Insight on AGB Mass-Loss and Dust Production from PNe

Abstract

The asymptotic giant branch (AGB) phase, experienced by low- and intermediate-mass stars (LIMSs), plays a crucial role in galaxies due to its significant dust production. Planetary nebulae (PNe) offer a novel perspective, providing valuable insights into the dust production mechanisms and the evolutionary history of LIMSs. We selected a sample of nine PNe from the Large Magellanic Cloud (LMC), likely originating from single stars. By modeling their spectral energy distributions (SEDs) with photoionization techniques, we successfully reproduced the observed photometric data, spectra, and chemical abundances. This approach enabled us to constrain key characteristics of the central stars (CSs), dust, and gaseous nebulae, which were then compared with predictions from stellar evolution models. By integrating observational data across ultraviolet (UV) to infrared (IR) wavelengths, we achieved a comprehensive understanding of the structure of the PNe in our sample. The results of the SED analysis are consistent with evolutionary models and previous studies that focus on individual components of the PN, such as dust or the gaseous nebula. Our analysis enabled us to determine the metallicity, the progenitor mass of the CSs, and the amount of dust and gas surrounding the CSs, linking these properties to the previous AGB phase. The PN phase provides critical insights into the physical processes active during earlier evolutionary stages. Additionally, we found that higher progenitor masses are associated with greater amounts of dust in the surrounding nebulae but lower amounts of gaseous material compared to sources with lower progenitor masses.

1. Introduction

The study of LIMSs, characterized by masses ranging from 1 to 8 ${\mathrm{M}}_{\odot }$, has witnessed growing interest from the scientific community with the aim of assessing the role that LIMSs play in enriching the interstellar medium. Indeed, during the AGB phase, the stars lose a significant amount of mass via stellar wind, and are therefore characterized by expanded circumstellar envelopes that are sites of substantial dust formation. The material ejected reflects the physical processes active throughout the AGB evolution, during which the stars significantly alter their surface chemical abundances [1,2]. For instance, the third dredge-up (TDU) [3] increases the surface abundance of ¹²C in stars with masses below ~$4 {\mathrm{M}}_{\odot }$, while hot bottom burning (HBB) [4] primarily depletes ¹²C and synthesizes ¹⁴N in stars with masses above ~$4 {\mathrm{M}}_{\odot }$.

Knowledge of the surface chemical composition, in particular the excess of C over O, is crucial for predicting the mineralogy of dust formed throughout the AGB phase. Indeed, stars that experience repeated TDU episodes can reach the carbon star stage, producing carbon-rich dust. In contrast, stars undergoing HBB, characterized by O-rich atmospheres, produce silicate-rich dust and are known as oxygen stars.

The material expelled during the AGB phase remains observable in the subsequent PN stage, marked by chemical signatures that act as precise indicators of the processes involved in stellar evolution, nucleosynthesis, and dust formation during the AGB phase [5,6]. Building on this foundation, Dell’Agli et al. [7] and Tosi et al. [8] investigated a range of spectral regions, from UV to IR wavelengths, to uncover key characteristics of the CS, as well as the dust and gas components within PNe. These findings were subsequently compared with ATON [9,10] theoretical model predictions, enabling the tracing of the progenitor mass of each source [8] and establishing connections between the physical conditions of the gas and dust in the PN environment and the final phases of the AGB stage [11].

The present manuscript aims to provide a comprehensive synthesis of the results gathered by our team, with the goal of constructing an evolutionary framework that describes the transition of low- and intermediate-mass stars (LIMSs) from the AGB phase to the PN stage. We begin with a brief introduction to the sample and methodology, followed by a discussion of new findings that build on the results presented in Tosi et al. [8].

2. Sample and Methodology

We selected targets from the Large Magellanic Cloud (LMC), which, due to its well-constrained distance [12], provides an optimal setting for accurately determining luminosities and initial stellar masses. We focused on sources exhibiting round or elliptical morphologies, as identified through Hubble Space Telescope (HST) imaging [13,14,15,16], to exclude close binary systems whose evolution might deviate significantly from that of single stars. Our final sample included seven LMC PNe with infrared spectra, enabling a direct evaluation of the dust present within the nebulae.

The analysis presented here is based on infrared spectra obtained using the Spitzer Infrared Spectrograph (IRS; LS07), ultraviolet spectra from the HST/Space Telescope Imaging Spectrograph (STIS, hereafter LS05) [17], and complementary photometric data [18,19,20]. Additionally, where available, carbon and oxygen abundance measurements were taken from the LS05, Leisy and Dennefeld [21], and Henry et al. [22]. We note that all the photometric data and the HST spectra were corrected for galactic foreground and LMC extinction using the parametrization provided in Stanghellini et al. [17].

The observational data points were essential for conducting photoionization and radiative transfer modeling using the spectral synthesis code CLOUDY v22.02 [23], facilitating the production of reliable synthetic SEDs. As described in Tosi et al. [8], through the reconstruction of the SED from the UV to the IR, we examined in detail the contributions of the CS, gaseous nebula, and warm dust peaking around 100 K.

The SED characterization provided crucial parameters for comparison with stellar evolution models, including the luminosity (L) and effective temperature of the central stars (T_eff), as well as the mineral composition of the surrounding warm dust (carbon- or oxygen-rich) and the amount of gas and warm dust surrounding the central star (${\mathrm{M}}_{\mathrm{gas}}$ and ${\mathrm{M}}_{\mathrm{dust}}$, respectively), along with their ratio (dust-to-gas ratio, ${\delta }_{\mathrm{C}}$). In this process, we leveraged insights from previous studies that analyzed the dust features of most of the sources in our sample [24,25,26]. These insights were instrumental in enhancing the reliability of our dust modeling, which, alongside our characterization of the CS and gas components, allowed us to develop an overall view of the entire PN.

