Extremely Metal-Poor Stars in the Milky Way Galaxy

Extremely Metal-Poor Stars
in the Milky Way Galaxy
Wako Aoki
National Astronomical Observatory of Japan
Extremely Metal-Poor (EMP) Stars
in the Milky Way Galaxy
Lowest metallicity stars ([Fe/H]～<-5)
●
Survey of EMP stars and statistics
●
Binary fraction
●
Lithium in EMP stars
●
Neutron-capture elements
●
UMP/HMP stars
Lowest metallicity stars
in the Milky Way
UMP/HMP stars
10 years from the discovery of
HE0107-5240 ([Fe/H]=-5.3)
The first “Hyper MetalPoor” (HMP) star
Christlieb et al. (2002)
[Fe/H]=-4.0
Beers & Christlieb (2005, ARAA)
[Fe/H]=-5.3
UMP/HMP stars
Search for metal-deficient stars in the Galaxy
Norris 2005
HE1327-2326
Bond (1981)
“Where is Population III”
HK survey
Discovery of HE0107-5240
([Fe/H]=-5.3)
UMP/HMP stars
nd
The 2 HMP star HE1327-2326
Frebel et al. (2005)
Solar
EMP
very weak Fe lines
→[Fe/H]=-5.4
detection of CH molecular
bands
→excess of carbon
HMP
UMP/HMP stars
Carbon-enhancement in HMP stars
The two HMP stars show large excesses of C, (N) and O …
low-mass star formation from C and O-rich cloud?
Aoki et al. (2006)
HE0557-4840
(Norris et al. 2007)
UMP/HMP stars
Discovery of the [Fe/H]～-5 star with
normal C abundance SDSS J1029+1729
Caffau et al. (2011, 2012)
[Fe/H]=-4.7 (1D LTE analysis) → Ultra-Metal-Poor (UMP) star
R～10,000
R～40,000
UMP/HMP stars
Normal C abundance of the UMP star
SDSS J1029+1729
SDSS J1029+1729
UMP/HMP stars
UMP/HMP stars found in the past decade
Object
V
[Fe/H]
(1D-LTE)
[C/Fe]
Teff
type
HE0107-5240
15.2
-5.3
4.0
5100
Giant
HE1327-2326
13.5
-5.6
4.2
6180 MSTO*
HE0557-4840
15.5
-4.8
1.6
4900
SDSS
J1029+1729
g=
16.9
-4.7
<0.9
* MSTO: main-sequence turn-off
Giant
5810 MSTO*
UMP/HMP stars
Absorption lines in HMP/UMP stars
Fe I 3860A
Mg I 5183A
HE1327-2326
Main-sequence
(6000K/log g=4.0)
[Fe/H]=-5
[Fe/H]=-6
[Mg/H]=-5
[Mg/H]=-6
Giant
(5000K/log g=2.0)
[Fe/H]=-5
[Fe/H]=-6
[Mg/H]=-5
[Mg/H]=-6
EMP searchers
Searches for EMP stars
and statistics
EMP searchers
EMP stars as constraints on low-mass
star formation in the early Galaxy
Questions on EMP stars:
-Where were EMP stars formed?
-Are they second generations of stars?
-Are they oldest stars in the Galaxy?
...
Statistics is required: metallicity distribution
function (MDF) is a key observable.
EMP searchers
What are required for observational
studies of EMP stars?
EMP stars are extremely rare in the solar
neighbourhood.
●
→wide-field & deep survey is required
Spectral lines are weak (depending on
temperature).
●
→high resolution spectroscopy is required.
EMP searchers
Large-scale surveys of metal-poor stars
●
●
HK survey
Beers et al. (1985, 1992, …)
e.g. BPS CS22892-052
Hamburg/ESO survey
Christlieb et al. (2001, …)
e.g. HE1327-2326
A plate image of objective
prism spectra
(courtesy T.C. Beers)
EMP searchers
Search for metal-poor stars by
Sloan Digital Sky Survey (SDSS)
The 2.5m telescope at
Apache Point
Observatory
SDSS spectroscopy:
R~1800
Covering 3900-9000A
14<V<20
●Metallicity estimate from Ca II
HK lines
●Standard stars in SDSS-I
●New surveys in SDSS-II
(SEGUE)→240,000 stars
●
High-resolution follow-up spectroscopy
with Subaru/HDS
Snap-shot spectroscopy
●R=36,000
●4030-6800A
●S/N～25-30
●～150 objects with V<16.5
cf. SDSS J1029+1729
(Caffau et al.) has V～16.7
[Fe/H]=-2.6
Turn-off star
[Fe/H]=-3.7
Turn-off star
Example: Mg triplet
around 5170A →
High S/N spectra with
R=60,000 for ~15
selected stars have
been obtained.
