necessary resubmission of phase II proposal due to failure of ACS instrument (revised) ----- Submission 1 (Thu May 18 19:32:02 GMT 2006) -----
Phase II Proposal 10807 (PhaseII) successfully submitted.
----- Submission 2 (Fri May 19 17:17:41 GMT 2006) -----
Phase II Proposal 10807 (PhaseII) successfully submitted.
----- Attempting Submission 3 (Mon Mar 19 23:18:35 GMT 2007) -----
Phase II Proposal 10807 (he2-90_phaseII_rev) successfully submitted.
----- Attempting Submission 4 (Thu Mar 22 16:36:14 GMT 2007) -----
Assigned ID: 444
----- Submission 1 (Fri Jan 27 21:46:22 GMT 2006) -----
Phase I Proposal 444 (he2-90) successfully submitted.
----- Submission 2 (Fri Jan 27 23:53:04 GMT 2006) -----
The knotty jet of He 2-90: An ideal laboratory for studying the formation and propagation of jets in dying starsPrevious WFPC2 observations have led to the serendipitous discovery of an extended, highly-collimated, ``pulsed" bipolar jet emanating from a compact planetary nebula, He 2-90. Subsequently, an average proper motion of the knots in the jet was measured, which together with radial velocities, enabled us to characterise the basic physical properties of the jet. The knotty jet in He 2-90 resembles other prominent examples of pulsed jets in young stellar objects or symbiotic stars, but is probably by far the best example yet of a non-relativistic, symmetric, jet in a ``clean" astrophysical environment.
The formation (acceleration and collimation) of jets is not fully understood, specially in the case of jets in dying stars. We now propose to re-image He 2-90 with WFPC2 and exploit the factor 3.5 longer time
baseline now available from the first-epoch observations in September 1999, in order to measure the proper motion of individual knots in the jet with unprecedented accuracy. These data will enable us to characterise the ejection history of the source, specially deviations from a constant period (latter is related to the binary period of the system), e.g., due to instabilities in the accretion mechanism. We will also be able to test if the ejection mechanism is symmetric: any deviation in the ejection history of the knots in the opposing jet beams, will indicate a magnetic field structure and/or the accretion disk which is not symmetric across the equatorial plane. We will also carry out deep imaging with the ACS/WFC camera in order to determine the shapes/sizes of a large number of knots. The shapes/sizes of the knots, and changes with distance from the source probe the strength of the magnetic field inside the jet. HRC imaging of the central source and jet on sub-arcsecond scales will be carried out to probe the magnetic field close to the jet source, and deviations from linearity in the jet-beam which may result from instabilities in the magnetic field. These data will allow us to significantly improve our existing 2-dimensional MHD model of the He2-90 jet, and/or provide impetus for new 3-dimensional models.WFPC2 imaging:
The image will be obtained with the Planetary Camera. The jet in He 2-90 was detected with a H-Alpha snapshot exposure (18 min in total) with the F656N filter (Sahai & Nyman 2000), a second epoch H-Alpha image with WFPC2 with the F656N filter to look for proper motion used a slightly higher exposure time of 20 min (Sahai et al. 2002). We would like to obtain now a third epoch H-Alpha images for a more accurate velocity mapping in this line. The image should be at least as deep, so we propose a total integration time of 35 min in this line (to roughly account for the slow degradation of the WFPC2 camera). The velocity mapping of the knots and determining the ejection history of the jet source is an important component of our study. We propose imaging with the H-Alpha filter to have comparable data from all three epochs.
We thus need 1 orbit for the WPFC2 imaging.
ACS / WFC imaging:
The WFC imaging will be used to search and resolve knots which are more distant to the jet source than those detected in the former WFPC2 imaging campaigns. The H-Alpha+[NII] filter F658N will be used. Ground based images have shown that the extent of the jet is about 4000, knots can be seen to a distance of 2500 (Fig.1); more distant knots can be expected. Sahai & Nyman (2000) found that the intensity of the knots decreases with r^-gamma and gamma = 2.2. The intensity of knot e' at about 7 '' was measured to 7.5 x10^-15 erg/s/cm^2, so the distant knot i' at 14'' is expected to have an intensity of about 1.6 x 10^-15 erg/s/cm^2. We propose to use a total integration time of 120 min, in order to determine the shape/structure of sufficiently distant knots. With a 120 min exposure, we can detect knot i' with a S/N ratio of about 20; observations with a S/N ratio of 10 can be done to an intensity value of 6 x10^-16 erg/s/cm^2, corresponding to a distance of about 19''. These values have been derived using the ACS exposure time calculator.
Assuming the periodical behavior of the jet ejection, the number of detected knots will increase by a factor of two. The intensity of the most distant detected knot is still high enough to resolve the shape of this knot, and therefore to reach the scientific goal of this campaign. Detailed investigations of the shapes of the knots will be used to determine the strength of the magnetic field inside the jet.
We thus need 3 orbits for the WFC imaging.
ACS / HRC imaging:
The HRC imaging will be used to resolve the innermost knots close to the central source and to measure the contrast in intensity between the knots and the inter-knot jet material. This contrast is expected to decrease with decreasing radius and can be used the estimate the strength of the magnetic field close to the jet source. We propose a total integration time of 40 min. The intensity of a knot at 100 is expected to be 5.5 x 10^-13 erg/s/cm^2, the S/N ratio is then 120, which is adequate for determining the knot to inter-knot intensity ratio accurately as well as characterise any deviations from non-linearity in the jet beam.
We thus need 1 orbit for the HRC imaging.
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