Welcome to Peredur Williams's home page


Research on Wolf-Rayet stars, especially members of colliding-wind binaries, and their carbon dust formation X-ray and non-thermal radio emission — such as the prototype WR 140, the long-period system WR 48a, and the maverick Apep. Colliding wind binary logo
The most massive hot stars are constantly losing mass in fast (1000-3000 km s-1) stellar winds which carry away ∼ 10-6 to 10-5 M⊙ y-1 (the higher mass loss coming from Wolf-Rayet stars), giving the winds significant kinetic power. When the stars are members of binary systems, "colliding-wind binaries", the winds crash into each other between the stars and some of this power is dissipated, leading to shocks, heating of the winds, strong X-ray emission, particle acceleration and sometimes even the formation of clouds of carbon dust if one of the stars is a WC-type Wolf-Rayet star.
If the two stars are in an elliptical orbit, the strength of the wind collision varies around the orbit, being most intense when the stars are closest, i.e. during periastron passage. The most recent periastron passage in the prototype, WR 140 (= HD 193793), occurred in late 2016, stimulating an on-going intensive multi-wavelength observing campaign to study its orbit and colliding-wind phenomena, such as X-ray emission and spectral line variations. Because its orbit is very elliptical (e ∼ 0.9), dust formation occurs for only a few weeks of its 7.93-year period, always at exactly the same phase.
It has become the protoype of Wolf-Rayet episodic dust makers, including WR 19 (P ∼ 10 y), WR 137 (P ∼ 13 y), WR 125 (P 28.1 y) in the Galaxy and HD 38030 (P ∼ 22 y) in the Large Magellanic Cloud. Related systems have been found in other galaxies.
A spectacular image of the dust shells made by WR 140 over the last century+ was observed with the JWST .

Critical configurations of WR 140 in 2024-25

On account of its high orbital eccentricity, e = 0.8993, the WR 140 system swings through most of its orbit in only one tenth of its period, as can be seen from the critical configurations around the 2024 periastron shown below, where f is the true anomaly of the O5 star relative to the WC7 star, ψ is the angle between our line of sight and the axis joining the stars (which would be the axis of symmetry of the wind-collision region in the absence of orbital motion), P.A. is the position angle (O5 star relative to the WC7 star) of this axis on the sky and r/a is the separation of the stars.

phase Orbital phenomenon f r/a P.A. ψ MJD Year Date
0.9556 conjunction: WC star behind 223 0.57 84 29 60506 2024.54 July 17
0.9966 quadrature 313 0.12 354 90 60626 2024.86 Nov 12
0.000 periastron passage 0 0.10 327 121 60636 2024.90 Nov 22
0.0030 conjunction: O star behind 42 0.12 264 151 60645 2024.91 Dec 1
0.0359 quadrature 133 0.49 174 90 60740 2025.18 Mar 6

More dust-making Wolf-Rayet stars, some variable or episodic and ten apparently constant dust makers, have been identified in a study of NEOWISE-R observations of WC type stars, which includes a summary of the properties of the variable WR dust emitters.

Publications: links from the SAO/NASA ADS to my publications
or, links to first-author publications only

Historical projects:
I am studying the third Astronomer Royal for Scotland, Ralph Copeland (1837-1905) and have published a biography in The Antiquarian Astronomer, 17, 28, 2023. He lived an adventurous early life, leaving England to join the Australian gold rush when he was 15 and then worked on a sheep farm. He returned to Britain, worked as a locomotive engineer and then went to the University of Göttingen, where he gained a PhD. He participated in the Second German North Polar expedition, working on a preliminary geodetic survey, and adding to the food supply with his rifle. On his return, he took positions in Ireland, first at Lord Rosse's observatory at Birr Castle and then at Dunsink. From 1876-1888, he worked at Lord Crawford's observatory at Dun Echt, Aberdeenshire, where he made extensive observations of comets and novae. He also made an expedition to study observing conditions in South America, making observations at Puno on Lake Titicaca and at Vincocaya (elev. 14,360 ft), documenting the conditions at high elevation and discovering a number of Wolf-Rayet stars from their spectacular spectra. In 1889, he was appointed Astronomer Royal for Scotland in succession to Charles Piazzi Smyth and played a major part in establishing the (new) Royal Observatory Edinburgh on Blackford Hill. He arranged for the motto below, NEC FASCES, NEC OPES, SOLA ARTIS SCEPTRA PERENNANT (only scientific achievements endure, not power nor wealth), with AD 1894 partly visible behind, to be engraved above the door of the Astronomer Royal's residence on the site.

Letters from Dun Echt: a networked observatory:
The Archives of the ROE preserve the outgoing letters of the private observatory (left) established by Lord Lindsay (who became 26th Earl of Crawford from 1880) at Dun Echt, providing a detailed picture of its development and operation during its twenty-year (1872-1892) existence. Nearly all were written by the two astronomers in charge, David Gill until mid-1876 followed by Ralph Copeland until the observatory merged with the ROE following Lord Crawford's benefaction. A survey (The Observatory, 144, 54, 2024) looks primarily at Gill's and Copeland's communications with other astronomers, both professional and amateur, to consider how the apparently isolated observatory extended its connections with the astronomical world.
Institute for Astronomy
Royal Observatory
Blackford Hill
Edinburgh
EH9 3HJ
United Kingdom 		Contact:
email: pmw [at] roe.ac.uk



2024 July