The Solar Neighborhood L Spectroscopic Discovery of K Dwarfs Younger Than 1 Gyr and New Binaries within 30 pc
2025-05-06
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The Solar Neighborhood L: Spectroscopic Discovery of K Dwarfs Younger Than 1 Gyr
and New Binaries within 30 pc
Hodari-Sadiki Hubbard-James
1,2,4
, D. Xavier Lesley
2,3
, Todd J. Henry
2,4
, Leonardo A. Paredes
1,2,4
, and
Azmain H. Nisak
1,4
1
Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30302, USA; hjames12@gsu.edu
2
RECONS Institute, Chambersburg, PA 17201, USA
3
Department of Physics, Southern Connecticut State University, New Haven, CT 06515, USA
Received 2022 February 10; revised 2022 August 25; accepted 2022 August 25; published 2022 October 5
Abstract
As part of a comprehensive effort to characterize the nearest stars, the CHIRON echelle spectrograph on the CTIO/
SMARTS 1.5 m telescope is being used to acquire high-resolution (R=80,000)spectra of K dwarfs within 50 pc.
This paper provides spectral details about 35 K dwarfs from five benchmark sets with estimated ages spanning 20
Myr–5.7 Gyr. Four spectral age and activity indicators are tested, three of which aligned with the estimated ages of
the benchmark groups—the Na Idoublet (5889.95 and 5895.92 Å), the Hαline (6562.8 Å), and the Li Iresonance
line (6707.8 Å). The benchmark stars are then used to evaluate seven field K dwarfs exhibiting variable radial
velocities for which initial CHIRON data did not show obvious companions. Two of these stars are estimated to be
younger than 700 Myr, while one exhibits stellar activity unusual for older K-dwarf field stars and is possibly
young. The four remaining stars turn out to be spectroscopic binaries, two of which are being reported here for the
first time with orbital periods found using CHIRON data. Spectral analysis of the combined sample of 42
benchmark and variable radial velocity stars indicates temperatures ranging from 3900 to 5300 K and metallicities
from −0.4 <[Fe/H]<+0.2. We also determine glog 4.5 4.7–=for main-sequence K dwarfs. Ultimately, this
study will target several thousand of the nearest K dwarfs and provide results that will serve present and future
studies of stellar astrophysics and exoplanet habitability.
Unified Astronomy Thesaurus concepts: K dwarf stars (876);Late-type dwarf stars (906);Solar neighbourhood
(1509);Spectroscopy (1558);Stellar activity (1580);Stellar ages (1581);Stellar associations (1582);Stellar
properties (1624)
1. Introduction
A key sample of stars to be explored for other planetary
systems includes the Sun’s slightly less massive, cooler cousins
known as K dwarfs. In the solar neighborhood, K dwarfs
account for ∼12% of all stars (Henry et al. 2006; with updates
at www.recons.org), and their longevity makes them ideal hosts
for habitable planets. A critical aspect in evaluating an
individual planet’s environment is the age of the star it orbits,
i.e., is the star young, adolescent, or mature? For example,
stellar youth is often associated with elevated ultraviolet
luminosity, chromospheric flares, and a general increase in
stellar activity, all factors linked to the magnetic dynamo at
the star’s core (Davenport et al. 2019). Skumanich (1972)
discussed the link between stellar age and activity, describing
how magnetic breaking would reduce the rotation speed of a
star and dampen its dynamo, causing flares to weaken
(Skumanich 1986). Consequently, young and active host stars
are unlikely to provide stable environments and present
challenges to astronomers attempting to estimate the locations
of habitable realms for orbiting planets (Segura et al. 2010).
Instead, post-adolescent hosts that have low levels of stellar
activity (Luger et al. 2015)are preferred, given that they
provide steadier, more durable conditions.
