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J/AZh/83/265  Optical spectral variability of IRAS 20508+2011 (Klochkova+, 2006)
Detection of spectral variability of the optical component of the IR-source
IRAS 20508+2011.
    Klochkova V.G., Panchuk V.E., Tavolganskaya N.S., Zhao G.
   <Astron. Zh. 83, 265 (2006)>
   =2006AZh....83..265K
   =2006ARep..50...232K
ADC_Keywords: Stars, atmospheres; Spectroscopy; Abundances


Abstract:
    Based on high-resolution spectra we revealed variability of the
    optical spectrum of the cool star identified with the IR source
    IRAS 20508+2011. Over the five years of our observations, the radial
    velocity derived from photospheric absorption lines varied in the
    interval Vr=15-30km/s. In the same time, the H(alpha) profile varied
    from an intense bell-shaped emission line with a small absorption to
    2-peaked emission with a central absorption feature below the
    continuum level. At all but one epoch, the positions of the metallic
    photospheric lines were systematically shifted relative to the
    H(alpha) emission: Delta Vr=Vr(met)-Vr(Halpha,emis)~-23km/s. The
    NaD doublet lines shown a complex profile with broad (half-width about
    120km/s) emission and photospheric absorption, as well as an
    interstellar component. We used model atmospheres to determine the
    physical parameters and chemical composition of the star's atmosphere:
    Teff=4800K, logg=1.5, Vt=4.0km/s and metallicity [Fe/H]=-0.36. We
    detected overabundances of oxygen [O/Fe]=+1.79 (with the ratio
    [C/O]~-0.9), and alpha-process elements, as well as a deficit of heavy
    metals. The totality of the parameters suggests that the optical
    component of IRAS 20508+2011 is an "O-rich" AGB star with luminosity
    Mv~-3mag that is close to its evolution transition to the post-AGB
    stage.
Objects:
    ---------------------------------------------------------------
       RA   (2000)   DE        Designation(s)
    ---------------------------------------------------------------
     20 53 08.68 +20 22 37.7   IRAS 20508+2011 = TYC 1655-558-1
    ---------------------------------------------------------------

File Summary:

       FileName      Lrecl  Records   Explanations

ReadMe            80        .   This file
table3.dat        40      375   Identification of lines and their intensities
                              in spectrum s25309 of IRAS 20508+2011,
                              along with the derived elemental abundances


Table 1: Spectral observations of IRAS 20508+2011 with the 6m SAO telescope
--------------------------------------------------------------------------------
 Spectrum    Date         UT     Delta_lambda, Å   Spectrograph
--------------------------------------------------------------------------------
 s25309    28.07.1999    21/50        4550-7930          PFES
 s28208    04.07.2000    23/13        4300-7820          PFES
 s28412    06.07.2000    23/35        4320-7825          PFES
 s40402    10.08.2003    23/05        5270-6760          NES
 s43223    27.08.2004    22/51        5270-6760          NES

Table 2: Mean heliocentric radial velocities for line groups and some individual
          lines derived from IRAS 20508+2011 spectra obtained on various dates.
          Uncertain values are given by colon.
--------------------------------------------------------------------------------
                                           RV, km/s
 Spectral features -------------------------------------------------------------
                     28.07.99  04.07.00   06.07.00   10.08.03   27.08.04
--------------------------------------------------------------------------------
 Absorption lines:
 -----------------
 metallic              15.5      29.8       27.1       20.3       29.9
 H(alpha)              23.1      38.6       40.7       21.4       38.0
 H(beta)               16.5      27.0       24.4
 H(gamma)                        27.6       23.7

 Emission lines:
 -----------------
 H(alpha)              39.3      51.6       57.1:      43.1       27.0
 I.S. NaI(D1,D2)       -4.6      -5.9       -5.5       -7.7       -7.3

Byte-by-byte Description of file: table3.dat

   Bytes Format Units   Label       Explanations

   1-  7  F7.2  0.1nm   Lambda      Wavelength expressed in Angstroem (Å)
  10- 13  A4    ---     Ion         Ion name and ionisation stage
  16- 20  F5.2  eV      EP          Excitation potential of the lower level
  23- 27  F5.2  [-]     loggf       Oscillator strength (loggf)
  30- 32  I3    0.1pm   EW          Equivalent width, in mÅ
  35- 40  F6.2  [-]    log(eps(X))  Chemical composition log(epsilon(X))
                                    (log(epsilon(H))=12.0)

(End)         Veta Avedisova [INASAN]                                26-Jan-2007