Magnetic Open Cluster Stars of a Peculiar Kind observed with HARPS-POL

by James Silvester, Uppsala University

Magnetic fields play a fundamental role in the atmospheric physics of a significant fraction of stars on the Hertzsprung–Russell diagram. The magnetic fields of the chemically peculiar magnetic A and B type stars (Ap/Bp) have quite different characteristics than, for example, cooler stars like the Sun. In these magnetic Ap/Bp stars the large-scale surface magnetic field is static on time-scales of at least many decades, and appears to be “frozen” into a rigidly rotating atmosphere. The magnetic field is globally organised, permeating the entire stellar surface, with a high field strength (typically of a few hundreds up to a few tens of thousands of gauss—by comparison the sun has a polar field strength of one to two gauss). These stars are so called “chemically peculiar” as a result of having peculiar abundances (amounts) of certain chemical elements compared to what is seen in the Sun or other solar type stars.

Shows a comparison between a solar abundance model spectrum (red dashes) and typical peculiar abundance spectrum (solid green) for a 13000 K star.
This figure shows a comparison between a solar abundance model spectrum (red dashes, T=5800 K) and typical peculiar abundance spectrum for a hotter star (solid green, T=13000 K).

It is thought that the presence of this magnetic field strongly influences energy and mass transport, and results in strong chemical abundance non-uniformities within the atmosphere. These uniformities can take the form of large abundance structures in certain layers in the atmosphere.

Originally the magnetic field geometries of these chemically peculiar Ap/BP stars were modelled in the context of a simple dipole field (think of a bar magnet stuck in the star). However, with the acquisition of increasingly sophisticated data, it has become clear that the large-scale field topologies exhibit important differences from a simple pure dipole model. Through the advent of high-resolution circular and linear polarisation spectroscopy we have found the presence of strong, small-scale complex field structures, which were completely unexpected based on earlier modelling.

How do we measure magnetic fields of Ap/Bp Stars

In 1897 Dutch physicist Pieter Zeeman discovered that in the presence of an external magnetic field; light is polarised circularly if viewed parallel to the direction of the magnetic field and is plane (or linearly) polarised if viewed perpendicular to the magnetic field. In addition, spectral lines in the presence of such a field can be split into discrete levels—the so called Zeeman effect—where the strength of the magnetic field is proportional to the width of the splitting within the spectral lines.

The figure below illustrates how both a spectral line and how the linear and circular polarisation signature can change in the presence of a strong magnetic field, in the case of when the field is either perpendicular or parallel to the line of sight of the observer.

Spectral line shapes in different polarization states.
Spectral line shapes in different polarization states.

We use these effects to study the magnetic field of stars with the aid of a spectropolarimeter. The form of the polarised light we receive tells us about the direction of the field on the surface of the star with respect to the observer, and the level of Zeeman splitting within a given spectral line or set of lines allows us to determine the strength of the magnetic field.

In cases where the polarisation signal is too weak to effectively measure in individual spectral lines,  we can use line averaging techniques to improve the signal by averaging all the lines in the spectrum showing polarisation signatures into one line.
The recent advances in tomographic imaging techniques and the new generation of spectropolarimeters such as ESPaDOnS (at the CFHT), NARVAL (at the TBL, Pic du Midi) and HARPS-pol (ESO 3.6 at La Silla) offer the opportunity to improve our understanding of the magnetic field of Ap/Bp stars by allowing us to map the magnetic field and chemical surface structure in quite some detail.  (For the finer details of how we map magnetic fields see the great article by Élodie Hébrard and Rakesh Yadav).

The extended magnetic field topology map for the Ap star HD 32633, the number in the right hand corner is the phase of rotation.
The extended magnetic field topology map for the Ap star HD 32633, the number in the right hand corner is the phase of rotation.

Even though we are able to very successfully map the magnetic fields of Ap/Bp stars, there are however some questions that remain open.  Notably the origin of these magnetic fields is not fully understood and importantly neither is the evolution of such magnetic fields and the atmospheric chemical structures with time. This is where Ap/Bp stars in open clusters comes in.

The Current Project – Observations of Cluster Ap/Bp Stars

The question of the evolution of the magnetic field and chemical surface structures in Ap/Bp stars can be investigated by studying these stars in open clusters.  An open cluster is a group of gravitationally bound stars, and whilst it is very difficult to get an precise age for an individual star, it is however possible to get a more precise age for an open cluster, because you have an ensemble of stars which are thought to be of a similar age. Therefore if you can confirm that a star is a true member of a cluster,  you have a much more reliable age for that star than if it was an individual star.

By studying the magnetic fields of stars in different open clusters with different ages, we can in essence look at Ap/Bp stars at different stages of evolution.  This allows us to investigate if the magnetic field complexity, or the form the magnetic field takes, varies as a function of age,  e.g does the magnetic field structure of an Ap/Bp star evolve with time?

The cluster NGC 6475 / M7. Image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. Image credits : ESO
The cluster NGC 6475 / M7. Image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at La Silla Observatory in Chile. Image credits : ESO

Our team, including astronomers based in Sweden, Canada and France, has begun to obtain observations of cluster Ap/Bp stars using the HARPSpol spectropolarimeter. By measuring the circular polarisation of these magnetic Ap/Bp stars and by obtaining measurements at all phases of rotation, we will be able to create magnetic and surface chemical maps for all the stars we observe.   It is hoped that the resulting maps from our target stars will give us insight into how the magnetic field geometries and chemical surface structures of Ap/Bp stars vary with age.   Having more information about the evolution of the magnetic field will also provide a powerful constraint for stellar evolution models.

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About the author. James Silvester is a postdoctoral researcher at Uppsala university in Sweden. He completed his BSc in Astrophysics at the University of Hertfordshire in 2004 and then moved to Canada to do an MSc (2007) and his PhD (2014) at Queen’s University in Kingston, Ontario.  The main focus of his research is to understand the magnetic fields of intermediate mass stars. James and his team use HARPS combined with a recently installed polarimeter (also known as HARPS-POL) to measure magnetic fields. His program overlapped with Pale Red Dot, and he was one of the observers that helped obtaining those Proxima spectra by the end of each night. Astrophotography is also among his hobbies.