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Register a new userW49A: A starburst triggered by expanding shells
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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 images 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.
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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.
We present the discovery of expanding spherical shells around low to intermediate-mass young stars in the Orion A giant molecular cloud using observations of $^{12}$CO (1-0) and $^{13}$CO (1-0) from the Nobeyama Radio Observatory 45-meter telescope. The shells have radii from 0.05 to 0.85 pc and expand outward at 0.8 to 5 km/s. The total energy in the expanding shells is comparable to protostellar outflows in the region. Together, shells and outflows inject enough energy and momentum to maintain the cloud turbulence. The mass-loss rates required to power the observed shells are two to three orders of magnitude higher than predicted for line-driven stellar winds from intermediate-mass stars. This discrepancy may be resolved by invoking accretion-driven wind variability. We describe in detail several shells in this paper and present the full sample in the online journal.
We present neutral hydrogen, ultraviolet, optical and near-infrared imaging, and optical spectroscopy, of Minkowskis Object (MO), a star forming peculiar galaxy near NGC 541. The observations strengthen evidence that star formation in MO was triggered by the radio jet from NGC 541. Key new results are the discovery of a 4.9E8 solar mass double HI cloud straddling the radio jet downstream from MO, where the jet changes direction and decollimates; strong detections of MO, also showing double structure, in UV and H-alpha; and numerous HII regions and associated clusters in MO. In UV, MO resembles the radio-aligned, rest-frame UV morphologies in many high redshift radio galaxies (HzRGs), also thought to be caused by jet-induced star formation. MOs stellar population is dominated by a 7.5 Myr-old, 1.9E7 solar mass instantaneous burst, with current star formation rate 0.52 solar masses per year (concentrated upstream from where the HI column density is high). This is unlike the jet-induced star formation in Centaurus A, where the jet interacts with pre-existing cold gas; in MO the HI may have cooled out of a warmer, clumpy intergalactic or interstellar medium as a result of jet interaction, followed by collapse of the cooling clouds and subsequent star formation (consistent with numerical simulations). Since the radio source that triggered star formation in MO is much less luminous, and therefore more common, than powerful HzRGs, and because the environment around MO is not particularly special in terms of abundant dense, cold gas, jet-induced star formation in the early universe might be even more prevalent than previously thought.
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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 analytically 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.
Based on imaging and spectroscopic data, we develop a 3-D model for the Huygens Region of the Orion Nebula. Theta1OriC , the hottest star in the Trapezium, is surrounded by a wind-blown Central Bubble that opens SW into the Extended Orion Nebula. Outside of this feature lies a layer of ionized gas at about 0.4 pc from Theta1OriC. Both of these features are moving rapidly away from Theta1OriC with an expansion age for the Central Bubble of only 15,000 yrs.
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