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Article

Tracing the Evolution of the Emission Properties of Carbon-Rich AGB, Post-AGB, and PN Sources

by
Silvia Tosi
1,2,3,* and
Ester Marini
3
1
Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00100 Roma, Italy
2
LNF, Laboratori Nazionali Fascati, Via Enrico Fermi, 54, 00044 Frascati, Italy
3
INAF, Observatory of Rome, Via Frascati 33, 00077 Monte Porzio Catone, Italy
*
Author to whom correspondence should be addressed.
Astronomy 2025, 4(1), 2; https://doi.org/10.3390/astronomy4010002
Submission received: 29 November 2024 / Revised: 10 January 2025 / Accepted: 16 January 2025 / Published: 20 January 2025
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Abstract

:
Understanding the transition from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PN) phase is crucial for advancing our knowledge of galaxy evolution and the chemical enrichment of the universe. In this manuscript, we analyze 137 carbon-rich, evolved low- and intermediate-mass stars (LIMSs) from both the Magellanic Clouds (MCs) and the Milky Way (MW). We focus on AGB, post-AGB, and PN sources, tracing the evolution of their emission through spectral energy distribution (SED) modeling. Consistent with previous studies, we observe that more evolved LIMSs exhibit cooler dust temperatures and lower optical depths. Amorphous carbon (amC) is the dominant dust species in all the evolutionary stages examined in this work, while silicon carbide (SiC) accounts for 5–30% of the total dust content. Additionally, we analyze color–color diagrams (CCDs) in the infrared using data from IRAC, WISE, and 2MASS, uncovering significant evolutionary trends in LIMS emission. AGB stars evolve from bluer to redder colors as they produce increasing amounts of dust. Post-AGB and PN sources are clearly differentiated from AGB stars, reflecting shifts in both effective stellar and dust temperatures as the stars transition through these evolutionary phases.

1. Introduction

Over the past few decades, significant progress has been made in understanding the evolution of sources with masses between 1 and 8 M ⊙ , commonly referred to as LIMSs. These stars have garnered increasing attention from the scientific community due to their pivotal role in enriching the interstellar medium (ISM) with gas and dust, reflecting the nucleosynthesis processes active during their evolution [1,2,3]. The significance of LIMSs arises from their substantial mass-loss rates during the AGB phase, which typically range between 10 − 8 and 10 − 5 M ⊙ yr − 1 [4]. This mass-loss results in the ejection of nearly their entire envelope, and combined with the low effective temperatures of AGB stars, creates ideal conditions for the condensation of gaseous molecules into solid particles [5], significantly contributing to the infrared (IR) emission observed in galactic environments [6].
Understanding the crucial role of LIMSs in the evolution of host galaxies [7,8] and, more broadly, in the chemical enrichment of the universe [9,10] requires a detailed study of the surface chemical composition of AGB stars. This composition is shaped by two key processes: the third dredge-up (TDU) [11] and hot bottom burning (HBB) [12]. During TDU, the convective envelope extends into regions affected by helium nucleosynthesis, enriching the surface with 12C. In stars with masses below 4 M ⊙ , repeated TDU episodes can lead to the formation of carbon stars, defined by a carbon-to-oxygen (C/O) ratio greater than one. Conversely, more massive LIMSs undergo HBB, a process involving proton-capture nucleosynthesis at the base of the convective envelope. HBB depletes 12C while producing 14N, preventing the formation of carbon stars. These changes in surface chemical composition significantly influence the mineralogy of dust produced during the AGB phase: carbon-rich dust (CRD) forms in stars with C/O ratios greater than one, while oxygen-rich dust (ORD) forms in stars with C/O ratios below unity [4].
Since the majority of dust is expelled during the late stages of the AGB phase [13,14,15], it remains detectable in subsequent evolutionary phases, including post-AGB stars [16,17] and PNe [18]. Recent studies by our group [19,20,21,22,23] have demonstrated that these final evolutionary stages of LIMS can provide valuable constraints on key uncertainties, such as the history of dust formation and mass-loss processes. Each study analyzed the SED of sources believed to have evolved as single stars from the MCs and the MW.
The analyses involved spectral and photometric data spanning ultraviolet (UV) to IR wavelengths, compared against synthetic SEDs. This methodology allowed us to reconstruct the evolutionary history of each object and identify the appropriate progenitor mass model for comparison with observational constraints. These efforts provided new insights into the consistency of observed dust properties with theoretical predictions, shedding light on the processes of dust and gas production from the late AGB to the PN stages. Specifically, consistent with findings for AGB stars [13,24,25], our studies revealed a strong correlation between the amount of observed dust and the progenitor’s mass, with the effective temperature further influencing this relationship in PN sources [19,21]. In addition, we found a discrepancy between the dust observed in the post-AGB sources and that expected to be produced during the AGB phase, highlighting the need for an upward revision of the mass-loss rates at the tip of the AGB phase for low-mass, metal-poor carbon stars (progenitor masses between ∼1 and 1.5 M ⊙ at the beginning of the AGB phase, and metallicities [Fe/H] < −0.7) [20].
Further advancements in this field are essential for a deeper understanding of AGB stars. Expanding the sample size is critical to generalizing these findings and better reconstructing the evolution of LIMSs. Nevertheless, a preliminary synthesis and comparison of existing results is feasible. As a first step in this direction, this manuscript aims to synthesize and compare key results obtained by our group, focusing on the sample of carbon-rich AGB, post-AGB, and PN objects. These sources have been characterized in terms of their progenitor masses and evolutionary histories in the following studies Marini et al. [14], Tosi et al. [20,21], Dell’Agli et al. [22,23]. In this work, we aim to establish evolutionary connections between them, tracing the evolution of their emission to provide a comprehensive perspective on how the SED evolves during the transition from the AGB to the PN phase and how this is reflected in the CCD analysis, leveraging photometric data from WISE and Spitzer’s IRAC bands. Extensive datasets from large surveys, such as Surveying the Agents of Galaxy Evolution (SAGE-LMC; Meixner et al. [26], SAGE-SMC Gordon et al. [27]), and the DUSTiNGS survey [28], have led to numerous investigations that have recognized IR CCDs as powerful tools for studying the evolutionary properties of evolved stars. Several investigations have employed synthetic spectra to interpret the positions of observed sources in IR diagrams and analyze their properties [29,30]. Such efforts have significantly advanced our understanding of the evolved stellar populations of the MCs and other Local Group galaxies [31,32]. Building on these foundations, previous studies have gained valuable insight into the mass, chemical composition, formation epochs, and dust production rates of individual sources [13,33,34,35] through detailed comparisons between the observed and theoretical distributions of AGB stars in IR CCDs. These investigations have inspired the analysis presented in this manuscript, which aims to trace the evolution of IR emission from the AGB to the PN phase.
In future work, we plan to expand the carbon-rich sample to include new, previously unexamined sources and incorporate an analysis of oxygen-rich stars. Furthermore, we aim to explore JWST diagrams based on synthetic magnitudes, which will allow us to predict the expected behavior of the sources under investigation once actual JWST photometric data become available.
The paper is organized as follows: In Section 2, we describe the selected sample and the methodology applied by Marini et al. [14], Tosi et al. [20,21], Dell’Agli et al. [22,23] to characterize the individual sources. The changes in the shape of the expected emission across the AGB to PN phases, as predicted by our modeling, are discussed in Section 3, where we illustrate the variation of the SED across the AGB to PN phases. The exploration of the CCD analysis is presented in Section 4.

