He2-90 in Centaurus

I would also like to dedicate this page to my fellow PNe observers in the Astronomical Society of New South Wales Inc., Mike Kerr (2006) and Scott Mellish (2011), who, since this was first written, have now both sadly passed away. Both had contributed much of their own skills and knowledge to this article in the NSP Series. There sad loss has been genuinely difficult for me and my fellow members of the ASNSWI, who miss their real camaraderie and fellowship.

I have selected another faint and difficult planetaries near the border of eastern Centaurus and Crux. As planetaries go, He2-90 may not rank as the most elegant, but it has the advantage of being quite near to one of the very best examples of an open star cluster — the absolutely fantastic and true southern observers delight, the Jewel Box / NGC 4755, which I like to term Gemma Australis Magnifica; and near the large Coal Sack dark nebula, just south east of the smallest constellation, Crux.

He2-90 / Sa2-90 / ESO 132-1 / PK 305+1.1 / Wray 16-125 / PN G305.1+01.4 (13097-6120) is placed 2°SEE from κ Crucis (NGC 4755), but it is not usually listed in any of the popular catalogues or atlases. First discovered by Karl Henize in 1964, it is often listed, like in electronic atlases like Megastar 5.0 — but under the alternative designation Sa2-90. It best found by using the double star J Cen / J Centauri / Δ133 (13227-6059), and then 1.6°W. He2-90s position is not marked in Sky Atlas 2000.0, but if you are using Uranometria 2000.0, the field of the planetary is marked as a tiny triangle of stars near the bottom of Map 432 and next to the RA line marked 13h 08m. The 10′×10′ field shows the STScI Image (Figure 1) of the small planetary, which is west of the triad of stars. This same triad can be also be seen in the attached field-finder chart in Figure 2. I attach this chart, as it is essential to find the PNe, which has a self-imposed 15th magnitude limit, with all stars of above 10th magnitude have there visual ones displayed.

He2-90 Observed by Scott
MellishHe2-90 is listed as 13.7p mag and subtends 10 arcsec. I could not see He2-90 directly using 30cm, but applying averted vision, reveals the small 4 arcsec star-like blip, which magically and unexpectedly disappeared in the O-III filter! I thought the visual magnitude to be about 14th, and this was confirmed once I had compared to the star chart.

I asked the late-Scott Mellish (Author of the series in the Journal Universe Sky-Sketchers Post Mortem) to observe this object with his 40cm. f/4.7 Dobsonian and 7mm Nagler. The observation was made under 7/10th seeing and transparency on 15th February 1999, where Scott states;

Almost gave up on this one. [He2-90 is] impossible to define as a planetary without a finder chart. Stellar in appearance, and remains so even in O-III filter, [and] even 8 arcsec. in size seems far too large. More like 2 arcsec.

This truly tiny and faint PNe lies in a profusely starry field. At first glance you might wonder why I even bothered looking for such an inelegant object. Honestly, the reason was that I found an interesting paper specifically on He2-90, and found it has characteristics quite unlike nearly all known planetaries — primarily it is very low [O-III] emissions. For example, if the relative intensity of the Hβ line is 100, then the emissions from the [O-III] line is O-III=205 (and the Hα=856) normally, values of [O-III] would be ten times this value. It is no wonder the O-III does not work as well as it should!

He2-90 Small
He2-90 Wide Field

Fig. 1. He2-90 : Wide Field Colour Image Aladin 12½′×12½′ (left) ;
Fig. 2. He2-90 : Enhanced Narrow Field Image 30″×30″ (right)

Studies of Unusual Planetary He2-90

A diameter stated in the ESO-Strasbourg Catalogue is less than 10 arcsec, while in the article PASP, 103, 275 (1991) states that the Hα image finds a circular 12 arcsec disk surrounded by a faint nebula shell.

One significant difficulty with observations of this PNe is that it lies a mere 21′NE from the northeast edge of the Coal Sack, euphemistically called the Black Magellanic Cloud, and the effect of extinction on the PNe in this region of the sky is uncertain. The most significant (and only) paper as (of 1999) on He2-90 was originally produced by Costa, et al. (1993).

It central star is 16.5v±0.25v and 15.6B±0.50B magnitude. It is believed the PNN is not very luminous, so that the ultraviolet energies that are required to illuminate the nebulosity are below par and are not very energetic — hence the poorness of the viewed PNe. Spectral measures also find the low carbon to nitrogen ratio of [C/N]=0.3, suggesting the original PNN progenitor mass is fairly small, perhaps around 0.6-0.7 M⊙. Again the spectra since the mid-1980s has very low abundances of elements such as Oxygen, Nitrogen, Sulphur, Neon and Argon, suggesting a low mass and/or an under-luminous star.

Not many of these objects like this are known, and include the PNe;

SwSt-1 / He2-377 / PK 001-06.2 / PN G (18162-3052) in Sagittarius.
Hu2-1 / ARO 100 / PK 051+09.1 / PN G (18498+2051) in Hercules.

