Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars I: Classifying Low-mass Host Stars Observed During Campaigns 1-7

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by  Courtney D. Dressing

Accepted to The Astrophysical Journal
PreprinttypesetusingLATEXstyleAASTeX6v.1.0
CHARACTERIZING K2 CANDIDATE PLANETARY SYSTEMS ORBITING LOW-MASS STARS I:
CLASSIFYING LOW-MASS HOST STARS OBSERVED DURING CAMPAIGNS 1-7
Courtney D. Dressing1,2,3, Elisabeth R. Newton4,5, Joshua E. Schlieder6, David Charbonneau7, Heather A.
Knutson1, and Andrew Vanderburg7,8
(Dated: January 4, 2017)
1DivisionofGeological&PlanetarySciences,CaliforniaInstituteofTechnology,Pasadena,CA91125
2NASASaganFellow
7 3
[email protected]
1
0 4DepartmentofPhysics,MassachusettsInstituteofTechnology,Cambridge,MA02139
2 5NationalScienceFoundationAstronomy&AstrophysicsPostdoctoralFellow
n 6NASAExoplanetScienceInstitute,CaliforniaInstituteofTechnology,Pasadena,CA91125
a 7Harvard-SmithsonianCenterforAstrophysics,Cambridge,MA02138
J
8NationalScienceFoundationGraduateResearchFellow
3

ABSTRACT
]
R
Wepresentnear-infraredspectrafor144candidateplanetarysystemsidentifiedduringCampaigns 1-7
S of the NASA K2 Mission. The goal of the survey was to characterize planets orbiting low-mass
.
h stars, but our IRTF/SpeX and Palomar/TripleSpec spectroscopic observations revealed that 49% of
p our targets were actually giant stars or hotter dwarfs reddened by interstellar extinction. For the
-
o 72starswithspectraconsistentwithclassificationascooldwarfs(spectraltypesK3-M4), werefined
r their stellar properties by applying empirical relations based on stars with interferometric radius
t
s measurements. Althoughourrevisedtemperaturesaregenerallyconsistentwiththosereportedinthe
a
Ecliptic Plane Input Catalog (EPIC), our revised stellar radii are typically 0.13R (39%) larger than
[ (cid:12)
the EPIC values, which were based on model isochrones that have been shown to underestimate the

1
radii of cool dwarfs. Our improved stellar characterizations will enable more efficient prioritization of
v
K2 targets for follow-up studies.
6
8 Keywords: planetary systems – planets and satellites: fundamental parameters – stars: fundamental
5 parameters – stars: late type – stars: low-mass – techniques: spectroscopic
0
0
1. 1. INTRODUCTION proposals.
0 The K2 mission design is particularly well-matched
Beginning in 2009, the NASA Kepler mission revolu-
7 forstudiesofplanetarysystemsorbitinglow-massstars.
tionizedexoplanetsciencebysearchingforplanetstran-
1
Although M dwarfs are intrinsically fainter than Sun-
: siting roughly 190,000 stars and detecting thousands of
v like stars, the prevalence of M dwarfs within the galaxy
planetcandidates(Boruckietal.2010,2011a,b;Batalha
i
X et al. 2013; Burke et al. 2014). The main Kepler mis- (e.g.,Henryetal.2006;Wintersetal.2015)ensuresthat
there are several thousand reasonably bright low-mass
r sion ended in 2013 when the second of four reaction
a stars per K2 field. Due to their smaller sizes and cooler
wheelsfailed,therebydestroyingtheabilityofthespace-
temperatures, these stars are relatively easy targets for
craft to point stably. Although the two-wheeled Kepler
planetdetectionfortwomainreasons. First,thetransit
was not able to continue observing the original targets,
depth is deeper for a given planet radius. Second, the
Ball Aerospace engineers and Kepler team members re-
habitable zones are closer to the stars, thereby increas-
alizedthatthetorquefromsolarpressurecouldbemiti-
ingboththegeometriclikelihoodthatplanetswithinthe
gated by selecting fields along the ecliptic plane. In this
habitable zone will appear to transit and the number of
new mode of operation (known as the K2 Mission), the
transits that could be observed during a single K2 cam-
spacecraft stares at 10,000 - 30,000 stars per field for
paign. For the coolest low-mass stars, the orbital peri-
roughly 80 days before switching to another field along
ods of planets within the habitable zone are even short
the ecliptic (Howell et al. 2014; Van Cleve et al. 2016).
enough that potentially habitable planets would transit
Unlike in the original Kepler mission, all K2 targets are
multiple times per campaign.
selected from community-driven Guest Observer (GO)
2 Dressing et al.
The “Small Star Advantage” of deeper transit depths KIC values with parameters of the nearest model star
and higher transit probabilities within the habitable selected from Yonsei-Yale isochrones (Demarque et al.
zone is partially offset by the challenge of identifying 2004), but those models noticeably underpredict the
samplesoflow-massstarsforobservation. Whenprepar- radii of low-mass stars (Boyajian et al. 2012).
ing for the original Kepler mission, Brown et al. (2011) Considering the non-planet candidate host stars,
conducted an extensive survey of the proposed field of Mannetal.(2012)acquiredmedium-resolution(1150(cid:46)
view to identify advantageous targets and determine R (cid:46) 2300) visible spectra of 382 putative low-mass
rough stellar properties. In contrast, the planning cy- dwarftargets. Usingthosestarsasa“trainingset,”they
cle for the K2 mission was too fast-paced to allow for found that the vast majority (96%±1%) of cool, bright
such methodical preparation. During the early days of (Kp<14) Kepler target stars were actually giants. For
the K2 mission, the official Ecliptic Plane Input Cat- fainter cool stars, giant contamination was much less
alog (EPIC) contained only coordinates, photometry, pronounced (7%±3%). For stars that were correctly
proper motions, and, when available, parallaxes. Pro- classified as dwarfs, Mann et al. (2012) found that the
posers therefore had to use their own knowledge of stel- KICtemperaturesweresystematically110Khotterthan
lar astrophysics to determine which stars were suitable thevaluesdeterminedbycomparingtheirspectratothe
for their investigations. BT-SETTL series of PHOENIX stellar models (Allard
More recently, Huber et al. (2016) updated the EPIC et al. 2011).
