AS5: Stars

Module title: Stars (AS5)

Module convenors:

Boris Gaensicke (Warwick) and Andrea Miglio (Birmingham)

Andrea Miglio: Lessons 1 to 5

After a brief recall of the equations governing stellar structure and evolution, we will discuss how the study and interpretation of stellar oscillation modes plays a fundamental role in constraining models of stars and models of stellar populations.
A description of the equations describing non-radial oscillation modes in stars will be followed by a detailed discussion on how the characteristics of the internal structure of stars determine the spectrum of normal oscillation modes. The evolution in time of the oscillation spectrum of a model star will be followed during the key stages of stellar evolution.

The topics to be covered are (Lessons 1 to 3):
- brief recall of the equations describing stellar structure
- evolution of the internal structure of stars, key physical processes at play and current uncertainties in the determination of stellar parameters
- method of small perturbations and equations of non-radial adiabatic stellar oscillations
- propagation diagrams and nature of normal modes in stars: acoustic, gravity and mixed modes - asymptotic approximation of pressure and gravity modes

Lesson 4 and 5
- case study: evolution of the surface properties, internal structure, and seismic properties of a 1-solar-mass star, from the main sequence to the red-giant stage using the stellar evolution code MESA.
- Seismology of stellar populations and its application to the study of clusters and the Milky Way's disk using space-based data from CoRoT and Kepler.

Boris Gaensicke: Lessons 6 to 10

Following on from investigating the internal structure of stars, we will look into the global properties of stellar populations.

- The star formation rate and the initial mass function (IMF) describe how much mass of the intestellar medium is converted into stars per unit time, and the relative number of stars formed per mass interval. Early investigations suggested that the IMF is proportional to M^-2.35 over a relatively large range in stellar mass, but drops of both to the high-mass and low-mass end.

- The initial-to-final mass relation, and the white dwarf luminosity function. Stars with masses up to ~10Msun will evolve into white dwarfs, those above that threshold will undergo core-collapse. Given that the Chandrasekhar mass limit is 1.4Msun, it is obvious that stars must lose subtantial amounts of mass during their late evolution. The observed mass distribution of white dwarfs is narrowly peaked around a mean of ~0.6Msun, with a long tail to higher masses, and represents a complex convolution of the IMF, stellar life times, and mass loss. Given that white dwarfs are no longer undergoing nuclear burning, and that their cooling rate is relatively well understood, the white dwarf luminosity function can be used to estimate the ages of individual stellar clusters, the galactic disk and the halo.

- Binary evolution. A substantial fraction of all stars are born as binary stars, and if the binary separation is sufficiently small they will interact as the more massive star evolves off the main sequence, often ensuing a common envelope phase. A myriad of different types of objects descend from binary evolution, including e.g. all known stellar black holes, milli-second pulsars, type Ia supernovae, and (probably) short GRBs.

- The companion mass function and the initial orbital separation distribution describe the statistical properties of the zero-age main sequence binary population, and the values adopted for those parameters has a strong effect on the outcome of stellar population models, e.g. it is currently not possible to firmly identify the binary nature of SN Ia progenitors. Constraining these parameters from observations is difficult and prone to large possible selection effects.

Assessment: using very basic assumptions on the IMF, stellar evolution, white dwarf cooling, and the initial-to-final mass relation, the students will write a program that computes the present-day white dwarf mass distribution and the white dwarf luminosity function.

Academic year: 
10/02/2015 - 09:00