化学組成で探る天の川銀河の進化

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化学組成で探る天の川銀河の進化
辻本拓司 (国立天文台)
4つのテーマに焦点




prompt Type Ia supernovae (2006~)
the IMF variation (classical issue)
stellar migration (2008~)
He-enriched stars in globular clusters (2004~)




Galactic thick/thin disk
Galactic bulge
Galactic halo
the Fornax dSph galaxy
天の川銀河研究会2012, 9月6−8日 in 鹿児島
Delay Time Distribution (DTD) of SNe Ia
Ia型超新星の寿命は短かった
observed results
theoretical models

from SN Ia surveys for extragalaxies
double-degenerate
scenario

Totani et al. 2008
Kirby et al. 2011
tIa~tGW  a4
fsep  a-1
Young progenitors
for SNe Ia are
dominant
(Mannucci et al. 2006;
Sullivan et al. 2006)
single-degenerate scenario
(Totani et al. 2008; Maoz et al. 2010)
about 70% of SNe Ia explodes with a time delay
within 1 Gyr
a significant impact on Galactic chemical evolution
Hachisu et al. 2008
DTD  tdelay1 for 0.1Gyr  tdelay 10Gyr
imply
T=(U2+V2)0.5
break in [a/Fe] among solar neighborhood stars
[Fe/H]
-1
apply
Galactic stars are now well kinematically
separated.
Pagel & Tautvaisiene 1995
thin disk
Yoshii et al. 1996
Toomre diagram
Venn et al. 2004
Drastic change in typical timescale
of SN Ia progenitors
~1 Gyr
~ 0.1Gyr
no break
no high a/Fe stars
no low Fe/H stars
thick disk
the presence of
break!
DTD discussion on a/Fe break should be assessed
by comparing the modeled chemical feature of
the thick disk with the corresponding observed one.
Updates of s-process yield in AGB stars
s-process元素はもう一つの元素(宇宙)時計
Mainly due to large uncertainties in convective mixing and 13C-pocket efficiencies,
the s-process nucleosynthesis allows a wide range for the level of a possible production.
Abundances of the surface of AGB
stars can be directly compared with
the nucleosynthesis results.
Theoretically
allowable
range
TT & Bekki 2011
The renewed picture of a SNIa clocking should
be tied up with another nucleosynthesis clock,
the s-process operating in an AGB star.
the best empirical Ba yield as a function
of stellar [Fe/H] so as to reproduce the Ba
evolution of thin disk stars
fails to reproduce the chemical
evolution of the thin disk
Busso et al. 2001
Chemical Evolution of Disks
✓Thick and thin disks are separately modeled.
see next
slide
✓First, thick disk is rapidly formed, and subsequently
thin disk is gradually formed.
✓Formation of two disks are connected in a sense that thin disk
stars start forming from a remaining gas of thick disk.
✓We examine the evolution of [Mg/Fe] and [Ba/Mg].
three nucleosynthesis clocks: SNe II, SNe Ia, AGB stars
not [Ba/Fe] so as to prevent the effect of s-processing
from being hidden by SN Ia contamination
✓We focus on the chemical evolution for [Fe/H] ≤ 0.
since the origin of metal-rich disk stars should be assessed
with an extra evolution factor such as stellar migration
Formation of the thick disk through minor merging
between the first generation of the Galactic thin disk
and a dwarf galaxy about ~10 Gyr ago
(Bekki &TT 2011, ApJ, 738, 4)
The thick disk can be regarded as a first disk which
is heated up by an ancient minor merger, that is
subsequently followed by the gradual formation
of a secondary disk, i.e., the thin disk.
no metallicity gradient
in the thick disk
obs.
Allende Prieto
et al. 2006
flattening of metallicity gradient Kinematic properties can
resulting from radial mixing
be also reproduced.
induced by minor merging
model
(but not a positive correlation..
: Spagna et al. 2010; Lee et al. 2011)
Chemical evolution of the thick disk
model parameters
 = 2 Gyr-1: SFR coefficient
DSF= 1.5 Gyr: SF duration
tin = 0.5 Gyr: timescale of an infall
A successful reproduction of the [Mg/Fe] feature
suggests that a new SN Ia DTD revealed by
extragalaxy studies is compatible with the Milky
Way case.
data from Bensby et al. 2005
Ruchti et al. 2011
Venn et al. 2004
an indication of pre-enrichment
including the s-processing
due to enriched bulge winds ?
or
by s-process elements from
fast-rotating massive stars ?
data from Bensby et al. 2005
Venn et al. 2004
TT & Bekki 2012
(Pignatari et al. 2008; Chiappini et al. 2011)
Chemical evolution of the thin disk
model parameters
 = 0.4 Gyr-1: SFR coefficient
DSF= 12 Gyr: SF duration
tin = 5 Gyr: timescale of an infall
The thin disk stars start forming from the thick disk's
remaining gas (corresponding to ~10 % of the original
gas) mixed with the infall gas accreted onto the disk.
data from Bensby et al. 2005
Venn et al. 2004

