Artificial guide star allows better view of globular cluster
Washington: Astronomers at the Southern Observatory for Astrophysical Research (SOAR) and the Cerro Tololo Inter-American Observatory (CTIO) have obtained sharp stellar images using a new instrument dubbed SAM, for SOAR Adaptive Module, which creates an artificial laser guide star.
By observing the globular cluster NGC 6496 using SAM, built by CTIO/NOAO-S, they have demonstrated the significant difference that sharp stellar images can make in our understanding of the properties of stars.
The instrument is mounted on the SOAR 4.1-meter telescope.
From the surface of the Earth, stars twinkle as their image wobbles around due to the effects of the Earth's atmosphere, rather like observing a penny on the bottom of a swimming pool.
By removing this wobble, using an adaptive optics system that utilizes a laser guide star, the stellar images are sharpened, and fainter stars appear.
The resulting stellar images allow astronomers to make more precise measures of the colours of the stars, and for a globular cluster, this translates into a better measurement of distance, age, and what astronomers call metallicity; how much the stars are enriched with elements that are heavier than hydrogen and helium. This, in turn, allows for better understanding of the stellar evolution of the stars in these dense clusters.
There are only about 150 known globular clusters in Milky Way, important because they represent some of the oldest objects in the galaxy. Because NGC 6496 is on the other side of the galactic center, it is seen through a thick layer of dust. The position of this globular cluster has made it difficult to determine its basic properties. For example, previous measurements of its distance do not agree well with each other.
Luciano Fraga, Andrea Kunder, and Andrei Tokovinin used the capabilities of SAM to sharpen star images to peer deep into this crowded cluster, obtaining more accurate results than done previously from the ground.
The authors find a distance of 32,600 light-years, an age of 10.5 billion years, and a value for metallicity that is much higher than in most globular clusters.
To do this, they measured over 7,000 stars in the cluster. Then they plotted the colour and brightness of each star, resulting in a diagram referred to as a colour-magnitude diagram. This diagram immediately tells astronomers a great deal about the evolutionary phase of the stars in the cluster.
Their work is described in a paper accepted for publication by the Astronomical Journal.