George's Random Astronomical Object

Object 41: BoBn 1

Podcast release date: 22 February 2021

Right ascension: 00:37:16.0

Declination: -13:42:59

Epoch: J2000

Constellation: Cetus

Corresponding Earth location: About 350 km northwest of Mocamedes, Angola, in the Atlantic Ocean

BoBn 1 is a planetary nebula in the constellation Cetus, which is a constellation named after a mythological whale or sea monster because, when you look at the constellation, the stars don't look anything like a whale or sea monster. Just to review, a planetary nebula like BoBn 1 has nothing to do with planets. Instead, a planetary nebula is the interstellar cloud of gas that forms when a star about the size of the Sun dies. Stars much larger than the Sun explode as supernovae, but most other stars simply blow away their outer gas layers over a time period of thousands of years. Quite a few of these look like bubbles of gas, and that might be an accurate description for the appearance of BoBn 1, although the best images of this object are extremely blurry.

BoBn 1 was named after G. O. Boeshaar and H. E. Bond, who discovered the planetary nebula in observations made in 1976 [1]. The Bo stands for Boesharr, and the Bn stands for Bond, although I think they should have used Bo for Bond, which would make this nebula's name BoBo 1. Anyway, they were originally trying to measure spectra for various stars and mistook BoBn 1 for a star. The object was identified as a planetary nebula by the fact that its spectrum did not look like a continuous rainbow of colours as would be expected for star but instead looked like a series of colours emitted at very specific wavelengths that appear as a collection of bright lines in a spectrum, which is what is expected for planetary nebulae in general.

Our galaxy contains over 1000 planetary nebulae [2,3,4,5], but BoBn 1 has the distinction of being one of the very few planetary nebulae associated with the halo of our galaxy. The Milky Way Galaxy is a spiral galaxy, and when most people think of the structure of a spiral galaxy, they think of the disk, which is the part that contains the spiral arms, and the sphere-like bulge of stars at the center of the disk. However, the Milky Way Galaxy and other spiral galaxies also have a large spherical halo. The halo is where you would find most of the dark matter in a spiral galaxy, and although it contains all of the globular clusters as well as a few other random stars, it looks mostly dark. When astronomers find stars that they can say are associated with the Milky Way's halo, they tend to get excited, so finding a planetary nebula in the halo makes astronomers really excited. At the time it was discovered, BoBn 1 was only the third planetary nebula ever found in the halo, and although a few more halo planetary nebulae have been found since, they are still quite rare. Because it's in the halo, BoBn 1 is located very far from Earth. Some of the distance measurements range from about 54000 to 95000 light years (16.5 to 29 kpc) [6,7,8,9], which is comparable to the diameter of the Milky Way's disk.

The stars in the halo, including the stars that have formed planetary nebulae, could come from a couple of different places. Some are randomly ejected from the disk, while others are from smaller galaxies that have fallen into the Milky Way. In the case of BoBn 1, it appears to be one of four planetary nebulae associated with an object called the Sagittarius Dwarf Spheroidal Galaxy [10]. As indicated by the name, this is a dwarf galaxy with a spherical shape that appears in the constellation Sagittarius as seen from Earth, and it is in orbit around the Milky Way. If you look on a map of the constellations in the sky, you will see that Sagittarius looks more like a teapot than the archer that it's supposed to depict and you will see that Sagittarius is nowhere near the constellation Cetus where BoBn 1 is located. However, even though the center of this dwarf galaxy is in Sagittarius, it is in the process of being shredded by the gravitational forces of the Milky Way, and a stream of stars from the dwarf galaxy can be found in the halo encircling the disk of our galaxy. BoBn 1 is in the part of the stream that passes through the constellation Cetus.

What makes halo stars, including halo stars that came from dwarf galaxies, particularly interesting is that they are much older than the stars found in the disk of the Milky Way. This means that the stars formed when the universe was much younger and contained far fewer of the elements heavier than hydrogen and helium (which include things that most other people would think of as light like carbon, nitrogen, and oxygen as well as elements that everyone agrees are heavy like iron). So, in general, if astronomers want to understand what stars were like in the early universe, they may spend time looking at halo stars.

