Carbon Star Formation Research Articles

C ii] 158 μm emission as an indicator of galaxy star formation rate

ABSTRACT Observations of local star-forming galaxies (SFGs) show a tight correlation between their singly ionized carbon line luminosity ($L_{\rm [C\, {\small II}]}$) and star formation rate (SFR), suggesting that $L_{\rm [C\, {\small II}]}$ may be a useful SFR tracer for galaxies. Some other galaxy populations, however, are found to have lower $L_{\rm [C\, {\small II}]}{}/{}\rm SFR$ than local SFGs, including the infrared (IR)-luminous, starburst galaxies at low and high redshifts as well as some moderately SFGs at the epoch of re-ionization (EoR). The origins of this ‘$\rm [C\, {\small II}]$ deficit’ is unclear. In this work, we study the $L_{\rm [C\, {\small II}]}$–SFR relation of galaxies using a sample of z = 0–8 galaxies with $M_*\approx 10^7-5\times 10^{11}\, \mathrm{M}_\odot$ extracted from cosmological volume and zoom-in simulations from the Feedback in Realistic Environments (fire) project. We find a simple analytic expression for $L_{\rm [C\, {\small II}]}$/SFR of galaxies in terms of the following parameters: mass fraction of $\rm [C\, {\small II}]$-emitting gas ($f_{\rm [C\, {\small II}]}$), gas metallicity (Zgas), gas density (ngas), and gas depletion time ($t_{\rm dep}{}={}M_{\rm gas}{}/{}\rm SFR$). We find two distinct physical regimes: $\rm H_2$-rich galaxies, where tdep is the main driver of the $\rm [C\, {\small II}]$ deficit and $\rm H_2$-poor galaxies where Zgas is the main driver. The observed $\rm [C\, {\small II}]$ deficit of IR-luminous galaxies and early EoR galaxies, corresponding to the two different regimes, is due to short gas depletion time and low gas metallicity, respectively. Our result indicates that the $\rm [C\, {\small II}]$ deficit is a common phenomenon of galaxies, and caution needs to be taken when applying a constant $L_{\rm [C\, {\small II}]}$-to-SFR conversion factor derived from local SFGs to estimate cosmic SFR density at high redshifts and interpret data from upcoming $\rm [C\, {\small II}]$ line intensity mapping experiments.

Hubble Space Telescope imaging of the compact elliptical galaxy M32 reveals a dearth of carbon stars

ABSTRACT We present new Hubble Space Telescope Wide-Field Camera 3/infrared medium-band photometry of the compact elliptical galaxy M32, chemically resolving its thermally pulsating asymptotic giant branch stars. We find 2829 M-type stars and 57 C stars. The carbon stars are likely contaminants from M31. If carbon stars are present in M32 they are so in very low numbers. The uncorrected C/M ratio is 0.020 ± 0.003; this drops to less than 0.007 after taking into account contamination from M31. As the mean metallicity of M32 is just below solar, this low ratio of C to M stars is unlikely due to a metallicity ceiling for the formation of carbon stars. Instead, the age of the asymptotic giant branch (AGB) population is likely to be the primary factor. The ratio of AGB to red giant branch stars in M32 is similar to that of the inner disc of M31 which contain stars that formed 1.5–4 Gyr ago. If the M32 population is at the older end of this age then its lack of C-stars may be consistent with a narrow mass range for carbon star formation predicted by some stellar evolution models. Applying our chemical classifications to the dusty variable stars identified with Spitzer, we find that the x-AGB candidates identified with Spitzer are predominately M-type stars. This substantially increases the lower limit to the cumulative dust-production rate in M32 to > 1.20 × 10−5${\rm M}_{\odot } \, {\rm yr}^{-1}$.

Understanding the evolution and dust formation of carbon stars in the Large Magellanic Cloud via the JWST

