The Solar Neighborhood LV: Spectral Characterization of an Equatorial Sample of 580 K Dwarfs

TL;DR

We present a high-resolution spectroscopic census of 580 nearby K dwarfs (3600–5500 K) within 33 pc, deriving $T_{ m eff}$, $[Fe/H]$, $\,\log g$, and $v\sin i$ with Empirical SpecMatch, while assessing chromospheric activity via $EW$(Hα) and youth via $EW$(Li I) and moving-group membership. A subset of 51 stars is identified as young/active (8.8%), with 19 showing Li I absorption and 36 with Hα activity, complemented by kinematic identifications aligning 7 Hyades and 4 AB Dor members, yielding 11 moving-group candidates and 8 spectroscopic outliers; together these yield a refined view of the solar-neighborhood K-dwarf population. Galactic context from Gaia DR3 and CHIRON radial velocities shows 80% thin-disk, 18.4% thick-disk, and 1 halo star, with metal-poor stars ($[Fe/H]\le -0.5$) comprising 4% and largely inactive. Cross-matching with NASA Exoplanet Archive reveals 44 planet-hosting K dwarfs (72 planets as of 2025-07), with giant planets preferentially around metal-rich hosts, while smaller planets span the full metallicity range. The resulting catalog identifies 529 mature, inactive K dwarfs as prime, long-lived targets for terrestrial planet searches and habitability assessments in the solar neighborhood, supported by a publicly available spectral gallery of high‑SNR CHIRON spectra. The work provides a foundation for understanding exoplanet demographics, stellar activity, and Galactic population structure as it pertains to habitability around nearby K dwarfs.

Abstract

We present a spectroscopic characterization of 580 K dwarfs within 33 pc, observed with the CHIRON echelle spectrograph (R=80,000) on the SMARTS 1.5m telescope. This volume-limited sample is part of the RKSTAR survey of $\sim$4400 K dwarf primaries within 50 pc. Using Empirical SpecMatch and the diagnostic lines H-alpha (6562.8 Angstrom) and Li I (6707.8 Angstrom), we derive stellar properties, activity status, and age indicators calibrated against 35 benchmark K dwarfs with ages from 20 Myr to 5 Gyr. We find that 7.4% (43 stars) exhibit signatures of youth and/or chromospheric activity: 19 stars show lithium absorption indicating ages $<$1 Gyr, and 36 display H$α$ emission. Kinematic analysis using BANYAN $Σ$ identifies 8 additional young stars through membership in the AB Doradus moving group and the Hyades cluster, bringing the total young/active population to 8.8% (51 stars). Stellar parameters span 3600--5500 K in \teff, $-$0.60 to $+$0.55 dex in [Fe/H], and $<$10 to $>$25 km s$^{-1}$ in $v\sin i$. A metal-poor population ([Fe/H] $\leq -$0.50 dex) comprises 4\% of the sample. Galactic kinematics place 80% in the thin disk and 18.4% in the thick disk, with one halo member (HD 134439). Young and active stars are predominantly thin disk members, with two thick disk exceptions. Cross-matching with NASA's Exoplanet Archive reveals only 7.5% (44 stars) host confirmed planets as of July 2025. Our results identify 529 mature, inactive K dwarfs as prime targets for terrestrial planet searches, providing a crucial resource for exoplanet habitability studies in the solar neighborhood.

Figures (20)

• Figure 1: Left: HR diagram displaying the RECONS sample of stars within 25 pc (grey dots),with confirmed exoplanet hosts as of June 2025 from the NASA Exoplanet Archive nasaexoplanetarchive highlighted in red. Dashed blue lines indicate the K0 and M0 dwarf boundaries used to select K dwarfs for this work. Right: HR diagram showing the same 25 pc sample in grey with our 580 K dwarfs (orange triangles) from the equatorial 33.3 pc survey. The sequence of objects above the main sequence indicates young and/or unresolved multiple stars, while subdwarfs appear along the lower envelope. Magnitude data are from $Gaia$ DR3 ($BP$) and 2MASS ($K=K_s$) with absolute magnitudes derived using $Gaia$ DR3 parallaxes.
• Figure 2: Left: Bowtie plot displaying declination (Dec) (circular direction) and distance (radial direction) for the survey sample of 580 K dwarfs. Right: Polar plot illustrating Right Ascension (R.A.) (circular direction) and distance (radial direction) for the survey sample of 580 K dwarfs. R.A. and Dec positions are based on J2000 coordinates, while distance values were derived from $Gaia$ DR3 parallax measurements.
• Figure 3: Examples of K dwarf spectra (blue) showing activity and age indicators with EW measurement methods. Voigt profiles (green), local continuum levels (yellow), and integration windows (purple) are shown. Top row: Li I absorption measured via Voigt fitting. Middle row: H$\alpha$ absorption measured via Voigt fitting. Bottom row: H$\alpha$ emission or filled-in profiles measured via the window method using specutils. Final EWs were computed by integrating flux over a nominal 2.1Å window (purple) and four additional windows at ±10% and ±20% of the nominal width (dashed red lines).
• Figure 4: Distribution of $[Fe/H]$ and $T_{eff}$ values determined via ESM for 571 of the 580 K dwarfs in our survey sample (orange plus symbols). The 215 stars within this $T_{eff}$ & $[Fe/H]$ range from the ESM library yee17 are shown as blue dots for comparison. The purple shaded regions highlight areas where the ESM library coverage is sparse, potentially affecting the reliability of derived parameters in these regions. The histogram on the y-axis shows the metallicity distribution of our sample in 0.1 dex bins, with a mean value of $-$0.02 dex.
• Figure 5: A sequence of spectra showing the effects of rotation on line profiles. The left panel displays a $\sim$60 Å window including H$\alpha$ at 6563 Å, while the right panel shows $\sim$50 Å window including Li I at 6707.8 Å and a Ca I line at 6717 Å. From bottom to top: $\epsilon$ Indi ($vsini$$<$ 2 km s$^{-1}$, slow rotator), RKS1855+2333 ($vsini$ = 15 km s$^{-1}$) and RKS0932-1111 ($vsini$ = 27 km s$^{-1}$) representing fast rotators from our survey sample of 580 K dwarfs, and AB Dor ($vsini$$>$ 50 km s$^{-1}$) and LO Peg ($vsini$$>$ 50 km s$^{-1}$) representing extremely fast rotators from the benchmark sample used to calibrate age and activity relationships, as described in hubbardjames22. The progressive line broadening and eventual smearing of spectral features with increasing rotation is clearly evident in both wavelength regions.