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Hubrig S.,Leibniz Institute for Astrophysics Potsdam | Scholler M.,European Southern Observatory | Kholtygin A.F.,Saint Petersburg State University | Gonzalez J.F.,Institute Ciencias Astronomicas | And 2 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Recent developments in observational techniques and theories revealed the potential significance of magnetic fields for stellar structure, evolution, and circumstellar environment. At present, the distribution of magnetic field strengths in massive stars from the zero-age main sequence to more evolved stages, which would shed light on the origin of the magnetic field, has not been studied. Aims. We searched for the presence of a magnetic field in the visually brightest early A-type supergiant HD 92207. Methods. Observations were obtained using the low-resolution spectropolarimetric mode of FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8 m Antu telescope of the VLT. For the mean longitudinal magnetic field measurements, we applied a linear regression analysis in two ways: using only the absorption hydrogen Balmer lines or using the entire spectrum including all available absorption lines. Results. A mean longitudinal magnetic field at a significance level of more than 3σ was detected in two out of three observations distributed over about one year. It is one of the rare cases where a field of about a few hundred Gauss is detected in an early A-type supergiant. All line profiles in the spectra of HD 92207 undergo distinct variations in radial velocities and intensities, probably caused by previously detected non-radial pulsations. © ESO, 2012. Source

Hubrig S.,Leibniz Institute for Astrophysics Potsdam | Fossati L.,University of Bonn | Carroll T.A.,Leibniz Institute for Astrophysics Potsdam | Castro N.,University of Bonn | And 11 more authors.
Astronomy and Astrophysics | Year: 2014

Aims. Recent magnetic field surveys in O-and B-type stars revealed that about 10% of the core-hydrogen-burning massive stars host large-scale magnetic fields. The physical origin of these fields is highly debated. To identify and model the physical processes responsible for the generation of magnetic fields in massive stars, it is important to establish whether magnetic massive stars are found in very young star-forming regions or whether they are formed in close interacting binary systems. Methods. In the framework of our ESO Large Program, we carried out low-resolution spectropolarimetric observations with FORS 2 in 2013 April of the three most massive central stars in the Trifid nebula, HD 164492A, HD 164492C, and HD 164492D. These observations indicated a strong longitudinal magnetic field of about 500-600 G in the poorly studied component HD 164492C. To confirm this detection, we used HARPS in spectropolarimetric mode on two consecutive nights in 2013 June. Results. Our HARPS observations confirmed the longitudinal magnetic field in HD 164492C. Furthermore, the HARPS observations revealed that HD 164492C cannot be considered as a single star as it possesses one or two companions. The spectral appearance indicates that the primary is most likely of spectral type B1-B1.5 V. Since in both observing nights most spectral lines appear blended, it is currently unclear which components are magnetic. Long-term monitoring using high-resolution spectropolarimetry is necessary to separate the contribution of each component to the magnetic signal. Given the location of the system HD 164492C in one of the youngest star formation regions, this system can be considered as a Rosetta Stone for our understanding of the origin of magnetic fields in massive stars. © 2014 ESO. Source

Jamet L.,Institute Astrofisica Of Andalucia Csic | Jamet L.,National Autonomous University of Mexico | Cervino M.,Institute Astrofisica Of Andalucia Csic | Luridiana V.,Institute Astrofisica Of Andalucia Csic | And 3 more authors.
Astronomy and Astrophysics | Year: 2010

Aims: We analyzed the resolved stellar population of the C component of the extremely metal-poor dwarf galaxy I Zw18 in order to evaluate its distance and star formation history as accurately as possible. In particular, we aimed at answering the question of whether this stellar population is young. Methods: We developed a probabilistic approach to analyzing high-quality photometric data obtained with the Advanced Camera for Surveys of the Hubble Space Telescope. This approach gives a detailed account of the various stochastic aspects of star formation. We carried out two successive models of the stellar population of interest, paying attention to how our assumptions could affect the results. Results: We found a distance to the C component of I Zw 18 as high as 27Mpc, a significantly higher value than those cited in previous works. The star formation history we inferred from the observational data shows various interesting features: A strong starburst that lasted for about 15 Myr, a more moderate one that occurred ≈100 Myr ago, a continuous process of star formation between both starbursts, and a possible episode of low level star formation at ages over 100 Myr. The stellar population studied is likely ≈125 Myr old, although ages of a few Gyr cannot be ruled out. Furthermore, nearly all the stars were formed in the last few hundreds of Myr. © 2010 ESO. Source

Glagolevskij Y.V.,Special Astrophysical Observatory | Chuntonov G.A.,Special Astrophysical Observatory | Shavrina A.V.,Main Astronomical Observatory
Astrophysics | Year: 2012

The temperature distribution with depth in the atmosphere of the magnetic star HD21699 is modelled for each observed rotational phase using Hδ line profiles obtained with a signal to noise ratio S/N = 1000. The observed temperature distributions were averaged within and outside the region of the magnetic spots. It appears that these average temperatures are equal to within the limits of error. This shows that the magnetic field does not affect the physical condition of the matter in the star and does not disturb its thermodynamic state. © 2012 Springer Science+Business Media, Inc. Source

Parnovsky S.L.,Taras Shevchenko National University | Izotova I.Y.,Taras Shevchenko National University | Izotov Y.I.,Main Astronomical Observatory
Astrophysics and Space Science | Year: 2013

We present the results of a statistical study of the star formation rates (SFR) derived from the Galaxy Evolution Explorer (GALEX) observations in the ultraviolet continuum and in the Hα emission line for a sample of about 800 luminous compact galaxies (LCGs). Galaxies in this sample have a compact structure and include one or several regions of active star formation. Global galaxy characteristics (metallicity, luminosity, stellar mass) are intermediate between ones of the nearby blue compact dwarf (BCD) galaxies and Lyman-break galaxies (LBGs) at high redshifts z>2-3. SFRs were corrected for interstellar extinction which was derived from the optical Sloan Digital Sky Survey (SDSS) spectra. We find that SFRs derived from the galaxy luminosities in the far ultraviolet (FUV) and near ultraviolet (NUV) ranges vary in a wide range from 0. 18 M⊙ yr-1 to 113 M⊙ yr-1 with median values of 3. 8 M⊙ yr-1 and 5. 2 M⊙ yr-1, respectively. Simple regression relations are found for luminosities L(Hα) and L(UV) as functions of the mass of the young stellar population, the starburst age, and the galaxy metallicity. We consider the evolution of L(Hα), L(FUV) and L(NUV) with a starburst age and introduce new characteristics of star formation, namely the initial Hα, FUV and NUV luminosities at zero starburst age. © 2012 Springer Science+Business Media Dordrecht. Source

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