Rotation and X-ray Emission from Protostars

The ASCA satellite has recently detected variable hard X-ray emission from two Class I protostars in the ρ Oph cloud, YLW15 (IRS43) and WL6, with a characteristic time scale ∼ 20h. In YLW15, the X-ray emission is in the form of quasi-periodic energetic flares, which we explain in terms of strong magnetic shearing and reconnection between the central star and the accretion disk. The flare modelling, based on the solar analogy, gives us access to the size of the magnetic structures, which in turn allows to calculate the rotation parameters of the star and the disk. In WL6, X-ray flaring 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 find that YLW15 is a fast rotator (near break-up), while WL6 rotates with a significantly longer period. We thus use X-ray flaring as a “clock” to measure the rotation of protostars. With the help of the mass-radius relation on the stellar “birthline”, we derive a mass M⋆ ∼ 2M⊙ and < ∼ 0.4M⊙ for the central stars of YLW15 and WL6 respectively. YLW15 thus appears as a future A star. On the long term, the magnetic interactions between the star and the disk results in magnetic braking and angular momentum loss of the star. The compared rotation behavior of YLW15 and WL6 confirms that for solar-mass stars their magnetic braking takes place on time scales tbr ∼ a few 105 yrs, i.e., of the same order as the estimated duration of the Class I protostar stage. The main parameter determining tbr turns out to be the stellar mass, so that close to the birthline there must be a mass-rotation relation, tbr ∝ ∼ M⋆, such that stars with M⋆ > ∼ 1− 2 M⊙ are fast rotators, while 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 YLW15 also naturally explain the observation of “superflares” of X-ray luminosities as high as LX > ∼ 10 33−34 erg s−1 during a few hours, while at the WL6 stage the lower X-ray luminosities are likely 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 YLW15, and perhaps also of other protostars, a hot coronal wind (T ∼ 106 K) may be responsible for the VLA thermal radio emission. This paper thus proposes the first clues to the magnetic properties of protostars, which govern their rotation status