Dig Sites of Stellar Archeology: Giant Stars in the Milky Way Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 113-120 WHAT DO RADIAL VELOCITY MEASUREMENTS TELL ABOUT RV TAURI STARS? Timur Şahin 1*, David L. Lambert 2, Oktay Solakcı 3 1 Department of Space Sciences and Technologies, Akdeniz University, 07058, Antalya, Türkiye 2 W. J. McDonald Observatory, The University of Texas, Austin, Texas, USA 3 Deparment of Physics, Akdeniz University, 07058, Antalya, Türkiye Abstract: In this study, we observed some selected RV Tauri stars spectroscopically for radial velocity monitoring. The high-resolution (R min ~55.000) spectra were obtained with the 2.1m Otto Struve telescope of McDonald Observatory to make abundance analysis of these stars possible. Ultimately the spectroscopic data obtained for these stars will not only be used for radial velocity measurements but also for abundance analysis hence for metallicity update in order to explain nature of observed anomalies in these stars. Here, we give a brief description of the project and present some preliminary results. Keywords: Stars: Post-AGB stars 1. INTRODUCTION RV Tauri stars are yellow super-giant stars located along the instability strip in the Hertzsprung-Russell (H-R) diagram and 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 with different chemical characteristics compared to the stars on the AGB stage where nuclear reactions and (3 rd ) dredge-up processes take place. Some RV Tauri stars show anomalies in abundances of the refractory elements (e.g. Fe, Mg, Si, Al, Ti, Ca); this has been called a Gas-Dust Winnowing Effect. In the process of gas and dust winnowing, the refractory elements condensed onto dust grains and are removed from the system by the radiation pressure. Such observed depletion in the abundances of refractory elements is a common property of RV Tauri stars, however, it is not a universal property. The details for the separation of * Corresponding Author: Tel: +90 242 3102260 Fax: +90 242 3102260 E-mail: timursahin@akdeniz.edu.tr 113
refractory elements in these stars are well explained and accepted in the literature. This separation process may have a little effect on some of these stars. There are also situations where this effect could be misidentified and taken into account as if it is caused by nuclear reactions. In the literature, there is no unique model to explain observed abundance anomalies in these stars. Furthermore, some RV Tauri stars show a perfect correlation between their observed abundance anomalies caused by gas and dust separation and predicted condensation temperatures. For some of these stars, the correlation is seen to be very weak. (we can think of a situation where Ca and Al abundances are too low but without significant depletion in abundances of S and Zn). For the stars in this second group, there seems to be a relation between abundance anomaly and first ionization potentials of the elements. In fact, such a relation is observed in the Solar Corona and is known as FIP (First Ionization Potential) effect. Our hypothesis is that a relation with condensation temperatures indicates a star in a binary system while a relation based on FIP would indicate a single star that experiences strong stellar winds [2]. In other words, the FIP effect should be dominant in a single star case. Such a clear definition will, of course, only be possible with a detailed spectroscopic abundance analysis of several post-agb stars. Continuous and long-term radial velocity monitoring is another important and powerful tool to use to reveal possible binary nature of an RV Tauri star. Distinguishing orbital and pulsational characteristics of such stars remains to be a difficult step that one has to take. Thus, in this context, we performed a (relatively) long-term radial velocity monitoring for 49 RV Tauri-like (i.e. RV Tau, W Vir, RR Lyr, SR type, post-agbs) IRAS stars. Our intent is to provide new velocity measurements with a typical accuracy of <0.5 km s 1 at the current epoch for selected IRAS stars. Here, we present preliminary results on the radial velocity monitoring of RV Tauri stars. We also provide information on a dedicated website specially compiled for the program stars. The website includes details on our RadVELAS code to compute heliocentric radial velocities ([3]) and wavelength calibrations, model parameters based on color-temperature calibrations to initiate 114
spectroscopic analysis, a snap shot view of the stellar spectrum of interest as well as the radial velocities reported in the literature for the program star (Figs. 1, 2, and 3). Figure 1. View of a top-level message on the RadVELAS website. Figure 2. Example view the website summary of AU Peg observations. 115