Our study aims to provide statistics on the youth and maturity of
the nearby K-dwarf population, here defined to be stars within
50 pc. This is part of a larger REsearch Consortium On Nearby
Stars (RECONS; www.recons.org)effort to characterize the closest
∼5000 K dwarfs. These stars are arguably among the best of all
stars to host habitable planets (Cuntz & Guinan 2016), and our
work will provide a list of key targets for exoplanet surveys from
the ground, as well as for both current and future exoplanet
atmosphere studies from space-based platforms, e.g., NASA’s
Hubble Space Telescope (HST)and James Webb Space Telescope
(JWST), as well as the European Space Agency’s upcoming
Twinkle mission (Mollière et al. 2017; Edwards et al. 2019).
Currently, the only star with a precise and fundamental
stellar age measurement is the Sun, determined via calculations
related to the decay of radioactive isotopes in meteorites
(Soderblom et al. 2014). All other age estimates are less
accurate and come from measuring semi-fundamental proper-
ties of stars, or are model derived and based on other scientific
inferences (Soderblom 2010; Soderblom et al. 2014). Popular
techniques used to estimate ages for G-, K-, and M-type dwarfs
include lithium depletion, utilized in Skumanich (1972), White
et al. (2007), López-Santiago et al. (2010), and Binks & Jeffries
(2014); kinematic motions in the Galaxy, highlighted in López-
Santiago et al. (2010)and Mamajek & Bell (2014); and
gyrochronology or stellar rotation studies, such as those of
Brandt & Huang (2015), Gossage et al. (2018), and Skumanich
(1972). These methods are often combined, in order to improve
the reliability of the age estimates (Soderblom 2010).
The Astronomical Journal, 164:174 (20pp), 2022 November https://doi.org/10.3847/1538-3881/ac8d6a
© 2022. The Author(s). Published by the American Astronomical Society.
4
Visiting Astronomer, Cerro Tololo Inter-American Observatory. CTIO is
operated by AURA, Inc., under contract to the National Science Foundation.
Original content from this work may be used under the terms
of the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s)and the title
of the work, journal citation and DOI.
1
In addition to age, other stellar parameters, such as effective
temperature, metallicity, surface gravity, and rotational velo-
city, come into play when estimating a planetary system’s
potential habitability. For example, an accurate measurement of
a star’s effective temperature is a prerequisite to setting the
boundaries of the so-called “Goldilocks Zone,”while metalli-
city may be an indicator of the amount of planet-building
material available in the system. A low surface gravity points to
a star that either has not yet reached the main sequence or has
evolved off of it, while estimating a star’s rotational velocity
gives insight into the stellar activity affecting any surrounding
planetary system (Cuntz & Guinan 2016; Brandt & Huang
2015). Overall, the assessment of any particular star as a host
of an orbiting planet requires observations that can be used
to evaluate vital stellar parameters like temperature, supple-
mented with additional information related to age, activity, and
multiplicity.
In this work we investigate the utility of spectral features at
optical wavelengths observed in K dwarfs that are potentially
linked to age—the Na Idoubletat5890/5896 Å,theHαline at
6563 Å,theLi
Iresonance line at 6708 Å, and one of the Ca II
infrared triplet lines at 8542 Å—to create a rubric for estimating
the ages and activity levels of these stars. We also utilize the
Empirical SpecMatch code of Yee et al. (2017)to derive stellar
parameters through spectral modeling. To accomplish these tasks,
we use high-resolution spectra (R=80,000)from the Small and
Moderate Aperture Research Telescope System (SMARTS)1.5 m
telescope and CHIRON echelle spectrograph for 35 K dwarfs in
five groups that have age estimates. Our rubric is then tested on an
initial set of seven nearby K dwarfs that exhibited variable radial
velocities (RVs)in early CHIRON data sets. These stars may be
particularly young or active, or the variations could be caused by
companions. Stellar parameters, including line equivalent widths
(EWs), temperatures, metallicities, surface gravities, and rotational
velocities, are evaluated for all 42 stars analyzed in this study. As
an ensemble, these stars constitute a benchmark sample of young
stars of known age, a set of nearby K dwarfs of moderate ages
determined using isochrones, and a number of new nearby young
star candidates, all observed methodically with the same
instrument and with parameters derived uniformly.
