We report precipitable water vapor (PWV) measurements made at Las Campanas Observatory using optical spectra of H2O lines obtained with the Magellan echelle spectrograph, and calculated using a robust technique that is accurate to 5%-10%. Calibration of the relationship between our PWV measurements and opacity values at 225 GHz was made possible by simultaneous observations with a tipping radiometer. Based on this calibration, we present Las Campanas Observatory wintertime precipitable water vapor statistics, measured using the tipping radiometer, during a 1.5 month campaign. The median value of 2.8 +/- 0.3 mm is consistent with that measured at the nearby La Silla Observatory during the VLT site survey. We conclude that in the Southern hemisphere winter months, we can expect good conditions for infrared observing (less than or similar to 1.5 mm) approximately 10% of the time at Las Campanas Observatory.

We report on the calibration of the relationship between precipitable water vapor and opacity at 225 GHz at Las Campanas Observatory as measured by a Tipping Radiometer. This relationship is a function of altitude and temperature and thus is highly dependent on location. We determine the relationship applicable at Las Campanas Observatory by using high-resolution Magellan Echelle spectra to measure the precipitable water vapor independently and absolutely. Temperature insensitive (between 220-300 K) lines allow the use of a single temperature atmospheric model as long as the lines are unsaturated. Absolute calibration was achieved by measuring the humidity in the path length of the McMath Solar telescope with a psycrometer [1]. We have expanded the method presented by Brault et al. (1975) with improved partition functions and additional lines. Based on this calibration, we present Southern hemisphere winter-time precipitable water vapor statistics for Las Campanas Observatory as measured during a two month campaign. We find that the median winter value of 2.8 +/- 0.3 mm is consistent with that measured at the nearby La Silla Observatory during the VLT site survey [21 and inconsistent (lower by a factor of approximately two) with estimates, also for La Silla, derived from GOES-8 satellite imagery and the European Centre for Medium-Range Weather Forecasting (ECMWF) meteorological numerical model 13]. Furthermore, in the Southern hemisphere winter months, we can expect good conditions for infrared observing (less than or similar to 1.5 mm) at the tenth percentile level. Further details can be found in Thomas-Osip et al. [4].

We present a detailed abundance analysis based on high-resolution and high signal-to-noise spectra of eight extremely metal-poor ( EMP) stars with [Fe/H] less than or similar to 3.5 dex, four of which are new. Only stars with 4900 K < T-eff < 5650 K are included. Two stars of the eight are outliers in each of several abundance ratios. The most metal-poor star in this sample, HE 1424-0241, has [Fe/H] similar to -4 dex and is thus among the most metal-poor stars known in the Galaxy. It has highly anomalous abundance ratios unlike those of any other known EMP giant, with very low Si, Ca, and Ti relative to Fe, and enhanced Mn and Co, again relative to Fe. Only ( low) upper limits for C and N can be derived from the nondetection of the CH and NH molecular bands. HE 0132-2429, another sample star, has excesses of N and Sc with respect to Fe. The strong outliers in abundance ratios among the Fe-peak elements in these C-normal stars, not found at somewhat higher metallicities ([Fe/H] similar to -3 dex), are definitely real. They suggest that at such low metallicities we are beginning to see the anticipated and long sought stochastic effects of individual supernova events contributing to the Fe-peak material within a single star. With spectra reaching well into the near-UV we are able to probe the behavior of copper abundances in such extreme EMP stars. A detailed comparison of the results of the analysis procedures adopted by our 0Z project compared to those of the First Stars VLT Large Project finds a systematic difference for [Fe/H] of similar to 0.3 dex, our values always being higher.

We present chemical abundances for O, Na, Mg, Al, Si, Ca, Ti, and Fe in eight red giants and one turnoff star in the metal-rich globular cluster 47 Tuc, based on spectroscopy with the Magellan Inamori Kyocera Echelle high-resolution spectrograph on the Magellan 6.5 m Clay telescope. A robust line by a line differential abundance analysis technique, relative to the K-giant Arcturus, was used to reduce systematic errors from atmospheric and atomic parameters. Our derived mean LTE [Fe/H] of -0.76 +/- 0.01 +/- 0.04 dex (random and systematic error, respectively) is more metal poor by about 0.1 dex than recent literature results. The chemical element ratios in this nearby globular cluster most closely resemble those of the Galactic bulge, although there is a non-negligible overlap with the composition of thick-disk stars. We find that the [Al/Fe] and [Na/Fe] ratios coincide with the upper boundary of the trends seen in the bulge and thick disk. There is only a small intrinsic scatter in the majority of the abundance ratios, indicating that 47 Tuc is mostly a rather chemically homogeneous system.

