J/ApJ/708/560 Spectroscopy of UMa II and Coma Ber (Frebel+, 2010)
High-resolution spectroscopy of extremely metal-poor stars in the least evolved
galaxies: Ursa Major II and Coma Berenices.
Frebel A., Simon J.D., Geha M., Willman B.
<Astrophys. J., 708, 560-583 (2010)>
=2010ApJ...708..560F 2010ApJ...708..560F
ADC_Keywords: Equivalent widths ; Abundances ; Stars, metal-deficient ;
Spectroscopy ; Galaxies, nearby
Keywords: early universe - galaxies: dwarf - Galaxy: halo - Local Group -
stars: abundances - stars: Population II
Abstract:
We present spectra of six metal-poor stars in two of the ultra-faint
dwarf galaxies orbiting the Milky Way (MW), Ursa Major II, and Coma
Berenices obtained with the Keck/High Resolution Echelle Spectrometer
(HIRES). These observations include the first high-resolution
spectroscopic observations of extremely metal-poor ([Fe/H]←3.0) stars
not belonging to the MW halo field star population. We obtain
abundance measurements and upper limits for 26 elements between carbon
and europium. The entire sample of stars spans a range of
-3.2<[Fe/H]←2.3, and we confirm that each galaxy contains a large
intrinsic spread of Fe abundances. A comparison with MW halo stars of
similar metallicities reveals substantial agreement between the
abundance patterns of the ultra-faint dwarf galaxies and the MW halo
for the light, α, and iron-peak elements (C to Zn). The
abundances of neutron-capture elements (Sr to Eu) in the ultra-faint
dwarf galaxies are extremely low, consistent with the most metal-poor
halo stars, but not with the typical halo abundance pattern at
[Fe/H]≳-3.0. Not only are our results broadly consistent with a
galaxy formation model that predicts that massive dwarf galaxies are
the source of the metal-rich component ([Fe/H]>-2.5) of the MW halo,
but they also suggest that the faintest known dwarfs may be the
primary contributors to the metal-poor end of the MW halo metallicity
distribution.
Description:
We observed the target stars (see table 1) with the HIRES spectrograph
on the Keck I telescope on 2008 February 22-24. We used a 7.0"x1.15"
slit, producing a spectral resolution of R=37000 over the wavelength
range from 4100 to 7200Å on the blue and green CCDs. Additional
observing details are summarized in Table 2.
We use two of the strong Mgb lines in the green part of the HIRES
spectra as well as three other Mg lines for our radial velocity
measurements. The standard deviation of the individual line
measurements is typically <0.5km/s.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
stars.dat 88 8 Photometry and other stellar parameters (table 1
and 5 of the paper)
table2.dat 92 8 Observing details
ew.dat 52 2191 Equivalent width measurements (table 4)
table6.dat 47 84 HIRES abundances of the UMa II stars
table7.dat 47 84 HIRES abundances of the Coma Ber stars
table9.dat 54 54 HIRES abundances of standard star HD 122563
--------------------------------------------------------------------------------
See also:
J/ApJS/191/352 : Abundances in stars of Milky Way dwarf satellites (Kirby+,
2010)
J/MNRAS/380/281 : Faint galactic satellites spectroscopic survey (Martin+,
2007)
Byte-by-byte Description of file: stars.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 A14 --- Gal Dwarf galaxy name (Coma Berenices or
Ursa Major II)
16- 19 A4 --- --- [SDSS]
21- 34 A14 --- SDSS Star SDSS designation (JHHMMSS+DDMMSS)
36- 44 A9 --- Name Star name
46 A1 --- f_Name [f] Non-member star (1)
48- 52 F5.2 mag Vmag ? V-band magnitude (2)
54- 58 F5.2 mag rmag ? SDSS-DR7 r-band magnitude (2)
60- 63 F4.2 mag g-r ? SDSS-DR7 g-r color index (2)
65- 69 F5.2 mag rMag ? Absolute r-band magnitude (2)
71- 74 I4 K Teff Effective temperature (3)
76- 78 F3.1 [cm/s2] logg Surface gravity
80- 84 F5.2 [-] [Fe/H] Fe/H abundance
86- 88 F3.1 km/s Vt Microturbulent velocity ξ
--------------------------------------------------------------------------------
Note (1):
f = UMa II-NM was observed but was later determined not to be a member of
UMa II.
