Soccer Balls in Interstellar Space

Figure 1: The structure of fullerene C₆₀ 
Credit: NAOJ

How much C₆₀ exists in space? Since C₆₀ is very stable, could there be much more C₆₀ in celestial objects than on Earth? To answer these queries, the team focused on a class of dying stars called "planetary nebulae" (Note 2), because cosmic C₆₀ has previously been detected in these objects. Otsuka analyzed all of the relevant spectroscopic data taken by the SST and analyzed spectral data from more than 300 planetary nebulae, checking each carefully for evidence of C₆₀ infrared wavelengths. The team identified C₆₀ in several planetary nebulae, including the first detection of C₆₀ in M1-11 (Figure 2). Otsuka also found evidence for C₆₀ in the archived spectra of M1-11 taken by the VLT and proceeded to estimate the total amount and the temperature of C₆₀ in M1-11. Using data obtained with the SST and AKARI, he also investigated the dust grain composition in M1-11 and obtained the amount in each component. Although M1-11 is rich in carbon-bearing molecules and minerals, C₆₀ makes up only 0.01 percent of the total dust mass. Therefore, he concluded that C₆₀ is very rare in the interstellar medium.

Figure 2: Detection of C₆₀ in M1-11. Arrows indicate the postions of the C₆₀ emissions at 17.3 and 18.9 microns. (Credit: NAOJ)

How, then, did C₆₀ form in space? Characterizing the physical conditions in the environments that contain C₆₀ is an important step in understanding this process. Otsuka applied the method of elemental abundance analysis to unveil the evolutionary status of M1-11's nebula and its progenitor star, which were largely unknown until his research. The team used the 8.2 m Subaru Telescope's High Dispersion Spectrograph (HDS) and the Okayama 1.88 m telescope's near infrared imager and spectrograph (ISLE) to precisely measure line strengths of chemical elements such as carbon (C), phosphorous (P), and krypton (Kr). Detailed analysis of the spectra (Figure 3) revealed the relative amounts (abundances) of eleven elements. The team found that M1-11 is a very young planetary nebula, which formed from material ejected by the star ~1000 years ago; and it evolved from a star 50% more massive than the Sun. The obtained values in M1-11, namely, the C₆₀ mass and temperature, the elemental composition of the gas in the nebula, the mass of the progenitor star, and the evolutionary status, are very similar to those seen in other planetary nebulae containing C₆₀, e.g., Tc1, where the cosmic C₆₀ was detected for the first time by the SST (Note 3). The team concluded that C₆₀ is likely to form in carbon-rich dusty objects such as M1-11. They believe that chemical processing and the destruction of HAC (hydrogenated amorphous carbon) by the radiation as well as the strong wind from the central star of a planetary nebula produce the cosmic C₆₀.

Figure 3: Detailed analysis of the spectra of M1-11 taken by the Subaru Telescope's High Dispersion Spectrograph (HDS) (upper panel) and the 1.88 m Okayama telescope's near-infrared imager and spectrograph (ISLE) (lower panel). The emission lines from the carbon (C) and the phosphorous (P) are indicated by the blue characters; those from hydrogen (Paβ), helium (He), and oxygen (O), are indicated by black characters. In the ISLE spectrum, molecular hydrogen (H₂) is also noted. (Credit: NAOJ)

Using the Subaru Telescope's Cooled Mid-Infrared Camera and Spectrometer (COMICS), Otsuka has recently started a new project to investigate the spatial distribution of C₆₀ in planetary nebulae to verify the C₆₀ formation scenario. As Otsuka stated, "These new Subaru observations will further clarify the C₆₀ formation process".

References:
The research paper on which this release is based was published in The Astrophysical Journal (Otsuka et al." The Detection of C₆₀ in the Well-Characterized Planetary Nebula M1-11" The Astrophysical Journal, 764:77 (20pp), 2013 February 10).
The authors are:

• Masaaki Otsuka (Academia Sinica Institute of Astronomy and Astrophysics [ASIAA]/Space Telescope Science Institute [STScI]), Taiwan/USA
• Franciska Kemper (ASIAA), Taiwan
• Siek Hyung (Chungbuk National University), Korea
• Benjamin A. Sargent (Rochester Institute of Technology [RIT]/STScI), USA
• Margaret Meixner (STScI), USA
• Akito Tajitsu (Subaru Telescope), USA
• Ken'shi Yanagisawa (Okayama Astrophysical Observatory [OAO]), Japan

Acknowledgements:
This research was supported by the following:

• Space Telescope Science Institute (STScI) GO-1129.01-A, DDRFD0101.90128, USA
• National Aeronautics and Space Administration (NASA) NAO-50-12595, USA
• National Science Council NSC100-2112-M-001-023-MY3, Taiwan
• National Research Foundation of Korea NRF-2011-005077, Korea

Notes:

1. A fullerene is any carbon molecule with the chemical formula C₆₀. It is patterned in a very specific way, like the pentagons and hexagons that make up the shape of a soccer ball. Named in homage to the designer, author, and inventor Buckminster Fuller, fullerenes resemble the geodesic domes that Fuller invented. C₆₀ is nicknamed a "buckyball", once again in honor of Buckminster Fuller.
2. Planetary nebulae are dying Sun-like stars that were initially one to eight times the mass of the Sun. They are clouds of glowing gas and dust surrounding a hot star (over 30,000 K).
3. See http://www.sciencemag.org/content/329/5996/1180.abstract