HM Sagittae

Lore & Background

HM Sagittae was discovered in 1975 by O. D. Dokuchaeva and colleagues when it increased in brightness by six magnitudes. It displays an emission line spectrum similar to a planetary nebula and was detected in the radio band in 1977. Unlike a classical nova, its optical brightness did not rapidly decrease, though it showed some variation. Observations in the infrared during 1978 showed it to be a very strong source with a spectrum consistent with a binary symbiotic system similar to V1016 Cyg. The cooler stellar component emits material that is ionized by a hot component, with emission from heated dust generated by the cooler star. By 1983, the infrared emission varied by a factor of 1.5 magnitudes in the K-band with a time scale of about 500 days. High resolution spectral examination in 1984 showed a bipolar outflow of matter at 200 km/s, with knots extending outward to 9 arcseconds. The nebula shows a bipolar, S-shaped morphology, similar to R Aqr.

Reader's Guide

The system consists of a central red giant star orbited by a compact object that accretes matter from the giant. The pair have an angular separation of 40±9 mas, aligned along a position angle of 130°±10°, with a physical separation estimated at 50 astronomical units. The giant component is most likely a Mira variable with a period of 527 days (as measured up to 1989) and is surrounded by a dusty shell composed mostly of silicates. The compact object is a hot white dwarf with 70% of the Sun's mass, orbited by an accretion disk. The 1975 nova-like outburst may have been generated by a burst of mass transfer from the giant to the white dwarf during periastron passage of an eccentric orbit, leading to a thermonuclear outburst. Winds from both stars collide to produce a shock region that is a source of ultraviolet emission. By 1985, a fading and increase in redness were observed, caused by dust obscuration. The hot component may inhibit dust formation around the giant except in the shadow region behind the star, which could explain individual dust obscuration events.

Did You Know?

• The system shows a bipolar outflow of matter at 200 km/s, observed in 1984.
• The white dwarf in the system has 70% of the mass of the Sun.