Carbon-Enhanced Metal-Poor (CEMP) in the Milky Way

KASI Seminar, May 27, 2015

Young Sun LeeChungnam National University

Outline

Part I CEMP Stars

Identification from Metal-Poor (MP) Stars Origin of CEMP Stars

Part 2 Exploring the Milky Way

Constraining the Halo’s Initial Mass Function (IMF) Halo Dichotomy with CEMP Stars Search for the Signature of the Nucleosynthesis of First

Generation of Stars (Pop III) Looking forward

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Metal-Poor (MP) Stars Efforts to searching for Very Metal-Poor

(VMP; [Fe/H] < -2.0) stars HK Survey of Beers and colleagues HES(Hamburg ESO) Survey of Christlieb and

colleagues These surveys identified several thousand VMP

stars Many tens of VMP stars from

Original Sloan Digital Sky Survey (SDSS) SEGUE (Sloan Extension for Galactic

Understanding and Exploration) I and II Ongoing SDSS (e.g., BOSS) LAMOST (Multi-Object fiber Spectroscopic

Telescope)

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Why We Need Large Numbers of MP Stars ?

Extremely MP stars have recorded the heavy element abundances produced in the first generations of stars Nucleosynthesis in the early universe

Determination of the frequency of C-enhanced or alpha-rich stars Initial mass function and star formation

history Identification of relatively rare objects(r-process / s-

process enhanced stars) amongst MP stars Site for production of neutron capture

elements Change in the nature of the Metallicity Distribution

Function (MDF) as a function of distance may reveal the assembly history of the MW

Known MP Stars – Pre and Post SDSS/SEGUE-1/SEGUE-2

Name Metallicity Pre Post

Metal-Poor (MP) [Fe/H] < -1.0 15,000 150,000+

Very Metal-Poor (VMP) [Fe/H] < -2.0 3,000 30,000+

Extremely Metal-Poor (EMP) [Fe/H] < -3.0 400 1000+

Ultra Metal-Poor (UMP) [Fe/H] < -4.0 6 20

Hyper Metal-Poor (HMP) [Fe/H] < -5.0 2 3

Mega Metal-Poor (MMP) [Fe/H] < -6.0 0 0

Until 2013 (Nomenclature; Beers & Christlieb 2005)

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Known MP Stars – Pre and Post SDSS/SEGUE-1/SEGUE-2

Name Metallicity Pre Post

Metal-Poor (MP) [Fe/H] < -1.0 15,000 150,000+

Very Metal-Poor (VMP) [Fe/H] < -2.0 3,000 30,000+

Extremely Metal-Poor (EMP) [Fe/H] < -3.0 400 1000+

Ultra Metal-Poor (UMP) [Fe/H] < -4.0 6 21

Hyper Metal-Poor (HMP) [Fe/H] < -5.0 2 4

Mega Metal-Poor (MMP) [Fe/H] < -6.0 0 1

Septa Metal-Poor (SMP) [Fe/H] < -7.0 0 1

Octa Metal-Poor (OMP) [Fe/H] < -8.0 0 0

Giga Metal-Poor (GMP) [Fe/H] < -9.0 0 0

Note that EMP stars probably include additional UMP, HMP, MMP, SMP, OMP, GMP stars, but wont be revealed as such until high-resolution follow-up conducted (contamination due to interstellar CaII K and/or carbon) 6

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Identification of Carbon-Enhanced Metal-Poor Stars

The HK and HES surveys revealed an unexpectedly large number of VMP stars with anomalously strong CH bands

Both surveys identified about 100 CEMP stars for [Fe/H] < -2.5

CH G band

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High-Resolution Follow-up

Detailed chemical-abundance analyses of VMP stars confirmed: Most VMP stars exhibit similar abundance

pattern But, there are peculiar objects with strong

enrichments or deficiencies of light elements such as C, N, O, Na, Mg, Si, etc.

