740 research outputs found
Mass estimates from stellar proper motions: The mass of Centauri
We lay out and apply methods to use proper motions of individual kinematic
tracers for estimating the dynamical mass of star clusters. We first describe a
simple projected mass estimator and then develop an approach that evaluates
directly the likelihood of the discrete kinematic data given the model
predictions. Those predictions may come from any dynamical modelling approach,
and we implement an analytic King model, a spherical isotropic Jeans equation
model and an axisymmetric, anisotropic Jeans equation model.We apply these
approaches to the enigmatic globular cluster omega Centauri, combining the
proper motion from van Leeuwen et al (2000) with improved photometric cluster
membership probabilities. We show that all mass estimates based on spherical
isotropic models yield , where our modelling allows us to show how the statistical precision of
this estimate improves as more proper motion data of lower signal-to-noise are
included. MLM predictions, based on an anisotropic axisymmetric Jeans model,
indicate for Cen that the inclusion of anisotropies is not important
for the mass estimates, but that accounting for the flattening is: flattened
models imply , 10%
lower than when restricting the analysis to a spherical model. The best current
distance estimates imply an additional uncertainty in the mass estimate of 12%.Comment: Accepted for publication in MNRA
The Milky Way's Stellar Disk
A suite of vast stellar surveys mapping the Milky Way, culminating in the
Gaia mission, is revolutionizing the empirical information about the
distribution and properties of stars in the Galactic stellar disk. We review
and lay out what analysis and modeling machinery needs to be in place to test
mechanisms of disk galaxy evolution and to stringently constrain the Galactic
gravitational potential, using such Galactic star-by-star measurements. We
stress the crucial role of stellar survey selection functions in any such
modeling; and we advocate the utility of viewing the Galactic stellar disk as
made up from `mono-abundance populations' (MAPs), both for dynamical modeling
and for constraining the Milky Way's evolutionary processes. We review recent
work on the spatial and kinematical distribution of MAPs, and lay out how
further study of MAPs in the Gaia era should lead to a decisively clearer
picture of the Milky Way's dark matter distribution and formation history.Comment: Astron. Astrophys. Rev., in pres
: A flexible chemical evolution model for abundance fitting - Do the Sun's abundances alone constrain chemical evolution models?
Elemental abundances of stars are the result of the complex enrichment
history of their galaxy. Interpretation of observed abundances requires
flexible modeling tools to explore and quantify the information about Galactic
chemical evolution (GCE) stored in such data. Here we present Chempy, a newly
developed code for GCE modeling, representing a parametrized open one-zone
model within a Bayesian framework. A Chempy model is specified by a set of 5-10
parameters that describe the effective galaxy evolution along with the stellar
and star-formation physics: e.g. the star-formation history, the feedback
efficiency, the stellar initial mass function (IMF) and the incidence of
supernova type Ia (SN Ia). Unlike established approaches, Chempy can sample the
posterior probability distribution in the full model parameter space and test
data-model matches for different nucleosynthetic yield sets. We extend Chempy
to a multi-zone scheme. As an illustrative application, we show that
interesting parameter constraints result from only the ages and elemental
abundances of Sun, Arcturus and the present-day interstellar medium (ISM). For
the first time, we use such information to infer IMF parameter via GCE
modeling, where we properly marginalize over nuisance parameters and account
for different yield sets. We find that of the IMF %
explodes as core-collapse SN, compatible with Salpeter 1955. We also constrain
the incidence of SN Ia per 10^3 Msun to 0.5-1.4. At the same time, this Chempy
application shows persistent discrepancies between predicted and observed
abundances for some elements, irrespective of the chosen yield set. These
cannot be remedied by any variations of Chempy's parameters and could be an
indication for missing nucleosynthetic channels. Chempy should be a powerful
tool to confront predictions from stellar nucleosynthesis with far more complex
abundance data sets.Comment: 19 pages, 17 figures, accepted for publication in A&A, python code:
https://github.com/jan-rybizki/Chemp
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