Using planetary nebulae to analyze the Galactic abundance gradient (a progress report)

1. Using planetary nebulae to analyze the Galactic abundance gradient(a progress report) Miriam Peña - Grażyna Stasińska - Sławomir Górny 1) Instituto de Astronomía UNAM, México - DAS, U. Chile 2) LUTH, Observatoire de Paris-Meudon, France 3) NCAC, CAMK, Toruń

2. introduction • Previous studies of Galactic abundance gradients using other indicators • Deharveng, Peña et al 2001 (HII regions) • Luck et al 2003 (Cepheids) • Daflon & Cunha 2005 (OB stars) • Previous studies of Galactic abundance gradients using PNe • Maciel & Quireza 1999 gradients flatten with galactocentric distance • Maciel et al. 2003 gradients vary with time • Main problems with using PNe • inhomogeneity of data sets • distances poorly known • contamination of nebular abundances due to stellar nucleosynthesis Here we wish to improve on the first two aspects

3. Comparison of published oxygen abundances • Some selected samples: • AC 83: Aller & Czyzak 83 • AK 87: Aller & Keyes 87 • KB94: Kingsburgh & Barlow 94 • Henry: compilation by Henry, Kwitter & Balick 2004 • data and methods are not always the same In some cases differences amount to over 0.5 dex

4. Galactic O/H « gradients »with published abundances and published distances • « gradients » with abundances from Henry et al. 2004 and RG calculated by different authors (different distance scales) • VZ 94 Van de Steen & Zijlstra 1994 • Phillips: Phillips 2002 • CKS: Cahn, Kaler & Stanghellini 1992 • a few point atRG~ 8 are artefacts (no distance given by those authors) Value of abundance gradient strongly depends on adopted distance scale

5. Increasing the sample of homogeneous observations Peña et al, in preparation  Spectrophotometric data of about a 100 galactic PNe. Low (4-6 A) and high (0.2-0.3 A) resolution spectra (2.1-m OAN, Mex.) treated in a consistent way  Good signal to noise for measuring the important diagnostic line ratios for Te ([OIII]4363/5007 and /or [NII]5755/6583) and N ([SII], [ArIV], [OII]) Data will be published as a catalogue of emission lines, physical conditions and chemical composition of observed PNe

6. Comparison of homogeneously recomputed abundances for different data samples • Data sets: • 1) Peña et al,2)Henry et al, 3) KB94, 4) Maciel (compilation Freitas Pacheco, Costa,…) • all the abundances have been recomputed with the classical empirical method using Tr[OIII] and ne[SII] with the atomic data collected by Stasińska (2005) • The results agree reasonably well but differences by 0.3 dex still occur occasionally For the following, we merge those 4 data samples

7. Our merged PN data sample • We have chosen several good, homogeneous and numerous data samples: • 1) Peña et al(2005),2)Henry 2004, 3) KB94, 4) Maciel (compilation) • In case of duplicate data, we chose them in the order 1, 2, 3, 4 • (Abundances recomputed with the classical empirical method using Te[OIII] and ne[SII] • with the atomic data collected by Stasińska (2005) Distribution of O/H vs RG (calculated with distances from VZ94)

8. A new way to estimate PN distancesStasińska et al (2005) • Basics of the method: interpolation in a large grid of photoionization models • Construction of the model grid (using the photoionization code PHOTO) • Free parameters: T* (3E4 -- 2.1E5 K), L* (3 --1000 Lsun ), Rin (3E16 -- 3E18 cm), nH (30 --1E5 cm-3), total Mneb (0.03 -- 3 Msun) • For each model, store quantities that do not depend on abundances and that can be compared to observables: [SII] 6717/6731, O++/O+, He++/H+, He+/H+ • For each model, the values of mV, theta, F(Hbeta) that correspond to various distances (0.5, 1, 2, …14 kpc) are computed • Check that the observables in the grid cover well the entire range of observed values in our sample of PNe • Interpolation in the grid • For each PN in our sample, the distance is found by interpolation, using an algorithm of locally weighted regression • The « kernel »is adjusted so that the median of the distances found for PNe within 10° from the Galactic center is 7.93 kpc

9. Comparison of Stasińska’s distances with others’ • strong correlation between Stasinska’s and other distance scales • also strong systematic differences

10. The merits of the new method • It does not depend on assumptions on a nebular mass or any other nebular or stellar property that is supposed to hold for all PNe • It does not depend on any assumption on stellar evolution • It can be refined with more realistic models for the nebulae or for the stellar atmospheres • Error bars can be determined A check of the method: For PNe within 10° of the Galactic center, the width of the distribution of distances is reasonable

11. Our present result for O/H versus RG (Stasińska et al in preparation) • A clear gradient is seen • it seems to level off in the inner parts of the galaxy • O/H in bulge PNe are more dispersed than in the disk (cf Górny, Stasińska et al 2004)

12. Still to do ! (Stasińska et al in preparation) • Check all the abundances (especially the « deviating points »), perhaps use Tr[NII] • Distinguish the various components of the PN population (bulge, disk, halo …) using position and radial velocity • Compute abundances of Ne, S, Ar using icfs interpolated from the model grid • Derive abundance gradients from the disk PNe taking into accounts errors in distances and in abundances • Estimate the progenitor masses and look for a possible time variation of the gradient

13. Still to do ! (Stasińska et al in preparation) • And what about a vertical abundance gradient in the Galaxy?