2. Characterizing the Sample: How to Determine the Stars’ Progenitors

2.1. Selection of the Sample and of the Observational Data

We collected a sample of sources likely to have evolved as single carbon stars, originating from both the MCs and the MW. The sample includes 110 LMC AGB stars from Marini et al. [14], 20 post-AGB stars from Tosi et al. [20,21], and 7 PNe from Tosi et al. [19] and Dell’Agli et al. [22]. A list of the sources, including their coordinates and respective parent galaxies, is provided in Table A1.
To characterize the emission of each source, we gathered relevant observational data. For the PNe, photometric data were compiled from various catalogs, including U, B, and V measurements reported by Reid [36] and Lasker et al. [37], WISE photometry from Cutri et al. [38], and mid-IR spectra from the Spitzer Infrared Spectrograph (IRS). Additionally, UV spectra were obtained using the HST/Space Telescope Imaging Spectrograph (STIS) [39]. For the post-AGB sources, we utilized Spitzer IR spectra from Volk et al. [40], SWS spectra from Sloan et al. [41], and photometric data spanning multiple bands, such as Massey’s CCD survey of the MCs [42], the LMC stellar catalog [43], and the Guide Star Catalog Version 2.3.2 [37]. Additional information came from the IRAC and MIPS bands in the SAGE-LMC catalogue [26], the J, H, and K bands from 2MASS [44], and WISE bands [45]. Similarly, for AGB stars, we leveraged the 2MASS JHKs [42], IRAC and MIPS photometric data [26], and spectra acquired using IRS available in the SAGE-Spec database [46], reduced by Jones et al. [47]. In their work, Jones et al. [47] also calculated the expected MIRI magnitudes used in this study by convolving the IRS data with the spectral response of the various filters.
The carbon-rich nature of post-AGB and PN sources was confirmed through chemical abundance studies. For post-AGB stars, the abundances of carbon, nitrogen, and oxygen were obtained from van Aarle et al. [48], De Smedt et al. [49], and Kamath et al. [17], while those for PNe were sourced from Stanghellini et al. [39], Leisy and Dennefeld [50], and Henry et al. [51]. AGB stars were identified as carbon-rich based on molecular absorption features of C 2 H 2 (acetylene) bands at 7.5 and 13.7 μm, as well as dust emission features at 11.3 μm and 30 μm, following the classification criteria outlined in Woods et al. [52].
In post-AGB stars, the single star assumption is supported by radial velocity studies [53,54] and by the selection of the sample, made to exclude the presence of a near-IR excess, a characteristic typically associated with binary evolution [16,54,55]. For PNe, we selected sources with round nebular shapes [56,57,58,59], which are generally associated with single-star evolution [60,61]. Among our sample, we also included PNe with elliptical shapes that can be reproduced by single-star evolution models [62], although the presence of binary systems cannot be entirely excluded. Similarly, for the AGB sample, a few binary systems cannot be ruled out. However, the inclusion of binary systems does not significantly affect the shape of the SED; rather, a companion could enhance the mass-loss mechanism through binary interactions, resulting in increased dust production in their outflows [63].

