37,459 research outputs found
Chemical Evolution of the Carina Dwarf Spheroidal
We explore a range of chemical evolution models for the Local Group dwarf
spheroidal (dSph) galaxy, Carina. A novel aspect of our work is the removal of
the star formation history (SFH) as a `free parameter' in the modeling, making
use, instead, of its colour-magnitude diagram (CMD)-constrained SFH. By varying
the relative roles of galactic winds, re-accretion, and ram-pressure stripping
within the modeling, we converge on a favoured scenario which emphasises the
respective roles of winds and re-accretion. While our model is successful in
recovering most elemental abundance patterns, comparable success is not found
for all the neutron capture elements. Neglecting the effects of stripping
results in predicted gas fractions approximately two orders of magnitude too
high, relative to that observed.Comment: Accepted for publication in PoS (Proceedings of Science): Nuclei in
the Cosmos XII (Cairns, Aug 2012); 6 pages; 4 figure
Type Ia Supernovae and the Value of the Hubble Constant
The methodology involved in deriving the Hubble Constant via the calibration
of the corrected peak luminosities of Type Ia supernovae (SNe) is reviewed. We
first present a re-analysis of the Calan-Tololo (C-T) and Center for
Astrophysics (CfA) Type Ia SN surveys. Bivariate linear least squares and
quadratic boot-strapped fits in peak apparent magnitude and light curve shape
are employed to correct this heterogeneous sample of peak apparent magnitudes,
resulting in an homogeneous (and excellent) secondary distance indicator: the
so-called corrected peak luminosity. We next provide an empirical calibration
for this corrected luminosity, using Cepheid-based distances for seven nearby
spiral galaxies host to Type Ia SNe. Included in this sample is the
spectroscopically peculiar SN 1991T (in NGC 4527), whose corrected peak
luminosity is shown to be indistinguishable from that of so-called ``normal''
SNe. A robust value of the Hubble Constant is derived and shown to be
H0=73+/-2(r)+/-7(s) km/s/Mpc.Comment: 10 pages, 2 figures, 2 tables, LaTeX (newpasp), also available at
http://www.swin.edu.au/astronomy/bgibson/publications.html, to appear in New
Cosmological Data and the Values of the Fundamental Parameters (IAU Symposium
#201), ed. A. Lasenby & A. Wilkinson, ASP Conference Series, in press (2001
Simulating a White Dwarf-dominated Galactic Halo
Observational evidence has suggested the possibility of a Galactic halo which
is dominated by white dwarfs (WDs). While debate continues concerning the
interpretation of this evidence, it is clear that an initial mass function
(IMF) biased heavily toward WD precursors (1 < m/Msol < 8), at least in the
early Universe, would be necessary in generating such a halo. Within the
framework of homogeneous, closed-box models of Galaxy formation, such biased
IMFs lead to an unavoidable overproduction of carbon and nitrogen relative to
oxygen (as measured against the abundance patterns in the oldest stars of the
Milky Way). Using a three-dimensional Tree N-body smoothed particle
hydrodynamics code, we study the dynamics and chemical evolution of a galaxy
with different IMFs. Both invariant and metallicity-dependent IMFs are
considered. Our variable IMF model invokes a WD-precursor-dominated IMF for
metallicities less than 5% solar (primarily the Galactic halo), and the
canonical Salpeter IMF otherwise (primarily the disk). Halo WD density
distributions and C,N/O abundance patterns are presented. While Galactic haloes
comprised of ~5% (by mass) of WDs are not supported by our simulations, mass
fractions of ~1-2% cannot be ruled out. This conclusion is consistent with the
present-day observational constraints.Comment: accepted for publication in MNRA
Observationally-Motivated Analysis of Simulated Galaxies
The spatial and temporal relationships between stellar age, kinematics, and
chemistry are a fundamental tool for uncovering the physics driving galaxy
formation and evolution. Observationally, these trends are derived using
carefully selected samples isolated via the application of appropriate
magnitude, colour, and gravity selection functions of individual stars;
conversely, the analysis of chemodynamical simulations of galaxies has
traditionally been restricted to the age, metallicity, and kinematics of
`composite' stellar particles comprised of open cluster-mass simple stellar
populations. As we enter the Gaia era, it is crucial that this approach
changes, with simulations confronting data in a manner which better mimics the
methodology employed by observers. Here, we use the \textsc{SynCMD} synthetic
stellar populations tool to analyse the metallicity distribution function of a
Milky Way-like simulated galaxy, employing an apparent magnitude plus gravity
selection function similar to that employed by the RAdial Velocity Experiment
(RAVE); we compare such an observationally-motivated approach with that
traditionally adopted - i.e., spatial cuts alone - in order to illustrate the
point that how one analyses a simulation can be, in some cases, just as
important as the underlying sub-grid physics employed.Comment: Accepted for publication in PoS (Proceedings of Science): Nuclei in
the Cosmos XIII (Debrecen, Jul 2014); 6 pages; 3 figure
Galactic Chemical Evolution Redux: Atomic Numbers 6 < Z < 15
Motivated by the inability of Galactic chemical evolution models to reproduce
some of the observed solar neighbourhood distribution of elements (and
isotopes) with atomic numbers 6 < Z < 15, we have revisited the relevant
stellar and Galactic models as part of an ambitious new program aimed at
resolving these long-standing discrepancies. Avoiding the use of (traditional)
parametric models for low- and intermediate-mass stellar evolution, we have
generated a new, physically self-consistent, suite of stellar models and
integrated the nucleosynthetic outputs into GEtool, our semi-analytical
galactic chemical evolution software package. The predicted temporal evolution
of several light- and intermediate-mass elements (and their isotopes) in the
solar neighbourhood - from carbon to phosphorus - demonstrate the efficacy of
the new yields in reconciling theory and observation.Comment: 4 pages, Accepted for publication in Nuclear Physics A (2005);
Proceedings of Nuclei in the Cosmic VII
Galactic Archaeology and Minimum Spanning Trees
Chemical tagging of stellar debris from disrupted open clusters and
associations underpins the science cases for next-generation multi-object
spectroscopic surveys. As part of the Galactic Archaeology project TraCD
(Tracking Cluster Debris), a preliminary attempt at reconstructing the birth
clouds of now phase-mixed thin disk debris is undertaken using a parametric
minimum spanning tree (MST) approach. Empirically-motivated chemical abundance
pattern uncertainties (for a 10-dimensional chemistry-space) are applied to
NBODY6-realised stellar associations dissolved into a background sea of field
stars, all evolving in a Milky Way potential. We demonstrate that significant
population reconstruction degeneracies appear when the abundance uncertainties
approach 0.1 dex and the parameterised MST approach is employed; more
sophisticated methodologies will be required to ameliorate these degeneracies.Comment: To appear in "Multi-Object Spectroscopy in the Next Decade: Big
Questions, Large Surveys and Wide Fields"; Held: Santa Cruz de La Palma,
Canary Islands, Spain, 2-6 Mar 2015; ed. I Skillen & S. Trager; ASP
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