Goswami Shejeelammal Goswami Purandardas 1]Indian Institute of Astrophysics,
Koramangala, Bangalore 560034, India 2] Universidade de Sao Paulo, Instituto
de Astronomia, Rua do Matao 1226, Cidade Universitaria, 05508-900, SP, Brazil
<EMAIL_ADDRESS>
#  On the potential of Carbon-Enhanced Metal-Poor stars for Galactic
Archaeology111Invited talk

Aruna  J  Partha Pratim  Meenakshi  [  [

( May 3, 2023)

###### Abstract

The low-mass metal-poor stars in the Galaxy that preserve in their atmosphere,
the chemical imprints of the gas clouds from which they were formed can be
used as probes to get insight into the origin and evolution of elements in the
early galaxy, early star formation and nucleosynthesis. Among the metal-poor
stars, a large fraction, the so-called carbon-enhanced metal-poor (CEMP) stars
exhibits high abundance of carbon. These stars exhibit diverse abundance
patterns, particularly for heavy elements, based on which they are classified
into different groups. The diversity of abundance patterns points at different
formation scenarios. Hence, accurate classification of CEMP stars and
knowledge of their distribution is essential to understand the role and
contribution of each group. While CEMP-s and CEMP-r/s stars can be used to get
insight into binary interactions at very low metallicity, CEMP-no stars can be
used to probe the properties of the first stars and early nucleosynthesis. To
exploit the full potential of CEMP stars for Galactic archaeology a
homogeneous analysis of each class is extremely important. Our efforts
towards, and contributions to providing an improved classification scheme for
accurate classification of CEMP-s and CEMP-r/s stars and in characterizing the
companion asymptotic giant branch (AGB) stars of CH, CEMP-no, CEMP-s and
CEMP-r/s binary systems are discussed. Some recent results obtained based on
low- and high-resolution spectroscopic analysis of a large number of potential
CH and CEMP star candidates are highlighted.

###### keywords:

Metal-Poor stars, Carbon stars, Nucleosynthesis, Abundances

##  1 Introduction

A number of large sky survey programs in the past, such as, HK survey (Beers
et al., 1985, 1992, 2007; Beers, 1999), Hamburg/ESO Survey (HES; Christlieb et
al. 2001a, b; Christlieb 2003; Christlieb et al. 2008), Sloan Digital Sky
Survey (SDSS; York et al. 2000), LAMOST (Large Sky Area Multi-Object Fiber
Spectroscopic Telescope) survey (Cui et al., 2012; Deng et al., 2012; Zhao et
al., 2012) were dedicated for identifying the most metal-poor stars. These
surveys revealed that a significant fraction ($\sim$20% of Very Metal-Poor
(VMP), [Fe/H]$<$$-$2) stars in the Galaxy are CEMP stars (Rossi et al., 1999;
Christlieb, 2003; Lucatello et al., 2005, 2006; Carollo et al., 2012) and that
the fraction of these stars increases with decreasing metallicity $\sim$40%
for [Fe/H]$<$$-$3 and $\sim$75% for [Fe/H]$<$$-$4 (Yong et al., 2013b; Lee et
al., 2013; Aoki et al., 2013; Placco et al., 2014; Frebel and Norris, 2015).

Beers and Christlieb (2005) first introduced four sub-classes of CEMP stars
considering the abundances of carbon and two n-capture elements, Ba (a
representative s-process element) and Eu (a representative r-process element).
Subsequently, various authors (Jonsell et al., 2006; Masseron et al., 2010;
Norris et al., 2010; Bisterzo et al., 2011; Bonifacio et al., 2015; Maeder and
Meynet, 2015; Yoon et al., 2016; Abate et al., 2016; Hansen et al., 2016;
Frebel, 2018; Hansen et al., 2019; Skúladóttir et al., 2020; Goswami et al.,
2021) put forward different criteria for the identification and classification
of the CEMP stars. The four primary sub-classes that are based on the level of
relative enrichment of neutron-capture elements are: CEMP-s (shows enhanced
abundances of s-process elements), CEMP-r (shows strong enhancement of
r-process elements), CEMP-r/s (shows enhancement of both s- and r-process
elements) and CEMP-no (does not show any enhanced abundance of neutron-capture
elements). The diverse abundance patterns point at different formation
scenarios for the respective sub-classes. The fractions and properties of CEMP
sub-classes provide a unique opportunity to probe the formation and evolution
of the Galactic halo and its building blocks. For this reason, the CEMP stars
alongside the halo tracers CH stars have been extensively studied in the last
few decades.