2.2. Input and Methodology Description

The sources presented in this manuscript were previously characterized in earlier work by our team [14,20,21,22,23], where a detailed analysis of the observational data outlined in Section 2.1 was performed. Specifically, these measurements were compared with synthetic SEDs calculated using two distinct codes: the radiative transfer code DUSTY [64] for AGB and post-AGB stars, and the photoionization code CLOUDY [65] for PNe. To construct the synthetic SEDs, we utilized different stellar atmosphere models tailored to the evolutionary stage of each source. Specifically, we employed COMARCS models [66] for AGB stars, Kurucz–Castelli models [67] for post-AGB stars, and models from Rauch [68] and Pauldrach et al. [69] for PNe central stars (CSs). For the latter, we also assumed a constant hydrogen density, with values reported by Stanghellini et al. [39]. The gas chemical composition followed the prescriptions of Aller and Czyzak [70] and Khromov [71], and the initial nebular radius was set to match the observed [OIII] radii from Shaw et al. [56]. Furthermore, we selected the built-in optical constants that best replicate the dust IR emission, specifically adopting Zubko et al. [72] for amC and Pegourie [73] for SiC in AGB and post-AGB stars. For the PNe, we applied Rouleau and Martin [74] for amC and Laor and Draine [75] for SiC.
To analyze the AGB sample, we adopted a two-step methodology: we first characterized the individual sources by comparing the spectra obtained by IRS and the synthetic SEDs, the latter being obtained by the modeling of the AGB evolution and of the dust formation process via the ATON code for stellar evolution (see Ventura et al. [76] for an exhaustive description of the numerical details of the code and the most recent updates).
This analysis led to the determination of the luminosities and the optical depth at λ = 10 μm ( τ 10 ) of the individual sources. These estimates are rather robust because the extinction of carbon stars is mostly due to solid carbon particles, with minimal dependency on other dust species. This step allowed the characterization of the stars considered in terms of the mass, formation epoch, and chemical composition of the progenitors. The second step involved refining the dust temperature ( T d ) and the details of the dust mineralogy to determine the percentages of the different dust species in the circumstellar envelope. This required a detailed analysis of the spectra collected with IRS, considering that the morphology of the different spectral features is extremely sensitive to the type and the quantity of the dust grains formed in the stellar wind.
For post-AGB and PN sources, the codes were iteratively run, adjusting the contributions from the central star, dust, and gas to achieve the best agreement with the observational data. By comparing the synthetic SEDs to the observations, we were able to derive key physical parameters: for post-AGB stars, we determined the stellar luminosity and effective temperature, while for PNe, we obtained the luminosity and effective temperature of the central star, as well as the ionized nebular mass. In both cases, we also determined dust properties, including mineralogy, T d , the relative distance of the inner boundary of the dusty region from the central star ( R ), and the amount of dust. The latter was quantified as the dust-to-gas ratio for PNe and as the τ 10 for post-AGB stars, depending on the SED modeling code employed. After identifying the best model, luminosities, effective stellar temperatures, and optical depths derived from the detailed analysis of SED morphology were further compared to the stellar models calculated with the ATON code, in order to derive the progenitor’s mass of each source. For the PN and post-AGB objects, the characterization was made even more robust thanks to the comparison of the observed carbon, oxygen, and nitrogen abundances with the predictions of the models.
A summary of the key physical parameters derived from the SED analysis is provided in Table A1, while examples of synthetic SEDs generated using the best set of parameters are shown in Figure 1. The selected sources represent each class of evolutionary stage: SSID 145 for AGB stars [14] (left panel), IRAS07134+1005 for the post-AGBs [20] (central panel), and SMP LMC 71 for the PNe [19] (right panel). The observed photometric data are depicted as blue squares, while the magenta line represents the HST/STIS spectrum and the green line the IR spectra: Spitzer/IRS for the AGB star and PN, and ISO/SWS for the post-AGB star. The spectra of the PNe are characterized by numerous emission lines, which enhance the flux observed in the photometric data. To account for this effect, in Tosi et al. [19], we derived synthetic photometric data that include both the continuum and all emission lines, allowing for a direct comparison with the observed data (open red circle in the right panel).
The procedure described in the present section, which enables a reliable determination of the progenitor mass, is crucial for linking the observations of an object to its correct evolutionary history, allowing for the testing of model predictions and the identification of new constraints.

3. Tracing the SED Evolution from the AGB to the PN Phase

Achieving a comprehensive understanding of the sources analyzed in this manuscript necessitates examining how their emission properties and the SED’s shape evolve throughout the transition from the AGB to the PN phase. This approach enables us to trace the evolutionary connections between these stages, using observed spectra to reveal how the physical and chemical characteristics of the circumstellar environment change over time. A detailed discussion of this transition, focusing on each component that contributes to the SED of evolved LIMS, is provided in the following subsections.

3.1. An Overall View of the CSs’ Dust and Gas Spectral Emission

Figure 2 shows the evolution of the SED for a LIMS transitioning through the AGB (green line), post-AGB (blue line), and PN (orange line) phases. As detailed in Section 2.2, the SEDs for the AGB and post-AGB phases are calculated using the radiative transfer code DUSTY, while the SED for the PN phase is modeled using the photoionization code CLOUDY.
Since most dust production occurs during the AGB phase, the dusty layers at this stage are located close to the central star, typically at distances where dust condensation is efficient, around 3–4 stellar radii [4]. This spatial proximity, combined with the star’s relatively low photospheric temperature (<5000 K), results in the overlap of the stellar and dust spectral emissions, with the dust emission becoming the dominant component of the SED.
Following the AGB stage, the CS enters the post-AGB phase and begins to contract, shifting its emission toward UV wavelengths. Simultaneously, radiation pressure from the star causes the dusty layer to detach from the stellar atmosphere, leading to a decrease in dust temperature. Our SED analysis results are consistent with this behavior, as demonstrated by the trend represented by the blue squares in the left panel of Figure 3, which shows how the distance of the dusty layer from the central star correlates with dust temperature.
As the dust temperature decreases, the dust emission peak shifts towards the mid-IR spectral region, producing a distinct separation between dust emission ( 3 ≲ λ [ μ m ] ≲ 100 , corresponding to the right peak of the post-AGB SED in Figure 2) and stellar emission ( 0.1 ≲ λ [ μ m ] ≲ 3 , left peak). This separation creates the characteristic double-peak shape of post-AGB SEDs, already investigated in the literature [16,54,77]. In addition to the shift to longer wavelengths, the dispersion of the dusty layer leads to lower post-AGB τ 10 values. This can be seen in the right panel of Figure 3, which displays the optical depth and luminosity values obtained through SED analysis for these classes of sources.
A similar scenario applies to PN sources, whose emission is characterized by three components: the CS, the gaseous nebula, and the dust [78]. The emission peak of the CS shifts toward bluer wavelengths compared to the post-AGB phase ( λ ≲ 0.3 μ m ), driven by the increasing effective temperature of the CS. Simultaneously, the dusty layer, which is now located farther from the star due to sustained radiation pressure from the previous phase, undergoes further cooling, causing the dust emission to shift to even redder wavelengths in the IR region ( λ ≳ 7 μ m , right peak of the blue line, Figure 2). This evolutionary scenario is supported by Figure 3, which shows that post-AGB stars (blue squares) are characterized by shorter distances between the dusty region and the central star, as well as higher dust temperatures, compared to PN sources (orange stars). In addition to the contributions from stellar and dust emission, a gaseous nebular component becomes prominent in the SED of the PN sources [78], primarily contributing in the wavelength range of 0.3 ≲ λ [ μ m ] ≲ 7 . This gaseous nebular emission exhibits a characteristic profile arising from a combination of processes discussed in detail by Brown and Mathews [79] and Zhang and Kwok [78].