Objects with enhanced carbon are the most common variety

Carbon-Enhanced Metal-Poor (CEMP) Stars

CEMP Carbon-Enhanced Metal-Poor (CEMP) Not Carbon-Extremely Metal-Poor CEMP defined by [Fe/H] < -1.0 and [C/Fe] >

+0.7 or [C/Fe] > +1.0 (Beers & Chrislieb 2005)

[C/Fe] Coin a term Carbonicity similar to Metallicity

(e.g., Carollo et al. 2012)

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Identifying CEMP Stars in SDSS/SEGUE

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Developed a program to estimate [C/Fe] from the SDSS/SEGUE stellar spectra using CH G band (Lee et al. 2013)

CEMP Stars in SDSS/SEGUE

Thousands of CEMP Stars Identified by SDSS/SEGUE

Lee et al. (2013) – CEMP stars from SDSS/SEGUE + Literature Sample12

The largest list of CEMP stars ever made

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Frequency of CEMP Stars Interestingly the fraction of CEMP stars

increases as the metallicity decreases This indicates that a large amount of

carbon was produced in the early history of the Milky Way

Then, a question arises “how?”

Lee et al. (2013)

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Patterns of n-capture Elements Another interesting aspect of CEMP

stars is that they have different enhancement of n-capture element

High-resolution spectroscopy of CEMP stars revealed different level of enhancement of n-capture elements s-process (e.g., Ba or Sr) rich r-process (e.g., Eu) rich r/s-process (Both Eu and Ba or Sr) rich No n-capture (neither Eu nor Ba)

Indicative of different astrophysical sites of carbon production at early times

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Subclasses of CEMP Stars CEMP Stars are further divided into

four more groups depending on the enhancement of the s-process element (Ba) and r-process element (Eu) CEMP-s and CEMP-n accounts for over 95

%

Beers & Christlieb ARAA (2005)

Origin of CEMP Stars

CEMP stars in the Galaxy likely have had multiple mechanisms to produce carbon

Each subclass displays distinct properties CEMP-s

Variation of radial velocity, indicating a binary system

Mostly discovered for [Fe/H] > -3.0Account for about 80 % of CEMP stars Origin: AGB binary mass transferCarbon and s-process elements are produced

during the AGB16

Origin of CEMP Stars – Con’t CEMP-no

Occur preferentially at the lowest metallicities ([Fe/H] < -3.0)

No radial velocity variation, so not in a binary 3 of the 4 stars known with [Fe/H] < -4.5Origin: A few mechanisms

- Rapidly rotating massive (50-100 Msun) MMP stars

- Faint SNe of intermediate mass (25-60 Msun) with mixing and fallback

CEMP-r, CEMP-r/sProbably, they are originated with supernova, but not well

known, and need more sample17

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Difference in [Fe/H] Distribution:CEMP-s vs. CEMP-no

CEMP-s

CEMP-no

Aoki et al. (2007) demonstrated that the CEMP-no stars occur preferentially at lower [Fe/H] than the CEMP-s stars

About 80% of CEMP stars are CEMP-s, 20% are CEMP-no

Global abundance patterns of CEMP-no stars is incompatible with AGB models at low [Fe/H]

Evidence of Faint SN Model: BD+44:493 – A 9th Magnitude Star Ito et al. (2009) report on discovery that BD+44 is

an [Fe/H] = -3.8, and carbon-enhanced and low nitrogen Light-element abundance pattern similar to those

for CEMP-no stars No RV variation at levels > 0.5 km/s over past 25

years Identified as CEMP-no star Compared the abundance pattern with faint SN

model with 25 Msun

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Abundance Pattern Compared to 25 MSun Mixing/Fallback Model

Low N, compared with some other CEMP-no stars with enhanced N => indication of the origin of faint SNe. Rapidly rotating scenario needs high [N/Fe] 20

Exploring the Milky Way with CEMP Stars

Constraining the Initial Mass Function (IMF) of the Galactic halo

Dual property of the halo with CEMP stars

Search for the signature of the Nucleosynthesis of First Generation of Stars (Pop III)

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Constraining the Halo’s IMF

It is possible to derive the mass of progenitors of CEMP-s stars by AGB model CEMP-s stars are produced in the range of M=1-8

