Old open clusters in the outer Galactic disk

Abstract

Context. The outer parts of the Milky Way disk are believed to be one of the main arenas where the accretion of external material in the form of dwarf galaxies and subsequent formation of streams is taking place. The Monoceros stream and the Canis Major and Argo over-densities are notorious examples. Understanding whether what we detect is the signature of accretion or, more conservatively, simply the intrinsic nature of the disk, represents one of the major goals of modern Galactic astronomy. Aims. We try to shed more light on the properties of the outer disk by exploring the properties of distant anti-center old open clusters. We want to verify whether distant clusters follow the chemical and dynamical behavior of the solar vicinity disk, or whether their properties can be better explained in terms of an extra-galactic population. Methods. VLT high resolution spectra have been acquired for five distant open clusters: Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73 and Berkeley 75. We derive accurate radial velocities to distinguish field interlopers and cluster members. For the latter we perform a detailed abundance analysis and derive the iron abundance [Fe/H] and the abundance ratios of several α elements. Results. Our analysis confirms previous indications that the radial abundance gradient in the outer Galactic disk does not follow the expectations extrapolated from the solar vicinity, but exhibits a shallower slope. By combining the metallicity of the five program clusters with eight more clusters for which high resolution spectroscopy is available, we find that the mean metallicity in the outer disk between 12 and 21 kpc from the Galactic center is [Fe/H] ≈ −0.35, with only marginal indications for a radial variation. In addition, all the program clusters exhibit solar scaled or slightly enhanced α elements, similar to open clusters in the solar vicinity and thin disk stars. Conclusions. We investigate whether this outer disk cluster sample might belong to an extra-galactic population, like the Monoceros ring. However, close scrutiny of their properties -location, kinematics and chemistry -does not convincingly favor this hypothesis. On the contrary, they appear more likely genuine Galactic disk clusters. We finally stress the importance to obtain proper motion measurements for these clusters to constrain their orbits.

The evolution of the Milky Way: new insights from open clusters

“…However, not all the investigators have arrived at similar measures of the radial metallicity gradient which may be partly related to the different methods adopted (see for example, Magrini et al 2009;Heiter et al 2014). Our current understanding suggests a gradient of about −0.06 dex kpc −1 (Friel et al 2002;Pancino et al 2010) to −0.20 dex kpc −1 (Frinchaboy et al 2013) in the radial range 5 to 10 kpc with a nearly flat trend (−0.02 dex kpc −1 ) beyond 13 kpc (Carraro et al 2007;Sestito et al 2008;Pancino et al 2010;Yong et al 2012;Frinchaboy et al 2013;Cantat-Gaudin et al 2016).…”

“…In recent years, modern high-resolution spectrographs and large reflectors have provided high-quality spectra of stars in OCs for secure measures of abundances, and extended abundance estimates to distant OCs in the direction of Galactic anti-centre (Carraro et al 2007;Yong et al 2005Yong et al , 2012Sestito et al 2006Sestito et al , 2008. However, not all the investigators have arrived at similar measures of the radial metallicity gradient which may be partly related to the different methods adopted (see for example, Magrini et al 2009;Heiter et al 2014).…”

Chemical gradients in the Milky Way from the RAVE data

“…In their model the whole disc forms simultaneously, and a metallicity gradient of ∼−0.11 dex kpc −1 is finally obtained because of the advection inwards of heavier elements released by SNe. Observational data (see Sestito et al 2008;Luck et al 2006;Carraro et al 2007;Yong et al 2012 among others) suggest that there are variations of the gradient along R: a steeper gradient in the inner disc (R 5 kpc) and a flatter gradient in the outer disc (R 10 kpc).…”

Abundances and physical parameters for stars in the open clusters NGC 5822 and IC 4756

“…BOCCE, with many singular or coordinated studies of stars of different spectral type and luminosity class, has significantly enhanced the database that can be employed to determine the Galactic abundance gradient (e.g., Carraro et al 2007;Magrini et al 2009;D'Orazi & Randich 2009;Paulson et al 2003). These studies aim mainly to investigate the evolution with time of several phenomena occurring in stars, such as the depletion of light elements and chromospheric activity, and global properties of Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, during the observing runs 073.D-0655 and 079.C-0131.…”