Range Of Galactocentric Distances Research Articles

Galactic rotation curve and spiral density wave parameters from 73 masers

Based on kinematic data on masers with known trigonometric parallaxes and measurements of the velocities of HI clouds at tangential points in the inner Galaxy, we have refined the parameters of the Allen-Santillan model Galactic potential and constructed the Galactic rotation curve in a wide range of Galactocentric distances, from 0 to 20 kpc. The circular rotation velocity of the Sun for the adopted Galactocentric distance $R_0=8$ kpc is $V_0=239\pm16$ km s$^{-1}$. We have obtained the series of residual tangential, $\Delta V_{\theta}$, and radial, $\Delta V_R,$ velocities for 73 masers. Based on these series, we have determined the parameters of the Galactic spiral density wave satisfying the linear Lin.Shu model using the method of periodogram analysis that we proposed previously. The tangential and radial perturbation amplitudes are $f_\theta=7.0\pm1.2$ km s$^{-1}$ and $f_R=7.8\pm0.7$ km s$^{-1}$, respectively, the perturbation wavelength is $\lambda=2.3\pm0.4$ kpc, and the pitch angle of the spiral pattern in a two-armed model is $i=-5.2^\circ\pm0.7^\circ$. The phase of the Sun $\chi_\odot$ in the spiral density wave is $-50^\circ\pm15^\circ$ and $-160^\circ\pm15^\circ$ from the residual tangential and radial velocities, respectively.

Barium in Cepheids: new data on the abundance distribution in the Galactic disc

Using an Non-Local Thermodynamic Equilibrium (NLTE) analysis and a homogenous spectral data base, we have determined Ba abundances in a sample of 210 Galactic Cepheids and related objects. We confirm the previous result reported in Andrievsky et al. that the barium distribution in the range of Galactocentric distances from 5 to 18 kpc shows very small gradient value. The value determined here – d[Ba/H]/dR ≈ −0.01 dex kpc−1 – is smaller than that for other s-process elements, such as La, Pr, Nd and Sm (Luck & Lambert), that typically show gradients of −0.03 dex kpc−1.

M31 GLOBULAR CLUSTER STRUCTURES AND THE PRESENCE OF X-RAY BINARIES

[Abridged] M31 has several times more globular clusters (GCs) than the Milky Way. It contains a correspondingly larger number of low mass X-ray binaries (LMXBs) associated with GCs, and can be used to investigate the GC properties which lead to X-ray binary formation. The best tracer of the spatial structure of M31 GCs is high-resolution imaging from the Hubble Space Telescope, and we have used HST data to derive structural parameters for 29 LMXB-hosting M31 GCs. These measurements are combined with structural parameters from the literature for a total of 41 (of 50 known) LMXB GCs and a comparison sample of 65 non-LMXB GCs. Structural parameters measured in blue bandpasses are found to show smaller core radii and higher concentrations than those measured in red bandpasses; this difference is enhanced in LMXB clusters and could be related to stellar population differences. Clusters with LMXBs show higher collision rates for their mass compared to those without LMXBs and collision rates estimated at the core radius show larger offsets than rates estimated at the half-light radius. These results are consistent with the dynamical formation scenario for LMXBs. A logistic regression analysis finds that, as expected, the probability of a GC hosting an LMXB increases with increasing collision rate and proximity to the galaxy center. The same analysis finds that P(LMXB) decreases with increasing GC mass at a fixed collision rate, although we caution that this could be due to sample selection effects. Metallicity is found to be a less important predictor of P(LMXB) than collision rate, mass, or distance, even though LMXB GCs have a higher metallicity on average. This may be due to the interaction of location and metallicity: a sample of M31 LMXBs with a greater range in galactocentric distance would likely contain more metal-poor GCs and make it possible to disentangle the two effects.

