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