14,746 research outputs found
The thickening of the thin disk in the third Galactic quadrant
In the third Galactic quadrant (180 < l < 270) of the Milky Way, the Galactic
thin disk exhibits a significant warp ---shown both by gas and young stars---
bending down a few kpc below the formal Galactic plane (b=0). This warp shows
its maximum at 240, in the direction of the Canis Major constellation. In a
series of papers we have traced the detailed structure of this region using
open star clusters, putting particular emphasis on the spiral structure of the
outer disk. We noticed a conspicuous accumulation of young star clusters within
2-3 kpc from the Sun and close to b=0, that we interpreted as the continuation
of the Local (Orion) arm towards the outer disk. While most clusters (and young
stars in their background) follow closely the warp of the disk, our decade-old
survey of the spiral structure of this region led us to identify three
clusters, Haffner~18(1 and 2) and Haffner~19, which remain very close to b=0
and lie at distances (4.5, 8.0, and 6.4 kpc) where most of the material is
already significantly warped. Here we report on a search for clusters that
share the same properties as Haffner~18 and 19, and investigate the possible
reasons for such an unexpected occurrence. We present UBVRI photometry of
5~young clusters, namely NGC~2345, NGC~2374, Trumpler~9, Haffner~20, and
Haffner~21, which also lie close to the formal Galactic plane. With the
exception of Haffner~20, in the background of these clusters we detected young
stars that appear close to b=0, and are located at distances up to 8 kpc from
the Sun, thus deviating significantly from the warp. These populations define a
structure that distributes over almost the entire third Galactic quadrant. We
discuss this structure in the context of a possible thin disk flaring, in full
similarity with the Galactic thick disk.Comment: 53 pages, 12 eps figures, in press in the Astronomical Journa
Obtaining a class of Type O pure radiation metrics with a cosmological constant, using invariant operators
Using the generalised invariant formalism we derive a class of conformally
flat spacetimes whose Ricci tensor has a pure radiation and a Ricci scalar
component. The method used is a development of the methods used earlier for
pure radiation spacetimes of Petrov types O and N respectively. In this paper
we demonstrate how to handle, in the generalised invariant formalism,
spacetimes with isotropy freedom and rich Killing vector structure. Once the
spacetimes have been constructed, it is straightforward to deduce their
Karlhede classification: the Karlhede algorithm terminates at the fourth
derivative order, and the spacetimes all have one degree of null isotropy and
three, four or five Killing vectors.Comment: 29 page
Invariant classification and the generalised invariant formalism: conformally flat pure radiation metrics, with zero cosmological constant
Metrics obtained by integrating within the generalised invariant formalism
are structured around their intrinsic coordinates, and this considerably
simplifies their invariant classification and symmetry analysis. We illustrate
this by presenting a simple and transparent complete invariant classification
of the conformally flat pure radiation metrics (except plane waves) in such
intrinsic coordinates; in particular we confirm that the three apparently
non-redundant functions of one variable are genuinely non-redundant, and easily
identify the subclasses which admit a Killing and/or a homothetic Killing
vector. Most of our results agree with the earlier classification carried out
by Skea in the different Koutras-McIntosh coordinates, which required much more
involved calculations; but there are some subtle differences. Therefore, we
also rework the classification in the Koutras-McIntosh coordinates, and by
paying attention to some of the subtleties involving arbitrary functions, we
are able to obtain complete agreement with the results obtained in intrinsic
coordinates. In particular, we have corrected and completed statements and
results by Edgar and Vickers, and by Skea, about the orders of Cartan
invariants at which particular information becomes available.Comment: Extended version of GRG publication, with some typos etc correcte
Type O pure radiation metrics with a cosmological constant
In this paper we complete the integration of the conformally flat pure
radiation spacetimes with a non-zero cosmological constant , and , by considering the case . This is a
further demonstration of the power and suitability of the generalised invariant
formalism (GIF) for spacetimes where only one null direction is picked out by
the Riemann tensor. For these spacetimes, the GIF picks out a second null
direction, (from the second derivative of the Riemann tensor) and once this
spinor has been identified the calculations are transferred to the simpler GHP
formalism, where the tetrad and metric are determined. The whole class of
conformally flat pure radiation spacetimes with a non-zero cosmological
constant (those found in this paper, together with those found earlier for the
case ) have a rich variety of subclasses with zero,
one, two, three, four or five Killing vectors
On the Symmetries of the Edgar-Ludwig Metric
The conformal Killing equations for the most general (non-plane wave)
conformally flat pure radiation field are solved to find the conformal Killing
vectors. As expected fifteen independent conformal Killing vectors exist, but
in general the metric admits no Killing or homothetic vectors. However for
certain special cases a one-dimensional group of homotheties or motions may
exist and in one very special case, overlooked by previous investigators, a
two-dimensional homethety group exists. No higher dimensional groups of motions
or homotheties are admitted by these metrics.Comment: Plain TeX, 7 pages, No figure
Growth-quality evaluation of Missouri-grown shortleaf pine (Pinus echinata, Mill.)
Digitized 2007 AES.Includes bibliographical references (pages 57-59)
Dimensionally Dependent Tensor Identities by Double Antisymmetrisation
Some years ago, Lovelock showed that a number of apparently unrelated
familiar tensor identities had a common structure, and could all be considered
consequences in n-dimensional space of a pair of fundamental identities
involving trace-free (p,p)-forms where 2p >= n$. We generalise Lovelock's
results, and by using the fact that associated with any tensor in n-dimensional
space there is associated a fundamental tensor identity obtained by
antisymmetrising over n+1 indices, we establish a very general 'master'
identity for all trace-free (k,l)-forms. We then show how various other special
identities are direct and simple consequences of this master identity; in
particular we give direct application to Maxwell, Lanczos, Ricci, Bel and
Bel-Robinson tensors, and also demonstrate how relationships between scalar
invariants of the Riemann tensor can be investigated in a systematic manner.Comment: 17 pages, 2 figure
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