# Anna Aret publications

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Author Title Year Journal/Proceedings
Kraus, M.; Kolka, I.; Aret, A.; Nickeler, D. H.; Maravelias, G.; Eenmäe, T.; Lobel, A.; Klochkova, V. G. A new outburst of the yellow hypergiant star ρ Cas 2019 Monthly Notices of the Royal Astronomical Society
2019, 483, 3792-3809
Abstract: Yellow hypergiants are evolved massive stars that were suggested to be in post-red supergiant stage. Post-red supergiants that evolve back to the blue, hot side of the Hertzsprung–Russell diagram can intersect a temperature domain in which their atmospheres become unstable against pulsations (the Yellow Void or Yellow Wall), and the stars can experience outbursts with short, but violent mass eruptions. The yellow hypergiant ρ Cas is famous for its historical and recent outbursts, during which the star develops a cool, optically thick wind with a very brief but high mass-loss rate, causing a sudden drop in the light curve. Here, we report on a new outburst of ρ Cas that occurred in 2013, accompanied by a temperature decrease of ∼3000 K and a brightness drop of 0.6 mag. During the outburst, TiO bands appear together with many low excitation metallic atmospheric lines characteristic for a later spectral type. With this new outburst, it appears that the time interval between individual events decreases, which might indicate that ρ Cas is preparing for a major eruption that could help the star to pass through the Yellow Void. We also analysed the emission features that appear during phases of maximum brightness and find that they vary synchronous with the emission in the prominent [Ca II] lines. We conclude that the occasionally detected emission in the spectra of ρ Cas, as well as certain asymmetries seen in the absorption lines of low to medium-excitation potential, are circumstellar in nature, and we discuss the possible origin of this material.
BibTeX:
@ARTICLE{2019MNRAS.483.3792K,
author = {{Kraus}, M. and {Kolka}, I. and {Aret}, A. and {Nickeler}, D.~H. and
{Maravelias}, G. and {Eenm{\"a}e}, T. and {Lobel}, A. and
{Klochkova}, V.~G.},
title = "{A new outburst of the yellow hypergiant star {\ensuremath{\rho}} Cas}",
journal = {Monthly Notices of the Royal Astronomical Society},
keywords = {stars: atmospheres, stars: individual: {\ensuremath{\rho}} Cas, stars: massive, supergiants, Astrophysics - Solar and Stellar Astrophysics},
year = "2019",
month = "Mar",
volume = {483},
number = {3},
pages = {3792-3809},
doi = {10.1093/mnras/sty3375},
archivePrefix = {arXiv},
eprint = {1812.03065},
primaryClass = {astro-ph.SR},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Kolka, I., Eenmäe, T., Laur, J., & Aret, A. Optical Variability of the Be X-Ray Binary PSR J2032+4127/MT91 213 around Periastron Passage 2017 Research Notes of the American Astronomical Society
2017, 1, id.37
BibTeX:
@ARTICLE{2017RNAAS...1...37K,
author = {{Kolka}, I. and {Eenm{\"a}e}, T. and {Laur}, J. and {Aret}, A.
},
title = "{Optical Variability of the Be X-Ray Binary PSR J2032+4127/MT91 213 around Periastron Passage}",
journal = {Research Notes of the American Astronomical Society},
year = 2017,
month = dec,
volume = 1,
eid = {37},
pages = {37},
doi = {10.3847/2515-5172/aa9f17},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}
Aret, A., Kraus, M., Kolka, I. & Maravelias, G. The yellow hypergiant - B[e] supergiant connection 2017 Stars: From Collapse to Collapse
Eds. Yu. Yu. Balega, D. O. Kudryavtsev, I. I. Romanyuk, and I. A. Yakunin
ASP Conference Series
2017, 510, 162
Abstract: B[e] supergiants and yellow hypergiants share a number of common properties regarding their circumstellar environments. Using the forbidden [O I] and [Ca II] lines as disk tracers, we suggest the presence of a Keplerian disk or ring around the yellow hypergiant V509 Cas and confirm the pole-on inner disk around V1302 Aql. These findings indicate a change in mass-loss behavior from spherical in cooler yellow hypergiants to axisymmetric in the hotter ones during the passage through the Yellow Void. The accumulation of material in the equatorial plane reminds of the disks of B[e] supergiants, supporting the suggestion that yellow hypergiants might appear as B[e] supergiants after they reach the blue edge of the yellow instability domain.
BibTeX:
@INPROCEEDINGS{2017ASPC..510..162A,
author = {{Aret}, A. and {Kraus}, M. and {Kolka}, I. and {Maravelias}, G.
},
title = "{The Yellow Hypergiant - B[e] Supergiant Connection}",
booktitle = {Stars: From Collapse to Collapse},
year = 2017,
series = {Astronomical Society of the Pacific Conference Series},
volume = 510,
archivePrefix = "arXiv",
eprint = {1611.06044},
primaryClass = "astro-ph.SR",
editor = {{Balega}, Y.~Y. and {Kudryavtsev}, D.~O. and {Romanyuk}, I.~I. and
{Yakunin}, I.~A.},
month = jun,
pages = {162},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Aret, A., Kolka, I., Kraus, M. & Maravelias, G. Similarities in the structure of the circumstellar environments of B[e] supergiants and yellow hypergiants 2017 The B[e] Phenomenon: Forty Years of Studies
Eds. A. Miroshnichenko, S. Zharikov, D. Korcakova, and M. Wolf
ASP Conference Series
2017, 508, 239
Abstract: Yellow Hypergiants (YHGs) and B[e] supergiants (B[e]SGs), though in different phases in their evolution, display many features in common. This is partly due to the fact that both types of objects undergo strong, often asymmetric mass loss, and the ejected material accumulates in shells, rings, or disk-like structures, giving rise to emission from warm molecules and dust. We performed an optical spectroscopic survey of northern Galactic emission-line stars aimed at identifying tracers for the structure and kinematics of circumstellar environments. We identified two sets of lines, [O I] and [Ca II], which originate from the discs of B[e]SGs. The same set of lines is observed in V1302 Aql and V509 Cas, which are both hot YHGs. While V1302 Aql is known to have a disc-like structure, the kinematical broadening of the lines in V509 Cas suggest a Keplerian disk or ring around this star alike their hotter B[e]SG counterparts.
BibTeX:
@INPROCEEDINGS{2017ASPC..508..239A,
author = {{Aret}, A. and {Kolka}, I. and {Kraus}, M. and {Maravelias}, G.
},
title = "{Similarities in the Structure of the Circumstellar Environments of B[e] Supergiants and Yellow Hypergiants}",
booktitle = {The B[e] Phenomenon: Forty Years of Studies},
year = 2017,
series = {Astronomical Society of the Pacific Conference Series},
volume = 508,
archivePrefix = "arXiv",
eprint = {1611.04490},
primaryClass = "astro-ph.SR",
editor = {{Miroshnichenko}, A. and {Zharikov}, S. and {Kor{\v c}{\'a}kov{\'a}}, D. and
{Wolf}, M.},
month = feb,
pages = {239},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Aret, A., Kraus, M., Kolka, I. & Maravelias, G. A new outburst of the yellow hypergiant star rho Cas 2017 The B[e] Phenomenon: Forty Years of Studies
Eds. A. Miroshnichenko, S. Zharikov, D. Korcakova, and M. Wolf
