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0830 |
Murray, Norman |
Rapid Star Formation |
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The work of Kennicutt established that, on galactic scales, the rate of star formation is slow. More quantitatively, only two percent of the galaxies gas is converted into stars in a disk dynamical time. This has lead to the notion that star formation on all scales below the disk size is slow. I will argue that the latter notion is incorrect. I will present observational evidence that, on the scale of giant molecular clouds (GMCs), five percent of the mass is turned into stars in a small fraction of the GMC dynamical time; if this were to continue for a dynamical time, 30-50 percent of the cloud would be consumed. I will present three dimensional hydrodynamical and magnetohydrodynamical simulations of turbulent GMCs and periodic boxes, both of which produce similarly rapid star formation. I will outline a simple analytic theory explaining these results, showing that the rate of star formation in either a GMC or a turbulently stirred periodic box is not constant, but rather accelerates, becoming more rapid with time. Finally, I will present observational evidence that the star formation in nearby clusters also accelerated, in a manner consistent with both the analytic theory and the numerical simulations. I will also explain how these results can be consistent with those of Kennicutt, via a combination of radiation and gas pressure produced by massive stars in the Milky Way, or by supernovae in much less massive galaxies like the SMC. |
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0845 |
van der Wiel, Matthijs |
The molecular environment surrounding two massive star-forming cores in NGC6334 |
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We present a study of highly excited gas in the molecular cloud NGC6334 (d~1.7 kpc), based on Herschel-SPIRE spectral imaging observations. Our fully sampled map resolves spatial scales down to ~0.2 pc and covers a projected area of ~2.5 x 1 pc, which includes two massive star-forming cores, sources 'I' and 'I(N)', separated by ~1 pc. Compared to the neighbouring core, source I is thought to be at a more evolved stage, being hotter and enjoying a very rich chemistry. It has been studied by numerous high spectral resolution surveys in the (sub)millimetre and far-infrared. Much less is known, however, about the extended molecular environment that both cores share. From our observations with the SPIRE Fourier Transform Spectrometer, with a frequency range of 447-1545 GHz and a spectral resolution of 1.2 GHz (R=400 to 1300), we extract maps of ten consecutive CO transitions (J=4-3 to 13-12). These lines trace gas temperatures between ~50 and 500 K, a range that has only opened up with the advent of the Herschel space observatory. By combining our observations with a non-LTE radiative transfer model, we derive a temperature, column density and number density at each pixel in our map. Exploiting the synergy between our data and the extremely high spectral resolution, but single-pixel dataset from the heterodyne spectrometer on Herschel, we study gas that is closely associated with each of the massive protostellar cores, as well as the surrounding gas mass that may in the future be pulled in to either of the protostellar accretion reservoirs. |
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0900 |
Broekhoven-Fiene, Hannah * |
A First Look at the Disk Population in the Auriga/California Giant Molecular Cloud |
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The Auriga-California Molecular Cloud (AMC) is a giant molecular cloud in the nearby Gould Belt of star-forming regions. Its membership in the Gould Belt was only recently identified; therefore, it is relatively unstudied compared to other well-known local clouds. Although it has a comparable mass and size to the Orion Molecular Cloud, it is forming far fewer stars. In fact the star formation in the AMC resembles more closely that of low-mass star-forming regions such as Taurus and Perseus. We use observations of the AMC with Spitzer, Herschel and SCUBA-2 to probe the disk population and compare it to the number of protostellar and starless cores in the cloud. We further use these data to constrain the age of the cloud and its star formation history. Comparison of our Spitzer results to the disk and YSO populations in Orion, Taurus and Perseus reveals that there is a high number of Class I and flat sources relative to Class II sources in the AMC, which suggests that the AMC is a relatively unevolved cloud in the Gould Belt. Finally, we describe our plans to search for evidence of grain growth in AMC disks using ALMA and the Jansky VLA (for which some time has been allocated). |
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0915 |
Johnstone, Doug |
Continuum Variability of Deeply Embedded Protostars as a Probe of Envelope Structure |
