J/ApJ/870/83 Abundances in the ultra-faint dwarf gal. GruI & TriII (Ji+, 2019)
Chemical abundances in the ultra-faint dwarf galaxies Grus I and Triangulum II:
neutron-capture elements as a defining feature of the faintest dwarfs.
Ji A.P., Simon J.D., Frebel A., Venn K.A., Hansen T.T.
<Astrophys. J., 870, 83-83 (2019)>
=2019ApJ...870...83J 2019ApJ...870...83J (SIMBAD/NED BibCode)
ADC_Keywords: Abundances; Equivalent widths; Spectra, optical;
Radial velocities
Keywords: galaxies: dwarf - galaxies: individual: (Gru I, Tri II) -
Local Group - nuclear reactions, nucleosynthesis, abundances -
stars: abundances
Abstract:
We present high-resolution spectroscopy of four stars in two candidate
ultra-faint dwarf galaxies (UFDs), GrusI (GruI) and TriangulumII
(TriII). Neither object currently has a clearly determined velocity
dispersion, placing them in an ambiguous region of parameter space
between dwarf galaxies and globular clusters (GCs). No significant
metallicity difference is found for the two GruI stars, but both stars
are deficient in neutron-capture elements. We verify previous results
that TriII displays significant spreads in metallicity and
[α/Fe]. Neutron-capture elements are not detected in our TriII
data, but we place upper limits at the lower envelope of Galactic halo
stars, consistent with previous very low detections. Stars with
similarly low neutron-capture element abundances are common in UFDs
but rare in other environments. This signature of low neutron-capture
element abundances traces chemical enrichment in the least massive
star-forming dark matter halos and further shows that the dominant
sources of neutron-capture elements in metal-poor stars are rare. In
contrast, all known GCs have similar ratios of neutron-capture
elements to those of halo stars, suggesting that GCs do not form at
the centers of their own dark matter halos. The low neutron-capture
element abundances may be the strongest evidence that GruI and TriII
are (or once were) galaxies rather than GCs, and we expect future
observations of these systems to robustly find nonzero velocity
dispersions or signs of tidal disruption. However, the nucleosynthetic
origin of this low neutron-capture element floor remains unknown.
Description:
Our program stars were observed between December 2015 and August 2017
from two telescopes with two different echelle spectrographs. Details
of the observations can be found in Table1.
The GruI stars were selected as the two brightest probable members of
GruI from Walker+ (2016, J/ApJ/819/53). We observed these stars with
the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the
Magellan-Clay telescope in 2017 August with the 1"0 slit, providing
resolution R∼28000 from ∼3900 to 5000Å on the blue arm and R∼22000
from ∼5000 to 9000Å on the red arm.
The two stars in TriII were observed with the Gemini Remote Access to
CFHT ESPaDOnS (Echelle SpectroPolarimetric Device for the Observation
of Stars at Canada-France-Hawaii Telescope) Spectrograph (GRACES).
These stars were selected as the brightest probable members of TriII
from Kirby+ (2015ApJ...814L...7K 2015ApJ...814L...7K) and Martin+ (2016ApJ...818...40M 2016ApJ...818...40M).