ASP Conference Series
2017, 508, 357
Abstract: Spectroscopic monitoring of the yellow hypergiant rho Cas revealed a new outburst in 2013, which is obvious from the development of TiO bands in the spectra. Also many atmospheric lines characteristic for a later spectral type appear. This spectroscopic outburst is in agreement with the photometric light curve, which displays a drop by about 0.6 mag during the same period.
BibTeX:
@INPROCEEDINGS{2017ASPC..508..357A,
author = {{Aret}, A. and {Kraus}, M. and {Kolka}, I. and {Maravelias}, G.
},
title = "{A New Outburst of the Yellow Hypergiant Star {$\rho$} Cas}",
booktitle = {The B[e] Phenomenon: Forty Years of Studies},
year = 2017,
series = {Astronomical Society of the Pacific Conference Series},
volume = 508,
archivePrefix = "arXiv",
eprint = {1611.04493},
primaryClass = "astro-ph.SR",
editor = {{Miroshnichenko}, A. and {Zharikov}, S. and {Kor{\v c}{\'a}kov{\'a}}, D. and
{Wolf}, M.},
month = feb,
pages = {357},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Maravelias, G., Kraus, M., Cidale, L., Arias, M. L., Aret, A., & Borges Fernandes, M. The circumstellar environments of B[e] Supergiants 2017 The Lives and Death-Throes of Massive Stars
Eds. J. J. Eldridge, J. C. Bray, L. A. S. McClelland, and L. Xiao
IAU Symposium
2017, 329, 421
Abstract: The evolution of massive stars encompasses short-lived transition phases in which mass-loss is more enhanced and usually eruptive. A complex environment, combining atomic, molecular and dust regions, is formed around these stars. In particular, the circumstellar environment of B[e] Supergiants is not well understood. To address that, we have initiated a campaign to investigate their environments for a sample of Galactic and Magellanic Cloud sources. Using high-resolution optical and near-infrared spectra (MPG-ESO/FEROS, GEMINI/Phoenix and VLT/CRIRES, respectively), we examine a set of emission features ([OI], [CaII], CO bandheads) to trace the physical conditions and kinematics in their formation regions. We find that the B[e] Supergiants are surrounded by a series of rings of different temperatures and densities, a probable result of previous mass-loss events. In many cases the CO forms very close to the star, while we notice also an alternate mixing of densities and temperatures (which give rise to the different emission features) along the equatorial plane.
BibTeX:
@INPROCEEDINGS{2017IAUS..329..421M,
author = {{Maravelias}, G. and {Kraus}, M. and {Cidale}, L. and {Arias}, M.~L. and
{Aret}, A. and {Borges Fernandes}, M.},
title = "{The circumstellar environments of B[e] Supergiants}",
keywords = {stars: circumstellar matter, stars: emission-line, Be, stars: mass loss, line: profiles,},
booktitle = {The Lives and Death-Throes of Massive Stars},
year = 2017,
series = {IAU Symposium},
volume = 329,
editor = {{Eldridge}, J.~J. and {Bray}, J.~C. and {McClelland}, L.~A.~S. and
{Xiao}, L.},
month = nov,
pages = {421-421},
doi = {10.1017/S1743921317002605},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Maravelias, G., Kraus, M., Aret, A., Cidale, L., Arias, M. L. & Borges Fernandes, M. B[e] Supergiants' circumstellar environment: disks or rings? 2017 The B[e] Phenomenon: Forty Years of Studies
Eds. A. Miroshnichenko, S. Zharikov, D. Korcakova, and M. Wolf
ASP Conference Series
2017, 508, 213
Abstract: B[e] Supergiants are a phase in the evolution of some massive stars for which we have observational evidence but no predictions by any stellar evolution model. The mass-loss during this phase creates a complex circumstellar environment with atomic, molecular, and dust regions usually found in rings or disk-like structures. However, the detailed structure and the formation of the circumstellar environment are not well-understood, requiring further investigation. To address that we initiated an observing campaign to obtain a homogeneous set of high-resolution spectra in both the optical and NIR (using MPG-ESO/FEROS, GEMINI/Phoenix and VLT/CRIRES, respectively). We monitor a number of Galactic B[e] Supergiants, for which we examined the [OI] and [CaII] emission lines and the bandheads of the CO and SiO molecules to probe the structure and the kinematics of their formation regions. We find that the emission from each tracer forms either in a single or in multiple equatorial rings.
BibTeX:
@INPROCEEDINGS{2017ASPC..508..213M,
author = {{Maravelias}, G. and {Kraus}, M. and {Aret}, A. and {Cidale}, L. and
{Arias}, M.~L. and {Borges Fernandes}, M.},
title = "{B[e] Supergiants' Circumstellar Environment: Disks or Rings?}",
booktitle = {The B[e] Phenomenon: Forty Years of Studies},
year = 2017,
series = {Astronomical Society of the Pacific Conference Series},
volume = 508,
archivePrefix = "arXiv",
eprint = {1610.00607},
primaryClass = "astro-ph.SR",
editor = {{Miroshnichenko}, A. and {Zharikov}, S. and {Kor{\v c}{\'a}kov{\'a}}, D. and
{Wolf}, M.},
month = feb,
pages = {213},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Aret, A., Kraus, M. & v Slechta, M. Spectroscopic survey of emission-line stars - I. B[e] stars 2016 Monthly Notices of the Royal Astronomical Society
2016, 456, 1424-1437
Abstract: Emission-line stars are typically surrounded by dense circumstellar material, often in form of rings or disc-like structures. Line emission from forbidden transitions trace a diversity of density and temperature regimes. Of particular interest are the forbidden lines of [O I] λλ6300,6364 and [Ca II] λλ7291,7324. They arise in complementary, high-density environments, such as the inner-disc regions around B[e] supergiants. To study physical conditions traced by these lines and to investigate how common they are, we initiated a survey of emission-line stars. Here, we focus on a sample of nine B[e] stars in different evolutionary phases. Emission of the [O I] lines is one of the characteristics of B[e] stars. We find that four of the objects display [Ca II] line emission: for the B[e] supergiants V1478 Cyg and 3 Pup, the kinematics obtained from the [O I] and [Ca II] line profiles agrees with a Keplerian rotating disc scenario; the forbidden lines of the compact planetary nebula OY Gem display no kinematical broadening beyond spectral resolution; the luminous blue variable candidate V1429 Aql shows no [O I] lines, but the profile of its [Ca II] lines suggests that the emission originates in its hot, ionized circumbinary disc. As none of the B[e] stars of lower mass displays [Ca II ] line emission, we conclude that these lines are more likely observable in massive stars with dense discs, supporting and strengthening the suggestion that their appearance requires high-density environments.
BibTeX:
@article{2016MNRAS.456.1424A,
author = {Aret, A. and Kraus, M. and v Slechta, M.},
title = {Spectroscopic survey of emission-line stars - I. B[e] stars},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2016},
volume = {456},
pages = {1424-1437},
doi = {http://dx.doi.org/10.1093/mnras/stv2758}
}

