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RISS 인기검색어
- 검색키워드
- 저자 : Surcis, G. (검색결과 3 건)
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EVOLUTION OF THE WATER MASER EXPANDING SHELL IN W75N VLA 2
Kim, Jeong-Sook,Kim, Soon-Wook,Kurayama, Tomoharu,Honma, Mareki,Sasao, Tesuo,Surcis, Gabriele,Cantó,, Jorge,Torrelles, José,M.,Kim, Sang Joon IOP Publishing 2013 The Astrophysical journal Vol.767 No.1
<P>We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H2O masers in the high-mass star-forming region of W75N, carried out with VLBI Exploration of Radio Astrometry (VERA) for three epochs in 2007 with an angular resolution of similar to 1 mas. We detected H2O maser emission toward the radio jet in VLA 1 and the expanding shell-like structure in VLA 2. The spatial distribution of the H2O masers detected with VERA and measured proper motions around VLA 1 and VLA 2 are similar to those found with previous VLBI observations in epochs 1999 and 2005, with the masers in VLA 1 mainly distributed along a linear structure parallel to the radio jet and, on the other hand, forming a shell-like structure around VLA 2. We have made elliptical fits to the VLA 2 H2O maser shell-like structure observed in the different epochs (1999, 2005, and 2007), and found that the shell is still expanding eight years after its discovery. From the difference in the size of the semi-major axes of the fitted ellipses in epochs 1999 (similar or equal to 71 +/- 1 mas), 2005 (similar or equal to 97 +/- 3 mas), and 2007 (similar or equal to 111 +/- 1 mas), we estimate an average expanding velocity of similar to 5 mas yr(-1), similar to the proper motions measured in the individual H2O maser features. A kinematic age of similar to 20 yr is derived for this structure. In addition, our VERA observations indicate an increase in the ellipticity of the expanding shell around VLA 2 from epochs 1999 to 2007. In fact, the elliptical fit of the VERA data shows a ratio of the minor and major axes of similar to 0.6, in contrast with an almost circular shape for the shell detected in 1999 and 2005 (b/a similar to 0.9). This suggests that we are probably observing the formation of a jet-driven H2O maser structure in VLA2, evolving from a non-collimated pulsed-outflow event during the first stages of evolution of a massive young stellar object (YSO). This may support predictions made earlier by other authors on this issue, consistent with recent magnetohydrodynamical simulations. We discuss possible implications of our results in the study of the first stages of evolution of massive YSOs.</P>
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Observing the onset of outflow collimation in a massive protostar
Carrasco-Gonzá,lez, C.,Torrelles, J. M.,Cantó,, J.,Curiel, S.,Surcis, G.,Vlemmings, W. H. T.,van Langevelde, H. J.,Goddi, C.,Anglada, G.,Kim, S.-W.,Kim, J.-S.,Gó,mez, J. F. American Association for the Advancement of Scienc 2015 Science Vol.348 No.6230
<P><B>Young stars grow up and narrow their focus</B></P><P>Stars are thought to grow by gathering spirals of material from a disk. If this is the case, to balance angular momentum, gas should flow out rapidly along the disk's rotation axis. Carrasco-Gonzalez <I>et al.</I> now seem to have glimpsed the “before” and “after” stages of the onset of such an outflow, over the course of just 18 years (see the Perspective by Hoare). Radio monitoring of the massive protostar W75N(B)-VLA2 reveals a transition from a spherical wind to a collimated one, giving critical insight into what happens as a massive star forms.</P><P><I>Science</I>, this issue p. 114; see also p. 44</P><P>The current paradigm of star formation through accretion disks, and magnetohydrodynamically driven gas ejections, predicts the development of collimated outflows, rather than expansion without any preferential direction. We present radio continuum observations of the massive protostar W75N(B)-VLA 2, showing that it is a thermal, collimated ionized wind and that it has evolved in 18 years from a compact source into an elongated one. This is consistent with the evolution of the associated expanding water-vapor maser shell, which changed from a nearly circular morphology, tracing an almost isotropic outflow, to an elliptical one outlining collimated motions. We model this behavior in terms of an episodic, short-lived, originally isotropic ionized wind whose morphology evolves as it moves within a toroidal density stratification.</P>
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