We combined data from two programs that both used the two-fiber
object+sky GRACES mode providing R∼40000 from ∼5000 to 10000Å.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
----------------------------------------------------------
22 56 42.4 -50 09 48 GruI = NAME Grus I
02 13 17.40 +36 10 42.4 TriII = NAME Tri II
----------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 89 10 Observing details of the four stars in the
ultra-faint dwarf galaxies (UFDs) GruI and TriII
table2.dat 136 289 Line measurements
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See also:
J/AJ/128/1177 : Galactic stellar abundances (Venn+, 2004)
J/AJ/130/2140 : Abundances and velocities in globular clusters (Pritzl+, 2005)
J/AJ/137/3100 : Radial velocities of 4 dSph galaxies (Walker+, 2009)
J/AN/331/474 : Abundances of metal-poor stars (Frebel, 2010)
J/ApJ/711/573 : Detailed abundances in a halo stellar stream (Roederer+, 2010)
J/ApJ/724/341 : Nucleosynthesis of massive metal-free stars (Heger+, 2010)
J/ApJ/767/134 : Abundances of red giant stars in UFD galaxies (Vargas+, 2013)
J/ApJ/769/57 : Equivalent widths of metal-poor stars (Frebel+, 2013)
J/ApJ/770/16 : Spectroscopic members of Segue 2 galaxy (Kirby+, 2013)
J/ApJ/779/102 : Metallicities of RGB stars in dwarf galaxies (Kirby+, 2013)
J/AJ/145/159 : LAMOST. II. ugriz photometry of 526 new quasars (Huo+, 2013)
J/A+A/562/A146 : Chemical abundances of 8 metal-poor stars (Ishigaki+, 2014)
J/A+A/565/A23 : Abundances of NGC5897 red giants (Koch+, 2014)
J/A+A/571/A47 : Extensive list of CH in stellar atmospheres (Masseron+, 2014)
J/AJ/147/136 : Stars very low metal abundance. VI. Abundances (Roederer+,2014)
J/ApJ/786/74 : EW measurements of 6 Segue 1 red giants (Frebel+, 2014)
J/ApJ/789/147 : Star formation histories of LG dwarf galaxies (Weisz+, 2014)
J/ApJ/797/21 : Carbon-enhanced metal-poor stars (Placco+, 2014)
J/A+A/574/A129 : The First CEMP star in the Sculptor dSph (Skuladottir+,2015)
J/ApJ/802/93 : Chemical abundance analysis 5 stars in Sculptor (Simon+, 2015)
J/ApJ/808/95 : Spectroscopy of Reticulum II (Simon+, 2015)
J/MNRAS/449/761 : EWs of metal-poor stars in UMiI dSph galaxy (Ural+, 2015)
J/MNRAS/449/3889 : Detailed abundances of 15 stars in NGC4833 (Roederer+, 2015)
J/AJ/151/82 : The 4 brightest red giants in UFD galaxy Ret 2 (Roederer+,2016)
J/ApJ/817/41 : Abundances of 4 metal-poor red giants in BooII (Ji+, 2016)
J/ApJ/819/53 : Magellan/M2FS spectroscopy of Tucana 2 & Grus 1 (Walker+, 2016)
J/ApJ/826/110 : Boo-127 and Boo-980 high-resolution spectra (Frebel+, 2016)
J/ApJ/830/93 : Abundances of Ret II brightest red giant members (Ji+, 2016)
J/MNRAS/458/L59 : DracoII stars gi magnitude & radial velocities (Martin+,2016)
J/A+A/604/A129 : Formation of MW halo and dwarf satellites (Mashonkina+, 2017)
J/ApJ/838/11 : Member stars in the MW satellite Tucana III (Simon+, 2017)
J/ApJ/838/44 : Abundances of the brightest member of Tuc III (Hansen+, 2017)
J/ApJ/838/83 : Radial velocities abundances membership in TriII (Kirby+, 2017)
J/ApJS/230/28 : The populations of Carina. II. Abundances (Norris+, 2017)
J/ApJ/852/99 : Abundances 3 metal-poor stars in Horologium I (Nagasawa+, 2018)
J/ApJ/855/83 : Abundances very metal-poor stars in Sagittarius (Hansen+, 2018)
J/ApJ/857/74 : Abundances of 7 new member stars in Tucana II (Chiti+, 2018)
J/ApJ/863/89 : GaiaDR2 PMs of stars in ultra-faint MW satellites (Simon, 2018)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 I1 --- Seq Star number (as in Table 2); column added by CDS
3- 10 A8 --- Name Star identification
12- 13 I2 h RAh [2/22] Hour of right ascension (J2000)
15- 16 I2 min RAm Minute of right ascension (J2000)
18- 21 F4.1 s RAs Second of right ascension (J2000)
23 A1 --- DE- Sign of declination (J2000)
24- 25 I2 deg DEd Degree of declination (J2000)
27- 28 I2 arcmin DEm Arcminute of declination (J2000)
30- 33 F4.1 arcsec DEs Arcseconde of declination (J2000)
35- 38 F4.1 mag Vmag [17/19] V magnitude
40- 50 A11 "date" Date Observation date
52- 54 I3 min Exp [60/430] Exposure time
56- 61 F6.1 km/s HRV [-397/-139] Helicocentric velocity (1)
63- 65 F3.1 km/s e_HRV [0.4/5]? Error on RV
67- 68 I2 --- S/N4500 [1/22]? Signal-to-noise ratio at 4500Å
per pixel (2)
70- 71 I2 --- S/N5300 [7/25]? Signal-to-noise ratio at 5300Å
per pixel (2)
73- 74 I2 --- S/N6500 [17/60]? Signal-to-noise ratio at 6500Å
per pixel (2)
76- 89 A14 --- Inst Spectrograph used
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Note (1): Velocity precision is computed with co-added spectra except for
TriII-46, where each visit is measured separately because of the
binary orbital motion.
