Technetium-rich M Stars: Prime diagnostics of recent third dredge-up events on the Asymptotic Giant Branch

TL;DR

This study analyzes high-resolution spectra of 39 M-type stars to investigate Technetium (Tc) enrichment and its relation to third dredge-up in AGB evolution. By identifying Tc-rich and Tc-poor stars through Tc I lines and examining TiO/ZrO band indices, the authors compare Tc-rich M stars with Tc-rich/poor S stars, using nucleosynthesis models (STAREVOL) to trace Tc and Zr across thermal pulses. Their results indicate Tc-rich M stars possess subsolar metallicities and, at roughly $1.0$–$2.0 M_igodot$, occupy HR diagram regions beyond the TDU onset, overlapping with Tc-rich S stars but exhibiting stronger TiO bands. Spatial and kinematic analyses suggest Tc-rich M stars are an older, low-mass AGB population, though the data reveal a complex picture with potential classification biases and a need for broader abundance studies to fully reconcile observations with nucleosynthesis predictions.

Abstract

Context. Technetium (Tc)-rich M-type stars have been known for over 45 years. However, the origin of Tc in these stars, particularly its detection without the concomitant detection of other s-process elements, typically produced during the asymptotic giant branch (AGB) s-process nucleosynthesis, remains poorly understood. Technetium was first identified in the spectra of S-type stars (which exhibit prominent ZrO bands) in 1952. The simultaneous enrichment of both Zr and Tc is well understood within the framework of s-process nucleosynthesis, which occurs during the AGB phase. However, despite being known for 45 years, Tc-rich M stars remain an enigma, as M-type stars are typically not enriched in heavy elements. Aims. This study aims at analyzing high-resolution spectra of a large sample of M-type stars to examine their spectral characteristics, and to compare their spectral properties with those of Tc-rich S-type stars in an attempt to understand the origin of their difference. Methods. We define a robust classification scheme to assign M stars to the Tc-rich or Tc-poor class. We compute nucleosynthesis models to trace the evolution of Zr and Tc abundances across successive thermal pulses during the AGB phase. We further analyze spectral indices measuring the depth of the TiO and ZrO bands as well as the wavelength of the Tc blend on both synthetic and observed spectra. Results. The Tc lines in Tc-rich M stars are similar to those in S stars. However, Tc-rich M stars exhibit stronger TiO bands than S stars while displaying similarly strong ZrO bands. Spectral synthesis, together with location in the HR diagram and spatial properties suggest that Tc-rich M stars may have slightly lower metallicity and lower masses than Tc-rich S stars.

Figures (10)

• Figure 1: Central wavelengths of the Tc I blends at $\lambda {4238}$ and $\lambda {4262}$ Tc I. The grey dashed lines delineate the boundaries between the Tc-poor zone (upper left rectangle) and the Tc-rich region Vaneck1999. Observed data points are shown as squares and triangles, corresponding to measurements of ($\lambda_{4238}$, $\lambda_{4262}$) in spectra of Tc-rich and Tc-poor M- and S-type stars, as labeled. Synthetic spectra were generated for two effective temperatures ($T_{\mathrm{eff}}$ = 2800 K and 4000 K), metallicities ([Fe/H] = 0.0 and -0.5), two microturbulence velocities ($\chi_t = 2$ and $3$ km/s), and a range of technetium abundances (as indicated by the color scale on the side). From these synthetic spectra, the central wavelengths of the $\lambda4238$ and $\lambda4262$ Tc I blends were measured. The results are represented as colored circles, connected by lines that group models with the same atmospheric parameters, as indicated by the labels.
• Figure 2: The band strength indices $B_{\mathrm{TiO}}$ and $B_{\mathrm{ZrO}}$ are shown for Tc-rich and Tc-poor M-type stars from this study, as well as for Tc-rich (intrinsic) and Tc-poor (extrinsic) S-type stars from abc2021. They are represented by filled or open squares and triangles, as indicated in the legend. Corresponding spectral indices were also computed from synthetic spectra based on model atmospheres with varying stellar parameters, including effective temperature (color-coded according to the colour bar), metallicity, C/O ratio, s-process enrichment [s/Fe], alpha-elements enrichment [$\alpha$/Fe] and microturbulence $\chi_t$. Band strengths derived from synthetic spectra are shown as crosses, with models of [Fe/H]$= 0$ connected by dashed lines and those with [Fe/H]$=-1$ connected by solid lines.
• Figure 3: Locations of the Tc-rich M stars AA Cam, OP Her and RX Lac (filled triangles) in the HR diagram. The HR diagrams also include STAREVOL evolutionary tracks corresponding to the closest available metallicities ([Fe/H] = –0.5 or –1.0) with [O/Fe] = 0 and 0.4, hence AA Cam and OP Her appear in the left subpanels, while RX Lac is shown in the right subpanels. The black dashed line represents the predicted onset of the TDU, that is, the lowest stellar luminosity following the first occurrence of a TDU episode. For comparison, Tc-rich S stars (empty triangles) from abc2021 and Tc-poor S stars (empty squares) from shetye2018 with metallicities around [Fe/H] $=-0.5$ are included.
• Figure 4: Nucleosynthesis predictions for Tc and Zr abundances during successive TDU events are shown as circles along the model tracks. Measured abundances in Tc-rich S stars (filled squares), from abc2021, are color-coded by C/O ratio. Blue crosses highlight the two Tc-rich MS stars (AA Cam and $o^1$ Ori) in their sample. The grey box highlights the expected locus of Tc-rich M stars, where the Tc range reflects the detection threshold for Tc-rich classification (see Fig. \ref{['fig:Tclinebarycenter']}), and the upper limit of the Zr range corresponds to the black dashed line in Fig. \ref{['fig:ZrOTiO']} with [s/Fe] = 0.2, which separates M-type from S-type stars.
• Figure 5: Cumulative distribution of the height above the Galactic plane |Z| (in kpc). The intrinsic and extrinsic S star samples are taken from Abia2022, as are N-type carbon stars. The solid lines correspond to the observed distributions and the dashed line to the fit corresponding to an exponentially-decreasing spatial $|Z|$ distribution. The meaning of colors are given in the insert.