Volume 541, May 2012
|Number of page(s)||15|
|Section||Interstellar and circumstellar matter|
|Published online||20 April 2012|
Comparison of Westerlund’s photographic plate #3117 with the interpolated and PSF-subtracted EMMI observation at the same phase (φ = 0.84). The field of view is 1.4′ × 1.4′, with north up and east to the left.
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CFHT/HRC image of the circumstellar nebula of RS Pup (left, 1992) and the same field observed with FORS (right, 2009). The field of view is 2′ × 2′, with north up and east to the left.
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Soon after his discovery of the nebula surrounding RS Pup, Westerlund obtained a series of photographic plates with the 74-inch Mount Stromlo telescope (located near Canberra, Australia) to search for the propagation of light echoes (Westerlund 1961). These plates were subsequently used by Havlen for his pioneer study of the nebula (Havlen 1972), and a detailed description of the plates can be found in this publication. For the present work, we digitized the original photographic plates, that were preserved by one of us (RJH) for almost 50 years, using a flatbed scanner at a resolution of 2400 dpi. This optical resolution is sufficient to resolve the film grain. We matched the astrometric WCS of the scanned plates with the EMMI images obtained in 2007 using a dozen of background stars spread over the image field. The proper motion of a few bright stars of the field is noticeable, however, the matching of the images is satisfactory.
We carefully blinked the Westerlund plates with the EMMI observations presented in Paper I. The EMMI images were interpolated at the phase of the Westerlund observations using the procedure described in Sect. 3. Among the 18 epochs covering the 1960–1963 period, we focused our comparison on three epochs that show particularly well the circumstellar nebula: 30 March 1962 (visual), 5 May 1962 (visual) and 26 February 1963 (blue and visual). These plates were obtained under very
good seeing conditions, and are the best of the series. We did not detect any change in the morphology of the nebula. This negative result was expected considering the fact that a 1″ angular displacement corresponds to a linear displacement of ≈2000 AU at the distance of RS Pup. Over 50 years, such a 1″ displacement would therefore require a very fast linear velocity of at least ≈200 km s-1.
The circumstellar nebula of RS Pup was observed on the nights of 4–6 January 1992, 9 April 1992, and 28 March 1993, using the HRC instrument installed at the prime focus of the Canada-France-Hawaii Telescope, for Michael J. Pierce & Douglas L. Welch. Exposures of 30 to 600 s were obtained in the B, V and R bands. We present in Fig. A.2 the best image in the V band (06-01-1992), compared to our second epoch observation with FORS (27-05-2009). The two observations do not correspond to the same phase, but we could not detect any change in the morphology of the nebula (apart from the photometric variations due to the echoes). However, we observe the apparent displacement of several stars in the field over the 17 years separating the two observations (an example is presented in Fig. A.3). The bright pair of point-like sources visible above the center of the FORS image (close to the central vertical white stripe) is an artefact of the instrument.
Displacement of a faint star in the field surrounding RS Pup between 1992 (left) and 2009 (right). The displacement of the brighter of the two stars near the center of the field is of ≈0.6″. The field of view is 30″ × 30″.
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© ESO, 2012
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