Precise Stellar Radial Velocities of an M Dwarf with a Michelson Interferometer and a Medium-Resolution Near-Infrared Spectrograph

Abstract

Precise near-infrared radial velocimetry enables efficient detection and transit verification of low-mass extrasolar planets orbiting M dwarf hosts, which are faint for visible-wavelength radial velocity surveys. The TripleSpec Exoplanet Discovery Instrument, or TEDI, is the combination of a variable-delay Michelson interferometer and a medium-resolution (R=2700) near-infrared spectrograph on the Palomar 200" Hale Telescope. We used TEDI to monitor GJ 699, a nearby mid-M dwarf, over 11 nights spread across 3 months. Analysis of 106 independent observations reveals a root-mean-square precision of less than 37 m s −1 for 5 minutes of integration time. This performance is within a factor of 2 of our expected photon-limited precision. We further decompose the residuals into a 33 m s −1 white noise component, and a 15 m s −1 systematic noise component, which we identify as likely due to contamination by telluric absorption lines. With further development this technique holds promise for broad implementation on medium-resolution near-infrared spectrographs to search for low-mass exoplanets orbiting M dwarfs, and to verify low-mass transit candidates. (Chabrier 2003). Combining the M V luminosity function for single main-sequence field stars (Wielen et al. 1983) with the distance modulus, one can calculate the relative number of single main-sequence stars in a magnitude-limited wide-field transit survey. Comparing the number of single, main sequence stars with M V from 2.5 to 8.5, corresponding to F, G and K dwarfs, to the number with M V from 8.5 to 18.5, corresponding to M-type dwarfs, one finds that F, G and K dwarfs outnumber M dwarfs in magnitude-limited wide-field survey by more than 1000 to 1. To overcome this limitation, the MEarth transit survey is individually targeting bright M dwarfs, rather than staring at a single field (Nutzman & Charbonneau 2008) at the expense of continuous coverage. Despite the challenges, both visible-wavelength radial velocity surveys and transit surveys have had success with M dwarfs. As of now, 27 planets have been detected around 21 M dwarfs, and two are known to transit their host stars: GJ 1214 b and GJ 436 b. All hosts have a spectral type earlier than M5. GJ 1214 b was initially detected by the MEarth transit survey, then confirmed by radial velocity measurements (Charbonneau et al. 2009), and GJ 436 b was initially discovered by a radial velocity survey (Butler et al. 2004) with the transit detection occurring later (Gillon et al. 2007). At this stage it is unclear whether the majority of future M dwarf transiting planets will be detected first by transit surveys then followed up with radial velocity measurements, or vice-versa, given that both individually target M dwarfs. However, the Kepler Mission is a sensitive wide-field transit survey observing approximately 2500 M dwarfs brighter than V = 14. If low-mass planets are common around M dwarfs, Kepler will potentially detect dozens of transiting candidates around M dwarfs which are bright enough for follow up radial veloci...

High-resolution broadband spectroscopy using externally dispersed interferometry at the Hale telescope: Part 1, data analysis and results

“…1) with a version called TEDI, (the T denotes the TripleSpec 16 spectrograph to which it was coupled). An earlier report 2 describes the Doppler velocimetry, and this report describes the spectroscopy. This is the first time multiple delays have been used on starlight to recreate a high-resolution spectrum.…”

“…A technique called externally dispersed interferometry 1 (EDI) can dramatically reduce the necessary size of spectrographs for Doppler radial velocimetry [1][2][3][4][5][6][7][8][9] and high-resolution spectroscopy. [10][11][12][13][14][15] Other workers have adopted the EDI method from our laboratories and demonstrated a Doppler planet detection.…”

“…Detailed instrument description is in Ref. 2 and instrument theory for a single delay spectroscopy is (1) heading for TripleSpec and uses a dichroic split input tracking system (3) to mixing in ThAr lamp (5) to feed a symmetric, collimated interferometry cavity (beamsplitter 10). One interferometer output (13) is dichroic split (14) to a sensitive chopped IR diode (15) for flux maximization and a fringe tracking camera (16) for cavity nulling.…”

Dramatic robustness of a multiple delay dispersed interferometer to spectrograph errors: how mixing delays reduces or cancels wavelength drift

“…1) with a version called TEDI, (the T denotes the TripleSpec 18 spectrograph to which it was coupled). An earlier report 7 describes the Doppler velocimetry, and this report describes the spectroscopy. Figure 2b is an example of EDI resolving a closely spaced ThAr line pair (red curve) otherwise unresolvable by the native spectrograph (green curve).…”

“…A novel technique called externally dispersed interferometry 1 (EDI) can dramatically reduce the necessary size of spectrographs for Doppler radial velocimetry [1][2][3][4][5][6][7][8][9] and high resolution spectroscopy, [10][11][12][13][14][15][16][17] and dramatically increase their robustness to PSF errors in the native spectrograph. Other workers have adopted the EDI method from our laboratories and demonstrated a Doppler planet detection.…”