Search

We present a spectroscopic study of a sample of 238 RR Lyrae stars, from the QUEST survey, located in the Galactic halo at distances between 4 and 20 kpc from the Sun. Combining their spatial position and kinematics we were able to identify sub-structures in this part of the halo. Some of those sub-structures may be associated with known halo features like the Virgo Overdensity, the Hercules-Aquila Cloud, and the Anticenter Stream.

We carried out a spectroscopic follow-up programme of the four new stellar stream candidates detected by Belokurov & Koposov in the outskirts of the Large Magellanic Cloud (LMC) using FORS2 (VLT). The medium-resolution spectra were used to measure the line-of-sight velocities, estimate stellar metallicities, and classify stars into Blue Horizontal Branch (BHB) and Blue Straggler (BS) stars. Using the 4-D phase-space information, we attribute approximately one half of our sample to the Magellanic Clouds, while the rest is part of the Galactic foreground. Only two of the four stream candidates are confirmed kinematically. While it is impossible to estimate the exact levels of MW contamination, the phase-space distribution of the entire sample of our Magellanic stars matches the expected velocity gradient for the LMC halo and extends as far as 33 deg (angular separation) or 29 kpc from the LMC centre. Our detections reinforce the idea that the halo of the LMC seems to be larger than previously expected, and its debris can be spread in the sky out to very large separations from the LMC centre. Finally, we provide some kinematic evidence that many of the stars analysed here have likely come from the Small Magellanic Cloud. [ABSTRACT FROM AUTHOR]

We report the discovery of a thin stellar stream – which we name the Jet stream – crossing the constellations of Hydra and Pyxis. The discovery was made in data from the Search for the Leading Arm of Magellanic Satellites (SLAMS) survey, which comprises deep g and r imaging for a 650 deg2 region above the Galactic disc performed by the CTIO Blanco + DECam. SLAMS photometric catalogues have been made publicly available. The stream is approximately 0.18 deg wide and 10 deg long, though it is truncated by the survey footprint. Its colour–magnitude diagram is consistent with an old, metal-poor stellar population at a heliocentric distance of approximately 29 kpc. We corroborate this measurement by identifying a spatially coincident overdensity of likely blue horizontal branch stars at the same distance. There is no obvious candidate for a surviving stream progenitor. [ABSTRACT FROM AUTHOR]

Context. Several works have found an increase of the abundances of the s-process neutron-capture elements in the youngest Galactic stellar populations. These trends provide important constraints on stellar and Galactic evolution and they need to be confirmed with large and statistically significant samples of stars spanning wide age and distance intervals. Aims. We aim to trace the abundance patterns and the time evolution of five s-process elements – two belonging to the first peak, Y and Zr, and three belonging to the second peak, Ba, La, and Ce – using the Gaia-ESO IDR5 results for open clusters and disc stars. Methods. From the UVES spectra of cluster member stars, we determined the average composition of clusters with ages >0.1 Gyr. We derived statistical ages and distances of field stars, and we separated them into thin and thick disc populations. We studied the time-evolution and dependence on metallicity of abundance ratios using open clusters and field stars whose parameters and abundances were derived in a homogeneous way. Results. Using our large and homogeneous sample of open clusters, thin and thick disc stars, spanning an age range larger than 10 Gyr, we confirm an increase towards young ages of s-process abundances in the solar neighbourhood. These trends are well defined for open clusters and stars located nearby the solar position and they may be explained by a late enrichment due to significant contribution to the production of these elements from long-living low-mass stars. At the same time, we find a strong dependence of the s-process abundance ratios on the Galactocentric distance and on the metallicity of the clusters and field stars. Conclusions. Our results, derived from the largest and most homogeneous sample of s-process abundances in the literature, confirm the growth with decreasing stellar ages of the s-process abundances in both field and open cluster stars. At the same time, taking advantage of the abundances of open clusters located in a wide Galactocentric range, these results offer a new perspective on the dependence of the s-process evolution on the metallicity and star formation history, pointing to different behaviours at various Galactocentric distances. [ABSTRACT FROM AUTHOR]

We present the status of an ongoing project to map the detailed chemical abundances of stars across the main body of the Sagittarius dwarf Spheroidal galaxy (Sgr dSph). The Sgr dSph is the closest known dwarf galaxy, and is being tidally destroyed by its interaction with the Milky Way (MW), leaving behind a massive stellar stream. Sgr dSph is a chemically outstanding object, with peculiar abundance ratios, clear center-outskirts abundance gradients, and spanning more than 3 orders of magnitude in metallicity. We present here detailed abundances from UVES@VLT spectra for more than 50 giants across 8 fields along the major and minor axes of Sgr dSph, and 5 more outside the galaxy main body, but possibly associated to its stellar stream. [ABSTRACT FROM AUTHOR]

