Molecular hydrogen line emission from photodissociation regions

The work presented in this thesis is dedicated to the study of the physical properties of photodissociation regions (PDRs), the surface layers of molecular clouds which are irradiated by ultraviolet radiation. The structure of PDRs is investigated with the development of an anlytical model which incorporates the essential heating and cooling mechanisms in a PDR. The main parameters in the model are the density and the incident ultraviolet radiation field, above the ambient value in the solar neighbourhood, impinging on the surface (Go) which dissociates the molecules in theaPDR. It is demonstrated that when the ratio (n/Go) is high (> 100 cm' ) the attenuation of ultraviolet photons is dominated by H2 self shielding which brings the HI/H2 transition zone close to the surface of the cloud (Av < 1). When the ratio is of order unity then the attenuation of ultraviolet photons is dominated by dust grains in the PDR. In this case, the HI/H2 transition zone occurs at a depth of Av ~ 2 - 3.Images of the PDR in the northern bar of M17 show that there is a spatial coincidence, accurate to ~ 1 arcsec, of the H2 and 3.28 fim emission regions (the 3.28 |im emission being a tracer of the hot edge of the PDR delineated by the Hll/HlC 'ltransition) placing a lower limit to the density in the clumps of 10 cm . This coincidence is also observed in other PDR sources (eg. NGC 2023) and can be readily explained if the sources are clumpy. It is not clear in the northern bar of M l7, where Go ~ 10^, whether shielding by dust or H2 molecules is dominating the attenuation of ultraviolet photons. A uniform, high density PDR model is sufficient to reproduce the observed H2 line intensity, however the images clearly reveal structures at the 2 arcsec level suggesting that a clumpy model is a realistic solution.Long slit K band spectroscopy measurements were taken in the northern bar of M l7, where up to 16 H2 lines were identified. Analysis of the data suggests that the emission can only be explained if the H2 molecules are being excited radiatively, rather than by shocks. The diagnostic line ratio of the H2 v=l-0 S(l) and 2-1 S(l) transitions is approximately 3 over the region observed implying that the lower levels of the molecule are slightly thermalised by the warm gas. The constancy of the ratio further implies that that the collisional deexcitation rate must be constant along this region. This may be taken to mean that the physical conditons do not change along the region and that we are observing the surface of the PDR. The profile of the molecular emission along the slit can be successfully modelled if it is assumed that the large scale surface geometry in the region can be described by a parabola.The ortho to para ratio of H2, measured in three PDR sources (M17 northern bar, NGC 2023 and Hubble 12), is less than the expected value for a hot (~ 300 K) gas in thermodynamic equilibrium (ie. 3). The measured values fall in the range 1.3 - 2.3. Modelling the ortho to para ratio using the rates of spin conversion of the H2 molecule do not satisfy the observations. Some other fonn of processing the ortho to para ratio must be occuring and three different models are considered. First, a different reformation mechanism is considered which allows the newly formed molecule to reside on the surface of the grain for a certain amount of time before it evaporates into the gas. The second model describes a dynamic PDR. Hot gas at the surface of the PDR is allowed to escape setting up an advancing photodissociation front. If the front advances in to the cooler gas before there is enough time for spin changing interactions to take place, then the measured ortho to para ratios will have values characteristic of the cooler (T ~ 50 - 150 K) gas. Both of these models can successfully reproduce the observed ortho to para ratios. The third model assumes that there is some processing of the ortho to para ratio during the fluorescent cascade, after it has been excited by ultraviolet photons. This model is not successful at reproducing the observed ortho to para ratios as the cross sections in the excited states are so large compared to the radiative decay rate that the ortho to para ratio is determined at the gas temperature, giving ratios ~ 3.Column densities calculated from the emission from NGC 2023 has shown an excess of emission above the pure fluorescent cascade in levels which are highly rotationally (J > 6) and vibrationally (v = 3 - 6) excited. In according with theoretical predictions, this may be the first direct evidence of the formation of H2 occuring in excited levels

SKA-VLBI Key Science Programmes

A significant fraction of the observing time with the two phase-I SKA telescopes (SKA1-LOW and SKA1-MID) will be spent on Key Science Projects led by member country scientists. The various SKA Science Working Groups, including the VLBI Focus Group are in the process of defining KSPs that are aligned with the High Priority Science Objectives of the SKA. At the moment it is not clear how the special observing mode of SKA-VLBI - when the SKA1 components are phased-up and included in VLBI networks - could be incorporated in KSPs. The VLBI community needs to be prepared by the time the KSP proposal calls are expected (mid-2020s). In this paper we outline the basic concept of SKA-VLBI, and some possibilities for us to engage in SKA KSPs.Comment: Presented at the 14th European VLBI Network Symposium and Users Meeting (EVN 2018), 8-11 October 2018, Granada, Spain. 7 pages 2x2 figures. Will appear in Proceedings of Scienc

