Browsing by Author "Walker, A."

Now showing 1 - 4 of 4
  • No Thumbnail Available
    Item
    On the Use of Field RR Lyrae as Galactic Probes. II. A New ΔS Calibration to Estimate Their Metallicity*
    (2021) Crestani, J.; Fabrizio, M.; Braga, V. F.; Sneden, C.; Preston, G.; Ferraro, I.; Iannicola, G.; Bono, G.; Alves-Brito, A.; Nonino, M.; D'Orazi, V.; Inno, L.; Monelli, M.; Storm, J.; Altavilla, G.; Chaboyer, B.; Dall'Ora, M.; Fiorentino, G.; Gilligan, C.; Grebel, E. K.; Lala, H.; Lemasle, B.; Marengo, M.; Marinoni, S.; Marrese, P. M.; Martinez-Vazquez, C. E.; Matsunaga, N.; Mullen, J. P.; Neeley, J.; Prudil, Z.; da Silva, R.; Stetson, P. B.; Thevenin, F.; Valenti, E.; Walker, A.; Zoccali, M.
    We performed the largest and most homogeneous spectroscopic survey of field RR Lyraes (RRLs). We secured 6300 high-resolution (HR, R similar to 35,000) spectra for 143 RRLs (111 fundamental, RRab; 32 first-overtone, RRc). The atmospheric parameters were estimated by using the traditional approach and the iron abundances were measured by using an LTE line analysis. The resulting iron distribution shows a well-defined metal-rich tail approaching solar iron abundance. This suggests that field RRLs experienced a complex chemical enrichment in the early halo formation. We used these data to develop a new calibration of the Delta S method. This diagnostic, based on the equivalent widths of Ca ii K and three Balmer (H-delta,H-gamma,H-beta) lines, traces the metallicity of RRLs. For the first time, the new empirical calibration: (i) includes spectra collected over the entire pulsation cycle; (ii) includes RRc variables; (iii) relies on spectroscopic calibrators covering more than three dex in iron abundance; and (iv) provides independent calibrations based on one/two/three Balmer lines. The new calibrations were applied to a data set of both SEGUE-SDSS and degraded HR spectra totalling 6451 low-resolution (R similar to 2000) spectra for 5001 RRLs (3439 RRab, 1562 RRc). This resulted in an iron distribution with a median eta = -1.55 0.01 and sigma = 0.51 dex, in good agreement with literature values. We also found that RRc are 0.10 dex more metal-poor than RRab variables, and have a distribution with a smoother metal-poor tail. This finding supports theoretical prescriptions suggesting a steady decrease in the RRc number when moving from metal-poor to metal-rich stellar environments.
  • No Thumbnail Available
    Item
    The RR Lyrae Instability Strip in the Split Horizontal Branch Globular Cluster NGC 6569
    (2015) Kunder, A.; Stetson, P. B.; Catelan, Marcio; Walker, A.; Cassisi, S.; Johnson, C.; Soto, M.
    The first calibrated broadband BVI time-series photometry is obtained for the RR Lyrae variable stars in the split horizontal branch (HB) cluster NGC 6569. As a result, a sizeable population of 27 RR Lyrae variables has been identified. An eclipsing binary in the RR Lyrae instability strip was also found, which is most likely a field star. The 12 fundamental-mode RR Lyrae variables (RR0) have an average period of [P]RR0= 0.57±0.02 d and the 15 first-overtone variables (RR1) have an average period of [P]RR1=0.30±0.01 d. The mean periods of the RR Lyrae stars are consistent with an Oosterhoff I designation, and the number ratio of RR1- to RR0-type variables, N1/NRR=0.56, is more consistent with an Oosterhoff II designation. Compared to the M3 variables, the NGC 6569 RR Lyrae variables are not shifted to a longer periods at a given amplitude, making it unlikely that these stars are He-enhanced. The similarity in the periods and amplitudes between the RR Lyrae stars in NGC 6569 and the bulge field RR Lyrae population may indicate similar formation conditions.
  • Thumbnail Image
    Item
    The rr lyrae variables and horizontal branch of NGC 6656 (m22)
    (2013) Kunder, A.; Stetson, P.; Cassisi, S.; Layden, A.; Bono, G.; Catelan, Márcio; Walker, A.; Paredes Alvarez, L.; Clem, J.; Matsunaga, N.; Salaris, M.; Lee, J.; Chaboyer, B.
  • Thumbnail Image
    Item
    Theoretical modeling of the RR Lyrae variables in NGC 1851
    (2015) Kunder, A.; Salaris, M.; Cassisi, S.; de Propris, R.; Walker, A.; Stetson, P. B.; Catelan, Marcio; Amigo, P.
    The RR Lyrae instability strip (IS) in NGC 1851 is investigated, and a model is presented which reproduces the pulsational properties of the RR Lyrae population. In our model, a stellar component within the IS that displays minor helium variations (Y∼0.248-0.270) is able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae variables. The RR Lyrae variables therefore may belong to an He-enriched second generation of stars. The RR Lyrae variables with a slightly enhanced helium (Y∼0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y∼0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables. %.