3,610 research outputs found
Wolf-Rayet stars in the Small Magellanic Cloud: I. Analysis of the single WN stars
Wolf-Rayet (WR) stars have a severe impact on their environments owing to
their strong ionizing radiation fields and powerful stellar winds. Since these
winds are considered to be driven by radiation pressure, it is theoretically
expected that the degree of the wind mass-loss depends on the initial
metallicity of WR stars. Following our comprehensive studies of WR stars in the
Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates
for all seven putatively single WN stars known in the SMC. Based on these data,
we discuss the impact of a low-metallicity environment on the mass loss and
evolution of WR stars. The quantitative analysis of the WN stars is performed
with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical
properties of our program stars are obtained from fitting synthetic spectra to
multi-band observations. In all SMC WN stars, a considerable surface hydrogen
abundance is detectable. The majority of these objects have stellar
temperatures exceeding 75 kK, while their luminosities range from 10^5.5 to
10^6.1 Lsun. The WN stars in the SMC exhibit on average lower mass-loss rates
and weaker winds than their counterparts in the Milky Way, M31, and the LMC. By
comparing the mass-loss rates derived for WN stars in different Local Group
galaxies, we conclude that a clear dependence of the wind mass-loss on the
initial metallicity is evident, supporting the current paradigm that WR winds
are driven by radiation. A metallicity effect on the evolution of massive stars
is obvious from the HRD positions of the SMC WN stars at high temperatures and
high luminosities. Standard evolution tracks are not able to reproduce these
parameters and the observed surface hydrogen abundances. Homogeneous evolution
might provide a better explanation for their evolutionary past.Comment: 18+12 pages; 22+8 figures; accepted for publication in A&
Strongly residual coordinates over A[x]
For a domain A of characteristic zero, a polynomial f over A[x] is called a
strongly residual coordinate if f becomes a coordinate (over A) upon going
modulo x, and f becomes a coordinate upon inverting x. We study the question of
when a strongly residual coordinate is a coordinate, a question closely related
to the Dolgachev-Weisfeiler conjecture. It is known that all strongly residual
coordinates are coordinates for n=2 . We show that a large class of strongly
residual coordinates that are generated by elementaries upon inverting x are in
fact coordinates for arbitrary n, with a stronger result in the n=3 case. As an
application, we show that all Venereau-type polynomials are 1-stable
coordinates.Comment: 15 pages. Some minor clarifications and notational improvements from
the first versio
The XMM-Newton EPIC X-ray Light Curve Analysis of WR 6
We obtained four pointings of over 100 ks each of the well-studied Wolf-Rayet
star WR 6 with the XMM-Newton satellite. With a first paper emphasizing the
results of spectral analysis, this follow-up highlights the X-ray variability
clearly detected in all four pointings. However, phased light curves fail to
confirm obvious cyclic behavior on the well-established 3.766 d period widely
found at longer wavelengths. The data are of such quality that we were able to
conduct a search for "event clustering" in the arrival times of X-ray photons.
