NGC 1252: a high altitude, metal poor open cluster remnant

TL;DR

This study combines photometry, proper motions, spectroscopy, and simulations to analyze NGC 1252, revealing it as a high-altitude, metal-poor open cluster remnant, one of the oldest and farthest from the Galactic plane.

Contribution

It provides a comprehensive multi-method analysis confirming NGC 1252 as a high-altitude open cluster remnant, a unique case among known clusters.

Findings

NGC 1252 is a metal-poor, high-altitude cluster about 1 kpc away.

It is approximately 3 billion years old, making it one of the oldest clusters nearby.

NGC 1252 is located nearly 900 pc below the Galactic plane.

Abstract

If stars form in clusters but most stars belong to the field, understanding the details of the transition from the former to the latter is imperative to explain the observational properties of the field. Aging open clusters are one of the sources of field stars. The disruption rate of open clusters slows down with age but, as an object gets older, the distinction between the remaining cluster or open cluster remnant (OCR) and the surrounding field becomes less and less obvious. As a result, finding good OCR candidates or confirming the OCR nature of some of the best candidates still remains elusive. One of these objects is NGC 1252, a scattered group of about 20 stars in Horologium. Here we use new wide-field photometry in the UBVI pass-bands, proper motions from the Yale/San Juan SPM 4.0 catalogue and high resolution spectroscopy concurrently with results from N-body simulations to decipher NGC 1252's enigmatic character. Spectroscopy shows that most of the brightest stars in the studied area are chemically, kinematically and spatially unrelated to each other. However, after analysing proper motions, we find one relevant kinematic group. This sparse object is relatively close (about 1 kpc), metal poor and is probably not only one of the oldest clusters (3 Gyr) within 1.5 kpc from the Sun but also one of the clusters located farthest from the disc, at an altitude of nearly -900 pc. That makes NGC 1252 the first open cluster that can be truly considered a high Galactic altitude OCR: an unusual object that may hint at a star formation event induced on a high Galactic altitude gas cloud. TW Hor and HD 20286 are unlikely to be part of NGC 1252. NGC 1252 17 is identified as an unrelated, Population II cannonball star moving at about 400 km/s.