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تسجيل مستخدم جديدTriggered Star Formation in the Environment of Young Massive Stars
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Recent observations with the Spitzer Space Telescope show clear evidence that star formation takes place in the surrounding of young massive O-type stars, which are shaping their environment due to their powerful radiation and stellar winds. In this work we investigate the effect of ionising radiation of massive stars on the ambient interstellar medium (ISM): In particular we want to examine whether the UV-radiation of O-type stars can lead to the observed pillar-like structures and can trigger star formation. We developed a new implementation, based on a parallel Smooth Particle Hydrodynamics code (called IVINE), that allows an efficient treatment of the effect of ionising radiation from massive stars on their turbulent gaseous environment. Here we present first results at very high resolution. We show that ionising radiation can trigger the collapse of an otherwise stable molecular cloud. The arising structures resemble observed structures (e.g. the pillars of creation in the Eagle Nebula (M16) or the Horsehead Nebula B33). Including the effect of gravitation we find small regions that can be identified as formation places of individual stars. We conclude that ionising radiation from massive stars alone can trigger substantial star formation in molecular clouds.
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W51A is one of the most active star-forming region in our Galaxy, which contains giant molecular clouds with a total mass of 10^6 Msun. The molecular clouds have multiple velocity components over ~20 km/s, and interactions between these components ha
ve been discussed as the mechanism which triggered the massive star formation in W51A. In this paper, we report an observational study of the molecular clouds in W51A using the new 12CO, 13CO, and C18O (J=1-0) data covering a 1.4x1.0 degree region of W51A obtained with the Nobeyama 45-m telescope at 20 resolution. Our CO data resolved the four discrete velocity clouds at 50, 56, 60, and 68 km/s with sizes and masses of ~30 pc and 1.0-1.9x10^5 Msun. Toward the central part of the HII region complex G49.5-0.4, we identified four C18O clumps having sizes of ~1 pc and column densities of higher than 10^23 cm^-3, which are each embedded within the four velocity clouds. These four clumps are distributed close to each others within a small distance of 5 pc, showing a complementary distribution on the sky. In the position-velocity diagram, these clumps are connected with each others by bridge features with intermediate intensities. The high intensity ratios of 13CO (J=3-2/J=1-0) also indicates that these four clouds are associated with the HII regions. We also found these features in other HII regions in W51A. The timescales of the collisions are estimated to be several 0.1 Myrs as a crossing time of the clouds, which are consistent with the ages of the HII regions measured from the size of the HII regions in the 21 cm continuum emissions. We discuss the cloud-cloud collision scenario and massive star formation in W51A by comparing with the recent observational and theoretical studies of cloud-cloud collision.
The star formation triggered in dense walls of expanding shells will be discussed. The fragmentation process is studied using the linear and non-linear perturbation theory. The influence of the energy input, the ISM distribution and the speed of soun
d is examined analytically and by numerical simulations. We formulate the condition for the gravitational fragmentation of expanding shells: if the total surface density of the disc is higher than a certain critical value, shells are unstable. This value depends on the energy of the shell and the sound speed in the ISM. As an example the formation of OB associations near the Sun will be discussed. We trace their orbits in the Milky Way to see where they have been born: 10 - 12 Myr ago progenitors of Scorpius-Centaurus OB associations and the Orion OB association resided together within a sheet-like region elongated in the $l = 20-200degrees direction, showing that the local OB associations may be formed as fragments of an expanding supershell.
Aims: We test the technique of spectro-astrometry as a potential method to investigate the close environment of massive young stars. Method: Archival VLT near infrared K band spectra (R=8900) of three massive young stellar objects and one Wolf-Raye
t star are examined for spectro-astrometric signatures. The young stellar objects display emission lines such as Brackett gamma, CO 2-0 and CO 3-1 that are characteristic of ionised regions and molecular disks respectively. Two of the sample sources also display emission lines such as NIII and MgII that are characteristic of high temperatures. Results: Most of the emission lines show spectro-astrometric signal at various levels resulting in different positional displacements. The shapes and magnitudes of the positional displacements imply the presence of large disk/envelopes in emission and expanding shells of ionised gas. The results obtained for the source 18006-2422nr766 in particular provide larger estimates (> 300AU) on CO emitting regions indicating that in MYSOs CO may arise from inner regions of extended dense envelopes as well. Conclusions: The overall results from this study demonstrate the utility of spectro-astrometry as a potential method to constrain the sizes of various physical entities such as disks/envelopes, UCHII regions and/or ionised shells in the close environment of a massive young star.
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