HR Delphini

Lore & Background

HR Delphini was discovered by George Alcock at 22:35 UT on 8 July 1967, after he had searched the sky for over 800 hours with binoculars. At discovery it had an apparent magnitude of 5.0. It reached a peak brightness of magnitude 3.5 on 13 December 1967, making it easily visible to the naked eye. Pre-outburst photographs taken with the Samuel Oschin telescope showed it as a roughly 12th magnitude star that might have been variable.

The nova was discovered 158 days before it reached peak brightness, and its light curve around the peak was very well observed. It had a very slow rise to maximum compared to other classical novae, taking 230 days to fade from its peak by 3 magnitudes, classifying it as a 'slow' nova. Its decline showed several brief outbursts, leading to its light curve being classified as type 'J' (for 'jitters'). In June 1970, it was detected in radio wavelengths at 3.7 cm and 11.1 cm with the Green Bank Interferometer, and at 1.95 cm with the Green Bank 140 foot telescope.

HR Delphini is a binary system with a donor star orbiting a white dwarf. The orbital period is 5.14 hours. The white dwarf mass is estimated between 0.6 and 0.75 solar masses, and the donor star is believed to be a main sequence star with mass between 0.52 and 0.58 solar masses and spectral type in the range K5V to M1V.

The system is surrounded by a bipolar nova remnant emission nebula, visible in Hα, Hβ, and forbidden lines of oxygen and nitrogen. The nebula is roughly ellipsoidal with a central ring around the equatorial region. It measured 3.7 × 2.5 arc seconds when discovered in 1981, and expanded to roughly 8.5 × 6.1 arc seconds when observed with the Hubble Space Telescope in 1997.

Reader's Guide

HR Delphini holds significance as one of the best-observed slow novae, with a light curve that was monitored from 158 days before peak through its extended decline. Its classification as a type 'J' nova—characterized by 'jitters' or brief outbursts during fading—provides a distinctive example of post-eruption variability. The detection of radio emission in 1970 at multiple wavelengths added to the understanding of the nova's ejecta and environment.

The resolved bipolar nebula, first seen in 1981 and later imaged by the Hubble Space Telescope, offers a detailed view of the expanding remnant structure, including a central ring. The binary parameters—orbital period of 5.14 hours, white dwarf mass between 0.6 and 0.75 solar masses, and a K5V to M1V donor star—contribute to models of nova systems and mass transfer. The pre-outburst variability suggested by archival photographs adds a layer of historical context to its eruption.