toincludestellarpropertiesfor138,600stars. Aftercom- In a following paper, Mann et al. (2013b) obtained
pletingthemessytasksofmatchingsourcesfrommulti- optical spectra of 123 putative low-mass stars hosting
plecatalogs,convertingthephotometrytostandardsys- 188planetcandidatesandNIRspectraforasmallersub-
tems, and enforcing quality cuts to discard low-quality setofhoststars. Flux-calibratingtheirspectraandcom-
photometry,Huberetal.(2016)usedtheGalaxia galac- paring them to BT-SETTL stellar models, they derived
tic model (Sharma et al. 2011) to generate synthetic re- a set of empirically-based relations to determine stellar
alizations of different K2 fields. They then determined effective temperatures from spectral indices measured
themostlikelyparametersforeachK2targetstargiven at visible and near-infrared wavelengths. Mann et al.
the available photometric and kinematic information. (2013b) also introduced a set of temperature–radius,
Whenpossible,theanalysisalsoincorporatedHipparcos temperature–mass, and temperature–luminosity rela-
parallaxes (van Leeuwen 2007) and spectroscopic esti- tionsbasedonthesampleofstarswithwell-constrained
mates of T , logg, and [Fe/H] from RAVE DR4 (Kor- radii, effective temperatures, and bolometric fluxes.
eff
dopatis et al. 2013), LAMOST DR1 (Luo et al. 2015), Focusing specifically on the coolest Kepler targets,
and APOGEE DR12 (Alam et al. 2015). Muirhead et al. (2012) re-characterized 84 cool Kepler
In all cases, Galaxia used Padova isochrones (Gi- Object of Interest (KOI) host stars by obtaining near-
rardi et al. 2000; Marigo & Girardi 2007; Marigo et al. infrared spectra with TripleSpec at the Palomar Hale
2008) to determine stellar properties. Aware that these Telescope. As explained in Rojas-Ayala et al. (2012),
isochrones tend to underpredict the radii of low-mass they estimatedtemperatures andmetallicitiesusing the
stars (Boyajian et al. 2012), Huber et al. (2016) there- H O-K2 index and the equivalent widths (EW) of the
2
fore warned that the EPIC radii of low-mass stars may Na I line at 2.210µm and the Ca I line at 2.260µm.
be up to roughly 20% too small. Given that 41% of se- Dependingonstellarmetallicity,theH O-K2indexsat-
2
lected K2 targets are low-mass M and K dwarfs (Huber uratesatapproximately3900K,sothisapproachcannot
etal.2016), improvingtheradiusestimates oflow-mass be used to characterize mid-K dwarfs. Muirhead et al.
K2 targets is important for maximizing the scientific (2012) then interpolated the temperatures and metal-
yield of the K2 mission. Both accurate characterization licities onto Dartmouth isochrones (Dotter et al. 2008;
of individual planet candidates and ensemble studies of Feiden et al. 2011) to estimate the radii and masses of
planetaryoccurrencedemandreliablestellarproperties. their target stars. In a follow-up analysis, Muirhead
Even during the more methodical Kepler era, the et al. (2014) expanded their sample to 103 cool KOI
properties of low-mass targets were frequently revised. host stars and updated their mass and radius estimates
Initially,Brownetal.(2011)characterizedallofthetar- using newer versions of the Dartmouth isochrones.
getsbycomparingmulti-bandphotometrytoCastelli& Both KOIs and non-KOIs need to be accurately char-
Kurucz (2004) stellar models. This approach worked acterized in order to use the Kepler data to investigate
well for characterizing Sun-like stars, but Brown et al. planet occurrence rates, which motivated Dressing &
(2011) cautioned that the Kepler Input Catalog (KIC) Charbonneau (2013) to refit the KIC photometry using
temperatures were untrustworthy for stars cooler than Dartmouth Stellar Evolutionary Models (Dotter et al.
3750K.Batalhaetal.(2013)laterimprovedtheclassifi- 2008; Feiden et al. 2011) to determine revised proper-
cationsformanyKepler targetsbyreplacingtheoriginal tiesfor3897dwarfscoolerthan4000K.Wethenusedthe
K2 Candidate Planetary Systems I: The Stars 3
revised stellar properties to investigate the frequency of ple. In this paper, we classify the subset of K2 target
planetary systems orbiting low-mass stars. stars that appear to be low-mass stars harboring plane-
Recognizingthatthestellarparametersinferredinthe tary systems. In the second paper in this series (Dress-
previousstudieswerebasedonstellarmodelsandthere- ing et al. 2016), we use our new stellar classifications to
fore likely to underestimate stellar radii, Newton et al. revisethepropertiesoftheassociatedplanetcandidates
(2015) revised the properties of cool KOI host stars by and identify intriguing systems for follow-up analyses.
employing empirical relations based on interferometri- In Section 2, we describe our observation procedures
cally characterized stars. Specifically, Newton et al. and conditions. We then discuss the target sample in
(2015)establishedrelationshipsbetweentheEWsofMg Section 3 and explain our data reduction and stellar
and Al features in H-band spectra from IRTF/SpeX characterization procedures in Section 4. Finally, we
and the temperatures, luminosities, and radii of low- address the implications of our results and conclude in
mass stars. Newton et al. (2015) found that the radii Section 5.
