71 research outputs found
Introduction to the GiNaC Framework for Symbolic Computation within the C++ Programming Language
The traditional split-up into a low level language and a high level language
in the design of computer algebra systems may become obsolete with the advent
of more versatile computer languages. We describe GiNaC, a special-purpose
system that deliberately denies the need for such a distinction. It is entirely
written in C++ and the user can interact with it directly in that language. It
was designed to provide efficient handling of multivariate polynomials,
algebras and special functions that are needed for loop calculations in
theoretical quantum field theory. It also bears some potential to become a more
general purpose symbolic package
Introduction to XLOOPS
The program package XLOOPS calculates massive one- and two-loop Feynman
diagrams. It consists of five parts:
i) a graphical user interface
ii) routines for generating diagrams from particle input
iii) procedures for calculating one-loop integrals both analytically and
numerically
iv) routines for massive two-loop integrals
v) programs for numerical integration of two-loop diagrams. The package
relies on the application of parallel space techniques. The treatment of tensor
structure and the separation of UV and IR divergences in analytic expressions
is described in this scheme. All analytic calculations are performed with
MAPLE. Two-loop examples taken from Standard Model calculations are presented.
The method has recently been extended to all two-loop vertex topologies,
including the crossed topology, graphs with divergent subloops and IR divergent
diagrams. This will be included in the XLOOPS package in the near future.Comment: 30 pages, LaTeX, uses epsfi
A companion candidate in the gap of the T Cha transitional disk
T Cha is a young star surrounded by a cold disk. The presence of a gap within
its disk, inferred from fitting to the spectral energy distribution, has
suggested on-going planetary formation. We observed T Cha in L' and K_s with
NAOS-CONICA, the adaptive optics system at the VLT, using sparse aperture
masking. We detected a source in the L' data at a separation of 62+-7 mas,
position angle of 78+-1 degrees, and a contrast of delta L' = 5.1+-0.2 mag. The
object is not detected in the Ks band data, which show a 3-sigma contrast limit
of 5.2 mag at the position of the detected L' source. For a distance of 108 pc,
the detected companion candidate is located at 6.7 AU from the primary, well
within the disk gap. If T Cha and the companion candidate are bound, the
comparison of the L' and Ks photometry with evolutionary tracks shows that the
photometry is inconsistent with any unextincted photosphere at the age and
distance of T Cha. The detected object shows a very red Ks-L' color for which a
possible explanation would be a significant amount of dust around it. This
would imply that the companion candidate is young, which would strengthen the
case for a physical companion, and moreover that the object would be in the
substellar regime, according to the Ks upper limit. Another exciting
possibility would be that this companion is a recently formed planet within the
disk. Additional observations are mandatory to confirm that the object is bound
and to properly characterize it.Comment: 4 pages, 4 figures; accepted for publication by A&
Observable Signatures of Planet Accretion in Red Giant Stars I: Rapid Rotation and Light Element Replenishment
The orbital angular momentum of a close-orbiting giant planet can be
sufficiently large that, if transferred to the envelope of the host star during
the red giant branch (RGB) evolution, it can spin-up the star's rotation to
unusually large speeds. This spin-up mechanism is one possible explanation for
the rapid rotators detected among the population of generally slow-rotating red
giant stars. These rapid rotators thus comprise a unique stellar sample
suitable for searching for signatures of planet accretion in the form of
unusual stellar abundances due to the dissemination of the accreted planet in
the stellar envelope. In this study, we look for signatures of replenishment in
the Li abundances and (to a lesser extent) 12C/13C, which are both normally
lowered during RGB evolution. Accurate abundances were measured from high
signal-to-noise echelle spectra for samples of both slow and rapid rotator red
giant stars. We find that the rapid rotators are on average enriched in lithium
compared to the slow rotators, but both groups of stars have identical
distributions of 12C/13C within our measurement precision. Both of these
abundance results are consistent with the accretion of planets of only a few
Jupiter masses. We also explore alternative scenarios for understanding the
most Li-rich stars in our sample---particularly Li regeneration during various
stages of stellar evolution. Finally, we find that our stellar samples show
non-standard abundances even at early RGB stages, suggesting that initial
protostellar Li abundances and 12C/13C may be more variable than originally
thought.Comment: Accepted for publication in the Astrophysical Journal. 29 pages in
emulateapj format, including 16 figures and 12 tables. Tables 4 and 8 are
provided in their entirety as plain text ancillary files (and will also be
available in the electronic edition of ApJ
Variability of the transitional T Tauri star T Chamaeleontis
We characterize the physical properties of T Chamaeleontis, a transitional T
Tauri star showing UX Ori-type variability, and of its associated disc, and
