Publish in OALib Journal

ISSN: 2333-9721

APC: Only $99


Any time

2019 ( 61 )

2018 ( 327 )

2017 ( 302 )

2016 ( 413 )

Custom range...

Search Results: 1 - 10 of 172080 matches for " Mary E. Oksala "
All listed articles are free for downloading (OA Articles)
Page 1 /172080
Display every page Item
Detection of a 1.59 h period in the B supergiant star HD 202850
Michaela Kraus,Sanja Tomic,Mary E. Oksala,Majda Smole
Physics , 2012, DOI: 10.1051/0004-6361/201219319
Abstract: Photospheric lines of B-type supergiants show variability in their profile shapes. In addition, their widths are much wider than can be accounted for purely by stellar rotation. This excess broadening is often referred to as macroturbulence. Both effects have been linked to stellar oscillations, but B supergiants have not been systematically searched yet for the presence of especially short-term variability caused by stellar pulsations. We have started an observational campaign to investigate the line profile variability of photospheric lines in a sample of Galactic B supergiants. These observations aim to improve our understanding of the physical effects acting in the atmospheres of evolved massive stars. We obtained four time-series of high-quality optical spectra for the Galactic B supergiant HD 202850. The spectral coverage of about 500 A around Halpha encompasses the Si II (6347 A, 6371 A), and the He I (6678 A) photospheric lines. The line profiles were analysed by means of the moment method. The time-series of the photospheric Si II and He I lines display a simultaneous, periodic variability in their profile shapes. Proper analysis revealed a period of 1.59 h in all three lines. This period is found to be stable with time over the observed span of 19 months. This period is much shorter than the rotation period of the star and might be ascribed to stellar oscillations. Since the star seems to fall outside the currently known pulsational instability domains, the nature of the discovered oscillation remains unclear.
Discovery of two new bright magnetic B stars: i Car and Atlas
Coralie Neiner,Bram Buysschaert,Mary E. Oksala,Aurore Blazere
Physics , 2015, DOI: 10.1093/mnrasl/slv128
Abstract: The BRITE (BRIght Target Explorer) constellation of nano-satellites performs seismology of bright stars via high precision photometry. In this context, we initiated a high resolution, high signal-to-noise, high sensitivity, spectropolarimetric survey of all stars brighter than V=4. The goal of this survey is to detect new bright magnetic stars and provide prime targets for both detailed magnetic studies and asteroseismology with BRITE. Circularly polarised spectra were acquired with Narval at TBL (France) and HarpsPol at ESO in La Silla (Chile). We discovered two new magnetic B stars: the B3V star i Car and the B8V component of the binary star Atlas. Each star was observed twice to confirm the magnetic detections and check for variability. These bright magnetic B stars are prime targets for asteroseismology and for flux-demanding techniques, such as interferometry.
Discovery of the first B[e] supergiants in M 31
Michaela Kraus,Lydia S. Cidale,Maria Laura Arias,Mary E. Oksala,Marcelo Borges Fernandes
Physics , 2013, DOI: 10.1088/2041-8205/780/1/L10
Abstract: B[e] supergiants (B[e]SGs) are transitional objects in the post-main sequence evolution of massive stars. The small number of B[e]SGs known so far in the Galaxy and the Magellanic Clouds indicates that this evolutionary phase is short. Nevertheless, the strong aspherical mass loss occurring during this phase, which leads to the formation of rings or disk-like structures, and the similarity to possible progenitors of SN1987A emphasize the importance of B[e]SGs for the dynamics of the interstellar medium as well as stellar and galactic chemical evolution. The number of objects and their mass loss behavior at different metallicities are essential ingredients for accurate predictions from stellar and galactic evolution calculations. However, B[e]SGs are not easily identified, as they share many characteristics with luminous blue variables (LBVs) in their quiescent (hot) phase. We present medium-resolution near-infrared K-band spectra for four stars in M 31, which have been assigned a hot LBV (candidate) status. Applying diagnostics that were recently developed to distinguish B[e]SGs from hot LBVs, we classify two of the objects as bonafide LBVs; one of them currently in outburst. In addition, we firmly classify the two stars 2MASS J00441709+4119273 and 2MASS J00452257+4150346 as the first B[e]SGs in M 31 based on strong CO band emission detected in their spectra, and infrared colors typical for this class of stars.
The galactic unclassified B[e] star HD 50138 III. The short-term line profile variability of its photospheric lines