of M dwarf planet candidates were typically 15% larger
thanpreviouslyestimatedintheHuberetal.(2014)cat- 2. OBSERVATIONS
alog, which contained a compilation of results from pre- WeconductedourobservationsusingtheSpeXinstru-
viousstudiesincludingDressing&Charbonneau(2013), ment on the NASA Infrared Telescope Facility (IRTF)
Muirhead et al. (2012, 2014), and Mann et al. (2013b). over 15 partial nights during the 2015A, 2015B, 2016A,
Accounting for the systematic effect of previously and 2016B semesters and the TripleSpec instrument
underestimated stellar radii, Dressing & Charbonneau on the Palomar 200” over four full nights during the
(2015) investigated low-mass star planet occurrence in 2016A semester. Eleven of our IRTF/SpeX nights
more detail by employing their own pipeline to detect were awarded to C. Dressing via programs 2015B068,
candidatesandmeasuresearchcompleteness. Usingthe 2016A066, and 2016B057; the remaining SpeX time
full four-year Kepler dataset, we found that the mean was provided by K. Aller, W. Best, A. Howard, and
numberofsmall(0.5−4R⊕)planetsperlateKorearly E. Sinukoff. All of our Palomar time was awarded to
M dwarf is 2.5±0.2 planets per star for orbital periods C. Dressing for program P08.
shorterthan200days. Withinthehabitablezone,wees- AsdetailedinTable1,ourobservingconditionsvaried
timated occurrence rates of 0.24+0.18 Earth-size planets fromphotometricnightstonightswithsignificantcloud
−0.08
and0.21+−00..1016 super-Earths(1.5−2R⊕)perstar. Those cover through which only our brightest targets were ob-
estimates agree well with rates derived in independent servable. As recommended by Vacca et al. (2003), we
studies(e.g.,Gaidos2013;Morton&Swift2014;Gaidos removed telluric features from our science spectra us-
et al. 2014, 2016). ing observations of A0V stars acquired under similar
InordertousetheK2datatoconductsimilarstudies observing conditions. Accordingly, we interspersed our
of planet occurrence rates and possibly investigate how science observations with observations of nearby A0V
the frequency of planetary systems orbiting low mass stars. When possible, these A0V stars were within 15◦
stars varies as a function of stellar mass, metallicity, or ofourtargetstarsandobservedwithinonehouratsim-
multiplicity,wefirstneedtocharacterizethestellarsam- ilar airmasses (difference <0.1 airmasses).
Table 1. Observing Conditions
Date Seeing Weather K2
Semester Instru Program (UT) Conditions Targetsa
2015A SpeX 989 Apr 16, 2015 0(cid:48).(cid:48)7−1(cid:48).(cid:48)0 Clear 2b
SpeX 989 May 5, 2015 0(cid:48).(cid:48)3−0(cid:48).(cid:48)8 Light wind, clear 5c
SpeX 981 June 13, 2015 0(cid:48).(cid:48)3−1(cid:48).(cid:48)0 Cirrus, patchy clouds 2d
2015B SpeX 057, 068 Aug 7, 2015 0(cid:48).(cid:48)5−1(cid:48).(cid:48)0 Clear at start; closed early due to high humidity 3e
SpeX 068 Sep 24, 2015 0(cid:48).(cid:48)5−1(cid:48).(cid:48)0 Patchy clouds cleared slightly overnight 20
SpeX 072 Oct 14, 2015 0(cid:48).(cid:48)4−1(cid:48).(cid:48)0 Cirrus 1f
SpeX 068 Nov 26, 2015 0(cid:48).(cid:48)5−2(cid:48).(cid:48)0 Patchy clouds; high humidity 16
SpeX 068 Nov 27, 2015 0(cid:48).(cid:48)6 Cirrus 16
Table 1 continued
4 Dressing et al.
Table 1 (continued)
Date Seeing Weather K2
Semester Instru Program (UT) Conditions Targetsa
2016A TSPEC P08 Feb 19, 2016 1(cid:48).(cid:48)2−2(cid:48).(cid:48)0 Cirrus clouds at start; moderately cloudy by morning 3
SpeX 066 Mar 4, 2016 0(cid:48).(cid:48)5−1(cid:48).(cid:48)0 Clear 10
SpeX 066 Mar 8, 2016 0(cid:48).(cid:48)5−1(cid:48).(cid:48)0 Thick, patchy clouds at sunset; thinner clouds by morning 12
SpeX 986 Mar 10, 2016 0(cid:48).(cid:48)9 Cirrus 5g
TSPEC P08 Mar 27, 2016 0(cid:48).(cid:48)9 Clear 15
TSPEC P08 Mar 28, 2016 0(cid:48).(cid:48)9−2(cid:48).(cid:48)1 Patchy clouds; closed early due to high humidity & fog 9
TSPEC P08 April 18, 2016 1(cid:48).(cid:48)1−1(cid:48).(cid:48)9 Clear 11
SpeX 066 May 5, 2016 0(cid:48).(cid:48)5−1(cid:48).(cid:48)0 Patchy clouds 11
SpeX 066 May 6, 2016 0(cid:48).(cid:48)3−0(cid:48).(cid:48)9 Clear 6
SpeX 066 June 7, 2016 0(cid:48).(cid:48)4−1(cid:48).(cid:48)0 Clear 8
2016B SpeX 057 Oct 26, 2016 0(cid:48).(cid:48)5−1(cid:48).(cid:48)4 Clear 5
a
We observed some stars twice on two different nights to assess the repeatability of our analysis.
b
Night awarded to Andrew Howard.
c
Night awarded to Andrew Howard, but observations obtained by Joshua Schlieder.
d
Observations obtained by Evan Sinukoff.
e
Includes one observation acquired by Will Best (Program 057) and two acquired by Courtney Dressing (Program 068).
f
Observations obtained by Kimberly Aller.
g
Night awarded to Andrew Howard, but observations obtained by Courtney Dressing.