CH stars ($-$2$\leq$[Fe/H]$\leq$$-$0.2) are characterized by strong CH and C2
molecular bands, C/O$>$1 and strong features due to the neutron-capture
elements (Keenan, 1942). These are mostly high velocity objects (Vr > 100 km
s-1), and members of the Galactic halo. A few CH stars are also found in
Globular clusters. Sub-giant CH stars have moderate velocities and they
exhibit old disk behaviour rather than the halo. These objects are believed to
be the progenitors of metal-deficient Ba stars. Long term radial velocity
study by McClure and Woodsworth (1990) showed that most of the CH stars are
binaries with companions that are now presumably white dwarfs. The heavy
s-process elements in giant CH stars are found to be more enhanced than the
light s-process elements (Karinkuzhi and Goswami, 2014, 2015), in agreement
with theoretical modelling of s-process in stars and its dependence on the
metallicity (Busso et al., 1999, 2001; Cristallo et al., 2016).

The CEMP-s stars are the metal-poor analogue of Ba and CH stars (Lucatello et
al., 2005; Abate et al., 2016). A number of long-term radial velocity
monitoring studies have shown that most of the CEMP-s stars are binaries
(Lucatello et al., 2005; Starkenburg et al., 2014; Jorissen et al., 2016;
Hansen et al., 2016), supporting the pollution from companion AGB stars.
Comparisons of observed abundances in the CEMP-s stars with theoretical model
predictions confirm the binary mass-transfer from the AGB companion (Bisterzo
et al., 2011; Placco et al., 2013; Hollek et al., 2015; Shejeelammal et al.,
2020; Shejeelammal and Goswami, 2021, 2022; Goswami et al., 2021; Goswami and
Goswami, 2022).

The CEMP-r/s stars show abundance enhancement in both slow (s-) and rapid
(r)-neutron capture process elements. Since most of the CEMP-r/s stars are
also found to be binaries just like CEMP-s stars, binary mass transfer from
the AGB companion is thought to be the reason for their origin as well,
however the presence of r-process component posed a challenge (Jonsell et al.,
2006; Herwig et al., 2011; Abate et al., 2016). It was proposed by Cowan and
Rose (1977) that the intermediate neutron-capture process the so-called
i-process, with neutron density between s- and r- process can produce both s-
and r- process elements at a single stellar site. Many studies have
successfully used model yields of i-process in low-mass low-metallicity AGB
stars to account for the observed abundance patterns in CEMP-r/s stars (Hampel
et al., 2016, 2019; Goswami et al., 2021; Shejeelammal et al., 2020;
Shejeelammal and Goswami, 2021, 2022). Although there are several suggestions
for the i-process sites, such as, super-massive AGB stars (Doherty et al.,
2015; Jones et al., 2016), low-mass low-metallicity stars (Campbell and
Lattanzio, 2008; Campbell et al., 2010; Cruz et al., 2013; Cristallo et al.,
2016), and rapidly accreting white dwarfs (Herwig et al., 2014; Denissenkov et
al., 2017) etc., the exact astrophysical site for the i-process is not yet
confirmed (Frebel, 2018; Koch et al., 2019).

CEMP-r stars are extremely rare. In spite of several studies on the formation
of CEMP-r stars, the origin of these stars are not yet clearly understood;
observations of more such stars are required to constrain their exact origin.

The CEMP-no stars with enhanced carbon abundance and no signatures of heavy
neutron-capture elements, are believed to be the most chemically primitive
objects presently known (Norris and Yong, 2019; Yoon et al., 2020). These
stars occupy the lowest metallicity tail ([Fe/H]$<$$-$3) of the Metallicity
Distribution Function (MDF) of the Galaxy (Yong et al., 2013a; de Bennassuti
et al., 2017; Yoon et al., 2019; Norris and Yong, 2019). Among the fourteen
known CEMP stars with [Fe/H]$<$$-$4.5, twelve stars belong to the CEMP-no
group. Being the oldest stars that can be directly observed at present,
valuable information can be derived from them, such as the formation and
nucleosynthesis pathways of the first stars, initial conditions for star and
galaxy formation (Klessen et al., 2012; Frebel and Norris, 2015; Bonifacio et
al., 2015; Hansen et al., 2016; Yoon et al., 2020, 2018), large galaxy
assembly (e.g. Carollo et al. (2007); Beers et al. (2012)), and early
nucleosynthesis (Hansen et al. (2011); Gil-Pons et al. (2021); Qian (2022)).
Further, these stars are expected to shed light on the physical processes
resulting in the transitions from massive Population III (Pop III) stars to
normal Population II (Pop II) stars (Salvadori et al., 2007; Hartwig et al.,
2015; de Bennassuti et al., 2017).

