5,661 research outputs found
Advances in mass-loss predictions
We present the results of Monte Carlo mass-loss predictions for massive stars
covering a wide range of stellar parameters. We critically test our predictions
against a range of observed mass-loss rates -- in light of the recent
discussions on wind clumping. We also present a model to compute the
clumping-induced polarimetric variability of hot stars and we compare this with
observations of Luminous Blue Variables, for which polarimetric variability is
larger than for O and Wolf-Rayet stars. Luminous Blue Variables comprise an
ideal testbed for studies of wind clumping and wind geometry, as well as for
wind strength calculations, and we propose they may be direct supernova
progenitors.Comment: 3 pages, 3 figures, to appear in the proceedings of workshop
'Clumping in Hot Star Winds', eds. W.-R. Hamann, A. Feldmeier, & L. Oskinov
Additive manufacturing technologies : 3D printing in organic synthesis
The manufacturing of a three-dimensional product from a computer-driven digital model (3D printing) has found extensive applications in several fields. Additive manufacturing technologies offer the possibility to fabricate ad hoc tailored products on demand, at affordable prices, and have been employed to make customized and complex items for actual sale. However, despite the great progress and the countless opportunities offered by the 3D printing technology, surprisingly a relatively limited number of applications have been documented in organic chemistry. This review will focus specifically on the exploitation of additive manufacturing technologies in the synthesis of organic compounds, and, in particular, on the use of 3D-printed catalysts and 3D printed reactors, and on the fabrication and use of 3D printed flow reactors
The antenna DSA 3 and its potential use for Radio Astronomy
The European Space Agency (ESA) will inaugurate its third Deep Space Antenna
(DSA 3) by the end of 2012. DSA 3 will be located in Argentina near the city of
Malarg"ue in the Mendoza province. While the instrument will be primarily
dedicated to communications with interplanetary missions, the characteristics
of its antenna and receivers will also enable standalone leading scientific
contributions, with a high scientific-technological return. We outline here
scientific proposals for a radio astronomical use of DSA 3.Comment: 4 pages, submitted as Proceedings for the BAA
Stereolithography 3D-Printed Catalytically Active Devices in Organic Synthesis
This article describes the synthesis of stereolithography (SLA) 3D-printed
catalyst-impregnated devices and their evaluation in the organocatalyzed Friedel\u2013Crafts alkylation
of N\u2013Me\u2013indole with trans--nitrostyrene. Using a low-cost SLA 3D printer and freeware design
software, dierent devices were designed and 3D-printed using a photopolymerizable resin containing
a thiourea-based organocatalyst. The architectural control oered by the 3D-printing process allows
a straightforward production of devices endowed with dierent shapes and surface areas, with
high reproducibility. The 3D-printed organocatalytic materials promoted the formation of the
desired product up to a 79% yield, although with longer reaction times compared to reactions under
homogeneous conditions
Evaluation of In-Batch and In-Flow Synthetic Strategies towards the Stereoselective Synthesis of a Fluorinated Analogue of Retro-Thiorphan
A stereoselective synthetic strategy for the preparation of trifluoromethylamine mimics of retro-thiorphan, involving a diastereoselective, metal-free catalytic step, has been studied in batch and afforded the target molecule in good yields and high diastereoselectivity. A crucial point of the synthetic sequence was the catalytic reduction of a fluorinated enamine with trichlorosilane as reducing agent in the presence of a chiral Lewis base. The absolute configuration of the key intermediate was unambiguously assigned by X-ray analysis. The synthesis was also investigated exploiting continuous flow reactions; that is, an advanced intermediate of the target molecule was synthesized in only two in-flow synthetic modules, avoiding isolation and purifications of intermediates, leading to the isolation of the target chiral fluorinated amine in up to an 87:13 diastereoisomeric ratio
Organocatalytic Michael addition to (D)-mannitol-derived enantiopure nitroalkenes: A valuable strategy for the synthesis of densely functionalized chiral molecules
Carbohydrates are abundant renewable resources and are a feedstock for green chemistry and sustainable synthesis of the future. Among the hexoses and the pentoses present in biomass, mannitol was selected in the present project as a valuable platform, directly available from the chiral pool, to build highly functionalized molecules. Starting from (R)-2,3-O-cyclohexylidene glyceraldehyde, which is easily prepared in a large scale from D-mannitol, an enantiopure chiral nitro alkene was prepared by reaction with nitromethane, and its reactivity studied. Organocatalytic Michael addition of dimethyl malonate, \u3b2-keto esters, and other nucleophiles on the nitro alkene afforded high stereoselectivity and densely functionalized chiral molecules, which were further synthetically developed, leading to five-membered lactones and bicyclic lactams. Preliminary studies showed that the metal-free catalytic reaction on the chiral nitro alkene can be performed under continuous flow conditions, thus enabling the use of (micro)mesofluidic systems for the preparation of enantiomerically pure organic molecules from the chiral pool
Deeper Chandra Follow-up of Cygnus TeV Source Perpetuates Mystery
A 50 ksec Chandra observation of the unidentified TeV source in Cygnus
reported by the HEGRA collaboration reveals no obvious diffuse X-ray
counterpart. However, 240 Pointlike X-ray sources are detected within or nearby
the extended TeV J2032+4130 source region, of which at least 36 are massive
stars and 2 may be radio emitters. That the HEGRA source is a composite, having
as counterpart the multiple point-like X-ray sources we observe, cannot be
ruled out. Indeed, the distribution of point-like X-ray sources appears
non-uniform and concentrated broadly within the extent of the TeV source
region. We offer a hypothesis for the origin of the very high energy gamma-ray
emission in Cyg OB2 based on the local acceleration of TeV range cosmic rays
and the differential distribution of OB vs. less massive stars in this
association.Comment: Substantially revised version; incorporates referee suggestions &
expanded discussio
The HI shell G132.6-0.7-25.3: A Supernova Remnant or an Old Wind-Blown Bubble?
Data from the Canadian Galactic Plane Survey reveal an abundance of HI shells
and arcs in the disk of our galaxy. While their shape is suggestive of stellar
winds or supernovae influence, very few of these structures have been examined
in detail thus far. A fine example is an HI shell in the outer Galaxy with no
continuum counterpart discovered in the survey's pilot project. Its size and
kinematics suggest that it was created by the winds of a single late-type O
star which has since evolved off the main sequence or by a supernova explosion.
A B1 Ia star at the centre of the shell, in projection, is a possible candidate
for energy source if the shell is assumed to be wind-blown. The shell's shape
implies a surprisingly small scale height of less than about 30 pc for the
surrounding gas if the elongation is due to evolution in a density gradient.Comment: 25 pages, 6 figures. Accepted for publication in the Astronomical
Journa
Organocatalytic alpha-trifluoromethylthiolation of silylenol ethers : Batch vs continuous flow reactions
This work describes the organocatalytic \uce\ub1-trifluoromethylthiolation of silylenol ethers using N-(trifluoromethylthio)saccharin as trifluoromethylthiolating reagent that is activated by the presence of catalytic amounts of a Lewis base. Tetrahydrothiophene was identified as the best organocatalyst and it was successfully employed to promote the synthesis of different \uce\ub1-trifluoromethylketones; the reaction has been performed under a traditional batch methodology and under continuous flow conditions. In general, yields obtained using the traditional batch process were higher than those observed when the reaction was performed under flow conditions. However, short reaction times, higher productivity and higher space time yields were observed when a flow system process was employed. Preliminary DFT calculations were also performed in order to elucidate the mechanism of the reaction
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