The information contained in

light :

A basic introduction to astronomical spectroscopy

(stars, galaxies,QSO)

National Astronomical Observatories of China, Beijing, 2011

G. Comte Observatoire de Marseille - Provence

& Laboratoire d'Astrophysique de Marseille March 2011

The Sun spectrum in the visible

Ca+ Mg0 Na0

CN CH FeH MgH

H H H (atm)

Most of innumerable thin lines are due to metals (Fe0, Ni0, Ti0, Co0 etc…)

1) Basic Physics

W « continuum »

()

« lines »

It is the distribution of this energy of this radiation along the frequency space (or wavelength space)

It is the histogram in energy of the photon distribution !

What is the SPECTRUM of a radiation source?

Sources with continuous spectra :

Continuum is produced either by thermal phenomena (i.e. depending on the source temperature, i.e. on the agitation of the particules) or by non-thermal phenomena..

1) Black-body radiation (stars)

« Any heated body radiates: to heat is to Increase the temperature, i.e. the agitation of atoms or molecules of the heated body

- the radiated energy distributionis continuous and described by Planck’s law

- It is asymetric in frequency On each side of a maximum of emissivity whose position only depends on the temperature. ( Wien’s law)

-The total energy amount emitted by a BB depends on T4 (Stephan’s law)

The surface layers (atmospheres) of stars approximatively radiate as black-bodies, enabling the definition of an «  effective  temperature» ( T of BB producing the same total energy per surface unit)

The presence of relatively cold atoms in the stellar atmosphere, intervening between the BB and the observer, modify this BB spectrum creating absorptions that are visibles as breaks, lines, bands …

W

nm

2) The bremstrahlung (or free-free continuum) (galaxy clusters, quasars)

In a completely ionized gas (plasma) electrons and ions coexist. (In astrophysical plasmas, ions are basically protons H+). The gas temperature (several thousand K at least) is such that these particles have quite high velocities, the electrons, because of their small mass, being much faster than the ions.

An electron traveling near an ion is attracted by the ion positive charge and its trajectory is deviated. The corresponding loss of kinetic energy is radiated as a photon. Statistically, in a large population of electrons, an emission of continuum, dependent on the electron temperature (mean velocity of the electrons, given by the Maxwell-Boltzmann law) is observed : this is called bremstrahlung (braking radiation) or free-free .

++

e-

3) A non–thermal source : the synchrotron radiation (quasars, SN remnants)An electron traveling at very high velocity (~ c) in a magnetic field follows a spiralling trajectory and emits a radiation called synchrotron radiation.

The radiation emitted by a large population of electrons is a continuum covering almost the whole electromagnetic spectrum (except a very very high energies)

The observed frequency distribution is much « flatter » than the blackbody one!

Matter – Radiation Interaction :Electromagnetic radiations meeting matter (atoms, molecules, ions, plasma) are reflected,, diffused, and absorbed. All these phenomena are quantum phenomena, described as photon – electron interaction .

- reflexion : sends back the wave energy in an unique direction, depending on incidence angle, without modification of the incident radiation spectrum. -diffusion : sends the wave energy in a beam of directions. The incident spectrum may be modified, some part of the incident energy being transferred to the diffusor, depending on the size of the diffusing particles. The diffusor is thus excited. -absorption : the wave energy is transferred to the absorbing matter whose atoms / molecules change of energy state. The absorbing matter will desexcite by radiation with a spectrum generally different from the incident one.

These phenomena will often coexist !!! incident photonenergy W1

diffused photon(energy W2 # W1)

atom

e-atom

e-

incident photonenergy W1

(excitation)

DIFFUSION ABSORPTION

Energy exchange between atoms and photons : absorption and emission

Ideal case: Atom with 2 energy levels

nucleus

Electron orbital A( minimum energy)

electron orbital B( maximum energy)

n.b.: intermediary orbitals are FORBIDDEN

permittedlevel B

permitted level A

energy

A photon interacting with the atom supposed in energy state A will be absorbed only if its energy is equal to the difference W between the energies of the permitted levels A and B. The frequency of such a photon is = W/h, its wavelength is : = c.h / W (« photon of absorption line »)

To return to energy state A, the atom excited in energy state B by the photon can only emit,, a photon of energy W (« photon of emission line »)

If the incident photon energy is larger than W’, the electron will be extracted from its orbital : the atom is IONIZED and takes a positive electric charge.. There is reorganization of surviving electrons orbitals into a new system with completely different energy levels.

