New insights into the use of Ultra Long Period Cepheids as cosmological standard candles

Abstract

Ultra Long Period Cepheids (ULPs) are pulsating variable stars with a period longer than 80d and have been hypothesized to be the extension of the Classical Cepheids (CCs) at higher masses and luminosities. If confirmed as standard candles, their intrinsic luminosities, ∼1 to ∼3 mag brighter than typical CCs, would allow to reach the Hubble flow and, in turn, to determine the Hubble constant, H0, in one step, avoiding the uncertainties associated with the calibration of primary and secondary indicators. To investigate the accuracy of ULPs as cosmological standard candles, we first collect all the ULPs known in the literature. The resulting sample includes 63 objects with a very large metallicity spread with 12 + log ([O/H]) ranging from 7.2 to 9.2 dex. The analysis of their properties in the VI period-Wesenheit plane and in the color-magnitude diagram (CMD) supports the hypothesis that the ULPs are the extension of CCs at longer periods, higher masses and luminosities, even if, additional accurate and homogeneous data and a devoted theoretical scenario are needed to get firm conclusions. Finally, the three M31 ULPs, 8-0326, 8-1498 and H42, are investigated in more detail. For 8-1498 and H42, we cannot confirm their nature as ULPs, due to the inconsistency between their position in the CMD and the measured periods. For 8-0326, the light curve model fitting technique applied to the available time-series data allows us to constrain its intrinsic stellar parameters, distance and reddening.

Combined spectroscopy and intensity interferometry to determine the distances of the blue supergiants P Cygni and Rigel

“…This value should be extended up to ∼50 Mpc with the James Webb Space Telescope (Riess et al 2009). To reach larger distances, a more recent method has been proposed based on Ultra Long Period Cepheids (e.g., see Fiorentino et al 2012;Musella et al 2021), which however needs to be further tested. As for the WLR method, following our recent results, we will engage a systematic intensity interferometric survey of a few tens of the closest and brightest OBA supergiants.…”

Inspecting the Cepheid Distance Ladder: the Hubble Space Telescope Distance to the SN Ia Host Galaxy NGC 5584

“…On the first rung, different studies (e.g., Nardetto 2018; Borgniet et al 2019;Kervella et al 2019b;Gallenne et al 2019;Anderson 2019;Hocdé et al 2020aHocdé et al , 2020bMusella et al 2021) have focused on enhancing our understanding of the different properties of Cepheid variables, and in a recent study, Breuval et al (2020) presented a new calibration of the periodluminosity (PL) relation for Milky Way (MW) Cepheids using their companion parallaxes from Gaia (Kervella et al 2019a). In addition, the high-precision measurement of the distance to the Large Magellanic Cloud (LMC) by Pietrzyński et al (2019) provides an accurate calibration of the Cepheid periodluminosity relation in this satellite galaxy of the MW.…”

Ultra Long Period Cepheids: Observation and Theory

“…For NGC 4151, in the literature, there is no reliable metallicity determination, whereas NGC 6814 has approximately a solar average abundance [34]. For the SH0ES sample, the V and I magnitudes tabulated are those computed by Musella et al [20] from the F555W and F814W magnitudes by Hoffmann et al [17] and the adopted distance moduli and individual metal abundances are the same as those adopted by Riess et al [16] and Hoffmann et al [17].…”

“…Musella et al [20] collected all the above mentioned ULPs to compare their properties in the color-magnitude diagram (CMD) and PW plane with the CC ones. Their sample included 63 objects, with a metallicity 12 + log(O/H) ranging from ∼7.2 to 9.2 dex and they found W V I = −2.15 log P − 4.89 with σ = 0.38, also in this case not confirming the flat slope by Bird et al [9].…”

“…The V and I for the M31_N ULP are taken by Ngeow et al [18]. For the M31_T ULP by Taneva et al [19], we do not have the I mean magnitude, but only the B, V and R mean magnitudes, and for this reason, it was separately discussed in Musella et al [20]. In this work, also the I mean magnitude is derived by adopting the well-defined relation between R and (V + I)/2 based on the Landolt standards used to calibrate the Johnson-Kron-Cousins photometric system [28][29][30].…”