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The Remarkable Chemical Compositions of Blue Metal-Poor Stars

George Preston on behalf of Chris Sneden friends & collaborators George Preston (Carnegie Observatories) John Cowan (University of Oklahoma) Ian Thompson, Steve Shectman (Carnegie Observatories. The Remarkable Chemical Compositions of Blue Metal-Poor Stars. Outline of the talk.

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The Remarkable Chemical Compositions of Blue Metal-Poor Stars

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  1. George Preston on behalf of Chris Sneden friends & collaborators George Preston (Carnegie Observatories) John Cowan (University of Oklahoma) Ian Thompson, Steve Shectman (Carnegie Observatories The Remarkable Chemical Compositions of Blue Metal-Poor Stars

  2. Outline of the talk • What are blue metal-poor (= BMP) stars? • Use of binary fractions to resolve BMPs into blue straggler and intermediate-age populations • Fundamental differences between blue stragglers in globular clusters and the halo field • Chemical compositions: general results • A new spectroscopic study: binaries versus single stars • Present & future observational/theoretical opportunities

  3. Identifying field BMP stars: Galactic disk star color-color relation BMP domain can be populated by metal-poor MS stars. It is almost empty in the solar neighborhood [Fe/H] ~ 3  [Fe/H] ~ 1 [Fe/H] ~ 0 de-blanketing (blue) vectors Data points are the B8-F0 stars in the Bright Star Catalog Preston et al. 1994

  4. BMP domain is a region in which isochrones for a wide range of ages and metallicities overlap in a tangled mess. MS isochrones Turnoffs for: [Fe/H] = 2.2 ages 3 7,10 Gy Isochrones of various [Fe/H] values and ages overlap in the BMP star domain SG isochrones Isochrones in the U-B versus B-V plane are from Green et al. (1987) Revised Yale Isochrones Preston & Sneden 2000

  5. BMP stars were readily identified by UBV photometryof stars found in the HK objective-prism survey Metal poor stars near turnoff BMPstars BHB MS [Fe/H]= 1 RHB MS [Fe/H]=0 “HK” Survey: Beers et al. 1985, 1992 Preston et al. 1994

  6. Blue metal-poor stars of the halo field have many physical characteristics of the blue stragglers first identified in globular clusters, but there is a problem: THERE ARE FAR TOO MANY OF THEM IN THE HALO FIELD! NGC 288 (Kaluzny 1996) AGB RHB RGB BHB BMP MS turnoff MS Common distance makes identification easy

  7. Resolution of the problem lay in a radial velocity survey:a surprisingly high proportion of BMP stars are members of spectroscopic binaries 1.5 km/s >60% of BMP’s are binaries with atypically long orbital periods and small mass functions. s < 1.5 km/s: 1/18 stars with orbits s > 1.5 km/s: 41/43 stars with orbits 1.5 km/s is an observational limit; more low-amplitude binaries may be hidden in the RV errors. Standard deviation of a RV measurement (km/s) Preston & Sneden 2000

  8. Typical orbital solutions based on radial velocity variations in the BMP sample >60% of BMP’s are binaries with unusually long orbital periods and unusually small mass functions. f(m) (K1)3P Preston & Sneden 2000

  9. We use binary fractions to resolve blue metal-poor stars into blue straggler and intermediate-age components. xxxxxxxxxxxxxxx n

  10. To estimate the BS fraction of BMP n(BS)/n(BMP) fBMP, fBS, and fIA must be “good numbers”. COMMENTS fBMP  Reliability limited only by accuracy of RV’s & duration of survey fBS Adopt Duquennoy & Mayor n(P) for primordial blue stragglers.  Assume that 13% of primordial blue straggler binaries with P < 5 d have merged.  Remainder (87%) must be mass-transfer binaries. fIA Adopt 0.15 as “universal binary fraction” for P<4000 d from: fDisk = 0.15 Duquennoy & Mayor 1991 fHalo = 0.14 Latham et al 1998

