Spectroscopy Team

Faculty of Physics

Department of Astrophysics Astronomy & Mechanics

UNIVERSITY OF ATHENS

 

 

RESEARCH TEAM
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Dr E. Danezis, Assistant Professor, Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens

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Dr E. Theodossiou, Assistant Professor, Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens

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Dr M. Stathopoulou, Lector, Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens

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Dr Th. Grammenos, Visiting Assistant Professor for Applied Mathematics and Theoretical Physics, Department of Mechanical Engineering , University of Thessaly

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Dr E. Lyratzi, PhD in Astrophysics, Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens

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Dr. A. Antoniou, PhD in Astrophysics, Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens

bullet G. Stamatogiannaki, Undergraduate student, Faculty of Physics, University of Athens
bullet D. Stathopoulos, Undergraduate student, Faculty of Physics, University of Athens

 

GENERAL REVIEW OF THE RESEARCH FIELD
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Satellite Absorption Components (SACs) - A new mathematical model for the density atmospherical regions which create the SACs in the spectra of Oe and Be stars (right-click here (186 KΒ .pdf file) and here (208 KΒ .pdf file) and select "Save Target As..." to download)

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DMVL A program for the reproduction of complex spectral line profiles  (see Satellite Absorption Components):  (click on the link to go to download page)

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Processing Stellar Data from the IUE Satellite through IDL with the IUERDAF package (18.1 Kb .pdf file, right-click here and select "Save Target As..." to download)

 

RESEARCH OF SPECTROSCOPY TEAM

Description of the problem

One of the most interesting and unsolved problems of astrophysics, until now, was the description of the physical laws that evolve in the environment of a group of peculiar stars, know as hot emission stars. Our scientific group studied this problem and succeeded in finding solutions to this problem. Here we will present the problem and the solution that we proposed. As we know now, stars of the same spectral type and luminosity class present the same absorption lines in their spectra.

Two Be stars of the same luminosity class present the same absorption lines in their spectra.

However, observations have shown that some hot emission stars (Oe and Be stars) and some quasars present some absorption lines that should not appear in their spectra, according to the classical physical theory. These peculiar spectral lines have been named Discrete Absorption Components (DACs).

In these figures we can see the comparison of Mg II resonance lines between the spectrum of a normal B star and the spectra of one active Be star. As we can observe the Be stars present some absorption components that do not appear in the spectrum of the classical B star (DACs).

Now we know that DACs are spectral lines of the same ion and the same wavelength as a main spectral line, which have different displacements in the spectrum, as they are created in different, independent density regions of matter, which rotate and move with different velocities (Danezis 1984, 1987, Danezis et al. 1991, 2003, 2005, 2007 και Lyratzi & Danezis 2004).
Another problem of many spectra of hot emission stars and quasars is that some spectral lines have complex profiles and could not be reproduced theoretically. This means that we could not know the physical conditions that exist in the density regions that create these lines.
In order to explain these peculiar profiles, we proposed the phenomenon of Satellite Absorption Components (SACs).

Very complex profile of the spectral lines C IV λλ 1548.187, 1550.772 Å that we can’t produce theoretically. This means that we could not know the physical conditions that exist in the high density regions that construct these spectral lines.

Our thought was quite simple and has as follows: if the density regions that create these lines rotate quickly and move slowly, then the spectral lines that they produce have large width and small displacement in the spectrum. As a result, the spectral lines are blended among themselves as well as with the main spectral line and thus they are not discrete In this way the observed profile in the spectrum is very complex.

Our suggestion

In order to establish this idea, we should be able to reproduce theoretically the spectral lines that have DACs or SACs. This means that we should calculate the line function of their complex profile. The line function relates the radiation intensity with the wavelength and includes as parameters the physical conditions that affect the construction of the spectral line profile. We accept that the values of the physical parameters, which give the best fit of the observed profile of the spectral line, describe the actual physical conditions of the regions which create the specific spectral line.
Such a theoretical reproduction of the observed profile of the spectral line was impossible, until 2003, when our scientific team proposed a line function, able to reproduce the complex structure of the density regions that exist in the environment of hot emission stars and some quasars.

The C IV UV doublet of the AGN PG 0946+301 (right). The values of radial displacements and the ratio of the line intensities suggest that the observed C IV shapes indicate the presence of a DACs phenomenon, similar with the DACs phenomenon that we can detect in the spectra of hot emission stars (such as HD 45910, left). The black line presents the observed spectral line's profile and the red one the model's fit.

Below, all the components which contribute to the observed features are presented separately.

The GR model

Our first step was to calculate a new distribution function, presupposing that the main reason of the broadening of the spectral lines is the quick rotation of the regions that create them. We named this distribution function “Rotation Distribution”.
In a better approximation, taking into account the random motions of the ions in the density regions where SACs and DACs are created, we calculated one more new distribution function, which included as reasons of the spectral line broadening both the rotation of the density regions and the random motions of the ions. We named this distribution function “Gaussian-Rotation Distribution (GR Distribution)”.
With GR model we are able to calculate the values of many physical parameters of the regions that create DACs and SACs.
The GR model has been tested and applied in a great number of spectral lines of hot emission stars and quasars and has given important results about kinematical and time evolution of stellar atmospheres of specific hot emission stars and quasars, as well as in statistical studies, in order to extract general conclusions. All these studies have been published in international refereed scientific journals and conference proceedings.

