title: Stellar physics. Observing the onset of outflow collimation in a massive protostar.
abstract: 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.
authors.affiliation: Centro de Radioastronomía y Astrofísica UNAM, Apartado Postal 3-72 (Xangari), 58089 Morelia, Michoacán, Mexico. carrasco@crya.unam.mx.
authors.forename: C
authors.initials: C
authors.lastname: Carrasco-González
authors.affiliation: Institut de Ciències de l'Espai (CSIC) and Institut de Ciències del Cosmos (UB)/Institut d'Estudis Espacials de Catalunya, Calle Martí i Franquès 1, 08028 Barcelona, Spain.
authors.forename: J M
authors.initials: JM
authors.lastname: Torrelles
authors.affiliation: Instituto de Astronomía (UNAM), Apartado 70-264, 04510 Mexico D.F., Mexico.
authors.forename: J
authors.initials: J
authors.lastname: Cantó
authors.affiliation: Instituto de Astronomía (UNAM), Apartado 70-264, 04510 Mexico D.F., Mexico.
authors.forename: S
authors.initials: S
authors.lastname: Curiel
authors.affiliation: Joint Institute for VLBI in Europe, Postbus 2, 79990 AA Dwingeloo, Netherlands.
authors.forename: G
authors.initials: G
authors.lastname: Surcis
authors.affiliation: Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden.
authors.forename: W H T
authors.initials: WH
authors.lastname: Vlemmings
authors.affiliation: Joint Institute for VLBI in Europe, Postbus 2, 79990 AA Dwingeloo, Netherlands. Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, Netherlands.
authors.forename: H J
authors.initials: HJ
authors.lastname: van Langevelde
authors.affiliation: Joint Institute for VLBI in Europe, Postbus 2, 79990 AA Dwingeloo, Netherlands. Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, Post Office Box 9010, 6500 GL Nijmegen, Netherlands.
authors.forename: C
authors.initials: C
authors.lastname: Goddi
authors.affiliation: Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain.
authors.forename: G
authors.initials: G
authors.lastname: Anglada
authors.affiliation: Korea Astronomy and Space Science Institute, 776 Daedeokdaero, Yuseong, Daejeon 305-348, Republic of Korea. Korea University of Science and Technology, 217 Gajeong-ro Yuseong-gu, Daejeon 305-350, Republic of Korea.
authors.forename: S-W
authors.initials: SW
authors.lastname: Kim
authors.affiliation: National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan.
authors.forename: J-S
authors.initials: JS
authors.lastname: Kim
authors.affiliation: Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain.
authors.forename: J F
authors.initials: JF
authors.lastname: Gómez