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Blue Straggler Star Popula.on in NGC 1261:
Evidence for a Triple BSS sequence and implica5ons for the Dynamical History of NGC 1261
Mirko Simunovic, Ph.D. Student
Ins.tute of Astrophysics, U. Catolica Chile Astronomisches Rechen Ins.tut, Heidelberg University
Mirko Simunovic Dance of Stars -‐ MODEST 2014 1
Co-‐Authors: Thomas Puzia, Alison Sills (McMaster)
• Complex Stellar Systems research group: Prof. Thomas Puzia
• Milky Way research group: Prof. Eva Grebel (see her talk at 16:00)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 2
Dynamics and BSS are related • Blue Straggler Stars (BSS) are
among the most massive stars in a GC.
• Strong dynamical events playing a major role in BSS forma.on
• They will respond notoriously to changes in the cluster poten.al.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 3
Dynamics and BSS are related
Mirko Simunovic Dance of Stars -‐ MODEST 2014 4
BSS observa.on
Dynamical State in a GC
BSS forma.on
Dynamics and BSS are related • The BSS frac.on radial
profile can be used to derive the cluster’s dynamical age (Ferraro et al. 2012)
• The radius at which BSS have sunk inwards will depend on the .me it takes the GC to reach energy equipar..on.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 5
Ferraro et al. 2012
Fun things happen on the dynamically oldest clusters
• The more evolved clusters will show a strong BSS central segrega.on
• Two special cases: M 30 (Ferraro et al. 2009) and NGC 362 (Dalessandro et al. 2013).
Mirko Simunovic Dance of Stars -‐ MODEST 2014 6
Ferraro et al. 2009 M 30
Fun things happen on the dynamically oldest clusters
• The more evolved clusters will show a strong BSS central segrega.on
• Two special cases: M 30 (Ferraro et al. 2009) and NGC 362 (Dalessandro et al. 2013).
Mirko Simunovic Dance of Stars -‐ MODEST 2014 7
Dalessandro et al. 2013 NGC 362
The occurrence of Double BSS Sequences in the CMD
Mirko Simunovic Dance of Stars -‐ MODEST 2014 8
Dalessandro et al. 2013 Ferraro et al. 2009
M 30
Dis.nct sequences suggest a single short forma.on event à Core Collapse
Mirko Simunovic Dance of Stars -‐ MODEST 2014 9
Dalessandro et al. 2013 Ferraro et al. 2009
M 30 Mass-‐transfer products Mass-‐transfer products
Collision products Collision products
*Tian et al. (2006) simula.ons predict BSS from case A (MS donor) mass-‐transfer binaries will populate a region whose fainter boundary is given by the ZAMS+0.75 mag in the V band.
Looking at NGC 1261
Mirko Simunovic Dance of Stars -‐ MODEST 2014 10
Credit: ESO
• Mass ≈ 3×105 M¤ (Boyles et al. 2011 )
• Rgal = 18.1 kpc (Harris catalog)
• [Fe/H] = -‐1.35 (Doler et al. 2012)
Looking at NGC 1261
Mirko Simunovic Dance of Stars -‐ MODEST 2014 11
5.05.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
0.5 1.0 1.5 2.0 2.5 3.0c
log(t_h)
Credit: ESO
Harris Catalog data
Looking at NGC 1261
Mirko Simunovic Dance of Stars -‐ MODEST 2014 12
Credit: ESO
Harris Catalog data
5.05.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
0.5 1.0 1.5 2.0 2.5 3.0c
log(t_h)
NGC362NGC1261
NGC7099
Paust et al. 2010 HST/ACS data
Looking at NGC 1261
Mirko Simunovic Dance of Stars -‐ MODEST 2014 13
Credit: ESO
Harris Catalog data
5.05.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
0.5 1.0 1.5 2.0 2.5 3.0c
log(t_h)
NGC362NGC1261
NGC7099
Paust et al. 2010 HST/ACS data
Leigh et al. 2011
Collec.ng Data for NGC 1261
• HST/ACS Globular Clusters Survey. High quality op.cal photometry in F606W (~V) and F814W (~I) bands for 65 GCs. (Sarajedini et al. 2007)
• Complementary F336W (~U) band HST/WFC3 photometry from Hubble Legacy Archive. PI: Piolo, Proposal ID: 13297. PSF photometry performed with DoPHOT (Alonso-‐Garcia et al. 2012)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 14
Paust et al. 2010 NGC 1261
Collec.ng Data for NGC 1261
• HST/ACS Globular Clusters Survey. High quality op.cal photometry in F606W (~V) and F814W (~I) bands for 65 GCs. (Sarajedini et al. 2007)
• Complementary F336W (~U) band HST/WFC3 photometry from Hubble Legacy Archive. PI: Piolo, Proposal ID: 13297. PSF photometry performed with DoPHOT (Alonso-‐Garcia et al. 2012)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 15
-55.25
-55.24
-55.23
-55.22
-55.21
-55.20
-55.19
-55.18
48.00 48.02 48.04 48.06 48.08 48.10 48.12RA
Dec
Final Photometric Catalog • We correct astrometry. Median
accuracy 0.002” between U and op.cal catalogs.
