Using Ultra Long Period Cepheids to Extend the Cosmic Distance Ladder to 100 Mpc and Beyond

Understand

We examine the properties of 17 long period (80-180 days) and very luminous (median absolute magnitude of M_I= -7.93 and M_V= -7.03) Cepheids to see if they can serve as an useful distance indicator.

• We find that these Ultra Long Period (ULP) Cepheids have a relatively shallow Period-Luminosity (PL) relation, so in fact they are more "standard candle"-like than classical Cepheids.
• In the reddening-free Wesenheit index, the slope of the ULP PL relation is ~10 times less steep than the standard PL relation for the SMC Cepheids.
• The scatter of our sample about the W_I PL relation is 0.22 mag, approaching that of classical Cepheids and Type Ia Supernovae.