existing Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) observations. We are thus able, for the first time, to constrain the absolute orientation of the binary orbit on the sky. An orbit with an inclination of i approx. 40deg, an argument of periapsis omega approx. 255deg, and a projected orbital axis with a position angle of approx. 312deg east of north provides the best fit to the observations, implying that the orbital axis is closely aligned in 3-1) space with the Homunculus symmetry axis, and that the companion star orbits clockwise on the sky relative to the primary
Constraints on decreases in Eta Carinae's mass loss from 3D hydrodynamic simulations of its binary colliding winds
Recent work suggests that the mass-loss rate of the primary star (Eta A) in
the massive colliding wind binary Eta Carinae dropped by a factor of 2-3
between 1999 and 2010. We present results from large- (r=1545au) and small-
(r=155au) domain, 3D smoothed particle hydrodynamic (SPH) simulations of Eta
Car's colliding winds for 3 Eta A mass-loss rates (2.4, 4.8, and 8.5 x 10^-4
M_sun/yr), investigating the effects on the dynamics of the binary wind-wind
collision (WWC). These simulations include orbital motion, optically thin
radiative cooling, and radiative forces. We find that Eta A's mass-loss rate
greatly affects the time-dependent hydrodynamics at all spatial scales
investigated. The simulations also show that the post-shock wind of the
companion star (Eta B) switches from the adiabatic to the radiative-cooling
regime during periastron passage. The SPH simulations together with 1D
radiative transfer models of Eta A's spectra reveal that a factor of 2 or more
drop in Eta A's mass-loss rate should lead to substantial changes in numerous
multiwavelength observables. Recent observations are not fully consistent with
the model predictions, indicating that any drop in Eta A's mass-loss rate was
likely by a factor < 2 and occurred after 2004. We speculate that most of the
recent observed changes in Eta Car are due to a small increase in the WWC
opening angle that produces significant effects because our line-of-sight to
the system lies close to the dense walls of the WWC zone. A modest decrease in
Eta A's mass-loss rate may be responsible, but changes in the wind/stellar
parameters of Eta B cannot yet be fully ruled out. We suggest observations
during Eta Car's next periastron in 2014 to further test for decreases in Eta
A's mass-loss rate. If Eta A's mass-loss rate is declining and continues to do
so, the 2014 X-ray minimum should be even shorter than that of 2009.Comment: 38 pages, 25 figures, 1 table. Accepted for publication in MNRA
On the influence of the companion star in Eta Carinae: 2D radiative transfer modeling of the ultraviolet and optical spectra
We present 2D radiative transfer modeling of the Eta Carinae binary system
accounting for the presence of a wind-wind collision (WWC) cavity carved in the
optically-thick wind of the primary star. By comparing synthetic line profiles
with HST/STIS spectra obtained near apastron, we show that the WWC cavity has a
strong influence on multi-wavelength diagnostics. This influence is regulated
by the modification of the optical depth in the continuum and spectral lines.
We find that H-alpha, H-beta, and Fe II lines are the most affected by the WWC
cavity, since they form over a large volume of the primary wind. These spectral
lines depend on latitude and azimuth since, according to the orientation of the
cavity, different velocity regions of a spectral line are affected. For 2D
models with orientation corresponding to orbital inclination angle 110deg < i <
140deg and longitude of periastron 210deg < omega < 330deg, the blueshifted and
zero-velocity regions of the line profiles are the most affected. These orbital
orientations are required to simultaneously fit the UV and optical spectrum of
Eta Car, for a half-opening angle of the cavity in the range 50-70deg. We find
that the excess P-Cygni absorption seen in H-alpha, H-beta and optical Fe II
lines in spherical models becomes much weaker or absent in the 2D models, in
agreement with the observations. The observed UV spectrum of Eta Car, dominated
by Fe II absorption lines, is superbly reproduced by our 2D cavity models.
Small discrepancies still remain, as H-gamma and H-delta absorptions are
overestimated by our models. We suggest that photoionization of the wind of the
primary by the hot companion star is responsible for the weak absorption seen
in these lines. Our CMFGEN models indicate that the primary star has a
mass-loss rate of 8.5x10e-4 Msun/yr and wind terminal velocity of 420 km/s
around the 2000 apastron.Comment: 20 pages, 14 figures, accepted for publication in MNRA