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Home , Blue straggler, NGC 290

Building a Comprehensive Picture of

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 don’t evolve as they “should”

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 transfer formation of blue straggler stars with the is helping fill in the gaps

N. M. Gosnell September 28, 2018 Open clusters are the ideal laboratory for studying stellar evolution

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)

APOD N. M. Gosnell September 28, 2018 Open clusters exist in the disk of our

VLT Telescopes, ESO/Y. Beletsky N. M. Gosnell September 28, 2018 Open clusters exist in the disk of our galaxy

M67 NGC 6819

Xanadu Observatory DSS

NGC 188 NGC 290

www.robgendlerastropics.com APOD

VLT Telescopes, ESO/Y. Beletsky N. M. Gosnell September 28, 2018 Created by A. Geller, Northwestern University

N. M. Gosnell September 28, 2018 Hertzsprung-Russell (H-R) diagrams organize stars by temperature (color) and (brightness)

Hot (blue) Cool (red)

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

N. M. Gosnell September 28, 2018 Hertzsprung-Russell (H-R) diagrams organize stars by temperature (color) and luminosity (brightness) Bright (large)

Hot (blue) Cool (red)

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

N. M. Gosnell September 28, 2018 H-R diagrams reveal different stages of typical stellar evolution Bright (large)

Horizontal Branch Giant Branch

Subgiants Sequence Hot (blue) Cool (red)

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

N. M. Gosnell September 28, 2018 H-R diagrams reveal different stages of typical stellar evolution Bright (large) Over time

Main Sequence Hot (blue) Cool (red)

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

N. M. Gosnell September 28, 2018 H-R diagrams reveal different stages of typical stellar evolution Bright (large)

Horizontal Branch Giant Branch

Subgiants White Dwarf Sequence Main Sequence Hot (blue) Cool (red)

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

N. M. Gosnell September 28, 2018 H-R diagrams reveal different stages of typical stellar evolution Bright (large)

Horizontal Branch Giant Branch

Subgiants White Dwarf Sequence Main Sequence Hot (blue) Cool (red) We have reliable models for how a single will evolve

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

N. M. Gosnell September 28, 2018 Open clusters are the ideal laboratory for studying stellar evolution

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

But which stars are Jewel Box Cluster (NGC 4755) members of the cluster?

APOD N. M. Gosnell September 28, 2018 Accurate cluster memberships are important for more detailed studies of stellar evolution

11 NGC 188: Bright -7 billion years old 12 -5500 light years away

13 1498 stars in the region of the cluster V

14

Magnitude

15 Group of hundreds to thousands of stars • born at the same time 16 Hot Cool • made from the same

Faint 0.4 0.6 0.8 1.0 1.2 1.4 StellarB - Vcolor material • all at the same distance Gosnell 2014, adapted from Geller et al. 2008

N. M. Gosnell September 28, 2018 Accurate cluster memberships are important for more detailed studies of stellar evolution

11 NGC 188: Bright -7 billion years old 12 -5500 light years away

13 1498 stars in the region of the cluster V

14

Magnitude

15 Group of hundreds to thousands of stars • born at the same time 16 Hot Cool • made from the same

Faint 0.4 0.6 0.8 1.0 1.2 1.4 StellarB - Vcolor material • all at the same distance Gosnell 2014, adapted from Geller et al. 2008

N. M. Gosnell September 28, 2018 The Astronomical Journal,148:38(20pp),2014August Milliman et al.

100 100

The Astronomical Journal,146:43(12pp),2013August 80 Platais et al. 80 Table 4 A Sample of the Catalog of Positions and Proper Motions in NGC 6819

