6,642 research outputs found
Plant canopy shape and the influences on UV exposures to the canopy
The solar spectra at selected sites over hemispherical, conical and pinnacle plant canopy models has been evaluated with a dosimetric technique. The irradiance at the sites varies by up to a factor of 0.31 compared to the irradiance on a horizontal plane. The biologically effective (UVBE) exposures evaluated with the dosimetric technique at sites over the plant canopy are up to 19% of that on a horizontal plane. Compared to a spectroradiometer, the technique provides a more practicable method of measuring the UVBE exposures at multiple sites over a plant canopy. Usage of a dosimeter at one site to provide the exposures at that site for different sun angles introduces an error of more than 50%. Knowledge of the spectra allowed the UV and UVBE exposures to be calculated at each site along with the exposures to the entire canopies. These were dependent on the sun angle and the canopy shape. For plant damage, the UVBE was a maximum of about 1.4 mJ cm-2/min. Compared to the hemispherical canopy, the UVBE exposure for generalised plant damage was 45% less for the pinnacle canopy and 23% less for the conical canopy. The canopy exposures could not be determined from measurements of the ambient exposure
A Black Hole of > 6 Solar Masses in the X-ray Nova XTE J1118+480
Observations of the quiescent X-ray nova XTE J1118+480 with the new 6.5-m MMT
have revealed that the velocity amplitude of the dwarf secondary is 698 +/- 14
km/s and the orbital period of the system is 0.17013 +/- 0.00010 d. The implied
value of the mass function, f(M) = 6.00 +/- 0.36 solar masses, provides a hard
lower limit on the mass of the compact primary that greatly exceeds the maximum
allowed mass of a neutron star. Thus we conclude that the compact primary is a
black hole. Among the eleven dynamically established black-hole X-ray novae,
the large mass function of XTE J1118+480 is rivaled only by that of V404 Cyg.
We estimate that the secondary supplies 34% +/- 8% of the total light at 5900A
and that its spectral type is in the range K5V to M1V. A double-humped I-band
light curve is probably due to ellipsoidal modulation, although this
interpretation is not entirely secure because of an unusual 12-minute offset
between the spectroscopic and photometric ephemerides. Assuming that the light
curve is ellipsoidal, we present a provisional analysis which indicates that
the inclination of the system is high and the mass of the black hole is
correspondingly modest. The broad Balmer emission lines (FWHM = 2300-2900 km/s)
also suggest a high inclination. For the range of spectral types given above,
we estimate a distance of 1.8 +/- 0.6 kpc.Comment: 4 pages, 2 figures, to appear in ApJ Letters; Minor changes to Fig 1
Gravitational wave bursts from cusps and kinks on cosmic strings
The strong beams of high-frequency gravitational waves (GW) emitted by cusps
and kinks of cosmic strings are studied in detail. As a consequence of these
beams, the stochastic ensemble of GW's generated by a cosmological network of
oscillating loops is strongly non Gaussian, and includes occasional sharp
bursts that stand above the ``confusion'' GW noise made of many smaller
overlapping bursts. Even if only 10% of all string loops have cusps these
bursts might be detectable by the planned GW detectors LIGO/VIRGO and LISA for
string tensions as small as . In the implausible case
where the average cusp number per loop oscillation is extremely small, the
smaller bursts emitted by the ubiquitous kinks will be detectable by LISA for
string tensions as small as . We show that the strongly
non Gaussian nature of the stochastic GW's generated by strings modifies the
usual derivation of constraints on from pulsar timing experiments. In
particular the usually considered ``rms GW background'' is, when G \mu \gaq
10^{-7}, an overestimate of the more relevant confusion GW noise because it
includes rare, intense bursts. The consideration of the confusion GW noise
suggests that a Grand Unified Theory (GUT) value is
compatible with existing pulsar data, and that a modest improvement in pulsar
timing accuracy could detect the confusion noise coming from a network of cuspy
string loops down to . The GW bursts discussed here might
be accompanied by Gamma Ray Bursts.Comment: 24 pages, 3 figures, Revtex, submitted to Phys. Rev.
