the remarkable chemical compositions of blue metal-poor stars
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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
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
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
Preston et al. 1994
Data points are the B8-F0 stars in the Bright Star Catalog
[Fe/H] ~ 0
[Fe/H] ~ 1
de-blanketing (blue) vectors
[Fe/H] ~ 3
BMP domain is a region in which isochrones for a wide range of ages and metallicities overlap in a tangled mess.
Preston & Sneden 2000
Isochrones of various [Fe/H] values and ages overlap in the BMP star domain
Isochrones in the U-B versus B-V plane are from Green et al. (1987)Revised Yale Isochrones
MS isochrones
SG isochrones
Turnoffs for: [Fe/H] = 2.2 ages 3 7,10 Gy
BMP stars were readily identified by UBV photometryof stars found in the HK objective-prism survey
Preston et al. 1994
BM
P st
ars
“HK” Survey: Beers et al. 1985, 1992
MS [Fe/H]=0
MS [Fe/H]= 1
RHB
Metal poor stars near turnoff
BHB
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)
Common distance makes identification easy
MS
RG
B
BH
B
RHB
AGB
MS turnoff
BMP
Resolution of the problem lay in a radial velocity survey:a surprisingly high proportion of BMP stars are members
of spectroscopic binaries
Preston & Sneden 2000
1.5 km/s
< 1.5 km/s: 1/18 stars with orbits
> 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.
>60% of BMP’s are binaries with atypically long orbital periodsand small mass functions.
Standard deviation of a RV measurement (km/s)
Typical orbital solutions based on radial velocity variations in the BMP sample
Preston & Sneden 2000
>60% of BMP’s are binaries with
unusually long orbital periodsand unusually small mass functions.
f(m) (K1)3P
We use binary fractions to resolve blue metal-poor stars into blue straggler and intermediate-age components.
xxxxxxxxxxxxxxx
n
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
Only this 13% of binaries with P4000d can merge in a Hubble time (Vilhu, O. 1982, A&Ap, 109, 17)
This is why we adoptfBS 0.87radial
velocitybinaries
visualbinaries
c.p.m.binaries
{
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
the blue straggler fraction of BMP stars is
fBMP fIA fBS
Binary Fraction (f) 0.60 0.15 0.87
nBS/nBMP = (fBMP fIA)/(fBS fIA) = 0.62
More than half of the blue metal-poor stars are blue stragglers.
BUT
Inserting our adopted binary fractions for the three populations
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~108yM(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:
Specific frequency of blue stragglers in globular clusters (~0.4) is one order-of-magnitude smaller than value in halo field (~4).
increasing cluster mass
Blue stragglers occur more frequently in less massive, loosely-bound clusters
4.5
4.0
halo field blue stragglers
Mateo, Harris, Nemec, & Olszewski
1990, Astronomical Journal, 100, 469
Mateo et al (1990) used estimates of merger time-scale (~5E+8 y)and blue straggler lifetime (7E+9 y)to conclude that the specific frequency of blue stragglers in NGC 5466 can be explained entirely by mergers of the cluster population of W Uma systems.
Mapelli et al. (2004) simulations of 47 Tuc data confirm that mergers produce the observed SGCBS
outside of the core.
Observed distribution(Ferraro et al 2004)
Collisional formation only
Collisions plus binary mergers
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.
Abundance analysis of Las Campanas high resolution spectra (R ~ 25,000) of BMP stars
We analyzed summed spectra.
Individual frames used to search for velocity variations have too small S/N to be employed in abundance work.
Preston & Sneden 2000
Vsini=40 km/s
[Fe/H]=-2.30
The abundance analysis
Use Fe-peak lines to derive atmosphere parametersInterpolated model atmospheres from Kurucz’s ATLAS gridStandard 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
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
at lowest [Fe/H]
Preston & Sneden 2000
Open circles denote stars with vesini > 24 km/s
lower accuracy
Orbital parameters for ordinary binary stars
... and for Carbon-rich & s process-rich binaries
Preston & Sneden 2000
The fraction of binaries with P > 25d & e < 0.15 is small.
The fraction of binaries with P > 25d & e < 0.15 is
larger.
Mass transfer during post-MS evolution circularizes orbits
25 d
Orbital parameters for ordinary binary stars
… and for BMP stars
Preston & Sneden 2000
The fraction of binaries with P > 25d & e < 0.15 is small.
The fraction of binaries with P > 25d & e < 0.15 is larger.
Mass transfer during post-MS evolution circularizes orbits
25 d
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
Radial velocities of the low metallicity BMP star sample:Some of them are binaries and others are not.
RV-constant stars Binary stars
Individual and mean spectra of low metallicity RV-constant and binary stars
High excitation O I lines are somewhat stronger in the binaries.
-capture elementsare ~ normal in the whole BMP sample
Original BMP sample: Preston & Sneden 2000
neutron exposure ~ constantUpper-envelope forn-capture elements
declines in the binaries as if neutron
exposure is ~ constant for all [Fe/H].
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
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
Preston & Sneden 2000
The “non-variable” spectrum is the mean of three stars
Mean abundances in the low-metallicity binary and RV-constant groups
Large values for C, Sr, & Ba
in the binaries indicate real star-to-star
differences
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
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
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
Pb-rich stars: a unique abundance signature?
Domain of the enhanced r-process metal-poor stars
Domain of the large s-process, lead-rich stars
Halo sample w [Fe/H]<-1.5
Evolutionary states of known lead-rich stars
Here is a new one:CS 22881-071, an
accidental discovery in a survey of 25 metal-poorred horizontal-branch
(RHB) stars
Preston et al. 2004
Most detailed n-capture abundance pattern of any lead-rich star?
Preston et al. 2004
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.
Recapitulate
BMP stars are in the “wrong” HR diagram place for metal-poor main sequence stars2/3 of BMP stars are BS binaries. 5/7 very metal-poor BMP binaries are rich in
s-process products → AGB mass transferCompanion stars must now be compact objectsPb discovered in one star; others must existThe 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?
The End
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