Although it is believed that stars are formed by a contraction of interstellar gas, there are still many mysteries left unsolved in the star formation process. For example, astronomers do not have a clear answer to the questions such as “How are the massive stars formed?”, “Are the formation processes of massive and low-mass stars identical?” This is partly because massive stars are formed far from the solar system and it is difficult to explore the vicinity of the baby massive stars at sufficient resolution.

One of the popular objects to study the formation of massive stars is “Orion KL” in the Orion nebula, which emits strong infrared light. Orion KL is the object closest to the solar system (about 1400 light years) where the stars eight times or more massive than the Sun are being formed. Since its discovery in 1967, this source has been observed by many astronomers. For example, it was selected as the first target of the Subaru Telescope (Figure 1).

The research group led by Tomoya Hirota, an assistant professor at NAOJ, have conducted high-resolution observations of water and SiO masers (strong radio emission similar to laser) in Orion KL with an NAOJ’s radio telescope network VERA [1].
In Orion KL, a peculiar baby star “Source I” is located whose detailed view is yet to be obtained. SiO maser emission was detected toward “Source I”, which is exceptional because the maser emission has been detected only in three sources, including “Source I”, other than old stars. Since SiO maser is often detected from the high-temperature gas around the old stars, the research group made a hypothesis that other molecular lines such as those emitted from high-temperature interstellar gas characteristic of the old stars, may be detected around Orion KL. They focused on the high-energy water maser emission (wavelength of 1.3 mm, frequency of 232 GHz) emanated from the high-temperature gas at the temperature of 3200 degrees Celsius. This line was not detected in Orion by observations with 10-m class radio telescopes, however, ALMA would be able to detect the line with the unprecedented sensitivity and resolution. To examine the feasibility of their idea, the research group started the data reduction of the ALMA Science Verification data of Orion KL [2].

The ALMA observations of Orion KL were conducted on January 20th, 2012 with 16 antennas. The research group analyzed the data around 232 GHz and sought the water maser emission.

As a result of the analysis, they found a clear radio emission in 232 GHz, which is consistent with the frequency of high-energy water maser. Obviously it is the first detection of the radio emission at this frequency in Orion KL. After careful inspection of the data and the molecular line database, they found that another line emission from Methyl formate molecule (HCOOCH3) is located very close to the water maser emission.

“If this is the observation with a single-dish telescope, we cannot distinguish the line emission from water and Methyl formate and it would be hard to derive new information. Thanks to the high resolution of the ALMA observations, however, it became possible to know the position where the line emission is strongly detected, and we can explore the detail.” says Hirota. Their inspection results show that the water maser and line emission from Methyl formate are emitted in different parts of Orion KL (Figure 2). The water maser is strongly detected around “Source I”, and it is proved that this maser is emitted from the high-temperature gas around “Source I”. This is the first detection of the high-energy water maser in star forming regions.

What is the exact source of this high-energy water emission in “Source I”? The SiO maser detected with VERA and lower-energy water maser give a hint. Comparison of those maser emissions obtained with other telescopes and the water maser emission obtained with ALMA shows that those emissions have the identical velocity (Figure 3). Astronomers proposed that the SiO maser is emitted from the gas disk around “Source I” and the low-energy water maser is emitted from the high-velocity jet emanated along the axis of the gas disk. Therefore, it is natural to conclude that the high-energy water emission detected with ALMA is also emitted from the disk or jet in the vicinity of “Source I” (Figure 4).

“Thanks to the high sensitivity and spatial resolution of ALMA, we successfully detected the high-energy water maser emission at a millimeter wavelength from a baby star Orion KL Source I.” says Hirota. “Only 16 antennas were used with the maximum baseline of 350 m in this Science Verification observation, however, only 20 minutes were needed to detect this weak water maser emission. We obtained a new tool to explore the vicinity of the high-temperature gas around baby stars with ALMA.”

Once completed, ALMA is equipped with 66 antennas with the maximum baseline of 18.5 km, which improves the spatial resolution 50 times better than this observation result. From this Science verification data, the water maser emission looks like a point source and it is impossible to investigate its internal structure. By combining the SiO maser seen with VERA and higher-resolution mm/submm emission with ALMA, the researchers expect to directly explore the gas disk and high-velocity jet around “Source I”. The Orion KL is one of the most mysterious objects since its discovery 50 years ago. It is expected that high-resolution observations with ALMA will unveil the nature of Orion KL and give a full picture of the massive star formation.

1. VERA (VLBI Exploration of Radio Astrometry) is the network of four 20-m radio telescopes located in Japan (Mizusawa, Iriki, Ogasawara, Ishigaki. See http://veraserver.mtk.nao.ac.jp/index.html ). The main goal of the VERA project is to construct the detailed map of our Milky Way Galaxy by measuring the precise position of many celestial objects. In order to measure the distances of those objects, the projects have observed a number of maser emission. VERA measured the precise distance to Orion KL.

2. The purpose of the Science Verification (SV) observation is to verify the ALMA’s observation capability and data quality. The targets for SV observations are selected from the well-observed objects with the existing telescopes to make the comparison possible. The observations and basic data analysis were done by the ALMA observatory staff, and the processed data are publicly available on the internet so that astronomers can use them for their research.

This study is partly supported by the JSPS Grant-in-Aid for Young Scientists (A) (24684011).

This research is reported in the paper “The first detection of the 232 GHz vibrationally excited H2O maser in Orion KL with ALMA” by Hirota et al. appeared in The Astrophysical Journal Letters 757, L1.

The research team is composed of T. Hirota, M.-K. Kim, and M. Honma (Mizusawa VLBI Observatory, National Astronomical Observatory of Japan).