Download Spectroscopy in stellar 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.
m=− log F λ
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( ‫ ) ע‬of the stellar atmosphere, which
is related to the absorption coefficient ( ‫ א( ע‬in the atmosphere as follows:
τ υ= ∫ χ υ dz
where z is the geometric depth and z0 is the stellar surface.
A stellar atmosphere will be optically thick when ζ ( ‫ ) ע‬is > 1
and it will be optically thin when ζ 1 < ( ‫( ע‬
An optically thick atmosphere emits practically like a blackbody B‫ע‬
while the emission in an optically thin atmosphere is :
I υ≈ I υ τ υ1 +B υ τ υ1
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:
2
πe g u
χ υ=
n l f ul φυ 1− e− hυkT
mc g l
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 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