3.2. Dust Features

While the models reported in Figure 2 are computed considering only amC, it is important to acknowledge that other dust species form in the winds of carbon stars, significantly influencing the IR spectral emission of evolved LIMSs. Understanding the mineralogy of the dust and the details of its spectral features is essential for interpreting the entire IR SED, as this provides valuable insights into the winds of carbon-rich sources and the mechanisms of dust formation. Therefore, for a reliable characterization of how the SED of LIMSs evolves through their different phases, it is essential to include accurate dust modeling. The presence of different dust species, such as amC and SiC, can alter the emission features of the SED. For instance, while amC produces featureless emission, SiC introduces a distinct emission feature at 11.3 μm [80,81].
To investigate the relative contributions of amC and SiC to the overall dust composition and their potential impact on the SEDs, we present in Figure 4 a density plot showing the relative percentages of amC and SiC in the total dust composition used in the SED models for the AGB and post-AGB sources analyzed in this study. According to Stanghellini et al. [39], only one carbon-rich PN in our sample exhibits evidence of SiC emission, which is why it has been excluded from this diagram. The probability density distribution in Figure 4 is normalized so that the sum of the areas of all bins equals 1.
As shown in Figure 4, the models indicate that most dust condenses into amC, with SiC accounting for 5% to 30% of the total dust content. Variations in SiC abundance arise from differences in dust formation histories. SiC production is particularly sensitive to the availability of silicon molecules, which are critical for its condensation and growth. Since silicon abundance is closely linked to stellar metallicity, environments with higher metallicity are capable of producing greater quantities of SiC grains compared to their lower-metallicity counterparts [82]. Therefore, the observed differences in SiC percentages between the AGB and post-AGB stars in this study likely reflect variations in their metallicities. To support this scenario, the left panel of Figure 5 shows the SiC abundance of the post-AGB sources plotted as a function of metallicity. This sample was selected because it includes a broad range of metallicities, reflecting the presence of sources from diverse galactic environments (see Table A1), specifically the MCs (open squares) and the MW (crossed open squares). In the figure, we differentiate sources whose dust emission was modeled using ISO/SWS spectra (orange squares), which provide a more robust analysis, from those without such measurements (blue squares). A rough trend between the SiC dust abundance and metallicity can be observed in all post-AGB sources, appearing more evident in the sources where the ISO spectrum is available. However, a more detailed investigation, incorporating new infrared spectra, is needed to confirm this scenario.
An additional explanation for this trend may arise from differences in the progenitor masses of the two samples. As shown in the right panel of Figure 5, most AGB stars in our sample exhibit luminosities exceeding 6500 L / L ⊙ , indicative of progenitor stars with relatively high initial masses (generally higher than 1.5 M ⊙ at the beginning of the AGB phase [14]). These massive AGB stars tend to produce significant amounts of dust, which leads to saturation in SiC production while amC continues to form, becoming increasingly dominant among the dust components [14,25]. This behavior leads to the lower relative abundance of SiC in the total dust observed in AGB stars compared to post-AGB phases, for which our previous studies derived progenitor masses generally equal to or lower than 1.5 M ⊙ [20,21]. Indeed, these lower-mass progenitors produce smaller amounts of dust, resulting in relatively higher fractions of SiC compared to the total dust content.
Other contributors to the overall dust emission in evolved LIMSs have also been investigated by various researchers, including the 21 μm and 30 μm features observed in SEDs [83]. The 21 μm feature remains unidentified but is thought to be associated with complex hydrocarbons [84]. In contrast, the 30 μm feature is likely attributed to magnesium sulfide (MgS) grains [85,86]. Beyond these, additional spectral features have been observed and are often linked to polycyclic aromatic hydrocarbons (PAHs) or very small hydrogenated amorphous carbon grains in the 3–12 μm range [87]. Furthermore, fullerenes such as C60 and C70 have been detected in some CRD sources [86,88], though their origins remain under debate.
Among the sources analyzed in the present manuscript, prior investigations have examined potential contributors to these features, including PAHs [86,89], fullerenes [90], and MgS [14,20,86]. However, achieving a comprehensive understanding of dust emission in evolved LIMSs will require future studies that systematically investigate all potential contributors to these features.

