The prevalence of weak magnetic fields in Herbig Ae stars: The case of PDS2

TL;DR

This study uses high-resolution spectropolarimetry to detect weak magnetic fields in the Herbig Ae star PDS2, revealing that such stars generally have much weaker magnetic fields than T Tauri stars, challenging existing accretion models.

Contribution

First high-resolution spectropolarimetric detection of a magnetic field in PDS2, providing new insights into magnetic field strengths in Herbig Ae/Be stars.

Findings

Detected a weak magnetic field of 33±5G in PDS2.

Most Herbig Ae/Be stars have magnetic fields weaker than 200G.

Magnetic fields in Herbig Ae/Be stars are significantly weaker than in T Tauri stars.

Abstract

Models of magnetically driven accretion and outflows reproduce many observational properties of T Tauri stars, but the picture is much less clear for the Herbig Ae/Be stars, due to the poor knowledge of their magnetic field strength and topology. The Herbig Ae star PDS2 was previously included in two magnetic studies based on low-resolution spectropolarimetric observations. Only in one of these studies the presence of a weak mean longitudinal magnetic field was reported. In the present study, for the first time, high-resolution HARPS spectropolarimetric observations of PDS2 are used to investigate the presence of a magnetic field. A firm detection of a weak longitudinal magnetic field is achieved using the multi-line singular value decomposition method for Stokes profile reconstruction (<B_z>=33+-5G). To gain better knowledge of typical magnetic field strengths in late Herbig Be and Herbig Ae stars, we compiled previous magnetic field measurements, revealing that only very few stars have fields stronger than 200G, and half of the sample possesses fields of about 100G and less. These results challenge our current understanding of the magnetospheric accretion in intermediate-mass pre-main sequence stars as they indicate that the magnetic fields in Herbig Ae/Be stars are by far weaker than those measured in T Tauri stars.