Different scenarios have been suggested by various authors for the origin of
CEMP-no stars (Hansen et al. 2015, Yoon et al. 2016 and references there in),
however, the exact origin of these stars still remains unknown. According to
some authors, the CEMP-no stars are generally single stars, suggesting an
intrinsic origin, i.e, formed from a pre-enriched natal cloud of ISM (Spite et
al., 2013; Starkenburg et al., 2014; Bonifacio et al., 2015; Hansen et al.,
2016; Yoon et al., 2016). Suggested progenitors for the CEMP-no stars also
include: (i) faint supernovae that underwent mixing and fall back. (Umeda and
Nomoto, 2003, 2005; Tominaga et al., 2014), (ii) extremely metal-poor fast
rotating massive stars or spinstars that enriched the ISM with C, N, and O
through strong stellar winds (Meynet et al., 2006; Frischknecht et al., 2012;
Maeder et al., 2015; Choplin et al., 2017), and (iii) i-process in massive Pop
III stars (Clarkson et al., 2018). A recent study by Arentsen et al. (2019)
have shown that the binary mass-transfer from an extremely metal-poor AGB
companion could also be a possible progenitor of the CEMP-no stars. If the
binary-mass-transfer scenario holds true for the origin of CEMP-no stars, this
will help to understand extremely metal-poor AGB stars and provide robust
observational constraints to the AGB nucleosynthesis and mass-transfer models
for metallicity [Fe/H]$<$$-$2.

Since most of the metal-poor stars in the halo trace their origin to the dwarf
satellite galaxies (Frebel and Norris, 2015; Frebel and Ji, 2023), they are
ideal tools to study the Galactic halo assembly. Based on the kinematic,
chemical, and dynamical properties of a sample of 644 CEMP stars, Zepeda et
al. (2023) have shown that the CEMP-no stars are not in-situ to the Galaxy,
but were originally born in satellite galaxies that were accreted and
disrupted by the Milky Way. This is also in support of the findings of Yoon et
al. (2019). Several authors have discussed the relevance and scope of
enhancing medium- and high-resolution spectroscopic studies of different sub-
groups of CEMP stars to further our understanding of early Galactic chemical
evolution and Galactic halo formation (eg. Hansen et al. 2015; Norris and Yong
2019; Yoon et al. 2019, 2020; Zepeda et al. 2023). Along this line, we had
conducted high-resolution spectroscopic studies of several potential CH and
CEMP star candidates identified from low-resolution spectroscopic studies with
the aim of deriving clues to their origin.

In section 2, we have discussed the limitations in classifying CEMP stars
based on low-resolution spectra. Shortcomings of current classification
criteria for CEMP-s and CEMP-r/s stars is discussed in section 3. A revised
classification scheme for identifying the CEMP-s and CEMP-r/s stars is also
presented in this section. In section 4, we have discussed about charaterizing
the companion AGBs of the CH, CEMP-s and CEMP-r/s stars. A discussion on
deriving clues for the origin of CEMP-r/s stars from abundance patterns is
presented in section 5. Observational evidence for multiple origins for CEMP-
no stars is discussed in section 6. The unique star HE 1005-1439 that shows
evidence of occurrence of dual nuclesynthesis is discussed in section 7. A few
critical issues in analysis of CEMP stars are discussed in section 8. The
conclusions are presented in section 9.

##  2 Limitations in classifying CEMP stars using low-resolution spectra

Due to the intrinsic weakness of the absorption features, using low-resolution
samples, it is difficult to distinguish between different sub-classes of CEMP
stars. This is illustrated in figure 1, where a few examples of low-resolution
(R $\sim$ 1300) spectra of CEMP stars obtained with HCT/HFOSC are shown.
Detail analysis of these spectra can be found in Goswami (2005); Goswami et
al. (2010). Visual inspection shows that the spectra look alike, and hence,
difficult to identify if they belong to different sub-classes. The spectra are
characterized by strong features of carbon molecules, and lines due to Ba and
Sr are only marginally detectable. However, high resolution spectroscopic
analysis have shown that HE 0319-0215 and HE 1305+0132 are CEMP-s star and HE
0017+0058 is a CEMP-r/s star (Goswami et al., 2021). Although the origin of
both these classes can be explained based on a binary picture with a now
invisible white dwarf companion, the progenitors AGBs have very different
nucleosynthetic and evolution history.

