V745 Sco
Credit: 3D Model: INAF-Osservatorio Astro. di Palermo/S.Orlando;
Illustrated model: NASA/CXC/M.Weiss
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For decades, astronomers have known about irregular outbursts from
the double star system V745 Sco, which is located about 25,000 light years
from Earth. Astronomers were caught by surprise when previous outbursts
from this system were seen in 1937 and 1989. When the system erupted on
February 6, 2014, however, scientists were ready to observe the event
with a suite of telescopes including NASA’s Chandra X-ray Observatory.
V745 Sco is a binary star system that consists of a red giant star and a white dwarf
locked together by gravity. These two stellar objects orbit so closely
around one another that the outer layers of the red giant are pulled
away by the intense gravitational force of the white dwarf. This
material gradually falls onto the surface of the white dwarf. Over time,
enough material may accumulate on the white dwarf to trigger a colossal
thermonuclear explosion, causing a dramatic brightening of the binary
called a nova. Astronomers saw V745 Sco fade by a factor of a thousand in optical light over the course of about 9 days.
Astronomers observed V745 Sco with Chandra a little over two weeks
after the 2014 outburst. Their key finding was it appeared that most of
the material ejected by the explosion was moving towards us. To explain
this, a team of scientists from the INAF-Osservatorio Astronomico di
Palermo, the University of Palermo, and the Harvard-Smithsonian Center
for Astrophysics constructed a three-dimensional (3D)
computer model of the explosion, and adjusted the model until it
explained the observations. In this model they included a large disk of
cool gas around the equator of the binary caused by the white dwarf
pulling on a wind of gas streaming away from the red giant.
The computer calculations showed that the nova explosion’s blast wave
and ejected material were likely concentrated along the north and south
poles of the binary system. This shape was caused by the blast wave
slamming into the disk of cool gas around the binary. This interaction
caused the blast wave and ejected material to slow down along the
direction of this disk and produce an expanding ring of hot, X-ray
emitting gas. X-rays
from the material moving away from us were mostly absorbed and blocked
by the material moving towards Earth, explaining why it appeared that
most of the material was moving towards us.
In the figure
(pictured above) showing the new 3D model of the explosion, the blast
wave is yellow, the mass ejected by the explosion is purple, and the
disk of cooler material, which is mostly untouched by the effects of the
blast wave, is blue. The cavity visible on the left side of the ejected
material (see the labeled version) is the result of the debris from the
white dwarf's surface being slowed down as it strikes the red giant.
Below is an optical image from Siding Springs Observatory in Australia.
Optical Image of V745 Sco
This image of V745 Sco (also known as Nova 1937) was taken on February 6, 2014 by S. O'Conner (OCN, St. Georges, Bermuda). Scale: 16 arcmin x 16 arcmin.
(Credit: S. O'Connor (OCN, St. Georges, Bermuda)
An extraordinary amount of energy was released during the explosion,
equivalent to about 10 million trillion hydrogen bombs. The authors
estimate that material weighing about one tenth of the Earth’s mass was
ejected.
While this stellar-sized belch was impressive, the amount of mass
ejected was still far smaller than the amount what scientists calculate
is needed to trigger the explosion. This means that despite the
recurrent explosions, a substantial amount of material is accumulating
on the surface of the white dwarf. If enough material accumulates, the
white dwarf could undergo a thermonuclear explosion and be completely
destroyed. Astronomers use these so-called Type Ia supernovas as cosmic distance markers to measure the expansion of the Universe.
The scientists were also able to determine the chemical composition
of the material expelled by the nova. Their analysis of this data
implies that the white dwarf is mainly composed of carbon and oxygen.
A 3D print of the model
was also created (pictured below). This 3D print was simplified and
printed in two parts, the blast wave (shown here in grey) and the
ejected material (shown here in yellow).
A paper describing these results was published in the February 1st,
2017 issue of the Monthly Notices of the Royal Astronomical Society and
is available online.
The authors are Salvatore Orlando from the INAF-Osservatorio
Astronomico di Palermo in Italy, Jeremy Drake from the
Harvard-Smithsonian Center for Astrophysics in Cambridge, MA and Marco
Miceli from the University of Palermo.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for V745 Sco:
Coordinates (J2000): RA 17h 55m 22.2s | Dec -33° 14´ 58.6"
Constellation: Scorpius
Instrument: ACIS
References: Orlando, S. et al., 2017, MNRAS, 464, 5003; arXiv:1610.05692
Distance Estimate: About 25,000 light years
Source: NASA’s Chandra X-ray Observatory