[Fe/H] and [Mg/Fe] decreases and increases,
respectively, owing to dilution by metal-poor
infalling gas from the halo.
This reverse evolution comes to an end when the
chemical enrichment by star formation exceeds the
effect of gas dilution, and subsequently an usual
evolutionary path appears.
data from Bensby et al. 2005
Venn et al. 2004
TT & Bekki 2012
the low Ba yield case
the absence of metal-poor thin disk stars as observed
The Galactic Bulge
the presence of two populations
two-peaked MDF
Babusiaux et al. (2010) studied
the correlation between
kinematics and metallicities
in Baade’s Window.
red clump stars in Baade’s window
Hill et al. 2011
two distinct populations
vertex deviation
microlensed dwarf and subgiant stars
an old spheroid or
a thick disc
a bar-like kinematics
[Fe/H] 0
Bensby et al. 2011
Formation of the Galactic bulge from a two-component stellar disk
(Bekki &TT 2011, MNRAS, 416, L60)
Two-component scenario
The first disk is disturbed by an ancient minor
merger, which induces a vertical growth of the
disk and transforms it into a thick disk, and
subsequently the thin disk starts to form with
an accompanying bar formation in the central
region.
cylindrical rotation
simultaneous
reproduction
vertical metallicity gradient
A vertical mixing induced
by a bar buckling functions
incompletely in a sense that
the high latitude region in
the thick disk is not well
mixed.
Note that in general, it is expected that a disk instability forming
the bulge induces a vertical mixing, which leads to erasing a
metallicity gradient along a minor axis.
銀河系バルジの起源は?
bar-induced?
YES
merger-built?
YES
metallicity gradient
cylindrical rotation
Zoccali et al. 2008
[Fe/H]=-0.03
[Fe/H]=-0.17
では、混在?
NO
[Fe/H]=-0.28
Minitti et al. 2005
Howard et al. 2009
single population
two-component bulge model
data from Hill et al. 2011
Bensby et al. 2011
metal-poor component
 = 4 Gyr-1
DSF = 1 Gyr
tin = 0.3 Gyr
IMF: x = -1.35
metal-rich component
 = 3 Gyr-1
DSF = 4 Gyr
tin = 1.5 Gyr
IMF: x = -1.05
✓A top-heavy IMF is indispensable to make a
metal-rich MDF as observed.
data from Bensby et al. 2011
Gonzalez et al. 2011
but, the color-magnitude
diagram is…
data from Bensby et al. 2011
TT & Bekki 2012
✓the enriched gas
an end result of chemical processing
associated with the halo formation or
the s-processing in massive stars ??
Bensby et al. 2011
✓A large age span
of bulge stars is
predicted.
x=-1.35
one-component bulge
red giants
in Baade window
(Fulbright et al. 2006;
-1
Zoccali et al. 2008)  = 2 Gyr
model
DSF = 2 Gyr
tin = 0.3 Gyr
IMF: x = -1.05
A top-heavy IMF is suggested.
The MDF with a Salptere IMF is entirely
skewed to a low metallicity. In addition,
the predicted [Mg/Fe] is lower than the
observed data in a metal-rich regime.
The predicted [Ba/Mg] exhibits a sharper
rise from a much lower metallicity than is
expected from the observation. This inconsistency is resolved by the model with a
flatter IMF.
TT & Bekki 2012
Halo vs. short-delayed SNe Ia
no indication of SNe Ia for the elemental abundance of halo stars,
exhibiting a plateau of [a/Fe] ratio over a whole metallicity range
implies
Halo stars must be rapidly formed in the Galactic building blocks
with a short timescale (~108yr), while an assembly of them finally
makes the stellar halo which exhibits an age span of a few Gyr.
to check if it is likely or not….
The s-process elements from AGB stars starting to release with a
timescale of a few 108yr are imprinted in the abundances of halo stars??
But,…..
The compatibility of the presence
of s-process among halo stars with
the short-delayed DTD can be also
understood consistently if s-process
elements are produced in fastrotating massive stars (Pignatari
et al. 2008; Chiappini et al. 2011).
s-process: no
[La/Eu] continues to
make a plateau
s-process: yes
[Pb/Eu] shows an upward trend
with an increasing [Fe/H]
Roederer et al. 2010
Unusual elemental feature of dwarf spheroidals
high s abundance
in the Fornax dSph
Letarte et al. 2010
low a/Fe ratio
previous results from six dSphs
Venn et al. 2004
[Fe/H]
the LMC, too!
also seen in the Sagitarrius dSph (Sbordone et al. 2007)
[Fe/H]
Pompeia et al. 2008
previous study
strong galactic wind model
occurrence of winds
stop the SF
no more r-process Eu
from SNe II
delayed s-process Ba
from AGB stars
[Fe/H]
[Fe/H]
Lanfranchi et al. 2008
proposed idea
GC’s data (Letarte et al. 2006)
Fe & a elements
s  process
Fe,a
s-process
8

10
50
stellar mass
(M)

~1.5-3 M
r-process
cut-off
~25-30 M
r-process Ba ([Ba/Eu]~-0.7)
[Fe/H]
Letarte et al. 2010
2
r  process
Fe,a

