6,869 research outputs found
Wind-wind collision in the eta Carinae binary system - III. The HeII 4686 line profile
We modeled the HeII 4686 line profiles observed in the eta Carinae binary
system close to the 2003.5 spectroscopic event, assuming that they were formed
in the shocked gas that flows at both sides of the contact surface formed by
wind-wind collision. We used a constant flow velocity and added turbulence in
the form of a gaussian velocity distribution. We allowed emission from both the
primary and secondary shocks but introduced infinite opacity at the contact
surface, implying that only the side of the contact cone visible to the
observer contributed to the line profile. Using the orbital parameters of the
binary system derived from the 7 mm light curve during the last spectroscopic
event (Paper II) we were able to reproduce the line profiles obtained with the
HST at different epochs, as well as the line mean velocities obtained with
ground based telescopes. A very important feature of our model is that the line
profile depends on the inclination of the orbital plane; we found that to
explain the latitude dependent mean velocity of the line, scattered into the
line of sight by the Homunculus, the orbit inclination should be close to 90
degrees, meaning that it does not lie in the Homunculus equatorial plane, as
usually assumed. This inclination, together with the relative position of the
stars during the spectroscopic events, allowed us to explain most of the
observational features, like the variation of the Purple Haze with the orbital
phase, and to conciliate the X-ray absorption with the postulated shell effect
used to explain the optical and UV light curves.Comment: to appear in the MNRA
Modeling the line variations from the wind-wind shock emissions of WR 30a
The study of Wolf-Rayet stars plays an important role in evolutionary
theories of massive stars. Among these objects, ~ 20% are known to be in binary
systems and can therefore be used for the mass determination of these stars.
Most of these systems are not spatially resolved and spectral lines can be used
to constrain the orbital parameters. However, part of the emission may
originate in the interaction zone between the stellar winds, modifying the line
profiles and thus challenging us to use different models to interpret them. In
this work, we analyzed the HeII4686\AA + CIV4658\AA blended lines of WR30a
(WO4+O5) assuming that part of the emission originate in the wind-wind
interaction zone. In fact, this line presents a quiescent base profile,
attributed to the WO wind, and a superposed excess, which varies with the
orbital phase along the 4.6 day period. Under these assumptions, we were able
to fit the excess spectral line profile and central velocity for all phases,
except for the longest wavelengths, where a spectral line with constant
velocity seems to be present. The fit parameters provide the eccentricity and
inclination of the binary orbit, from which it is possible to constrain the
stellar masses.Comment: accepted for publication in the MNRA
The true nature of the alleged planetary nebula W16-185
We report the discovery of a small cluster of massive stars embedded in a NIR
nebula in the direction of the IRAS15411-5352 point source, which is related to
the alleged planetary nebula W16-185. The majority of the stars present large
NIR excess characteristic of young stellar objects and have bright counterparts
in the Spitzer IRAC images; the most luminous star (IRS1) is the NIR
counterpart of the IRAS source. We found very strong unresolved Brgamma
emission at the IRS1 position and more diluted and extended emission across the
continuum nebula. From the sizes and electron volume densities we concluded
that they represent ultra-compact and compact HII regions, respectively.
Comparing the Brgamma emission with the 7 mm free-free emission, we estimated
that the visual extinction ranges between 14 and 20 mag. We found that only one
star (IRS1) can provide the number of UV photons necessary to ionize the
nebula.Comment: 30 pages, 15 figures, 2 tables V3: minor grammatical changes. Figure
4 is available in pdf file. Accepted for publication in AJ, April / 200
Resolving Gamma-Ray Burst 000301C with a Gravitational Microlens
The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic,
short time-scale variability that is difficult to reconcile with the standard
relativistic shock model. We interpret the observed light curves as a
microlensing event superimposed on power-law flux decays typical of afterglows.
