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Register a new userEmbedded, Accreting Disks in Massive Star Formation
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Kaitlin M. Kratter
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Recent advances in our understanding of massive star formation have made clear the important role of protostellar disks in mediating accretion. Here we describe a simple, semi-analytic model for young, deeply embedded, massive accretion disks. Our approach enables us to sample a wide parameter space of stellar mass and environmental variables, providing a means to make predictions for a variety of sources that next generation telescopes like ALMA and the EVLA will observe. Moreover we include, at least approximately, multiple mechanisms for angular momentum transport, a comprehensive model for disk heating and cooling, and a realistic estimate for the angular momentum in the gas reservoir. We make predictions for the typical sizes, masses, and temperatures of the disks, and describe the role of gravitational instabilities in determining the binarity fraction and upper mass cut-off.
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Most analytic work to date on protostellar disks has focused on those in isolation from their environments. However, observations are now beginning to probe the earliest, most embedded phases of star formation, during which disks are rapidly accreting from their parent cores and cannot be modeled in isolation. We present a simple, one-zone model of protostellar accretion disks with high mass infall rates. Our model combines a self-consistent calculation of disk temperatures with an approximate treatment of angular momentum transport via two mechanisms. We use this model to survey the properties of protostellar disks across a wide range of stellar masses and evolutionary times, and make predictions for disks masses, sizes, spiral structure, and fragmentation that will be directly testable by future large-scale surveys of deeply embedded disks. We define a dimensionless accretion-rotation parameter which, in conjunction with the disks temperature, controls the disk evolution. We track the dominant mode of angular momentum transport, and demonstrate that for stars with final masses greater than roughly one solar mass, gravitational instabilities are the most important mechanism as most of the mass accumulates. We predict that binary formation through disk fission, fragmentation of the disk into small objects, and spiral arm strength all increase in importance to higher stellar masses.
We study the orbital decay of a pair of massive black holes (BHs) with masses 5 * 10^5 and 10^7 M_sun, using hydrodynamical simulations of circumnuclear disks (CNDs) with the alternating presence of sub-grid physics such as radiative cooling, star formation, supernova feedback, BH accretion and feedback. In the absence of such processes, the orbit of the secondary BH decays over timescales of ~10 Myr to the center of the CND, where the primary BH resides. When strong dissipation operates in CNDs, fragmentation into massive objects the size of giant molecular clouds and with densities in the range 10^4 - 10^7 amu / cm^3 occurs, causing stochastic torques and hits that can eject the secondary BH from the midplane. Outside the plane, the low-density medium provides only weak drag, and the BH return is governed by inefficient dynamical friction. In rare cases, clump-BH interactions can lead to a faster decay. Feedback processes lead to outflows, but do not change significantly the overall density of the CND midplane. However, with a spherically distributed BH feedback a hot bubble is generated behind the secondary, which almost shuts off dynamical friction, a phenomenon we dub wake evacuation, leading to delays in the decay of possibly ~0.3 Gyr. We discuss the non-trivial implications on the discovery space of the eLISA telescope. Our results suggest the largest uncertainty in predicting BH merger rates lies in the potentially wide variety of galaxy host systems, with different degrees of gas dissipation and heating, yielding decay timescales from ~10 to ~300 Myr.
We present a Herschel far-infrared study towards the rich massive star- forming complex G305, utilising PACS 70, 160 {mu}m and SPIRE 250, 350, and 500 {mu}m observations from the Hi-GAL survey of the Galactic plane. The focus of this study is to identify the embedded massive star-forming population within G305, by combining far-infrared data with radio continuum, H2O maser, methanol maser, MIPS, and Red MSX Source survey data available from previous studies. By applying a frequentist technique we are able to identify a sample of the most likely associations within our multi-wavelength dataset, that can then be identified from the derived properties obtained from fitted spectral energy distributions (SEDs). By SED modelling using both a simple modified blackbody and fitting to a comprehensive grid of model SEDs, some 16 candidate associations are identified as embedded massive star-forming regions. We derive a two-selection colour criterion from this sample of log(F70/F500)geq 1 and log(F160/F350)geq 1.6 to identify an additional 31 embedded massive star candidates with no associated star-formation tracers. Using this result we can build a picture of the present day star-formation of the complex, and by extrapolating an initial mass function, suggest a current population of approx 2 times 10^4 young stellar objects (YSOs) present, corresponding to a star formation rate (SFR) of 0.01-0.02 Modot yr^-1. Comparing this resolved star formation rate, to extragalactic star formation rate tracers (based on the Kennicutt-Schmidt relation), we find the star formation activity is underestimated by a factor of geq 2 in comparison to the SFR derived from the YSO population.
We present deep, wide-field J, H and Ks images taken with IRIS2 on the Anglo Australian Telescope, towards the massive star formation region G305.2+0.2. Combined with 3.6, 4.5, 5.8 and 8.0 micron data from the GLIMPSE survey on the Spitzer Space Telescope, we investigate the properties of the embedded stellar populations. After removing contamination from foreground stars we separate the sources based on their IR colour. Strong extended emission in the GLIMPSE images hampers investigation of the most embedded sources towards the known sites of massive star formation. However, we find a sizeable population of IR excess sources in the surrounding region free from these completeness effects. Investigation reveals the recent star formation activity in the region is more widespread than previously known. Stellar density plots show the embedded cluster in the region, G305.24+0.204, is offset from the dust emission. We discuss the effect of this cluster on the surrounding area and argue it may have played a role in triggering sites of star formation within the region. Finally, we investigate the distribution of IR excess sources towards the cluster, in particular their apparent lack towards the centre compared with its immediate environs.
The 25 years following the serendipitous discovery of megamasers have seen tremendous progress in the study of luminous extragalactic H$_2$O emission. Single-dish monitoring and high resolution interferometry have been used to identify sites of massive star formation, to study the interaction of nuclear jets with dense molecular gas and to investigate the circumnuclear environment of active galactic nuclei (AGN). Accretion disks with radii of 0.1--3 pc were mapped and masses of nuclear engines of order 10$^{6}$--10$^{8}$ M$_{odot}$ were determined. So far, $sim$50 extragalactic H$_2$O maser sources have been detected, but few have been studied in detail.
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