2.1. IRTF/SpeX ized IRTF calibration sequence every few hours during
ourobservationsandensuringthateachregionofthesky
For our SpeX observations, we selected the
0(cid:48).(cid:48)3×15”slitandobservedinSXDmodetoobtainmod- hadaseparatesetofcalibrationframes. Thecalibration
sequence includes flats taken using an internal quartz
erateresolution(R≈2000)spectra(Rayneretal.2003,
lamp and wavelength calibration spectra acquired using
2004). Due the SpeX upgrade in 2014, our spectra in-
an internal thorium-argon lamp.
cludeenhancedwavelengthcoveragefrom0.7-2.55µm.
WecarriedoutallofourobservationsusinganABBA
nod pattern with the default settings of 7(cid:48).(cid:48)5 separation 2.2. Palomar/TripleSpec
betweenpositionsAandBand3(cid:48).(cid:48)75separationbetween We acquired our TripleSpec observations using the
either pointing and the ends of the slit. For all targets fixed 1” × 30” slit, which yields simultaneous cover-
except close binary stars, we aligned the slit with the age between 1.0 and 2.4 µm at a spectral resolution of
parallactic angle to minimize systematic effects in our 2500−2700 (Herter et al. 2008). In order to decrease
reduced spectra; for binary stars, we rotated the slit so the effect of bad pixels on the detector, we adopted the
that the sky spectra acquired in the B position would 4-position ABCD nod pattern used by Muirhead et al.
befreeofcontaminationfromthesecondstarorsothat (2014)ratherthanthe2-positionABBApatternweused
spectra from both stars could be captured simultane- for our SpeX observations. With the exception of dou-
ously. We scaled the exposure times for our targets and ble star systems for which we altered the slit rotation
repeatedtheABBAnodpatternasrequiredsothatthe to place both stars in the slit when possible, we left the
resulting spectra would have S/N of 100−200 per res- slit in a fixed East-West orientation. We calibrated our
olution element. spectra using dome darks and dome flats acquired at
We calibrated these spectra by running the standard- both the beginning and end of the night.
K2 Candidate Planetary Systems I: The Stars 5
Table 2. Targets Observed by K2 Campaign
Campaign Total Classification in This Paper
Field RA Dec Galactic Targets Cool Hotter
Number (hh:mm:ss) (dd:mm:ss) Latitude (◦) Observed Dwarfs1 Dwarfs Giants
1 11:35:46 +01:25:02 +59 10 9 (90%) 1 (10%) 0 (0%)
2 16:24:30 −22:26:50 +19 8 0 (0%) 4 (50%) 4 (50%)
3 22:26:40 −11:05:48 −52 12 6 (50%) 5 (42%) 1 (8%)
4 03:56:18 +18:39:38 −26 24 10 (42%) 10 (42%) 4 (17%)
5 08:40:38 +16:49:47 +32 41 27 (66%) 13 (32%) 1 (2%)
6 13:39:28 −11:17:43 +50 34 16 (47%) 12 (36%) 6 (18%)
7 19:11:19 −23:21:36 −15 17 6 (35%) 4 (24%) 7 (41%)
1−7 ··· ··· ··· 146 74 (51%) 49 (34%) 23 (16%)
1
Two K2 targets (EPIC 211694226 and EPIC 212773309) have nearby companions that may or may not
be physically associated. We classified 74 cool dwarfs in 72 systems.
3. TARGET SAMPLE targets. Neither pipeline considers stars observed as
part of “super-stamps.” Due to the heterogenous na-
The objective of our observing campaign was to de-
ture of the K2 target lists and the limited information
termine the properties of K2 target stars and assess
providedintheEPICduringearlyK2campaigns,these-
the planethood of associated planet candidates. Conse-
lected target sample is heavily biased. As noted by Hu-
quently,ourtargetswereselectedfromlistsofK2planet
ber et al. (2016), the K2 target lists are biased toward
candidatescompiledbyA.VanderburgandtheK2 Cal-
cool dwarfs. Overall, the set of stars observed during
ifornia Consortium (K2C2). These early target lists are
Campaigns 1–8 consisted primarily of K and M dwarfs
preliminaryversionsofplanetcandidatecatalogssuchas
(41%), F and G dwarfs (36%), and K giants (21%), but
those published in Vanderburg et al. (2016) and Cross-
thegiantfractionwashigherforfieldsclosetothegalac-
field et al. (2016).
tic plane (see Table 2) than for fields at higher galactic
Of the 144 K2 targets observed, 99 (69%) appear in
latitude (Huber et al. 2016). Many Guest Observers
unpublished lists provided by A. Vanderburg, 28 (19%)
used a magnitude cut when proposing targets, which
were published in the Vanderburg et al. (2016) cata-
may have increased the representation of multiple star
log, and 77 (53%) were reported in previously unpub-
systems within the selected sample.
lished lists generated by K2C2. (These totals sum to
Due to the design of the K2 mission, our K2 targets
>100% due to partial overlap between the Vanderburg
wereconcentratedindistinctfieldsoftheskyeachspan-
and K2C2 candidate lists.) The K2C2 planet candi-
ning roughly 100 square degrees. We note the number
dates from K2 Campaigns 0 − 4 were later published
of targets observed from each campaign in Table 2. As
in Crossfield et al. (2016). Although we did not consult
showninFigure1,themagnitudedistributionofourK2
thesecatalogsforinitialtargetselection,ourtargetsam-
targets ranged from 6.2−13.1 in Ks, with a median Ks
ple also contains 46 systems from Barros et al. (2016),
magnitude of 10.8. In the Kepler bandpass (similar to
26 stars from Pope et al. (2016), 5 stars from Foreman-
V-band),ourtargetshadbrightnessesofKp=9.0−16.3
Mackey et al. (2015), 5 stars from Montet et al. (2015),
and a median brightness of Kp=13.5.
and 4 stars from Adams et al. (2016).
