4,269 research outputs found
Methanol in the sky with diamonds
The present of gas phase methanol in dense interstellar molecular clouds was established by radio detection of its rotational emission lines. However, the position, width, and profile of a absorption band near 1470 cm(exp -1) in the IR spectra of many dense molecular clouds strongly suggests that solid methanol is an important component of interstellar ices. In an attempt to better constrain the identification of 1470 cm(exp -1) feature, we began a program to search for other characteristic absorption bands of solid state methanol in the spectra of objects known to produce this band. One such feature is now identified in the spectra of several dense molecular clouds and its position, width, and profile fit well with those of laboratory H2O:CH3OH ices. Thus, the presence of methanol-bearing ices in space is confirmed
ALFA & 3D: integral field spectroscopy with adaptive optics
One of the most important techniques for astrophysics with adaptive optics is
the ability to do spectroscopy at diffraction limited scales. The extreme
difficulty of positioning a faint target accurately on a very narrow slit can
be avoided by using an integral field unit, which provides the added benefit of
full spatial coverage. During 1998, working with ALFA and the 3D integral field
spectrometer, we demonstrated the validity of this technique by extracting and
distinguishing spectra from binary stars separated by only 0.26". The
combination of ALFA & 3D is also ideally suited to imaging distant galaxies or
the nuclei of nearby ones, as its field of view can be changed between
1.2"x1.2" and 4"x4", depending on the pixel scale chosen. In this contribution
we present new results both on galactic targets, namely young stellar objects,
as well as extra-galactic objects including a Seyfert and a starburst nucleus.Comment: SPIE meeting 4007 on Adaptive Optical Systems Technology, March 200
The Highly Dynamic Behavior of the Innermost Dust and Gas in the Transition Disk Variable LRLL 31
We describe extensive synoptic multi-wavelength observations of the
transition disk LRLL 31 in the young cluster IC 348. We combined four epochs of
IRS spectra, nine epochs of MIPS photometry, seven epochs of cold-mission IRAC
photometry and 36 epochs of warm mission IRAC photometry along with multi-epoch
near-infrared spectra, optical spectra and polarimetry to explore the nature of
the rapid variability of this object. We find that the inner disk, as traced by
the 2-5micron excess stays at the dust sublimation radius while the strength of
the excess changes by a factor of 8 on weekly timescales, and the 3.6 and
4.5micron photometry shows a drop of 0.35 magnitudes in one week followed by a
slow 0.5 magnitude increase over the next three weeks. The accretion rate, as
measured by PaBeta and BrGamma emission lines, varies by a factor of five with
evidence for a correlation between the accretion rate and the infrared excess.
While the gas and dust in the inner disk are fluctuating the central star stays
relatively static. Our observations allow us to put constraints on the physical
mechanism responsible for the variability. The variabile accretion, and wind,
are unlikely to be causes of the variability, but both are effects of the same
physical process that disturbs the disk. The lack of periodicity in our
infrared monitoring indicates that it is unlikely that there is a companion
within ~0.4 AU that is perturbing the disk. The most likely explanation is
either a companion beyond ~0.4 AU or a dynamic interface between the stellar
magnetic field and the disk leading to a variable scale height and/or warping
of the inner disk.Comment: Accepted to ApJ. 10 pages of text, plus 11 tables and 13 figures at
the en
On groups and counter automata
We study finitely generated groups whose word problems are accepted by
counter automata. We show that a group has word problem accepted by a blind
n-counter automaton in the sense of Greibach if and only if it is virtually
free abelian of rank n; this result, which answers a question of Gilman, is in
a very precise sense an abelian analogue of the Muller-Schupp theorem. More
generally, if G is a virtually abelian group then every group with word problem
recognised by a G-automaton is virtually abelian with growth class bounded
above by the growth class of G. We consider also other types of counter
automata.Comment: 18 page
Open-String Actions and Noncommutativity Beyond the Large-B Limit
In the limit of large, constant B-field (the ``Seiberg-Witten limit''), the
derivative expansion for open-superstring effective actions is naturally
expressed in terms of the symmetric products *n. Here, we investigate
corrections around the large-B limit, for Chern-Simons couplings on the brane
and to quadratic order in gauge fields. We perform a boundary-state computation
in the commutative theory, and compare it with the corresponding computation on
the noncommutative side. These results are then used to examine the possible
role of Wilson lines beyond the Seiberg-Witten limit. To quadratic order in
fields, the entire tree-level amplitude is described by a metric-dependent
deformation of the *2 product, which can be interpreted in terms of a deformed
(non-associative) version of the Moyal * product.Comment: 30 pages, harvma
Constraints on the Space Density of Methane Dwarfs and the Substellar Mass Function from a Deep Near-Infrared Survey
We report preliminary results of a deep near-infrared search for
methane-absorbing brown dwarfs; almost five years after the discovery of Gl
229b, there are only a few confirmed examples of this type of object. New J
band, wide-field images, combined with pre-existing R band observations, allow
efficient identification of candidates by their extreme (R-J) colours.
Follow-up measurements with custom filters can then confirm objects with
methane absorption. To date, we have surveyed a total of 11.4 square degrees to
J~20.5 and R~25. Follow-up CH_4 filter observations of promising candidates in
1/4 of these fields have turned up no methane absorbing brown dwarfs. With 90%
confidence, this implies that the space density of objects similar to Gl 229b
is less than 0.012 per cubic parsec. These calculations account for the
vertical structure of the Galaxy, which can be important for sensitive
measurements. Combining published theoretical atmospheric models with our
observations sets an upper limit of alpha <= 0.8 for the exponent of the
initial mass function power law in this domain.Comment: 11 pages + 2 figures To be published in Astrophysical Journal Letter
Some improvements in the method of the weakly conjugate operator
We present some improvements in the method of the weakly conjugate operator,
one variant of the Mourre theory. When applied to certain two-body Schroedinger
operators, this leads to a limiting absorption principle that is uniform on the
positive real axis.Comment: 11 page
Sandy Feet
Sgt. Brady is a former armed forces member with a transtibial amputation. He walks on sand frequently for work and leisure, but experiences severe pain and inconvenience because his current prosthesis is not meant for use on sand. Sand Foot V2 is a prosthesis that allows Sgt. Brady to walk on sand for long periods of time without issue
Ultra-low threshold CW Triply Resonant OPO in the near infrared using Periodically Poled Lithium Niobate
We have operated a CW triply resonant OPO using a PPLN crystal pumped by a
Nd:YAG laser at 1.06 micron and generating signal and idler modes in the 2-2.3
micron range. The OPO was operated stably in single mode operation over large
periods of time with a pump threshold as low as 500 microwatts.Comment: 7 pages, 5 figures, submitted to JEOS
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