Observations were obtained by Costa et al. (1993) on 11th May 1991 and 4th April 1992, using the 1.6-metre Cassegrain at the National Astrophysical Observatory, at Brasópolis in Brazil. They obtained the spectrograph resolution of 0.7nm, with the spectra surrounding the planetary was taken elsewhere by A. Damineli on 6th June 1992 surrounding the red Hα region was accurate to about 0.04nm.

Graph 2 shows the line intensities of the major emissions between 372.7nm. and 733.0nm, using the data from Table 2 of Costa et al., p.187 (1993). A comparison spectra of the standard” PNe is also given. This clearly shows the deficiency in both [O-III] and spectra with the other lightweight noble gases. (Compare this with the information on He2-111 (NSP 16), which has this effect reversed and an excess of O-III.)

Measuring the Hβ flux and the diameter of about 6 arcsec, enabling the calculation of distance using the Shklovsky Method, whose distance is about 1.5kpc. (This is presently (1999) one of the only estimations for this object!) Measuring the slight blue-shift of the sharp spectral lines of SII and NII, finds that the radial velocity of He2-90 moves towards us at −31 km.s-1. Also from the spectra finds the expansion gas velocity is presently at a pedestrian 3 km.s-1.

Gleise et al. (1989) has determined the Zanstra temperature (TZ) of 50,000K, and later by Kaler & Jacoby (1991), who determined 51,000K. Latest measures of the PNN of He2-90, say these values maybe about 15% too high.

It appears the that the atmosphere surrounding the PNN is fairly compact and dense, with the luminosity being some one thousand times more luminous than the Sun. [log(L★/L⊙) is 3.0] This confirmed by observations by the IRAS satellite and in the near infrared (NIR). IRAS observations in 1982 at 100 mm makes He2-90 is a strong energies or flux PNe at this wavelength, and supports the NIR observations of the D classification in 1987 — where the major of visual brightness in these wavelengths come from circumstellar dust — an uncommon property with PNe. (See NSP 21: NGC 6445 that also shows significant dustiness.) Hα profiles produced by Damineli (1992) shows the PNN superwind velocity of 1.050km.s-1.

Note: A discussed in earlier parts of the NSP series, the velocity of this outflow emanates from the PNN itself. As the gas radial travels away from the PNN, its mass collides with the much slower 10.2 km.s-1 atmosphere loss that once occurred during the Asymptotic Giant Branch (AGB) stage, and causes the illumination of gaseous structures that we see in the telescope.

Using Damindis (1992) data, Costa et al. (1993) infer the PNN radii as around 0.38R⊙ or 500,000 km., and the local speed of sound” at the top of the photosphere being about 15 km.s-1. For a comparison, the speed of sound at sea level on Earth is about 0.3 km.s-1 and across the solar photosphere about 1.2 km.s-1.

Overall, the past evolution of He2-90 and the cause of the very low abundances remains particularly uncertain, though it is likely produced by either the low mass progenitor or under luminous star.

References to He2-90

  1. Costa, de Freitas Pacheco and Maciel He2-90 : a southern planetary nebulae with low metal abundances.”, A&A., 276, 184 (1993).
  2. Damindi, , (1992)
  3. Gleise, et al., , A&A., 222, 237 (1989)
  4. Kaler, J., Jacoby, , AJ., 372, 215 (1991)
  5. PASP, 103, 275 (1991)

He2-90 Update: 23rd June 1999

Attached is a processed image (Figure 3 — immediately below) by the late-Mike Kerr which discusses the weird nature of this planetary. There are few images that I know of that look like this! The image shows two quite unusual features;

1) The stunning red colouration.

2) The polar "spikes".

There is obviously unusual characteristics with this object, which reflects the low [O-III] abundance and the low PNN temperature.

From a private communication by e-mail on the 21st June 1999 Mike said;

Ive got an image for you of He2-90. The image is 100 arc-seconds square with north up and east to the left.
I produced this true-colour image using the narrow band image process I described at the last (ASNSW) Technical Meeting. The images are from the Innsbruck University database and are Hα and [O-III] images taken with ESOs 3.5-metre New Technology Telescope in Chile. I have applied a logarithmic scale to the images to compress the dynamic range and better show both the bright and faint parts of the object. I assume the two spikes are not part of the object but are diffraction spikes from the telescope, which have been enhanced because of my processing.
At the Hα wavelength(s), He2-90 appears to have a bright, circular central disk about 6″ in diameter surrounded by a fainter circular halo about 14″ in diameter. [O-III] emission is limited to a central, stellar-sized disk about 2.5″ in diameter. This would seem to match pretty well with Scotts observation. All in all, quite a strange planetary!

He2-90 Processed by Mike 
Figure 3 : He2-90 Imaged by Mike Kerr


Last Update : 24th October 2011

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