Of course, real atoms have many many energy levels, and therefore many spectral lines exist !

Forbidden region

Forbidden region

W

(ionisation)

W’

Absorption and re-emission of photons :

Line spectra.

absorption

emission

molecular absorptions

Emission and absorption in diluted gaseous matter.

Ideal thermalsource (black body)

Spectrum in absorption

Continuous spectrum Spectrum in emission

Depending on the observing direction , the spectrum resulting from matter- radiation interaction is not the same! Atoms in « box » b absorb light quanta from source s whose frequencies correspond to finite differences of energy between their energy levels ( dark lines A, B, C …). Then, they re-emit these radiations upon desexcitation, but this re-emission is isotropic. (all directions, none favoured) : along the direction s b a, there is only a modest amount of emission which cannot compensate the energy loss due to absorption. Along direction b e, there is no background source and only emission is visible, although it is not brighter than along b a

s b

e

a

Doppler – Fizeau effect

frequency 0

wavelength 0

frequency

wavelength < 0

fréquence <

V

V

long. d'onde > 0

1) Source at rest w.r.t. observer

2) Source moving to observer (V is negative)

3) Source moving away from observer (V is positive)

In cases 2 and 3 the radial velocity is (for V<<c) V = c . ( – 0 ) / 0

Spectral lines are NOT infinitely narrow !!!

3 reasons for broadening :

1) Uncertainty principle in QM shows that the energy value of an energy level in an atom cannot be EXACTLY defined. Thus an energy level has a finite width in energy.

This translates into a broadening of the line (i.e. a small extent in the frequency range of

the photons corresponding to the transition which can be described by a Lorentz function.

2) Temperature : thermal broadening is present in any source whose temperature is larger than 0 K . It is due to the statistical distribution velocities of the atoms of the source with respect to the observer. The velocity distribution is described by the Maxwell-Boltzmann statistic and the corresponding broadening is a Gaussian function.

3) The collisional broadening depends on the density of the matter, and is due to interaction of electron electric fields with the environment.

Lines normally (at infinite resolution) must exhibit a profile : Voigt = Lorentz * Gauss

But the instrumentation, with finite resolution, smoothes most lines into observed gaussians.

T small, the gas is « cold »

densityDoppler broadening àf spectral lines ("thermal«  broadening)

T large, gas is « hot »

density

What about molecules ???…

Atoms associate themselves to form molecules by coupling peripheral electrons into « chemical links », which, on the quantum mechanics point of view, correspond to common orbitals surrounding the multi-atom system. This considerably enlarge the ability to interact with the environment (matter and radiation around). Molecules are submitted to collisional excitations and radiative excitations. They become OSCILLATORS with at least 2 degrees of freedom :

- rotation- vibration

And they may interact with incident photons : - electronic transitions + excitation of the two other modes

vibration

rotation

Molecules are quantic objects so the rotative oscillator (rotation) as well as the linear oscillator (vibration) have only discrete states corresponding to well-defined energy levels. Jumping to one state to another one is achieved by transitions that are either photon emission (when there is energy loss) or photon absorption (when there is energy)

Energy differences between levels in rotation : some milli - electrons-volts :

---> rotation lines generally are in the millimetric (sometimes centimetric) radio range in vibration : some centi - to deci - electrons-volts:

---> vibration lines are in the IR.

Vibration and rotation work as variations of inertial moment of the molecule : that is the way in which molecular spectra are theoretically predicted .

Electronic transitions:

The electron set of a molecule results from the « fusion » of the electronic sets of the molecule’s atoms. The set of energy levels of the molecule is specific to it, and has nothing to do with the energy levels of the individual atoms entering the molecule. Interactions photon – molecular electrons may take place with the usual quantum conditions:

excitation --> absorption of photon of energy = energy differencebetween two levels

desexcitation --> emission of a photon whose energy = energy differencebetween two levels (not necessarily the same)

Molecules are fragile:

They may be easily destroyed by energetic radiations (UV, X), by temperature (when collisions with atoms and ions are too energetic), by chemical reactions with other molecules or ions , by cosmic rays particles (relativistic electrons, protons, muons,…)

They are very abundant in planetary atmospheres, comets, and in cold interstellar gas.

