Abundances of Galactic Anticenter Planetary Nebulae and the Oxygen Abundance Gradient in the Galactic Disk

Abstract

We have obtained spectrophotometric observations of 41 anticenter planetary nebulae (PNe) located in the disk of the Milky Way. Electron temperatures and densities, as well as chemical abundances for He, N, O, Ne, S, Cl, and Ar were determined. Incorporating these results into our existing database of PN abundances yielded a sample of 124 well-observed objects with homogeneously determined abundances extending from 0.9 to 21 kpc in galactocentric distance. We performed a detailed regression analysis which accounted for uncertainties in both oxygen abundances and radial distances in order to establish the metallicity gradient across the disk to be 12 + log(O/H) = (9.09 ± 0.05) − (0.058 ± 0.006) × R g , with R g in kpc. While we see some evidence that the gradient steepens at large galactocentric distances, more objects toward the anticenter need to be observed in order to confidently establish the true form of the metallicity gradient. We find no compelling evidence that the gradient differs between Peimbert Types I and II, nor is oxygen abundance related to the vertical distance from the galactic plane. Our gradient agrees well with analogous results for H ii regions but is steeper than the one recently published by Stanghellini & Haywood over a similar range in galactocentric distance. A second analysis using PN distances from a different source implied a flatter gradient, and we suggest that we have reached a confusion limit which can only be resolved with greatly improved distance measurements and an understanding of the natural scatter in oxygen abundances.

Evidence of quasi-chemically homogeneous evolution of massive stars up to solar metallicity

“…For HII regions (Balser et al 2011), we provide two values: one from the azimuthally averaged gradient (Zhii) and one from the relation established in the azimuthal range which corresponds to the star's galactic longitude (Zhiib). For Planetary nebulae, we use the relation of Henry et al (2010). The O/H and Fe/H determinations give consistent results.…”

Chemical Cartography With Apogee: Large-Scale Mean Metallicity Maps of the Milky Way Disk

“…Open cluster measurements (Yong et al 2012;Frinchaboy et al 2013) have found a flattening of the radial gradient at large Galactocentric radii, while observations with Cepheid variables (Luck & Lambert 2011;Lemasle et al 2013) find that the slope does not change in the outer disk. In the inner Galaxy, Henry et al (2010) find that the slope of the radial gradient is shallower than at the solar circle using planetary nebula, but Cepheid observations (Luck & Lambert 2011;Genovali et al 2013) find that the gradient is steepest in the inner Galaxy. These previous studies have generally been limited in sample size to less than a few hundred objects, and generally target objects close to the plane of the Galaxy.…”

“…Various tracers have been used including Cepheid variables (e.g., Lemasle et al 2013), planetary nebulae (e.g., Henry et al 2010;Stanghellini & Haywood 2010), H ii regions (e.g., Balser et al 2011), open clusters (e.g., Carrera & Pancino 2011;Frinchaboy et al 2013), B stars (e.g., Daflon & Cunha 2004;Daflon et al 2009), and surveys of main sequence stars (e.g., Cheng et al 2012b for the Sloan Extension for Galactic Understanding and Exploration (SEGUE; Yanny et al 2009) of the Sloan Digital Sky Survey (SDSS; York 2000); Nordström et al 2004 for the GenevaCopenhagen Survey (GCS; Boeche et al 2013) for the Radial Velocity Experiment (Steinmetz et al 2006)). Near the solar circle, the measured amplitude of the Milky Way radial gradient ranges from −0.04 dex kpc −1 in [S/H] in OB stars (Daflon et al 2009) to −0.099 dex kpc −1 in [Fe/H] in main sequence stars between 4 < Gyr < 6 from the GCS (Nordström et al 2004).…”

Further exploration of Galactic disk abundance gradients

“…Uncertainties in 12+log(X/H) were adjusted using a scaling factor f (X ) until the fit accounted for all of the scatter in the data. This optimized fit yields χ 2 = 1 (see Henry et al 2010). Fits and resultant abundance gradients are shown in Table 1.…”