WitheachK2datarelease, weinitiallyprioritizedob-
The Vanderburg and K2C2 catalogs contain all of
servationsofstarsharboringsmallplanetcandidates(es-
the planet candidates detected by the corresponding
timated planet radius < 4R ) and systems that could
pipeline(K2SFFandTERRA,respectively)intheK2light ⊕
potentiallybewell-suitedforhigh-precisionradialveloc-
curves of stars proposed as individual Guest Observer
6 Dressing et al.
4.1. Initial Classification
30 Ks
After completing the Spextool reduction, we used an
Kp
25 interactive Python-based plotting interface to compare
20 our spectra to the spectra of standard stars from the
t
n IRTF Spectral Library (Rayner et al. 2009). We al-
u
o15
C lowed each model spectrum to shift slightly in wave-
10 lengthspacetoaccommodatedifferencesinstellarradial
velocities. Considering the J, H, and K bandpasses in-
5
dependently, we assessed the χ2 of a fit of each model
0 spectrum to our data and recorded the dwarf and giant
6 8 10 12 14 16
Magnitude models with the lowest χ2.
We then considered the target spectrum holistically
Figure 1. Magnitudedistributionofourfulltargetsamplein andassignedasingleclassificationtothestar. Although
the Kepler bandpass (Kp; blue) and Ks (red). Our targets the focus of this analysis was to characterize planetary
havemedianbrightnessesofKs=10.8andKp=13.5. The systemsorbitinglow-massdwarfs,ourtargetsampledid
Kepler bandpass extends from roughly 420 nm to 900 nm
include contamination from hotter and evolved stars.
with maximum response at 575 nm (Van Cleve & Caldwell
2016);theKsbandpassiscenteredat2.159µm(Cohenetal. We list the 23 giants and 49 hotter dwarfs in Tables 3
2003). and 4, respectively. We did not include either group
in the more detailed analyses described in Section 4.2.
ity observations (host star brighter than V = 12.5 and
For the purposes of identifying contamination, we re-
estimated radial velocity semi-amplitude K > 2 m/s).
jected all stars that we visually classified as giants or
Once we had exhausted the those targets, we worked
dwarfswithspectraltypesearlierthanK3. Table4also
down the target list and observed increasingly fainter
includes all stars for which the Newton et al. (2015)
host stars harboring larger planets. Our goal was to se-
routines yielded estimated temperatures above 4800K
lect late K dwarfs and M dwarfs, but the initial stellar
orradiilargerthan0.8R (seeSection4.2). Wedisplay
(cid:12)
classifications were uncertain, particularly for the first
the reduced spectra for all targets in Appendix A. We
K2fieldswhentheHuberetal.(2016)EPICstellarcat-
have also posted our spectra and stellar classifications
alog was not yet available. To ensure that few low-mass
on the ExoFOP-K2 follow-up website.1
stars were excluded from our analysis, we adopted le-
Figure 2 displays the spectral type distribution of the
nient criteria when selecting potential target stars. Our
stars in the selected cool dwarf sample. The sample
rough guidelines were J −K > 0.5 and, for stars with
includes stars with spectral types between K3 and M4,
coarseinitialtemperatureestimates,temperaturecooler
withamedianspectraltypeofM0. Thesespectraltypes
than 4900K. Concentrating on the brightest targets bi-
are rather coarse visual assignments (±1 subclass), so
ased our sample towards giant stars and binary stars.
the spike at M3V may be a quirk of the particular tem-
Similarly, our selected J − K color-cut also boosted
platestarsusedforspectraltypeassignmentratherthan
thegiantfractionbyexcludinghotterdwarfswithbluer
a true feature of the distribution. Due to the small
J − K colors without discarding giant stars with ex-
sample size, the spike can also be explained by Poisson
tremely red J − K colors. The binary boost due to
counting errors.
prioritizing bright targets may have been partially off-
set by our avoidance of stars with nearby companions
4.2. Detailed Stellar Characterization
detected in follow-up adaptive optics images.
For the stars that were visually identified as dwarfs
4. DATA ANALYSIS & with spectral types of K3 or later, we used a series
STELLAR CHARACTERIZATION of empirical relations to refine the stellar classifica-
tion. We began by using the publicly-available, IDL-
We performed initial data reduction using the
basedtellrv2 andnirew3 packagesdevelopedbyNew-
publicly-available Spextool pipeline (Cushing et al.
ton et al. (2014, 2015) to shift each spectrum to the
2004) and a version customized for use with TripleSpec
stellar rest frame on an order-by-order basis, measure
data (available upon request from M. Cushing). Both
the equivalent widths of key spectral features, and es-
versions of the pipeline include the xtellcor telluric
correctionpackage(Vaccaetal.2003). Asrecommended
in the Spextool manual, we selected the Paschen δ line
1 https://exofop.ipac.caltech.edu/k2/
at1.005µmwhengeneratingtheconvolutionkernelused
toapplytheobservedinstrumentalprofileandrotational 2 https://github.com/ernewton/tellrv
broadening to the Vega model spectrum. 3 https://github.com/ernewton/nirew
K2 Candidate Planetary Systems I: The Stars 7
20
) 1.0 Hotter Dwarf Cool Dwarf
(

W
15
E 0.5

t
unt10 Firs
o 0.0
C -
W
E
5 d 0.5
n
o
c Same Instrument
0 K3 K4 K5 K7 M0 M1 M2 M3 M4 Se 1.0 Different Instrument
Visually Assigned Spectral Type 0 2 4 6 8 10
First EW ( )
Figure 2. Distributionofvisuallyassignedspectraltypesfor
the 74 stars in our cool dwarf sample.
