during the last stages of their evolution, low mass stars lose much of their mass in the form of a low velocity, molecular wind. at the end of this stage, the central temperature of the star increases illuminating and ionizing this expanding circumstellar shell. also, a high velocity but low density wind from the central star gives rise to the formation of a planetary nebula. the study of pn is important because, the mass ejected by low mass stars dominate the return of material to the ism. moreover, these stellar sources are the principal source of interstellar dust grains. recent studies of pne have revealed that the neutral atomic and molecular gas can be dominant component of pne. the helix (ngc 7293) is the closest planetary nebula with a massive envelope of molecular gas (dvirgul130 pc). it is far larger in angular size than any other of this class, and offers the best opportunity to explore the large and small scale structure of a planetary nebula in great detail. we propose a comprehensive study of the properties of the neutral and ionized gas in the helix nebula taking full advantage of the unique capabilities of the iso instruments to probe low surface brightness emission regions. the proposed observations will provide detailed information on the ionization structure of the nebula, the most important cooling lines of the photon dominated regions (pdrs) at the ionized-neutral interface, the nature of the dust particles, and the distribution of the dust emission from mid to far-infrared wavelengths. these observations are fundamental for understanding the physical conditions in the envelopes of planetary nebulae and will provide, at unmatched linear resolution, detailed insights into basic processes which are of general importance in the interstellar medium.
Instrument
CAM01 , PHT22
Temporal Coverage
1996-05-02T07:53:01Z/1997-12-09T04:24:05Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the world's first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