Similarity of SACs phenomenon in Oe star’s HD 34656 spectrum and in AGNs' PG 1254+047 spectrum. The black line presents the observed spectral line's profile and the red one the model's fit. We also present all the components which contribute to the observed features, separately.

Below, all the components which contribute to the observed features are presented separately.

Summary of results

In brief, the results of our study are as follows:

  1. We were able to explain the origin of DACs phenomenon and to propose the SACs phenomenon in order to explain the complex structure of the observed profiles of many spectral lines that are created in the environment of hot emission stars.

  2. We observed the same phenomenon in the spectra of quasars and we proposed that the origin of this phenomenon is the same as in the case of hot emission stars.

  3. We constructed a new model (GR model) able to reproduce theoretically the observed complex spectral line profiles. In order to do so, we calculated for the first time a line function through the solution of the radiation transfer equation.

  4. We calculated two new distribution functions (Rotation distribution and Gauss-Rotation distribution).

  5. We constructed a first version of software, in order to reproduce the observed spectral lines and to calculate some physical parameters.

  6. We applied successfully the GR model in a great number of spectral lines of hot emission stars and quasars.

Calculating a number of parameters for the first time, we are able to study the physical laws that exist in the regions where the spectral lines that present DACs and SACs are created.

 

RECENT PUBLICATIONS OF SPECTROSCOPY TEAM

  1. Lyratzi, E., Danezis, E., Popović, L. Č., Antoniou, A., Dimitrijević, M. S. & Stathopoulos, D.: Studying the complex BAL profiles in the BALQSOs spectra, Proceedings of 25th Summer School and International Symposium on the Physics of Ionized Gases (SPIG 2010), held on August, 30 - September, 3, 2010, Donji Milanovac, Serbia.

  2. Lyratzi, E., Danezis, E., Popović, L. Č., Dimitrijević, M. S. & Antoniou, A.: Broad Absorption Lines with DACs and SACs in the spectra of PG 0946+301 and PG 1254+047, Proceedings of “JENAM 2010 The European Week of Astronomy and Space Science”, held on September 6-10, 2010, Lisbon, Portugal.

  3. Antoniou, A., Danezis, E., Lyratzi, E., Popović, L. Č. & Dimitrijević, M. S.: A statistical study of the UV Mg II resonance lines’ parameters in 20 Be stars, Proceedings of “JENAM 2010 The European Week of Astronomy and Space Science”, held on September 6-10, 2010, Lisbon, Portugal.

  4. Danezis, E., Lyratzi, E., Antoniou, A., Popović, L. Č., Dimitrijević, M. S.: Studying the complex spectral line profiles in the spectra of hot emission stars and quasars, Proceedings of the “1st Workshop: Astrophysical winds and disks, Similar phenomena is stars and quasars”, held September 3-8, 2009, Platamonas, Greece. Edited by Emmanuel Danezis and Evangelia Lyratzi (under publication).

  5. Lyratzi, E., Danezis, E., Popović, L. Č., Dimitrijević, M. S., Antoniou, A. & Stathopoulos, D.: Kinematic properties of the Broad Absorption Line Regions in the spectra of quasars, Proceedings of the “1st Workshop: Astrophysical winds and disks, Similar phenomena is stars and quasars”, held September 3-8, 2009, Platamonas, Greece. Edited by Emmanuel Danezis and Evangelia Lyratzi (under publication).

  6. Antoniou, A., Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S. & Theodossiou, E.: Studying the location of SACs and DACs regions in the environment of hot emission stars, Proceedings of the “1st Workshop: Astrophysical winds and disks, Similar phenomena is stars and quasars”, held September 3-8, 2009, Platamonas, Greece. Edited by Emmanuel Danezis and Evangelia Lyratzi (under publication).

  7. Lyratzi, E., Danezis, E., Popović, L. Č., Dimitrijević, M. S., Antoniou, A.: Ways of creation of DACs and SACs in the spectra of PG 0946+301 and PG 1254+047, 2010, ASPC, 424, 308, Proceedings of the 9th International Conference of the Hellenic Astronomical Society, held September 20-24, 2009 in Athens, Greece. Edited by Kanaris Tsinganos, Despina Hatzidimitriou and Titos Matsakos.

  8. Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S., Antoniou, A.: Investigating DACs/SACs Phenomena in Hot Emission Stars and Quasars, 2010, ASPC, 424, 305, Proceedings of the 9th International Conference of the Hellenic Astronomical Society, held September 20-24, 2009 in Athens, Greece. Edited by Kanaris Tsinganos, Despina Hatzidimitriou and Titos Matsakos.