Haucke, M., Tomić, S., Cidale, L., Kraus, M. & Aret, A. What do we know about mass ejection in B Supergiant Stars? 2016 Boletin de la Asociacion Argentina de Astronomia La Plata Argentina
2016, 58, 171-173
Abstract: In this paper we present a H spectroscopic monitoring of B supergiants (BSGs). Among the preliminary results, we present the wind properties of HD 41117 (62 Ori) and suggest the pulsation mechanism to describe its variability.
BibTeX:
@article{2016BAAA...58..171H,
author = {Haucke, M. and Tomić, S. and Cidale, L. and Kraus, M. and Aret, A.},
title = {What do we know about mass ejection in B Supergiant Stars?},
journal = {Boletin de la Asociacion Argentina de Astronomia La Plata Argentina},
year = {2016},
volume = {58},
pages = {171-173},
url = {http://www.astronomiaargentina.org.ar/b58/2016baaa...58...171H.pdf}
}

Kraus, M., Cidale, L. S., Arias, M. L., Maravelias, G., Nickeler, D. H., Torres, A. F., Borges Fernandes, M., Aret, A., Curé, M., Vallverdú, R. & Barbá, R. H. Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73 2016 Astronomy & Astrophysics
2016, 593, A112
Abstract: Context. B[e] supergiants are evolved massive stars, enshrouded in a dense wind and surrounded by a molecular and dusty disk. The mechanisms that drive phases of enhanced mass loss and mass ejections, responsible for the shaping of the circumstellar material of these objects, are still unclear.
Aims: We aim to improve our knowledge on the structure and dynamics of the circumstellar disk of the Large Magellanic Cloud B[e] supergiant LHA 120-S 73.
Methods: High-resolution optical and near-infrared spectroscopic data were obtained over a period of 16 and 7 yr, respectively. The spectra cover the diagnostic emission lines from [Ca ii] and [O i], as well as the CO bands. These features trace the disk at different distances from the star. We analyzed the kinematics of the individual emission regions by modeling their emission profiles. A low-resolution mid-infrared spectrum was obtained as well, which provides information on the composition of the dusty disk.
Results: All diagnostic emission features display double-peaked line profiles, which we interpret as due to Keplerian rotation. We find that the profile of each forbidden line contains contributions from two spatially clearly distinct rings. In total, we find that LHA 120-S 73 is surrounded by at least four individual rings of material with alternating densities (or by a disk with strongly non-monotonic radial density distribution). Moreover, we find that the molecular ring must have gaps or at least strong density inhomogeneities, or in other words, a clumpy structure. The optical spectra additionally display a broad emission feature at 6160-6180 Å, which we interpret as molecular emission from TiO. The mid-infrared spectrum displays features of oxygen- and carbon-rich grain species, which indicates a long-lived, stable dusty disk. We cannot confirm the previously reported high value for the stellar rotation velocity. He i λ 5876 is the only clearly detectable pure atmospheric absorption line in our data. Its line profile is strongly variable in both width and shape and resembles of those seen in non-radially pulsating stars. A proper determination of the real underlying stellar rotation velocity is hence not possible.
Conclusions: The existence of multiple stable and clumpy rings of alternating density recalls ring structures around planets. Although there is currently insufficient observational evidence, it is tempting to propose a scenario with one (or more) minor bodies or planets revolving around LHA 120-S 73 and stabilizing the ring system, in analogy to the shepherd moons in planetary systems.

Based on observations: (1) with the 1.52-m and 2.2-m telescopes at the European Southern Observatory (La Silla, Chile), under the programme 076.D-0609(A) and under the agreement with the Observatório Nacional-MCT (Brazil); (2) at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), under the programmes GS-2004B-Q-54, GS-2010B-Q-31, and GS-2012B-Q-90; (3) at Complejo Astronómico El Leoncito operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba and San Juan (Visiting Astronomer: A.F.T.); and (4) with the du Pont Telescope at Las Campanas Observatory, Chile, under the programme CNTAC 2008-02 (Visiting Astronomer: R.B. and A.F.T.).Presented spectroscopic data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/593/A112

BibTeX:
@article{2016A&A...593A.112K,
author = {Kraus, M. and Cidale, L.~S. and Arias, M.~L. and Maravelias, G. and Nickeler, D.~H. and Torres, A.~F. and Borges Fernandes, M. and Aret, A. and Curé, M. and Vallverdú, R. and Barbá, R.~H.},
title = {Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73},
journal = {aap},
year = {2016},
volume = {593},
pages = {A112},
doi = {http://dx.doi.org/10.1051/0004-6361/201628493}
}

Kraus, M., Haucke, M., Cidale, L. S., Venero, R. O. J., Nickeler, D. H., Németh, P., Niemczura, E., Tomić, S., Aret, A., Kubát, J., Kubátová, B., Oksala, M. E., Curé, M., Kamiński, K., Dimitrov, W., Fagas, M. & Polińska, M. Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198 478 2015 Astronomy & Astrophysics
2015, 581, A75
Abstract: Context. Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the star's evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind.
Aims: To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind.
Methods: We modeled the H, He i, Si ii, and Si iii lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations.
Results: The Hα line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7-2 on different timescales. We also observe changes in the ionization rate of Si ii and determine a multiperiodic oscillation in the He i absorption lines, with periods ranging from a few hours to 22.5 days.
Conclusions: We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of α Cyg variables.

Based on observations taken with the Perek 2m telescope at Ondřejov Observatory, Czech Republic, and the Poznan Spectroscopic Telescope 2 at the Winer Observatory in Arizona, USA.Tables 1 and 2, Figs. 3 to 7 are available in electronic form at http://www.aanda.org

BibTeX:
@article{2015A&A...581A..75K,
author = {Kraus, M. and Haucke, M. and Cidale, L.~S. and Venero, R.~O.~J. and Nickeler, D.~H. and Németh, P. and Niemczura, E. and Tomić, S. and Aret, A. and Kubát, J. and Kubátová, B. and Oksala, M.~E. and Curé, M. and Kamiński, K. and Dimitrov, W. and Fagas, M. and Polińska, M.},
title = {Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198 478},
journal = {aap},
year = {2015},
volume = {581},
pages = {A75},
doi = {http://dx.doi.org/10.1051/0004-6361/201425383}
}

Aret, A. & Kraus, M. Forbidden Calcium Lines as Disc Tracers 2015 The Physics of Evolved Stars: A Conference Dedicated to the Memory of Olivier Chesneau
Eds. E. Lagadec, F. Millour and T. Lanz
EAS Publications Series
2015, 71, 231-233
BibTeX:
@inproceedings{2015EAS....71..231A,
author = {Aret, A. and Kraus, M.},
title = {Forbidden Calcium Lines as Disc Tracers},
booktitle = {The Physics of Evolved Stars: A Conference Dedicated to the Memory of Olivier Chesneau},
Editor = {Lagadec, E. and Millour, F. and Lanz, T.},
year = {2015},
Series  = {EAS Publications Series},
volume = {71},
pages = {231-233},
doi = {http://dx.doi.org/10.1051/eas/1571052}
Archiveprefix  = {arXiv},
Eprint = {1509.00772},
}

