Recent Spitzer Space Telescope Infrared Array Camera (IRAC) surveys of the Galactic Plane have revealed a new class of objects that are characterized by bright extended emission in the IRAC 4.5 micron band. Over 300 of these objects (designated EGOs=extended green objects, for the common coding of the 4.5 micron band as green in 3-color IRAC images) have been catalogued by Dr. Edward Churchwell (University of Wisconsin - Madison) and his team. Analysis of the mid-infrared spectral energy distributions (SEDs) of EGOs indicates that they are massive young stellar objects (MYSOs). The mechanism for exciting the 4.5 micron emission, thought to be shocked H2 or CO gas in supersonic outflows, suggests that EGOs are MYSOs in a state of active accretion. The investigators will carry out a comprehensive set of multiwavelength studies designed to characterize the properties of the newly discovered EGOs and to place constraints on theories of massive star formation; in particular how accretion, outflows and clustering operate simultaneously.
Very little is currently known about EGOs other than the mid-Infrared properties derived from Spitzer data. The studies that will be carried out under this project will characterize the outflow properties, luminosities, masses, and clustering properties of a well-chosen sample of 30 EGOs, via high angular resolution studies at near-Infrared through centimeter wavelengths. These include an Extended-Very Large Array (EVLA) search for 44 GHz Class I CH3OH masers, known to be associated with molecular outflows, which will be used in conjunction with lower-resolution molecular line surveys (HCO+ and SiO) to test the hypothesis of an outflow origin for the IRAC 4.5 micron emission and measure outflow properties. An EVLA search for 6.7 GHz Class II CH3OH masers, known to be associated with MYSOs, will be used along with simultaneously obtained high-resolution 3.6 cm and 7 mm continuum data to establish the mass range of EGOs. Kinematic distances will be determined for all the sources in the sample, permitting the luminosity, stellar mass, and envelope accretion rate to be extracted from their SEDs using a grid of protostar models. These follow-up observations on this new class of objects will add much needed information on this heretofore hidden stage of high-mass protostellar evolution.
In addition to the primary research goals of better understanding the nature of MYSOs, this project will support the training of one full time graduate student and several undergraduate students over its duration as well as contributing to public outreach through several local organizations, including the University of Wisconsin Space Place, the off-campus outreach center of the Department of Astronomy, and a weekly astronomy radio program.
Most sun-like stars are born in the presence of their much larger and more energetic cousins: massive stars, which will end their lives in supernova explosions. Even while they are young, massive stars inject tremendous amounts of energy into their environments, heating, churning, and ionizing the surrounding gas. How star formation happens in such messy environments is still poorly understood. Massive stars evolve quickly, so it is difficult to catch them "in the act" of formation. The goal of this project was to do so: to study a unique new set of sources thought to be actively growing young massive stars. The first goal of this project was to determine the nature of these objects, which were identified in mid-infrared surveys of the Galactic Plane. Through Very Large Array observations of two types of masers that trace different physical conditions, we showed that our targets were in fact much-sought young massive stars that were still growing, throwing gas off into their surroundings as they did so. Such outflows affect the environment of the forming massive star, and so its future growth: a form of "feedback." We studied these outflows in detail using the Submillimeter Array and the Combined Array for Research in Millimeter Astronomy, telescopes that allow direct observations of the molecular gas. We also searched for evidence that strong ultraviolet light from the nascent massive stars had ionized the surrounding gas (again using the Very Large Array). The results of our observations to date indicate that for these very young massive stars, the outflows are the more energetic, and more important, form of feedback. The young massive stars identified and studied in this project will be prime targets for the Atacama Large Millimeter/Submillimeter Array (ALMA), which recently "opened its eyes" and began science observations in Chile. Broader impacts of this project included preparing graduate students to use this revolutionary new facility by providing training in the science and techniques of (sub)millimeter astronomy and building collaborations between universities and the national centers that will support ALMA. Work is ongoing on the comprehensive multiwavelength dataset that we have assembled on this unique sample of young massive stars (and to which we continue to add), and additional publications are in preparation. We have recently begun to expand our search for these young, actively growing massive stars beyond the inner Galaxy to its outer reaches. We plan to engage the public in this search through the Milky Way Project citizen science project.
