1,197 research outputs found
A census of massive stars in NGC 346. Stellar parameters and rotational velocities
Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small
Magellanic Cloud has been combined with that for 116 targets from the
VLT-FLAMES Survey of Massive Stars. Spectral classification yields a sample of
47 O-type and 287 B-type spectra, while radial-velocity variations and/or
spectral multiplicity have been used to identify 45 candidate single-lined
systems, 17 double-lined systems, and one triple-lined system. Atmospheric
parameters (T and log) and projected rotational velocities
(sin) have been estimated using TLUSTY model atmospheres; independent
estimates of sin were also obtained using a Fourier Transform method.
Luminosities have been inferred from stellar apparent magnitudes and used in
conjunction with the T and sin estimates to constrain stellar
masses and ages using the BONNSAI package. We find that targets towards the
inner region of NGC 346 have higher median masses and projected rotational
velocities, together with smaller median ages than the rest of the sample.
There appears to be a population of very young targets with ages of less than 2
Myr, which have presumably all formed within the cluster. The more massive
targets are found to have lower sin consistent with previous studies.
No significant evidence is found for differences with metallicity in the
stellar rotational velocities of early-type stars, although the targets in the
SMC may rotate faster than those in young Galactic clusters. The rotational
velocity distribution for single non-supergiant B-type stars is inferred and
implies that a significant number have low rotational velocity (10\%
with <40 km/s), together with a peak in the probability distribution at
300 km/s. Larger projected rotational velocity estimates have been
found for our Be-type sample and imply that most have rotational velocities
between 200-450 km/s.Comment: Accepted by A&
Chemical abundances and winds of massive stars in M31: a B-type supergiant and a WC star in OB10
We present high quality spectroscopic data for two massive stars in the OB10
association of M31, OB10-64 (B0Ia) and OB10-WR1 (WC6). Medium resolution
spectra of both stars were obtained using the ISIS spectrograph on the William
Hershel Telescope. This is supplemented with HST-STIS UV spectroscopy and KeckI
HIRES data for OB10-64. A non-LTE model atmosphere and abundance analysis for
OB10-64 is presented indicating that this star has similar photospheric CNO, Mg
and Si abundances as solar neighbourhood massive stars. A wind analysis of this
early B-type supergiant reveals a mass-loss rate of M_dot=1.6x10^-6
M_solar/yr,and v_infty=1650 km/s. The corresponding wind momentum is in good
agreement with the wind momentum -- luminosity relationship found for Galactic
early B supergiants. Observations of OB10W-R1 are analysed using a non-LTE,
line-blanketed code, to reveal approximate stellar parameters of log L/L_solar
\~ 5.7, T~75 kK, v_infty ~ 3000 km/s, M_dot ~ 10^-4.3 M_solar/yr, adopting a
clumped wind with a filling factor of 10%. Quantitative comparisons are made
with the Galactic WC6 star HD92809 (WR23) revealing that OB10-WR1 is 0.4 dex
more luminous, though it has a much lower C/He ratio (~0.1 versus 0.3 for
HD92809). Our study represents the first detailed, chemical model atmosphere
analysis for either a B-type supergiant or a WR star in Andromeda, and shows
the potential of how such studies can provide new information on the chemical
evolution of galaxies and the evolution of massive stars in the local Universe.Comment: 17 pages, 14 figures, MNRAS accepted version, some minor revision
Infrared properties of Active OB stars in the Magellanic Clouds from the Spitzer SAGE Survey
We present a study of the infrared properties of 4922 spectroscopically
confirmed massive stars in the Large and Small Magellanic Clouds, focusing on
the active OB star population. Besides OB stars, our sample includes yellow and
red supergiants, Wolf-Rayet stars, Luminous Blue Variables (LBVs) and
supergiant B[e] stars. We detect a distinct Be star sequence, displaced to the
red, and find a higher fraction of Oe and Be stars among O and early-B stars in
the SMC, respectively, when compared to the LMC, and that the SMC Be stars
occur at higher luminosities. We also find photometric variability among the
active OB population and evidence for transitions of Be stars to B stars and
vice versa. We furthermore confirm the presence of dust around all the
supergiant B[e] stars in our sample, finding the shape of their spectral energy
distributions (SEDs) to be very similar, in contrast to the variety of SED
shapes among the spectrally variable LBVs.Comment: 5 pages, 1 figure, to appear in the proceedings of the IAUS 272 on
"Active OB stars: structure, evolution, mass loss and critical limits"
(Paris, July 19-23, 2010), Cambridge University Press. Editors C. Neiner, G.
Wade, G. Meynet and G. Peter
Interpretation of UV Absorption Lines in SN1006
We present a theoretical interpretation of the broad silicon and iron UV
absorption features observed with the Hubble Space Telescope in the spectrum of
the Schweizer-Middleditch star behind the remnant of Supernova 1006. These
features are caused by supernova ejecta in SN1006. We propose that the
redshifted SiII2 1260 A feature consists of both unshocked and shocked SiII.
