Statistics – Methodology
Scientific paper
Jul 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006hst..prop10837u&link_type=abstract
HST Proposal ID #10837. Cycle 15
Statistics
Methodology
Scientific paper
We propose to reobserve the spherical circumstellar shells of an evolved star, HD 179821. The nature of this object, whether it is an intermediate-mass post-asymptotic giant branch star or a massive super-giant, has been debated over years. The past investigations scrutinized the properties of the object and its circumstellar shells. This enigmatic object possesses characteristics of a post-AGB star and a yellow hyper-giant, and there is no evidence that unambiguously defines the nature of this object. The circumstellar shell of HD 179821 was imaged in the previous WFPC2 observations during cycle 6, and its multiple concentric arcs extending out to about 8 arcsec were revealed. The purpose of the proposed HST observations is to obtain the second-epoch images of this source and perform proper-motion measurements of the shells in order to directly determine the distance to this object, thereby establishing its incredibly elusive nature once and for all. It has been nearly 10 years since the last observations. Given the object's rather high expansion velocity of 34 km/s, the expected angular expansion is 72 milli-arcsec if the object is located at 1 kpc and 12 milli-arcsec if 6 kpc. These values are well above the smallest material motion we can measure, 2 to 3 milli-arcsec, in our methodology that has been proven in our recent proper-motion measurements on a proto-planetary nebula, the Egg Nebula, using the archived HST/NICMOS near-IR continuum data. We estimate that the new distance measurement is of about 10% error. Once we obtain the distance to HD 179821, we will construct a radiative transfer model of the object to constrain physical parameters of the star and shells with a greater certainty. Then, we can confront theories of the stellar evolution with the newly obtained revelation. In addition, we can examine if all shells are moving at the same speed, since there are multiple shells around HD 179821. It is typically assumed that the expansion of the circumstellar shells is constant over the lifetime of the nebula. Our results will immediately test the validity of this one of the most fundamental assumptions in the stellar outflows.
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