9,447 research outputs found
Stabilization of high-temperature superconducting A15 phase LaH below 100 GPa
High pressure plays a crucial role in the field of superconductivity.
Compressed hydride superconductors are leaders in the race for a material that
can conduct electricity without resistance at high or even room temperature.
Different synthetic paths under pressure will drive the formation of different
polyhydrides. In the present work, through precise control of the synthesis
pathway, we have discovered new lanthanum superhydride, cubic A15-type
LaH, with lower stabilization pressure compared to the reported
LaH. Superconducting LaH was obtained by laser
heating of LaH with ammonia borane at about 120 GPa. Transport measurements
reveal the maximum critical temperature (onset) = 105 K at
118 GPa, as evidenced by the sharp drop of electrical resistance and the
displacement of superconducting transitions in applied magnetic fields.
Extrapolated upper critical field (0) of LaH is
about 33 T at 114 GPa in agreement with theoretical estimates. Discovered
lanthanum hydride is a new member of the A15 family of superconductors with
exceeding the boiling point of liquid nitrogen
Metallic surface states in a correlated d-electron topological Kondo insulator candidate FeSb2
The resistance of a conventional insulator diverges as temperature approaches
zero. The peculiar low temperature resistivity saturation in the 4f Kondo
insulator (KI) SmB6 has spurred proposals of a correlation-driven topological
Kondo insulator (TKI) with exotic ground states. However, the scarcity of model
TKI material families leaves difficulties in disentangling key ingredients from
irrelevant details. Here we use angle-resolved photoemission spectroscopy
(ARPES) to study FeSb2, a correlated d-electron KI candidate that also exhibits
a low temperature resistivity saturation. On the (010) surface, we find a rich
assemblage of metallic states with two-dimensional dispersion. Measurements of
the bulk band structure reveal band renormalization, a large
temperature-dependent band shift, and flat spectral features along certain high
symmetry directions, providing spectroscopic evidence for strong correlations.
Our observations suggest that exotic insulating states resembling those in SmB6
and YbB12 may also exist in systems with d instead of f electrons
ALMA reveals sequential high-mass star formation in the G9.62+0.19 complex
Stellar feedback from high-mass stars (e.g., H{\sc ii} regions) can strongly
influence the surrounding interstellar medium and regulate star formation. Our
new ALMA observations reveal sequential high-mass star formation taking place
within one sub-virial filamentary clump (the G9.62 clump) in the G9.62+0.19
complex. The 12 dense cores (MM 1-12) detected by ALMA are at very different
evolutionary stages, from starless core phase to UC H{\sc ii} region phase.
Three dense cores (MM6, MM7/G, MM8/F) are associated with outflows. The
mass-velocity diagrams of outflows associated with MM7/G and MM8/F can be well
fitted with broken power laws. The mass-velocity diagram of SiO outflow
associated with MM8/F breaks much earlier than other outflow tracers (e.g., CO,
SO, CS, HCN), suggesting that SiO traces newly shocked gas, while the other
molecular lines (e.g., CO, SO, CS, HCN) mainly trace the ambient gas
continuously entrained by outflow jets. Five cores (MM1, MM3, MM5, MM9, MM10)
are massive starless core candidates whose masses are estimated to be larger
than 25 M_{\sun}, assuming a dust temperature of 20 K. The shocks from
the expanding H{\sc ii} regions ("B" \& "C") to the west may have great impact
on the G9.62 clump through compressing it into a filament and inducing core
collapse successively, leading to sequential star formation. Our findings
suggest that stellar feedback from H{\sc ii} regions may enhance the star
formation efficiency and suppress the low-mass star formation in adjacent
pre-existing massive clumps.Comment: Accepted to Ap
A Submillimetre Search for Cold Extended Debris Disks in the Beta Pictoris Moving Group
The Beta Pictoris Moving Group is a nearby stellar association of young
(12Myr) co-moving stars including the classical debris disk star beta Pictoris.
Due to their proximity and youth they are excellent targets when searching for
submillimetre emission from cold, extended, dust components produced by
collisions in Kuiper-Belt-like disks. They also allow an age independent study
of debris disk properties as a function of other stellar parameters. We
observed 7 infrared-excess stars in the Beta Pictoris Moving Group with the
LABOCA bolometer array, operating at a central wavelength of 870 micron at the
12-m submillimetre telescope APEX. The main emission at these wavelengths comes
from large, cold dust grains, which constitute the main part of the total dust
mass, and hence, for an optically thin case, make better estimates on the total
dust mass than earlier infrared observations. Fitting the spectral energy
distribution with combined optical and infrared photometry gives information on
the temperature and radial extent of the disk. From our sample, beta Pic,
HD181327, and HD172555 were detected with at least 3-sigma certainty, while all
others are below 2-sigma and considered non-detections. The image of beta Pic
shows an offset flux density peak located near the south-west extension of the
disk, similar to the one previously found by SCUBA at the JCMT. We present SED
fits for detected sources and give an upper limit on the dust mass for
undetected ones. We find a mean fractional dust luminosity f_dust=11x10^{-4} at
t=12Myr, which together with recent data at 100Myr suggests an f_dust propto
t^{-alpha} decline of the emitting dust, with alpha > 0.8.Comment: 11 pages, 3 figures, 3 tables; accepted for publication in Astronomy
& Astrophysic
Discovery of New Eunicellins from an Indonesian Octocoral Cladiella sp.
