[Paper Review] Spectral analysis of multi mode pulsating sdB stars II. Feige 48, KPD 2109+4401 and PG 1219+534

U. Heber, I. N. Reid+1 more|arXiv (Cornell University)|Sep 11, 2000

Stellar, planetary, and galactic studies5 citations

TL;DR

This study presents a detailed spectral analysis of three multi-mode pulsating subdwarf B (sdB) stars—Feige 48, KPD 2109+4401, and PG 1219+534—using high-resolution Keck HIRES spectra. Employing line-blanketed NLTE and LTE model atmospheres, the authors determine atmospheric parameters, element abundances, and rotational velocities, finding consistent temperature estimates via Balmer and helium line fitting for Feige 48 and KPD 2109+4401, but a significant discrepancy for PG 1219+534, where ionization equilibria suggest higher temperatures than line profile fitting. The key contribution is the confirmation of solar iron abundance in these stars despite strong diffusion processes, supporting theoretical predictions.

ABSTRACT

Three members of the new class of pulsating sdB stars (sdBV or EC 14026 stars) are analysed from Keck HIRES spectra using line blanketed NLTE and LTE model atmospheres. Atmospheric parameters (Teff, log g, He/H), metal abundances and rotational velocities are determined. As is typical for sdB stars, all programme stars are found to be helium deficient, with a He abundance ranging from 1/80 solar for Feige 48 to 1/3 solar for PG 1219+534, probably due to diffusion. Most metals are also depleted. The abundances of C, O, Ne, Mg, Al and Si in the high gravity programme stars KPD 2109+4401 and PG 1219+534 are considerably lower than in the lower gravity stars Feige 48 and PG 1605+072 which could be explained by an equilibrium between gravitational settling and radiative levitation. Surprisingly iron is solar to within error limits in all programme stars irrespective of their gravity, confirming predictions from diffusion calculations. The metal lines are very sharp and allow the microturbulent velocity to be constrained to be lower than 5km/s (KPD2109+4401, PG 1219+534). Also the projected rotational velocities have to be very low (vrot sini<10km/s). For Feige 48 the limits are even tighter (vmicro<=3km/s, vrot sini<=5km/s).

Motivation & Objective

• To determine precise atmospheric parameters (Teff, log g, He/H) for three multi-mode pulsating sdB stars using high-resolution spectroscopy.
• To investigate element abundances, particularly iron, in relation to theoretical diffusion models in extreme horizontal branch stars.
• To assess the impact of non-LTE (NLTE) effects on gravity and temperature determinations in hot, high-gravity stars.
• To constrain microturbulent and rotational velocities from sharp spectral lines, informing asteroseismic modeling.
• To resolve discrepancies in temperature estimates derived from different spectral indicators, especially in PG 1219+534.

Proposed method

• High-resolution echelle spectra (R ≈ 100,000) were obtained with the Keck HIRES spectrograph covering 3700–5200 Å.
• Line-blanketed NLTE and LTE model atmospheres were used to fit observed Balmer and helium lines, enabling consistent temperature and gravity determination.
• Ionization equilibrium of He, N, and Si was used as a temperature diagnostic, cross-checked with Balmer line profile fitting.
• Abundance analysis was performed using equivalent widths and synthetic spectrum fitting, with attention to microturbulent velocity and rotational broadening.
• Projected rotational velocities (vsini) were constrained by fitting sharp spectral lines, with upper limits derived from line broadening.
• Interstellar Ca II K absorption lines were modeled with multiple Gaussian components to correct for foreground absorption.

Experimental results

Research questions

• RQ1 What are the effective temperatures, gravities, and helium abundances of Feige 48, KPD 2109+4401, and PG 1219+534 as derived from spectral fitting?
• RQ2 Why do ionization equilibria of nitrogen and helium yield significantly higher effective temperatures than Balmer line fitting for PG 1219+534?
• RQ3 How do NLTE and LTE model atmospheres compare in determining gravity and temperature for these hot, high-gravity stars?
• RQ4 To what extent are metal abundances depleted in these sdB stars, and why is iron found to be solar despite strong diffusion processes?
• RQ5 What are the upper limits on microturbulent velocity and projected rotational velocity, and how do they affect asteroseismic modeling?

Key findings

• For Feige 48 and KPD 2109+4401, temperature estimates from Balmer line fitting and ionization equilibria of He, N, and Si agree within ±300 K, indicating consistent atmospheric parameters.
• For PG 1219+534, ionization equilibria of nitrogen and helium yield effective temperatures of 36,800 K and 34,400 K, respectively, significantly higher than the 33,200 K derived from Balmer line profile fitting.
• NLTE model atmospheres yield gravities that are up to 0.1 dex lower than LTE results, indicating a systematic NLTE correction in high-gravity sdB stars.
• All three stars are helium deficient, with He/H ranging from 1/80 solar (Feige 48) to 1/3 solar (PG 1219+534), likely due to gravitational settling and diffusion processes.
• Metal abundances of C, O, Ne, Mg, Al, and Si are significantly depleted in high-gravity stars (KPD 2109+4401 and PG 1219+534), consistent with equilibrium between gravitational settling and radiative levitation.
• Iron abundance is solar within error limits in all three stars, confirming theoretical predictions from Charpinet et al. (1997) that diffusion can produce solar iron abundances in pulsating sdB star envelopes.