[Fe/H]=-3.5
giant
Metallicity ([Fe/H]) from Subaru spectra
and comparison with SDSS estimates
The current version of
SSPP derives metallicity
that agrees with the
estimate from high
resolution spectroscopy
(no systematic offset).
However, significant
scatter remains in EMP
stars.
Metallicity distribution function of
(kinematically selected) halo stars
Ryan & Norris (1991)
EMP sample was too small
Carney et al. (1994)
EMP searchers
Metallicity Distribution Function (MDF) estimated
from Hamburg/ESO survey + medium-resolution
(R=2000) spectroscopy
Schörck et al. (2009)
Li et al. (2010)
drop at
[Fe/H]=-3.5
Selection bias
EMP searchers
Comparisons with chemical evolution models
Comparison with models
including modifications for
simple models by Pranzos
(2003, 2008)
Comparisons with models
assuming critical metallicity for
low-mass star formation by
Salvadori et al. (2007)
Li et al. (2010)
EMP searchers
Comparison of the MDFs of the SDSS/Subaru
sample with the HES result (Li et al. 2010)
HES
Selection
bias
Tail of MDF in [Fe/H]<-3.6? (or Drop at [Fe/H]=-3.8?)
EMP searchers
Metallicity Distribution Function estimated
from the SDSS/Subaru sample for [Fe/H]<-3
Our estimate of MDF agrees with the HES estimate for
[Fe/H]>-3.5, but existence of stars with [Fe/H]<-3.5 in our
sample suggests a tail at the lowest metallicity end.
●
cf. recent discovery of [Fe/H]=-5 star (SDSS J1029+1729)
with no carbon-excess by Caffau et al. (2011)
Further calibration is required between high-resolution and
medium-resolution spectroscopy.
●
EMP searchers
Fraction of CEMP stars and their nature
9 of the 25 giants are carbon-rich
→fraction of CEMP stars is 36% at [Fe/H]～ -3
cf. Carollo et al. 2011
●
→only 3 of the 9 CEMP giants show excesses of sprocess elements (“CEMP-s”). Dominant source of
carbon-excesses at [Fe/H]=-3 may not be AGB
stars (but supernovae?).
10 of the 108 main-sequence turn-off stars are
carbon-rich. They are highly C-rich and 9 of them
are s-process-enhanced.
← “moderately carbon-enhanced” stars have not
been detected due to weaker CH features in
warm stars
●
Binary
Binary fraction in EMP stars
(based on SDSS/Subaru sample)
Binary
Methods of binary searches
Spectroscopic: radial velocity variations
double-lined binary
●High resolution imaging: speckle, interferometry, ...
●
Raestegaev (2010)
Period → Separation
Double-lined spectroscopic binaries
in the SDSS/Subaru sample
3 stars are double-lined
spectroscopic binaries
among 108 turn-off stars
●
SDSS J0817+2841
([Fe/H]=-2.9)
SDSS J1108+1747
([Fe/H]=-3.2)
triple-system!
Aoki et al. (in prep)
Binary
Binary
Detectability of double-lined
spectroscopic binary
(1)Radial velocity difference
Spectroscopy with R=36000 → line width is ～10km/s
… about 50% of binaries with P<1000 days can be
detected by single epoch spectroscopy
wavelength(A)
Detectability of double-lined
spectroscopic binary
Binary
(2)Luminosity ratios
S/N～25 spectroscopy → the secondary's line is
detected if that is about 1/5 of the primary's line
… about 60% of binary pairs of main-sequence turn-off
stars can be detected.
○
×
Binary
Fraction of double-lined spectroscopic
binaries among EMP stars
3 binaries are found among 108 turn-off stars
(～3%).
●
Given the detectability estimated for the sample
of turn-off stars, the fraction could be as high as
10%.
●
The orbital period of binaries detectable by our
observations is shorter than 1000 days.
●
Binary
Estimate of binary fraction from
CEMP-s fraction
CEMP-s stars are likely formed by mass transfer from
AGB companion, which would be low-mass (1-2Msun?).
→ CEMP-s stars are binaries of low-mass star pair.
●
The fraction of CMEP-s stars are about 10% (or higher)
at [Fe/H]～-3.
●
The carbon-excesses are significant ([C/Fe]>+2) in
most objects. → separation of the binary could be short
(P<1000 days? … dependent on the mixing (dilution) in
the secondary).