  9. Antoniou, A., Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S., Stathopoulos, D.: A New Approach of the GR Model, 2010, ASPC, 424, 187, Proceedings of the 9th International Conference of the Hellenic Astronomical Society, held September 20-24, 2009 in Athens, Greece. Edited by Kanaris Tsinganos, Despina Hatzidimitriou and Titos Matsakos.

  10. Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S., Antoniou, A.: Interpreting the complex line profiles in the stellar spectra, 2009, NewAR, 53, 214

  11. Lyratzi, E., Popović, L. Č., Danezis, E., Dimitrijević, M. S., Antoniou, A.: Kinematics of the broad absorption line region in QSOs: Rotation and random motion, 2009, NewAR, 53, 179.

  12. Antoniou, A., Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S., Theodosiou, E., Stathopoulos, D.: A study of the atmospheric structure of AX Mon (HD 45910), 2009, CoAst, 159, 119, Proceedings of “JENAM 2008 Symposium No 4: Asteroseismology and Stellar Evolution”, held on September 8-12, 2008, edited by Sonja Schuh and Gerald Handler.

  13. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: Kinematical parameters in the coronal and post-coronal regions of the Oe stars, 2008, POBeo, 84, 443, 2008, JPhCS, 133a, 2028, 24th Summer School and International Symposium on the Physics of Ionized Gases, (SPIG 2008), August 25-29, 2008, Novi Sad, Serbia.

  14. Lyratzi, E., Danezis, E., Popović, L. Č., Dimitrijević, M. S. & Antoniou, A.: DACs and SACs in the UV Spectrum of the Quasar PG 0946+301, 2008, AIPC, 1058, 323, 19th International Conference on Spectral Line Shapes, (19 ICSLS), June 15 - 20, 2008, Valladolid, Spain.

  15. Lyratzi, E., Danezis, E., Popović, L. Č., Dimitrijević, M. S. & Antoniou, A.: Kinematics of Broad Absorption Line Regions of PG 1254+047, 2008, AIPC, 1058, 320, 19th International Conference on Spectral Line Shapes, (19 ICSLS), June 15 - 20, 2008, Valladolid, Spain.

  16. Antoniou, A., Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S., Theodosiou, E. & Katsavrias, G.: A Study of the Structure of Different Ionization Potential Regions in the Atmosphere of AX Mon (HD 45910), 2008, AIPC, 1058, 317, 19th International Conference on Spectral Line Shapes, (19 ICSLS), June 15 - 20, 2008, Valladolid, Spain.

  17. Antoniou, A., Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S., Theodosiou, E. & Stathopoulos, D.: AX Mon (HD 45910) Kinematical Parameters in the Fe II Spectral Lines as a Function of the Excitation Potential, 2008, AIPC, 1058, 314, 19th International Conference on Spectral Line Shapes, (19 ICSLS), June 15 - 20, 2008, Valladolid, Spain.

  18. Danezis, E., Lyratzi, E., Popović, L. Č. & Dimitrijević, M. S., Antoniou, A.: Similarity Between DACs/SACs Phenomena in Hot Emission Stars and Quasars Absorption Lines, 2008, AIPC, 1058, 305, 19th International Conference on Spectral Line Shapes, (19 ICSLS), June 15 - 20, 2008, Valladolid, Spain.

  19. Lyratzi, E., Danezis, E., Popović, L. Č., Dimitrijević, M. S. & Antoniou, A.: DACs and SACs phenomena in the C IV emitting regions of QSOs, 2008, POBeo, 84, 483, 24th Summer School and International Symposium on the Physics of Ionized Gases, (SPIG 2008), August 25-29, 2008, Novi Sad, Serbia.

  20. Danezis, E., Lyratzi, E., Popović, L. Č., Antoniou, A. & Dimitrijević, M. S.: The very broad lines in the UV spectra of hot emission stars - A possible explanation, 2008, POBeo, 84, 463, 24th Summer School and International Symposium on the Physics of Ionized Gases, (SPIG 2008), August 25-29, 2008, Novi Sad, Serbia.

  21. Antoniou, A., Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S., Theodossiou, E. & Stathopoulos, D.: A study of AX Mon (HD 45910) kinematical parameters of the Fe II density regions, 2008, POBeo, 84, 455, 24th Summer School and International Symposium on the Physics of Ionized Gases, (SPIG 2008), August 25-29, 2008, Novi Sad, Serbia.

  22. Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S. & Antoniou, A.: DACs/SACs phenomena in astrophysical spectra, Joint European and National Astronomical Meeting (JENAM 2008), September 8-12, 2008, Vienna, Austria.

  23. Lyratzi, E., Danezis, E., Popović, L. Č., Dimitrijević, M. S. & Antoniou, A.: An analysis of C IV and Si IV lines in the spectra of the quasars PG 0946+301 and PG 1254+047, Joint European and National Astronomical Meeting (JENAM 2008), September 8-12, 2008, Vienna, Austria.

