590 research outputs found
Time evolution of spin state of radical ion pair in microwave field: An analytical solution
The paper reports an exact solution for the problem of spin evolution of
radical ion pair in static magnetic and resonant microwave field taking into
account Zeeman and hyperfine interactions and spin relaxation. The values of
parameters that provide one of the four possible types of solution are
analysed. It is demonstrated that in the absence of spin relaxation, besides
the zero field invariant an invariant at large amplitudes of the resonant
microwave field can be found. The two invariants open the possibility for
simple calculation of microwave pulses to control quantum state of the radical
pair. The effect of relaxation on the invariants is analysed and it is shown
that changes in the high field invariant are induced by phase relaxation.Comment: 18 pages, 7 figure
The Early Evolution of Massive Stars: Radio Recombination Line Spectra
Velocity shifts and differential broadening of radio recombination lines are
used to estimate the densities and velocities of the ionized gas in several
hypercompact and ultracompact HII regions. These small HII regions are thought
to be at their earliest evolutionary phase and associated with the youngest
massive stars. The observations suggest that these HII regions are
characterized by high densities, supersonic flows and steep density gradients,
consistent with accretion and outflows that would be associated with the
formation of massive stars.Comment: ApJ in pres
An oral delivery system for controlling white spot syndrome virus infection in shrimp using transgenic microalgae
White spot disease (WSD) is a longstanding and serious viral disease of various shrimp species that has caused high mortality rates for many decades. Currently, there is no practical method to control this disease. Therefore, we have explored the development of a novel vaccine-based method to control this disease using transgenic algae. During infection by white spot syndrome virus (WSSV), the interaction between viral envelope proteins and cell surface protein receptors on target cells is the key step of viral entry and replication. Hence, transgenic lines of the green microalga Chlamydomonas reinhardtii harbouring a WSSV VP28 viral envelope protein were created as an oral delivery system for vaccinating shrimp. Two separate transplastomic lines containing wild-type and codon optimized gene sequences for VP28 were evaluated for recombinant protein levels. Only the codon optimized line gave rise to detectable VP28 in western blot analysis, which demonstrated that optimization for chloroplast codon bias improved the efficiency of expression and that the gene design produced a favourable RNA secondary structure with suitable free energy for translation. In addition, bile salt and acid tolerance tests demonstrated that this transgenic Chlamydomonas can tolerate mildly acidic (pH 5.0) conditions and 0.30% bile salts. These features indicated that algal cells are suitable for delivering viral antigens through a shrimp's digestive system. In WSSV infection experiments, the highest survival rate (87%) was recorded in shrimps fed with the codon optimized VP28 line mixed into their feed, indicating that this line could be employed in the control of WSSV spread in shrimp populations. This algal strategy offers a new, efficient, fast and less labour-intensive method for the control of other diseases in aquatic animals through oral delivery
The primordial Helium-4 abundance determination: systematic effects
By extrapolating to O/H = N/H = 0 the empirical correlations Y-O/H and Y-N/H
defined by a relatively large sample of ~ 45 Blue Compact Dwarfs (BCDs), we
have obtained a primordial 4Helium mass fraction Yp= 0.2443+/-0.0015 with dY/dZ
= 2.4+/-1.0. This result is in excellent agreement with the average Yp=
0.2452+/-0.0015 determined in the two most metal-deficient BCDs known, I Zw 18
(Zsun/50) and SBS 0335-052 (Zsun/41), where the correction for He production is
smallest. The quoted error (1sigma) of < 1% is statistical and does not include
systematic effects. We examine various systematic effects including collisional
excitation of Hydrogen lines, ionization structure and temperature fluctuation
effects, and underlying stellar HeI absorption, and conclude that combining all
systematic effects, our Yp may be underestimated by ~ 2-4%. Taken at face
value, our Yp implies a baryon-to-photon number ratio eta = 4.7x10^-10 and a
baryon mass fraction Omega_b h^2_{100} = 0.017+/-0.005 (2sigma), consistent
with the values obtained from deuterium and Cosmic Microwave Background
measurements. Correcting Yp upward by 2-4% would make the agreement even
better.Comment: 12 pages, 5 PS figures, to appear in "Matter in the Universe", ed P.