Maravelias, G., Kraus, M. & Aret, A. Disk Tracing for B[e] Supergiants in the Magellanic Clouds 2015 The Physics of Evolved Stars: A Conference Dedicated to the Memory of Olivier Chesneau
Eds. E. Lagadec, F. Millour and T. Lanz
EAS Publications Series
2015, 71, 229-230
Abstract: B[e] supergiants are evolved massive stars with a complex circumstellar environment. A number of important emission features probe the structure and the kinematics of the circumstellar material. In our survey of Magellanic Cloud B[e] supergiants we focus on the [OI] and [CaII] emission lines, which we identified in four more objects.
BibTeX:
@inproceedings{2015EAS....71..229M,
author = {Maravelias, G. and Kraus, M. and Aret, A.},
title = {Disk Tracing for B[e] Supergiants in the Magellanic Clouds},
booktitle = {The Physics of Evolved Stars: A Conference Dedicated to the Memory of Olivier Chesneau},
Editor = {Lagadec, E. and Millour, F. and Lanz, T.},
year = {2015},
Series  = {EAS Publications Series},
volume = {71},
pages = {229-230},
doi = {http://dx.doi.org/10.1051/eas/1571051}
}

Sapar, L., Sapar, A., Poolamäe, R. & Aret, A. Diffusional separation of calcium isotopes in chemically peculiar stellar atmospheres 2014 Baltic Astronomy
2014, 23, 171-187
Abstract: Diffusional separation of calcium isotopes in the atmospheres of hot chemically peculiar stars is studied. In addition to the usual radiative acceleration effect, the light-induced drift is taken into account. We propose that microturbulence in stable stellar atmospheres is generated by the interaction between plasma particles and radiative flux. Formulae for the microturbulent velocity and microturbulence diffusion coefficient are derived. Data on isotopic and hyperfine splitting of the calcium spectral lines have been collected as an input file. The equilibrium Ca isotope concentrations are found in model computations, iteratively correcting the radiative acceleration values. The general picture of Ca isotope stratification is found to be similar to our previous results obtained for Hg isotopes: dominating overabundance of the heaviest isotope. Diffusional stratification of Ca isotope concentrations in atmospheres of late B and early A spectral types are computed and visualized in figures. The isotope abundances on the inner boundary surface were fixed to be the solar ones. The computed Ca II infrared triplet line profiles are compared with the observed line profiles in a high-dispersion spectrum of HD 175640.
BibTeX:
@article{2014BaltA..23..171S,
author = {Sapar, L. and Sapar, A. and Poolamäe, R. and Aret, A.},
title = {Diffusional separation of calcium isotopes in chemically peculiar stellar atmospheres},
journal = {Baltic Astronomy},
year = {2014},
volume = {23},
pages = {171-187},
}

Aret, A. & Kraus, M. Optical spectroscopy of stars with disks 2013 Massive Stars: From alpha to Omega
2013, id.138
Abstract: Observational data on structure and kinematics of circumstellar disks are crucial in order to place constraints on theories of formation and evolution of massive stars. While molecular and dust spectroscopy provides information about the outer disk, several strategic optical emission lines are good probes of density and temperature structure of hot gaseous regions close to the star. Forbidden emission lines are especially valuable disk tracers, because they are optically thin, and therefore their profiles reflect the kinematics within their formation region. Gas diagnostics using forbidden [O I] lines is well known. Recently we have discovered also forbidden [Ca II] lambdalambda7291, 7324 lines in spectra of B[e] supergiants, which trace even hotter regions closer to the star than [O I] lines. This discovery inspired us to start a spectroscopical survey of stars surrounded by high-density disc-like structures in order to obtain constraints on the physical conditions under which the [Ca II] lines appear. Our sample consists of B[e] stars, Yellow Hypergiants, Herbig Ae/Be stars, T-Tauri stars, and Be stars. Medium-resolution spectra of 63 stars in three different wavelength regions, i.e., around Halpha, in the region of the [Ca II] lambdalambda7291, 7324 lines, and in the region of the Ca II infrared triplet have been obtained during years 2011 and 2012 using the Coude spectrograph attached to the the 2-m Perek telescope (Ondrejov, Czech Republic). Additionally, more than 10 Be stars were inspected for presence of [Ca II] lines. We present preliminary results of our spectroscopic survey.
BibTeX:
@INPROCEEDINGS{2013msao.confE.138A,
author = {{Aret}, A. and {Kraus}, M.},
title = "{Optical spectroscopy of stars with disks}",
booktitle = {Massive Stars: From alpha to Omega},
year = 2013,
month = jun,
eid = {138},
pages = {138},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Kraus, M., Cidale, L. S., Arias, M. L., Torres, A. F., Aret, A., Fernandes, M. B., Muratore, F., Cure, M. & Oksala, M. E. Tracing the mass-loss history of B[e] supergiants 2013 Massive Stars: From alpha to Omega
2013, id.160
Abstract: The post-main sequence evolution of massive stars encompasses several phases with strong, often eruptive mass-loss events, including the puzzling B[e] supergiants (B[e]SG). Stars in this group are surrounded by disks which are cool and dense, and give rise to a complex chemistry, producing molecules and dust. The original idea was that these disks have been formed via a steady, but slow, high density equatorially confined wind. However, recent observations revealed that the circumstellar material is located in detached disks or rings, sometimes even multiple rings, favouring a scenario in which mass loss happens episodically rather than smoothly. Furthermore, time-resolved observations of these disk or ring structures implied a high variability in density and kinematics. Some Galactic B[e]SGs were recently found to be in binaries, and in a few cases, the disks are circumbinary instead of circumstellar. We have initiated an observing campaign using high-resolution optical and near-infrared spectroscopy aimed at studying the structure and kinematics of the circumstellar material of B[e]SGs. While in the optical spectral range several forbidden emission lines can be used as ideal tracers for the ionized and neutral atomic disk regions close to the star, near-infrared spectra host band emission from molecules such as CO, which are excellent indicators for the disk conditions at larger distances. Here we present first results and discuss possible mass-loss history scenarios for some of these enigmatic stars.
BibTeX:
@INPROCEEDINGS{2013msao.confE.160K,
author = {{Kraus}, M. and {Cidale}, L.~S. and {Arias}, M.~L. and {Torres}, A.~F. and
{Aret}, A. and {Fernandes}, M.~B. and {Muratore}, F. and {Cure}, M. and
{Oksala}, M.~E.},
title = "{Tracing the mass-loss history of B[e] supergiants}",
booktitle = {Massive Stars: From alpha to Omega},
year = 2013,
month = jun,
eid = {160},
pages = {160},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Kraus, M., Oksala, M. E., Nickeler, D. H., Muratore, M. F., Borges Fernandes, M., Aret, A., Cidale, L. S. & de Wit, W. J. Molecular emission from GG Carinae's circumbinary disk 2013 Astronomy & Astrophysics
2013, 549, A28
Abstract: Context. 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. Aims: 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. Methods: 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. Results: 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 ± 1 km s-1. The CO ring has a column density of (5 ± 3) × 1021 cm-2 and a temperature of 3200 ± 500 K. In addition, the material is chemically enriched in 13C, 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 extrema would be required.
Based on observations collected with the ESO VLT Paranal Observatory under programs 384.D-0613(A) and 088.D-0442(B).
BibTeX:
@article{2013A&A...549A..28K,
author = {Kraus, M. and Oksala, M.~E. and Nickeler, D.~H. and Muratore, M.~F. and Borges Fernandes, M. and Aret, A. and Cidale, L.~S. and de Wit, W.~J.},
title = {Molecular emission from GG Carinae's circumbinary disk},
journal = {Astronomy & Astrophysics},
year = {2013},
volume = {549},
pages = {A28},
doi = {http://dx.doi.org/10.1051/0004-6361/201220442}
}

