ALMA Catches a Dying Star Mid-Transformation and Finds a Ring From Its Last Outburst
Radio images of IRAS 06530 reveal a vast ring, spiral arcs and a young central nebula, but none of the fast jets that theory says should have shaped them.
Astronomers using the Atacama Large Millimeter/submillimeter Array, or ALMA, have caught a dying star in the middle of a dramatic transformation. The radio data reveal a giant ring that appears to record the star's final outburst. The research was posted to the arXiv preprint server on Sept. 18 and has been accepted for publication in The Astrophysical Journal.
Most stars in the sun's mass range do not end in supernova explosions. They shed their outer layers and become planetary nebulae, the colorful shells that show up in deep-space photographs. The star in this study, IRAS 06530, is a carbon-rich object in the stage just before that, called a preplanetary nebula. It had already been imaged by Hubble, which made it a good target for a closer look.
There is a long-standing puzzle behind the work. Red giants blow off slow, rounded shells of gas, yet most planetary nebulae and their predecessors are not spherical. They can be bipolar, multipolar or elliptical. The leading explanation is that fast, narrow jets, probably launched through interaction with a binary companion, carve the slow wind from the inside out during the brief preplanetary phase. To test simulations of that process, astronomers need high-resolution measurements of the masses and motions of the gas in a representative sample of objects. The study, led by Raghvendra Sahai of NASA's Jet Propulsion Laboratory, is part of a survey called PANORAMA that aims to supply them.
The team tracked carbon monoxide and other molecules to learn how dense, warm and massive each region is, and used the gas speeds to work out how old each structure is and how fast the star was shedding material. From the star's brightness, they estimated its original mass at roughly 1.6 to 3.4 times the sun's.
The images show three main structures. The first is a large circular ring with a radius of about 16,950 times the distance from Earth to the sun. The team says it is the first evidence of the star's last brief flare-up near the end of its life. Mass loss slowed for about 2,000 years after the flare-up, then picked up again as the star moved into its next phase. At the center sit a two-lobed nebula with a dense, doughnut-shaped torus, formed within the last 700 years or so. The torus has unusual carbon, oxygen and nitrogen ratios that standard models cannot explain, possibly pointing to explosive nuclear burning. Between them lies a fainter set of curved arcs that holds most of the system's mass, far more than the ring or the central nebula. The arcs resemble the spirals seen around some bloated red giants with binary companions and hint at a hidden companion roughly 155 to 340 times the Earth-sun distance from the star.
The surprise is what is missing. If fast jets from a companion shaped the center, spectra should show gas moving faster than 50 kilometers per second (31 miles per second), as in typical preplanetary nebulae. The team found nothing faster than 16.5 km/s (10.3 miles per second) and no compact dusty disk at the star's position. The authors suggest a short-lived disk may once have launched brief jets before dissipating. They also say a companion at such a wide separation would interact strongly only if its orbit is elongated enough to bring it close at times. Accretion onto a companion could still power the outflow, even at about 75 times the Earth-sun distance. The result leaves IRAS 06530 as a layered system with a history astronomers are still working out.