• We keep sources with separa.ons less than 0.02”, i.e. about 0.5 pixels of ACS camera.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 16
-55.25
-55.24
-55.23
-55.22
-55.21
-55.20
-55.19
-55.18
48.00 48.02 48.04 48.06 48.08 48.10 48.12RA
Dec
-2x10
0500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0.0 0.5 1.0 1.5 2.0Separation (arcsec)
Cou
nt
Selec.ng BSS candidates • We select BSS candidates photometrically
based on their loca5on in the (V-‐I) vs I color-‐magnitude diagram. We use the region defined by Leigh et al. (2011).
Mirko Simunovic Dance of Stars -‐ MODEST 2014 17
Selec.ng BSS candidates • We select BSS candidates photometrically
based on their loca5on in the (V-‐I) vs I color-‐magnitude diagram. We use the region defined by Leigh et al. (2011).
Mirko Simunovic Dance of Stars -‐ MODEST 2014 18
Selec.ng BSS candidates • We select BSS candidates photometrically
based on their loca5on in the (V-‐I) vs I color-‐magnitude diagram. We use the region defined by Leigh et al. (2011).
• The photometry reveals the presence of what seems to be a double sequence. We iden.fy the Blue-‐BSS and Red-‐BSS.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 19
Selec.ng BSS candidates • We select BSS candidates photometrically
based on their loca5on in the (V-‐I) vs I color-‐magnitude diagram. We use the region defined by Leigh et al. (2011).
• The photometry reveals the presence of what seems to be a double sequence. We iden.fy the Blue-‐BSS and Red-‐BSS.
• We detect 7 BSS candidates all roughly located similarly bluer than the B-‐BSS and extending in the same magnitude range.
• Preliminary evidence for a third component. We iden.fy them as extremelyB-‐BSS (eB-‐BSS).
Mirko Simunovic Dance of Stars -‐ MODEST 2014 20
Selec.ng BSS candidates • We select BSS candidates photometrically
based on their loca5on in the (V-‐I) vs I color-‐magnitude diagram. We use the region defined by Leigh et al. (2011).
• The photometry reveals the presence of what seems to be a double sequence. We iden.fy the Blue-‐BSS and Red-‐BSS.
• We detect 7 BSS candidates all roughly located similarly bluer than the B-‐BSS and extending in the same magnitude range.
• Preliminary evidence for a third component. We iden.fy them as extremelyB-‐BSS (eB-‐BSS).
• The BSS distribu.on in the region of the sequence is clearly bimodal at least.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 21
Looking for contaminants. • The U band photometry allows for
detec.on of poten.al cooler star blending or EHB+MS blend.
• Two B-‐BSSs and two eB-‐BSSs cannot be dis.nguished from normal stars.
• These stars are removed from sta.s.cal and popula.on tests.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 22
Looking for contaminants. • The U band photometry allows for
detec.on of poten.al cooler star blending or EHB+MS blend.
• Two B-‐BSSs and two eB-‐BSSs cannot be dis.nguished from normal stars.
• These stars are removed from sta.s.cal and popula.on tests.
• Variability can be an explana.on. U and I images are taken 7 years apart. SX Phe stars and W Uma stars are common among BSSs. Have been found in M 30 and NGC 362. (Ferraro et al. 2009; Dalessandro et al. 2013)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 23
Looking for contaminants. • The U band photometry allows for
detec.on of poten.al cooler star blending or EHB+MS blend.
• Two B-‐BSSs and two eB-‐BSSs cannot be dis.nguished from normal stars.
• These stars are removed from sta.s.cal and popula.on tests.