Two new 11-hydroxyeunicellin diterpenoids, cladieunicellin F (1) and (–)-solenopodin C (2), were isolated from an Indonesian octocoral Cladiella sp. The structures of eunicellins 1 and 2 were established by spectroscopic methods, and eunicellin 2 was found to be an enantiomer of the known eunicellin solenopodin C (3). Eunicellin 2 displayed inhibitory effects on the generation of superoxide anion and the release of elastase by human neutrophils. The previously reported structures of two eunicellin-based compounds, cladielloides A and B, are corrected in this study
A possible architecture of the planetary system HR 8799
HR8799 is a nearby A-type star with a debris disk and three planetary
candidates recently imaged directly. We undertake a coherent analysis of
various portions of observational data on all known components of the system.
The goal is to elucidate the architecture and evolutionary status of the
system. We try to further constrain the age and orientation of the system,
orbits and masses of the companions, as well as the location of dust. From the
high luminosity of debris dust and dynamical constraints, we argue for a rather
young system's age of <50Myr. The system must be seen nearly, but not exactly,
pole-on. Our analysis of the stellar rotational velocity yields an inclination
of 13-30deg, whereas i>20deg is needed for the system to be dynamically stable,
which suggests a probable inclination range of 20-30deg. The spectral energy
distribution is naturally reproduced with two dust rings associated with two
planetesimal belts. The inner "asteroid belt" is located at ~10AU inside the
orbit of the innermost companion and a "Kuiper belt" at >100AU is just exterior
to the orbit of the outermost companion. The dust masses in the inner and outer
ring are estimated to be ~1E-05 and 4E-02 M_earth, respectively. We show that
all three planetary candidates may be stable in the mass range suggested in the
discovery paper by Marois et al. 2008 (between 5 and 13 Jupiter masses), but
only for some of all possible orientations. Stable orbits imply a double
(4:2:1) mean-motion resonance between all three companions. We finally show
that in the cases where the companions themselves are orbitally stable, the
dust-producing planetesimal belts are also stable against planetary
perturbations.Comment: 12 pages, 14 figures, 4 tables, accepted to be published in Astronomy
& Astrophysics (May 20, 2009
A near-infrared interferometric survey of debris disc stars. II. CHARA/FLUOR observations of six early-type dwarfs
High-precision interferometric observations of six early-type main sequence
stars known to harbour cold debris discs have been obtained in the
near-infrared K band with the FLUOR instrument at the CHARA Array. The measured
squared visibilities are compared to the expected visibility of the stellar
photospheres based on theoretical photospheric models taking into account
rotational distortion, searching for potential visibility reduction at short
baselines due to circumstellar emission. Our observations bring to light the
presence of resolved circumstellar emission around one of the six target stars
(zeta Aql) at the 5 sigma level. The morphology of the emission source cannot
be directly constrained because of the sparse spatial frequency sampling of our
interferometric data. Using complementary adaptive optics observations and
radial velocity measurements, we find that the presence of a low-mass companion
is a likely origin for the excess emission. The potential companion has a
K-band contrast of four magnitudes, a most probable mass of about 0.6 Msun, and
is expected to orbit between about 5.5 AU and 8 AU from its host star assuming
a purely circular orbit. Nevertheless, by adjusting a physical debris disc
model to the observed Spectral Energy Distribution of the zeta Aql system, we
also show that the presence of hot dust within 10 AU from zeta Aql, producing a
total thermal emission equal to 1.69 +- 0.31% of the photospheric flux in the K
band, is another viable explanation for the observed near-infrared excess. Our
re-interpretation of archival near- to far-infrared photometric measurements
shows however that cold dust is not present around zeta Aql at the sensitivity
limit of the IRS and MIPS instruments onboard Spitzer, and urges us to remove
zeta Aql from the category of bona fide debris disc stars.Comment: 14 pages, accepted for publication in A&
Warm dusty discs: Exploring the A star 24um debris population
(Abridged) Studies of debris discs have shown that most systems are analogous
to the EKB. In this study we aim to determine how many IRAS 25um excesses
towards A stars are real, and investigate where the dust lies. We observe with
TIMMI2, VISIR, Michelle and TReCS a sample of A and B-type main sequence stars
reported as having mid-IR excess. We constrain the location of the debris
through combined modelling of the emission spectrum and a modelling technique
designed to constrain the radial extent of emission in mid-IR imaging. We
independently confirm the presence of warm dust around 3 of the candidates:
HD3003, HD80950 and eta Tel. For the binary HD3003 a stability analysis
indicates the dust is either circumstellar and lying at ~4 AU with the binary
orbiting at >14AU, or the dust lies in an unstable location; there is some
evidence for temporal evolution of its excess emission on a ~20 year timescale.
For 7 of the targets we present quantitative limits on the location of dust
around the star. We demonstrate that the disc around HD71155 must have
spatially distinct components at 2 and 60AU. We model the limits of current
instrumentation and show that most of the known A star debris discs which could
be readily resolved at 18um on 8m instruments have been resolved. Limits from
unresolved imaging can help distinguish between competing models of the disc
emission, but resolved imaging is key to the determination of the disc
location. Modelling of the detection limits for extended emission can be useful
for targeting future observational campaigns. MIRI on the JWST will be able to
resolve most of the known A star debris disc population. METIS on the E-ELT
will provide the opportunity to explore the hot disc population more thoroughly
by detecting extended emission where calibration accuracy limits disc detection
through photometry, reaching levels below 1 zodi for stars at <10pc.Comment: Accepted for publication in Astronomy and Astrophysic
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