2. Sample Selection
There are two samples of K dwarfs targeted in this effort—a
primary sample of (presumably)field stars and a benchmark
comparison sample of stars with estimated ages. The Hertz-
sprung–Russell (H-R)diagrams presented in Figure 1(left
panel is an overview of stars within 25 pc; right panel is a
zoom-in of the K-dwarf region)show that the majority of field
K dwarfs and those from older groups are located on the main
sequence, whereas the younger stars in the benchmark sample
are sometimes located above the main sequence. Here we
define K dwarfs as having Gaia EDR3 absolute BP (hereafter
B
G
)magnitudes of M5.0 8.5
BG=-(equivalent to 5.8–8.8 in
absolute Johnson Vphotometry)and colors of B
G
–K=2.0–4.0
(1.9–3.7 in V–Kcolor).
5
These ranges are based on work done
by the RECONS group and Eric Mamajek’s spectral notes.
6
In the right panel of Figure 1it is apparent that stars of the β
Pic moving group, the youngest sample examined here with
ages ∼20 Myr (see Table 1), lie clearly above the main
sequence. As evident in their spectra (described in Section 5),
these pre-main-sequence stars exhibit significant chromo-
spheric activity. They are larger and brighter because they are
still settling onto the main sequence, as their outer layers are
contracting and their internal temperatures are increasing. With
ages of ∼40 Myr, K-dwarf members of the Tuc-Hor group are
also noticeably above the main sequence but are elevated less
than βPic K dwarfs. By ages of ∼145 Myr, members of the
AB Dor moving group are found at positions indistinguishable
from the main sequence. The Hyades cluster, field stars, and
RV variable (RVV)stars all lie firmly on the main sequence,
with the exception of the dwarf BD +05 2529, which has
Figure 1. Left: H-R diagram highlighting the members of the benchmark sample targeted to map spectral features related to age and activity for K dwarfs. Points are
color-coded as in the legend for βPic, Tuc-Hor, AB Dor, Hyades, field K dwarfs, and RV variable dwarfs (RVV). Smaller gray points represent stars in the RECONS
25 pc sample. Right: zoomed-in view to focus on the stars investigated in this paper, with the same background stars present. Magnitude information is obtained from
Gaia EDR3 (BP =B
G
)and Two Micron All Sky Survey (2MASS; K
s
=K). Absolute B
G
magnitudes were derived using Gaia EDR3 parallax measurements.
Guidelines for spectral types are given in both panels.
5
GAIA EDR3 Documentation: Relationships with other photometric systems
6
http://www.pas.rochester.edu/~emamajek
2
The Astronomical Journal, 164:174 (20pp), 2022 November Hubbard-James et al.
recently been found to be a spectroscopic binary (Sperauskas
et al. 2019)and is confirmed via our CHIRON data. Thus, only
for ages less than ∼50 Myr can young K dwarfs be identified
via their positions on the H-R diagram, at least relative to the
ensemble of mixed stars in the solar neighborhood.
2.1. Primary Sample of Field K Dwarfs
Of the ∼5000 K-dwarf systems
7
in the full 50 pc sample, the
first author’s Ph.D. work focuses on the ∼1200 systems within
40 pc and lying in the equatorial band of the sky from decl.
+30°to −30°. These stars have been selected using Gaia
parallax measurements (Gaia Collaboration et al. 2016,2018;
Lindegren et al. 2018)and continue to be vetted to provide,
ultimately, a volume-limited and volume-complete sample.
Sample construction continues because parallaxes may change
slightly in future Gaia Data Releases and new K dwarfs may be
added because solutions are not yet available. Some stars will
be removed because the goal is to include systems with
K-dwarf primaries, i.e., if white dwarfs are found, the system
will be dropped from the primary sample because the white
dwarf progenitor was originally more massive. We note that
systems with white dwarf primaries are useful for age
determinations, so these systems will continue to be considered
for age calibration work but not included in the statistics for
K-dwarf samples and their companions. There are more than
1200 K dwarfs in the equatorial 40 pc sample, of which more
than 95% have been observed at least once between 2017 June
and 2021 December.
