AKARI Highlights I: A supergiant making a splash in a cosmic river
- AKARI revealed the shock wave at the interface between stellar wind and interstellar matters -
A river of the interstellar medium meanders through deep space. When stars cross these rivers, they make astronomically large splashes. AKARI, the Japan Aerospace Exploration Agency (JAXA) infrared astronomical satellite with the European Space Agency (ESA) participation, has obtained a high-resolution image of Betelgeuse; a bright red supergiant in the constellation Orion located about 640 light year from the Earth, making such a big splash as the star goes across a cosmic river.
AKARI's Far-Infrared Surveyor (FIS) instrument imaged Betelgeuse and its surroundings in four different bands at 65, 90, 140, and 160 micrometers (1 micrometer = 1/1000 millimeter). Figure 1 shows a false-colour image (blue = 65 micrometer image, green = 90 micrometer image, red = 140 micrometer image). Betelgeuse appears bluish-white in the middle, and there is an arc-like structure in greenish-yellow that surrounds the supergiant. This arc-like structure is the splash that Betelgeuse is making.
Figure 1: The three colour composite image of Betelgeuse and surrounding composed from images at 65 (Blue), 90 (Green) and 140 (Red) micrometers taken by the AKARI's Far-Infrared Surveyor (FIS). The arc-like structure in the upper left direction from the star is the bow shock due to collision of stellar wind and the interstellar matter in the direction of star's motion (from lower right to upper left). The diameter of the bow shock is about three light years. The blue-white linear structure from upper left to lower right through the star is due to an instrumental effect.
Space is not void, and this seemingly empty space is actually filled with matter called the interstellar medium that consists of tenuous gas and tiny solid particles known as "dust". Stars spew out gas and dust in a flow called a stellar wind as they age. These stellar winds eventually bump into the interstellar medium, and they get mixed together. At the interface between the stellar wind and the interstellar medium, physical conditions such as density and pressure change dramatically, ending up in a phenomenon called a shock. In Figure 1, Betelgeuse is moving in space from lower right to upper left angled into the plane of the sky, and therefore, there is a shock taking place in the direction into which Betelgeuse is moving. This arc-like shock is usually called a "bow shock".
In fact, the existence of this bow shock around Betelgeuse was known by observations made with the first-ever infrared astronomy satellite, IRAS, launched 25 years ago by the US, the Netherlands, and UK consortium. However, IRAS data did not have enough resolution to know the details of interactions at the shock interface. AKARI's data, having better spatial resolution, allowed researchers to study detailed structure of the shock for the first time.
The shape of a bow shock can be calculated exactly using a shock theory based on the conditions of the shock. By analyzing the shape of the bow shock around Betelgeuse, researchers were able to figure out that there is a strong flow of the interstellar medium around Betelgeuse and that Betelgeuse is actually going across this cosmic river of the interstellar medium. This flow originates from star forming regions in the Orion's Belt and is flowing at 11 km/s from lower right to upper left almost along the plane of the sky in Figure 1. Betelgeuse is crossing this river at 30 km/s, while spewing out its wind at 17 km/s. Overall, the combined motion of Betelgeuse and its wind is forcing its way through this cosmic river, just like a ship making wakes by its bow when going across a river. Figure 2 shows how the bow shock structure is oriented with respect to Betelgeuse, the flow of the interstellar medium, and the Earth.
Figure 2: An illustration of bow shock. Discontinuity of density and pressure of the matter appears at the boundary where the stellar wind from Betelgeuse collides into the interstellar matter. Betelgeuse moves in the space from lower right to upper left in this figure.
Stars condense out of the interstellar medium when they are born. On the other hand, old stars like Betelgeuse spew out matter into the surrounding space, ever-enriching the interstellar medium. This enrichment process is repeated by generations of stars for eons. As a result, the universe keeps evolving chemically, providing enough matter to form stars, planets, and eventually, life, as we know it. AKARI has already found a number of bow shock cases like Betelgeuse. Future investigations into these processes will help us further our understanding of the cosmic recycling of matter.
This research has been conducted by Dr. Toshiya Ueta at the University of Denver, Dr. Hideyuki Izumiura, at the Okayama Astrophysical Observatory/National Astronomical Observatory of Japan, and Dr. Issei Yamamura, at Institute of Space and Aeronautical Sciences/Japan Space Exploration Agency, in collaboration with researchers at the University of Tokyo, National Observatory of Japan, and Kyoto University.