Note (2): S/N values for TriII stars were determined after co-adding.
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Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 A5 --- Ion Atomic species
7- 13 F7.2 0.1nm lambda [3961/8195] Rest wavelength (λ)
15- 18 F4.2 eV EP [0/4.5]? Excitation potential (χ)
20- 24 F5.2 [-] loggf [-5.7/0.65]? log statistical weight*oscillator
strength
26- 30 F5.1 0.1pm EW1 [22/311]? Measured equivalent width, GruI-032
32- 34 A3 --- f_EW1 syn: abundance from spectrum synthesis
36- 39 F4.1 0.1pm e_EW1 [5/66]? Uncertainty in EW1 (σ(EW))
41- 41 A1 --- l_logeps1 Limit flag on logeps1
43- 47 F5.2 [-] logeps1 [-2.5/5.7]? Abundance derived from this line,
GruI-032 (logε)
49- 52 F4.2 [-] e_logeps1 [0.1/1.2]? Uncertainty on logeps1
(σ(logε))
54- 58 F5.1 0.1pm EW2 [20/280]? Measured equivalent width, GruI-038
60- 62 A3 --- f_EW2 syn: abundance from spectrum synthesis
64- 67 F4.1 0.1pm e_EW2 [5/79]? Uncertainty in EW2 (σ(EW))
69- 69 A1 --- l_logeps2 Limit flag on logeps2
71- 75 F5.2 [-] logeps2 [-1.8/5.7]? Abundance derived from this line,
GruI-038 (logε)
77- 80 F4.2 [-] e_logeps2 [0.1/1.5]? Uncertainty on logeps2
(σ(logε))
82- 86 F5.1 0.1pm EW3 [16/209]? Measured equivalent width, TriII-40
88- 90 A3 --- f_EW3 syn: abundance from spectrum synthesis
92- 95 F4.1 0.1pm e_EW3 [4/34]? Uncertainty in EW3 (σ(EW))
97- 97 A1 --- l_logeps3 Limit flag on logeps3
99-103 F5.2 [-] logeps3 [-2.3/5.1]? Abundance derived from this line,
TriII-40 (logε)
105-108 F4.2 [-] e_logeps3 [0.1/0.5]? Uncertainty on logeps3
(σ(logε))
110-114 F5.1 0.1pm EW4 [52/207]? Measured equivalent width, TriII-46
116-118 A3 --- f_EW4 syn: abundance from spectrum synthesis
120-123 F4.1 0.1pm e_EW4 [12/70]? Uncertainty in EW4 (σ(EW))
125-125 A1 --- l_logeps4 Limit flag on logeps4
127-131 F5.2 [-] logeps4 [-0.06/6.2]? Abundance derived from this line,
TriII-46 (logε)
133-136 F4.2 [-] e_logeps4 [0.2/1.1]? Uncertainty on logeps4
(σ(logε))
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History:
From electronic version of the journal
(End) Prepared by [AAS], Coralie Fix [CDS] 11-Feb-2020