a a a a a b c c d e ID R.A. (J2000) Decl. (J2000) g g r µx µy σµ σµ 60σest Pµ Xt Yt Plates ndel 60 ′ ′ − ′ x y 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 0 − − 159195 294.936317 39.821684 13.905 1.128 7.55 0.39 0.82 0.82 0.82 0 17.812 22.444 00001 0 − − 160120 295.124249 39.822393 16.125 1.336 10.94 0.75 0.70 0.7040 0.70 0 9.151 22.430 00001 0 40 − − − − Completeness (%) 162002 295.053344 39.823850 15.594 0.799 6.94 4.22 0.70 0.70 0.70 0 12.418 22.334 00001 0 Completeness (%) − − − 162175 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 Kinematic information separates− cluster− members− 20 20 Notes. a 1 Unitsfrom in mas yr− the. field population b Proper-motion membership probability in percent. c Tangential coordinates in arcmin with zeropoint at the center of NGC 6819. 0 0 d Encoded number of plates, tuuvz,wheret is McDonald; uu is Yale; v is Hale; z is Mt. Wilson (see Table 10). 5 10 15 20 25 30 11 12 13 14 15 16 e Number of rejected datasets.Proper motions Radial velocitiesRadius (arcmin) V (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 regarding Figure 4. Percentage of stars in our sample that have three or more RV observations and either no P information or a P 4% with respect to distance from the its formCompare and content.) modern images with photographic Requires at least 3 spectra of every star, and µ µ ! plates from 50–70 years ago clusteryears center of (left) coverage and V magnitude to obtain (right). binary memberships

140 Table 3 RV Distribution Gaussian Fit Parameters for Cluster and Field RV Distributions Cluster Member Distribution 120 Parameter Cluster Field Field Population Stars in cluster 100 Ampl. (number) 112.99 8.81 RV (km s 1)2.4514.59 − − σ (km s 1)1.0225.79 80 Stars in cluster −

