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Astron. Astrophys. 343, 536-544 (1999)

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3. Results

3.1. S 61: the coronographic images

We present the final coronographic H[FORMULA] image of S 61 nebula in Fig. 1, where North is up and East to the left. This image has been obtained by aligning and adding up all the individual frames, for a total exposure time of 900 s. In this image, we haven't placed the star under the occulting mask, which would otherwise cover a significant portion of the nebula. The apodizing mask within the coronograph suppresses diffraction from the telescope spiders and allows us to achieve good contrast in the nebular region even without using the occulting bar. We find that the S 61 nebula is characterized by a spherical shape with uneven surface brigthness and a mild axisymmetry at the center.

[FIGURE] Fig. 1. Coronographic image of the nebula surrounding S 61, taken in the H[FORMULA] filter. North is up, East at left. In this image, the star is not occulted by the coronographic mask. A scale bar of [FORMULA] is included for reference.

We measure a shell diameter of [FORMULA] which translates to 1.8 pc, assuming a distance to the LMC of 51.2 kpc (Panagia et al. 1992). The H[FORMULA] emission is brightest in a region which peaks about [FORMULA] to the north of the star (0.3 pc), and displays a horn shape very similar to the inner nebula resolved around the galactic LBV candidate HD168625 (Hutsemekers et al. 1994; Nota et al. 1996a).

The S 61 nebula is also very reminiscent of the nebula recently discovered around the Ofpe/WN9 star S119 (Nota et al. 1994; Smith et al. 1998). In terms of linear size it is slightly smaller, the S119 nebula is 1.9 [FORMULA] 2.1 pc, and similar to the nebula around the galactic LBV AG Carinae, which is 1.1 [FORMULA] 1.0 pc (Nota et al. 1995).

The H[FORMULA] flux was measured on the image after subtraction of the central source. The reddened H[FORMULA] flux measured from the image is 1.3[FORMULA]10-12 erg s- 1 cm-2. In order to deredden the nebular measured flux, we have used the value E(B-V) = 0.21, obtained from the spectra (cf. Sect. 3.2). Dereddening the measured H[FORMULA] flux, we obtain an integrated flux of 2.4 [FORMULA]10- 12 erg s-1 cm-2.

The mass of ionized gas in S 61 may be calculated from the integrated, extinction free emission line luminosity and electron density. Using the relation:

[EQUATION]

we derive an ionized gas mass of 4.0 [FORMULA], where ne=400 cm-3, Te=6120 K (cf. Sect. 3.2) and d=51.2 kpc.

3.2. S 61: the spectra

In January 1996, we took spectra in the 3380-5280 Å and 6280-6870 Å bandpasses at three different pointings on the nebula: on the star, and at [FORMULA] North and South respectively. In July 1998 we acquired spectra in the ranges 4520-5820 Å and 6280-6870 Å for the position at [FORMULA] North from the star. The long-slit was aligned East-West, as shown in Fig. 2. We extracted both blue and red spectra for the four observed positions. Extended nebular lines are detected: in the blue region, [OII] [FORMULA]3726, H[FORMULA], H[FORMULA], H[FORMULA] and H[FORMULA] ([NII] [FORMULA]5755 only at [FORMULA] North), while in the red range [NII] [FORMULA]6548, 6584, H[FORMULA] and [SII] [FORMULA]6717, 6731. The spectra, extracted by coadding over the spatial extent of the nebular lines, are presented in Fig. 3. Two wavelength regions are displayed: the first is centered on H[FORMULA] (left panel), and the second includes the HeI [FORMULA]6678 stellar line and the [SII] [FORMULA]6717, 6731 nebular emissions (right panel). The presence of stellar H[FORMULA] and HeI [FORMULA]6678 in the spectra at [FORMULA] North, [FORMULA] North and [FORMULA] South is probably due to contamination from the central star. A strong HeI [FORMULA]6678 line is present in the pointing at [FORMULA] South, while the line is marginally detected in the pointing at [FORMULA] North.

[FIGURE] Fig. 2. Coronographic image of the nebula surrounding S 61, on which we have superimposed the four slit positions used in our observations. A scale bar of [FORMULA] is included for reference.

[FIGURE] Fig. 3. Spectra of the S 61 nebula for the pointings at [FORMULA] North, [FORMULA] North, on the the central star, and at [FORMULA] South, in the wavelength range from 6530 Å to 6610 Å (left panel) and 6666 Å to 6750 Å (right panel).

