1,372 research outputs found
The structure of radiative shock waves. V. Hydrogen emission lines
We considered the structure of steady-state plane-parallel radiative shock
waves propagating through the partially ionized hydrogen gas of temperature T_1
= 3000K and density 1e-12 gm/cm^3 <= \rho_1 <= 1e-9 gm/cm^3. The upstream Mach
numbers range within 6 <= M_1 <= 14. In frequency intervals of hydrogen lines
the radiation field was treated using the transfer equation in the frame of the
observer for the moving medium, whereas the continuum radiation was calculated
for the static medium. Doppler shifts in Balmer emission lines of the radiation
flux emerging from the upstream boundary of the shock wave model were found to
be roughly one-third of the shock wave velocity. The gas emitting the Balmer
line radiation is located at the rear of the shock wave in the hydrogen
recombination zone where the gas flow velocity in the frame of the observer is
approximately one-half of the shock wave velocity. The ratio of the Doppler
shift to the gas flow velocity of 0.7 results both from the small optical
thickness of the shock wave in line frequencies and the anisotropy of the
radiation field typical for the slab geometry. In the ambient gas with density
of \rho_1 >= 1e-11 gm/cm^3 the flux in the H-alpha frequency interval reveals
the double structure of the profile. A weaker H-beta profile doubling was found
for \rho_1 >= 1e-10 gm/cm^3 and U_1 <= 50 km/s. The unshifted redward component
of the double profile is due to photodeexcitation accompanying the rapid growth
of collisional ionization in the narrow layer in front of the discontinuous
jump.Comment: 13 pages, 13 figures, LaTeX, accepted for publication in A
Pulsational instability of yellow hypergiants
Instability of population I (X=0.7, Y=0.02) massive stars against radial
oscillations during the post-main sequence gravitational contraction of the
helium core is investigated. Initial stellar masses are in the range from
65M_\odot to 90M_\odot. In hydrodynamic computations of self-exciting stellar
oscillations we assumed that energy transfer in the envelope of the pulsating
star is due to radiative heat conduction and convection. The convective heat
transfer was treated in the framework of the theory of time-dependent turbulent
convection. During evolutionary expansion of outer layers after hydrogen
exhaustion in the stellar core the star is shown to be unstable against radial
oscillations while its effective temperature is Teff > 6700K for
Mzams=65M_\odot and Teff > 7200K for mzams=90M_\odot. Pulsational instability
is due to the \kappa-mechanism in helium ionization zones and at lower
effective temperature oscillations decay because of significantly increasing
convection. The upper limit of the period of radial pulsations on this stage of
evolution does not exceed 200 day. Radial oscillations of the hypergiant resume
during evolutionary contraction of outer layers when the effective temperature
is Teff > 7300K for Mzams=65M_\odot and Teff > 7600K for Mzams=90M_\odot.
Initially radial oscillations are due to instability of the first overtone and
transition to fundamental mode pulsations takes place at higher effective
temperatures (Teff > 7700K for Mzams=65M_\odot and Teff > 8200K for
Mzams=90M_\odot). The upper limit of the period of radial oscillations of
evolving blueward yellow hypergiants does not exceed 130 day. Thus, yellow
hypergiants are stable against radial stellar pulsations during the major part
of their evolutionary stage.Comment: 20 pages, 7 gigures. Accepted for publication in Astronomy Letter
The Cleo III Ring Imaging Cherenkov Detector
The CLEO detector has been upgraded to include a state of the art particle
identification system, based on the Ring Imaging Cherenkov Detector (RICH)
technology, in order to take data at the upgraded CESR electron positron
collider. The expected performance is reviewed, as well as the preliminary
results from an engineering run during the first few months of operation of the
CLEO III detector.Comment: 5 pages, 2 Figures Talk given by M. Artuso at 8th Pisa Meeting on
Advanced Detectors, May 200
The CLEO-III Ring Imaging Cherenkov Detector
The CLEO-III Detector upgrade for charged particle identification is
discussed. The RICH design uses solid LiF crystal radiators coupled with
multi-wire chamber photon detectors, using TEA as the photosensor, and
low-noise Viking readout electronics. Results from our beam test at Fermilab
are presented.Comment: Invited talk by R.J. Mountain at ``The 3rd International Workshop on