Context. Carbon stars have been, and still are, extensively studied. Given their complex internal structure and their peculiar chemical composition, they are living laboratories in which we can test stellar structure and evolution theories of evolved stars. Furthermore, they are the most relevant dust manufacturers, thus playing a crucial role in the evolution of galaxies. Aims. We aim to study the dust mineralogy of the circumstellar envelope of carbon stars in the Large Magellanic Cloud (LMC) to achieve a better understanding of the dust formation process in the outflow of these objects. We intend to investigate the expected distribution of carbon stars in the observational planes built with the filters of the Mid-Infrared Instrument (MIRI) mounted onboard the James Webb Space Telescope (JWST) to select the best planes allowing an exhaustive characterisation of the stars. Methods. We compared the synthetic spectral energy distributions, obtained by modelling asymptotic giant branch stars and the dust formation process in the wind, with the spectra of carbon stars in the LMC, taken with the Infrared Spectrograph onboard the Spitzer Space Telescope. From the detailed comparison between synthetic modelling and observation we characterise the individual sources and derive the detailed mineralogy of the dust in the circumstellar envelope. Results. The sample of stars considered here is composed of stars of diverse mass, formation epoch, degree of obscuration, and metallicity. We find that precipitation of MgS on SiC seeds is common to all non-metal-poor carbon stars. Solid carbon is the dominant dust component, with percentages above 80% in all cases; a percentage between 10% and 20% of carbon dust is under the form of graphite, the remaining being amorphous carbon. Regarding the observational planes based on the MIRI filters, the colour-magnitude ([F770W]–[F1800W], [F1800W]) plane allows the best understanding of the degree of obscuration of the stars, while the ([F1800W]–[F2550W], [F1800W]) diagram allows better discrimination among stars of different metallicities.

Shaping the initial-final mass relation of white dwarfs with AGB outflows

AbstractA recent analysis of a few carbon-oxygen white dwarfs in old open clusters of the Milky Way (MW) identified a kink in the initial-final mass relation (IFMR), located over a range of initial masses, 1.65 ≲ Mi/M⊙ ≲ 2.10, which unexpectedly interrupts the commonly assumed monotonic trend. The proposed interpretation links this observational fact to the formation of carbon stars and the modest outflows (with mass loss rate < 10−7M⊙/yr) that are expected as long as the carbon excess remains too low to produce dust grains in sufficient amount. Under these conditions the mass of the carbon-oxygen core can grow more than is generally predicted by stellar models. We discuss these new findings also in light of a new systematic follow-up investigation, based on Gaia EDR3, of evolved giants (13 carbon stars, 3 S stars and 4 M stars) belonging to intermediate-age open clusters.

Carbon star formation as seen through the non-monotonic initial–final mass relation

The initial–final mass relation (IFMR) links the birth mass of a star to the mass of the compact remnant left at its death. While the relevance of the IFMR across astrophysics is universally acknowledged, not all of its fine details have yet been resolved. A new analysis of a few carbon–oxygen white dwarfs in old open clusters of the Milky Way led us to identify a kink in the IFMR, located over a range of initial masses, 1.65 ≲ Mi/M⊙ ≲ 2.10. The kink’s peak in white dwarf mass of about 0.70−0.75 M⊙ is produced by stars with Mi ≈ 1.8−1.9 M⊙, corresponding to ages of about 1.8−1.7 Gyr. Interestingly, this peak coincides with the initial mass limit between low-mass stars that develop a degenerate helium core after central hydrogen exhaustion, and intermediate-mass stars that avoid electron degeneracy. We interpret the IFMR kink as the signature of carbon star formation in the Milky Way. This finding is critical to constraining the evolution and chemical enrichment of low-mass stars, and their impact on the spectrophotometric properties of galaxies. An analysis of the relation between a star’s initial mass and its final mass (as a white dwarf) reveals a kink in the initial mass range 1.65–2.10 M⊙. This kink appears to correspond to the minimum mass required for carbon star formation in the Milky Way at solar metallicity.

Population Synthesis of Helium White Dwarf–Red Giant Star Mergers and the Formation of Lithium-rich Giants and Carbon Stars

Abstract The formation histories of lithium-rich and carbon-rich red giants are not yet understood. It has been proposed that the merger of a helium-core white dwarf with a red giant branch (RGB) star might provide a solution. We have computed an extended grid of post-merger evolution models and combined these with predictions of binary-star population synthesis. The results strongly support the proposal that the merger of a helium white dwarf with an RGB star can provide the progenitors of both lithium-rich red clump stars and early-R carbon stars. The distribution of post-merger models in T eff, log g, log L, the surface abundances of lithium and carbon, and the predicted space densities agree well with the observed distributions of these parameters for Li-rich and early-R stars in the Galaxy.

