PFS

As a postdoc, I've been involved with the Planet Finder Spectrograph at Magellan II/Clay (PIs: Stephen Shectman and Jeff Crane). The California-Carnegie Planet Search represents one of, if not the largest, longest-running ground-based exoplanet search in history. Dr. Paul Butler of Carnegie pioneered the precision Doppler velocity technique of planet detection -- measuring the wobble of a star caused by the gravitational attraction of an orbiting object, from which the mass and orbital period can be derive -- in the early 1990's, and has since helped discover or confirm over 300 exoplanets. In 2010, the Carnegie Planet Finder Spectrograph was commissioned on the 6.5m Magellan Clay telescope at Las Campanas Observatory in Chile, where Carnegie has a 50% share of telescope time. The purpose of PFS is to use high-precision RV measurements to monitor ~500 of the closest (<100 pc) least-active FGKM stars to find terrestrial-mass planets, hopefully some of which are in habitable zones where liquid water could exist on the surface. These targets are most amenable to future follow-up observations with large ground- or space-based telescopes. PFS uses the traditional iodine cell techinque to imprint known spectral features in each stellar spectrum, providing a simultaneous solution for the wavelength scale, point spread function, and Doppler shift of the star. Most of our observations are taken through a 0.5'' slit and R~76,000, and span wavelengths from the Ca H&K lines at 3968a and 3933A to a bit beyond H-alpha at 6562A.

Left plots: From Anglada-Escude et al. (2014). Phase folded Doppler curves to the reported signals around Kapteyn's star with the other signal removed (HARPS are red circles, HIRES are brown diamonds and PFS are blue squares). The maximum likelihood solution is depicted as a black line. Right plots: From Wittenmyer et al. (2014). Radial velocities and fit for GJ 832c, a super-Earth orbiting near the inner edge of the habitable zone of GJ 832, an M dwarf previously known to host a Jupiter analog in a nearly-circular 9.4-year orbit (the signal of the outer planet has been removed). AAT – green, HARPS – red, PFS – blue. Bottom: Same, but the AAT data have been omitted from the plot to more clearly show the low-amplitude signal.

Above is Figure 6 from Van Eylen et al. (2016), a paper confirming the orbit (4.6 days), radius (8.3+/- 1.1 R_earth), and mass (50.3+9.7/-9.4 M_earth) of K2-39b (EPIC 206247743b). It shows the radial velocity observations over time (left) and phased (right), along with the best fitting model as solid line, and the residuals after subtracting the model. The internal RV uncertainties are indicated by the black error bars. The gray error bars include an additional “stellar jitter” term added to account for our incomplete knowledge of stellar activity, such that the minimum reduced chi-squared for the data obtained by each spectrograph is close to unity. This is a super cool result because it's the shortest-period planet orbiting a subgiant star known to date, and these planets are rare in the first place. Vincent's paper explains why this discovery is important for understanding planet formation around subgiant stars.

PFS was the first American RV spectrograph to achieve 1 ms−1 precision, and on very quiet (not magnetically active) stars it has reached ∼0.6 ms−1, putting it on par with HARPS planet search capabilities (e.g., Lovis et al. 2005). With its current precision, and the cadence that the Carnegie team obtains with 4-5 ten-night runs/year, PFS is able to detect terrestrial mass habitable planets (at 0.1-0.2 AU) around M dwarfs, which experience greater gravitational effects of smaller planets, and have closer-in habitable zones. Recent PFS detections include a system with three habitable super-Earths (Anglada-Escude et al. 2013), a system with one sub-Saturn mass and one super-Earth mass planet (Arriagada et al. 2013), and two super-Earth mass planets, one of which is in the habitable zone, around Kapteyn’s star (Anglada-Escude et al. 2014), one of the oldest and closest exoplanet systems discovered to date. Plus there's the long period, giant planets we detected with PFS alone in 2016! PFS observations also contribute to confirmation of transiting planet candidates and mass measurements of planets detected via transit (e.g., Dai et al. 2015; Hartman et al. 2015; Jordan et al. 2014; Zhou et al. 2014; Van Eylen et al. 2016).

Above: PFS Selfie in December 2014. Below: Me helping to fix some suspicious valves and install a new fan in PFS in April 2016. Also pictured is Jeff Crane, instrument co-PI. Check out our time lapse video here!

Above: Below: Me helping to mount PFS on Magellan II in June 2015.

Below: PFS Team, minus Ian Thompson and Matias Diaz, at dinner in Pasadena in May 2014.

In 2017B, PFS will undergo three important upgrades to improve instrument performance and our ability to detect small planets around nearby stars. Stay tuned for updates! You can read more about my observing and instrumentation adventures at Las Campanas Belles.

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