Re‐determining the parameters of the Galactic spiral density wave from data on masers with trigonometric parallaxes

AbstractThe parameters of the Galactic spiral wave are re‐determined using a modified periodogram (spectral) analysis of the galactocentric radial velocities of 58 masers with known trigonometric parallaxes, proper motions, and line‐of‐site velocities. The masers span a wide range of galactocentric distances, 3 < R < 14 kpc, which, combined with a large scatter of position angles θ of these objects in the Galactic plane required an accurate account of logarithmic dependence of spiral‐wave perturbations on both the galactocentric distance and position angle. A periodic signal was detected corresponding to a spiral density wave with wavelength of λ = 2.4 ± 0.4 kpc, peak velocity of wave perturbations of fR = 7.5 ± 1.5 km s–1, phase of the Sun of χ⊙ = –160 ± 15°, and pitch angle of –5.5 ± 1°. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

AHerschel[C ii] Galactic plane survey

[Abridged] The [CII] 158um line is an important tool for understanding the life cycle of interstellar matter. Ionized carbon is present in a variety of phases of the interstellar medium, including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions (PDRs). The Galactic Observations of Terahertz C+ (GOTC+) project surveys the [CII] line over the entire Galactic disk with velocity-resolved observations using the Herschel/HIFI instrument. We present the first longitude-velocity maps of the [CII] emission for Galactic latitudes b=0deg, +-0.5deg, and +-1.0deg. [CII] emission is mostly associated with spiral arms, mainly emerging from Galactocentric distances between 4 and 10 kpc. We estimate that most of the observed [CII] emission is produced by dense PDRs (47%), with smaller contributions from CO-dark H2 gas (28%), cold atomic gas (21%), and ionized gas (4%). Atomic gas inside the Solar radius is mostly in the form of cold neutral medium (CNM), while the warm neutral medium (WNM) gas dominates the outer galaxy. The average fraction of CNM relative to total atomic gas is 43%. We find that the warm and diffuse CO-dark H2 is distributed over a larger range of Galactocentric distances (4-11 kpc) than the cold and dense H2 gas traced by 12CO and 13CO (4-8kpc). The fraction of CO-dark H2 to total H2 increases with Galactocentric distance, ranging from 20% at 4 kpc to 80% at 10 kpc. On average, CO-dark H2 accounts for 30% of the molecular mass of the Milky Way. When the CO-dark H2 component is included, the radial distribution of the CO-to-H2 conversion factor is steeper than that when only molecular gas traced by CO is considered. Most of the observed [CII] emission emerging from dense PDRs is associated with modest far-ultraviolet fields in the range chi0~1-30.

Barium abundances in Cepheids

We derived the barium atmospheric abundances for a large sample of Cepheids, comprising 270 stars. The sample covers a large range of galactocentric distances, from about 4 to 15 kpc, so that it is appropriated to investigate the existence of radial barium abundance gradients in the galactic disc. In fact, this is the first time that such a comprehensive analysis of the distribution of barium abundances in the galactic disc is carried out. As a result, we conclude that the Ba abundance distribution can be characterized by a zero gradient. This result is compared with derived gradients for other elements, and some reasons are briefly discussed for the independence of the barium abundances upon galactocentric distances.

The chemical abundance in M31 from H iiregions

We have obtained multi-slit spectroscopic observations from 3700A to 9200A with LRIS at the Keck I telescope for 31 HII regions in the disk of the Andromeda galaxy (M31), spanning a range in galactocentric distance from 3.9 kpc to 16.1 kpc. In 9 HII regions we measure one or several auroral lines ([OIII]4363, [NII]5755, [SIII]6312, [OII]7325), from which we determine the electron temperature (Te) of the gas and derive chemical abundances using the 'direct Te-based method'. We analyze, for the first time in M31, abundance trends with galactocentric radius from the 'direct' method, and find that the Ne/O, Ar/O, N/O and S/O abundance ratios are consistent with a constant value across the M31 disc, while the O/H abundance ratio shows a weak gradient. We have combined our data with all spectroscopic observations of HII regions in M31 available in the literature, yielding a sample of 85 HII regions spanning distances from 3.9 kpc to 24.7 kpc (0.19 - 1.2 R25) from the galaxy center. We have tested a number of empirical calibrations of strong emission line ratios. We find that the slope of the oxygen abundance gradient in M31 is -0.023+/-0.002 dex/kpc, and that the central oxygen abundance is in the range 12+log(O/H) = 8.71 - 8.91 dex (i.e. between 1.05 and 1.66 times the solar value, for 12+log(O/H)_solar=8.69), depending on the calibration adopted. The HII region oxygen abundances are compared with the results from other metallicity indicators (supergiant stars and planetary nebulae). The comparison shows that HII region O/H abundances are systematically ~0.3 dex below the stellar ones. This discrepancy is discussed in terms of oxygen depletion onto dust grains and possible biases affecting Te-based oxygen abundances at high metallicity.

