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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 (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), 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) 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) 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: ----------------------------------------------------- RA (ICRS) DE Designation(s) ----------------------------------------------------- 10 07 03.2 +16 04 25 Segue 1 = NAME Segue I ----------------------------------------------------- File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table1.dat 112 6 Observing Details table2.dat 126 505 Equivalent width measurements of the Segue 1 stars
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 --- 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
Note (1): The S/N is measured per ∼33 mÅ pixel (MIKE spectra) and ∼20 mÅ pixel (HIRES spectrum).
Byte-by-byte Description of file: table2.dat
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
Note (1): In units of milli-Angstroms.
History: From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 21-Jul-2017
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