Šurlan, B., Hamann, W.-R., Aret, A., Kubát, J., Oskinova, L. M. & Torres, A. F. Macroclumping as solution of the discrepancy between Halpha and P v mass loss diagnostics for O-type stars 2013 Astronomy & Astrophysics
2013, 559, A130
Abstract: Context. Recent studies of O-type stars have demonstrated that discrepant mass-loss rates are obtained when different diagnostic methods are employed. Fitting the unsaturated UV resonance lines (e.g., P v) gives drastically lower values than obtained from the Halpha emission. Wind inhomogeneity (so-called "clumping") may be the main cause of this discrepancy. Aims: In a previous paper, we presented 3D Monte-Carlo calculations for the formation of scattering lines in a clumped stellar wind. In the present paper we select five O-type supergiants (from O4 to O7) and test whether the reported discrepancies can be resolved this way. Methods: In the first step, the analyses started with simulating the observed spectra with Potsdam Wolf-Rayet (PoWR) non-LTE model atmospheres. The mass-loss rates are adjusted to fit to the observed Halpha emission lines best. For the unsaturated UV resonance lines (i.e., P v) we then applied our 3D Monte-Carlo code, which can account for wind clumps of any optical depths ("macroclumping"), a non-void interclump medium, and a velocity dispersion inside the clumps. The ionization stratifications and underlying photospheric spectra were adopted from the PoWR models. The properties of the wind clumps were constrained by fitting the observed resonance line profiles. Results: Our results show that with the mass-loss rates that fit Halpha (and other Balmer and He ii lines), the UV resonance lines (especially the unsaturated doublet of P v) can also be reproduced with no problem when macroclumping is taken into account. There is no need to artificially reduce the mass-loss rates or to assume a subsolar phosphorus abundance or an extremely high clumping factor, unlike what was claimed by other authors. These consistent mass-loss rates are lower by a factor of 1.3 to 2.6, compared to the mass-loss rate recipe from Vink et al. Conclusions: Macroclumping resolves the previously reported discrepancy between Halpha and P v mass-loss diagnostics.
Based on observations collected with the Perek 2-m Telescope of the Ondrejov Observatory, Czech Republic.Based on observations taken at the Complejo Astronómico El Leoncito (CASLEO), operated under an agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina, the Secretaría de Ciencia y Tecnología de la Nación, and the National Universities of La Plata, Córdoba, and San Juan. Appendix A is available in electronic form at http://www.aanda.org
BibTeX:
@article{2013A&A...559A.130S,
author = {v Surlan, B. and Hamann, W.-R. and Aret, A. and Kubát, J. and Oskinova, L.~M. and Torres, A.~F.},
title = {Macroclumping as solution of the discrepancy between Halpha and P v mass loss diagnostics for O-type stars},
journal = {Astronomy & Astrophysics},
year = {2013},
volume = {559},
pages = {A130},
doi = {http://dx.doi.org/10.1051/0004-6361/201322390}
}

Šurlan, B., Hamann, W.-R., Kubát, J., Aret, A. & Oskinova, L. M. Macroclumping Resolves Discrepancy Between H-$and P V Mass-loss Diagnostics 2013 Massive Stars: From alpha to Omega 2013, id.48 Abstract: For investigating the reported differences between H-alpha and P V mass-loss diagnostics, we simultaneously analysed the UV and optical spectra of five O-type stars. For this purpose we utilized the Potsdam Wolf-Rayet (PoWR) model atmospheres code, which is a full non-LTE line blanketing code, and a 3-D Monte Carlo radiative transfer code for clumped hot-star winds. The latter is suitable to calculate resonance lines, and can account for optically thick clumps (macroclumping), a non-void inter-clump medium, and arbitrary values for the onset of clump formation and the velocity dispersion inside clumps. Our results show that with the mass-loss rates derived by fitting the optical spectrum (mainly H-alpha, H-beta, and H-gamma lines) it is possible to simultaneously fit the observed P V resonance lines without any reduction of mass-loss rates, contrary to what was usually concluded. The main conclusion from our work is that macroclumping resolves the previously reported discrepancy between H-alpha and P V mass-loss diagnostics. BibTeX: @INPROCEEDINGS{2013msao.confE..48S, author = {{{\v S}urlan}, B. and {Hamann}, W.-R. and {Kub{\'a}t}, J. and {Aret}, A. and {Oskinova}, L.~M.}, title = "{Macroclumping Resolves Discrepancy Between H-{$\alpha\$} and P V Mass-loss Diagnostics}",
booktitle = {Massive Stars: From alpha to Omega},
year = 2013,
month = jun,
eid = {48},
pages = {48},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Aret, A., Kraus, M., Muratore, M. F. & Borges Fernandes, M. A new observational tracer for high-density disc-like structures around B[e] supergiants 2012 Monthly Notices of the Royal Astronomical Society
2012, 423, 284-293
Abstract: 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 [O I] lines, we discovered the [Ca II] lambdalambda7291, 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 [O I] lambda5577 line-forming region. The line profiles of both the [O I] and the [Ca II] 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 LHA 120-S 22 might have a spiral arm rather than a disc. Based on observations collected with the ESO 2.2 m telescope in La Silla, Chile, under programme 076.D-0609(A).
BibTeX:
@article{2012MNRAS.423..284A,
author = {Aret, A. and Kraus, M. and Muratore, M.~F. and Borges Fernandes, M.},
title = {A new observational tracer for high-density disc-like structures around B[e] supergiants},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2012},
volume = {423},
pages = {284-293},
doi = {http://dx.doi.org/10.1111/j.1365-2966.2012.20871.x}
}

Muratore, M. F., de Wit, W. J., Kraus, M., Aret, A., Cidale, L. S., Borges Fernandes, M., Oudmaijer, R. D. & Wheelwright, H. E. Unveiling the Structure and Kinematics of B[e] Stars' Disks From FEROS and CRIRES Spectra 2012 Circumstellar Dynamics at High Resolution
Eds. A. Carciofi and Th. Rivinius
ASP Conference Series
2012, 464, 67
Abstract: We are investigating the circumstellar material for a sample of B[e] stars using high spectral resolution data taken in the optical and near-infrared regions with ESO/FEROS and ESO/CRIRES spectrographs, respectively. B[e] stars are surrounded by dense disks of still unknown origin. While optical emission lines from [O I] and [Ca II] reflect the disk conditions close to the star (few stellar radii), the near-infrared data, especially the CO band emission, mirror the characteristics in the molecular part of the disk farther away from the star (several AU). Based on our high resolution spectroscopic data, we seek to derive the density and temperature structure of the disks, as well as their kinematics. This will allow us to obtain a better understanding of their structure, formation history and evolution. Here we present our preliminary results.
BibTeX:
@inproceedings{2012ASPC..464...67M,
author = {Muratore, M.~F. and de Wit, W.~J. and Kraus, M. and Aret, A. and Cidale, L.~S. and Borges Fernandes, M. and Oudmaijer, R.~D. and Wheelwright, H.~E.},
title = {Unveiling the Structure and Kinematics of B[e] Stars' Disks From FEROS and CRIRES Spectra},
booktitle = {Circumstellar Dynamics at High Resolution},
year = {2012},
Editor = {{Carciofi}, A.~C. and {Rivinius}, T.},
Series = {ASP Conference Series},
volume = {464},
pages = {67},
Archiveprefix = {arXiv},
Eprint = {1212.4798},
url = {http://aspbooks.org/custom/publications/paper/464-0067.html}
}

