Discovery of a Pair of Classical Cepheids in an Invisible Cluster Beyond the Galactic Bulge

TL;DR

This paper reports the discovery of a pair of classical Cepheid variables behind the Galactic bulge, indicating a hidden young open cluster in a region previously inaccessible due to heavy extinction.

Contribution

First identification of a classical Cepheid pair in an unseen cluster behind the Galactic bulge using near-infrared photometry, revealing a new hidden stellar structure.

Findings

Cepheids are located at 11.4 kpc, less than 1 pc from the Galactic plane.

The pair is likely part of a young open cluster.

The cluster is associated with the Far 3 kpc Arm behind the bulge.

Abstract

We report the discovery of a pair of extremely reddened classical Cepheid variable stars located in the Galactic plane behind the bulge, using near-infrared time-series photometry from the VVV Survey. This is the first time that such objects have ever been found in the opposite side of the Galactic plane. The Cepheids have almost identical periods, apparent brightnesses and colors. From the near-infrared Leavitt law, we determine their distances with ~1.5% precision and ~8% accuracy. We find that they have a same total extinction of A(V)~32 mag, and are located at the same heliocentric distance of <d>=11.4+/-0.9 kpc, and less than 1 pc from the true Galactic plane. Their similar periods indicate that the Cepheids are also coeval, with an age of ~48+/-3 Myr, according to theoretical models. They are separated by an angular distance of only 18.3", corresponding to a projected separation of ~1 pc. Their position coincides with the expected location of the Far 3 kpc Arm behind the bulge. Such a tight pair of similar classical Cepheids indicates the presence of an underlying young open cluster, that is both hidden behind heavy extinction and disguised by the dense stellar field of the bulge. All our attempts to directly detect this "invisible cluster" have failed, and deeper observations are needed.