Figure 3. Repeatability of our equivalent width measure-
mentswhenusingthesameinstrumentforbothobservations
(magenta points) or different instruments for each observa-
timate stellar properties. Specifically, the packages em- tion (navy points). The 75 data points plotted here are the
ploy empirically-based relations linking the equivalent EW measured for five Mg and Al features in thirty spectra
widths of H-band Al and Mg features to stellar tem- of fifteen candidate low-mass dwarfs (two observations per
star). Eight stars were later classified as cool dwarfs (large
peratures, radii, and luminosities (Newton et al. 2015).
circles;spectraltypesK7,M0,andM1)andsevenwereclas-
These relations are appropriate for stars with spectral sifiedashotterdwarfs(smallpoints). Forreference,thegray
types between mid-K and mid-M (i.e., temperatures of dashedlinemarkszerodifferencebetweenthetwoEWmea-
surements.
3200 − 4800K, radii of 0.18 < R < 0.8R , and lu-
(cid:63) (cid:12)
minosities of −2.5 < logL/L < −0.5). The relations
(cid:12)
werecalibratedusingIRTF/SpeXspectra(Newtonetal. from one realization to the next. Several of our spec-
2015) so we downgraded the Palomar/TSPEC spectra tra have SNR of less than 200, which was the threshold
to match the lower resolution of IRTF/SpeX data be- usedin theNewton etal. (2014)study. Accordingly, we
fore applying the relations. We note that neglecting altered measure hband to calculate the temperatures,
the change in resolution can lead to systematic 0.1˚A luminosities, and radii for each realization of the noise
differences in the measured EW due to variations in and report the 50th, 16th, and 84th percentiles as the
theamountofcontaminationincludedinthedesignated best-fit values, lower error bars, and upper error bars,
wavelength interval (Newton et al. 2015). As shown in respectively.
Figure 3, we find generally consistent equivalent widths Our changes significantly improve the reproducibil-
inspectraacquiredondifferentoccasionsevenifthetwo ity of temperature, luminosity, and radius estimates for
observations used separate instruments under variable starswithlowerSNRspectra. Forexample,werepeated
observing conditions. Specifically, the median absolute the classification of the M2 dwarf EPIC 206209135 five
difference in equivalent widths for the five cool dwarfs times using both the original and modified versions
withrepeatedmeasurementsusingthesameinstrument of measure hband. For each classification, we deter-
was 0.2˚A(0.9σ). The median absolute difference for the mined parameter errors by generating 1000 noise real-
three cool dwarfs with measurements from different in- izations. The original code yielded estimated tempera-
struments was 0.3˚A(1.9σ). turesrangingfrom3267−3461K,radiiof0.32−0.35R ,
(cid:12)
Intheoriginalformulationofthemeasure hbandstel- and −1.94≤logL/L ≤−1.85. The variations in the
(cid:12)(cid:63)
larcharacterizationroutine,theerrorsonstellarparam- assigned temperatures and luminosities of 194 K and
eters are determined via a Monte Carlo simulation in 0.09log L were significantly larger than the individ-
(cid:12)
which multiple realizations of noise are added to the ual error estimates of 85 K and 0.06log L and the
(cid:12)
spectra and the equivalent widths of features are re- spread in assigned radii of 0.03R was equal to the in-
(cid:12)
measured. Theerrorsarethendeterminedbycombining dividual radius errors. In comparison, our new method
therandomerrorsintheresultingEWswiththeintrinsic found T = 3360±87 K, R = 0.33±0.03R , and
eff (cid:63) (cid:12)
scatter in the relations. This approach yields useful er- logL = −1.87±0.06log L in all cases. Due to the
(cid:63) (cid:12)
rors, but the adopted stellar parameters are taken from asymmetry of the resulting temperature and radius dis-
a single realization of the noise. For high SNR spec- tributions for some stars, we also report separate upper
tra, variations in the simulated noise might not lead to and lower error bounds instead of forcing the errors to
large changes in stellar properties, but for lower SNR be symmetric in all cases. (EPIC 2106209135 is an ex-
spectra the estimated properties can differ considerably ample of a star with naturally symmetric errors.)
8 Dressing et al.
We confirmed that our cool dwarf classifications were H-band based temperatures display considerable scat-
repeatable by comparing our parameter estimates for ter and are systematically lower than the three other
the fifteen stars observed on two different observing estimates (the temperatures based on the Newton et al.
runs. Figure4revealssatisfactoryagreementinthetem- (2015) routines, the J-band temperatures, and the K-
perature and radius estimates for the eight stars cooler band temperatures). This discrepancy, which is most
than 4800K, the designated upper limit for our cool noticeable for stars hotter than 4000K, is likely caused
dwarf sample. Our results for the seven hotter stars by saturation of the index as the continuum flattens
are less consistent, but the relations from Newton et al. for hotter stars. The J-band temperatures also display
(2015) are not valid at those temperatures. large scatter, but they are more centered along a one-
to-one relation than the H-band estimates. Due to the
much tighter correlation observed between the K-band
First Observation temperaturesandtheEW-basedtemperatureestimates,
1.4
) Second Observation we adopt the K-band temperatures as the “Mann tem-
dii K2 peratures” for our stars. We also see discrepancies for
a SpeX TSPEC
R 1.2 stars with T < 3500. There are three stars for which
eff
r
a thetemperatureinferredusingtheNewtonetal.(2015)
ol
S relationsislargerthanthatinferredfromtheJ,HandK
( 1.0
s band temperatures, The error bars in the temperature
u K3
di K7 inferredfromtheNewtonetal.(2015)relationsarealso
Ra0.8 large. This is caused by the disappearance of the Mg
r K5 and Al features in the coolest dwarf stars, which tends
a M0
ell0.6 toresultinanoverestimateofTeff. Alisweakeratlower
t
S metallicity,consistentwiththiseffectonlybeingseenin
M1
metal-poor stars at the limits of the calibration.