Figure 1:  Low resolution (R $\sim$ 1300) spectra of a few CEMP stars. The
spectra characterized by strong carbon molecular features look alike and
difficult to classify them into different CEMP sub-classes based on the low-
resolution spectra. Follow-up high resolution spectroscopic studies have
confirmed the objects HE 0319$-$0215 and HE 1305+0132 as CEMP-s stars and HE
0017+0058 as a CEMP-r/s star.

##  3 Classification of CEMP stars: shortcomings of the current classification
criteria and a revised classification scheme for identifying CEMP-s and
CEMP-r/s stars

Among the CEMP stars, distinguishing the CEMP-s and CEMP-r/s stars is crucial
in order to understand the various astrophysical and nucleosynthetic processes
responsible for the abundance patterns of the two sub-classes. The object HD
145777 in our sample, particularly posed a challenge in this regard. This
object could be classified as a CEMP-r/s star based on the classification
schemes of Beers and Christlieb (2005), Frebel (2018), and Hansen et al.
(2019), and as a CEMP-s star if the classification criteria of Abate et al.
(2016) are adopted. Based on a parametric model based analysis we have found
that in order to fit the observed abundances, a model with a neutron density
of 1010 cm-3 is required (Goswami et al., 2021) which is towards the higher
limit of the neutron density required for s-process nucleosynthesis. To remove
the uncertainties and confusion regarding its classification, we have
revisited the classification schemes of CEMP-s and CEMP-r/s stars found in the
literature. From a detailed investigation we have seen that none of the
existing classification criteria could clearly distinguish the CEMP-s and
CEMP-r/s stars. We have examined if abundance ratios such as [hs/ls] (where hs
implies heavy s-process elements and ls the light s-process elements),
[Sr/Ba], [Ba/Eu], [La/Eu], [La/Ce], [As/Ge] and [Se/Ge] could be used as
classifiers to distinguish the CEMP-s and CEMP-r/s stars effectively. We note
that the [hs/ls] peaks at different values giving higher values for CEMP-r/s
stars than CEMP-s stars. However, the CEMP-s and the CEMP-r/s stars are also
found to show an overlap in the range 0.0 $<$ [hs/ls] $<$1.5. Hence, this
ratio alone cannot be used as a definitive classifier of CEMP-s and CEMP-r/s
stars. Based on our investigation, we suggest that an object satisfying the
criteria 0.0 $<$ [Ba/Eu] $<$ 1.0 and 0.0 $<$ [La/Eu] $<$ 0.5 can be classified
as a CEMP-r/s star; however, if [La/Eu] = 0.6 $\pm$ 0.1 the condition [Eu/Fe]
$>$ 1.0 also needs to be satisfied for the star to be a CEMP-r/s star. We
proposed, the best criteria to distinguish the CEMP-s and CEMP-r/s stars as:

  * •

CEMP: [C/Fe] $\geq$ 0.7

  * •

CEMP-r/s: [Ba/Fe] $\geq$ 1.0, [Eu/Fe] $\geq$ 1.0

    * i)

0.0 $\leq$ [Ba/Eu] $\leq$ 1.0 and/or 0.0 $\leq$ [La/Eu] $\leq$ 0.7;

  * •

CEMP-s: [Ba/Fe] $\geq$ 1.0

    * i.)

[Eu/Fe] $<$ 1.0, [Ba/Eu] $>$ 0.0 and/or [La/Eu] $>$ 0.5;

    * ii.)

[Eu/Fe] $\geq$ 1.0, [Ba/Eu] $>$ 1.0 and/or [La/Eu] $>$ 0.7.

This scheme of classification based on the abundance ratios of three key
neutron-capture elements barium, lanthanum and europium have proven to be
highly effective in distinguishing the CEMP-s and CEMP-r/s stars. A critical
investigation on the CEMP stars formation scenarios and a thorough discussion
on the CEMP stars classification criteria along with the new classification
scheme put forward by us are presented and discussed at length in Goswami et
al. (2021).

##  4 Characterizing the companion AGB stars of CH, CEMP-s and CEMP-r/s stars

We have investigated the mass of companion AGB stars using several diagnostics
such as C, N, O, Na, Mg abundances, [hs/ls] ratio, and [Rb/Zr] ratio. The
carbon isotopic ratio 12C/13C an important mixing indicator is also measured
whenever possible. Na and Mg enrichment is expected to observe if the
s-process over abundance is resulting from the neutrons produced during the
convective thermal pulses through the reaction 22Ne($\alpha$,n)25Mg. Several
abundance ratios of certain key elements are also used as diagnostics to
understand the characteristic properties of the companion AGB stars as
discussed in Shejeelammal et al. (2021); Shejeelammal and Goswami (2021,
2022); Purandardas and Goswami (2021b); Goswami et al. (2021).