We have derived from VIMOS spectroscopy the radial velocities for a sample of 71 stars selected from CFHT/Megacam photometry around the Galactic globular cluster NGC 7492. In the resulting velocity distribution, it is possible to distinguish two relevant non-Galactic kinematic components along the same line of sight: a group of stars at 〈vr〉 ∼ 125 km s−1 which is compatible with the velocity of the old leading arm of the Sagittarius tidal stream, and a larger number of objects at 〈vr〉 ∼ −110 km s−1 that might be identified as members of the trailing wrap of the same stream. The systemic velocity of NGC 7492 set at vr ∼ −177 km s−1 differs significantly from that of both components, thus our results confirm that this cluster is not one of the globular clusters deposited by the Sagittarius dwarf spheroidal in the Galactic halo, even if it is immersed in the stream. A group of stars with 〈vr〉 ∼ − 180 km s−1 might be comprised of cluster members along one of the tidal tails of NGC 7492. [ABSTRACT FROM AUTHOR]

We take advantage of the deep and wide coverage of the VLT Survey Telescope (VST) ATLAS survey to study the line-of-sight structure of the Sagittarius (Sgr) stellar stream in the Southern hemisphere, only ∼40° away from the progenitor. We use photometrically selected subgiant branch (SGB) stars to reveal a complex debris morphology of the trailing arm and detect at least two clear peaks in the SGB distance modulus distribution. The separation between the two line-of-sight components is at least 5 kpc at the edge of the VST ATLAS footprint, but appears to change along the stream, which allows us to conclude that these detections correspond to two physically independent stellar structures, rather than amix of co-distant stellar populations within a single stream. Our discovery of a fork in the Sgr trailing arm is verified using blue horizontal branch stars and our distance measurements are calibrated using RR Lyrae stars from the Catalina Real-Time Transient Survey. Comparing with numerical simulations of the Sgr dwarf disruption, the more distant of the two components in the fork matches perfectly with the track of the trailing debris. However, no obvious counterpart exists in the simulation for the closer line-of-sight component. [ABSTRACT FROM AUTHOR]

We present a study of the kinematics of 101 stars observed with Visible MultiObject Spectrograph around Whiting 1, a globular cluster embedded in the Sagittarius tidal stream. The obtained velocity distribution shows the presence of two wraps of that halo substructure at the same heliocentric distance as that of the cluster and with well-differentiated mean radial velocities. The most prominent velocity component seems to be associated with the trailing arm of Sagittarius with (vr) ~ -130 km s-1, which is consistent with the velocity of Whiting 1. This result supports that this globular cluster was formed in Sagittarius and recently accreted by the Milky Way. The second component with (vr) ~ 120km s-1 might correspond to the leading arm of Sagittarius, which has been predicted by numerical simulations but with no conclusive observational evidence of its existence presented so far. This detection of the old leading wrap of Sagittarius in the Southern hemisphere may be used to confirm and further constrain the models for its orbit and evolution. [ABSTRACT FROM AUTHOR]

Context. The tidal disruption of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) is producing the most prominent substructure in the Milky Way (MW) halo, the Sagittarius Stream. Aside from field stars, it is suspected that the Sgr dSph has lost a number of globular clusters (GC). Many Galactic GC are thought to have originated in the Sgr dSph. While for some candidates an origin in the Sgr dSph has been confirmed owing to chemical similarities, others exist whose chemical composition has never been investigated. Aims. NGC 5053 and NGC 5634 are two of these scarcely studied Sgr dSph candidate-member clusters. To characterize their composition we analyzed one giant star in NGC 5053, and two in NGC 5634. Methods. We analyze high-resolution and signal-to-noise spectra by means of the MyGIsFOS code, determining atmospheric parameters and abundances for up to 21 species between O and Eu. The abundances are compared with those of MW halo field stars, of unassociated MW halo globulars, and of the metal-poor Sgr dSph main body population. Results. We derive a metallicity of [Fe II/H] = -2:26 ± 0:10 for NGC 5053, and of [Fe I/H] = 1:99 ± 0:075 and -1:97 ± 0:076 for the two stars in NGC 5634. This makes NGC 5053 one of the most metal-poor globular clusters in the MW. Both clusters display an α enhancement similar to the one of the halo at comparable metallicity. The two stars in NGC 5634 clearly display the Na-O anticorrelation widespread among MW globulars. Most other abundances are in good agreement with standard MW halo trends. Conclusions. The chemistry of the Sgr dSph main body populations is similar to that of the halo at low metallicity. It is thus difficult to discriminate between an origin of NGC 5053 and NGC 5634 in the Sgr dSph, and one in the MW. However, the abundances of these clusters do appear closer to that of Sgr dSph than of the halo, favoring an origin in the Sgr dSph system. [ABSTRACT FROM AUTHOR]

