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L155 the ROTATION of SUBPOPULATIONS in Q The Astrophysical Journal, 661:L155–L158, 2007 June 1 ൴ ᭧ 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE ROTATION OF SUBPOPULATIONS IN q CENTAURI1 E. Pancino,2 A. Galfo,3 F. R. Ferraro,3 and M. Bellazzini2 Received 2007 March 1; accepted 2007 April 19; published 2007 May 8 ABSTRACT We present the first result of the Ital-FLAMES survey of red giant branch (RGB) stars in q Cen. Radial velocities with a precision of ∼0.5 km sϪ1 are presented for 650 members of q Cen observed with FLAMES- GIRAFFE at the Very Large Telescope. We found that stars belonging to the metal-poor (RGB-MP), metal- intermediate (RGB-MInt), and metal-rich (RGB-a) subpopulations of q Cen are all compatible with having the same rotational pattern. Our results appear to contradict past findings by Norris et al., who could not detect any rotational signature for metal-rich stars. The slightly higher precision of the present measurements and the much larger sample size, especially for the stars richer in metals, appear as the most likely explanations for this discrepancy. The result presented here weakens the body of evidence in favor of a merger event in the past history of q Cen. Subject headings: globular clusters: individual (NGC 5139) — techniques: radial velocities Online material: machine-readable table 1. INTRODUCTION richer in metals tend to be concentrated in the cluster center, which is dynamically hotter. The giant globular cluster (GC) q Centauri is one of the most studied objects in the Milky Way since the 1960s, thanks 2. OBSERVATIONS AND DATA TREATMENT to its many peculiar properties. As its main anomaly, q Cen A sample of ∼700 red giants was selected from the wide field ∼ shows a wide abundance spread ( 1 dex) not only in the light photometry originally published by Pancino et al. (2000) and Pan- elements but also in the iron-peak elements (Norris & Da Costa cino (2003) to derive accurate chemical abundances of stars span- 1995; Norris et al. 1996; Suntzeff & Kraft 1996; Smith et al. ning the whole metallicity range of q Cen. Special care was taken 2000), pointing toward a chemical evolution history more rem- to include a large fraction of high-metallicity stars, which are the iniscent of a dwarf galaxy than a genuine GC. Moreover, its less studied up to now. All program stars are distributed within structural, kinematical, and dynamical properties appear atyp- 15Ј from the cluster center. Figure 1 shows the program stars, ical for a Galactic GC, having properties more similar to dwarf marked on the B versus (B Ϫ I ) color-magnitude diagram (CMD) elliptical galactic nuclei (Mackey & van den Bergh 2005) and of Pancino et al. (2000). Following the classification scheme of being also an elongated (Geyer et al. 1983; Pancino et al. 2003), Pancino et al. (2000, 2003), we divide the stars into three groups: rotating system (Merritt et al. 1997) that is not completely the metal-poor population (RGB-MP) comprising 75% of the clus- relaxed dynamically (Anderson 1997; Ferraro et al. 2006). ter giants, with [Fe/H] ∼ Ϫ1.6; the metal-intermediate population Among the most recent, puzzling results, we list the detection (RGB-MInt) comprising 20% of the cluster giants, with [Fe/H] ∼ of complex substructures in all evolutionary sequences, including Ϫ1.2; and the metal-rich or anomalous stars (RGB-a) comprising the red giant branch (RGB; Lee et al. 1999; Pancino et al. 2000; 5% of the cluster giants, with [Fe/H] Ӎ Ϫ0.6 (Pancino et al. 2002). Sollima et al. 2005a), the subgiant branch and turnoff region Sample sizes are shown in Table 2 below. (Ferraro et al. 2004; Sollima et al. 2005b), and the main sequence Observations were done with FLAMES (Pasquini et al. 2002) (Bedin et al. 2004; Piotto et al. 2005). These studies have raised at the ESO VLT in Paranal, Chile, between 2003 May 22 and 28, new questions that need to be answered, such as the debated age within the Guaranteed Observing Time of the Ital-FLAMES Con- differences among subpopulations (Hilker & Richtler 2000; sortium. FLAMES was used in Medusa-combined mode, feeding Hughes & Wallerstein 2000; Sollima et al. 2005b), the possibility 8 fibers to UVES and 132 fibers to GIRAFFE. To ensure the that some subpopulations possess an extremely high helium con- maximum homogeneity in data quality and treatment, here we tent (Norris 2004; Piotto et al. 2005), and others. present the first results obtained with GIRAFFE only, using the More related to the present Letter, we point out a very in- high-resolution (R Ӎ 22,500) grating HR13 (6120–6395A˚ ) and teresting study on the correlations between the chemical and reaching a