J/ApJ/955/129ivo://CDS.VizieR/J/ApJ/955/129doi:10.26093/cds/vizier/ba-xv-41CDSWang S.Necib L.Ji A.P.Ou X.Lisanti M.de los Reyes M.A.C.,Strom A.L.Truong M.2026-07-02T12:00:42Z2026-01-07T09:36:10ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frchemical-abundancesstellar-associationsradial-velocityvisible-astronomyspectroscopyline-intensitiesNyx is a nearby, prograde, and high-eccentricity stellar stream physically contained in the thick disk, but its origin is unknown. Nyx could be the remnant of a disrupted dwarf galaxy, in which case the associated dark matter substructure could affect terrestrial dark matter direct-detection experiments. Alternatively, Nyx could be a signature of the Milky Way's disk formation and evolution. To determine the origin of Nyx, we obtained high- resolution spectroscopy of 34 Nyx stars using Keck/HIRES and Magellan/MIKE. A differential chemical abundance analysis shows that most Nyx stars reside in a metal-rich ([Fe/H]>-1) high-{alpha} component that is chemically indistinguishable from the thick disk. This rules out the originally suggested scenario that Nyx is the remnant of a single massive dwarf galaxy merger. However, we also identify 5 substantially more metal-poor stars ([Fe/H]~-2.0) whose chemical abundances are similar to those of the metal-weak thick disk. It remains unclear how stars that are chemically identical to the thick disk can be on such prograde, high-eccentricity orbits. We suggest two most likely scenarios: that Nyx is the result of an early minor dwarf galaxy merger, or that it is a record of the early spin-up of the Milky Way disk- although neither perfectly reproduces the chemodynamic observations. The most likely formation scenarios suggest that future spectroscopic surveys should find Nyx-like structures outside of the solar neighborhood.2023ApJ...955..129Whttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/ApJ/955/129CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toIII/279 : RAVE 5th data release (Kunder+, 2017)V/154 : Sloan Digital Sky Surveys (SDSS), Release 16 (DR16) (Ahumada+, 2020)III/286 : APOGEE-2 DR17 final allStar catalog (Abdurro'uf+, 2022)I/357 : Gaia DR3 Part 3. Non-single stars (Gaia Collaboration, 2022)J/A+A/410/527 : Abundances in the Galactic disk (Bensby+, 2003)J/ApJ/617/1091 : La and Eu abundances in 85 stars (Simmerer+, 2004)J/AJ/128/1177 : Galactic stellar abundances (Venn+, 2004)J/A+A/433/185 : Abundance analysis of 102 F and G dwarfs (Bensby+, 2005)J/A+A/464/201 : RGB stars in Sagittarius streams (Monaco+, 2007)J/A+A/465/815 : Abundances of Sgr dSph stars (Sbordone+, 2007)J/A+A/511/L10 : Abundances and space velocities of 94 stars (Nissen+, 2010)J/ApJ/737/9 : RAVE sp. data of stars in the thick disk (Ruchti+, 2011)J/AJ/141/175 : Abundances in M15 RGB/RHB stars (Sobeck+, 2011)J/ApJ/778/149 : Abundances for 3 stars in Sgr dSph (McWilliam+, 2013)J/A+A/562/A71 : Abundances of solar neighbourhood dwarfs (Bensby+, 2014)J/A+A/586/A49 : r- & s- process elements in the MW disk (Battistini+, 2016)J/A+A/598/A100 : Abundances of disk giants: O, Mg, Ca and Ti (Jonsson+, 2017)J/ApJ/865/129 : Abundance analysis of HD 222925 (Roederer+, 2018)J/A+A/631/A113 : Abundances of disk & bulge giants (Forsberg+, 2019)J/A+A/625/A141 : Abundances of disk and bulge giants (Lomaeva+, 2019)J/AJ/160/181 : Chemical abundances in red giants with Magellan (Ji+, 2020)J/ApJ/889/27 : Abundances of 11 stars in Carina II and III (Ji+, 2020)J/ApJ/891/39 : LAMOST DR3 VMP stars of the Gal. halo (Yuan+, 2020)J/MNRAS/506/150 : The GALAH+ Survey DR3 (Buder+, 2021)J/A+A/645/A19 : Arcturus Lab (line list) (Fanelli+, 2021)J/A+A/665/A46 : High-precision abund. of the Helmi streams (Matsuno+, 2022)J/other/Nat/603.599 : 247014 subgiant stars data (Xiang+, 2022)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/ApJ/955/129https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/ApJ/955/129http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/ApJ/955/129https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/ApJ/955/129https://vizier.iucaa.in/viz-bin/votable?-source=J/ApJ/955/129http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/ApJ/955/129GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/ApJ/955/129/table1 (Observations)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/ApJ/955/129/table1?https://vizier.iucaa.in/viz-bin/conesearch/J/ApJ/955/129/table1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/ApJ/955/129/table1?GETtext/xml+votable180.050000true10.7451178537.380430040.005555555555555556Cone search capability for table J/ApJ/955/129/table3 (Stellar parameters)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/ApJ/955/129/table3?https://vizier.iucaa.in/viz-bin/conesearch/J/ApJ/955/129/table3?