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

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

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

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 at RG~ 8 are artefacts (no distance given by those authors)

Value of abundance gradient strongly depends on adopted distance scale

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

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

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)

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

Comparison of Stasińska’s distances with others’

• strong correlation between Stasinska’s and other distance scales

• also strong systematic differences

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

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)

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

Still to do ! (Stasińska et al in preparation)

• And what about a vertical abundance gradient in the Galaxy?