Detached dust shell around wolf rayet star WR60 6 in the young stellar cluster VVV CL036

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(1)The Astronomical Journal, 147:18 (8pp), 2014 January C 2014.. doi:10.1088/0004-6256/147/1/18. The American Astronomical Society. All rights reserved. Printed in the U.S.A.. DETACHED DUST SHELL AROUND WOLF–RAYET STAR WR60-6 IN THE YOUNG STELLAR CLUSTER VVV CL036∗ J. Borissova1 , M. S. N. Kumar2 , P. Amigo1 , A.-N. Chené3 , R. Kurtev1 , and D. Minniti4,5,6 1. Departamento de Fı́sica y Astronomı́a, Universidad de Valparaı́so, Av. Gran Bretaña 1111, Playa Ancha, Casilla 5030, Chile; [email protected] 2 Centro de Astrofı́sica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal 3 Gemini Observatory, Northern Operations Center, 670 North A’ohoku Place Hilo, HI 96720, USA 4 Pontificia Universidad Católica de Chile, Facultad de Fı́sica, Departamento de Astronomı́a y Astrofı́sica, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile 5 Vatican Observatory, I-V00120 Vatican City State, Italy 6 Departamento de Ciencia Fisicas, Universidad Andres Bello, Santiago, Chile Received 2013 April 23; accepted 2013 October 24; published 2013 December 10. ABSTRACT The discovery of a detached dust shell around the Wolf–Rayet (WR) star WR60-6 in the young stellar cluster VVV CL036 is reported. This shell is uncovered through the Spitzer–MIPS 24 μm image, where it appears brightest, and it is invisible at shorter wavelengths. Using new APEX observations and other data available from the literature, we have estimated some of the shell parameters: the inner and outer radii of 0.15 and 0.90 pc, respectively; the overall systemic velocity of the molecular 12 CO(3 → 2) emission of −45.7 ± 2.3 km s−1 ; an expansion velocity of the gas of 16.3 ± 1 km s−1 ; the dust temperature and opacity of 122 ± 12 K and 1.04, respectively; and an age of 2.8 × 104 yr. The WR star displays some cyclic variability. The mass computed for the WR60-6 nebula indicates that the material was probably ejected during its previous stages of evolution. In addition, we have identified a bright spot very close to the shell, which can be associated with the Midcourse Space Experiment source G312.13+00.20. Key words: circumstellar matter – infrared: stars – open clusters and associations: individual (VVV CL036) – stars: massive – stars: Wolf–Rayet Online-only material: color figures, supplemental data. mass and the composition of the gas (CNO abundances) are very uncertain. Here we report the discovery of a new WR nebula in a newly discovered cluster VVV CL036. In the next sections, we will briefly describe the nebula and its morphology, and discuss its possible origin.. 1. INTRODUCTION Massive stars are known to dump a tremendous amount of energy, momentum, and chemically enriched material into the surrounding interstellar medium (ISM) by means of their strong stellar winds. In particular, the evolved He-core-burning Wolf–Rayet (WR) stars have wind densities that are of an order of a magnitude higher than their progenitors, the massive O stars. In the last few years, the mass-loss rates of O stars have been revised strongly downward (Fullerton et al. 2006; Puls et al. 2008). This highlights the key role played by episodes of extreme mass loss in the intermediate evolutionary phase before they become WRs, such as luminous blue variable (LBV) and red supergiant (RSG) star phases. During these phases, the wind is relatively slow. In the rapid transition toward the WR phase, the mass loss goes down, but the wind speed goes up by an order of magnitude and reaches 2000–4000 km s−1 on average (Crowther 2007). As this fast wind encounters its slow predecessor, it sweeps the old material in a moving shell, called a WR nebula. A significant fraction of the Galactic WR stars may be surrounded by nebulae (Miller & Chu 1993; Marston 1997; Wachter et al. 2010), but not all of them are ejecta nebulae. Studies of WR nebulae play a central role in evaluating the total amount of mass lost and understanding