Building a Comprehensive Picture of Stellar Evolution

Building a Comprehensive Picture of Stellar Evolution

Natalie M. Gosnell Assistant Professor, Colorado College

September 28, 2018 St. Olaf College

Physics Colloquium

AIDA: R. Chromik

What am I going to talk about today?

A large fraction of stars don’t evolve as they “should”

N. M. Gosnell September 28, 2018

Current understanding of stellar evolution is incomplete, so we need observations to improve our models

Blue straggler stars provide the largest handle on this population of stars

My work on mass transfer formation of blue straggler stars with the Hubble Space Telescope is helping fill in the gaps

Open clusters are the ideal laboratory for studying stellar evolution

APOD

Group of hundreds to thousands of stars • born at the same time • made from the same

material • all at the same distance

Jewel Box Cluster (NGC 4755)


VLT Telescopes, ESO/Y. Beletsky

Open clusters exist in the disk of our galaxy


https://500px.com/ybeletsky

NGC 6819

DSS

APOD

NGC 290

www.robgendlerastropics.com

NGC 188

Open clusters exist in the disk of our galaxyM67

Xanadu Observatory

VLT Telescopes, ESO/Y. Beletsky


https://500px.com/ybeletsky

Created by A. Geller, Northwestern University


Hot (blue) Cool (red)

Hertzsprung-Russell (H-R) diagrams organize stars by temperature (color) and luminosity (brightness)

Movie credit: NASA, ESA, J. Anderson, & R. van der Marel (annotations mine)


Hertzsprung-Russell (H-R) diagrams organize stars by temperature (color) and luminosity (brightness)


faint (small)

Bright (large)



H-R diagrams reveal different stages of typical stellar evolution

Main Sequence

faint (small)

Bright (large)


Subgiants

Giant BranchHorizontal Branch


White Dwarf Sequence



Main Sequence

Over time

faint (small)

Bright (large)





Main Sequence

Subgiants

Giant Branch

faint (small)

Bright (large)



Horizontal Branch




Main Sequence

Subgiants

Giant Branch

faint (small)

Bright (large)



We have reliable models for how a single star will evolve

Horizontal Branch



Open clusters are the ideal laboratory for studying stellar evolution

APOD



Jewel Box Cluster (NGC 4755)But which stars are

members of the cluster?


Accurate cluster memberships are important for more detailed studies of stellar evolution

NGC 188: -7 billion years old -5500 light years away

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Gosnell 2014, adapted from Geller et al. 2008





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eNGC 188:

-7 billion years old -5500 light years away

1498 stars in the region of the cluster

Hot Cool



Kinematic information separates cluster members from the field population

The Astronomical Journal, 148:38 (20pp), 2014 August Milliman et al.

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Figure 4. Percentage of stars in our sample that have three or more RV observations and either no Pµ information or a Pµ ! 4% with respect to distance from thecluster center (left) and V magnitude (right).

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RV DistributionCluster Member DistributionField Population

Figure 5. Histogram of the RV distribution of single stars, e/i < 4, withthree or more RV observations. Also plotted are the Gaussian distributionssimultaneously fit to the cluster, the large peak at a mean velocity of 2.4 km s−1

(blue dotted line), and the field (orange dashed line).(A color version of this figure is available in the online journal.)

because they require better observing conditions (clearer skies,dimmer moon, etc.) and longer exposures to meet our signal-to-noise requirements.

4.4. RV Membership Probabilities

The RV membership probability of a given star, PRV, listedin Table 2 is calculated using the equation:

PRV (v) = Fcluster(v)Ffield(v) + Fcluster(v)

, (3)

where Fcluster and Ffield are separate Gaussian functions fit to thecluster and field-star populations using our sample of single starswith three or more RV observations (Figure 5). The parametersfor these Gaussian fits are shown in Table 3. With the observationand completion of more single stars these values have shiftedonly slightly from the numbers published in Hole et al. (2009)and we continue to use the membership threshold of PRV ! 50%adopted by Hole et al. (2009) to identify cluster members. We

Table 3Gaussian Fit Parameters for Cluster and Field RV Distributions

Parameter Cluster Field

Ampl. (number) 112.99 8.81RV (km s−1) 2.45 −14.59σ (km s−1) 1.02 25.79

Table 4Number of Stars Within Each Classification

Classification N Stars

SM 566SN 1381BM 93BN 79BLM 20BLN 172BU 22U 1562

estimate from the cluster and field Gaussian functions a field starcontamination of 13% at this membership threshold (Figure 5).Interestingly, using Pµ < 4% as a criterion for non-membership,we find a field-star contamination of 12% ± 2% among starswith PRV ! 50%.

