M.Otsuka's homepage

Background image:  imaginative picture of fullerene C60 molecules distributing into interstellar medium from a planetary nebula. 

credit: National Astronomical Observatory of JAPAN (国立天文台)

Dr. Masaaki OTSUKA / 大塚雅昭

Project-specified Assistant Professor

Okayama Observatory, Kyoto University

Asakuchi, Okayama 719-0232, Japan

E-mail: otsuka [at] kusastro.kyoto-u.ac.jp 

Education

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Honors, Awards

Research Grants (as PI)

Research Interests

During the last stages of stellar evolution, stars eject materials enriched with atoms, molecules, and dust grains into circumstellar shells, and eventually these materials are returned back to the interstellar medium of the host galaxies. An investigation on atomic/molecule gas and dust grains ejected from evolved stars increases our understanding of stellar evolution, and chemical evolution of galaxies, and ultimately provides useful information for the formation of the solar system and the origin of our life.

My research goal is to investigate atomic and molecular gas and dust grains in evolved stars and galaxies, and understand both evolution of stars and galaxies. For that purpose, I have been with enthusiasm to study elemental abundances, dust, and molecules based on observational data obtained from 8-m class telescopes such as Subaru and infrared space telescopes such as Spitzer and Herschel. I have contributed to my research committee. 

Below, I introduce my highlights on fullerene C60 in Galactic planetary nebulae (my discovery of C60 in a planetary nebula, see my story)  and discovery of significant cold dust mass in the supernova SN1987A in the Large Magellanic Cloud.

keywords: Interstellar and circumstellar medium (atomic gas, molecular gas, dust grains), Stellar evolution, late stars (e.g., asymptotic giant branch stars, planetary nebulae, supernovae and their remnants), Elemental abundances, Diffuse interstellar bands (DIBs), Galaxy chemical evolution

Highlight 1. Physical Properties of the Fullerene C60-containing Galactic Planetary Nebulae

Figure 1: [LEFT] Detection of C60 in M1-11. Arrows indicate the positions of the C60 emissions at 17.3 and 18.9 μm (Credit: NAOJ). [RIGHT] The location of the C60-containing PNe (blue symbols) in the Milky Way (Credit: Otsuka et al. 2014, MNRAS, 437, 2577).

Since their laboratory discoveries in 1985, particularly C60 have drawn considerable interest from astrochemists looking for them in circumstellar/interstellar conditions. Fullerenes are extremely stable and easily form in laboratories on the earth, so it had been thought that they should exist in interstellar space. However, the first confirmed detection of cosmic fullerenes was only recently reported in the C-rich planetary nebula (PN) Tc1 with mid-infrared (mid-IR) Spitzer Space Telescope. Figure 1 left shows the C60 emission lines in the PN M1-11 (Otsuka et al. 2013, ApJ, 764, 77O). So far, C60 has been found in about 20-30 objects in the Milky Way and the Magellanic Clouds. However, it is still unclear what kind of environment is required to form fullerenes and how fullerenes are excited in the circumstellar/interstellar space.

To answer these fundamental and important questions, we newly found out 6 fullerene-containing PNe out of ALL Spitzer spectra of  over 300 Galactic PNe, and we investigated elemental abundances, dust grains/molecules, and the central stars (Otsuka, M. et al. 2014, MNRAS, 437, 2577O). Our interesting findings are (1) the chemical abundances of C60-containing PNe can be explained by asymptotic giant branch (AGB) star nucleosynthesis models for initially 1.5-2.5 solar mass stars with 1/5 solar metallicity, and the progenitors are an older population, (2) most of C60-containing PNe are outside the solar circle, consistent with low metallicity value (Figure 1 right).

After the infrared space telescope James Webb Space Telescope (JWST) are launched, I will do spectroscopic survey with JWST/MIRI (mid-IR instrument)/NIRSpec (0.6-5 μm spectrograph) and investigate C+60  (i.e., ionized fullerene C60) and C60 bands (i.e., neutral C60).

Highlight 2. Dust Production in Supernovae and Their Remnants; Detection of Significant Cold Dust in SN1987A

Figure 2: The spectral energy distribution of SN 1987A. Previously, Spitzer Space Telescope detected warm dust around the object. This dust formed before the explosion, but as shock waves impacted pre-existing dust grains, they heated up. Using Herschel Space Observatory, we detected the far-IR thermal radiation from cold dust that formed after the explosion (Credit: ESA/NASA-JPL/UCL/STScI).

In order to understand both stellar and galaxy evolution, we have to understand supernovae (SNe) as well. SNe influence atomic gas, dust, and molecules in interstellar matter, and they control the star formation rate. SN blast wave destroys interstellar matter. SNe play a critical role in material recycling in galaxies.

