The Case for an Ultra-High Precision Radial Velocity Spectrograph at Gemini Jacob L

Radial Velocity Spectrograph Bean et al.

The Case for an Ultra-High Precision Radial Velocity Spectrograph at Gemini Jacob L. Bean1, Andrew Szentgyorgyi1, Drake Deming2, & David Latham1 1 Harvard-Smithsonian Center for Astrophysics 2 NASA/Goddard Space Flight Center contact: [email protected]

We advocate here for the implementation of a next-generation optical spectrograph capable of measuring differential radial velocities to a precision of 9 cm s−1, which is equal to the velocity semi-amplitude induced on the Sun by the Earth. The primary anticipated use for this instrument is the discovery and characterization of planets with masses less than 10 M⊕, including potentially habitable planets, as part of both standalone surveys, and in conjunc- tion with transit surveys and transit spectroscopy efforts. The instrument would ideally be installed at the Gemini North telescope to enable Kepler follow-up, and so as to not conﬂict with similar planned instruments on large telescopes in the southern hemisphere.

Background Doppler spectroscopy has arguably been the most important observational technique in the short history of the field of exoplanet science, and the technique could continue to play a critical role in this field despite the advances made with other methods. One reason for this is that radial velocity surveys yield a more statistically complete sample of exoplanets than any other technique. Another reason for the ongoing importance of the radial velocity method is that radial velocity measurements are needed to confirm and measure the masses of planet candidates identified by transit searches. Transiting planets are the only planets for which masses and radii can be measured, and these properties together provide potent insight to the structure and bulk compositions of the planets. The fortuitous geometry of transiting planets also allows their spectra to be obtained in transmission, emission, and reflection via temporal rather than spatial sampling. In contrast, direct imaging could potentially yield the masses and spectra of planets, but will never be able to deliver model-independent radii for planets. Therefore, the combination of transit and orbital Doppler spectroscopy provides the most complete picture of exoplanet characteristics that can be obtained short of actually sending a satellite to another system.

The need for a next generation radial velocity instrument One of the main goals of the ﬁeld of exoplanet science now is to ascertain the frequency, and physical and orbital characteristics of low-mass planets (deﬁned as those with masses at least below 10 M⊕ and especially below 5 M⊕ for the purposes of this document), including those in the habitable zones of main sequence stars. The success of the combination of the radial velocity and transit techniques to study planets so far suggests that a “dynamics-based” approach could effectively address this goal over the next 10 years (Charbonneau & Deming 2007). The main aspects of this approach are:

• The combination of the transit and radial velocity methods to identify and deliver masses and radii for a large, representative sample of low-mass planets so that the range of physical properties of these planets can be ascertained.

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• The combination of the transit and radial velocity methods to identify a few low-mass planets around bright stars for which atmospheric studies can be carried out with the techniques of transit spectroscopy. This includes the search for molecular features in the atmospheres of potentially habitable planets around M dwarfs. • The use of the radial velocity method on its own to obtain a statistically complete census of low-mass planets in the solar neighborhood so that the frequency, orbital parameters, and host star properties of low-mass planets can be investigated.

The essential technology components for this approach are of course radial velocity, transit survey, and transit spectroscopy facilities that can obtain the required sensitivities. The recent launch of Kepler and its demonstrated capabilities (Borucki et al. 2010) indicates that the transit side of the statistics of low-mass planets is on firm ground. Indeed, Kepler has already identified hundreds of low-mass planet candidates in orbital periods out to a few tens of days. As the mission continues, it is likely that more transiting planet candidates will be identified at longer periods, and Kepler ultimately aims to detect a transiting Earth analog. The MEarth ongoing ground-based transit search targeting M dwarfs has already found the first transiting super- Earth around a nearby star (Charbonneau et al. 2009), and this has opened the door for the first atmospheric characterization of a low-mass planet. The proposed missions TESS1 and PLATO2 suggest that the appropriate development of transit identification of low-mass planets around bright stars is also progressing. In addition, the planned capabilities of JWST will enable atmospheric studies of transiting low-mass planets in the habitable zones around bright stars (Deming et al. 2009). Given the ongoing and expected progress on the discovery of transiting planets, the key miss- ing technology component in the dynamics-based approach to studying low-mass exoplanets are appropriate radial velocity facilities. In the last 15 years the techniques for Doppler spectroscopy have progressed to the point that precisions of 1 – 2 m s−1 are routinely obtained on bright stars (e.g., Howard et al. 2009; Bouchy et al. 2009), and precisions of 60 – 80 cm s−1 have been obtained in a few select cases (e.g., Mayor et al. 2009). This level of precision enables the detection of planets with masses down to approximately 5 M⊕ with periods up to a few tens of days around solar-type stars, and down to a few M⊕ in similar periods around M dwarfs. Figure 1 shows the known planets detected with the radial velocity method and the detection limits corresponding to different levels of precision. Despite the past progress on Doppler spectroscopy techniques, the radial velocity detection of low-mass planets orbiting in the habitable zones of solar-type stars requires an approximately order of magnitude improvement over existing capabilities. In addition, the most interesting planets already identified with Kepler, to say nothing of those planets still to be identified during the nominal mission, can not be confirmed and weighed with existing facilities because the radial velocity sig- nals are too small and/or the host stars are too faint (note that Kepler looks at a field from +37.5◦ to +52.5◦ in declination). TESS and PLATO will have the same problem if they fly. Furthermore, the recent super-Earth planet discovered by the MEarth facility was only possible to be confirmed and have its mass measured because it was observable by southern hemisphere facilities. Exclusively northern hemisphere candidates identified by MEarth in the future will be extremely challenging to perform the necessary follow-up radial velocity measurements for, primarily because of the faint- ness of their host stars. Ultimately, the lack of follow-up capabilities for ongoing and future transit

