The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog I: Data

0.86 MB·English

by  Andrew A. West

Accepted for Publicationin AJ
PreprinttypesetusingLATEXstyleemulateapjv.6/22/04
THE SLOAN DIGITAL SKY SURVEY DR7 SPECTROSCOPIC M DWARF CATALOG I: DATA
Andrew A. West1,2,10, Dylan P. Morgan2, John J. Bochanski3,9, Jan Marie Andersen2, Keaton J. Bell4, Adam F.
Kowalski4, James R. A. Davenport4, Suzanne L. Hawley4, Sarah J. Schmidt4, David Bernat5, Eric J. Hilton4,
Philip Muirhead5, Kevin R. Covey2,5,6, Ba´rbara Rojas-Ayala5, Everett Schlawin5, Mary Gooding7, Kyle
Schluns2, Saurav Dhital8, J. Sebastian Pineda3, David O. Jones2
Accepted forPublication inAJ
ABSTRACT
1
1 We present a spectroscopic catalog of 70,841 visually inspected M dwarfs from the seventh data
0 release of the Sloan Digital Sky Survey (SDSS). For each spectrum, we provide measurements of the
2 spectraltype,anumberofmolecularbandheads,andthe Hα, Hβ,Hγ,Hδ andCaIIKemissionlines.
In addition, we calculate the metallicity-sensitive parameter ζ and identify a relationship between ζ
n
and the g −r and r −z colors of M dwarfs. We assess the precision of our spectral types (which
a
J were assigned by individual examination), review the bulk attributes of the sample, and examine
the magnetic activity properties of M dwarfs, in particular those traced by the higher order Balmer
0
2 transitions. Our catalogis cross-matchedto Two MicronAll Sky Survey(2MASS) infrareddata,and
containsphotometricdistancesforeachstar. Lastly,weidentifyeightnewlate-typeMdwarfsthatare

] possibly within 25 pc of the Sun. Future studies will use these data to thoroughly examine magnetic
R activityandkinematicsinlate-typeMdwarfsandexaminethe chemicalanddynamicalhistoryofthe
S local Milky Way.
. Subject headings: stars: low-mass — brown dwarfs — stars: activity — stars: late-type — stars:
h
p abundances — Galaxy: kinematics and dynamics
-
o
r 1. INTRODUCTION (Covey et al. 2008b; Bochanski et al. 2010).
t
s Over the past decade, wide-field, deep astronomical M dwarfs have main sequence lifetimes that are con-
a siderablylongerthanthe ageofthe Universeandcanbe
surveys have providedan unprecedented statisticalplat-
[ usedtotracethe evolutionofbothstellarpropertiesand
form for studying the Universe (e.g. Sloan Digital Sky
the Milky Way disks. West et al. (2006, 2008) showed
2 Survey, Two Micron All Sky Survey, 2dF, UKIRT In-
v frared Deep Sky Survey). For the lowest-mass and most that magnetic activity (as traced by Hα) in M dwarfs
2 populous stars in the Milky Way (M dwarfs), the Sloan decreases with age and that M dwarfs appear to have
8 DigitalSkySurvey(SDSS)hasyieldedphotometricsam- finite activity lifetimes from ∼1-2 Gyr for early-type M
0 dwarfs (M0-M3) to ∼7-8 Gyr for later type stars (M5-
ples that exceed 30 million stars (e.g. Bochanski et al.
1 M7). These results are important for the habitability of
2010)and spectroscopicsamples of over40,000M and L
. extrasolar planets orbiting M dwarfs (e.g. Charbonneau
1 dwarfs (West et al. 2008). Previous SDSS studies have
et al. 2009), as active stars may disrupt planetary at-
0 elucidated the mean properties of low-mass stars (Haw-
1 ley et al. 2002; West et al. 2004, 2005; Bochanski et al. mospheres (Segura et al. 2010). The Hα emission does
1 2007b;Schmidtetal.2010b),theirmagneticactivityand nothavesufficientenergytosignificantlyaffectplanetary
: systems but has been found to be correlate with X-ray
v flaringpropertiesasafunctionofmass(Westetal.2004,
emission (Reid et al. 1995b; Covey et al. 2008a), which
i 2008;Kowalskietal.2009;Kruseetal.2010;Hiltonetal.
X can interact with extrasolar planet atmospheres.
2010),aswellasthestructureandkinematicsofthelocal
While Hα is the most commonly studied emission line
r Milky Way thin and thick disks (Bochanskiet al.2007a;
a inMdwarfs,thehigher-energyhydrogenBalmerandCa
Juri´cetal.2008;Fuchsetal.2009;Bochanskietal.2010)