  24. Antoniou, A., Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S. & Katsavrias, G.: The structure of atmospherical layers of AX Mon (HD 45910), Joint European and National Astronomical Meeting (JENAM 2008), September 8-12, 2008, Vienna, Austria.

  25. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: Investigation of the post coronal density regions of Oe stars, through the N V UV resonance lines, 2007, AIPC, 938, 210, VI Serbian Conference on Spectral Line Shapes in Astrophysics (VI SCSLSA), June 2007, Sremski Karlovci, Serbia.

  26. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: The N IV density regions in the spectra of 20 Oe stars, 2007, AIPC, 938, 206, VI Serbian Conference on Spectral Line Shapes in Astrophysics (VI SCSLSA), June 2007, Sremski Karlovci, Serbia.

  27. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: Long term variability of the coronal and post-coronal regions of the Oe star HD 149757 (zeta Oph), 2007, AIPC, 938, 198, VI Serbian Conference on Spectral Line Shapes in Astrophysics (VI SCSLSA), June 2007, Sremski Karlovci, Serbia.

  28. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: A statistical study of physical parameters of the C IV density regions in 20 Oe stars, 2007, AIPC, 938, 194, VI Serbian Conference on Spectral Line Shapes in Astrophysics (VI SCSLSA), June 2007, Sremski Karlovci, Serbia.

  29. Lyratzi, E., Danezis, E., Dimitrijević, M. S., Popović, L. Č., Nikolaidis, D. & Antoniou, A.: The Evolution of Some Physical Parameters in the DACs/SACs Regions in Be Stellar Atmospheres, 2007, AIPC, 938, 176, VI Serbian Conference on Spectral Line Shapes in Astrophysics (VI SCSLSA), June 2007, Sremski Karlovci, Serbia.

  30. Danezis, E., Antoniou, A., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S. & Nikolaidis, D.: The DACs and SACs Effects From Stars to Quasars. Some First General Notices, 2007, AIPC, 938, 119, VI Serbian Conference on Spectral Line Shapes in Astrophysics (VI SCSLSA), June 2007, Sremski Karlovci, Serbia. (Προσκεκλημένη ομιλία).

  31. Chatzichristou, E. T., Danezis, E., Popović, L. Č., Lyratzi, E. & Dimitrijević, M. S.: Studying the complex absorption and emission lines in AGN spectra, 2007, AIPC, 938, 65, VI Serbian Conference on Spectral Line Shapes in Astrophysics (VI SCSLSA), June 2007, Sremski Karlovci, Serbia.

  32. Danezis, E., Nikolaidis D., Lyratzi, E., Popović, L. Č., Dimitriević, M. S., Antoniou, A. & Theodosiou, E.: A new model for the structure of DACs and SACs regions in Oe and Be stellar atmospheres, 2007, PASJ, 59, 827.

  33. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: Hyper Ionization Phenomena In The N V Region Of 20 Oe Stars, 2006, IAUJD, 4E, 7, 2007, uasb.conf, 155, XXVIth IAU General Assembly, 2006, August 14-25, Prague, Czech Republic.

  34. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: Hyper Ionization Phenomena In The N IV Region Of 20 Oe Stars, 2006, IAUJD, 4E, 6, 2007, uasb.conf, 149, XXVIth IAU General Assembly, 2006, August 14-25, Prague, Czech Republic.

  35. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: Hyper Ionization Phenomena In The C IV Region Of 20 Oe Stars, 2006, IAUJD, 4E, 5, 2007, uasb.conf, 143, XXVIth IAU General Assembly, 2006, August 14-25, Prague, Czech Republic.

  36. Lyratzi, E., Danezis, E., Antoniou, A., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: The complex structure of the Si IV λλ 1393.755, 1402.77 Å regions of 68 Be-type stars, 2006, IAUJD, 4E, 10, 2007, uasb.conf, 137, XXVIth IAU General Assembly, 2006, August 14-25, Prague, Czech Republic.

  37. Lyratzi, E., Danezis, E., Antoniou, A., Nikolaidis, D., Popović, L. Č. & Dimitrijević, M. S.: Study of Hα regions in 120 Be-type stars, 2006, IAUJD, 4E, 9, 2007, uasb.conf, 131, XXVIth IAU General Assembly, 2006, August 14-25, Prague, Czech Republic.

  38. Danezis, E., Antoniou, A., Lyratzi, E., Nikolaidis, D., Popović, L. Č., Dimitrijević, M. S. & Theodossiou, E.: Long term variability of the coronal and post-coronal C IV region of the Oe star HD 93521, 2006, IAUJD, 4E, 4, 2007, uasb.conf, 113, XXVIth IAU General Assembly, 2006, August 14-25, Prague, Czech Republic.

  39. Nikolaidis, D., Danezis, E., Lyratzi, E., Popović, L. Č., Dimitrijević, M. S., Antoniou, A. & Theodossiou, E.: A New Approach For DACs And SACs Phenomena In The Atmospheres Of Hot Emission Stars, 2006, IAUJD, 4E, 11, 2007, uasb.conf, 107, XXVIth IAU General Assembly, 2006, August 14-25, Prague, Czech Republic.