4.5. Membership Classification of Stars

Stars in our RV survey of NGC 6819 can fall into one of eightdifferent membership classifications determined by membershipprobability, number of observations, and variability. The numberof stars in each membership classification is listed in Table 4. Forsingle stars and velocity variable stars without completed orbitalsolutions we calculate PRV from the mean velocity, RV, and forbinary stars with completed orbital solutions we use the center-of-mass velocity, γ , from the orbital solution. For stars withoutPµ information, membership is determined by PRV alone.

Single member (SM). Stars that have e/i < 4, PRV ! 50%,and Pµ ! 4%.

Single non-member (SN). Stars that have e/i < 4, and PRV <50% or Pµ < 4%.

6

Milliman et al. 2014

Proper motions Compare modern images with photographic

plates from 50–70 years ago

Radial velocities Requires at least 3 spectra of every star, and

years of coverage to obtain binary memberships

The Astronomical Journal, 146:43 (12pp), 2013 August Platais et al.

Table 4A Sample of the Catalog of Positions and Proper Motions in NGC 6819

ID R.A. (J2000) Decl. (J2000) g′ g′ − r ′ µxa µy

a σµxa σµy

a σesta Pµ

b Xtc Yt

c Platesd ndele

155298 294.890863 39.818837 14.272 0.510 10.61 9.19 0.75 0.75 0.75 0 −19.908 −22.605 00001 0159195 294.936317 39.821684 13.905 1.128 7.55 0.39 0.82 0.82 0.82 0 −17.812 −22.444 00001 0160120 295.124249 39.822393 16.125 1.336 −10.94 −0.75 0.70 0.70 0.70 0 −9.151 −22.430 00001 0162002 295.053344 39.823850 15.594 0.799 6.94 −4.22 0.70 0.70 0.70 0 −12.418 −22.334 00001 0162175 294.948913 39.823970 15.238 0.425 6.30 −1.14 0.70 0.70 0.70 0 −17.231 −22.309 00001 0

Notes.a Units in mas yr−1.b Proper-motion membership probability in percent.c Tangential coordinates in arcmin with zeropoint at the center of NGC 6819.d Encoded number of plates, tuuvz, where t is McDonald; uu is Yale; v is Hale; z is Mt. Wilson (see Table 1).e Number of rejected datasets.

(This table is available in its entirety in machine-readable and Virtual Observatory (VO) forms in the online journal. A portion is shown here for guidance regardingits form and content.)

Figure 3. Vector-point diagram in the area of NGC 6819. A tight clumpat µx = µy = 0.0 mas yr−1 indicates the location of cluster members, closelysurrounded by a slightly offset and slanted distribution of field stars.

stars, Φf , which adequately represent the chosen local sample.The parameters of these distributions can be estimated in situand prior to calculating:

Pµ = Φc

Φc + Φf

. (3)

For technical details, we refer the reader to Kozhurina-Plataiset al. (1995) and Platais et al. (2003). There are, however, acouple of simplifications in the case of NGC 6819. In terms ofradial distance, the entire inner area of r < 10′ was consideredas a local sample for all targets within this area. For the outerarea with r > 10′, membership probabilities were calculated forstars with g′ < 17 and their local sample confined to the outerarea. In both cases we used a 2 mag window, centered on thetarget’s magnitude, to constrain the local sample. Thus, a localsample can include around one hundred stars up to more than onethousand, depending on target’s spatial location and brightness.The division of the inner and outer local samples is dictated bya sharp decrease in the accuracies of proper motions beyondr = 10′ and a reduced limiting magnitude (see Table 1). Priorto finding the distribution parameters, the VPD was rotated by

Figure 4. Distribution of Pµ as a function of magnitude. The decline ofmaximum Pµ toward fainter magnitudes is a consequence of steadily growingluminosity function of field stars and an increase of proper motion errors. Atg′ > 20.5 the separation between the cluster and field stars is poor and that isindicated by a flat and unrealistically high distribution of Pµ. The secondarydistribution peaking at g′ ∼16 (max Pµ = 63%) is for the stars outside theradius r = 10′.