the dust and gas chemistry as a function of progenitor mass. It is still unknown as to how and when such nebulae form. Also, very little is known about the mechanism involved in the giant eruptions observed in some LBVs, probably causing many of the WR nebulae. Finally, important quantities such as the nebular dust. 2. THE OBJECT AND ITS MORPHOLOGY The ESO Public Survey VISTA Variables in the Vı́a Láctea (VVV) provides deep multi-epoch infrared (IR) observations for the Galactic bulge and adjacent regions of the disk (Minniti et al. 2010; Saito et al. 2012). In this survey, nearly 150 new open clusters and cluster candidates (Borissova et al. 2011a, 2011b) have been discovered so far. By combining the VVV near-infrared photometry with the follow-up nearinfrared spectroscopy, six young clusters were found, each containing at least one newly discovered WR star (Chené et al. 2013a). Around one of these WR stars, namely Obj 9 of the cluster VVV CL036, we found a spectacular detached dust shell (Figure 1) strongly emitting at 24 μm. Using the nomenclature proposed in the VIIIth catalog of WR stars of Crowther et al.,7 we attributed to WR star VVV CL036-9 a WR number WR60aa. However, in the latest version of the catalog (2013 June), the authors recommended switching from alphabetical to numerical identification. Following this recommendation, hereafter we will call our object WR60-6. We gathered the images and measurements available for this target in various archives and combined them with the existing literature studies in order to analyze the properties and. ∗ Based on observations gathered as part of observing programs 179.B-2002, VIRCAM, VISTA at ESO, Paranal Observatory, NTT at ESO, and APEX C-090.F-9705B-2012.. 7. 1. http://pacrowther.staff.shef.ac.uk/WRcat/filter.php, ver. 2012.

(2) The Astronomical Journal, 147:18 (8pp), 2014 January. Borissova et al.. Figure 1. Three-color Ks , GLIMPSE 8 μm, and MIPS 24 μm images of the region around Wolf–Rayet star WR60-6. (A color version of this figure is available in the online journal.). shell is very similar to the average value of 0. 7 for 24 newly discovered shells in MIPS 24 μm (Wachter et al. 2010). It can be seen from Figure 1 that the shell is not homogeneous, instead it is composed of 10–12 clumps extending outward in the form of fingers. The fingers point toward the WR star, which can be taken as additional evidence that this is the main exciting star. At α2000.0 = 14h 09m 09.s 54, δ2000.0 = −61o 16 33. 69, we identify a compact and bright spot, with a radius Rspot ∼ 30 , which can be associated with the Midcourse Space Experiment (MSX) source G312.1315+00.1991 (hereafter G312.13+00.20). The G312.13+00.20 region is an AKARI (Murakami et al. 2007) and IRAS source and also shows weak emission at 4.5 μm. Such characteristics are usually associated with the ionized and shocked gas in the high-mass star formation regions, and in very young massive stars.. origin of the shell. New CO(3–2) molecular line observations at 345.79 GHz using the APEX telescope were also obtained. The object is not detected at wavelengths shorter than 3 μm (Two Micron All Sky Survey (2MASS) and VVV, Figure 2) or on the Southern Hα Sky Survey images (Russeil et al. 1998), and thus cannot be associated with any warm ionized interstellar gas around VVV CL036 or dense molecular clouds nearby. The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 8.0 μm (Churchwell et al. 2009; Benjamin et al. 2003), Wide-field Infrared Survey Explorer (WISE) 12 μm (Wright et al. 2010), and Herschel 70 μm (Poglitsch et al. 2010) images show some weak emission; however, the shell structure disappears at the longer Herschel wavelengths, indicating an absence of cooler dust at the sensitivity and resolution of Herschel PACS and SPIRE (Griffin et al. 2010) detectors (Figure 2). Therefore, the best image available to outline the morphology of the object is that of MIPS 24 μm. In Figure 1, we display the above features using a three-color composite. The three-color image is produced by coding the Ks -band image from VVV survey as blue, Spitzer–Infrared Array Camera (IRAC) 8 μm image as green, and Spitzer–MIPS 24 μm image as red. The dust shell is almost circular and is approximately centered on the WR star, offset by ΔR.A. = 3. 