A survey of CO and its isotope lines for possible cloud-cloud collision candidates

In the 12CO (J=1−0) survey of 1331 cold IRAS sources, 214 sources show profiles with multiple peaks and are selected as cloud-cloud collision candidates. In January 2005, 201 sources were detected with 12CO(1−0), 13CO(1−0), and C18O(1−0) emissions by the 13.7 m telescope at Purple Mount Observatory. This is the first survey of CO and its isotope lines directed toward possible cloud-cloud collision regions. According to the statistics of the 201 sources in the Galactic distribution, the 201 sources show a similar distribution to the parent sample (1331 cold IRAS sources). These sources are located over a wide range of Galactocentric distances, and are partly associated with the star forming region. Based on preliminary criteria which describe the spectral properties of the possible cloud-cloud collision region, the 201 sources are classified into four types by the fit of the spectral profiles between the optically thick and thin lines toward each source. The survey is focused on possible cloud-cloud collision regions, and gives some evidence to help us with selecting the target region. We will continue the process of mapping and studying multi-wavelength observations for the selected region in the future.

Microlensing Surveys for Exoplanets

Unlike most other planet-detection techniques, gravitational microlensing does not rely on detection of photons from either the host or the planet. Rather, planets are discovered by their gravitational perturbation of light from a more distant source. I review the fundamental concepts of microlensing planet searches and discuss their practical application. I show how the strengths and peculiarities of the method flow from the basic manner in which planets are discovered. In particular, microlensing is sensitive to very low-mass planets on wide orbits and free-floating planets, and can be used to search for planets orbiting host stars with a broad range of masses and Galactocentric distances. However, microlensing events are rare and cannot be predicted in advance, the majority of the host stars are extremely faint, and the planetary signals typically last less than a day. These strengths motivate microlensing searches as powerful, complementary probes of unexplored parameter space that have already provided important constraints on the demographics of planets beyond the “snow line.” However, the real-world challenges associated with the practical application of the method have driven the organization and evolution of the microlensing field and will continue to drive future developments from next-generation ground-based experiments through possible future space-based missions.

Redetermination of galactic spiral density wave parameters based on spectral analysis of masers radial velocities

To redetermine the Galactic spiral density wave parameters, we have performed a spectral (Fourier) analysis of the radial velocities for 44 masers with known trigonometric parallaxes, proper motions, and line-of-sight velocities. The masers are distributed in a wide range of Galactocentric distances $(3.5<R<13.2$ kpc) and are characterized by a wide scatter of position angles $\theta$ in the Galactic XY plane. This has required an accurate allowance for the dependence of the perturbation phase both on the logarithm of the Galactocentric distances and on the position angles of the objects. To increase the significance of the extraction of periodicities from data series with large gaps, we have proposed and implemented a spectrum reconstruction method based on a generalized maximum entropy method. As a result, we have extracted a periodicity describing a spiral density wave with the following parameters from the maser radial velocities: the perturbation amplitude $f_R = 7.7^{+1.7}_{-1.5}$ km s$^{-1}$, the perturbation wavelength $\lambda=2.2^{+0.4}_{-0.1}$ kpc, the pitch angle of the spiral density wave $i=-5^{+0.2^\circ}_{-0.9^\circ}$, and the phase of the Sun in the spiral density wave $\chi_\odot= -147^{+3^\circ}_{-17^\circ}$.

GAS METALLICITIES IN THE EXTENDED DISKS OF NGC 1512 AND NGC 3621. CHEMICAL SIGNATURES OF METAL MIXING OR ENRICHED GAS ACCRETION?

