10,252 research outputs found
Measurement of the Nodal Precession of WASP-33 b via Doppler Tomography
We have analyzed new and archival time series spectra taken six years apart
during transits of the hot Jupiter WASP-33 b, and spectroscopically resolved
the line profile perturbation caused by the Rossiter-McLaughlin effect. The
motion of this line profile perturbation is determined by the path of the
planet across the stellar disk, which we show to have changed between the two
epochs due to nodal precession of the planetary orbit. We measured rates of
change of the impact parameter and the sky-projected spin-orbit misalignment of
yr and
~ yr, respectively,
corresponding to a rate of nodal precession of
~ yr. This is only the
second measurement of nodal precession for a confirmed exoplanet transiting a
single star. Finally, we used the rate of precession to set limits on the
stellar gravitational quadrupole moment of
.Comment: Published in ApJL. 5 pages, 3 figures. Corrected error in the
calculation of J_
Possible Observational Criteria for Distinguishing Brown Dwarfs from Planets
The difference in formation process between binary stars and planetary
systems is reflected in their composition as well as their orbital
architecture, particularly orbital eccentricity as a function of orbital
period. It is suggested here that this difference can be used as an
observational criterion to distinguish between brown dwarfs and planets.
Application of the orbital criterion suggests that with three possible
exceptions, all of the recently-discovered substellar companions discovered to
date may be brown dwarfs and not planets. These criterion may be used as a
guide for interpretation of the nature of sub-stellar mass companions to stars
in the future.Comment: LaTeX, 11 pages including 2 figures, accepted for publication in the
Astrophysical Journal Letter
Failure mechanisms of graphene under tension
Recent experiments established pure graphene as the strongest material known
to mankind, further invigorating the question of how graphene fails. Using
density functional theory, we reveal the mechanisms of mechanical failure of
pure graphene under a generic state of tension. One failure mechanism is a
novel soft-mode phonon instability of the -mode, whereby the graphene
sheet undergoes a phase transition and is driven towards isolated benzene rings
resulting in a reduction of strength. The other is the usual elastic
instability corresponding to a maximum in the stress-strain curve. Our results
indicate that finite wave vector soft modes can be the key factor in limiting
the strength of monolayer materials
Searching for Planets in the Hyades V: Limits on Planet Detection in the Presence of Stellar Activity
We present the results of a radial velocity survey of a sample of Hyades
stars, and discuss the effects of stellar activity on radial velocity
measurements. The level of radial velocity scatter due to rotational modulation
of stellar surface features for the Hyades is in agreement with the predictions
of Saar & Donahue (1997)- the maximum radial velocity rms of up to ~50 m/s,
with an average rms of ~16 m/s. In this sample of 94 stars, we find 1 new
binary, 2 stars with linear trends indicative of binary companions, and no
close-in giant planets. We discuss the limits on extrasolar planet detection in
the Hyades and the constraints imposed on radial velocity surveys of young
stars.Comment: To appear in the June 2004 issue of A
Recommended from our members
Degradation of contaminants of emerging concern by UV/H2O2 for water reuse: Kinetics, mechanisms, and cytotoxicity analysis.
Advanced oxidation using UV and hydrogen peroxide (UV/H2O2) has been widely applied to degrade contaminants of emerging concern (CECs) in wastewater for water reuse. This study investigated the degradation kinetics of mixed CECs by UV/H2O2 under variable H2O2 doses, including bisphenol A, estrone, diclofenac, ibuprofen, and triclosan. Reverse osmosis (RO) treated water samples from Orange County Water District's Groundwater Replenishment System (GWRS) potable reuse project were collected on different dates and utilized as reaction matrices with spiked additions of chemicals (CECs and H2O2) to assess the application of UV/H2O2. Possible degradation pathways of selected CECs were proposed based on high resolution mass spectrometry identification of transformation products (TPs). Toxicity assessments included cytotoxicity, aryl hydrocarbon receptor-binding activity, and estrogen receptor-binding activity, in order to evaluate potential environmental impacts resulting from CEC degradation by UV/H2O2. Cytotoxicity and estrogenic activity were significantly reduced during the degradation of mixed CECs in Milli-Q water by UV/H2O2 with high UV fluence (3200 mJ cm-2). However, in GWRS RO-treated water samples collected in April 2017, the cytotoxicity and estrogen activity of spiked CEC-mixture after UV/H2O2 treatment were not significantly eliminated; this might be due to the high concentration of target CEC and their TPs, which was possibly affected by the varied quality of the secondary treatment influent at this facility such as sewer-shed and wastewater discharges. This study aimed to provide insight on the impacts of post-UV/H2O2 CECs and TPs on human and ecological health at cellular level
The CN Isotopic Ratios In Comets