5 and Δdecl. = 4. 1. To estimate the inner and outer radii of the shell, we fit circles on the two borders, which are defined where the flux density reaches a value equal to two times the background level. The fit was done “by eye” with consideration of the flux density levels. The inner radius is Ri = 10 , inside which the region around the WR star seems empty of gas and dust, and the outer radius is Ro = 1 . Note that this mean radius of the WR60-6. 3. NEW CO J = 3–2 OBSERVATIONS Submillimeter observations were performed using the 12 m APEX telescope located on Llano de Chajnantor, Chile, during 2012 October 21–22. We used the APEX-2 heterodyne receiver, centered at the 12 CO(3 → 2) line (345.79 GHz). The telescope beam size at this frequency is 18. 2 (FWHM). The back end was the Radiometer Physics GmbH (RPG) company extended bandwidth fast Fourier transform spectrometer, which provided a channel separation of 76.3 kHz and a total bandwidth of 2.5 GHz with 32,768 spectral channels. We mapped the emission in on-the-fly mode, covering an area of 5 × 4 centered on the WR60-6 star (α2000.0 = 14h 09m 04.s 3δ2000.0 = −61o 15 53. 0), which is spatially larger in extent than that of the shell detected by the MIPS 24 μm dust 2.

(3) The Astronomical Journal, 147:18 (8pp), 2014 January. Borissova et al.. Figure 2. Three-color images of the surrounding region of WR60-6 as follows from left to right: VVV J, H , Ks ; WISE 3.6 μm, 4.6 μm, and 12 μm; GLIMPSE 4.5 μm, GLIMPSE 5.8 μm, and GLIMPSE 8.0 μm; Herschel 70 μm; Herschel 70 μm, 160 μm, and 250 μm (30 × 30 ); and Herschel 250 μm, 350 μm, and 500 μm (30 × 30 ). The orientation and size of the images are shown in the pictures; the red cross marks the position of the WR60-6 star. (A color version of this figure is available in the online journal.). Figure 3. Observed integrated spectra of the 12 CO(3 → 2) line emission over the WR shell region taken with the APEX telescope.. emission map. The spacing between adjacent points on a regular grid was 20 . The observed intensities were converted to a main-beam brightness temperature scale by TMB = TA /ηMB , where ηMB = 0.74, following the information provided in the APEX8 Web site. The molecular line data were reduced accord-. ing to the standard procedure using CLASS90 and GREG from the GILDAS9 software package. Figure 3 shows the 12 CO(3 → 2) spectrum integrated over the whole region observed with the APEX telescope. Multiple 9. 8. The Grenoble Image and Line Data Analysis Software is developed by IRAM and it is available at http://www.iram.fr/IRAMFR/GILDAS/. http://www.apex-telescope.org/telescope/efficiency/index.php. 3.

(4) Borissova et al.. DEC. The Astronomical Journal, 147:18 (8pp), 2014 January. RA Figure 4. Contour maps of velocity-integrated ambient molecular emission of the region, overlaid on a MIPS 24 μm dust continuum emission map (gray scale). The range of integration in vlsr is from −41 to −51 km s−1 . (A color version of this figure is available in the online journal.). width of 12 CO(3 → 2) ranges from 14 to 27 km s−1 with a mean value of 21.5 ± 3 km s−1 . Figures 4 and 5 show the distribution of the peak intensity in Tmb (K), and the integrated intensity of the 12 CO(3 → 2) . Despite the low S/N and resolution, in general, the 12 CO(3 → 2) distribution follows the MIPS 24 μm structure. The peak of the integrated intensity is calculated as 53.6 K.. velocity components are clearly visible on the spectrum and closely resemble the Hα (Russeil et al. 1998) and 13 CO(3 → 2) (Urquhart et al. 2007) components. The identification of the peaks is as follows. The velocity interval between 0 and −10 km s−1 , with peaks at −2 and −8 km s−1 , is a foreground cloud, as it appears all over the entire mapped field. The