A. Introduction

I saw my first image of this target, the Toby Jug Nebula, years ago, just as I was starting to get back into astronomy. I was immediately taken with its dramatic look. I had never seen a yellow nebula nor nebulae with as finely defined and symmetric a structure as this one. Then in 2019, I read that amateurs had discovered a theretofore unknown pair of red bubbles around the yellow nebula (https://iopscience.iop.org/article/10.3847/2515-5172/aac6c7).

That blew my mind entirely.

First, what an amazing thing: the biggest telescopes in the world operated by professional astronomers from the finest sites had been used to study this target, but it was an amateur that discovered a large new structural element. What could be more inspiring for the rest of us? Second, what an aesthetic enhancement to an already beautiful target.

The European Southern Obervatory has made this target one of the members of their Cosmic Gems program (https://www.eso.org/public/outreach/gems/), so I am not the only one convinced of its particular beauty.

The target is inaccessible from the northern hemisphere and so was out of reach of my imaging efforts at the time. Once my system in Chile was up and running, this was top of the list.

I hope you enjoy!

B. Target Description

IC 2220 is a rare bipolar (double lobed) reflection nebula first detected in 1908 in photographs taken from Arequipa. Located about 1,200 light years from earth in the southern constellation Carina (the Ship’s Keel), it is estimated to be roughly 3,900 years old. Which means its birth coincides with the Bronze Age on earth, a fitting coincidence given the nebula’s color. Its major axis is roughly 1.3 light years in length. The mass of the material in the nebula is estimated at one of our suns (not including the Ha bubble (CCD Surface Photometry of IC 2220 1991)).

There are 3 basic types of reflection nebulae:

Newly formed stars illuminating their dusty birth places (for example, the Horsehead and the Iris Nebulae).
Dense interstellar clouds illuminated by chance encounters with luminous stars (the Pleaides).
Nebulae created when an evolving AGB star loses dust, creating an illuminated envelope around itself.

IC 2220 is a member of this third class. This stage in the life cycle of a star is a transition stage as the star becomes a protoplanetary nebula. Studying stars in transition can improve our understanding of the development of the beautiful, complex shapes of planetary nebulae, so such objects are much sought out by astrophysicists. However, the extreme brevity of this transitional stage means that these objects are extraordinarily rare – only a handful of AGB stars with bilpolar reflection nebulae have been discovered.

There are several key structural elements of IC2220:

the central AGB star,
the “yellow” reflection nebula itself
the fluffy red bubbles of hydrogen that surround IC2220; these are an emissions nebula

C. The Major Structural Elements of IC2220

1. The Central Star

The central star of IC 2220 , known by the catchy moniker HR 3216, was first found on a Schmidt plate taken in 1971 at the Cerritos Tololo observatory in Chile. I note the location only because you can literally see it from Obstech, the observatory from which my image was taken.

The star is an M3 red giant, five times the size of the sun with a current mass of 2.2 of our suns. There is no definitive determination of its initial mass (i.e. before it lost the material that makes up the nebula), because the full extent (and therefore mass) of the nebula is not well understood and it is also unclear if all the mass in the nebula came from this star.

Despite being only 50 million years old, it is in a much later stage of life than the sun, its greater mass having resulted in it burning through its fuel supply more quickly. Its temperature is now ~3,700K, much cooler than our sun’s 10,000k.

An irregular variable star, it shows up to 0.9 magnitudes of variability over an 11-15 year period. This variability is believed to be due to the clouds of dust and gas ejected from the star itself passing through the line of site between the earth and the star. Even during a single night, magnitude variations can reach 0.1 (Infrared interferometric imaging of the compact dust disk around the AGB star HR3126 with the bipolar Toby Jug Nebula).

Early studies in the 1970's showed that the distance of HR 3216 and the nebula were about 375 pc, the same distance as the open cluster NGC 2516 which is 1.5’ to the south. This (and several other factors) prompted the speculation that the central star of IC2220 was a former member of the cluster (Dachs, Isserstedt and Rahe).

You can see IC2220 and the cluster in this remarkable widefield image from the ESO:

Later studies using Gaia data have validated these distance estimates, though I could not find any conclusive determinations on the matter of the star’s former membership in the cluster (The dipole nebula IC 2220, a southern reflection nebula around the variable red giant HD 65750).

Why is this star interesting?

Brief background on the lifecycle of low mass stars

Stars that start out with initial masses less than roughly eight of our suns have a particular life cycle. They evolve into Asymptotic Giant Branch (AGB) stage stars, expanding and shedding their outer layers, creating the structures for protoplanetary nebulae (PPNe) and then planetary nebulae (PNe) as the stars themselves become white dwarves.

The mechanisms for this shedding of mass are a combination of stellar wind and massive thermal pulses. Stars exist in a dynamic equilibrium between two forces: gravity driving them to contract, counteracted by the heat of the fusion at their core causing them to expand. As a star burns through the hydrogen at its core and can no longer sustain hydrogen fusion, the thermal pressure counteracting gravity decreases, gravity causes the core to contract, increasing the temperature of the core until the thermal pressure from the core is high enough to overcome the contractional impulse of gravity, the outer layers of the stars then expand and cool, and the star grows into a red giant.