• Variability can be an explana.on. U and I images are taken 7 years apart. SX Phe stars and W Uma stars are common among BSSs. Have been found in M 30 and NGC 362. (Ferraro et al. 2009; Dalessandro et al. 2013)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 24
W Uma BSS in NGC 362, Dalessandro et al. 2013
ΔU≈0.3 mag
NGC 1261 has BSS sequences.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 25
NGC 362. Dalessandro et al. 2013
M 30 Ferraro et al. 2009
Comparison with Collision Models. • We use collision models from Sills et al.
(2009). • We are limited to Z=0.001 ([Fe/H]=-‐1.27)
and no alpha enhancement. • The models represents collisions of pairs of
stars with masses between 0.4 and 0.8 solar masses which then reach equilibrium.
• We adjust for distance modulus and reddening as found in Doler et al. (2012)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 26
Comparison with Collision Models. • We use collision models from Sills et al.
(2009). • We are limited to Z=0.001 ([Fe/H]=-‐1.27)
and no alpha enhancement. • The models represents collisions of pairs of
stars with masses between 0.4 and 0.8 solar masses which then reach equilibrium.
• We adjust for distance modulus and reddening as found in Doler et al. (2012)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 27
Comparison with Collision Models. • We use collision models from Sills et al.
(2009). • We are limited to Z=0.001 ([Fe/H]=-‐1.27)
and no alpha enhancement. • The models represents collisions of pairs of
stars with masses between 0.4 and 0.8 solar masses which then reach equilibrium.
• We adjust for distance modulus and reddening as found in Doler et al. (2012)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 28
Comparison with Collision Models. • We use collision models from Sills et al.
(2009). • We are limited to Z=0.001 ([Fe/H]=-‐1.27)
and no alpha enhancement. • The models represents collisions of pairs of
stars with masses between 0.4 and 0.8 solar masses which then reach equilibrium.
• We adjust for distance modulus and reddening as found in Doler et al. (2012)
Mirko Simunovic Dance of Stars -‐ MODEST 2014 29
1.1 M¤
1.5 M¤ 1.3 M¤
1.0 M¤
What can we learn about the R-‐BSS sequence?
• We use 0.25 Gyr old isochrones (~ZAMS)
from Dartmouth data base (Doler et al. 2008).
• We find that the R-‐BSSs populate a region bracketed by ZAMS+0.45 and ZAMS+0.75 mag limits.
• The ZAMS+0.75 mag boundary does not well represent the loca.on of the R-‐BSS sequence in NGC 1261.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 30
What can we learn about the R-‐BSS sequence?
• The R-‐BSSs in NGC 1261 are binaries with q
Results so far… • We have a prely good chance of having
detected a new double BSS sequence in NGC 1261.
• The B-‐BSS and R-‐BSS sequence are similar in shape and extension to what has been found in M 30 and NGC 362.
• We have evidence of a third BSS popula.on, presumable much younger.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 32
-‐R-‐BSSs are binaries with q
The segrega.on of BSSs -‐> Dynamics • We construct cumula.ve radial profiles. The
center of gravity of NGC 1261 given by ACS data analysis of Goldsbury et al. (2010).
• We find the full BSS sample more concentrated than SGB stars. This is expected.
• We find the B-‐BSS popula.on more centrally concentrated than the R-‐BSS popula.on. (K-‐S test p-‐value=0.33-‐0.14) – Collision BSS forma.on more concentrated? – BSS binary profile evolved radially outwards?
• M30 and NGC362 show the opposite. – What can this tell us about BSS forma.on in them?
• Are collision products expected to be distributed along the cluster similarly?. This could suggest they are simply following the density profile.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 33
eB-BSS
The segrega.on of BSSs -‐> Dynamics
Mirko Simunovic Dance of Stars -‐ MODEST 2014 34
eB-BSS
NGC 362. Dalessandro et al. 2013
M 30 Ferraro et al. 2009
The segrega.on of BSSs -‐> Dynamics
Mirko Simunovic Dance of Stars -‐ MODEST 2014 35
eB-BSS
NGC 362. Dalessandro et al. 2013
M 30 Ferraro et al. 2009
Milone et al. (2012): Binary frac5ons profiles based on HST/ACS data
The segrega.on of BSSs -‐> Dynamics
Mirko Simunovic Dance of Stars -‐ MODEST 2014 36
eB-BSS
NGC 362. Dalessandro et al. 2013
M 30 Ferraro et al. 2009
Introducing a wide-‐field catalog • We use the wide-‐field photometric catalog produced by Kravtsov et al. (2010). • 1.3m Warsaw Telescope UBVI photometry in a 14’x14’ field of view centered in NGC
1261. • Kravtsov et al. (2010) select BSS candidates based on their loca5on on all CMDs.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 37
Kravtsov et al. (2010)
Merging inner and outer catalog
• We combine both catalogs at 3.8 rc .