Cool Dwarfs
0.4 Newton et al. (2015) also compared temperature es-
6000 5500 5000 4500 4000 3500
timates derived using their empirical relations with
Stellar Effective Temperature (K)
those based on the Mann et al. (2013b) temperature-
sensitive indices. They found large standard devia-
Figure 4. Repeatability of parameter estimates for the sub-
tions of σ =140 K and σ =170 K in J-band and
sample of fifteen stars with two observations. The data ∆T ∆T
points mark the estimated temperatures and radii found H-band,respectively,betweentemperaturesdetermined
by applying the Newton et al. (2015) EW relations to the using each method, which they attributed to telluric
firstobservations(circles)andsecondobservations(squares)
contamination. In contrast, the standard deviation be-
of each star. The colors differentiate between observations
made using SpeX on the IRTF (blue) and TSPEC on the tweenthe Newton etal. (2015) estimates andthe Mann
Palomar 200” (red). The thick gray lines connect the two et al. (2013b) K-band estimates was only σ = 90 K,
∆T
classificationsforeachstar. Theclusterofpointsnear4350K
suggestingthattheK-bandrelationislesscontaminated
and0.7R containsfiveK7dwarfsobservedtwiceeach. The
(cid:12)
whiteboxindicatestheboundariesofourcooldwarfsample: by telluric features.
For our sample of stars, the agreement between the
T <4800 K, R <0.8R .
eff (cid:63) (cid:12)
two methods is much worse: we measure standard de-
viations of 278 K, 311 K, and 162 K for the temper-
4.2.1. Stellar Effective Temperature
ature differences between the EW-based estimates and
For comparison, we also determined stellar effec- the estimates based on the J-band, H-band, and K-
tive temperatures using the J-, H-, and K-band bandspectralindices,respectively. Themediantemper-
temperature-sensitiveindicesandrelationspresentedby ature differences are 13 K, 143 K, and 64 K for J-band,
Mannetal.(2013b). Wethenappliedthetemperature– H-band, and K-band, respectively, with the EW-based
metallicity–radius relation from Mann et al. (2015) to estimateshigherthanthespectralindex-basedestimate
assign stellar radii. Next, we determined luminosities for H- and K-band and lower for J-band. The signifi-
and masses from the estimated stellar effective temper- cantly poorer agreement is likely due to the differences
atures using relations 7 & 8 from Mann et al. (2013b). between the Newton et al. (2015) stellar sample and
These relations are based on stars with effective tem- our stellar sample. The Newton et al. (2015) sample
peratures between 3238K and 4777K and radii between was dominated by mid- and late-M dwarfs with effec-
0.19R and 0.78R . tive temperatures between 3000 K and 3500 K. In con-
(cid:12) (cid:12)
In Figure 5, we plot the temperature estimates gen- trast, our targets are primarily late K dwarfs and early
erated using the Newton et al. (2015) pipeline against M dwarfs.
thosefromtheMannetal.(2013b)relations. TheMann For an additional check on our stellar classifications,
K2 Candidate Planetary Systems I: The Stars 9
5000 J Band H Band K Band
0.45
)4500 0.30
K ]
( H
ff 0.15 e/
e4000 F
T [
n 0.00 n
n n
Ma3500 0.15Ma
0.30
3000
0.45
3000 3500 4000 4500 5000 3000 3500 4000 4500 5000 3000 3500 4000 4500 5000
Newton Teff (K) Newton Teff (K) Newton Teff (K)
Figure 5. Comparison of temperatures derived using EW-based estimates from Newton et al. (2015) and spectral indices from
Mannetal.(2013b)inJ-band(left),H-band(middle),andK-band(right). Pointswithintheshadedregionliewithin150Kof
a one-to-one relation (solid line). All points are color-coded by [Fe/H] as indicated by the colorbar.
weappliedtheH O-K2index-spectraltyperelationcal-
2 5000
ibratedbyNewtonetal.(2014)toestimatenear-infrared 1
4800
spectral types. The H O-K2 index (Rojas-Ayala et al. e
2 p 0 4600
2012) provides an estimate of the level of water absorp- y )
T K
tion in an M dwarf spectrum by measuring the shape of al 1 4400f (
the spectrum between 2.07µm and 2.38µm. Higher val- tr 4200ef
c 2 T
uesindicatelowerH2Oopacityandthereforehottertem- pe 4000n
peratures. The H2O-K2 index is the second-generation 2 S 3 3800wto
version of the H O-K index introduced by Covey et al. K 3600e
2 - 4 N
O
(2010) and uses slightly different portions of the spec- 3400
2
H 5
trum to avoid contamination from atomic lines in early 3200
Mdwarfs. Theindexisgravity-insensitiveforstarswith 6 M4 M2 M0 K5 K3 3000
Visually Assigned Spectral Type
effective temperatures between 3000K and 3800K and
metallicity-insensitive for stars cooler than 4000K. The
H O-K2indexsaturatesnear4000K,sotheseindexmea- Figure 6. Numerical spectral types automatically derived
2
fromtheH O-K2indexversusourvisuallydeterminedspec-
surementsandspectraltypesarenotvalidforthehotter 2
traltypes. ThepointsarecolorcodedbasedontheEW-based
stars in our sample.
temperatureestimateresultingfromtheNewtonetal.(2015)
As shown in Figure 6, our visually assigned spectral relations. Thegrayshadedregiondenotesspectraltypesthat
types and the index-based spectral types agree well for fallwithinonespectraltypeofaone-to-onerelation. Forref-
erence, the rainbow shading also denotes the spectral type
stars cooler than roughly 3800K. Above this tempera-
ranges. We assigned visual spectral types at integer values,
ture, the index-based spectral types plateau near M1 but the points are horizontally offset for clarity.
due to the inapplicability of the index for the earli-
est M dwarfs. The saturation of the H O-K2 index
2
is highlighted in Figure 7, which provides an alterna- pleareM4dwarfs,sowedidnotneedtotransitionfrom
tive comparison of our spectral type assignments and the metallicity relations for K7−M5 dwarfs provided by
temperature estimates. In the left panel, we show that Mann et al. (2013a) to the relations for M4.5−M9.5
ourvisually-assignedspectraltypesdisplaytheexpected dwarfsfromMannetal.(2014). Wecalculatedmetallic-
correlation with temperature throughout the spectral ities using H-band and K-band spectra separately and
type range of our sample. In contrast, the index-based comparetheresultingdistributionsof[Fe/H]and[M/H]
spectral types deviate from the expected correlation for in Figure 8. On average, a typical star in our cool
starsearlierthanM1V.Welistthevisually-assignedand dwarf sample has near-solar metallicity. Averaging the
index-based spectral types for the cool dwarf sample in H-band and K-band estimates for each star, we obtain
Table 5. median metallicities of [Fe/H]= 0.02 and [M/H]= 0.00.
