X-Ray Luminosity of Stars in NGC 752: Finding the Magnetic Flux of Young Stars

Charlotte Christensen

Mentor: Theodore Simon

The sun's magnetic field varies in intensity both as it ages and over short periods of time. The eleven-year cycle, during which time the magnetic flux peaks and the field switches direction, is studied because of its clear pattern.  In 1955, Parker proposed that a hydromagntic dynamo caused by differential rotation and convection near the surface of the star changes the poloidal magnetic field into a toridal one and so produces this cycle.  The exact physics of this dynamo and how universal it is, however, are still being researched.  Knowledge of how the mass and age of a given star affect the strength and time variability of its magnetic field is necessary to produce a robust theory of the dynamo.  In this study we focused on how the age of the star affects the magnetic activity, in particular, if and when a dynamo develops and how the slower rotation of the star as it ages affects the magnetic field.

To study the magnetic fields of young stars, we used the Chandra X ray telescope to measure the coronal luminosities of stars in the NGC 752 open cluster.   The coronal luminosity of a star, which is bright enough to shine strongly in X rays, is proportional to the amount of magnetic flux and so may be used as a proxy.  In addition, the range of coronal luminosities for stars of a given mass is proportional to the range of magnetic activity a star of that mass may experience on timescales corresponding to a magnetic cycle. Members of NGC 752 are all 2 Gyr old, an age that has not yet been studied and at which it is hypothesized the sun's dynamo was first being developed. We found an average coronal luminosity of 3 × 10²⁸ ergs/sec for G-type stars in the cluster.   This is evidence for an exponential decay in average magnetic flux as the star ages.  Further observations of stars in this region indicating their stellar type and probability of cluster membership will allow us to include more data from more stars and perhaps draw conclusions about magnetic field time variability at this point in a stars life.