1. The Chemical Signatures of First-Generation Stars
Timothy C. Beers
University of Notre Dame
The  Chemical  Signatures  of   First-Generation   Stars
SDSS

2. Expected Signatures
First-generation objects of high mass presumably formed from
metal-free gas
Lived short lives (Myrs not Gyrs)
Exploded
Distributed (pre or post explosion) their nucleosynthetic products
Next-generation objects formed from the gas polluted by first-generation objects
A wider range of masses allowed, perhaps including stars with main-sequence lifetimes > a Hubble time
Further star formation (Pop II) contributed additional material, and diluted the signatures of first/next-generation stars
We should look for a characteristic set of abundance signatures ONLY found among the lowest metallicity stars

3. Expected Signatures
Among the lowest metallicity stars, there are two basic “families”
Stars with enhanced abundances of carbon (CEMP stars), and other light elements (including the lowest [Fe/H] star yet discovered), and lack of over-abundances of neutron-capture elements (CEMP-no stars)
Associated with production by “faint SNe” – progenitors with mass on the order of Mo undergoing mixing and fallback, or rapidly rotating mega metal-poor (MMP; [Fe/H] < -6.0) stars, both of which eject large amount of CNO, but little heavy metals. Low-mass stars formed with the help of C, O cooling
Stars with “normal” carbon and light-element abundances, apparently formed with cooling other than C, O – perhaps by silicates ?
Possibly associated with production by first-generation objects of very high mass ( Mo), which produce large amounts of heavy metals, but little carbon

4. Frequencies of CEMP Stars Based on Stellar Populations
Carbon-Enhanced Metal-Poor (CEMP) stars have been recognized to be an important stellar component of the halo system
CEMP star frequencies are:
20% for [Fe/H] < -2.5
30% for [Fe/H] < EMP
40% for [Fe/H] < -3.5
75% for [Fe/H] < UMP
100% for [Fe/H] < HMP
But Why ? – Atmospheric/Progenitor or Population Driven ?
Carollo et al. (2012,2014) suggest the latter

5. Cumulative Frequencies of CEMP Stars from SDSS/SEGUE

6. Cumulative Frequencies of CEMP-no (ONLY) Stars from SDSS/SEGUE, with Luminosity Corrections
Placco et al. (2014)

7. Exploration of Nature’s Laboratory for Neutron-Capture Processes
Beers & Christlieb ARAA (2005)

8. The UMP/HMP Stars are (Almost) ALL CEMP-no Stars
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
or CEMP-r/s, 20% are
CEMP-no
Global abundance
patterns of CEMP-no
stars incompatible
with AGB models
at low [Fe/H]

9. CEMP-no Stars are Associated with UNIQUE Light-Element Abundance Patterns (Aoki et al. 2002)
CS : [Fe/H] = -3.8; [C/Fe] = +1.9
Harbingers of Things to Come!

10. Last but Definitely NOT Least… (Christlieb et al. 2002; Frebel et al
HE [Fe/H] = -5.3 [C/Fe] = +3.9
It is the SAME pattern among the light elements !

11. BD+44:493 – A 9th Magnitude Messenger from the Early Universe
Ito et al. (2009) report on discovery that BD+44 is an [Fe/H] = -3.8, CEMP-no star; more detailed observations by Ito et al. (2013)
Light-element abundance patterns similar to those for other CEMP-no stars
Previous RV monitoring by Carney et al. indicate no variation at levels > 0.5 km/s over past 25 years
The same is true for other CEMP-no stars with available RV monitoring (Hansen et al. 2013, and in prep; Norris et al. 2013)

12. Something You Don’t Often See
An Object of COSMOLOGICAL Significance with Diffraction Spikes

13. Abundance Pattern Compared to 25 Mo Mixing/Fallback Model
Ito et al. (2013) : Note the low N, compared with some other CEMP-no stars with enhanced N

