The Gaia DR2andthe Database of Metal-Poor Stars(Sokendai / NAOJ)

In collaboration with Takuma Suda (Univ. of Tokyo), Wako Aoki (NAOJ)
Shimako Yamada, Yutaka Katsuta (Hokkaido Univ.) et al.

Good memories of stars
Stars remember their birth conditions
Stellar kinematics
assumptions- negligible stellar interactions- slowly changing gravitational potential with symmetry

There are should be conserved quantities

Stellar chemical abundances
abundance of 10Gyr-old stars= abundance of ISM 10Gyr ago,reflecting star formation before constantStars

the Galaxymetalabundance production

To understand the halo formation fromchemo-dynamical analysis of (very)
metal-poor stars
halodisk
Why metal-poor stars?
How do we study and where can we getchemical abundances and kinematics?

Metal-Poor Stars
metal-poor = old(the Universe started without metals)

Very Metal-poor stars ([Fe/H]<-2.0)- almost as old as the Galaxy (~10Gyr old)- mostly found in the Galactic stellar halo

Tell us about the very beginning ofthe Galaxy/halo formation
𝑁𝑋
Note: [X/Y]= log−𝑁𝑌𝑁𝑋log
𝑁𝑌 ⊙Chemical Abundances
How do we get chemical abundance of stars?
Absorption lines in stellar spectra
Requirements- high resolution (R>20,000)- high S/N (S/N>20)[Å]

Expensive observation

SAGA database Suda et al. (2008, 2011, 2017) Yamada et al. (2013)
Stellar Abundances for Galactic Archaeology Database- Compilation of abundance measurements for very
metal-poor stars in literatures- >1300 very metal-poor stars in >300 literatures

Stellar kinematics
All theGaia
BeforeAfterstars
GaiaDR2DR2,on April 25, 2018
Gaia
Before Gaia
Sun© S. BRUNIER/ESO, ESA

Gaia: space telescope for astrometry
We are now able to explore chemo-dynamics ofvery metal-poor stars for the first timeKinematics and abundances in halo
Stellar kinematics in the halo

5Gyr after satellite accretion in N-body simulation by Jean-Baptiste et al. (2017)

Efaintersatellites
satellites[Fe/H][Ba/Fe][Mg/Fe]

LzAll stars
in-situ stars
accreted stars
data from SAGA database
Stellar chemical abundances
Ba: r-process element
Mg: α-element
Milky Way

[Fe/H]

Results
Global property
Two components

Substructures

Global property of metal-poor stars[Fe/H]<-1.5
retrograde prograde

prograde

retrograde

disk rotation
disk

disk stars

No overdensity
associated with the disk
There would be a metallicity above which disk
formation has started

Abundances of prograde/retrograde halo
Metallicity difference[Mg/Fe][Ba/Fe]Lack of abundance difference at [Fe/H]<-2
The metal-poor component has formed in a similar
way between prograde/retrograde haloKnown Substructures in the Halo

12
3

4
Helmi+17

What are the origins of the substructures?

[Mg/Fe]Mg abundance of substructures
1234All stars in the database / stars in a substructure

Prograde substructures have higher [Mg/Fe],meaning they have formed in Milky Way
Similar picture in Nissen&Schuster 10

123 4

[Ba/Fe]Ba abundance of substructures
Large scatter at low metallicity
12
34

All stars in the database / stars in a substructure

A possible signature of “[Ba/Fe]” jump
Similar feature in a dwarf galaxy

12
Tsujimoto, Matsuno, et al. (2017)

3 4

Conclusion
First chemo-dynamical analysis for a large
number of very metal-poor stars are conducted
Indications to the early phase of the Galaxy formation- Disk formation has started above a certain metallicity- The metal-poor component of the prograde/retrograde
haloes seems to have formed in a similar way- The most prograde kinematic substructure might be a
mixture of stars formed in Milky Way and those formed
in a dwarf galaxy

FutureGaia DR2 are excellent!!
we could utilize a large potion of stars in SAGA database
Still, any chemical signatures are, at most, week
Limitations:
Abundance precision
Systematic errors could have blurred signatures
LAMOST-Subaru survey of 400 metal-poor stars(H.-N. Li, W. Aoki, T. Matsuno et al. )Number of stars
cf. 400 stars in 20 Subaru nights
abundance determination from lower-res. spectra