As the core continues to contract and gets hotter, it eventually becomes hot enough to start the fusion of helium. The star will eventually use up the helium as well. Then begins a series of alternating residual hydrogen/helium shell ignition events, creating thermal “pulses” (see https://en.wikipedia.org/wiki/Asymptotic_giant_branch or this interesting lecture https://youtu.be/NQaU_ZJlHZ0?feature=shared).

All AGB stars lose mass through their stellar winds, but these thermal pulses produce periods of even higher mass loss and can result in detached shells of material around the star. It was presumably pulses such as this that created the Fluffy Red Bubble.

It is this mass loss that has created the other structural elements of this nebula discussed below.

Very Rare

AGB stars develop bipolar or more complex structures as they evolve from the AGB stage through the PPNe stage to become the incredibly beautiful planetary nebulae that we observe in the sky. The actual dynamics that lead to these complex structures are not well understood and so astrophysicists need more observational data. Of particular interest are objects in the transitional phases between states – AGB stars evolving into PPNe and PPNe evolving into PNe.

Here is the rub, though: these transitional phases are incredibly brief in the cosmic sense, and so transitional objects such as these are very rare. Only a handful of red giants surrounded by self-generated reflection nebulae (i.e. AGB stars transitioning to protoplanetary nebulae) are known to exist.

IC2220 is one of these few. Moreover it is very close to us at ~1,200 light years and has created a large (~1 light-year long) nebula, enabling us to study it in detail.

2. The Toby Jug Structure

A Brief Cultural Diversion

Let’s start with that name. I needed to look up the definition of a Toby Jug. I enjoy the intellectual elements of astronomy but could never have anticipated some of the places this hobby would send me!

Per the website of the American Toby Jug Museum (!!!!) (https://www.tobyjugmuseum.com/toby-jugs), a Toby Jug (a “Toby”) is a ceramic pitcher modeled in the form of a person or animal. The original Tobies date back to the 1760's. A true Toby must be in the form of a full-bodied character (a jug comprised of only the head or face is a Character Jug). There are competing theories as to the origin of the name "Toby Jug", but per the Museum (which I choose to treat as the definitive authority on the subject) the name most likely comes from a notorious 18th century Yorkshire drinker, Henry Elwes, who was known as "Toby Fillpot" (a fantastic name in and of itself for a drinker) and was inspired by an old English drinking song, "The Brown Jug", which paid tribute to Toby Fillpot, whose ashes were made into Toby's jug; the popular verses were first published in 1761. I looked for an online performance of the song without success. However, I did find this documentary video on Toby Jugs, for those so inclined: https://www.youtube.com/watch?v=

IC 2220 was nicknamed the Toby Jug Nebula by British astronomers Paul Murdin, David Allen and David Malin who took some of the early photos of the object (Murdin, P. Allen, D., Malin, D., 1979, Catalogue of the Universe, Book Club Associates, Cambridge University Press.). Comparing my images to a Toby Jug, I personally could not see the resemblance. So I tracked down the image taken by Murdin, Allen and Malin, and all became clear:

So, here is the crazy thing.

If you stop to think about it for a moment, three astronomers have in effect acted like the Greek gods, taking a man who was “famous for drinking 2,000 gallons of strong stingo beer from his silver tankard” and immortalizing him in the sky as an astral object like a Greek hero.

This is a such a wonderful hobby!

Back to the Science

Origin of the Toby Jug Nebula

As discussed above, the nebula was created at least in part by mass loss from the central AGB star. Studies (e.g. (Infrared interferometric imaging of the compact dust disk around (2020)) show that while the nebula itself has a mass of roughly one of our suns (a sol), the star is losing mass through a bipolar mass outflow at a rate of 2-4 x 10-7 sol/year. This material is moving away from the star at a rate of 54km/s. From the size of the nebula and the rate of dispersion of the ejected mass, astronomers have concluded that the current mass-loss rate is too low to have created the nebula we see today, implying that there must have been an episode of higher rate mass loss in the past (The Toby Jug Nebula - A Bipolar and Biconical Nebula (1981)). It has also been suggested that nebular mass may have been contributed by a companion star (see below).

Shape

The early images of this object, such as the one above, captured only a portion of the nebula, which led to its comparison to a Toby Jug. Later, deeper images show its fuller extent, and that shape is frequently described in the academic literature as resembling a butterfly.

Perhaps. To me, it looks like a golden spider or a golden Egyptian scarab.

Regardless of your preferred visual analogy here, it is clear that this is a bipolar nebula, and at least one study characterizes it as likely biconical as well (The Toby Jug Nebula - A Bipolar and Biconical Nebula (1981)). To envision this, imagine looking at two cones with their pointed ends joined at the central star viewed from the side. The long filaments that extend to either side of the star make up the surface of the cones

What gave rise to the shape? The answer is a very definitive “we don’t really know”.

