Anomalous Cepheids in the Large Magellanic Cloud

Abstract

Context. The properties of variable stars can give independent constraints on the star formation history of the host galaxy, by determining the age and metallicity of the parent population. Aims. We investigate the pulsation properties of 84 Anomalous Cepheids (ACs) detected by the OGLE-III survey in the Large Magellanic Cloud (LMC), in order to understand the formation mechanism and the characteristics of the parent population they came from. Methods. We used an updated theoretical pulsation scenario to derive the mass and the pulsation mode of each AC in the sample. We also used a Kolmogorov-Smirnov test to analyze the spatial distribution of the ACs, in comparison with that of other groups of variable stars, and connect their properties with the star formation history of the LMC. Results. We find that the mean mass of ACs is 1.2 ± 0.2 M . We show that ACs do not follow the same spatial distribution of classical Cepheids. This and the difference in their period-luminosity relations further support the hypothesis that ACs are not the extension to low luminosity of classical Cepheids. The spatial distribution of ACs is also different from that of bona-fide tracers of the old population, such as RR Lyrae stars and population II Cepheids. We therefore suggest that the majority of ACs in the LMC are made of intermediate-age (1-6 Gyr), metal-poor single stars. Finally, we investigate the relation between the frequency of ACs and the luminosity of the host galaxy, disclosing that purely old systems follow a very tight relation and that galaxies with strong intermediate-age and young star formation tend to have an excess of ACs, in agreement with their hosting ACs formed via both single and binary star channels.

The period-luminosity and period-radius relations of Type II and anomalous Cepheids in the Large and Small Magellanic Clouds

“…ACs fill a space on the PL-diagram above the RRL and T2Cs by ∼0.5 to ∼2 magnitudes (as the period increases), but do not reach into the CC region. Caputo et al (2004) investigated the possibility that they do continue to the PL relation of CCs, but Fiorentino & Monelli (2012) concluded that they are metal-poor stars that would not evolve into the CC IS region.…”

“…Quoting the results of Vassiliadis & Wood (1993), Wallerstein (2002) gives the initial mass of the brighter T2Cs (the RVT) to be around 1 M ⊙ . In the case of ACs pulsation models have been considered by various authors to find masses in the range 1.3-2.2 M ⊙ (for Z = 0.0001 and 0.0004, Bono et al (1997b), Marconi et al (2004)), or, specifically 1.2 ± 0.2 M ⊙ for the ACs in the LMC (Fiorentino & Monelli 2012). Recently, Martínez-Vázquez et al (2016) find ∼ 1.5 M ⊙ for four ACs in the Sculptor dSph galaxy.…”

Life in the Outer Limits: Insights on Hierarchical Assembly from Stellar Halos in the Local Universe

“…Theoretical investigations into the origins of Anomalous Cepheids indicate these stars are either (1) evolved, intermediate-age (< 5 Gyr) stars, or (2) the evolved products of mass transfer in old (> 10 Gyr) binary systems, but with no definitive means to distinguish either mechanism (Bono et al 1997). Nevertheless, the observational evidence better supports the interpretation that Anomalous Cepheids are metal-poor, intermediate age variables: (1) the spatial distribution of Anomalous Cepheids in the LMC shows they are not coincident with the older stellar populations (Fiorentino & Monelli 2012), (2) (blue), M31 data for And01, And03, And02 and And06 (green) and data for Leo T (red) Gallart et al 2004). In comparison to theoretical predictions, the scatter is attributed to the mass of each individual star.…”

“…Even in well-studied systems like the Large Magellanic Cloud (LMC), such tracers have proven invaluable for proper reconstruction of the SFH (Fiorentino & Monelli 2012). At M31 distances, the Cepheid-type variables are particularly useful due to their intrinsic luminosity -several magnitudes brighter than the horizontal branch (HB) -and can be efficiently photometered from the ground -unlike RR Lyrae which require HST temporal monitoring at M31 distance (Sandage & Tammann 2006;Mateo et al 1995).…”

“…While relatively rare, Anomalous Cepheids have been found in all dwarf galaxies that have been monitored for variable stars (Pritzl et al 2005), with virtually all of the ∼200 known Anomalous Cepheids being members of LG dwarf galaxies Sandage & Tammann 2006;Gallart et al 2004;Clementini et al 2012;Fiorentino & Monelli 2012). Only one Anomalous Cepheid has been identified in a globular cluster, that being NGC 5466 (Zinn & King 1982).…”