Spectroscopy in Stellar Astrophysics Alberto Rebassa Mansergas ASTROPHYSICS

Spectroscopy in Stellar Astrophysics Alberto Rebassa Mansergas

ASTROPHYSICS : studies the physics of stars, stellar systems and interstellar material. LIGHT!! PHOTOMETRY Measures the amount of electromagnetic energy received from a celestial object. SPECTROSCOPY Studies the nature of the celestial objects by analyzing the light they produce TELESCOPES SPECTRUM

The interstellar medium and the Earth’s atmosphere absorb part of the light generated in the stars. An additional problem is the turbulence. In the surface we only can observe the visible and some IR wavelengths. To observe in other wavelengths it is necessary to go to the space (the interstellar medium problem still remains!) The best places are Chile, Hawaii and the Canary Islands. -Mountains -High quality of the sky -Surrounded by sea

The spectrograph is the instrument which disperses the light in the whole wavelength interval. The dispersed light impacts the CCD camera and we get the spectrum of the celestial object. .

Reduction process Calibration process So what can we say about a spectrum? What are the “lines” and where do they come from?

FORMATION OF THE LINES The crucial parameter is the optical depth ζ( ) ע of the stellar atmosphere, which is related to the absorption coefficient ) א) ע in the atmosphere as follows: where z is the geometric depth and z 0 is the stellar surface. A stellar atmosphere will be optically thick when ζ ( ) ע is > 1 and it will be optically thin when ζ ( > ( ע 1 An optically thick atmosphere emits practically like a blackbody B ע while the emission in an optically thin atmosphere is :

The formation of the lines is given when the optically thin atmosphere becomes optically thick due to changes in the absorption coefficient א Emission line formation Absorption line formation

And why does the absorption coefficient increase? Due to the bound- bound processes in the atmosphere! The electrons are continuously changing from one atom level to another. The absorption coefficient in these processes is severely dependent on the probability of these transitions: The most important factors are nl and ful

Thus the lines in a spectrum are formed due to the transitions in the atoms of the atmosphere. In the stars the most common element is the H. Typical lines in the spectrum of stars are furthermore the H lines.

SPECTRA OF THE STARS There are 7 spectral types of stars: O H (weak), He+, C++, N++, O++, Si+++ B H (moderate), He, C+, N+, O+, Si++, Mg+ A H (very strong), O+, Si+, Mg+, Ca+, Ti+, Fe F H (strong), Ca+, Cr+, Fe+, Sr+ G H (moderate), Ca+ (strong), Fe+, Cr+ K H (weak), Ca+, Fe, Cr, Ti. O bands M H (very weak), Ca, Fe, Cr, Ti. O bands (stronger) Each spectral type has furthermore a characteristic spectrum

SPECTRA OF CATACLYSMIC VARIABLES AND WHITE DWARF- MAIN SEQUENCE BINARIES CVs are binary stars composed of a white dwarf and a low mass companion. There is mass transfer from the low mass companion to the WD and an accretion disc is normally formed. The orbital periods range between around 80 minutes and 14 hours. WDMS binaries are composed of a WD and a MS star. The orbital separation is wider and there is no mass transfer. The orbital periods can be as longer as thousands of days. WDMS “are” the progenitors of the CVs.

WDMS CV