Magnetic Properties of Young Stars in the TW Hydrae Association

TL;DR

This study measures magnetic fields in young stars of the TW Hydrae Association using IR spectra, revealing how stellar magnetic properties evolve from 2 to 10 million years.

Contribution

It provides the first direct measurements of magnetic fields in TWA stars and compares their evolution with stars in Taurus, enhancing understanding of stellar magnetic development.

Findings

Magnetic field strength increases with stellar age.

Total magnetic flux decreases as stars contract.

No excess broadening in magnetically insensitive lines confirms magnetic origin.

Abstract

We present an analysis of infrared (IR) echelle spectra of five stars in the TW Hydrae Association (TWA). We model the Zeeman broadening in four magnetic-sensitive \ion{Ti}{1} lines near $2.2 \mu$m and measure the value of the photospheric magnetic field averaged over the surface of each star. To ensure that other broadening mechanisms are properly taken into account, we also inspect several magnetically insensitive CO lines near $2.3 \mu$m and find no excess broadening above that produced by stellar rotation and instrumental broadening, providing confidence in the magnetic interpretation of the width of the \ion{Ti}{1} lines. We then utilize our results to test the relationship between stellar magnetic flux and X-ray properties and compare the measured fields with equipartition field values. Finally, we use our results and recent results on a large sample of stars in Taurus to discuss the potential evolution of magnetic field properties between the age of Taurus ($\sim$2 Myrs) and the age of TWA ($\sim$10 Myrs). We find that the average stellar field strength increases with age; however, the total unsigned magnetic flux decreases as the stars contract onto the main-sequence.