251 research outputs found
On the Mass-Loss Rates of Massive Stars in the Low-Metallicity Galaxies IC 1613, WLM and NGC 3109
We present a spectroscopic analysis of VLT/X-Shooter observations of six
O-type stars in the low-metallicity (Z ~ 1/7 Z\odot) galaxies IC 1613, WLM and
NGC 3109. The stellar and wind parameters of these sources allow us, for the
first time, to probe the mass-loss versus metallicity dependence of stellar
winds below that of the Small Magellanic Cloud (at Z ~ 1/5Z\odot) by means of a
modified wind momentum versus luminosity diagram. The wind strengths that we
obtain for the objects in WLM and NGC 3109 are unexpectedly high and do not
agree with theoretical predictions. The objects in IC 1613 tend towards a
higher than expected mass-loss rate, but remain consistent with predictions
within their error bars. We discuss potential systematic uncertainties in the
mass-loss determinations to explain our results. However, if further
scrutinization of these findings point towards an intrinsic cause for this
unexpected sub-SMC mass-loss behavior, implications would include a higher than
anticipated number of Wolf-Rayet stars and Ib/Ic supernovae in low-metallicity
environments, but a reduced number of long-duration gamma-ray bursts produced
through a single-star evolutionary channel.Comment: 9 pages, 3 figures; accepted for publication in The Astrophysical
Journal Letter
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
A dearth of short-period massive binaries in the young massive star forming region M17: Evidence for a large orbital separation at birth?
The formation of massive stars remains poorly understood and little is known
about their birth multiplicity properties. Here, we investigate the strikingly
low radial-velocity dispersion measured for a sample of 11 massive pre- and
near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive
star forming region M17 to obtain first constraints on the multiplicity
properties of young massive stellar objects. Methods: We compute the RV
dispersion of synthetic populations of massive stars for various multiplicity
properties and we compare the simulated sigma_rv distributions to the observed
value. We specifically investigate two scenarios: a low binary fraction and a
dearth of short-period binary systems. Results: Simulated populations with low
binary fractions (f_bin = 0.12_{-0.09}^{+0.16}) or with truncated period
distributions (P_cutoff > 9 months) are able to reproduce the low sigma_rv
observed within their 68%-confidence intervals. Parent populations with f_bin >
0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level.
Both constraints are contrast with the high binary fraction and plethora of
short-period systems found in few Myr-old, OB-type populations. To explain the
difference, the first scenario requires a variation of the outcome of the
massive star formation process. In the the second scenario, compact binaries
must form later on, and the cut-off period may be related to physical
length-scales representative of the bloated pre-main-sequence stellar radii or
of their accretion disks. Conclusions: If the obtained constraints are
representative of the overall properties of massive young stellar objects, our
results may provide support to a formation process in which binaries are
initially formed at larger separations, then harden or migrate to produce the
typical (untruncated) power-law period distribution observed in few Myr-old OB
binaries.Comment: 5 pages; Accepted for publication in Astronomy and Astrophysics
Letter
The mass of the very massive binary WR21a
We present multi-epoch spectroscopic observations of the massive binary
system WR21a, which include the January 2011 periastron passage. Our spectra
reveal multiple SB2 lines and facilitate an accurate determination of the orbit
and the spectral types of the components. We obtain minimum masses of
and for the two components of
WR21a. Using disentangled spectra of the individual components, we derive
spectral types of O3/WN5ha and O3Vz~((f*)) for the primary and secondary,
respectively. Using the spectral type of the secondary as an indication for its
mass, we estimate an orbital inclination of and
absolute masses of and , in
agreement with the luminosity of the system. The spectral types of the WR21a
components indicate that the stars are very young (12 Myr), similar to the
age of the nearby Westerlund 2 cluster. We use evolutionary tracks to determine
the mass-luminosity relation for the total system mass. We find that for a
distance of 8 kpc and an age of 1.5 Myr, the derived absolute masses are in
good agreement with those from evolutionary predictions.Comment: 7 pages, 4 figures; accepted for publication in MNRA
Development and Evaluation of an Adaptive Digital Module on Enzyme Kinetics
An adaptive module on basic enzyme kinetics was developed for first- and second-year university students. The module offers more assignments to students who have less knowledge of the theory than to more advanced students. The aim of the research was to investigate what influence students’ backgrounds have on their use and appreciation of this module. Both freshmen and second-year students showed a large variation in the number of assignments they needed to perform in order to finish the module, indicating that the module’s adaptive feature was exploited by all the students. Findings indicated that the prior knowledge was of influence of students’ motivation and perception of difficulty of the modul
RCW36: characterizing the outcome of massive star formation
Massive stars play a dominant role in the process of clustered star
formation, with their feedback into the molecular cloud through ionizing
radiation, stellar winds and outflows. The formation process of massive stars
is poorly constrained because of their scarcity, the short formation timescale
and obscuration. By obtaining a census of the newly formed stellar population,
the star formation history of the young cluster and the role of the massive
stars within it can be unraveled. We aim to reconstruct the formation history
of the young stellar population of the massive star-forming region RCW 36. We
study several dozens of individual objects, both photometrically and
spectroscopically, look for signs of multiple generations of young stars and
investigate the role of the massive stars in this process. We obtain a census
of the physical parameters and evolutionary status of the young stellar
population. Using a combination of near-infrared photometry and spectroscopy we
estimate ages and masses of individual objects. We identify the population of
embedded young stellar objects (YSO) by their infrared colors and emission line
spectra. RCW 36 harbors a stellar population of massive and intermediate-mass
stars located around the center of the cluster. Class 0/I and II sources are
found throughout the cluster. The central population has a median age of 1.1
+/- 0.6 Myr. Of the stars which could be classified, the most massive ones are
situated in the center of the cluster. The central cluster is surrounded by
filamentary cloud structures; within these, some embedded and accreting YSOs
are found. Our age determination is consistent with the filamentary structures
having been shaped by the ionizing radiation and stellar winds of the central
massive stars. The formation of a new generation of stars is ongoing, as
demonstrated by the presence of embedded protostellar clumps, and two exposed
jets.Comment: 18 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
The properties of single WO stars
The enigmatic oxygen sequence Wolf-Rayet (WO) stars represent a very late
stage in massive star evolution, although their exact nature is still under
debate. The spectra of most of the WO stars have never been analysed through
detailed modelling with a non-local thermodynamic equilibrium expanding
atmosphere code. Here we present preliminary results of the first homogeneous
analysis of the (apparently) single WOs.Comment: 2 pages, 1 figure, Proceedings IAU Symposium No. 307, 2014, 'New
windows on massive stars: asteroseismology, interferometry, and
spectropolarimetry
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
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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