523 research outputs found
Factorizations of tropical and sign polynomials
In this text, we study factorizations of polynomials over the tropical hyperfield and the sign hyperfield, which we call `tropical polynomials' and `sign polynomials', respectively. We classify all irreducible polynomials in either case. We show that tropical polynomials factor uniquely into irreducible factors, but that unique factorization fails for sign polyomials. We describe division algorithms for tropical and sign polynomials by linear terms that correspond to roots of the polynomials
The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM and NGC 3109
Massive stars likely played an important role in the reionization of the
Universe, and the formation of the first black holes. Massive stars in
low-metallicity environments in the local Universe are reminiscent of their
high redshift counterparts. In a previous paper, we reported on indications
that the stellar winds of low-metallicity O stars may be stronger than
predicted, which would challenge the current paradigm of massive star
evolution. In this paper, we aim to extend our initial sample of six O stars in
low-metallicity environments by four. We aim to derive their stellar and wind
parameters, and compare these to radiation-driven wind theory and stellar
evolution models. We have obtained intermediate-resolution VLT/X-Shooter
spectra of our sample of stars. We derive the stellar parameters by fitting
synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic
fitting algoritm. We compare our parameters to evolutionary tracks and obtain
evolutionary masses and ages. We also investigate the effective temperature
versus spectral type calibration for SMC and lower metallicities. Finally, we
reassess the wind momentum versus luminosity diagram. The derived parameters of
our target stars indicate stellar masses that reach values of up to 50
. The wind strengths of our stars are, on average, stronger than
predicted from radiation-driven wind theory and reminiscent of stars with an
LMC metallicity. We discuss indications that the iron content of the host
galaxies is higher than originally thought and is instead SMC-like. We find
that the discrepancy with theory is lessened, but remains significant for this
higher metallicity. This may imply that our current understanding of the wind
properties of massive stars, both in the local universe as well as at cosmic
distances, remains incomplete.Comment: Accepted for publication in Astronomy and Astrophysics. 10 pages, 8
figure
The VLT-FLAMES Tarantula Survey
We present a number of notable results from the VLT-FLAMES Tarantula Survey
(VFTS), an ESO Large Program during which we obtained multi-epoch
medium-resolution optical spectroscopy of a very large sample of over 800
massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC).
This unprecedented data-set has enabled us to address some key questions
regarding atmospheres and winds, as well as the evolution of (very) massive
stars. Here we focus on O-type runaways, the width of the main sequence, and
the mass-loss rates for (very) massive stars. We also provide indications for
the presence of a top-heavy initial mass function (IMF) in 30 Dor.Comment: 7 Figures, 8 pages. Invited talk: IAUS 329: "The Lives and
Death-Throes of Massive Stars
The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries
The initial distribution of spin rates of massive stars is a fingerprint of
their elusive formation process. It also sets a key initial condition for
stellar evolution and is thus an important ingredient in stellar population
synthesis. So far, most studies have focused on single stars. Most O stars are
however found in multiple systems. By establishing the spin-rate distribution
of a sizeable sample of O-type spectroscopic binaries and by comparing the
distributions of binary sub-populations with one another as well as with that
of presumed single stars in the same region, we aim to constrain the initial
spin distribution of O stars in binaries, and to identify signatures of the
physical mechanisms that affect the evolution of the massive stars spin rates.
We use ground-based optical spectroscopy obtained in the framework of the
VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial
rotational velocities (\vrot) for components of 114 spectroscopic binaries in
30 Doradus. The \vrot\ values are derived from the full-width at half-maximum
(FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we
derive based on previous line analysis methods applied to single O-type stars
in the VFTS sample. The overall \vrot\ distribution of the primary stars
resembles that of single O-type stars in the VFTS, featuring a low-velocity
peak (at \vrot < 200 kms) and a shoulder at intermediate velocities (200 <
\vrot < 300 kms). The distributions of binaries and single stars however
differ in two ways. First, the main peak at \vrot \sim100 kms is broader and
slightly shifted toward higher spin rates in the binary distribution compared
to that of the presumed-single stars. Second, the \vrot distribution of
primaries lacks a significant population of stars spinning faster than 300 kms
while such a population is clearly present in the single star sample.Comment: 16 pages, 16 figures, paper accepted in Astronomy & Astrophysic
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