X-Ray Emissions In Young Metal-Poor Stars Similar To Metal-Rich Stars
Lawrence LeBlond for redOrbit.com – Your Universe Online
Astronomers have found that a region of space known as the Wing has fewer “metals” (elements with more than two protons in the nucleus) compared to most other areas within our own Milky Way galaxy. There are also relatively lower amounts of gas, dust and stars in the Wing compared to the Milky Way.
This knowledge makes the Wing an excellent candidate for the study of the life cycle of stars and the gas lying between them. Not only are the conditions that exist in the Wing typical for dwarf irregular galaxies like the Small Magellanic Cloud (SMC), they also mimic ones that would have existed in the early Universe.
Star formation near the tip of the Wing occurs mainly in a relatively small region known as NGC 602, which contains at least three star clusters. One of these clusters, NGC 602a, is similar in age, mass and size to the Orion Nebula Cluster. Astronomers have been studying NGC 602a to see if its young stars have different properties when metals are few and far between. For the record, astronomers call all elements that are heavier than hydrogen and helium “metals.”
Using NASA´s Chandra X-ray Observatory, which was launched in July 1999, astronomers discovered extended X-ray emission from the two most densely populated regions in NGC 602a. This extended X-ray cloud likely comes from the population of young, low-mass stars in the cluster, which have been previously spotted by infrared surveys from Spitzer and optical surveys from Hubble.
The emission is not likely to be just hot gas blown away by massive stars, because the low metal content implies that these stars should have weak winds. By not detecting X-ray emission from the most massive star in NGC 602a, astronomers believe their observations make a strong case in their favor, because X-ray emission is an indicator of the strength of winds from massive stars. They have not detected any individual low-mass stars, but overlapping emission from several thousand stars is bright enough to be observed.
The results of their Chandra observations imply that young, metal-poor stars in NGC 602a produce X-rays similar to the stars with much higher metal content found in the Orion cluster. Based on their observations, the astronomers theorize that just like X-ray properties being similar in young stars from different environments, other related properties — including formation and evolution of planet-forming disks — are also likely to be similar.
The astronomers´ combined X-ray, optical and infrared data also revealed, for the first time outside the Milky Way, objects representative of an even younger stage of evolution of a star. These “young stellar objects” are only thousands of years old and are still embedded in the pillar of gas and dust from which stars form, such as the “Pillars of Creation” in the Eagle Nebula.
The results of this study have been published in the March 1 issue of The Astrophysical Journal. The lead author of the paper is Lidia Oskinova from the University of Potsdam, Germany. Her colleagues consist of researchers from several institutions across the globe, including those from the US, UK, Spain, Mexico, China, Belgium, Brazil, and elsewhere.