The more robust (CN, CH, C2 , TiO, ZrO, MgH, FeH, CaH, BaO, CaO, etc…)

are found in moderate temperature stellar atmospheres.

Sun spectrum,as observed with an echelle grating - © NOAO - Kitt Peak Observatory

2) The Spectra of Normal Stars

Na I D2 et D1

Sun spectrum - © NOAO - Kitt Peak Observatory

Sun spectrum - © NOAO - Kitt Peak Observatory

Spectral types at end of XIX century (R.P. Secchi) (Bibl. Observatoire de Paris)

u.v. violet blue green yellow red near i.r.

Ca+ H H H Mg0 Na0 H

O5

B0

B5

A1

A5

F0

G0

K0

K5

M0

M5

Ca0 CH TiO TiO TiO TiO TiO

Spectral types for normal main sequence stars (dwarfs)

He0 He0 He+ He+ He0

T* / TSun

Observation of thousands of stellar spectra has been used to derive the basic principles of the classification system universally adopted, the MK (Morgan-Keenan)

- Lines of hydrogen : are present in hot stars, their maximum visibility is for surface temperatures around 10 000 K (Véga, Sirius…), their intensities decline in colder stars.

- Lines of helium : only in hot or very hot stars. (T >= 12000 - 40000 K) (Rigel)

- Metal lines: Fe, Ti, Cr, etc., in very large numbers in solar-type stars and colder stars. The presence of ions of different ionisation potential enable a thorough temperature classification. ( Fe+ versus Fe0, Ti+ versus Ti0 etc…

- Molecular bands and lines: the colder is the star, the more numerous molecular signatures will appear in its atmosphere. In the Sun, ( 5800 K) CN, CH, MgH and C2 are present, but TiO, VO, ZrO, CaH, CaO, appear only in very cold stars, while H2O is only visible in latest M stars and brown dwarfs T< 2700 K.

n.b.: Solar spots are much colder than the surrounding photospheric plasma and many molecules and oxydes are found in the spectrum of solar spots (like TiO) although they should be immediately destroyed elsewhere.

The « effective » temperature of a starThe effective temperature of a star of radius R* is defined as the temperature of the blackbody of same radius that emits the same total flux.

L * = 4 R*2 Teff

4

Sun spectrum in the visible

5790 K BB spectrum

Temperature sequence for Main-sequence stars (dwarfs) # 1 (Pickles stellar library)

Temperature sequence for Main-sequence stars (dwarfs) # 2 (Pickles stellar library)

O6 V

B0.5 V

B5 V

A0 V

He II

He I

H

H

H

H

Temperature sequence from O to G for main sequence stars (1)

40 000 K

25 000 K

17 000 K

9 500 K

A5 V

F0 V

F5 V

G0 V

Ca I

CH

H

H

H

H

H

H

H

H

Temperature sequence from O to G for main sequence stars (2)

7 500 K

6 100 K7 000 K

6 600 K

The coldest stars and brown dwarfs:

No need to observe at wavelengths shorter than 7000 A !

Sun spectrum in the visible light

Ca+ Mg0 Na0

CN CH FeH MgH

H H H (atm)

(Most very thin lines are due to metals (Fe0, Ni0, Ti0, Co0 etc…) )

Spectral lines contain a wealth of informations:1) position in wavelength for a given element (atom, ion, isotope … ): - Doppler effect --->> radial velocity of the source w.r.t. observer2) identification of chemical elements in the source, and their derivatives (ions, molecules…)3) energy subtracted to continuum in absorption lines:

- abundance of theélément or derivative in the absorbing layer

- thermodynamical conditions in absorbing layer (T, pressure)

4) Intensity ratios of lines from a same element or ion : - thermodynamical conditions, ionisation degree, depth of line formation in source, etc… (radiative transfer)5) detailed analysis of line profile : thermodynamical conditions , hydrodynamics of plasma6) special effects due to magnetic field (Zeeman effect, Hanle polarisation, etc…

