Great progress has been made in recent years in detecting massive planets orbiting other stars. These exciting discoveries are an important step toward understanding the evolution of other solar systems and whether or not they may harbor life. Tantalizing as these discoveries are, however, they are not conclusive. Astronomers have yet to find a planet in another solar system that could support life as we know it. To do so would require finding smaller solid (not gaseous) earth-like planets orbiting a star within the "habitable zone" where the distance of the planet from the star is such that the surface temperature of the planet can sustain liquid water.
There are at least four ways to detect a planet orbiting a star. By far the most successful means to date has been to very carefully monitor the velocity of the star. If the velocity tends to vary regularly about its average value, then it is possible that we are measuring the reflex velocity of the star as a planet moves around it in its orbit. This is a very exacting measurement to make and it relies on extremely accurate velocity measurements made with a spectrograph. Dr. Steve Osterman of the University Colorado is designing a very stable and accurate calibration mechanism to improve the radial velocity measurements for spectra taken not in visible light, but in the infrared. Infrared light is very useful in the study of cooler and smaller stars which are more numerous than hot stars. Finding planets around these stars will give us a much clearer picture than we currently have of how planets are actually formed and evolve. Dr. Osterman's wavelength calibration scheme uses a very rapidly pulsed laser to imprint a regularly spaced calibration "comb" of lines of precisely known wavelengths onto the spectrum of the star. This new technique is expected to improve the velocity determination from the current standard enough to detect earth-like planets in the habitable zone. Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.
Introduction and Overview: One of the most active fields of astronomy in the last decade has been the detection and characterization of extrasolar planets. Modern astronomical instrumentation has matured to the level where it is possible to detect super-earth planets by observing the time dependent Doppler shift in the light coming from the host star, a method referred to as radial velocity measurements. However, the precision of these measurements is limited by many factors, notably including wavelength calibration techniques, with ultimate precision many times too coarse to observe an earthlike planet orbiting a solar type star. We proposed to address this issue in two ways; first, by creating a much more precise wavelength calibration source, a laser frequency comb, and second, by optimizing the comb’s wavelength range for observing low mass M-Stars, where a terrestrial planet in the habitable zone would create a much larger wobble in the host star than in the case of objects orbiting more massive (i.e. sun-like) stars. Our project goals were to create a laser frequency comb for observations in the near infrared (NIR) H-Band (~1.45-1.75 μm) and test the comb in the field with an astronomical spectrograph. We accomplished these goals by August, 2010, including field tests with the Pathfinder instrument a the Hobby-Eberly Telescope. We then exceeded our goals by further developing the comb to increase the line spacing, to increase the instrument bandwidth, and by pushing the output to much shorter wavelengths in order to support the Patfinder follow-on instrument, the Habitable zone Planet Finder. The Laser Frequency Comb: A laser frequency comb is a device that uses a femtosecond pulse laser to create an array of precisely spaced laser modes whose frequencies are given as fn=nfrep+f0 . This relationship is exact, with n being an integer, frep the laser repetition rate and f0 the offset frequency. frep and f0 can be precisely controlled both in the lab and in the field so that the absolute uncertainty in any line is less than one part in 1011. The lines emitted by the Er:fiber laser we use are so tightly spaced that they cannot be resolved by a typical astronomical spectrograph. To increase the line spacing to a level where they can be resolved we use Fabry-Pérot interferometers to increase the line spacing by a factor of 50 to 100 (Figure 1). Results: In the first phase of this project, we developed the H-band comb as a stand-alone instrument that could be operated at a remote facility. We transported our comb to the Hobby-Eberly Telescope at the McDonald Observatory and, collaborating with the Pennsylvania State University Pathfinder Spectrograph team, we were able to demonstrate the utility of our comb in obtaining precise radial velocity measurements from astronomical objects (Figures 2 & 3). Upon completion of this demonstration we began modifying the comb so that it could provide a calibration signal at shorter wavelengths with greater line spacing and greater instrumental autonomy to support the Habitable Planet Finder (HPF) spectrograph, Penn State’s follow-on to the Pathfinder spectrograph. By implementing a combination of erbium and ytterbium fiber amplifiers along with non-liner fiber for broadening the band pass and polarization maintaining fiber to increase spectral stability, we have been able to produce an instrument that can support the full bandpass of the HPF, now under construction at Penn State (0.8-1.3 μm). We have demonstrated autonomous operation of over 46 days with the instrument operating nominally 98% of the time. Funding for this project has enabled development of technology and collaboration with astronomers building NIR radial velocity spectrographs to pave the way forward in the deployment of the comb as a turnkey calibration system, to be built under NSF/ATI grant 13110875.