A Dramatic Decrease in Carbon Star Formation in M31

Abstract We analyze resolved stellar near-infrared photometry of 21 Hubble Space Telescope (HST) fields in M31 to constrain the impact of metallicity on the formation of carbon stars. Observations of nearby galaxies show that carbon stars are increasingly rare at higher metallicity. Models indicate that carbon star formation efficiency drops due to the decrease in dredge-up efficiency in metal-rich thermally pulsing Asymptotic Giant Branch (TP-AGB) stars, coupled to a higher initial abundance of oxygen. However, while models predict a metallicity ceiling above which carbon stars cannot form, previous observations have not yet pinpointed this limit. Our new observations reliably separate carbon stars from M-type TP-AGB stars across 2.6–13.7 kpc of M31's metal-rich disk using HST WFC3/IR medium-band filters. We find that the ratio of C to M stars (C/M) decreases more rapidly than extrapolations of observations in more metal-poor galaxies, resulting in a C/M that is too low by more than a factor of 10 in the innermost fields and indicating a dramatic decline in C star formation efficiency at metallicities higher than [M/H] ≈ −0.1 dex. The metallicity ceiling remains undetected, but must occur at metallicities higher than what is measured in M31's inner disk ([M/H] ≳ +0.06 dex).

Asymptotic giant branch and super-asymptotic giant branch stars: modelling dust production at solar metallicity

We present dust yields for asymptotic giant branch (AGB) and super--asymptotic giant branch (SAGB) stars of solar metallicity. Stars with initial mass $1.5~M_{\odot} \leq M_{\rm ini} \leq 3~M_{\odot}$ reach the carbon star stage during the AGB phase and produce mainly solid carbon and SiC. The size and the amount of the carbon particles formed follows a positive trend with themass of the star; the carbon grains with the largest size ($a_{\rm C} \sim 0.2\mu$m) are produced by AGB stars with $M_{\rm ini} = 2.5-3~M_{\odot}$, as these stars are those achieving the largest enrichment of carbon in the surface regions. The size of SiC grains, being sensitive to the surface silicon abundance, keeps around $a_{\rm SiC} \sim 0.1\mu$m. The mass of carbonaceous dust formed is in the range $10^{-4} - 5\times 10^{-3}~M_{\odot}$, whereas the amount of SiC produced is $2\times 10^{-4} - 10^{-3}~M_{\odot}$. Massive AGB/SAGB stars with $M_{\rm ini} > 3~M_{\odot}$ experience HBB, that inhibits formation of carbon stars. The most relevant dust species formed in these stars are silicates and alumina dust, with grain sizes in the range $0.1\mu m < a_{\rm ol} < 0.15\mu$m and $a_{\rm Al_2O_3} \sim 0.07\mu$m, respectively. The mass of silicates produced spans the interval $3.4\times 10^{-3}~M_{\odot} \leq M_{\rm dust} \leq 1.1\times 10^{-2}~M_{\odot}$ and increases with the initial mass of the star.

Dust from asymptotic giant branch stars: relevant factors and modelling uncertainties

The dust formation process in the winds of Asymptotic Giant Branch stars is discussed, based on full evolutionary models of stars with mass in the range $1$M$_{\odot} \leq$M$\leq 8$M$_{\odot}$, and metallicities $0.001 < Z <0.008$. Dust grains are assumed to form in an isotropically expanding wind, by growth of pre--existing seed nuclei. Convection, for what concerns the treatment of convective borders and the efficiency of the schematization adopted, turns out to be the physical ingredient used to calculate the evolutionary sequences with the highest impact on the results obtained. Low--mass stars with M$\leq 3$M$_{\odot}$ produce carbon type dust with also traces of silicon carbide. The mass of solid carbon formed, fairly independently of metallicity, ranges from a few $10^{-4}$M$_{\odot}$, for stars of initial mass $1-1.5$M$_{\odot}$, to $\sim 10^{-2}$M$_{\odot}$ for M$\sim 2-2.5$M$_{\odot}$; the size of dust particles is in the range $0.1 \mu$m$\leq a_C \leq 0.2\mu$m. On the contrary, the production of silicon carbide (SiC) depends on metallicity. For $10^{-3} \leq Z \leq 8\times 10^{-3}$ the size of SiC grains varies in the range $0.05 \mu {\rm m} < {\rm a_{SiC}} < 0.1 \mu$m, while the mass of SiC formed is $10^{-5}{\rm M}_{\odot} < {\rm M_{SiC}} < 10^{-3}{\rm M}_{\odot}$. Models of higher mass experience Hot Bottom Burning, which prevents the formation of carbon stars, and favours the formation of silicates and corundum. In this case the results scale with metallicity, owing to the larger silicon and aluminium contained in higher--Z models. At Z=$8\times 10^{-3}$ we find that the most massive stars produce dust masses $m_d \sim 0.01$M$_{\odot}$, whereas models of smaller mass produce a dust mass ten times smaller. The main component of dust are silicates, although corundum is also formed, in not negligible quantities ($\sim 10-20\%$).

