J/ApJ/786/74 EW measurements of 6 Segue 1 red giants (Frebel+, 2014)
Segue 1: an unevolved fossil galaxy from the early universe.
Frebel A., Simon J.D., Kirby E.N.
<Astrophys. J., 786, 74 (2014)>
=2014ApJ...786...74F 2014ApJ...786...74F (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Equivalent widths ; Stars, population II ;
Stars, giant ; Photometry, SDSS ; Abundances
Keywords: early universe - galaxies: dwarf - Galaxy: halo - Local Group -
stars: abundances - stars: Population II
Abstract:
We present Magellan/MIKE and Keck/HIRES high-resolution spectra of six
red giant stars in the dwarf galaxy Segue 1. Including one additional
Segue 1 star observed by Norris et al. (2010ApJ...722L.104N 2010ApJ...722L.104N),
high-resolution spectra have now been obtained for every red giant in
Segue 1. Remarkably, three of these seven stars have metallicities
below [Fe/H]=-3.5, suggesting that Segue 1 is the least chemically
evolved galaxy known. We confirm previous medium-resolution analyses
demonstrating that Segue 1 stars span a metallicity range of more than
2 dex, from [Fe/H]=-1.4 to [Fe/H]=-3.8. All of the Segue 1 stars
are α-enhanced, with [α/Fe]∼0.5. High α-element
abundances are typical for metal-poor stars, but in every previously
studied galaxy [α/Fe] declines for more metal-rich stars, which
is typically interpreted as iron enrichment from supernova Ia. The
absence of this signature in Segue 1 indicates that it was enriched
exclusively by massive stars. Other light element abundance ratios in
Segue 1, including carbon enhancement in the three most metal-poor
stars, closely resemble those of metal-poor halo stars. Finally, we
classify the most metal-rich star as a CH star given its large
overabundances of carbon and s-process elements. The other six stars
show remarkably low neutron-capture element abundances of [Sr/H]←4.9
and [Ba/H]←4.2, which are comparable to the lowest levels ever
detected in halo stars. This suggests minimal neutron-capture
enrichment, perhaps limited to a single r-process or weak s-process
synthesizing event. Altogether, the chemical abundances of Segue 1
indicate no substantial chemical evolution, supporting the idea that
it may be a surviving first galaxy that experienced only one burst of
star formation.
Description:
We observed five of our six target stars with the MIKE spectrograph
(Bernstein et al. 2003SPIE.4841.1694B 2003SPIE.4841.1694B) on the Magellan-Clay telescope
in 2010 March and May, and 2011 March. Observing conditions during
these runs were mostly clear, with an average seeing of 0.8" to 1.0".
MIKE spectra have nearly full optical wavelength coverage from
∼3500-9000 Å. A 1.0"x5" slit yields a spectral resolution of ∼22000
in the red and ∼28000 in the blue wavelength regime. We observed the
final star in the sample, SDSS J100742+160106, with the HIRES
spectrograph (Vogt et al. 1994SPIE.2198..362V 1994SPIE.2198..362V) on the Keck I telescope
on 2010 April 1. The observations were obtained with a 1.15"x7" slit
(providing a spectral resolution of 37500), the kv389 blocking filter,
and a total integration time of 3.6 h.
Objects:
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RA (ICRS) DE Designation(s)
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10 07 03.2 +16 04 25 Segue 1 = NAME Segue I
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 112 6 Observing Details
table2.dat 126 505 Equivalent width measurements of the Segue 1
stars
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See also:
J/ApJ/692/1464 : Spectroscopy of Segue 1 (Geha+, 2009)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 19 A19 --- Name SDSS star's name (JHHMMSS+DDMMSS)
21- 22 I2 h RAh Hour of Right Ascension (J2000)
24- 25 I2 min RAm Minute of Right Ascension (J2000)
27- 30 F4.1 s RAs Second of Right Ascension (J2000)
32 A1 --- DE- Sign of the Declination (J2000)
33- 34 I2 deg DEd Degree of Declination (J2000)
36- 37 I2 arcmin DEm Arcminute of Declination (J2000)
39- 42 F4.1 arcsec DEs Arcsecond of Declination (J2000)
44- 76 A33 --- Obs.date UT Date(s) of the observation
78- 81 F4.2 arcsec Slit Slit width
83- 86 F4.1 h Texp Exposure time
88- 92 F5.2 mag gmag SDSS g band magnitude
94- 98 F5.3 mag E(B-V) Reddening
100-101 I2 --- S/N5300 Signal-to-noise ratio at 5300 Å (1)
103-104 I2 --- S/N6000 Signal-to-noise ratio 6000 Å (1)
106-112 A7 --- Name1 Star's name as written in table2.dat file
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Note (1): The S/N is measured per ∼33 mÅ pixel (MIKE spectra) and ∼20 mÅ
pixel (HIRES spectrum).
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 A4 --- ID Element
6- 13 F8.3 0.1nm Wave Wavelength; Angstroms
15- 18 F4.2 eV ExPot ? Excitation potential
20- 25 F6.3 [-] log(gf) ? Log oscillator strength
27- 32 F6.2 10-13m EW-J100714 ? Equivalent width of
SDSS J100714+160154 (1)
34 A1 --- f_EW-J100714 [s] Flag indicating spectrum synthesis used
36- 40 F5.2 [-] eps-J100714 ? Log epsilon abundance of
SDSS J100714+160154
42- 47 F6.2 10-13m EW-J100710 ? Equivalent width of
SDSS J100702+155055 (1)
49 A1 -- f_EW-J100710 [s] Flag indicating spectrum synthesis used
51 A1 --- l_eps-J100710 [<] Limit flag on eps-J100710
52- 56 F5.2 [-] eps-J100710 ? Log epsilon abundance of
SDSS J100702+155055
60- 65 F6.2 10-13m EW-J100702 ? Equivalent width of
SDSS J100702+155055 (1)
67 A1 --- f_EW-J100702 [s] Flag indicating spectrum synthesis used
69 A1 --- l_eps-J100702 [<] Limit flag on eps-J100702
70- 75 F6.2 [-] eps-J100702 ? Log epsilon abundance of
SDSS J100702+155055
77- 82 F6.2 10-13m EW-J100742 ? Equivalent width of
SDSS J100742+160106 (1)
84 A1 --- f_EW-J100742 [s] Flag indicating spectrum synthesis used
86 A1 --- l_eps-J100742 [<] Limit flag on eps-J100742
87- 92 F6.2 [-] eps-J100742 ? Log epsilon abundance of
SDSS J100742+160106
94- 99 F6.2 10-13m EW-J100652 ? Equivalent width of
SDSS J100652+160235 (1)
101 A1 --- f_EW-J100652 [s] Flag indicating spectrum synthesis used
103 A1 --- l_eps-J100652 [<] Limit flag on eps-J100652
104-109 F6.2 [-] eps-J100652 ? Log epsilon abundance of
SDSS J100652+160235
111-116 F6.2 10-13m EW-J100639 ? Equivalent width of
SDSS J100639+160008 (1)
118 A1 --- f_EW-J100639 [s] Flag indicating spectrum synthesis used
120 A1 --- l_eps-J100639 [<] Limit flag on eps-J100639
121-126 F6.2 [-] eps-J100639 ? Log epsilon abundance of
SDSS J100639+160008
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Note (1): In units of milli-Angstroms.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 21-Jul-2017