The Carina Project. Ix. On Hydrogen and Helium Burning Variables

Abstract

We present new multiband (UBVI) time-series data of helium burning variables in the Carina dwarf spheroidal galaxy. The current sample includes 92 RR Lyrae-six of them are new identifications-and 20 Anomalous Cepheids, one of which is new identification. The analysis of the Bailey diagram shows that the luminosity amplitude of the first overtone component in double-mode variables is located along the long-period tail of regular first overtone variables, while the fundamental component is located along the short-period tail of regular fundamental variables. This evidence further supports the transitional nature of these objects. Moreover, the distribution of Carina double-mode variables in the Petersen diagram (P 1 /P 0 versus P 0 ) is similar to metal-poor globulars (M15, M68), to the dwarf spheroidal Draco, and to the Galactic Halo. This suggests that the Carina old stellar population is metal-poor and affected by a small spread in metallicity. We use trigonometric parallaxes for five field RR Lyrae stars to provide an independent estimate of the Carina distance using the observed reddening free Period-Wesenheit [PW, (BV)] relation. Theory and observations indicate that this diagnostic is independent of metallicity. We found a true distance modulus of μ = 20.01 ± 0.02 (standard error of the mean) ± 0.05 (standard deviation) mag. We also provided independent estimates of the Carina true distance modulus using four predicted PW relations (BV, BI, VI, BVI) and we found: μ = (20.08 ± 0.007 ± 0.07) mag, μ = (20.06 ± 0.006 ± 0.06) mag, μ = (20.07 ± 0.008 ± 0.08) mag, and μ = (20.06 ± 0.006 ± 0.06) mag. Finally, we identified more than 100 new SX Phoenicis stars that together with those already known in the literature (340) make Carina a fundamental laboratory for constraining the evolutionary and pulsation properties of these transitional variables.

On THE RR LYRAE STARS IN GLOBULARS. IV. Ω CENTAURI OPTICAL UBVRI PHOTOMETRY*

“…The current empirical slopes for the optical PW relations agree quite well with similar estimates recently provided by Coppola et al (2015) for more than 90 RRLs of the Carina dSph. They found slopes of −2.7 [global, PW(V, B-V )] and −2.6 [global, PW(V, B-I)].…”

“…Following Braga et al (2015) and Marconi et al (2015) we evaluated the PL relations for RRc, RRab and for the global (All) sample. In the global sample the RRc were "fundamentalized," i.e., we adopted = + P P log log 0.127 Iben & Huchra 1971;Rood 1973;Cox et al 1983;Di Criscienzo et al 2004;Coppola et al 2015). The coefficients, their errors and the standard deviations of the empirical PL relations are listed in Table 7.…”

“…A similar approach was adopted by Martínez-Vázquez et al (2015) and by Coppola et al (2015) to estimate the metallicity distribution of RRLs in Sculptor and in Carina, respectively. The absolute I-band magnitudes (M I ) of RRLs were estimated using the true distance modulus (μ = 13.70 ± 0.02 mag) based on theoretical PW relations (see Section 7).…”

Recent progress in the theoretical modelling of Cepheids and RR Lyrae stars

“…This theoretical framework was applied with success to observational samples, e.g. in the Galactic globular cluster M4 (see [6]) and in the dwarf spheroidal galaxy Carina (see [9]). An important new prediction of these models was an analytical relation to infer double mode RR Lyrae masses as a function of the period ratio and the metallicity.…”

Faint (and bright) variable stars in the satellites of the Milky Way

“…Dwarf Cepheid stars have been searched so far only in three extra-Galactic systems: the LMC ( [11]), Fornax ( [15]) and Carina ( [16][17][18]). Our study in Carina ( [17]) has raised several interesting points: (i) Carina is very rich in dwarf Cepheid stars; (ii) There seems to be a fundamental difference between the dwarf Cepheid population in dSph galaxies and the Galactic field.…”