60 Table 4 Number of Stars Within Each Classification

Number of Objects 40 Classification N Stars Number of stars Stars in field of galaxy SM 566 Figure20 4. Distribution of Pµ as a function of magnitude. The decline of maximum Pµ toward fainter magnitudes is a consequence of steadily growing SN 1381 movement (y) Proper luminosity function of field stars and an increase of proper motion errors. At 0 BM 93 g′ > 20.5theseparationbetweentheclusterandfieldstarsispoorandthatis BN 79 indicated by-60 a flat and unrealistically-40 -20 high distribution0 of Pµ.Thesecondary20 40 BLM 20 distribution peaking at g′ 16 (maxRVPµ (km/s)63%) is for the stars outside the Proper motion movement (x) radius r 10 . ∼= (km/s) BLN 172 = ′ Figure 3. Vector-point diagram in the area of NGC 6819. A tight clump FigureMilliman 5. Histogramet al. 2014 of the RV distribution of single stars, e/i < 4, with BU 22 1 at µPlataisx µy &0 Gosnell.0masyr− etindicates al. 2013 the location of cluster members, closely three or more RV observations. Also plotted are the Gaussian distributions U1562 surrounded= by= a slightly offset and slanted distribution of field stars. simultaneously fit to the cluster, the large peak at a mean velocity of 2.4 km s 1 30◦ to align the field distribution along the R.A. and decl. axes. − (blue− dotted line), and the field (orange dashed line). At g′ 16, the center of field-star distribution in the VPD is only (A color version= of1 this figure is available in the online journal.) stars, Φf ,whichadequatelyrepresentthechosenlocalsample. 0.8 mas yr− away from the center of the cluster distribution. 1 estimate from the cluster and field Gaussian functions a field star N.The M. parameters Gosnell of these distributions can be estimated in situ The corresponding Gaussian dispersions are September 2.5 mas yr− 28,for 2018 1 contamination of 13% at this membership threshold (Figure 5). and prior to calculating: becausefield stars they along require the better minor observing axis and 3.6 conditions mas yr− (cleareralong the skies, major axis, but only 0.4 mas yr 1 in either axis for cluster stars, Interestingly, using Pµ < 4% as a criterion for non-membership, dimmer moon, etc.) and longer− exposures to meet our signal-to- Φc illustrating how unlike the two distributions are. We note that we find a field-star contamination of 12% 2% among stars Pµ . (3) noise requirements. with P 50%. ± = Φc + Φf at fainter magnitudes (g′ 20) the distributions are separated RV ! 1 = by 2.5 mas yr− , but the cluster dispersion also grows up to 14.4. RV Membership Probabilities For technical details, we refer the reader to Kozhurina-Platais 1masyr− . 4.5. Membership Classification of Stars ∼ et al. (1995)andPlataisetal.(2003). There are, however, a TheThe RV formal membership sum of membership probability probabilities of a given indicates star, P a total,listed couple of simplifications in the case of NGC 6819. In terms of of 2500 cluster members down to g 22.5(V 22) withRV the Stars in our RV survey of NGC 6819 can fall into one of eight in Table∼ 2 is calculated using the equation:′ = ∼ radial distance, the entire inner area of r<10′ was considered aforementioned qualification on the outer sample. The reliability different membership classifications determined by membership as a local sample for all targets within this area. For the outer of our proper motions is shown by comparing the maximum Fcluster(v) probability, number of observations, and variability. The number area with r>10′,membershipprobabilitieswerecalculatedfor Pµ as a functionPRV of(v) magnitude (Figure 4). The, maximum Pµ (3) of stars in each membership classification is listed in Table 4.For = F (v)+F (v) stars with g′ < 17 and their local sample confined to the outer remains above 90% at g′ field< 18 butcluster then steadily drops to single stars and velocity variable stars without completed orbital area. In both cases we used a 2 mag window, centered on the 40% at g 20 for the stars with r<10 .Fortheouter ∼ ′ = ′ solutions we calculate P from the mean velocity, RV, and for target’s magnitude, to constrain the local sample. Thus, a local wherestars,Fcluster the maximumand FfieldPareµ is separate only 63% Gaussian at g 16 functions.3anddropsto fit to the RV ′ = sample can include around one hundred stars up to more than one cluster40% and at g field-star′ 18. In populations the outer parts using of our cluster sample the membership of single stars binary stars with completed orbital solutions we use the center- thousand, depending on target’s spatial location and brightness. withprobabilities three or= more are only RV observations useful for identifying (Figure additional5). The parameters cluster of-mass velocity, γ , from the orbital solution. For stars without The division of the inner and outer local samples is dictated by formembers these Gaussian on the fits main are sequence, shown in which Table in3. a With CMD the is observation the most Pµ information, membership is determined by PRV alone. asharpdecreaseintheaccuraciesofpropermotionsbeyond densely populated feature of a cluster. and completion of more single stars these values have shifted Single member (SM).Starsthathavee/i < 4, PRV ! 50%, r 10′ and a reduced limiting magnitude (see Table 1). Prior only slightlyThe integrity from the of our numbers new cluster published memberships in Hole et are al. best (2009) and P 4%. to= finding the distribution parameters, the VPD was rotated by illustrated by provisional CMDs of stars separated into the likely µ ! and we continue to use the membership threshold of PRV ! 50% Single non-member (SN).Starsthathavee/i < 4, and PRV < adopted6 by Hole et al. (2009)toidentifyclustermembers.We 50% or Pµ < 4%. 6 Accurate cluster memberships are important for more detailed studies of stellar evolution

11 NGC 188: Bright -7 billion years old 12 -5500 light years away

13 1498 stars in the region of the cluster V

14

Magnitude

15 Group of hundreds to thousands of stars • born at the same time 16 Hot Cool • made from the same

Faint 0.4 0.6 0.8 1.0 1.2 1.4 StellarB - Vcolor material • all at the same distance Gosnell 2014, adapted from Geller et al. 2008

N. M. Gosnell September 28, 2018 Accurate cluster memberships are important for more detailed studies of stellar evolution