The chemical composition of carbon stars. The R-type stars

The aim of this work is to shed some light on the problem of the formation of carbon stars of R-type from a detailed study of their chemical composition. We use high-resolution and high signal-to-noise optical spectra of 23 R-type stars selected from the Hipparcos catalogue. The chemical analysis is made using spectral synthesis in LTE and state-of-the-art carbon-rich spherical model atmospheres. We derive their CNO content (including the carbon isotopic ratio), average metallicity, lithium, and light (Sr, Y, Zr) and heavy (Ba, La, Nd, Sm) s-element abundances. The observed properties of the stars (galactic distribution, kinematics, binarity, photometry and luminosity) are also discussed. Our analysis shows that late-R stars are carbon stars with identical chemical and observational characteristics than the normal (N-type) AGB carbon stars. We confirm the results of the sole previous abundance analysis of early-R stars by Dominy (1984, ApJS, 55, 27), namely: they are carbon stars with near solar metallicity showing enhanced nitrogen, low carbon isotopic ratios and no s-element enhancements. In addition, we have found that early-R stars have Li abundances larger than expected for post RGB tip giants. We also find that a significant number (aprox. 40 %) of the early-R stars in our sample are wrongly classified, being probably classical CH stars and normal K giants. In consequence, we suggest that the number of true R stars is considerably lower than previously believed. We briefly discuss the different scenarios proposed for the formation of early-R stars. The mixing of carbon during an anomalous He-flash is favoured, although no physical mechanism able to trigger that mixing has been found yet. The origin of these stars still remains a mystery.

Nuclear Reaction Rates and Carbon Star Formation

We have studied how the third dredge-up and the carbon star formation in low-mass asymptotic giant branch stars depend on certain key nuclear reaction rates. From a set of complete stellar evolution calculations of a 2 M☉ model with Z = 0.01 including mass loss, we find that varying either the 14N(p, γ)15O or the triple-α reaction rate within their uncertainties as given in the NACRE compilation results in dredge-up and yields that differ by a factor of 2. Model tracks with a higher rate for the triple-α rate and a lower rate for the 14N(p, γ)15O reaction both show more efficient third dredge-up. New experimental results for the 14N(p, γ)15O reaction rates are surveyed, yielding a rate that is about 40% lower than the tabulated NACRE rate and smaller than NACRE's lower limit. We discuss the possible implications of the revised nuclear reaction rate for stellar evolution calculations that aim to reproduce the observed carbon star formation at low mass, which requires efficient third dredge-up.

Carbon star populations in systems with different metallicities: statistics in Local Group galaxies

We present evolutionary population synthesis models for the study of the cool and luminous intermediate age stellar populations in resolved galaxies with particular emphasis on carbon star populations. We study the effects of the star formation history, the age and the metallicity on the populations of intermediate mass stars. In the case of instantaneous bursts, we confirm that lower metallicity results in higher contributions of carbon stars to the total star number, and in higher number ratios of carbon stars to late-type M stars. Chemically consistent models are used to study the effect of the star formation history on the relations between carbon star population properties and global parameters of the parent galaxy (age, metallicity). Our models are able to account, for the first time, for those correlations, as observed in the galaxies of the Local Group. For stellar populations older than about 1 Gyr, the properties of carbon star populations are linked to the current metallicity in a way that is quite independent of the star formation scenario. The number ratio of carbon stars to late-type M stars forms a metallicity sequence along which stellar populations with very different star formation histories are found. For the same populations, we find that both the mean bolometric luminosity of carbon stars and their normalized number to the luminosity of the parent galaxy are quite independent of metallicity over a large range in metallicity. This is in good agreement with the observational constraints. The observed statistics of carbon star populations can be interpreted by two principal effects: (i) the carbon star formation efficiency is higher in metal-poor systems, (ii) the typical star formation time-scale along the Hubble sequence of galaxies is much longer than the typical time-scale for the production of carbon stars at any metallicity.