velocity component at −31 km s−1 can also be connected with a nearby cloud due to its kinematic distance of 2.2 kpc. The velocity component between −40 and −60 km s−1 is related to the shell. Although the resolution and signal-to-noise ratio (S/N) of our data are lower than those of the Urquhart et al. (2007) 13 CO(3 → 2) spectrum, we can identify similar peaks at −41, −45, −53, and −59 km s−1 . From the Gaussian fit over this interval, the averaged 12 CO(3 → 2) spectrum peaks at −46.7 ± 4.5 km s−1 , very similar to that of the 12 CO(3 → 2) emission taken from the central WR star WR60-6 (−46.4 km s−1 ). The mean radial velocity of the G312.13+00.20 spot was measured as −44.8 ± 1.3 km s−1 , while the shell shows −44.9 ± 1.9 km s−1 . Thus, we adopt the average value of the above listed systemic velocity measurements, −45.7 ± 2.3 km s−1 , as the overall systemic velocity of the molecular emissions in this region. The errors are calculated by adding quadratically the individual errors of different measurements to the error of the mean value. The line. 4. DISTANCE TO THE NEBULA The kinematic distance to the nebula can be obtained from its proximity to known H ii regions as well as radial velocity measurements of the gas via the kinematic ambiguity solution. In our case, the WR nebula is very close to the source G312.112+00.314, which is a complex dominated by a bright H ii region, as observed by Caswell & Haynes (1987), who reported a H2 CO velocity of −49 km s−1 . Using this reference, the near and far distances are calculated as 3.8 ± 0.89 kpc and 9.6 ± 1.25 kpc, respectively. The reported Hα systematic and absolute velocities of −47 and −49 km s−1 of the G312+0.2 region (Russeil et al. 1998) correspond to 3.4 ± 0.9 kpc near kinematic distance. Our APEX observations give a systematic velocity of the observed region of −45.7 ± 2.3 km s−1 , which corresponds to 3.2 ± 0.7 kpc. The stellar distance 4.

(5) The Astronomical Journal, 147:18 (8pp), 2014 January. Borissova et al.. Figure 5. Distribution of the peak intensity (left panel) in Tmb (K) and the integrated intensity of the 12 CO(3 → 2). (A color version of this figure is available in the online journal.). calculated from spectral parallaxes of several stars in the cluster VVV CL036, as well as isochrone fitting (Chené et al. 2013a), is 2.0 ± 1.0 kpc. Thus, we will adopt an average distance of 3.1 ± 0.8 kpc to the WR nebula. With this assumption, the measured inner and outer angular radii of the shell translate to projected dimensions of 0.15 and 0.90 pc, respectively. The radius of the compact and bright spot associated with the MSX source G312.13+00.20 will then be 0.45 pc.. and also those obtained from the literature. The literature measurements were transformed to Jansky (Jy) by the Hubble Space Telescope NICMOS Units Conversion calculator,10 and also by using conversion equations given in Web sites of the surveys. No additional color corrections are made, because according to the Web sites of the GLIMPSE, WISE, Herschel, and AKARI surveys, the deviations are usually 2%–5%. This uncertainty is included in the error estimates. The total mass of the dust in the system (shell, WR star, and the G312.13+00.20 spot) is calculated to be Mdust = 0.15 M , following Equation (1) of Paron et al. (2012; see also Rosolowsky et al. 2010). The authors assume that the continuum emission originates only in the dust and depends on the distance to the object in kpc, the emission flux in Jy, and the dust temperature in K. This equation also assumes an excitation temperature of 20 K. In our case, we used the flux densities measured from MIPS 24 μm, the adopted distance of 3.1 kpc, and the dust temperature of 122 K (see the next paragraph). Assuming a gas-to-dust ratio of 100, the estimated dust mass implies a gas mass of 15 M . The spectral energy distributions (SEDs; see Figure 6) of the central star WR60-6 and the surrounding shell were fitted using the VOSA (virtual observatory) SED Analyzer (Bayo et al. 2008). This tool11 was developed by