We used the [NII] [FORMULA]6584 line to derive the radial velocity map of the nebula. In order to further improve the S/N in the line, we binned the spectrum by a factor of 2 along the spatial direction (corresponding to [FORMULA] since 1 pixel = [FORMULA]), and extracted an individual spectrum from each bin. We measured the peak wavelengths of the [NII] [FORMULA]6584 line in each extracted spectrum using a multi-gaussian line fitting routine, and derived the radial expansion velocity of the nebula as a function of position with respect to the central star for the pointings on the star and at [FORMULA] North and South, where the S/N ratio is higher. The spectral range between 6280 Å and 6870 Å is characterized by an instrumental line profile of 1 Å FWHM, corresponding to a velocity resolution of 46 km s-1. The observed velocity distributions, referred to the heliocentric system, are plotted in Fig. 4 for the selected slit positions (filled dots). In Fig. 4 we also report the systemic velocity of the star, measured to be 287 km s-1 from the HeI [FORMULA]6678 line. The velocity split in the nebula is well detected in the spectra taken with the slit positioned onto the star and at [FORMULA] South, while only a blueward motion is revealed at [FORMULA] North, located at [FORMULA] [FORMULA] to the East of the star. A comparison with the coronographic image suggests that this motion is possibly representative of the northern, bright arc. We conclude therefore that the nebula is most likely a hollow expanding shell, and in order to derive its kinematical properties, we have fitted the velocity distributions observed on the star and at [FORMULA] South using a simple velocity law derived by Solf & Carsenty (1982) for the case of a spherically expanding shell:

[EQUATION]

where [FORMULA] is the heliocentric radial velocity at the position [FORMULA] of our sample and [FORMULA] are the spatial coordinates of the central star. The model is presented in Fig. 4 (open dots) and indicates an expansion velocity of 28 km s-1 with a nebular radius of [FORMULA]. The center of the expansion coincides with the velocity of the star at [FORMULA] = 287 km s-1 so that the nebula appears kinematically associated with the central star. From these kinematical properties we infer an age of approximately 30000 yrs, comparable to the dynamical age of the nebulae around R127 (Smith et al. 1998) and S119 (Nota et al. 1995).

[FIGURE] Fig. 4. Radial velocity maps as a function of distance from the star obtained for S 61. From the top to the bottom, the maps correspond to the different pointing positions, namely [FORMULA] North, on the star and [FORMULA] South. Radial velocities are in km s-1 while distances are in arcsecs. The filled dots represent the velocity data, and the cross indicates the radial velocity of the star. The open dots are the modelled velocity law for an expanding shell to the velocity distributions observed on the star and at [FORMULA] South (Solf & Carsenty 1982). The fits are centered on the star systemic velocity and have been obtained assuming a nebular radius of [FORMULA] and an expansion velocity of 28 km s-1.

In addition to the kinematical properties, the nebular emission lines provide information on the physical and chemical characteristics of the nebula. Unfortunately, the spectra obtained in 1996 with EMMI in the blue region could not be reliably flux calibrated: therefore, we list in Table 2 the observed line fluxes of only the red spectra taken in 1996 and the line fluxes of both blue and red spectra, obtained in 1998. We have deblended the stellar and nebular contributions to the H[FORMULA] and H[FORMULA] lines observed for each slit position by using a multigaussian fit. The resulting individual stellar and nebular fluxes are listed in Table 3. The accuracy of the fitting procedure is estimated to be [FORMULA] 5[FORMULA], and well within the uncertainty of the flux calibration. Following Osterbrock's notation for Case B recombination, and assuming an electron temperature Te = 5000 K, which well approximates the nebular physical conditions, we have used the H[FORMULA]/H[FORMULA] ratio to calculate the extinction coefficient c[FORMULA] at the [FORMULA] North position. We have obtained c[FORMULA] = 0.33. In order to derive from this value the reddening E(B-V), we have fitted the c[FORMULA] and corresponding E(B-V) values reported in Table 3 of Kaler & Lutz (1985) with a polynomial of second order. The fit gives an E(B-V) value of 0.21 which fairly agrees with the E(B-V) = 0.15 obtained by Pasquali et al. (1997) from the comparison between model atmospheres and the observed spectral energy distribution of S 61.