We show that the Galactic bulge and disk share a similar, strong, decline in [O/Mg] ratio with [Mg/H]. The similarity of the [O/Mg] trend in these markedly different populations suggests that the strong decline is due to a metallicity-dependent modulation of the stellar yields from massive stars by mass loss from winds, related to the Wolf-Rayet phenomenon, as proposed by McWilliam & Rich in 2004. We have modified existing models for the chemical evolution of the Galactic bulge and the solar neighborhood with the inclusion of metallicity-dependent oxygen yields from theoretical predictions for massive stars that include mass loss by stellar winds. Our results significantly improve the agreement between predicted and observed [O/Mg] ratios in the bulge and disk above solar metallicity; however, a small zero-point normalization problem remains to be resolved. The zero-point shift indicates that either the semi-empirical yields of Francois et al. obtained in 2004 need adjustment, or that the bulge initial mass function ( IMF) is not quite as flat as found by Ballero et al. in 2007. Our result removes a previous inconsistency between the interpretation of [O/Fe] and [Mg/Fe] ratios in the bulge, and confirms the conclusion that the bulge formed more rapidly than the disk, based on the overabundances of elements produced by massive stars. We also provide an explanation for the long-standing difference between [alpha/Fe] and [O/Fe] trends among disk stars more metal rich than the Sun.

To compare the globular clusters (GCs) associated with the Sagittarius Galaxy (Sgr) we report the results obtained from new high-resolution spectra of red giant stars in Terzan 8 and Arp 2, collected with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph at Las Campanas Observatory. For Ter 8 we find [Fe I/H] = -2.37 +/- 0.04 and [Fe II/H] = -2.30 +/- 0.07, while for Arp 2 [Fe I/H] = -1.77 +/- 0.04 and [Fe II/H] = -1.89 +/- 0.07. Other elements, usually of interest, show the following results. For oxygen, we obtain [O/Fe] = 0.71 +/- 0.17 for Ter 8 and [O/Fe] = 0.21 +/- 0.22 for Arp 2. For the alpha-elements (Mg, Si, Ca, Ti), the mean values are 0 ''.37 +/- 0.14 dex for Ter 8 and 0.31 +/- 0.11 for Arp 2. These values are not too different from the Galactic GCs at a similar [Fe/H] value. Regarding Cu, we find quite underabundant results for both clusters: [Cu/Fe] = -0.96 for Ter 8 and [Cu/Fe] = -0.93 for Arp 2. The heavy s-process elements Ba and Nd show no excesses in Ter 8 and Arp 2. Our only r-process element, Eu, shows an excess of 0.45 dex in Arp 2, based on the only useful line at 6645 angstrom.

Aims. We model the chemical evolution of manganese relative to iron in three different stellar systems: the Solar neighbourhood, the Galactic bulge, and the Sagittarius dwarf spheroidal galaxy, and we compare our results with recent and homogeneous observational data sets.

We report on the abundance analysis of two red giants in the faint Hercules dwarf spheroidal (dSph) galaxy. These stars show a remarkable deficiency in the neutron-capture elements, while the hydrostatic alpha-elements (O, Mg) are strongly enhanced. Our data indicate [Ba/Fe] and [Mg/Fe] abundance ratios of less than or similar to - 2 and similar to + 0.8 dex, respectively, with essentially no detection of other n-capture elements. In contrast to the only other dSph star with similar abundance patterns, Dra 119, which has a very low metallicity at [Fe/H] = -2.95 dex, our objects, at [Fe/H] similar to -2.0 dex, are only moderately metal-poor. The measured ratio of hydrostatic/explosive alpha-elements indicates that high-mass (similar to 35 M-circle dot) Type II supernovae progenitors are the main, if not only, contributors to the enrichment of this galaxy. This suggests that star formation and chemical enrichment in the ultrafaint dSphs proceeds stochastically and inhomogeneously on small scales, or that the IMF was strongly skewed to high-mass stars. The neutron capture deficiencies and the [Co/Fe] and [Cr/Fe] abundance ratios in our stars are similar to those in the extremely low metallicity Galactic halo. This suggests that either our stars are composed mainly of the ejecta from the first, massive, Population III stars ( but at moderately high [Fe/H]), or that SN ejecta in the Hercules galaxy were diluted with similar to 30 times less hydrogen than typical for extreme metal-poor stars.

Context. The evolution of C and O abundances in the Milky Way can impose strong constraints on stellar nucleosynthesis and help in understanding the formation and evolution of our Galaxy.