423 research outputs found
Electronic Portfolios for Scientists
Electronic portfolios (ePortfolios) are electronic versions of paper based portfolios. They are increasingly applied in education. Software for building and maintaining ePortfolios is emerging; open specifications for the exchange of ePortfolios exist. They show the potential to serve as a standard tool for documenting achievements in lifelong learning. In this paper we explore the potential of ePortfolios for scientists
Management of Informal Mathematical Knowledge – Lessons Learned from the Trial-Solution Project
Four nearby L dwarfs
We present spectroscopic, photometric and astrometric observations of four
bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our
spectroscopic data extend to wavelengths shortward of 5000\AA in the L0 dwarf
2MASSJ0746+2000 and the L4 dwarf 2MASSJ0036+1840, allowing the identification
of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I
4227\AA and Na I 5890/5896\AA are extremely strong, with the latter having an
equivalent width of 240\AA in the L4 dwarf. By spectral type L5, the D lines
extend over \AA and absorb a substantial fraction of the flux emitted
in the V band, with a corresponding effect on the (V-I) broadband colour. The
KI resonance doublet at 7665/7699\AA increases in equivalent width from
spectral type M3 to M7, but decreases in strength from M7 to L0 before
broadening substantially at later types. These variations are likely driven by
dust formation in these cool atmospheres.Comment: to appear in AJ, January 2000; 27 pages, including 3 tables and 7
figures embedded in the tex
Theoretical search for Chevrel phase based thermoelectric materials
We investigate the thermoelectric properties of some semiconducting Chevrel
phases. Band structure calculations are used to compute thermopowers and to
estimate of the effects of alloying and disorder on carrier mobility. Alloying
on the Mo site with transition metals like Re, Ru or Tc to reach a
semiconducting composition causes large changes in the electronic structure at
the Fermi level. Such alloys are expected to have low carrier mobilities.
Filling with transition metals was also found to be incompatible with high
thermoelectric performance based on the calculated electronic structures.
Filling with Zn, Cu, and especially with Li was found to be favorable. The
calculated electronic structures of these filled Chevrel phases are consistent
with low scattering of carriers by defects associated with the filling. We
expect good mobility and high thermopower in materials with the composition
close to (Li,Cu)MoSe, particularly when Li-rich, and recommend this
system for experimental investigation.Comment: 4 two-column pages, 4 embedded ps figure
Multicentric extra-abdominal desmoid tumors arising in bilateral lower limbs
Extra-abdominal desmoid tumors preferentially affect the shoulders, arms, backs, buttocks, and thighs of young adults. Multicentric occurrence is rather rare but seems to be another distinctive feature of extra-abdominal desmoid tumors. In this article we report a rare case of multicentric extra-abdominal desmoid tumors arising in bilateral lower limbs
L Dwarfs and the Substellar Mass Function
Analysis of initial observations from near-infrared sky surveys has shown
that the resulting photometric catalogues, combined with far-red optical data,
provide an extremely effective method of finding isolated, very low-temperature
objects in the general field. Follow-up observations have already identified
more than 25 sources with temperatures cooler than the latest M dwarfs. A
comparison with detailed model predictions (Burrows & Sharp) indicates that
these L dwarfs have effective temperatures between ~2000\pm100 K and 1500\pm100
K, while the available trigonometric parallax data place their luminosities at
between 10^{-3.5} and 10^{-4.3} L_solar. Those properties, together with the
detection of lithium in one-third of the objects, are consistent with the
majority having substellar masses. The mass function cannot be derived
directly, since only near-infrared photometry and spectral types are available
for most sources, but we can incorporate VLM/brown dwarf models in simulations
of the Solar Neighbourhood population and constrain Psi(M) by comparing the
predicted L-dwarf surface densities and temperature distributions against
observations from the DENIS and 2MASS surveys. The data, although sparse, can
be represented by a power-law mass function, Psi(M) ~ M^{-alpha}, with 1 <
alpha < 2. Current results favour a value nearer the lower limit. If alpha =
1.3, then the local space density of 0.075 > M/M_solar > 0.01 brown dwarfs is
0.10 systems pc^{-3}. In that case brown dwarfs are twice as common as
main-sequence stars, but contribute no more than ~15% of the total mass of the
disk.Comment: To appear in Astrophysical Journal (20 August 1999). 44 Pages. For