the Spanish Virtual Observatory in order to build the spectral energy distributions of different kinds of objects, to determine physical properties (such as effective temperature, surface gravity, and luminosity) of each object of interest by comparing its SED with those derived from a set of theoretical models. The best-fitting model parameters are provided as a result. In our case, for the shell, the best fit was achieved (96% probability) for a blackbody with a mean temperature of 122 ± 12 K. The dust opacity is calculated as 1.04 following Liu. 5. PARAMETERS OF THE WR60-6 SHELL AND ITS CENTRAL STAR The expansion velocity vexp of the shell is defined as (Δv)/2 +2 km s−1 , where Δv = v2 − v1 is the velocity interval through which the shell is visible (Benaglia & Cappa 1999). The expansion velocity measured from the maximum line width of the 12 CO(3 → 2) emission is 15.5 km s−1 . The velocity interval of the 13 CO(3 → 2) observations of the G312.13+00.20 (Caswell & Haynes 1987) gives vexp = 17 km s−1 . Thus, we adopted a mean value of 16.3 ± 1 km s−1 . Aperture photometry was performed on the available GLIMPSE, WISE, and Herschel images, providing a good measurement of the nebular flux, with typical uncertainties 15% in the extracted flux densities. These measurements are performed on the flux calibrated images (see the GLIMPSE, WISE, and Herschel Web sites for more details) and by using an aperture that included the total flux from the shell. Thus, we did not perform any aperture and color corrections. The MIPS 24 μm data was reprocessed using the MOPEX software by ingesting fbcd frames. Aperture photometry was performed using the standard MOPEX–APEX single frame pipeline on the final mosaic by using default settings and an aperture of 7 radius (no background annulus was used). No additional aperture or color corrections are made. The 24 μm flux densities are 0.745 mJy ± 7 μJy for the WR star, and the flux density in the shell is 145 ± 65 mJy. The flux densities are tabulated in Table 1 and include our measurements. 10 11. 5. http://www.stsci.edu/hst/nicmos/tools/conversion_form.html http://svo.cab.inta-csic.es/svo/theory/vosa/.

(6) The Astronomical Journal, 147:18 (8pp), 2014 January. Borissova et al.. Table 1 The Measured and Taken from the Literature Fluxes for G312.13+00.20 Spot, WR60-6 Star, and WR60-6 Dust Shell Filter ID VVV Z VVV Y 2MASS J 2MASS H 2MASS K WISE W1 IRAS I1 IRAS I2 WISE W2 IRAS I3 IRAC I4 MSX A AKARI IRC S9W WISE W3 WISE W4 MIPS M1 IRAS 60 AKARI FIS N60 MIPS M2 AKARI FIS WIDE-S AKARI FIS WIDE-L AKARI FIS N160. Wavelength (μm). FSPOT (Jy). Err FSP. FWR (Jy). Err FWR. FSHELL (Jy). Err FSH. 0.888 1.032 1.235 1.662 2.159 3.353 3.55 4.439 4.603 5.731 7.872 8.28 8.61 11.561 22.088 23.675 61.48 66.73 71.42 89.2 145 163.06. ... ... ... 0.005 0.010 0.093 ... ... 0.08 ... ... 0.26 0.39 0.41 0.76 0.83 48.8 47.9 48.11 27.89 41.62 54.59. ... ... ... 0.002 0.005 0.01 ... ... 0.01 ... ... 0.04 0.08 0.06 0.11 0.12 7.32 9.58 7.22 5.58 8.32 10.92. 0.84 1.5 2.6 2.1 1.7 0.84 0.74 0.49 0.59 0.35 0.24 ... ... 0.12 0.0427 0.0008 ... ... ... ... ... .... 0.09 0.17 0.36 0.25 0.21 0.10 0.08 0.05 0.07 0.05 0.03 ... ... 0.01 0.00 ... ... ... ... ... ... .... ... ... ... ... ... ... ... ... ... ... 0.022 ... ... 0.084 0.114 0.145 ... ... 0.077 ... ... .... ... ... ... ... ... ... ... ... ... ... 0.01 ... ... 0.01 0.02 0.07 ... ... 0.02 ... ... .... The WR60-6 star was classified as WN6 on the basis of low-resolution K-band spectra (Chené et al. 2013a). The SED of the star is constructed using the data available in the literature (2MASS, VVV, IRAC, and WISE). In this case, the best fit (99% probability) in the VOSA SED Analyzer was achieved with the TLUSTY OSTAR2002+BSTAR2006 grid. The merged files use the BSTAR2006 models for effective temperatures up to 30,000 K and the OSTAR2002 models for higher temperatures (Lanz & Hubeny 2003, 2007). This model estimates the temperature of the star as Teff = 30,000 K and the bolometric luminosity as Lbol = 4.23 105 L at 3.1 kpc. According