Msun But, efficiently in M=1-3 Msun

CEMP stars are dominated by CEMP-s (over 80 %)

Constraints on the IMF of the Galactic halo AGB population synthesis model predicts the

CEMP frequencyCompare that with the observation (e.g., Suda et

al. 2013; Lee et al. 2014)22

Lee et al. (2014) compared AGB population synthesis model prediction with that from SDSS/SEGUE dataThe transition of the IMF occurred between [Fe/H] = -2.5 and -1.5

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Transition time

Constraining the Halo’s IMF

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Dichotomy of the Galactic Halo

Structural components of the stellar populations Bulge / Thin Disk / Thick Disk (MWTD) /

Halo New results from SDSS revealed (Carollo et

al. 2007, 2010)

Inner Outer

Distance (R) < 10-15 kpc > 15-20 kpc

Rotational Velocity (Vφ)

~0-50 km/s -40 to -70 km/s

Distribution Eccentric orbits (oblate shape)

More spherical shape

[Fe/H] -1.6 -2.2

Origin Dissipative collapse Accreted

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This dichotomy should appear in other chemical elements as in [Fe/H]

[Mg/Fe] is higher in the inner halo by 0.1 index than

the outer halo (Roederer 2009)

What about [C/Fe]?

Carollo et al. (2012) more studies the distribution of

[C/Fe] in the inner and outer halo

Contract in Other Elements?

Spatial Distribution of [C/Fe]

It shows that [C/Fe] continuously changes from low to high as |Z| increases (not expected for single halo)

Lee et at. in preparation

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Global CEMP Fraction vs. |Z|

Clear increase of f (CEMP) with |Z| (not expected for single halo)

Lee et at. in preparation27

Carollo et al. 2012

Inner/Outer Halo CEMP Fractions

f (CEMP)OH ~ 2 x f (CEMP) IH <[C/Fe]> roughly constant IH/OH

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Carollo et al. (2012)

What Do These Imply ?

The distribution of CEMP stars indicates that there is likely to be more than one source of C production at low metallicity, and that the difference can be associated with assignment to inner/outer halo

It is speculated that the majority of CEMP stars associated with the inner halo will be CEMP-s, while those associated with the outer halo will be CEMP-no

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Fraction of CEMP-no and CEMP-s in the Inner/Outer Halo

Sample of 183 stars with high-resolution spectroscopy Include about 50 CEMP stars

Need more sample, it is underway with SDSS/SEGUE

Carollo et al. 201430

Inner halo

Outer halo

Connection with Ultra-faint Dwarf Galaxies Ultra-faint SDSS dwarf galaxies possess lots of

CEMP stars, some of which have low n-capture abundances

Building block of the outer halo?

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SEGUE1 (Frebel et al. 2014) Three of the 7 giants that have

[Fe/H] < -3.5 are CEMP stars All three are CEMP-no stars Concluded that disrupted UF

dSph galaxies could account for the CEMP-no stars found in the outer halo of the MW

Search for Nucleosynthesis Production of First Generation Stars (Pop III)

Different initial mass produces different chemical abundance pattern

Chemical abundance pattern leads to the mass of the progenitor Model versus Observation

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Expected Chemical Signatures

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Nu

mb

er

per

Mass

B

in

MCEMP-s CEMP-no

Enhanced Light elements

Carbon-normalNormal light elements

Tumlinson 2002

As if Right on Cue …

Nature – March 2014

A single low-energy, iron-poor supernova as

the source of metals in the star SMSS

J031300.36-670839.3

S. C. Keller, M. S. Bessell, A. Frebel, A. R. Casey, M. Asplund, H. R. Jacobson, K. Lind, J. E. Norris, D. Yong, A. Heger, Z. Magic, G. S. Da Costa, B. P. Schmidt, & P. Tisserand

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Discovery of a Star with [Fe/H] = -7.1

Announcement of the discovery of a star with metallicity [Fe/H] < -7.1 More than 10,000,000 times lower than the Sun

And of course, it is a CEMP-no star, with the same light element abundance pattern as other CEMP-no stars