Sapar, A., Aret, A., Sapar, L. & Poolamäe, R. Modelling of mercury isotope separation in CP stellar atmospheres: Results and problems 2009 New Astronomy Reviews
2009, 53, 240-245
Abstract: Formation of anomalous isotope abundances in the atmospheres of chemically peculiar (CP) stars can be explained by light-induced drift (LID). This effect is additional to the radiative acceleration and appears due to systematic asymmetry of radiative flux in partly overlapping isotopic spectral line profiles. LID causes levitation of an isotope with a red-shifted spectral line and sinking of an isotope with a blue-shifted line, generating thus diffusive separation of isotopes. We have studied diffusion of mercury as a typical well-studied isotope-rich heavy metal. Our model computations show that in mercury-rich quiescent atmospheres of CP stars LID causes levitation of the heavier mercury isotopes and sinking of the lighter ones. Precise quantitative modelling of the process of isotope separation demands very high-resolution computations and the high-precision input data, including data on hyperfine and isotopic splitting of spectral lines, adequate line profiles and impact cross-sections. Presence of microturbulence and weak stellar winds can essentially reduce the effect of radiative-driven diffusion.
BibTeX:
@article{2009NewAR..53..240S,
author = {Sapar, A. and Aret, A. and Sapar, L. and Poolamäe, R. },
title = {Modelling of mercury isotope separation in CP stellar atmospheres: Results and problems},
journal = {New Astronomy Reviews},
year = {2009},
volume = {53},
pages = {240-245},
doi = {http://dx.doi.org/10.1016/j.newar.2009.08.010}
}

Aret, A., Sapar, A., Poolamäe, R. & Sapar, L. SMART -- a computer program for modelling stellar atmospheres 2008 The Art of Modeling Stars in the 21st Century
Eds. L. Deng and K.L. Chan
IAU Symposium
2008, 252, 41-42
Abstract: Program SMART (Spectra and Model Atmospheres by Radiative Transfer) has been composed for modelling atmospheres and spectra of hot stars (O, B and A spectral classes) and studying different physical processes in them (Sapar & Poolaäe 2003, Sapar et al. 2007). Line-blanketed models are computed assuming plane-parallel, static and horizontally homogeneous atmosphere in radiative, hydrostatic and local thermodynamic equilibrium. Main advantages of SMART are its shortness, simplicity, user friendliness and flexibility for study of different physical processes. SMART successfully runs on PC both under Windows and Linux.
BibTeX:
@INPROCEEDINGS{2008IAUS..252...41A,
author = {{Aret}, A. and {Sapar}, A. and {Poolam{\"a}e}, R. and {Sapar}, L.
},
title = "{SMART   a computer program for modelling stellar atmospheres}",
keywords = {Radiative transfer, stars: atmospheres},
booktitle = {The Art of Modeling Stars in the 21st Century},
year = 2008,
series = {IAU Symposium},
volume = 252,
archivePrefix = "arXiv",
eprint = {1310.1815},
primaryClass = "astro-ph.SR",
editor = {{Deng}, L. and {Chan}, K.~L.},
month = oct,
pages = {41-42},
doi = {10.1017/S1743921308022394},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Sapar, A., Aret, A., Sapar, L. & Poolamäe, R. A Pan-Spectral Method of Abundance Determination 2008 Precision Spectroscopy in Astrophysics
Eds. N.C. Santos, L. Pasquini, A.C.M. Correia, and M. Romanielleo
ESO Astrophysics Symposia
2008, 22, 145-148
Abstract: We propose a new method for determination of element abundances in stellar atmospheres aimed for the automatic processing of high-quality stellar spectra. The pan-spectral method is based on weighted cumulative line-widths Q_lambda = int_lambda _0 ^lambda | dR_lambda /dZ | (1 - R_lambda )dlambda , where R lambda is residual flux and Z is abundance of studied element. Difference in quantities Q lambda found from synthetic and observed spectra gives a correction to the initial abundance. Final abundances are then found by rapidly converging iterations. Calculations can be made for many elements simultaneously and do not demand supercomputers.
BibTeX:
@INPROCEEDINGS{2008psa..conf..145S,
author = {{Sapar}, A. and {Aret}, A. and {Sapar}, L. and {Poolam{\"a}e}, R.
},
title = "{A Pan-Spectral Method of Abundance Determination}",
booktitle = {Precision Spectroscopy in Astrophysics},
year = 2008,
archivePrefix = "arXiv",
eprint = {0712.3222},
editor = {{Santos}, N.~C. and {Pasquini}, L. and {Correia}, A.~C.~M. and
{Romaniello}, M.},
pages = {145-148},
doi = {10.1007/978-3-540-75485-5_32},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Sapar, A., Aret, A., Poolamäe, R. & Sapar, L. Segregation of isotopes of heavy metals due to light-induced drift: results and problems 2008 Contributions of the Astronomical Observatory Skalnaté Pleso
2008, 38, 273-278
Abstract: Atutov and Shalagin (1988) proposed light-induced drift (LID) as a physically well understandable mechanism to explain the formation of isotopic anomalies observed in CP stars. We have generalized the theory of LID and applied it to diffusion of heavy elements and their isotopes in quiescent atmospheres of CP stars. Diffusional segregation of isotopes of chemical elements is described by the equations of continuity and diffusion velocity. Computations of evolutionary sequences for the abundances of mercury isotopes in several model atmospheres have been made, using the Fortran 90 program SMART composed by the authors. Results confirm predominant role of LID in separation of isotopes.
BibTeX:
@ARTICLE{2008CoSka..38..273S,
author = {{Sapar}, A. and {Aret}, A. and {Poolam{\"a}e}, R. and {Sapar}, L.
},
title = "{Segregation of isotopes of heavy metals due to light-induced drift: results and problems}",
journal = {Contributions of the Astronomical Observatory Skalnate Pleso},
archivePrefix = "arXiv",
eprint = {0712.3462},
keywords = {processes: diffusion, stars: atmospheres, stars: chemically peculiar},
year = 2008,
month = apr,
volume = 38,
pages = {273-278},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Sapar, A., Aret, A., Sapar, L. & Poolamäe, R. Formulae for study of light-induced drift diffusion in CP star atmospheres 2008 Contributions of the Astronomical Observatory Skalnaté Pleso
2008, 38, 445-446
Abstract: We present formulae suitable for computing LID acceleration and corresponding diffusive segregation of isotopes in atmospheres of CP stars.
BibTeX:
@ARTICLE{2008CoSka..38..445S,
author = {{Sapar}, A. and {Aret}, A. and {Sapar}, L. and {Poolam{\"a}e}, R.
},
title = "{Formulae for study of light-induced drift diffusion in CP star atmospheres}",
journal = {Contributions of the Astronomical Observatory Skalnate Pleso},
archivePrefix = "arXiv",
eprint = {0712.3456},
keywords = {diffusion, stars: atmospheres, stars: chemically peculiar},
year = 2008,
month = apr,
volume = 38,
pages = {445-446},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Sapar, A. A., Aret, A. V., Sapar, L. J. & Poolamäe, R. I. The separation of isotopes of chemical elements in the atmospheres of CP stars due to light-induced drift 2007 Spectroscopic Methods in Modern Astrophysics
Eds. L. Mashonkina and M. Sachkov
Janus-K, 2007, 220-235
Abstract: Present paper is devoted to the discussion of main features of diffusion of chemical elements and their isotopes in the atmospheres of chemically peculiar (CP) stars, including the separation of isotopes of chemical elements in these atmospheres due to light-induced drift (LID). Diffusion is effective only in the case of quiescent stellar atmospheres, i.e. in the atmospheres where the large-scale motions (convection, meridional circulation, stellar %wind) are lacking or are extremely weak. Main equations are given and their physical meaning has been explained. Numerical computations have been carried out for the evolutionary segregation of mercury isotopes in the atmospheres of HgMn stars. Computational results of the evolutionary modelling are illustrated by the figures. The cases where the initial abundance of Hg is solar, exceeds it by 3 dex and 6 dex are studied. The initial mixture of isotopes has been taken to be corresponding to the terrestrial one. Also the case where the initial mercury abundance and isotopic mixture correspond to the observations of HR 7775 is discussed. It has been found that evolution of stellar atmospheres results in sedimentation of lighter isotopes of heavy elements and finally in the strong prevalence of the heaviest isotope, in the present case of the ^204Hg.
BibTeX:
@inproceedings{2007smma.conf..220S,
author = {Sapar, A.~A. and Aret, A.~V. and Sapar, L.~J. and Poolamäe, R.~I.},
title = {The separation of isotopes of chemical elements in the atmospheres of CP stars due to light-induced drift},
booktitle = {Spectroscopic Methods in Modern Astrophysics},
Editor = {{Mashonkina}, L. and {Sachkov}, M.},
publisher = {Janus-K},
year = {2007},
pages = {220-235},
note = {Proceedings of the conference held 13--15 September 2006, Moscow, Russia}
}

