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How high-mass stars form remains unclear currently. Calculation suggests that the radiation pressure of a forming star can halt spherical infall, preventing its further growth when it reaches 10 M$_{odot}$. Two major theoretical models on the further growth of stellar mass were proposed. One model suggests the mergence of less massive stellar objects, and the other is still through accretion but with the help of disk. Inflow motions are the key evidence of how forming stars further gain mass to build up massive stars. Recent development in technology has boosted the search of inflow motion. A number of high-mass collapse candidates were obtained with single dish observations, mostly showed blue profile. The infalling signatures seem to be more common in regions with developed radiation pressure than in younger cores, which opposes the theoretical prediction and is also very different from that of low mass star formation. Interferometer studies so far confirm such tendency with more obvious blue profile or inverse P Cygni profile. Results seem to favor the accretion model. However, the evolution tendency of the infall motion in massive star forming cores needs to be further explored. Direct evidence for monolithic or competitive collapse processes is still lack. ALMA will enable us to probe more detail of gravity process.
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We present an evolutionary sequence of models of the photoionized disk-wind outflow around forming massive stars based on the Core Accretion model. The outflow is expected to be the first structure to be ionized by the protostar and can confine the expansion of the HII region, especially in lateral directions in the plane of the accretion disk. The ionizing luminosity increases as Kelvin-Helmholz contraction proceeds, and the HII region is formed when the stellar mass reaches ~10-20Msun depending on the initial cloud core properties. Although some part of outer disk surface remains neutral due to shielding by the inner disk and the disk wind, almost the whole of the outflow is ionized in 1e3-1e4 yr after initial HII region formation. Having calculated the extent and temperature structure of the HII region within the immediate protostellar environment, we then make predictions for the strength of its free-free continuum and recombination line emission. The free-free radio emission from the ionized outflow has a flux density of ~(20-200)x(nu/10GHz)^p mJy for a source at a distance of 1 kpc with a spectral index p~0.4-0.7, and the apparent size is typically ~500AU at 10GHz. The H40alpha line profile has a width of about 100km/s. These properties of our model are consistent with observed radio winds and jets around forming massive protostars.
Using the HPC ressources of the state of Baden-Wurttemberg, we modelled for the first time the luminous burst from a young massive star by accretion of material from its close environment. We found that the surroundings of young massive stars are shaped as a clumpy disk whose fragments provoke outbursts once they fall onto the protostar and concluded that similar strong luminous events observed in high-mass star forming regions may be a signature of the presence of such disks.
It has been known since many decades that galaxy interactions can induce star formation (hereafter SF) enhancements and that one of the driving mechanisms of this enhancement is related to gas inflows into the central galaxy regions, induced by asymmetries in the stellar component, like bars. In the last years many evidences have been accumulating, showing that interacting pairs have central gas-phase metallicities lower than those of field galaxies, by {sim} 0.2-0.3 dex on average. These diluted ISM metallicities have been explained as the result of inflows of metal-poor gas from the outer disk to the galaxy central regions. A number of questions arises: Whats the timing and the duration of this dilution? How and when does the SF induced by the gas inflow enrich the circumnuclear gas with re-processed material? Is there any correlation between the timing and strength of the dilution and the timing and intensity of the SF? By means of Tree-SPH simulations of galaxy major interactions, we have studied the effect that gas inflows have on the ISM dilution, and the effect that the induced SF has, subsequently, in re-enriching the nuclear gas. In this contribution, we present the main results of this study.
Context: The morphology of massive star formation in the central regions of galaxies is an important tracer of the dynamical processes that govern the evolution of disk, bulge, and nuclear activity. Aims: We present optical imaging of the central regions of a sample of 73 spiral galaxies in the H alpha line and in optical broad bands, and derive information on the morphology of massive star formation. Methods: We obtained images with the William Herschel Telescope, mostly at a spatial resolution of below one second of arc. For most galaxies, no H alpha imaging is available in the literature. We outline the observing and data reduction procedures, list basic properties, and present the I-band and continuum-subtracted H alpha images. We classify the morphology of the nuclear and circumnuclear H alpha emission and explore trends with host galaxy parameters. Results: We confirm that late-type galaxies have a patchy circumnuclear appearance in H alpha, and that nuclear rings occur primarily in spiral types Sa-Sbc. We identify a number of previously unknown nuclear rings, and confirm that nuclear rings are predominantly hosted by barred galaxies. Conclusions: Other than in stimulating nuclear rings, bars do not influence the relative strength of the nuclear H alpha peak, nor the circumnuclear H alpha morphology. Even though our selection criteria led to an over-abundance of galaxies with close massive companions, we do not find any significant influence of the presence or absence of a close companion on the relative strength of the nuclear H alpha peak, nor on the H alpha morphology around the nucleus.
We performed a survey in the SiO $J=5rightarrow4$ line toward a sample of 199 Galactic massive star-forming regions at different evolutionary stages with the SMT 10 m and CSO 10.4 m telescopes. The sample consists of 44 infrared dark clouds (IRDCs), 86 protostellar candidates, and 69 young HII regions. We detected SiO $J=5rightarrow4$ line emission in 102 sources, with a detection rate of 57%, 37%, and 65% for IRDCs, protostellar candidates, and young HII regions, respectively. We find both broad line with Full Widths at Zero Power (FWZP) $>$ 20 kms and narrow line emissons of SiO in objects at various evolutionary stages, likely associated with high-velocity shocks and low-velocity shocks, respectively. The SiO luminosities do not show apparent differences among various evolutionary stages in our sample. We find no correlation between the SiO abundance and the luminosity-to-mass ratio, indicating that the SiO abundance does not vary significantly in regions at different evolutionary stages of star formation.
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