[Paper Review] Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements

T. Ryabchikova, Mikhail Sachkov+11 more|ArXiv.org|Nov 28, 2006

Stellar, planetary, and galactic studies27 references36 citations

TL;DR

This study presents the first direct measurement of pulsation wave propagation in the atmosphere of the roAp star HD 24712 using simultaneous high-resolution spectroscopy and photometry from MOST. It reveals phase shifts between radial velocity and photometric pulsation maxima for different elements, indicating varying line formation depths, and derives a pulsation wave speed slightly below the sound speed, providing new constraints on atmospheric structure and pulsation dynamics in roAp stars.

ABSTRACT

We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m/s) for REE lines and the Halpha core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.

Motivation & Objective

• To investigate the atmospheric structure of the rapidly oscillating Ap star HD 24712 using high-resolution spectroscopic observations.
• To link radial velocity variations with photometric light variations observed by the MOST satellite to understand pulsation dynamics.
• To determine the propagation speed of pulsation waves in the outer atmosphere of a roAp star for the first time.
• To assess differences in pulsation amplitudes and phases across spectral lines of various elements to infer atmospheric stratification.
• To test the hypothesis of rare-earth element (REE) concentration above the hydrogen line-forming layers using phase and amplitude variations near the Balmer jump.

Proposed method

• Acquired time-resolved spectroscopic data from multiple ground-based telescopes (CFHT, NOT, ESO, TNG) and simultaneous photometric data from the Canadian MOST space telescope.
• Performed radial velocity (RV) measurements on individual spectral lines of various elements (REE, Hα, Mg, Si, Ca, Fe-peak, Nd, Eu, Tb) across multiple observing seasons (2001–2004).
• Conducted Fourier analysis on RV and photometric light curves to identify pulsation frequencies and extract amplitudes and phases.
• Calculated phase shifts between photometric maxima and RV maxima for different ions to infer line formation depths in the stellar atmosphere.
• Prewhitened RV data with dominant pulsation frequencies to isolate residual signals and improve spectral resolution.
• Used the observed phase lags and amplitude variations to estimate the pulsation wave speed and assess atmospheric stratification.

Experimental results

Research questions

• RQ1 What is the propagation speed of the pulsation wave in the outer atmosphere of HD 24712, and how does it compare to the sound speed?
• RQ2 How do the radial velocity amplitudes and phases of spectral lines formed in different atmospheric layers vary across different chemical elements?
• RQ3 Do spectral lines of rare-earth elements (REEs) formed before and after the Balmer jump show different pulsation amplitudes or phases, supporting the hypothesis of REE concentration above the H-line formation region?
• RQ4 What is the relationship between photometric variations and radial velocity variations for different ions, and what does this reveal about atmospheric structure?
• RQ5 Can radial velocity measurements of individual spectral lines be used to perform 3D tomography of the pulsating atmosphere of HD 24712?

Key findings

• The pulsation wave propagation speed in the outer atmosphere of HD 24712 was directly estimated at approximately 90% of the local sound speed, indicating subsonic propagation.
• Radial velocity amplitudes for REE lines and the Hα core were found to be large (150–400 m s⁻¹), while lines of Mg, Si, Ca, and Fe-peak elements showed nearly constant velocities with minimal pulsation amplitude.
• Phase shifts between photometric and radial velocity pulsation maxima were directly measured for different ions, indicating that lines form at different atmospheric depths.
• No significant difference in pulsation amplitudes or phases was found between REE lines formed before and after the Balmer jump, supporting the hypothesis that REEs are concentrated in the upper atmosphere above the H-line formation region.
• The observed phase lags and amplitude variations across different ions provide strong evidence for a 3D atmospheric structure, enabling the potential for 3D tomography of the pulsating atmosphere.
• The simultaneous MOST photometry and spectroscopy allowed for the first direct calibration of atmospheric dynamics with photometric light curves, enhancing the accuracy of pulsation modeling.