Quantitative spectroscopy : the « equivalent width » of a line

Giant stars, dwarf stars : the luminosity classes

At a given surface temperature, stars may have different luminosity:

Sirius A : type A1 T = 9940 K L = 25 suns ====>> radius = 1.7 solar radius «  dwarf »

Deneb : type A2 T = 8400 K L = 250 000 suns ====>> radius = 250 solar radii « supergiant »

Sun

Sun

Sirius A

Deneb

(Radii may be easily computed using Stefan’s law)

Gravity at the star "surface" : g = G . M / R2

(n.b. : it is the acceleration to which a small mass at the surface is submitted)

(M : star mass , R : star radius, G : gravitation constant)

In solar units:

Sun : (M = 1 ; R = 1) g = 1. G

Sirius A : ( M = 2 solar masses; R = 1.7 solar radius) g = 0.69 G

Deneb : (M = 25 solar masses; R = 250 solar radii) g = 0.0004 G

With ~ T similar, gravity in Sirius photosphere is 1700 times larger than in Deneb photosphere !

Hence, the gas pressure (weight of the column of stellar atmosphere above the layer where an absorption line is formed) is 1700 times larger….. And, therefore, matter has a much larger density…

More density of matter ===>> perturbations of electric field from the peripheral electrons of atoms exerted by the neighbouring atoms

===> modifications of energy absorption levels

===> Stark broadening of lines (H and light ions)

Less density of matter ===>> interatomic collision rate lower

===> modification of survival rate of ions before recombination (ex., Sr+, Ba+ et Fe+ ) and reinforced abundance of these ions in giants / supergiants

Low gravity <======> lines narrower ionic abundances slightly different

[Atlas MK]

The gravity effect at Teff = 10 000 K

Extreme gravity effect : a white dwarf of type DA

Cold giant stars

The metal abundance :

At a given effective temperature, a metal-rich star will show a spectrum with deep and contrasted absorption lines, while a metal-poor star (e.g. old Milky Way halo star, or some globular cluster star) will show faint absorption lines.

This is especially true for cold stars which show many metal lines; the classification as metal-poor stars is much more difficult for hot stars which do not exhibit rich metallic spectra because of their temperature.

The metal content is expressed as:

[Fe/H]star = log10 { (Fe/H)star / (Fe/H) Sun }

do not miss the brackets !!!

angstroms

(Elodie stellar library)

Giants: metal abundance sequence at 6000K (window 1)

(Elodie stellar library)

Giants: metal abundance sequence at 6000K (window 2)

angstroms

Summary :

A stellar spectrum is representative of a set of 3 principal « atmospheric parameters » that describe the surface plasma of the star :

- the effective temperature

- the surface gravity (expressed in log g , with g in cm.s-2)

- the metal abundance

n.b: a fourth parameter discriminates between stars that have had subtle evolutionary differences or have formed in different regions of a galaxy, it is the ratio of alpha-elements over iron.

3) Some spectroscopic

curiosities in stars

(there are many others…) :

spectrograph

This side of the star comes towards

Absorption lines are blue-shifted by Doppler effect

No motion with respect to observer : lines are at their reference position

This side of the star goes away from us

Absorption lines are red-shifted by Doppler effect

The spectrograph integrates all the light in its slit : lines are broadened

Spectrum of stars in fast rotation

Th rotation effect resolved on the Sun (n.b. : very slow rotator) between E and W sides of the photosphere.

B stars : slow rotators (left ) and fast ones (right )

Ramspeck, Heber & Moehler, 2001, Astron. & Astrophys. 378, 907

Spectroscopic binary stars

A1 V

K0 IV

?

HH H H H H H H (atm)

FeI+CN CaII CH MgI NaI (atm)

N.b. : mock spectrum fabricated using two « true » spectra superimposed with Photoshop)

K0 Seen from Earth, these two stars :-have an angular separation too small to be distinguished

- have such orbital parameters that they do not eclipse each other

At usual angular resolution (1") they look like a single star, and since their brightnesses are additive (no eclipse !) their spectrum appears as COMPOSITE

Couple "physical " pair of stars (tied by gravitation)

N.B. : special investigation methods at very highresolution (speckle interferometry) may show directly the existence of two stars and provide an angular separation measurement.

A0

How to recognise and understand a spectroscopic binary ?

