Bat Symbol, PN G318.4+41.4, PK 318+41.1, ESO 577-24
Virgo
R.A.: 13h40m41.3s
Dec.: -19°52'55"
Mag 13.1
Breaking with the current run on OOTW as galaxies this week’s challenge is Abell 36 in Virgo. Also known as the Bat Symbol this is a large but relatively faint planetary nebula. It was first discovered by George Abell on POSS plates and included first of all in his 1955 paper and then catalogued as number 36 in the classic 1966 paper on faint old planetary nebula. Abell 36 has a relatively bright central star at a magnitude of 11.3. Although its magnitude and spectral type (sdO) are well known there seems to be a lot of doubt about the central star’s temperature and I have seen estimates as anything between 93000K and 113000K.
The nebula’s distance also seems to be unknown to any accuracy. The mean distance seems to be around 640pc but I have seen distances for it as low as 800 light years (245pc). The early distance measures came from Hipparcos parallaxes but they were very poor as the CS is near the magnitude limit of Hipparcos. I guess with such a bright central star it may be possible to dig a parallax out of GAIA DR2 (Wouter?) (My attempt at this via Aladin gives a parallax for the central star of 2.2902mas which would correspond to a distance of 437pc approximately, if I have got my sums right 😊 ). The nebula itself appears to be a spheroid with various arcs projected on it from a later ejection event that is highly collimated. As these events are tilted it suggested that there is a continually precessing source at the centre so possibly a binary star, although none has been observed. The nebula is surrounded by a very faint large halo (approx. 5 degrees) of ionised material. This does not appear to have come from the PN itself but is ionised local ISM. The halo is also barrel shaped like the nebula and suggests the PN is optically thin to the ionising radiation so it leaks out to ionise the local ISM. The central star in Abell 36 is off centre and this could be due to the interaction of the nebula with the ISM as it ages.
PanSTARRS image of A36
From northern latitudes the nebula never rises very high and my only view of it from home (51N) is as a faint patch with my old 20” Obsession with the telescope pretty much pointing horizontal. It did respond well to an OIII filter. I must admit I have not tried it with the 22" as skies from the UK appear to be getting worse low down. I suspect it may be much easier to see from further south. Its size is variously given as 6’ or 8’. I was surprised to see it on the Astronomical League’s Planetary Nebula list when the brighter PN IC 972 in Virgo is not.
Reply With Quote