Rotation and X‐Ray Emission from Protostars

Abstract

The ASCA satellite has recently detected variable hard X-ray emission from two class I protostars in the o Oph cloud, YLW 15 (IRS 43) and WL 6, with a characteristic timescale of D20 hr. In YLW 15, the X-ray emission is in the form of quasi-periodic energetic Nares, which we explain in terms of strong magnetic shearing and reconnection between the central star and the accretion disk. The Nare modeling, based on the solar analogy, gives us access to the size of the magnetic structures, which in turn allows us to calculate the rotation parameters of the star and the disk. In WL 6, X-ray Naring is rotationally modulated and appears to be more like the solar-type magnetic activity ubiquitous on T Tauri stars. On the basis of these observations, we Ðnd that YLW 15 is a fast rotator (near break-up), while WL 6 rotates with a signiÐcantly longer period. We thus use X-ray Naring as a clock II to measure the rota- tion of protostars. With the help of the mass-radius relation on the stellar birthline,II we derive masses of and for the central stars of YLW 15 and WL 6, respectively. YLW 15 thus M * D 2 M _ (0.4 M _ appears to be a future A star. In the long term, the magnetic interactions between the star and the disk result in magnetic braking and angular momentum loss of the star. A comparison of the rotation behav- ior of YLW 15 and WL 6 conÐrms that for solar-mass stars magnetic braking takes place on timescales few ) 105 yr, i.e., of the same order as the estimated duration of the class I protostar stage. The t br D a main parameter determining turns out to be the stellar mass, so that close to the birthline there must t br be a mass-rotation relation, such that stars with are fast rotators, while t br D P M * , M * Z 1E2 M _ their lower mass counterparts have had the time to spin down and reach synchronous rotation with the inner surrounding accretion disk. The rapid rotation and strong star-disk magnetic interactions of YLW 15 also naturally explain the observation of superNares II of X-ray luminosities as high as L X Z ergs s~1 during a few hours, while at the WL 6 stage the lower X-ray luminosities are likely 1033E1034 to be of purely stellar origin. The mass-rotation relation through magnetic braking may also explain why so few class I protostars have been detected in X-rays to date, and why they all lie in clusters. In the case of YLW 15, and perhaps also in other protostars, a hot coronal wind (T D 106 K) may be responsible for the VLA thermal radio emission. This paper thus proposes the Ðrst clues to the magnetic properties of protostars, which govern their rotation status and evolution. Subject headings : stars : Nare E stars : individual (YLW 15, WL 6) E stars : magnetic Ðelds E stars : preEmain-sequence E stars : rotation E X-rays : stars