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Stars appear to be assembled in large growth spurts; however the evidence for this hypothesis is circumstantial. Directly studying the accretion at the earliest phases of stellar growth is challenging because young stars are deeply embedded in optically thick envelopes, which have spectral energy distributions that peak in the far-IR, where observations are difficult. We consider the feasibility of detecting accretion outbursts from these younger stars by investigating the timescales for how the protostellar envelope responds to changes in the emission properties of the central source. The envelope heats up in response to an outburst, brightening at all wavelengths and with the emission peak moving to shorter wavelengths. The timescale for this change depends on the time for dust grains to heat and re-emit photons and the time required for the energy to escape the inner, optically thick portion of the envelope. We find that the dust response time is much shorter than the photon propagation time and thus the timescale over which the emission varies is set by time delays imposed by geometry. These times are hours to days near the peak of the spectral energy distribution and weeks to months in the sub-mm. The ideal location to quickly detect continuum variability is therefore in the mid- to far-IR, near the peak of the spectral energy distribution, where the change in emission amplitude is largest. Searching for variability in sub-mm continuum emission is also feasible, though with a longer time separation and a weaker relationship between the amount of detected emission amplitude and change in central source luminosity. Such JCMT, CCAT, or ALMA observations would constrain accretion histories of protostars and would help to trace the disk/envelope instabilities that lead to stellar growth. |
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0930 |
Maddison, Sarah |
Radial dust migration in the TW Hydra protoplanetary disk |
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During the formation and evolution of protoplanetary disks, dust grains grow in size mostly via coagulation. Dust in gas-rich disks experience aerodynamic drag which ensures that large grains (~1mm and larger) rapidly decouple from the gas, settle to the disk midplane and simultaneously migrate inward towards the protostar. This concentration of the dust significantly increases the differential velocity between dust of different sizes and hence amplifies coagulation, which in turn increases planetary formation efficiency. However, the "radial drift barrier" requires some halting mechanism, else cm-sized dust grains are expected to be accreted on to the central protostar before they can grow to form planetary building blocks. While these dynamical processes can be studied numerically, they are poorly constrained by observations. At a distance of just 56 pc, TW Hydra is the closest T Tauri star and offers a unique opportunity to study dust radial migration. The nearly face-on disk has been well studied across the electromagnetic spectrum and angularly resolved from visible scattered light to 3 cm thermal emission. With an estimated age of 10 Myr, the TW Hya disk is expected to display both vertical and radial particle size segregation. Here we present new Australia Telescope Compact Array 7 mm observations of TW Hya to complement 7 mm VLA observations. Our results clearly demonstrate that the large pebble-sized grains are strongly peaked towards the centre of the disk compared to the 870 micron emission, suggesting separate dust populations are needed to explain these observations. While there is clear evidence of radial size sorting of the dust, the cause of the inner disk cavity near 4 AU remains unclear. Whatever the cause, the gas pressure maxima in the inner disk ensures that dust up to at least 1mm will remain in the disk for at least 10 Myr in the case of TW Hya. |
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0945 |
Alexandersen, Mike * |
The first known Uranian Trojan and the frequency of temporary giant-planet co-orbitals. |
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Trojan objects share a planet's orbit, never straying far from the triangular Lagrangian points, 60 degrees ahead of or behind the planet. We report the first discovery of a Uranian Trojan, in CFHT Megacam imaging taken for an outer Solar System survey. The newly discovered object (with IAU Minor Planet Center designation 2011 QF99) oscillates around the Uranian L4 Lagrange point for >70 kyr and remains co-orbital (in 1:1 resonance) for ~1 Myr before becoming a Centaur. Instead of being a primordial Trojan, it must be a temporary co-orbital. We construct a steady-state Centaur model, supplied from the transneptunian region, to investigate the frequency and duration of temporary co-orbital captures, finding that at any time large fractions (0.5% and 1.9%) of the population will be Uranian and Neptunian co-orbitals, respectively. We show for the first time that the high co-orbital Centaur fraction (~3%) in the IAU Minor Planet Center database is that expected under transneptunian steady-state supply. |
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