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public:seminar_abstracts [2018/06/11 10:17]
Samuel Grant
public:seminar_abstracts [2019/01/14 17:04] (current)
Krishna Prasad
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 ====== ARC Seminar Abstracts ====== ====== ARC Seminar Abstracts ======
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 +===Energetic particles and star formation =====
 +** Dr Donna Rodgers-Lee** (Hertfordshire)- ARC Seminar Wednesday 16 January 2019 3 pm
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 +There is a growing body of observational evidence to suggest that energetic particles are produced during the low-mass star formation process. Both the young low-mass stars themselves and the jets they power appear to be capable of accelerating particles up to ~GeV energies. These low-energy cosmic rays may be important in determining the ionisation rate in star-forming regions and subsequently the chemical and dynamic evolution of protoplanetary disks.
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 +Here, based on the assumption that young low-mass stars accelerate protons to ~GeV energies, I will discuss our recent work focusing on the ionising effect of these energetic particles in protoplanetary disks, as well as further ways of investigating their transport properties. I will also describe on-going observational efforts to identify further evidence of non-thermal emission from young stellar objects. ​  
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 +===Revealing the Evolution of Comet Nuclei with Ground Photometric Observations =====
 +** Dr Rosita Kokotanekova** (ESO)- ARC Seminar Wednesday 12 December 2018 3 pm
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 +The complex nature of comets has earned them a spot among the most interesting objects in the Solar System. Comets are believed to still preserve information about the physical conditions in the protoplanetary disk. At the same time, they also bear signatures of the epoch of planetary migration ~4 billion years ago, of the time spent in the outer solar system, as well as of their recent activity. In the past three decades, a great progress in untangling the intricate history of comets has come from the in-situ studies during a series of space missions which culminated with the Rosetta mission between 2014 and 2016. However, with no plans for space missions to further comets in the next couple of decades, we have to rely more heavily on telescope observations to reveal new clues on the unanswered questions in cometary science. ​
 +In this talk, I will present results from our effort to study Jupiter-family comet nuclei and their source populations in the Centaur region and the Kuiper Belt from the ground. This work has demonstrated that photometric observations of the rotation and surface properties of comet nuclei can be key for understanding their evolution.
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 +===Internal Gravity Waves in Massive Stars =====
 +** Dr Tamara Rogers** (Newcastle University)- ARC Seminar Wednesday 05 December 2018 1 pm
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 +Internal Gravity Waves (IGW) can lead to angular momentum transport and chemical mixing in stellar interiors. In this talk I will  present numerical simulations of these waves in massive stars and discuss how they might contribute to the understanding of a variety of observational mysteries.  ​
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 +===Unveiling extreme dusty star-formation in the distant Universe =====
 +** Dr Julie Wardlow** (Lancaster University)- ARC Seminar Wednesday 28 November 2018 3.30 pm
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 +In recent years the high-redshift Universe has been increasingly opened to scrutiny at far-infrared wavelengths,​ where cool dust emission from star-formation dominates. The dusty star-forming galaxies (DSFGs), selected at these wavelengths likely represent an important, but short-lived phase in the growth of massive galaxies. These DSFGs often have star-formation rates in excess of ~1000 solar masses per year and are confirmed beyond z~6, although their redshifts and high dust contents make them faint and difficult to study at other wavelengths. I will present results probing their nature and the triggering mechanisms of their immense star-formation rates, using data from ALMA and other leading facilities.
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 +===Transverse Wave Induced Kelvin-Helmholtz Rolls in Spicules =====
 +** Dr Patrick Antolin** (University of St. Andrews)- ARC Seminar Wednesday 21 November 2018 3pm
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 +In addition to their jet-like dynamic behavior, spicules usually exhibit strong transverse speeds, multi-stranded structure, and heating from chromospheric to transition region temperatures. In this work we first analyze Hinode and IRIS observations of spicules and find different behaviors in terms of their Doppler velocity evolution and collective motion of their sub-structure. Some have a Doppler shift sign change that is rather fixed along the spicule axis, and lack coherence in the oscillatory motion of strand-like structure, matching rotation models, or long- wavelength torsional Alfvén waves. Others exhibit a Doppler shift sign change at maximum displacement and coherent motion of their strands, suggesting a collective magnetohydrodynamic (MHD) wave. 