  11. Only this 13% of binaries with P4000d can merge in a Hubble time (Vilhu, O. 1982, A&Ap, 109, 17) This is why we adopt fBS 0.87 visual binaries radial velocity binaries { c.p.m. binaries

  12. To estimate the BS fraction of BMP n(BS)/n(BMP) fBMP, fBS, and fIA must be “good numbers”. COMMENTS fBMP  Reliability limited only by accuracy of RV’s & duration of survey fBS Adopt Duquennoy & Mayor n(P) for primordial field blue stragglers.  Assume that 13% of primordial blue straggler binaries with P < 5 d have merged.  Remainder (87%) must be mass-transfer binaries. fIA Adopt 0.15 as “universal binary fraction” for P<4000 d from fDisk = 0.15 Duquennoy & Mayor 1991 fHalo = 0.14 Latham et al 1998

  13. Inserting our adopted binary fractions for the three populations fBMPfIAfBS Binary Fraction (f) 0.60 0.15 0.87 the blue straggler fraction of BMP stars is nBS/nBMP = (fBMPfIA)/(fBSfIA) = 0.62 More than half of the blue metal-poor stars are blue stragglers. BUT

  14. Halo field blue stragglers (FBS) are a different breed. S = Specific frequency = (BMP)/(HB) BMP = 300 kpc-3 SBMP = 6.7 HB = 45 kpc-3 BS~108yM(parent pop) SHFBS = 0.62*6.7 = 4.2 Specific frequency of halo field blue stragglers exceeds specific frequency of blue stragglers in globular clusters by factor 10:

  15. Specific frequency of blue stragglers in globular clusters (~0.4) is one order-of-magnitude smaller than value in halo field (~4). Blue stragglers occur more frequently in less massive, loosely-bound clusters  4.5  halo field blue stragglers 4.0 increasing cluster mass            

  16. Mateo et al (1990) used estimates of merger time-scale (~5E+8 y) and blue straggler lifetime (7E+9 y) to conclude thatthe specific frequency of blue stragglers in NGC 5466 can be explained entirely by mergersof the cluster population of W Uma systems. Mateo, Harris, Nemec, & Olszewski 1990, Astronomical Journal, 100, 469

  17. Mapelli et al. (2004) simulations of 47 Tuc data confirm that mergers produce the observed SGCBS outside of the core. Collisional formation only Observed distribution (Ferraro et al 2004) Collisions plus binary mergers

  18. TO SUMMARIZE We use binary fractions to resolve BMPs into two populations: 40% intermediate-age, metal-poor stars (IA) 60% old metal-poor blue stragglers (FBS) A small fraction of HFBS are formed by merger of close pairs. The rest must be formed by McCrea mass transfer, because there are no collisions in the halo. GCBS are formed primarily by collisions (in core) and mergers (everywhere) of the small portion (10%) of primordial binaries that survive disruption by encounters. By this reasoning we understand why the specific frequency of HFBS exceeds that of GCBS by an order-of-magnitude.

  19. Abundance analysis of Las Campanas high resolution spectra (R ~ 25,000) of BMP stars Vsini=40 km/s We analyzed summed spectra. Individual frames used to search for velocity variations have too small S/N to be employed in abundance work. [Fe/H]=-2.30 Preston & Sneden 2000

  20. The abundance analysis • Use Fe-peak lines to derive atmosphere parameters • Interpolated model atmospheres from Kurucz’s ATLAS grid • Standard LTE analysis with the MOOG code • Teff from Fe I abundances with excitation potential • log g from Fe I versus Fe II abundances • vt from Fe I abundances with EW • Basic results: (1) overall metallicities, (2) Teff’s in 6700-7500K range, (3) main-sequence gravities • Limited # of lines → abundances of 8 elements

