Sunday, April 12, 2015

Merging Stars

The remains of Nova Vul 1670, the "new star" that was seen in the year 1670. Observations of the molecular gas and its composition in the nebula find strong evidence that the nova was the result of the merger of two stars. The image shows visible light (blue), dust seen at submillimeter wavelengths (green), and molecular emission at submillimeter wavelengths (red). Credit: APEX, SMA, Kamins

Astronomers have known for decades that the merger of two normal stars is a frequent and astronomically important phenomenon. In globular clusters, for example, with as many as several million stars gravitationally bound together, collisions often occur between stars, producing stars that are more massive, hotter, and bluer than usual. In star forming clusters, mergers of small stars have been proposed as a way to form massive young stars, and computer simulations lend some support to this idea. Not least, some kinds of novae -- stars that suddenly brighten and were once thought to be “new” stars -- are the result of stellar mergers or near-mergers.

The variable star CK Vulpeculae (Nova Vul 1670) had a bright outburst in 1670-1672 and then dimmed. No counterpart was seen until 1982 when a nebula was found at its location, presumably a remnant of the outburst of 1670. The star itself remains undetected, presumably hidden behind a heavy dust layer ejected in that outburst. The nebula itself has been of interest to astronomers for decades because it is rich in molecular gas. CfA astronomer Nimesh Patel and his colleagues studied Nova Vul 1670 and its chemical composition using two millimeter telescopes capable of measuring its molecular constituents in detail, the Submillimeter Array and the Atacama Pathfinder Experiment (APEX).

The scientists report in the latest issue of Nature that Nova Vul 1670 is not only rich in molecular species, its gas has dramatically unusual isotopic abundances (that is, the atoms present, carbon, oxygen and nitrogen in particular, have extra neutrons in their nuclei). Element synthesis in stars is well understood, and produces specific isotopic ratios; in the solar system, for example, the ratio of carbon with an atomic number of 12 to carbon 13 is 89, but in Nova Vul 1670 it is ten times less. Similarly low ratios were found for nitrogen and oxygen isotopes.

The astronomers conclude that the atoms in Nova Vul 1670 were not produced in a normal stellar furnace, nor for that matter even in a furnace operating under very different conditions. Neither could they identify any kind of explosive event that would produce these ratios. The team argues that the most likely scenario is the violent merger in 1670 of two stars; the event ejected inner parts of the stars into the nebula, exposing the ashes from earlier stages of nuclear burning, and mixing them with more processed material. People watching the nova in 1670 were no doubt amazed at the appearance of a "new star". Imagine what their reaction would have been to find out it was actually the merger of two stars.


"Nuclear ashes and outflow in the eruptive star Nova Vul 1670," Tomasz Kaminski, Karl M. Menten, Romuald Tylenda, Marcin Hajduk, Nimesh A. Patel & Alexander Kraus, Nature, online version, 23 March 2015.