 +By comparing with an idealized 3D MHD simulation combined with radiative transfer modeling, we analyze the role of transverse MHD waves and associated instabilities in spicule-like features. We find that transverse wave induced Kelvin– Helmholtz (TWIKH) rolls lead to coherence of strand-like structure in imaging and spectral maps, as seen in some observations. The rapid transverse dynamics and the density and temperature gradients at the spicule boundary lead to ring-shaped Mg II k and Ca II H source functions in the transverse cross-section,​ potentially allowing IRIS to capture the Kelvin–Helmholtz instability dynamics. Twists and currents propagate along the spicule at Alfvénic speeds, and the temperature variations within TWIKH rolls, produce the sudden appearance/​disappearance of strands seen in Doppler velocity and in Ca II H intensity. However, only a mild intensity increase in higher-temperature lines is obtained, suggesting there is an additional heating mechanism at work in spicules. ​
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 +===The origin of the highest energy particles in the universe: A "​Whodunnit"​ =====
 +** Dr James Matthews** (Oxford)- ARC Seminar Wednesday 7 November 2018 1pm
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 +The origin of ultra-high energy cosmic rays (UHECRs) - protons and nuclei striking our atmosphere with energies extending beyond 1e20 eV - has been an open question for decades. In this talk I will review what we know about UHECRs from an observational and theoretical perspective,​ focusing particularly on diffusive shock acceleration. I will discuss the physical requirements for acceleration,​ before exploring whether radio galaxies might meet these requirements. Aided by hydrodynamic simulations,​ I will show that shocks can be formed in backflows in the radio lobe and that these shocks are better accelerators to ultra-high energies than the relativistic termination shock. I will then discuss a model in which giant-lobed radio galaxies such as Centaurus A and Fornax A act as slowly-leaking UHECR reservoirs, with the UHECRs being accelerated during a more powerful past episode. These radio galaxies may also explain the observed hotspots in the UHECR data, but deflections in Galactic and extragalactic magnetic fields make robust associations difficult. ​
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 +===A Nanoflare Explanation for Periodic Variations in Flare Stars  --  A Solar Physicist'​s Adventure in Single Pixel Land =====
 +** Christopher Dillon** (QUB)- ARC Seminar Wednesday 31 October 2018 3pm
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 +Several studies have documented periodic and quasi-periodic signals from the time series of dMe flare stars and other stellar sources. Such periodic signals, observed within quiescent phases (i.e., devoid of larger-scale microflare or flare activity), range in period from 1-1000 seconds and hence have been tentatively linked to ubiquitous p-mode oscillations generated in the convective layers of the star. As such, most interpretations for the observed periodicities have been framed in terms of magneto-hydrodynamic wave behavior. However, we propose that a series of continuous nanoflares, based upon a power-law distribution,​ can provide a similar periodic signal in the associated synthetic time series. Monte Carlo simulations,​ embodying the nanoflare signals and modelled noise profiles, produces a time series consistent with previous observations of dMe flare star lightcurves. Through an examination of nanoflare decay timescales and differing power-law indices, we provide evidence that periodic signals found in stellar time series can be explained by low-energy nanoflares embedded within the noise envelope of a stellar lightcurve. Phenomena traditionally considered a consequence of wave behaviour may be described by a number of high frequency but discrete nanoflare energy events.
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 +===ATLAS Shrugged: The search for gravitational wave counterparts with an asteroid survey =====
 +** Owen McBrien** (QUB)- ARC Seminar Wednesday 24 October 2018 3pm
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 +The Laser Interferometer Gravitational-wave Observatory (LIGO) has completed two observing runs between 12th September 2015 and 25th August 2017 and found a host of gravitational wave (GW) signals owing to compact binary coalescences of massive bodies and atypical bursts of unknown origin. In the wake of any of these GW triggers however, large portions of the sky must be combed with optical instruments in the hope of identifying a visible counterpart to the GW source. This is realisable with high-cadence,​ full time surveys such as ATLAS, the Asteroid Terrestrial-impact Last Alert System. I will present this ATLAS search for optical counterparts to several LIGO burst triggers as well as the ATLAS search for kilonovae following the discovery of AT2017gfo last August and discuss how this will inform our follow-up campaign to triggers announced during the third LIGO observing run next year.
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 +===Concerning type Ibc supernovae =====
 +** Dr Simon Prentice** (QUB)- ARC Seminar Wednesday 17 October 2018 3pm
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 +Type Ibc supernovae (aka stripped-envelope supernovae) are the final act of massive stars that have been stripped of their He and/or H envelopes. These explosions are amongst the most energetic in the Universe, and can even power gamma-ray bursts. However, despite many well observed examples, their origins are not well understood. Stellar evolution predicts they arise from very massive stars, but this is inconsistent with the observations. I will present analysis of the largest data set of SE-SNe light curves to date, and physical parameters derived from these. I will show that these objects are diverse in their properties and that the results favour "​low"​ high mass stars, stripped through binary interaction,​ as the progenitors for these events.
