1,491 research outputs found
Braking the Gas in the beta Pictoris Disk
(Abridged) The main sequence star beta Pictoris hosts the best studied
circumstellar disk to date. Nonetheless, a long-standing puzzle has been around
since the detection of metallic gas in the disk: radiation pressure from the
star should blow the gas away, yet the observed motion is consistent with
Keplerian rotation. In this work we search for braking mechanisms that can
resolve this discrepancy. We find that all species affected by radiation force
are heavily ionized and dynamically coupled into a single fluid by Coulomb
collisions, reducing the radiation force on species feeling the strongest
acceleration. For a gas of solar composition, the resulting total radiation
force still exceeds gravity, while a gas of enhanced carbon abundance could be
self-braking. We also explore two other braking agents: collisions with dust
grains and neutral gas. Grains surrounding beta Pic are photoelectrically
charged to a positive electrostatic potential. If a significant fraction of the
grains are carbonaceous (10% in the midplane and larger at higher altitudes),
ions can be slowed down to satisfy the observed velocity constraints. For
neutral gas to brake the coupled ion fluid, we find the minimum required mass
to be 0.03 M_\earth, consistent with observed upper limits of the
hydrogen column density, and substantially reduced relative to previous
estimates. Our results favor a scenario in which metallic gas is generated by
grain evaporation in the disk, perhaps during grain-grain collisions. We
exclude a primordial origin for the gas, but cannot rule out the possibility of
its production by falling evaporating bodies near the star. We discuss the
implications of this work for observations of gas in other debris disks.Comment: 19 pages, 12 figures, emulateapj. Accepted for publication in Ap
First NACO observations of the Brown Dwarf LHS 2397aB
Observations of the standard late type M8 star LHS 2397aA were obtained at
the ESO-VLT 8m telescope ``Yepun'' using the NAOS/CONICA Adaptive Optics
facility. The observations were taken during the NACO commissioning, and the
infrared standard star LHS 2397aA was observed in the H, and Ks broad band
filters. In both bands the brown dwarf companion LHS2397aB was detected. Using
a program recently developed (Bouy et al., 2003) for the detection of stellar
binaries we calculated the principal astrometric parameters (angular binary
separation and position angle P.A.) and the photometry of LHS 2397aA and LHS
2397aB. Our study largely confirms previous results obtained with the
AO-Hokupa'a facility at Gemini-North (Freed et al., 2003); however a few
discrepancies are observed.Comment: 5 page
Comment on "Large energy gaps in CaC6 from tunneling spectroscopy: possible evidence of strong-coupling superconductivity"
Comment on "Large energy gaps in CaC6 from tunneling spectroscopy: possible
evidence of strong-coupling superconductivity
A library of near-infrared integral field spectra of young M-L dwarfs
We present a library of near-infrared (1.1-2.45 microns) medium-resolution
(R~1500-2000) integral field spectra of 15 young M6-L0 dwarfs, composed of
companions with known ages and of isolated objects. We use it to (re)derive the
NIR spectral types, luminosities and physical parameters of the targets, and to
test (BT-SETTL, DRIFT-PHOENIX) atmospheric models. We derive infrared spectral
types L0+-1, L0+-1, M9.5+-0.5, M9.5+-0.5, M9.25+-0.25, M8+0.5-0.75, and
M8.5+-0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B,
DH Tau B, CT Cha b, and HR7329B, respectively. BT-SETTL and DRIFT-PHOENIX
models yield close Teff and log g estimates for each sources. The models seem
to evidence a 600-300+600 K drop of the effective temperature at the M-L
transition. Assuming the former temperatures are correct, we derive new mass
estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4,
OTS 44, and Cha1109 lay inside or at the boundary of the planetary mass range.