###  4.1 The [hs/ls] ratio as an indicator of neutron source

The [hs/ls] ratio is a useful indicator of neutron source in the former
companion AGB star. At higher neutron exposures, more second peak (hs)
s-process elements are produced over the first peak (ls), resulting in high
[hs/ls] ratio. As the neutron exposure increases with decreasing metallicity,
[hs/ls] ratio increases with decreasing metallicity, showing anti-correlation
with metallicity. The anti-correlation of [hs/ls] suggest the operation of
13C($\alpha$, n)16O neutron source, since 13C($\alpha$, n)16O is found to be
anti-correlated with metallicity (Clayton, 1988; Wallerstein et al., 1997).
Hence positive values for [hs/ls] ratio could be seen in low-mass, low
metallicity AGB stars (Goriely and Mowlavi, 2000; Busso et al., 2001);
whereas, in the case of massive AGB stars (5 - 8M⊙), where the neutron source
is 22Ne($\alpha$, n)25Mg, negative values for this ratio is observed (Goriely
and Siess, 2005; Karakas, 2010; Karakas et al., 2012; van Raai et al., 2012;
Karakas and Lattanzio, 2014). The 22Ne($\alpha$, n)25Mg source has smaller
neutron exposure compared to the 13C($\alpha$, n)16O source. Hence, in the
stars where 22Ne($\alpha$, n)25Mg operates, we expect a lower [hs/ls] ratio.
The lower neutron exposure of the neutrons produced from the 22Ne source
together with the predictions of low [hs/ls] ratio in massive AGB star models
have been taken as the evidence of operation of 22Ne($\alpha$, n)25Mg in
massive AGB stars. A Mg enrichment is expected in the stars where this
reaction takes place. None of the CEMP stars studied by us in Shejeelammal et
al. (2021); Shejeelammal and Goswami (2021, 2022) shows such an enrichment,
ruling out the possibility of 22Ne($\alpha$, n)25Mg reaction as a possible
neutron source with respect to [hs/ls] ratio. All the CEMP-r/s stars and
CEMP-s stars (except HE 0920$-$0506 and HE 1354$-$2257), studied in the above
mentioned studies show positive values of [hs/ls] indicating low-mass of the
companion AGB stars.

###  4.2 Rb as a probe of neutron density at the s-process site: the [Rb/Zr]
ratio

In addition to the [hs/ls] ratio, the abundance of rubidium can also provide
clues to the mass of the companion AGB stars. The AGB star models predict
higher Rb abundances for massive AGB stars where the neutron source is
22Ne($\alpha$,n)25Mg reaction (Abia et al., 2001; van Raai et al., 2012).
Detailed nucleosynthesis models for the stars with masses between 5 - 9 M⊙ at
solar metallicity predict [Rb/(Sr,Zr)]$>$0 (Karakas et al., 2012). A positive
value of [Rb/Sr] or [Rb/Zr] ratio indicates a higher neutron density, whereas
a negative value indicates a low neutron density with 13C($\alpha$, n)16O
reaction acting as the neutron source. A comprehensive discussion on Rb as a
probe of the mass of the companion AGB star can be found in Shejeelammal et
al. (2021); Shejeelammal and Goswami (2021). In these studies the authours
have found negative values for [Rb/Zr] and [Rb/Sr] ratios for stars for which
these ratios could be estimated. The observed [Rb/Fe] and [Zr/Fe] ratios and
the abundances of Rb and Zr are found to be consistent with the range normally
observed in the low-mass AGB stars, that indicate low-mass nature of the
companion AGB stars.

###  4.3 Determination of the mass of the companion AGB stars from a
parametric model based analysis using FRUITY

Detail procedure of the parametric model based analysis to derive the mass of
the companion AGB stars have been discussed at length in several of our papers
(Shejeelammal and Goswami, 2021, 2022; Goswami et al., 2021; Purandardas and
Goswami, 2021b; Goswami and Goswami, 2023). The mass of the AGB stars
responsible for the observed abundances of the stars reported in these papers
are derived by comparing the observed abundance with the predicted abundance
from FRUITY models (Cristallo et al., 2009, 2011, 2015) for the heavy elements
(Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm and Eu). The best fitting mass of the
companion AGB is obtained by fitting the observed abundance with the
parametric model function of Husti et al. (2009) by minimizing the $\chi^{2}$
value. All the stars in these studies are found to have low-mass AGB
companions with M $\leq$ 3 M⊙.