[TABLE]

Table 2. Observed nebular fluxes for S 61 (in units of 10-13 erg s-1 cm- 2)



[TABLE]

Table 3. Deblended stellar and nebular fluxes in H[FORMULA] and H[FORMULA] lines (in units of 10- 13 erg s-1 cm-2) for S 61


We have measured the N+/S+ ratio in order to qualitatively evaluate the nitrogen abundance in S 61. The values are in the last column of Table 4 and range between 34 and 40, with a mean of 36 [FORMULA] 3. In this respect, S 61 nebula closely resembles R127 (also in Table 5) and P Cygni at N+/S+ = 33 [FORMULA] 5 (Johnson et al. 1992) and, therefore, it is nitrogen enriched at the same extent of other LBV nebulae. A more detailed chemical analysis relies on the electron temperature and density of the nebula. First, we have scaled the line fluxes measured at [FORMULA] North to H[FORMULA] = 100, and dereddened them using c[FORMULA] = 0.33 (cf. Table 4). Second, we have used the [SII]6717/6731 ratio to determine ne and the ([NII]6548+6584)/[NII]5755 ratio to evaluate Te. With the IRAF package NEBULAR (TEMDEN routine) we have derived Te = 6120 K and ne = 400 cm-3. Finally, we have calculated the ionic abundance from each N and S line using the IONIC routine in the same package. The ionic abundances obtained for each element have been scaled by the corresponding line fluxes and added up to derive the total element abundance. Table 5 summarizes the final N and S abundances obtained for the position at [FORMULA] North in units of Log(X/H) + 12 and compares them to the abundances derived for two established LBVs (AG Car and R127), one Ofpe/WN9 (S119), and to the mean abundances found for HII regions. In the case of S119, we have listed two sets of abundances, derived assuming Te = 5600 K (first row) and Te = 6800 K (second row). These two values are reported from the ionic abundances calculated by Smith et al. (1998) due to the non-detection of the [NII] [FORMULA]5755 line. Within the uncertainty of about 0.2 dex in the chemical composition, we may conclude that S 61 nebula not only is very similar to S119 but also matches the typical properties of LBV nebulae, by having a N overabundance of [FORMULA] 1 dex and a S underabundance of [FORMULA] 0.5 dex with respect to a typical HII region.


[TABLE]

Table 4. Dereddened line fluxes for the [FORMULA] North position, in units of H[FORMULA] = 100 erg s-1 cm- 2



[TABLE]

Table 5. S 61 nebular abundances, compared to other LBVs, in units of Log(X/H) + 12.
Notes:
(1) Smith et al. 1997 (2) Smith et al. 1998 (3) Shaver et al. 1983


3.3. BE 381: the coronographic images

In Fig. 5 we show the final H[FORMULA] image of the region around BE 381 with North up and East to the left. For this observation, the central star has been placed under the coronographic wedge, to achieve maximum enhancement of the circumstellar surroundings. It is evident from the figure that BE 381 lies in a diffuse HII region which appears denser at the NE and to the SE. Two diffuse arches in the image suggest the possibility of a shell structure around BE 381. The exact nature of these two structures is not clear, however, since the eastern component may be associated with the extended diffuse nebulosity seen towards the NE. The arches define a shell of diameter [FORMULA] which translates into [FORMULA] 3.2 pc at the LMC distance of 51.2 kpc (Panagia et al. 1992), with the structure to the east of BE 381 being much brighter than the western arch. The star appears to be located slightly offcenter with respect to the two arches, at a distance of [FORMULA] (1.3 pc) from the eastern arch, and of [FORMULA] (1.8 pc) from the western arch.

[FIGURE] Fig. 5. Coronographic image of BE 381 circumstellar region in the light of H[FORMULA] (North is up and East left) with the star occulted by the coronograph bar. A scale bar of [FORMULA] is included for reference.

3.4. BE 381: the spectra

Fig. 6 shows the observed slit positions projected on the image of BE 381. Because of the poor S/N ratio achieved in the blue spectral region, we have analysed only the spectra taken in the region 6280-6870 Å. In these longslit spectra, we detect the nebular lines of [NII] [FORMULA]6548, 6584, H[FORMULA] and [SII] [FORMULA]6717, 6731. In the spectrum centered on the star, we detect strong stellar HeI [FORMULA]6678 emission (Fig. 7). The integrated line fluxes (not corrected for reddening) are listed in Table 6. The nebular lines of [NII] and [SII] are spatially extended. The emission is stronger in correspondence with the brighter eastern arch, and it extends up to [FORMULA] [FORMULA] (3.5 pc) from the star. On the western side of the star the emission is rather faint and seems to extend over [FORMULA] [FORMULA] (2.1 pc).

[FIGURE] Fig. 6. Observed slit positions projected on the coronographic image of BE 381 (North is up, East left). From the top: [FORMULA] North, on the star and [FORMULA] South. A scale bar of [FORMULA] is included for reference.

[FIGURE] Fig. 7. Spectra of the BE 381 nebula for the pointings at [FORMULA] North, on the the central star, and at [FORMULA] South, in the wavelength range between 6530 Å and 6760 Å.