4. AGB to PN Transition in IR Observational Diagrams

In this section, we present CCDs derived from IRAC, WISE, and 2MASS observations of the stars in our sample, emphasizing the evolutionary trends followed by the sources as they progress from the AGB phase through the post-AGB to the PN stages. During this transition, significant changes occur in the shape of the IR emission, primarily driven by the shift in dust emission toward longer wavelengths (Section 3), leading to a redistribution of sources at different evolutionary stages within the investigated CCDs. These distinct positions highlight the diagnostic potential of such diagrams in characterizing the complex physical and chemical processes that govern the evolution of evolved stars.
As mentioned in the introduction, several studies have highlighted the utility of IR CCDs as powerful tools for probing the evolutionary properties of evolved stars. In particular, the use of the ([3.6]–[4.5], [5.8]–[8.0]) CCD is particularly effective for analyzing such sources. Previous research has shown that this diagram can distinguish between different populations of evolved stars in the LMC. For example, Dell’Agli et al. [33] showed that the extreme AGB stars in the LMC trace a diagonal sequence in this diagram, with the reddest stars representing the final stages of the AGB evolution. Additionally, other studies have further confirmed the effectiveness of IRAC colors for analyzing and classifying evolved stars, providing strong support for the application of such diagrams in our work. For instance, McQuinn et al. [91] examined the IRAC colors of variable stars in the galaxy M33, identifying a significant population of AGB stars and distinguishing between oxygen-rich and carbon-rich types based on their [3.6]–[4.5] colors. Further, Marigo et al. [92] also made significant contributions by modeling the mid-infrared color distribution of AGB stars in various environments, showing how these stars’ IRAC colors can be used to distinguish different evolutionary stages and chemical compositions. These findings further reinforce the value of the ([3.6]–[4.5], [5.8]–[8.0]) CCD as a valuable tool for analyzing and characterizing our sample and, more generally, evolved stars.
In line with these previous works, the left panel of Figure 6 illustrates the distribution of the sample stars in the ([3.6]–[4.5], [5.8]–[8.0]) diagram, with triangles representing AGB stars, squares indicating post-AGBs, and stars corresponding to PNe. Filled squares represent post-AGB stars from the MCs, while open crossed squares identify those from the MW. The full lines represent the evolutionary model for an AGB star with a progenitor mass of 3.0 M ⊙ (magenta line) and 2.0 M ⊙ (yellow line).
Among the AGB sample, redder [3.6]–[4.5] colors are typically associated with sources that originate from higher-mass progenitors, as these stars reach higher optical depths. This trend can be attributed to an increased number of TDU episodes in more massive stars, which enhance dust production. As a result, dust emission shifts to longer wavelengths (as shown in Figure 2), due to the larger amount of dust formed in the stellar envelope [93,94,95]. This behavior is evident when comparing the evolutionary tracks of stars with initial masses of 3.0 M ⊙ and 2.0 M ⊙ at metallicity Z = 0.008 from Marini et al. [14], and is consistent with the findings of Dell’Agli et al. [13]. The AGB evolution of these stars was recently studied in detail by Marini et al. [14], who highlighted the significant accumulation of carbon at the stellar surface and intense dust production, particularly in the final phases of the AGB. At this stage, the stars achieve the highest infrared excesses and reddest IR colors in their evolution. In contrast, bluer colors are typical of objects that have only recently begun dust production, marking the earlier phases of the AGB. This evolutionary pattern leads to the obscuration sequence traced by AGB stars in the CCDs, as observed in Figure 6, where sources in earlier stages, characterized by [3.6]–[4.5] and [5.8]–[8.0] ∼0, gradually shift to redder colors as they evolve toward the later AGB phases, with [3.6]–[4.5] ∼3.0 and [5.8]–[8.0] ∼2.5, in agreement with the findings of Dell’Agli et al. [13].
We note that the post-AGB and PNe sources occupy a defined region in this diagram, distinct from the sequence traced by AGB stars. This distribution can be attributed to the discussion offered in Section 3.1 regarding the evolution of the overall emission from the AGB to the PNe phase: the increase in the effective temperature shifts the CS’s emission peak toward shorter wavelengths compared to the AGB phase, while the greater distance and lower dust temperature move the dust emission peak toward longer wavelengths with respect to the AGB stars. This is clearly illustrated in Figure 7, where a color coding is applied to represent the effective stellar temperatures (left panel) and dust temperatures (right panel). As shown in the figure, the IRAC colors are sensitive to changes in the dust and effective stellar temperatures, which characterize the phases following the AGB stage, making the post-AGB and PNe sources deviate from the obscuration pattern traced by AGB stars and populate the CCD zone with 0 < [3.6]–[4.5] < 1 and 1.7 < [5.8]–[8.0] < 2.5.
However, it remains unclear whether post-AGB and PN objects could follow a trajectory similar to that of AGB stars, progressively moving toward redder colors. Confirming such a trend and identifying its physical drivers would require analysis of a larger sample.
We note that three AGB sources are located off the evolutionary sequence traced by the AGB sample. According to the interpretation provided by Marini et al. [14], these stars (SSID 349, SSID 125, and SSID 190, previously classified as AGB stars) are likely beginning their contraction toward the post-AGB phase. This finding is based on the cool dust temperature ( ∼ 350 K; see the right panel of Figure 7) required to fit the observed spectra, suggesting that the dust layer is moving away from the system. It is also consistent with the low optical depths ( τ 10 ∼ 2–3) observed, which may result from decreased gas densities due to expansion. A more detailed discussion of these objects can be found in Marini et al. [14]. Based on this understanding, the location of SSID 65 in the CCD, which is situated in the same region of the CCD occupied by the stars SSID 125 and SSID 190, suggests that this source may also be an early post-AGB star. If this picture is correct, it is possible to delineate an evolutionary sequence within this CCD, connecting the AGB stars to the subsequent stages of evolution. This sequence is traced by the entire sample, providing insight into the progression from the AGB phase to the PN phase. To help visualize the suggested evolution, we have added a magenta dashed line in the figure, which represents an extension of the 3.0 , M ⊙ model. However, confirming this trend would require the development of new evolutionary tracks. This task is challenging due to the difficulties in defining accurate mass-loss rates [4], which result in the absence of a reliable description of the wind dynamics during the transition from the AGB to the PN phase.
Another CCD diagram, similar to the one shown in the left panel of Figure 6, can assist in characterizing sources at different evolutionary stages by analyzing the variation of [3.6]–[4.5] as a function of J–Ks. When [3.6]–[4.5] data are unavailable, the W1–W2 vs. J–Ks diagram provides an alternative. This is illustrated in the right panel of Figure 6, where sources at various evolutionary stages occupy distinct regions, allowing for clear differentiation between them. AGB stars typically show larger J–Ks values than post-AGBs and PNe, with AGB stars exceeding J–Ks > 1.0. The only exceptions are SSID 125 and 190, which display lower J–Ks values, aligning closely with the post-AGB sample. For post-AGB stars, these bands typically correspond to the region of the SED where the minimum occurs between the peak due to the star and the peak due to the dust (see Section 3). As discussed in Tosi et al. [21], the SED morphology in this region is primarily governed by the dust temperature, with a secondary influence from the star’s effective temperature, resulting in the broad spread in the W1–W2 color observed in the right panel of Figure 6. For PNe, the filters primarily detect emission from the nebular gas. This nebular emission also influences the behavior in the WISE and IRAC filters, resulting in observed [3.6]–[4.5] (and/or W1–W2) values ranging from approximately 0.5 to 1.0 in our sample. As mentioned for the left panel, a larger sample would be necessary to confirm these findings, identify potential trends, and better understand their underlying physical origins.
Additional investigation of CCDs beyond those used in the present manuscript is necessary to derive further insight into the final evolutionary stages of LIMSs. As shown in Suh [96], the combination of IRAS, AKARI, MSX, and 2MASS filters can help trace the emission variations of sources transitioning through the AGB, post-AGB, and PN phases. To study this transition, they made several key assumptions necessary for calculating evolutionary models extending into the PN phase. These models demonstrated that post-AGB tracks generally align with the observed positions of PNe, supporting our findings that CCDs can reliably trace the final evolutionary sequences of LIMS.

