[Paper Review] The CARMENES search for exoplanets around M dwarfs. First visual-channel radial-velocity measurements and orbital parameter updates of seven M-dwarf planetary systems

Trifon Trifonov, M. Kürster+167 more|arXiv (Cornell University)|Oct 4, 2017

Stellar, planetary, and galactic studiesPhysics and Astronomy77 references61 citations

TL;DR

The paper presents first optical radial-velocity measurements from CARMENES for seven M-dwarf planet hosts, updates orbital parameters, and compares CARMENES performance to HARPS and HIRES, confirming planets in most systems and revealing new signals in GJ15 A and GJ1148.

ABSTRACT

Context: The main goal of the CARMENES survey is to find Earth-mass planets around nearby M-dwarf stars. Seven M-dwarfs included in the CARMENES sample had been observed before with HIRES and HARPS and either were reported to have one short period planetary companion (GJ15A, GJ176, GJ436, GJ536 and GJ1148) or are multiple planetary systems (GJ581 and GJ876). Aims: We aim to report new precise optical radial velocity measurements for these planet hosts and test the overall capabilities of CARMENES. Methods: We combined our CARMENES precise Doppler measurements with those available from HIRES and HARPS and derived new orbital parameters for the systems. Bona-fide single planet systems are fitted with a Keplerian model. The multiple planet systems were analyzed using a self-consistent dynamical model and their best fit orbits were tested for long-term stability. Results: We confirm or provide supportive arguments for planets around all the investigated stars except for GJ15A, for which we find that the post-discovery HIRES data and our CARMENES data do not show a signal at 11.4 days. Although we cannot confirm the super-Earth planet GJ15Ab, we show evidence for a possible long-period ($P_{ m c}$ = 7025$_{-629}^{+972}$ d) Saturn-mass ($m_{ m c} \sin i$ = 51.8$_{-5.8}^{+5.5}M_\oplus$) planet around GJ15A. In addition, based on our CARMENES and HIRES data we discover a second planet around GJ1148, for which we estimate a period $P_{ m c}$ = 532.6$_{-2.5}^{+4.1}$ d, eccentricity $e_{ m c}$ = 0.34$_{-0.06}^{+0.05}$ and minimum mass $m_{ m c} \sin i$ = 68.1$_{-2.2}^{+4.9}M_\oplus$. Conclusions: The CARMENES optical radial velocities have similar precision and overall scatter when compared to the Doppler measurements conducted with HARPS and HIRES. We conclude that CARMENES is an instrument that is up to the challenge of discovering rocky planets around low-mass stars.

Motivation & Objective

• Motivate the search for Earth-mass planets around nearby M-dwarfs using precise optical RVs.
• Provide new precise optical RV measurements from CARMENES for seven known planet-hosting M dwarfs.
• Combine CARMENES data with HIRES and HARPS to refine orbital parameters and test planetary signals.
• Assess CARMENES RV performance against HARPS and HIRES and demonstrate its capability for discovering rocky planets around low-mass stars.

Proposed method

• Collect precise optical radial velocities with CARMENES VIS channel (R = 94,600) and derive RVs with the SERVAL pipeline.
• Combine CARMENES RVs with archival HIRES and HARPS data without applying jitter modeling, treating unknown noise as RV scatter.
• Use GLS (Generalized Lomb-Scargle) periodograms to identify significant periodic signals and bootstrap to estimate FAP levels.
• Model bona fide single-planet systems with Keplerian fits; model multi-planet systems with self-consistent N-body dynamics in Jacobi coordinates.
• Apply a nightly zero-point (NZP) correction to CARMENES RVs to mitigate instrument systematics and improve combined fits.

Experimental results

Research questions

• RQ1 Can CARMENES optical RV measurements confirm or refute previously reported planets around seven M-dwarf hosts when combined with HIRES and HARPS data?
• RQ2 What are the updated orbital parameters for these systems under a joint Keplerian (single-planet) and N-body (multi-planet) modeling framework?
• RQ3 How does the precision and scatter of CARMENES VIS RVs compare with HARPS and HIRES for these targets?
• RQ4 Are there new planetary signals in these systems revealed by the combined dataset, and are they dynamically stable?

Key findings

• CARMENES optical RVs show similar precision and scatter to HARPS and HIRES for these stars.
• GJ 15 A: the 11.4 day signal is not recovered; a possible long-period Saturn-mass companion (P ≈ 7030 days, m sin i ≈ 51.8 M⊕) is suggested rather than the previously claimed Ab.
• GJ 1148: a second planet (GJ 1148 c) is discovered with P ≈ 532.6 days, e ≈ 0.342, m sin i ≈ 68.1 M⊕.
• GJ 15 A, GJ 176, GJ 436, GJ 536, GJ 581, and GJ 876: the combined data confirm or strongly support the existence of planets around these stars in the study, with orbital parameters refined.
• NZP correction improves the rms of the time series by ~25% on average for the seven systems, demonstrating the usefulness of nightly zero-point calibration.
• The paper demonstrates that CARMENES is capable of contributing to the discovery and characterization of rocky planets around low-mass stars when combined with existing HARPS/HIRES data.