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Astron. Astrophys. 363, 671-674 (2000)

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3. Kinematical analysis

The long-slit spectrum of RX Pup at P.A.=[FORMULA]o is presented in the upper panel of Fig. 1. Its most notable feature is the spatially extended emission at the wavelengths of the [NII ]658.3 nm and (fainter) [NII ]654.8 nm doublet lines. This emission extends up to [FORMULA]3".2 from the centre toward P.A.=[FORMULA]o, and to [FORMULA]2".4 (albeit fainter) in the opposite direction.

[FIGURE] Fig. 1. Above : the long-slit spectrum at P.A.=[FORMULA]o. The spatial direction runs vertical, while wavelengths increase along the horizontal axis. Below : the integrated spectral profile obtained by spatial binning in the inner 6".5, with two different intensity cuts to highlight both the low and high intensity regions.

In the lower box of Fig. 1, we plot the spatially integrated spectrum that includes all the emission from the core as well as from the extended [NII ] components. Several features can be recognised:

  • the very bright and double-peaked H[FORMULA] emission which is typical of symbiotic stars (cf. van Winckel et al. 1993), but which has developed in RX Pup only in the last decade (M99). The two peaks of H[FORMULA] have an heliocentric velocity [FORMULA][FORMULA]-20 km s-1 and [FORMULA] km s-1, while the central absorption has [FORMULA][FORMULA]0 km s-1. Asymmetrical, slowly decreasing wings span the full observed spectral region;

  • a broad emission bump (labelled `Bump' in Fig. 1) which is redwards of the H[FORMULA] peak by a few 100 km s-1. The heliocentric velocity of the point where the bump intensity abruptly turns down is [FORMULA] km s-1. This bump is present in the spectra presented by M99, but fainter.

  • broad [NII ] lines corresponding to the spatially extended emission, and which are resolved into at least two components.

  • a narrow peak (labelled `FeII?'), at an heliocentric wavelength of 658.64 nm and with a FWHM of 13 km s-1 (corrected for the instrumental broadening). This narrow line was not visible in recent spectra taken by other authors (Mikolajewska, private communication). We can exclude that the peak is [NII ] emission at [FORMULA]=+135 km s-1, since the [NII ]654.8 nm companion line is not observed (after scaling to the relative line intensities of the doublet and corrected for the instrumental responses at the two wavelengths). Neither is the peak an H[FORMULA] component at [FORMULA][FORMULA]+1100 km s-1, because its width is not consistent with any reasonable temperature for the ionized gas (for T=10000 K, a thermal broadening of 22 km s-1 FWHM is expected for the hydrogen lines). The only two possibilities left are: i) the peak is (recently developed) FeII emission as observed in RR Tel (Crawford et al. 1999list the presence of FeII emission at [FORMULA] nm and with a FWHM of 13 km s-1); ii) it is an instrumental artifact (possibly contamination from an adjacent order of the echelle grating). Since this narrow peak is not relevant for the discussion of the extended outflow of RX Pup, we do not discuss it further in this paper.

Unlike the spectrum at P.A.=[FORMULA]o, the spectra at the other position angles ([FORMULA]o, [FORMULA]o, and -30o) do not show any obvious sign of spatially extended emission, confirming the results of P90 that the [NII ] nebula of RX Pup is mainly elongated toward P.A.=[FORMULA]o.

The analysis of the spatial point-spread function (PSF) of the spectra as a function of wavelength, however, shows that in all the observed position angles the FWHM of the PSF is slightly but systematically increased for the [NII ] lines as compared with the H[FORMULA] peaks, the `Bump', and the `continuum' regions covered by the present spectra. We have therefore done the following exercise. In each spectrum, we extracted the empirical PSF in a region far from the [NII ] wavelengths. By scaling this PSF to the observed peak intensity at each wavelength, we have then grown along the dispersion axis a 2-D `core spectrum' of RX Pup. This conservatively assumes that the contribution of the spatially extended emission to the peak intensity at all wavelengths is negligible. The residuals of the subtraction of this core spectrum from the original would then highlight the possible presence of spatially extended emission. In the subtracted spectra, the emission from the continuum, the H[FORMULA] line, the `Bump' as well as from the `FeII?' features is fully, that is correctly, removed; we compute that the residuals in these spectral regions are just the photon noise of the original spectra. At the [NII ] wavelengths, however, a pattern appears which is clearly spatially extended (Fig. 2), with blueshifted emission to the east (P.A.=[FORMULA]o, [FORMULA]o, [FORMULA]o), and redshifted emission to the west (P.A.=-75o, -120o, -30o). The velocity difference between the peaks of the blue-red/east-west residuals, as measured by Gaussian fitting, is between 135 and 160 km s-1, depending on the position angle under consideration. The average redshift of the two velocity components is [FORMULA]=[FORMULA][FORMULA]5 km s-1. In the following, we adopt this value as the heliocentric systemic velocity of the RX Pup system. This is not far from the value of [FORMULA]=[FORMULA][FORMULA]10 km s-1 estimated for the hot component of the system (M99, also Mikolajewska, private communication). Corrected to the Local Standard of Rest, the adopted systemic velocity amounts to [FORMULA] km s-1, which corresponds to a kinematical distance of [FORMULA] kpc from the Sun, assuming that RX Pup moves around the Galactic centre according to the circular rotation curve of the Galaxy. In this paper, following M99 we will adopt a distance of 1.8 kpc to RX Pup.

[FIGURE] Fig. 2. The [NII ]658.3 nm spectra of RX Pup, after subtraction of the unresolved core spectrum (see text). [FORMULA] are the observed radial velocities corrected for the adopted systemic velocity. Successive levels of the contour plots increment of a factor [FORMULA].

The marginally resolved E-W kinematical pattern is roughly symmetric with respect to the direction of the jet-like nebula at P.A.=[FORMULA]o; this is confirmed by the fact that at this latter angle the residuals in the innermost regions of the spectrum do not show any clear blue-/red-shifted components on either side of the central object. The extent of this E-W nebula is about 1".5, including smearing due to seeing. Using the recipe of Bedding & Zijlstra (1994), its deconvolved radius is between 0:005 and 0:008, depending on the assumed intrinsic geometry of the nebular model (hollow shell, disc, or uniform sphere). Due to the fact that the emission is only marginally resolved, and considering the simplistic model assumptions about its actual geometry, the figures above should be treated with caution. Note also that the [NII ] emission is likely to be more prominent between P.A.=[FORMULA]o and [FORMULA]o (and on the opposite side), as suggested by the higher intensity of the residuals in the spectra at those position angles, as well as by their slightly larger velocity split.

Coming now to the jet-like feature at P.A.=[FORMULA]o which was imaged by P90 (Fig. 2, upper left box), its radial velocities were measured by multi-Gaussian fitting at successive positions along the slit. We found that the velocity of this collimated feature, with respect to the adopted systemic velocity, decreases from about -80 km s-1 at a distance [FORMULA]0".9 from the centre, to -50 km s-1 at [FORMULA]2".6, and to 0 km s-1 at [FORMULA]3".2. On the other side of the object, there is a fainter, narrow component at about the systemic velocity extending out to [FORMULA]-2".4, while a broad component with a FWHM of [FORMULA]300  km s-1 can be followed out to [FORMULA]2" on both sides of the nebula. It is not clear whether the latter feature is just the low-intensity tails of the innermost E-W velocity pattern described in the previous paragraph.

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Online publication: December 11, 2000