11 NGC 188: Bright -7 billion years old 12 -5500 light years away

13 1498 stars in the region of the cluster V 14 ➔ 473 members

Magnitude

15 Group of hundreds to thousands of stars

• born at the same time 16 ✔ Hot Cool ✔• made from the same Faint 0.4 0.6 0.8 1.0 1.2 1.4 StellarB - Vcolor material • all at the same distance Gosnell 2014, adapted from Geller et al. 2008 ✔

N. M. Gosnell September 28, 2018 Cluster color-magnitude diagrams are filled with alternative-pathway stellar products

11 large, hot large, cool Bright 12 NGC 188

13

Giant Branch V V V 14

Magnitude single star model 15 Branch Main 16 Sequence small, hot small, cool Faint 0.4 0.6 0.8 1.0 1.2 1.4 1.6 StellarB - Vcolor

Gosnell 2014, adapted from Geller et al. 2008

N. M. Gosnell September 28, 2018 Approximately half of all stars are in binary (or higher-order) systems

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

Casey Reed

James Lombardi David A. Hardy & PPARC N. M. Gosnell September 28, 2018 Cluster color-magnitude diagrams are filled with alternative-pathway stellar products

11 large, hot large, cool Open cluster Bright 12 NGC 188

13 equal-mass binary model

Giant Branch V V V 14

Magnitude single star model 15 Subgiant Branch Main 16 Sequence small, hot small, cool Faint 0.4 0.6 0.8 1.0 1.2 1.4 1.6 StellarB - Vcolor

Gosnell 2014, adapted from Geller et al. 2008

N. M. Gosnell September 28, 2018 Cluster color-magnitude diagrams are filled with alternative-pathway stellar products

11 large, hot NGC 188 large, cool Open cluster Bright Yellow giants 12 NGC 188 Contact binaries

13 Blue stragglers

X-ray sources V V V 14

Magnitude

15 Sub-subgiants

16 small, hot small, cool Faint 0.4 0.6 0.8Stellar1.0 color1.2 1.4 1.6 B - V

Gosnell 2014, adapted from Geller et al. 2008

N. M. Gosnell September 28, 2018 25% of evolved stars in old open clusters do not follow single-star evolutionary models

Stellar products (blue stragglers, yellow giants, sub-subgiants, W UMa) 25% Giants

59% 16% Subgiants

N. M. Gosnell September 28, 2018 25% of evolved stars in old open clusters do not follow single-star evolutionary models

Stellar products (blue stragglers, yellow giants, sub-subgiants, W UMa) 25% Giants

59% Subgiants 16%

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

N. M. Gosnell September 28, 2018 25% of evolved stars in old open clusters do not follow single-star evolutionary models

Stellar products (blue stragglers, yellow giants, sub-subgiants, W UMa) 25% Giants

59% Subgiants 16%

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

N. M. Gosnell September 28, 2018 25% of evolved stars in old open clusters do not follow single-star evolutionary models

Stellar products (blue stragglers, yellow giants, sub-subgiants, W UMa) 25% Giants

59% Subgiants 16%

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

N. M. Gosnell September 28, 2018 Current theoretical models of stellar populations cannot make accurate stellar products

Many areas of astrophysics rely on models.

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

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

N. M. Gosnell September 28, 2018 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

N. M. Gosnell September 28, 2018 Blue stragglers are found in every evolved and old stellar population

Open Clusters Globular Clusters Direct N-body modelling of stellar populations 631 Dwarf

Add mass, but how? Magnitude Magnitude Magnitude

M67

Figure 1. The number of blue stragglers relative to the number of MS stars in the two magnitudes below the turn-off as a function of the population age. The stars represent the open cluster data of Ahumada & Lapasset 51995), with the M67 point an open symbol. The dashed line represents our population synthesis with a 50 per cent binary population, using the para- meters of PS6 5see Section 3). The solid line represents our M67 N-body simulation 5see Section 6; note that the log-scale does not clearly indicate the length of the simulation).Stellar color Stellar color Stellar color

