The VLT-FLAMES Tarantula Survey

Abstract

Whether massive stars ( > ∼ 30 M ) can occasionally form in relative isolation (e.g. in clusters with M < 100 M ) or if they require a large cluster of lower-mass stars around them is a key test in the differentiation of star-formation theories as well as how the initial mass function of stars is sampled. Previous attempts to find O-type stars that formed in isolation were hindered by the possibility that such stars are merely runaways from clusters, i.e., their current isolation does not reflect their birth conditions. We introduce a new method to find O-type stars that are not affected by such a degeneracy. Using the VLT-FLAMES Tarantula Survey and additional high resolution imaging we have identified stars that satisfy the following constraints: 1) they are O-type stars that are not detected to be part of a binary system based on radial velocity (RV) time series analysis; 2) they are designated spectral type O7 or earlier; 3) their velocities are within 1σ of the mean of OB-type stars in the 30 Doradus region, i.e. they are not runaways along our line-of-sight; 4) the projected surface density of stars does not increase within 3 pc towards the O-star (no evidence for clusters); 5) their sight lines are associated with gaseous and/or dusty filaments in the interstellar medium (ISM); and 6) if a second candidate is found in the direction of the same filament with which the target is associated, both are required to have similar velocities. With these criteria, we have identified 15 stars in the 30 Doradus region, which are strong candidates for being high-mass stars that have formed in isolation. Additionally, we employed extensive Monte Carlo stellar cluster simulations to confirm that our results rule out the presence of clusters around the candidates. Eleven of these are classified as Vz stars, possibly associated with the zero-age main sequence. We include a newly discovered Wolf-Rayet star as a candidate, although it does not meet all of the above criteria.

STELLAR CLUSTERINGS AROUND “ISOLATED” MASSIVE YSOs IN THE LMC

“…As pointed out by Bressert et al (2012), the term "isolated high-mass star formation" can be unclear. Specifically, these authors suggested three possible criteria that may suggest a high-mass star is not isolated: 1) a high-mass star is forming with other high-mass stars in a molecular cloud; 2) the formation of a high-mass star may be triggered by another high-mass star; and 3) a high-mass star was gravitationally bound (within ∼3 pc) with another high-mass star sometime in the past.…”

“…Specifically, these authors suggested three possible criteria that may suggest a high-mass star is not isolated: 1) a high-mass star is forming with other high-mass stars in a molecular cloud; 2) the formation of a high-mass star may be triggered by another high-mass star; and 3) a high-mass star was gravitationally bound (within ∼3 pc) with another high-mass star sometime in the past. Bressert et al (2012) were specifically interested in criterion 3, the least restrictive of the criteria, and found 15 candidates in the 30 Doradus region that may satisfy this criterion. This study is more concerned with the most restrictive of the criteria, criterion 1, and therefore it is more akin to the investigations of field O-stars by de Wit et al (2004de Wit et al ( , 2005 and Zinnecker & Yorke (2007), who suggested that 4-10% of all Galactic O-stars are not runaways, but formed in isolation.…”

“…Based on the first criterion for isolation proposed by Bressert et al (2012), a high-mass star forms in isolation if it is not member of an OB association or of a runaway population. This criterion extended to MYSOs requests that the isolated source should be forming away from an OB association, as well as of any GMC.…”

2dF-AAOmega spectroscopy of massive stars in the Magellanic Clouds

“…Evans & Howarth 2008), the formation of massive stars in relative isolation (e.g. Lamb et al 2010;Bressert et al 2012;Oey et al 2013), the structure of stellar clusters (e.g. Hénault-Brunet et al 2012a,b), the properties of the interstellar medium in the Clouds (e.g.…”

Star formation in the Gaia era

“…Gaia will provide an opportunity to study these issues in greater detail, as well as address questions such of whether OB associations are expanding star clusters or whether they form with low densities , and whether massive stars can form in isolated or low-density environments, or if their dynamics can explain their formation elsewhere (Bressert et al, 2012). …”