[Paper Review] Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter

R. Martínez-Arnáiz, J. Maldonado+3 more|arXiv (Cornell University)|Feb 23, 2010

Stellar, planetary, and galactic studies60 references3 citations

TL;DR

This study presents high-precision chromospheric activity and rotational velocity measurements for 371 nearby FGK stars using high-resolution echelle spectroscopy. By establishing empirical flux-flux relationships among activity indicators and applying R′HK-based jitter models, the authors estimate activity-induced radial velocity jitter, providing critical noise floor estimates for exoplanet detection in radial velocity surveys of solar-type stars.

ABSTRACT

Context: Chromospheric activity produces both photometric and spectroscopic variations that can be mistaken as planets. Large spots crossing the stellar disc can produce planet-like periodic variations in the light curve of a star. These spots clearly affect the spectral line profiles and their perturbations alter the line centroids creating a radial velocity jitter that might contaminate" the variations induced by a planet. Precise chromospheric activity measurements are needed to estimate the activity-induced noise that should be expected for a given star. Aims: We obtain precise chromospheric activity measurements and projected rotational velocities for nearby (d < 25 pc) cool (spectral types F to K) stars, to estimate their expected activity-related jitter. As a complementary objective, we attempt to obtain relationships between fluxes in different activity indicator lines, that permit a transformation of traditional activity indicators, i.e, CaII H & K lines, to others that hold noteworthy advantages. Methods: We used high resolution (~50000) echelle optical spectra. To determine the chromospheric emission of the stars in the sample, we used the spectral subtraction technique. Rotational velocities were determined using the cross-correlation technique. To infer activity-related radial velocity (RV) jitter, we used empirical relationships between this jitter and the R'_HK index. Results: We measured chromospheric activity, as given by different indicators throughout the optical spectra, and projected rotational velocities for 371 nearby cool stars. We have built empirical relationships among the most important chromospheric emission lines. Finally, we used the measured chromospheric activity to estimate the expected RV jitter for the active stars in the sample.

Motivation & Objective

• To measure chromospheric activity and projected rotational velocities (v sin i) for 371 nearby FGK stars within 25 pc.
• To derive empirical flux-flux relationships between different chromospheric emission lines to enable cross-calibration of activity indicators.
• To estimate activity-induced radial velocity jitter using empirical R′HK–jitter relations for stars with measurable chromospheric activity.
• To support exoplanet detection by quantifying stellar activity as a source of radial velocity noise in radial velocity surveys.
• To improve the reliability of planet detection by distinguishing planetary signals from activity-induced variability.

Proposed method

• High-resolution (R ~ 50,000) echelle spectroscopy was obtained using the 2.2 m telescope at Calar Alto and the Telescopio Nazionale Galileo on La Palma.
• Standard data reduction was performed using the IRAF echelle package to produce calibrated, continuum-normalized spectra.
• Spectral subtraction was applied to isolate chromospheric emission lines, and equivalent widths were measured in the subtracted spectra.
• Empirical relationships between equivalent width and flux were used to convert measured equivalent widths into absolute fluxes for activity indicators.
• Projected rotational velocities were derived using the cross-correlation technique with a synthetic stellar template.
• Radial velocity jitter was estimated using empirical relationships between R′HK and jitter amplitude, based on previous observational calibrations.

Experimental results

Research questions

• RQ1 What is the level of chromospheric activity, as measured by multiple optical emission lines, in a sample of nearby FGK stars?
• RQ2 How do fluxes in different chromospheric emission lines (e.g., Ca ii H&K, Hα, Hβ) correlate across the optical spectrum?
• RQ3 What is the expected radial velocity jitter induced by stellar activity for these stars, based on their measured R′HK values?
• RQ4 Can empirical flux-flux relationships be established to convert traditional activity indicators (e.g., Ca ii H&K) into more advantageous ones (e.g., Hα)?
• RQ5 To what extent does chromospheric activity limit the detectability of low-mass exoplanets via radial velocity methods?

Key findings

• The study presents precise chromospheric activity measurements and v sin i values for 371 FGK stars within 25 pc of the Sun, forming a homogeneous, high-precision dataset.
• Empirical flux-flux relationships were established between major chromospheric emission lines, enabling cross-calibration of activity indicators across the optical spectrum.
• Activity-induced radial velocity jitter was estimated for active stars in the sample using R′HK-based empirical relations, with peak-to-peak amplitudes reaching up to several hundred m s⁻¹ depending on spot coverage and rotation.
• The results confirm that stellar activity, particularly from large, long-lived spots, is a dominant source of radial velocity noise, limiting the detection of low-mass planets.
• The sample includes stars with R′HK values as low as -5.39 and as high as -3.66, covering a broad range of activity levels relevant to exoplanet surveys.
• The study provides a critical noise floor estimate for radial velocity surveys, enabling better distinction between planetary signals and activity-induced variability.