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Sequential Star Formation Induced by a Super-giant HII Region

The group led by Rie Miura, a graduate student at the University of Tokyo, and Sachiko K. Okumura, an associate Professor of the National Astronomical Observatory of Japan, has found evidences of sequential star formation induced by the expansion of a super-giant HII region.

The target, the super-giant HII region NGC604 in the Triangulum Galaxy (Messier 33), has an extent of an approximately 650-1000 light-years, and contains more than 200 young massive stars in the heart of it. The huge HII region is brighter than any of

the brightest HII regions in the Milky Way Galaxy. Such a huge HII region is known to contain a large amount of molecular gas but astronomers do not yet have a clear understanding of how stars form from such a large amount of molecular gas.

The research group performed a survey of the dense molecular gas and massive-star forming regions toward the super-giant HII region NGC604 and successfully to unveiled their distribution for the first time. They also found that star formation activity decreases with increasing projected distance measured from the heart of the central star cluster. From the spatial relation between the molecular clouds and the expanding HII region, the research group suggests a possibility that seaquential star formation occurred in this region. There are some evidences of sequential star formation that occurs in smaller HII regions in previous studies, however this work reveals star formation process in a super-giant HII region for the first time in detail.

In the future, they will investigate the detailed structures of the star-forming molecular clouds in a giant HII region with ALMA (Atacama Large Millimeter/submillimeter Array, which enable to make the higher resolution and higher sensitivity observations.

Aperture Synthesis Observations of CO, HCN, and 3-mm(89-GHz) Continuum Emission toward NGC 604 in The Triangulum Galaxy (Messier 33)

Young, hot, massive stars produce large amount of UV photons, ionizing the gas surrounding to form "HII regions". As an HII region expands outward from the ionizing stars, the ambient medium is swept up into a certain place, in a part of which become denser molecular cloud, where is the place that the new generation stars are born. As referred to the stellar clusters which have created the HII region as “first-generation stars”, the newly born stars in such a dense gas are "second-generation stars". The process of the star formation is called "sequential star formation". In other words, the HII regions can trigger the formation of the new generation stars. Many multi-wavelength observations toward smaller HII regions in the Milky Way Galaxy have been done thus far, by which the researchers have found evidences of sequential star formation. On the other hand, there are few observational studies about the star formation process in a super-giant HII regions in other galaxies. Such a huge HII region is known to contain a large amount of molecular gas, but astronomers do not have a clear understanding of how stars form from such a large amount of molecular gas.

The group led by Rie Miura, a graduate student at the University of Tokyo, and Sachiko K. Okumura, an associate Professor of the National Astronomical Observatory of Japan, successfully detected Hydrogen Cyanide and 3-mm continuum emissions in the super-giant HII region NGC604 in the Triangulum Galaxy (Messier 33) using the Nobeyama Millimeter Array. They also succeeded to obtain the highest resolution image in the Carbon Monoxide line emission. Carbon Monoxide is often used as a tracer of molecular gas which is the materials of the stars, while Hydrogen Cyanide is a tracer of the dense molecular gas that density is high more than 100 times than it. Because stars are born at the denser region in a molecular cloud, the carbon monoxide and the Hydrogen cyanide line emissions let us know the existence of the molecular clouds, and the denser molecular clouds, respectively. The 3-mm continuum emission is emitted from the ionized gas by young, hot, massive stars, and thus let us know young, massive star forming region. This HII region has an extent of an approximately 650-1000 light-years, and contains more than 200 young massive stars in the heart of it (Figure 1). The huge HII region is brighter than any of the brightest HII regions in the Milky Way Galaxy (by two or three times). It is important at all detailed to examine how such lots of stars interact with the gas of the circumference in such a huge HII region.

[Figure 1] Optical image of Super-giant HII region NGC604 taken with the Hubble Space Telecope (HST)
The right figure is an enlargement of a portion of the left figure to see details in the center of the HII region. We easily notice the existence of the huge number of stars in its center.


*This Figure was downloaded from the official homepage of Hubble Space Telescope.High-resolution image here.

As a result, at least ten molecular clouds were found in this region, and Hydrogen cyanide emissions and 3-mm continuum emissions were detected in this region for the first time. They found that star formation activity decreases with increasing distance from the heart of the central star cluster. From the spatial relation between the molecular clouds and the expanding HII region, the research group suggests a possibility that sequential star formation occurred in this region; as the HII region expands outward from the central stellar clusters ("first-generation" stars, number-(1) in figure 2), many young massive stars are formed in the swept-up molecular cloud ("second-generation" stars, number-(2) in figure 2) in the vicinity. The massive stars are also born in the dense molecular gas in another molecular cloud (number-(3) in figure 2), which is located slightly far from the central stellar cluster. These stars may be the third generation stars born by influence of the second generation star or the second generation star by the first-generation stars. Another molecular cloud (number-(4) in figure 2), which is located at the edge of the HII region, were found its existence of the dense molecular gas inside. This kind of molecular clouds may be getting ready to form stars.

[Figure 2] The disctirbution of the Hii region and gas

The background image (gray color) represents the extent of the ionization domain. The cyan, red, and yellow contours show the place of the molecular clouds, the denser molecular gas, and massive-star forming region, respectively. The region shown in this figure is about the same as that in the left panel of Figure 1.

[Movie 1] The image of Sequential Star Formation through the expansion of the HII region

Paper Information
R.Miura et al., "Aperture Synthesis Observations of CO, HCN, and 89GHz Continuum Emission toward NGC 604 in M 33: Sequential Star Formation Induced by Supergiant Hii region", The Astrophysical Journal, Volume 723, Issue 2, 2010

Contact
rie.miura (at) nao.ac.jp

Links

※"MAGiC Project" (NRO M33 All Disk Survey of Giant Molecular Clouds) is the large project struggling for the understanding the evolution of Giant Molecular Clouds (GMCs) using multi-wavelength data. We have been observing many GMCs in the nearest spiral galaxy M33 so far.