Consequences of a thermonuclear explosion and double star a system about 3,400 light-years away was witnessed Hubble Space Telescope.
HM Sagittae, or HM Sge for short, is what is known as a symbiotic system in which white dwarf feeds the companion red giant star. The stolen material forms an accretion disk swirling around the white dwarf. If too much material from the disk falls onto a white dwarf at once, the pressure and temperature increase so much that a thermonuclear explosion occurs on the surface of the white dwarf.
Although this explosion is not enough to destroy the white dwarf in a supernova, it releases enough energy to cause the system to brighten in what is called a “nova”.
Between April and September 1975, HM Sge went nova in the constellation Sagitta, Sagittarius. It became clear in night sky by six magnitudes from magnitude +17 (visible only to telescopes with an aperture greater than about 305 mm/12 inches) to magnitude +10.5, at which point it became more easily visible to telescopes with a smaller aperture equal to about 102 mm/4 inches , allowing amateur astronomers to observe it. This clarification is equivalent to an increase in v luminosity 250 times.
Related: “Vampire” neutron star explosions are associated with jets moving at near-light speeds
Since it became nova, HM Sge has been breaking the rules. Most novae fade after a few days; HM Sge remained at its peak brightness for years, until the mid-1980s, before slowly fading, punctuated by more notable dimming events. Even now it has only weakened to magnitude +12.
“In 1975, HM Sge went from being a nondescript star to something that every astronomer in the field was looking at, and at some point the flurry of activity slowed down,” said Ravi Sankrit of the Space Telescope Science Institute (STScI). declaration.
“Symbiotic stars like HM Sge are rare here galaxyand witnessing a nova-like explosion is even rarer,” Steven Goldman, also of STScI, added in a statement. “This unique event is a decades-long treasure for astrophysicists.”
Observations over the years of many telescopes have tried to get to the bottom of what is happening in HM Sge. Now, Goldman and Sankrit and their team have obtained new results based on Hubble Space Telescope observations in 2021 and data collected by NASAis now defunct SOFIA (Stratospheric Observatory for Infrared Astronomy), which contained an infrared telescope in the rear Boeing 747 aircraft, in 2021 and 2022.
The onset of dimming with respect to the system in 1985 has until now been at least partially attributed to the behavior of the red giant star. It’s called Mira variable (after the prototype of the Mira class — omicron Ceti — in the constellation Cetus, the Whale) and endures periodic pulsations roughly every 534 days. The start of the system’s dimming in the mid-1980s was attributed to one of two things. It could be caused by a larger-than-typical loss of mass from the red giant associated with its pulsations, which would create a dust outflow that blocks some of the light, or it could be the result of the 90-year, non-circular orbit of the white dwarf and red giant around each other. which further distances them from each other, reducing the amount of material flowing between them. Currently, the distance between the two components of the system is approximately 40 astronomical units (AU), where 1 AU is defined as the average distance between Earth and our sun, 149.6 million kilometers (93 million mi). For comparison, Neptune is 30 AU from the Sun.
Hubble’s observations also revealed a strong emission line from ionized magnesium. This emission line was not present in the spectra of HM Sge from 1990, when the white dwarf’s temperature was 200,000 degrees Celsius (about 400,000 degrees Fahrenheit). For highly ionized magnesium to exist in large quantities, the temperature of the white dwarf must have risen to 250,000 degrees Celsius (about 450,000 degrees Fahrenheit) during that time. This makes it one of the hottest white dwarfs known, despite the overall decrease in system brightness. What is causing this increase in temperature is currently a mystery.
In addition, SOFIA was able for the first time to detect emission lines from water vapor in the disc in a symbiotic binary star and use its signal as a proxy to measure the properties of the accretion disc. The water molecules appear to be moving at 29 kilometers per second, which is attributed to their speed flowing around the edge of the disk.
However, most of the emission lines in the spectrum of HM Sge weaken compared to 1990, indicating that the system is slowly changing and evolving, perhaps as the red giant and white dwarf move further apart.
Goldman and Sankrit’s team conclude that the HM Sge system settled to a “new normal” relatively quickly after the nova explosion in 1975, with only a slow decrease in brightness on average over the years (there was some increase and decrease in brightness, both in optical and infrared and not always the same time, again attributed to red giant behavior). The overall dimming can continue at a slow rate for many more years until the white dwarf and red giant come closer together in their orbits, increasing the amount of material flowing between the two and sparking another nova.
Finally, a white dwarf is a preview of what fate has in store for a red giant companion. Both were once like the sun stars in a binary system, one star is slightly more massive than the other. The more massive star used up its nuclear fuel more quickly and evolved into a red giant, eventually shedding its diffuse outer envelope to reveal its exposed, inert core—a white dwarf. The other star evolved slightly more slowly, but now follows the same path as its sibling, first evolving into a red giant and then a million years later into a white dwarf.
The gravitational jolt that the red giant’s transformation will cause could pull the two white dwarfs together. One day, if they collide, they will explode as Type Ia supernovabut that won’t happen for hundreds of millions or maybe even billions of years.
Findings from HST and SOFIA were published in The Astrophysical Journal.