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The longest period (32.5 y) dust-maker WR 48a

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Colliding-wind binaries
WR104 pinwheel
Episodic dust-makers
WR140 orbital motion
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Stellar Winds in Interaction
Co-ordinates J2000
R.A. 13 12 39.65
Dec. -62 42 55.8
8.9"N and 2.3"E
of CPD -62°3058
X-ray source
WR 48a is a most luminous X-ray source (Zhekov et al. 2011) — further evidence for strongly colliding winds.

The Wolf-Rayet dust maker with the longest period appears to be WR 48a, whose period inferred from recurrence of maxima in its infrared (IR) emission and IR image of its dust is about 32.5 years. It was the first dust-making Wolf-Rayet star to be identified from its strong dust emission in the IR by Danks et al. (1979) in their survey of the H II region G305.5 +0.0. They observed WR 48a to be undergoing an infrared outburst similar to that undergone in 1977 (Williams et al. 1978) by the prototype episodic dust maker HD 193793 (= WR 140). Danks et al. tracked the rise of the IR emission to a maximum in 1979 and its initial fading.
Right: Infrared image of WR 48a and the two star clusters Danks 1 and Danks 2 in the G305 star-forming region extracted from the near-IR Zoomable Mosaic of the Milky Way built from the VISTA VVV and UKIDSS GPS infrared surveys. This extract is about 1⁄8° wide.
(The dark spots in the middles of the brightest star images, including WR 48a, are a consequence of saturation in the images.)
The clusters and the G305 star formation region have been the subjects of many studies, e.g. Davies et al. (2012). They are located about 3.5 kpc from the Earth.

The tracking of the fading dust emission by IR photometry was extended to 1986 by Williams, van der Hucht & Thé (1987), who also argued from the approximately equal strengths of the C III 5696-Å and C IV 5808-Å spectral classification lines in their spectrum that WR 48a was a WC8 type star. Further spectroscopy (Williams et. at. 2012) revealed that the binary companion to the WC8 star was about 3× (1.2 mag.) brighter than the WC8 star in the visible and not an ordinary OB star but a more luminous emission-line star. The classification of the companion was refined using higher resolution spectroscopy to a WN8h type WR star by Zhekov et al. (2014).
The IR photometry was continued (Williams et. at. 2012) until the IR emission had returned to the level at which it was first observed by Danks et al. in 1978.
Right: Long-term light curves of WR 48a in the near-infrared H, K and L bands from Williams et. at. (2012). Besides the long-term variation, the light curves show secondary episodes of dust formation, e.g. in 1990, 1994 and 2000, resembling those observed in another episodic dust maker, WR 137 (Williams et al. 2001). H, K and L/L' light curves
Right:
It is possible to monitor the temperature and luminosity of the newly formed dust using its (H-L) colour and K magnitude. In 1978-1979, the luminosity increased while the constant (H-L) colour implies a constant temperature, that at which new dust was forming. After 1979, the luminosity fell and (H-L) grew redder, indicating that the dust was cooling as it moved away from the stars and was not being replenished at the same rate. After 1997, the luminosity started rising again and the blueness of (H-L) was the signature of new, hot dust. Also evident are the episodes e.g. 1990, 1994, when (H-L) suddenly got bluer for a short time, indicating short bursts of dust formation.
Below:
From infrared photometry at a range of wavelengths, we form spectral energy distributions (SEDs) to study the luminosity and temperature of the dust. The figure compares the SEDs for three epochs: the 1979 maximum, when the luminosity is highest and the dust hottest as new dust is condensing, in 1996 around minimum, when the luminosity is lower and the dust is cooler as condensation had all but ceased, and in 2006, when the IR flux had started rising again, and the SED shows that there is more hot, new dust than in 1996.
(See Williams et. at. 2012 for details.).


The WR 48a system makes enough dust for it to be imaged, even at its distance of about 3.5 kpc. In 2004, Marchenko & Moffat (2007) (their figure 2; the image labelled "WR 48a" in their figure 3 is really of WR 112) observed a 12-µm image of WR 48a using TReCS on Gemini, which showed the dust emission to have the form of a "pinwheel" like that observed around WR 104 by Tuthill, Monnier & Danchi (1999), but having a higher inclination.
Below is another 12-µm image of WR 48a, observed in 2006 with the same instrumentation and retrieved from the Gemini public data archive (PI Moffat), which shows a very similar structure. The overlaid pinwheel (inclined 60°, line of nodes E--W) is purely suggestive; what is significant is the spacing between the two turns: 2.5 and 2.6 arc seconds. At the distance of WR 48a, this is consistent with the dust expanding for 32.5 years at 1300 km/s, which is reasonable for the wind terminal velocity of a WC8 star.
12-µm image of WR 48a, observed in 2006 with TReCS on Gemini S. In order to show the faint emission, the intensity scale for this image was set using histogram equalisation. The image is poor along the E-W axis owing to an artefact of the detector: the apparent breaks in the structure may not be real.
rev: 25 November 2015
Peredur Williams
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[1] A. C. Danks  et al. A&A 118, 301, 1983
[2] P. M. Williams, K. A. van der Hucht & P. S. Thé A&A 182, 91, 1987
[3] P. M. Williams et al. MNRAS 185, 467, 1978
[4] B. Davies et al. MNRAS 419, 1871, 2012
[5] P. M. Williams et al. MNRAS 420, 2026, 2012
[6] S. A. Zhekov et al. MNRAS 445, 1663, 2014.
[7] P. M. Williams et al. MNRAS 324, 156, 2001.
[8] S. V Marchenko & A. F. J. Moffat ASP Conf. Series 367, 213, 2007
[9] P. G. Tuthill, J. D. Monnier & W. C. Danchi Nature 398, 487, 1999
[10] S. A. Zhekov, M. Cagné & S. Skinner ApJ 727, L17, 2011.