Modelling of intermediate age stellar populations III. Effects of dust-shells around AGB stars

In this paper we present single stellar population models of intermediate age stellar populations where dust- enshrouded Asymptotic Giant Branch (AGB) stars are introduced. The formation of carbon stars is also accounted for, and is taken to be a function of both initial stellar mass and metallicity. The effects of the dusty envelopes around AGB stars on the optical/near-infrared spectral energy distribution were introduced using semi-empirical models where the mass-loss and the photospheric chemistry determine the spectral properties of a star along the AGB sequence. The spectral dichotomy between oxygen-rich stars and carbon stars is taken into account in the modelling. We have investigated the AGB sequence morphology in the near-infrared colour-magnitude diagram as a function of time and metallicity. We show that this diagram is characterized by three morphological features, occupied by optically bright oxygen- rich stars, optically bright carbon stars, and dust-enshrouded oxygen rich and carbon stars respectively. Our models are able to reproduce the distribution of the three AGB subtype stellar populations in colour-colour diagrams. Effects of dusty envelopes on the luminosity function are also investigated. We have extended our investigations to the integrated spectro-photometric properties of stellar populations. We find that the contribution of AGB stars to the near-infrared integrated light decreases, making optical/near-infrared colours of intermediate age populations bluer than what is expected from pure photospheric emission models.

First spectroscopic evidence for carbon stars outside the local group: Properties of a massive star cluster in NGC 7252

We present near-infrared (1 2:3m) spectroscopy of the massive intermediate age star cluster W3 in the merger remnant and proto-elliptical galaxy NGC 7252, obtained with the NTT telescope. This cluster has an age when the integrated near-infrared properties of a stellar population are dominated by the cool and luminous Asymptotic Giant Branch (AGB). We compare the data with instantaneous burst model predictions from new evolutionary synthesis models that include: (i) the computation of the evolution through the thermally pulsing AGB (TP-AGB) for low- and intermediate-massive stars, with the initial mass and metallicity dependent formation of carbon stars; (ii) spectroscopic data from a new stellar library in which dierences between static red giants, variable oxygen rich TP-AGB stars and carbon stars are accounted for. The new evolu- tionary model predicts that the contribution of carbon rich stars to the luminosities in the near-IR passbands is a strong function of metallicity. The comparison of the data to the models clearly shows that carbon stars are present: for the first time, carbon rich star spectral features are thus detected directly outside the Local Group galaxies. Good fits to the available optical/near-IR photometry and the near-IR spectrum of NGC 7252-W3 are found for an age of 300-400 Myr and AV ' 0:6 0:8. The models show that these parameters depend weakly on the model metallicity in the range of Z=Z= 0:4 1, with higher likelihood for solar metallicity models. At solar metallicity, a mixture of carbon rich and oxygen rich stars is predicted. The strength of the near-IR molecular bands that originated from oxygen rich AGB stars can be used to constrain the absolute Te scale of these objects, i.e. a relation between colour and Te. We found that, in the framework of our set of evolutionary tracks, the data are more consistent with the temperature scale calibrated on Long Period Variables than on giant stars. At a given colour, variable AGB stars have a lower Te than static (or quasi-static) M giants.

The pulsation modes and masses of carbon-rich long period variables

Following our study of the carbon-rich giants in the HR diagram and of their luminosity function (Paper III), we investigate the pulsation data of the long period variables (LPVs) included in our sample. Pulsation modes (fundamental, over- tone(s)) for carbon LPVs are identified in the period-radius diagram, making use of observed bi-periodicity in a small subsample of those stars, and of comparison to models. Mean pulsation masses are then deduced from theoretical PMR-relations, with due attention paid to a possible bias while averaging. Mean (present) pulsation masses (0:6 4:0 M) are found to increase along the group sequence HC5 to CV6, with still larger masses possibly associated with cool extreme CV7-objects with strong mass loss and thick circumstellar shells. This is con- sistent with the 0.8-4 M range of initial masses found in Paper III for the majority of carbon-rich giants aected by mass loss during their evolution. The pulsation masses found for a few HC-stars (M 0:8 M) are consistent with their low initial masses (Mi < 1:1 M); as inferred from their thick disk membership (age' 11 Gyr?) and locus in the HR diagram. A mean pulsation mass of'0:6 M is found for the three population II Cepheids in the sample. A mass-luminosity diagram is proposed for the Galactic carbon giants. The data from observations is found consistent with theoretical predictions from AGB modeling, specially the third dredge-up (TDU) through thermal pulses (TP) with a carbon star formation line (CSFL) for TP-AGB stars. It appears that the CV-giants are close to the tip and end of their evolutionary tracks in the TP-AGB of the HR diagram. It is confirmed that this end shifts toward lower eective temperatures and higher luminosities, with increasing masses. It is shown that the C/O abundance ratios do correlate with eective temperatures, according to three distinct distributions (halo CH stars, thick disk HC-stars, and thin disk CV-stars). The mean stellar density decreases along the HC5-CV7 sequence, while the surface gravity remains nearly constant at about 0.5 CGS unit logg' 0:3; 5 10 3 SI : The nature of (thin disk) CV-stars as TP-AGB objects being confirmed, the discussion is focused on (thick disk) HC-stars since the origin of these old low-mass giants remains unclear. Unpredicted extra mixing on RGB and/or E-AGB is favored. Evolution from (old, low O/H) dwarf carbon stars is also considered since observations of metal-poor stars and recent calculations point to large supersolar (C=Fe) ratios in Population III objects and contamination through rapid cycling in the interstellar medium.

