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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
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.