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Register a new userProper Motion Dispersions of Red Clump Giants in the Galactic Bulge: Observations and Model Comparisons
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Added by
Nicholas Rattenbury
Publication date
2007
fields
Physics
and research's language is
English
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Red clump giants in the Galactic bulge are approximate standard candles and hence they can be used as distance indicators. We compute the proper motion dispersions of RCG stars in the Galactic bulge using the proper motion catalogue from the second phase of the Optical Gravitational Microlensing Experiment (OGLE-II, Sumi et al. 2004) for 45 fields. The proper motion dispersions are measured to a few per cent accuracy due to the large number of stars in the fields. The observational sample is comprised of 577736 stars. These observed data are compared to a state-of-the-art particle simulation of the Galactic bulge region. The predictions are in rough agreement with observations, but appear to be too anisotropic in the velocity ellipsoid. We note that there is significant field-to-field variation in the observed proper motion dispersions. This could either be a real feature, or due to some unknown systematic effect.
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We present a study of the luminosity density distribution of the Galactic bar using number counts of red clump giants (RCGs) from the OGLE-III survey. The data were recently published by Nataf et al. (2013) for 9019 fields towards the bulge and have $2.94times 10^6$ RC stars over a viewing area of $90.25 ,textrm{deg}^2$. The data include the number counts, mean distance modulus ($mu$), dispersion in $mu$ and full error matrix, from which we fit the data with several tri-axial parametric models. We use the Markov Chain Monte Carlo (MCMC) method to explore the parameter space and find that the best-fit model is the $E_3$ model, with the distance to the GC is 8.13 kpc, the ratio of semi-major and semi-minor bar axis scale lengths in the Galactic plane $x_{0},y_{0}$, and vertical bar scale length $z_0$, is $x_0:y_0:z_0 approx 1.00:0.43:0.40$ (close to being prolate). The scale length of the stellar density profile along the bars major axis is $sim$ 0.67 kpc and has an angle of $29.4^circ$, slightly larger than the value obtained from a similar study based on OGLE-II data. The number of estimated RC stars within the field of view is $2.78 times 10^6$, which is systematically lower than the observed value. We subtract the smooth parametric model from the observed counts and find that the residuals are consistent with the presence of an X-shaped structure in the Galactic centre, the excess to the estimated mass content is $sim 5.8%$. We estimate the total mass of the bar is $sim 1.8 times 10^{10} M_odot$. Our results can be used as a key ingredient to construct new density models of the Milky Way and will have implications on the predictions of the optical depth to gravitational microlensing and the patterns of hydrodynamical gas flow in the Milky Way.
We have used the AAOMEGA spectrograph to obtain R $sim 1500$ spectra of 714 stars that are members of two red clumps in the Plaut Window Galactic bulge field $(l,b)=0^{circ},-8^{circ}$. We discern no difference between the clump populations based on radial velocities or abundances measured from the Mg$b$ index. The velocity dispersion has a strong trend with Mg$b$-index metallicity, in the sense of a declining velocity dispersion at higher metallicity. We also find a strong trend in mean radial velocity with abundance. Our red clump sample shows distinctly different kinematics for stars with [Fe/H] $<-1$, which may plausibly be attributable to a minority classical bulge or inner halo population. The transition between the two groups is smooth. The chemo-dynamical properties of our sample are reminiscent of those of the Milky Way globular cluster system. If correct, this argues for no bulge/halo dichotomy and a relatively rapid star formation history. Large surveys of the composition and kinematics of the bulge clump and red giant branch are needed to define further these trends.
Oxygen and zinc in the Galactic bulge are key elements for the understanding of the bulge chemical evolution. Oxygen-to-iron abundance ratios provide a most robust indicator of the star formation rate and chemical evolution of the bulge. Zinc is enhanced in metal-poor stars, behaving as an $alpha$-element, and its production may require nucleosynthesis in hypernovae. Most of the neutral gas at high redshift is in damped Lyman-alpha systems (DLAs), where Zn is also observed to behave as an alpha-element. The aim of this work is the derivation of the alpha-element oxygen, together with nitrogen, and the iron-peak element zinc abundances in 417 bulge giants, from moderate resolution (R~22,000) FLAMES-GIRAFFE spectra. For stars in common with a set of UVES spectra with higher resolution (R~45,000), the data are intercompared. The results are compared with literature data and chemodynamical models.
The Milky Way bulge is an important tracer of the early formation and chemical enrichment of the Galaxy. The abundances of different iron-peak elements in field bulge stars can give information on the nucleosynthesis processes that took place in the earliest supernovae. Cobalt (Z=27) and copper (Z=29) are particularly interesting.We aim to identify the nucleosynthesis processes responsible for the formation of the iron-peak elements Co and Cu. Methods. We derived abundances of the iron-peak elements cobalt and copper in 56 bulge giants, 13 of which were red clump stars. High-resolution spectra were obtained using FLAMES-UVES at the ESO Very Large Telescope by our group in 2000-2002, which appears to be the highest quality sample of high-resolution data on bulge red giants obtained in the literature to date. Over the years we have derived the abundances of C, N, O, Na, Al, Mg; the iron-group elements Mn and Zn; and neutron-capture elements. In the present work we derive abundances of the iron-peak elements cobalt and copper. We also compute chemodynamical evolution models to interpret the observed behaviour of these elements as a function of iron. The sample stars show mean values of [Co/Fe]~0.0 at all metallicities, and [Cu/Fe]~0.0 for [Fe/H]>-0.8 and decreasing towards lower metallicities with a behaviour of a secondary element. We conclude that [Co/Fe] varies in lockstep with [Fe/H], which indicates that it should be produced in the alpha-rich freezeout mechanism in massive stars. Instead [Cu/Fe] follows the behaviour of a secondary element towards lower metallicities, indicating its production in the weak s-process nucleosynthesis in He-burning and later stages. The chemodynamical models presented here confirm the behaviour of these two elements (i.e. [Co/Fe] vs. [Fe/H]~constant and [Cu/Fe] decreasing with decreasing metallicities).
We seek to constrain the formation of the Galactic bulge by means of analysing the detailed chemical composition of a large sample of red clump stars in Baades window. We measure [Fe/H] in a sample of 219 bulge red clump stars from R=20000 resolution spectra obtained with FLAMES/GIRAFFE at the VLT, using an automatic procedure, differentially to the metal-rich local reference star muLeo. For a subsample of 162 stars, we also derive [Mg/H] from spectral synthesis around the MgI triplet at 6319A. The Fe and Mg metallicity distributions are both asymmetric, with median values of +0.16 and +0.21 respectively. The iron distribution is clearly bimodal, as revealed both by a deconvolution (from observational errors) and a Gaussian decomposition. The decomposition of the observed Fe and Mg metallicity distributions into Gaussian components yields two populations of equal sizes (50% each): a metal-poor component centred around [Fe/H]=-0.30 and [Mg/H]=-0.06 with a large dispersion and a narrow metal-rich component centred around [Fe/H]=+0.32 and [Mg/H]=+0.35. The metal poor component shows high [Mg/Fe] ratios (around 0.3) whereas stars in the metal rich component are found to have near solar ratios. Babusiaux et al. (2010) also find kinematical differences between the two components: the metal poor component shows kinematics compatible with an old spheroid whereas the metal rich component is consistent with a population supporting a bar. In view of their chemical and kinematical properties, we suggest different formation scenarios for the two populations: a rapid formation timescale as an old spheroid for the metal poor component (old bulge) and for the metal rich component, a formation over a longer time scale driven by the evolution of the bar (pseudo-bulge).
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