Hen : The Garden Sprinkler Nebula Angels Riera Universitat Politècnica de Catalunya

The central star is classified as Be star (Henize 1976, Lamers et al. 1998). Evolutionary stage: transition from a AGB star to a PN (Parthasarathy & Pottasch 1989, Riera et al. 1995). Jets: Optical imaging from the ground (Riera et al. 1995) and with the HST/WFC2 (Bobrowsky et al. 1995, Borkwoski, Blondin & Harrington 1997). Optical spectroscopy from the ground (Bobrowsky et al. 1995, Riera et al. 1995), and with the HST/STIS (Borkowski & Harrington 2001; Riera et al. 2003). Proper motions (Borkowski & Harrington 2001, Riera et al. 2002, 2003). H  m image show the presence of molecular H 2 in the jet (Harrington et al IAU Symp. 191, p. 509). X-ray emission (knot NW1) (Guerrero, Sahai, p.c.).

Central region is probably a dusty disk (or torus). Thermal emission from hot dust in the NIR (García-Lario et al. 1997, Rodrigues et al. 2003), optical polarization (Rodrigues et al. 2003), and HI P-Cygni profiles indicate circumstellar dust close to the central star and ongoing post-AGB mass loss (Riera et al. 1995, Sánchez Contreras & Sahai 2001). Circumstellar shell expansion: seen in CO (Knapp et al. 1995, Bujarrabal et al. 2001), and in OH maser emission (Bobrowsky et al. 1995, Zijlstra et al. 2001). Radio continuum indicates the presence of a compact ionized region surrounding the central B star (Knapp et al. 1995).

F555w blue F502N green F656N yellow-orange F673N orange-red F814W red

Wavelength (Å) Kinematics of the Outflow  Large radial velocities (up to 900 km s -1 ).  Decrease in radial velocity moving outwards in a step-like fashion. The radial velocity decreases by 300 km s -1 from the innermost regions to the intermediate knots.  Within knot NW1, we observe an abrupt change of 145 km s -1 in just 0.90 arcsec.  Knot SE2: the spectrum displays a highly marked asymmetric shape with two intensity maxima. The high- velocity maxima occur 0.15 arcsec farther away from the central star. This is very similar to the velocity structure usually observed in HH objects.

Proper motion measurements KnotV T (km/s)PA (º) SE1a51194 SE1b SE1c SE NW1a NW1b NW1c98315 NW NW Adopted distance 5.8 Kpc

Line excitation mechanism: shocks (100 →200 km s -1 ) STIS G430L λ c = 4300 Å G750L λc = 7751 Å

 Line excitation mechanism: V s = 100 → 200 km s -1  Emission line profiles: double-peaked profiles and extraordinarily line widths (exceeding 500 km s -1 ).  High proper motions  Step-like variations of the radial velocities along the jet ↕ HH emission is associated with shocks produced in outflows. Bow shock-like structures form as a result of a variability of the ejection velocity (Raga et al. 1990). From the observed radial velocities 1-D analytical model of Raga et al. (1990) predicts: Shock velocity V s ~ 200 km s -1 Tangential velocities ~ 475 km s -1 Timescales for the variability of the velocity τ ~ 100 years.

Time-dependent ejection velocity model (poster Riera, Velázquez & Raga): 3D numerical simulations computed with YGUAZÚ-A adaptative grid code which integrates the 3D gasdynamic equation + a system of transport/rate equation for 17 atomic/ionic species. Ejection velocity variability: V j = V 0 + V 1 sin (2 π (t – t 0 ) / τ) + a t Precession: half opening angle α and period τ p Parameters: V 0 = 400 km s -1 V 1 = 150 km s -1 τ = 120 yrs a= 1 km s -1 yr -1 α= 7.5º τ p = 1500 yrs

 Formation of highly collimated jets in PPNe: - toroidal magnetic fields? (García-Segura et al. 1999, García-Segura & López 2000) - accretion disk? (Soker & Livio 1994, Blackman et al. 2001).  Physical origin of the periodic variability: - instabilities in the gas-dust coupling in a radiation preassure-driven outflow? (Deguchi 1977) - influence of a binary companion? (Harpaz et al. 1997, Mastrodemos & Morris 1997). - magnetic cycle leading to coronal ejection-like events? (Soker 2000, Blackman et al. 2001) - MHD instabilities in a magnetized outflow (García-Segura 1997).  Origin of precession: - influence of a binary companion? - induced by irradiation of the disk by a central source? (Livio & Pringle 1997).