Massive Star Formation and Missing Supernova Relics of the Milky Way


Glostar Image


A new view of star formation in our home galaxy has revealed some previously hidden secrets. Astronomers using two of the world’s most powerful radio telescopes have made a detailed and sensitive survey of a large segment of the Milky Way, detecting previously unseen tracers of massive star formation, a process that dominates galactic ecosystems and solving the mystery of the galaxy’s missing supernova remnants. 

The scientists combined the capabilities of the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) and the 100-meter Effelsberg Telescope in Germany –one of the largest fully steerable radio telescopes on galaxy–to produce high-quality data that will serve researchers for years to come.

The GLOSTAR Survey –Unveils Formation of Massive Stars

Stars with more than about ten times the mass of our Sun are important components of the Galaxy and strongly affect their surroundings. However, understanding how these massive stars are formed has proved challenging for astronomers. In recent years, this problem has been tackled by studying the Milky Way at a variety of wavelengths, including radio and infrared. This new survey, called GLOSTAR (Global view of the Star formation in the Milky Way), was designed to take advantage of the vastly improved capabilities that an upgrade project completed in 2012 gave the VLA to produce previously unobtainable data.

GLOSTAR has excited astronomers with new data on the birth and death processes of massive stars, as well on the tenuous material between the stars. The GLOSTAR team of researchers has published a series of papers in the journal Astronomy & Astrophysics reporting initial results of their work, including detailed studies of several individual objects.

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Detected Telltale Tracers

The survey detected telltale tracers of the early stages of massive star formation, including compact regions of hydrogen gas ionized by the powerful radiation from young stars, and radio emission from methanol (wood alcohol) molecules that can pinpoint the location of very young stars still deeply shrouded by the clouds of gas and dust in which they are forming.

The survey also found many new remnants of supernova explosions — the dramatic deaths of massive stars. Previous studies had found fewer than a third of the expected number of supernova remnants (SNRs) in the Milky Way. The GLOSTAR observations can detect smaller and fainter SNRs than previous surveys. In the region it studied, GLOSTAR more than doubled the number found using the VLA data alone, with more expected to appear in the Effelsberg data. 

Mystery of the Missing Supernova Remnants

“This is an important step to solve this long standing mystery of the missing supernova remnants,” said Rohit Dokara, at the Max Planck Institute for Radioastronomy (MPIfR) and lead author on a paper about the remnants.

“We believe that this is an observational bias against small and/or faint SNRs.  Deeper and high-res surveys are likely to detect more and more remnants,” wrote Dakara to The Daily Galaxy in reply to why earlier studies had detected only a third of the expected supernova remnants.”

The GLOSTAR team combined data from the VLA and the Effelsberg telescope to obtain a complete view of the region they studied. The multi-antenna VLA — an interferometer — combines the signals from 27 widely-separated dishes, each with a diameter of 25 meters, to make images with very high resolution that show small details. The VLA interferometer has four different configurations: the most extended baseline can reveal small structures while the most compact configuration, where all 27 dishes are within 600 meters of the center, can probe large-scale structures. However, even in its most compact configuration, the VLA cannot detect the largest structures in the sky. 

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The space between two antennas of an interferometer must be small to detect large scale structures: the smaller the closest spacing, the larger the structures it can detect,” explained Dokara in his email to The Daily Galaxy. “We cannot get closer than the antenna dish size, so there’s always large scale structures that are missing from an image made by an interferometer.” This is why the team also used the 100-meter-diameter Effelsberg telescope, which provided data on structures larger than those the VLA could detect, making the image complete.

“This clearly demonstrates that the Effelberg telescope is still very crucial, even after 50 years of operation,” said Andreas Brunthaler of MPIfR, project leader and first author of the survey’s overview paper.

Visible light is strongly absorbed by dust, which radio waves can readily penetrate. Radio telescopes are essential to revealing the dust-shrouded regions in which young stars form.

The results from GLOSTAR, combined with other radio and infrared surveys, “offers astronomers a nearly complete census of massive star-forming clusters at various stages of formation, and this will have lasting value for future studies,” said team member William Cotton, of the National Radio Astronomy Observatory (NRAO), who is an expert in combining interferometer and single-telescope data.

Detection of Atomic and Molecular Spectral Lines

“GLOSTAR is the first map of the Galactic Plane at radio wavelengths that detects many of the important star formation tracers at high spatial resolution. The detection of atomic and molecular spectral lines is critical to determine the location of star formation and to better understand the structure of the Galaxy,” said Dana Falser, also of NRAO.

Moreover, the GLOSTAR survey is providing a more accurate census of the types, sizes, chemical abundances, luminosities, and kinematics of SNRs. In the email to The Daily Galaxy, Dokara wrote, “SNRs are the major source of distribution of metals that were produced inside a star (“chemical enrichment”).  Their shockwaves heat up the volume around it, increase turbulent pressure, and create/destroy dust.  All these “feedback” processes affect the star formation itself, and this of course once again affects the birth of new SNRs.  Ideally we would like to have a complete catalog of SNRs to understand the star formation history of our Milky Way better.”

The initiator of GLOSTAR, the MPIfR’s Karl Menten, added, “It’s great to see the beautiful science resulting from two of our favorite radio telescopes joining forces.”

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Rohit Dokara and NRAO

Image credit: GLOSTAR image, using data from both the VLA and the Effelsberg radio telescope, shows a segment of the Milky Way’s disk, revealing previously unseen tracers of massive star formation. Brunthaler et al., Sophia Dagnello, NRAO/AUI/NSF.


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