Among the observed stars is a subset of 300 stars with
repeated CHIRON observations targeted in an RV survey for
companions (Paredes et al. 2021). We selected seven stars that
exhibited RV variations of 50 m s
−1
or more that did not appear
to be due to companions in the available CHIRON data as of
2018 December; instead, the variable velocities may be
indicative of activity related to youth. Here we investigate
these seven because they are promising candidates to be nearby
young stars, and we evaluate them in comparison with the
features examined in the spectra of the young stars in the
benchmark sample.
2.2. Benchmark Sample of K Dwarfs with Age Estimates
A supplementary benchmark sample of ∼100 K dwarfs
includes those with age estimates. These stars have been taken
from moving groups, associations, or clusters, plus a handful of
field stars within 25 pc that have ages determined via isochrone
fitting. Table 1lists the various subsets used to construct the
benchmark sample and the estimated ages of each group. The
four associations utilized here are the βPictoris moving group
(βPic, age 24 ±3 Myr), the Tucana-Horologium association
(Tuc-Hor, 45 ±4 Myr), the AB Doradus moving group (AB
Dor, 145 19
50
Myr), and the Hyades cluster (750 ±100 Myr;
Bell et al. 2015; Brandt & Huang 2015). The small set of field
K dwarfs within 25 pc having age estimates made via model
isochrone fits have ages of 0.3–5.7 Gyr.
K-dwarf members of the βPic, Tuc-Hor, and AB Dor groups
were identified using the Bell et al. (2015)bona fide
membership list and confirmed through the Gagne et al.
(2018)web-based BANYAN Σcode,
8
in combination with
Gaia DR2 parallax, proper-motion, and RV data. This yielded a
larger sample of K dwarfs than Gagne et al. (2018), which
listed bona fide members before the Gaia DR2 release. Hyades
membership was determined using K dwarfs in the Gagne et al.
(2018)bona fide list with all 47 stars checked using BANYAN
Σand updated Gaia DR2 data.
To date, 44 members of the benchmark sample have been
observed using CHIRON, with 35 of these spectra having
sufficient signal-to-noise ratios (S/Ns)for detailed spectral
analysis. The discrepancy between the number of observed and
the number of measured spectra mainly stems from the poor
S/N achieved for fainter (V>12), usually cooler stars. The
EWs acquired from the set of 35 stars proved to be sufficient
for identifying spectral age and activity trends.
Table 1
Moving Groups, Associations, and Clusters Providing K Dwarfs for the Benchmark Sample
Group Name R.A. (J2000)Decl. (J2000)Distance (pc)
a
Age Members
b,c
K Dwarfs Observed
βPictoris moving group ∼14 30 ∼−42 00 ∼30 24 ±3 Myr
d
97 19 11
Tuc-Hor association ∼02 36 ∼−52 03 ∼40 45 ±4 Myr
d
176 18 10
AB Doradus moving group ∼05 28 ∼−65 26 ∼33 145 19
50
Myr
d
84 24 8
Hyades cluster ∼04 26 ∼+15 52 ∼42 750 ±100 Myr
d
177 47 10
Field K dwarfs Various Various <25 0.3–5.7 Gyr L10 5
o
2
Eri 04 15 16.3 −07 39 10 5 4.3 Gyr
e
LLL
HD 50281 06 52 18.1 −05 10 25 9 1.9 Gyr
f
LLL
20 Crt 11 34 29.5 −32 49 53 10 4.6 Gyr
e
LLL
PX Vir 13 03 49.7 −05 09 43 22 0.3 Gyr
g
LLL
òInd 22 03 21.7 −56 47 10 4 3.7–5.7 Gyr
h
LLL
Notes.
a
Distance from Gaia EDR3.
b
Membership list for βPic, Tuc-Hor, and AB Dor: Bell et al. (2015).
c
Membership list for Hyades: Gagne et al. (2018).
d
Gagne et al. (2018).
e
Mamajek & Hillenbrand (2008).
f
Luck (2017).
g
Stanford-Moore et al. (2020).
h
Feng et al. (2019).
