217 research outputs found
A note on the metallization of compressed liquid hydrogen
We examine the molecular-atomic transition in liquid hydrogen as it relates
to metallization. Pair potentials are obtained from first principles molecular
dynamics and compared with potentials derived from quadratic response. The
results provide insight into the nature of covalent bonding under extreme
conditions. Based on this analysis, we construct a schematic
dissociation-metallization phase diagram and suggest experimental approaches
that should significantly reduce the pressures necessary for the realization of
the elusive metallic phase of hydrogen.Comment: 11 pages, 4 figure
Strongly extreme points and approximation properties
We show that if is a strongly extreme point of a bounded closed convex
subset of a Banach space and the identity has a geometrically and topologically
good enough local approximation at , then is already a denting point. It
turns out that such an approximation of the identity exists at any strongly
extreme point of the unit ball of a Banach space with the unconditional compact
approximation property. We also prove that every Banach space with a Schauder
basis can be equivalently renormed to satisfy the sufficient conditions
mentioned. In contrast to the above results we also construct a non-symmetric
norm on for which all points on the unit sphere are strongly extreme, but
none of these points are denting.Comment: 14 page
Structure and phase boundaries of compressed liquid hydrogen
We have mapped the molecular-atomic transition in liquid hydrogen using first
principles molecular dynamics. We predict that a molecular phase with
short-range orientational order exists at pressures above 100 GPa. The presence
of this ordering and the structure emerging near the dissociation transition
provide an explanation for the sharpness of the molecular-atomic crossover and
the concurrent pressure drop at high pressures. Our findings have non-trivial
implications for simulations of hydrogen; previous equation of state data for
the molecular liquid may require revision. Arguments for the possibility of a
order liquid-liquid transition are discussed
A Statistical Survey of Peculiar L and T Dwarfs in SDSS, 2MASS, and WISE
We present the final results from a targeted search for brown dwarfs with
unusual near-infrared colors. From a positional cross-match of SDSS, 2MASS and
WISE, we have identified 144 candidate peculiar L and T dwarfs. Spectroscopy
confirms that 20 of the objects are peculiar or are candidate binaries. Nine of
the 420 objects in our sample are young (200 Myr; 2.1%) and another 8
(1.9%) are unusually red with no signatures of youth. With a spectroscopic
color of 2.58 0.11 mag, one of the new objects, the L6 dwarf
2MASS J03530419+0418193, is among the reddest field dwarfs currently known and
is one of the reddest objects with no signatures of youth known to date. We
have also discovered another potentially very low gravity object, the L1 dwarf
2MASS J00133470+1109403, and independently identified the young L7 dwarf 2MASS
J00440332+0228112, first reported by Schneider and collaborators. Our results
confirm that signatures of low gravity are no longer discernible in low to
moderate resolution spectra of objects older than 200 Myr. The 1.9% of
unusually red L dwarfs that do not show other signatures of youth could be
slightly older, up to 400 Myr. In this case a red color may be
more diagnostic of moderate youth than individual spectral features. However,
its is also possible that these objects are relatively metal-rich, and so have
an enhanced atmospheric dust content.Comment: 27 pages, 7 figures, 5 tables, accepted to A
The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass
Upon its discovery in 2006, the young L7.5 companion to the solar analog HD
203030 was found to be unusual in being 200 K cooler than older late-L
dwarfs. HD 203030B offered the first clear indication that the effective
temperature at the L-to-T spectral type transition depends on surface gravity:
now a well-known characteristic of low-gravity ultra-cool dwarfs. An initial
age analysis of the G8V primary star indicated that the system was 130--400 Myr
old, and so the companion between 12--31 . Using moderate
resolution near-infrared spectra of HD 203030B, we now find features of very
low gravity comparable to those of 10--150 Myr-old L7--L8 dwarfs. We also
obtained more accurate near infrared and {\sl Spitzer}/IRAC photometry, and
find a color of mag---comparable to those
observed in other young planetary-mass objects---and a luminosity of
log() dex. We further reassess the
evidence for the young age of the host star, HD 203030, with a more
comprehensive analysis of the photometry and updated stellar activity
measurements and age calibrations. Summarizing the age diagnostics for both
components of the binary, we adopt an age of 100 Myr for HD 203030B and an age
range of 30--150 Myr. Using cloudy evolutionary models, the new companion age
range and luminosity result in a mass of 11 with a range of 8--15
, and an effective temperature of K.Comment: 12 pages, 7 figures, accepted for publication in A
Tetrahedral clustering in molten lithium under pressure
A series of electronic and structural transitions are predicted in molten
lithium from first principles. A new phase with tetrahedral local order
characteristic of bonded materials and poor electrical conductivity is
found at pressures above 150 GPa and temperatures as high as 1000 K. Despite
the lack of covalent bonding, weakly bound tetrahedral clusters with finite
lifetimes are predicted to exist. The stabilization of this phase in lithium
involves a unique mechanism of strong electron localization in interstitial
regions and interactions among core electrons. The calculations provide
evidence for anomalous melting above 20 GPa, with a melting temperature
decreasing below 300 K, and point towards the existence of novel low-symmetry
crystalline phases.Comment: 5 pages, 5 figure
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