16 research outputs found
Massive stars at low metallicity: Evolution and surface abundances of O dwarfs in the SMC
We study the evolution, rotation, and surface abundances of O-type dwarfs in
the Small Magellanic Cloud. We analyzed the UV and optical spectra of
twenty-three objects and derived photospheric and wind properties. The observed
binary fraction of the sample is ~ 26%, which is compatible with more
systematic studies, if one considers that the actual binary fraction is
potentially larger owing to low-luminosity companions and that the sample
excluded obvious spectroscopic binaries. The location of the fastest rotators
in the H-R diagram indicates that these could be several Myr old. The offset in
the position of these fast rotators compared with the other stars confirms the
predictions of evolutionary models that fast-rotating stars tend to evolve more
vertically in the H-R diagram. Only one star of luminosity-class Vz, expected
to best characterize extreme youth, is located on the ZAMS, the other two stars
are more evolved. The distribution of nitrogen abundance of O and B stars
suggests that the mechanisms responsible for the chemical enrichment of slowly
rotating massive stars depends only weakly on the star's mass. We confirm that
the group of slowly rotating N-rich stars is not reproduced by the evolutionary
tracks. Our results call for stronger mixing in the models to explain the range
of observed N abundances. All stars have an N/C ratio as a function of stellar
luminosity that matches the predictions of the stellar evolution models well.
More massive stars have a higher N/C ratio than the less massive stars. Faster
rotators show on average a higher N/C ratio than slower rotators. The N/O
versus N/C ratios agree qualitatively well with those of stellar evolution
models. The only discrepant behavior is observed for the youngest two stars of
the sample, which both show very strong signs of mixing, which is unexpected
for their evolutionary status.Comment: Accepted for publication in A&A (43 pages, 60 figures
The Astropy Problem
The Astropy Project (http://astropy.org) is, in its own words, "a community
effort to develop a single core package for Astronomy in Python and foster
interoperability between Python astronomy packages." For five years this
project has been managed, written, and operated as a grassroots,
self-organized, almost entirely volunteer effort while the software is used by
the majority of the astronomical community. Despite this, the project has
always been and remains to this day effectively unfunded. Further, contributors
receive little or no formal recognition for creating and supporting what is now
critical software. This paper explores the problem in detail, outlines possible
solutions to correct this, and presents a few suggestions on how to address the
sustainability of general purpose astronomical software
NOMINAL VALUES FOR SELECTED SOLAR AND PLANETARY QUANTITIES: IAU 2015 RESOLUTION B3
In this brief communication we provide the rationale for and the outcome of the International Astronomical Union (IAU) resolution vote at the XXIXth General Assembly in Honolulu, Hawaii, in 2015, on recommended nominal conversion constants for selected solar and planetary properties. The problem addressed by the resolution is a lack of established conversion constants between solar and planetary values and SI units: a missing standard has caused a proliferation of solar values (e.g., solar radius, solar irradiance, solar luminosity, solar effective temperature, and solar mass parameter) in the literature, with cited solar values typically based on best estimates at the time of paper writing. As precision of observations increases, a set of consistent values becomes increasingly important. To address this, an IAU Working Group on Nominal Units for Stellar and Planetary Astronomy formed in 2011, uniting experts from the solar, stellar, planetary, exoplanetary, and fundamental astronomy, as well as from general standards fields to converge on optimal values for nominal conversion constants. The effort resulted in the IAU 2015 Resolution B3, passed at the IAU General Assembly by a large majority. The resolution recommends the use of nominal solar and planetary values, which are by definition exact and are expressed in SI units. These nominal
values should be understood as conversion factors only, not as the true solar/planetary properties or current best estimates. Authors and journal editors are urged to join in using the standard values set forth by this resolution in
future work and publications to help minimize further confusion
Evidence for quasi-homogeneous evolution of massive stars
Rotation affects the evolution of massive stars. The paths followed by massive stars in the HR diagram are different depending on the initial rotation velocity. The transport of chemical species freshly produced in the core is also more efficient in rotating stars. In the case of very fast initial rotation, massive stars are expected to follow a peculiar evolution: instead of evolving off the main sequence by its cool side, stars become hotter and hotter and evolve blueward of the ZAMS. The reason is the very efficient chemical mixing: stars evolve quasi homogeneously. In this talk, we will present evidence for the existence of such an evolution from the analysis of early-type H-rich WN stars in the Magellanic Clouds and the Galaxy. The stellar parameters are determined by spectroscopic analysis and are used to place the stars in the HR diagram: they are on the hot side of the ZAMS. In addition, they still contain a large fraction of hydrogen in their atmosphere. This cannot be explained by standard evolutionary tracks. Quasi homogeneous evolution is able to reproduce all these properties. We will discuss the implications of these results in the context of the formation of long-soft gamma ray bursts and the collapsar model