7
Defined to have a K-dwarf primary, plus any additional lower-mass stellar,
brown dwarf, or planetary companions.
8
BANYAN Σ-http://www.exoplanetes.umontreal.ca/banyan/.
3
The Astronomical Journal, 164:174 (20pp), 2022 November Hubbard-James et al.
3. Observations and Data Reduction
All spectra used in this study were acquired between 2017
June and 2020 March using the CHIRON echelle spectrograph at
the SMARTS 1.5 m telescope described in Tokovinin et al.
(2013)and Paredes et al. (2021). We utilized CHIRON’s slicer
mode to attain spectra with resolution R=80,000, with each
spectrum split into 59 orders and covering a wavelength range of
4150–8800 Å. Each observation consisted of a single exposure of
900 s (stars with V<10.5)or 1200 s (V>10.5). This was done
to ensure that we attained an S/N greater than 30 for all spectra.
Each observation was followed by a single ThAr lamp exposure
of 0.4 s that was used for wavelength calibration. Included with
each night’s observations were two sets of calibration frames
integral to the data reduction process; one set is taken each
afternoon before nighttime operations begin, and another after all
observations have been completed for the night.
All CHIRON data are reduced using a customized data
reduction pipeline described in Tokovinin et al. (2013), with
additional details specific to our program in Paredes et al.
(2021). The pipeline is currently run by members of the
RECONS team, with CHIRON spectra reduced and distributed
to dozens of research teams for over 1000 nights by the end of
2021. Briefly, each spectrum is bias-corrected and flat-fielded
using quartz lamp calibrations to remove electronic readout noise
and to correct for individual pixel sensitivities. After removing
cosmic rays from the spectrum, profile order extraction is
performed using an extraction algorithm based on the REDUCE
package by Piskunov & Valenti (2002). Finally, each spectrum
with extracted orders is matched with its closest ThAr calibration
frame to obtain the sampled wavelength solution.
Once the basic pipeline reductions are done, the S/N per
pixel is calculated between 6717 and 6720 Åusing the method
described in Tokovinin et al. (2013). Spectra with an S/N less
than 30 are omitted from the present analysis because their EW
measurements are often unreliable for the spectral features of
interest. Each order is then trimmed at the edges to eliminate
poor signal portions of the orders, and the spectra are
normalized and flattened using a MATLAB script. Spectra
are then shifted twice: (1)first by applying a barycentric
velocity correction, and (2)then shifted to zero velocity relative
to the Sun. Spectral analysis of specific lines was then carried
out using the methods listed below.
4. Spectral Analysis
4.1. Line Selection
Four spectral features have been selected to create a rubric to
evaluate ages and activity for K stars—Hαat 6563 Å,theNa
I
doublet at 5890 and 5896 Å,LiIat 6708 Å,andCaII at 8452 Å.
Plots of the spectral regions containing the four selected lines for
all 42 stars are shown in Figures 2and 3.
The most direct information about stellar age for K dwarfs can
be gleaned from a Li Iresonance line at 6707.8 Å. Depletion of
lithium in late-type dwarfs has been well documented in previous
studies, such as Soderblom & Jones (1993), White et al. (2007),
López-Santiago et al. (2010), and Binks & Jeffries (2014).Itis
proposed that lithium is destroyed as a young K star settles onto
the main sequence through a process similar to the proton–proton
chain reaction, with the end product being two helium atoms and
the release of energy (Soderblom 2010). The decrease in the EW
of the Li Iline has been associated with increased age and is used
in our study of K dwarfs as the most direct age marker
(Soderblom et al. 2014). However, using the Li Ifeature is
somewhat limiting because the majority of lithium is depleted
within the first 200 Myr for dwarfs of type late G through early
M(Soderblom et al. 2014).ThetrendintheLi
Iλ6707.8 line
strength is also temperature dependent, fading faster for cool,
late-type dwarfs (K8V–M9V), as shown by Riedel et al. (2017),
who find an absence of Li Ifeatures in M-dwarf members of
moving associations with age estimates of only 50 Myr.