−30◦ to align the field distribution along the R.A. and decl. axes.At g′ = 16, the center of field-star distribution in the VPD is only0.8 mas yr−1 away from the center of the cluster distribution.The corresponding Gaussian dispersions are 2.5 mas yr−1 forfield stars along the minor axis and 3.6 mas yr−1 along themajor axis, but only 0.4 mas yr−1 in either axis for cluster stars,illustrating how unlike the two distributions are. We note thatat fainter magnitudes (g′ = 20) the distributions are separatedby 2.5 mas yr−1, but the cluster dispersion also grows up to∼1 mas yr−1.

The formal sum of membership probabilities indicates a totalof ∼2500 cluster members down to g′ = 22.5 (V ∼ 22) with theaforementioned qualification on the outer sample. The reliabilityof our proper motions is shown by comparing the maximumPµ as a function of magnitude (Figure 4). The maximum Pµ

remains above 90% at g′ < 18 but then steadily drops to∼40% at g′ = 20 for the stars with r < 10′. For the outerstars, the maximum Pµ is only 63% at g′ = 16.3 and drops to40% at g′ = 18. In the outer parts of cluster the membershipprobabilities are only useful for identifying additional clustermembers on the main sequence, which in a CMD is the mostdensely populated feature of a cluster.

The integrity of our new cluster memberships are bestillustrated by provisional CMDs of stars separated into the likely

6

Platais & Gosnell et al. 2013

Proper motion movement (x)

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Stars in cluster

Stars in field of galaxy

Stars in cluster





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➔ 473 members

✔✔

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Cluster color-magnitude diagrams are filled with alternative-pathway stellar products

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single star model

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small, coolsmall, hot

large, hot large, cool Open cluster NGC 188

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James Lombardi

NASA/AEI/ZIB/M. Koppitz and L. Rezzolla

David A. Hardy & PPARC

Casey Reed

Approximately half of all stars are in binary (or higher-order) systems



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25% of evolved stars in old open clusters do not follow single-star evolutionary models

25%

16%59%

Giants

Subgiants

Stellar products (blue stragglers, yellow giants,

sub-subgiants, W UMa)


25%

16%59%


Giants

Stellar products excluded from stellar evolution studies for many years • Possible field contamination • Thought to be rare or anomalous

Subgiants




25%

16%59%


Giants

Stellar products are numerous and form a key subpopulation of evolved stars!!

Subgiants




25%

16%59%


Giants

My work focuses on this 25% of stars in order to build a more comprehensive picture of stellar evolution

Subgiants




Current theoretical models of stellar populations cannot make accurate stellar products

Many areas of astrophysics rely on stellar population models.

In order to match theory and reality we need to fix the physics in these models that creates stellar products.

Evolved populations in these models (the most luminous stars and easiest to observe) are inaccurate and incomplete.


How do we build a more comprehensive picture?

Use synergy of observations and theoretical models to determine the formation of stellar products

Once we know how they formed, we can model future evolution

Add new insights into stellar population models


Blue stragglers are found in every evolved and old stellar population

Ferraro et al. 1999

Globular Clusters

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Open Clusters

M67

Momany 2014

Dwarf Galaxies

Fig. 6.2 From left to right the panels show the colour-magnitude diagram of NGC 6362 [57], LeoIIdwarf galaxy [25], and Fornax dwarf galaxy [28]. The ellipse approximately traces the BSS regionin NGC 6362 and LeoII, whereas for the Fornax dwarf contamination by the young stars forbidsreliable BSS estimates.

blue plume population falls within the BSS limits in globular clusters. On the otherhand, the right panel of Fig.6.2 summarises the BSS–young stars ambiguity in dwarfgalaxies. The Fornax dwarf galaxy is known to host a recent star formation episode,that occurred some 200 Myr ago [69], and the diagram from the HST survey [28]shows that in such cases one cannot properly reach the ancient MS turn-off levelwithout the inclusion of contaminant young stars. Thus, the selection of a dwarfgalaxies sample for BSS studies must filter out all those galaxies that do not allow aclear detection of the ancient MS turn-off level.

