Ensemble study of Post-AGB stars with dusty disks.

Student thesis: Master's Thesis

Abstract

The fi nal evolution of low- and intermediate-mass stars is a rapid transition from the Asymptotic
Giant Branch (AGB) over the post-AGB phase towards the Planetary Nebula phase (PNe),
before the stellar remnant cools down as a White Dwarf (WD). Over this relative short phase,
the circumstellar geometry, mass-loss mode and e ective temperature of the central star changes
considerably, yet it's luminosity remains constant. The general picture is rather well established
for single star evolution. In the circumstellar environment of a cool giant, dust can form and is
expelled in a slow, dense stellar wind, resulting in a spherically symmetric AGB envelope. At the
end of the AGB, a phase of high mass loss, called the super wind, quickly depletes the envelope.
The star structure changes, the radius shrinks and as more of the stellar core is disclosed, the e ective
temperature rises, which will generate a line-driven wind. This fast wind is generated and will
soon collide with the circumstellar material ejected by the super-wind, causing shock-ionization.
As the e ective temperature of the central star reaches 30000K, the circumstellar gas will also
be photo-ionized: a Planetary Nebula is formed. A main goal for astrophysics of evolved stars is
understanding the driving mechanisms that explain the morphological and kinematical di erences
between the spherical AGB circumstellar envelopes and their evolved counterparts displaying a
panoply of shapes and structures, which are not understood from rst principles. The discovery
that even cool Post-AGB stars, even well before they may generate a fastwind, show a wide variety
in shapes and structure, presents an even greater challenge to explain.
We are thus far from a complete evolutionary picture of evolved stars. A possible link to binarity
even complicates this task. For a few decades, stellar evolutionary specialists have attempted to
explain these observations in a consistent model. Given the high binarity fraction among the
observed objects, there is growing evidence that the processes at work in these systems, are driven
by binarity, or may be at least in
uenced by a companion. In this thesis we will focus on a
homogeneous study of a set of well selected Post-AGB stars of proven or suspected binary nature.
As spectroscopic techniques are still the main observational tool to identify binary stars, the
commissioning of the HERMES spectrograph, mounted on the 1.2m telescope at La Palma, is a
true opportunity. In this thesis, the radial velocities of 31 Post-AGB star, of the original sample by
de Ruyter et al. (2006), will be analyzed. We aim to nd clear evidence of binarity and constrain,
or even determine the orbital parameters and their errors. These will help to place this class of
objects in an consistent evolutionary framework.
This thesis is organized as follows: the rst chapter deals with the evolution of single stars that
have left the quiescent main sequence phase. A focus on structural changes over the Post-AGB
phase is presented. The second chapter summarizes the wide range of physical processes relevant
in evolved objects. The third chapter deals with two case-studies, that outline the methodology.
The process of data-gathering and data-reduction is described and radial velocities are determined.
An in-detail analysis shows how to determine the orbital elements and derive stellar parameters thereof. In the fourth chapter, these techniques will be applied to a homogeneous sample of known
Post-AGB stars. This will enable us to make a population synthesis, which results in a clear picture
of the orbital characteristics of binary Post-AGB stars. This class of objects is then compared to
their possible progenitors, the M Giants, and their possible descendants, the Barium and Symbiotic
stars. Finally, we will summarize our main conclusions with respect to this eld of research and
outline future challenges.
Date of Award30 Jun 2010
Original languageEnglish
SupervisorHans Van Winckel (Promotor)

Keywords

  • Binaries
  • Stellar evolution
  • Post-AGB stars

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