  40. Lyratzi, E., Danezis, E., Popović, L. Č., Dimitijević, M. S. & Nikolaidis D.: The complex structure of the MgII λλ 2795.523, 2802.698 A regions of 64 Be stars, 2007, PASJ, 59, 357.

  41. Danezis, E., Popović, L. Č., Lyratzi, E. & Dimitriević, M. S.: The peculiar absorption and emission phenomena from stars to quasars, 2006, AIPC, 876, 373, 23rd Summer School and International Symposium on the Physics of Ionized Gases, (SPIG 2006), 2006, August 28 - September 1, Kopaonik, Serbia (Προσκεκλημένη ομιλία).

  42. Danezis, E., Lyratzi, E., Nikolaidis, D., Antoniou, A., Popović, L. Č. & Dimitrijević, M. S.: The SACs Broadening, 23rd Summer School and International Symposium on the Physics of Ionized Gases (SPIG 2006), August 28 - September 1, 2006, Kopaonik, Serbia.

  43. Lyratzi, E., Danezis, E., Nikolaidis, D., Antoniou, A., Popović, L. Č., Dimitrijević, M. S. & Theodossiou, E.: The Photospheric And The Respective Si IV Regions' Rotational Velocities In 27 Be Stars, 23rd Summer School and International Symposium on the Physics of Ionized Gases, (SPIG 2006), August 28 - September 1, 2006, Kopaonik, Serbia.

  44. Nikolaidis, D., Danezis, E., Lyratzi, E., Popović, L. Č., Antoniou, A. Dimitrijević, M. S. & Theodossiou, E.: A Study Of DACs And SACs Regions In The Atmospheres Of Hot Emissions Stars, 23rd Summer School and International Symposium on the Physics of Ionized Gases, (SPIG 2006), August 28 - September 1, 2006, Kopaonik, Serbia.

  45. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č., Dimitrijević, M. S. & Theodossiou, E.: The Photospheric And The Respective C IV Regions' Rotational Velocities In 20 Oe Stars, 23rd Summer School and International Symposium on the Physics of Ionized Gases, (SPIG 2006), August 28 - September 1, 2006, Kopaonik, Serbia.

  46. Lyratzi, E., Danezis, E., Nikolaidis, D., Antoniou, A., Popović, L. Č. & Dimitijević, M. S.: A new approach for the structure of Ha regions in 120 Be stars, 2006, AIPC, 848, 402, RECENT ADVANCES IN ASTRONOMY AND ASTROPHYSICS: 7th International Conference of the Hellenic Astronomical Society, September 2005, Kefallinia Island, Greece.

  47. Danezis, E., Nikolaidis, D., Lyratzi, E., Popović, L. Č., Dimitijević, M. S. & Antoniou, A.: A new modelling approach for DACs and SACs regions in the atmospheres of hot emission stars, 2006, AIPC, 848, 380, RECENT ADVANCES IN ASTRONOMY AND ASTROPHYSICS: 7th International Conference of the Hellenic Astronomical Society, September 2005, Kefallinia Island, Greece.

  48. Antoniou, A., Danezis, E., Lyratzi, E., Nikolaidis, D., Popović, L. Č., Dimitijević, M. S. & Theodossiou, E.: Long Term Variability of the Radial Velocities in the Coronal and Post-coronal Regions of the Oe Star HD 93521, 2006, AIPC, 848, 370, RECENT ADVANCES IN ASTRONOMY AND ASTROPHYSICS: 7th International Conference of the Hellenic Astronomical Society, September 2005, Kefallinia Island, Greece.

  49. Lyratzi, E., Danezis, E., Nikolaidis, D., Popović, L. Č., Dimitriević, M. S., Theodossiou, E. & Antoniou, A.: A new approach for the structure of Ηα regions in 120 Be stars, 2005, MSAIS, 7, 114, 5th Serbian Conference on Spectral Lines Shapes in Astrophysics (V SCSLSA), June 2005, Vrsać, Serbia. (Προσκεκλημένη ομιλία).

  50. Danezis, E., Nikolaidis, D., Lyratzi, E., Popović, L. Č., Dimitriević, M. S., Theodossiou, E. & Antoniou, A.: A new modelling approach for DACs and SACs regions in the atmospheres of hot emission stars, 2005, MSAIS, 7, 107, 5th Serbian Conference on Spectral Lines Shapes in Astrophysics (V SCSLSA), June 2005, Vrsać, Serbia. (Προσκεκλημένη ομιλία).