II transitions are also present in the optical spectra of
and the low-mass initial mass and luminosity functions
active stars. The higher energy emission lines appear to
1 Correspondingauthor: [email protected] trace different temperature regions in the chromosphere
2 Department of Astronomy, Boston University, 725 Common- and can be used to characterize the upper atmospheres
wealthAve,Boston,MA02215 of active M dwarfs (Walkowicz & Hawley 2009, and ref-
3 MIT Kavli Institute for Astrophysics and Space Research, 77
erences therein). However, lacking large samples of low-
Massachusetts Ave,Cambridge,MA02139-4307
4 Department of Astronomy, University of Washington, Box mass stars with spectroscopic coverage across the entire
351580, Seattle, WA98195 optical bandpass, it is still not clear how the various
5 Department of Astronomy, Cornell University, 610 Space Sci- activity indicators trace each other in active M dwarfs
encesBuilding,Ithaca, NY14853
6 HubbleFellow (Rauscher & Marcy 2006; Walkowicz & Hawley 2009).
7 WellsCollege,DepartmentofMathematical andPhysicalSci- Westetal.(2008,hereafterW08)alsoshowedthatthe
ences,170MainStreet,Aurora,NY13026 ratio of CaH/TiO molecular indices (a quantity that is
8 DepartmentofPhysicsandAstronomy,VanderbiltUniversity, likely related to metallicity; Gizis 1997) decreased with
6301Stevenson Center,Nashville,TN37235
9 Astronomy and Astrophysics Department, The Pennsylvania height in the Galactic disk (a proxy for age). This tan-
StateUniversity,525DaveyLab,UniversityPark,PA16802 talizing result suggests that M dwarfs canbe used to re-
10 Visiting Investigator, Department of Terrestrial Magnetism, construct the chemical evolution of the local Milky Way
CarnegieInstituteofWashington, 5241BroadBranchRoad,NW, disk. However,the inability to assignaccuratemetallici-
Washington, DC20015
2 West et al.
ties to individual M dwarfs limits a detailed exploration presentationandcharacterizationofanewspectroscopic
of Galactic chemical evolution. Several recent studies sample, we have included a general magnetic activity
have made progressin calibrating the M dwarf metallic- analysis as well as a discussion about the future use of
ity scale. Rojas-Ayala et al. (2010) used infrared spec- this sample to probe Galactic chemical evolution. We
troscopy of M dwarfs in wide binaries with higher-mass describe our sample selection techniques in Section 2,
stars to calibrate infrared spectral features with metal- specifically discussing our manual inspection of all M
licity. In the optical, L´epine et al. (2007) developed a dwarf candidates. Section 3 gives the mean properties
metallicity-dependent quantity ζ, which is a relationbe- of the sample, including a new activity analysis of the
tween the CaH and TiO molecular indices and defined hydrogenBalmerandCaII Kemissionfeatures. Section
as: 3 also contains a list of candidate M dwarfs within 25
pc and explores how the metallicity sensitive parameter
1−TiO5 ζ varies as a function of stellar parameters. We discuss
ζ = , (1)
1−[TiO5] our results and future use of our new sample in Section
Z⊙
4.
where
2. DATAANDSAMPLESELECTION
[TiO5]Z⊙ = −0.164(CaH2+CaH3)3 The SDSS provides a large, uniform photometric and
+0.670(CaH2+CaH3)2 (2) spectroscopicdatasetfromwhichto extracthigh-quality
samples of low-mass dwarfs (Gunn et al. 1998; Fukugita
−0.118(CaH2+CaH3)−0.050.
et al. 1996; Hogg et al. 2001; Gunn et al. 2006; Ivezi´c
UsingwidebinariesconsistingofanMdwarfandahigher et al. 2004; Pier et al. 2003; Smith et al. 2002; Tucker
mass star, Woolf et al. (2009) showed that the ζ index et al. 2006). We used the DR7 CasJobs12 tool to select
is a good discriminant between high and low-metallicity 116,161 M dwarf candidate objects with SDSS spectra
([Fe/H] =0 and [Fe/H] = -1 respectively) for early-type that 1) fell within the typical color range for M and L
M dwarfs (∼M0-M3). While ζ is a useful tool for ex- dwarfs(r−i>0.42andi−z >0.24;W08,Kowalskietal.
aminingtherelativemetallicity amongstars,finerdeter- 2009); 2) had SDSS spectral classifications of “STAR”
minations of an absolute metallicity from ζ will require or “STAR LATE”; and 3) had radial velocities smaller
additional calibration. than 1500 km s−1. Kowalski et al. (2009) found that
BeforetheSDSS(Yorketal.2000),thelargestspectro- the color cuts used in previous studies did not extend
scopic samples of M dwarfs contained only a few thou- blue enough(r−i in particular)to include allofthe M0
sand individual stars (Reid et al. 1995a) and were pri- dwarfs. Our new (bluer) color selection corrects for this
marily focused on the red portion of the optical spec- previous oversight.
trum (∼6000-8000 ˚A). The spectroscopic catalogs of M Thespectraforall116,161candidateswerevisuallyin-
dwarfsfromthe firstfewSDSSdatareleasesquicklysur- spected and assigned spectral types using the Hammer
passed previous records for sample size (Hawley et al. spectral typing facility (Covey et al. 2007). Low signal-
2002; West et al. 2004) and had spectral coverage that to-noise ratio (SNR) spectra (SNR <3 at ∼8300 ˚A) and
spanned the entire optical band (∼3900-9300 ˚A) at low extragalactic interlopers were removed during visual in-
resolution(R∼1800). W08presentedaspectroscopiccat- spection, resulting in 109,639 objects. We also removed
alog of more than 44,000M dwarfs from the SDSS Data stellar spectra that were not identified as M dwarfs dur-
Release 5 (DR5), more than doubling the M dwarfspec- ing visual inspection as well as those spectra that were
troscopic tally. Due to the large number of spectra in duplicatedinoursample(including6771spectrafor2661
the DR5 sample, spectral types were assigned using the M dwarfs that were taken on different days) and arrived
Hammer11automaticspectraltypingfacility(Coveyetal. at the 70,841 spectra of M dwarfs in our DR7 catalog.
2007). While the Hammer spectral types are generally While many of the objects that we removed were K or
good to within ±1 sub–type, there have been recent in- L dwarfs, others were white dwarf–M dwarf (WD-dM)
dications that there may be systematic offsets for some pairs and low-metallicity subdwarfs. All of the L dwarfs
of the M dwarf spectral types (see Section 2). There- that were removedfrom our DR7 sample were cataloged
fore, to ensure the highest quality sample, it is impor- and analyzed by Schmidt et al. (2010b). Many of the
tant to visually inspect eachspectrum. There have been other objects that were not included in our DR7 sample
twosubsequentSDSSdatareleasessincetheW08sample will be presented in future studies. Not all of the WD-
that include over 50,000 additional M dwarf candidates dMpairswereremovedduringthevisualinspection. We
andcontainmanysightlinesatlowGalacticlatitudes(as used the color cuts from Smolˇci´c et al. (2004, u−g <
partofthe SEGUE survey;Yanny etal.2009). The new 2, g −r > 0.3, r − i > 0.7, σu,g,r,i < 0.1) to identify
lines of sight extend both the radial and vertical extent additional WD-dM pairs (that avoided manual detec-
of the M dwarf sample, and provide a larger statistical tion). All of the 497 objects that matched these color
platform from which to probe the structure, kinematics criteria are flagged as “WDM” in our DR7 catalog. We
andevolutionofthe Milky Way usingits smalleststellar also defined a clean photometric sample using the SDSS
constituents. photometric processing flags (SATURATED, PEAK-
Here we present the latest spectroscopic catalog of M CENTER, NOTCHECKED, PSF FLUX INTERP, IN-
dwarfsfromDataRelease7(DR7;Abazajianetal.2009) TERP CENTER, BAD COUNTS ERROR were all set
of the SDSS. While the main thrust of this paper is the to zero in the r, i, and z bands)13, which resulted in
11 The Hammer software is available for public download at: 12 http://casjobs.sdss.org/
http://astro.washington.edu/users/slh/hammer 13 http://www.sdss.org/dr7/products/catalogs/flags.html
SDSS DR7 M Dwarfs 3
65,277 M dwarfs. While our catalog contains all of the SpT < M5 SpT ≥ M5
400 50
70,84 visually inspected M dwarf spectra, the “GOOD-
PHOT”and“WDM”flagscanbeusedtoobtainsamples 40
300
thatinclude goodphotometryandremovepossible WD-
dM pairs respectively. mber200 mber30
Radial velocities (RVs) were measured by cross- Nu Nu20
correlatingeach spectrum with the appropriateBochan-
ski et al. (2007b) M dwarf template. This method has 100 10
been shown to produce uncertainties ranging from 7-10
kms−1 (Bochanskietal.2007b). Allofthe DR7 objects 0 0
-2 -1 0 1 2 -2 -1 0 1 2
were cross-matched to the USNO-B/SDSS proper mo- Hammer - Average Visual Spectral Type Hammer - Average Visual Spectral Type
tioncatalog(Munn etal. 2004,2008),identifying 39,151 Fig. 1.— The distribution of differences between the Hammer
M dwarfs with good proper motions14. Distances to automatic spectral types and the mean of the visually inspected
spectraltypesforMdwarfswithvisualtypesearlierthanM5(left)
each star were calculated using the M vs. r−z color-
r and M5 and later (right). While there is good agreement for the
magnituderelationgiveninBochanskietal.(2010). Our early-typeM dwarfs,thereis asystematic offset inthe automatic
calculated distances have uncertainties of ∼20%, arising typesassignedbytheHammer;theHammerassignsanearliertype
mostly from the intrinsic spread of the main sequence. ∼38%ofthetime.
