765 research outputs found
The Last Stages of Star Formation in dEs?
A significant fraction of Virgo cluster early-type dwarf galaxies have blue central colours caused by recent or ongoing star formation. A spectral analysis shows that even in their centers, stellar mass is dominated by an old population. These galaxies are an unrelaxed cluster population that possibly formed from morphological transformation of late-type galaxie
Star Formation History in two fields of the Small Magellanic Cloud Bar
The Bar is the most productive region of the Small Magellanic Cloud in terms
of star formation but also the least studied one. In this paper we investigate
the star formation history of two fields located in the SW and in the NE
portion of the Bar using two independent and well tested procedures applied to
the color-magnitude diagrams of their stellar populations resolved by means of
deep HST photometry. We find that the Bar experienced a negligible star
formation activity in the first few Gyr, followed by a dramatic enhancement
from 6 to 4 Gyr ago and a nearly constant activity since then. The two examined
fields differ both in the rate of star formation and in the ratio of recent
over past activity, but share the very low level of initial activity and its
sudden increase around 5 Gyr ago. The striking similarity between the timing of
the enhancement and the timing of the major episode in the Large Magellanic
Cloud is suggestive of a close encounter triggering star formation.Comment: 30 pages, 22 figures, accepted for publication in Ap
Structural Parameters of Seven SMC Intermediate-Age and Old Star Clusters
We present structural parameters for the seven intermediate-age and old star
clusters NGC121, Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in
the Small Magellanic Cloud. We fit King profiles and Elson, Fall, and Freeman
profiles to both surface-brightness and star count data taken with the Advanced
Camera for Surveys aboard the Hubble Space Telescope. Clusters older than 1 Gyr
show a spread in cluster core radii that increases with age, while the youngest
clusters have relatively compact cores. No evidence for post core collapse
clusters was found. We find no correlation between core radius and distance
from the SMC center, although consistent with other studies of dwarf galaxies,
some relatively old and massive clusters have low densities. The oldest SMC
star cluster, the only globular NGC121, is the most elliptical object of the
studied clusters. No correlation is seen between ellipticity and distance from
the SMC center. The structures of these massive intermediate-age (1-8 Gyr) SMC
star clusters thus appear to primarily result from internal evolutionary
processes.Comment: 16 pages, 13 figure
The many faces of early-type dwarf galaxies
We present a study of 413 early-type dwarf (dE) galaxies in the Virgo Cluster with SDSS multicolour imaging. Several dE subclasses show systematic differences in shape, clustering properties, and stellar content. We find different colour-magnitude relations for nucleated and non-nucleated dEs, as well as for dE samples populating regions of different densitie
Ages and Luminosities of Young SMC/LMC Star Clusters and the recent Star Formation History of the Clouds
In this paper we discuss the age and spatial distribution of young
(age1Gyr) SMC and LMC clusters using data from the Magellanic Cloud
Photometric Surveys. Luminosities are calculated for all age-dated clusters.
Ages of 324 and 1193 populous star clusters in the Small and the Large
Magellanic Cloud have been determined fitting Padova and Geneva isochrone
models to their resolved color-magnitude diagrams. The clusters cover an age
range between 10Myr and 1Gyr in each galaxy. For the SMC a constant distance
modulus of = 18.90 and a metallicity of Z = 0.004 were adopted. For
the LMC, we used a constant distance modulus of = 18.50 and a
metallicity of Z = 0.008. For both galaxies, we used a variable color excess to
derive the cluster ages. We find two periods of enhanced cluster formation in
both galaxies at 160Myr and 630Myr (SMC) and at 125Myr and 800Myr (LMC). We
present the spatially resolved recent star formation history of both Clouds
based on young star clusters. The first peak may have been triggered by a close
encounter between the SMC and the LMC. In both galaxies the youngest clusters
reside in the supergiant shells, giant shells, the inter-shell regions, and
toward regions with a high H content, suggesting that their formation
is related to expansion and shell-shell interaction. Most of the clusters are
older than the dynamical age of the supergiant shells. No evidence for cluster
dissolution was found. Computed V band luminosities show a trend for fainter
magnitudes with increasing age as well as a trend for brighter magnitudes with
increasing apparent cluster radii.Comment: 17 pages, 14 figures, Accepted for publication in A&
Reverse dynamisation: a modern perspective on Stephan Perrenâs strain theory
The present review acknowledges the tremendous impact of Stephan Perrenâs strain theory, considered with respect to the earlier contributions of Roux and Pauwels. Then, it provides further insight by examining how the concept of reverse dynamisation extended Perrenâs theory within a modern context. A key factor of this more contemporary theory is that it introduces variable mechanical conditions at different time points during bone healing, opening the possibility of manipulating biology through mechanics to achieve the desired clinical outcome. The discussion focusses on the current state of the art and the most recent advances made towards optimising and accelerating bone regeneration, by actively controlling the mechanical environment as healing progresses. Reverse dynamisation utilises a very specific mechanical manipulation regimen, with conditions initially flexible to encourage and expedite early callus formation. Once callus has formed, the mechanical conditions are intentionally modified to create a rigid environment under which the soft callus is quickly converted to hard callus, bridging the fracture site and leading to a more rapid union. The relevant literature, principally animal studies, was surveyed to provide ample evidence in support of the effectiveness of reverse dynamisation. By providing a modern perspective on Stephan Perrenâs strain theory, reverse dynamisation perhaps holds the key to tipping the balance in favour of a more rapid and reliable union when treating acute fractures, osteotomies, non-unions and other circumstances where it is necessary to regenerate bone
- âŠ