اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدEjection History of the IRAS 04166+2706 Molecular Jet
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The high-velocity molecular jet driven by Class 0 protostar IRAS 04166+2706 exhibits a unique saw-tooth velocity pattern. It consists of a series of well-aligned symmetric knots with similar averaged speeds, whose speeds at peaks of emission decreases roughly linearly away from the origin. Recent ALMA observations of knots R6 and B6 reveal kinematic behavior with expansion velocity increasing linearly from the axis to the edge. This pattern can be formed by a spherically expanding wind with axial density concentration. In this picture, the diverging velocity profile naturally possesses an increasing expansion velocity away from the axis, resulting in a tooth-like feature on the position-velocity diagram through projection. Such geometric picture predicts a correspondence between the slopes of the teeth and the outflow inclination angles, and the same inclination angle of 52$^circ$ of the IRAS 04166+2706 can generally explain the whole pattern. Aided by numerical simulations in the framework of unified wind model by Shang et al. (2006), the observed velocity pattern can indeed be generated. A proper geometrical distribution of the jet and wind material is essential to the reconstruction the ejection history of the system.
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$Aims.$ We study the relation between the jet and the outflow in the IRAS 04166+2706 protostar. This Taurus protostar drives a molecular jet that contains multiple emission peaks symmetrically located from the central source. The protostar also drive
s a wide-angle outflow consisting of two conical shells. $Methods.$ We have used the Atacama Large Millimeter/submillimeter Array (ALMA) interferometer to observe two fields along the IRAS 04166+2706 jet. The fields were centered on a pair of emission peaks that correspond to the same ejection event, and were observed in CO(2-1), SiO(5-4), and SO(65-54). $ Results.$ Both ALMA fields present spatial distributions that are approximately elliptical and have their minor axes aligned with the jet direction. As the velocity increases, the emission in each field moves gradually across the elliptical region. This systematic pattern indicates that the emitting gas in each field lies in a disk-like structure that is perpendicular to the jet axis and is expanding away from the jet. A small degree of curvature in the first-moment maps indicates that the disks are slightly curved in the manner expected for bow shocks moving away from the IRAS source. A simple geometrical model confirms that this scenario fits the main emission features. $Conclusions.$ The emission peaks in the IRAS 04166+2706 jet likely represent internal bow shocks where material is being ejected laterally away from the jet axis. While the linear momentum of the ejected gas is dominated by the component in the jet direction, the sideways component is not negligible, and can potentially affect the distribution of gas in the surrounding outflow and core.
Context: IRAS 04166+2706 in Taurus is one of the most nearby young stellar objects whose molecular outflow contains a highly collimated fast component. Methods: We have observed the IRAS 04166+2706 outflow with the IRAM Plateau de Bure interferomet
er in CO(J=2-1) and SiO(J=2-1) achieving angular resolutions between 2 and 4. To improve the quality of the CO(2-1) images, we have added single dish data to the interferometer visibilities. Results: The outflow consists of two distinct components. At velocities <10 km/s, the gas forms two opposed, approximately conical shells that have the YSO at their vertex. These shells coincide with the walls of evacuated cavities and seem to result from the acceleration of the ambient gas by a wide-angle wind. At velocities >30 km/s, the gas forms two opposed jets that travel along the center of the cavities and whose emission is dominated by a symmetric collection of at least 7 pairs of peaks. The velocity field of this component presents a sawtooth pattern with the gas in the tail of each peak moving faster than the gas in the head. This pattern, together with a systematic widening of the peaks with distance to the central source, is consistent with the emission arising from internal working surfaces traveling along the jet and resulting from variations in the velocity field of ejection. We interpret this component as the true protostellar wind, and we find its composition consistent with a chemical model of such type of wind. Conclusions: Our results support outflow wind models that have simultaneously wide-angle and narrow components, and suggest that the EHV peaks seen in a number of outflows consist of internally-shocked wind material.
We present Expanded Very Large Array (EVLA) water maser observations at 22 GHz toward the source IRAS 18113-2503. Maser components span over a very high velocity range of ~500 km/s, the second largest found in a Galactic maser, only surpassed by the
high-mass star forming region W49N. Maser components are grouped into a blue and a redshifted cluster, separated by 0.12. Further mid-IR and radio data suggest that IRAS 18113-2503 is a post-AGB star, thus a new bona fide member of the rare class of water fountains. It is the evolved object with the largest total velocity spread in its water masers, and with the highest velocity dispersion within its red- and blue-shifted lobes (~170 km/s). The large total velocity range of emission probably indicates that IRAS 18113-2503 has the fastest jet among the known water fountain stars. On the other hand, the remarkably high velocity dispersion within each lobe may be interpreted in terms of shocks produced by an episode of mass ejection whose velocity increased up to very high values or, alternatively, by projection effects in a jet with a large opening angle and/or precessing motions.
Water fountains are evolved stars showing early stages of collimated mass loss during transition from the asymptotic giant branch, providing valuable insight into the formation of asymmetric planetary nebulae. We report the results of multi-epoch VLB
I observations, which determine the spatial and three-dimensional kinematic structure of H2O masers associated with the water fountain IRAS 18113-2503. The masers trace three pairs of high-velocity (~150-300 km/s) bipolar bow shocks on a scale of 0.18 (~2000 au). The expansion velocities of the bow shocks exhibit an exponential decrease as a function of distance from the central star, which can be explained by an episodic, jet-driven outflow decelerating due to drag forces in a circumstellar envelope. Using our model, we estimate an initial ejection velocity ~840 km/s, a period for the ejections ~10 yr, with the youngest being ~12 yr old, and an average envelope density within the H2O maser region n(H2) ~ 10^6 cm^(-3). We hypothesize that IRAS 18113-2503 hosts a binary central star with a separation of ~10 au, revealing novel clues about the launching mechanisms of high-velocity collimated outflows in water fountains.
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