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Barium Enhanced Blue Stragglers in Open Cluster NGC 6819
Katelyn Milliman
University of Wisconsin-MadisonK. Milliman, R. Mathieu, & S. Schuler 2015
MotivationNon-standard stars are ubiquitous.Blue stragglers, yellow giants, sub-subgiants are not anomalous or rare.
NASA/ESA
MotivationBlue Straggler Formation• Stellar collision during dynamical
encounter.
• Merger in hierarchical triple.
• Mass transfer from an evolved companion.
Nucleosynthesis within the giant
ends up on the surface of the
blue straggler.
Red Giant (Case B) à C, N, O
AGB (Case C) à s-process, barium
NASA/ESA
Previous BSS Discoveries
47 Tuc; Ferraro et al. 2006
M30; Lovisi et al. 2013b
Carbon and oxygen anomaliesà CNO burning à mass transfer from RGB star.
NGC 188 Results
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4M2 ( MO • )
0
20
40
60
80
100
frequ
ency
(%)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4M2 ( MO • )
0
20
40
60
80
100
frequ
ency
(%)
Geller & Mathieu 2011
Gosnell et al. 2015Geller et al. 2009
Case C mass transfer with AGB donors à look for barium.
• P ~1,000 days• M2 peak 0.5 Msun• White dwarf
detections• HST, COS spectra
NGC 6819
0.5 1.0 1.5 2.0(V-I)
16
15
14
13
12
11
V
• Open cluster
• 2.5 Gyr, [Fe/H] = +0.09
• Observations in July 2014
• Hydra at WIYN 3.5 m
R ~19,000
• Ba II at 5853.7 Å
Results: Enhanced BSS!
5848 5850 5852 5854 5856 5858 5860Wavelength
0.8
1.0
1.2
1.4
1.6
1.8
Rela
tive
Flux
PRIMARY
SECONDARY
[Ba/Fe]= +1.32
[Ba/Fe]= +0.02
One Double-lined binary(flux from secondary)
Period = 300 dayseccentricity = 0.23
L2/L1 = 0.35q = 0.79
Results: Enhanced BSS!
5852 5853 5854 5855 5856Wavelength
0.7
0.8
0.9
1.0
1.1
Rela
tive
Flux
WOCS 5006
5852 5853 5854 5855 5856Wavelength
0.7
0.8
0.9
1.0
1.1
Rela
tive
Flux
WOCS 9003
5852 5853 5854 5855 5856Wavelength
0.7
0.8
0.9
1.0
1.1
Rela
tive
Flux
WOCS 8021
5852 5853 5854 5855 5856Wavelength
0.7
0.8
0.9
1.0
1.1
Rela
tive
Flux
WOCS 10006
Four “Single” StarsRed = solar bariumBlue = fits with
enhanced barium
Results
-0.3 -0.2 -0.1 -0.0 0.1 0.2 0.3[Fe/H]
-0.5
0.0
0.5
1.0
1.5
[Ba/Fe]
Error bars represent range in values from adjusting parameters.
Temperature: ± 200Klog (g): ± 0.2 dexvt: +0.5 km/s (at least)
NLTE estimated at 0.05 dex(Korotin et al. 2011)
Narrow CMD Location
Between evolutionary tracks for1.75 MSun and 1.95 MSun
0.5 1.0 1.5 2.0(V-I)
16
15
14
13
12
11
V
-0.3 -0.2 -0.1 -0.0 0.1 0.2 0.3[Fe/H]
-0.5
0.0
0.5
1.0
1.5[Ba/Fe]
No RV Companions?• RV observations over 24 years!
• Very solid, unvarying RVs.
• Monte Carlo simulation we detect:
50% of binaries at 15,500 days
(780 AU, 42 years)
25% of binaries at 40,000 days
(1,500 AU, 109 years)
à LONG period or single
0.5 1.0 1.5 2.0(V-I)
16
15
14
13
12
11
V
Dynamical Encounters?• Case C expect 1000-day, BS-WD
• Hard-soft limit 105 – 106 days.
