Blue straggler stars beyond the Milky Way On the origin of blue straggler stars in Magellanic Cloud clusters

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1 Blue straggler stars beyond the Milky Way On the origin of blue straggler stars in Magellanic Cloud clusters Chengyuan Li (Macquarie University) Weijia Sun, Richard de Grijs, Licai Deng

2 Blue straggler stars are baby stars in old stellar systems. Formation Channels: Direct stellar collisions Binary evolution (mass transfer, merger)

Why Magellanic Cloud clusters? Advantage: They cover a wide age range. They suffer minor effect of extinction. They are not so dense like Galactic globular clusters.

7 Why Magellanic Cloud clusters? Advantage: They cover a wide age range. They suffer minor effect of extinction. They are not so dense like Galactic globular clusters. Disadvantage: Field contamination is poorly constrained. Crowding due to large distance. Instrument: HST, ACS/WFC & UVIS/WFC3 Filtered in B, V & I band (central wavelength from 435nm to 814nm). Resolution of ~80 mas in optical, equal to ~0.02 pc at LMC. NGC 2173

8 Surface Density Profile ebsss (arcsec 2 ) 10 0 BSSs RGB+AGB+RC MS PARSEC model (Bressan et al. 2012) logr (arcsec) Model parameters: Age (log(t/yr)), Metallicity (Z), Extinction (Av), Distance modulus ((m-m)0). Structure parameters: Core size (rc) Half-light radius (rhl) Dynamical parameters: Cluster Mass (M) Cluster Core Mass (MC) Central mass density (ρ0) Core velocity dispersion ( σ0 )

9 Nbss,c vs. Mcore Knigge et al This work Knigge et al This work A sub-linear relationship between the number of core BSSs and the core mass is revealed.

10 Nbss,c vs. γ Pooley & Hut (2006)

11 Why sub-linear? Nbss fbinmcore However, Milone et al Leigh et al based on the datasets from Milone et al. 2012

(Macquarie University), Licai Deng, Richard de Grijs, Antonino P. Milone Most young massive clusters in the Large Magellanic Cloud have bimodal or extended main sequences.

In this work, we examine this scenario through looking at the radial segregation of blue main sequence stars in cluster NGC 1850.

We suggest that this is because the blue main sequence stars are low mass-ratio binaries, for which the dynamical disruption is efficient in the cluster central region.

The tidal interaction from their binary components has decelerated their rotations, making them looks bluer than normal stars in NGC 1850 (most main sequence stars in NGC 1850 should be fast rotating

12 Why sub-linear? Nbss fbinmcore Dynamical implication of low mass-ratio binaries in young massive clusters --- new insights into the secondary main sequence in cluster NGC 1850 Author: Yujiao Yang, Chengyuan Li (Macquarie University), Licai Deng, Richard de Grijs, Antonino P. Milone Most young massive clusters in the Large Magellanic Cloud have bimodal or extended main sequences. It is suggested that the secondary main sequence stars are decelerated stars which hide a binary component. In this work, we examine this scenario through looking at the radial segregation of blue main sequence stars in cluster NGC We find that the blue main sequence stars are less segregated than normal stars and are anti-correlated to high massratio binaries in radial. We suggest that this is because the blue main sequence stars are low mass-ratio binaries, for which the dynamical disruption is efficient in the cluster central region. 3 Scan the QR code to obtain the manuscript or contact Chengyuan Li through chengyuan.li@mq.edu.au 1 It is suggested that most blue main sequence stars hide a binary component. The tidal interaction from their binary components has decelerated their rotations, making them looks bluer than normal stars in NGC 1850 (most main sequence stars in NGC 1850 should be fast rotating stars) QUESTION: What is the radial behaviour of these secondary main sequence stars (blue dots) compare to genuine high mass-ratio binaries (pink dots)? NGC 1850 is a young massive cluster (about 100 million years) located in the Large Magellanic Cloud galaxy. 2 If these secondary main-sequence stars are single stars, they should have the same radial profile like normal main sequence stars (green dots). 4 Highly anti-correlated! MAIN RESULT: We find that the blue main-sequence stars are less segregated than normal stars. In addition, it exhibits an anticorrelated radial profile (blue line) to mass-ratio binaries (red line) IMPLICATIONS: 1, Blue main sequence stars are low mass-ratio binaries. Most of them have been disrupted through three-body interactions in the cluster central region. NGC 1850 has a secondary (blue) main sequence which cannot be explained by a simple-stellar population (right-bottom) 2, The radial anti-correlation between blue main sequence stars and high mass-ratio binaries may indicate that the dynamical massexchange process among these binaries is ongoing. Milone et al. 2012

13 Nbss,c vs. γ Pooley & Hut (2006) Binary disruption Simulation data derived from Chatterjee et al. (2013)

14 Take-home message The main formation channel for blue straggler stars in Magellanic Cloud clusters, is binary evolution. Future work: 1, binary fractions in MC clusters. 2, mass segregation of blue straggler stars. 3, origin of bifurcation of blue straggler stars in young globular clusters

The Multiple Origin of Blue Stragglers. Hagai Perets Technion Israel Institute of Technology November 8, 2012

The Multiple Origin of Blue Stragglers Hagai Perets Technion Israel Institute of Technology November 8, 2012 Overview BSS formation channels and their predictions The triple origin of blue stragglers (TRI)