The advent of space-based astronomy has allowed us to observe the cosmos in new portions of the electromagnetic spectrum. In particular, X-ray telescopes like the Chandra X-ray Observatory have revealed high-energy emission from nearly all types of astronomical objects including the Sun, comets, stars, black holes, supernovae, and the most distant quasars and clusters of galaxies.
In most cases the X-rays are produced by very hot gas whose temperature exceeds 1 million degrees. X-ray emission from stars like the Sun is produced in a corona of magnetically confined plasma. X-ray images of the Sun reveal the complexity of the magnetic fields which confine and heat the gas in the Sun's outer atmosphere.
The Sun and planets were formed 4.5 billion years ago along with many other stars in a massive star-forming region, like most stars in our Milky Way Galaxy. But star formation goes on today. We study these massive star-forming regions to understand the evolution of the massive stars and the not-so-massive stars like the Sun.
Many of the massive star systems have revealed themselves to be copious, sometimes variable x-ray emitters. We now know that many of the most massive stars live in close binary systems. The winds from the individual stars collide in a wind-collision zone producing x-rays. On some stars like theta 1 Ori C, the central star of the Orion Nebula, the x-rays are produced through the interaction of the wind with a large-scale stellar magnetic field.
Young stars like those found in star-forming regions and young open clusters are very active at X-ray wavelengths, typically 1000 times brighter in X-rays than the Sun. Active star-forming regions contain hot massive stars on or near the hydrogen-burning main sequence and cooler, less-massive protostars that have yet to reach the main-sequence. X-ray observations of the Orion Nebula cluster and the rho Ophiuchus cloud, two of the best-studied sites of ongoing star-formation, show high X-ray activity and X-ray temperatures in excess of 20 million K from both groups of stars. X-ray flares on protostars have been observed to decay over a period of a few hours to many days. The X-ray flare data are a good probe of the density and geometry of the magnetic fields. There is growing evidence that the magnetic fields extend far above the star's photosphere creating a magnetosphere of hot, dense plasma.
Large-scale magnetic fields are thought to play a central role in the collapse of the parent molecular cloud, in the maintenance of a circumstellar disk, in mass accretion from a disk, and in mass loss from a wind and/or a bipolar outflow. The interaction of a contracting, rotating young star, magnetic fields, a disk, and a wind will determine the evolution of stellar angular momentum. In particular, stars with more massive disks in the pre-main-sequence phase will arrive on the main sequence as slow rotators. The evolution of disks, rotation, and coronal heating in young solar-type stars will determine X-ray and ultraviolet photoionizing radiation levels present during the formation planets, and later, of planetary atmospheres. For stars that are just arriving on the main-sequence, X-ray and extreme-ultraviolet light curves and spectra show rapid variability and coronal temperatures of up to 25 million K.
In the research projects below, we explore the x-ray and infrared emission from newly born stars in a number of star-forming regions through out the Milky Way. If you would like to learn more, please contact me here.
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