  51. Popović, L. Č., Dimitrijević, M. S., Mediavilla, E., Danezis, E., Lyratzi, E., Bon, E., Ilić, D., Jovanović, P., Theodossiou, E. & Dačić, M.: Some spectroscopic methods for astrophysical plasma research, 2004, AIPC, 740, 497, 22nd Summer School and International Symposium on the Physics of Ionized Gases (SPIG 2004), 2004, August, 23 - 27, National Park "TARA", Serbia and Montenegro. (Προσκεκλημένη ομιλία) (right click here and select "Save Target As..." to download .pdf file)

  52. Lyratzi, Ε., & Danezis, E.: SACs and DACs phenomena in the atmospheres of hot emission stars, 2004, AIPC, 740, 458, 22nd Summer School and International Symposium on the Physics of Ionized Gases (SPIG 2004), 2004, August, 23 - 27, National Park "TARA", Serbia and Montenegro. (Προσκεκλημένη ομιλία) (right click here and select "Save Target As..." to download .pdf file)

  53. Danezis, E., Koutroumanou, M., Lyratzi, Ε., Theodossiou, E., Stathopoulou, M., Nikolaidis, D., Antoniou, A., Drakopoulos, C. & Soulikias, A.: Discontinuous structure of SiIV regions in the Oe star's λ Orionis atmosphere, 2004, hell.conf, 117, 6th Astronomical Conference of the Hellenic Astronomical Society, September 2005, Athens, Greece.

  54. Lyratzi, Ε., Danezis, E., Stathopoulou, M., Theodossiou, E., Nikolaidis, D., Antoniou, A., Drakopoulos, C., Soulikias, A. & Koutroumanou, M.: Discontinuous phenomena in SiIV regions of BeV stars, 2004, hell.conf, 116L, 6th Astronomical Conference of the Hellenic Astronomical Society, September 2005, Athens, Greece.

  55. Danezis, E., Antoniou, A., Lyratzi, Ε., Theodossiou, E., Stathopoulou, M., Nikolaidis, D., Drakopoulos, C., Soulikias, A. & Koutroumanou, M.: Discontinuous structure and SACs phenomena in the Oe star's HD 175754 atmosphere, 2004, hell.conf, 116D, 6th Astronomical Conference of the Hellenic Astronomical Society, September 2005, Athens, Greece.

  56. Lyratzi, Ε., Danezis, E., Stathopoulou, M., Theodossiou, E., Nikolaidis, D., Antoniou, A., Drakopoulos, C., Soulikias, A. & Koutroumanou, M.: Discontinuous phenomena in MgII regions of BeV stars, 2004, hell.conf, 115L, 6th Astronomical Conference of the Hellenic Astronomical Society, September 2005, Athens, Greece.

  57. Danezis, E., Lyratzi, Ε., Nikolaidis, D., Theodossiou, E., Stathopoulou, M., Drakopoulos, C. & Soulikias, A.: Discontinuous structures in early type stars' atmospheres, 2004, hell.conf, 115D, 6th Astronomical Conference of the Hellenic Astronomical Society, September 2005, Athens, Greece.

  58. Lyratzi, Ε., Danezis, E., Popović, L. Č., Dimitrijević, M. S., Nikolaidis, D., Soulikias, A., Antoniou, A. & Stathopoulou, M.: The complex structure of the SiIV λλ 1393.73, 1402.73 A regions of 57 BeV stars, JENAM 2004, Spetember, 13-17, 2004, Granada, Spain. (right click here and select "Save Target As..." to download .pdf file)

  59. Danezis, E., Antoniou, A., Popović, L. Č., Dimitrijević, M. S., Lyratzi, Ε., Nikolaidis, D., Soulikias, A. & Theodossiou, E.: Satellite Absorption Components of the UV spectral lines SiIV, CIV, NV, and NIV in the atmosphere of the Oe star HD 175754, JENAM 2004, Spetember, 13-17, 2004, Granada, Spain. (right click here and select "Save Target As..." to download .pdf file)

  60. Danezis, E., Lyratzi, Ε., Popović, L. Č., Dimitrijević, M. S., Christou, G., Soulikias, A. & Antoniou, A.: Satellite Absorption Components of the UV spectral lines SiIV, CIV, NV, and NIV in the atmosphere of the Oe star HD 66811 (ζPup), JENAM 2004, September, 13-17, 2004, Granada, Spain. (right click here and select "Save Target As..." to download .pdf file).

  61. Lyratzi, Ε., Danezis, E., Popović, L. Č., Dimitrijević, M. S., Nikolaidis, D., Thiedossiou, E., Soulikias, A. & Antoniou, A.: The complex structure of the MgII λλ 2795.523, 2802.698 A regions of 64 Be stars, 17th International Conference on Spectral Line Shapes (17th ICSLS), June 21-25, 2004, Paris, France.

  62. Lyratzi, Ε., Danezis, E., Popović, L. Č., Dimitrijević, M. S., Nikolaidis, D., Theiodossiou, E., Soulikias, A. & Antoniou, A.: The complex structure of the SiIV λλ 1393.73, 1402.73 A regions of 42 BeV stars, 17th International Conference on Spectral Line Shapes (17th ICSLS), June 21-25, 2004, Paris, France.

  63. Popović, L. Č., Danezis, E., Lyratzi,E., Dimitrijević, M. S. & Theodossiou E.: A model for the Broad Absorption Line Region in BAL-QSOs, The conference of Euro-asian astronomical society, Russian Astronomical conference, July, 2 - 10, 2004, Moscow, Russia.