(Abridged) We have obtained spectra of 135 HII regions located in the inner and extended disks of the spiral galaxies NGC 1512 and NGC 3621, spanning the range of galactocentric distances 0.2-2 x R25 (from 2-3 kpc to 18-25 kpc). We find that the excitation properties of nebulae in the outer (R>R25) disks are similar to those of the inner disks, but on average younger HII regions tend to be selected in the bright inner disks. Reddening by dust is not negligible in the outer disks, and subject to significant large-scale spatial variations. For both galaxies the radial abundance gradient flattens to a constant value outside of the isophotal radius. The outer disk O/H abundance ratio is highly homogeneous, with a scatter of only ~0.06 dex. Based on the excitation and chemical (N/O ratio) analysis we find no compelling evidence for variations in the upper initial mass function of the ionizing clusters of extended disks. The O/H abundance in the outer disks of the target galaxies corresponds to 35% of the solar value (or higher, depending on the metallicity diagnostic). This conflicts with the notion that metallicities in extended disks of spiral galaxies are necessarily low. The observed metal enrichment cannot be produced with the current level of star formation. We discuss the possibility that metal transport mechanisms from the inner disks lead to metal pollution of the outer disks. Gas accretion from the intergalactic medium, enriched by outflows, offers an alternative solution.

Looking for tidal streams around Galactic globular clusters

The picture of building the Milky Way halo from merging protogalactic fragments is considered the local manifestation of the hierarchical galaxy formation process. In this scenario, some observational evidences have suggested that the outer young Galactic halo globular cluster population might be associated (or even the nuclei) to tidal disrupted dwarf spheroidals, now extinct galaxies. If this hypothesis is true, these systems might be surrounded by a distinct and still detectable stellar population. We have carried out a systematic observation of Galactic globulars covering the galactocentric distance range 10< RGC <40kpc in both hemispheres. We have used wide field instruments both in La Palma and in La Silla observatories to obtain deep photometry of wide areas around these globulars to unveil the possible remnants of their progenitor dwarf galaxies.

Outer density profiles of 19 Galactic globular clusters from deep and wide-field imaging

Using deep photometric data from WFC@INT and WFI@ESO2.2m we measure the outer number density profiles of 19 stellar clusters located in the inner region of the Milky Way halo (within a Galactocentric distance range of 10-30 kpc) in order to assess the impact of internal and external dynamical processes on the spatial distribution of stars. Adopting power-law fitting templates, with index $-\gamma$ in the outer region, we find that the clusters in our sample can be divided in two groups: a group of massive clusters ($ \ge 10^5 $ M_sun) that has relatively flat profiles with $2.5 < \gamma < 4$ and a group of low-mass clusters ($ \le 10^5 $ M_sun), with steep profiles ($\gamma > 4$) and clear signatures of interaction with the Galactic tidal field. We refer to these two groups as 'tidally unaffected' and 'tidally affected', respectively. Our results also show a clear trend between the slope of the outer parts and the half-mass density of these systems, which suggests that the outer density profiles may retain key information on the dominant processes driving the dynamical evolution of Globular Clusters.

A CHEMICAL ABUNDANCE STUDY OF 10 OPEN CLUSTERS BASED ON WIYN-HYDRA SPECTROSCOPY

We present a detailed chemical abundance study of evolved stars in 10 open clusters based on Hydra multi-object echelle spectra obtained with the WIYN 3.5m telescope. From an analysis of both equivalent widths and spectrum synthesis, abundances have been determined for the elements Fe, Na, O, Mg, Si, Ca, Ti, Ni, Zr, and for two of the 10 clusters, Al and Cr. To our knowledge, this is the first detailed abundance analysis for clusters NGC 1245, NGC 2194, NGC 2355 and NGC 2425. These 10 clusters were selected for analysis because they span a Galactocentric distance range Rgc~9-13 kpc, the approximate location of the transition between the inner and outer disk. Combined with cluster samples from our previous work and those of other studies in the literature, we explore abundance trends as a function of cluster Rgc, age, and [Fe/H]. The [Fe/H] distribution appears to decrease with increasing Rgc to a distance of ~12 kpc, and then flattens to a roughly constant value in the outer disk. Cluster average element [X/Fe] ratios appear to be independent of Rgc, although the picture for [O/Fe] is more more complicated by a clear trend of [O/Fe] with [Fe/H] and sample incompleteness. Other than oxygen, no other element [X/Fe] exhibits a clear trend with [Fe/H]; likewise, there does not appear to be any strong correlation between abundance and cluster age. We divided clusters into different age bins to explore temporal variations in the radial element distributions. The radial metallicity gradient appears to have flattened slightly as a function of time, as found by other studies. There is also indication that the transition from the inner disk to the outer disk occurs at different Galactocentric radii for different age bins. (Abridged.)