14. HST/STIS Observations of Be and B in BD+44 at the Lowest Levels Measured for Metal-Poor Stars

15. Dust in the Early Universe (Watson et al. 2015 – Nature)
This image, taken by the Hubble Space Telescope, shows a part of the galaxy cluster, Abell 1689, whose gravitational field amplifies the distant galaxy behind it. The distant dust-filled galaxy in zoom in the box, at a redshift z ~ 7.5

16. The Carbon-Low and Carbon-High Bands
Recent observational evidence strongly suggests that CEMP-no stars are
associated with “low” absolute C abundances (A(C) ~ 6.5), while CEMP-s
stars are associated with “high” absolute C abundances (A(C) ~ 8.25)
This recognition may provide a more fundamental definition of CEMP-no
stars, rather than simply [Ba/Fe] < 0 (which may take on higher values)
Spite et al. (2013)

17. The Carbon-Low and Carbon-High Bands
Similar results seen in the HERES sample (Barklem et al. 2005), as
summarized by Placco (private communication)
Placco et al. based on HERES Sample

18. The Carbon-Low and Carbon-High Bands
Similarly from Hansen et al. (2015)
Note that A(C) has been
corrected for luminosity
effects following Placco et al. (2014)
CEMP-no ONLY 
Including stars from
Yong et al. (2013) 

19. The Carbon-Low and Carbon-High Bands
Similarly from Bonifacio et al. (2015)

20. The Distribution of A(C) for the Carbon-Low and Carbon-High Bands (Bonifacio et al. 2015)

21. Bottom Line We have observed (!) the nucleosynthesis products
CEMP stars in the Galaxy likely have had multiple sources of
carbon production
CEMP-s in AGB stars
CEMP-no in massive ( Mo) rapidly rotating MMP stars
CEMP-no in intermediate high-mass (25-50 Mo) “faint” SNe
CEMP-no stars occur preferentially at the lowest metallicities,
including all but 2 of the 20 stars known with [Fe/H] < -4.0
CEMP stars are found in substantial numbers in the ultra-faint
SDSS dwarf galaxies, some of which have low n-capture abundances
High-z DLA systems exhibit similar abundance patterns as
CEMP-no stars
Discovery of “dusty galaxies” at high-z
We have observed (!) the nucleosynthesis products
of first generation stars (Pop III)

22. 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 and the ~ 5-10 million medium-res spectra coming from LAMOST
We need bright stars to obtain Be, B with HST/STIS and CUBES (Be)
High-resolution follow-up spectroscopy of a core sample of CEMP stars, in order to assign classifications based on heavy elements, and to determine CNO, alpha elements, and other light-element abundances
Radial velocity monitoring of CEMP stars, in order to determine binary nature, as well as characterize correlations between chemical patterns and nature of the detected binary
Full numerical GCE models, taking into account the effects of local mixing, in order to match frequencies of CEMP stars and C-normal stars, as well as Li-depletion phenomenon for very metal-poor stars

23. RV Monitoring – Hansen et al. (in prep): rI / rII

24. RV Monitoring – Hansen et al. (in prep): CEMP-s and CEMP-r/s

25. RV Monitoring – Hansen et al. (in prep): CEMP-no

26. New Surveys for JINA-CEE
Medium-res follow-up of HES VMP and carbon-selected stars using “bad weather” time on Gemini 8m telescopes – some 2000 stars
Medium-resolution follow-up of MP candidates from Australian SkyMapper Southern Sky Survey on the AAT 3.9m telescope – some 70,000 stars
High-resolution follow-up with various 8m telescopes (Magellan) ongoing for both samples of stars

27. New Surveys for JINA-CEE
Medium-resolution follow-up of [Fe/H] < -2.0 stars from RAVE, with various 4m and 8m telescopes, in order to identify the important subset of BRIGHT CEMP stars among them – some 1000 stars (some with B ~ 10)
Medium-resolution follow-up of disk and halo stars of the MW, with the LAMOST 4m telescope in China – some 6 Million stars
High-resolution follow-up with various 8m telescopes ongoing for both samples of stars

28. Medium-Res Spectra of RAVE Stars with [Fe/H] < -2.0

29. [C/Fe] vs. [Fe/H] (Medium-Res Results)