Magnetic Fields Near the Young Stellar Object IRAS 16293-2422

1. Magnetic Fields Near the Young Stellar Object IRAS 16293-2422 M. J Claussen (NRAO), A. P. Sarma (E. Kentucky Univ), H.A. Wootten (NRAO), K. B. Marvel (AAS), B. A. Wilking (UMSL) Observation and Results We used the Very Long Baseline Array (VLBA) to observe the water masers in IRAS 16293-2422 on October 8, 1999. The angular resolution of the VLBA observations is 2.8 by 0.7 milli-arcseconds (mas). Many maser spots were detected spread over a region of approximately 600 mas (~ 80 AU). The distribution of masers is similar to that which we have seen previously. We also observed masers and the 1.3 cm continuum emission from IRAS 16293 using the Very Large Array (VLA) in its BnA configuration on June 18, 1998. The angular resolution of the VLA observations was 300 by 230 mas. In the figure to the right we show the 1.3 cm image. We have clearly detected the 1.3 cm continuum emission from IRAS 16293 A1, A2 and B as has been noted previously. We measured the absolute position of the water masers in the VLBA data, using the technique of fringe-rate mapping, to an accuracy of about 50 mas in declination (and somewhat better in right ascension). In the figure further to the right we show the positions of the water masers and the positions of components A1 and A2. The (0,0) position of this figure is that of the brightest maser component. Crosses show the positions of the A1 and A2 continuum components. No correction has been made for proper motions of the continuum components between these two epochs. Zeeman Splitting Detected Shown directly to the right is a spectrum of the strongest water maser feature (at an LSR velocity of +2.5 km/s) which is found within a projected distance of 20 AU of component A1. The velocity of this feature is not much different than that of the ambient cloud material of +3.7 km/s, so it is not clear if this maser feature is participating in the outflow from IRAS16293 or some other kinematic structure. The spectra show that we have detected the Zeeman effect in this feature. A fit of the Stokes V profile to the derivative of the Stokes I profile gives a line-of-sight magnetic field of about 40 milliGauss. Such a very strong field compared to the interstellar field must be important for the dynamics of the star-forming process and have implications for the ejection and collimation of the wind. Indeed the magnetic pressure completely dominates the thermal pressure for reasonable temperatures in the region. Summary We have detected Zeeman splitting in the water masers toward the young stellar object within 20 AU of one of the YSOs comprising the object IRAS 16293-2422. The splitting corresponds to a line-of-sight magnetic field of 40 mG. Whether or not the magnetic field we detected is in the outflow or perhaps closer to the accreting protostar requires further VLBA monitoring and polarimetry of the masers in this interesting source. The figures to the left show the relative positions between the water masers and the radio continuum sources. The crosses give an estimate of the error on the position of the continuum sources. The water masers in the far southwest delineate a curved, bow-shock structure that appears to be moving in the plane of the sky to the southwest with a velocity 20 km/s (Murphy et al. 1999). The maser circles are color-coded with their LSR velocity: Magenta: -3.2 to -0.9 km/s Blue: -0.9 to +1.4 km/s Cyan: +1.4 to +3.5 km/s Green: +3.5 to +5.8 km/s Red: +5.8 to +8.2 km/s Introduction IRAS 16293-2422 is a well-studied Class 0 protostellar object in the small dark cloud L1689N found in the nearby (120-160 pc) rho Ophiuchus molecular cloud. IRAS 16293 is a known protobinary system, with a projected separation of 840 AU (Mundy et al. 1992). A powerful molecular outflow is found emanating from IRAS 16293. The outflow is complex and shows a quadrupolar structure (Walker et al. 1988). It has been suggested that there are two protostars driving this outflow, although identification of the powering sources has been difficult. IRAS 16293 A, the southeast component of the protobinary, appears to drive the NE-SW outflow. In the radio continuum, this source divides into two peaks. Large proper motions of these three radio sources (A1, A2, and B) reported by Loinard (2002) add to the confusion of whether or not the A1 and A2 radio continuum peaks are separate protostars. Proper motions of water masers surrounding IRAS 16293 A have been measured; in general the masers trace the NE-SW outflow on very small scales (100 AU). However, there are several maser structures which do not fit well with the outflow. Magnetic fields are expected to be found in the outflows and the accretion disks around these low-mass young stellar objects since the stellar core collapses from molecular clouds that are supported by large-scale magnetic fields. Observations of the magnetic field strength within a few AU of the driving sources, however, are hard to make. We report here on the measurement of Zeeman splitting of a strong water maser feature near the IRAS 16293 A1 protostellar source. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.