The sharp red edge of the line at 7070 km/s indicates the position of the
reverse shock, while its Gaussian blue edge reveals shocked Si with a mean
velocity of 5050 km/s and a dispersion of 1240 km/s, implying a reverse shock
velocity of 2860 km/s. The measured velocities satisfy the energy jump
condition for a strong shock, provided that all the shock energy goes into
ions, with little or no collisionless heating of electrons. The line profiles
of the SiIII and SiIV absorption features indicate that they arise mostly from
shocked Si. The total mass of shocked and unshocked Si inferred from the SiII,
SiIII and SiIV profiles is M_Si = 0.25 \pm 0.01 Msun on the assumption of
spherical symmetry. Unshocked Si extends upwards from 5600 km/s. Although there
appears to be some Fe mixed with the Si at lower velocities < 7070 km/s, the
absence of FeII absorption with the same profile as the shocked SiII suggests
little Fe mixed with Si at higher (before being shocked) velocities. The column
density of shocked SiII is close to that expected for SiII undergoing steady
state collisional ionization behind the reverse shock, provided that the
electron to SiII ratio is low, from which we infer that most of the shocked Si
is likely to be of a fairly high degree of purity, unmixed with other elements.
We propose that the ambient interstellar density on the far side of SN1006 is
anomalously low compared to the density around the rest of the remnant. ThisComment: 24 pages, with 8 figures included. Accepted for publication in the
Astrophysical Journa
Experimental behaviour of a steel structure under natural fire
Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.http://www.sciencedirect.com/science/article/B6V37-4KN5C4D-1/1/8f781d0c96159d54029bef7c9ec451b
Experimental behaviour of a steel structure under natural fire
Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.http://www.sciencedirect.com/science/article/B6V37-4KN5C4D-1/1/8f781d0c96159d54029bef7c9ec451b
Quantum device fine-tuning using unsupervised embedding learning
Quantum devices with a large number of gate electrodes allow for precise
control of device parameters. This capability is hard to fully exploit due to
the complex dependence of these parameters on applied gate voltages. We
experimentally demonstrate an algorithm capable of fine-tuning several device
parameters at once. The algorithm acquires a measurement and assigns it a score
using a variational auto-encoder. Gate voltage settings are set to optimise
this score in real-time in an unsupervised fashion. We report fine-tuning times
of a double quantum dot device within approximately 40 min
Synthetic spectra of H Balmer and HeI absorption lines. II: Evolutionary synthesis models for starburst and post-starburst galaxies
We present evolutionary stellar population synthesis models to predict the
spectrum of a single-metallicity stellar population, with a spectral sampling
of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which
are optimized for galaxies with active star formation, synthesize the profiles
of the hydrogen Balmer series (Hb, Hg, Hd, H8, H9, H10, H11, H12 and H13) and
the neutral helium absorption lines (HeI 4922, HeI 4471, HeI 4388, HeI 4144,
HeI 4121, HeI 4026, HeI 4009 and HeI 3819) for a burst with an age ranging from
1 to 1000 Myr, and different assumptions about the stellar initial mass
function. Continuous star formation models lasting for 1 Gyr are also
presented. The input stellar library includes NLTE absorption profiles for
stars hotter than 25000 K and LTE profiles for lower temperatures. The
temperature and gravity coverage is 4000 K <Teff< 50000 K and 0.0< log g$< 5.0,
respectively.
The models can be used to date starburst and post-starburst galaxies until 1
Gyr. They have been tested on data for clusters in the LMC, the super-star
cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical
NGC 205 and a luminous "E+A" galaxy. The full data set is available for
retrieval at http://www.iaa.es/ae/e2.html and at
http://www.stsci.edu/science/starburst/, or on request from the authors at
[email protected]: To be published in ApJS. 48 pages and 20 figure
Rotational velocities of single and binary O-type stars in the Tarantula Nebula
Rotation is a key parameter in the evolution of massive stars, affecting
their evolution, chemical yields, ionizing photon budget, and final fate. We
determined the projected rotational velocity, , of 330 O-type
objects, i.e. 210 spectroscopic single stars and 110 primaries in
binary systems, in the Tarantula nebula or 30 Doradus (30\,Dor) region. The
observations were taken using VLT/FLAMES and constitute the largest homogeneous
dataset of multi-epoch spectroscopy of O-type stars currently available. The
most distinctive feature of the distributions of the
presumed-single stars and primaries in 30 Dor is a low-velocity peak at around
100\,. Stellar winds are not expected to have spun-down the
bulk of the stars significantly since their arrival on the main sequence and
therefore the peak in the single star sample is likely to represent the outcome
of the formation process. Whereas the spin distribution of presumed-single
stars shows a well developed tail of stars rotating more rapidly than
300\,, the sample of primaries does not feature such a
high-velocity tail. The tail of the presumed-single star distribution is
attributed for the most part -- and could potentially be completely due -- to
spun-up binary products that appear as single stars or that have merged. This
would be consistent with the lack of such post-interaction products in the
binary sample, that is expected to be dominated by pre-interaction systems. The
peak in this distribution is broader and is shifted toward somewhat higher spin
rates compared to the distribution of presumed-single stars. Systems displaying
large radial velocity variations, typical for short period systems, appear
mostly responsible for these differences.Comment: 6 pages, 3 figures, Proceedings IAU Symposium No. 307, 2014, 'New
windows on massive stars: asteroseismology, interferometry, and
spectropolarimetry
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