Note (2): The r magnitudes and g-r colors were obtained from the updated SDSS
DR7 (Abazajian et al. 2009, Cat. II/294), while the V magnitudes were
determined from g and r using the conversions given by Smith et al.
(2002AJ....123.2121S 2002AJ....123.2121S).
Note (3): Temperature are rounded to the nearest 10K.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- Name Star name
11- 12 I2 h RAh Hour of right ascension (J2000)
14- 15 I2 min RAm Minute of right ascension (J2000)
17- 21 F5.2 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- 34 F5.2 arcsec DEs Arcsecond of declination (J2000)
36- 44 F9.1 d JD Julian Date of observation
46- 49 F4.2 h Exp Exposure time
51- 53 I3 --- S/N5 5000Å signal to noise ratio (4)
55- 57 I3 --- S/N6 6000Å signal to noise ratio (4)
59- 61 I3 --- S/N65 6500Å signal to noise ratio (4)
63- 68 F6.1 km/s RVH HIRES radial velocity
70- 72 F3.1 km/s e_RVH RVH uncertainty
74- 79 F6.1 km/s RVD ? DEIMOS radial velocity
81- 83 F3.1 km/s e_RVD ? RVD uncertainty
85- 92 A8 --- Comm Comment
--------------------------------------------------------------------------------
Note (4): The S/N measurements are for ∼22mÅ ("blue" CCD), ∼26mÅ
("green" CCD), and ∼28mÅ ("green" CCD) pixel sizes, respectively.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: ew.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- Name Star name
11- 14 A4 --- Ion Species identification
16- 22 F7.2 0.1nm lambda Wavelength in Angstroms
24- 27 F4.2 eV ExPot ? Excitation potential
29- 35 F7.3 [-] log(gf) ? Log of the oscillator strength
37 A1 --- l_EW Limit flag on EW
38- 42 F5.1 0.1pm EW ? Equivalent width in milli-Angstroms
44 A1 --- f_EW [s] s: indicates a spectrum synthesis measurement
46 A1 --- l_log(e) Limit flag on log(e)
47- 52 F6.2 [-] log(e) ? logε abundance (H=12)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table6.dat table7.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- Name Star name
11- 15 A5 --- Ion Species identification
17 A1 --- l_log(e) Limit flag on loge
18- 22 F5.2 [-] log(e) logε abundance (X)
24 A1 --- f_log(e) [a] distorted lines (1)
26 A1 --- l_[X/H] Limit flag on [X/H]
27- 31 F5.2 [Sun] [X/H] X/H abundance
33 A1 --- l_[X/Fe] Limit flag on [X/Fe]
34- 38 F5.2 [Sun] [X/Fe] ? X/Fe abundance (G1)
40- 42 I3 --- N Number of spectra used
44- 47 F4.2 [-] Sig ? Abundance uncertainty
--------------------------------------------------------------------------------
Note (1):
a = The Zn lines are somewhat distorted; this may lead to an overestimated
Zn abundance.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table9.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- Name Star name
11- 15 A5 --- Ion Species identification
17- 20 F4.2 [-] log(e)X0 ? Log ε abundance (X)0
22 A1 --- l_log(e) Limit flag on loge
23- 27 F5.2 [-] log(e) ? Log ε abundance (X)
29 A1 --- f_log(e) [b] neutron capture element (1)
31 A1 --- l_[X/H] Limit flag on [X/H]
32- 36 F5.2 [Sun] [X/H] ? X/H abundance
38 A1 --- l_[X/Fe] Limit flag on [X/Fe]
39- 43 F5.2 [Sun] [X/Fe] ? X/Fe abundance (G1)
45- 47 I3 --- N ? Number of spectra used
49- 52 F4.2 [-] Sig ? Abundance uncertainty
54 A1 --- ref [cd] source of the data: c=this paper,
d=Aoki et al. (2007ApJ...660..747A 2007ApJ...660..747A)
--------------------------------------------------------------------------------
Note (1): b = Neutron-capture abundances are taken from Honda et al.
(2006ApJ...643.1180H 2006ApJ...643.1180H).
--------------------------------------------------------------------------------
Global notes:
Note (G1): [X/Fe] ratios are computed with [FeI/H] abundances of the
respective stars. Solar abundances have been taken from Asplund et al.
(2005ASPC..336...25A 2005ASPC..336...25A). For abundances measured from only one line, we
adopt a nominal uncertainty of 0.20dex.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 02-Feb-2012