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Chemistry in the High expansion-velocity C-rich evolved star AFGL2233. Isotopic ratios, peculiarities and evolutionary status

G. Quintana-Lacaci, M. Agúndez, L. Velilla-Prieto, J. Alcolea, A. Castro-Carrizo, J. P. Fonfría, J. Cernicharo

TL;DR

This study re-evaluates AFGL2233, a rare high-expansion-velocity carbon star, using a comprehensive 3 mm/1 mm line survey with IRAM 30m, Herschel/HIFI, and interferometric maps to map the molecular inventory and isotopic composition of its circumstellar envelope. A Gaia DR3 distance of 1.236 kpc sets a luminosity of about $2\times10^{4}\,L_\odot$ and supports a massive initial mass of $M_{init}\approx4.5$–$9\,M_\odot$, consistent with a massive AGB or super-AGB evolutionary state. The chemistry is broadly C-rich but shows notable NH$_3$, H$_2$O, and SiN enhancements, and a wide range of isotopic ratios ($^{12}$C/$^{13}$C from 7 to 55) with region-dependent variations. LVG and rotational-diagram analyses reveal a multi-shell CSE with hot and cold CO components, a hollow-shell morphology for C$_2$H and C$_3$N, and high abundances for several N- and O-bearing species, suggesting either residual HBB processing, shock-induced chemistry, or binary-driven photochemistry. Overall, AFGL2233 likely represents a complex, high-mass evolved star whose current C-rich chemistry coexists with indicators of nitrogen enrichment and possible binary interaction, underscoring the need for high-resolution mappings to unravel its evolutionary history.

Abstract

High expansion velocity carbon stars (HVCs) are a rare class of evolved stars whose circumstellar envelopes (CSEs) combine C-rich chemistry with unusually high expansion velocities typical of O-rich massive evolved stars. AFGL2233 has been proposed as a high-mass evolved object that exhausted hot-bottom burning. Studying its chemistry is essential to understand the nature and evolution of these objects. We characterize the chemical composition and isotopic ratios of the CSE of AFGL2233 and investigate chemical peculiarities, including the presence of N- and O-bearing species in a C-rich environment. We carried out a complete line survey at 3 mm and 1 mm using the IRAM 30m telescope, complemented by Herschel/HIFI FIR observations and interferometric maps of SiO, C2H, and HCN. Molecular emission was analyzed using rotational diagrams and radiative transfer modeling under the LVG approximation. Column densities and fractional abundances were derived for more than 30 molecular species, including isotopologues, and compared with other evolved stars. The Gaia DR3 distance of 1.236 kpc implies a luminosity of ~2 Lsun, consistent with an initial mass of 4.5-9 Msun. The molecular inventory confirms C-rich chemistry but reveals unusually high abundances of NH3, H2O, and SiN. The isotopic ratios vary among species, with 12C/13C ranging from 7 to 55. The C2H/C4H ratio is abnormally high compared with C-rich AGB stars. The presence of SiN and high NH3 may indicate N-enrichment or the influence of a companion. AFGL2233 is likely a high-mass AGB or super-AGB star with a complex evolutionary history involving nucleosynthesis, shocks, and possible binary interaction.

Chemistry in the High expansion-velocity C-rich evolved star AFGL2233. Isotopic ratios, peculiarities and evolutionary status

TL;DR

This study re-evaluates AFGL2233, a rare high-expansion-velocity carbon star, using a comprehensive 3 mm/1 mm line survey with IRAM 30m, Herschel/HIFI, and interferometric maps to map the molecular inventory and isotopic composition of its circumstellar envelope. A Gaia DR3 distance of 1.236 kpc sets a luminosity of about and supports a massive initial mass of –, consistent with a massive AGB or super-AGB evolutionary state. The chemistry is broadly C-rich but shows notable NH, HO, and SiN enhancements, and a wide range of isotopic ratios (C/C from 7 to 55) with region-dependent variations. LVG and rotational-diagram analyses reveal a multi-shell CSE with hot and cold CO components, a hollow-shell morphology for CH and CN, and high abundances for several N- and O-bearing species, suggesting either residual HBB processing, shock-induced chemistry, or binary-driven photochemistry. Overall, AFGL2233 likely represents a complex, high-mass evolved star whose current C-rich chemistry coexists with indicators of nitrogen enrichment and possible binary interaction, underscoring the need for high-resolution mappings to unravel its evolutionary history.

