Early Results from a Multi Object Doppler Planet Survey

The first DFDI instrument used for planet searches was the Exoplanet Tracker (ET) that was commissioned at the KPNO 0.9-m Coude Feed/2.1-m telescope in late 2003. ET was used for a planet survey of ^150 solar-type stars with V = 7.6-9 in 2004-2006, and since Fall 2006 the instrument has been available to the public.

The instrument Doppler precision over a few hours' baseline was measured in April 2007; the results are shown in Fig. 2.5. The measurement errors are consistent with photon noise limits. The total measured detection efficiency, including the telescope, seeing, fiber, instrument and detector losses, is 18% under typical seeing conditions (1.5 arcsec) at the KPNO Coude Feed/2.1 m. This efficiency is about four times higher than that reached with the state-of-the-art echelle Doppler instrument HARPS on the ESO 3.6 meter telescope (Pepe et al 2002). One new planet, orbiting the V = 8.05 G8V star HD 102195, has been discovered by ET (Ge et al. 2006; see Fig. 2.6).

The first DFDI instrument designed for the MARVELS survey at the SDSS telescope was constructed at the University of Florida in 2005-2006 with support from the W.M. Keck Foundation and designated as the W.M. Keck Exoplanet Tracker, or Keck ET. It was commissioned at the SDSS telescope in spring 2006. Figure 2.7 shows ET fiber plugging in the SDSS fiber cartridge and the ET setup on an optical bench in a thermally controlled instrument enclosure. The Keck ET is based upon the design of the single-object KPNO ET. The Keck ET consists of eight subsystems: the multi-object fiber feed, the iodine cell, the fixed-delay interferometer system, the slit, the collimator, the grating, the camera, and the 4k x 4k CCD. The instrument contains four auxiliary subsystems for interferometer control, instrument calibration, photon flux monitoring, and thermal control. The instrument is fed with 60 fibers of 200 pm core diameters, which are coupled to 180 pm core diameter short fibers from the 2.5-m telescope; the latter measurement corresponds to 3" on the sky at f /5. The spectral resolution for the spectrograph is

100 S/N per pixel

Fig. 2.5. Doppler precision measurements with KPNO ET in April 2007. The dotted and long dashed lines represent the photon noise limits for 36 UMa (V = 4.8, F8V) and HIP 48455 (V = 3.9, K2III), respectively. The RMS RV measurement errors are consistent with the photon noise limits. The best photon limiting precision is 1.4 m/s and 2.4 m/s for HIP 48455 (V = 3.9, K2III) and 36 UMa, respectively.

100 S/N per pixel

Fig. 2.5. Doppler precision measurements with KPNO ET in April 2007. The dotted and long dashed lines represent the photon noise limits for 36 UMa (V = 4.8, F8V) and HIP 48455 (V = 3.9, K2III), respectively. The RMS RV measurement errors are consistent with the photon noise limits. The best photon limiting precision is 1.4 m/s and 2.4 m/s for HIP 48455 (V = 3.9, K2III) and 36 UMa, respectively.

-100

-100

Orbital Phase

Fig. 2.6. The first planet discovered with a prototype of the MARVELS spectrograph, a single-fiber instrument operating on the Kitt Peak 2.1-m telescope (Ge et al. 2006a). Points show the measured radial velocity of the star against the orbital phase, with one repetition. The smooth curve shows the best-fit orbit, which is nearly circular with a period of 4.11 days.

Orbital Phase

Fig. 2.6. The first planet discovered with a prototype of the MARVELS spectrograph, a single-fiber instrument operating on the Kitt Peak 2.1-m telescope (Ge et al. 2006a). Points show the measured radial velocity of the star against the orbital phase, with one repetition. The smooth curve shows the best-fit orbit, which is nearly circular with a period of 4.11 days.

Fig. 2.7. left: An SDSS fiber cartridge with ET fibers plugged ready for multi-object ET observations. right: Part of the setup of the Keck ET multi-object Doppler instrument in April 2006.
Fig. 2.8. left: 59 stellar fringes recorded by the 4kx4k CCD of Keck ET. The brightest star is V = 8 and the fainest star is V = 12. right: Expanded fringing spectra of the central region. Fringes can be clearly seen.

fl=5,100 and the wavelength coverage is 900 A, centered at ~ 5400 A. Details of the instrument design can be found in Ge et al. (2006b), Wan et al. (2006), and Zhao & Ge (2006). Figure 2.8 shows the spectral format on the 4kx4k CCD detector.

The current Keck ET has one spectrograph and one 4k x 4k CCD camera to capture only one of the two interferometer outputs, and has a 5.5% detection efficiency from the telescope to the detector without the iodine cell under the typical seeing condition at Apache Point Observatory (APO). (The instrument will be upgraded to capture both interferometer outputs and have better throughput in Spring 2008 before the MARVELS survey starts in July 2008.) The instrument can record 59

objects in a single exposure (a slight modification planned this fall will increase this to 60 spectra). The instrument Doppler precision was measured with the day sky scattered light, which offers a stable, homogeneous RV source for simultaneously calibrating the instrument performance for all of the sky fibers. The sky spectra had an average signal-to-noise ratio of ^150 per pixel, or a total of 1.8x109 photons. The average RMS error over a few hours of RV measurements for the 59 fibers is 6.3±1.3 m/s in 2006 November; the corresponding average photon-limit error is 5.5±0.5 m/s. Figure 2.9 shows the RV accuracy of the sky measurements as a function of the recorded signal: the short term RMS errors are consistent with the photon-noise limit errors. Figure 2.10 shows a comfirmation of the planet HD 209458b using the Keck ET in fall 2006.

The instrument's long term precision has been measured using the sky scattered light during two extended periods: 45 days in fall 2006, and 150 days in winter/spring 2007. The RMS RV measurement errors for these periods, after photon noise errors are subtracted, are 11.7 ± 2.7 m/s and 11.3 ± 2.5 m/s, respectively. Figure 2.11 shows the RV measurements with one of the fibers in 2007. These measurement errors are mainly caused by inhomogeneous illumination of the slit, image aberration, and the interferometer comb aliasing (sampling on the detector).

The instrument's long term RV stability has been derived from sky observations between December 2006 to May 2007. The average RV offsets of later months compared to the first month data are all within ±2.0 m/s.

Fig. 2.9. Doppler precision measurements with Keck ET on 4 April 2007. The solid line is the photon noise limit. The triangle dots represent the average RMS error of the day sky RV measurements of the 59 fibers.

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