3,873 research outputs found
Quantization of the massive gravitino on FRW spacetimes
In this article we study the quantization and causal properties of a massive
spin 3/2 Rarita-Schwinger field on spatially flat Friedmann-Robertson-Walker
(FRW) spacetimes. We construct Zuckerman's universal conserved current and
prove that it leads to a positive definite inner product on solutions of the
field equation. Based on this inner product, we quantize the Rarita-Schwinger
field in terms of a CAR-algebra. The transversal and longitudinal parts
constituting the independent on-shell degrees of freedom decouple. We find a
Dirac-type equation for the transversal polarizations, ensuring a causal
propagation. The equation of motion for the longitudinal part is also of
Dirac-type, but with respect to an `effective metric'. We obtain that for all
four-dimensional FRW solutions with a matter equation of state p = w rho and w
in (-1,1] the light cones of the effective metric are more narrow than the
standard cones, which are recovered for the de Sitter case w=-1. In particular,
this shows that the propagation of the longitudinal part, although non-standard
for w different from -1, is completely causal in cosmological constant, dust
and radiation dominated universes.Comment: 6 pages, 1 figure; published in PR
Covariant Symplectic Structure and Conserved Charges of New Massive Gravity
We show that the symplectic current obtained from the boundary term, which
arises in the first variation of a local diffeomorphism invariant action, is
covariantly conserved for any gravity theory described by that action.
Therefore, a Poincare invariant 2-form can be constructed on the phase space,
which is shown to be closed without reference to a specific theory. Finally, we
show that one can obtain a charge expression for gravity theories in various
dimensions, which plays the role of the Abbott-Deser-Tekin (ADT) charge for
spacetimes with non-constant curvature backgrounds, by using the diffeomorphism
invariance of the symplectic 2-form. As an example, we calculate the conserved
charges of some solutions of New Massive Gravity (NMG) and compare the results
with the previous works.Comment: 18 pages, No figures, RevTEX4.1; ver 2: minor corrections, version
accepted for publication in Physical Review
Algebraic and geometric structures in string backgrounds
We give a brief introduction to the study of the algebraic structures -- and their geometrical interpretations -- which arise in the BRST construction of a conformal string background. Starting from the chiral algebra \cA of a string background, we consider a number of elementary but universal operations on the chiral algebra. From these operations we deduce a certain fundamental odd Poisson structure, known as a Gerstenhaber algebra, on the BRST cohomology of \cA. For the 2D string background, the correponding G-algebra can be partially described in term of a geometrical G-algebra of the affine plane \bC^2. This paper will appear in the proceedings of {\it Strings 95}
Detailed compositional analysis of the heavily polluted DBZ white dwarf SDSS J073842.56+183509.06: A window on planet formation?
We present a new model atmosphere analysis of the most metal contaminated
white dwarf known, the DBZ SDSS J073842.56+183509.06. Using new high resolution
spectroscopic observations taken with Keck and Magellan, we determine precise
atmospheric parameters and measure abundances of 14 elements heavier than
helium. We also report new Spitzer mid-infrared photometric data that are used
to better constrain the properties of the debris disk orbiting this star. Our
detailed analysis, which combines data taken from 7 different observational
facilities (GALEX, Gemini, Keck, Magellan, MMT, SDSS and Spitzer) clearly
demonstrate that J0738+1835 is accreting large amounts of rocky
terrestrial-like material that has been tidally disrupted into a debris disk.
We estimate that the body responsible for the photospheric metal contamination
was at least as large Ceres, but was much drier, with less than 1% of the mass
contained in the form of water ice, indicating that it formed interior to the
snow line around its parent star. We also find a correlation between the
abundances (relative to Mg and bulk Earth) and the condensation temperature;
refractory species are clearly depleted while the more volatile elements are
possibly enhanced. This could be the signature of a body that formed in a lower
temperature environment than where Earth formed. Alternatively, we could be
witnessing the remains of a differentiated body that lost a large part of its
outer layers.Comment: 16 pages, 17 figures, accepted for publication in The Astrophysical
Journa
Batalin-Vilkovisky Integrals in Finite Dimensions
The Batalin-Vilkovisky method (BV) is the most powerful method to analyze
functional integrals with (infinite-dimensional) gauge symmetries presently
known. It has been invented to fix gauges associated with symmetries that do
not close off-shell. Homological Perturbation Theory is introduced and used to
develop the integration theory behind BV and to describe the BV quantization of
a Lagrangian system with symmetries. Localization (illustrated in terms of
Duistermaat-Heckman localization) as well as anomalous symmetries are discussed
in the framework of BV.Comment: 35 page
Swap Bribery
In voting theory, bribery is a form of manipulative behavior in which an
external actor (the briber) offers to pay the voters to change their votes in
order to get her preferred candidate elected. We investigate a model of bribery
where the price of each vote depends on the amount of change that the voter is
asked to implement. Specifically, in our model the briber can change a voter's
preference list by paying for a sequence of swaps of consecutive candidates.