Improved synthetic TP-AGB models

We present recent improvements and results of an extensive analysis of the TP-AGB phase performed by means of a synthetic model (Marigo 1998a, b; Marigo et al. 1998a, b). The improvements concern: i) the use of a homogeneous and accurate set of analytical relations (Wagenhuber &amp; Groenewegen 1998); ii) a new treatment of envelope burning in the most massive TP-AGB stars (M &gt; 3.5M⊙) to account for the possible break-down of the core mass-luminosity relation; iii) a better treatment of the third dredge-up to infer if and when the process takes place.Extensive calculations of synthetic TP-AGB models have been carried out over the mass range (0.8M⊙ ÷ 5M⊙) and for three sets of initial metallicity (Z = 0.019, Z = 0.008, Z = 0.004). The formation of carbon stars is investigated addressing the following issues: a) the reproduction of the observed luminosity functions of carbons stars in both Magellanic Clouds, and b) the formation of very bright and optically obscured carbon stars.

Nucleosynthesis in AGB Stars

We give a qualitative review of the nucleosynthesis occurring in AGB stars. We summarise some new calculations of intermediate mass stars which include all thermal pulses until the star is about to leave the AGB, as well as a detailed nucleosynthesis network. We will show that hot bottom burning delays, rather than prevents, the formation of carbon stars; those that form are not optically visible.

The Formation of Carbon Stars in the Magellanic Clouds from Mass Transfer in Close Binaries

The presence of the carbon stars in the MC with luminosities higher or lower than predicted for thermally-pulsing (TP) AGB stars, can be explained by processes that happen during the early AGB (E–AGB) stage. I examine this assumption by means of a population simulation technique. I find that there must be TP–AGB C and S stars in the MC that formed as a result of mass transfer in binary systems. Their presence may influence the age determinations of MC clusters.

Carbon stars in populations of different metallicity

Our current knowledge of carbon stars in the Local Group and beyond, is discussed. Although many carbon stars and late M-stars have been identified in external galaxies, a coherent understanding in terms of the chemical evolution- and star-formation-rate history of a galaxy is still largely lacking. Issues that need to be addressed are: 1) for some of the larger galaxies only a small fraction in area has been surveyed so far, 2) surveys have been conducted using different techniques, and may be incomplete in bolometric magnitude, 3) only for some galaxies is there information about the late M-star population, 4) not all galaxies in the Local Group have been surveyed, 5) only for a sub-set of stars are bolometric magnitudes available.From the existing observations one can derive the following: the formation of carbon stars is both a function of metallicity and star formation. In galaxies with a similar star-formation-rate history, there will be relatively more carbon stars formed in the system with the lower metallicity. On the other hand, the scarcity of AGB-type carbon stars in some systems with the lowest metallicity indicates that these galaxies have had a low, if any, star-formation-rate history over the last few Gyrs.

Prevention of High-Luminosity Carbon Stars by Hot Bottom Burning

CNO abundances are presented for theoretical models of asymptotic giant branch stars of 4, 5, and 6 M ○. with Z=0.02 and Z=0.001. Self-consistent stellar evolutionary models were computed, starting on the pre-main sequence and terminating after a series of helium shell flashes on the asymptotic giant branch (AGB). The 5 and 6 M ○. cases encountered hot bottom burning, with temperatures at the base of the convective envelope reaching (60-80)×10 6 K for Z=0.02 and (80-100)×10 6 K for Z=0.001. A luminosity boundary near M C bol ∼−6.4 is predicted; for brighter luminosities, hot bottom burning will prevent carbon star formation (these will be S stars, with 3≤ 12 C/ 13 C≤18, and for a period they will be super lithium-rich)