Lecture on spectroscopy and applications (Brno 9.02.17) Stephane Vennes Astronomical Institute Czech Academy of Sciences Spectroscopy and applications 1 9/02/2017
Syllabus: Physical description: Atoms and molecules; light properties-energy and polarization: Temperature, magnetic and abundance effects. Spectrographs; basic concepts. Explore some astrophysical contexts. Instrumental capabilities: Wavelength range and resolving power; integral field; echelle. Multi-wavelength astrophysics from the ultraviolet to the infrared (IR). With examples and applications. Spectroscopy and applications 2 9/02/2017
Physics 1.1 Temperature, Z, B In the following we will use white dwarf properties to illustrate some physical properties of stars. White dwarfs are compact stars with a fully degenerate core (C, O, Ne, ?). However, their atmospheres exhibit a range of ``classical’’ phenomena. Temperature effects as in OBA stars, but with more extreme abundance variations, and stronger magnetic fields (kG to GG). Surface abundance ranges from pure H, He, to C and O with extreme metallicity variations. Spectroscopy and applications 3 9/02/2017
Physics 1.2 Temperature, Z, B Spectroscopy and applications 4 9/02/2017
Physics 1.3 Temperature, Z, B Spectroscopy and applications 5 9/02/2017
Physics 1.4 Temperature, Z, B Spectroscopy and applications 6 9/02/2017
Physics 1.5 Temperature, Z, B Spectroscopy and applications 7 9/02/2017
Physics 1.6 Temperature, Z, B Spectroscopy and applications 8 9/02/2017
Physics 1.7 Temperature, Z, B Spectroscopy and applications 9 9/02/2017
Physics 1.8 Temperature, Z, B Spectroscopy and applications 10 9/02/2017
Physics 1.9 Temperature, Z, B DO: HeII lines DB: HeI lines DA: strong to weak HI lines DC: weak to no HeI lines DZ: weak to no HeI lines but metal lines DQ: weak to no HeI lines but C2/CN/CH molecular vibrational bands Spectroscopy and applications 11 9/02/2017
Physics 2.1 Zeeman effect l = angular momentum m l = magnetic moment: m l l , l 1 ,..., 0 ,..., l 1 , l The allowed transitions follow the selection m l =0, 1 In this example, the Zeeman triplet (normal Zeeman) splits at: 7 2 4 . 67 10 B ( g m g m ) B s i i j j Where i/j are lower/upper levels. B s is mean surface B. Spectroscopy and applications 12 9/02/2017
Physics 2.1 Zeeman effect Lower level 4s 1/2 (g=2) m 1 / 2 , 1 / 2 l Upper level 4p ½ (g=2/3) m 1 / 2 , 1 / 2 l The allowed transitions follow the selection m l =0, 1 The anomalous Zeeman multiplet splits in 4 components at: E ( eV ) 0 . 0058 B ( g m g m ) B s i i j j Where i/j are lower/upper levels. B s is mean surface B. Spectroscopy and applications 13 9/02/2017
Physics 2.1 Zeeman effect Lower level 4s 1/2 (g=2) m 1 / 2 , 1 / 2 l Upper level 4p ½ (g=2/3) m 3 / 2 , 1 / 2 , 1 / 2 , 3 / 2 l The allowed transitions follow the selection m l =0, 1 The anomalous Zeeman multiplet splits in 6 components at: E ( eV ) 0 . 0058 B ( g m g m ) B s i i j j Where i/j are lower/upper levels. B s is mean surface B. Spectroscopy and applications 14 9/02/2017
Physics 2.2 Zeeman effect Observed behaviour: The line intensity ( and )in absorption) and polarization ( ) depends on viewing angle (to field orientation): The components are at maximum intensity at 90 with nil circular polarization and full linear polarization. The contrast between and intensity constrains a key geometric parameter, the field inclination relative to viewer. Spectroscopy and applications 15 9/02/2017
Physics 2.2 Zeeman effect Model atmosphere and spectral synthesis: Cool white dwarf without (red) and with a magnetic field (blue 163 kG). Model computations applicable to cool (<3000K, GRASSE) and hot white dwarfs (>100000K, TLUSTY). LTE/non-LTE; convective/non- convective; Teff/log(g) from Eddington limit up to 9.5. Includes metallicity (Z) and low magnetic fields (| B |<10 MG). Spectroscopy and applications 16 9/02/2017