5. Conclusions

In this manuscript, we compiled a sample of 137 carbon-rich sources from the MCs and the MW, previously investigated by our team, with the goal of establishing an evolutionary connection among their emission characteristics. Building on the analyses from our earlier studies, we examined the evolution of the SED from the AGB to the PN phases, with a particular emphasis on the role of dust emission in relation to the CSs. Our findings indicate that more evolved LIMSs are characterized by dust layers increasingly detached from the CSs, leading to cooler temperatures and lower optical depths, as expected.
The SED modeling incorporated contributions from key dust species, specifically amC and SiC. Differences between the individual stars in both the AGB and post-AGB samples were attributed to variations in stellar mass and metallicity, reflecting the distinct environments of the MCs and the MW. The evolution of the SED was further examined using color–color diagrams ([3.6]–[4.5], [5.8]–[8.0]), and (W1–W2 (or [3.6]–[4.5] when WISE data were unavailable), J–Ks). In the first diagram, in agreement with previous findings, we observed a reddening trend in the AGB stars, spanning from the less obscured sources to those evolving through the most advanced AGB stages, when more dust is produced, that are characterized by redder colors. In addition, we found a likely evolutionary sequence, starting with AGB sources and continuing through post-AGB and PN sources, which exhibit higher [5.8]–[8.0] and lower J–Ks colors compared to the AGB sample. We attribute this divergence to the detachment of the dusty layer from the central star in the more evolved phases and, in the case of PNe, to the contribution of nebular gas emission. Consistent with this evolutionary trend, we identified two sources previously classified as AGB stars that align with the post-AGB sample, likely representing stars that have completed their AGB phase and are now beginning their contraction towards the post-AGB phase. A similar division between AGB and post-AGB/PN sources is also observed in the second diagram, where the more evolved objects show higher [5.8]–[8.0] and lower J–Ks colors relative to AGB stars. Once again, the two sources thought to have transitioned from the AGB phase align with the post-AGB sample.
Future investigations will expand the carbon-rich sample to include previously unexamined sources, such as AGB stars and PNe from the MW, and incorporate the analysis of oxygen-rich stars to assess whether the CCD diagrams used in this manuscript can reliably trace the final evolutionary sequences of all LIMSs. Additionally, we aim to determine whether an evolutionary sequence similar to that found in the Spitzer colors can also be observed in JWST diagrams. To this end, we plan to investigate JWST color–color diagrams based on synthetic magnitudes, which will help us predict the expected behavior of the studied sources once actual JWST photometric data become available. This will provide further insights into the evolution of these sources through different observational phases.

Author Contributions

Writing—original draft preparation, S.T.; writing—review and editing, E.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