A truncated IMF in dwarf galaxies
observationally
Meurer et al. 2009
Low-surface brightness galaxies
massive O-type stars
have
FHa
A low
ratio
FFUV
less massive stars
a smaller number of
very massive stars
theoretically
A high mass end of the IMF depends on the mass of the star clusters.
In the low density environment, the formation of massive star clusters is suppressed.
Kroupa & Weidner 2003, Pflamm-Altenburg & Kroupa 2008
Their model predictions have been shown to be consistent with the observed trend
for the Ha-to-FUV flux ratio (Lee et al. 2009).
Model result
Ba s-process yield
no metallicity dependence
Fnx dSph
for low metallicity
Busso’s results
the Galaxy
Cescutti et al. 2006
Fnx dSph case: Mu=25 M with ΔSF=1.5 Gyr
A rapid enrichment
is implied by
3 Mo
1.5 Mo
Coleman & de Jong 2008
stars with age > 10 Gyr
TT 2011
これまで化学進化の分野では、ほとんど無視されていたが…….
惑星を持つ太陽近傍星はmetal-rich
惑星を持つ星の
ほとんどが[Fe/H]>0
のmetal-richな星。
Santos et al. 2003
化学進化の描像
およそ20%の星が太陽よりmetal-rich
([Fe/H]>0)
重元素量
基本的に重元素は時間と
ともに増えていくもの
0
Nordstrom et al. 2004
重元素量(metallicity)
時間
時間
metal-richな星は太陽近傍
の化学進化の終着点
ではなさそう。。
?
時間
Cepheids
OB stars
H II regions
have [Fe/H]~ 0
Metal-richな星は簡単には作れない
重元素量頻度分布(ADF)からわかること
[Fe/H]present=+0.03
the key features of the ADF
[Fe/H]present=+0.4 ・the deficiency of metal-poor stars
ー the G-dwarf problem ー
・[Fe/H]peak = -0.2 ~ -0.1
&
・[Fe/H]present > +0.2
+0.4まではありそう
The predicted ADFs are biased to
metal-rich as compared with the
observations.
([Fe/H]~+0.3-0.4)
Metal-richな星は進化の終着点
Tsujimoto 2007
とは考えにくい。
Stellar migration (Radial mixing)I
stellar migration due to resonant
scattering with transient spiral arms
Sellwood & Binney 2002
Roskar et al. 2008
final
explains a large scatter in
age-metallicity relation
initial
Two-dimensional histogram of final particle radii
vs. particle formation radii
Roskar et al. 2008
Stellar migration (Radial mixing)II
stellar migration due to resonant
scattering with transient spiral arms
Sellwood & Binney 2002
Roskar et al. 2008
predicts a steepening of
abundance gradient
reproduce metal-rich stars
present
past
Roskar et al. 2008
w/migration
w/o migration
flattening
Maciel et al. 2006
Daflon & Cunha 2004
金属量勾配の時間変化 I
flattening
現在
現在
obs.
GCE models
predict a steepening (Chiappini et al. 2001)
abundance gradient
or a flattening (Hou et al. 2001)
from R~4 kpc to R~14 kpc
-0.1 dex kpc-1
-0.04 dex kpc-1
The predicted change in abundance
in the last several Gyr
gradient in the last several Gyr is
<0.02 dex kpc-1 - too small -
金属量勾配の時間変化 II
old stars
old open clusters
time
young stars
Cepheids
TT, Bland-Hawthorn, & Freeman 2010
Yong et al. 2006
銀河系を見る限り、steepeningの証拠はなさそう
(過去ほど、勾配が緩やか)
double main-sequence in w Cen
HST reveals
VLT reveals
photometric result
blue MS
spectroscopic result
red MS Bedin et al. 2004
fbMS~ 0.2-0.3
the origin of bMS
1. Y~0.4
2. very low metallicity ([Fe/H]<-2)
Piotto et al. 2005
bMS stars are more metal-rich
than rMS stars.
D[Fe/H]~0.3
super helium-rich stars in GCs
47 Tucanae
NGC 2808
Milone et al. 2012
Milone et al. 2012
Y=025/0.32/0.38
the origin of
He-rich stars
✓GC is not a single population!
NGC 6566
(note: a spread in [Fe/H]:w Cen, M22, Terzan 5, NGC 2419
a subgiant-branch split: NGC 1851,
M22, 47 Tuc,…..)
likely, the formation
from AGB ejecta ✓the presence of super He-rich stars
only in massive GCs?
>17% halo stars from disintegrated GCs (Martell et al. 2011)
seen for metal-rich stars in the bulge (Nataf & Gould 2012)
まとめ
 大きく認識を変えたいこと
✓Ia型超新星は結構早く爆発する
~1億年
✓球状星団(の一部)は単一の星の種族ではない
ヘリウム過剰星の存在
 今後注視していきたいディスク銀河進化のドライバー
✓stellar migration (星の動径方向の大移動)
但し、過大評価の可能性もある
 IMFの普遍性問題
✓バルジ、近傍矮小銀河にIMF variationの証拠
top-heavy top-light

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