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Searching for the oldest stars with the world’s

largest telescopes

Prof. Anna FrebelPhysics DepartmentAstrophysics Division

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The very first stars in the Universe

NASA/WMAP science team

artist’s impression

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A long time ago...

Today!

cosmic time scale

“Big Bang”

Larson & Bromm 2001

second and latergenerations of starsFirst stars

First galaxiestoday’sgalaxies

0 years 13.7 billion years

...not to scale

We want to find the stars of the second and early generations

of stars!

Scientific American (Bromm + Larsen 2001)

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Chemical Evolution We are made of stardust! ! Old stars contain fewer elements (e.g. iron) than younger stars

We look for the stars with the least amounts of elements

heavier than H and He!We call them

metal-poor stars!

TU Berlin

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Astronomer’s periodic table

All other elements combined

Metals “Z” ~ 1,4%

“X” ~ 71,5% “Y” ~ 27%

Hydrogen1

H1.0079

Helium2

He4.0026

In theUniverse

today

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The Milky Way

Halo

Bulge Disk

Metal-poorhalo stars

Dwarf galaxies

!! !

! !!

!!

!!

Low-mass stars (M < 1 M!)! Lifetimes > 10 billion yrs! they are still around!

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Star spectra... we can explore the

atmosphere of a star!

despite their very large distances from us!

A picture is worth a thousand words

But a spectrum is worth a thousand pictures!! :)

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The cosmic chemical barcode

Mg

Mg Na

Mg

Ca CH H

A. FrebelThe cosmic chemical barcode

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Line strength ⇒ Abundance of element

Existance of line ⇒ Element present in star

A. Frebel

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Chemical analysis of stars

The position and strength of the absorption lines tell us about the chemical composition of the star

prism, spectrograph...

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Taking a spectroscopic look

“Loo

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Gal

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Abundances are derived from integrated absorption line strengths

Sun

most iron-poor star

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[Fe/H]NLTE = !5.2 Christlieb et al. (2002), Nature 419, 904Christlieb et al. (2004), ApJ 603, 708Bessell et al. (2004), ApJ 612, L61

[Fe/H]NLTE = !5.4 Frebel, Aoki et al. 2005, Nature 434, 871

Frebel et al. 2006, ApJ 638, L17 Aoki, Frebel et al. 2006, ApJ 639, 897

Frebel et al. 2008, ApJ 684, 588

HE 0107"5240Red giant(5200K)

HE 1327"2326Subgiant(6180K)

The most iron-deficient stars known

Masses: 0.6 - 0.8 M!

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is in there?

HE1327-2326(most Fe-poor star)

100 x lessthan Earth’s iron coreStar is a million

times bigger than earth (300,000 more massive)

Earth

Earth’s iron core

not to scale

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What can we learn from old halo stars?

Low-mass stars (M < 1 M!)! Lifetimes: >10 billion years => still around! ________________________ Using metal-poor stars to reconstruct:!Origin and evolution of chemical elements !Lower limit to the age of the Universe

... and to provide constraints !Nature of the first stars & first supernovae!Assembly of galaxies like the Milky Way

APOD

Metal-poor stars are a great tool for near-field cosmology because they are the local equivalent of

the high-redshift Universe!

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SkyMapper is taking shallow data now!Provides new metal-poor halo stars now

and will ultimately also find more dwarf galaxies!

1.3m telescope Siding Spring ObservatoryAustraliaPI: Brian Schmidt

Skymapper telescope

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Fig. 1.— Portions of the MIKE spectra for four stars in our sample around the Ca II H and K lines.Stellar parameters and metallicities determined in our analysis are also indicated. Note the variations in linestrength with decreasing [Fe/H] (top to bottom).

Fe I abundance with excitation potential (E.P.),log g by matching Fe I and Fe II abundances,microturbulence (vt) by removal of any slope ofFe I abundance with reduced equivalent width(REW). As has long been discussed in the liter-ature, such spectroscopically-determined param-eters can differ greatly from values determinedvia other methods (e.g., photometry, theoreticalisochrones). Spectroscopic effective temperaturesare generally cooler than photometric tempera-tures, for example, due to the relatively smallnumber of Fe lines in metal-poor stars and/ordepartures from local thermodynamic equilibrium(LTE; Johnson 2002; Cayrel et al. 2004; Lai et al.

2008; Hollek et al. 2011). Too-cool temperaturestranslates into smaller log g and larger vt valuesthan would be found using photometric tempera-tures.

We have adopted the effective temperature cor-rection presented in Frebel et al. (2013) that placesspectroscopically-determined temperatures on ascale similar to that found by photometric tem-perature methods. This calibration is appropri-ate for the program stars as it was obtained usingMIKE spectra, and the majority of the programstars span the temperature range for which the cal-

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Magellan/MIKE results:

2012 Pilot sample

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Fig. 1.— Portions of the MIKE spectra for four stars in our sample around the Ca II H and K lines.Stellar parameters and metallicities determined in our analysis are also indicated. Note the variations in linestrength with decreasing [Fe/H] (top to bottom).

Fe I abundance with excitation potential (E.P.),log g by matching Fe I and Fe II abundances,microturbulence (vt) by removal of any slope ofFe I abundance with reduced equivalent width(REW). As has long been discussed in the liter-ature, such spectroscopically-determined param-eters can differ greatly from values determinedvia other methods (e.g., photometry, theoreticalisochrones). Spectroscopic effective temperaturesare generally cooler than photometric tempera-tures, for example, due to the relatively smallnumber of Fe lines in metal-poor stars and/ordepartures from local thermodynamic equilibrium(LTE; Johnson 2002; Cayrel et al. 2004; Lai et al.