  21. Abundances from the Las Campanas BMP high resolution survey Normal results () compared to other Pop II halo stars: (1) Mg, Ca, Ti ↑ (2) Mn ↓ (3) Sr, Ba: large s at lowest [Fe/H] Open circles denote stars with vesini > 24 km/s  lower accuracy Preston & Sneden 2000

  22. Mass transfer during post-MS evolution circularizes orbits Orbital parameters for ordinary binary stars ... and for Carbon-rich & s process-rich binaries The fraction of binaries with P > 25d & e < 0.15 is small. 25 d The fraction of binaries with P > 25d & e < 0.15 is larger. Preston & Sneden 2000

  23. Mass transfer during post-MS evolution circularizes orbits Orbital parameters for ordinary binary stars … and for BMP stars The fraction of binaries with P > 25d & e < 0.15 is small. 25 d The fraction of binaries with P > 25d & e < 0.15 is larger. Preston & Sneden 2000

  24. Followup high resolution BMP study • 5 BMP binaries, 5 BMP RV-constant stars • Las Campanas echelle, with new CCD detector → higher S/N data • Original goal: a comparative Li abundance study • Oops!: Li undetected in all 10 stars! • Much more interesting: look at the 3/5 stars in each group with [Fe/H] < -2

  25. Radial velocities of the low metallicity BMP star sample:Some of them are binaries and others are not. RV-constant stars Binary stars

  26. Individual and mean spectra of low metallicity RV-constant and binary stars High excitation O I lines are somewhat stronger in the binaries.

  27. a-capture elementsare ~ normal in the whole BMP sample Upper-envelope for n-capture elements declines in the binaries as if neutron exposure is ~ constant for all [Fe/H]. neutron exposure ~ constant Original BMP sample: Preston & Sneden 2000

  28. Carbon species in the spectra of BMP RV-constant stars and binary CS 29497-030 CH & C I respond VERY differently to changes in temperature & gravity! The “non-variable” spectrum is the mean of three stars

  29. Neutron-capture species in BMP RV-constant stars and binary CS 29497-030 (another lead-rich star) Note similarity of lines for Fe-peak elements The “non-variable” spectrum is the mean of three stars Preston & Sneden 2000

  30. Mean abundances in the low-metallicity binary and RV-constant groups Large s values for C, Sr, & Ba in the binaries indicate real star-to-star differences

  31. Abundances in CS 29497-030 and the RV-constant stars Abundances of C, O, and n-capture elements are new; other abundances are from Preston & Sneden 2000

  32. We know of several very lead-rich stars Abundances are normalized to CS 29497-030 at Ba, or La, or both Normalizations are simple vertical shifts Sneden et al. 2003

  33. s-process predictions versus abundances in lead-rich stars Mean observed abundances are computed after normalizations Neutron/seed ratio is the main variable in the theoretical computations Arbitrary normalization between theory and observation at Ba & La

  34. Pb-rich stars: a unique abundance signature? Halo sample w [Fe/H]<-1.5 Domain of the enhanced r-process metal-poor stars Domain of the large s-process, lead-rich stars

  35. Evolutionary states of known lead-rich stars Here is a new one: CS 22881-071, an accidental discovery in a survey of 25 metal-poor red horizontal-branch (RHB) stars Preston et al. 2004

  36. Most detailed n-capture abundance pattern of any lead-rich star? Other stuff about CS 22881-071: (1) It is relatively carbon-rich (like all other lead stars) (2) It is an RR Lyrae star(P=0.59 d) for heaven’s sake! (3) Is it in a binary? It ought to be! Time will tell. Preston et al. 2004

  37. Recapitulate • BMP stars are in the “wrong” HR diagram place for metal-poor main sequence stars • 2/3 of BMP stars are BS binaries. • 5/7 very metal-poor BMP binaries are rich in s-process products → AGB mass transfer • Companion stars must now be compact objects • Pb discovered in one star; others must exist • The Pb-rich turn-off stars must have experienced AGB mass transfer? How does this happen? • Question: How can mass transfer be so efficient in (now) widely separated pairs?

  38. The End

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