Extremely Metal-Poor Stars in the Milky Way Halo

Extremely Metal-Poor Stars in the Milky Way Halo

Extremely Metal-Poor Stars
in the Milky Way Halo
Shuzenji
“Chikurin-no-komichi”
Wako Aoki
National Astronomical Observatory of Japan
Extremely Metal-Poor (EMP) stars
Solar
EMP
HMP
Spectra of the Sun and
EMP/HMP stars
(Aoki et al. 2006)
Beers & Christlieb (2005, ARAA)
What can we learn from Extremely
Metal-Poor (EMP) stars?
(1) Individual nucleosynthesis processes by
first/early generations of stars
(2) Constraints on low-mass star formation at
lowest metallitiy and formation of the Milky
Way halo
(3) Constraints on low-mass star evolution (Li
abundances, mixing processes, binary
interaction)
Extremely Metal-Poor (EMP) stars: [Fe/H]<-3
●[Fe/H]=log(n[Fe]/n[H])-log(n[Fe]/n[H])sun
●
EMP stars as constraints on nucleosynthesis
processes by first/early generations of stars
Averaged abundance pattern of EMP stars and
prediction by supernova nucleosynthesis models
Solar abundance
ratios
Tominaga et al. (2007)
Chemical abundance measurements for halo stars
with the Subaru Telescope (the past decade)
Spectroscopic Studies of Extremely Metal-Poor Stars with the
Subaru High Dispersion Spectrograph (I-V)
Honda et al. (2004a,b), Aoki et al. (2005, 2007), Honda et al.
(2011)
●
Neutron-capture elements in very metal-poor stars
Honda et al. (2006, 2007)
●
Carbon-Enhanced Metal-Poor (CEMP) stars (I-III)
Aoki et al. (2007, 2006, 2008)
●
●
“Hyper Metal-Poor star” HE1327-2326, and related objects
Frebel et al. (2005), Aoki et al. (2006) , Ito et al. (2009)
These studies mostly focus on deriving (detailed)
chemical abundance patterns of individual stars to
provide constraints on nucleosynthesis processes
Chemical abundances of individual stars
(1)carbon-rich “Hyper Metal-Poor (HMP)” star HE1327-2326
[Fe/H]=-5.6
[C/Fe]=+4
Average of extremely
metal-poor stars
Frebel et al.
(2005)
Aoki et al.
(2006)
Origin of this abundance pattern:
“faint supernovae”? / mass-loss from massive star?
/mass transfer from AGB?
Chemical abundances of individual stars
(1)carbon-rich stars – related to the HMP star?
Carbon-enhanced EMP star BD+44 493 ([Fe/H]=-3.7):
another evidence for “faint supernovae”
Abundance ratios (X/Fe) relative to the solar value
The normal Ba abundance, the high O/C, and the low N/C exclude the AGB and massive rotating stars as the progenitor
→ Faint supernova scenario is the remaining possibility.
Atomic Number
Ito et al. (2009, ApJL)
Ito et al. (in prep.)
Chemical abundances of individual stars
(2)Detailed abundance patterns of neutron-capture
elements
Large enhancements of light neutron-capture elements (e.g. Sr, Y)
→representing a (still unknown) neutron-capture process
HD88609 ([Fe/H]=-3.0)
Honda et al. 2007
n-capture elements in dwarf galaxies: Honda et al., this conference
What can we learn from Extremely
Metal-Poor (EMP) stars?
(1) Individual nucleosynthesis processes by
first/early generations of stars
(2) Constraints on low-mass star formation at
lowest metallitiy and formation of the Milky
Way halo
(3) Constraints on low-mass star evolution
(mixing processes, binary interaction)
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.
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.
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)
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=30,000
●4030-6800A
●S/N～25-30
●～150 objects
Example: Mg triplet
around 5170A →
High S/N spectra with
R=60,000 for ~15
selected stars have
been obtained.
[Fe/H]=-2.6
Turn-off star
[Fe/H]=-3.7
Turn-off star
[Fe/H]=-3.5
giant
Metallicity ([Fe/H]) from Subaru spectra
and comparison with SDSS estimates
Extremely metal-poor
stars are very
efficiently selected
from SDSS spectra, even
though the correlation is
not good for the earlier
version of SDSS pipeline
(SSPP).
●
EMP
(estimates in 2008)
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.
(estimates by the latest SSPP)
Metallicity distribution function of
(kinematically selected) halo stars
Ryan & Norris (1991)
EMP sample was too small
Carney et al. (1994)
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
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)
High resolution spectra are required for
accurate metallicity estimates for EMP stars
(in particular for the low metallicity end)
-weakness of absorption lines
-“outliers” in abundance patterns (e.g. carbon-excesses)
→measurements of abundance ratios (C/Fe, Mg/Fe
etc.) are required for metallicity estimates.
Metallicity distribution of
the SDSS/Subaru sample
Metallicity is determined from Fe lines in high
resolution spectra obtained with Subaru (137 stars)
Selection
bias
Whole sample
Carbonenhanced stars
Homogeneity of the sample:
no trend exists for V magnitude and temperature
turn-off stars
giants
(+4 cool dwarfs)
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?)
Metallicity Distribution Function estimated
from the SDSS/Subaru sample for [Fe/H]<-3
A trend 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 with no carbon-excess
by Caffau et al. (2011)
Further calibration is required between high-resolution and
medium-resolution spectroscopy.
●
Other topics from the high resolution
follow-up study with Subaru for the
SDSS sample:
Aoki et al. (in prep)
Fraction of CEMP stars
●
Alpha elements
●
Fraction of CEMP stars and their nature
8 of the 25 giants are carbon-rich
→fraction of CEMP stars is ～30% at [Fe/H]=-3
cf. Carollo et al. 2011
●
→only 3 of the 8 CEMP giants show excesses of sprocess elements. Dominant source of carbonexcesses at [Fe/H]=-3 is not AGB stars (but
supernovae?).
11 of the 108 main-sequence turn-off stars are
carbon-rich
← “moderately carbon-enhanced” stars have not
been detected due to weaker CH features in
warm stars
●
Mg/Fe ratios
No significant scatter (within the relatively large errors)
●Low [Mg/Fe] at [Fe/H]=-2.5 ??
●
□ C-rich object
Summary
High resolution abundance studies with the Subaru
Telescope for individual objects
→Chemical abundance patterns constraining nucleosynthesis
models
●
Abundance studies of large sample of EMP stars based on
SDSS/SEGUE to understand the nature of EMP stars
●
→metallicity distribution function, in particular its low
metallicity end
→fraction of carbon-enhanced stars and their nature
Future prospect:
Prime-focus of the Subaru Telescope
Current FOV :
30 arcmin x 30 arcmin
●
Current instruments:
Suprime-Cam (opt. Camera)
FMOS (NIR fiber spectrograph)
●
A new prime-focus camera (Hyper
Suprime-Cam: HSC) is planned to
be installed in 2011-2012. The FOV
is 1.4 degφ
●
Plan of Prime-Focus Spectrograph
(PFS): multi-object, wide-field
spectroscopy!
●
→M. Chiba, this conference
More future (2019～) ---High resolution spectroscopy with
Thirty Meter Telescope (TMT)