Physics 2.3 Zeeman effect Intermediate-dispersion spec- troscopy ESO VLT/Xshooter: NLTT 53908 (2 Gyr) and NLTT10480 (4 Gyr) are two magnetic and polluted white dwarfs. High incidence of magnetism in this class of objects (33%) suggests that all old white dwarfs are magnetic. CaH&K show anomalous Zeeman effect: quadruplet and sextuplet, 4 and 6 discrete values for (g i m i -g j m j ) instead of 3. Spectroscopy and applications 17 9/02/2017
Physics 2.4 Zeeman effect Basic configuration for the measurement of circularly polarized light: o eo o eo 1 V f f f f o eo o eo I 2 f f f f 45 45 Spectroscopy and applications 18 9/02/2017
Spectroscopy 1.1 The main ingredients of spectroscopy: F ( ): The intrinsic (model or template) astrophysical I. intensity spectrum measured at Earth (star, galaxies, HII regions, any source), I ( ): The instrument response (sensitivity or throughput, and II. instrument profile or resolution, slit loss ...), T ( ): Atmospheric transmittance, III. Other astrophysical effects might require special attention IV. such as stellar rotation G ( ). For example assuming a non-rotating stellar model F ( ), V. the observed count spectrum of a rotating star is the result of the convolution: C ( ) [ T ( ) F ( )] G ( ) I ( ) Spectroscopy and applications 19 9/02/2017
Spectroscopy 1.2 Mathematical convolution applied to rotation: L F ( ) F G F ( ) G ( ) d L Where L is calculated at maximum velocity (edge of stellar disc ... next slide). And applied to the instrument profile: C ( ) F I F ( ) I ( ) d 0 Where it is sufficient to integrate such that and is the instrumental resolution (studied next). ...and remember convolution is commutative and associative ... Spectroscopy and applications 20 9/02/2017
Spectroscopy 1.3 Measurement of stellar rotation is a major application of astrophysical spectroscopy. In the convolution integral L ( ) ( ) ( ) F F G F G d L G( - ) is given by Gray (1976, 1992, 2005, 2008): 2 1 / 2 2 G ( ) c [ 1 ( / ) ] c [ 1 ( / ) ] 1 L 2 L Where L is the largest observed wavelength shift at the surface of a star rotating at a projected velocity v sin( i ): v sin( i ) L c In observing stellar spectra, a measurement of v sin( i ) is one of the results hoped for... Spectroscopy and applications 21 9/02/2017
Spectroscopy 1.4 Measurement of stellar rotation: The parameters c 1 and c 2 contain a major physical ingredient, the limb-darkening coefficient ... The intensity of emitted light decreases from centre to limb (see Mihalas 1978, Stellar Atmospheres). In 2 1 / 2 2 G ( ) c [ 1 ( / ) ] c [ 1 ( / ) ] 1 L 2 L 2 ( 1 ) 1 , 2 c c ( 1 / 3 ) 2 ( 1 / 3 ) A value =0 corresponds to a uniformly illuminated disc and =0.6 is a representative empirical and theoretical value with the limb 60% darker than the centre. The next slide displays the function G in terms c 1 and c 2 . Spectroscopy and applications 22 9/02/2017
Spectroscopy 1.5 Measurement of stellar rotation: 2 1 / 2 2 G ( ) c [ 1 ( / ) ] c [ 1 ( / ) ] 1 L 2 L 2 ( 1 ) c , c 1 2 ( 1 / 3 ) 2 ( 1 / 3 ) . Spectroscopy and applications 23 9/02/2017
Spectroscopy 1.6 -G( ) movie Spectroscopy and applications 24 9/02/2017
Spectroscopy 1.7 -CaK movie Spectroscopy and applications 25 9/02/2017
Spectrographs 1.1 A simple spectrograph design: Spectroscopy and applications 26 9/02/2017
Spectrographs 1.2 Another simple design: Focal lengths: Slit-to-collimator f coll Camera-to-CCD f cam Spectroscopy and applications 27 9/02/2017
Spectrographs 1.2 Another simple design: Important angles: Collimator-to-camera: (fixed) Incident (collimator-to- grating normal GN): i Reflected (relative to GN): r Blaze angle Diffracted envelope: Spectroscopy and applications 28 9/02/2017
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