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 +===High resolution ALMA imaging of SMGs =====
 +** Dr Bitten Gullberg** (Durham University)- ARC Seminar Wednesday 3 October 2018 1pm
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 +I will present high resolution (0.03"​) rest-frame 160micron continuum maps of four z~4.5 sub-mm galaxies (SMGs) selected from the ALMA-LESS and ALMA-UDS surveys. These cycle 3 observations resolve the gas and dust within the ISM of these galaxies on 0.2-1kpc scales. The data reveal an apparent range of morphologies. Though the continuum morphologies appear to be: smooth and compact or extended and ‘clumpy’,​ comparison with simulations reveal that all four sources are consistent with exponential discs. From the morphologies and dynamics of the gas and dust, I will show that these SMGs are most likely to contain dust discs which are smooth on scales of ~200pc. These four SMGs are also bright [CII] emitters and by considering the L([CII])/​L(FIR) ratio as a function of the star-formation rate surface density I will revisit the [CII] deficit, which these measurement suggest, is consistent with local processes. I will present some of the physical drivers that may be responsible for this trend and can give rise to the properties found in the densest regions of SMGs.
  
 ===Dwarf Spheroidal Galaxies: Cosmological probes on our doorstep ===== ===Dwarf Spheroidal Galaxies: Cosmological probes on our doorstep =====
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 The infrared (IR) spectra of objects associated with dust and gas – including evolved stars, reflection nebulae, the interstellar medium (ISM), star-forming regions, and galaxies out to redshifts of z ∼ 3 – are dominated by emission bands at 3.3, 6.2, 7.7, 8.6 and 11.2 μm, the so-called unidentified infrared (UIR) bands. They are generally attributed to the IR fluorescence of Polycyclic Aromatic Hydrocarbon molecules (PAHs) UV pumped by nearby massive stars. Hence, the UIR band strengths are used to determine the star formation rate in galaxies, one of the key indicators for understanding galaxy formation and evolution. To date, PAHs are among the largest and most complex molecules known in space and emit up to 10% of the total power output of star-forming galaxies. The infrared (IR) spectra of objects associated with dust and gas – including evolved stars, reflection nebulae, the interstellar medium (ISM), star-forming regions, and galaxies out to redshifts of z ∼ 3 – are dominated by emission bands at 3.3, 6.2, 7.7, 8.6 and 11.2 μm, the so-called unidentified infrared (UIR) bands. They are generally attributed to the IR fluorescence of Polycyclic Aromatic Hydrocarbon molecules (PAHs) UV pumped by nearby massive stars. Hence, the UIR band strengths are used to determine the star formation rate in galaxies, one of the key indicators for understanding galaxy formation and evolution. To date, PAHs are among the largest and most complex molecules known in space and emit up to 10% of the total power output of star-forming galaxies.
 Space-based telescopes such as the Infrared Space Observatory (ISO) and the Spitzer Space Telescope revealed the richness of the PAH spectrum and provided extensive evidence for significant variability in the PAH spectrum from source to source and spatially within sources. In this talk, I will focus on the PAH properties in the reflection nebula NGC2023. I will present spectral maps of NGC2023 obtained with the SL and SH mode of the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. These observations clearly illustrate that the detailed characteristics of the PAH emission features vary across the reflection nebula and that different sets of PAH bands correlate spatially across the nebula. In particular, we conclude that at least 2 spatially distinct components contribute to the 7–9 μm PAH emission. We interpret these differences in spatial behavior in terms of variations in PAH characteristics such as size, charge and structure with the changing environment across the nebula. Space-based telescopes such as the Infrared Space Observatory (ISO) and the Spitzer Space Telescope revealed the richness of the PAH spectrum and provided extensive evidence for significant variability in the PAH spectrum from source to source and spatially within sources. In this talk, I will focus on the PAH properties in the reflection nebula NGC2023. I will present spectral maps of NGC2023 obtained with the SL and SH mode of the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. These observations clearly illustrate that the detailed characteristics of the PAH emission features vary across the reflection nebula and that different sets of PAH bands correlate spatially across the nebula. In particular, we conclude that at least 2 spatially distinct components contribute to the 7–9 μm PAH emission. We interpret these differences in spatial behavior in terms of variations in PAH characteristics such as size, charge and structure with the changing environment across the nebula.
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public/seminar_abstracts.1528712236.txt.gz · Last modified: 2018/06/11 10:17 by Samuel Grant

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