To enhance our efforts to estimate ages, we also consider
spectral lines resulting from stellar activity. Increased activity
in the chromospheres of late-type stars has been linked with age
since the publication of Skumanich (1972)five decades ago, so
a comprehensive literature search was done to find spectral
lines that might be used as activity, and presumably age,
markers for K dwarfs. Candidate lines need to be located within
CHIRON’s spectral range of 4150–8800 Å, and therefore the
popular Ca II H and K lines at 3968 and 3934 Åthat trace
chromospheric activity are excluded. For this study, we have
identified the Hαline at 6563 Åand one line of the Ca II
infrared triplet at 8542 Åas activity tracers; both lines exhibit
core emission or filled-in profiles when a K dwarf’s chromo-
sphere is active (Montes & Martin 1998). The other two Ca II
infrared triplet lines at 8498 and 8662 Åare omitted because
the orders produced by CHIRON’s slicer mode are truncated at
longer wavelengths and miss both lines.
Surface gravity diagnostic lines have also been proposed as
age markers for K dwarfs (Soderblom 2010). The idea is that
younger stars with ages <100 Myr are still contracting and
have bloated atmospheres compared to their older counterparts
already on the main sequence. Thus, the younger star’s larger
radius at the same mass results in a lower surface gravity, and
this can be revealed via relatively narrower spectral lines. In
effect, increased opacity in a fully contracted main-sequence
star leads to more atomic collisions and interactions in its
atmosphere, resulting in a wider absorption feature with
broader wings—this process is called pressure broadening.
The Na Idoublet lines at 5889.95 and 5895.92 Åare very
sensitive to pressure broadening. An increase in the EW of the
Na Idoublet feature (EW[Na ID])is therefore theorized to
accompany an increase in age (Soderblom 2010).
4.2. Equivalent Width Measurements
To carry out the spectroscopic analysis of age and activity
for the sample stars, we measured the EWs of both Na Ilines at
5889.95 and 5895.92 Å, the Hαline at 6563 Å, the unresolved
Li doublet at 6707.8 Å, and the Ca II infrared triplet line at
8542 Å, using the SPLAT-VO software, which is distributed by
the Starlink Project (Škoda et al. 2014). We compared SPLAT-
VO to other comparable methods for measuring EWs,
including the SPLOT package in IRAF, specutils using Python,
and VOSpec from the European Space Agency. All methods
resulted in similar EWs for a test sample of K dwarfs, and we
decided to use SPLAT-VO owing to its user-friendly interface.
All 42 K dwarfs were analyzed, including 35 from the
benchmark sample and the 7 RVV stars. A 20 Åwindow was
created for the Na Idoublet feature, and a 10 Åwindow was
created around the centers of the Li, Hα, and Ca II lines, with
pseudocontinuum fits made across each window using the
normalized data before carrying out the EW measurements.
SPLAT-VO uses the ABLINE technique to fit a Gaussian,
4
The Astronomical Journal, 164:174 (20pp), 2022 November Hubbard-James et al.
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TheSolarNeighborhoodL:SpectroscopicDiscoveryofKDwarfsYoungerThan1GyrandNewBinarieswithin30pcHodari-SadikiHubbard-James1,2,4,D.XavierLesley2,3,ToddJ.Henry2,4,LeonardoA.Paredes1,2,4,andAzmainH.Nisak1,41DepartmentofPhysicsandAstronomy,GeorgiaStateUniversity,Atlanta,GA30302,USA;hjames12@gsu.edu2RECONSIn...
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