The proper motions and distances were combined with
the RVs to produce 3-dimensional space motions for the
DR7 M dwarfs. Although we include the standard U,
SDSSsamplethatweredeterminedautomaticallybythe
V, W space motions in our catalog, we caution that the
“Hammer”spectraltypingalgorithm(Coveyetal.2007).
U, V, W velocities are in a Cartesian coordinate sys-
Thebiaswasdetectedasasystematicoffsetforlate-type
tem that may not be appropriate for stars at apprecia-
stars,whoseautomaticclassificationwasoften1subtype
ble distances from the Sun. We therefore also include
earlier than determined via visual inspection (see Fig-
the Galactic radial (R), tangential (Θ) and vertical (Z)
ure 1). We thus decided that visual inspection would
cylindrical components of the position and velocity for
producethemostreliableandprecisespectroscopicsam-
each star for which we have 3D space motions.
ple. We visually inspected all 116,161 M dwarf candi-
We also matched our catalog to the 2MASS point
dates andmanuallyassignedspectraltypes. The sample
source catalog (Cutri et al. 2003), matching only to
′′ was divided among 17 individuals15 who used the man-
unique 2MASS counterparts within 5 of the SDSS po-
ual“eyecheck”mode of the Hammer (v. 1 2 5)to assign
sition that do not fall within the boundaries of an ex-
spectral types and remove non-M dwarf interlopers.
tended source (gal contam =0). To ensure that we
Figure 1 shows the difference between the spectral
used only high quality 2MASS photometric data, we
types automaticallydetermined by the Hammer and the
applied additional cuts to each of the J, H, and K
S mean visual inspection. For early-type M dwarfs (left
bands. If the source was not detected (rd flg =0),
panel), the Hammer and the spectral typers agree most
nominallydetected(rd flg=6),wasdetectedbutunre-
ofthetime. However,for∼38%ofthelate-typeMdwarfs
solved(rd flg=9),orhadcontaminated/confusedpho-
(right panel), the Hammer assigns spectral types 1 sub-
tometry(cc flag6=0)ina particularband, the 2MASS
typeearlierthantheaveragehumanspectraltyper. This
data were not included. This resulted in 57,956 2MASS
confirms that while the Hammer generates automatic
counterparts with J, H and K magnitudes and their
S spectral types within the quoted ±1 subtype accuracy
uncertainties that were included in the catalog.
(Coveyet al. 2007),there is a systematic offset of 1 sub-
We used the 2MASS photometry to investigate any
type for the late-type stars, and justifies our effort to
possible M giant contamination. Bessell & Brett (1988)
manually inspect more than 116,000 candidates.
foundthat MgiantsandMdwarfsseparateinJ−H vs.
Eachspectral typer also examined a control sample of
H−K color space due to differences in H O absorption
2 1000spectra, of which 638 were M dwarfs. We used this
in their atmospheres. By comparing the DR7 J − H
controlsampleto assessthe qualityandreliabilityofthe
vs. H −K color-color diagram with that of Bessell &
visually inspected sample by quantifying the variations
Brett (1988), we find that no more than 0.5% of our
among the 17 individual typers, and by comparing the
samplecouldbegiants. Inaddition,Coveyetal.(2008b)
median visual type for each star to its automatic Ham-
conducted a complete magnitude-limited spectroscopic
surveyofa12◦fieldintheSDSSfootprintandconcluded mer type. The results of the control sample show that
there is excellent agreement between all of the spectral
that the giant contamination rate was less than 2% for
typers with a large fraction of the stars being assigned.
stars redder than a spectral type of K5.
Almostallofthevisualclassificationsinthecontrolsam-
pleagreedtowithin±1subtype ofthe medianvalue;for
2.1. Spectral Typing most of the stars, the dispersion in visual classifications
PreviouslargespectroscopicsamplesofMdwarfshave was<0.4 subtypes. To ensure that there were no major
relied on automated spectral typing due to the quantity systematics in the spectral types assigned by any indi-
of time required to manually inspect tens of thousands vidual, we examined the mean difference between each
of candidate spectra. Systematic discrepancies were re- typer’s visual classifications of the control sample spec-
cently identified in the spectral types from the W08 tra and the median classifications of the entire group.
We divided the sample into the early types (< M5) and
14 GoodpropermotionsaredefinedasthosewithMATCH=1,
DIST22>7,SIGRA<1000,SIGDEC <1000and(NFIT =6or 15Theorderoftheco-authorswasbasedonthenumberofspec-
(NFIT=5and(O<2orJ<2)))(Munnetal.2004). traexamined.
4 West et al.
late types (≥ M5) to investigate any spectral type de- automatically measuring activity in a subsample of 300
pendence in typing quality: all of the spectral typers M dwarfs (that spanned a range of SNR and spectral
clustered near the median values with no individual dis- types) using 10 different EW activity thresholds. Addi-
playing a systematic offset larger than 0.2 subtypes. tionally, each spectrum was visually inspected and the
activity state for all five emission lines was determined.
