Star Formation studies: an Universal Approach
M. S. Nanda Kumar
Stars are the fundamental constituents of our galaxy contributing to much of its observed mass. The understanding of star formation is closely connected with our understanding of the formation of planets as well as that of the universe. Stars are found to exist in a wide variety of sizes and masses that are classified into OBAFGKMRNS types. An universal theory of star formation should successfully explain the formation of all these types of stars and also the variety of modes in which they are found. Though the formation of sun-like stars are fairly well understood, the current theoritical and observational advancements are just sufficient to address the problem of star formation in its universality. In the following lectures I will review the current status of star formation studies and discuss future issues. Through out the course I shall emphasise the observational aspects and techniques of the physics of star formation.
Session 1: Galactic Star Formation: An overview
13 November 2002
The discovery of dark patches on the sky by E. E. Bernard in 1916 led to the identification of dust clouds in the sky. The association of these dark clouds with enigmatic variable stars in the constellation of Taurus and with emission blobs in NGC1999 were providing a hazy picture of their interconnection. On the other hand, though many molecules such as CH, CN were discovered in the interstellar medium as early as 1937, the much expected discovery of molecular hydrogen had to wait until 1970. Many innovative methods and the combined view of optical, infrared and radio astronomers together brought about the discovery of molecules such as OH, CO, NH3 and H2 in the interstellar and dust clouds. These rather slow progress of early discoveries from 1900-1980 tied up several loose ends in our understanding of the star formation process and its relation to the galaxy structure. The resulting complete picture has rapidly increased the understanding of the star formation phenomenon in the last two decades making it one of the current topics of astrophysical interest. In this lecture I shall provide an historical summary of related events and show how the process of star formation is linked to various other processes in the galaxy and to the galactic structure itself.
Session 2: Star formation I: Molecular clouds to cores
27 November 2002
In this lecture we shall first derive the physical properties and behaviour of a large mass of gas M, of density rho (with a power law) in hydrostatic equilibrium under the influence of boyles law and newtons laws of gravity. We shall see that this results in the well known criteria of Bonnor-Ebert and Jeans for the critical mas of gravitational collapse. Next, we will show that adding angular momentum and magnetic field will naturally yield the ideas of magnetic braking and ambipolar diffusion. We will then look at the observational properties of molecular clouds and see how the laws we derived compare with observations. We will study the Larsons laws for molecular clouds, the heirarchial structure of GMC's and its implications and how the structure extends to clumps and cores. We shall then discuss the observational probes such as molecular lines, dust extinction methods and understand their positive and negative aspects. In particular we shall see this in the light of molecular differentiation and its application to the study of starless cores. We shall finally discuss the current problems in studying core structures and infall.
Session 3: Star formation II: Young stellar objects
11 December 2002
In this session we will review the Isothermal collapse of a prestellar core leading to formation of shock fronts and an hydrostatic core, resulting density and velocity distributions. The need to sustain collapse until the ignition of nuclear fusion and the presence of angular momentum that is continuously working against gravity in the presence of a magnetic field will lead to the idea of magnetocentrifugally driven flows from YSOs. Thus we will discuss Shu's model which will take us to outflows and HH objects. We shall relate the important results from outflow observations to the picture of protostellar collapse we have seen. Then we will see how to study protostars through its infrared and millimeter spectral energy distribution and indirectly through outflows and HH objects. We shall briefly review the status of disk studies and how it connects with the rest of the picture.
Session 4: Formation of massive stars, substellar objects and clusters
7 January 2003
I shall review the current state of massive star formation by examining most recent observational evidences. We shall see that massive stars are invariably associated with clusters or associations of low mass stars. In this background we will study the powerful technique of color-color diagrams to evaluate the physical properties of stars in a cluster. I shall also discuss stellar surface density maps and thier use in studying star clusters. A quick review of the current state of proto-brown dwarfs and observational results will be made. Finally I shall discuss the latest theoritical models on cluster formation and supporting observations. We shall close with a review of interesting problems for further investigation in star formation.