1,497 research outputs found
Sharp lines in the absorption edge of EuTe and PbEuTe in high magnetic fields
The optical absorption spectra in the region of the \fd transition energies
of epitaxial layers of of EuTe and \PbEuTe, grown by molecular beam epitaxy,
were studied using circularly polarized light, in the Faraday configuration.
Under \sigmam polarization a sharp symmetric absorption line (full width at
half-maximum 0.041 eV) emerges at the low energy side of the band-edge
absorption, for magnetic fields intensities greater than 6 T. The absorption
line shows a huge red shift (35 meV/T) with increasing magnetic fields. The
peak position of the absorption line as a function of magnetic field is
dominated by the {\em d-f} exchange interaction of the excited electron and the
\Euion spins in the lattice. The {\em d-f} exchange interaction energy was
estimated to be eV. In \PbEuTe the same absorption line
is detected, but it is broader, due to alloy disorder, indicating that the
excitation is localized within a finite radius. From a comparison of the
absorption spectra in EuTe and \PbEuTe the characteristic radius of the
excitation is estimated to be \AA.Comment: Journal of Physics: Condensed Matter (2004, at press
Radiative transfer effects on Doppler measurements as sources of surface effects in sunspot seismology
We show that the use of Doppler shifts of Zeeman sensitive spectral lines to
observe wavesn in sunspots is subject to measurement specific phase shifts
arising from, (i) altered height range of spectral line formation and the
propagating character of p mode waves in penumbrae, and (ii) Zeeman broadening
and splitting. We also show that these phase shifts depend on wave frequencies,
strengths and line of sight inclination of magnetic field, and the polarization
state used for Doppler measurements. We discuss how these phase shifts could
contribute to local helioseismic measurements of 'surface effects' in sunspot
seismology.Comment: 12 pages, 4 figures, Accepted for publication in the Astrophysical
Journal Letter
On the Formation Height of the SDO/HMI Fe 6173 Doppler Signal
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the mag- netic field in
the solar photosphere. It observes the full solar disk in the Fe I absorption
line at 6173\AA . We use the output of a high-resolution 3D, time- dependent,
radiation-hydrodynamic simulation based on the CO5BOLD code to calculate
profiles F({\lambda},x,y,t) for the Fe I 6173{\AA} line. The emerging profiles
F({\lambda},x,y,t) are multiplied by a representative set of HMI filter
transmission profiles R_i({\lambda},1 \leq i \leq 6) and filtergrams
I_i(x,y,t;1 \leq i \leq 6) are constructed for six wavelengths. Doppler
velocities V_HMI(x,y,t) are determined from these filtergrams using a
simplified version of the HMI pipeline. The Doppler velocities are correlated
with the original velocities in the simulated atmosphere. The cross-
correlation peaks near 100 km, suggesting that the HMI Doppler velocity signal
is formed rather low in the solar atmosphere. The same analysis is performed
for the SOHO/MDI Ni I line at 6768\AA . The MDI Doppler signal is formed
slightly higher at around 125 km. Taking into account the limited spatial
resolution of the instruments, the apparent formation height of both the HMI
and MDI Doppler signal increases by 40 to 50 km. We also study how
uncertainties in the HMI filter-transmission profiles affect the calculated
velocities.Comment: 15 pages, 11 Figure
Cool Companions to White Dwarfs from the 2MASS Second Incremental Data Release
We present near-infrared magnitudes for all white dwarfs (selected from the
catalog of McCook & Sion) contained in the 2 Micron All Sky Survey Second
Incremental Data Release(2MASS 2IDR). We show that the near-IR color-color
diagram is an effective means of identifying candidate binary stars containing
a WD and a low mass main sequence star. The loci of single WDs and WD + red
dwarf binaries occupy distinct regions of the near-IR color-color diagram. We
recovered all known unresolved WD + red dwarf binaries located in the 2IDR sky
coverage, and also identified as many new candidate binaries (47 new candidates
out of 95 total). Using observational near-IR data for WDs and M-L dwarfs, we
have compared a sample of simulated WD + red dwarf binaries with our 2MASS
data. The colors of the simulated binaries are dominated by the low mass
companion through the late-M to early-L spectral types. As the spectral type of
the companion becomes progressively later, however, the colors of unresolved
binaries become progressively bluer. Binaries containing the lowest mass
companions will be difficult to distinguish from single WDs solely on the basis