I have an interest in the origin of significant dust detected in high-z galaxies. At the present epoch in the Galaxy, both asymptotic giant branch (AGB) stars and core-collapsed SNe (CCSNe) are thought to be the important dust producers. In early galaxies, AGB stars are not likely to contribute significantly to dust production because such low-mass stars proceed too slowly toward the AGB to produce dust within 1 Gyr. However, massive stars can evolve into CCSNe in timespans under 20 Myr. Theoretical models predict that a CCSN evolved from a 20 solar mass star has to make at least ∼0.1-1 solar mass of dust in order to be a viable source for the dust found in high-z galaxies. However, there was a large discrepancy between the model predicted SN dust mass (~0.1 solar mass) and the estimated SN dust mass using mid-IR data (~1/10000 times solar mass).

From these reasons, therefore, since 2008 when I was a postdoc fellow  at the Space Telescope Science Institute, I have studied dust formation in SNe. By our Herschel far-IR observation, we resolved this discrepancy by our novel discovery of significant cold dust mass in the famous SN, SN1987A in the Large Magellanic Cloud (∼0.4-0.7 solar mass, Figure 2, Matsuura, Dwek, Meixner, Otsuka et al. 2011, Science, 333, 1258M). This result is strong evidence that CCSNe are significant dust producers in galaxies.

I still continue studying SN dust formation using mid-R instruments attached to ground-based telescope (i.e., Subaru/COMICS and VLT/VISIR). I am an active member of the JWST SN research consortium.

Recent research

Chemical evolution of galaxies is the direct consequence of material cycling between stellar mass-loss and ISM. However, it is not well understood “when, in which directions, how much mass containing what types of atomic/molecular gas and dust did stars eject into ISM”. To answer this vital question is very important to understand both stellar and galaxy evolutions. Therefore, using multiwavelength 3-D spectroscopic data of Galactic PNe, I am recently studying (1) spatial distributions of elemental abundances and gas/dust masses and (2) PN shaping process. 

This study is supported by the 3.8-m Seimei Telescope/KOOLS-IFU and so on. Below, I introduce the recent results of the planetary nebula IC2165 using KOOLS-IFU (Otsuka, 2022, MNRAS, 511, 4774-4800).

About 1 arcsec spatial resolution maps of IC2165 delivered by Seimei/KOOLS-IFU

Figure 3:  Selected 2-D emission-line images of IC2165. In each panel, spatial resolution evaluated by the Gaussian PSF FWHM measured from the PSF deconvolved standard stars is indicated by the filled grey circle with the radius of a half of Gaussian PSF FWHM. ~1 arcsec  spatial resolution!

Highly-resolved spatial-distributions of ionic and elemental abundances of IC2165 by Seimei/KOOLS-IFU

Figure 4: The ionic and elemental abundance maps of He, C, and O. The black lines on each panel are the intensity contours of the observed H I 4861 A emission line map. Elemantal abundances are not uniformly distributed.

Dust and Gas mass distributions of IC 2165 solely generated from Seimei/KOOLS-IFU

Figure 5. (from left to right) The gas-to-dust mass ratio, gas, and dust mass maps, respectively. These maps are genenated from the optical KOOLS-IFU emission line maps (IR data are not used). The grey lines on each panel are the intensity contours of the observed H I 4861 A emission line map. The maps are masked with a circular area of the radius of 4.0 arcsec.

These mass maps are similar regarding their increase with increasing distance from the central star. The gas and dust masses seem to concentrate in the elliptical nebula’s minor axis, which would be regarded as the equatorial plane. Such mass distributions may be related to the nonisotropic mass loss during the AGB phase and nebula shaping. Note that the gas-to-dust mass ratio is not constant in spatial.

Publications

95 papers, including 59 refereed papers are publications of the following major journals; Science, Nature Astronomy, MNRAS, ApJ, ApJL, ApJS, AJ, A&A, PASP, and PASJ. Amongst 59 refereed papers, 19 papers have been published as the 1st author (as of 2023/04). 

My h-index is 22 (12 as the first author).

Media Release

Awarded Observation Time (as PI)

References

Dr. Francisca Kemper (ESO, ASIAA)

Research Fellow, former supervisor, collaborator

Dr. You-Hua Chu (ASIAA)

Distinguished Research Fellow, former director of ASIAA,  collaborator

 Dr. Margaret Meixner

NASA SOFIA director, former supervisor, collaborator

Prof. Toshiya Ueta

U. of Denver, Associate professor, Collaborator

Prof. Hideyuki Izumiura

NAOJ, PhD supervisor, formar supervisor