USING ULTRA LONG PERIOD CEPHEIDS TO EXTEND THE COSMIC DISTANCE LADDER TO 100 Mpc AND BEYOND

Abstract

We examine the properties of 18 long period (80-210 days) and very luminous (median absolute magnitude of M I = −7.86 and M V = −6.97) Cepheids to see if they can serve as a 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 consistent with zero. The scatter of our sample about the W I PL relation is 0.23 mag, approaching that of classical Cepheids and Type Ia Supernovae. We expect this scatter to decrease as bigger and more uniform samples of ULP Cepheids are obtained. We also measure a nonzero period derivative for one ULP Cepheid (SMC HV829) and use the result to probe evolutionary models and mass loss of massive stars. ULP Cepheids' main advantage over classical Cepheids is that they are more luminous, and as such show great potential as stellar distance indicators to galaxies up to 100 Mpc and beyond.

Convection, granulation, and period jitter in classical Cepheids

“…This suggests that long-period Cepheids, such as l Car and RS Pup, will have more relative period jitter than short-period Cepheids such as δ Cep and V1154 Cyg. Furthermore, this period jitter may complicate distance estimates from ultra-long-period Cepheids (P > 80 days) (Bird et al 2009;Fiorentino et al 2012). …”

Application of Cepheids to Distance Scale: Extending to Ultra-Long Period Cepheids

“…Therefore, these long period Cepheids were excluded from extra-galactic distance scale applications. However, Bird et al (2009) proposed Cepheids with period longer than 80 days follow a different Leavitt Law, and called them the ultralong period Cepheids (ULPCs). ULPCs are indeed Cepheids with higher masses than their shorter period Cepheids, with masses in the range of ∼ 12 to ∼ 20 M (Bird et al, 2009;Fiorentino et al, 2012).…”

“…However, Bird et al (2009) proposed Cepheids with period longer than 80 days follow a different Leavitt Law, and called them the ultralong period Cepheids (ULPCs). ULPCs are indeed Cepheids with higher masses than their shorter period Cepheids, with masses in the range of ∼ 12 to ∼ 20 M (Bird et al, 2009;Fiorentino et al, 2012). ULPCs are also intrinsically ∼ 1 to ∼ 3 magnitudes brighter than the shorter period Cepheids (as targeted by HST Key Project) suggesting that it is possible to probe the distance to galaxies in the Hubble flow (> 100 Mpc) by using ULPCs 2 , and hence deriving the H 0 in a "singlestep" without the calibration of secondary distance indicators (Bird et al, 2009;Fiorentino et al, 2012).…”

“…Further details on the follow-up observations and results can be found in Ngeow et al (2014 -submitted). Figure 1 presents the fitting of these two ULPCs in M31 with two period-Wesenheit functions available from Bird et al (2009) andFiorentino et al (2012), at which can be used to determine the distance modulus to M31. The derived distance moduli as shown in Figure 1, albeit with large standard deviations, are consistent with the recommended value of 24.46 ± 0.10 given in de Grijs & Bono (2014).…”

Asteroseismology, standard candles and the Hubble Constant: what is the role of asteroseismology in the era of precision cosmology?

“…The Ultra Long Period Cepheids (ULPs) are fundamental-mode Cepheid-like pulsators with P 80 days (see Bird et al 2009 for an extensive discussion), and were first identified in the Magellanic Clouds (Freedman et al 1985). They are much brighter (M I from -7 to -9 mag) than short-period Cepheids, hence HST is able to observe them up to distances of 100 Mpc.…”