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تسجيل مستخدم جديدTriggered star formation in expanding shells
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We discuss fragmentation processes which induce star formation in dense walls of expanding shells. The influence of the energy input, the ISM scale-height and speed of sound in the ambient medium is tested. 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. The value of the critical density depends on the energy of the shell and the sound speed in the ISM.
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S. Ehlerova
, J. Palous (Astronomical Institute
, Academy of Sciences ofn the Czech Republic
2000
We discuss the induced star formation in dense walls of expanding shells. The fragmentation process is studied using the linear perturbation theory. The influence of the energy input, the ISM distribution and the ISM speed of sound is examined analyt
ically and by numerical simulations. We formulate the universal condition for the gravitational fragmentation of expanding shells: if the total surface density of the disk is higher than a certain critical value, shells are unstable. The value of the critical density depends on the energy of the shell and the sound speed in the ISM.
W49A is a giant molecular cloud which harbors some of the most luminous embedded clusters in the Galaxy. However, the explanation for this starburst-like phenomenon is still under debate. Methods. We investigated large-scale Spitzer mid-infrared imag
es together with a Galatic Ring Survey 13CO J = 1-0 image, complemented with higher resolution (~ 11 arcsec) 13CO J = 2-1 and C18O J = 2-1 images over a ~ 15 x 13 pc^2 field obtained with the IRAM 30m telescope. Two expanding shells have been identified in the mid-infrared images, and confirmed in the position-velocity diagrams made from the 13CO J = 2-1 and C18O J = 2-1 data. The mass of the averaged expanding shell, which has an inner radius of ~ 3.3 pc and a thickness of ~ 0.41 pc, is about 1.9 x 10^4 M*. The total kinetic energy of the expanding shells is estimated to be ~ 10^49 erg which is probably provided by a few massive stars, whose radiation pressure and/or strong stellar winds drive the shells. The expanding shells are likely to have a common origin close to the two ultracompact Hii regions (source O and source N), and their expansion speed is estimated to be ~ 5 km/s, resulting in an age of ~ 3-7 x 10^5 years. In addition, on larger (~ 35 x 50 pc^2) scales, remnants of two gas ejections have been identified in the 13CO J = 1 - 0 data. Both ejections seem to have the same center as the expanding shells with a total energy of a few times 10^50 erg. The main driving mechanism for the gas ejections is unclear, but likely related to the mechanism which triggers the starburst in W49A.
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.
Galaxy pairs provide a potentially powerful means of studying triggered star formation from galaxy interactions. We use a large cosmological N-body simulation coupled with a well-tested semi-analytic substructure model to demonstrate that the majorit
y of galaxies in close pairs reside within cluster or group-size halos and therefore represent a biased population, poorly suited for direct comparison to ``field galaxies. Thus, the frequent observation that some types of galaxies in pairs have redder colors than ``field galaxies is primarily a selection effect. We select galaxy pairs that are isolated in their dark matter halos with respect to other massive subhalos (N=2 halos) and a control sample of isolated galaxies (N=1 halos) for comparison. We then apply these selection criteria to a volume-limited subset of the 2dF Galaxy Redshift Survey with M_Bj <= -19 and obtain the first clean measure of the typical fraction of galaxies affected by triggered star formation and the average elevation in the star formation rate. We find that 24% (30.5%) of these L^star and sub-L^{star} galaxies in isolated 50 (30) kpc/h pairs exhibit star formation that is boosted by a factor of >~ 5 above their average past value, while only 10% of isolated galaxies in the control sample show this level of enhancement. Thus, 14% (20 %) of the galaxies in these close pairs show clear triggered star formation. The isolation criteria we develop provide a means to constrain star formation and feedback prescriptions in hydrodynamic simulations and a very general method of understanding the importance of triggered star formation in a cosmological context. (Abridged.)
We present causal and positional evidence of triggered star formation in bright-rimmed clouds in OB associations, e.g., Ori OB1, and Lac OB1, by photoionization. The triggering process is seen also on a much larger scale in the Orion-Monoceros Comple
x by the Orion-Eridanus Superbubble. We also show how the positioning of young stellar groups surrounding the H II region associated with Trumpler 16 in Carina Nebula supports the triggering process of star formation by the collect-and-collapse scenario.
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