2.2. Measured Quantities
By comparing the results of our manual inspection with
Aspartofouranalysiswemeasuredanumberofspec- the 10 different automatic routines, we determined the
tral lines and molecular features in each M dwarf spec- EW thresholds that most accurately reproduced our vi-
trum. Allofthespectralmeasurementsweremadeusing sual activity classifications. The EW activity threshold
theRVcorrectedspectra. TheTiO1,TiO2,TiO3,TiO4, for each of the five emission lines can be found in Table
TiO5, TiO8, CaOH, CaH1, CaH2, and CaH3 molecu- 1. We found that the previous Hα activity limit of 1 ˚A
larbandheadindices andtheir formaluncertainties were (West et al. 2004) was slightly too high and excluded a
measured using the Hammer with the molecular band- number of good detections. We have therefore reduced
headsasdefinedby Reidetal.(1995a)andGizis(1997). the Hα EW activity criterion to 0.75 ˚A.
We also measured the chromospheric hydrogen Balmer Recently, Kruse et al. (2010, hereafter K10) investi-
and Ca II lines that are associated with magnetic activ- gatedthetime variabilityofSDSSMdwarfsanddefined
ity. We expanded the Hα analysis of West et al. (2004, their Hα activity thresholds based on the median EW
2008)to include Hβ, Hγ, Hδ and Ca II K (Hǫ and Ca II values that were detected at a 3σ confidence level as a
HareblendedinSDSSdataandwerenotincludedinour function of spectral type. Although the K10 threshold
sample). All of the line measurements were made by in- EWs provide a robust way to select a clean sample, it
tegratingoverthe specific line region(8 ˚A wide centered unduly biases the sample toward the most active stars
onthe line) andsubtracting offthe meanflux calculated at later spectral types. For example, our DR7 catalog
fromtwoadjacentcontinuumregions(ContinuumAand contains a large number of active M6 dwarfs (that have
ContinuumB inTable 1). Equivalentwidths (EW)were Hα emission lines with peak values that are more than
computed for each line by dividing the integrated line 5 times larger than the noise) that have EWs below the
flux by the mean continuum value (as in previous stud- 3˚A threshold used in the K10 study. In addition, the
ies, we define the EW to be positive for emission lines; activity fractions presented in K10 (see their Figure 4)
Hawleyetal.1996;Westetal.2004). Thelow-resolution include these active stars (with EW < 3˚A) in the “in-
of SDSS spectra does not affect our ability to measure active” component of the activity fraction denominator.
accuratelinevalues,withtheexceptionofCaIIK.Dueto ThisexplainswhytheactivityfractionsreportedbyK10
theintrinsicCaIIKabsorptionpresentinMdwarfphoto- are smaller than previous determinations (West et al.
spheres,weakactivityappearsasanemissionpeakinthe 2004, 2008) and indicates a serious selection bias in the
absorption line. This weak emission cannot be resolved K10 results.
in SDSS spectra. However strong CaII K emission over- For all of the active stars in the sample we computed
whelms the absorption and can be easily recoveredfrom theratioofluminosityintheemissionlineascomparedto
low-resolutionspectra(Walkowicz& Hawley 2009). The the bolometric luminosity (L /L ). We followed the
line bol
linesandcontinuumregions(invacuumwavelengths)can methodsofHall(1996),Walkowiczetal.(2004)andWest
be found in Table 1. Formal uncertainties on the EWs &Hawley(2008)whoderivedχfactorsfortheHydrogen
for each line were also computed. BalmerandCaII chromosphericlinesasafunctionofM
Similar to the West et al. (2004, 2008) samples, we dwarf spectral type. The χ factor uses empirical results
defined magnetically active stars as those that had de- forthebolometricluminositiesofMdwarfsasafunction
tectable emission lines in their spectra. Previously, this of color and/or spectral type (Leggett et al. 1996, 2001)
hadbeendonesolelyforHα. Tobeclassifiedasactivein and relates the EW of a line to the fraction of the bolo-
aspecific emissionline, the followingfour criteriahadto metric luminosity emitted by the line. The L /L
line bol
be satisfied: 1) the EW of the line was larger than some values were computed by multiplying the EW of each
minimum value (see below); 2) the EW value must be 3 active star by the appropriate χ value. Formal uncer-
times the uncertainty; 3) the signalto noise ratio (SNR) tainties werecomputed for eachL /L value andare
line bol
inthecontinuummustbelargerthan3;and4)theheight included in the final database.
ofthe spectrallinemustbe largerthan3times the noise Wealsocomputedthemetalsensitiveparameterζ,de-
inthecontinuum. Starswereclassifiedasinactiveifthey fined by L´epine et al. (2007), which uses a combination
met criterion#3 and had no detectable emission. Crite- ofthe TiO5,CaH2,andCaH3molecularbandindices to
rion #3 preferentially selects brighter stars at any given separatethe sampleintodifferentmetallicity classes(see
colororspectraltype. Thisselectionbiasshouldnothave Equations1&2). ThisissimilartotheGizis(1997)clas-
alargeeffectour resultssince weexamine the properties sification system but was re-calibrated using wide com-
of our sample in a Galactic context and both active and monpropermotionpairsthatwereassumedtobe atthe
inactive stars with low SNR should be removed. How- samemetallicity. Starswithsolarmetallicity([Fe/H]=0)
ever, if active stars are intrinsically more luminous than have ζ values ∼1 and stars with [Fe/H]=-1 have ζ ∼0.4
their inactive counterparts (at the same color; see Sec- (Woolf et al.2009). Although there is considerable scat-
tion3.2below),criterion#3mayservetoincludeslightly ter in the [Fe/H] versus ζ relation at high-metallicities,
more active stars in our resulting analyses. this parameter is very useful for finding and classify-
Because the strength of the stellar continuum changes ing low-metallicity stars that are likely members of the
dramatically from CaII K to Hα in M dwarfs, the EW Galactic halo.