signal-to-noise ratio of Ӎ50–100 pixelϪ1, depending kinematical properties of q Cen carried out by Norris et al. on the star magnitude. (1997) in which stars in the cluster were divided into two groups Data were reduced with the GIRAFFE BLDRS (Base-Line Data based on their Ca abundance. They showed that stars poorer Reduction Software),4 which includes cosmic-ray removal, bias in metals rotate the same as the majority of stars in the cluster subtraction, flat-field correction, wavelength calibration, and pixel do, while stars richer in metals do not show any sign of rotation. resampling. The version of the pipeline that we used does not They also showed that the radial velocity dispersion appears include interorder background or sky subtraction, but these have to decrease with metallicity, a very strange behavior since stars no significant effect on radial velocity determinations. Radial velocities were determined using DAOSPEC5 (P. B. 1 Based on data obtained with the Giraffe-FLAMES facility of ESO Very Stetson & E. Pancino 2007, in preparation), a program that Large Telescope during the Ital-FLAMES GTO program 71.D-0217(A). Also based on data from the VALD and GEISA databases. automatically measures equivalent widths of absorption lines 2 INAF–Bologna Observatory, via Ranzani 1, I-40127 Bologna, Italy; [email protected]. 4 Available at http://girbldrs.sourceforge.net. 3 Astronomy Department, Bologna University, via Ranzani 1, I-40127 Bo- 5 Available at http://cadcwww.hia.nrc.ca/stetson/daospec/ and http://www.bo logna, Italy. .astro.it/∼pancino/projects/daospec.html. L155 L156 PANCINO ET AL. Vol. 661 correction using the rvcorrect task within IRAF.8 The re- sulting heliocentric radial velocities have a typical uncertainty of 0.5 km sϪ1 and are reported in Table 1. Contaminating field stars were easily eliminated since their -38.5kmsϪ1, in very good agree ע average velocity is Ϫ4.0 ment with Galactic model predictions for disk stars (Robin et al. 2003) and very different from the typical velocity of q Cen. ≤ Ϫ1 After removing all stars withVr 190 km s , the final sample p contains 649 cluster members, with an averageVr 233.4 and p Ϫ1 jr 13.2 km s . Star by star comparisons with other catalogs yield a very good agreement. For instance, the average radial velocity difference of the 136 stars in common with Mayor et Ϫ1 ע p al. (1997) isDVr 0.4 1.4 km s , while for the 53 stars in ע p Ϫ common with Suntzeff & Kraft (1996) it is DVr 0.5 1.8 km sϪ1, and for the 382 stars in common with Reijns et al. Ϫ1 ע p Ϫ (2006) it isDVr 2.4 4.6 km s . 3. RESULTS We used the photometric definition of populations described in § 2, together with our radial velocity measurements, to con- struct rotation curves for subpopulations in q Centauri. To this aim, we first deprojected right ascension and declination into Fig. 1.—Wide-field CMD of q Cen from Pancino et al. (2000). Stars ob- XY- and -coordinates with the following relations (see also served with GIRAFFE are highlighted: RGB-MP stars with dark gray circles, van de Ven et al. 2006), suited for extended objects that are RGB-MInt with light gray diamonds, and RGB-a stars with white triangles. not close to the celestial equator: p Ϫ Ϫ in high-resolution spectra. Radial velocities are determined by X r00cos d sin (a a ), the cross-correlation of the measured line centers and a set of laboratory wavelengths. We used 150 clean and unblended lines 6 p Ϫ Ϫ of the most common species obtained from the VALD (Vienna Y r00[sin d cos d cos d sin d 0cos (a a 0)], Atomic Line Database; Kupka et al. 1999). Observed radial velocities are the average of the velocity obtained for each line where r p 10,800/ is the scale factor to have XЈ and YЈ in ͱ 0 p after a 3 j clipping, and the associated uncertainty isj/ n , arcminutes. where n is the number of lines used. Typical uncertainties are As a second step, we searched for the orientation of the Ϫ1 of the order of 0.13 km s . rotation axis v that maximizes the amplitude of the rotation Since radial velocities were not the main goal of the ob- signal. We found a relatively broad maximum aroundv p 0 , serving program, simultaneous calibration lamps were not and thus, for simplicity, we adopt a rotation axis aligned with taken during the observations. We used laboratory wavelengths the north-south direction, which corresponds to the YЈ axis and ˚ of the telluric lines of the 6300A O2 absorption band from the to the minor isophotal axis (Geyer et al. 1983; Pancino et al. GEISA7 (Gestion et Etude des Informations Spectroscopique 2003). We therefore plotted radial velocities of each subpop- Atmospheriques; Jaquinet-Husson et al.
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