[TABLE]

Table 6. Observed nebular fluxes for BE 381 (in erg s-1 cm-2)


We have used both the H[FORMULA] and [NII] [FORMULA]6584 lines to derive the kinematical information. In order to improve the S/N in our data, we have binned the spectrum by 4 pixels ([FORMULA]), along the spatial direction and extracted the binned spectra. We have measured both H[FORMULA] and [NII] [FORMULA]6584 peak wavelength, and calculated their corresponding radial velocities, which have then been referred to the heliocentric system of reference and averaged. The values derived from the two different lines were averaged; the mean standard deviation associated with the averaged velocities is small ([FORMULA] 3 km s-1 with a peak standard deviation of 6 km s-1), and well within the spectral resolution of 46 km s-1 of our observing configuration.

The averaged radial velocities are plotted as a function of position with respect to the star in Fig. 8 for the pointings at [FORMULA] North, on the star, and [FORMULA] South (filled dots). In the figure, the cross represents the star radial velocity, which has been determined to be 267.3 km s-1 from the average of the H[FORMULA] and HeI [FORMULA]6678 lines. The data are characterized by a resolution of FWHM = 1 Å, corresponding to a velocity of 46 km s-1 at H[FORMULA]. No velocity structure is present in the pointings at [FORMULA] North or South; both distributions are flat, averaged around 258 [FORMULA] 3 km s-1 and 263 [FORMULA] 3 km s-1, respectively, possibly indicating the overall motion of the underlying interstellar region. In the map obtained with the slit on the star there appears to be a mild distortion to the otherwise flat distribution, between [FORMULA] to the east and [FORMULA] to the west, which may be interpreted as a blueward motion of the region surrounding BE 381 with a velocity [FORMULA] 20 km s-1 with respect to the star.

[FIGURE] Fig. 8. Radial velocity map of BE 381 as a function of distance from the star. The radial velocities are reduced to the heliocentric frame of reference. They are plotted, in units of km s-1, as a function of position (in arcseconds) with respect to the star. The cross indicates the radial velocity of the star.

Within the errors, this result is in broad agreement with Nota et al. (1996b) who suggested the presence of two different velocity components in the H[FORMULA] profile of BE 381, obtained with high resolution echelle spectroscopy. These components were believed to be the signature of a shell expanding at about 30 km s-1. Given the lower spectral resolution of our data compared to Nota et al. (1996b) it is plausible that we do not resolve the second component but just detect the overall motion.

In addition to the kinematics, from the nebular spectra we can also derive some basic chemical information: we have measured the [SII] 6717/6731 ratio for positions [FORMULA] North and [FORMULA] South in order to estimate the gas density. We have assumed an electron temperature Te = 10000 K, and we have derived (using the IRAF NEBULAR package) a mean density of [FORMULA] 30 cm-3 and 120 cm-3 at [FORMULA] North and South, respectively. It is interesting to notice that Nota et al. (1996b) estimated a gas density of 800 cm-3 from the [SII] lines in the echelle spectrum taken on the star. For their echelle measurements, they used a slit of size [FORMULA] [FORMULA] [FORMULA], which most likely missed both nebular arches. Their density measurements therefore refer to a circumstellar region very close to the star. We have searched for [SII] emission in the spectrum taken with the slit centered onto the star in order to resolve this apparent discrepancy, but the contamination from the stellar source is large and no [SII] lines are detected at that position.

We have also extracted from the longslit frames the brightest nebular region, corresponding to the eastern arch and repeated the analysis. We find that the eastern circumstellar region is not homogeneous: the gas density varies from 120 cm-3 at [FORMULA] N to 240 cm-3 at [FORMULA] South. Unfortunately, the low S/N ratio of the spectra does not allow us to determine the gas density of the fainter arch to the west of BE 381. Since the nebular H[FORMULA] flux is not available and the H[FORMULA] flux is strongly contaminated by the stellar wind, we are not able to derive exact chemical abundances for the nebula around BE 381. Nevertheless, we can use the N+/S+ ratio to establish the nebular content of nitrogen. We have obtained a mean N+/S+ ratio of 2.3 at [FORMULA] North and 1.5 at [FORMULA] South, respectively. For the eastern circumstellar region alone the ratio N+/S+ results to be 1.9 in the North position and 1.2 in the South. These values are comparable, within the errors, to the averaged N+/S+ ratio of 3 derived for classical HII regions by Shaver et al. (1983). Alternatively, ejected nebulae such as LBV nebulae are characterized by N+/S+ value ranging between 33 (i.e. P Cygni, Johnson et al. 1992) and 80 (HR Carinae, Nota et al. 1997). Therefore, we may rule out a stellar origin for the shell surrounding BE 381.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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