thresholds used in criterion #1 are different for each Ourcatalogcontainingallofthemeasuredquantitiesis
line. We experimentally derived the EW thresholds by
SDSS DR7 M Dwarfs 5
TABLE 1
Emission Lines
Line Central ContinuumA ContinuumB ActivityLimit
Wavelength (˚A) (˚A) (˚A) EW(˚A)
Hα 6564.66 6555.0–6560.0 6570.0–6575.0 0.75
Hβ 4862.69 4840.0–4850.0 4875.0-4885.0 1.00
Hγ 4341.69 4310.0–4330.0 4350.0–4370.0 1.00
Hδ 4102.90 4075.0–4095.0 4110.0–4130.0 1.50
CaIIK 3934.78 3952.8–3956.0 3974.0–3976.0 1.50
Note. — All of thewavelengths are given in vacuum units
publicly availableonthe Vizier16 site orcanbe obtained
by contacting the corresponding author. The individual
spectra are available from the SDSS DR7 website17. As 10000
with previous SDSS spectroscopic catalogs of low-mass
stars, we remind the community that these data do not
1000
represent a complete sample and that the complicated DR7 Sample
GOODPHOT = 1
SDSSspectraltargeting(seeSection3.1)introducesava- GOODPM = 1
rietyofselectioneffects. However,ournewsamplecovers 100
alargerangeofvaluesformanyofthephysicalattributes
of the M dwarfs, including parameters that are sensitive
10
to activity, metallicity, and Galactic motion, making ac-
curate activity, kinematic, and chemical analyses possi-
ble. In addition, because some of the derived quantities 1
M0 M1 M2 M3 M4 M5 M6 M7 M8 M9
are computed by automatic routines, values for a small Spectral Type
percentage (∼4%; West et al. 2004) of individual stars
may be incorrect; this should not affect large statistical Fig. 2.—Thedistributionofspectraltypesinthespectroscopic
SDSSDR7Mdwarfcatalog(solid). Thedottedlinerepresentsthe
results. Users are nevertheless cautioned to understand
stars for which good quality photometry exists (GOODPHOT =
the origin of specific data products before using them 1) and the dashed line indicates the stars for which good proper
indiscriminately. motionsexist(GOODPM=1). Duetotheshallowdepthsandblue
sensitivityof theUSNO-Bcatalog, thereareonlyasmallnumber
ofmeasuredpropermotionsofthelatest-type Mdwarfs.
3. RESULTS
Figure2showsthedistributionofspectraltypesforthe
DR7 low-massstar sample. We included the entire visu-
allyconfirmedsample(solid),thestarsforwhichwehave objects in the SDSS footprint, the added uncertainties
goodphotometry(asindicatedbyourGOODPHOTflag; to the distances are typically less than 5%. We are cur-
dotted), and the stars for which we have 3D kinematic rently determining the amount of extinction needed to
information (as set by our GOODPM flag; dashed). A reproduce an unreddened spectrum for each object in
majorityofthelate-typeMdwarfsareremovedfromthe our DR7 sample, and will report on that analysis in a
kinematic sample because they are too faint to be de- future paper (Jones et al. 2011).
tected in the USNO-B catalog. Future studies will mea- We computed median colors for each subtype of the
sure proper motions for the late-type M dwarfs in the DR7 sample, applying stringent cuts to include only
DR7 catalogandcreate a morerepresentativekinematic those stars with the highest quality photometry. To be
sample. includedinthemediancolorcalculation,SDSSstarshad
tohaveGOODPHOT=1,r-bandextinction<0.05mag-
3.1. Median Colors nitudes, and r, i and z-band magnitude uncertainties
<0.05 magnitudes. Median colors incorporating 2MASS
AllofthemagnitudesincludedinourDR7catalogand
magnitudeswerecalculatedfromthesubsetofstarswith
used to compute the median colors were extinction cor-
J, H, and K uncertainties <0.05. The median colors
rected using the Schlegel et al. (1998) line-of-sight dust S
and standard deviations (in parentheses) are shown in
maps(andthedustmodelofCardellietal.1989,andR
V
Table 2. Due to the systematic offset in the Hammer
=3.1). We provideextinctioncorrectionsforeachband
automatic spectral types for late-type M dwarfs, our av-
in the catalog so that observed magnitudes can be eas-
erage colors are bluer than previously reported (W08,
ily reproduced. Because of the close proximity of many
Kowalski et al. 2009). We include the g −r colors but
of the M dwarfs in the sample, the entire Schlegel et al.
warn that they are likely metallicity dependent (West
(1998)extinctionislikelyanoverestimateformanylines-
et al. 2004; Bochanski et al. 2007b, see also Section 3.3
of-sight. The uncertainty in the extinction correction
below).
introduces a small uncertainty in the derived distances.
Figure 3 shows the r versus r − z Hess diagram for
However, given the small amount of extinction toward
the M dwarfs in the SDSS DR7 sample. The gaps in
16 http://vizier.u-strasbg.fr/cgi-bin/VizieR color space reflect the non-uniform sampling due to the
17 http://www.sdss.org/dr7 spectral targeting algorithm. To quantify the effect of
6 West et al.
TABLE 2
Median Colors
Sp. Type NSDSSa N2MASSb g−rc r−i i−z z−J J−H H−KS
M0 915 259 1.31(0.16) 0.56(0.08) 0.33(0.06) 1.20(0.25) 0.64(0.07) 0.16(0.07)
M1 699 230 1.39(0.16) 0.73(0.10) 0.41(0.08) 1.32(0.41) 0.62(0.08) 0.19(0.08)
M2 1078 689 1.40(0.13) 0.96(0.10) 0.53(0.08) 1.26(0.18) 0.60(0.07) 0.22(0.08)
M3 1220 831 1.41(0.13) 1.13(0.11) 0.61(0.08) 1.30(0.24) 0.59(0.06) 0.23(0.06)
M4 814 522 1.46(0.15) 1.33(0.13) 0.71(0.09) 1.37(0.15) 0.59(0.06) 0.25(0.06)
M5 328 269 1.52(0.13) 1.62(0.21) 0.90(0.13) 1.47(0.12) 0.59(0.06) 0.28(0.07)
M6 359 401 1.56(0.15) 1.92(0.14) 1.05(0.07) 1.61(0.08) 0.60(0.06) 0.31(0.08)
M7 160 352 1.58(0.16) 2.09(0.19) 1.14(0.10) 1.71(0.10) 0.60(0.08) 0.34(0.07)
M8 6 118 1.64(0.12) 2.56(0.24) 1.41(0.13) 1.93(0.13) 0.63(0.07) 0.39(0.08)
M9 4 77 1.87(0.74) 2.70(0.20) 1.71(0.18) 2.15(0.14) 0.69(0.09) 0.42(0.07)
Note. — The median and rms scatter (in parentheses) of the extinctioncorrected SDSS-2MASS colors for each M dwarf
spectraltype. u−g colorswerenotincludedduetothelargeuncertaintiesintheu-bandphotometryformostofthesample
Mdwarfs. Theg−rcolorsareincludedbutarelikelymetallicitydependent(Westetal.2004;Bochanskietal.2007b).
aTheSDSSnumbersreflectthestarsthathavegoodSDSSphotometry(theGOODPHOTflagsetto1)andhaver,iandz
photometricuncertainties<0.05magnitudes.
bThe2MASSnumbersindicatethenumberofstarsthathavez,J,H andKS-banduncertainties<0.05magnitudes.
cTheg−rcolorswerecalculatedusingstarswithgoodSDSSphotometry(theGOODPHOTflagsetto1)andgandr-band
photometricuncertainties<0.05magnitudes.
