A highly abnormal massive star mass function in the Orion Nebula cluster and the dynamical decay of trapezium systems

Pflamm-Altenburg

2010

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Using archival Spitzer Space Telescope data, we identified for the first time a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the detection of their bow shocks. The geometry of detected bow shocks allows us to infer the direction of motion of the associated stars and to determine their possible parent clusters and associations. One of the identified runaway stars, AzV 471, was already known as a high-velocity star on the basis of its high peculiar radial velocity, which is offset by 40 km s −1 from the local systemic velocity. We discuss implications of our findings for the problem of the origin of field OB stars. Several of the bow shockproducing stars are found in the confines of associations, suggesting that these may be "alien" stars contributing to the age spread observed for some young stellar systems. We also report the discovery of a kidney-shaped nebula attached to the early WN-type star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure created owing to the interaction between the stellar wind and the ambient interstellar medium.

Section: Discussionmentioning

confidence: 99%

“…Pflamm-Altenburg & Kroupa 2006;Moeckel & Bate 2010). The ejected (runaway) stars form the population of field OB stars that end their lives in supernova explosions hundreds of parsecs from their birthplaces.…”

Section: Introductionmentioning

confidence: 99%

Massive runaway stars in the Small Magellanic Cloud

Gvaramadze

Pflamm-Altenburg

2010

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“…Gvaramadze 2009). The only viable alternative is that the massive stars were ejected in the field via dynamical three-or four-body encounters (Poveda et al 1967;Leonard & Duncan 1990;Kroupa 1998;Pflamm-Altenburg & Kroupa 2006;Gvaramadze, Gualandris & Portegues Zwart 2008, 2009). Naturally, less massive (late B-type) stars are also ejected from their birth clusters by dynamical interactions (e.g.…”

Section: Discussionmentioning

confidence: 99%

“…An obvious alternative to this interpretation is that the massive field stars were actually formed in a clustered environment and subsequently ejected from their birth sites via dynamical processes (e.g. Clarke & Pringle 1992;Pflamm-Altenburg & Kroupa 2006); i.e., the field massive stars are runaway stars (Walborn et al 2002;Brandl et al 2007). The large separations from the possible parent clusters and the young (∼2 Myr) ages of the very massive field stars imply that their (transverse) velocities should be as high as ∼50−100 km s −1 (Walborn et al 2002).…”

Section: Very Massive Field Stars As Runawaysmentioning

confidence: 99%

Massive runaway stars in the Large Magellanic Cloud

Gvaramadze

Pflamm-Altenburg

2010

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The origin of massive field stars in the Large Magellanic Cloud (LMC) has long been an enigma. The recent measurements of large offsets (∼100 km s −1 ) between the heliocentric radial velocities of some very massive (O2-type) field stars and the systemic LMC velocity provides a possible explanation of this enigma and suggests that the field stars are runaway stars ejected from their birthplaces at the very beginning of their parent cluster's dynamical evolution. A straightforward way to prove this explanation is to measure the proper motions of the field stars and to show that they are moving away from one of the nearby star clusters or OB associations. This approach is, however, complicated by the long distance to the LMC, which makes accurate proper motion measurements difficult. We used an alternative approach for solving the problem (first applied for Galactic field stars), based on the search for bow shocks produced by runaway stars. The geometry of detected bow shocks would allow us to infer the direction of stellar motion, thereby determining their possible parent clusters. In this paper we present the results of a search for bow shocks around six massive field stars that have been proposed as candidate runaway stars. Using archival Spitzer Space Telescope data, we found a bow shock associated with one of our programme stars, the O2 V((f*)) star BI 237, which is the first-ever detection of bow shocks in the LMC. Orientation of the bow shock suggests that BI 237 was ejected from the OB association LH 82 (located at 120 pc in projection from the star). A by-product of our search is the detection of bow shocks generated by four OB stars in the field of the LMC and an arc-like structure attached to the candidate luminous blue variable R81 (HD 269128). The geometry of two of these bow shocks is consistent with the possibility that their associated stars were ejected from the 30 Doradus star-forming complex. We discuss implications of our findings for the problem of the origin of runaway stars and the early dynamical evolution of star clusters.

“…In this case, the initial density may have been one to two orders of magnitude higher than that today. Pflamm-Altenburg & Kroupa (2006) investigate the decay of the ONC core. Models with initially 40 massive stars (>600 M ), which is the expected number if stars in the ONC, were selected from the canonical IMF at birth, in a region of 0.025 pc end up with about the observed number of massive objects (10 stars above 5 M ) and display a Trapezium structure.…”

Section: Orion Nebula Clustermentioning

confidence: 99%

Inverse dynamical population synthesis

Marks

2012

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240

302

Binary populations in young star clusters show multiplicity fractions both lower and up to twice as high as those observed in the Galactic field. We follow the evolution of a population of binary stars in dense and loose star clusters starting with an invariant initial binary population and a formal multiplicity fraction of unity, and demonstrate that these models can explain the observed binary properties in Taurus, ρ Ophiuchus, Chamaeleon, Orion, IC 348, Upper Scorpius A, Praesepe, and the Pleiades. The model needs to consider solely different birth densities for these regions. The evolved theoretical orbital-parameter distributions are highly probable parent distributions for the observed ones. We constrain the birth conditions (stellar mass, M ecl , and half-mass radius, r h ) for the derived progenitors of the star clusters and the overall present-day binary fractions allowed by the present model. The results compare very well with properties of molecular cloud clumps on the verge of star formation. Combining these with previously and independently obtained constraints on the birth densities of globular clusters, we identify a weak stellar mass -half-mass radius correlation for cluster-forming cloud clumps, r h /pc ∝ (M ecl /M ) 0.13 ± 0.04 . The ability of the model to reproduce the binary properties in all the investigated young objects, covering present-day densities from 1−10 stars pc −3 (Taurus) to 2 × 10 4 stars pc −3 (Orion), suggests that environment-dependent dynamical evolution plays an important role in shaping the present-day properties of binary populations in star clusters, and that the initial binary properties may not vary dramatically between different environments.