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Register a new userOn the use of field RR Lyrae as Galactic probes: IV. New insights into and around the Oosterhoff dichotomy
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We discuss the largest and most homogeneous spectroscopic dataset of field RR Lyrae variables (RRLs) available to date. We estimated abundances using both high-resolution and low-resolution ({Delta S} method) spectra for fundamental (RRab) and first overtone (RRc) RRLs. The iron abundances for 7,941 RRLs were supplemented with similar literature estimates available, ending up with 9,015 RRLs (6,150 RRab, 2,865 RRc). The metallicity distribution shows a mean value of <[Fe/H]> = -1.51pm0.01, and {sigma}(standard deviation)= 0.41 dex with a long metal-poor tail approaching [Fe/H] = -3 and a sharp metal-rich tail approaching solar iron abundance. The RRab variables are more metal-rich (<[Fe/H]>ab = -1.48pm0.01, {sigma} = 0.41 dex) than RRc variables (<[Fe/H]>c = -1.58pm0.01, {sigma} = 0.40 dex). The relative fraction of RRab variables in the Bailey diagram (visual amplitude vs period) located along the short-period (more metal-rich) and the long-period (more metal-poor) sequences are 80% and 20%, while RRc variables display an opposite trend, namely 30% and 70%. We found that the pulsation period of both RRab and RRc variables steadily decreases when moving from the metal-poor to the metal-rich regime. The visual amplitude shows the same trend, but RRc amplitudes are almost two times more sensitive than RRab amplitudes to metallicity. We also investigated the dependence of the population ratio (Nc/Ntot) of field RRLs on the metallicity and we found that the distribution is more complex than in globular clusters. The population ratio steadily increases from ~0.25 to ~0.36 in the metal-poor regime, it decreases from ~0.36 to ~0.18 for -1.8 < [Fe/H] < -0.9 and it increases to a value of ~0.3 approaching solar iron abundance.
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We collected the largest spectroscopic catalog of RR Lyrae (RRLs) including $approx$20,000 high-, medium- and low-resolution spectra for $approx$10,000 RRLs. We provide the analytical forms of radial velocity curve (RVC) templates. These were built using 36 RRLs (31 fundamental -- split into three period bins -- and 5 first overtone pulsators) with well-sampled RVCs based on three groups of metallic lines (Fe, Mg, Na) and four Balmer lines (H$_alpha$, H$_beta$, H$_gamma$, H$_delta$). We tackled the long-standing problem of the reference epoch to anchor light curve and RVC templates. For the $V$-band, we found that the residuals of the templates anchored to the phase of the mean magnitude along the rising branch are $sim$35% to $sim$45% smaller than those anchored to the phase of maximum light. For the RVC, we used two independent reference epochs for metallic and Balmer lines and we verified that the residuals of the RVC templates anchored to the phase of mean RV are from 30% (metallic lines) up to 45% (Balmer lines) smaller than those anchored to the phase of minimum RV. We validated our RVC templates by using both the single- and the three-phase points approach. We found that barycentric velocities based on our RVC templates are two-three times more accurate than those available in the literature. We applied the current RVC templates to Balmer lines RVs of RRLs in the globular NGC~3201 collected with MUSE at VLT. We found the cluster barycentric RV of $V_{gamma}$=496.89$pm$8.37(error)$pm$3.43 (standard deviation) km/s, which agrees well with literature estimates.
