CHEMICAL ABUNDANCE ANALYSIS OF RC CANDIDATE STAR HD (46 LMi) : PRELIMINARY RESULTS
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1 Dig Sites of Stellar Archeology: Giant Stars in the Milky Way Ege Uni. J. of Faculty of Sci., Special Issue, 2014, CHEMICAL ABUNDANCE ANALYSIS OF RC CANDIDATE STAR HD (46 LMi) : PRELIMINARY RESULTS Betül Civelekler * Istanbul University Astronomy and Space Sciences Department, Istanbul, Türkiye Abstract: In this study, elemental abundances in the atmosphere of HD (K0 III) will be analyzed. This star is selected from Catalog of Red Clump (RC) candidates in Tycho-2. It is a RC candidate with a probability of 0.68 from photometry. The star was observed on March 30-31, 2012 with the RTT 150 telescope of TÜBİTAK National Observatory, using its Coudé spectrograph. The resolving power was and the S/N ~ 200. The analysis of the star is at the stage of equivalent width measurements and line identification. Keywords: red clump; abundance analysis; HD INTRODUCTION After the main sequence, low-mass stars ascend to the Red Giant Branch (RGB) with a degenerate He-core. After the He-flash phase, degeneracy is removed and the star begins quiescent He-core burning. These low-mass stars form a dense aggregation to the left from the RGB in H-R diagram, and become Red Clump (RC) giants. All the low-mass stars have similar core masses at the beginning of He- burning. Hence, they have similar luminosities and thus their absolute magnitudes are nearly constant. Due to this fact they are very useful indicators of ages and distances for clusters and galaxies. It is necessary to investigate these RC stars to better understand mixing processes in evolved low-mass stars. Moreover, it s important to separate which stars of the RC region in the HR diagram are H-core and which are in the He-core burning stages of evolution. The stars that are ascending for the first time to the RGB and returning back from the Helium Flash phase occupy the same region in the Color Magnitude Diagram. * Corresponding Author: betulcivelekler@gmail.com 145
2 Thus it is very difficult to distinguish these stars only using photometric criteria. This distinction may be made by the spectroscopic analysis of a star. The atmospheric compositions of He-core burning stars are affected by interior nucleosynthesis and envelope convection in RGB. For example, separation from first-ascent giants is possible to investigate carbon isotope abundances. At the same time, elemental abundances of RC stars in general can play important role in the study of Galactic kinematics and chemistry. 2. OBSERVATıON AND REDUCTıON The star 46 LMi has been selected from Catalog of Red Clump candidates in Tycho- 2 as a Red Clump (RC) candidate star, with 0.68 probability of this classification from photometry. This is the only RC candidate star in the catalog that has suitable brightness (V=3.82 m ) for high quality spectroscopic observations from TÜBİTAK National Observatory. The star was observed with RTT 150 telescope of TÜBİTAK National Observatory using the Coudé echelle spectrograph in March 30-31, The resolving power is and the S/N ~ 200. Wavelength range is 3000 Å Å. Bias and dark subtraction has been applied to the CCD frames by using Maxim DL software [1]. Then a mask is created for spectrum extraction with the aid of DECH 95 image processing software [2]. This reduction program passes a polynomial fit to marked points in order to determine the location of each spectral order, taking into account the given order aperture width (see Fig. 1). This process allows one to inspect each order visually. During the process the program recalculates counts of frames vertically according to the marked points and adds these counts to the fit in the center of each order and thus improves the S/N in the resulting spectrum. The DECH 20 [2] program is used for continuum fitting. Because the program allows visual inspection of the reduction steps, the user can intervene if necessary. For example, one can apply smooth splines to determine the position of continuum points easily. All these steps are applied to the seven original observations of 46 LMi, and then the spectra were combined to improve the total S/N, which we estimate to be about 200. A dispersion curve was created using a ThAr lamp. The flat correction procedure was 146
3 applied only to orders affected by CCD fringing. After subtraction of cosmic ray events, the spectrum was normalized to the continuum. Figure 1. Creating spectrum mask for the original CCD observation frame. 3. MEASUREMENT OF EQUIVALENT WIDTHS Before the measurement of equivalent widths, a line list was obtained from the VALD [3] database, adopting approximate T eff (4750 K) and log g (3.0) values of the star in the literature for wavelength range of our spectra. Because the star is a K giant, its spectrum has numerous molecular bands. Many metal lines are blended with these molecular bands, which make the line identification process and equivalent width measurements more complicated. For proper line identification and measurement of equivalent width, a synthetic spectrum was generated with SYNTHE [4, 5] program. Thus, the positions of molecular bands and other significant contaminants were determined (see Fig.2). ATLAS9 [4, 5] was used for model atmosphere calculations. 147
4 Figure 2. Comparison of the observed spectrum with synthetic spectrum. For equivalent width measurements, the spectool package in IRAF [6] was used. First, a line profile was obtained for each component of a blended feature. Then, a Gaussian fit to each line was applied to yield the measured equivalent widths (see Fig. 3). 148
5 Figure 3. Measurement of equivalent width of the λ 8526 Fe I line. In the top panel we show the output of the IRAF spectool multi-line fit. The bottom panel shows a comparison between synthetic and observed spectra of this line. The measured EW is 103 må. 149
6 Our aim is to measure numerous lines visually by separating blends and then detailed analysis of the 46 LMi spectrum with the aid of spectrum synthesis and equivalent width technique. Detailed atmospheric abundance analyses for RC giants are rare. Therefore, our study would be an important source for the related literature. REFERENCES [1] [2] [3] [4] R. L. Kurucz, 1993, CD-ROM 13, 18 [5] L. Sbordone, P. Bonifacio, F. Castelli, and R. L. Kurucz, ATLAS and SYNTHE under Linux, Memorie della Societa Astronomica Italiana Supplement, Vol.5, pp , [6] 150
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