Each swap may have a different price; the price of a bribery is the sum of the
prices of all swaps that it involves. We prove complexity results for this
model, which we call swap bribery, for a broad class of election systems,
including variants of approval and k-approval, Borda, Copeland, and maximin.Comment: 17 page
Molecules in the transition disk orbiting T Cha
We seek to establish the presence and properties of gas in the circumstellar
disk orbiting T Cha, a nearby (d~110 pc), relatively evolved (age ~5-7 Myr) yet
actively accreting 1.5 Msun T Tauri star. We used the APEX 12 m radiotelescope
to search for submillimeter molecular emission from the T Cha disk, and we
reanalyzed archival XMM-Newton spectroscopy of T Cha to ascertain the
intervening absorption due to disk gas along the line of sight to the star
(N_H). We detected submillimeter rotational transitions of 12CO, 13CO, HCN, CN
and HCO+ from the T Cha disk. The 12CO line appears to display a double-peaked
line profile indicative of Keplerian rotation. Analysis of the CO emission line
data indicates that the disk around T Cha has a mass (M_disk,H_2 = 80 M_earth)
similar to, but more compact (R_disk, CO~80 AU) than, other nearby, evolved
molecular disks (e.g. V4046 Sgr, TW Hya, MP Mus) in which cold molecular gas
has been previously detected. The HCO+/13CO and HCN/13CO, line ratios measured
for T Cha appear similar to those of other evolved circumstellar disks (i.e. TW
Hya and V4046 Sgr), while the CN/13CO ratio appears somewhat weaker. Analysis
of the XMM-Newton data shows that the atomic absorption toward T Cha is
1-2 orders of magnitude larger than toward the other nearby T Tauri with
evolved disks. Furthermore, the ratio between atomic absorption and optical
extinction N_H/A_V toward T Cha is higher than the typical value observed for
the interstellar medium and young stellar objects in the Orion Nebula Cluster.
This may suggest that the fraction of metals in the disk gas is higher than in
the interstellar medium. Our results confirm that pre-main sequence stars older
than ~5 Myr, when accreting, retain cold molecular disks, and that those
relatively evolved disks display similar physical and chemical properties.Comment: Accepted for publication on A&
Counting Giant Gravitons in AdS_3
We quantize the set of all quarter BPS brane probe solutions in global AdS_3
\times S^3 \times T^4/K3 found in arxiv:0709.1168 [hep-th]. We show that,
generically, these solutions give rise to states in discrete representations of
the SL(2,R) WZW model on AdS_3. Our procedure provides us with a detailed
description of the low energy 1/4 and 1/2 BPS sectors of string theory on this
background. The 1/4 BPS partition function jumps as we move off the point in
moduli space where the bulk theta angle and NS-NS fields vanish. We show that
generic 1/2 BPS states are protected because they correspond to geodesics
rather than puffed up branes. By exactly quantizing the simplest of the probes
above, we verify our description of 1/4 BPS states and find agreement with the
known spectrum of 1/2 BPS states of the boundary theory. We also consider the
contribution of these probes to the elliptic genus and discuss puzzles, and
their possible resolutions, in reproducing the elliptic genus of the symmetric
product.Comment: 47 pages; (v2) references and minor clarifications adde
An accurate distance to 2M1207Ab
In April 2004 the first image was obtained of a planetary mass companion (now
known as 2M1207 b) in orbit around a self-luminous object different from our
own Sun (the young brown dwarf 2MASSW J1207334-393254, hereafter 2M1207 A).
2M1207 b probably formed via fragmentation and gravitational collapse, offering
proof that such a mechanism can form bodies in the planetary mass regime.
However, the predicted mass, luminosity, and radius of 2M1207 b depend on its
age, distance, and other observables such as effective temperature. To refine
our knowledge of the physical properties of 2M1207 b and its nature, we
obtained an accurate determination of the distance to the 2M1207 A and b system
by measurements of its trigonometric parallax at the milliarcsec level. With
the ESO NTT/SUSI2 telescope, in 2006 we began a campaign of photometric and
astrometric observations to measure the trigonometric parallax of 2M1207 A. An
accurate distance ( pc) to 2M1207A was measured. From distance and
proper motions we derived spatial velocities fully compatible with TWA
membership. With this new distance estimate, we discuss three scenarios
regarding the nature of 2M1207 b: (1) a cool ( K) companion of mass
M, (2) a warmer ( K) and heavier (
M) companion occulted by an edge-on circum-secondary disk or (3) a
hot protoplanet collision afterglow.Comment: 5 pages, 3 figures, accepted for publication as letter in A&A,
6/11/200
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