Shock emission in the bipolar post-AGB star IRAS 16594-4656
Van de Steene G.C., van Hoof P.A.M.
published in: A&A 406, 773 (2003)
In this paper we study the near-infrared emission spectrum of
IRAS 16594-4656, a bipolar post-AGB star with spectral type B7 and
no observed ionization. Using optical and near-infrared photometry we
determined the total extinction towards this object to be
AV = 7.5 +/- 0.4 mag and derived a distance of 2.2 +/- 0.4 kpc,
assuming a luminosity of 10,000 solar luminosities. The near-infrared spectrum
shows strong H2 emission lines and some typical metastable shock
excited lines such as [Fe II] 1.257 & 1.644 μm. We
determined the rotational and vibrational excitation temperatures, as
well as the ortho-to-para ratio of the molecular hydrogen. Based on
these we argue that the H2 emission is mainly collisionally
excited. Line ratios indicate that the H2 emission originates in a
approximately 25 km/s C-type shock. On the other hand, the metastable
lines, and especially the [Fe II] emission lines, indicate the
presence of a approximately 75 km/s J-type shock. Hence we postulate
that the H2 emission originates where the stellar wind (with an
observed terminal velocity of approximately 126 km/s) is funneled
through an equatorial density enhancement, impinging almost
tangentially upon the circumstellar material. The [Fe II]
emission either occurs along the walls of the bipolar lobes where the
transverse shock velocity would be higher, or could originate much closer
to the central star in shocks in the post-AGB wind itself, or possibly
even an accretion disk. Further high resolution near-infrared spectra
are currently being obtained to confirm the proposed geometry and
kinematics.
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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium
email: p DOT vanhoof AT oma DOT be