However, we fail to detect any such clustering. One possibility is that X-rays
are generated in a stationary shock structure. In this context we favor a
co-rotating interaction region (CIR) and present a phenomenological model for
X-rays from a CIR structure. We show that a CIR has the potential to account
simultaneously for the X-ray variability and constraints provided by the
spectral analysis. Ultimately, the viability of the CIR model will require both
intermittent long-term X-ray monitoring of WR 6 and better physical models of
CIR X-ray production at large radii in stellar winds.Comment: to appear in Ap
Broad P V Absorption in the BALQSO, PG 1254+047: Column Densities, Ionizations and Metal Abundances in BAL Winds
This paper discusses the detection of P V 1118,1128 and other broad
absorption lines (BALs) in archival HST spectra of the low-redshift BALQSO, PG
1254+047. The P V identification is secured by excellent redshift and profile
coincidences with the other BALs, such as C IV 1548,1550 and Si IV 1393,1403,
and by photoionization calculations showing that other lines near this
wavelength, e.g. Fe III 1123, should be much weaker than P V. The observed BAL
strengths imply that either 1) there are extreme abundance ratios such as [C/H]
>~ +1.0, [Si/H] >~ +1.8 and [P/C] >~ +2.2, or 2) at least some of the lines are
much more optically thick than they appear. I argue that the significant
presence of P V absorption indicates severe line saturation, which is disguised
in the observed (moderate-strength) BALs because the absorber does not fully
cover the continuum source(s) along our line(s) of sight. Computed optical
depths for all UV resonance lines show that the observed BALs are consistent
with solar abundances if 1) the ionization parameter is at least moderately
high, log U >~ -0.6, 2) the total hydrogen column density is log N_H(cm-2) >~
22.0, and 3) the optical depths in strong lines like C IV and O VI 1032,1038
are >~25 and >~80, respectively. These optical depths and column densities are
at least an order of magnitude larger than expected from the residual
intensities in the BAL troughs, but they are consistent with the large
absorbing columns derived from X-ray observations of BALQSOs. The outflowing
BALR, at velocities from -15,000 to -27,000 km/s in PG 1254+047, is therefore a
strong candidate for the X-ray absorber in BALQSOs.Comment: 16 pages (LaTeX) plus 8 pages of figures in one file
(pg1254_figs.ps.gz), in press with Ap
Using BBN in cosmological parameter extraction from CMB: a forecast for Planck
Data from future high-precision Cosmic Microwave Background (CMB)
measurements will be sensitive to the primordial Helium abundance . At the
same time, this parameter can be predicted from Big Bang Nucleosynthesis (BBN)
as a function of the baryon and radiation densities, as well as a neutrino
chemical potential. We suggest to use this information to impose a
self-consistent BBN prior on and determine its impact on parameter
inference from simulated Planck data. We find that this approach can
significantly improve bounds on cosmological parameters compared to an analysis
which treats as a free parameter, if the neutrino chemical potential is
taken to vanish. We demonstrate that fixing the Helium fraction to an arbitrary
value can seriously bias parameter estimates. Under the assumption of
degenerate BBN (i.e., letting the neutrino chemical potential vary), the
BBN prior's constraining power is somewhat weakened, but nevertheless allows us
to constrain with an accuracy that rivals bounds inferred from present
data on light element abundances.Comment: 14 pages, 4 figures; v2: minor changes, matches published versio
Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars
Context. Metal-poor massive stars are supposed to be progenitors of certain
supernovae, gamma-ray bursts and compact object mergers, potentially
contributing to the early epochs of the Universe with their strong ionizing
radiation. However, they remain mainly theoretical as individual spectroscopic
observations of such objects have rarely been carried out below the metallicity
of the SMC.
Aims. This work aims at exploring what our state-of-the-art theories of
stellar evolution combined with those of stellar atmospheres predict about a
certain type of metal-poor (0.02 Z) hot massive stars, the chemically
homogeneously evolving ones, called TWUIN stars.
Methods. Synthetic spectra corresponding to a broad range in masses (20-130
M) and covering several evolutionary phases from the zero-age
main-sequence up to the core helium-burning stage were computed.
Results. We find that TWUIN stars show almost no emission lines during most
of their {core hydrogen-burning} lifetimes. Most metal lines are completely
absent, including nitrogen. During their core helium-burning stage, lines
switch to emission and even some metal lines (oxygen and carbon, but still
almost no nitrogen) show up. Mass loss and clumping play a significant role in
line-formation in later evolutionary phases, particularly during core
helium-burning. Most of our spectra are classified as an early O type giant or
supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning
phase.
Conclusions. An extremely hot, early O type star observed in a
low-metallicity galaxy could be the outcome of chemically homogeneous evolution
and therefore the progenitor of a long-duration gamma-ray burst or a type
Ic supernova. TWUIN stars may play an important role in reionizing the Universe
due to their being hot without showing prominent emission lines during the
majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres
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