●
Binary
Binaries with short periods (P<1000
days) are preferable in EMP stars (?)
Estimates from double-lined spectroscopic binaries and
CEMP-s　→ 20% (or more) are short period binaries(!?)
Raestegaev (2010)
Lithium
Lithium in EMP stars
Lithium
The Lithium problem:
The discrepancy between Li abundances in
metal-poor stars (Spite plateau value) and
prediction of big-bang nucleosynthesis models
Asplund et al. (2006)
Lithium
Depletion and scatter of Li in EMP stars
Asplund et al. (2006), Sbordone et al. (2010), Aoki et al. (2009)
Lithium in the Cosmos
Feb. 27-29, 2012, Paris
Lithium
Further measurements for EMP stars
New measurements for SDSS/Subaru sample based on
high S/N follow-up spectroscopy.
SDSS data
○Bonifacio et al. (2012)
●Aoki et al. (in prep)
Lithium
Li measurements for binaries: strong evidence
for Li depletion in main-sequence stars
Extremely metal-poor ([Fe/H]=-3.6), doublelined spectroscopic binary CS22876-032
Gonzalez-Hernandez et al. (2008)
A: 6500K, A(Li)=2.22
B: 5900K, A(Li)=1.75
B
A
Lithium
Li in the very metal-poor ([Fe/H]=-2.5),
double-lined spectroscopic binary G166-45
A: 6350K, A(Li)=2.23 +/- 0.06,
B: 5830K, A(Li)=2.11+/- 0.05
Aoki et al. (2012)
Lithium
Li measurements for binaries: strong evidence
for Li depletion in main-sequence stars
EMP binary CS22876-032 → low Li in the secondary (by 0.4 dex)
VMP binary G166-45 → the difference is small (0.1 dex)
CMB+SBBN
Lithium
Further Li depletion in UMP/HMP stars...
Bonifacio et al. (2012)
n-capture
Neutron-capture elements:
Abundance scatter and
origins of heavy elements
n-capture
Origins of heavy neutron-capture
elements in EMP stars
Sr and Ba (singly ionized species Sr II and Ba II) are
detectable even in EMP stars
●
Sr and Ba are efficiently produced by s-process, but sprocess in AGB stars would not have contributed to EMP
stars (except for binary stars).
●
Sr: near the 1st abundance peak of neutron capture
elements, representing the “weak r-process”?
●
Ba: near the 2nd abundance peak of neutron-capture
elements, representing the “main r-process”.
●
n-capture
Trend and scatter in abundance ratios
of Sr and Ba from SAGA database
(Carbon-enhanced stars are excluded)
r-process-enhanced stars
Very large scatter in [Sr/Fe] and in [Ba/Fe] in [Fe/H]<-2.5.
●A group of stars show very high [Ba/Fe] at [Fe/H]=-3.. Such
stars are not found in the [Sr/Fe] diagram.
●
n-capture
Trend and scatter in Sr/Ba abundance
ratios of from SAGA database
“weak r-process”
Sr-enhanced
(“weak r-process”
-enhanced)
main r-process
A tentative definition of Sr-enhanced stars:
[Sr/Ba]> +0.5 → “weak r-process”-enhanced stars
n-capture
Metallicity distribution of Sr-enhanced stars
All stars (non C-rich)
Number
[Sr/Ba]>+0.5
[Fe/H]=-3.5
[Fe/H]
n-capture
Distributions of Sr and Ba abundances in
metal-poor stars: Summary
Scatter in Sr/Ba (←weak-r/main-r) increases with
decreasing metallicity
●
The fraction of Sr-enhanced stars ([Sr/Ba]>+0.5)
is 30-40% at [Fe/H]～-3.0.
→the progenitors of EMP stars in this metallicity
range (very massive stars?) are the sources of
weak r-process
●
No Sr-enhanced star is found in [Fe/H]<-3.5.
●
→the progenitors of [Fe/H]<-3.5 stars are different from
those of [Fe/H]>-3.5 stars?
Extremely Metal-Poor Stars
in the Milky Way Galaxy
Summary
4 HMP/UMP stars have been discovered in the past decade.
They already show diversity in chemical compositions.
●
New surveys are enlarging the sample of EMP stars,
contributing to deriving statistics (e.g.MDF and CEMP fraction).
●
Fraction of binary of EMP stars probably having short periods
(P<1000 datys?) is 20% or higher.
●
The Li abundance trend in EMP stars (compared to VMP stars)
is almost established, constraining the solution for the Li
problem.
●
Origins of n-capture elements in lowest metallicity range
([Fe/H]<-3.5) should be different from those for [Fe/H]～-3.
●