of their near-IR colors.Comment: 18 pages, including 2 figures, accepted for publication in Ap
3D-Guided Face Manipulation of 2D Images for the Prediction of Post-Operative Outcome after Cranio-Maxillofacial Surgery
Cranio-maxillofacial surgery often alters the aesthetics of the face which can be a heavy burden for patients to decide whether or not to undergo surgery. Today, physicians can predict the post-operative face using surgery planning tools to support the patient\u27s decision-making. While these planning tools allow a simulation of the post-operative face, the facial texture must usually be captured by another 3D texture scan and subsequently mapped on the simulated face. This approach often results in face predictions that do not appear realistic or lively looking and are therefore ill-suited to guide the patient\u27s decision-making. Instead, we propose a method using a generative adversarial network to modify a facial image according to a 3D soft-tissue estimation of the post-operative face. To circumvent the lack of available data pairs between pre- and post-operative measurements we propose a semi-supervised training strategy using cycle losses that only requires paired open-source data of images and 3D surfaces of the face\u27s shape. After training on "in-the-wild" images we show that our model can realistically manipulate local regions of a face in a 2D image based on a modified 3D shape. We then test our model on four clinical examples where we predict the post-operative face according to a 3D soft-tissue prediction of surgery outcome, which was simulated by a surgery planning tool. As a result, we aim to demonstrate the potential of our approach to predict realistic post-operative images of faces without the need of paired clinical data, physical models, or 3D texture scans
Seeing-Induced Errors in Solar Doppler Velocity Measurements
Imaging systems based on a narrow-band tunable filter are used to obtain
Doppler velocity maps of solar features. These velocity maps are created by
taking the difference between the blue- and red-wing intensity images of a
chosen spectral line. This method has the inherent assumption that these two
images are obtained under identical conditions. With the dynamical nature of
the solar features as well as the Earth's atmosphere, systematic errors can be
introduced in such measurements. In this paper, a quantitative estimate of the
errors introduced due to variable seeing conditions for ground-based
observations is simulated and compared with real observational data for
identifying their reliability. It is shown, under such conditions, that there
is a strong cross-talk from the total intensity to the velocity estimates.
These spurious velocities are larger in magnitude for the umbral regions
compared to the penumbra or quiet-sun regions surrounding the sunspots. The
variable seeing can induce spurious velocities up to about 1 km/s It is also
shown that adaptive optics, in general, helps in minimising this effect.Comment: 14 page
Infrared Candidates for the Intense Galactic X-ray Source GX 17+2
We present new astrometric solutions and infrared Hubble Space Telescope
observations of GX 17+2 (X1813-140), one of the brightest X-ray sources on the
celestial sphere. Despite 30 years of intensive study, and the existence of a
strong radio counterpart with a sub-arcsecond position, the object remains
optically unidentified. The observed X-ray characteristics strongly suggest
that it is a so-called "Z-source," the rare but important category that
includes Sco X-1 and Cyg X-2. Use of the USNO-A2.0 catalog enables us to
measure the position of optical and infrared objects near the radio source to
sub-arcsecond precision within the International Celestial Reference Frame, for
direct comparison with the radio position, which we also recompute using modern
calibrators. With high confidence we eliminate the V~17.5 star NP Ser, often
listed as the probable optical counterpart of the X-ray source, as a candidate.
Our HST NICMOS observations show two faint objects within our 0.5" radius 90%
confidence error circle. Even the brighter of the two, Star A, is far fainter
than expected (H~19.8), given multiple estimates of the extinction in this
field and our previous understanding of Z sources, but it becomes the best
candidate for the counterpart of GX 17+2. The probability of a chance
coincidence of an unrelated faint object on the radio position is high.
However, if the true counterpart is not Star A, it is fainter still, and our
conclusion that the optical counterpart is surprisingly underluminous is but
strengthened.Comment: 15 pages including 3 figures and 3 tables. Accepted for publication
in The Astrophysical Journa
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