##  5 Origin of CEMP-r/s stars: clues from the abundance pattern and
parametric model based analysis

A comprehensive discussion on the parametric model based analysis and the
results obtained for the stellar sample studied by us can be found in (Goswami
et al., 2021; Shejeelammal and Goswami, 2021, 2022). The overlap of [hs/ls]
ratios between the CEMP-s and CEMP-r/s stars (Goswami et al., 2021) indicates
that the origin of both the groups owe a common astrophysical site and process
but likely under different conditions. The higher [hs/ls] ratio of the
CEMP-r/s stars compared to the CEMP-s stars indicate a higher neutron exposure
for their formation than the classical s-process. The tight correlation
between the observed [Eu/Fe] and [hs/Fe] ratios in CEMP-r/s stars (Goswami et
al., 2021) points towards a single stellar site where both the s- and r-
process elements could be produced simultaneously. A higher neutron densities
than that required for the classical s-process, of the order of 1015 \- 1017
cm-3, intermediate between the s- and r-process neutron densities, could be
achieved when a substantial amount of hydrogen rich-material is mixed into the
intershell region of the evolved red-giant stars (Proton Ingestion Episode,
PIE) undergoing helium shell flash (Cowan and Rose, 1977). There are several
sites proposed to host the PIEs in order for the i-process nuclesynthesis to
take place, however the astrophysical site for occurrence of i-process still
remains a topic of debate.

We have used the i-process model yields [X/Fe], from Hampel et al. (2016) and
compared with the observed [X/Fe] of the CEMP-r/s stars in our sample for the
neutron densities ranging from 109 to 1015 n cm-3. We have considered only the
neutron-capture elements for the comparison. The neutron-density responsible
for the observed abundances in the star is derived by fitting the observed
abundance with the dilution factor incorporated parametric model function:

X = Xi . (1-d) + X⊙ . d

where X represents the final abundance, Xi the i-process abundance, d is the
dilution factor and X⊙ is the solar-scaled abundance. Here d is a free
parameter that can be varied to find the best fit between the model and the
observational data for each constant neutron density. The best fit model is
found using $\chi^{2}$min method. Some examples of the best fits obtained for
the CEMP-r/s stars are shown in Figure 2.

Figure 2: Parametric model fits for CEMP-r/s stars. Solid curves represent the
best fit for the parametric model function. The points with error bars
indicate the observed abundances.

##  6 Observational evidence for multiple origins for CEMP-no stars

The differences in the morphology of CEMP-no stars on the Yoon - Beers diagram
(A(C) - [Fe/H] diagram), leading to two distinct groups, Group II and Group
III CEMP-no stars suggest multiple formation pathways for CEMP-no stars (Yoon
et al., 2016). The two CEMP-no stars HE 2148$-$2039 and HE 2155$-$2043 studied
by us (Purandardas and Goswami, 2021a) are also found to provide observational
evidence for multiple origins for CEMP-no stars. These two objects are found
to have similar metallicity ([Fe/H] of $-$ 3.32 and $-$3.46 respectively) and
enhanced carbon with [C/Fe] values 1.27 and 2.05 respectively. While Mg is
found to be under abundant in HE 2148$-$2039 with [Mg/Fe] = $-$0.35, it is
highly enhanced in HE 2155$-$2043 with [Mg/Fe] = 1.57, indicating different
origins as far as the ${\alpha}$-elements are concerned. As far as the heavy
elements are concerned, while HE 2148$-$2039 shows a very strong feature of Ba
II 4934.06 Å and a weak feature of Sr II 4215.52 Å , in contrast, the object
HE 2155$-$2043 shows a strong feature of Sr II 4215.52 Å and a weak feature of
Ba II 4934.06 Å (Figure 3). The estimated [Sr/Fe] and [Ba/Fe] are ($-$2.02,
$-$0.84) for HE 2148$-$2039 and ($-$0.04, $-$1.64) for HE 2155$-$2043\. These
differences clearly indicate different origins for these two objects. The
locations of these two stars in the absolute carbon abundance, A(C) versus
[Fe/H] diagram, show that HE 2148$-$2039 is a CEMP-no Group II object and HE
2155$-$2043 is a CEMP-no Group III object (Purandardas and Goswami, 2021a).