Figure8alsodisplaysdistributionsofthedifferencesbe-
4.2.2. Stellar Metallicities
tween the H-band and K-band metallicity estimates;
We estimated [Fe/H] and [M/H] using the relations theyagreeatthe1σlevel. Althoughourcooldwarfsam-
from Mann et al. (2013a). The latest stars in our sam- ple includes 11 mid-K dwarfs, we restricted our metal-
10 Dressing et al.
5000
Boyajian 0.45
4500 Newton 0.30 H]
) Mann K /
K 0.15 e
( F
eff 4000 0.00 n [
T 0.15n
3500 a
M
0.30
3000 M4 M2 M0 K5 K3 M4 M2 M0 K5 K3 0.45
4 2 0 2 4 4 2 0 2 4
Visually Assigned Spectral Type H O-K2 Spectral Type
2
Figure 7. Temperatures from Newton et al. (2015) EW-based relation (circles) and Mann et al. (2013b) K-band relation
(squares) versus visually assigned spectral type (left) and automatically assigned H O-K2 index-based spectral types (right).
2
Forreference,theblacklineshowsthespectraltypesandtemperaturesreportedbyBoyajianetal.(2012)forinterferometrically-
characterizedstars. NotethatBoyajianetal.(2012)reporttemperaturesathalfspectraltypesbetweenM0andM4. Allpoints
are color-coded by [Fe/H] as indicated by the colorbar.
licity analysis to the 63 cool dwarfs with spectral types constrained properties, the expected temperatures and
of K7 or later. radii of M5.5−M3 dwarfs are 3054−3412 K and 0.14−
0.41R , respectively (Boyajian et al. 2012). Although
(cid:12)
4.2.3. Stellar Radii
these stars were visually classified as M3 or M4 dwarfs,
We infer stellar radius using the methods from New- the Newton et al. (2015) routines assigned them high
tonetal.(2015)andMannetal.(2015). Theformerare temperatures of 3594 − 3869 K because the Al-a EW
derived directly from the EWs. The latter use T and measured in their spectra were below the lower limit of
eff
metallicity to estimate radii indirectly; for T we use the calibration sample (see Table 7 for EW measure-
eff
the K-band temperatures (which we refer to as “Mann ments). The Mann routines assigned the stars cooler
temperatures”, see Section 4.2.1). The Mann et al. temperaturesof3276−3317K.Duetothebetteragree-
(2015) temperature-metallicity-radius relation is valid ment between the Mann temperatures and expected
for stars with temperatures between 2700K and 4100K, temperatures of mid-M dwarfs, we chose to adopt the
butmanyofthestarsinoursamplearehotterthanthis Mann et al. classifications for those three stars.
upper limit. For the stars for which the Mann et al. 4.2.4. Stellar Luminosities
(2015) relations yield temperatures hotter than 4100K,
We compared the stellar luminosities estimated us-
we instead compare the Newton et al. (2015) radii to
ing the EW-based relation from Newton et al. (2015) to
the radii estimated by applying the temperature-radius
those found using the temperature-luminosity relation
relationprovidedinEquation8ofBoyajianetal.(2012)
from Mann et al. (2013b). Due to the functional nature
using the Mann temperatures.
of the Mann et al. (2013b) relation, the Mann values
We display the resulting radius estimates in Figure 9.
followed a single track whereas the Newton values dis-
The Mann et al. (2015) methodology and the Newton
played scatter about that relation. Ignoring the three
et al. (2015) routines yield similar radii: the median
mid-M dwarfs that are too cool for the Newton rela-
radius difference is 0.01R (the Mann radii are larger)
(cid:12) tions, the luminosity differences (Newton - Mann) have
andthestandarddeviationofthedifferencesis0.06R .
(cid:12) a median value of 0.008L and a standard deviation of
For comparison, the median reported radius errors are (cid:12)
0.05L . Thescatterincreasesastemperatureincreases.
0.03R fortheNewtonetal.(2015)valuesand0.05R (cid:12)
(cid:12) (cid:12) Dividing the sample into stars hotter and cooler than
fortheMannetal.(2015)values. Lookingatthehotter
4000K,theluminositydifferencesforcoolersamplehave
stars, the median difference between the Newton radii
a median value of 0.005L and a standard deviation of
andBoyajianetal.(2012)radiiisonly0.002R andthe (cid:12)
(cid:12) 0.03L while the hotter sample has a median value of
standard deviation of the difference is 0.05R (cid:12)
(cid:12) 0.034L andastandarddeviationof0.07L . Intheleft
As shown in Figure 10, the primary reason why (cid:12) (cid:12)
panel of Figure 11, we display the adopted luminosities
the temperature agreement looks worse for the coolest
as a function of effective temperature.
stars is because three cool stars (EPIC 211817229,
4.2.5. Stellar Masses
EPIC 211799258, and EPIC 211826814) have signif-
icantly different parameters using the two methods. The Newton et al. (2015) relations do not include
Based on the sample of stars with interferometrically masses,sowecomputedthemassesforallstarsusingthe