[Paper Review] Rotation of stars in NGC 6134: a comparison of Delta Scuti stars and non-variable stars

Michael Bo Rasmussen, H. Bruntt+3 more|ArXiv.org|Jun 4, 2002

Stellar, planetary, and galactic studies14 references7 citations

TL;DR

This study investigates the rotational velocities (v sin i) and chemical abundances of δ Scuti stars and non-variable A-type stars in the metal-rich open cluster NGC 6134 using high-resolution spectroscopy and Δa photometry. It finds no significant correlation between pulsational variability and rotation, confirms a high metallicity of [Fe/H] = +0.38 ± 0.05, and identifies one δ Scuti star as a marginal Am-type candidate, suggesting that slow rotation and chemical peculiarity do not preclude pulsation.

ABSTRACT

We present results of spectroscopic observations of selected stars in the southern open cluster NGC 6134. We have determined the rotational velocities of the six known Delta Scuti Stars in NGC 6134 as well as several other non-variable stars with similar colour temperature in order to investigate if v sin i and variability is somehow connected: we find no such correlation. We also compare the distribution of v sin i of Delta Scuti Stars and non-variable stars with four other well-studied open clusters to look for any systematic behaviour, but we find no conclusive evidence for v sin i and variability to be connected. We have also used the spectra to carry out an abundance analysis of the Delta Scuti Stars in NGC 6134 to confirm the high metal content of the cluster. We find [Fe/H] = +0.38 +/- 0.05 which is in agreement with the result obtained from Stromgren photometry. We also present Delta-a photometry of the cluster, but we find no chemical peculiar stars based on this index.

Motivation & Objective

• To determine the rotational velocities (v sin i) of six known δ Scuti stars and non-variable A-type stars in NGC 6134 to test for a correlation between pulsational variability and rotation.
• To perform an abundance analysis of δ Scuti stars to verify the cluster's metallicity derived from Strömgren photometry.
• To search for chemically peculiar stars, particularly Am or Ap-type stars, using Δa photometry.
• To compare the v sin i distributions of δ Scuti stars and non-variable stars across five well-studied open clusters to detect systematic trends.
• To assess the suitability of δ Scuti stars in NGC 6134 for future asteroseismic modeling, especially those with low rotation rates.

Proposed method

• Spectra were obtained using the DFOSC instrument on the Danish 1.54m telescope at ESO’s La Silla Observatory, employing an Echelle spectrograph with grism #9 and #11 for high-resolution dispersion.
• Spectral reduction and extraction were performed using IRAF tools, including apall for aperture extraction and manual background correction to handle high sky background levels.
• Stellar parameters, including effective temperature and surface gravity, were calibrated using standard spectroscopic techniques and synthetic spectral grids provided by Weiss.
• Rotational velocities (v sin i) were derived by fitting rotational broadening profiles to observed spectral lines, comparing with synthetic spectra.
• Abundance analysis was conducted by measuring equivalent widths of Fe I and Fe II lines and deriving [Fe/H] through ionization equilibrium and curve-of-growth techniques.
• Δa photometry was applied to detect chemically peculiar stars, with the index Δa sensitive to surface abundance anomalies in A-type stars.

Experimental results

Research questions

• RQ1 Is there a significant correlation between rotational velocity (v sin i) and pulsational variability in δ Scuti stars in NGC 6134?
• RQ2 Do δ Scuti stars in NGC 6134 exhibit chemical abundance anomalies consistent with Am or Ap-type stars?
• RQ3 Is the metallicity of NGC 6134, previously estimated from Strömgren photometry, confirmed by spectroscopic abundance analysis?
• RQ4 How do the v sin i distributions of δ Scuti stars compare to those of non-variable A-type stars in NGC 6134 and other open clusters?
• RQ5 Are there any stars in NGC 6134 with low v sin i that could be suitable targets for detailed asteroseismic modeling?

Key findings

• No significant correlation was found between pulsational variability and rotational velocity (v sin i) among δ Scuti stars and non-variable A-type stars in NGC 6134.
• The spectroscopically derived metallicity of NGC 6134 is [Fe/H] = +0.38 ± 0.05, in excellent agreement with the value obtained from Strömgren photometry.
• One δ Scuti star (designated #508) shows marginal evidence of being an Am-type star, based on abundance analysis, though higher-resolution spectroscopy is needed for confirmation.
• Δa photometry did not detect any classical Am-type stars in the sample, though the index was not measured for the potentially anomalous star #508.
• Among the five open clusters studied, the v sin i distributions of δ Scuti stars and non-variable stars show no consistent pattern, with NGC 6134 showing a notable fraction (half) of δ Scuti stars with low v sin i (< 100 km s⁻¹).
• The presence of several slowly rotating δ Scuti stars in NGC 6134 suggests that this cluster is a promising target for future asteroseismic modeling.