We combine the empirical luminosities of the M9.5-L0 sources to the Teff to
derive semi-empirical radii estimates that do not match "hot-start"
evolutionary models predictions at 1-3 Myr. We use complementary data to
demonstrate that atmospheric models are able to reproduce the combined optical
and infrared spectral energy distribution, together with the near-infrared
spectra of these sources simultaneously. But the models still fail to represent
the dominant features in the optical. This issue casts doubts on the ability of
these models to predict correct effective temperatures from near-infrared
spectra alone. We advocate the use of photometric and spectroscopic data
covering a broad range of wavelengths to study the properties of very low mass
young companions to be detected with the planet imagers (Subaru/SCExAO,
LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).Comment: 27 pages, 14 tables, 19 figures, accepted for publication in
Astronomy & Astrophysic
Rigidity transitions and constraint counting in amorphous networks: beyond the mean-field approach
Subj-class: Disordered Systems and Neural NetworksComment: 12 pages, revtex, 3 figure
Using the Sun to estimate Earth-like planets detection capabilities. III. Impact of spots and plages on astrometric detection
Stellar activity is a potential important limitation to the detection of low
mass extrasolar planets with indirect methods (RV, photometry, astrometry). In
previous papers, using the Sun as a proxy, we investigated the impact of
stellar activity (spots, plages, convection) on the detectability of an
Earth-mass planet in the habitable zone (HZ) of solar-type stars with RV
techniques. We extend here the detectability study to the case of astrometry.
We used the sunspot and plages properties recorded over one solar cycle to
infer the astrometric variations that a Sun-like star seen edge-on, 10 pc away,
would exhibit, if covered by such spots/bright structures. We compare the
signal to the one expected from the astrometric wobble (0.3 {\mu}as) of such a
star surrounded by a one Earth-mass planet in the HZ. We also briefly
investigate higher levels of activity. The activity-induced astrometric signal
along the equatorial plane has an amplitude of typ. less than 0.2 {\mu}as
(rms=0.07 {\mu}as), smaller than the one expected from an Earth-mass planet at
1 AU. Hence, for this level of activity, the detectability is governed by the
instrumental precision rather than the activity. We show that for instance a
one Earth-mass planet at 1 AU would be detected with a monthly visit during
less than 5 years and an instrumental precision of 0.8 {\mu}as. A level of
activity 5 times higher would still allow such a detection with a precision of
0.35 {\mu}as. We conclude that astrometry is an attractive approach to search
for such planets around solar type stars with most levels of stellar activity.Comment: Letter accepted by A&
Electrochemical Cell with Improved Water or Gas Management
An electrochemical cell having a water/gas porous separator prepared from a polymeric material and one or more conductive cell components that pass through, or are located in close proximity to, the water/gas porous separator, is provided. The inventive cell provides a high level of in-cell electrical conductivity
An Economic Evaluation of Soybean-Based Biodiesel Production on Commercial Farms in the Soybean-Producing Regions of KwaZulu-Natal: Some Preliminary Results
Global biofuel production has risen substantially in recent years, driven primarily by government support for biofuel industries. The stated motivations for these initiatives are numerous and have varied over time. Soybeans are the only field crop produced in sufficient quantities in KwaZulu-Natal (KZN) that the South African (SA) industrial biofuel strategy identifies as a potential biodiesel feedstock. Preliminary results from a mixed integer linear programming model support the notion of Funke et al. (2009), who contend that the incentives and commitments outlined by the industrial biofuel strategy are inadequate to both establish and sustain a domestic biodiesel industry.Industrial biofuels strategy, soybeans, biodiesel, KwaZulu-Natal, mixed integer linear programming, Crop Production/Industries, Resource /Energy Economics and Policy,
Scanning Tunneling Spectroscopy on the novel superconductor CaC6
We present scanning tunneling microscopy and spectroscopy of the newly
discovered superconductor CaC. The tunneling conductance spectra, measured
between 3 K and 15 K, show a clear superconducting gap in the quasiparticle
density of states. The gap function extracted from the spectra is in good
agreement with the conventional BCS theory with = 1.6 0.2
meV. The possibility of gap anisotropy and two-gap superconductivity is also
discussed. In a magnetic field, direct imaging of the vortices allows to deduce
a coherence length in the ab plane 33 nm
- âŠ