J/ApJ/955/129 High-resolution abundances of the Nyx stream (Wang+, 2023)
High-resolution chemical abundances of the Nyx stream.
Wang S., Necib L., Ji A.P., Ou X., Lisanti M., de los Reyes M.A.C.,
Strom A.L., Truong M.
<Astrophys. J., 955, 129 (2023)>
=2023ApJ...955..129W 2023ApJ...955..129W
ADC_Keywords: Abundances; Associations, stellar; Radial velocities;
Spectra, optical; Equivalent widths
Keywords: Stellar abundances ; Stellar kinematics ; Milky Way formation ;
Dark matter
Abstract:
Nyx 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-α 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.
Description:
We select 34 out of 94 stars from Necib+ 2020NatAs...4.1078N 2020NatAs...4.1078N (erratum
in 2022NatAs...6..866N 2022NatAs...6..866N) with a velocity cluster with the highest
probabilities of belonging to the Nyx velocity cluster. This biases
our sample toward higher velocities and a smaller velocity dispersion
compared to the full Nyx sample.
We obtained high-resolution spectroscopy of these 34 Nyx stars:
28 stars on 2020 August 1, with the 0.5" slit using the Keck/HIRES
spectrograph (R∼67k at 4100Å), and 6 stars on 2021 January 3 or
2021 July 28 (Nyx122147) with the 0.5" slit using the Magellan/MIKE
spectrograph (R∼50k/40k on the blue/red arm of MIKE). The HIRES
spectra range from 3650 to 8160Å, and the MIKE spectra range from
3300 to 9400Å.
We determine chemical abundances differentially (see Section 3.4), so
we select two stars from the Bensby+ (2014, J/A+A/562/A71) thick-disk
sample to use as abundance references: the metal-rich reference star
HIP88622, and the metal-poor reference star HIP7162. HIP88622 was
observed on 2020 August 1 with the 0.5" slit using the Keck/HIRES
spectrograph (signal-to-noise ratio (S/N) per pixel ∼250 at 6500Å).
HIP7162 was observed on 2022 June 30 with the 0.5" slit using the
MIKE/Magellan spectrograph (S/N∼114 at 6500Å).
See Section 2 for further explanations.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 103 34 Observations
table2.dat 89 3133 Line measurements
table3.dat 60 36 Stellar parameters
table4.dat 68 382 Stellar abundances
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See also:
III/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)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Nyx star name (1)
11- 29 I19 --- Gaia Gaia DR3 Source id
31- 42 F12.8 deg RAdeg [10.7/357] Gaia DR3 Right Ascension (ICRS)
at Epoch=2016.0
44- 55 F12.8 deg DEdeg [-71.4/54] Gaia DR3 Declination (ICRS)
at Epoch=2016.0
57- 66 A10 "Y/M/D" Date Observation Date (UT)
68- 72 A5 --- Inst Instrument ("HIRES" or "MIKE")
74- 78 F5.2 mag Gmag [8.88/12.81] Gaia DR3 G-band magnitude
80- 83 I4 s Texp [70/4085] Exposure time
85- 87 I3 --- SNR [121/248] Signal-to-noise of spectrum at 6500Å
89- 93 F5.1 km/s HRV [-16.7/27.4] Heliocentric radial velocity (2)
95- 98 F4.2 --- S [0.95/1] Neural network score (3)
100-103 F4.2 --- Prob [0.6/0.97] Probability of belonging to Nyx
velocity distribution (4)
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Note (1): The coolest and most metal-rich Nyx185206 is removed from the
differential abundance analysis because blending issues affect almost
all elements of the star.
Note (2): Radial velocities were measured by cross-correlating with a MIKE
spectrum of the metal-poor giant HD122563 around Mgb. For stars that
were observed with multiple exposures, we report the heliocentric
velocity measured in the middle of the observation. The formal
velocity uncertainties are 0.1-0.2km/s, but past experience shows that
the typical systematic error on the velocity due to wavelength
calibration and template mismatches is 1.0km/s. See Section 2.
