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Register a new userHigh-resolution spectroscopy of RGB stars in the Sagittarius Streams. I. Radial velocities and chemical abundances
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L. Monaco
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Context. The Sagittarius (Sgr) dwarf spheroidal galaxy is currently being disrupted under the strain of the Milky Way. A reliable reconstruction of Sgr star formation history can only be obtained by combining core and stream information. Aims. We present radial velocities for 67 stars belonging to the Sgr Stream. For 12 stars in the sample we also present iron (Fe) and $alpha$-element (Mg, Ca) abundances. Methods. Spectra were secured using different high resolution facilities: UVES@VLT, [email protected], and SARG@TNG. Radial velocities are obtained through cross correlation with a template spectra. Concerning chemical analysis, for the various elements, selected line equivalent widths were measured and abundances computed using the WIDTH code and ATLAS model atmospheres. Results. The velocity dispersion of the trailing tail is found to be $sigma$=8.3$pm$0.9 km s$^{-1}$, i.e., significantly lower than in the core of the Sgr galaxy and marginally lower than previous estimates in the same portion of the stream. Stream stars follow the same trend as Sgr main body stars in the [$alpha$/Fe] vs [Fe/H] plane. However, stars are, on average, more metal poor in the stream than in the main body. This effect is slightly stronger in stars belonging to more ancient wraps of the stream, according to currently accepted models of Sgr disruption.
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Elemental abundances of carbon, nitrogen, oxygen, magnesium, aluminum, and silicon are presented for a sample of twelve rapidly rotating OB star (v sin i > 60 km s^-1) members of the Cep OB2, Cyg OB3 and Cyg OB7 associations. The abundances are derived from spectrum synthesis, using both LTE and non-LTE calculations. As found in almost all previous studies of OB stars, the average abundances are slightly below solar, by about 0.1 to 0.3 dex. In the case of oxygen, even with the recently derived low solar abundances the OB stars are closer to, but still below, the solar value. Results for the 9 Cep OB2 members in this sample can be combined with results published previously for 8 Cep OB2 stars with low projected rotational velocities to yield the most complete set of abundances, to date, for this particular association. These abundances provide a clear picture of both the general chemical and individual stellar evolution that has occurred within this association. By placing the Cep OB2 stars studied in an HR diagram we identify the presence of two distinct age subgroups, with both subgroups having quite uniform chemical abundances. Two stars are found in the older subgroup that show significant N/O overabundances, with both stars being two of the most massive, the most evolved, and most rapidly rotating of the members studied in Cep OB2. These characteristics of increased N abundances being tied to high mass, rapid rotation, and an evolved phase are those predicted from models of rotating stars which undergo rotationally driven mixing.
We present chemical abundances in a sample of luminous cool stars located within 30 pc of the Galactic Center. Abundances of carbon, nitrogen, oxygen, calcium, and iron were derived from high-resolution infrared spectra in the H- and K-bands. The abundance results indicate that both [O/Fe] and [Ca/Fe] are enhanced respectively by averages of +0.2 and +0.3 dex, relative to either the Sun or the Milky Way disk at near solar Fe abundances. The Galactic Center stars show a nearly uniform and nearly solar iron abundance. The mean value of A(Fe) = 7.59 +- 0.06 agrees well with previous work. The total range in Fe abundance among Galactic Center stars, 0.16 dex, is significantly narrower than the iron abundance distributions found in the literature for the older bulge population. Our snapshot of the current-day Fe abundance within 30 pc of the Galactic Center samples stars with an age less than 1 Gyr; a larger sample in time (or space) may find a wider spread in abundances.
We present chemical abundances and radial velocities of six HII regions in the extremely metal-poor star-forming dwarf galaxy DDO 68. They are derived from deep spectra in the wavelength range 3500 - 10,000 {AA}, acquired with the Multi Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT). In the three regions where the [O III]$lambda$4363 {AA} line was detected, we inferred the abundance of He, N, O, Ne, Ar, and S through the direct method. We also derived the oxygen abundances of all the six regions adopting indirect method calibrations. We confirm that DDO 68 is an extremely metal-poor galaxy, and a strong outlier in the luminosity - metallicity relation defined by star-forming galaxies. With the direct-method we find indeed an oxygen abundance of 12+log(O/H)=7.14$pm$0.07 in the northernmost region of the galaxy and, although with large uncertainties, an even lower 12+log(O/H)=6.96$pm$0.09 in the tail. This is, at face value, the most metal-poor direct abundance detection of any galaxy known. We derive a radial oxygen gradient of -0.06$pm$0.03 dex/kpc (or -0.30 dex $R_{25}^{-1}$) with the direct method, and a steeper gradient of -0.12$pm$0.03 dex/kpc (or -0.59 dex $R_{25}^{-1}$) from the indirect method. For the $alpha$-element to oxygen ratios we obtain values in agreement with those found in other metal-poor star-forming dwarfs. For nitrogen, instead, we infer much higher values, leading to log(N/O)$sim-1.4$, at variance with the suggested existence of a tight plateau at $-1.6$ in extremely metal poor dwarfs. The derived helium mass fraction ranges from Y=0.240$pm$0.005 to Y=0.25$pm$0.02, compatible with standard big bang nucleosynthesis. Finally, we measured HII region radial velocities in the range 479$-$522 km/s from the tail to the head of the comet, consistent with the rotation derived in the HI.
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