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The Mass Transfer Formation of Blue Stragglers as Revealed by

White Dwarf Companions

March 27, 2014Stellar Tango at the Rockies

Collaborators: Bob Mathieu (UW-‐Madison), Aaron Geller (Northwestern), Alison Sills (McMaster), Nathan Leigh (Alberta), Christian Knigge (Southampton)

Natalie GosnellUniversity of Wisconsin–Madison

(Fall ’14 → University of Texas at Austin)

Thursday, March 27, 14

Blue Straggler Formation Scenarios

1. Inner binary merger driven by Kozai mechanism (Perets & Fabrycky 2009)

2. Stellar collision during dynamical encounter (Leigh & Sills 2011)

3. Mass transfer from primary on giant branch (Chen & Han 2008)

Lombardi et al. 2011

Aaron Geller

Blondin, Richards, & Malinowski


Blue straggler

Blue straggler binary (SB1)


NGC 188 Cluster Members

•Over 15 years of radial velocity data, including orbital solutions

•Complete down to V=16.5

•3D membership determination

11

12

13

14

15

16

V (m

ag)

0.4 0.6 0.8 1.0 1.2 1.4 1.6B–V (mag)

Age: 7 GyrDistance: 2 kpc

Mathieu & Geller 2009

Blue straggler



1.61.41.21.00.80.60.4

BSS : 80% binaries

MS : 23% binaries


0 1 2 3 4

log[P (d)]

1.0

0.8

0.6

0.4

0.2

0.0

a

bEcce

ntric

ity

1.0

0.8

0.6

0.4

0.2

0.0

0 1 2 3 4

Main Sequence

Blue Stragglers

Mathieu & Geller 2009

Main Sequence

Blue Stragglers

Blue straggler binaries are different, and suggest that companions are white dwarfs

•Binary properties different than main sequence population•Long-period (some circular!) binaries•Secondary mass distribution peaks at 0.5 M⊙

log Period (days)

Ecce

ntri

city

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40

20

40

60

80

100

Freq

uenc

y (%

)

Companion mass (M�)

Blue Straggler Secondary Mass Distribution

Geller & Mathieu 2012


Search for white dwarf companions with HST

•ACS/SBC far-UV photometry to detect white dwarf companions to the blue stragglers in NGC 188

•Awarded 41 orbits in Cycle 19 (data obtained ~13 months ago)

•Can use presence of white dwarf(s) to map out specific histories for this population of blue stragglers


NGC 188 presents a unique opportunity to study potential white dwarf companions

•Old cluster (7 Gyr) with solar metallicity has cooler blue stragglers

•White dwarfs detectable as photometric UV excess (not possible in globular clusters or younger open clusters)

•Use of derived bandpasses isolates bluest UV flux (see Dieball et al. 2005)

1200 1400 1600 1800 200022

21

20

19

18

17

1200 1400 1600 1800 2000Wavelength (Å)

22

21

20

19

18

17

STM

AG

White dwarf: 13,000 K Blue Straggler: 6,500 K

F140N

F150N

F165LP

Blue straggler spectra: UVBLUE (Rodríguez-Merino et al. 2005)White dwarf models: P. Bergeron, priv. comm.


0 1 2 3 4

24

22

20

18

0 1 2 3 4

F150N - F165LP

24

22

20

18F

150N

99%

11000 13250 15500 17750 20000

White Dwarf Temperature (K)

WOCS 4348

WOCS 4540

WOCS 5379

90%

70%

50%

30%

HST photometry reveals white dwarf companions

Single Blue Stragg

lers

6,000 K

6,500 K

Blue straggler models: UVBLUEWhite dwarf models: Bergeron

Gosnell et al. 2014, ApJL, 783, L8


UV excess straightforwardly explained by white dwarf companion

1200 1400 1600 180022

21

20

19

18

17

1200 1400 1600 1800Wavelength (Å)

22

21

20

19

18

17

STM

AG

1400 1600 18001400 1600 1800Wavelength (Å)

1400 1600 1800 20001400 1600 1800 2000Wavelength (Å)

1200 1200

WOCS 4540 WOCS 5379 WOCS 4348WD: 18,000 K BSS: 6,500 K WD: 17,000 K BSS: 6,000 K WD: 13,000 K BSS: 6,500 K

F140N

F150N

F165LP

F140N

F150N

F165LP

F140NF150N

F165LP

Gosnell et al. 2014, ApJL, submitted

•Detected UV excess cannot be explained by a normal blue straggler spectrum alone, but well-matched by relatively hot white dwarfs•Derived narrow bandpasses provide best determination of UV flux and estimate of white dwarf temperature from photometry alone•White dwarf temperature correlates to a white dwarf age


UV excess straightforwardly explained by white dwarf companion

1200 1400 1600 180022

21

20

19

18

17

1200 1400 1600 1800Wavelength (Å)