Figure 2. CMD for M67 5NGC 2682) using photometric data taken from Fig. 6.2 From left to right the panels show the colour-magnitude diagram of NGC 6362 [57], LeoII the most obvious scenarios involving binary evolution. A case A the Open ClusterHurley Database 5OCD; et Mermilliodal. 2001 1996). Circles show dwarf galaxy [25], and dwarf galaxyMomany [28]. The ellipse 2014 approximately traces the BSS region mass transfer scenario 5Kippenhahn, Weigert & Hoffmeister 1967) probable members P $ 80 per cent) as indicated by proper motion in NGC 6362 and LeoII, whereas for the Fornax dwarf contamination by the young stars forbids Ferraro et al. 1999 mb reliable BSS estimates. involves a main-sequence star filling its and studies, and triangles show stars of less certain membership. Open symbols transferring mass to its companion, a less massive main-sequence are spectroscopic binaries. Stars identified as blue stragglers in the OCD star, followed by coalescence of the two stars as the orbit shrinks are plotted as stars. Open squares represent the seven RS CVn candidates blue plume population falls within the BSS limits in globular clusters. On the other owing to angular momentum loss. The result is a more massive identified by Belloni, Verbunt & Mathieu 51998, hereafter BVM). hand, the right panel of Fig.6.2 summarises the BSS–young stars ambiguity in dwarf main-sequence star that is rejuvenated relative to other stars of the Asterisks are the six X-ray sources examined by van den Berg, Verbunt & Mathieu 51999, hereafter MVB), excluding S1082 which has already galaxies. The Fornax dwarf galaxy is known to host a recent episode, same mass and thus evolves to become a blue straggler. This is an been plotted as a blue straggler. The cross represents the triple system that occurred some 200 Myr ago [69], and the diagram from the HST survey [28] efficient method of producing single blue stragglers, provided that observed by Mathieu, Latham & Griffin 51990, hereafter MLG). All the shows that in such cases one cannot properly reach the ancient MS turn-off level a large population of close binaries exists in the cluster. Case B $ special stars that we have highlighted have Pmb 80 per cent. Also plotted without the inclusion of contaminant young stars. Thus, the selection of a dwarf mass transfer involves a main-sequence star accreting material 5full line) is an isochrone at t 4160 Myr with Z 0:02; m 2 M 9:7 ˆ ˆ ˆ galaxies sample for BSS studies must filter out all those galaxies that do not allow a from a more evolved companion and thus could be a likely and E B 2 V 0:015: explanation for blue stragglers in short-period spectroscopic †ˆ clear detection of the ancient MS turn-off level. binaries, such as F190. Blue stragglers in long-period binaries In this regards, one should bear in mind that the fainter end of the BSS sequence extends to 0.6 magnitude below the ancient MS turn-off level (e.g. the case of N. M. couldGosnell be produced by case C mass transfer when the primary is an 30 per cent. Additionally, the probability of encounters depends on ⇠ September 28, 2018 5AGB) star that has lost much of its mass. the density of the cluster. It is also possible that perturbations from M55 by [34]). Given the small number statistics of the BSS stars in dwarf galaxies, Wind in binaries that initially have fairly large periods passing stars may induce an eccentricity in a previously circular could also be responsible for such systems. orbit. As discussed by Leonard 51996), it is unlikely that any one In all these cases, except perhaps wind accretion, the binary formation mechanism dominates, and in the case of the diverse orbits should be circularized by tides before and during mass blue straggler population of M67 it seems probable that all the transfer. Other scenarios are needed to explain the binaries in above scenarios play a role. eccentric orbits, and this is where the effects of dynamical The aim of this paper is to compare N-body models with interactions in a cluster environment become important. Physical observations of M67 to investigate the incidence and distribution stellar collisions during binary±binary and binary±single inter- of blue stragglers 5BSs), and in so doing to constrain the nature of actions can produce blue stragglers in eccentric orbits, as well as the primordial binary population. In Section 2 we give an allowing the possibility of an existing blue straggler being overview of the observational data, in terms of individual stellar exchanged into an eccentric binary. According to Davies 51996), populations and overall cluster parameters. We describe the details encounters between binaries and single stars become important of our binary population synthesis in Section 3 and use it to when the binary fraction in the core exceeds about 5 per cent and constrain the parameters of the various distributions involved, with binary±binary encounters dominate if the fraction is greater than a view to maximizing the number of blue stragglers produced. The

q 2001 RAS, MNRAS 323, 630±650 Observations and theory settle on two major formation mechanisms for blue straggler stars