Ring Imaging Cherenkov Detectors," a research workshop of the Israel Science
Foundation, Ein-Gedi, Dead-Sea, Israel, Nov. 15-20, 1998, 14 pages, 9 figure
Spectropolarimetry of R Coronae Borealis in 1998--2003: Discovery of Transient Polarization at Maximum Brightness
We present an extended optical spectropolarimetry of R CrB from 1998 January
to 2003 September. The polarization was almost constant in the phase of maximum
brightness, being consistent with past observations. We detected, however,
temporal changes of polarization ( %) in 2001 March and August, which
were the first detection of large polarization variability in R CrB near
maximum brightness. The amplitude and the position angle of the `transient
polarization' were almost constant with wavelength in both two events. There
was a difference by about 20 degrees in the position angle between the two
events. Each event could be explained by light scattering due to short-lived
dust puff occasionally ejected off the line of sight. The flatness of the
polarization against the wavelength suggests that the scatterer is a mixture of
dust grains having various sizes. The rapid growth and fading of the transient
polarization favors the phenomenological model of dust formation near the
stellar photosphere (e.g., within two stellar radii) proposed for the time
evolution of brightness and chromospheric emission lines during deeply
declining periods, although the fading timescale can hardly be explained by a
simple dispersal of expanding dust puff with a velocity of km s
. Higher expansion velocity or some mechanism to destroy the dust grains
should be needed.Comment: 22 pages, 10 figures, accepted for publication in A
The Cleo Rich Detector
We describe the design, construction and performance of a Ring Imaging
Cherenkov Detector (RICH) constructed to identify charged particles in the CLEO
experiment. Cherenkov radiation occurs in LiF crystals, both planar and ones
with a novel ``sawtooth''-shaped exit surface. Photons in the wavelength
interval 135--165 nm are detected using multi-wire chambers filled with a
mixture of methane gas and triethylamine vapor. Excellent pion/kaon separation
is demonstrated.Comment: 75 pages, 57 figures, (updated July 26, 2005 to reflect reviewers
comments), to be published in NIM
The Contribution of Magnetic Nanoparticles to Ferrogel Biophysical Properties
Iron oxide gamma-Fe(2)O(3 )magnetic nanoparticles (MNPs) were fabricated by laser target evaporation technique (LTE) and their structure and magnetic properties were studied. Polyacrylamide (PAAm) gels with different cross-linking density of the polymer network and polyacrylamide-based ferrogel with embedded LTE MNPs (0.34 wt.%) were synthesized. Their adhesive and proliferative potential with respect to human dermal fibroblasts were studied. At the same value of Young modulus, the adhesive and proliferative activities of the human dermal fibroblasts on the surface of ferrogel were unexpectedly much higher in comparison with the surface of PAAm gel. Properties of PAAm-100 + gamma-Fe2O3 MNPs composites were discussed with focus on creation of a new generation of drug delivery systems combined in multifunctional devices, including magnetic field assisted delivery, positioning, and biosensing. Although exact applications are still under development, the obtained results show a high potential of LTE MNPs to be applied for cellular technologies and tissue engineering. PAAm-100 ferrogel with very low concentration of gamma-Fe2O3 MNPs results in significant improvement of the cells' compatibility to the gel-based scaffold
Rest-Frame R-band Lightcurve of a z~1.3 Supernova Obtained with Keck Laser Adaptive Optics
We present Keck diffraction limited H-band photometry of a z~1.3 Type Ia
supernova (SN) candidate, first identified in a Hubble Space Telescope (HST)
search for SNe in massive high redshift galaxy clusters. The adaptive optics
(AO) data were obtained with the Laser Guide Star facility during four
observing runs from September to November 2005. In the analysis of data from
the observing run nearest to maximum SN brightness, the SN was found to have a
magnitude H=23.9 +/- 0.14 (Vega). We present the H-band (approximately
rest-frame R) light curve and provide a detailed analysis of the AO photometric
uncertainties. By constraining the aperture correction with a nearby (4"
separation) star we achieve 0.14 magnitude photometric precision, despite the
spatially varying AO PSF.Comment: 11 pages, 8 figures, Accepted for Publication in AJ Updated the
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