217 research outputs found
Atmospheres and Winds of PN Central Stars
The progress over the last years in modelling the atmospheres and winds of PN
central stars is reviewed. We discuss the effect of the inclusion of the
blanketing by millions of metal lines in NLTE on the diagnostics of
photospheric and stellar wind lines, which can be used to determine stellar
parameters such as effective temperature, gravity, radius, mass loss rate and
distance. We also refer to recent work on the winds of massive O-type stars,
which indicates that their winds are possibly inhomogeneous and clumped. We
investigate implications from this work on the spectral diagnostics of PN
central stars and introduce a method to determine wind clumping factors from
the relative strengths of Halpha and HeII 4686. Based on new results we discuss
the wind properties of CSPN.Comment: 8 pages, 12 figures; Proceedings, IAU Symposium No. 234, 2006,
"Planetary Nebulae in our Galaxy and Beyond", M.J. Barlow and R.H. Mendez,
ed
Age Dating Stellar Populations in the Near Infrared: An absolute age indicator from the presence/absence of red supergiants
The determination of age is a critical component in the study of a population
of stellar clusters. In this letter we present a new absolute age indicator for
young massive star clusters based on J-H colour. This novel method identifies
clusters as older or younger than 5.7 +/- 0.8 Myr based on the appearance of
the first population of red supergiant stars. We test the technique on the
stellar cluster population of the nearby spiral galaxy, M83, finding good
agreement with the theoretical predictions. The localisation of this technique
to the near-IR promises that it may be used well into the future with space--
and ground--based missions optimised for near-IR observations.Comment: 5 pages, 5 figures. Accepted to MNRAS, November 201
The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
The quantitative study of the physical properties and chemical abundances of
large samples of massive blue stars at different metallicities is a powerful
tool to understand the nature and evolution of these objects. Their analysis
beyond the Milky Way is challenging, nonetheless it is doable and the best way
to investigate their behavior in different environments. Fulfilling this task
in an objective way requires the implementation of automatic analysis
techniques that can perform the analyses systematically, minimizing at the same
time any possible bias.
As part of the ARAUCARIA project we carry out the first quantitative
spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55
at 1.94 Mpc away. By applying the methodology developed in this work, we derive
their stellar parameters, chemical abundances and provide a characterization of
the present-day metallicity of their host galaxy.
Based on the characteristics of the stellar atmosphere/line formation code
FASTWIND, we designed and created a grid of models for the analysis of massive
blue supergiant stars. Along with this new grid, we implemented a spectral
analysis algorithm. Both tools were specially developed to perform fully
consistent quantitative spectroscopic analyses of low spectral resolution of
B-type supergiants in a fast and objective way.
We present the main characteristics of our FASTWIND model grid and perform a
number of tests to investigate the reliability of our methodology. The
automatic tool is applied afterward to a sample of 12 B-type supergiant stars
in NGC55, deriving the stellar parameters and abundances. The results indicate
that our stars are part of a young population evolving towards a red supergiant
phase. The derived chemical composition hints to an average metallicity similar
to the one of the Large Magellanic Cloud, with no indication of a spatial trend
across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&
The ARAUCARIA Project: VLT-FORS spectroscopy of blue supergiants in NGC 3109 - Classifications, first abundances and kinematics
We have obtained multi-object spectroscopy of luminous blue supergiants in
NGC 3109, a galaxy at the periphery of the Local Group at ~1.3 Mpc. We present
a detailed catalog including finding charts, V and I magnitudes, spectral
classifications, and stellar radial velocities. The radial velocities are seen
to trace the rotation curves obtained from studies of the HI gas. From
quantitative analysis of eight B-type supergiants we find a mean oxygen
abundance of 12+log(O/H) = 7.76 +/-0.07 (1-sigma systematic uncertainty), with
a median result of 7.8. Given its distance, we highlight NGC 3109 as the ideal
example of a low metallicity, dark-matter dominated, dwarf galaxy for
observations with the next generation of ground-based extremely large
telescopes (ELTs).Comment: 42 pages, 19 figures (most reduced in size). Accepted for publication
in ApJ. A copy with full res. & embedded figures is at
http://www.roe.ac.uk/~cje/ngc3109.ps.g
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