Mass Transfer

Collision

Astronomy Magazine: Roen Kelly N. M. Gosnell September 28, 2018 Observations and theory settle on two major formation mechanisms for blue straggler stars

Mass Transfer

Collision

Astronomy Magazine: Roen Kelly N. M. Gosnell September 28, 2018 Stars can collide in cluster environment during gravitational interactions

Created by A. Geller, Northwestern University

N. M. Gosnell September 28, 2018 Stars can collide in a cluster environment during dynamical encounters

binary system

triple system

Courtesy of Aaron Geller N. M. Gosnell September 28, 2018 We can model the collision of the two stars in detail

0.8 solar

0.7 solar masses

James Lombardi N. M. Gosnell September 28, 2018 Observations and theory settle on two major formation mechanisms for blue straggler stars

Mass Transfer

Collision

Astronomy Magazine: Roen Kelly N. M. Gosnell September 28, 2018 Mass transfer occurs when one star overflows its sphere of gravitational influence Main sequence primary companion

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

(to approximate scale)

N. M. Gosnell September 28, 2018 Mass transfer occurs when one star overflows its sphere of gravitational influence Red Giant Main sequence primary companion

Blue straggler White dwarf

The redcompanion giant transfers becomes its a envelopeblue straggler, to the and companion, the red whichgiant leaves gains behindmass a white dwarf

(to approximate scale)

N. M. Gosnell September 28, 2018 We can model mass transfer, but we lack excellent observational constraints

Blondin, Richards, & Malinowski N. M. Gosnell September 28, 2018 I use the Hubble Space Telescope to study mass transfer formation of blue straggler stars 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)

NASA N. M. Gosnell September 28, 2018 Open cluster NGC 188 provides an ideal environment for studying blue straggler stars

•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 N. M. Gosnell September 28, 2018 Detecting a white dwarf companion tells us the blue straggler formed through mass transfer

In the past Today Red Giant White Dwarf

Accreting Main Sequence Blue Straggler companion

NASA/ESA, A. Feild (STScI)

N. M. Gosnell September 28, 2018 NGC 188 provides unique opportunity to detect white dwarf companions

BS: 6,500 K

18 • Old cluster (7 Gyr) has cooler, 140N WD: 18,000 K less massive blue stragglers

20 150N • White dwarfs detectable as STMAG far-ultraviolet excess (not

log (Detected Flux) possible in younger open 22 WD: 12,000 K BS: 6,000 K clusters)

165LP 24 1400140 1601600 1800180 2000200 Wavelength Wavelength (nm)

N. M. Gosnell September 28, 2018 Some blue stragglers have significant photometric far-ultraviolet excess

Gosnell et al. 2014, 2015

Bright

19

20

99.73% 21

95.45% F150N 68%

Magnitude 22 99.73%

23 95.45% Expected blue

99.73% (hot) straggler (warm)emission Faint 24 0 1 2 3 4 StellarF150N - F165LPcolor

N. M. Gosnell September 28, 2018 Some blue stragglers have significant photometric far-ultraviolet excess

Gosnell et al. 2014, 2015

Bright hot white dwarf 11000 12750 14500 16250 18000 19 White Dwarf Temperature (K)

20 cool white dwarf

99.73% 21

95.45% F150N 68%

Magnitude 22 99.73%

23 95.45% Expected blue

99.73% (hot) straggler (warm)emission Faint 24 0 1 2 3 4 StellarF150N - F165LPcolor

