Abstract
Abstract Disk winds are thought to play a critical role in star birth. As winds extract excess angular momentum from accretion disks, matter in the disk can be transported inward to the star to fuel mass growth. However, observational evidence of wind carrying angular momentum has been very limited. We present Submillimeter Array (SMA) observations of the young star MWC 349A in the H26α and H30α recombination lines. The high signal-to-noise ratios made possible by the maser emission process allow us to constrain the relative astrometry of the maser spots to milli-arcsecond precision. Previous observations of the H30α line with the SMA and the Plateau de Bure interferometer (PdBI) showed that masers are distributed in the disk and wind. Our new high-resolution observations of the H26α line reveal differences in spatial distribution from that of the H30α line. H26α line masers in the disk are excited in a thin annulus with a radius of about 25 au, while the H30α line masers are formed in a slightly larger annulus with a radius of 30 au. This is consistent with expectations for maser excitation in the presence of an electron density variation of approximately R −4. In addition, the H30α and H26α line masers arise from different parts in the wind. This difference is also expected from maser theory. The wind component of both masers exhibits line-of-sight velocities that closely follow a Keplerian law. This result provides strong evidence that the disk wind extracts significant angular momentum, thereby facilitating mass accretion in the young star.
Highlights
The formation of a flattened disk surrounding a young stellar object is a natural outcome of a rotating and collapsing molecular core
If visibility phases are well determined across the spectral line, centroid fitting to the maser emission yields astrometry with precisions better than the spatial resolution
The ability to measure the position is limited by the phase noise in the spectrum, which in turn is related to the signal-to-noise ratio (SNR) in the data
Summary
The formation of a flattened disk surrounding a young stellar object is a natural outcome of a rotating and collapsing molecular core. Beuther et al 2005; Zhang 2005; Cesaroni et al 2007), may play a pivotal role in the mass growth of protostars by extracting excess angular momentum in the infalling matter through winds and outflows along the polar direction (Arce et al 2007; Zhang et al 2001, 2005; Qiu et al 2008). In theoretical models of star formation, magneto hydrodynamic winds extract angular momentum from the disk (Shu et al 2000; Konigl & Pudritz 2000). While theories on protostellar wind do not agree on the location at which the wind is launched, the role it plays in shedding angular momentum is a common feature in all theoretical models
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