1see http://space.mit.edu/TESS 2see http://www.lesia.obspm.fr/perso/claude-catala/plato web.html

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Figure 1: Comparison of planet masses and orbital periods with radial velocity detection limits. The ﬁlled circles are the known exoplanets detected with the radial velocity method. The crosses are the eight Solar System planets. The lines give the planet mass corresponding to a velocity −1 −1 semi-amplitude of 1 m s (dashed lines) and 9 cm s (solid lines) for a 1.0 M⊙ star (blue lines) and a 0.2 M⊙ star (orange lines). searches suggests that the future atmospheric characterization of low-mass transiting planets with JWST hinges critically on the development of new radial velocity capabilities, especially in the northern hemisphere

Instrument requirements Clearly, a next-generation specialized radial velocity instrument is needed. We advocate for a such an instrument with an achievable precision given by the velocity semi-amplitude induced on the Sun by the Earth: 9 cm s−1. This is the minimum precision needed to detect a true Earth analog as part of a survey (i.e., not an “all eggs in one basket” intense campaign on one or two stars), and would also at least technically enable detection of analogs of all the other Solar System planets except Mercury and Mars. In addition, this level of precision would enable confirmation and mass measurement for many candidate transiting low-mass planets identified by Kepler and other existing or planned surveys to be accomplished as part of routine operations. We acknowledge the possibility that ultimately the radial velocity technique may be limited by astrophysical noise sources like pulsations, granulation, and magnetic field effects, and that 9 cm s−1 may be difficult to impossible to obtain in practice. However, stellar variability at this level is poorly understood, and current instruments have been used up to their technical limits (significantly less than 1 m s−1 for HARPS) for quiet stars without reaching the astrophysical noise floor. Furthermore, many people in the community feel that long-term precisions of at least about

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30 cm s−1 can be obtained for solar-type stars with an appropriate observing strategy3. In light of this, we propose that a next generation radial velocity instrument should be designed so that the technical issues are not the limiting factor in approaching the detection of an Earth analog. An 8 m class telescope is required for an instrument designed to obtain a new level of sensitivity because obtaining just a photon-limited precision of 9 cm s−1 requires a huge number of counts to be recorded. For example, the HARPS instrument on the 3.6 m telescope at La Silla (the current world-leader in efficient high-precision radial velocity measurements) would require 100 minutes to obtain an adequate number of counts for a V = 7.5 solar-type star. This is limiting for large-scale follow-up of low-mass transiting planets identified by bright star transit searches (e.g., TESS and Plato), and impractical for radial velocity only survey work. With an 8 m telescope, the needed exposure times drop to 20 minutes, which makes the above programs feasible. Additionally, the most interesting planets identified by Kepler will orbit stars with magnitudes ranging between 11 and 14, and even higher. Achieving even 1 m s−1 for a large sample of the Kepler candidates is not possible with the best northern hemisphere radial velocity facility now (HIRES on Keck) because of its low efficiency. With a HARPS style instrument on an 8 m telescope 1 m s−1 radial velocity precision could be obtained for a V = 12.5 solar-type star in 20 minutes. This would allow large-scale confirmation and mass measurement for low-mass planet candidates that are already known from Kepler, but that can’t be fully studied at the moment. Furthermore, the low noise floor of the proposed instrument means that a select sample of Kepler candidates requiring sub m s−1 precision could simply receive more intense scrutiny. In the photon-limited regime the only constraint is the amount of telescope time deemed appropriate to spend on a given planet. An 8 m class telescope is also needed to take advantage of the opportunity offered by M dwarf stars. Because of their low-mass, it is possible to detect planets around these stars with lower masses for a given level of radial velocity precision, and because of their small size, it is also possible to detect and study the atmospheres of smaller transiting planets for a given level of pho- tometric precision. In addition, their low intrinsic luminosity brings the habitable zone inwards to periods where there is a much higher likelihood of observing transits. All of these factors taken together make M dwarfs the prime candidates for hosting the first habitable planet for which spectra could be obtained (using the techniques of transit spectroscopy, Deming et al. 2009). With the proposed instrument, 1 m s−1 could be obtained in 15 minutes or less for M dwarfs out to V magnitudes of 13.1. This is 2.5 magnitudes deeper than what can be done with HARPS and, therefore, significantly increases the numbers of M dwarfs for which the sensitivity to low-mass planets can be obtained. For example, the TESS mission is expected to discover many tens of super-Earth planets around M dwarfs brighter than 13.1, but likely fewer than six around M dwarfs brighter than 10.6. Being able to go deeper with the proposed instrument over existing facilities is critical for increasing the chance that a subset of the planets that can be confirmed and have their masses measured are in the habitable zones of their host stars. These planets could be the prime targets of atmospheric investigations using JWST, so it is imperative to be able to detect them within the same timescale that JWST is expected to be operational.

