7,247 research outputs found
PPl 15: The First Brown Dwarf Spectroscopic Binary
PPl 15 is the first object to have been confirmed as a brown dwarf by the
lithium test (in 1995), though its inferred mass was very close to the
substellar limit. It is a member of the Pleiades open cluster. Its position in
a cluster color-magnitude diagram suggested that it might be binary, and
preliminary indications that it is a double-lined spectroscopic binary were
reported by us in 1997. Here we report on the results of a consecutive week of
Keck HIRES observations of this system, which yield its orbit. It has a period
of about 5.8 days, and an eccentricity of 0.4+/-0.05. The rotation of the stars
is slow for this class of objects. Because the system luminosity is divided
between 2 objects with a mass ratio of 0.85, this renders each of them an
incontrovertible brown dwarf, with masses between 60-70 jupiters. We show that
component B is a little redder than A by studying their wavelength-dependent
line ratios, and that this variation is compatible with the mass ratio. We
confirm that the system has lithium, but cannot support the original conclusion
that it is depleted (which would be surprising, given the new masses). This is
a system of very close objects which, if they had combined, would have produced
a low mass star. We discuss the implications of this discovery for the theories
of binary formation and formation of very low mass objects.Comment: Latex, 18 pages, 4 figures, submitted to Astron.
Ternary inorganic compounds containing carbon, nitrogen, and oxygen at high pressures
Ternary C_{x}N_{y}O_{z} compounds are actively researched as novel high
energy density and ultrahard materials. Although some synthesis work has been
performed at ambient conditions, very little is known about the high pressure
chemistry of of C_{x}N_{y}O_{z} compounds. In this work, first principles
variable-composition evolutionary structure prediction calculations are
performed with the goal of discovering novel mixed C_{x}N_{y}O_{z} materials at
ambient and high pressure conditions. By systematically searching ternary
variable composition crystalline materials, the full ternary phase diagram is
constructed in the range of pressures from 0 to 100 GPa. The search finds the
C_{2}N_{2}O crystal containing extended covalent network of C, N, and O atoms,
having space group symmetry Cmc2_{1}, and stable above just 10 GPa. Several
other novel metastable (CO)_{x}-(N)_{y} crystalline compounds discovered during
the search, including two polymorphs of C_{2}NO_{2} and two polymorphs of
C_{3}N_{2}O_{3} crystals are found to be energetically favorable compared to
polymeric carbon monoxide (CO) and nitrogen. Predicted new compounds are
characterized by their Raman spectra and equations of state
A representative sample of Be Stars I: Sample Selection, Spectral Classification and Rotational Velocities
We present a sample of 58 Be stars containing objects of spectral types O9 to
B8.5 and luminosity classes III to V. We have obtained 3670 - 5070 Angstrom
spectra of the sample which are used to derive spectral types and rotational
velocities. We discuss the distribution of spectral types and rotational
velocities obtained and conclude that there are no significant selection
effects in our sample.Comment: 10 Pages, 9 Figures, Accepted for publication in A&A
Novel Potassium Polynitrides at High Pressures
Polynitrogen compounds have attracted great interest due to their potential
applications as high energy density materials. Most recently, a rich variety of
alkali polynitrogens (R_{x}N_{y}; R=Li, Na, and Cs) have been predicted to be
stable at high pressures and one of them, CsN_{5} has been recently
synthesized. In this work, various potassium polynitrides are investigated
using first-principles crystal structure search methods. Several novel
molecular crystals consisting of N_{4} chains, N_{5} rings, and N_{6} rings
stable at high pressures are discovered. In addition, an unusual nitrogen-rich
metallic crystal with stoichiometry K_{2}N_{16} consisting of a planar
two-dimensional extended network of nitrogen atoms arranged in fused eighteen
atom rings is found to be stable above 70 GPa. An appreciable electron transfer
from K to N atoms is responsible for the appearance of unexpected chemical
bonding in these crystals. The thermodynamic stability and high pressure phase
diagram is constructed. The electronic and vibrational properties of the
layered polynitrogen K_{2}N_{16} compound are investigated, and the
pressure-dependent IR-spectrum is obtained to assist in experimental discovery
of this new high-nitrogen content material
Pentazole and Ammonium Pentazolate: Crystalline Hydro-Nitrogens at High Pressure
Two new crystalline compounds, pentazole (N_{5}H) and ammonium pentazolate
(NH_{4})(N_{5}), both featuring cyclo-{\rm N_{5}^{-}} are discovered using
first principles evolutionary search of the nitrogen-rich portion of the
hydro-nitrogen binary phase diagram (N_{x}H_{y}, x\geqy) at high pressures.
Both crystals consist of the pentazolate N_{5}^{-} anion and ammonium
NH_{4}^{+} or hydrogen H^{+} cations. These two crystals are predicted to be
thermodynamically stable at pressures above 30 GPa for (NH_{4})(N_{5}) and 50
GPa for pentazole N_{5}H. The chemical transformation of ammonium azide
(NH_{4})(N_{3}) mixed with di-nitrogen (N_{2}) to ammonium pentazolate
(NH_{4})(N_{5}) is predicted to become energetically favorable above 12.5 GPa.
To assist in identification of newly synthesized compounds in future
experiments, the Raman spectra of both crystals are calculated and mode
assignments are made as a function of pressure up to 75 GPa
Stabilized gas laser oscillators Final report, 24 Jun. 1964 - 21 Jun. 1966
Phase-locking scheme for frequency-stabilized gas laser oscillator
Gamma Ray Bursts in the Era of Rapid Followup
We present a status report on the study of gamma-ray bursts (GRB) in the era
of rapid follow-up using the world's largest robotic optical telescopes - the
2-m Liverpool and Faulkes telescopes. Within the context of key unsolved issues
in GRB physics, we describe (1) our innovative software that allows real-time
automatic analysis and interpretation of GRB light curves, (2) the novel
instrumentation that allows unique types of observations (in particular, early
time polarisation measurements) and (3) the key science questions and
discoveries to which robotic observations are ideally suited, concluding with a
summary of current understanding of GRB physics provided by combining rapid
optical observations with simultaneous observations at other wavelengths.Comment: 20 pages, 12 figures; Review article accepted for publication in
Advances in Astronomy, special issue 'Robotic Astronomy (Hindawi Publishing
Corporation)
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