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/ApJ/955/129/table3?GETtext/xml+votable180.050000true10.744513937.38051220.0055555555555555566/2168 2179 2232 2256 2610 8767 9326 9559 12444 12489 12753 12775 13150 13267 14652 14809 14946 15252 16952 19155 19290 19455 19689 19992 20003 20134 25023 29808 31768 32752 33147 33152 34937 35647 36768 39062 390641.9867573869263685e-19 3.916500758235022e-19https://cdsarc.cds.unistra.fr/viz-bin/moc/J/ApJ/955/129?format=asciiOpticaldefaultJ/ApJ/955/129/table1ObservationsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameNyx star name (1)meta.id;meta.maincharprimaryGaiaGaia DR3 Source idmeta.id.crossRA_ICRS[10.7/357] Gaia DR3 Right Ascension (ICRS) at Epoch=2016.0degpos.eq.ra;meta.mainDE_ICRS[-71.4/54] Gaia DR3 Declination (ICRS) at Epoch=2016.0degpos.eq.dec;meta.mainDateObservation Date (UT)'Y:M:D'time.epoch;obsnullableSimbadSimbad column added by the CDSmeta.ref.urlcharInstInstrument ("HIRES" or "MIKE")meta.id;instrcharGmag[8.88/12.81] Gaia DR3 G-band magnitudemagphot.mag;em.optfloatTexp[70/4085] Exposure timestime.duration;obs.exposureintSNR[121/248] Signal-to-noise of spectrum at 6500{AA}stat.snrintHRV[-16.7/27.4] Heliocentric radial velocity (2)km/sspect.dopplerVeloc;pos.heliocentricfloatS[0.95/1] Neural network score (3)stat.fit.goodnessfloatProb[0.6/0.97] Probability of belonging to Nyx velocity distribution (4)phys.veloc;stat.probabilityfloatNlNumber of line measurements for this star (link to Table 2)meta.numberintnullableNabundNumber of abundance measurements for this star (link to Table 4)meta.numberintnullableJ/ApJ/955/129/table2Line measurementsNameNyx star namemeta.id;meta.maincharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintlambda[4449.14/7699] Wavelength0.1nmem.wlfloatIon[11.0/63.1] Species IDphys.atmol.ionStagefloatchi[0/5.11] Excitation potentialphys.atmol.excitationfloatloggf[-5.97/0.57] Oscillator strengthphys.atmol.oscStrengthfloatEW[2/326]? Equivalent width, Angstroms (1)0.1mspect.line.eqWidthfloatnullablee_EW[0.2/6]? Equivalent width uncertainty0.1mstat.error;spect.line.eqWidthfloatnullableFWHM[0.06/0.6] Width at half maximum, Angstroms0.1nmspect.line.widthfloatlogeps[-0.38/7.63]? Measured abundance of the reference star (2)phys.abundfloatnullabledi[-0.9/1.3]? Line-by-line abundance difference relative to the reference starphys.abund;arith.difffloatnullableei[0/0.4] Statistical uncertaintystat.error;phys.abund;arith.difffloatdelTeff[-0.18/0.42] Stellar parameter abundance difference (effective temperature)phys.temperature.effective;arith.difffloatdellogg[-0.22/0.25] Stellar parameter abundance difference (surface gravity)phys.gravity;arith.difffloatdelVt[-0.16/0.04] Stellar parameter abundance difference (microturbulence)phys.veloc.microTurb;arith.difffloatdel([M/H])[-0.07/0.14] Stellar parameter abundance difference (metallicity)phys.abund.Z;arith.difffloatJ/ApJ/955/129/table3Stellar parameterse_logg[0.15/0.31] Uncertainty in logglog(cm.s**-2)stat.error;phys.gravityfloatvt[0.79/1.7] microturbulent velocitykm/sphys.veloc.microTurbfloate_vt[0.08/1.05] Uncertainty in vtkm/sstat.error;phys.veloc.microTurbfloat[M/H][-2.26/-0.21] Model metallictyphys.abund.Zfloate_[M/H][0.11/0.14] Uncertainty in [M/H]stat.error;phys.abund.Zfloat[A/Fe][0.25/0.4] {alpha} element differential Fe abundancephys.abund.Fe;arith.difffloatSimbadNameSimbad column added by the CDSmeta.idcharnullablerecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameNyx star name and reference stars (1)meta.id;meta.maincharprimaryTeff[4290/6501] Surface effective temperatureKphys.temperature.effectiveinte_Teff[68/280] Uncertainty in TeffKstat.error;phys.temperature.effectiveintlogg[1.54/4.54] log, surface gravity, cgslog(cm.s**-2)phys.gravityfloat[Fe/H][-2.2/-0.13] differential Fe abundancephys.abund.Fe;arith.difffloat_RARight Ascension (J2000) from SIMBAD (not part of the original data)degpos.eq.ra;meta.main_DEDeclination (J2000) from SIMBAD (not part of the original data)degpos.eq.dec;meta.mainJ/ApJ/955/129/table4Stellar abundancesNameNyx star namemeta.id;meta.maincharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintElElementphys.atmol.elementcharNl[1/25] Number of linesmeta.numberfloat[X/H]d[-2.53/0.63]? Differential abundance relative to the solar abundance (1)phys.abund;arith.difffloatnullable[X/Fe]d[-0.36/0.95]? Differential abundancephys.abund;arith.difffloatnullablee_[X/Fe]d[0.06/0.35] Differential abundance uncertaintystat.error;phys.abund;arith.difffloat[X/H]ref[-2.23/-0.1]? Reference abundance relative to the solar abundancephys.abundfloatnullablee_[X/H]ref[0.01/0.18]? Uncertainty of the reference abundance relative to the solar abundancestat.error;phys.abundfloatnullable[X/Fe]ref[-0.22/0.4]? Reference abundancephys.abundfloatnullablee(rand)[0/0.1] Uncertainty due to the random spectrum noisestat.error;instr.det.noisefloate(sys)[0/0.3] Systematic uncertaintystat.error.sysfloate(sp)[0.03/0.33] Uncertainty due to stellar parametersstat.errorfloat