One early (1973) study I found noted that the filaments of the nebula resemble the structure of a magnetic dipole field centered on the red giant star at its center, suggesting that their structure may be due to a magnetic field generated by the star (https://ui.adsabs.harvard.edu/abs/1973A%26A....23..241D/abstract).

https://en.wikipedia.org/wiki/Magnetic_dipole

After going down a rabbit hole of research on that topic, I could find no studies that offered direct support for that idea for this object. While the authors of that study were not wrong as to the resemblance, since a dipole magnetic field seems to be bipolar by nature, that may just be a comparison of similar structures rather than an explanation of causation.

The 1981 study referenced above speculated that the biconical shape of the nebula could be explained if there a circumstellar disk, oriented orthogonally to the major axis of the nebula, with the stellar material being channeled out from the disk. They noted that they had no direct evidence for this explanation, and recommended follow up observations.

If they are right, it might look something like this (with a star in the center of the disk):

Forty years later, someone did in fact follow up and make additional observations (Infrared interferometric imaging of the compact dust disk around (2020). Astronomers theorize that the bipolar structures around red giant stars in this phase of their life may form due to interactions with a binary companion star. Several attempts have been made to find such a companion star for HR3126, and while not completely definitive have determined that it is very unlikely there is any companion currently. However, instead of a companion star, astronomers observed an extremely compact elliptical ring-like structure around the central star, which is assumed to be the inner rim of an equatorial dust disk with a radius of ~ 2au. Analysis of the dust ring suggests that the dust is composed of silicate grains as large as 4um (larger than the pixel size in many of our cameras!) The “image” take of the dust ring is below:

It has been suggested that the red giant shredded a former binary companion, triggering the formation of the disk and the nebula.

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Color of the Yellow Nebula

IC 2220 is an unusual reflection nebula in that its illuminating star is very red. Yet the nebula is obviously yellow. Substantial differential scattering of the shorter wavelength (blue) components of the light coming from the star by the nebular dust itself is part of the answer.

3. The Fluffy Red Bubbles

I should hasten to say that I am solely responsible for this appellation for these structures. As mentioned in the introduction, recent imaging by amateurs with a Ha filter revealed two sets of very faint, bubble-like nebulosities with diameters of 10’ and 15’ (Discovery of a New, Bubble-like Nebula around IC 2220) that surround IC2220. These outer nebulae are suggestive of episodic mass-loss events preceding the formation of the yellow bipolar nebula.

While faint these bubbles are very extensive. Interestingly, once you know to look for them, they are visible even in the (sufficiently deep) purely RGB images (that is, without the use of Ha enhancement). See the details below from my own paritally processed RGB images:

On the theory of “lets just see”, I took a substantial amount of data in the OIII and SII bands, but sadly found nothing in these wavelengths.

4. The General Environment

Widefield images of this area show that there is a substantial cloud of integrated flux nebula in the area, with one end of the cloud just reaching the tip of the eastern side of the Fluffy Red Bubbles around IC2220. In my image the dust clouds seem to wrap around and curl over the Ha structures, like clouds curling around red mountain peaks.

5. Small Galaxies

There are a huge number of galaxies in this field, several of which are of sufficient size to actually show some detail and warranted some individualized attention in processing. One such collection:

My favorite, though, is the one nestled amongst the alpine peaks of the eastern end of the Fluffy Red Bubble:

D. Data and Acquisition

Total time collected was ~75 hours. Seeing was reasonable during this period, so I was able to set a maximum FWHM cutoff of 1.4” and still have 7 or more hours across every filter, ~44 hours in total.

Measured FWHM of the unprocessed master lum is 1.19” and after BlurX is 0.57”. The limiting magnitude of the master lum is 24.97, which I believe is a personal record.

Note the Ha was acquired with a 65mm x 65mm filter, but I struggled to create that as a new equipment item in Astrobin's equipment database and so recorded it as 50mm x 50mm.

E. Processing

[b] [/b]

Color calibration. Done using SPCC[b] [/b]with average spiral galaxy as the white reference.
Stretching. Still using the LRGB technique recommended by Promper and Hayes and described in the PI official instructional videos, which has largely resolved the color saturation issues with which I had been struggling.
StarX. In a departure from the difficulties I have had with recent images, StarX worked like a breeze, easily separating the stars from the nebula and small galaxies. I did notice that I got fewer artifacts around the bright stars sitting in front of the nebula if I removed the stars before stretching rather than after, and so took that approach.
NoiseX. After repeated advice about not succumbing to the temptation to overdo NoiseX, I am using it in a more restrained fashion. If you have feedback on the level of residual noise in this image, please let me know.
Contrast Enhancement. I used a significant amount of HDRMT to bring detail back into the very bright region of the nebula around the central star while trying to still create a natural brightness gradient radiating out from the star into the nebula. This was followed by gentle applications of LHE at kernal sizes of 40 and 150. A very modest application of MMT through a range mask was also beneficial.
Curves. I fiddled around a lot with curves, but only for the K channel – no saturation or color adjustments. The challenge here was to find a balance between the sky background, the bright reflection nebula, the dim emissions nebula and the interstellar dust in the frame.
Ha. I used the Night Photon PhotemetricContinuumSubtraction script to create a starless, continuum subtracted Ha image, and then stretched it. I added it to the LRGBimage using pixel math, 4 parts to the red, 1 part to the blue.