  64. Danezis, E., Lyratzi, Ε., Nikolaidis, D., Stathopoulou, M., Theodossiou, E., Drakopoulos, C., Soulikias, A. & Antoniou, A.: On modeling SACs regions in early type atmospheres, 2003, POBeo, 76, 179, IV Serbian Conference on Spectral Lines Shapes (IV SCSLS), 2003, October 10-15, Arandjelovac, Serbia.

  65. Danezis, E., Antoniou, A., Lyratzi, Ε., Theodossiou, E., Stathopoulou, M., Nikolaidis, D., Drakopoulos, C., Soulikias, A. & Koutroumanou, M.: The complex coronal structure of the Oe star HD 175754, 2003, POBeo, 76, 175, IV Serbian Conference on Spectral Lines Shapes (IV SCSLS), 2003, October 10-15, Arandjelovac, Serbia.

  66. Lyratzi, Ε., Danezis, E., Stathopoulou, M., Theodossiou, E., Nikolaidis, D., Drakopoulos, C. & Soulikias, A.: The complex structure of the MgII regions of 40 Be V stars, 2003, POBeo, 76, 27, IV Serbian Conference on Spectral Lines Shapes (IV SCSLS), 2003, October 10-15, Arandjelovac, Serbia (Προσκεκλημένη ομιλία).

  67. Danezis, E., Lyratzi, E., Nikolaides, D., Theodossiou, E., Stathopoulou, M., Christou, G., Drakopoulos, C., Kosionidies, A. & Soulikias, A.: Applications of a New Model of Density Layers of Matter in the Expanding Gaseous Envelope of Oe and Be Stars, 2003, IAUS, 210P, E29, 210th Symposium of the International Astronomical Union, June 2002, Uppsala, Sweden.

  68. Danezis, E., Nikolaides, D., Lyratzi, E., Stathopoulou, M., Theodossiou, E., Kosionidies, A., Drakopoulos, C., Christou, G. & Koutsouris, P.: A new model of density layers of matter to the expanding gaseous envelope of Oe and Be stars, 2003, IAUS, 210P, E3, 210th Symposium of the International Astronomical Union, June 2002, Uppsala, Sweden.

  69. Danezis E., Nikolaidis D., Lyratzi Ε., Stathopoulou M., Theodossiou E., Kosionidis A., Drakopoulos C., Christou G. & Koutsouris P.: Α New Model for the Structure of the DACs Regions in the Oe and Be stars, 2003, Ap&SS, 284, 1119. (right click here and select "Save Target As..." to download .pdf file)

  70. Danezis, E., Lyratzi, Ε., Nikolaidis, D., Theodossiou, E., Stathopoulou, M., Drakopoulos, C. & Soulikias, A.: SACs phenomena in early type stars, JENAM 2003, August 25 - 31, 2003 Budapest, Hungary.

  71. Lyratzi, Ε., Danezis, E., Stathopoulou, M., Theodossiou, E., Nikolaidis, D., Antoniou, A., Drakopoulos, C., Soulikias, A. & Koutroumanou, M.: SACs phenomena in MgII regions of 40 BeV stars, JENAM 2003, August 25 - 31, 2003 Budapest, Hungary.

  72. Lyratzi, Ε., Danezis, E., Stathopoulou, M., Theodossiou, E., Nikolaidis, D., Antoniou, A., Drakopoulos, C., Soulikias, A. & Koutroumanou, M.: SACs phenomena in SiIV regions of 42 BeV stars, JENAM 2003, August 25 - 31, 2003 Budapest, Hungary. (right click here and select "Save Target As..." to download .pdf file)

  73. Danezis, E., Antoniou, A., Lyratzi, Ε., Theodossiou, E., Stathopoulou, M., Nikolaidis, D., Drakopoulos, C., Soulikias, A. & Koutroumanou, M.: The complex structure of hyperionization regions of the Oe star HD 175754, JENAM 2003, August 25 - 31, 2003 Budapest, Hungary. (right click here and select "Save Target As..." to download .pdf file)

  74. Danezis, E., Koutroumanou, M., Lyratzi, Ε., Theodossiou, E., Stathopoulou, M., Nikolaidis, D., Antoniou, A., Drakopoulos, C. & Soulikias, A.: The complex structure of SiIV regions of the Oe star λ Orionis, JENAM 2003, August 25 - 31, 2003 Budapest, Hungary. (right click here and select "Save Target As..." to download .pdf file)

  75. Lyratzi E., Danezis E., Nikolaidis D., Theodossiou E., Stathopoulou M., Drakopoulos C., Soulikias A. & Koutsouris P.: The complex structure of the expanding gaseous envelope of the Be star HD45910 (AX Mon), 210th Symposium of the International Astronomical Union, June 2002, Uppsala, Sweden. (right click here and select "Save Target As..." to download .pdf file)