Further exploration of Galactic disk abundance gradients

AbstractWe examine the abundance gradient in the Milky Way disk via homogeneously determined data for 124 Galactic planetary nebulae (PNe). We present recent results from a detailed regression analysis of the O gradient. With O, Ne, S, Cl, and Ar available and a range of galactocentric distance (Rg) from 0.9 to 21 kpc, we present additional exploration of the disk radial gradient by statistically analyzing a series of short segments of increasing average Rg.

Old open clusters and the Galactic metallicity gradient: Berkeley 20, Berkeley 66 and Tombaugh 2★

To study the crucial range of Galactocentric distances between 12 and 16 kpc, where little information is available, we have obtained VI CCD imaging of Berkeley 20 and BVI CCD imaging of Berkeley 66 and Tombaugh 2, three distant, old open clusters. Using the synthetic colour magnitude diagram (CMD) technique with three types of evolutionary tracks of different metallicities, we have determined age, distance, reddening and indicative metallicity of these systems. The CMD of Be 20 is best reproduced by stellar models with a metallicity about half of solar (Z=0.008 or 0.01), in perfect agreement with high resolution spectroscopic estimates. Its age is between 5 and 6 Gyr from stellar models with overshooting and between 4.3 and 4.5 Gyr from models without it. The distance modulus from the best fitting models is always (m-M)0=14.7 (corresponding to a Galactocentric radius of about 16 kpc), and the reddening E(B-V) ranges between 0.13 and 0.16. A slightly lower metallicity (Z ~ 0.006) appears to be more appropriate for Be 66. This cluster is younger, (age of 3 Gyr), and closer, (m-M)0=13.3 (i.e., at 12 kpc from the Galactic centre), than Be 20, and suffers from high extinction, 1.2 < E(B-V) < 1.3, variable at the 2-3 per cent level. Finally, the results for To 2 indicate that it is an intermediate age cluster, with an age of about 1.4 Gyr or 1.6-1.8 Gyr for models without and with overshooting, respectively. The metallicity is about half of solar (Z=0.006 to 0.01), in agreement with spectroscopic determinations. The distance modulus is (m-M)0=14.5, implying a distance of about 14 kpc from the Galactic centre; the reddening E(B-V) is 0.31-0.4, depending on the model and metallicity, with a preferred value around 0.34.

ABUNDANCES OF GALACTIC ANTICENTER PLANETARY NEBULAE AND THE OXYGEN ABUNDANCE GRADIENT IN THE GALACTIC DISK

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 0.9-21 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+/-.05) - (0.058+/-.006) x Rg, with Rg 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 (2010) 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. Finally, a consideration of recently published chemical evolution models of the Galactic disk suggests that reconciling the current range in published oxygen gradients is necessary for adequately constraining parameters such as the surface density threshold for star formation and the characteristic timescale for disk formation.

Galactic parameters from masers with trigonometric parallaxes

Spatial velocities of all currently known 28 masers having trigonometric parallaxes, proper motion and line-of-site velocities are reanalyzed using Bottlinger's equations. These masers are associated with 25 active star-forming regions and are located in the range of galactocentric distances 3<R<14 kpc. To determine the Galactic rotation parameters, we used the first three Taylor expansion terms of angular rotation velocity {\Omega} at the galactocentric distance of the Sun R0=8 kpc. We obtained the following solutions: {\Omega}o=-31.0 +/- 1.2 km/s/kpc, {\Omega}o'=4.46 +/- 0.21 km/s/kpc^2, {\Omega}o"=-0.876 +/- 0.067 km/s/kpc^3, Oort constants: A=17.8 +/- 0.8 km/s/kpc, B=-13.2 +/- 1.5 km/s/kpc and circular velocity of the Solar neighborhood rotation Vo=248 +/- 14 km/s. Fourier analysis of galactocentric radial velocities of masers VR allowed us to estimate the wavelength {\lambda}=2.0 +/- 0.2 kpc and peak velocity f_R=6.5 +/- 2 km/s of periodic perturbations from the density wave and velocity of the perturbations 4 +/- 1 km/s near the location of the Sun. Phase of the Sun in the density wave is estimated as {\chi}o ~ -130^o +/- 10^o. Taking into account perturbations evoked by spiral density wave we obtained the following non-perturbed components of the peculiar Solar velocity with respect to the local standard of rest (LSR) (Uo,Vo,Wo)LSR=(5.5,11,8.5) +/- (2.2,1.7,1.2) km/s.