22

21

20

19

18

17

STM

AG

1400 1600 18001400 1600 1800Wavelength (Å)

1400 1600 1800 20001400 1600 1800 2000Wavelength (Å)

1200 1200

WOCS 4540 WOCS 5379 WOCS 4348WD: 18,000 K BSS: 6,500 K WD: 17,000 K BSS: 6,000 K WD: 13,000 K BSS: 6,500 K

F140N

F150N

F165LP

F140N

F150N

F165LP

F140NF150N

F165LP

Gosnell et al. 2014, ApJL, submitted

•Detected UV excess cannot be explained by a normal blue straggler spectrum alone, but well-matched by relatively hot white dwarfs•Derived narrow bandpasses provide best determination of UV flux and estimate of white dwarf temperature from photometry alone•White dwarf temperature correlates to a white dwarf age

246 Myr68 Myr 77 Myr


3 detections consistent with all 14 single-lined binary blue stragglers being formed through mass transfer

0 2 4 6 8 10 12Number of detections

0.00

0.05

0.10

0.15

0.20

0.25

Freq

uenc

y (%

)

0 500 1000 1500 2000 2500BS Age Since MT (Myr)

0.00

0.05

0.10

0.15

0.20

Freq

uenc

y (%

)

Age distribution of mass transfer-formed blue stragglers from N-body model of NGC 188,

using BSE (Geller et al. 2013)

Monte Carlo sampling of age/temp distribution given our

observational design

12,000 K

Gosnell et al. 2014, ApJL

3.4±1.5 detections expected


Previous in-depth investigation of these post-mass transfer binaries and the cluster provide key to mass transfer modelsBinaries:Solved binary orbits •period, eccentricity, mass function

Broadband photometry •ultraviolet to J, H, K bands

NGC 188:AgeMetallicityDistanceMembershipN-body models

} Can define stellar population at any age

Have the pieces necessary to observationally constrain mass transfer models of these specific systems

11

12

13

14

15

16

V (m

ag)

0.4 0.6 0.8 1.0 1.2 1.4 1.6B–V (mag)

0 1 2 3 4

24

22

20

18

0 1 2 3 4

F150N - F165LP

24

22

20

18

F150N

99%

11000 13250 15500 17750 20000

White Dwarf Temperature (K)

WOCS 4348

WOCS 4540

WOCS 5379

90%

70%

50%

30%


What is the mass transfer history of these binaries?

What do we know?•Mass transfer ended recently (< 300 Myr ago)•Initial primary star must be ~1.17 M⊙

•Resulting blue straggler must have MBS > 1.12 M⊙ (Mturnoff)

•Final orbital period matches observed period


Basic mass transfer modeling can create plausible blue straggler-white dwarf binaries

Using Binary-Star Evolution (BSE) code (Hurley et al. 2002)

Initial binary: 1.18 M⊙ primary and 1.11 M⊙ secondary with Porb=1685 days

Evolves for 6.752 Gyr: • primary is now AGB and mass is 0.91 M⊙ due to wind loss

Overflows the Roche lobe, transferring mass onto the secondary

Mass transfer ends leaving blue straggler and white dwarf:• CO white dwarf = 0.57 M⊙

• Blue straggler = 1.29 M⊙

• Porb = 1168 days (Pobs = 1168 ± 8 days)• WD age = 246 Myr ~ 13,000 K


NASA 1200 1250 1300 1350 1400 1450Wavelength (Å)

0

1

2

3

4

f h (1

0ï��

erg

cm

ï� s

ï� Å

ï�)

S1040:

log (g) = 6.7

Teff

= 16,135 K

M = 0.235 M~

Landsman et al. 1997

Will measure masses of white dwarfs directly with COS spectra this year (September 2014)

•Cycle 21 COS spectra of Ly-α region of white dwarf companions to measure masses and temperatures

•Provide additional constraints on mass transfer history to feed more complicated mass transfer modeling efforts


Three binaries test three different mass transfer scenarios

WOCS 5379: Porb = 120 days ⇒ RGB Roche lobe overflow

(Case B)

WOCS 4348: Porb = 1168 days ⇒ AGB Roche lobe overflow

(Case C)

WOCS 4540: Porb = 3030 days ⇒ BHL wind accretion inBSE, but perfect test case for “Case D” accretion

BSE scenarios completed (Gosnell et al. 2014), will move onto sophisticated modeling with COS measurements as constraints


Summary

•Blue straggler formation inherently tied to binary evolution

•Three NGC 188 blue stragglers have young, hot white dwarf companions that formed through recent mass transfer

•These binaries provide important constraints for mass transfer modeling efforts

•Using first-order modeling tools (e.g. BSE) we can successfully model the formation of these systems, but more modeling for the future!


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