This article is based on work from European Cooperation in Science and Technology (COST) Action NanoSpace, CA21126, supported by COST. EM acknowledges support from the INAF research project LBT–Supporto Arizona Italia.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Main physical parameters of the AGB, post-AGB, and PN sources analyzed in this manuscript. The columns present (1) source ID, (2) sample, (3,4) coordinates, (5) stellar luminosity, (6) stellar effective temperature, (7) amC optical depth at 10 μm, (8) amC dust temperature, and (9) reference paper where the source is discussed.
Table A1. Main physical parameters of the AGB, post-AGB, and PN sources analyzed in this manuscript. The columns present (1) source ID, (2) sample, (3,4) coordinates, (5) stellar luminosity, (6) stellar effective temperature, (7) amC optical depth at 10 μm, (8) amC dust temperature, and (9) reference paper where the source is discussed.
Source IDSampleRAJ2000DEJ2000L/L⊙Teff [K] τ 10 Td [K]Ref.
AGB
SSID 18LMC73.4345− 66.1961740021001.70009001
SSID 167LMC85.3363−69.07891000020002.05009001
SSID 4197LMC76.2703−68.96361170021001.45009001
SSID 4812LMC90.6882−67.37851200021001.10009001
SSID 4519LMC83.014−69.2918670024000.520010801
SSID 4401LMC81.161−70.3992640026001.140010001
SSID 4556LMC83.5668−70.38131160021001.000010001
SSID 4722LMC85.1503−69.88051200021000.900010801
SSID 4391LMC80.9098−66.89021000022000.560010801
SSID 4411LMC81.3313−71.06731100025000.190010801
SSID 145LMC83.6727−69.4419600027000.188010801
SSID 181LMC86.158−67.6161740026000.190010801
SSID 4589LMC83.9156−65.3323850025000.300010801
SSID 4491LMC82.5162−66.8234790022000.980010001
SSID 4435LMC81.5965−69.189660026000.950010001
SSID 4252LMC78.2131−69.6306665022001.000010001
SSID 4003LMC68.8503−66.947570023001.000010001
SSID 4240LMC77.8078−67.6045940022001.000010001
SSID 4062LMC73.4433−68.27041100021001.050010801
SSID 4402LMC81.1821−71.7907740022001.050010001
SSID 4759LMC86.0569−69.7384710022001.10009001
SSID 4238LMC77.7936−67.86951170021001.150010001
SSID 140LMC83.2786−70.5095850022001.20009001
SSID 4309LMC79.7344−67.75131150021001.20009001
SSID 4001LMC68.2391−69.4426785022001.250010001
SSID 4228LMC77.582−69.8309950022000.650010801
SSID 4155LMC75.2684−66.21121130022000.850010801
SSID 4758LMC85.9002−70.1764800023000.850010801
SSID 4150LMC75.1346−72.1506690023000.700010801
SSID 4052LMC73.0841−68.7249700023000.650010801
SSID 4251LMC78.1337−69.26111130022000.620010801
SSID 80LMC79.5136−68.8307800030000.024010801
SSID 4476LMC82.1856−66.23471300027000.101010801
SSID 51LMC76.7895−68.98061100028000.041010801
SSID 4432LMC81.5834−69.6936850033000.005110801
SSID 55LMC77.1097−68.5208710026000.096010801
SSID 4448LMC81.7698−69.6379650029000.031010801
SSID 126LMC82.6875−68.35811200028000.061010801
SSID 4717LMC85.086−66.2456610022001.000010001
SSID 4562LMC83.7239−70.4902950022000.970010001
SSID 4000LMC67.1257−69.5139780023000.790010801
SSID 4478LMC82.1942−71.3202670023000.790010801
SSID 4421LMC81.4664−68.77621090022000.680010801
SSID 4600LMC84.1005−72.6924570023000.850010801
SSID 4225LMC77.5−69.936950024000.230010801
SSID 4211LMC77.1068−68.9002870026000.180010801
SSID 4016LMC71.8171−68.4072910022000.700010801
SSID 4244LMC77.9111−66.85271050022000.510010801
SSID 4256LMC78.3184−68.7361930023000.410010801
SSID 4012LMC70.7389−70.2071650024000.380010801
SSID 4469LMC82.1675−66.2317590025000.235010801
SSID 4034LMC72.7957−69.3373990025000.235010801
SSID 98LMC80.675−69.2572530025000.330010801
SSID 4334LMC80.0808−66.5966550026000.380010801
SSID 4463LMC82.0478−70.5663610027000.210010801
SSID 7LMC72.669−68.9719560028000.050010801
SSID 4604LMC84.1531−69.7895960025000.230010801
SSID 4442LMC81.649−69.1397660025000.265010801
SSID 4293LMC79.3624−68.9163675022000.950010801
SSID 4510LMC82.8048−66.1614970025000.285010801
SSID 4004LMC69.3448−68.4177970022000.670010801
SSID 4447LMC81.712−69.5269550024000.514010801
SSID 4481LMC82.2025−69.8004650023000.700010801
SSID 4593LMC84.0054−66.7776570023000.950010801
SSID 4241LMC77.8314−68.70761070021000.940010801
SSID 4488LMC82.2818−66.97091100028000.035010801
SSID 4206LMC76.6465−70.2806875021001.06009001
SSID 4783LMC87.7082−71.3933970021001.050010801
SSID 4339LMC80.2518−69.3486560023000.900010801
SSID 4736LMC85.2424−69.8869700025000.285010801
SSID 4780LMC87.5281−71.76761050022000.520010801
SSID 4408LMC81.2738−70.1697865022000.600010801
SSID 4093LMC73.8286−68.7751540025000.580010801
SSID 4067LMC73.5067−65.0811950025000.270010801
SSID 156LMC84.8748−69.9656550027000.128010801
SSID 66LMC78.2769−69.1629520025000.320010801
SSID 141LMC83.3275−66.01113500NaN0.291010801
SSID 60LMC77.6182−68.7421390026000.170010801
SSID 4385LMC80.7881−69.2964540026000.225010801
SSID 4002LMC68.4322−70.1642530025000.450010801
SSID 4479LMC82.1965−66.2373540024000.700010801
SSID 4692LMC84.966−70.0214535024001.200010001
SSID 4565LMC83.7647−69.8793600024000.750010801
SSID 103LMC81.022−68.3006360028000.075010801
SSID 3LMC71.6131−68.7963500027000.100010801
SSID 4037LMC72.8146−68.69451330026000.180010801
SSID 4154LMC75.2538−67.58991450022000.540010801
SSID 4811LMC90.6293−67.2131340021001.130010001
SSID 4100LMC73.9123−67.81961460021001.100010001
SSID 4779LMC87.4858−70.88671660021001.20009001
SSID 4246LMC78.0035−70.53991400020001.70009001
SSID 4794LMC89.1616−67.89281720020002.05009001
SSID 4776LMC87.287−71.53531740028000.088010801
SSID 4513LMC82.9205−69.65551590022000.620010801
SSID 4575LMC83.862−69.87441180025000.331010801
SSID 4021LMC72.3269−69.88721550022000.601010801
SSID 9LMC72.9192−68.793515024003.40008701
SSID 4308LMC79.7016−69.5596670024005.60008001
SSID 4489LMC82.4079−72.8314500024004.30008501
SSID 4781LMC87.6091−69.93421000019005.20008001
SSID 4185LMC76.0233−68.3945500024006.20008001
SSID 4171LMC75.6312−68.0934810021006.30007201
SSID 65LMC78.2576−69.5642650024007.10006801
SSID 4299LMC79.5488−70.5075940019005.30006501
SSID 4415LMC81.4193−70.1409400024006.60007501
SSID 190LMC87.2504−70.55621130020003.40002251
SSID 125LMC82.6853−71.7167820020002.30002251
SSID 4109LMC74.1339−68.880827000NaN0.370010801
SSID 4540LMC83.2344−68.213526000NaN0.060110801
SSID 4451LMC81.8506−69.662528800NaN0.910010001
Post-AGB
IRAS04296+3429MW68.2374014134.60344627600072720.0072802
IRAS06530 −0213MW36.6741184362.35611398690078090.00472002
IRAS07134+1005MW109.042744549.99665253550074850.0062102
IRAS08143−4406MW124.01257847−44.26794331540070130.0021502
IRAS08281−4850MW127.41895483−49.00119212790074620.0072802
IRAS12360−5740MW189.72126697−57.94218386630072730.00351802
IRAS13245−5036MW201.90386260−50.86837655710090370.0042202
IRAS14325−6428MW219.14313660−64.69197718703572560.0071902
IRAS14429−4539MW298.21959045−17.03065000900095790.00853602
IRAS19500−1709MW300.4979456132.79242099710082390.00522202
IRAS22272+5435MW353.1865722962.06362764600053250.00872502
IRAS23304+6147MW221.55738007−45.86812587640062760.00852002
J050632.10−714229.8LMC076.633750−71.708278600076000.0032403
J051848.84−700247.0LMC079.703500−70.046389670060000.0082403
J053250.69−713925.8LMC083.211208−71.657167520060000.0062503
J052220.98−692001.5LMC080.587417−69.333750450057500.0152203
J003643.94−723722.1SMC009.183083−72.622806650075000.0022503
J004114.10−741130.1SMC010.308750−74.191694580057500.0072803
J004441.03−732136.0SMC011.170958−73.360000850060000.0023203
J005803.08−732245.1SMC014.512833−73.3791941200065000.0252803
PN
SMP LMC 4LMC070.84117−71.50246400105000−984, 5
SMP LMC 25LMC076.59962−69.0552470060000−1374, 5
SMP LMC 34LMC077.57108−68.80623000107000−1204, 5
SMP LMC 66LMC082.17083−67.5609390046000−1354, 5
SMP LMC 71LMC082.63825−70.74375300164000−1304, 5
SMP LMC 102LMC097.38717−68.05912800140000−1064, 5
SMP LMC 18LMC075.92762−70.1131150050000−1234, 5
Notes: 1—Marini et al. [14], 2—Tosi et al. [20], 3—Tosi et al. [21], 4—Tosi et al. [19], 5—Dell’Agli et al. [22].