We provide the largest and most homogeneous sample of $alpha$-element (Mg, Ca, Ti) and iron abundances for field RR Lyrae (RRLs, 162 variables) by using high-resolution spectra. The current measurements were complemented with similar abundances available in the literature for 46 field RRLs brought to our metallicity scale. We ended up with a sample of old (t$ge$ 10 Gyr), low-mass stellar tracers (208 RRLs: 169 fundamental, 38 first overtone, 1 mixed mode) covering three dex in iron abundance (-3.00$le$[Fe/H]$le$0.24). We found that field RRLs are $sim$0.3 dex more $alpha$-poor than typical Halo tracers in the metal-rich regime, ([Fe/H]$ge$-1.2) while in the metal-poor regime ([Fe/H]$le$-2.2) they seem to be on average $sim$0.1 dex more $alpha$-enhanced. This is the first time that the depletion in $alpha$-elements for solar iron abundances is detected on the basis of a large, homogeneous and coeval sample of old stellar tracers. Interestingly, we also detected a close similarity in the [$alpha$/Fe] trend between $alpha$-poor, metal-rich RRLs and red giants (RGs) in the Sagittarius dwarf galaxy as well as between $alpha$-enhanced, metal-poor RRLs and RGs in ultra faint dwarf galaxies. These results are supported by similar elemental abundances for 46 field Horizontal Branch (HB) stars. These stars share with RRLs the same evolutionary phase and the same progenitors. This evidence further supports the key role that old stellar tracers play in constraining the early chemical enrichment of the Halo and, in particular, in investigating the impact that dwarf galaxies have had in the mass assembly of the Galaxy.
We performed the largest and most homogeneous spectroscopic survey of field RR Lyraes (RRLs). We secured $approx$6,300 high resolution (HR, R$sim$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 CaII K and three Balmer (H$_{delta,gamma,beta}$) 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; iv) provides independent calibrations based on one/two/three Balmer lines. The new calibrations were applied to both SEGUE-SDSS and degraded HR spectra totalling 6,451 low resolution (LR, R$sim$2,000) spectra for 5,001 RRLs (3,439 RRab, 1,562 RRc). This resulted in an iron distribution with a median of -1.55$pm$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.
An examination of the period-V amplitude relation for RRab stars (fundamental mode pulsators) with `normal light curves in the Oosterhoff type I clusters M3 and M107 and in the Oosterhoff type II clusters M9 and M68 reveals that the V amplitude for a given period is not a function of metal abundance. Rather, it is a function of the Oosterhoff type. This result is confirmed by published observations of RRab stars in M4, M5 and M92. A method devised by Jurcsik and Kovacs has been used to determine whether the light curve of an RRab star is `normal or `peculiar. Although M3 is considered to belong to the Oosterhoff type I group, it has three bright RRab stars that seem to fit the period-amplitude relation for Oosterhoff type II RRab stars. There is evidence that these bright stars are in a more advanced evolutionary state than the other RRab stars in M3, thus leading to the conclusion that the Oosterhoff dichotomy is due to evolution. Our result gives support to the Lee, Demarque & Zinn hypothesis that most RR Lyrae variables in Oosterhoff type I clusters are ZAHB objects while those in the Oosterhoff type II clusters are more evolved. This may have important implications for the derived ages of Oosterhoff type II clusters. If their RR Lyrae variables have all evolved away from the ZAHB, then their ages have been overestimated in studies that assume they are ZAHB objects.
Ultra Long Period Cepheids (ULPs) are pulsating variable stars with a period longer than 80d and have been hypothesized to be the extension of the Classical Cepheids (CCs) at higher masses and luminosities. If confirmed as standard candles, their intrinsic luminosities, 1 to 3 mag brighter than typical CCs, would allow to reach the Hubble flow and, in turn, to determine the Hubble constant, H_0, in one step, avoiding the uncertainties associated with the calibration of primary and secondary indicators. To investigate the accuracy of ULPs as cosmological standard candles, we first collect all the ULPs known in the literature. The resulting sample includes 63 objects with a very large metallicity spread with 12 + log([O/H]) ranging from 7.2 to 9.2 dex. The analysis of their properties in the VI period-Wesenheit plane and in the color-magnitude diagram (CMD) supports the hypothesis that the ULPs are the extension of CCs at longer periods, higher masses and luminosities, even if, additional accurate and homogeneous data and a devoted theoretical scenario are needed to get firm conclusions. Finally, the three M31 ULPs, 8-0326, 8-1498 and H42, are investigated in more detail. For 8-1498 and H42, we cannot confirm their nature as ULPs, due to the inconsistency between their position in the CMD and the measured periods. For 8-0326, the light curve model fitting technique applied to the available time-series data allows us to constrain its intrinsic stellar parameters, distance and reddening.
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