Spectroscopic Sleuthing: Unmasking Chemically Peculiar δ Scuti Stars

In this study, Kahraman Alıçavuş and collaborators set out to verify whether certain δ Scuti stars, pulsating stars of types A–F, are truly “chemically peculiar,” meaning they have unusual surface element abundances compared to the Sun. Earlier classifications often relied on limited data, which can be misleading. The team selected ten stars previously flagged as chemically peculiar, ensuring that each had both public high-resolution spectra and high-quality light curves from NASA’s TESS mission.

Chemical Peculiarity in Context

The authors first reviewed the concept of chemical peculiarity, which includes metallic-line A stars (Am), Ap stars with strong magnetic fields, and λ Bootis stars with metal deficiencies. These differences arise from processes like atomic diffusion or accretion of interstellar material. Because δ Scuti pulsations occur in the same temperature range as many of these stars, they are an excellent laboratory for studying how surface chemistry might affect stellar oscillations.

Data Collection and Spectral Classification

The team gathered high-resolution spectra from instruments like HARPS, SOPHIE, and ELODIE, and checked for contamination or binarity. They performed spectral classification, looking at hydrogen, calcium, and metal lines separately, before conducting a more detailed atmospheric parameter analysis. Using both photometric color indices and spectral fitting, they determined each star’s effective temperature, surface gravity, and projected rotation speed. The most definitive test came from measuring abundances of individual elements by matching synthetic spectra to observed ones.

Pulsation Analysis

Next, they used TESS light curves with 120-second cadence to search for oscillations. All ten stars showed δ Scuti-type pulsations, but five also had low-frequency γ Doradus-type oscillations, making them “hybrid” pulsators. The team calculated the pulsation constant (Q) for the dominant frequency in each star, which helps constrain the type of oscillation mode present.

Determining Fundamental Parameters

By placing the stars on the Hertzsprung–Russell diagram and comparing them with evolutionary models, the authors estimated their masses (1.5–2.5 times the Sun’s) and ages (from ~0.1 to 1.6 billion years). All were located inside or near the cool edge of the δ Scuti instability strip.

Key Findings on Chemical Peculiarity

The main result was that only three stars, AU Scl and FG Eri (both Am stars) and HZ Vel (a λ Bootis star), were confirmed as chemically peculiar. The other seven were chemically normal despite prior claims. One of the Am stars, FG Eri, stood out as an unusual case because of its position on the H–R diagram and its relatively large pulsation constant, suggesting it may be a rare transitional object.

Implications and Conclusions

This work demonstrates the necessity of detailed spectroscopic abundance analysis for correctly identifying chemically peculiar stars. It also reinforces recent evidence that chemically peculiar A stars can indeed pulsate like δ Scuti stars, challenging earlier ideas that such pulsations would be suppressed. These findings add carefully verified examples to the growing catalog of pulsating chemically peculiar stars, which are valuable for testing theories of stellar structure and evolution.

Source: Alıçavuş