427 research outputs found
AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY
We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (ÎĽ = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe
Extinction Maps Toward The Milky Way Bulge: Two-Dimensional And Three-Dimensional Tests With APOGEE
Galactic interstellar extinction maps are powerful and necessary tools for Milky Way structure and stellar population analyses, particularly toward the heavily reddened bulge and in the midplane. However, due to the difficulty of obtaining reliable extinction measures and distances for a large number of stars that are independent of these maps, tests of their accuracy and systematics have been limited. Our goal is to assess a variety of photometric stellar extinction estimates, including both two-dimensional and three-dimensional extinction maps, using independent extinction measures based on a large spectroscopic sample of stars toward the Milky Way bulge. We employ stellar atmospheric parameters derived from high-resolution H-band Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra, combined with theoretical stellar isochrones, to calculate line-of-sight extinction and distances for a sample of more than 2400 giants toward the Milky Way bulge. We compare these extinction values to those predicted by individual near-IR and near+mid-IR stellar colors, two-dimensional bulge extinction maps, and three-dimensional extinction maps. The long baseline, near+mid-IR stellar colors are, on average, the most accurate predictors of the APOGEE extinction estimates, and the two-dimensional and three-dimensional extinction maps derived from different stellar populations along different sightlines show varying degrees of reliability. We present the results of all of the comparisons and discuss reasons for the observed discrepancies. We also demonstrate how the particular stellar atmospheric models adopted can have a strong impact on this type of analysis, and discuss related caveats.NSF Astronomy & Astrophysics Postdoctoral Fellowship AST-1203017Physics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA) PHY 08-22648U.S. National Science FoundationAlfred P. Sloan FoundationParticipating InstitutionsU.S. Department of Energy Office of Science ANR-12-BS05-0015-01Astronom
APOGEE DR14/DR15 Abundances in the Inner Milky Way
We present an overview of the distributions of 11 elemental abundances in the
Milky Way's inner regions, as traced by APOGEE stars released as part of SDSS
Data Release 14/15 (DR14/DR15), including O, Mg, Si, Ca, Cr, Mn, Co, Ni, Na,
Al, and K. This sample spans ~4000 stars with R_GC<4 kpc, enabling the most
comprehensive study to date of these abundances and their variations within the
innermost few kiloparsecs of the Milky Way. We describe the observed abundance
patterns ([X/Fe]-[Fe/H]), compare to previous literature results and to
patterns in stars at the solar Galactic radius, and discuss possible trends
with DR14/DR15 effective temperatures. We find that the position of the
[Mg/Fe]-[Fe/H] "knee" is nearly constant with R_GC, indicating a well-mixed
star-forming medium or high levels of radial migration in the early inner
Galaxy. We quantify the linear correlation between pairs of elements in
different subsamples of stars and find that these relationships vary; some
abundance correlations are very similar between the alpha-rich and alpha-poor
stars, but others differ significantly, suggesting variations in the
metallicity dependencies of certain supernova yields. These empirical trends
will form the basis for more detailed future explorations and for the
refinement of model comparison metrics. That the inner Milky Way abundances
appear dominated by a single chemical evolutionary track and that they extend
to such high metallicities underscore the unique importance of this part of the
Galaxy for constraining the ingredients of chemical evolution modeling and for
improving our understanding of the evolution of the Galaxy as a whole.Comment: Submitted to AAS Journals; revised after referee repor
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Mapping The Interstellar Medium With Near-Infrared Diffuse Interstellar Bands