• The combined SGB radial profile shows fair completeness.
• We use the reference SGB popula.on to calculate the BSS frac5on radial profile, i.e. NBSS/NSGB.
Mirko Simunovic Dance of Stars -‐ MODEST 2014 38
010
20
30
40
50
60
70
80
90
0 2 4 6 8 10 12 14 16r/r_c
Cou
nt
SGB radial distribution
BSS Frac.on Radial Profile
• We find a central peak in NBSS/NSGB
without clear signs of a subsequent secondary peak.
• BSS appear to have all (or most) experienced mass segrega.on.
• NGC 1261 would classify as a dynamically old GC (Ferraro et al. 2012).
Mirko Simunovic Dance of Stars -‐ MODEST 2014 39
BSS Frac.on Radial Profile
Mirko Simunovic Dance of Stars -‐ MODEST 2014 40
Dalessandro et al. 2013 NGC 362
NGC 1261: Dynamically Young or Old?
Mirko Simunovic Dance of Stars -‐ MODEST 2014 41
• The core appears extended. Concentra.on c ≈ 1.2
• The central stellar density is low. ρ=2.22 L¤/pc3 (Paust et al. 2010)
• The binary frac.on profile lacks central segrega.on.(Milone et al. 2012)
• The half-‐mass relaxa5on 5me is short: log(th) ≈ 8
• The occurrence of a double BSS sequence. (Core-‐collapse)
• The BSS affected by segrega.on
5.05.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
0.5 1.0 1.5 2.0 2.5 3.0c
log(t_h)
NGC362NGC1261
NGC7099
concentra.on
Log(t h)
NGC 1261: Dynamically Young or Old?
Maybe is both
Mirko Simunovic Dance of Stars -‐ MODEST 2014 42
NGC 1261: Dynamically Young or Old?
Mirko Simunovic Dance of Stars -‐ MODEST 2014 43
• Monte Carlo dynamical models (Heggie & Giersz 2008) and N-‐body models (Hurley & Shara 2012): GCs can go through core-‐collapse and then stay or pass through a long-‐lived post-‐core-‐collapse bounce state.
• M 4 went through core collapse at 8 Gyr and then roughly stayed in a non-‐collapsed (but centrally more concentrated) state for another ~2-‐3 Gyr due to binary burning in the core. (Heggie & Giersz 2008)
arcmin Milone et al. 2012
fbq>0.5
NGC 1261: Dynamically Young or Old?
Mirko Simunovic Dance of Stars -‐ MODEST 2014 44
• Monte Carlo dynamical models (Heggie & Giersz 2008) and N-‐body models (Hurley & Shara 2012): GCs can go through core-‐collapse and then stay or pass through a long-‐lived post-‐core-‐collapse bounce state.
• M 4 went through core collapse at 8 Gyr and then roughly stayed in a non-‐collapsed (but centrally more concentrated) state for another ~2-‐3 Gyr due to binary burning in the core. (Heggie & Giersz 2008)
arcmin Milone et al. 2012
fbq>0.5
Conclusions
Mirko Simunovic Dance of Stars -‐ MODEST 2014 45
1. The inner BSS popula.on of NGC 1261 has a double sequence in the CMD, much similar to what’s found in M30 and NGC 362.
2. We find photometric evidence for a third bluer BSS popula5on.
3. Collision models are able to reproduce the B-‐BSSs as a 2 Gyr old popula.on and the eB-‐BSSs as a 200 Myr old popula.on.
4. The dynamical state of NGC 1261 is unclear, one possible scenario: – NGC 1261 could have gone through core-‐collapse or some similar phase about 2 Gyr
ago, and since then it has been bouncing around through core oscilla5ons, occasionally crea.ng more BSSs during short .me-‐scale processes -‐one in par.cular 0.2 Gyr ago-‐, and also likely burning some of its core binaries up.
– NGC 1261 is currently in a post core-‐collapsed state which, according to different simula.ons, may look as an unevolved GC.