Chandra Projects
The Pleiades
ρ Ophiuchus
M 16
NGC 2024
θ¹Orionis C

Research Interests

Research Interests

• X-ray imaging spectroscopy of star-forming regions
• X-ray imaging spectroscopy of supernova remnants in the Large Magellanic Cloud
• X-ray and EUV spectroscopy of late-type stellar coronae
• X-ray photometry of stars in young open clusters
• X-ray emission from O, B, and A stars
• X-ray, EUV, optical, and radio monitoring of flare stars

The advent of space-based astronomy has allowed us to observe the cosmos in new portions of the electromagnetic spectrum. In particular, X-ray observatories have revealed high-energy emission from nearly all types of astronomical objects including the Sun, comets, stars, black holes, supernovae, and the most distant clusters of galaxies. In most cases the X-rays are produced by very hot gas whose temperature exceeds 1 million degrees Kelvin. 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.

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. Microflare simulations suggest that continuous low-level flaring may be the dominant coronal heating mechanism on these 100-300 million year-old solar-type stars. Although microflares are seen on the Sun, they are not the dominant source of heating in the solar corona.