Design considerations The basic concept of the instrument we propose would likely have to be based on the HARPS approach – that is, a high-resolution optical echelle spectrograph that is highly-stabilized. Highly-

3See for example many of the presentations at a recent workshop: http://exoplanets.astro.psu.edu/ workshop/program.html

Page 4 of 5 Radial Velocity Spectrograph Bean et al. stabilized means exquisite temperature and pressure control and no moving parts. The instrument should be fiber fed so that it can be bench-mounted on a gravity invariant platform. This also has the advantage in that the fibers would provide (or at least play an important role in) the necessary image scrambling to eliminate variations in the spectrograph illumination as a source of radial velocity error. A single arm spectrograph could deliver spectra from roughly 400 – 800 nm, where the lower cutoff is set by considering the drop in reflectance from the silver-coated telescope mirrors, and red coverage would be important for enabling efficient targeting of M dwarfs. The nominal resolving power would have to be approximately 150 000, but it should be possible to include a “high-efficiency” mode option that would downgrade this to offer improved efficiency for generic stellar spectroscopy. Reaching the goal of 9 cm s−1 radial velocity precision will certainly necessitate some signif- icant advances over HARPS. In particular, the environmental control, calibration strategy (ThAr lamps and iodine cells are likely not sufficient), and image scrambling would need to be improved. These issues should be studied in detail during a Phase A-type study. However, there are already promising candidates for solutions to all of these problems. The cost of the instrument would probably be in the neighborhood of 15 million US dollars including hardware, manpower, and all necessary software. The instrument would ideally be located on Gemini-North to enable Kepler follow-up, and so as to not conflict with similar planned instruments on large telescopes in the southern hemisphere (e.g., ESPRESSO on the VLT and G-CLEF for the GMT).

Context in the US Decadal Survey The recently released pre-publication report from the 2010 US Decadal Survey4 gives high priority to both the science goals discussed here, and the speciﬁc approach of utilizing high-precision radial velocity measurements to pursue these goals. In particular, the report states “NASA and NSF should support an aggressive program of ground-based high-precision radial velocity surveys” (p. 1-8), “[radial velocity] surveys will need new spectrometers and signiﬁcant time allocation on 8- 10m class telescopes” (p. D-2), and “the committee recommends ... combined exploitation of the current Kepler mission ... and a vigorous ground-based research program” (p. 1-23). Furthermore, the report supports the effort to reach an unprecedented radial velocity precision of approximately 9 cm s−1 (p. 7-8). The Decadal Survey committee has also made these recommendations in light of anticipated funding constraints. Therefore, it seems possible that such an effort as we have proposed could have the necessary support from the US side of the Gemini partnership.

References Borucki, W. J. et al. 2010, arXiv:1006.2799 Bouchy, F. et al. 2009, A&A, 496, 527 Charbonneau, D. & Deming, D. 2007, arXiv:0706.1047 Charbonneau, D. et al. 2009, Nature, 462, 891 Deming, D. et al. 2009, PASP, 121, 952 Howard, A. et al. 2009, ApJ, 696, 75 Mayor, M, et al. 2009, A&A, 493, 639

4http://www.nap.edu/catalog.php?record id=12951

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