Figure 3:  While Ba II 4934.08 Å is marginally detectable in HE 2155$-$2043,
this feature appears strong in HE 2048$-$2039\. On the other hand, Sr II
4215.52 Å feature is strong in HE 2155$-$2043 and weak in HE 2148$-$2039\.
Both the objects are of similar metallicity [Fe/H] $<$ $-$3.3 and [C/Fe] $>$
1.0.

##  7 HE 1005-1439: A unique star with dual nucleosynthesis signatures

This object with a unique abundance pattern that shows dual nucleosynthesis
signatures was first reported in Goswami and Goswami (2022). The star was
found to be extremely metal-poor with [Fe/H] = $-$ 3.03 and heavily enriched
with neutron-capture elements. The observed elemental abundances of the object
could not be explained based on theoretical s-, r-, or i-process model
predictions alone. We have performed a parametric-model-based analysis of the
abundances of the heavy elements which indicated that the star’s surface
chemical composition is being influenced by similar contributions from both
the s- and i-processes. This forms a new class of object of distinct abundance
pattern with dual nucleosynthesis signatures.

The estimated radial velocities of the object from several epochs showed
variations indicating the presence of a binary companion. Based on this
observation, we proposed a formation scenario for HE 1005-1439 involving mass
transfer from a now extinct AGB companion where both i- and s-process
nucleosynthesis took place during various stages of the AGB evolution with
proton ingestion episodes (PIEs) triggering i-process followed by s-process
AGB nucleosynthesis with a few third-dredge-up episodes. A detailed study on
this object and the significance of the results obtained are presented in
Goswami and Goswami (2022).

##  8 A few critical issues in analysis of CEMP stars

To understand the role of CEMP stars on Galactic chemical evolution, accurate
knowledge of the CEMP fractions among the metal-poor stars of the Galactic
halo is vital. The fraction is found to have different values based on the
adopted definition of CEMP stars, i.e., either [C/Fe] $>$ 1.0 (Beers &
Chrislieb 2005), or [C/Fe] ${\geq}$ 0.7 Aoki et al. (2007). A compilation of
published CEMP fractions and samples of Galactic halo stars from the past 25
years, revealed that they are not all consistent with each other (Arentsen et
al. 2022). Significant differences were noticed between various surveys when
comparing their trends of [Fe/H] versus [C/Fe] and their distributions of CEMP
stars. These discrepancies were primarily attributed to the differences in the
analysis procedures and criteria adopted by different groups.

The application of the empirical correction to the [C/Fe] values based on the
evolutionary stage of a star and its metallicity is essential for a
homogeneous analysis and interpretation. Hence, in order to have accurate
carbon estimates the necessary evolutionary correction to the observed carbon
abundance values needs to be applied.

Most of the known CEMP stars are giants or sub-giants. The stars on the red-
giant branch (RGB) undergo internal mixing and hence alters the surface CN
abundances (eg. Aoki et al. 2007; Gratton et al. 2000; Placco et al. 2014).
Placco et al. (2014) derived [C/Fe] corrections based on evolutionary stage of
a star and its metallicity. The application of the empirical correction to the
[C/Fe] values from Aguado et al. (2019) resulted in reducing the total number
of CEMP stars from 64 to 12 (Arentsen et al., 2022). Based on the analysis of
a sample of 505 metal-poor ([Fe/H] ${\leq}$$-$2) stars with no enhancement of
neutron-capture elements from the literature, Placco et al. (2014) have shown
that the frequency of CEMP-no stars ([C/Fe] $\geq$ 0.7) in the Galaxy is
increased by $\sim$ 11% for [Fe/H] $\leq$ –3 when proper correction required
for the carbon depletion is applied to the observed [C/Fe]. A comparison of
the [C/Fe] ratios obtained using corrected and non-corrected carbon abundance
as a function of log g is shown in figure 4 for CEMP stars from the literature
(Placco et al. 2014 and references therein) and our program stars.

Since CEMP-s and CEMP-no stars have very different progenitors, their relative
fraction provide insight into the different physical processes at different
metallicity. Hence, identification of genuine CEMP-no stars and CEMP-s and
accurate knowledge of CEMP-s and CEMP-no fractions among CEMP stars is
crucial.