Jetzer, K. Pretzl and R. von Steiger, Kluwer, Dordrecht (2002
The young age of the extremely metal-deficient blue compact dwarf galaxy SBS 1415+437
We use Multiple Mirror Telescope (MMT) spectrophotometry and Hubble Space
Telescope (HST) Faint Object Spectrograph (FOS) spectra and Wide Field and
Planetary Camera 2 (WFPC2) V and I images to study the properties and
evolutionary status of the nearby (D = 11.4 Mpc) extremely metal-deficient blue
compact dwarf (BCD) galaxy SBS 1415+437=CG 389. The oxygen abundance in the
galaxy is 12+log(O/H)=7.60+/-0.01 or Zsun/21. The helium mass fraction in SBS
1415+437 is Y=0.246+/-0.004 which agrees with the primordial helium abundance
determined by Izotov & Thuan using a much larger sample of BCDs. The
alpha-elements-to-oxygen abundance ratios (Ne/O, S/O, Ar/O) are in very good
agreement with the mean values for other metal-deficient BCDs and are
consistent with the scenario that these elements are made in massive stars. The
Fe/O abundance ratio is ~2 times smaller than the solar ratio. The Si/O ratio
is close to the solar value, implying that silicon is not significantly
depleted into dust grains. The values of the N/O and C/O ratios imply that
intermediate-mass stars have not had time to evolve in SBS 1415+437 and release
their nucleosynthesis products and that both N and C in the BCD have been made
by massive stars only. This sets an upper limit of ~100 Myr on the age of SBS
1415+437. The (V-I) color of the low-surface-brightness component of the galaxy
is blue (<0.4 mag) indicative of a very young underlying stellar population.
The (V-I) - I color-magnitude diagrams of the resolved stellar populations in
different regions of SBS 1415+437 suggest propagating star formation from the
NE side of the galaxy to the SW. All regions in SBS 1415+437 possess very blue
spectral energy distributions (SED). We find that the ages of the stellar
populations in SBS 1415+437 to range from a few Myr to 100 Myr.Comment: 25 pages, 12 PS and 5 JPG figures, to appear in Ap
Internal Dynamics of the Hypercompact H II Region G28.20-0.04N
High resolution (0.15") Very Large Array observations of 7 mm continuum and
H53a line emission toward the hypercompact H II region G28.20-0.04N reveal the
presence of large-scale ordered motions. We find a velocity gradient of 1000
km/s/pc along the minor axis of the continuum source. Lower resolution
(1.0"-2.3") radio recombination line observations indicate a systematic
increase of line width from H30alpha to H92alpha. Under the assumption that the
H30alpha line does not suffer significant pressure broadening, we have
deconvolved the contributions of statistical broadening (thermal, turbulent,
and pressure) from large-scale motions. The pressure broadening of the
H53alpha, H76alpha, and H92alpha lines implies an electron density of 6.9E+06,
8.5E+05, and 2.8E+05 cm^(-3), respectively.Comment: 12 pages, 2 figures, 1 table, Accepted for publication in Ap
Spectrophotometric Observations of Blue Compact Dwarf Galaxies: Mrk 370
We present results from a detailed spectrophotometric analysis of the blue
compact dwarf galaxy (BCD) Mrk 370, based on deep UBVRI broad-band and Halpha
narrow-band observations, and long-slit and two-dimensional spectroscopy of its
brightest knots. The spectroscopic data are used to derive the internal
extinction, and to compute metallicities, electronic density and temperature in
the knots. By subtracting the contribution of the underlying older stellar
population, modeled by an exponential function, removing the contribution from
emission lines, and correcting for extinction, we can measure the true colors
of the young star-forming knots. We show that the colors obtained this way
differ significantly from those derived without the above corrections, and lead
to different estimates of the ages and star-forming history of the knots. Using
predictions of evolutionary synthesis models, we estimate the ages of both the
starburst regions and the underlying stellar component. We found that we can
reproduce the colors of all the knots with an instantaneous burst of star
formation and the Salpeter initial mass function with an upper mass limit of
100 solar masses. The resulting ages range between 3 and 6 Myrs. The colors of
the low surface brightness component are consistent with ages larger than 5
Gyr. The kinematic results suggest ordered motion around the major axis of the
galaxy.Comment: 26 pages with 14 figures; accepted for publication in Ap
The Primordial Abundance of He4: An Update
We include new data in an updated analysis of helium in low metallicity
extragalactic HII regions with the goal of deriving the primordial abundance of
He4 (Y_P). We show that the new observations of Izotov et al (ITL) are
consistent with previous data. However they should not be taken in isolation to
determine (Y_P) due to the lack of sufficiently low metallicity points. We use
the extant data in a semi-empirical approach to bounding the size of possible
systematic uncertainties in the determination of (Y_P). Our best estimate for
the primordial abundance of He4 assuming a linear relation between He4 and O/H
is Y_P = 0.230 \pm 0.003 (stat) based on the subset of HII regions with the
lowest metallicity; for our full data set we find Y_P = 0.234 \pm 0.002 (stat).