In general, a relativistic GRB shock appears on the sky as a thin ring
expanding at a superluminal speed. Initially the ring is small relative to its
angular separation from the lens and so its flux is magnified by a constant
factor. As the ring grows and sweeps across the lens its magnification reaches
a maximum. Subsequently, the flux gradually recovers its unlensed value. This
behavior involves only three free parameters in its simplest formulation and
was predicted theoretically by Loeb & Perna (1998). Fitting the available
R-band photometric data of GRB 000301C to a simple model of the microlensing
event and a broken power-law for the afterglow, we find reasonable values for
all the parameters and a reduced chi^2/DOF parameter of 1.48 compared with 2.99
for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8
days after the burst. The entire optical-IR data imply a width of the GRB ring
of order 10% of its radius, similar to theoretical expectations. The angular
resolution provided by microlensing is better than a micro-arcsecond. We infer
a mass of approximately 0.5 M_Sun for a lens located half way to the source at
z_s=2.04. A galaxy 2'' from GRB 000301C might be the host of the stellar lens,
but current data provides only an upper-limit on its surface brightness at the
GRB position.Comment: to appear in the ApJ Letters, 13 pages, 3 figures (one additional
figure included); all data used for the fits available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB000301C/ and through WWW at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
Non-LTE, Relativistic Accretion Disk Fits to 3C~273 and the Origin of the Lyman Limit Spectral Break
We fit general relativistic, geometrically thin accretion disk models with
non-LTE atmospheres to near simultaneous multiwavelength data of 3C~273,
extending from the optical to the far ultraviolet. Our model fits show no flux
discontinuity associated with a hydrogen Lyman edge, but they do exhibit a
spectral break which qualitatively resembles that seen in the data. This break
arises from relativistic smearing of Lyman emission edges which are produced
locally at tens of gravitational radii in the disk. We discuss the possible
effects of metal line blanketing on the model spectra, as well as the
substantial Comptonization required to explain the observed soft X-ray excess.
Our best fit accretion disk model underpredicts the near ultraviolet emission
in this source, and also has an optical spectrum which is too red. We discuss
some of the remaining physical uncertainties, and suggest in particular that an
extension of our models to the slim disk regime and/or including nonzero
magnetic torques across the innermost stable circular orbit may help resolve
these discrepancies.Comment: Accepted for publication in Ap
Thickness dependent magnetotransport in ultra-thin manganite films
To understand the near-interface magnetism in manganites, uniform, ultra-thin
films of La_{0.67}Sr_{0.33}MnO_3 were grown epitaxially on single crystal (001)
LaAlO_3 and (110) NdGaO_3 substrates. The temperature and magnetic field
dependent film resistance is used to probe the film's structural and magnetic
properties. A surface and/or interface related dead-layer is inferred from the
thickness dependent resistance and magnetoresistance. The total thickness of
the dead layer is estimated to be for films on NdGaO_3 and for films on LaAlO_3.Comment: 11 pages, 4 figure
Ferromagnetic resonance study of polycrystalline Fe_{1-x}V_x alloy thin films
Ferromagnetic resonance has been used to study the magnetic properties and
magnetization dynamics of polycrystalline FeV alloy films with
. Films were produced by co-sputtering from separate Fe and V
targets, leading to a composition gradient across a Si substrate. FMR studies
were conducted at room temperature with a broadband coplanar waveguide at
frequencies up to 50 GHz using the flip-chip method. The effective
demagnetization field and the Gilbert damping
parameter have been determined as a function of V concentration. The
results are compared to those of epitaxial FeV films
The effects of hemodynamic lag on functional connectivity and behavior after stroke
Stroke disrupts the brain's vascular supply, not only within but also outside areas of infarction. We investigated temporal delays (lag) in resting state functional magnetic resonance imaging signals in 130 stroke patients scanned two weeks, three months and 12 months post stroke onset. Thirty controls were scanned twice at an interval of three months. Hemodynamic lag was determined using cross-correlation with the global gray matter signal. Behavioral performance in multiple domains was assessed in all patients. Regional cerebral blood flow and carotid patency were assessed in subsets of the cohort using arterial spin labeling and carotid Doppler ultrasonography. Significant hemodynamic lag was observed in 30% of stroke patients sub-acutely. Approximately 10% of patients showed lag at one-year post-stroke. Hemodynamic lag corresponded to gross aberrancy in functional connectivity measures, performance deficits in multiple domains and local and global perfusion deficits. Correcting for lag partially normalized abnormalities in measured functional connectivity. Yet post-stroke FC-behavior relationships in the motor and attention systems persisted even after hemodynamic delays were corrected. Resting state fMRI can reliably identify areas of hemodynamic delay following stroke. Our data reveal that hemodynamic delay is common sub-acutely, alters functional connectivity, and may be of clinical importance
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