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Figure 1. SED analysis of the AGB star SSID 145 from Marini et al. [14] (left panel), the post-AGB star IRAS07134+1005 from Tosi et al. [20] (central panel), and the PN SMP LMC 71 from Tosi et al. [19] (right panel). The black lines represent the synthetic spectra calculated using the radiative transfer code DUSTY and the ionization code CLOUDY. The spectra of SSID 145 and SMP LMC 71, obtained from Spitzer/IRS, as well as the ISO/SWS spectrum for IRAS07134+1005, are shown in green. The magenta line is the HST/STIS spectrum. Observed photometry is represented by blue squares, while the red open circle indicates the synthetic photometry derived by Tosi et al. [19].
Figure 1. SED analysis of the AGB star SSID 145 from Marini et al. [14] (left panel), the post-AGB star IRAS07134+1005 from Tosi et al. [20] (central panel), and the PN SMP LMC 71 from Tosi et al. [19] (right panel). The black lines represent the synthetic spectra calculated using the radiative transfer code DUSTY and the ionization code CLOUDY. The spectra of SSID 145 and SMP LMC 71, obtained from Spitzer/IRS, as well as the ISO/SWS spectrum for IRAS07134+1005, are shown in green. The magenta line is the HST/STIS spectrum. Observed photometry is represented by blue squares, while the red open circle indicates the synthetic photometry derived by Tosi et al. [19].
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Figure 2. Evolution of the SED from an AGB star (green line), a post-AGB star (blue line), and a PN source (orange line).
Figure 2. Evolution of the SED from an AGB star (green line), a post-AGB star (blue line), and a PN source (orange line).
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Figure 3. Left panel: Variation of the logarithmic distance of the dusty layer relative to the central star as a function of dust temperature. The orange stars represent PN sources, the open blue squares correspond to post-AGB stars from the MCs, and the open crossed squares represent post-AGB sources from the MW. Right panel: Optical depth and stellar luminosity for AGB (green triangles) and post-AGB (blue squares) stars. Open blue squares and open crossed squares represent post-AGB sources from the MCs and the MW, respectively. These values were obtained through SED fitting by Marini et al. [14], Tosi et al. [19,20,21], Dell’Agli et al. [22], as described in Section 2.2.
Figure 3. Left panel: Variation of the logarithmic distance of the dusty layer relative to the central star as a function of dust temperature. The orange stars represent PN sources, the open blue squares correspond to post-AGB stars from the MCs, and the open crossed squares represent post-AGB sources from the MW. Right panel: Optical depth and stellar luminosity for AGB (green triangles) and post-AGB (blue squares) stars. Open blue squares and open crossed squares represent post-AGB sources from the MCs and the MW, respectively. These values were obtained through SED fitting by Marini et al. [14], Tosi et al. [19,20,21], Dell’Agli et al. [22], as described in Section 2.2.
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Figure 4. Density plot showing the percentages of amC and SiC relative to the total dust content derived from the DUSTY SED modeling. The green and magenta bars represent the SiC content in AGB and post-AGB stars, respectively, while the orange and blue bars correspond to the amC content in the same groups.
Figure 4. Density plot showing the percentages of amC and SiC relative to the total dust content derived from the DUSTY SED modeling. The green and magenta bars represent the SiC content in AGB and post-AGB stars, respectively, while the orange and blue bars correspond to the amC content in the same groups.
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Figure 5. Left panel: SiC abundance relative to the total dust content as a function of metallicity for post-AGB stars. Open squares represent sources from the MCs, while crossed open squares correspond to those from the MW. Post-AGB stars analyzed using ISO/SWS measurements are shown in orange, whereas those without such measurements are depicted in blue. Right panel: SiC abundance relative to the total dust content as a function of the luminosity derived from the SED modeling. The color coding indicates the percentage of amC relative to the total dust, with open triangles representing AGB stars and filled squares representing post-AGB objects.
Figure 5. Left panel: SiC abundance relative to the total dust content as a function of metallicity for post-AGB stars. Open squares represent sources from the MCs, while crossed open squares correspond to those from the MW. Post-AGB stars analyzed using ISO/SWS measurements are shown in orange, whereas those without such measurements are depicted in blue. Right panel: SiC abundance relative to the total dust content as a function of the luminosity derived from the SED modeling. The color coding indicates the percentage of amC relative to the total dust, with open triangles representing AGB stars and filled squares representing post-AGB objects.
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Figure 6. The distribution based on photometric data of the sample sources is shown in the CCD ([3.6]–[4.5], [5.8]–[8.0]) (left panel) and in the CCD built using the [3.6]–[4.5] (open symbols, or W1–W2, when IRAC data are unavailable) and the J–Ks colors (right panel). Green triangles represent AGB stars, blue squares represent post-AGB stars, and orange stars represent PN sources. The filled blue squares represent post-AGB sources from the MCs, while the crossed blue squares correspond to those from the MW. The magenta and orange lines in the left panel represent the evolutionary models for AGB stars with masses of 3.0 M ⊙ and 2.0 M ⊙ , respectively, while the magenta, dashed line refers to the hypothetical extension of the 3.0 M ⊙ model, as described in Section 4.
Figure 6. The distribution based on photometric data of the sample sources is shown in the CCD ([3.6]–[4.5], [5.8]–[8.0]) (left panel) and in the CCD built using the [3.6]–[4.5] (open symbols, or W1–W2, when IRAC data are unavailable) and the J–Ks colors (right panel). Green triangles represent AGB stars, blue squares represent post-AGB stars, and orange stars represent PN sources. The filled blue squares represent post-AGB sources from the MCs, while the crossed blue squares correspond to those from the MW. The magenta and orange lines in the left panel represent the evolutionary models for AGB stars with masses of 3.0 M ⊙ and 2.0 M ⊙ , respectively, while the magenta, dashed line refers to the hypothetical extension of the 3.0 M ⊙ model, as described in Section 4.
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Figure 7. Same CCD of Figure 6, with color coding applied to represent the effective stellar temperatures (left panel) and dust temperatures (right panel) of the sample sources.
Figure 7. Same CCD of Figure 6, with color coding applied to represent the effective stellar temperatures (left panel) and dust temperatures (right panel) of the sample sources.
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Tosi, S.; Marini, E. Tracing the Evolution of the Emission Properties of Carbon-Rich AGB, Post-AGB, and PN Sources. Astronomy 2025, 4, 2. https://doi.org/10.3390/astronomy4010002

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Tosi S, Marini E. Tracing the Evolution of the Emission Properties of Carbon-Rich AGB, Post-AGB, and PN Sources. Astronomy. 2025; 4(1):2. https://doi.org/10.3390/astronomy4010002

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Tosi, Silvia, and Ester Marini. 2025. "Tracing the Evolution of the Emission Properties of Carbon-Rich AGB, Post-AGB, and PN Sources" Astronomy 4, no. 1: 2. https://doi.org/10.3390/astronomy4010002

APA Style

Tosi, S., & Marini, E. (2025). Tracing the Evolution of the Emission Properties of Carbon-Rich AGB, Post-AGB, and PN Sources. Astronomy, 4(1), 2. https://doi.org/10.3390/astronomy4010002

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