related preprints, see
http://www.ipac.caltech.edu/2mass/overview/ldwarfs.htm
Discovery of a Brown Dwarf Companion to Gliese 570ABC: A 2MASS T Dwarf Significantly Cooler than Gliese 229B
We report the discovery of a widely separated (258\farcs3\pm0\farcs4) T
dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was
initially identified from the Two Micron All Sky Survey (2MASS). Its
near-infrared spectrum shows the 1.6 and 2.2 \micron CH absorption bands
characteristic of T dwarfs, while its common proper motion with the Gl 570ABC
system confirms companionship. Gl 570D (M = 16.470.07) is nearly a
full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and
estimates of L = (2.80.3)x10 L_{\sun} and T = 75050
K make it significantly cooler and less luminous than any other known brown
dwarf companion. Using evolutionary models by Burrows et al. and an adopted age
of 2-10 Gyr, we derive a mass estimate of 5020 M for this object.Comment: 13 pages, 2 figures, 2 tables, accepted by ApJ
High contrast optical imaging of companions: the case of the brown dwarf binary HD-130948BC
High contrast imaging at optical wavelengths is limited by the modest
correction of conventional near-IR optimized AO systems.We take advantage of
new fast and low-readout-noise detectors to explore the potential of fast
imaging coupled to post-processing techniques to detect faint companions to
stars at small separations. We have focused on I-band direct imaging of the
previously detected brown dwarf binary HD130948BC,attempting to spatially
resolve the L2+L2 benchmark system. We used the Lucky-Imaging instrument
FastCam at the 2.5-m Nordic Telescope to obtain quasi diffraction-limited
images of HD130948 with ~0.1" resolution.In order to improve the detectability
of the faint binary in the vicinity of a bright (I=5.19 \pm 0.03) solar-type
star,we implemented a post-processing technique based on wavelet transform
filtering of the image which allows us to strongly enhance the presence of
point-like sources in regions where the primary halo dominates. We detect for
the first time the BD binary HD130948BC in the optical band I with a SNR~9 at
2.561"\pm 0.007" (46.5 AU) from HD130948A and confirm in two independent
dataset that the object is real,as opposed to time-varying residual speckles.We
do not resolve the binary, which can be explained by astrometric results
posterior to our observations that predict a separation below the NOT
resolution.We reach at this distance a contrast of dI = 11.30 \pm 0.11, and
estimate a combined magnitude for this binary to I = 16.49 \pm 0.11 and a I-J
colour 3.29 \pm 0.13. At 1", we reach a detectability 10.5 mag fainter than the
primary after image post-processing. We obtain on-sky validation of a technique
based on speckle imaging and wavelet-transform processing,which improves the
high contrast capabilities of speckle imaging.The I-J colour measured for the
BD companion is slightly bluer, but still consistent with what typically found
for L2 dwarfs(~3.4-3.6).Comment: accepted in A\&
The Spectra of T Dwarfs I: Near-Infrared Data and Spectral Classification
We present near-infrared spectra for a sample of T dwarfs, including eleven
new discoveries made using the Two Micron All Sky Survey. These objects are
distinguished from warmer (L-type) brown dwarfs by the presence of methane
absorption bands in the 1--2.5 \micron spectral region. A first attempt at a
near-infrared classification scheme for T dwarfs is made, based on the
strengths of CH and HO bands and the shapes of the 1.25, 1.6, and 2.1
\micron flux peaks. Subtypes T1 V through T8 V are defined, and spectral
indices useful for classification are presented. The subclasses appear to
follow a decreasing T scale, based on the evolution of CH and
HO bands and the properties of L and T dwarfs with known distances.
However, we speculate that this scale is not linear with spectral type for cool
dwarfs, due to the settling of dust layers below the photosphere and subsequent
rapid evolution of spectral morphology around T 1300--1500 K.
Similarities in near-infrared colors and continuity of spectral features
suggest that the gap between the latest L dwarfs and earliest T dwarfs has been
nearly bridged. This argument is strengthened by the possible role of CH as
a minor absorber shaping the K-band spectra of the latest L dwarfs. Finally, we
discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue
near-infrared colors (J-K = 0.24) due to suppression of the 2.1
\micron peak. The feature is likely caused by enhanced collision-induced
H absorption in a high pressure or low metallicity photosphere.Comment: 74 pages including 26 figures, accepted by ApJ v563 December 2001;
full paper including all of Table 3 may be downloaded from
http://www.gps.caltech.edu/~pa/adam/classification ;also see submission
010844
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