ASTRONOMERS WATCH UNFOLDING SAGA OF MASSIVE STAR FORMATION
A pair of images of a young star, made 18 years apart, has revealed a dramatic difference that is providing astronomers with a unique, “real-time” look at how massive stars develop in the earliest stages of their formation.
The astronomers used the Karl G. Jansky Very Large Array (VLA) in the USA to study a massive young star called W75N(B)-VLA 2, some 4200 light-years from Earth. They compared an image made in 2014 with an earlier VLA image from 1996.
“The comparison is remarkable,” said Carlos Carrasco-Gonzalez of the National Autonomous University of Mexico, leader of the research team. The 1996 image shows a compact source of a hot, ionised wind ejected from the young star. The 2014 image shows that ejected wind deformed into a distinctly elongated outflow.
The new observations were triggered by results obtained with the European VLBI Network (EVN) in 2009 when a team of astronomers, led by Gabriele Surcis of the Joint Institute for VLBI in Europe (JIVE), measured for the first time the orientation of the large-scale magnetic field in the star-forming region where the massive young star is located. Surcis explains “We witnessed the magnetic field around W75N(B)-VLA2 develop a preferred direction aligned with the large scale magnetic field in the region. Now we also find the outflow in the same direction, indicating that magnetic forces are important in the formation of this star”
“And we are seeing this dramatic change in real time, so this object is providing us an exciting opportunity to watch the developments over the next few years, as this very young star develops the characteristic bipolar outflow morphology”, explains Huib van Langevelde, director of JIVE and professor at Leiden University.
The scientists believe the young star is forming in a dense, gaseous environment, and is surrounded by a doughnut-shaped, dusty torus. The star has episodes in which it ejects a hot, ionised wind for several years. First, that wind can expand in all directions, forming a spherical shell around the star. Later, the wind hits the dusty torus, which slows it. Wind expanding outward along the poles of the torus, where there is less resistance, moves more quickly, resulting in an elongated shape for the outflow.
“The evolution that we observed over only 18 years, follows the models very accurately”, adds Ciriaco Goddi from Radboud University Nijmegen. Theoretical models were developed to explain why nearly-spherical expansion of such outflows had been seen with young stars much more massive than the Sun, when narrower outflows were expected based on observations of less-massive, Sun-like stars at similar stages of development. W75N(B)-VLA 2 is estimated to be about 8 times more massive than the Sun. The more-uniform outflows are seen in massive young stars in the first few thousand years of their lives, the stage at which W75N(B)-VLA 2 is thought to be.
“Our understanding of how massive young stars develop is much less complete than our understanding of how Sun-like stars develop,” Surcis said. “It’s going to be really great to be able to watch one as it changes. We expect to learn a lot from this object with the proposed monitor programme on the European VLBI Network and other telescopes” he added.
Article
The international team of astronomers that publish their results in Science come from Mexico, the Netherlands, Sweden, Spain, Korea, and Japan. The details of the article: Observing the onset of outflow collimation in a massive protostar by C. Carrasco-González, J. M. Torrelles, J. Cantó, S. Curiel, G. Surcis, W. H. T. Vlemmings, H. J. van Langevelde, C. Goddi, G. Anglada, S.-W. Kim, J.-S. Kim, J. F. Gómez. Science, In: Science, vol. 348, no. 6230, 3 April 2015. DOI: http://www.sciencemag.org/lookup/doi/10.1126/science.aaa7216
VLBI
VLBI is an astronomical method by which multiple radio telescopes distributed across great distances observe the same region of sky simultaneously. Data from each telescope is sent to a central “correlator” to produce images with higher resolution than the most powerful optical telescopes. The authors from JIVE used the VLBI technique to discover a change in the magnetic field associated with W75N(B)-VLA2.
JIVE
The Joint Institute for VLBI in Europe (JIVE, www.jive.nl) is a scientific foundation with a mandate to support the operations of the European VLBI Network (EVN, www.evlbi.org). For this purpose it maintains, operates and develops the EVN data correlator, a powerful supercomputer that combines the signals from radio telescopes located across the planet.
IMAGES….Observing the onset of outflow collimation in a massive protostar: 3D hydrodynamic simulation and visualization (generated in the Shape software) of an episodic, short-lived, originally isotropic outflow ejection (Picture 1) from the massive protostar W75N(B)-VLA 2, whose morphology evolves into a collimated outflow (Picture 2) as it expands within a toroid of dust and gas. Image Credit: Wolfgang Steffen, Instituto de Astronomía, UNAM.
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