State of the Field: Extreme Precision Radial Velocities (1602.07939v2)

Abstract: The Second Workshop on Extreme Precision Radial Velocities defined circa 2015 the state of the art Doppler precision and identified the critical path challenges for reaching 10 cm/s measurement precision. The presentations and discussion of key issues for instrumentation and data analysis and the workshop recommendations for achieving this precision are summarized here. Beginning with the HARPS spectrograph, technological advances for precision radial velocity measurements have focused on building extremely stable instruments. To reach still higher precision, future spectrometers will need to produce even higher fidelity spectra. This should be possible with improved environmental control, greater stability in the illumination of the spectrometer optics, better detectors, more precise wavelength calibration, and broader bandwidth spectra. Key data analysis challenges for the precision radial velocity community include distinguishing center of mass Keplerian motion from photospheric velocities, and the proper treatment of telluric contamination. Success here is coupled to the instrument design, but also requires the implementation of robust statistical and modeling techniques. Center of mass velocities produce Doppler shifts that affect every line identically, while photospheric velocities produce line profile asymmetries with wavelength and temporal dependencies that are different from Keplerian signals. Exoplanets are an important subfield of astronomy and there has been an impressive rate of discovery over the past two decades. Higher precision radial velocity measurements are required to serve as a discovery technique for potentially habitable worlds and to characterize detections from transit missions. The future of exoplanet science has very different trajectories depending on the precision that can ultimately be achieved with Doppler measurements.

• The paper presents how innovations such as HARPS, HARPS-N, and laser frequency combs improve calibration stability toward 10 cm/s precision.
• The paper details the use of Gaussian processes and Bayesian frameworks to disentangle stellar jitter from true Keplerian signals.
• The paper advocates for international collaboration and advanced telescopic investments to support future exoplanet detection missions.

Extreme Precision Radial Velocities: Current Status and Challenges

The paper "State of the Field: Extreme Precision Radial Velocities," authored by Debra A. Fischer and a broad collaboration, offers a detailed account of the advancements and challenges faced by contemporary extreme precision radial velocity (EPRV) techniques in astronomy. With an emphasis on reaching a measurement precision of 10 cm s$^{-1}$, the document systematically evaluates the instrumental developments, the statistical approaches for data analysis, and the enduring obstacles primarily posed by stellar activity to enable precise Doppler measurements.

Key Achievements in Instrumentation

Prominent among the strides in instrumentation is the use of HARPS, a spectrograph that has pioneered stability via a vacuum-enclosed design with a stringent temperature control of $17^\circ \pm 0.01^\circ$ C. Its counterpart, HARPS-N, enhances these design elements, emphasizing fiber-fed systems that increase illumination stability through octagonal fibers and a double scrambler setup. These instruments characterize the state of the art, reaching a single measurement precision of about 0.8 m s$^{-1}$ at a signal-to-noise ratio (SNR) of 200.

Significant progress has been made with the integration of laser frequency combs (LFCs) as sophisticated wavelength calibrators. These systems promise a consistent and stable absolute wavelength reference across broad spectroscopic bandwidths, crucial for achieving the targeted 10 cm s$^{-1}$ precision. The adoption of LFCs represents a paradigm shift from traditional thorium-argon lamps, especially given the emerging challenges in acquiring quality ThAr sources.

Challenges and Prospective Solutions

Despite the advancements in instrumentation, a substantial limitation to precision remains the stellar 'jitter'—velocity perturbations arising from stellar activity. The challenge lies in differentiating these photospheric velocities from true center-of-mass Keplerian motions. Existing funnels like spectral line bisectors and the full width half maximum (FWHM) of cross-correlation functions have proven insufficient for quieter stars due to their low rotation speeds. Progress demands novel statistical approaches, such as those employing Gaussian processes (GP) to model stellar jitter alongside Keplerian signals, allowing for superior disentangling of the noise from the astrophysical velocity signals.

The community has initiated collaborative endeavors, like the RV fitting challenge, which leverage Bayesian approaches with red noise models to refine detection capabilities of low amplitude signals. Outcomes indicate that while a Bayesian framework, particularly those including red noise models, provides better signal recovery, only a fraction of signals below 1 m s$^{-1}$ are currently detectable with confidence.

Future Trajectories and Implications

The paper suggests that reaching the 10 cm s$^{-1}$ precision threshold requires a coordinated international investment in instrumentation and methodology. A call is made for dedicated medium to large aperture telescopes equipped with optical to near-infrared dual spectrometers, high resolving power, and state-of-the-art wavelength calibration. This trajectory would significantly benefit ongoing programs like TESS and future missions, such as PLATO and CHEOPS, by providing vital mass measurements required for characterizing discovered exoplanets.

Moreover, the implications extend beyond mere detection into broader astrophysical applications. Discovering and characterizing habitable zone planets in the solar neighborhood could benefit planned direct imaging missions by honing target lists and providing accurate prior data on planetary masses and orbits.

In conclusion, the quest for extreme precision in radial velocity measurements is pivotal for the next decade of exoplanet science, bridging the gap between detection and characterization of Earth analogs and guiding future direct imaging endeavors. Continued international collaboration and iterative advancements in both instrument technology and noise modeling stand as the keystones for unlocking the next frontier of discovery in exoplanetary science.