372 research outputs found
Identification of red supergiants in nearby galaxies with mid-IR photometry
The role of episodic mass loss in massive star evolution is one of the most
important open questions of current stellar evolution theory. Episodic mass
loss produces dust and therefore causes evolved massive stars to be very
luminous in the mid-infrared and dim at optical wavelengths. We aim to increase
the number of investigated luminous mid-IR sources to shed light on the late
stages of these objects. To achieve this we employed mid-IR selection criteria
to identity dusty evolved massive stars in two nearby galaxies. The method is
based on mid-IR colors, using 3.6 {\mu}m and 4.5 {\mu}m photometry from
archival Spitzer Space Telescope images of nearby galaxies and J-band
photometry from 2MASS. We applied our criteria to two nearby star-forming dwarf
irregular galaxies, Sextans A and IC 1613, selecting eight targets, which we
followed up with spectroscopy. Our spectral classification and analysis yielded
the discovery of two M-type supergiants in IC 1613, three K-type supergiants
and one candidate F-type giant in Sextans A, and two foreground M giants. We
show that the proposed criteria provide an independent way for identifying
dusty evolved massive stars, that can be extended to all nearby galaxies with
available Spitzer/IRAC images at 3.6 {\mu}m and 4.5 {\mu}m.Comment: 8 pages, 4 figures, A&A in pres
Accurate fundamental parameters and distance to a massive early-type eclipsing binary in the Danks 2 cluster
We present a study of the properties of the O-type, massive eclipsing binary
2MASS J13130841-6239275 located in the outskirts of the Danks 2 cluster in the
G305 star-forming complex, using near-infrared spectroscopy from VLT/ISAAC. We
derive the masses and radii to be 24.50.9 M and 9.20.1
R for the primary and 21.70.8 M and 8.70.1
R for the secondary component. In addition, we evaluate the
sensitivity of our parameters to the choice of the spectral features used to
determine the radial velocities. Both components appear to be main-sequence
O6.5O7 type stars at an age of 5 Myr, which is in agreement with the
age of the cluster. A high visual extinction of A=11.90.1 mag is
reported, which is likely attributed to the cold molecular gas contaminating
the north-east region of the cluster. By fitting the spectral energy
distribution of the system to the available photometry, we
determine a distance to the system of 3.520.08 kpc with a precision of
2, which is the most well-determined distance to the Danks 2 cluster and
the host complex reported in the literature.Comment: 13 pages, 9 figures, 6 tables. Accepted for publication in A&
Ionic conductivity and thermal stability of magnetron-sputtered nanocrystalline yttria-stabilized zirconia
Thermally stable, stoichiometric, cubic yttria-stabilized zirconia (YSZ) thin-film electrolytes have been synthesized by reactive pulsed dc magnetron sputtering from a Zr-Y (80/20 at. %) alloy target. Films deposited at floating potential had a ‹111› texture. Single-line profile analysis of the 111 x-ray diffraction peak yielded a grain size of ~20 nm and a microstrain of ~2% regardless of deposition temperature. Films deposited at 400 °C and selected bias voltages in the range from -70 to -200 V showed a reduced grain size for higher bias voltages, yielding a grain size of ~6 nm and a microstrain of ~2.5% at bias voltages of -175 and -200 V with additional incorporation of argon. The films were thermally stable; very limited grain coarsening was observed up to an annealing temperature of 800 °C. Temperature-dependent impedance spectroscopy analysis of the YSZ films with Ag electrodes showed that the in-plane ionic conductivity was within one order of magnitude higher in films deposited with substrate bias corresponding to a decrease in grain size compared to films deposited at floating potential. This suggests that there is a significant contribution to the ionic conductivity from grain boundaries. The activation energy for oxygen ion migration was determined to be between 1.14 and 1.30 eV. Original Publication:M. Sillassen, Per Eklund, M. Sridharan, N. Pryds, N. Bonanos and J. Bottiger, Ionic conductivity and thermal stability of magnetron-sputtered nanocrystalline yttria-stabilized zirconia, 2009, Journal of Applied Physics, (105), 10, 104907.http://dx.doi.org/10.1063/1.3130404Copyright: American Institute of Physicshttp://www.aip.org
The most general axially symmetric electrovac spacetime adimitting separable equations of motion
We obtain the most general solution of the Einstein electro - vacuum equation
for the stationary axially symmetric spacetime in which the Hamilton-Jacobi and
Klein - Gordon equations are separable. The most remarkable feature of the
solution is its invariance under the duality transformation involving mass and
NUT parameter, and the radial and angle coordinates. It is the general solution
for a rotating (gravitational dyon) particle which is endowed with both
gravoelectric and gravomagnetic charges, and there exists a duality
transformation from one to the other. It also happens to be a transform of the
Kerr - NUT solution. Like the Kerr family, it is also possible to make this
solution radiating which asymptotically conforms to the Vaidya null radiation.Comment: 9 pages, RevTex, Accepted by Class. Quantum Grav. Title, Abstract and
some expressions have been modified, typos corrected. The solution and main
result remain unaltere
The VLT-FLAMES Tarantula Survey XXIII. Two massive double-lined binaries in 30 Doradus
Aims. We investigate the characteristics of two newly discovered short-period, double-lined, massive binary systems in the Large
Magellanic Cloud, VFTS 450 (O9.7 II–Ib + O7::) and VFTS 652 (B1 Ib + O9: III:).
Methods. We perform model-atmosphere analyses to characterise the photospheric properties of both members of each binary (denoting the “primary” as the spectroscopically more conspicuous component). Radial velocities and optical photometry are used to estimate the binary-system parameters.
Results. We estimate Teff = 27 kK, log g = 2.9 (cgs) for the VFTS 450 primary spectrum (34 kK, 3.6: for the secondary spectrum); and Teff = 22 kK, log g = 2.8 for the VFTS 652 primary spectrum (35 kK, 3.7: for the secondary spectrum). Both primaries show surface nitrogen enrichments (of more than 1 dex for VFTS 652), and probable moderate oxygen depletions relative to reference LMC abundances. We determine orbital periods of 6.89 d and 8.59 d for VFTS 450 and VFTS 652, respectively, and argue that the primaries must be close to filling their Roche lobes. Supposing this to be the case, we estimate component masses in the range ∼20–50 M⊙.
Conclusions. The secondary spectra are associated with the more massive components, suggesting that both systems are high-mass analogues of classical Algol systems, undergoing case-A mass transfer. Difficulties in reconciling the spectroscopic analyses with the light-curves and with evolutionary considerations suggest that the secondary spectra are contaminated by (or arise in) accretion disks
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
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