Akdeniz Üniversitesi

Akdeniz University High Resolution Stellar Spectroscopy Group - HRSS

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High Resolution Spectroscopy of Highly Evolved Stars such as HD 179821:

       Unambiguous assignment of a F­ G supergiant to the proper evolutionary path is not always immediately possible, even when a wide variety of observational techniques are applied and the electromagnetic spectrum is well sampled. For instance, HD   179821,   also   known   as   V1427   Aql   and   IRAS   19114+0002,   remains   a   supergiant   of uncertain evolutionary status despite a lengthy literature and frequent investigations into its origin.   Zacs   et   al.   (1996)   and   Reddy   &   Hrivnak   (1999)   used   their   measurements   of overabundances of s­process nuclides to depict its origin as a post­AGB star. Others have  used the star’s distance implied by its radial velocity and characteristics of its circumstellar gas and dust shell to conclude that the star is a massive supergiant: see, for example, Jura & Werner (1999) and Jura, Velusamy, & Werner (2001), Oudmaijer et al. (2009).

Some references:

"HD 179821 (V1427 Aql, IRAS 19114+0002) – a massive post-red supergiant star?" (pdf)

               













Fig. Line profile variations in the spectrum of HD 179821.

             

Fig. Line profile variations in the spectrum of HD 179821.

             

Fig. Hyrogen alpha variation in the spectrum of HD 179821.

            

Fig. Line profile variations in the spectrum of HD 179821 for low and high excitation lines.

            

Fig. Line profile variations in the spectrum of HD 179821 for BaII line at 6142 A.

 


 

High Resolution Spectroscopy of Post-AGB stars:

Why studying Post-AGB stars?

       Stellar spectroscopy allows us to determine the properties and chemical compositions of the stars. From this information for stars of different ages in the Milky Way, it is possible to reconstruct the chemical evolution of the Galaxy, as well as the origin of the elements heavier than boron, created mainly in stellar interiors. It is also possible to study star formation, and the formation of the Galaxy, from the signature of the Galactic potential in the stellar orbits, as well as the distributions of mass, ages and the abundance of heavy elements.

HD102870

Obtaining the high-resolution spectra is also necessary for studies of chemical compositions that clearly require advanced and efficient instrumentation. This is particularly true for research that calls for large stellar samples, and that demands the observation of hundreds of sources simultaneously. Efficiency requires that the data processing and analysis are performed in an automated way.

chi

The interpretation of stellar  spectra is based on physical models of the stellar atmospheres, from where the light that we observe originates. The main ingredients for building such model atmospheres are fluid dynamics, and the properties of atoms, ions and molecules, especially with regard to their interactions with radiation from the stellar interior. Once we have a plausible model, it is possible to compute in detail how the radiation propagates through the stellar atmosphere. The emergent spectrum can then be iteratively compared with observations to refine the best matching model.

Post-AGB stars, at AGB evolutionary stage, produce a considerable fraction of the heavy elements present in the interstellar medium which set the basis for the formation of a new generation of stars. High-resolution spectrographs operating on large telescopes allow us to directly measure the abundance of these elements. Although the spectra of AGB stars are dominated by molecular bands, often preventing the analysis of individual atomic lines, post-AGB stars provide a unique opportunity to monitor chemical history of their progenitors (AGB stars).

In our
newly forming High Resolution Stellar Spectroscopy Group at Space Sciences and Technologies Department in Akdeniz University, Antalya-Turkey, we aim to develope tools for data reduction and the analysis of spectroscopic observations (see the section: Tools), in particular for the determination of chemical abundances in stars in an automated way. That means less time for computation and more time for science.

postagb

Some references:

  • "High Resolution Echelle Spectroscopy of Two High Proper Motion Stars: HD 102870 and BD+20 3603", 2014arXiv1407.5375O
  • "Spectroscopic followup of three bright halo stars selected from SDSS and GALEX photometry", 2013arXiv1303.4853S
  • "High-resolution optical spectroscopy of the F supergiant protoplanetary nebula IRAS 18095+2704", 2011MNRAS.410..612S
  • "Abundance analyses of helium-rich subluminous B stars", 2010MNRAS.409..582N
  • "Abundance analyses of three helium-rich sdB stars", 2010Ap&SS.329..119N
  • "High-resolution optical spectroscopy of a newly discovered post-AGB star with a surprising metallicity in the globular cluster M79", 2009MNRAS.398.1730S
  • "An abundance analysis of a chemically peculiar B star - JL 87", 2007A&A...465..541A

Variability and Chemistry of Post-AGB stars

     During AGB stage, post-AGB stars lose mass at a rather high rate. Thus, they are among the major contributors of heavy elements to the interstellar medium for the formation of new stars and planets. AGB stars are important sources of elements produced via the s-process and of dust which forms in the outer part of their atmosphere. AGB stars are also the precursors of beautiful, mostly a-spherical PNe, but the transition is poorly understood: is the shape the result of binary interaction or is it due to intrinsic properties of the AGB progenitor? Recent results indicate that circumstellar envelopes already become highly a-spherical during the AGB phase, possibly under the influence of a strong magnetic field. Polarimetry of a large sample of stars can provide important clues about when the AGB star envelopes become spherical, as polarisation is a measure of asymmetry of the scattering particles around the star.

Some references:

 Radial velocity measurements of RV Tauri like IRAS objects   (Project Page)

       RV Tauri stars are yellow super-giant stars located along the instability strip in the Hertzsprung-Russell (H-R) diagram. They are characterized by shallow and deep minima with periods ranging from 20 to 150 days. When compared to AGB stars, they are seen to be located in the higher temperature region of the H-R diagram and they show different chemical characteristics compared to the stars on the AGB stage at which nuclear reactions and dredge-up (3rd) processes take place (Van Winckel 2003). Moreoever, some of these stars show anomalies in abundances of the refractory elements (e.g. Fe, Mg, Si, Al, Ti, Ca; Gas-Dust Winnowing Effect; the stars with solar CNO abundances and [Fe/H]=-4.8 ).

The evolution of posts-AGB stars including RV Tauri stars are very fast so that statistical studies on these stars are rare and, most of the time, are not enough and very valuable.

radial

Kinematics and Orbits:

The study of unevolved late-type stars belonging to different Galactic populations (the halo, the thick and thin disks) with large proper motions is fundamental for our knowledge of the Galaxy’s stellar content as well as for reconstructing its chemical evolution.

toomre

The Toomre energy diagram for program stars from (Nissen & Schuster
2010) and (Bensby et al. 2005) with 54 HPM stars from the ELODIE library (in pink).

Late-type, metal-poor stars (the F to K type dwarfs such as BD +20 3603) with narrow and less blended spectral lines in their spectra are excellent probes at medium resolution of the chemical history of the earliest stellar populations. Large ongoing surveys of such stars, such as HERMES/GALAH, ESO-Gaia and RAVE depend on accurately calibrated stellar parameters from such stars.

orbit1

Computed orbit for ELODIe HPM star HD 000693.

We are currently undertaking a program of determining abundances for a large number of elements from High Proper Motion (here after HPM) stars and corellating these with the stellar Galactic orbital parameters.

orbit2

Computed orbit for ELODIe HPM star HD 006755. The star seems to have a highly eccentric orbit with e=0.47.

orbit3

 Computed orbit for ELODIe HPM star HD 043318.

Some references:

  • "High Resolution Echelle Spectroscopy of Two High Proper Motion Stars: HD 102870 and BD+20 3603", 2014arXiv1407.5375O

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