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 receivedfrom a celestial object.

SPECTROSCOPYStudies the nature of the celestial objects by analyzing the light theyproduce

SPECTRUMTELESCOPESm=− log F λ

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 wavelengthsit is necessary to go to the space(the interstellar medium problemstill remains!)

The best places are Chile, Hawaiiand the Canary Islands.

-Mountains -High quality of the sky-Surrounded by sea

The spectrograph is the instrument which disperses the light in the wholewavelength interval. The dispersed light impacts the CCD camera and weget the spectrum of the celestial object.

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So what can we say about a spectrum? Whatare the “lines” and where do they come from?

Reduction process

Calibration process

FORMATION OF THE LINES

The crucial parameter is the optical depth ζ( ע ) of the stellar atmosphere, whichis related to the absorption coefficient ( ע )א in the atmosphere as follows:

where z is the geometric depth and z0 is the stellar surface.

A stellar atmosphere will be optically thick when ζ ( ע ) is > 1and it will be optically thin when ζ ( ע ) < 1

An optically thick atmosphere emits practically like a blackbody Bעwhile the emission in an optically thin atmosphere is :

τ υ=∫ χ υ dz

Iυ≈Iυ τυ1+Bυ τυ1

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

Absorption line formation

Emission 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:

χ υ=πe2

mcn lgug lf ul φυ 1−e−hυkT

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, He+, C++, N++, O++, Si+++

B H (moderate), He, C+, N+, O+, Si+, Si++, Mg+

A H (very strong), O+, Si+, Mg+, Ca+, Ti+, Fe

F H (strong), Ca+, Cr, Cr+, Fe, Fe+, Sr+

G H (moderate), Ca+ (strong), Fe, Fe+, Cr, Cr+

K H (weak), Ca, Ca+, Fe, Cr, TiO bands

M H (very weak), Ca, Fe, Cr, TiO bands (stronger)

Each spectral type has furthermore a characteristic spectrum

SPECTRA OF CATACLYSMIC VARIABLES ANDWHITE 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 periodscan be as longer as thousands of days. WDMS “are” the progenitors of the CVs.

WDMS

CV