Although spectro-astrometry proved to yield good results, there are still several issues which can be improved during future studies. First of all, a significant improvement of the spatial resolution can be achieved if the instrumental artefacts could be removed com-pletely. To get more information on the instrumental PSF, imaging the 2D-PSF in the slit plane simultaneously with the registration of the longslit spectrum could hint at the general form of the PSF. This would at least record the instrumental influences which take place before the light passes the spectrograph. The usage of a gas absorption cell does not necessarily improve the determination of the instrumental profile for spectro-astrometry.
Only if there are no telluric absorption lines and the target is expected to show kinematic spectro-astrometric signatures, gas cell reference lines may lead to better removal results.
Current plans to build special instrumentation (Wiedemann, G.; private communications) are expected to solve the problem of instrumental artefacts: if exposures at anti-parallel slit orientations are performed simultaneously and through the same optical path, the removal should become very accurate. Such an instrument will have to split the incoming light, rotate one of the light beams by 180◦ and disperse both light beams by a single longslit spectrograph. In this way, the full potential of spectro-astrometry could be ex-ploited – however, at the cost of the need for special instrumentation.
To be able to put strong constraints on the best-fitting object configuration, future obser-vations of complex sources should apply as many independent slit orientations as possible.
5.2 Outlook 79
Observing a target at various wavelength regions also allows confirm or exclude certain un-derlying source configurations. Furthermore, a more sophisticated approach to modelling the target SEDs with synthetic spectra is necessary. As uncertainties in the synthetic spec-tra directly spec-translate into an uncertainty in the source geometry, accurate model specspec-tra are required. In particular, very cool giant star atmospheres to date are not yet fully understood.
Observing more objects with shorter integration times (and NDIT=1) will lead to possibly thousands of exposures. As a manual inspection and analysis will virtually be impossible, a better automation of the programmed data reduction and analysis code will be mandatory.
Spectro-astrometry will profit in the future from increasing telescope sizes and instrument capabilities. An increasing spatial resolution of a telescope/adaptive optics system directly translates into better spatial resolution in the spectro-astrometric quantities, cf. Eq. (2.6).
Furthermore, larger telescopes will increase the S/N on the target which further pushes the attainable spatial resolution.
Nomenclature
(N)LTE (Non) Local Thermal Equilibrium
AGB Asymptotic Giant Branch
APD Avalanche Photo Diode
CADARS Catalogue of Apparent Diameters and Absolute Radii of Stars
CCD Charge Coupled Device
CHARM2 Catalogue of High Angular Resolution Measurements CMOS Complementary MetalOxide Semiconductor
CPU Central Processing Unit
CRIRES CRyogenic high-resolution IR Echelle Spectrograph DIT integration time per exposure
ESO European Southern Observatory; with the full name being “European organization for astronomical research in the southern hemisphere”
FITS Flexible Image Transport System
FWHM Full Width at Half Maximum
GAIA Global Astrometric Interferometer for Astrophysics
GPL General Public License
HST/STIS Hubble Space Telescope / Space Telescope Imaging Spectrograph IRAF Image Reduction and Analysis Facility
ISAAC Infrared Spectrometer And Array Camera MACAO Multi-Applications Curvature Adaptive Optics
NACO NAOS-CONICA: Nasmyth Adaptive Optics System – Near-Infrared Imager and Spectrograph
NDIT Number of (DIT-) exposures to be averaged before writing the FITS-file
PSF Point Spread Function
RV Radial Velocity
SED Spectral Energy Distribution SNR,S/N Signal to Noise Ratio
UVES Ultraviolet and Visual Echelle Spectrograph
VLT Very Large Telescope
1.1 Diffraction pattern of a grating . . . 3
1.2 Simple spectrograph setup . . . 4
1.3 Diffraction pattern of a straight edge . . . 5
1.4 2D-diffraction pattern of a circular aperture . . . 6
1.5 MACAO Strehl ratio plot . . . 8
1.6 CRIRES instrument setup . . . 10
1.7 Exemplary Fortrat diagram . . . 15
1.8 Point source and binary visibilities . . . 18
2.1 Spectro-astrometric spatial signatures . . . 23
2.2 Spectro-astrometric kinematic signatures . . . 24
2.3 Spatial profile fit errors . . . 27
2.4 Position spectrum extraction: Gaussian fitting and Tukey’s biweight . . . . 29
2.5 Spectro-astrometric artefacts for an elliptical PSF . . . 32
2.6 Artefact removal, χ2-landscape . . . 34
2.7 χ2-landscapes: C(λ) andW(λ) . . . 35
2.8 One spot simulations: detectability . . . 38
2.9 Two spots simulations: detectability . . . 39
3.1 Trace correction . . . 51
3.2 Odd-even effect . . . 52
3.3 Image distortions . . . 53
4.1 CRIRES data: spatial profile . . . 59
4.2 Artefact variations . . . 60
4.3 Telluric standard star SEDs . . . 61
4.4 RS Vir: raw and corrected C(λ) andW(λ) . . . 62
4.5 RS Vir: χ2-landscape . . . 63
4.6 RS Vir: best-fitting PSF configurations . . . 64
4.7 RS Vir: observed and synthetic SEDs . . . 65
4.8 TW Oph: raw and correctedC(λ) and W(λ) . . . 67
4.9 TW Oph: best-fitting PSF configuration . . . 68
4.10 TW Oph: χ2-landscape . . . 68
4.11 TW Oph: correctedC(λ) andW(λ) for second position angle . . . 69
4.12 TW Oph: observed and synthetic SEDs . . . 70
4.13 TW Oph: synthetic CO line depths . . . 71
4.14 TW Oph: spot configuration fits toC(λ) and W(λ) . . . 71
4.15 TW Oph: best-fitting spot model images . . . 72
4.16 α Cen A: raw and corrected C(λ), target and telluric SEDs . . . 74
4.17 α Cen A: best-fitting PSF configuration . . . 74
LIST OF FIGURES 83
4.18 α Cen A: observed and synthetic SEDs . . . 75
A.1 Tukey’s biweight: ζ-function and weighting function . . . 85
A.2 α Centauri A: SED . . . 86
A.3 RS Vir: SED . . . 87
A.4 TW Oph: SED . . . 88
3.1 RS Vir and TW Oph: basic target parameters . . . 44
3.2 Observing summary . . . 46
3.3 α Cen A: basic target parameters . . . 46
4.1 RS Vir: elliptical PSF configurations . . . 63
4.2 RS Vir: two-satellite PSF configurations . . . 64
4.3 RS Vir: single spot detection limits . . . 66
4.4 TW Oph: two-satellite PSF configurations . . . 68
4.5 TW Oph: single spot configurations . . . 72
4.6 TW Oph: single spot detection limits . . . 73
4.7 α Cen A: two-satellite PSF configurations . . . 75
Appendix A
Figures
Figure A.1: Theζ-function (left panel) and the weighting functionζ/z (right panel) of Tukey’s biweight. Here, the cut-off is chosen to bea= 1. The weights forz≥1 are zero. Hence, any data point outside|z|<1 does not contribute at all.
Figure A.2: Spectral energy distribution ofαCentauri A in the M-band, detectors one to four of CRIRES. Overplotted is the SED of the telluric standard HR6084.
87
Figure A.3: Spectral energy distribution of RS Vir in the K-band, detectors one to four of CRIRES. Overplotted is the SED of the telluric standard HD121263.
Figure A.4: Spectral energy distribution of TW Oph in the K-band, detectors one to four of CRIRES. Overplotted is the SED of the telluric standard HR173300.