probe possible effects of disc clearing processes. Different spectral
diagnostics are examined, based on a rich collection of optical high- and
low-resolution spectra. We determine radial and projected rotational
velocities, and measure equivalent widths of the Li I (6708 A) line and of the
most prominent emission lines (e.g. Ha, Hb and [OI] 6300A); we analyse shape
changes of photospheric lines via bisector-method, while variability in Ha and
Hb is inspected through line-profile correlation matrices. The strength of the
Ha and Hb emission is highly variable and well correlated with that of the [OI]
lines, as well as with Av variations of over three magnitudes. Variations up to
nearly 10 km/s in the radial velocity of the star are measured on analogous
time-scale, but with no apparent periodicity. SED modelling confirms the
existence of a gap in the disc. Variable circumstellar extinction is pointed
out as responsible for the conspicuous variations observed in the stellar
continuum flux and for concomitant changes in the emission features by contrast
effect. Clumpy structures, incorporating large dust grains and orbiting the
star within a few tenths of AU, obscure episodically the star and, eventually,
part of the inner circumstellar zone, while the bulk of the hydrogen lines
emitting zone and outer low-density wind region traced by the [OI] remain
unaffected. Coherently with this scenario, the detected radial velocity changes
are also explainable in terms of clumpy materials transiting and partially
obscuring the star.Comment: 38 pages, 32 figures. To appear in the Astronomy and Astrophysics
main journa
Influence of solvent granularity on the effective interaction between charged colloidal suspensions
We study the effect of solvent granularity on the effective force between two
charged colloidal particles by computer simulations of the primitive model of
strongly asymmetric electrolytes with an explicitly added hard sphere solvent.
Apart from molecular oscillating forces for nearly touching colloids which
arise from solvent and counterion layering, the counterions are attracted
towards the colloidal surfaces by solvent depletion providing a simple
statistical description of hydration. This, in turn, has an important influence
on the effective forces for larger distances which are considerably reduced as
compared to the prediction based on the primitive model. When these forces are
repulsive, the long-distance behaviour can be described by an effective Yukawa
pair potential with a solvent-renormalized charge. As a function of colloidal
volume fraction and added salt concentration, this solvent-renormalized charge
behaves qualitatively similar to that obtained via the Poisson-Boltzmann cell
model but there are quantitative differences. For divalent counterions and
nano-sized colloids, on the other hand, the hydration may lead to overscreened
colloids with mutual attraction while the primitive model yields repulsive
forces. All these new effects can be accounted for through a solvent-averaged
primitive model (SPM) which is obtained from the full model by integrating out
the solvent degrees of freedom. The SPM was used to access larger colloidal
particles without simulating the solvent explicitly.Comment: 14 pages, 16 craphic
Opioid-induced inhibition of the human 5-HT and noradrenaline transporters in vitro: link to clinical reports of serotonin syndrome
Opioids may inhibit the 5-HT transporter (SERT) and the noradrenaline transporter (NET). NET inhibition may contribute to analgesia, and SERT inhibition or interactions with 5-HT receptors may cause serotonergic toxicity. However, the effects of different opioids on the human SERT, NET and 5-HT receptors have not been sufficiently studied.; We determined the potencies of different opioids to inhibit the SERT and NET in vitro using human transporter-transfected HEK293 cells. We also tested binding affinities at 5-HT; 1A; , 5-HT; 2A; and 5-HT; 2C; receptors. Additionally, we assessed clinical cases of the serotonin syndrome associated with each opioid reported by PubMed and a World Health Organization database.; Dextromethorphan, l(R)-methadone, racemic methadone, pethidine, tramadol and tapentadol inhibited the SERT at or close to observed drug plasma or estimated brain concentrations in patients. Tapentadol was the most potent NET inhibitor. Pethidine, tramadol, l(R)-methadone, racemic methadone, dextromethorphan and O-desmethyltramadol also inhibited the NET. 6-Monoacetylmorphine, buprenorphine, codeine, dihydrocodeine, heroin, hydrocodone, hydromorphone, morphine, oxycodone and oxymorphone did not inhibit the SERT or NET. Fentanyl interacted with 5-HT; 1A; receptors and methadone, pethidine and fentanyl with 5-HT; 2A; receptors, in the low micromolar range. Opioids most frequently associated with the serotonin syndrome are tramadol, fentanyl, tapentadol, oxycodone, methadone and dextromethorphan.; Some synthetic opioids interact with the SERT and NET at potentially clinically relevant concentrations. SERT inhibition by tramadol, tapentadol, methadone, dextromethorphan and pethidine may contribute to the serotonin syndrome. Direct effects on 5-HT; 1A; and/or 5-HT; 2A; receptors could be involved with methadone and pethidine
Computer-algebraische und analytische Methoden zur Berechnung von Vertexfunktionen im Standardmodell
Das Standardmodell der elektroschwachen Wechselwirkung hatin den vergangenen Jahrzehnten beachtliche Erfolge erzielt.Die Suche nac
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