Marcelo Borges Fernandes,Michaela Kraus,Dieter H. Nickeler,Peter De Cat,Patricia Lampens,Claudio Bastos Pereira,Mary E. Oksala
Physics , 2012, DOI: 10.1051/0004-6361/201220166
Abstract: HD 50138 presents the B[e] phenomenon, but its nature is not clear yet. This star is known to present spectral variations, which have been associated with outbursts and shell phases. We analyze the line profile variability of HD 50138 and its possible origin, which provide possible hints to its evolutionary stage, so far said to be close to the end of (or slightly beyond) the main sequence. New high-resolution spectra of HD 50138 obtained with the HERMES spectrograph over several nights (five of them consecutively) were analyzed, allowing us to confirm short-term line profile variability. Our new data show short-term variations in the photospheric lines. On the other hand, purely circumstellar lines (such as [O I] lines) do not show such rapid variability. The rotational velocity of HD 50138, V_rot = 90.3 +- 4.3 km/s, and the rotation period, P = 3.64 +- 1.16 d, were derived from the He II 4026A photospheric line. Based on the moment method, we confirm that the origin of this short-term line profile variability is not stellar spots, and it may be caused by pulsations. In addition, we show that macroturbulence may affect the profiles of photospheric lines, as is seen for B supergiants. The location of HD 50138 at the end of (or slightly beyond) the main sequence, the newly detected presence of line profile variability resembling pulsating stars, and macroturbulence make this star a fascinating object.
Pulsations in the Late-type B Supergiant Star HD 202850
Sanja Tomic,Michaela Kraus,Mary Oksala
Physics , 2013, DOI: 10.1017/S1743921313015263
Abstract: HD 202850 is a late B-type supergiant. It is known that photospheric lines of such stars vary. Due to macroturbulence the lines are much wider than expected. Macroturbulence has been linked to stellar pulsations. It has been reported that there are several B supergiants that undergo pulsations. In our previous work, we detected a pulsational period of 1.59 hours in this object from data taken with the Ondrejov 2-m telescope. We continued to investigate this object and we took several time series with the DAO 1.2-m telescope. Our new data suggest that there may be some additional pulsational periods in this star. We present our new results in this poster.
Magnetospheres of massive stars across the EM spectrum
V. Petit,S. P. Owocki,M. E. Oksala,the MiMeS Collaboration
Physics , 2011,
Abstract: Magnetic massive stars -- which are being discovered with increasing frequency -- represent a new category of wind-shaping mechanism for O and B stars. Magnetic channeling of these stars' radiation-driven winds, the Magnetically Confined Wind Shock paradigm, leads to the formation of a shock-heated magnetosphere, which can radiate X-rays, modify UV resonance lines, and create disks of Halpha emitting material. The dynamical properties of these magnetospheres are well understood from a theoretical point of view as an interplay between the magnetic wind confinement and rotation. However, the manifestations of magnetospheres across the spectrum may be more complex and various than first anticipated. On the other hand, recent advances in modeling these magnetospheres provide a key to better understand massive star winds in general. We will summarize the coordinated observational, theoretical, and modeling efforts from the Magnetism in Massive Star Project, addressing key outstanding questions regarding magnetosphere manifestations across the spectral domain.
Discovery of SiO band emission from Galactic B[e] supergiants
Michaela Kraus,Mary Oksala,Lydia Cidale,Maria Laura Arias,Andrea Torres,Marcelo Borges Fernandes
Physics , 2015, DOI: 10.1088/2041-8205/800/2/L20
Abstract: B[e] supergiants (B[e]SGs) are evolved massive stars in a short-lived transition phase. During this phase, these objects eject large amounts of material, which accumulates in a circumstellar disk-like structure. The expelled material is typically dense and cool, providing the cradle for molecule and dust condensation and for a rich, ongoing chemistry. Very little is known about the chemical composition of these disks, beyond the emission from dust and CO revolving around the star on Keplerian orbits. As massive stars preserve an oxygen-rich surface composition throughout their life, other oxygen-based molecules can be expected to form. As SiO is the second most stable oxygen compound, we initiated an observing campaign to search for first-overtone SiO emission bands. We obtained high-resolution near-infrared L-band spectra for a sample of Galactic B[e]SGs