We map the distribution and properties of the Milky Way's interstellar medium as traced by diffuse interstellar bands (DIBs) detected in near-infrared stellar spectra from the SDSS-III/APOGEE survey. Focusing exclusively on the strongest DIB in the H band, at lambda similar to 1.527 mu m, we present a projected map of the DIB absorption field in the Galactic plane, using a set of about 60,000 sightlines that reach up to 15 kpc from the Sun and probe up to 30 mag of visual extinction. The strength of this DIB is linearly correlated with dust reddening over three orders of magnitude in both DIB equivalent width (Wpm) and extinction, with a power law index of 1.01 +/- 0.01, a mean relationship of W-DIB/A(v) = 0.1 angstrom mag(-1) and a dispersion of similar to 0.05 angstrom mag(-1) at extinctions characteristic of the Galactic midplane. These properties establish this DIB as a powerful, independent probe of dust extinction over a wide range of Av values. The subset of about 14,000 robustly detected DIB features have a W-DIB distribution that follows an exponential trend. We empirically determine the intrinsic rest wavelength of this transition to be lambda(0) = 15 272.42 angstrom and use it to calculate absolute radial velocities of the carrier, which display the kinematical signature of the rotating Galactic disk. We probe the DIB carrier distribution in three dimensions and show that it can be characterized by an exponential disk model with a scale height of about 100 pc and a scale length of about 5 kpc. Finally, we show that the DIB distribution also traces large-scale Galactic structures, including the Galactic long bar and the warp of the outer disk.NSF Astronomy & Astrophysics Postdoctoral Fellowship AST-1203017NSF AST-1109665Alfred P. Sloan FoundationNational Science FoundationU.S. Department of Energy Office of ScienceUniversity of ArizonaBrazilian Participation GroupBrookhaven National LaboratoryUniversity of CambridgeCarnegie Mellon UniversityUniversity of FloridaFrench Participation GroupGerman Participation GroupHarvard UniversityInstituto de Astrofisica de CanariasMichigan State/Notre Dame/JINA Participation GroupJohns Hopkins UniversityLawrence Berkeley National LaboratoryMax Planck Institute for AstrophysicsMax Planck Institute for Extraterrestrial PhysicsNew Mexico State UniversityNew York UniversityOhio State UniversityPennsylvania State UniversityUniversity of PortsmouthPrinceton UniversitySpanish Participation GroupUniversity of TokyoUniversity of UtahVanderbilt UniversityUniversity of VirginiaUniversity of WashingtonYale UniversitySpanish Ministry of Economy and Competitiveness AYA-2011-27754McDonald Observator
A Young Planet Search in Visible and IR Light: DN Tau, V836 Tau, and V827 Tau
In searches for low-mass companions to late-type stars, correlation between
radial velocity variations and line bisector slope changes indicates
contamination by large starspots. Two young stars demonstrate that this test is
not sufficient to rule out starspots as a cause of radial velocity variations.
As part of our survey for substellar companions to T Tauri stars, we identified
the ~2 Myr old planet host candidates DN Tau and V836 Tau. In both cases,
visible light radial velocity modulation appears periodic and is uncorrelated
with line bisector span variations, suggesting close companions of several
M_Jup in these systems. However, high-resolution, infrared spectroscopy shows
that starspots cause the radial velocity variations. We also report unambiguous
results for V827 Tau, identified as a spotted star on the basis of both visible
light and infrared spectroscopy. Our results suggest that infrared follow up
observations are critical for determining the source of radial velocity
modulation in young, spotted stars.Comment: Accepted for publication in the Astrophysical Journal Letter
SMASHing the LMC: A Tidally-induced Warp in the Outer LMC and a Large-scale Reddening Map
We present a study of the three-dimensional (3D) structure of the Large
Magellanic Cloud (LMC) using ~2.2 million red clump (RC) stars selected from
the Survey of the MAgellanic Stellar History. To correct for line-of-sight dust
extinction, the intrinsic RC color and magnitude and their radial dependence
are carefully measured by using internal nearly dust-free regions. These are
then used to construct an accurate 2D reddening map (165 square degrees with
~10 arcmin resolution) of the LMC disk and the 3D spatial distribution of RC
stars. An inclined disk model is fit to the 2D distance map yielding a best-fit
inclination angle i = 25.86(+0.73,-1.39) degrees with random errors of +\-0.19
degrees and line-of-nodes position angle theta = 149.23(+6.43,-8.35) degrees
with random errors of +/-0.49 degrees. These angles vary with galactic radius,
indicating that the LMC disk is warped and twisted likely due to the repeated
tidal interactions with the Small Magellanic Cloud (SMC). For the first time,
our data reveal a significant warp in the southwestern part of the outer disk
starting at rho ~ 7 degrees that departs from the defined LMC plane up to ~4
kpc toward the SMC, suggesting that it originated from a strong interaction
with the SMC. In addition, the inner disk encompassing the off-centered bar
appears to be tilted up to 5-15 degrees relative to the rest of the LMC disk.
These findings on the outer warp and the tilted bar are consistent with the
predictions from the Besla et al. simulation of a recent direct collision with
the SMC.Comment: 25 pages, 15 figures, published in Ap
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