to the evolutionary tracks of Ekström et al. (2012), the mass of the star is between 35 and 40 M . Such estimated parameters are in agreement with CMFGEN (Hillier & Miller 1998) detailed modeling of the near-infrared spectrum of the star (Chené et al. 2013b), which yields Teff = 33,890 K, log(L/L ) = 5.5, and a stellar mass of 32 M . We also examined the photometric variability of WR60-6. We have retrieved from the Cambridge Astronomical Unit Web site the 89 Ks -band magnitudes available as of now, as well as the magnitudes of the star 2 MASS J14090484-6115287. The latter object has approximately the same Ks magnitude as WR60-6 and can be used as a comparison star because it does not show any signatures of periodic variability (see Figure 7, upper panel). The Lafler–Kinman (Lafler & Kinman 1965) “theta” statistics method was applied to search for cycles or periods. It can be seen that the WR60-6 star displays some kind of cyclic variability. A tentative cycle of P = 0.43244 days was found, but more data are necessary to confirm its reality.. 102. 10. Flux (Jy). 1. 10-1. 10-2. 10-3. 10-4 0.1. 1.0. 10.0 λ (μm). 100.0. Figure 6. Spectral energy distribution of WR60-6 (blue), the WR60-6 shell (green), and G312.13+00.20 (red). (A color version of this figure is available in the online journal.). et al. (2012). The virial mass Mvir = 2465 M of the gas in the whole area, as defined by 12 CO(3 → 2) APEX observations, is calculated following Ungerechts et al. (2000). If we assume approximately 20 km s−1 space velocity for the shell and assume that the material we see of about 0.9 pc in width is spread uniformly from mass loss, then that process must have continued for about 2.8 × 104 yr. Our data are not deep enough to model the properties of the shell in more detail.. 6. DISCUSSION The morphological classification scheme of WR nebulae was proposed by Chu (1981, 1991), who basically divided them in two classes: wind-blown bubbles (W type) and ejecta-type nebulae (E type). The E-type nebulae were defined as those that were likely to contain processed ejecta via a giant eruption 6.

(7) The Astronomical Journal, 147:18 (8pp), 2014 January. Borissova et al.. 9.5. example, a one-sided arc may sometimes be seen. Finally, the mixed nebular phase ends the cycle, with no definite morphology or one-to-one correspondence between optical and IR images. This corresponds to the last stage, when the circumstellar nebula begins to dissolve into the ISM (the description of the phases is taken from Nazé et al. 2012). The WR60-6 nebulae can be associated with the second phase of the above proposed classification scheme. The morphology of the object is similar to those discovered by Wachter et al. (2010; 2 MASS J11441803–6245210 and 2 MASS J16321298–4750358) and Guerrero et al. (2012; WR 8). Unfortunately, with the present data it is not possible to carry out a thorough chemical analysis of the nebular abundances and some spectroscopic observations are planned to clarify this point.. 10.0. 7. SUMMARY. 7 8. KS. 9 10 11 12. Ks. 0. 200. 400 600 800 MJD-55260 (days). 1000. 1200. In this work, we reported the discovery of a new detached nebula around the Wolf–Rayet star WR60-6. Through the analysis of submillimeter observations of 12 CO(3 → 2) emission in the region, we have determined a velocity of the system of −45.7 ± 2.3 km s−1 . The IR SED of the shell is best fit by a blackbody function with a temperature of 122 ± 12 K. The shell is estimated to be 2.8 × 104 yr old. The central star, WR60-6, displays variability in the Ks band that is possibly cyclic with a tentative period of 0.4324 days. The estimated mass of dust in the nebular shell and the similar stellar and nebular velocities lead to the conclusion that the material in the shell was most likely ejected in previous stages of the evolution of WR60-6.. 10.5 11.0 0.0. 0.5. 1.0 PHASE. 1.5. 