The first Δa observations of three globular clusters

Abstract

Globular clusters are main astrophysical laboratories to test and modify evolutionary models. Thought to be rather homogeneous in their local elemental distribution of members, results suggest a wide variety of chemical peculiarities. Besides different main sequences, believed to be caused by different helium abundances, peculiarities of blue horizontal-branch stars and on the red giant branch were found. This whole zoo of peculiar objects has to be explained in the context of stellar formation and evolution. The tool of ∆a photometry is employed in order to detect peculiar stars in the whole spectral range. This three filter narrow band system measures the flux distribution in the region from 4900 to 5600Å in order to find any peculiarities around 5200Å. It is highly efficient to detect classical chemically peculiar stars of the upper main sequence, Be/Ae, shell and metal-weak objects in the Milky Way and Magellanic Clouds. We present ∆a photometry of 2266 stars from 109 individual frames for three globular clusters (NGC 104, NGC 6205, and NGC 7099). A comparison with published abundances, for three horizontal-branch stars, only, yield an excellent agreement. According to the 3σ detection limit of each globular cluster, about 3% of the stars lie in abnormal regions in the diagnostic diagrams. The first observations of three widely different aggregates give very promising results, which will serve as a solid basis for follow-up observations including photometric as well as spectroscopic studies.

Δa Photometric Survey of the Small Magellanic Cloud

“…This is a very small percentage compared to the value of the Milky Way (up to 15%). However, an incidence that low in the LMC had already been discovered (Paunzen et al 2014), and supports the fossil theory as the origin of the stable stellar magnetic field; the global magnetic field of the SMC is much weaker than in the Milky Way. It is possible that the lower global metallicity also plays a role, though that has not yet been confirmed by theory.…”

“…Since the 1970s CP and other non-standard stars in the Galactic field and in open clusters have been targets of photoelectric observations, and more latterly of CCD Δa photometry, leading to numerous publications. Observations have since been extended to globular clusters and to the Large Magellanic Cloud to search for all types of non-standard objects (Paunzen et al 2014).…”

A search for peculiar stars in the open cluster Hogg 16

“…Although variability or binarity might interfere the results, the photometric system can be considered as an efficient pre-selection tool in particular for mCP stars. The system was not only applied to Galactic field stars, open clusters (Netopil et al 2007) and globular clusters (Paunzen et al 2014a), but also to clusters in the Large Magellanic Cloud (Paunzen et al 2006a), where photometric mCP star detections are also confirmed by spectroscopy (Paunzen et al 2011).…”