  76. Lyratzi E., Danezis E., Nikolaides D., Theodossiou E., Stathopoulou M., Kyriakopoulou A. & Soulikias A.: The complex structure of Ha region in the gaseous envelope of Be V stars, 210th Symposium of the International Astronomical Union, June 2002, Uppsala, Sweden. (right click here and select "Save Target As..." to download .pdf file)

  77. Danezis E., Lyratzi E., Nikolaides D., Theodossiou E., Stathopoulou M., Christou G. & Soulikias A.: The complex structure of MgII region in the gaseous envelope of Be V stars, 210th Symposium of the International Astronomical Union, June 2002, Uppsala, Sweden. (right click here and select "Save Target As..." to download .pdf file)

  78. Danezis E., Lyratzi E., Christou G., Theodossiou E., Stathopoulou M., Nikolaides D. & Kyriakopoulou A.: Hyperionisation and emission phenomena in the coronal an post-coronal region of the moving atmosphere of the Oe star HD66811 (ζPup), 210th Symposium of the International Astronomical Union, June 2002, Uppsala, Sweden. (right click here and select "Save Target As..." to download .pdf file)

  79. Lyratzi, E., Danezis, E., Nikolaidis, D., Theodossiou, E., Stathopoulou, M., Drakopoulos, Ch. & Bourma, P.: Application of a new model of density shells of matter to the expanding gaseous envelope of the star HD 45910 (AXMonocerotis), 2001, hell.confE, 67, 5th Hellenic Astronomical Conference, September 2001, Crete, Greece. (right click here and select "Save Target As..." to download .pdf file)

  80. Danezis, E., Nikolaides, D., Lyratzi, E., Stathopoulou, M., Theodossiou, E., Kyriakopoulou, A., Christou, G., Kossionides, A. & Drakopoulos, C.: The complex structure of the expanding gaseous envelope of the star HD 164794 (9Sagittarii), 2001, hell.confE, 66, 5th Hellenic Astronomical Conference, September 2001, Crete, Greece. (right click here and select "Save Target As..." to download .pdf file)

  81. Danezis, E., Lyratzi, E., Theodossiou, E., Nikolaidis, D., Stathopoulou, M., Kiriakopoulou, A., Ghristou, G. & Drakopoulos, Ch.: Application of a new model of density shells of matter to the expanding gaseous envelope of the star HD 175754, 2001, hell.confE, 65, 5th Hellenic Astronomical Conference, September 2001, Crete, Greece. (right click here and select "Save Target As..." to download .pdf file)

  82. Danezis, E., Nikolaidis, D., Lyratzi, E., Stathopoulou, M., Theodossiou, E., Kosionidis, Α., Drakopoulos, Ch. & Soulikias, A.: Phenomena of hyperionization and emission in the gaseous envelope of the star λ Orionis, 2001, hell.confE, 64, 5th Hellenic Astronomical Conference, September 2001, Crete, Greece. (right click here and select "Save Target As..." to download .pdf file)

  83. Danezis, E., Christou, G., Stathopoulou, M., Theodossiou, E., Nikolaidis, D., Lyratzi, E., Drakopoulos, Ch., Bourma, E. & Soulikias, A.: Phenomena of hyperionization and emission in the gaseous envelope of the star HD 66811 (ζ Puppis), 2001, hell.confE, 63, 5th Hellenic Astronomical Conference, September 2001, Crete, Greece. (right click here and select "Save Target As..." to download .pdf file)

  84. Danezis E., Kiriakopoulou A., Nikolaidis D., Theodossiou E., Stathopoulou M., Lyratzi E., Ghristou G. & Kosionidis A.: A final consideration of the developing velocity fields in the gaseous envelope of the star HD 50138, 2001, hell.confE, 62, 5th Hellenic Astronomical Conference, September 2001, Crete, Greece. (right click here and select "Save Target As..." to download .pdf file)

  85. Danezis E., Nikolaidis D., Theodossiou E., Kosionidis A., Stathopoulou M., Lyratzi V., Draakopoulos C. & Bourma P.: A simple model for the complex structure of moving atmospheric shells in Oe and Be stars. The example of HD 36861, some first conclusions. JENAM 2000, May 29-June 3, 2000 Moscow, Russia. (click here for Abstract)

  86. Danezis E., Nikolaidis D., Theodossiou E., Kosionidis A., Stathopoulou M., Lyratzi V., Draakopoulos C. & Bourma P.: A simple model for the complex structure of moving atmospheric shells in Oe and Be stars. The example of HD 66811, some first conclusions. JENAM 2000, May 29-June 3, 2000 Moscow, Russia. (click here for Abstract)

  87. Danezis E., Nikolaidis D., Theodossiou E., Kosionidis A., Stathopoulou M., Lyratzi V., Draakopoulos C. & Bourma P.: A simple model for the complex structure of moving atmospheric shells in Oe and Be stars. The example of HD 152408, some first conclusions. JENAM 2000, May 29-June 3, 2000 Moscow, Russia. (click here for Abstract)

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