Ultra Long Period Cepheids: a primary standard candle out to the Hubble flow

Abstract

The cosmological distance ladder crucially depends on classical Cepheids (with P=3-80 days), which are primary distance indicators up to 33 Mpc. Within this volume, very few SNe Ia have been calibrated through classical Cepheids, with uncertainty related to the non-linearity and the metallicity dependence of their period-luminosity (PL) relation. Although a general consensus on these effects is still not achieved, classical Cepheids remain the most used primary distance indicators. A possible extension of these standard candles to further distances would be important. In this context, a very promising new tool is represented by the ultra-long period (ULP) Cepheids (P \geq 80 days), recently identified in star-forming galaxies. Only a small number of ULP Cepheids have been discovered so far. Here we present and analyse the properties of an updated sample of 37 ULP Cepheids observed in galaxies within a very large metallicity range of 12+log(O/H) from ~7.2 to 9.2 dex. We find that their location in the colour(V-I)-magnitude diagram as well as their Wesenheit (V-I) index-period (WP) relation suggests that they are the counterparts at high luminosity of the shorter-period (P \leq 80 days) classical Cepheids. However, a complete pulsation and evolutionary theoretical scenario is needed to properly interpret the true nature of these objects. We do not confirm the flattening in the studied WP relation suggested by Bird et al. (2009). Using the whole sample, we find that ULP Cepheids lie around a relation similar to that of the LMC, although with a large spread (~0.4 mag).

8 pages, 4 figures, accepted for publication in Astrophysics & Space Science

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• The Star Formation History of the very metal poor BCD IZw18 from HST/ACS data
• Large Magellanic Cloud Near-Infrared Synoptic Survey. II. The Wesenheit relations and their application to the Distance scale
• Variability of Red Supergiants in M31 from the Palomar Transient Factory
• Convection, granulation and period jitter in classical Cepheids
• Combined spectroscopy and intensity interferometry to determine the distances of the blue supergiants P Cygni and Rigel
• Probing the nature of the pre-merging system Hickson Compact Group 31 through IFU data
• The mid-infrared Leavitt Law for Classical Cepheids in the Magellanic Clouds
• VI-Band Follow-Up Observations of Ultra-Long-Period Cepheid Candidates in M31
• New insights into the use of Ultra Long Period Cepheids as cosmological standard candles
• Ultra Long Period Cepheids as standard candles from Gaia to Rubin-LSST
• Optical and near-infrared pulsation properties of RR Lyrae and Population II Cepheid variables in the Messier 15 globular cluster