On the Use of Field RR Lyrae as Galactic Probes. I. The Oosterhoff Dichotomy Based on Fundamental Variables*

Abstract

We collected a large data set of field RR Lyrae stars (RRLs) by using catalogues already available in the literature and Gaia DR2. We estimated the iron abundances for a sub-sample of 2,382 fundamental RRLs (∆S method: Ca II K, Hβ, Hγ and Hδ lines) for which are publicly available medium-resolution SDSS-SEGUE spectra. We also included similar estimates available in the literature ending up with the largest and most homogeneous spectroscopic data set ever collected for RRLs (2,903). The metallicity scale was validated by using iron abundances based on high resolution spectra for a fundamental field RRL (V Ind), for which we collected X-shooter spectra covering the entire pulsation cycle. The peak ([Fe/H]=-1.59±0.01) and the standard deviation (σ=0.43 dex) of the metallicity distribution agree quite well with similar estimates available in the literature. The current measurements disclose a well defined metal-rich tail approaching Solar iron abundance. The spectroscopic sample plotted in the Bailey diagram (period vs luminosity amplitude) shows a steady variation when moving from the metal-poor ([Fe/H]=-3.0/-2.5) to the metal-rich ([Fe/H]=-0.5/0.0) regime. The smooth transition in the peak of the period distribution as a function of the metallicity strongly indicates that the longstanding problem of the Oosterhoff dichotomy among Galactic globulars is the consequence of the lack of metal-intermediate clusters hosting RRLs. We also found that the luminosity amplitude, in contrast Corresponding author: Michele Fabrizio michele.fabrizio@ssdc.asi.it * Partially based on observations collected under ESO programmes 297.D-5047 (PI. G. Bono) and 083.B-0281 (PI. D. Romano). arXiv:1908.02064v2 [astro-ph.SR] 7 Aug 2019 M. Fabrizio et al.with period, does not show a solid correlation with metallicity. This suggests that period-amplitudemetallicity relations should be cautiously treated.

Humps and bumps: the effects of shocks on the optical light curves of fundamental-mode RR Lyrae stars

“…For the Oo II group the median value of the amplitude of the hump is equal to (12 ± 5) · 10 −2 , and for the Oo I population its value is (10 ± 5) · 10 −2 . The median value for the amplitude o the bump is again similar for both Oo types, (6 ± 1) · 10 −2 (Oo I stars) and (5 ± 2) · 10 −2 (Oo II stars), which is in agreement with the population effect of the Oosterhoff dichotomy as suggested by Fabrizio et al (2019).…”

“…Several possible solutions emerged over the past decades, e.g., increased helium abundance for RR Lyrae stars in Oosterhoff typeİI (Oo II) clusters Sandage 1981) or a different direction in crossing the instability strip (van Albada & Baker 1973). In more recent years some progress has been made suggesting that population and/or metallicity effects might be the cause of the Oosterhoff dichotomy, e.g., Catelan (2009); Lee et al (2014); Fabrizio et al (2019), especially among field RR Lyrae variables.…”

Evolutionary and pulsation properties of Type II Cepheids

“…A.1 show that field TIICs cover a broad range in metallicity, from [Fe/H]∼-2 to [Fe/H]∼0.3. The distribution appears to be similar to field RRLs (Magurno et al 2018;Fabrizio et al 2019), suggesting a similar evolutionary channel. Moreover, BLHs, WVs and RVTs appear to have very similar metallicity distributions.…”

On the Use of Field RR Lyrae as Galactic Probes. III. The α-element Abundances*

“…Continuum normalization and Doppler-shift corrections were made using the National Optical Astronomy Observatory libraries for IRAF 2 (Image Reduction and Analysis Facility, Tody 1993). Further information about the sample selection and radial velocity studies for the spectra analysed in this work can be found in Fabrizio et al (2019); Bono et al (2020), andCrestani et al (2021).…”