Sapar, A. A., Poolamäe, R. I., Sapar, L. J. & Aret, A. V. Computation of model atmospheres and spectra of hot stars by programme SMART: results and problems 2007 Spectroscopic Methods in Modern Astrophysics
Eds. L. Mashonkina and M. Sachkov
Janus-K, 2007, 236-254
Abstract: Program SMART has been composed for computation of model atmospheres of hot stars of arbitrary chemical composition and their synthetic spectra, and also for study of different physical processes in stellar atmospheres. Main advantages of SMART are shortness, simplicity, user friendliness and flexibility for study of different physical processes. Lack of subroutines for computing convection and molecular absorption in the present version of SMART means that it is applicable only to effective temperatures exceeding 9000 K. A new pan-spectral method for determination of chemical composition of stellar atmospheres has been proposed. The method is based on a weighted cumulative width of spectral lines for an element or ion studied. We applied this method to find abundances of 12 elements in the atmosphere of chemically peculiar star HD 175640 observed by Castelli and Hubrig (2004). Our results are in good agreement with the results by Castelli and Hubrig, who analyzed observations using software by Kurucz (1993, 1997).
BibTeX:
@inproceedings{2007smma.conf..236S,
author = {Sapar, A.~A. and Poolamäe, R.~I. and Sapar, L.~J. and Aret, A.~V.},
title = {Computation of model atmospheres and spectra of hot stars by programme SMART: results and problems},
booktitle = {Spectroscopic Methods in Modern Astrophysics},
Editor = {{Mashonkina}, L. and {Sachkov}, M.},
publisher = {Janus-K},
year = {2007},
pages = {236-254},
note = {Proceedings of the conference held 13--15 September 2006, Moscow, Russia}
}

Sapar, A., Aret, A. & Poolamäe, R. The role of light-induced drift in diffusion of heavy metals and their isotopes in CP stars: an example of mercury 2005 Element Stratification in Stars: 40 Years of Atomic Diffusion
Eds. G. Alecian, O. Richard, and S. Vauclair
EAS Publications Series
2005, 17, 341-344
Abstract: Main theoretical formulae and some computational results of evolutionary segregation of Hg isotopes due to light-induced drift in the atmospheres of chemically peculiar stars have been represented.
BibTeX:
@inproceedings{2005EAS....17..341S,
author = {Sapar, A. and Aret, A. and Poolamäe, R.},
title = {The role of light-induced drift in diffusion of heavy metals and their isotopes in CP stars: an example of mercury},
booktitle = {Element Stratification in Stars: 40 Years of Atomic Diffusion},
Editor = {{Alecian}, G. and {Richard}, O. and {Vauclair}, S.},
year = {2005},
Series = {EAS Publications Series},
volume = {17},
pages = {341-344},
doi = {http://dx.doi.org/10.1051/eas:2005137}
}

Aret, A. & Sapar, A. Light-Induced Drift in Atmospheres of CP Stars 2003 Modelling of Stellar Atmospheres
Eds. N. Piskunov, W.W. Weiss, and D.F. Gray
IAU Symposium
2003, 210, B4
Abstract: The efficiency of the light–induced drift (LID) for formation of anomalous abundance of mercury and its isotopes in the atmospheres of HgMn stars has been studied. Generation of elemental abundance anomalies in quiescent atmospheres of chemically peculiar (CP) stars can generally be explained by the mechanism of diffusive segregation of chemical elements due to oppositely directed gravitational and radiative forces. It has been shown that formation of the observed isotopic anomalies of Hg, and eventually also of other heavy elements, can be successfully explained by a diffusion due to LID. The observed ratios of isotopes can be used to estimate the evolutionary stages of CP stars.
BibTeX:
@INPROCEEDINGS{2003IAUS..210P..B4A,
author = {{Aret}, A. and {Sapar}, A.},
title = "{Light-Induced Drift in Atmospheres of CP Stars}",
booktitle = {Modelling of Stellar Atmospheres},
year = 2003,
series = {IAU Symposium},
volume = 210,
editor = {{Piskunov}, N. and {Weiss}, W.~W. and {Gray}, D.~F.},
pages = {B4},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Aret, A. & Sapar, A. Light-induced drift for Hg isotopes in chemically peculiar stars 2002 Astronomische Nachrichten
2002, 323, 21-30
Abstract: In the present paper the abundance anomalies of mercury and its isotopes in the atmospheres of HgMn stars have been studied. Observations have shown strongly anomalous isotopic composition of Hg, Pt, Tl and He in the atmospheres of such CP stars. Generation of elemental abundance anomalies in quiescent atmospheres of CP stars can generally be explained by the mechanism of diffusive segregation of elements due to oppositely directed gravitational and radiative forces. It has been shown that the formation of the observed isotopic anomalies can be successfully explained by a diffusion mechanism called the light-induced drift (LID). The observed ratios of isotopes also enable to estimate the evolutionary stages of CP stars.
BibTeX:
@article{2002AN....323...21A,
author = {Aret, A. and Sapar, A.},
title = {Light-induced drift for Hg isotopes in chemically peculiar stars},
journal = {Astronomische Nachrichten},
year = {2002},
volume = {323},
pages = {21-30},
doi = {http://dx.doi.org/10.1002/1521-3994(200203)323:1<21::AID-ASNA21>3.0.CO;2-4}
}

