5,123 research outputs found
Two New Species of Leafblight Fungi on Kalmia Latifolia
The evergreen shrub, Kalmia latifolia L., commonly known as mountain laurel, calico bush, or sheep-kill, grows widely on rocky, acid soils in the eastern United States. Whether growing in its natural habit or in cultivation, mountain laurel appears to be equally subject to attack by fungi. The following account characterizes and discusses two of these fungi. One of them has not been described previously and additional observations have been made regarding the developmental morphology of the other one.
Both pathogens are Pyrenomycetes, one a Physalospora and the other a Diaporthe. Each produces a leafblight disease. Tiny brown discolorations on young leaves characterize the early stages of attack by both organisms. These small lesions gradually enlarge and become irregular brown spots that may encompass the major portion of the leaf surface. The invaded tissues are darkest near the margins of the lesions, but a reddish zone lies between the darker border and the surrounding green tissues. Severely attacked leaves are deformed and shed prematurely.
The reproductive structures of the Physalospora occur on the lower surface and begin to develop before the leaves are shed. The pycnidial stromata of the Diaporthe elevate the epidermis and caticle, and consequently produce grayish spots on the leaf surface. Both fungi continue to develop after the leaves have fallen, and since the mycelia extend beyond the margins of the lesions, perithecia ultimately may occupy most of the leaf surface. [excerpt
Negative differential conductance induced by spin-charge separation
Spin-charge states of correlated electrons in a one-dimensional quantum dot
attached to interacting leads are studied in the non-linear transport regime.
With non-symmetric tunnel barriers, regions of negative differential
conductance induced by spin-charge separation are found. They are due to a
correlation-induced trapping of higher-spin states without magnetic field, and
associated with a strong increase in the fluctuations of the electron spin.Comment: REVTEX, 4 pages including 3 figures; Accepted for publication on
Physical Review Letter
On the radial density profile of intracluster gas tracing the isothermal dark halo with a finite core
The cusped NFW universal density profile suggested by typical CDM models has
been challenged in recent years by the discoveries of the soft cores for a
broad range of masses from dwarf galaxies to clusters of galaxies. It is thus
desirable that a new, analytic model would instead become available for
virialized dark halos. One promising candidate is probably the empirical
density profile proposed by Burkert, which resembles an isothermal profile with
a constant core in the inner region and matches the NFW profile at large radii.
Meanwhile, such a revised dark halo (RDH) profile has turned out a great
success on galactic scales. This stimulates us to apply the RDH profile to more
massive systems like clusters of galaxies. In this paper we have made an
attempt to derive the radial density profile of intracluster gas from the RDH
profile, and compare it with those revealed by X-ray observations and inferred
from the NFW profile. It is shown that the RDH predicted gas density can be
well represented by the conventional beta model with a typical beta parameter
of 0.7-0.9. Fitting the theoretically predicted X-ray surface brightness
profile to an ensemble of 45 X-ray clusters, we find that the RDH and NFW
profiles become to be almost indistinguishable. Yet, the RDH profile allows us
to work out straightforwardly the central dark matter density of clusters from
X-ray measurements. Using the 45 clusters, we find that the central densities
of clusters have an average value of ~0.01 solar mass pc^{-3}, in agreement
with the result estimated on galactic scales, which reinforces the claim for
the presence of the soft halo cores over the entire mass range.Comment: 22 pages, 7 figures, to appear in ApJ v543 Nov10, 2000 issu
A dual output polarimeter devoted to the study of the Cosmic Microwave Background
We have developed a correlation radiometer at 33 GHz devoted to the search
for residual polarization of the Cosmic Microwave Background (CMB). The two
instruments`s outputs are linear combination of two Stokes Parameters (Q and U
or U and V). The instrument is therefore directly sensitive to the polarized
component of the radiation (respectively linear and circular). The radiometer
has a beam-width oif 7 or 14 deg, but it can be coupled to a telescope
increasing the resolution. The expected CMB polarization is at most a part per
milion. The polarimeter has been designed to be sensitive to this faint signal,
and it has been optimized to improve its long term stability, observing from
the ground. In this contribution the performances of the instrument are
presented, together with the preliminary test and observations.Comment: 12 pages, 6 figures, in print on the Proc. SPIE Conf. - August 200
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