N. M. Gosnell September 28, 2018 Some blue stragglers have significant photometric far-ultraviolet excess

Gosnell et al. 2014, 2015

Bright hot white dwarf 11000 12750 14500 16250 18000 19 White Dwarf Temperature (K)

20 cool white dwarf

99.73% 21

95.45% F150N 68%

Magnitude 22 99.73%

23 95.45% Expected blue

99.73% (hot) straggler (warm)emission Faint 24 0 1 2 3 4 StellarF150N - F165LPcolor

N. M. Gosnell September 28, 2018 Many blue stragglers have detected white dwarf companions

12

18,500 16,000 13,500 11,000 Bright Temperature of WD companion (K) Non-velocity variable 13 Single-lined binaries Double-lined binaries 5350

4540 14 V

V

4348 1888 2679

Magnitude 15 4230 5379

16

Faint 0.4 0.6 0.8 1.0 1.2 StellarBB –- VcolorV Gosnell et al. 2015

N. M. Gosnell September 28, 2018 Many blue stragglers have detected white dwarf companions

12

18,500 16,000 13,500 11,000 Bright Temperature of WD companion (K) Non-velocity variable 13 Single-lined binaries Double-lined binaries 5350

4540 14

V Each white dwarf will set

V 4348 the mass transfer timeline 1888 2679

Magnitude 15 4230 5379

16

Faint 0.4 0.6 0.8 1.0 1.2 StellarBB –- VcolorV Gosnell et al. 2015

N. M. Gosnell September 28, 2018 We can determine how long ago a white dwarf formed by measuring the temperature

20,000 White Dwarf Cooling Curve

15,000

10,000

5,000 Temperature of WD (Kelvin) 2 4 6 8 Time since WD formed (billions of years)

Based on models by P. Bergeron

N. M. Gosnell September 28, 2018 We can determine how long ago a white dwarf formed by measuring the temperature

20,000 White Dwarf Cooling Curve

White dwarfs cool over time, 15,000 Teff = 12,000 K time = 0.4 Gyr like coals after a fire goes out

Teff = 8,000 K 10,000 time = 2.5 Gyr Teff = 6,000 K time = 4.8 Gyr 5,000 Temperature of WD (Kelvin) 2 4 6 8 Time since WD formed (billions of years)

Based on models by P. Bergeron

N. M. Gosnell September 28, 2018 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)

N. M. Gosnell September 28, 2018 We can determine how long ago a white dwarf formed by measuring the temperature

20,000 White Dwarf Cooling Curve This white dwarf formed only 15,000 100 million years ago!

10,000 ➔ mass transfer ended 100 Myr ago

5,000 Temperature of WD (Kelvin) 2 4 6 8 Time since WD formed (billions of years)

Based on models by P. Bergeron

N. M. Gosnell September 28, 2018 Observationally-established mass transfer histories constrain models of mass transfer physics 110 Myr ago Today

Mass of primary star: Mass of WD: 1.2 M⊙ 0.5 M⊙ Mass of ??? secondary star:

1.0 M⊙ ⊙ Amount of mass Mass of blue transferred: Binary0.3 M period: Binary period: 1100 days straggler: 1.3 M⊙ 1600 days

NASA/ESA, A. Feild (STScI) We know starting and ending points, now we need to fix the mass transfer physics in the middle

N. M. Gosnell September 28, 2018 Observationally-established mass transfer histories constrain models of mass transfer physics 110 Myr ago Today

Mass of primary star: Mass of WD: 1.2 M⊙ 0.52 M⊙ Mass of secondary star:

1.0 M⊙ ⊙ Amount of mass Mass of blue transferred: Binary0.3 M period: Binary period: 1100 days straggler: 1.3 M⊙ 1600 days

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

N. M. Gosnell September 28, 2018 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 not yet population models

N. M. Gosnell September 28, 2018 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

N. M. Gosnell September 28, 2018