SDSS spectroscopic targeting algorithms on the median
12
colors, we conducted the following test. We began with
the straw man assumption that the median r−i, i−z
and r−z color-spectraltype relations reported in Table
14
2arerepresentativeofthe underlyingstellarpopulation.
We then used these quantities, along with the observed
color spread in each spectral type bin, to construct a 16
synthetic stellar population with a uniform distribution
in both magnitude and spectral type. We then applied
photometric cuts to the smooth underlying distribution, 18
replicatingtheSDSSspectroscopicselectionandproduc- r
ing a synthetic “observed” sample that is highly struc-
tured in color-magnitude space. The median colors and 20
spreads were then calculated as a function of spectral
type from this synthetic sample and compared to the
measured colors in Table 2. In general, the colors of the 22
syntheticstarsagreetowithin0.02magsoftheobserved
sample (an agreementthat would not be expected if the
24
underlying population were different from the observed
SDSS stars ) and we conclude that the colors presented
in Table 2 are not severely affected by the spectroscopic -1 0 1 2 3 4 5 6
selection. r-z
3.2. Activity
Fig. 3.—Therversusr−zHessdiagramfortheMdwarfsinthe
Figure 4 shows the Hα activity fraction as a function
SDSSDR7sample. Thegapsincolorspacereflectthenon-uniform
of M dwarf spectral type for stars with data quality suf- sampling due to the spectral targeting algorithm. The irregular
ficient to measure Hα emission (and GOODPHOT =1 samplinghasaminimaleffectonthemediancolorsderivedinthe
and WDM =0). Our results are in good agreementwith DR7sample.
previousstudies (Westetal.2004,W08),butwithmuch
lower uncertainties due to the ∼59,000 M dwarfs used
to generate Figure 4 (7952 are Hα active and 51,034are pendent on the location of the samples in the Galaxy.
Hα inactive). There are a few small differences between Because M dwarfs have finite activity lifetimes and are
Figure 4 and the Hα activity fractions reported by W08 dynamicallyheatedintheGalacticdiskastheyage,their
thatarelikelydue tothe changesinsomeofthe spectral activity state is correlated with position in the Galaxy.
types(seeabove)andthedifferentGalacticsightlinesin- OneofthereasonsthattheMdwarfsinoursamplehave
cluded in the DR7 sample. Because of the lower SNR in muchsmalleractivityfractionsthanthosestudiednearby
the blue portion of M dwarfs, fewer stars were available (e.g. Hawley et al. 1996; Gizis et al. 2000), is that the
tomeasureactivityforthehigherorderBalmerandCaII SDSS volume is muchlargerthanthose usedin previous
Kemissionlines. Table3 givesthe numberofactiveand catalogs and is concentrated on the north galactic cap,
inactive stars for each emission line indicator that had yielding a much older stellar population. This is par-
the necessary quality for our activity analysis. ticularly clear for the early-type M dwarfs, which have
AsdiscussedinW08,theactivityfractionishighlyde- mediandistancesgreaterthan500pc,andthereforeages
SDSS DR7 M Dwarfs 7
enough to measure (i.e. are sensitive to) Hβ (top left),
Hγ (top right), Hδ (bottom left) and CaII K (bottom
1.0
right) activity as well as the emission lines redder than
0.8 407 135 that specific tracer. The activity fractions for all of the
linesareinexcellentagreementwitheachotherandindi-
e
n Activ 0.6 4011 3295 caamteoutnhtatoafltlifimveedeumriisnsgiotnhleinleifsetairmeeproofdaunceMdfdowrathrfe.sTamhies
o
cti 0.4 3252 new result is complementary to previous studies that
a
Fr found strong correlations among different emission line
0.2 7972 strengthsinMdwarfs(Rauscher&Marcy2006;Walkow-
10562 8728 9801 10350 icz & Hawley 2009). Although the relative emission line
0.0 strengthsforindividualstarsmayfluctuateovertime(see
Cincunegui et al. 2007),the mean values of the emission
M0 M1 M2 M3 M4 M5 M6 M7 M8 M9
Spectral Type lines are well correlated in the large DR7 sample. The
unprecedentedsizeandGalacticdistributionofourspec-
Fig. 4.—TheHαmagneticactivityfractionasafunctionofspec- troscopicsampleconfirmsthatnotonlythestrength,but
tral type. Error bars were calculated from the binomial distribu-
the duration of activity appears to be similar for all of
tionandthenumberaboveeachdatapointindicatesthenumbers
ofstarsinthatbin. Seetextformoredetails. the Balmer and CaII activity indicators.
Figure6 alsodemonstratesa possible (andimportant)
selection effect present in magnitude-limited or activity-
TABLE 3 selected samples. A close examination of Figure 6 (and
Activity Indicators Figure5)revealsthattheactivityfractionsarelargerfor
the stars that are sensitive to the bluer emission lines.
Line Nactive Ninactive ActiveFraction Thistrendwasnotexpectedsincethestarsusedtocom-
Hα 7952 51034 0.13 putethe activityfractionweredrawnfromthesamevol-
Hβ 2236 35144 0.06 umes in each of the four panels. The only difference was
Hγ 1175 18816 0.06
that the stars selected for the bluer emission line analy-
Hδ 528 14664 0.03
CaIIK 620 9053 0.06 seswererequiredtohavehigherSNR.Withthisinmind,
we can explain the different activity fractions as follows.
The actual spread in absolute magnitude (or luminos-
ity) at a given spectral type can be quite significant.18
that are considerably older than their shortactivity life- Someofthisspreadmaybeduetodifferencesinthephys-
times (∼ 1-2 Gyr). We reiterate the warning from W08 ical properties of the stars. Hawley et al. (1996) showed
that activity fractions in M dwarfs must be discussed in that magnetically active stars are brighter in MV than
the proper Galactic context. their inactive counterparts. Recently, Bochanski et al.