An HST/WFPC Survey of Bright Young Clusters in M31. II. Photometry of Less Luminous Clusters in the Fields
We report on the properties of 89 low mass star clusters located in the
vicinity of luminous young clusters (blue globulars) in the disk of M31. 82 of
the clusters are newly detected. We have determined their integrated magnitudes
and colors, based on a series of Hubble Space Telescope Wide Field/Planetary
Camera 2 exposures in blue and red (HST filters F450W and F814W). The
integrated apparent magnitudes range from F450W = 17.5 to 22.5, and the colors
indicate a wide range of ages. Stellar color-magnitude diagrams for all
clusters were obtained and those with bright enough stars were fit to
theoretical isochrones to provide age estimates. The ages range from 12 Myr to
>500 Myr. Reddenings, which average E(F450 - F814) = 0.59 with a dispersion of
0.21 magnitudes, were derived from the main sequence fitting for those
clusters. Comparison of these ages and integrated colors with single population
theoretical models with solar abundances suggests a color offset of 0.085
magnitudes at the ages tested. Estimated ages for the remaining clusters are
based on their measured colors. The age-frequency diagram shows a steep decline
of number with age, with a large decrease in number per age interval between
the youngest and the oldest clusters detected.Comment: 20 pages, 9 figure
Spectroscopic Discovery of the Supernova 2003dh Associated with GRB 030329
We present early observations of the afterglow of the Gamma-Ray Burst (GRB)
030329 and the spectroscopic discovery of its associated supernova SN 2003dh.
We obtained spectra of the afterglow of GRB 030329 each night from March 30.12
(0.6 days after the burst) to April 8.13 (UT) (9.6 days after the burst). The
spectra cover a wavelength range of 350 nm to 850 nm. The early spectra consist
of a power-law continuum (F_nu ~ nu^{-0.9}) with narrow emission lines
originating from HII regions in the host galaxy, indicating a low redshift of
z=0.1687. However, our spectra taken after 2003 Apr. 5 show broad peaks in flux
characteristic of a supernova. Correcting for the afterglow emission, we find
the spectrum of the supernova is remarkably similar to the type Ic `hypernova'
SN 1998bw. While the presence of supernovae have been inferred from the light
curves and colors of GRB afterglows in the past, this is the first direct,
spectroscopic confirmation that a subset of classical gamma-ray bursts
originate from supernovae.Comment: published by ApJ Letters; additional material avilable at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
Evaluating the quality of evidence from a network meta-analysis
Systematic reviews that collate data about the relative effects of multiple interventions via network meta-analysis are highly informative for decision-making purposes. A network meta-analysis provides two types of findings for a specific outcome: the relative treatment effect for all pairwise comparisons, and a ranking of the treatments. It is important to consider the confidence with which these two types of results can enable clinicians, policy makers and patients to make informed decisions. We propose an approach to determining confidence in the output of a network meta-analysis. Our proposed approach is based on methodology developed by the Grading of Recommendations Assessment, Development and Evaluation (GRADE) Working Group for pairwise meta-analyses. The suggested framework for evaluating a network meta-analysis acknowledges (i) the key role of indirect comparisons (ii) the contributions of each piece of direct evidence to the network meta-analysis estimates of effect size; (iii) the importance of the transitivity assumption to the validity of network meta-analysis; and (iv) the possibility of disagreement between direct evidence and indirect evidence. We apply our proposed strategy to a systematic review comparing topical antibiotics without steroids for chronically discharging ears with underlying eardrum perforations. The proposed framework can be used to determine confidence in the results from a network meta-analysis. Judgements about evidence from a network meta-analysis can be different from those made about evidence from pairwise meta-analyses. © 2014 Salanti et al
Quintessence, the Gravitational Constant, and Gravity
Dynamical vacuum energy or quintessence, a slowly varying and spatially
inhomogeneous component of the energy density with negative pressure, is
currently consistent with the observational data. One potential difficulty with
the idea of quintessence is that couplings to ordinary matter should be
strongly suppressed so as not to lead to observable time variations of the
constants of nature. We further explore the possibility of an explicit coupling
between the quintessence field and the curvature. Since such a scalar field
gives rise to another gravity force of long range (\simg H^{-1}_0), the solar
system experiments put a constraint on the non-minimal coupling: |\xi| \siml
10^{-2}.Comment: 9 pages, a version to be published in Phys.Rev.
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