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Astron. Astrophys. 357, 951-956 (2000)

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4. Discussion

It follows from our analysis of the RW Aur spectra that the primary of this triple system is a source of outflowing material, where strong absorption lines of singly ionised metals originate. Physical parameters, geometry and the origin of this outflow (warm wind) are discussed below. The strong infrared excess in the RW Aur A spectrum (Ghez et al. 1997) suggests that an opaque accretion disk surrounds the star, so we interpret the observational data in the frame of the accretion disk/wind paradigm.

We have found above that the wind gas temperature is definitely below [FORMULA] K. It follows from calculations of Arnaud & Raymond (1992)that the abundance of Fe I should exceed 10% in the region of Fe II line formation, if the ionization of iron atoms is due to electron collisions. The Fe I 2788.1 line is much weaker than the Fe II 2783.7 line, which means that [FORMULA] is significantly less than [FORMULA]. If levels of the neutral iron have LTE populations, this can occur only when [FORMULA]. Stellar H I [FORMULA] quanta are responsible for the ionization excess of iron in the wind as well as the atoms of elements with an ionization potential below 10.2 eV. Hydrogen [FORMULA] quanta can ionize also sulfur atoms, explaining the absence of a deep absorption feature in the blue wing of the Si IV 1402.8 line that can be produced by the strong resonance S I 1401.5 line.

Brown et al. (1984) and Lamzin (1999) found that the [FORMULA] luminosity of T Tau and RU Lup is [FORMULA]. The equivalent width of the RW Aur [FORMULA] line is [FORMULA] Å, which means that the [FORMULA] luminosity is [FORMULA], so the luminosity of the star in [FORMULA] line probably exceeds [FORMULA] erg s-1. Strong fluorescent lines of Fe II and [FORMULA] indicate that the RW Aur A [FORMULA] luminosity is indeed very large. Only the [FORMULA] lines, observed in RW Aur UV spectra, are excited by quanta with wavelengths redshifted relative to the [FORMULA] line up to +490 km s-1 as in the case of R(1) and P(3) lines. It occurs because the blue wing of stellar [FORMULA] line is almost completely absorbed inside the wind. This absorption is probably more stronger than the one of the blue wings of the Mg II h and k lines (Fig. 1). At the same time the observed Fe II fluorescent lines can be excited by quanta from the blue wing of the [FORMULA] line if they originate in the wind. Indeed the wavelengths of pumping transitions producing the Fe II 1539.05 and 1534.84 lines are shifted, relative to the center of the [FORMULA] line, to -470 km s-1 and -980 km s-1 respectively.

The stellar radiation of the Ly-series lines should strongly populate the excited levels of the hydrogen in the wind, explaining the presence of strong absorption lines of the Balmer and Paschen series. The photoionization of hydrogen from excited levels by Ly-series lines is a valuable source of heating of the wind and (along with collisional ionization) production of free electrons in spite of the relatively low gas temperature. Radiation pressure produced by [FORMULA] quanta can also play an important role in the initial acceleration of the wind matter (Lamzin 1999).

We estimate the hydrogen particle column density [FORMULA] (cm-2) in the warm wind along the line of sight from the optical depth of the Fe II 2783.69 line [FORMULA] transition). The relative LTE population of the [FORMULA] level [FORMULA] is near [FORMULA] at [FORMULA] K (and [FORMULA] times less at [FORMULA] K). If [FORMULA] we find from Eq. (1), with [FORMULA] that the optical depth [FORMULA] in the center of the Fe II 2783.69 line is [FORMULA]. As above mentioned, the line looks strong enough, so [FORMULA] definitely exceeds 1, and therefore [FORMULA]. Our estimation is very conservative and the real value of [FORMULA] could be an order of magnitude larger. It exceeds significantly the IS hydrogen column density, which can be derived from [FORMULA] relation by Vrba & Rydgren (1985)with [FORMULA] (Ghez et al. 1997). We estimate from Cruddace et al. (1974)data that the stellar X-ray flux below [FORMULA] keV should be absorbed by the stellar wind, unfortunately there are no suitable X-ray data to check this conclusion.

As above mentioned, the level population of the Mg I metastable [FORMULA] term is large enough to produce subordinate absorption lines of the uv 6 multiplet. The electron density of the wind exceeds the critical density of the [FORMULA] transition with respect to the Mg I ] 4571.1 line. Adopting atomic data by Mauas et al. (1988)we found [FORMULA]. We derive from the [FORMULA] inequality that the extension of the wind region along the line of sight is less than [FORMULA].

The observed gas outflow originates in the immediate vicinity of the star, presumably near the boundary between the stellar magnetosphere and an accretion disk. Gas acceleration up to [FORMULA] km s-1 occurs at distance [FORMULA] cm, but the radial velocity of the jet matter is 4 times larger (Hirth et al. 1994), so the acceleration continues at larger distance. We have no reasons to exclude that there is also a gas outflow from more extended regions of the disk. We can only say that it should be much cooler than the observed one. It is not possible to deduce from our data if the Fe I and Fe II absorption lines form in the same or different regions of the wind.

Unfortunately, we can say almost nothing about the emission lines of "high temperature" ions originating presumably in an accretion shock, as well as the Mg II and H I [FORMULA] lines (Lamzin 1998). The O IV] 1401.2 line looks a bit stronger than the S IV] 1406.0 line, in agreement with the accretion shock model predictions (Lamzin & Gomez de Castro 1998), but a quantitative comparison is impossible due to the poor S/N ratio of the spectrum. Gomez de Castro & Lamzin (1999)have estimated the infall gas density [FORMULA] by IUE data in agreement with the wind gas density found by us.

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Online publication: June 5, 2000