Free Access
Volume 542, June 2012
Article Number A49
Number of page(s) 14
Section Galactic structure, stellar clusters and populations
Published online 05 June 2012

Online material

Appendix A: Hubble archival data

We make use of HST images that were retrieved through the Hubble Legacy Archive web service. The images came from a collection proposals and programs. A table is provided below to summarise the different data and their properties which are important for this paper.

Table A.1

Details of the HST data used in this paper.

Appendix B: Probability test for chance alignment of runaway stars with filaments

Currently, there are no high-resolution, continuous, large field-of-view maps of the 30 Doradus region that show where the star forming filamentary gas and dust are. In the coming year this deficiency will be resolved as the Herschel Space Observatory survey maps of the LMC, HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) (Meixner et al. 2010), will be publicly released. By combining the HERITAGE maps with the data presented in this paper one could reject the likelihood of O-stars being runaways based on probability. We propose that the definition of a filament from Herschel observations is best defined by the 2nd differential maps Molinari et al. (2010) introduced, where a physically relevant radius from the peaks of the filaments can bear association to the stars.

To do this, we take O-stars similar to the ones discussed in this paper and assume that they are runaways. Using the radial velocity measurements from the Tarantula Survey, we

approximate that there are roughly N number of runaways in total. Then we place a circular boundary around R136 at distance R, where R = Rboundary − Rcluster, and has area Aboundary. Within R distance, let’s assume that gas/dust filaments covers an area Afilament, such that Afilament ≤ Aboundary. Refer to Fig. B.1 for a visual context of the scenario we have now discussed.

With the above conditions, what is the probability that n of N runaway O-stars line up with filamentary structures in the line-of-sight? For simplicity, assume that the N O-stars are randomly distributed within the boundary such that the probability of an O-star lining up with a filament can be expressed as ρ. (B.1)To see what probability n of N (n ≤ N) O-stars is in the line-of-sight with filaments we invoke the binomial probability equation as shown below, where we assume that each line-of-sight event is independent of one another. (B.2)

thumbnail Fig. B.1

A diagram showing how the probabilistic method should be conceptualised. The radius R = Rboundary − Rcluster. All the stars outside of the cluster are initially assumed to be runaways for the binomial probability problem to calculate the likelihood of multiple alignment events between the runaways and the filaments (grey clouds). The filaments at these scales,  >5 pc, will not be affected by a single O-star such that nothing remains.

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© ESO, 2012

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