- The spectrum shows (generally …) signatures of two different stellar atmospheres (in T especially : coexistence of lines normally found in largely different types at a given luminosity class is suspect. Ex : Fe0 strong AND Fe+ strong in a dwarf)

- The stars are in a physical pair : variations of the projection of their orbital velocities (except in very peculiar case ) are easily detectables in good resolution spectrography.

(ex., Doppler velocity mesured on Fe 0 lines will be different from that measured on Fe+ lines and this difference will exhibit periodical and regular variations with time).

These measures allow to estimate the orbital parameters and to derive the mass ratio of the components.

© E. van den Besselaar _ Université de Nimègue

Composite of a hot white dwarf and a red dwarf M4 V

Emission line stars

B  « e »  stars

normal B5 star

Tau (Be)

Lyr (Be)

WR 140 (WR/WC)

H

H He H

CIII CIII H

© Christian Buil

Emission and absorption in diluted gaseous matter.

Ideal thermalsource (black body)

Spectrum in absorption

Continuous spectrum Spectrum in emission

Depending on the observing direction , the spectrum resulting from matter- radiation interaction is not the same! Atoms in « box » b absorb light quanta from source s whose frequencies correspond to finite differences of energy between their energy levels ( dark lines A, B, C …). Then, they re-emit these radiations upon desexcitation, but this re-emission is isotropic. (all directions, none favoured) : along the direction s b a, there is only a modest amount of emission which cannot compensate the energy loss due to absorption. Along direction b e, there is no background source and only emission is visible, although it is not brighter than along b a

s b

e

a

When emission lines are visible in a stellar spectrum, the reason is that ONE SEES SOMTHING ELSE THAN THE PHOTOSPHERE SIMULTANEOUSLY WITH THE PHOTOSPHERE

- a more diluted plasma, - excited by radiation fluxes (or particles --> collisions) not directed

towards the observer

Generally, it is an ENVELOPE, +/- extended, very close to the star ("circumstellar") where the matter is orders of magnitudes less dense than in the photosphere.

Close to us:the solarCHROMOSPHERE and its « flash-spectrum »seen during total eclipses.(discovery of Helium)

This envelope may be very "dynamical", with violent ejections of matter (« winds" of Wolf-Rayet and Of stars) leading to large mass loss : emission lines show Doppler broadening due to large expansion velocities.

March 1970 eclipse

4) The spectrum of

interstellar matter :

warm ionized gas

The emission spectrum of ionized hydrogen regions (HII regions)

HII regions emit a light that is completely dominated, in the visible and near-infrared ranges, by a spectrum of emission lines.

One identifies lines of hydrogen and helium,on one part, that are identical to those produced in the laboratory when using « spectral lamps » (gas electric discharge tubes)

On another part, specific lines are also observed, that cannot be reproduced in the laboratory, whose origin has long been remained mysterious. These lines are « forbidden » and produced by heavy elements as oxygen, nitrogen, sulfur, neon, argon, … in various stages of ionisation.

Emission lines of interstellar gas in the visible

Formation of the hydrogen emission spectrumin «  HII regions » (« capture – cascade »)

Energyof H atom

n

1

2

3

4

56

Lines of Lymanseries (uv)

Lines of Balmerseries(visible)

Lines of Paschenseries(near ir )

(ionisation : 13,6 eV)

In the ionized plasma of the HII region, protons capture electrons on all the possible energy levels. Recombined atoms lose their energy by emittingradiative quanta « capture-cascade » process).The recombined atoms are immediately reionized by far-ultraviolet photons emitted by the excitating stars.

This process produce PERMITTED LINES

(Only thefirst lines ofeach series have been represented, they are noted

Formation of « forbidden lines » spectrum of metal ions

Ground energy level

Ion: O+ Ion: O++Metastable energy levelsE

xcitation

by

collisions

(In dark red:Fine structure transitions In the far-infrared)

NGC 5236 (M83) - VLT

NGC 1316 (Fornax A) - VLT

NGC 1512 - HST

5) The Spectra of Galaxies

The “cosmological” (redshift)

Hubble, Humason, and Mayall, found that the light from distant galaxies shows a spectrum similar to that of red giant stars affected by a shift always towards the red (low frequencies, long wavelengths)

