Carbon in Spiral Galaxies fromHubble Space TelescopeSpectroscopy

Abstract

We present measurements of the gas-phase abundance ratio C/O in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space T elescope. The ratios of C to O increase systematically with O/H in both galaxies, from log C/O B [0.8 at log O/H \ [4.0 to log C/O B [0.1 at log O/H \ [3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, conÐrming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemicalevolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much Ñatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important e †ect on the yields and thus on the evolution of carbon and oxygen abundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-population ages much younger than those of the inner disks.

Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution★

“…The peculiar position of NGC 2363 is indicated in Figure 6 because it will be discussed later. The figure shows the wellknown C/O vs. O/H correlation found in previous works by, for example, Garnett et al (1995Garnett et al ( , 1999) from observations of CELs or Esteban et al (2002Esteban et al ( , 2009) from observations of RLs. Figure 6 includes the time evolution of the abundances predicted by the chemical evolution models of the Milky Way disc presented by Esteban et al (2013).…”

On the Cosmic Origins of Carbon and Nitrogen

“…Consensus toward this claim in the case of carbon has been building for several years. For example, work by Prantzos et al (1994), Garnett et al (1999), and Gustafsson et al (1999) show that carbon is largely produced by massive stars, to which we now add our support. In the case of nitrogen, the predicted large IMS yields in conjunction with the large extant database of planetary nebula abundances suggest strongly that the production of this element is dominated by IMS.…”

“…3 Metallicity mass fraction, where z =0.02 Garnett et al (1995Garnett et al ( , 1997Garnett et al ( , 1999, Izotov & Thuan (1999;I), and Kobulnicky & Skillman (1998, K). The symbols M and S show the positions of the Galactic H II region M8 and the sun, respectively.…”

“…The sun's position (Grevesse et al 1996) is shown with an 'S'. Garnett et al (1999) calculated two C/O ratios each for six objects, using two different reddening laws. The common values of 12+log(O/H) for these pairs are: 8.…”

“…The literature on the origin of carbon has recently been reviewed by Gustafsson et al (1999), and Garnett et al (1999). The former conclude that their own stellar results are "consistent with carbon enrichment by superwinds of metal-rich massive stars, but inconsistent with a main origin of carbon in low mass stars", which is pretty much echoed by the latter authors who state that the behavior of C/O ratios as a function of O/H is "best explained [by] ... effects of stellar mass loss on massive star yields".…”