• Hard becomes harder.More likely to shrink binary.
• Energy equipartition.Kick out white dwarf and leave blue straggler in a binary.
0.5 1.0 1.5 2.0(V-I)
16
15
14
13
12
11
VA. M. Geller
Dynamical Encounters?• Case C = 1000-day, BS-WD
• Hard-soft limit 105 – 106 days.
• Hard becomes harder.More likely to shrink binary.
• Energy equipartition.Kick out white dwarf and leave blue straggler in a binary.
A. M. Geller 0 .2 .4 .6 .8 1
−20
0
20
ORBITAL PHASER
AD
IAL
VE
LO
CIT
Y (
km s
− 1
)
STAR: 4004
29−Mar−2013
P = 297.168061 ± 0.090010 days
γ = +2.526 ± 0.260 km s−1
KA = 18.147 ± 0.468 km s−1
KB = 22.931 ± 0.420 km s−1
e = 0.227 ± 0.017
ω A = 88.88 ± 4.59 deg
T = 51907.048 ± 3.709
q = 0.791 ± 0.027
a A sin i = 72.2 ± 2.0 Gm
a B sin i = 91.3 ± 1.9 Gm
a sin i = 234.9 ± 4.1 Ro
M A sin 3 i = 1.101 ± 0.056 Mo
M B sin 3 i = 0.871 ± 0.051 Mo
NA = 38 / NB = 21
σ A = 2.10 km s −1
σ B = 1.56 km s −1
∆Τ = 47670.97 − 55823.80
= 8152.82 days
= 27.4 cycles
One Double-lined binary(flux from secondary)
Period = 300 dayseccentricity = 0.23
L2/L1 = 0.35q = 0.79
≈ 1
Universal Formation Channel?
0 1 2 3 4log (Period [days])
0.0
0.2
0.4
0.6
0.8
1.0
Ecce
ntric
ity
Four Open Cluster BSS: NGC 7789, NGC 6819, M67, NGC 188
Jorissen et al. 2015
Universal Formation Channel?
0 1 2 3 4log (Period [days])
0.0
0.2
0.4
0.6
0.8
1.0
Ecce
ntric
ity
Four Open Cluster BSS: NGC 7789, NGC 6819, M67, NGC 188
Jorissen et al. 2015
14% 80%
These BSS are Old
F.R.U.I.T.Y models Cristallo et al. 2009
-0.3 -0.2 -0.1 -0.0 0.1 0.2 0.3[Fe/H]
-0.5
0.0
0.5
1.0
1.5
[Ba/Fe]
+1.0 dex barium à 2 Msun star à age = 1 Gyr(naïve, simplified picture)
Age Limit for Barium Marker
0 1000 2000 3000BS Age Since MT (Myr)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Freq
uenc
y
Geller et al. 2013
Lifetime 1.5 MSun ≈ 3 GyrLongest lived BSS ≈ 2 Gyr
Age limit ≈ 5 Gyr
Cristallo et al. 2009
N-body Models AGB Models
(Solar metallicity)
NGC 188 (7 Gyr) Results
0.4 0.6 0.8 1.0 1.2 1.4 1.6(B-V)
16
15
14
13
12
11V
-0.3 -0.2 -0.1 -0.0 0.1 0.2 0.3[Fe/H]
-0.5
0.0
0.5
1.0
1.5
[Ba/Fe]
Summary• 5 barium enriched BSS in OC
NGC 6819 (2.5 Gyr).• Three are ~1 Gyr old.• Occupy a narrow region of CMD.• None have evidence of WD
companions.Periods over 15,500 days.Formation? -0.3 -0.2 -0.1 -0.0 0.1 0.2 0.3
[Fe/H]
-0.5
0.0
0.5
1.0
1.5
[Ba/Fe]
0.5 1.0 1.5 2.0(V-I)
16
15
14
13
12
11
V
0 1 2 3 4log (Period [days])
0.0
0.2
0.4
0.6
0.8
1.0
Ecce
ntric
ity