2. OBSERVATIONS High-resolution spectroscopic observations of 49 RV Tauri type stars of the Northern sky listed in Table 1 were conducted in the period from 2008 to 2010. We made use of the 2.1 m Struve reflector telescope with the CCD-equipped Sandiford Cassegrain echelle spectrograph ([1]) at the W. J. McDonald Observatory. A typical spectrum has a spectral resolving power of about 55,000. Th-Ar comparison spectra were taken right before and after every program star observation. The quality of the data was monitored by using observations of the day sky and observing one or two RV-standard stars (from the Astronomical Almanac) per night. The adopted set up of the spectrograph enabled us to cover the 4800 5600 Å spectral range. The signalto-noise ratio (S/N) ranges between 30 and 250 per pixel, not only changing with the blaze function within echelle orders but also star brightness between echelle orders. Figure 3. Example view of the website temperatures and example spectrum. 116
3. DATA REDUCTION AND RADIAL VELOCITIES The observed (raw) spectra were reduced from two-dimensional to echelle format in a standard way with the ECHELLE package of NOAO IRAF1 using IRAF-CL scripts andin-house developed IDL routines. The reduction of science frames consisted of the standard steps of bias correction, cosmic-ray removal, flat fielding, scattered light subtraction, wavelength calibration, continuum normalization (see Fig. 2) and correction for the heliocentric velocity. The internal accuracy of the wavelength calibration via the Th-Ar lamp spectra was on average 0.002 Å (and not worse than 0.004 Å), which corresponds to an RV accuracy of order 109 m s 1 (and not worse than 218 ms 1 ). The blaze correction was performed in an automated manner using in-house IDL package RadVELAS, which is mainly designed to obtain spectroscopic radial velocities from echelle and/or long-slit spectra of RV Tauri type post-agb stars. Several processes including continuum normalization and merging are performed in an automated manner, without need for a user interaction. A continuum-normalized spectrum is cross-correlated with a mask that can be an either a stellar spectrum or a synthetic spectrum, which is degraded to the resolution of input stellar spectrum. Coefficients for fitted polynomials and peak value of crosscorrelation function are reported in a separate log file. Merged output spectra are also recorded as an SP2 file allowing one to further process the result spectrum in IDL (see [3] for details). Special care was taken during concatenation of spectral orders to a single spectrum. 4. PRELIMINARY RESULTS For this radial velocity monitoring project, all available radial velocity data in the literature for the program stars are compiled and merged with fresh radial velocities obtained with RadVELAS at the current epoch mainly to distinguish orbital and pulsational characteristics of the program stars. Here, in this section, we present IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation. 117
example radial velocity curves for selected program stars: TX Del (Fig 4), IX Cas (Fig 5), and CP Cep (Fig 6). A summary of the orbital parameters are also presented (Table 1, 2, and 3). Radial velocities for the whole sample await publication. The dedicated webpage is not public yet but will be available soon. Figure 4. Example radial velocity curve for TX Del (W Vir type). Eccentricity: 0.12+-0.06 Semi-Amplitude: 15+-1 km/s Systemic Velocity: 13+-1 km/s Orbital Period: 133.5 +-0.7 days Pulsational Period: 6 days Table 1. Summary of orbital parameters for TX Del. Figure 5. Example radial velocity curve for IX Cas (W Vir type). 118
Eccentricity: 0.05+-0.06 Semi-Amplitude: 32+-3 km/s Systemic Velocity: -102+-2 km/s Orbital Period: 110 +-1 days Pulsational Period: 9 days Table 2. Summary of orbital parameters for IX Cas. Figure 6. Example radial velocity curve for CP Cep (δ Cep type). Eccentricity: 0.25+-0.04 Semi-Amplitude: 21+-1 km/s Systemic Velocity: -41+-1 km/s Orbital Period: 18 +-1 days Pulsational Period: 9 days Table 3. Summary of orbital parameters for CP Cep. 119
ACKNOWLEDGMENT TS acknowledge the financial support by TUBITAK (TÜBİTAK TBAG- 1001; Project No: 111T219). Thanks to Dr. Ahmet DERVIS for his help in obtaining orbital parameters. REFERENCES [1] J. K. McCarthy, B. A. Sandiford, D. Boyd, and J. Booth, "The Sandiford 2.1-m Cassegrain echelle spectrograph for McDonald Observatory - Optical and mechanical design and performance", Publ. Astron. Soc. Pac., vol 105, pp. 881-893, 1993. [2] N. K. Rao and B. E. Reddy, E. B., "High-resolution spectroscopy of the high galactic latitude RV Tauri star CE Virginis,", Mon. Not. Roy. Astron. Soc., vol. 357, pp. 235-241, 2005. [3] T. Şahin, Radial Velocity Analysis Software RadVELAS," XVIII. National Astronomy and Space Sciences Meeting, İnönü Üniv., Malatya, Inonu University Publications, 2012. 120