2.0. Figure 7. Ks light curves of WR60-6 (red) and the comparison star (blue). The top panel shows the Julian Date of observations versus Ks magnitudes of the WR60-6 star and that of the comparison star. The errors of the photometry are overplotted. The bottom panel shows the light curve of WR60-6 folded on the tentative period of P = 0.432443 days. (A color version and FITS images of this figure are available in the online journal.). from the progenitor star. The W-type nebulae have been created because of the high-mass-loss rates, which naturally formed circumstellar gaseous nebulae. In our case, the relatively little amount of gas (15 M ; see the previous section) and the similar stellar and nebular velocities show that we most likely have an ejecta-type nebula, where the material was ejected in some of the previous phases of the evolution of WR60-6. Then, this dense circumstellar medium is photoionized by the hot WR star and swept up by its fast WR wind. The question is about the progenitor of the ejected nebula. According to Meynet et al. (2011), for a single massive star with a mass between 30 and 40 M , the star can possibly go through the three phases of its evolution: RSG, LBV, and WR star. After spending a normal life as an O star on the main sequence, the star evolves toward cooler temperatures, becoming an LBV or RSG star. These stars undergo strong mass loss (up to 10−3 –10−4 M yr−1 ) through winds and episodic and non-spherical ejections of material (Humphreys 2010), and thus peel off parts of their stellar envelope to form a nebulae. WR60-6 has an initial mass of around 35 M and therefore most likely has experienced a red or a yellow supergiant phase instead of an LBV phase before becoming a WR star. Recently, Guerrero et al. (2012) proposed an additional morphological classification of the WR nebulae on the basis of 35 nebulae associated with WRs. They proposed three phases of morphology of the WR nebulae. In the first one, WR nebulae appear as complete shells or bubbles. This corresponds to the star just entering the WR stage, when its powerful wind sweeps up the previous slow and dense winds (from, e.g., LBV or RSG stages). The second phase is the clumpy phase. At this point, the nebulae display knots of gas and dust connected by partial shells and arcs. It corresponds to an age of a few 104 yr, when instabilities break down the swept-up shell. The stellar motion through the ISM has an impact on the morphology, for. We gratefully acknowledge use of data from the ESO Public Survey program ID 179.B-2002 taken with the VISTA telescope, data products from the Cambridge Astronomical Survey Unit, and funding from the FONDAP Center for Astrophysics 15010003, the BASAL CATA Center for Astrophysics and Associated Technologies PFB-06, and the MILENIO Milky Way Millennium Nucleus from the Ministry of Economy´s ICM grant P07-021-F. We thank the anonymous referee for help in improving this work. Support for J.B. is provided by Fondecyt Regular No. 1120601. M.S.N.K. is supported by a Ciência 2007 contract, funded by FCT (Portugal) and POPH/FSE (European Commission). Support for P.A. is provided by the ALMA-CONICYT project number 31110002. A.-N.C. is supported by GEMINICONICYT project number 32110005 and FONDECYT number 1120601. D.M. is supported by Fondecyt No. 1130196. This paper made use of information from the following facilities: the Red MSX Source survey database at www.ast.leeds.ac.uk/RMS, which was constructed with support from the Science and Technology Facilities Council of the UK; the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA; and the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This publication makes use of VOSA, developed under the Spanish Virtual Observatory project supported from the Spanish MICINN through grant AyA2008-02156. In the present paper, we are discussing observations performed with the ESA Herschel Space Observatory (Pilbratt et al. 2010), in particular employing Herschel’s large telescope and powerful science payload to do photometry using the PACS (Poglitsch et al. 2010) and SPIRE (Griffin et al. 2010) instruments. 7.

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