The Origin of Massive O-type Field Stars. Part I: A Search for Clusters

TL;DR

This study investigates the origins of 43 massive O-type field stars by searching for nearby stellar clusters, finding most are isolated and suggesting many may have been ejected from clusters.

Contribution

It provides the first observational analysis of the cluster environment around O-type field stars using multi-scale density maps, revealing most are isolated.

Findings

Only 12% of the stars are associated with small-scale clusters.

Most O-type field stars are isolated, not in clusters.

Supports the hypothesis of dynamical ejection for many field stars.

Abstract

We present a study aimed at clarifying the birthplace for 43 massive O-type field stars. In this first paper we present the observational part: a search for stellar clusters near the target stars. We derive stellar density maps at two different resolving scales, viz. 0.25pc and 1.0pc from NTT and TNG imaging and the 2MASS catalogue. These scales are typical for cluster sizes. The main result is that the large majority of the O-type field population are isolated stars: only 12% (5 out of 43) of the O-type field stars is found to harbour a small-scale stellar cluster. We review the literature and aim at characterizing the stellar field of each O-type field star with the emphasis on star formation and the presence of known young stellar clusters. An analysis of the result of this paper and a discussion of the O-type field population as products of a dynamical ejection event is presented in an accompanying paper.