In this regards, one should bear in mind that the fainter end of the BSS sequenceextends to ⇠ 0.6 magnitude below the ancient MS turn-off level (e.g. the case ofM55 by [34]). Given the small number statistics of the BSS stars in dwarf galaxies,

Stellar color Stellar color Stellar color

Mag

nitud

e

Mag

nitud

e

Mag

nitud

eAdd mass, but how?


Observations and theory settle on two major formation mechanisms for blue straggler stars

Astronomy Magazine: Roen Kelly

Mass Transfer

Collision


Stars can collide in cluster environment during gravitational interactions

Created by A. Geller, Northwestern University


Stars can collide in a cluster environment during dynamical encounters

Courtesy of Aaron Geller

triple system

binary system


We can model the collision of the two stars in detail

0.8 solar masses

0.7 solar masses

James Lombardi


Observations and theory settle on two major formation mechanisms for blue straggler stars

Astronomy Magazine: Roen Kelly

Mass Transfer

Collision


Main sequence companion

Mass transfer occurs when one star overflows its sphere of

gravitational influence

These stars have similar masses, but the slightly more massive one evolves faster and becomes a red giant

Main sequence primary

Red Giant primary

(to approximate scale)N. M. Gosnell September 28, 2018

Mass transfer occurs when one star overflows its sphere of

gravitational influence Red Giant

primary

White dwarfBlue straggler

The red giant transfers its envelope to the companion, which gains mass

(to approximate scale)

The companion becomes a blue straggler, and the red giant leaves behind a white dwarf

Main sequence companion


We can model mass transfer, but we lack excellent observational constraints

Blondin, Richards, & MalinowskiN. M. Gosnell September 28, 2018

I use the Hubble Space Telescope to study mass transfer formation of blue straggler stars

NASA

HST sensitive in the ultraviolet where white dwarf companions will be brighter than the blue straggler

Awarded a total of 53 orbits, or ~80 hours, of HST time

Discovered white dwarf companions of blue stragglers (Gosnell et al. 2014)

Majority of open cluster blue stragglers form through mass transfer (Gosnell et al. 2015)


•Relatively close (5500 light years away) •Very evolved (7 billion years old) •Extremely well-studied (Platais et al. 2003; Sarajedini et al. 1999; Geller et al. 2008, 2009, 2013; Mathieu & Geller 2009; Geller & Mathieu 2011, 2012, Gosnell et al. 2014, 2015)

Digitized Sky Survey

Open cluster NGC 188 provides an ideal environment for studying blue straggler stars


Detecting a white dwarf companion tells us the blue straggler formed through mass transfer

NASA/ESA, A. Feild (STScI)

Red Giant

Accreting Main Sequence companion

Blue Straggler

White Dwarf

TodayIn the past


NGC 188 provides unique opportunity to detect white dwarf companions

1400 1600 1800 200024

22

20

18

1400 1600 1800 2000Wavelength

24

22

20

18

STM

AG

BS: 6,500 K

BS: 6,000 K

WD: 18,000 K

WD: 12,000 K

140N

150N

165LP

• Old cluster (7 Gyr) has cooler, less massive blue stragglers

• White dwarfs detectable as far-ultraviolet excess (not possible in younger open clusters)

140 160 180 200Wavelength (nm)

log

(Det

ecte

d Flu

x)


0 1 2 3 424

23

22

21

20

19

0 1 2 3 4F150N - F165LP

24

23

22

21

20

19F1

50N

99.73%

99.73%

99.73%

95.45

%

95.45%

68%

Expected blue

straggler emissio

n

Gosnell et al. 2014, 2015

Bright

Faint

Mag

nitud

e

Stellar color

Some blue stragglers have significant photometric far-ultraviolet excess

(warm)(hot)


0 1 2 3 424

23

22

21

20

19

0 1 2 3 4F150N - F165LP

24

23

22

21

20

19F1

50N

99.73%

99.73%

99.73%

95.45

%

95.45%

68%

11000 12750 14500 16250 18000White Dwarf Temperature (K)

