Discovery of the Longest-Period Classical Cepheid in the Milky Way

TL;DR

This paper reports the discovery of the longest-period classical Cepheid in the Milky Way, expanding understanding of such stars and their distribution within our galaxy.

Contribution

It presents the first ultra long period Cepheid in the Milky Way, identified through extensive photometric and radial velocity data, and discusses implications for galactic structure.

Findings

Discovered a Cepheid with a 78.14-day period, the longest in the Milky Way.

Confirmed the star as a fundamental-mode classical Cepheid using multi-band data.

Estimated the star's distance at approximately 4.47 kpc.

Abstract

We report the discovery of the classical Cepheid OGLE-GD-CEP-1884 (= GDS_J1535467-555656) with the longest pulsation period known in our Galaxy. The period of 78.14 d is nearly 10 d longer than that of the previous record-holding Cepheid, S Vulpeculae, and thus, OGLE-GD-CEP-1884 can be categorized as the first ultra long period Cepheid in the Milky Way. This star is present in the ASAS-SN and Gaia DR3 catalogs of variable stars, but it has been classified as a long-period variable in those catalogs. Based on more than 10 years of the photometric monitoring of this star carried out by the OGLE project in the I and V bands and a radial velocity curve from the Gaia Focused Product Release, we unequivocally demonstrate that this object is a fundamental-mode classical Cepheid. By employing the mid-infrared period-luminosity relation, we determine the distance to OGLE-GD-CEP-1884 (4.47 +- 0.34 kpc) and place it on the Milky Way map, along with about 2400 other classical Cepheids. We also discuss the potential of finding additional ultra long period Cepheids in our Galaxy.