Overlapping abundance gradients and azimuthal gradients related to the spiral structure of the Galaxy

Abstract

The connection between some features of the metallicity gradient in the Galactic disc, best revealed by Open Clusters and Cepheids, and the spiral structure, has been explored. The step-like abrupt decrease in metallicity at 8.5 kpc (with R0= 7.5 kpc, or at 9.5 kpc if R0= 8.5 kpc is adopted) is well explained by the corotation ring-shaped gap in the density of gas, which isolates the internal and external regions of the disc one from the other. This solves the long-standing problem of a lack of understanding of the different chemical characteristics of the inner and outer parts of the disc. The time required to build up the metallicity difference between the two sides of the step is a measure of the minimal lifetime of the present grand-design spiral pattern structure, of the order of 3 Gyr. The plateaux observed on both sides of the step are interpreted in terms of the large-scale radial motion of the stars and of the gas flow induced by the spiral structure. The star formation rate revealed by the density of open clusters is maximum in the Galactic radial range from 6 to 12 kpc (with an exception of a narrow gap at corotation), coinciding with the region where the four-arms mode is allowed to exist. We argue that most of the old open clusters situated at large Galactocentric radii were born in this inner region where conditions more favourable for star formation are found. The ratio of α-elements to Fe of the sample of Cepheids does not vary appreciably with the Galactic radius, which reveals a homogeneous history of star formation. Different arguments are forwarded to show that the usual approximations of chemical evolution models, which assume fast mixing of metallicity in the azimuthal direction and ignore the existence of the spiral arms, are poor ones

CCD UBV(RI)C photometry of twenty open clusters

“…As discussed in the literature, this is caused by inhomogeneous chemical enhancements in the Galactic disc (Haywood , 2008;Jacobson et al , 2011), orbital diffusion, and radial migration and mixing of OCs (Schönrich & Binney , 2009). These OCs have probably originated from differing Galactic radii and/or from various star forming regions (Lepine et al , 2011). 7.…”

“…12, a global abundance gradient of +0.002 ± 0.022 dex/kpc is calculated indicating that there is no correlation between [Fe/H] and R GC (kpc) over this wide range. However, the works of Twarog et al (1997), Magrini et al (2009), Lepine et al (2011), andLepine (2013) have shown that abundance gradients correlating [Fe/H] with R GC for OCs should take into account a possible discontinuity at R GC ≈ 8-10 kpc. Figure 13 shows an age-metallicity plot from the corresponding values of our 20 OCs plus those from Dias catalogue.…”

The almost ubiquitous association of 6.7-GHz methanol masers with dust★

“…The entry shock experienced by the interstellar medium (ISM) gas entering a spiral arm should only exist inside the co-rotation radius, where there is a differential velocity between the spiral pattern speed and the orbital rotation speed of the Galactic ISM. Outside this radius (thought to be just beyond the solar circle in the Milky Way; Lépine et al 2011), supernovae may be the dominant mechanism determining the state of the ISM and, hence, star formation (e.g., Kobayashi et al 2008).…”

The prevalence of star formation as a function of Galactocentric radius

“…Sodroski et al (1997) derive a dust temperature gradient with RGC, owing in part to the metallicity gradient in the Galactic disc (Lépine et al 2011) and to variations in the strength of the interstellar radiation field with RGC. The links between these trends and star formation, however, remain tenuous.…”