First things first – massive stars in W3Main are not born coeval
Using the new LUCI-1 instrument at the worlds largest telescope, the Large Binocular Telescope (LBT) in Arizona, astronomers from German institutes detected for the first time that the massive stars in the Galactic star forming region W3Main are not born simultaneously. This discovery has important consequences for our general understanding of the star formation process and the simple picture where stars are born in just a single event is under debate.
Over the recent years evidence has been presented that it is more complex in most regions. From this result we now know that stars can form over a time span of several million years
| Background information | Image download |
Massive stars, as massive as 10 to 100 times the sun, always form together in groups or clusters within dense clouds of dust and gas, so-called molecular clouds. One example of such an environment in our own galaxy, the Milky Way, is W3Main at a distance of 6000 light years. W3Main is part of a larger star forming region in the northern constellation Cassiopeia.
The assumption that the stars in such regions form simultaneously is being questioned for a long time and it could be possible that the first formed stars may initiate or at least affect the formation of the later generation of stars. The only way to figure out the actual history and the time span of star formation would be to accurately measure the ages of the massive stars inside.
Peering through the dark – making the invisible visible
Such observations are challenging, since massive stars form deeply embedded in molecular clouds, thus enshrouded in a dense and apparently impenetrable curtain of dust and gas. This makes their formation and first stages of their life invisible for telescopes working at optical wavelengths. Fortunately, observations at longer, near-infrared wavelengths lift the curtain and reveal the hidden stars by penetrating the dust.
The new LUCI-1 instrument – a multi-mode near-infrared spectro-imager at the LBT – is perfectly suited to study these young and recently formed massive stars and to try to measure their ages. For comparison, Figure 1 shows the LUCI-1 image of W3Main in the near-infrared (top left) and an image taken in visible light (top right). While the latter shows only a few stars, the LUCI-1 image, in contrast, reveals a populous group of young stars.
LUCI-1 spectra – like fingerprints of the stars
Images, however, only provide information about the brightness and color of stars and are, therefore, of limited use in determining stellar properties. Spectra, on the other hand, enable a much better characterization, since they are like stellar fingerprints. In a spectrum, stellar light is fanned out into its individual colors (wavelengths), as visible in a rainbow revealing a lot of spectral lines, characteristic of their chemical and physical condition. This allows a determination of its spectral type from which luminosity and temperature can accurately be derived. Comparison with models of stellar evolution results in a precise age estimate.
Getting spectra of a representative number of the distant and therefore faint stars in W3Main requires long observations to collect enough light, even with the largest telescopes. Now a group of scientists, led by Dr. Arjan Bik of the Max-Planck Institute for Astronomy in Heidelberg, could obtain high quality spectra of the 16 most massive stars in W3Main simultaneously (see Figure 2 for details) by using the infrared multi-object spectroscopy (MOS) mode of LUCI-1 at the LBT.
"For the first time we were able to derive ages of several massive stars inside W3Main and derive the time span over which the stars in W3Main are forming" said Dr. Arjan Bik, the leading author of the related scientific paper published in the January issue of the renowned Astrophysical Journal. "The oldest stars in the star forming region turn out to be already 2 million years old, while other stars are even younger and still forming and deeply embedded in their birth cocoons."
This result confirms that star formation in regions where massive stars form can be more complex than a single episode and stars can form over several million years. We now know that the stars in W3 main formed over a time span, starting 2 million years ago and continuing until today. This also suggests that the mutual interaction between the stars formed first and those still forming is a very important ingredient of the star formation process.
Backgroundinformation
The original scientific paper was published in the ASTROPHYSICAL JOURNAL, 744, 87 (January 10, 2012)
The Large Binocular Telescope (LBT) is a collaboration among the Italian astronomical community (National Institute of Astrophysics – INAF), The University of Arizona, Arizona State University, Northern Arizona University, the LBT Beteiligungsgesellschaft in Germany (Max-Planck-Institut für Astronomie in Heidelberg, Zentrum für Astronomie der Universität Heidelberg, Astrophysikalisches Institut in Potsdam, Max-Planck-Institut für Extraterrestrische Physik in Munich, and Max-Planck-Institut für Radioastronomie in Bonn), The Ohio State University and Research Corporation (Ohio State University, University of Notre Dame, University of Minnesota, and University of Virginia).
The LBT is located on Mount Graham (Arizona, USA) at 3200m above sea level. It utilizes two giant 8.4 meter mirrors on one mount making the LBT the largest single telescope in the world. With the adaptive optics system (to correct for atmospheric turbulences) and in interferometric mode (for combining the light paths of both 8.4 m mirrors), the LBT will achieve the resolution of a 22.8-meter telescope in its final increment (ten times better than the Hubble Space Telescope).
LUCI is an acronym for Large Binocular Telescope Near-infrared Utility with Camera and Integral Field Unit.
LUCI-1 is a remarkable new multi-purpose instrument with great flexibility combining a large field of view with a high resolution. It provides three exchangeable cameras for imaging and spectroscopy in different resolutions according to observational requirements. Besides its outstanding imaging capability which presently makes use of 18 high-quality filters, LUCI-1 allows the simultaneous spectroscopy of about two dozen objects in the infrared through laser-cut slit-masks. For highest flexibility the masks can be changed even at the cryogenic temperatures, through the innovative development of a unique robotic mask grabber which places the individual masks with absolute precision into the focal plane.
LUCI-1 and its twin LUCI-2 are mounted at the Bent Gregorian focus points of the LBT's two giant 8.4-meter (27.6 foot) diameter telescope mirrors. Each instrument is cooled to a chilly –213 degrees Celsius in order to observe in the near-infrared wavelength range. Near-infrared observations are essential for understanding the formation of stars and planets in our galaxy as well as revealing the secrets of the most distant and very young galaxies.
The LUCI – instruments have been built by a consortium of five German institutes led by the Center for Astronomy of Heidelberg University (Landessternwarte Heidelberg, LSW) together with the Max Planck Institute for Astronomy in Heidelberg (MPIA), the Max Planck Institute for Extraterrestrial Physics in Garching (MPE), the Astronomical Institute of the Ruhr-University in Bochum (AIRUB) as well as the University of Applied Sciences in Mannheim (Hochschule Mannheim).
Mittels der Verwendung des infraroten Multi-Objekt-Spektroskopie-Modus (MOS) von LUCI-1 am LBT konnten die Spektren gleichzeitig gemessen werden. Die gestrichelten Linien zeigen jene Details der Spektren, die für die Klassifikation der Sterne wichtig sind. Damit konnten die Astronomen die Altersbestimmung der Sterne vornehmen.
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