Publications

Read the abstracts of my papers, posters and talks, and click through for download or request links.

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Peer-Reviewed Papers

White et al. 2015, MNRAS, accepted

Turbulent mixing layers in supersonic protostellar outflows, with application to DG Tauri

arXiv pre-print

Turbulent entrainment processes may play an important role in the outflows from young stellar objects at all stages of their evolution. In particular, lateral entrainment of ambient material by high-velocity, well-collimated protostellar jets may be the cause of the multiple emission-line velocity components observed in the microjet-scale outflows driven by classical T Tauri stars. Intermediate-velocity outflow components may be emitted by a turbulent, shock- excited mixing layer along the boundaries of the jet. We present a formalism for describing such a mixing layer based on Reynolds decomposition of quantities measuring fundamental properties of the gas. In this model, the molecular wind from large disc radii provides a continual supply of material for entrainment. We calculate the total stress profile in the mixing layer, which allows us to estimate the dissipation of turbulent energy, and hence the luminosity of the layer. We utilize MAPPINGS IV shock models to determine the fraction of total emission that occurs in [Fe II] 1.644 {\mu}m line emission in order to facilitate comparison to previous observations of the young stellar object DG Tauri. Our model accurately estimates the luminosity and changes in mass outflow rate of the intermediate-velocity component of the DG Tau approaching outflow. Therefore, we propose that this component represents a turbulent mixing layer surrounding the well-collimated jet in this object. Finally, we compare and contrast our model to previous work in the field.


White et al. 2014b, MNRAS, 442, 28-42

Multi-epoch Sub-arcsecond [Fe II] Spectroimaging of the DG Tau Outflows with NIFS. II. On the Nature of the Bipolar Outflow Asymmetry

arXiv pre-printADS entry

The origin of bipolar outflow asymmetry in young stellar objects (YSOs) remains poorly understood. It may be due to an intrinsically asymmetric outflow launch mechanism, or it may be caused by the effects of the ambient medium surrounding the YSO. Answering this question is an important step in the development of outflow launch models. We have investigated the bipolar outflows driven by the T Tauri star DG Tauri on scales of hundreds of AU, using the Near-infrared Integral Field Spectrograph (NIFS) on Gemini North. The approaching outflow consists of a well-collimated jet, nested within a lower-velocity disc wind. The receding outflow is composed of a single-component bubble-like structure. We analyse the kinematics of the receding outflow using indicative models, and determine that it is an expanding, receding bubble. We propose that this bubble forms because the receding counterjet from DG Tau is obstructed by a clumpy ambient medium above the circumstellar disc surface, based on similarities between this structure and those found in the modeling of active galactic nuclei outflows. There is evidence of interaction between the obscured counterjet and clumpy ambient material, which we attribute to the large molecular envelope around the DG Tau system. An analytical model of a momentum-driven bubble is shown to be consistent with our observations. We conclude that the bipolar outflow from DG Tau is intrinsically symmetric, and the observed asymmetries are due to environmental effects. This mechanism can potentially be used to explain the observed bipolar asymmetries in other YSO outflows.


White et al. 2014a, MNRAS, 441, 1681-1707

Multi-epoch Sub-arcsecond [Fe II] Spectroimaging of the DG Tau Outflows with NIFS. I. First data epoch

arXiv pre-printADS entry

Investigating the outflows emanating from young stellar objects (YSOs) on sub-arcsecond scales provides important clues to the nature of the underlying accretion–ejection process occurring near the central protostar.We have investigated the structures and kinematics of the outflows driven by the YSO DG Tauri, using the Near-infrared Integral Field Spectrograph (NIFS) on Gemini North. The blueshifted outflow shows two distinct components in [Fe II] 1.644 µm emission, which are separated using multicomponent line fitting. Jet parameters are calculated for the high-velocity component. A stationary recollimation shock is observed, in agreement with previous X-ray and far-ultraviolet observations. The presence of this shock indicates that the innermost streamlines of the high-velocity component are launched at a very small radius, 0.01–0.15 au, from the central star. The jet accelerates and expands downstream of the recollimation shock; the ‘acceleration’ is likely a sign of velocity variations in the jet. No evidence of rotation is found, and we compare this non-detection to previous counterclaims. Moving jet knots, likely the result of the jet velocity variations, are observed. One of these knots moves more slowly than previously observed knots, and the knot ejection interval appears to be non-periodic. An intermediate-velocity component surrounds this central jet, and is interpreted as the result of a turbulent mixing layer along the jet boundaries generated by lateral entrainment of material by the high-velocity jet. Lateral entrainment requires the presence of a magnetic field of strength a few mG or less at hundreds of au above the disc surface, which is argued to be a reasonable proposition. In H2 1–0 S(1) 2.1218 µm emission, a wide-angle, intermediate-velocity blueshifted outflow is observed. Both outflows are consistent with being launched by a magnetocentrifugal disc wind, although an X-wind origin for the high-velocity jet cannot be ruled out. The redshifted outflow of DG Tau takes on a bubble-shaped morphology, which will be discussed in a future paper.


Talks

Conference

Star Formation Through Spectroimaging at High Angular Resolution, Taipei, Jul 2011

Sub-arcsecond Observations and Models of the Outflow from the Young Stellar Object DG Tauri

Abstract not available.


Colloquia

University of Amsterdam; STARPLAN, Copenhagen, Jul 2013

NIFS Spectroimaging of the DG Tauri Outflows: New Insights into Protostellar Outflows

Abstract not available.


Posters

Protostars and Planets VI, Heidelberg, 15-20 Jul 2013

Multi-epoch Spectroimaging of the DG Tauri Outflows with NIFS

The outflows from young stellar objects provide important clues to the nature of the underlying accretion-ejection mechanism. We present unique high-resolution multi-epoch spectroimaging data of the outflows from the young stellar object DG Tauri, obtained using the Near-infrared Integral Field Spectrograph (NIFS) on Gemini North. These data reveal the presence of recollimation shocks, jet acceleration, entrainment, and bipolar outflow asymmetry, which we model to create a picture of the DG Tau system.