Aims. We study how diffuse interstellar bands (DIBs) measured toward distance-distributed target stars can be used to locate dense interstellar (IS) clouds in the Galaxy and probe a line-of-sight (LOS) kinematical structure, a potentially useful tool when gaseous absorption lines are saturated or not available in the spectral range. Cool target stars are numerous enough for this purpose. Methods. We devised automated DIB-fitting methods appropriate for cool star spectra and multiple IS components. The data were fitted with a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profiles. The initial number of DIB components and their radial velocity were guided by HI 21 cm emission spectra, or, when available in the spectral range, IS neutral sodium absorption lines. For NaI, radial velocities of NaI lines and DIBs were maintained linked during a global simultaneous fit. In parallel, stellar distances and extinctions were estimated self-consistently by means of a 2D Bayesian method from spectroscopically-derived stellar parameters and photometric data. Results. We have analyzed Gaia-ESO Survey (GES) spectra of 225 stars that probe between ~2 and 10 kpc long LOS in five different regions of the Milky Way. The targets are the two CoRoT fields, two open clusters (NGC 4815 and λ Vel), and the Galactic bulge. Two OGLE fields toward the bulge observed before the GES are also included (205 target stars). Depending on the observed spectral intervals, we extracted one or more of the following DIBs: λλ 6283.8, 6613.6, and 8620.4. For each field, we compared the DIB strengths with the Bayesian distances and extinctions, and the DIB Doppler velocities with the HI emission spectra. Conclusions. For all fields, the DIB strength and the target extinction are well correlated. For targets that are widely distributed in distance, marked steps in DIBs and extinction radial distance profiles match each other and broadly correspond to the expected locations of spiral arms. For all fields, the DIB velocity structure agrees with HI emission spectra, and all detected DIBs correspond to strong NaI lines. This illustrates how DIBs can be used to locate the Galactic interstellar gas and to study its kinematics at the kpc scale, as illustrated by Local and Perseus Arm DIBs that differ by ≳~30 km s-1, in agreement with HI emission spectra. On the other hand, if most targets are located beyond the main absorber, DIBs can trace the differential reddening within the field. [ABSTRACT FROM AUTHOR]

Context. The current and planned high-resolution, high-multiplexity stellar spectroscopic surveys, as well as the swelling amount of underutilized data present in public archives, have led to an increasing number of efforts to automate the crucial but slow process of retrieving stellar parameters and chemical abundances from spectra. Aims. We present MyGIsFOS1, a code designed to derive atmospheric parameters and detailed stellar abundances from medium- to high-resolution spectra of cool (FGK) stars. We describe the general structure and workings of the code, present analyses of a number of well-studied stars representative of the parameter space MyGIsFOS is designed to cover, and give examples of the exploitation of MyGIsFOS very fast analysis to assess uncertainties through Monte Carlo tests. Methods. MyGIsFOS aims to reproduce a "traditional" manual analysis by fitting spectral features for different elements against a precomputed grid of synthetic spectra. The lines of Fe I and Fe II can be employed to determine temperature, gravity, microturbulence, and metallicity by iteratively minimizing the dependence of Fe i abundance from line lower energy and equivalent width, and imposing Fe I-Fe II ionization equilibrium. Once parameters are retrieved, detailed chemical abundances are measured from lines of other elements. Results. MyGIsFOS replicates closely the results obtained in similar analyses on a set of well-known stars. It is also quite fast, performing a full parameter determination and detailed abundance analysis in about two minutes per star on a mainstream desktop computer. Currently, its preferred field of application are high-resolution and/or large spectral coverage data (e.g., UVES, X-shooter, HARPS, Sophie). [ABSTRACT FROM AUTHOR]

We have completed a spectroscopic study of the “12.4 hr clump”, the second largest substructure in the Quasar Equatorial Survey Team (QUEST) catalog. First discovered as an over-density of RR Lyrae stars (Vivas et al. 2001, ApJL 554 33), the region containing the “12.4 hr clump” has generated much interest (Newberg et al. (2002), Martinez-Delgado et al. (2007), Juric et al. (2008), amongst many others). Our first spectroscopic study of this clump revealed the presence of a sharp peak in the radial velocity histogram for the candidate stars (Duffau et al. 2006). The combination of this result and metal abundance estimates for the sample was then interpreted as a signature of the presence of a stellar stream within the clump. This sub-structure was named the “Virgo Stellar Stream” (VSS), given its location in the direction of the Virgo Constellation, at approximately 20 kpc from the Sun. Several other groups have studied this region and have suggested that the over-density containing the VSS could extend to larger areas of the sky (outside QUEST's observing range). We present the complete spectroscopic follow up of the clump candidates present in QUEST and the composite of the studies we performed along the same l.o.s., including data at brighter magnitudes (Vivas et al. 2008). Our study confirmed the nature of the VSS, revealed its likely extent within the QUEST survey and defined a number of its relevant properties. [ABSTRACT FROM PUBLISHER]

We present VLT spectroscopy of 13 RR Lyrae variables from the QUEST survey that lie in the leading arm of the tidal stream from Sagittarius dSph galaxy. We observed these stars to measure their radial velocities and metal abundances and to complete a sample of stars from the overdensity detected by QUEST in this area of the sky. The full sample contains 42 stars, 29 observed with VLT and 13 observed with GEMINI, we report here the results from the VLT observations so far. We conclude that the new stars and stars reported before by Vivas, Zinn and Gallart (2005) are consistent with belonging to Sgr. We discuss the importance of the final third of the sample, the GEMINI targets, which will provide important constrains on models of the Sgr streams that assume different dark matter halo shapes. [ABSTRACT FROM PUBLISHER]