Topics in precision astrophysical spectroscopy

Abstract

Applying advances in optical frequency metrology to astronomy has enabled extremely precise spectroscopic measurements of stars. Precision spectroscopy of stars can inform us not only about stellar physical processes but also about the existence of extrasolar planets. An indispensable tool for carrying out these measurements is the optical frequency comb. Using a turn-key Ti:sapphire optical frequency comb, we report the generation of a 16-GHz visible frequency comb suited to astrophysical spectroscopy (i.e., an “astro-comb”). The light from this source is used to calibrate the HARPS-N astrophysical spectrograph for precision radial velocity measurements. The comb-calibrated spectrograph achieves a stability of ∼ 20 kHz within half an hour of averaging time. We also use the astro-comb as a reference for measurements of solar spectra obtained with a compact telescope and as a tool to study intrapixel sensitivity variations on the spectrograph detector. After the initial tests of the astro-comb, we describe the design and testing of a dispersion-engineered tapered photonic crystal fiber which is capable of producing a broad, flat, visible supercontinuum spectrum. Using 30 fs, 100 pJ pulses from our Ti:sapphire laser, we obtain an output spectrum that is flat to within 3 dB over the range 490-690 nm with a blue tail extending below 450 nm. This greatly extends the range of applicability of the astro-comb. Finally, we present a remote-control interface for the astro-comb and outline future automation prospects. Beyond the detection of extrasolar planets, we show that the precision radial velocity technique is sensitive enough to observe the acceleration of stars in the galactic potential. Since the bulk of the matter in the galaxy is invisible, stellar acceleration measurements would inform us about the dark matter density distribution in the Milky Way. This knowledge is crucial to both our understanding of the standard cosmological model and for grounding direct and indirect searches for the particles comprising dark matter. Current measurements of galactic dark matter content rely on model assumptions to infer the forces acting upon stars from the distribution of observed velocities. Using the precision radial velocity method instead, we can measure the change in the velocity of an ensemble of stars over time, thereby providing a direct probe of the local gravitational potential in the galaxy. Using numerical simulations, we develop a realistic strategy to observe the differential accelerations of stars in our galactic neighbourhood with next-generation telescopes, at the level of 10^-8 cm/s^2. Our simulations show that detecting accelerations at this level with an ensemble of 10^3 stars requires the effect of stellar noise on radial velocity measurements to be reduced to < 10 cm/s. The measured stellar accelerations may then be used to extract the local dark matter density and morphological parameters of the density profile. Many extrasolar planets have already been discovered, so it is natural to ask whether if any of them are habitable. In an effort to answer this question, we can search for the absorption signatures of oxygen, which is crucial to life on Earth, in the atmospheric spectra of exoplanets. This is an extremely challenging measurement to do with ground-based surveys, and one obvious systematic that threatens the quality of the data is the variability of the inevitable spectral background resulting from the Earth's oxygen absorption features. Using four hours of high-resolution atmospheric spectra obtained with a Fourier transform spectrometer, we examine correlations of line parameters with environmental parameters to try to assess whether this contribution may be modelled out if in-situ monitoring of the atmosphere is required.