Seeing a planetary nebula that formed from a very old star like BoBn 1 is very interesting because it is potentially an example of how these heavier elements were created when the universe was much younger and stars had far fewer heavy elements to begin with. BoBn 1 contains a lot of elements such as fluorine, rubidium, krypton, xenon, and barium that were created in a fusion process in which the nuclei of the elements get larger by slowly capturing neutrons over time, with slightly less than half of the captured neutrons forming protons through radioactive decay processes after they get captured [11,12]. This fusion process would have taken place before the planetary nebula formed, but the nebula itself represents the stage where these elements get dispersed into the interstellar medium and when they are easy to detect from Earth.

Interestingly, the large amounts fluorine in BoBn 1 along with large amounts of carbon and nitrogen imply that the planetary nebula formed from a binary star system [11,12]. One star would have been about 50% more massive than the Sun, and the other would have been slightly smaller than the Sun. The larger star would have transformed into a red giant first, and when it did this, it would have become large enough to swallow the smaller star, leading to the two stars merging and also leading to the production of extra fluorine in particular but also extra carbon and nitrogen, and all of this would have happened before the object finally became a planetary nebula. Presumably, the remnant from the planetary nebula is the core of the merged stars, which would have become a white dwarf consisting of carbon and oxygen that cannot be fused into heavier elements because the star is not large enough to trigger fusion. However, the planetary nebula is so far away that this white dwarf would be very difficult to see.

References

[1] Boeshaar, G. O. and Bond, H. E., Chemical abundances of a new halo planetary nebula., 1977, Astrophysical Journal, 213, 421

[2] Acker, A. et al., The Strasbourg-ESO Catalogue of Galactic Planetary Nebulae. Parts I, II., 1992

[3] Parker, Quentin A. et al., The Macquarie/AAO/Strasbourg Halpha Planetary Nebula Catalogue: MASH, 2006, Monthly Notices of the Royal Astronomical Society, 373, 79

[4] Miszalski, Brent et al., MASH-II: more planetary nebulae from the AAO/UKST Halpha survey, 2008, Monthly Notices of the Royal Astronomical Society, 384, 525

[5] Frew, David J. and Parker, Quentin A., Planetary Nebulae: Observational Properties, Mimics and Diagnostics, 2010, Publications of the Astronomical Society of Australia, 27, 129

[6] Hawley, S. A. and Miller, J. S., Improved abundances in three halo planetary nebulae., 1978, Astrophysical Journal, 220, 609

[7] Kingsburgh, Robin L. and Barlow, M. J., Distances for galactic planetary nebulae using mean [O II] doublet ratio electron densities., 1992, Monthly Notices of the Royal Astronomical Society, 257, 317

[8] Mal'Kov, Yu. F., A self-consistent determination of the distances, physical parameters, and chemical composition for a large sample of galactic planetary nebulae: The distances and parameters of central stars and the optical depths of envelopes, 1997, Astronomy Reports, 41, 760

[9] Henry, R. B. C. et al., Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV. Synthesis and the Sulfur Anomaly, 2004, Astronomical Journal, 127, 2284

[10] Zijlstra, Albert A. et al., The planetary nebula population of the Sagittarius dwarf spheroidal galaxy, 2006, Monthly Notices of the Royal Astronomical Society, 369, 875

[11] Otsuka, Masaaki et al., Detection of Fluorine in the Halo Planetary Nebula BoBn 1: Evidence for a Binary Progenitor Star, 2008, Astrophysical Journal Letters, 682, L105

[12] Otsuka, Masaaki et al., The Origin and Evolution of the Halo PN BoBn 1: From a Viewpoint of Chemical Abundances Based on Multiwavelength Spectra, 2010, Astrophysical Journal, 723, 658

Podcast and Website: George J. Bendo

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© George Bendo 2021. See the acknowledgments page for additional information.

Last update: 20 February 2021