Ondřejov, Czech Republic
Ondřejov, Czech Republic

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Vennes S.,Astronomickyustav | Kawka A.,Astronomickyustav
Astrophysical Journal Letters | Year: 2012

We report the identification of the double-degenerate system NLTT16249 that comprises a normal, hydrogen-rich (DA) white dwarf and a peculiar, carbon-polluted white dwarf (DQ) showing photospheric traces of nitrogen. We disentangled the observed spectra and constrained the properties of both stellar components. In the evolutionary scenario commonly applied to the sequence of DQ white dwarfs, both carbon and nitrogen would be dredged up from the core. The C/N abundance ratio (50) in the atmosphere of this unique DQ white dwarf suggests the presence of unprocessed material (14N) in the core or in the envelope. Helium burning in the DQ progenitor may have terminated early on the red giant branch after a mass-ejection event leaving unprocessed material in the core, although current mass estimates do not favor the presence of a low-mass helium core. Alternatively, some nitrogen in the envelope may have survived an abridged helium-core burning phase prior to climbing the asymptotic giant branch. Based on available data, we estimate a relatively short orbital period (P ≲ 13hr) and ongoing spectroscopic observations will help determine precise orbital parameters. © 2012. The American Astronomical Society. All rights reserved.

Kawka A.,Astronomickyustav | Vennes S.,Astronomickyustav | Nemeth P.,Florida Institute of Technology | Kraus M.,Astronomickyustav | Kubat J.,Astronomickyustav
Monthly Notices of the Royal Astronomical Society | Year: 2010

We report the discovery of two new hot, hydrogen-rich subdwarfs (sdB) in close binary systems. The hot subdwarfs, GALEX J0321+4727 and GALEX J2349+3844, were selected from a joint optical-ultraviolet catalogue of hot subluminous stars based on GSC2.3.2 and the Galaxy Evolution Explorer all-sky survey. Using high-dispersion spectra of the Hα core, obtained using the 2-m telescope at Ondřejov Observatory, we measured the radial velocities of the sdB primaries and determined orbital periods of 0.265 84 ± 0.000 04 d and 0.462 49 ± 0.000 07 d for GALEX J0321+4727 and GALEX J2349+3844, respectively. The time series obtained from the Northern Sky Variability Survey with an effective wavelength near the R-band show that GALEX J0321+4727 is a variable star (Δm= 0.12 mag), while no significant variations are observed in GALEX J2349+3844. The period of variations in GALEX J0321+4727 coincides with the orbital period and the variability is probably caused by a reflection effect on a late-type secondary star. Lack of photometric variations in GALEX J2349+3844 probably indicates that the companion is a white dwarf star. Using all available photometry and spectroscopy, we measured the atmospheric properties of the two sdB stars and placed limits on the mass and luminosity of the companion stars. © 2010 The Authors. Journal compilation © 2010 RAS.

Petit V.,West Chester University | Owocki,University of Delaware | Wade,Royal Military College of Canada | Cohen D.H.,Swarthmore College | And 11 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

Building on results from the Magnetism in Massive Stars (MiMeS) project, this paper shows how a two-parameter classification of massive-star magnetospheres in terms of the magnetic wind confinement (which sets the Alfv́en radius RA) and stellar rotation (which sets the Kepler co-rotation radius RK) provides a useful organization of both observational signatures and theoretical predictions. We compile the first comprehensive study of inferred and observed values for relevant stellar and magnetic parameters of 64 confirmed magnetic OB stars with Teff> 16 kK. Using these parameters, we locate the stars in the magnetic confinement-rotation diagram, a log-log plot of RK versus RA. This diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with RA > RK, versus dynamical magnetospheres (DM), with RK > RA. We show how key observational diagnostics, like the presence and characteristics of Ha emission, depend on a star-s position within the diagram, as well as other parameters, especially the expected wind mass-loss rates. In particular, we identify two distinct populations of magnetic stars with Ha emission: namely, slowly rotating O-type stars with narrow emission consistent with a DM, and more rapidly rotating B-type stars with broader emission associated with a CM. For O-type stars, the high mass-loss rates are sufficient to accumulate enough material for line emission even within the relatively short free-fall time-scale associated with a DM: this high mass-loss rate also leads to a rapid magnetic spindown of the stellar rotation. For the B-type stars, the longer confinement of a CMis required to accumulate sufficient emitting material from their relatively weak winds, which also lead to much longer spindown time-scales. Finally, we discuss how other observational diagnostics, e.g. variability of UV wind lines or X-ray emission, relate to the inferred magnetic properties of these stars, and summarize prospects for future developments in our understanding of massive-star magnetospheres©2012 The Authors.

Aret A.,Astronomickyustav | Aret A.,Tartu Observatory | Kraus M.,Astronomickyustav | Muratore M.F.,National University of La Plata | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

The disc formation mechanism of B[e] supergiants is one of the puzzling phenomena in massive star evolution. Rapid stellar rotation seems to play an important role for the non-spherically symmetric mass-loss leading to a high-density disc- or ring-like structure of neutral material around these massive and luminous objects. The radial density and temperature structure as well as the kinematics within this high-density material are, however, not well studied. Based on the high-resolution optical spectra of a sample of B[e] supergiants in the Magellanic Clouds we especially searched for tracers of the kinematics within their discs. Besides the well-known [Oi] lines, we discovered the [Caii] λλ7291, 7324 lines which can be used as a complementary set of disc tracers. We find that these lines originate from very high density regions, located closer to the star than the [Oi] λ5577 line-forming region. The line profiles of both the [Oi] and the [Caii] lines indicate that the discs or rings of high-density material are in Keplerian rotation. We estimate plausible ranges of disc inclination angles for the sample of B[e] supergiants and suggest that the star LHA120-S22 might have a spiral arm rather than a disc. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Liermann A.,University of Potsdam | Liermann A.,Max Planck Institute for Radio Astronomy | Kraus M.,Astronomickyustav | Schnurr O.,University of Sheffield | And 2 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

We report on the first detection of 13C enhancement in two B[e] supergiants (B[e]SGs) in the Large Magellanic Cloud. Stellar evolution models predict the surface abundance in 13C to strongly increase during main-sequence and post-main-sequence evolution of massive stars. However, direct identification of chemically processed material on the surface of B[e]SGs is hampered by their dense, disc-forming winds, hiding the stars. Recent theoretical computations predict the detectability of enhanced 13C via the molecular emission in 13CO arising in the circumstellar discs of B[e]SGs. To test this potential method and to unambiguously identify a post-main-sequence B[e] SG by its 13CO emission, we have obtained high-quality K-band spectra of two known B[e] SGs in the Large Magellanic Cloud, using the Very Large Telescope's Spectrograph for INtegral Field Observation in the Near-Infrared (VLT/SINFONI). Both stars clearly show the 13CO band emission, whose strength implies a strong enhancement of 13C, in agreement with theoretical predictions. This first ever direct confirmation of the evolved nature of B[e]SGs thus paves the way to the first identification of a Galactic B[e]SG. © 2010 The Authors. Journal compilation © 2010 RAS.

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