Note (3): Star's neural network score (S) from Ostdiek+ 2020A&A...636A..75O 2020A&A...636A..75O
Note (4): Probability of belonging to the Nyx velocity distribution
from Necib+ 2020NatAs...4.1078N 2020NatAs...4.1078N
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Nyx star name
11- 17 F7.2 0.1nm lambda [4449.14/7699] Wavelength
19- 22 F4.1 --- Ion [11.0/63.1] Species ID
24- 27 F4.2 --- chi [0/5.11] Excitation potential
29- 33 F5.2 [-] loggf [-5.97/0.57] Oscillator strength
35- 39 F5.1 0.1m EW [2/326]? Equivalent width, Angstroms (1)
41- 43 F3.1 0.1m e_EW [0.2/6]? Equivalent width uncertainty
45- 48 F4.2 0.1nm FWHM [0.06/0.6] Width at half maximum, Angstroms
50- 54 F5.2 --- logeps [-0.38/7.63]? Measured abundance of the
reference star (2)
56- 60 F5.2 --- di [-0.9/1.3]? Line-by-line abundance
difference relative to the reference star
62- 65 F4.2 --- ei [0/0.4] Statistical uncertainty
67- 71 F5.2 --- delTeff [-0.18/0.42] Stellar parameter abundance
difference (effective temperature)
73- 77 F5.2 --- dellogg [-0.22/0.25] Stellar parameter abundance
difference (surface gravity)
79- 83 F5.2 --- delVt [-0.16/0.04] Stellar parameter abundance
difference (microturbulence)
85- 89 F5.2 --- del([M/H]) [-0.07/0.14] Stellar parameter abundance
difference (metallicity)
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Note (1): Equivalent widths of lines used in spectral synthesis are
set to blanks (nulls).
Note (2): The reference star is Arcturus for a metal-rich giant,
HIP88622 for a metal-rich dwarf and HIP7162 for a metal-poor star.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Nyx star name and reference stars (1)
11- 14 I4 K Teff [4290/6501] Surface effective temperature
16- 18 I3 K e_Teff [68/280] Uncertainty in Teff
20- 23 F4.2 [cm/s2] logg [1.54/4.54] log, surface gravity, cgs
25- 28 F4.2 [cm/s2] e_logg [0.15/0.31] Uncertainty in logg
30- 33 F4.2 km/s vt [0.79/1.7] microturbulent velocity
35- 38 F4.2 km/s e_vt [0.08/1.05] Uncertainty in vt
40- 44 F5.2 [-] [M/H] [-2.26/-0.21] Model metallicty
46- 49 F4.2 [-] e_[M/H] [0.11/0.14] Uncertainty in [M/H]
51- 54 F4.2 [-] [A/Fe] [0.25/0.4] α element differential
Fe abundance
56- 60 F5.2 [-] [Fe/H] [-2.2/-0.13] differential Fe abundance
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Note (1): HIP88622 is the metal-rich reference star, and
HIP7162 is the metal-poor reference star.
For the reference star Arcturus, high-resolution spectra were
obtained from Hinkle+ 2000vnia.book.....H 2000vnia.book.....H
See Section 2.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- Name Nyx star name
11- 14 A4 --- El Element
16- 19 F4.1 --- Nl [1/25] Number of lines
21- 25 F5.2 [-] [X/H]d [-2.53/0.63]? Differential abundance
relative to the solar abundance (1)
27- 31 F5.2 [-] [X/Fe]d [-0.36/0.95]? Differential abundance
33- 36 F4.2 [-] e_[X/Fe]d [0.06/0.35] Differential abundance
uncertainty
38- 42 F5.2 [-] [X/H]ref [-2.23/-0.1]? Reference abundance relative
to the solar abundance
44- 47 F4.2 [-] e_[X/H]ref [0.01/0.18]? Uncertainty of the reference
abundance relative to the solar abundance
49- 53 F5.2 [-] [X/Fe]ref [-0.22/0.4]? Reference abundance
55- 58 F4.2 --- e(rand) [0/0.1] Uncertainty due to the random
spectrum noise
60- 63 F4.2 --- e(sys) [0/0.3] Systematic uncertainty
65- 68 F4.2 --- e(sp) [0.03/0.33] Uncertainty due to stellar
parameters
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Note (1): Reference solar abundances can be found in
Asplund+ 2009ARA&A..47..481A 2009ARA&A..47..481A
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 02-Dec-2025