[Paper Review] Understanding the origin of CEMP-no stars through ultra-faint dwarfs

M. Rossi, Stefania Salvadori+2 more|arXiv (Cornell University)|Feb 20, 2023

Stellar, planetary, and galactic studies4 citations

TL;DR

This study develops a data-calibrated, multi-source chemical evolution model for Boötes I to trace the origins of CEMP-no stars, identifying distinct chemical signatures from Pop III and Pop II stars. It shows that A(C) > 6 CEMP-no stars are primarily enriched by Pop II AGB stars, while A(C) ≤ 6 stars are likely true Pop III descendants with constant [C/Mg], offering a new diagnostic to distinguish their origins via [C/Mg] ratios.

ABSTRACT

The origin of Carbon Enhanced Metal-Poor (CEMP-no) stars with low abundances of neutron-capture elements is still unclear. These stars are ubiquitous, found primarily in the Milky Way halo and ultra-faint dwarf galaxies (UFDs). To make a major step forward, we developed a data-calibrated model for Böotes I that simultaneously includes all carbon sources: supernovae and asymptotic giant branch (AGB) stars both from first (Pop III) stars, and subsequent normal star formation (Pop II). We demonstrate that each of these sources leave a specific chemical signature in the gas, allowing us to identify the origin of present day CEMP-no stars through their location in the A(C)-[Fe/H] diagram. The CEMP stars with A(C)>6 are predominantly enriched by AGB Pop II stars. We identify a new class of 'moderate CEMP-s' stars with A(C)~ 7 and 0<[Ba/Fe]<+1 , imprinted by winds from AGB stars. True Pop III descendants are predicted to have A(C)<6 and a constant [C/Mg] with [Fe/H], in perfect agreement with observations in Böotes I and the Milky Way halo. For the first time we now have a complete picture of the origins of CEMP-no stars which can and will be verified with future observations.

Motivation & Objective

• To resolve the unresolved origin of Carbon-Enhanced Metal-Poor (CEMP-no) stars, especially in ultra-faint dwarf galaxies (UFDs).
• To determine whether CEMP-no stars originate from Pop III or Pop II stellar populations, and which enrichment mechanisms (SNe or AGB winds) are responsible.
• To distinguish between CEMP-no stars enriched by first-generation (Pop III) stars versus second-generation (Pop II) stars using chemical diagnostics.
• To validate the model against observed CEMP-no stars in Boötes I and the Milky Way halo, enabling future observational tests.

Proposed method

• A semi-analytical, data-calibrated chemical evolution model simulates star formation and enrichment in Boötes I from virialization to present day.
• The model includes multiple carbon sources: Pop III SNe, Pop III AGB stars, Pop II SNe, and Pop II AGB stars, each with distinct nucleosynthetic yields.
• The model tracks enrichment history using initial mass functions and gas cooling, with star formation proportional to available gas mass.
• It predicts the A(C)-[Fe/H] and [C/Mg]-[Fe/H] distributions of CEMP-no stars based on dominant enrichment sources.
• The model is calibrated using observed stellar metallicities and abundances from Boötes I and the Milky Way halo.
• It uses [C/Mg] as a key diagnostic to differentiate between Pop III-only and Pop II-dominated enrichment.

Figure 1: The A(C)-[Fe/H] diagram of our simulated stellar populations (1000 realisations) of Boötes I. The black line corresponds to [C/Fe] = +0.7. Yellow star symbols identify the observed CEMP-no stars in Boötes I, while the green and blue ellipses correspond to the CEMP-no groups by (Yoon et al.

Experimental results

Research questions

• RQ1 What is the dominant enrichment source (Pop III or Pop II) for CEMP-no stars with A(C) > 6 in ultra-faint dwarfs?
• RQ2 How do the chemical signatures of CEMP-no stars differ when enriched by Pop III versus Pop II stars, particularly in [C/Mg] and [Fe/H] space?
• RQ3 Can the observed bimodality in A(C)-[Fe/H] and [C/Mg]-[Fe/H] diagrams be explained by distinct stellar populations and enrichment mechanisms?
• RQ4 What is the origin of CEMP-no stars with A(C) ≤ 6, and do they represent true descendants of Pop III stars?
• RQ5 Can [C/Mg] be used as a reliable diagnostic to distinguish between Pop III and Pop II enrichment in CEMP-no stars?

Key findings

• CEMP-no stars with A(C) > 6 are predominantly enriched by Pop II AGB stars, with a strong correlation between [C/Mg] and [Fe/H] that increases at lower metallicities.
• A new class of moderate CEMP-s stars is predicted with A(C) ≈ 7 and 0 < [Ba/Fe] < +1, imprinted by AGB winds without binary mass transfer.
• True Pop III descendants are predicted to have A(C) < 6 and a constant [C/Mg] with [Fe/H], matching observations in Boötes I and the Milky Way halo.
• Stars with [Fe/H] ≤ -4 enriched by Pop II AGB stars alone show >95% contribution, resulting in high [C/Mg] and low Mg abundance.
• The [C/Mg] ratio is a key diagnostic: constant [C/Mg] indicates Pop III-only enrichment, while increasing [C/Mg] with decreasing [Fe/H] indicates Pop II AGB dominance.
• The model predicts that low-Fe ( [Fe/H] < -4 ) Pop III-imprinted CEMP-no stars likely exist in UFDs, though they remain observationally hidden.

Figure 2: The A(C) versus [Fe/H] diagram for different CEMP-no stellar populations. Colors highlight CEMP-no stars enriched to a level higher than $75\%$ by different stellar populations: in the columns we distinguish between Pop III (left) and Pop II stars (right) and in each panel we differentiate