Using the Palomar Transient Factory to Search for Ultra-Long-Period Cepheid Candidates in M31

TL;DR

This study searches for ultra-long-period Cepheids in M31 using Palomar Transient Factory data, identifying three candidates including one new discovery, which can help calibrate cosmic distance measurements beyond 100 Mpc.

Contribution

The paper presents the first comprehensive search for ULPCs in M31 using PTF data, discovering a new ULPC candidate and refining period measurements of known candidates.

Findings

Identified 3 ULPC candidates in M31, including one new discovery.

Found that some candidates previously reported have shorter periods than measured in this study.

Determined that several other long-period variables are not ULPCs.

Abstract

Ultra-long-period Cepheids (ULPCs) are important in distance-scale studies due to their potential for determining distance beyond ~100 Mpc. We performed a comprehensive search for ULPCs in M31, a local benchmark to calibrate the distance ladders. We use data from the Palomar Transient Factory (PTF), which has imaged M31 using a 1.2-m telescope equipped with a ~7.26 deg2 field-of-view (FOV) camera, usually with daily sampling, since the beginning of 2010. The large FOV, together with the regular monitoring, enables us to probe ULPCs in the bulge, disk, and even out to the halo of M31. Using a difference imaging analysis technique, we found and characterized 3 promising ULPC candidates based on their luminosities, amplitudes and Fourier parameters. The mean absolute magnitude for these 3 ULPC candidates, calibrated with latest M31 distance, is M_R=-6.47mag. Two out of the 3 ULPC candidates have been reported in literature, however their published periods from Magnier et al. are about half of the periods we found in this work. The third ULPC candidate is a new discovery. We studied 5 other candidates and determined that they are probably Mira-like or ultra-long-period variables, but not ULPCs.