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Science Spotlight from this week's observations

Proposal ID = 12519
Principle Investigator =  Raghvendra Sahai -  Jet Propulsion Laboratory 
Title = " Newly Discovered LMC Preplanetary Nebulae as Probes of Stellar
 	  Evolution"
Time = Jun 9, 2012, 21:52:01 - 22:30:54
Targets = J045622.62-663656.8
Instrument =  ACS/WFC

Background:

Stars live most of their lives on what is called the "Main Sequence". During this stage they have nearly constant luminosity and color, sedately converting hydrogen to helium in their core via the process of nuclear fusion. Once the hydrogen is gone, the star readjusts its structure as it burns heavier elements such as helium, carbon, oxygen, ..., eventually running out of nuclear fuel. During these later stages in the evolution of a star, the color and luminosity changes wildly. One of these intermediate stages is called the Asymptotic Giant Branch (AGB). During this phase, the stars envelope is roughly spherical. Shortly thereafter, intermediate mass stars become Planetary Nebula (PN). PN's have a wide range of bizarre, generally bipolar appearances. The question is what causes this dramatic morphological transformation from a spherical to aspherical morphology. A leading theory is that the central "star" is actually a pair of stars, with the bipolar morphology being related to the binary nature of the central engine. This snapshot proposal aims to identify stars in the Large Magellanic Clouds (LMC) in the transition phase from AGB to PN, in order to test this theory, and to learn more about the late-stages of evolution in intermediate-mass stars.

Paraphrasing from the abstract:

Pre-Planetary nebulae (PPNs), dying stars in transition between the AGB and planetary nebula (PN) evolutionary phases, hold the key to our understanding of the late evolutionary stages of (1-8) solar mass stars. Imaging surveys of Galactic young PNs and PPNs with HST show that the fraction of round objects is insignificant, whereas bipolar and multipolar morphologies abound, implying a dramatic but poorly understood transformation of the mass-loss process during the PPN phase. But the lack of reliable distances for most of these objects means that many key properties of these objects such as luminosity, dynamical time-scales, and masses cannot be determined, making it impossible to address many fundamental questions related to their evolution. In fact, a small sub-class of Galactic PPNs appear to be apparently very long-lived (with dynamical ages of >6000 years), providing a challenge for single-star stellar evolution theory because the latter predicts a very short lifetime for this phase (about 1000 yr or less). We therefore propose a pilot SNAPshot imaging survey of a small sample of spectroscopically-confirmed PPNs in the LMC with ACS/WFC. The well-known distance to the LMC will enable us to determine their sizes (and thus nebular ages) and luminosities unambiguously, providing compelling tests of stellar evolution models.


You can find most of this information and more from the following webpage: http://www.stsci.edu/hst/ by entering "12519" in the Prop. ID box.