The disk around the brown dwarf KPNO Tau 3

We present submillimeter observations of the young brown dwarfs KPNO Tau 1, KPNO Tau 3, and KPNO Tau 6 at 450 micron and 850 micron taken with the Submillimeter Common-User Bolometer Array on the James Clerke Maxwell Telescope. KPNO Tau 3 and KPNO Tau 6 have been previously identified as Class II objects hosting accretion disks, whereas KPNO Tau 1 has been identified as a Class III object and shows no evidence of circumsubstellar material. Our 3 sigma detection of cold dust around KPNO Tau 3 implies a total disk mass of (4.0 +/- 1.1) x 10^{-4} Msolar (assuming a gas to dust ratio of 100:1). We place tight constraints on any disks around KPNO Tau 1 or KPNO Tau 6 of <2.1 x 10^{-4} Msolar and <2.7 x 10^{-4} Msolar, respectively. Modeling the spectral energy distribution of KPNO Tau 3 and its disk suggests the disk properties (geometry, dust mass, and grain size distribution) are consistent with observations of other brown dwarf disks and low-mass T-Tauri stars. In particular, the disk-to-host mass ratio for KPNO Tau 3 is congruent with the scenario that at least some brown dwarfs form via the same mechanism as low-mass stars.Comment: 18 pages (preprint format), 3 figures, published in Ap

High sensitivity VLBI with SKA

The Square Kilometre Array (SKA), with the aim of achieving a collecting area of one square kilometre, will be the world's largest radio telescope. A scientific collaboration between 12 countries (with more to join), it will consist of one Observatory with 2 telescopes located in South Africa and Australia. The telescope deployment is planned in two phases, but even in its first stage (SKA1) it will already enable transformational science in a broad range of scientific objectives. The inclusion of SKA1 in the Global VLBI networks (SKA-VLBI) will provide access to very high angular resolution to SKA science programmes in anticipation of the science to be realized with the full telescope deployment (SKA2). This contribution provides an overview of the SKA Observatory VLBI capability, the key operational concepts and outlines the need to update the science use cases.Comment: 11 pages, 6 figures, conferenc

Toward Resolving the Outflow Engine: An Observational Perspective

Jets from young stars represent one of the most striking signposts of star formation. The phenomenon has been researched for over two decades and there is now general agreement that such jets are generated as a by-product of accretion; most likely by the accretion disk itself. Thus they mimic what occurs in more exotic objects such as active galactic nuclei and micro-quasars. The precise mechanism for their production however remains a mystery. To a large degree, progress is hampered observationally by the embedded nature of many jet sources as well as a lack of spatial resolution: Crude estimates, as well as more sophisticated models, nevertheless suggest that jets are accelerated and focused on scales of a few AU at most. It is only in the past few years however that we have begun to probe such scales in detail using classical T Tauri stars as touchstones. Application of adaptive optics, data provided by the HST, use of specialised techniques such as spectro-astrometry, and the development of spectral diagnostic tools, are beginning to reveal conditions in the jet launch zone. This has helped enormously to constrain models. Further improvements in the quality of the observational data are expected when the new generation of interferometers come on-line. Here we review some of the most dramatic findings in this area since Protostars and Planets~IV including indications for jet rotation, i.e. that they transport angular momentum. We will also show how measurements, such as those of width and the velocity field close to the source, suggest jets are initially launched as warm magneto-centrifugal disk winds. (abridged)Comment: 14 pages, 8 figures, contributed chapter for Planets and Protostars V meeting (October 2005

Observatory/data centre partnerships and the VO-centric archive: The JCMT Science Archive experience

We present, as a case study, a description of the partnership between an observatory (JCMT) and a data centre (CADC) that led to the development of the JCMT Science Archive (JSA). The JSA is a successful example of a service designed to use Virtual Observatory (VO) technologies from the start. We describe the motivation, process and lessons learned from this approach.Comment: Accepted for publication in the second Astronomy & Computing Special Issue on the Virtual Observatory; 10 pages, 5 figure