20 research outputs found
A Search for Nitrogen-Enhanced Metal-Poor Stars
Theoretical models of very metal-poor intermediate-mass Asymptotic Giant
Branch (AGB) stars predict a large overabundance of primary nitrogen. The very
metal-poor, carbon-enhanced, s-process-rich stars, which are thought to be the
polluted companions of now-extinct AGB stars, provide direct tests of the
predictions of these models. Recent studies of the carbon and nitrogen
abundances in metal-poor stars have focused on the most carbon-rich stars,
leading to a potential selection bias against stars that have been polluted by
AGB stars that produced large amounts of nitrogen, and hence have small [C/N]
ratios. We call these stars Nitrogen-Enhanced Metal-Poor (NEMP) stars, and
define them as having [N/Fe] > +0.5 and [C/N] < -0.5. In this paper, we report
on the [C/N] abundances of a sample of 21 carbon-enhanced stars, all but three
of which have [C/Fe] < +2.0. If NEMP stars were made as easily as
Carbon-Enhanced Metal-Poor (CEMP) stars, then we expected to find between two
and seven NEMP stars. Instead, we found no NEMP stars in our sample. Therefore,
this observational bias is not an important contributor to the apparent dearth
of N-rich stars. Our [C/N] values are in the same range as values reported
previously in the literature (-0.5 to +2.0), and all stars are in disagreement
with the predicted [C/N] ratios for both low-mass and high-mass AGB stars. We
suggest that the decrease in [C/N] from the low-mass AGB models is due to
enhanced extra-mixing, while the lack of NEMP stars may be caused by
unfavorable mass ratios in binaries or the difficulty of mass transfer in
binary systems with large mass ratios.Comment: 14 pages, 7 figures, to be published in Ap
Stellar Chemical Signatures And Hierarchical Galaxy Formation
ABRIDGED: To compare the chemistries of stars in the Milky Way dSph galaxies
with stars in the Galaxy, we have compiled a large sample of Galactic stellar
abundances from the literature. As found in previous studies, the [alpha/Fe]
ratios of most stars in the dSph galaxies are generally lower than similar
metallicity Galactic stars in this extended sample. Our kinematically selected
stars confirm that this is true for all major stellar components of the Galaxy,
including the halo, thin disk, and thick disk. There is marginal overlap in the
low [alpha/Fe] ratios between dSph stars and Galactic halo stars on extreme
retrograde orbits, but this is not supported by other element ratios, like
[Ba/Y]. Thus, the full chemical signature of most of the dSph stars are
distinct from the main components of the Galaxy. This result rules out
continuous merging of low mass galaxies similar to these dSph satellites during
the formation of the Galaxy, which is discuss further. The new environments
offered by the dSph galaxies also allow us to examine fundamental assumptions
related to nucleosynthesis. The metal-poor stars ([Fe/H] < -1.8) in the dSph
galaxies have [alpha/Fe] ratio that are consistent with predictions from the
alpha-process (alpha-rich freeze out). This may also explain the very low Y
abundances, and could suggest a separate r-process site for this light (first
peak r-process) element. In stars with higher metallicities ([Fe/H] > -1.8),
contributions from the s-process are expected; [Ba/Y] is still much higher in
the dSph stars than similar metallicity Galactic stars though, consistent with
s-process contributions from only the low metallicity AGB stars in dSph
galaxies. Finally, the Na-Ni trend in Galactic halo stars is confirmed, but
discuss this in terms of nucleosynthesis, and not the accretion of dSphs.Comment: Accepted for Sept 2004 Astronomical Journal; 26 pages, 9 figures, 3
table
Mass Segregation in Young Magellanic Clouds Star Clusters: Four Clusters observed with HST
We present the results of our investigation on the phenomenon of mass
segregation in young star clusters in the Magellanic Clouds. HST/WFPC2
observations on NGC 1818, NGC 2004 & NGC 2100 in the Large Magellanic Cloud and
NGC 330 in the Small Magellanic Cloud have been used for the application of
diagnostic tools for mass segregation: i) the radial density profiles of the
clusters for various mass groups and ii) their mass functions (MFs) at various
radii around their centres. All four clusters are found to be mass segregated,
but each one in a different manner. Specifically not all the clusters in the
sample show the same dependence of their density profiles on the selected
magnitude range, with NGC 1818 giving evidence of a strong such relation and
NGC 330 showing only a hint of the phenomenon. NGC 2004 did not also show any
significant signature of mass segregation in its density profiles. The MFs
radial dependence provides clear proof of the phenomenon for NGC 1818, NGC 2100
and NGC 2004, while for NGC 330 it gives only indications. An investigation on
the constraints introduced by the application of both diagnostic tools is
presented. We also discuss the problems related to the construction of a
reliable MF for a cluster and their impact on the investigation of the
phenomenon of mass segregation. We find that the MFs of these clusters as they
were constructed with two methods, are comparable to Salpeter's IMF. A
discussion is given on the dynamical status of the clusters and a test is
applied on the equipartition among several mass groups in them. Both showed
that the observed mass segregation in the clusters is of primordial nature.Comment: A&A Accepted, 20 pages, 9 Figures, Version with language errors and
typos correcte
Foster Carers Who Care for Children with Challenging Behaviour: A Total Population Study
Given the concern that those undertaking the foster care task in the UK may not have all the skills and capacities necessary to care for the special needs of looked after children, particularly those with challenging behaviour, it is essential to gain a deeper insight into what carers themselves own by way of personal background, experience and perception about their role. This is important, for if we are to 'professionalise' the carer role we need to know more about those in whom we would seek to build capacities in order to make them 'professional'. Thus we need to know more about carer attitudes, motives and experience in order to design the right sort of capacity-building and not simply to think that a 'one size fits all' response of training and support will suffice. Andrew Pithouse, Kathy Lowe and Jan Hill-Tout examine carer characteristics as found in a total population of carers (n = 115) in four neighbouring Welsh unitary authorities caring for all children (n = 114) considered to exhibit challenging behaviour. Selective findings are presented and connected to other relevant UK research. Key categories include the carers' social background, tenure in fostering, children fostered, their training, their perception of the fostering role within the local authority, their views on support from social services and their experience of stress. A concluding section identifies those areas where capacity-building in carers is most pressing