This redshift is due to the apparent recession velocity of distant objects created by the cosmological expansion Universe expansion. For “low” recession velocities, the effect is undistinguishable from the usual Doppler-Fizeau

effect and the formula is the same ! Moderate redshifts [ z = ( – 0 ) / 0 ] correspond to apparent à des recession velocities proportional to the distance of the object (linear Hubble’s law). (But this is no more valid for high redshifts). Attention : apparent recession velocity is a fictitious velocity : the galaxies do not move the ones with respect to each other (except in very modest quantities : “peculiar velocities”) , but the geometry of space that inflates ! The redshift IS NOT A DOPPLER EFFECT !! As example, apparent recession velocities are supra-luminic for z ~= 1, and many objects are observed with z > 2, 3, 4 etc… !

(redshift z)

observer

galaxy

Spectrum of stellar populations in galaxies

1) Individual star spectra add to each other in intensity

2) Each star moves with respect to the observer. The radial component of its velocity shifts the spectrum of the star by Doppler effect. The addition of these numerous shifted spectra result in a broadened line spectrum (blurring by velocity dispersion )

3) The redshift due to cosmological Universe expansion globally shifts (and extends) the blurred spectrum as a whole.

Spectrum of template star

Observed spectrum

Starburst galaxy : «HII» -like spectrum

« Normal » galaxy, withno stellar formation: lightdominated by red giant stars

Sloan

Digital

Sky

Survey

Cold giant stars

Composite spectrum of an evolved galaxy :

a mix of old cold giants G0 III to M2 III

(Stellar population synthesis)

Galaxy spectra

Most galaxy spectra are easily recognised as being the superimposition of:

A)- the spectrum of a luminous stellar population in which cold evolved giants (types G0-G5 to M0) dominate (age > 5 Gyr). This population may be (or not) accompanied by the specific spectral signature of a much younger and massive, bluer population, in which A to F stars of main sequence dominate, showing deep absorption lines of hydrogen.

B)- the emission spectrum of interstellar gas ionized by UV photons from hot, young, massive stars. This component traces recent star formation. It will be totally absent in galaxies devoid of interstellar matter and star formation (ellipticals, lenticulars).

Both components are affected by the redshift, and by Doppler effects due to internal motions projected on the direction of observation, and by peculiar motions of the galaxy as a whole (e.g. faloff onto a cluster center). Internal motions of the stars « blur » the spectral lines in absorption (velocity dispersion). They may also be affected by internal interstellar extinction and reddening !

6) Spectra of Active Galaxy Nuclei and Quasars (AGN and QSO)

Elliptical galaxy

Quasars are extremely active nuclei of galaxies that are, aside from their center, normal (in general). The energy source is the gravitational attraction exerted by a supermassive black hole (SMBH, several 107 solar masses in an event radius smaller than the internal Solar System) on the neighbouring matter, stars, stellar clusters, interstellar matter.

This matter orbits with high velocity around the SMBH, with intense heating, shocks and ionization, due to friction and tidal forces, and finally is swallowed by the accreting BH.

This occurs in a very small area at the center of the galaxy (a few parsecs across) emitting an intense spectrum of emission with broad lines (due to extreme velocity dispersion of the turbulent gas: typically 10000 to 20000 km/s) and strong uv continuum, mostly of bremstrahlung.

Synchrotron radiation may also be present, and this is diffused by free electrons and considerable amounts of dust.

Depending on projection effects and the direction of observation, the type of AGN / QSO is different.

Finally, the light of sufficiently distant QSO may have to cross interstellar matter of foreground galaxies ( one or several) that leaves its imprint as very thin and deep absorption lines with redshift(s) smaller than the emission redshift of the QSO.

a question of perspective

Seyfert type II

Seyfert type II

Blazar

Blazar

Seyfert type I,Quasar

Seyfert type I,Quasar

Seyfert type I,Quasar

Seyfert type I,Quasar

a typical Seyfert 2 spectrum

Composite spectrum of quasar in the rest frame

A high redshift QSO

Lyman « forest »

912 A cutoff

Ly a emission line

a QSO with multiple absorption systems

QSO G1 G2 observer

z em z abs, 1 z abs, 2

Interpretation of multiple absorption redshift systems