17,054 research outputs found
CO Infrared Phonon Modes in Interstellar Ice Mixtures
CO ice is an important reservoir of carbon and oxygen in star and planet
forming regions. Together with water and CO, CO sets the physical and
chemical characteristics of interstellar icy grain mantles, including
desorption and diffusion energies for other ice constituents. A detailed
understanding of CO ice spectroscopy is a prerequisite to characterize
CO interactions with other volatiles both in interstellar ices and in
laboratory experiments of interstellar ice analogs. We report laboratory
spectra of the CO longitudinal optical (LO) phonon mode in pure CO ice
and in CO ice mixtures with HO, CO, O components. We show that the
LO phonon mode position is sensitive to the mixing ratio of various ice
components of astronomical interest. In the era of JWST, this characteristic
could be used to constrain interstellar ice compositions and morphologies. More
immediately, LO phonon mode spectroscopy provides a sensitive probe of ice
mixing in the laboratory and should thus enable diffusion measurements with
higher precision than has been previously possible
Measurement of the Spatial Cross-Correlation Function of Damped Lyman Alpha Systems and Lyman Break Galaxies
We present the first spectroscopic measurement of the spatial
cross-correlation function between damped Lyman alpha systems (DLAs) and Lyman
break galaxies (LBGs). We obtained deep u'BVRI images of nine QSO fields with
11 known z ~ 3 DLAs and spectroscopically confirmed 211 R < 25.5
photometrically selected z > 2 LBGs. We find strong evidence for an overdensity
of LBGs near DLAs versus random, the results of which are similar to that of
LBGs near other LBGs. A maximum likelihood cross-correlation analysis found the
best fit correlation length value of r_0 = 2.9^(+1.4)_(-1.5) h^(-1)Mpc using a
fixed value of gamma = 1.6. The implications of the DLA-LBG clustering
amplitude on the average dark matter halo mass of DLAs are discussed.Comment: 12 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
CO diffusion and desorption kinetics in CO ices
Diffusion of species in icy dust grain mantles is a fundamental process that
shapes the chemistry of interstellar regions; yet measurements of diffusion in
interstellar ice analogs are scarce. Here we present measurements of CO
diffusion into CO ice at low temperatures (T=11--23~K) using CO
longitudinal optical (LO) phonon modes to monitor the level of mixing of
initially layered ices. We model the diffusion kinetics using Fick's second law
and find the temperature dependent diffusion coefficients are well fit by an
Arrhenius equation giving a diffusion barrier of 300 40 K. The low
barrier along with the diffusion kinetics through isotopically labeled layers
suggest that CO diffuses through CO along pore surfaces rather than through
bulk diffusion. In complementary experiments, we measure the desorption energy
of CO from CO ices deposited at 11-50 K by temperature-programmed
desorption (TPD) and find that the desorption barrier ranges from 1240 90
K to 1410 70 K depending on the CO deposition temperature and
resultant ice porosity. The measured CO-CO desorption barriers demonstrate
that CO binds equally well to CO and HO ices when both are compact. The
CO-CO diffusion-desorption barrier ratio ranges from 0.21-0.24 dependent on
the binding environment during diffusion. The diffusion-desorption ratio is
consistent with the above hypothesis that the observed diffusion is a surface
process and adds to previous experimental evidence on diffusion in water ice
that suggests surface diffusion is important to the mobility of molecules
within interstellar ices
Current International Policies on Plant Breeders’ Rights
Established and supported under the Australian Government’s Cooperative Research Centre Progra
NASA-JSC antenna near-field measurement system
Work was completed on the near-field range control software. The capabilities of the data processing software were expanded with the addition of probe compensation. In addition, the user can process the measured data from the same computer terminal used for range control. The design of the laser metrology system was completed. It provides precise measruement of probe location during near-field measurements as well as position data for control of the translation beam and probe cart. A near-field range measurement system was designed, fabricated, and tested
Observation of magnetization reversal and negative magnetization in a double perovskite compound Sr2YbRuO6
Detailed magnetic properties of the compound Sr2YbRuO6 are presented here.
The compound belongs to the family of double perovskites forming a monoclinic
structure. Magnetization meas-urements reveal clear evidence for two components
of magnetic ordering aligned opposite to each other, leading to a magnetization
reversal, compensation temperature (T* = 34 K) and neg-ative magnetization at
low temperatures and low magnetic fields. Heat capacity measurements
corroborate the presence of two components in the magnetic ordering and a
noticeable third anomaly at low temperatures (~15 K) which cannot be attributed
the Schottky effect. The calcu-lated magnetic entropy is substantially lower
than that expected for the ground states of the or-dered moments of Ru5+ and
Yb3+, indicating the presence of large crystal field effects and/ or
in-complete magnetic ordering and/or magnetic frustrations well above the
magnetic ordering. An attempt is made to explain the magnetization reversal
within the frameworks of available models.Comment: 15 pages text, 6 figures Journal-ref: J.Phys.:Condens.Matter
20(2008)23520
Dust Production of Comet 21P/Giacobini-Zinner Using Broadband Photometry
Comet 21P/Giacobini-Zinner is a Jupiter family comet, approximately 2 km in diameter, and is established to be the parent of the Draconids, a meteor shower known to outburst. In 1933 and 1946 up to 10,000 meteors per hour were reported for the Draconids, and 2011 saw a minor Draconid outburst. Meteor stream modeling/forecasting being a primary focus for the NASA Meteoroid Environment Office, it was decided to monitor 21P for three purposes: firstly to find the apparent and absolute magnitude with respect to heliocentric distance; second to calculate Af , a quantity that describes the dust production rate and is used in models to predict the activity of the Draconids; and thirdly to detect possible increases in cometary activity, which could correspond to future Draconid meteor outbursts. A similar study was done for 21P during its 2004-2006 close approach to the Sun in which apparent and absolute magnitudes were found with various heliocentric distances, as well as the dust production. At 2.32 AU from the Sun, 21P possessed an apparent magnitude of 17.05 and Af of 83 cm, and an apparent magnitude of 15.91 and Af of 130.66 cm at 1.76 AU from the sun
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