arXiv subjects
Publications and source records attributed to T. J. O'Brien.
We have recently begun a search for Classical Novae in M31 using three years of multicolour data taken by the POINT-AGAPE microlensing collaboration with the 2.5m Isaac Newton Telescope (INT) on La Palma. This is a pilot program leading to the use of the Liverpool Telescope (LT) to systematically search for and follow novae of all speed classes in external galaxies to distances up to around 5Mpc.
The association between the Pencil nebula (RCW 37, NGC 2736), the Vela X-ray fragment D/D' and the recently discovered new X-ray supernova remnant (RX J0852.0-4622) in Vela is investigated. Recently published Chandra images of D/D' are compared with optical images of RCW 37 and confirm the close association of the two objects. New optical line profiles of RCW 37 from an extended slit position passing through this unusual optical nebula are presented. They reveal a partial velocity ellipse with expansion velocities of around 120 km/s. Various scenarios for the origin of the nebula are considered and the evidence of a link with RX J0852.0-4622 is reviewed. A funnel of gas similar to those in the Crab and DEM34a SNRs is not ruled out but a more plausible explanation may be that a `wavy sheet' is responsible. We suggest that RX J0852.0-4622 is located within the older larger Vela SNR and that some of the X-ray gas from RX J0852.0-4622 has collided with the dense HI wall of the older remnant. This gives rise to the morphology and velocity structure of the optical emission and explains the unusual X-ray emission from this portion of the supernova remnant. If our hypothesis is correct, a distance prediction of 250 +/- 30 pc can be made, based on recent measurements of the distance to the old Vela SNR. This is at the lower end of the range of distances quoted in the literature would confirm unusual nature of this young nearby supernova remnant.
Presented here are high angular resolution MERLIN 5 GHz (6 cm) continuum observations of the binary proplyd system, LV 1 in the Orion nebula, which consists of proplyd 168--326SE and its binary proplyd companion 168--326NW (separation 0.4 arcsec). Accurate astrometric alignment allows a detailed comparison between these data and published HST PC Halpha and [Oiii] images. Thermal radio sources coincide with the two proplyds and originate in the ionized photoevaporating flows seen in the optical emission lines. Flow velocities of approx 50 km/s from the ionized proplyd surfaces and \geq 100 km/s from a possible micro-jet have been detected using the Manchester Echelle spectrometer. A third radio source is found to coincide with a region of extended, high excitation, optical line emission that lies between the binary proplyds 168--326SE/326NW . This is modelled as a bowshock due to the collision of the photoevaporating flows from the two proplyds. Both a thermal and a non-thermal origin for the radio emission in this collision zone are considered.
In this paper we report on the first five out of eleven observations in our programme of Hubble Space Telescope (HST) imaging of old nova shells. We present new WFPC2 images of the shells around FH Ser and V533 Her taken in the F656N (Halpha+[NII]) filter. We also show long-slit spectra taken using the William Herschel Telescope (WHT) in La Palma for these objects in the same spectral range. The shell around FH Ser is found to be a prolate ellipsoid of ellipticity 1.3 +/- 0.1 inclined at 62 +/- 4 deg to the line of sight. The shell has an equatorial ring which is found to be due to increased emission in the two [NII] lines rather than Halpha. The expansion velocity, systemic velocity and distance are found to be 490 +/- 20 km/s, -45 km/s and 950 +/- 50 pc. The origin of the [NII] equatorial ring is discussed in the context of a photoionization feature resulting from aspherical illumination by the central source rather than a simple density enhancement. It is possible however that the ring is also in part due to an extremely localised increase in the nitrogen abundance. Similar imaging and spectroscopy of the nova V533 Her reveal a shell of radius 5 +/- 0.7 arcsec with an axial ratio of 1.2 +/- 0.2, expansion velocity of 850 +/- 150 km/s and distance 1250 +/- 300 pc. The shells around BT Mon, DK Lac and V476 Cyg were not detected with HST.
We have investigated how several parameters can affect the results of a collision between an extragalactic jet and a dense, intergalactic cloud, through a series of hydrodynamic simulations. Such collisions are often suggested to explain the distorted structures of some radio jets. However, theoretical studies of this mechanism are in conflict over whether it can actually reproduce the observations. The parameters are the Mach number, and the relative densities of the jet and the cloud to the ambient medium. Using a simple prescription we have produced synthetic radio images for comparison with observations. These show that a variety of structures may be produced from simple jet-cloud collisions. We illustrate this with a few examples, and examine the details in one case. In most cases we do not see a clear, sustained deflection. Lighter jets are completely disrupted. The most powerful jets produce a hotspot at the impact which outshines any jet emission and erode the cloud too quickly to develop a deflected arm. It appears that moderate Mach numbers and density contrasts are needed to produce bends in the radio structure. This explains the apparent conflict between theoretical studies, as conclusions were based on different values of these parameters. Shocks are produced in the ambient medium that might plausibly reproduce the observed alignment of the extended emission line regions with the radio axis.
We present an optical imaging study of 20 southern-sky nova remnants which has resulted in the discovery of four previously unknown nova shells -- V842 Cen, RR Cha, DY Pup and HS Pup. The study has also revealed previously unobserved features in three other known shells -- those of BT Mon, CP Pup and RR Pic. The images of BT Mon, V842 Cen, RR Cha, DY Pup and HS Pup have been processed using several deconvolution algorithms (Richardson-Lucy, maximum entropy and clean) in addition to straightforward point-source subtraction in an attempt to resolve the shells from the central stars. The use of four different methods enables us to make a qualitative judgement of the results. Notably, the shell of RR Pic displays tails extending outwards from clumps in the main ejecta similar to those previously detected in DQ Her.
Recently, it has been claimed that the recurrent nova T Pyx exhibits oppositely directed jets of ejecta apparent in features seen in H alpha emission. Here we demonstrate that these features are in fact emission in the [N II] lines which lie either side of H alpha and arise from the expanding shell associated with this object rather than from collimated jets. We estimate an expansion velocity along a line of sight through the centre of the shell of about 500 km/s.