This comprehensive perspective was essential for comparisons with the results from stellar evolution models, enabling a deeper understanding of the relationships between observed data and theoretical predictions. To facilitate robust comparisons with stellar evolution models, we first needed to map the evolutionary path of each CS. This process began by determining the metallicity of each source, achieved by comparing observed oxygen abundances with predictions from the ATON stellar evolution code [9]. We then inferred the progenitor mass using a combined approach: first, by comparing observed carbon abundances with ATON model predictions (see Figure 7 [8]), and second, by matching the luminosity and effective temperature derived from SED modeling to ATON evolutionary tracks (see Figure 5 [8]). After establishing the evolutionary history, we were able to compare observed dust masses to theoretical values, leveraging the AGB dust formation model incorporated in ATON through the formalism of [10,27].

3. Discussion

Through photoionization modeling and comparisons with ATON stellar evolution models, we identified seven sources that exhibit signs of repeated TDU episodes and the presence of carbon-rich dust (see [8] for more details on this procedure). In the left panel of Figure 1, we explore the relationship between the progenitor’s mass ($\mathrm{M}/{\mathrm{M}}_{\odot }$, measured at the beginning of the AGB phase), the dust-to-gas ratio of warm dust (${\delta }_{\mathrm{C}}$), and the effective temperature (T_eff). Different symbols represent the varying metallicities of each PN in our sample (triangles, $\mathrm{Z}=8\times {10}^{-3}$; circles, $\mathrm{Z}=4\times {10}^{-3}$; and squares, $\mathrm{Z}=2\times {10}^{-3}$).

Several studies have demonstrated that more massive carbon-rich stars can produce larger amounts of dust compared to their lower mass counterparts, due to the greater number of TDU episodes during the AGB phase [28,29,30]. Given that the dust observed in the PN phase reflects prior AGB evolution [7], we would expect to see a similar relationship within our sample. Consistent with this expectation, we observe that more massive stars are characterized by higher values of ${\delta }_{\mathrm{C}}$ (see left panel, Figure 1). However, this behavior appears to be influenced by additional mechanisms. For instance, considering two sources of similar metallicity, such as SMP LMC 71 and SMP LMC 4, we would expect the more massive star to exhibit higher dust quantities during the PN phase based on their mass loss histories. As illustrated by the color-coding in Figure 1, the sources with the most significant dust reduction also have the highest T_eff, suggesting the possibility of active dust destruction events that considerably diminished the amount of dust surrounding the central star in our hottest sources. Various mechanisms, such as spallation and sputtering, have been proposed by the scientific community to explain the observed lower amounts of dust compared to model predictions [31,32]. Nevertheless, as highlighted by Otsuka et al. [33], incorporating the emission from cold dust, peaking around 20 K, into the SED modeling could yield a higher dust-to-gas ratio. This approach could narrow the gap between observed dust quantities and model predictions.

Given the larger envelopes typical of more massive AGB stars, we might expect more massive PNe to exhibit greater gas masses. However, the right panel of Figure 1 reveals the opposite: the nebular mass surrounding these more massive sources is the lowest within the sample. Possible explanations include efficient mass loss during the AGB phase, which left minimal traces of the gas ejected during this period, or a weakening in the emission explored in the present manuscript, which makes detection challenging. In contrast, this reduction in the gaseous nebula emission is not observed in lower mass stars, for which the SED analysis indicates gas masses consistent with those expected to be produced during the final thermal pulse.

Further insights can be drawn from Figure 2, which illustrates the amount of carbon-rich dust (M_dust) in the PNe of the sample as a function of the progenitor’s mass, with different colors representing various metallicities. Solid squares depict the dust quantities derived from SED modeling, while open symbols correspond to predictions from the ATON stellar evolution models. Specifically, the open circle represents the total dust produced throughout the entire AGB phase, the open triangle indicates the dust produced during the last two thermal pulses, and the open square reflects the dust generated during the final thermal pulse.

As shown in Figure 2, sources with metallicities above $\mathrm{Z}>2\times {10}^{-3}$ and higher progenitor masses tend to exhibit greater dust masses. However, the two hottest sources in our sample (SMP LMC 102 and SMP LMC 71) show dust masses that are significantly lower than those predicted by synthetic models. Regarding the metal-poor sources (Z $=2\times {10}^{-3}$), the M_dust vs. M trend appears to deviate, except for SMP LMC 18, which, as discussed in Tosi et al. [8], underwent a late thermal pulse that altered its dust composition. We propose that the larger amounts of dust observed in SMP LMC 25 and SMP LMC 34, compared to ATON predictions, could have a similar explanation to that discussed in Tosi et al. [34] and Tosi et al. [35]. In those studies, reproducing the observed dust around metal-poor, low-mass post-AGB stars necessitated increasing the mass loss rate at the end of the AGB phase by a factor of three or four, typically reaching values of $3-4\times {10}^{-5}{\mathrm{M}}_{\odot }/$yr. This enhanced mass loss rate is attributed to the higher availability of carbon for condensation into carbonaceous dust, which is related to the lower oxygen abundance due to the lower metallicity compared to metal-rich sources.

We conclude that the dust and gas surrounding the central star in PNe reflect the star’s previous evolutionary history, providing valuable insights into key aspects of LIMSs, such as the mass loss and dust production history. For a more detailed discussion of the results presented in this study, refer to Dell’Agli et al. [7], Tosi et al. [8], and Ventura et al. [11].