The various emission lines measured in our spectra (2011, hereafter Paper II) found that both active and
are formed at slightly different locations in the chro- highermetallicityMdwarfs(manystarsareboth)appear
mosphere, suggesting that the strength of one emission tobebrighterinMr atagivencolororspectraltype. Be-
line may not necessarilypredict the strength of another. cause the bluer emission lines that we are examining in
We therefore examinedhow the various activity induced Figure 6 require higher SNR spectra to accurately mea-
emission lines trace each other as a function of spectral sure activity, the M4 dwarfs at a given distance that
type and absolute distance from the Galactic plane (a meetthe SNRcriterionarepreferentiallymoreluminous
proxy for age). Figure 5 shows the Hα (diamonds) and than other stars in the same distance bin. If activity is
Hβ (asterisks) activity fractions for M2-M7 dwarfs as a correlated with luminosity, our SNR selection will bias
function of Galactic height. Only the Hα activity frac- our sample towardcontaining a higher fraction of active
tions for stars that could have detected Hβ emission are stars.
included. This limits the sample to the brighter stars at In addition to activity fractions, we also investigated
eachspectraltype thatmeetthe SNRcriterionanddoes the median luminosity in each emission line as a func-
not include all of the M dwarfs that are sensitive to Hα tion of spectral type. Previous studies have investigated
emission. Spectral types earlier than M2 and later than how the Hα luminosity changes as a function of spectral
M7 were not included due to an insufficient sample size type (Hawley et al. 1996; Gizis et al. 2000; Burgasser
of active stars. As seen in previous studies, the mag- et al. 2002; West et al. 2004). These studies found that
netic activity fractions decrease as a function of Galac- thefractionofluminosityemittedinHα isconstantover
tic height, confirming an age-activity relationship; stars the range M0-M5 and decreases by almost an order of
closer to the Plane are statistically younger and more magnitudefor late-typeMdwarfs. However,mostofthe
likely to be active. Figure 5 also demonstrates that Hα previousstudiesconcentratedonHαanddidnotinclude
and Hβ trace each other extremely well for the entire similar analyses for the other Balmer or CaII lines, and
DR7 sample, implying that the Hβ activity lifetimes are those that did were based on a small sample of stars
essentially the same as those for Hα. (Hawley & Pettersen 1991;West & Hawley 2008).
Theotheractivitytracersalsoappeartocorrelatewith
Hα emission. Figure 6 shows the activity fractions for 18 Recent studies have confirmed that the spread in absolute
magnitudeasafunctionofcolorismuchsmaller(Bochanskietal.
all five emission lines in M4 dwarfs as a function of ver-
2010, 2011; Faherty et al. 2011). This is the primary reason that
tical distance from the Galactic plane. Each panel plots ourdistances areestimatedfromcolorsandnotspectraltypes.
the activity fractions for stars where the data are good
8 West et al.
M2 M3 M4
0.4
0.08 0.25
0.20 Hα 0.3
e 0.06 e Hβ e
v v v
cti cti 0.15 cti
A A A
n n n 0.2
o 0.04 o o
cti cti cti
a a 0.10 a
Fr Fr Fr
0.02 0.1
0.05
0.00 0.00 0.0
0 200 400 600 800 0 100 200 300 400 0 50 100 150 200 250 300
Absolute Vertical Distance from Plane (pc) Absolute Vertical Distance from Plane (pc) Absolute Vertical Distance from Plane (pc)
M5 M6 M7
1.0 1.0
0.8
0.8 0.8
e e e
v 0.6 v v
cti cti cti
n A n A 0.6 n A 0.6
o o o
cti 0.4 cti cti
a a a
Fr Fr 0.4 Fr 0.4
0.2
0.2 0.2
0.0 0.0 0.0
0 50 100 150 200 0 20 40 60 80 100 120 0 20 40 60 80 100 120
Absolute Vertical Distance from Plane (pc) Absolute Vertical Distance from Plane (pc) Absolute Vertical Distance from Plane (pc)
Fig. 5.—The HαandHβ activity fractions as a functionof absolute vertical distance fromthe Galactic plane forM2–M7dwarfs. For
this comparison, only the stars with data good enough to measure Hβ were included. Error bars indicate the binomial errors in the Hα
fractions – the Hβ uncertainties are similar. The Hβ activity fractions are in excellent agreement with the Hα data, indicating that Hβ
activityhasthesamedurationinMdwarfs.
EachpanelofFigure7showsthemedianluminosityof cause they span a large range of distance and contain a
a specific activity tracer (as compared to the bolometric significantnumberofstarswithζ uncertainty<0.1. Fig-
luminosity of the stars) for active stars sensitive to that ure 8 shows a clear decrease in ζ for both spectral types
particular emission line. All of the activity tracers ex- withincreasingGalacticheight. Thediscrepanciesinthe
hibit similar behavior,with the relative luminosity stay- nearbybinsreflectthescatterinthehigh-metallicityend
ingconstantfortheearly-typeMdwarfsandthenfalling of the ζ vs. metallicity relation (Woolf et al. 2009). The
to lower values at or around a spectral type of M5. The results shown in Figure 8 suggesta significant change in
medianvaluesandtheupperandlowerquartilesthatare themedianmetallicityofMdwarfs(andbyextensionthe
plotted in Figure 7 are also given in Table 4. The bluer Galaxy) as a function of age, with the oldest stars (that
emission lines produce smaller fractional luminosities as are farther from the Plane) having the lowest metallic-
expected,andareconsistentwithpreviousBalmerdecre- ity. Similartrendshavebeenseeninboththe radialand
mentstudies(Hawley&Pettersen1991;Bochanskietal. vertical directions from previous spectroscopic samples
2007b; West & Hawley 2008). W08 found evidence for of higher mass stars (Nordstr¨om et al. 2004;Ivezi´c et al.
a slight decrease in L /L as a function of Galactic 2008). However, the higher density and longer main se-
Hα bol
height, but the effect is small and not included in our quencelifetimes ofMdwarfsmayprovideabetter probe
analysis. ofthemetalcontent(andevolution)ofthelocalGalactic
disk. The ζ values of ∼0.6 in the most distant bins of
3.3. Metallicity Sensitive Features Figure 8 have metallicities of [Fe/H]∼-0.7 according to
Metallicity continues to be one of the most elusive the wide binary analysis of Woolf et al. (2009).19 We
physicalquantitiestomeasureinMdwarfs. Recentwork note that the photometric distances derived in our sam-
usinginfraredspectroscopyandimprovedphotometryfor plearelikelyaffectedbymetallicityandthatthefarthest
M dwarfs in wide binaries may soon lead to accurate bins inFigure8 (thathavelowermetallicities)arecloser
metallicity determinations forlargesamples ofMdwarfs than shown due to fainter Mr magnitudes (Paper II).