Our aim is to determine the isotopic ratios (12)C/(13)C and (14)N/(15)N in a variety of comets and link these measurements to the formation and evolution of the solar system. The (12)C/(13)C and (14)N/(15)N isotopic ratios are measured for the CN radical by means of high-resolution optical spectra of the R branch of the B-X (0, 0) violet band. 23 comets from different dynamical classes have been observed, sometimes at various heliocentric and nucleocentric distances, in order to estimate possible variations of the isotopic ratios in parent molecules. The (12)C/(13)C and (14)N/(15)N isotopic ratios in CN are remarkably constant (average values of, respectively, 91.0 +/- 3.6 and 147.8 +/- 5.7) within our measurement errors, for all comets whatever their origin or heliocentric distance. While the carbon isotopic ratio does agree with the terrestrial value (89), the nitrogen ratio is a factor of two lower than the terrestrial value (272), indicating a fractionation in the early solar system, or in the protosolar nebula, common to all the comets of our sample. This points towards a common origin of the comets independently of their birthplaces, and a relationship between HCN and CN.NSFAstronom
Lead Precipitation Fluxes at Tropical Oceanic Sites Determined From ^(210)Pb Measurements
Concentrations of lead, ^(210)Pb, and ^(210)Po were measured in rain selected for least influence by local sources of contamination at several tropical and subtropical islands (Enewetak; Pigeon Key, Florida; and American Samoa) and shipboard stations (near Bermuda and Tahiti). Ratios expressed as ng Pb/dpm ^(210)Pb in rain were 250–900 for Pigeon Key (assuming 12% adsorption for ^(210)Pb and no adsorption for lead), depending on whether the air masses containing the analyzed rain came from the Caribbean or from the continent, respectively; about 390 for the northern Sargasso Sea downwind from emissions of industrial lead in North America; 65 for Enewetak, remote from continental emissions of industrial lead in the northern hemisphere; and 14 near Tahiti, a remote location in the southern hemisphere where industrial lead emissions to the atmosphere are much less than in the northern hemisphere. (The American Samoa sample yielded a higher ratio than Tahiti; the reason for this is not clear but may be due to local Pb sources.) The corresponding fluxes of lead to the oceans, based on measured or modeled ^(210)Pb precipitation fluxes, are about 4 ng Pb/cm^2y for Tahiti, 10 for Enewetak, and 270 for the Sargasso Sea site, and between 110 to 390 at Pigeon Key
Large excess of heavy nitrogen in both hydrogen cyanide and cyanogen from comet 17P/Holmes
From millimeter and optical observations of the Jupiter-family comet
17P/Holmes performed soon after its huge outburst of October 24, 2007, we
derive 14 N/15N = 139 +/- 26 in HCN, and 14N/15N = 165 +/- 40 in CN,
establishing that HCN has the same non-terrestrial isotopic composition as CN.
The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp)
after a reanalysis of previously published measurements. These results are
compatible with HCN being the prime parent of CN in cometary atmospheres. The
15N excess relative to the Earth atmospheric value indicates that N-bearing
volatiles in the solar nebula underwent important N isotopic fractionation at
some stage of Solar System formation. HCN molecules never isotopically
equilibrated with the main nitrogen reservoir in the solar nebula before being
incorporated in Oort-cloud and Kuiper-belt comets. The 12C/13C ratios in HCN
and CN are measured to be consistent with the terrestrial value.Comment: Accepted for publication in the Astrophysical Journal (Letters) 4
page
Spitzer observations of the Hyades: Circumstellar debris disks at 625 Myr of age
We use the Spitzer Space Telescope to search for infrared excess at 24, 70,
and 160 micron due to debris disks around a sample of 45 FGK-type members of
the Hyades cluster. We supplement our observations with archival 24 and 70
micron Spitzer data of an additional 22 FGK-type and 11 A-type Hyades members
in order to provide robust statistics on the incidence of debris disks at 625
Myr of age an era corresponding to the late heavy bombardment in the Solar
System. We find that none of the 67 FGK-type stars in our sample show evidence
for a debris disk, while 2 out of the 11 A-type stars do so. This difference in
debris disk detection rate is likely to be due to a sensitivity bias in favor
of early-type stars. The fractional disk luminosity, L_dust/L*, of the disks
around the two A-type stars is ~4.0E-5, a level that is below the sensitivity
of our observations toward the FGK-type stars. However, our sensitivity limits
for FGK-type stars are able to exclude, at the 2-sigma level, frequencies
higher than 12% and 5% of disks with L_dust/L* > 1.0E-4 and L_dust/L* > 5.0E-4,
respectively. We also use our sensitivity limits and debris disk models to
constrain the maximum mass of dust, as a function of distance from the stars,
that could remain undetected around our targets.Comment: 33 pages, 11 figures, accepted by Ap
Fact Sheet no.1: Nonpoint Source Pollution and Water Quality of Northwest Arkansas
Preservation of a clean, safe, and biologically diverse environment can capture public attention as few other issues can. Indeed, recent international cooperative efforts indicate that there is an increasing global desire to pass on to future generations an environment at least equal in quality to the one we inherited
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