Figure 4: Panel (a): Absolute carbon abundance A(C) with respect to
metallicity [Fe/H] are plotted for CEMP stars. Black circles represent Group I
CEMP-s and CEMP-r/s stars, blue crosses Group II CEMP-no stars, and the green
stars represent the Group III CEMP-no stars from the literature (Yoon et al.
2016 and references therein). Red triangles represent CEMP stars from
Shejeelammal et al. (2021), Shejeelammal & Goswami (2021), Shejeelammal &
Goswami (2022), and Shejeelammal & Goswami (2023, in preparation). Red
rectangles represent CEMP stars from Purandardas et al. (2019), Purandardas &
Goswami (2021a), and Purandardas & Goswami (2021b). The red circles represent
CEMP stars from Goswami et al. (2021), Goswami & Goswami (2022), and Goswami &
Goswami (2023). The black dashed lines indicate - high and low carbon bands at
A(C)$\sim$7.96 & 6.28. The solid line corresponds to [C/Fe] = 0.70 and the
long-dash dot line corresponds to [C/Fe] = 0. Panel (b): [C/Fe] ratio as a
function of log g for CEMP stars from the literature (Placco et al. 2014 and
references therein). The observed [C/Fe] for CEMP-no stars are represented by
blue filled circles, the CEMP-s and CEMP-r/s stars by green filled circles,
and the corrected [C/Fe] by black open circles. Red trinagles indicate CEMP
stars from Shejeelammal et al. (2021), Shejeelammal & Goswami (2021),
Shejeelammal & Goswami (2022) and Shejeelammal & Goswami (2023, in
preparation). Red rectangles represent CEMP stars from Purandardas et al.
(2019), Purandardas & Goswami (2021a) and Purandardas & Goswami (2021b). Red
circles represent CEMP stars from Goswami et al. (2021), Goswami & Goswami
(2022) and Goswami & Goswami (2023). The filled symbols are used for observed
values and the open symbols are used for the corrected values. The dashed
horizontal line corresponds to [C/Fe] = 0.7, and the vertical dot-dashed line
corresponds to log g = 1.8.

The G-band of CH is commonly used as carbon abundance diagnostics in CEMP
stars. Most of these estimates are based on LTE assumption. In a recent study,
Popa et al. (2023) have explored the sensitivity of this feature to NLTE
effects considering the case of the Sun and red-giant stars. They found that
in the statistical equilibrium of the CH molecule this effect is significant,
increases with decreasing metallicity, and cannot be neglected for precision
spectroscopic analysis of C abundances. The C abundance is under-estimated if
LTE is assumed. The non-LTE corrections to C abundance inferred from the CH
band range from +0.04 dex to 0.2 dex for a red giant with metallicity [Fe/H] =
$-$4.0.

Sample selection is also crucial while determining CEMP fractions. Only stars
of similar evolutionary phases should be used to compare CEMP samples to
ensure similar evolutionary paths, similar systematic errors and to minimize
biases.

##  9 Concluding Remarks

Some highlights of the results obtained from low- and high-resolution
spectroscopic analysis of CEMP stars are presented. A new classification
scheme put forward by Goswami et al. (2021) clearly distinguishes CEMP-s and
CEMP-r/s stars. Detailed abundance analysis of CH, CEMP-s and CEMP-r/s stars
re-confirms pollution from low-mass AGB companion. Parametric model based
analysis performed for CH, CEMP-s and CEMP-r/s stars also confirm low-mass for
their former companion AGB stars. Our analysis confirms that a modified
s-process the so-called i-process, is responsible for the observed abundance
pattern in CEMP-r/s stars. The i-process models could successfully reproduce
the observed abundance in CEMP-r/s stars.

Peculiar abundances of HE 1005-1439 present first ever observational evidence
of the occurrence of dual nucleosynthesis in the companion AGB star, the
i-process followed by s-process in various stages of the AGB evolution, with
PIEs triggering i-process followed by s-process during third-dredge-up
episodes.

Surface chemical composition and spectral signatures of two extremely metal-
poor CEMP-no stars HE 2148$-$2039 and HE 2155$-$2043 reaffirm multiple origins
for CEMP-no stars.

Knowing the fraction of CEMP stars relative to carbon-normal stars is
important for interpretation of populations of metal-poor stars. However,
classification and hence the fraction of each class is not possible to
estimate accurately based on low-resolution spectroscopy.

The fraction of CEMP stars as a function of metallicity, how it changes with
the different classes of CEMP stars and different stellar evolutionary phases
are some of the outstanding questions that are yet to find clear answers. More
homogeneous analysis of larger samples are needed to unravel the full
potential of CEMP stars for Galactic Archaeology.

###### Acknowledgements.

AG acknowledges the support received from the Belgo-Indian Network for
Astronomy & Astrophysics project BINA - 2 (DST/INT/Belg/P-02 (India) and
BL/11/IN07 (Belgium)).

ORCID Identification of the authors -

0000-0002-8841-8641 (Aruna Goswami)

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