Expected blue

straggler emissio

n

cool white dwarf

hot white dwarf


Bright

Stellar colorFaint

Mag

nitud

e


(warm)(hot)


0 1 2 3 424

23

22

21

20

19

0 1 2 3 4F150N - F165LP

24

23

22

21

20

19F1

50N

99.73%

99.73%

99.73%

95.45

%

95.45%

68%

11000 12750 14500 16250 18000White Dwarf Temperature (K)

cool white dwarf

hot white dwarf

Expected blue

straggler emissio

n


Bright M

agnit

ude

Stellar colorFaint


(warm)(hot)


Many blue stragglers have detected white dwarf companions

Gosnell et al. 2015

0.4 0.6 0.8 1.0 1.2

16

15

14

13

12

0.4 0.6 0.8 1.0 1.2B - V

16

15

14

13

12

V

18,500 16,000 13,500 11,000Temperature of WD companion (K)

18882679

4230

4348

4540

5350

5379

B – V

V

Non-velocity variableSingle-lined binariesDouble-lined binaries

Stellar color

Mag

nitud

e

Bright

Faint


Many blue stragglers have detected white dwarf companions

Gosnell et al. 2015

0.4 0.6 0.8 1.0 1.2

16

15

14

13

12

0.4 0.6 0.8 1.0 1.2B - V

16

15

14

13

12

V

18,500 16,000 13,500 11,000Temperature of WD companion (K)

18882679

4230

4348

4540

5350

5379

B – V

V

Non-velocity variableSingle-lined binariesDouble-lined binaries

Stellar color

Mag

nitud

e

Bright

Faint

Each white dwarf will set the mass transfer timeline


We can determine how long ago a white dwarf formed by measuring the temperature

Based on models by P. Bergeron

White Dwarf Cooling Curve

Time since WD formed (billions of years)

2 4 6 8

20,000

15,000

10,000

5,000

Tem

pera

ture

of

WD

(K

elvi

n)



Teff = 12,000 K time = 0.4 Gyr

Teff = 8,000 K time = 2.5 Gyr Teff = 6,000 K

time = 4.8 Gyr

White dwarfs cool over time, like coals after a fire goes out




2 4 6 8

20,000

15,000

10,000

5,000

Tem

pera

ture

of

WD

(K

elvi

n)


Measure white dwarf temperature accurately and precisely with ultraviolet spectroscopy using HST

Geocoronal lines from the glow of Earth’s atmosphere

Teff = 17,300 ± 200 K MWD = 0.52 ± 0.01 M⊙

HST spectrum of white dwarf companion

Gosnell et al. 2017 (in prep)




2 4 6 8

20,000

15,000

10,000

5,000

This white dwarf formed only 100 million years ago!

➔ mass transfer ended 100 Myr ago

Tem

pera

ture

of

WD

(K

elvi

n)




Observationally-established mass transfer histories constrain models of mass transfer physics

Mass of primary star: 1.2 M⊙

Mass of secondary star:

1.0 M⊙

Mass of WD: 0.5 M⊙

Mass of blue straggler: 1.3 M⊙

Amount of mass

transferred: 0.3 M⊙

Today110 Myr ago

Binary period: 1600 days


???


We know starting and ending points, now we need to fix the mass transfer physics in the middle


Observationally-established mass transfer histories constrain models of mass transfer physics

Today110 Myr ago


We know starting and ending points, now we need to fix the mass transfer physics in the middle, using MESA (Paxton et al. 2010)

Mass of primary star: 1.2 M⊙

Mass of secondary star:

1.0 M⊙

Mass of WD: 0.52 M⊙

Mass of blue straggler: 1.3 M⊙

Amount of mass

transferred: 0.3 M⊙




How do we build a more comprehensive picture?

Use synergy of observations and theoretical models to determine the formation of stellar products

Once we know how they formed, we can model future evolution

Add new insights into stellar population models

✔

(✔)not yet


Summary and Implications

25% of evolved stars follow alternative pathways in stellar evolution

Current understanding of stellar evolution is incomplete, so we need observations to improve our models

Blue straggler stars provide the largest handle on this population of stellar products

Constraints from white dwarf companions outline the formation history of blue stragglers and will improve stellar population models in the future


Building a Comprehensive Picture of Stellar Evolution

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