Aret, A. & Sapar, A. Isotopic Anomalies in CP Stars 2000 European Astronomy at the Turn of the Millennium. JENAM2000 Abstracts
2000, 71
BibTeX:
@inproceedings{2000JENAM..71,
author = {Aret, A. and Sapar, A.},
title = {Isotopic Anomalies in CP Stars},
booktitle = {European Astronomy at the Turn of the Millennium. JENAM2000 Abstracts},
year = {2000},
pages = {71}
}

Aret, A. & Sapar, A. Isotopic Anomalies in CP Stars 1999 Odessa Astronomical Publications
1999, 12, 162-165
Abstract: In the present paper we analyze the anomalies in the atmospheres of HgMn stars. The abundance anomalies include both overabundances and underabundances of heavy elements. Recent observations show strongly anomalous isotopic composition of Hg, Pt, Tl and of He. Generation of abundance anomalies in quiescent atmospheres of CP stars is successfully explained by the mechanism of diffusional segregation of elements due to oppositing gravitational and radiative forces, but the formation of isotopic anomalies is not yet well explained. New diffusion mechanism called light-induced drift (LID), added to the one of radiative acceleration, successfully explains the observed isotopic anomalies. We have refined the theory of LID and applied it to CP star atmospheres. The results of computations confirm the important role of LID for diffusive segregation of isotopes.
BibTeX:
@article{1999OAP....12..162A,
author = {Aret, A. and Sapar, A.},
title = {Isotopic Anomalies in CP Stars},
journal = {Odessa Astronomical Publications},
year = {1999},
volume = {12},
pages = {162-165}
}

Aret, A. & Sapar, A. Modelling of Light-Induced Drift and Radiative Accelerations in Atmospheres of CP Stars 1998 Prospects of the Astronomy and Astrophysics for the New Millennium. JENAM98 Late Abstracts
1998, 12
BibTeX:
@inproceedings{1998JENAM..12,
author = {Aret, A. and Sapar, A.},
title = {Modelling of Light-Induced Drift and Radiative Accelerations in Atmospheres of CP Stars},
booktitle = {Prospects of the Astronomy and Astrophysics for the New Millennium. JENAM98 Late Abstracts},
year = {1998},
pages = {12}
}

Aret, A. & Sapar, A. Particle diffusion in atmospheres of CP stars 1998 Contributions of the Astronomical Observatory Skalnaté Pleso
1998, 27, 329-331
Abstract: We give concisely the formulae governing diffusion of chemical elements and their isotopes in quiescent stellar atmospheres, due to electrostatic, gravitational and radiation fields and to impacts between particles. Isotope segregation of heavy elements due to light-induced drift is emphasized.
BibTeX:
@ARTICLE{1998CoSka..27..329A,
author = {{Aret}, A. and {Sapar}, A.},
title = "{Particle diffusion in atmospheres of CP stars}",
journal = {Contributions of the Astronomical Observatory Skalnate Pleso},
eprint = {astro-ph/9805150},
keywords = {STARS: CHEMICALLY PECULIAR, DIFFUSION},
year = 1998,
month = apr,
volume = 27,
pages = {329-331},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Sapar, A. & Aret, A. The formation of chemical peculiarities in stellar atmospheres 1995 Astronomical and Astrophysical Transactions
1995, 7, 1-27
Abstract: Generalized equations of dynamics for plasma components describing formation of chemical peculiarities in stellar atmospheres due to gravitational, electrostatic, magnetostatic and radiation fields acting on colliding plasma particles have been derived. The equations describing both diffusion and drift phenomena can be treated as intercoupled equations of Fokker–Planck type. It has been shown that in stellar atmospheres there will be generated the electrostatic field which is important for separation of light elements and their isotopes. The highly anisotropical diffusion in upper atmospheric layers of magnetic stars has been demonstrated. For interaction of plasma components with radiation field besides the usual upwards directed radiation flux generated acceleration, there is a more complicated light induced drift due to selective resonance-like absorption of radiation by thermally moving plasma particles with definite momentum values. The light induced drift can be directed both upwards or downwards and can be dominant mechanism in chemical element separation.
BibTeX:
@article{1995AAT....7....1S,
author = {Sapar, A. and Aret, A.},
title = {The formation of chemical peculiarities in stellar atmospheres},
journal = {Astronomical and Astrophysical Transactions},
year = {1995},
volume = {7},
number = {1},
pages = {1-27}
}

Aret, A. & Sapar, A. Element diffusion in atmospheres of CP stars 1995 Stellar Surface Structure
Ed. Klaus G. Strassmeier
IAU Symposium
1995, 176, 142-144
BibTeX:
@INPROCEEDINGS{1995IAUS..176P.142A,
author = {{Aret}, A. and {Sapar}, A.},
title = "{Element diffusion in atmospheres of CP stars}",
booktitle = {Stellar Surface Structure. Poster proceedings of IAU Symposium 176},
editor = {Strassmeier, K.~G.},
publisher = {University of Vienna},
year = 1995,
series = {IAU Symposium},
volume = 176,
pages = {142},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Aret, A. & Sapar, A. Generalized Diffusion Theory of Chemical Stratification in Stellar Atmospheres 1994 22nd General Assembly of IAU, Astronomy Posters Abstracts
Twin Press, 1994, 240
BibTeX:
@inproceedings{1994GA22..240,
author = {Aret, A. and Sapar, A.},
title = {Generalized Diffusion Theory of Chemical Stratification in Stellar Atmospheres},
booktitle = {22nd General Assembly of IAU, Astronomy Posters Abstracts},
publisher = {Twin Press},
year = {1994},
pages = {240}
}

Sapar, A., Aret, A. & Poolamäe, R. Panspectral Continuous Interstellar Opacity 1994 22nd General Assembly of IAU, Astronomy Posters Abstracts
Twin Press, 1994, 176
BibTeX:
@inproceedings{1994GA22..176,
author = {Sapar, A. and Aret, A. and Poolamäe, R.},
title = {Panspectral Continuous Interstellar Opacity},
booktitle = {22nd General Assembly of IAU, Astronomy Posters Abstracts},
publisher = {Twin Press},
year = {1994},
pages = {176}
}

Altukhov, A. M., Aret, A. V., Belov, A. V., Dorman, L. I., Eroshenko, E. A., Oleneva, V. A. & Rudnev, I. F. Cosmic ray anisotropy and gradient in different solar wind structures 1991 Akademiia Nauk SSSR, Izvestiia, Seriia Fizicheskaia
1991, 55, 1927-1929
BibTeX:
@article{1991IzSSR..55.1927A,
author = {Altukhov, A.~M. and Aret, A.~V. and Belov, A.~V. and Dorman, L.~I. and Eroshenko, E.~A. and Oleneva, V.~A. and Rudnev, I.~F.},
title = {Cosmic ray anisotropy and gradient in different solar wind structures},
journal = {Akademiia Nauk SSSR, Izvestiia, Seriia Fizicheskaia},
year = {1991},
volume = {55},
number = {10},
pages = {1927-1929}
}


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