Both values are entirely consistent with our previous results. We discuss the
implications of these values for standard big bang nucleosynthesis (SBBN),
particularly in the context of recent measurements of deuterium in high
redshift, low metallicity QSO absorption-line systems.Comment: 26 pages, latex, 6 ps figure
Deflection of ice cover caused by an underwater body moving in channel
Deflections and strains in an ice cover of a frozen channel caused by an underwater body moving under the ice with a constant speed along the channel are studied. The channel is of rectangular cross section, the fluid in the channel is inviscid and incompressible. The ice cover is clamped to the channel walls. The ice cover is modeled by a thin viscoelastic plate. The underwater body is modeled by a three-dimensional dipole. The intensity of the dipole is related to the speed and size of the underwater body. The problem is considered within the linear theory of hydroelasticity. For small deflections of the ice cover the velocity potential of the dipole in the channel is obtained by the method of images in leading order without account for the deflection of the ice cover. The problem of moving dipole in the channel with rigid walls provides the hydrodynamic pressure on the upper boundary of the channel, which corresponds to the ice cover. This pressure distribution does not depend on the deflection of the ice cover in the leading approximation. The deflections of the ice and strains in the ice plate are independent of time in the coordinate system moving together with the dipole. The problem is solved numerically using the Fourier transform, method of the normal modes and the truncation method for infinite systems of algebraic equations
Spectrophotometric investigations of Blue Compact Dwarf Galaxies: Markarian 35
We present results from a detailed spectrophotometric analysis of the blue
compact dwarf galaxy Mrk 35 (Haro 3), based on deep optical (B,V,R,I) and
near-IR (J,H,K) imaging, Halpha narrow-band observations and long-slit
spectroscopy. The optical emission of the galaxy is dominated by a central
young starburst, with a bar-like shape, while an underlying component of stars,
with elliptical isophotes and red colors, extends more than 4 kpc from the
galaxy center. High resolution Halpha and color maps allow us to identify the
star-forming regions, to spatially discriminate them from the older stars, and
to recognize several dust patches. We derive colors and Halpha parameters for
all the identified star-forming knots. Observables derived for each knot are
corrected for the contribution of the underlying older stellar population, the
contribution by emission lines, and from interstellar extinction, and compared
with evolutionary synthesis models. We find that the contributions of these
three factors are by no means negligible and that they significantly vary
across the galaxy. Therefore, careful quantification and subtraction of
emission lines, galaxy host contribution, and interstellar reddening at every
galaxy position, are essential to derive the properties of the young stars in
BCDs. We find that we can reproduce the colors of all the knots with an
instantaneous burst of star formation and the Salpeter initial mass function
with an upper mass limit of 100 M_solar. In all cases the knots are just a few
Myr old. The underlying population of stars has colors consistent with being
several Gyr old.Comment: 21 pages, 13 figures. Accepted for publication in ApJ, tentatively
scheduled for the ApJ November 1, 2007 v669n1 issu
- âŠ