The stellar population content of the thick disc and halo of the Milky Way analogue NGC 891

We present deep VI images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope, covering three fields in the north–east side of the edge–on disc galaxy NGC 891. The observed fields span a wide range of galactocentric distances along the eastern minor axis, extending from the plane of the disc to 12 kpc, and out to ∼25 kpc along the major axis. The photometry of individual stars reaches ∼2.5 mag below the tip of the red giant branch. We use the astrophotometric catalogue to probe the stellar content and metallicity distribution across the thick disc and spheroid of NGC 891. The colour–magnitude diagrams of thick disc and spheroid population are dominated by old red giant branch stars with a wide range of metallicities, from the sparsely populated metal–poor tail at [Fe/H]∼−2.4 dex, up to about half–solar metallicity. The peak of the metallicity distribution function of the thick disc is at −0.9 dex. The inner parts of the thick disc, within ∼14 kpc along the major axis show no vertical colour/metallicity gradient. In the outer parts, a mild vertical gradient of Δ(V−I)0/Δ|Z| = 0.1 ± 0.05 kpc−1 or less than 0.1 dex kpc−1 is detected, with bluer colours or more metal–poor stars at larger distances from the plane. This gradient is, however, accounted for by the mixing with the metal–poor halo stars. No metallicity gradient along the major axis is present for thick–disc stars, but strong variations of about 0.35 dex around the mean of [Fe/H]=−1.13 dex are found. The properties of the asymmetric metallicity distribution functions of the thick–disc stars show no significant changes in both the radial and the vertical directions. The stellar populations situated within the solar–cylinder–like distances show strikingly different properties from those of the Galaxy populating similar distances. This suggests that the accretion histories of both galaxies have been different. The spheroid population, composed of the inner spheroid and the halo, shows remarkably uniform stellar population properties. The median metallicity of the halo stellar population shows a shallow gradient from about −1.15 dex in the inner parts to −1.27 dex at 24 kpc distance from the centre, corresponding to ∼13reff. Similar to the thick–disc stars, large variations around the mean relation are present.

THE FLAT OXYGEN ABUNDANCE GRADIENT IN THE EXTENDED DISK OF M83

We have obtained deep multi-object optical spectra of 49 HII regions in the outer disk of the spiral galaxy M83 (=NGC 5236) with the FORS2 spectrograph at the Very Large Telescope. The targets span the range in galactocentric distance between 0.64 and 2.64 times the R25 isophotal radius (5.4-22.3 kpc), and 31 of them are located at R>R25, thus belonging to the extreme outer disk of the galaxy, populated by UV complexes revealed recently by the GALEX satellite. In order to derive the nebular chemical abundances, we apply several diagnostics of the oxygen abundance, including R23, [NII]/[OII] and the [OIII]4363 auroral line, which was detected in four HII regions. We find that, while inwards of the optical edge the O/H ratio follows the radial gradient known from previous investigations, the outer abundance trend flattens out to an approximately constant value. The latter varies, according to the adopted diagnostic, between 12+log(O/H)=8.2 and 12+log(O/H)=8.6 (i.e. from approximately 1/3 the solar oxygen abundance to nearly the solar value). An abrupt discontinuity in the radial oxygen abundance trend is also detected near the optical edge of the disk. These results are tentatively linked to the flat gas surface density in the outskirts of the galaxy, the relatively unevolved state of the extended disk of M83, and the redistribution of chemically enriched gas following a past galaxy encounter.