(Woolf et al. 2009; Johnson & Apps 2009; Rojas-Ayala Regardless,thisstrongtrendinmetallicitydemonstrates
et al. 2010). However, the L´epine et al. (2007) ζ pa- the utility of large spectroscopic samples of M dwarfs
rameter is currently the best indicator of metallicity in (suchastheonepresentedinthispaper)forunderstand-
optical M dwarf spectra. Using the DR7 M dwarf sam- ing the chemical evolution of the Milky Way. However,
ple,weexaminedhowζ changesasafunctionofGalactic
height. Figure 8 shows median ζ values for M1 and M2 19 The Woolf et al. (2009) [Fe/H] – ζ calibration was lim-
ited to ∼M0-M3dwarfs and has considerable scatter at the high-
dwarfs as a function of absolute height above the Galac-
metallicityendoftherelation.
tic plane. These specific spectral types were chosen be-
SDSS DR7 M Dwarfs 9
Sensitive to Hβ Activity Sensitive to Hγ Activity
0.8 0.8
e e
v v
cti 0.6 cti 0.6
A A
n n
o 0.4 o 0.4
cti cti
a a
r 0.2 r 0.2
F F
0.0 0.0
0 50 100 150 200 250 0 50 100 150 200 250
Absolute Vertical Distance from Plane (pc) Absolute Vertical Distance from Plane (pc)
Sensitive to Hδ Activity Sensitive to CaII K Activity
0.8 0.8
Hα
e e Hβ
ctiv 0.6 ctiv 0.6 HHγδ
A A CaIIK
n n
o 0.4 o 0.4
cti cti
a a
r 0.2 r 0.2
F F
0.0 0.0
0 50 100 150 200 250 0 50 100 150 200 250
Absolute Vertical Distance from Plane (pc) Absolute Vertical Distance from Plane (pc)
Fig. 6.—TheHα,Hβ,Hγ,HδandCaIIKactivityfractionsforM4dwarfsasafunctionofabsoluteverticaldistancefromtheGalactic
plane. Each panel includes only the stars where the data are good enough to be sensitive to that emission line (as well as all of the
redderemissionlines). UncertaintiesareincludedforHαbutaresimilarforalloftheemissionlines. Theactivityfractionshavethesame
magnitude anddecreasingtrendwithGalacticheight foralllines. Theactivity fractionsforstarssensitivetoHδ andCaIIK activityare
slightlyhigherthanforstarssensitivetoHβ andHγ.
TABLE 4
Luminosity inActivity Tracer Emission Lines
Spectral LHα/Lbol LHβ/Lbol LHγ/Lbol LHδ/Lbol LCaIIK/Lbol
Type (×10−4) (×10−4) (×10−4) (×10−4) (×10−4)
M0 1.27+−00..3291 0.85+−00..2197 0.50+−00..1038 0.76+−00..1496 0.33+−00..0171
M1 1.51+−00..4395 0.81+−00..2185 0.50+−00..2103 0.39+−00..0096 0.45+−00..2114
M2 1.42+−00..4355 0.77+−00..2264 0.44+−00..1155 0.32+−00..0089 0.34+−00..1088
M3 1.36+−00..4422 0.67+−00..2129 0.35+−00..1121 0.26+−00..1005 0.30+−00..1145
M4 1.44+−00..6437 0.61+−00..2292 0.35+−00..1143 0.29+−00..0190 0.28+−00..1111
M5 0.81+−00..2252 0.33+−00..1018 0.37+−00..1101 0.26+−00..0077 0.37+−00..0195
M6 0.57+−00..2103 0.22+−00..0076 0.13+−00..0022 0.08+−00..0022 0.17+−00..0045
M7 0.19+−00..0085 0.06+−00..0031 0.07+−00..0003 0.06+−00..0023 0.05+−00..0033
M8 0.27+−00..1027 0.14+−00..0065 ··· ··· ···
M9 0.20+−00..0056 0.12+−00..1047 ··· ··· ···
Note. —Bolometricallyscaledluminositieswerederivedusingstars
thatshowactivityineachemissionline. Reporteduncertaintiesarethe
25th and 75th quartile values.
additional calibration is required to convert quantities M dwarfs, like the one presented here, can help corre-
such as ζ into precise metallicities. late photometric properties of M dwarfs to spectroscop-
PreviousSDSS studieshaveshownthatthe g−r color ically derived physical parameters. Figure 9 shows the
of M dwarfs appears to correlate with metallicity (West g −r colors of M dwarfs as a function of ζ for M1-M4
et al. 2004; L´epine & Scholz 2008). Future large photo- dwarfs. Although there is a trend of decreasing ζ with
metricsurveyssuchasLSST(Ivezicetal.2008)willrely redder g−r color, the change in g−r is different across
heavily on the ability to classify objects based on their spectraltypes. Therefore,g−r coloraloneisnota good
broadbandcolors alone. A large spectroscopic sample of tracerofMdwarfmetallicityacrosstherangeofMdwarf
10 West et al.
10-4
ol
b
L
/
α
LH 10-5 Sensitive to Hα Activity

10-4
ol
b
L
/
β
LH 10-5 Sensitive to Hβ Activity

10-4
ol
b
L
/
γH
L 10-5 Sensitive to Hγ Activity

10-4
ol
b
L
/
δ
LH 10-5 Sensitive to Hδ Activity

10-4
ol
b
L
/
K
aII 10-5
C
L Sensitive to CaII K Activity
10-6
M0M1M2M3M4M5M6M7M8M9
Fig. 7.—TheratiooflineluminositytobolometricluminosityasafunctionofspectraltypeforHα,Hβ,Hγ,HδandCaIIK.Thehigher
orderBalmerandCaIIKemissionlineshaveprogressivelylowerluminositythantheHαlines. Alloftheemissionlinesshowadropinthe
luminosityratioforlate-typeMdwarfs.
spectral types. However, when we examined the ζ val- fit that relates ζ to the g−r and r−z colors:
ues as a function of both g −r and r −z (a proxy for
spectral type; see Table 2), we found a strong trend of
decreasingζ diagonaltotheg−r,r−z axisandperpen- ζ =1.04−0.98(g−r)2−0.07(r−z)2+1.07(g−r)
dicular to the stellar locus (Covey et al. 2007). Figure −0.53(r−z)+0.63(g−r)(r−z). (3)
10 highlights a region of color space that can be used
to probe the metallicity content of M dwarfs and obtain Equation 3 is valid over the ranges shown in Figure 10
low-metallicitymembersofthe Galactichalofromsolely and has typical uncertainties of 10-20%.
broadband photometry. Using a Levenberg-Marquardt
minimization technique, we derived a two-dimensional 3.4. Nearby Stars