2008; Hollek et al. 2011). Too-cool temperaturestranslates into smaller log g and larger vt valuesthan would be found using photometric tempera-tures.

We have adopted the effective temperature cor-rection presented in Frebel et al. (2013) that placesspectroscopically-determined temperatures on ascale similar to that found by photometric tem-perature methods. This calibration is appropri-ate for the program stars as it was obtained usingMIKE spectra, and the majority of the programstars span the temperature range for which the cal-

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Flux through that line = metallicity indicator !

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What does SkyMapper observe? The filter set

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Textmillions

hundreds

dozens

Three Observational

Steps to Find Old Stars

SkyMapper

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Skymapper &

Candidate metal-poor stars selected with high efficiency...

observed with the 6.5m Magellan telescopeat Las Campanas Observatory, Chile

• SkyMapper can only provide candidate metal-poor stars• High-resolution spectroscopy (R>20,000) is required to confirm

metal-deficiency and to carry out chemical abundance analysis

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at Las Campanas Observatory

in Chile

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• Movies are available on youtube• https://www.youtube.com/channel/

UC3cyRVDoePNf_rLQlwKpdeg

• And on my website• annafrebel.com => science

communication => Science — it’s a human thing!

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Videos about observing with

Magellan: youtube and

annafrebel.com

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A true second-generation star

[Fe/H] = -4.2

[Fe/H] = -5.2

[Fe/H] < -7.0

Kelle

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2014

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Interstellar Ca

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Second/early generation stars

Implications for the yields of first supernova => low Fe yields required

We learn about the explosion mechanisms and energies, and masses of the first stars!

Great prospects for finding more second-generation stars => Ultimate diagnostic for studying the first stars

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METAL–POOR STARS 5

Fig. 1.— [Fe/H] for the most metal-poor star then known as a function of epoch. The symbols denote the abundance determinedby the authors, while the horizontal lines refer, approximately, to currently accepted values. (The abundances are based onone-dimensional, Local Thermodynamic Equilibrium model atmosphere analysis. See Section 3.1.)

time and target faintness, it was common that not allstars could be observed. In the original candidate listin the magnitude range 13.0 ! B ! 17.5 there were∼3700 red giants of which about 1700 were observed atmedium-resolution (Schörck et al. 2009), together with∼3400 near-main-sequence-turnoff stars, of which ∼700have follow-up spectroscopy (Li et al. 2010). There isalso a bright sample of ∼1800 stars having B < 14.5,for all of which medium resolution spectra were obtainedby Frebel et al. (2006). From these samples, the mostmetal-poor candidates were selected for high-resolutionobservation. Various considerations determined whethera star was ultimately observed. These include telescopetime allocations, observability and weather conditionsduring observing time, target brightness, reliability ofthe medium-resolution result, science questions to be ad-dressed, and of course the preliminary metallicity of thestar. Given these limitations, fainter stars remain unob-served on the target lists due to time constraints.To this point the discussion has been confined to sur-

veys that have concentrated on discovering candidatemetal-poor stars with B ! 17.5, with follow-up medium-resolution spectroscopy complete in most cases to onlysomewhat brighter limits. Surveys that reach to con-siderably fainter limits are the Sloan Digital Sky Survey(SDSS) and the subsequent SEGUE-I and II surveys (seehttp://www.sdss.org), which have obtained spectra withresolving power R ∼ 2000, and are also proving to be aprolific source of metal-poor stars. In a sample of some400,000 stars, SDSS/SEGUE has discovered 26,000 starswith spectra having S/N > 10, and [Fe/H] < –2.0 (basedon these intermediate-resolution spectra), while some 400

have [Fe/H] < –3.0.The search for metal-poor stars remains a very active

field, with several exciting projects coming to comple-tion, currently in progress, and planned. This matterwill be further discussed in Section 7.

2.3. High-Resolution, High S/N Follow-Up Spectroscopy

The final observational step in the discovery pro-cess is spectroscopy of the most significant objects(e.g., most metal-poor, or most chemically peculiar)at very high resolving power (R ∼ 104 – 105) andS/N " 100, in order to reveal the fine detail re-quired for the determination of parameters such as accu-rate chemical abundances, isotope ratios, and in somecases stellar ages. This is best achieved with 6 –10m telescope/échelle spectrograph combinations – cur-rently HET/HRS, Keck/HIRES, Magellan/MIKE, Sub-aru/HDS, and VLT/UVES.In order to give the reader a feeling for both the role

of increased resolution and the manner in which decreas-ing metallicity affects the observed flux, Figure 2 showsthe increase in spectroscopic detail between intermediate(R ∼ 1600) and high (R ∼ 40000) resolving power forfour metal-poor red giants of similar effective tempera-ture (Teff) and surface gravity (log g) as metal abundancedecreases from [Fe/H] = –0.9 to –5.4 (for HE 0107–5240,the most metal-poor giant currently known).

2.4. Census of the Most Metal-Poor Stars

This section presents a census of stars having [Fe/H]< –3.0 and for which detailed high-resolution, high S/N ,published abundance analyses are available. The data

Discovering the most metal-poor stars

met

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Frebel & Norris (2013)

SM03

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Fig. 7.— Same as Figure 6, but for the alpha elements.

8

black open circles: 190 literature stars re-analyzed by Yong et al. 2013blue squares: this study

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"CDM hierarchical structure formation model

Comprehensive understanding of galaxy formation

Spectroscopic observations

of stars 6.5m Magellan Telescopes, Chile

at the cutting edge

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The oldest stars are just like diamonds:

- They are rare- They are difficult to come by - They contain a lot of carbon- They last (almost) forever - They are good for many occasions/applications- They make you happy!

Little Diamonds in the sky...

“Old stars are a girl’s best friend!”

With our new survey & large telescopes we are continuing this treasure hunt !