Object Of The Week August 06, 2017: HM Sagittae

HM Sagittae, Nova Sge 1975, PK 053-03 2

Symbiotic star

Constellation: Sagitta
RA: 19h 41m 57.086s
DEC: +16º 44' 39.94""

Mag: 11.10 (V)

"Wait. WHAT? This is a deep sky forum so what is a star doing here?" You may think this right now. Well, hold on. Yes this is a star, or actually two (see below), but it has some properties that mark it as a deeps sky object as well. As a matter of fact, Simbad lists it primarily as a nova but also as symbiotic star and planetary nebula.

Symbiotic stars have been mentioned already several times on this forum and two already have been (part of) object of the week: R Aquarii and Haro 1-36. As a recap, symbiotic stars are very close binary stars usually comprising of a white dwarf and a late sequence red star. They are indicated by the exhibition of spectral lines of the highly contrasting temperatures of the red giant (~3000 K) and the white dwarf (~20 000 K). The white dwarf gains mass from stellar wind from the red giant or gravitational streaming. Thank you Wikipedia!

Kent Wallace's new book "Visual Observations of Planetary Nebulae" states: In April 1975 this star brightened from around m=16 to m=11 showing emission lines in September 1975, roughly a 100 times increase in brightness in about five months.

Currently, according to AAVSO, the star is around m=12.2 so the magnitude quoted above from Simbad is the maximum brightness that this star reached in 1975.

In 1999 or 2000 the Hubble Space Telescope was pointed at HM Sge to produce this image:

Attachment 2624

The first time the object was visually observed was on 14 October 1990 by Jack Marling using a 17.5" F/4.5 reflector. I observed this star on July 29, 2017, and my notes read

"Visible at 256x and 320x. Stellar. Responds very well to OIII, less to UHC and hardly to H-Beta. I didn't try higher magnifications due to poor seeing and lots of wind."


As always,

"Give it a go and let us know!
Good luck and great viewing!"

Interesting object, for sure. Here's an observation I made in 1989 (a year before Jack Marling):

17.5" (9/2/89): at 220x; bright, stellar object, estimate V = 10.5. Appears as a large magnitude contrast double star with a mag 14 companion just 15" S. Located in a rich star field with two mag 11 stars 1.7' N and 3.2' ENE. These stars are fainter using the OIII filter but HM Sagittae has a good contrast gain with the filter.

Note: Listed as a possible proto-planetary in 1976 although recent analysis shows it to be a mag 11 symbiotic star system.

Pity that Kent didn't know about your observation, Steve!

Oh, I can't take credit for the first. HM Sagittae was discussed in an Appendix to the 1979 Webb Society Handbook Volume 2 on "Planetary and Gaseous Nebulae" and there is reference to using a direct vision spectroscope or prism, along with observations using an 8½-inch reflector and a prism, probably in the mid- or late-1970's.

With enough power, and correspondingly good seeing to allow it to be useful, HM Sge is non-stellar. My notes from August, 2010 on a night I rated at 9/10 seeing:
"Tiny p.n., stellar at most powers. Starts showing a hint of a disc at 769x. Definitely non-stellar at 1099x and 1539x.
Responds well to UHC filter."