with reported CO band emission. We clearly detect emission from the SiO first-overtone bands in CPD-52 9243 and indications for faint emission in HD 62623, HD 327083, and CPD-57 2874. From model fits, we find that in all these stars the SiO bands are rotationally broadened with a velocity lower than observed in the CO band forming regions, suggesting that SiO forms at larger distances from the star. Hence, searching for and analyzing these bands is crucial for studying the structure and kinematics of circumstellar disks, because they trace complementary regions to the CO band formation zone. Moreover, since SiO molecules are the building blocks for silicate dust, their study might provide insight in the early stage of dust formation.
Molecular emission from GG Car's circumbinary disk
Michaela Kraus,Mary Oksala,Dieter Nickeler,Florencia Muratore,Marcelo Borges Fernandes,Anna Aret,Lydia Cidale,Willem-Jan de Wit
Physics , 2012, DOI: 10.1051/0004-6361/201220442
Abstract: The appearance of the B[e] phenomenon in evolved massive stars such as B[e] supergiants is still a mystery. While these stars are generally found to have disks that are cool and dense enough for efficient molecule and dust condensation, the origin of the disk material is still unclear. We aim at studying the kinematics and origin of the disk in the eccentric binary system GG Car, whose primary component is proposed to be a B[e] supergiant. Based on medium- and high-resolution near-infrared spectra we analyzed the CO-band emission detected from GG Car. The complete CO-band structure delivers information on the density and temperature of the emitting region, and the detectable 13CO bands allow us to constrain the evolutionary phase. In addition, the kinematics of the CO gas can be extracted from the shape of the first 12CO band head. We find that the CO gas is located in a ring surrounding the eccentric binary system, and its kinematics agrees with Keplerian rotation with a velocity, projected to the line of sight, of (80\pm 1) km/s. The CO ring has a column density of (5\pm 3)x10^21 cm^-2 and a temperature of 3200\pm 500 K. In addition, the material is chemically enriched in 13CO, which agrees with the primary component being slightly evolved off the main sequence. We discuss two possible scenarios for the origin of the circumbinary disk: (i) non-conservative Roche lobe overflow, and (ii) the possibility that the progenitor of the primary component could have been a classical Be star. Neither can be firmly excluded, but for Roche lobe overflow to occur, a combination of stellar and orbital parameter extremawould be required.
The Magnetism in Massive Stars project: first HARPSpol discoveries
E. Alecian,R. Peralta,M. E. Oksala,C. Neiner,the MiMeS collaboration
Physics , 2012,
Abstract: In the framework of the Magnetism in Massive Stars (MiMeS) project, a HARPSpol Large Program at the 3.6m-ESO telescope has recently started to collect high-resolution spectropolarimetric data of a large number of Southern massive OB stars in the field of the Galaxy and in many young clusters and associations. In this contribution, we present details of the HARPSpol survey, the first HARPSpol discoveries of magnetic fields in massive stars, and the magnetic properties of two previously known magnetic stars.
Modeling the Magnetosphere of the B2Vp star sigma Ori E
M. E. Oksala,G. A. Wade,R. H. D Townsend,O. Kochukhov,S. Owocki
Physics , 2010, DOI: 10.1017/S1743921311010143
Abstract: This paper presents results obtained from Stokes I and V spectra of the B2Vp star sigma Ori E, observed by both the Narval and ESPaDOnS spectropolarimeters. Using Least- Squares Deconvolution, we investigate the longitudinal magnetic field at the current epoch, including period analysis exploiting current and historical data. sigma Ori E is the prototypical helium-strong star that has been shown to harbor a strong magnetic field, as well as a magnetosphere, consisting of two clouds of plasma forced by magnetic and centrifugal forces to co-rotate with the star on its 1.19 day period. The Rigidly Rotating Magnetosphere (RRM) model of Townsend & Owocki (2005) approximately reproduces the observed variations in longitudinal field strength, photometric brightness, Halpha emission, and various other observables. There are, however, small discrepancies between the observations and model in the photometric light curve, which we propose arise from inhomogeneous chemical abundances on the star's surface. Using Magnetic Doppler Imaging (MDI), future work will attempt to identify the contributions to the photometric variation due to abundance spots and due to circumstellar material.
Page 1 /172080
Display every page Item

Copyright © 2008-2017 Open Access Library. All rights reserved.