Abstract

High expansion velocity carbon stars (HVCs) are a rare class of evolved stars whose circumstellar envelopes (CSEs) combine C-rich chemistry with unusually high expansion velocities typical of O-rich massive evolved stars. AFGL2233 has been proposed as a high-mass evolved object that exhausted hot-bottom burning. Studying its chemistry is essential to understand the nature and evolution of these objects. We characterize the chemical composition and isotopic ratios of the CSE of AFGL2233 and investigate chemical peculiarities, including the presence of N- and O-bearing species in a C-rich environment. We carried out a complete line survey at 3 mm and 1 mm using the IRAM 30m telescope, complemented by Herschel/HIFI FIR observations and interferometric maps of SiO, C2H, and HCN. Molecular emission was analyzed using rotational diagrams and radiative transfer modeling under the LVG approximation. Column densities and fractional abundances were derived for more than 30 molecular species, including isotopologues, and compared with other evolved stars. The Gaia DR3 distance of 1.236 kpc implies a luminosity of ~2 Lsun, consistent with an initial mass of 4.5-9 Msun. The molecular inventory confirms C-rich chemistry but reveals unusually high abundances of NH3, H2O, and SiN. The isotopic ratios vary among species, with 12C/13C ranging from 7 to 55. The C2H/C4H ratio is abnormally high compared with C-rich AGB stars. The presence of SiN and high NH3 may indicate N-enrichment or the influence of a companion. AFGL2233 is likely a high-mass AGB or super-AGB star with a complex evolutionary history involving nucleosynthesis, shocks, and possible binary interaction.
Paper Structure (26 sections, 8 equations, 34 figures, 8 tables)

This paper contains 26 sections, 8 equations, 34 figures, 8 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. HIFI Data
  4. IRAM 30m data
  5. IRAM PdBI data
  6. 2019 results revisited. Gaia DR3.
  7. Spectral energy distribution (SED).
  8. Molecular emission model.
  9. Results
  10. Structure of the ejecta
  11. Line identification
  12. Abundance and temperature estimate
  13. CO & $^{13}$CO.
  14. HCN, HC$_3$N, CN and HNC
  15. C$_3$N & $^{13}$CCCN.
  16. ...and 11 more sections

Figures (34)

  • Figure 1: Spectra of the integrated flux obtained with IRAM PdBI towards AFGL 2233.
  • Figure 2: SED fitting obtained with the new distance estimate for AFGL 2233. Green and blue curves correspond to the two components, and the red curve to the sum of both components.
  • Figure 3: Top-left: CO $J=1-0$ line profile obtained with the IRAM 30m telescope for AFGL 2233. Top-right: CO $J=2-1$ line profile obtained with the IRAM 30m telescope for AFGL 2233. Bottom-left: Azimuthally averaged profile of the CO $J=1-0$ molecular emission of the central velocity channel obtained towards AFGL 2233 with PdBI. Bottom-right: Azimuthally averaged profile of the CO $J=2-1$ molecular emission of the central velocity channel obtained towards AFGL 2233 with PdBI. The black lines correspond to the observations and the red lines to the results of the model.
  • Figure 4: Azimuthal averaged intensity of the different transitions observed with PdBI for the central velocity channel, normalized to its maxium intensity.
  • Figure 5: Fitting of the HCN transitions observed, taking into account the IR-pumping.
  • ...and 29 more figures