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TOPIC: Monoceros Stream

Blobrana

L

Posts: 131433

Date: Jul 12 08:44 2012

Monoceros ring	Permalink

Title: Comments on the "Monoceros" affair Authors: M. Lopez-Corredoira, A. Moitinho, S. Zaggia, Y. Momany, G. Carraro, P. L. Hammersley, A. Cabrera-Lavers, R. A. Vazquez This is a brief note to comment on some recent papers addressing the Monoceros ring. In our view, nothing new was delivered on the matter: No new evidence or arguments are presented which lead to think that the over-densities in Monoceros must not be due to the flared thick disc of the Milky Way. Again, we restate that extrapolations are easily misleading and that a model of the Galaxy is not the Galaxy. Raising and discussing exciting possibilities is healthy. However, enthusiasm should not overtake and produce strong claims before thoroughly checking simpler and more sensible possibilities within their uncertainties. In particular, claiming that a reported structure, such as the Monoceros Ring, is not Galactic (an exciting scenario) should not be done without rejecting the possibility of being due to the well established warped and flared disc of the Milky Way (simpler). Read more (16kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: May 17 00:07 2012

Monoceros Overdensity	Permalink

Title: Slicing The Monoceros Overdensity with Suprime-Cam Authors: Blair C. Conn, Noelia E. D. Noël, Hans-Walter Rix, R. R. Lane, G. F. Lewis, M. J. Irwin, N. F. Martin, R. A. Ibata, A. Dolphin, S. Chapman We derive distance, density and metallicity distribution of the stellar Monoceros Overdensity (MO) in the outer Milky Way, based on deep imaging with the Subaru Telescope. We applied CMD fitting techniques in three stripes at galactic longitudes: l=130°, 150°, 170°; and galactic latitudes: +15 < b° < +25 . The MO appears as a wall of stars at a heliocentric distance of ~ 10.1 ±0.5 kpc across the observed longitude range with no distance change. The MO stars are more metal rich ([Fe/H] ~ -1.0) than the nearby stars at the same latitude. These data are used to test three different models for the origin of the MO: a perturbed disc model, which predicts a significant drop in density adjacent to the MO that is not seen; a basic flared disc model, which can give a good match to the density profile but the MO metallicity implies the disc is too metal rich to source the MO stars; and a tidal stream model, which bracket the distances and densities we derive for the MO, suggesting that a model can be found that would fully fit the MO data. Further data and modelling will be required to confirm or rule out the MO feature as a stream or as a flaring of the disc. Read more (1023kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: May 7 19:14 2012

Monoceros Stream	Permalink

Title: The Metallicity of the Monoceros Stream Authors: Aaron M. Meisner, Anna Frebel, Mario Juric, Douglas P. Finkbeiner We present low-resolution MMT Hectospec spectroscopy of 594 candidate Monoceros stream member stars. Based on strong colour-magnitude diagram overdensities, we targeted three fields within the stream's footprint, with 178 deg < l < 203 deg and -25 deg < b < 25 deg. By comparing the measured iron abundances with those expected from smooth Galactic components alone, we measure, for the first time, the spectroscopic metallicity distribution function for Monoceros. We find the stream to be chemically distinct from both the thick disk and halo, with [Fe/H] = -1, and do not detect a trend in the stream's metallicity with Galactic longitude. Passing from b = +25 deg to b = -25 deg the median Monoceros metallicity trends upward by 0.1 dex, though uncertainties in modelling sample contamination by the disk and halo make this a marginal detection. In each field, we find Monoceros to have an intrinsic [Fe/H] dispersion of 0.10-0.22 dex. From the CaII K line, we measure [Ca/Fe] for a subsample of metal poor program stars with -1.1 < [Fe/H] < -0.5. In two of three fields, we find calcium deficiencies qualitatively similar to previously reported [Ti/Fe] underabundances in Monoceros and the Sagittarius tidal stream. Further, using 90 spectra of thick disk stars in the Monoceros pointings with b ~ ±25 deg, we detect a 0.22 dex north/south metallicity asymmetry coincident with known stellar density asymmetry at R_GC ~ 12 kpc and \|Z\| ~ 1.7 kpc. Our median Monoceros [Fe/H] = -1.0 and its relatively low dispersion naturally fit the expectation for an appropriately luminous M_V ~ -13 dwarf galaxy progenitor. Read more (485kb, PDF) __________________

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