Stars and their evolution play an important role in our Universe. Stars form by the collapse of large clouds of gas and dust in the universe. In our Milky Way, there are some regions of star formation even today. We can study those regions of star formation to understand how stars and planets form and how massive stars influence their environment. Once they are fully formed (“on the main sequence”), both very massive and low-mass stars continue to emit X-rays and other high-energy radiation. We study the formation and evolution of stars of all masses, and in particular their X-ray properties.
The birth of stars, when clumps of molecular gas contract to form a protostar, sets the conditions for which planets and potentially life may form. For most stars, this phase is characterized by the formation of a circumstellar disk composed of gas and dust where the central protostar builds mass over time by accreting material from its disk. This stage of stellar evolution also exhibits outflows, local gas and dust being accelerated away from the system, in the form of jets and hot winds.
X-ray grating spectroscopy allows us to measure the temperature and density of the plasma that generates the X-rays. Spectra from low-mass stars (similar to our Sun) have a number of emission lines, in particular they have lines from oxygen and neon that is highly ionized and have only two electrons left (Ne IX and O VII). In most stars, these lines are in the so-called “coronal limit” because their X-ray are formed in the corona, a thin, outer layer around the star, that is magnetically heated. In this low-density plasma, the “f”-line of the Ne IX and O VII triplet is strong, and the “i” line is weak (blue spectrum on the right in the figure). However, in accreting stars like TW Hya, the “f” line is weak and the “i” line is strong (red spectrum, left in the figure). Here, mass falls in from the circumstellar disk onto the star. As it hits the stellar surface, a strong shock forms, which heats the gas to X-ray emitting temperatures. This extraordinary spectrum of TW Hya was first observed with the Chandra/HETG by our group and we continue to study accreting stars.
More massive stars evolve faster, but they also wield a lot more influence about their environment through their massive winds and strong radiation fields that can evaporate dust and circumstellar disks of lower mass stars in a star forming region. The Orion Nebula Cluster (ONC) is the closest region of massive star formation and we have spent several weeks of Chandra/HETG observing time on this region. Because there are so many stars, both massive and lower-mass, the grating spectra sometimes overlap in the image, but as the sky rotates around over the course of the year, Chandra catches the ONC at different angles. The image shows a superposition of many different observations at different times and with different roll angles. Knowing where the brightest stars are, we can extract the spectra and filter out regions with overlap. This is not a simple task, but it allows us observe grating spectra of several dozen stars at the same time!