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High-resolution Spectrum

The high-resolution spectrum from Magellan shows lack of detectable Fe lines ( Keller et al., Nature 2014)

Stellar parameters:Teff = 5100log g = 2.3[Fe/H] < -7.1[C/Fe] ~ +4.5

UMP

HMP

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Close-up of Ca II K Region

A comparison of the spectrum of SMSS 0313-6708 with other UMP ([Fe/H] < -4) and HMP ([Fe/H] < -5) stars in the regions of CaII K. 37

Observed Elemental Abundance Pattern

Note singular detections of C, Mg, and Ca (Keller et al. 2014)38

● 　 SMSS 0313-6708 　　　　　　 Faint SN model of 60 Msun star Pollution from a single SN

Evidence for Something Missing

Not all stars with [Fe/H] < -2.5 are carbon-enhanced, in particular for the abundance range -3.5 < [Fe/H] < -2.5 Including at least one star, with [Fe/H] ~ -5.0, and a

number with [Fe/H] ~-4.0, without the detection of the chemical signature of CEMP-no stars

Where did they come from?

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Aoki et al. (Science, Aug. 22, 2014)

A chemical signature of first-generation very massive stars

W. Aoki, N. Tominaga, T. C. Beers, S. Honda, Y. S. Lee

Abstract: Numerical simulations of structure formation in the early universe predict the formation of some fraction of stars with several hundred solar masses. No clear evidence of supernovae from such very massive stars has, however, yet been found in the chemical compositions of Milky Way stars. We report on an analysis of a very metal-poor star SDSS J001820.5-093939.2, which possesses elemental-abundance ratios that differ significantly from any previously known star. This star exhibits low [alpha-element Fe] ratios and large contrasts between the abundances of odd and even element pairs, such as scandium/titanium and cobalt/nickel. Such features have been predicted by nucleosynthesis models for supernovae of stars more than 140 times as massive as the Sun, suggesting that the mass distribution of first-generation stars might extend to 100 solar masses or larger.

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Abundance Patterns Like No Other

SDSS J0018-0939 Cool MS star with [Fe/H] = -2.5,

NOT carbon-enhanced, and with elemental-abundance ratios unlike any previously studied very low-metallicity star.

Abundance ratios between adjacent odd- and even-element pairs are very low: [Na/Mg] = -0.56, [Sc/Ti] < -0.99, [Co/Ni] = -0.77

n-capture elements are quite low compared to other VMP stars: [Sr/Fe] < -1.8, [Ba/Fe] < -1.3.

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Comparison to Standard Supernova Model

炭素

● 　 SDSS J0018-0939 　▲ 　 Comparison star (G39-36, [Fe/H]=-2.1) Core collapse supernova model Does not reproduce C, Na, Mg, Al, Si, and Co abundances

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Comparison to Other Supernova Models

● 　 SDSS J0018-0939 　　　　　　 Pair Instability Supernova (PISN) with M=130 Msun 　　　　　 Core-collapse very-massive star model, M=1000 Msun

Mass distribution of first-generation stars might extend to 100 Msun or larger

The Path Forward

Expansion of numbers of identified CEMP stars, in particular with [Fe/H] < -2.5, which include both CEMP-s and CEMP-no stars, both from HK/HES, SDSS/SEGUE, APOGEE, and the ~ 8 million medium-res spectra coming from LAMOSTFor detailed dual property of the Galactic haloCEMP Fraction as a function of [Fe/H] Connection with ultra-faint dwarf galaxies

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The Path Forward – Con’t Detailed chemical abundance analysis from high-

resolution follow-up of VMP ([Fe/H] < -3.0) stars Establishment of the frequency of such objects as SDSS

J0018-0939 or SMSS J0313, based on high-resolution spectroscopic surveys of the many thousands of stars known with [Fe/H] < -2.5

Refinement of various SN models (PISN or very massive core collapse) with observed abundance pattern

Gemini Korean time is a good opportunity for the high-resolution follow-up of EMP stars

Lots of faint targets in SDSS/SEGUE/BOSSMostly too faint (g > 17) for 8-10m class telescopeReally good targets for GMT

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