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Metallicity of ionized gas in the Irr galaxy IC10
Egorov O.V.1, Lozinskaya T.A.1, Moiseev A.V.2
1 2

Stern berg Ast rono mical Insti tut e, Mo scow, Rus sia

Special Ast rop hysical Obs er vato ry of Rus sian Acad emy of Sci ence, Nizhnii A rkhyz, Karac hai-Ch erkessia, Rus sia

Introduction
The dwarf irregular galaxy IC10 is the nearest starburst galaxy at the distance 800 kpc. HА and [SII] line ima ges o f this galaxy show a giant complex of multiple shells and supeshells with sizes ran gin g from 50 to 500-800 pc (see Fig. 1). The stellar population of the galaxy and anomalously high space density of WR stars are indicative of almost "synchronous" and short burst of ongoing star formation, which, however, must have spread throughout most of the galaxy. It is the place where the currently most bona fide hypernova remnant - the Synchrotron Supershell - has been identified [1].

Fig. 1a,b. Location of slit spectrograms on the HА-line image of IC10. The circles indicate WR stars.


Observations
The observations of the galaxy were performed at the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. Five spectrograms named by their position angles - PA0, PA45, PA132, PA268 and PA331- were taken with SCORPIO focal reducer operating in the slit spectrograph mode. Figures 1a and b show the locations of the slit spectrograms on the HА-line image of IC10 with the arc second marks superimposed. The scale along the slit and spectral resolution were equal to 0,36"/pixel and about 5-9 A, respectively.

Relative abun dances
Relative abundances of oxygen are derived using three methods. First one is the theoretical dependence of 12+log(O/H) from [OII]К3727 and [OIII]К4959+5007 line intensities divided by HБ line intensity, and from electronic density and temperature [2]. Second one is the revised calibration of 12+log(O/H) as a function of [OII] and [OIII] line intensities [3]. And third method [4] is the dependence of 12+log(O/H) from ratios log([OII]К3727)/log([OIII] К5007) and log([NII] К6583)/log([SII] К6717+6731). We have defined relative abundances of nitrogen and sulfur, using a method resulted in [2]. Figures 2, 3, 4 show the distributions of the inferred 12+log(O/H), log(N/O), log(S/O) values along the slits; we averaged the abundances over HII re gions from the catalog of Hodge and Lee [5]. The results are also summarized in the Table1. Columns 1, 2, 3, 4, 5 and 6 give the name of the region according to [5]; the position on the corresponding slit, and the inferred 12+log(O/H) (fir st and second methods only), log(N/O), log(S/O) values, respectively.

Position along the slit, arcsec

Fig. 2. Relative oxygen abundance along the slits. Solid line marks a f irst method [2] of abundance determination, dotted line ­ a second method [3] and dashed line ­ a third method [4].


Position along the slit, arcsec

Position along the slit, arcsec

Fig. 3. Relative nitrogen abundance along the slits.

Fig. 2. Relative sulf ur abundance along the slits.

Table. 1.Relative abundances of O, N, S.


Disc ussion and results.
We reveal strong variations of interstellar extinction between different regions, which explain the discrepancies between early extinction and distance estimates for IC10. The observed variations may be due to "local" extinction in the central star-forming re gion, which resides in the direction of the densest cloud of neutral and molecular gas and is possibly partially embedded in it. We adopt E(B-V) = 0.95m (our paper [6] in press) in this work. Our results provide the currently most detailed O, N and S relative abundance data for the galaxy. Our measurements show the oxygen abundance to vary over a wide range from one region to another. The mean abundance averaged over the regions studied is 12 + log(O/H) = 8.17 ± 0.35, that correspond to metallicity Z=0.18 ± 0.14Z . The oxygen abundance in the brightest region HL111 is, on the average, equal to 12+log(O/H)=8.25 ±0.19.
The line intensity ratio of I([SII])/I(HА) = 0.7­1.0 in the Synchrotron Supershell (SS in table and at the figures) is indicative of ga s e mission behind the shock. The latter is also evidenced by higher I([NII]6548+6583)/I(HА) 0.25 and I([OII]3727)/I([OIII]5007) ratios and lower I([OIII]5007)/I(HБ) 1.58 0.15 ratio compared to the neighboring HL106 bright

HII re gion. The inferred estimate of oxygen abundance in the Synchrotron Supershell remains uncertain, because the compact hypernova remnant -- IC 10 X-1 -- is a binary consisting of a black hole and WR M17 star. Ionizing radiation of the WR star may be important in its neighborhood. On the other hand, shock must be taken into account for the technique used for HII to be correctly applicable in our case. We therefore derive provisional oxygen abundance estimates using three methods while being fully aware of the incorrectness of both approaches. Using the first method we find 12+log(O/H)=8 .22 ± 0.21, 12+log(O/H)=8 .04 ± 0.43 and 12+log(O/H)=8 .19 ± 0.39 based on the spectrograms of PA132, PA0 and PA331 interpreted in terms of the HII re gion model. Our observations agree best with the models of [7] for the radiation of gas behind a shock propagating at a vel ocity of about 100 km/s ([8]) if we adopt 12+log(O/H) 8.15 for models of pure shock excitation or 12+log(O/H) 8.35 for models that allow shock pre-ionization of gas. You can find more detailed resul ts of our work in our paper [6].

This work is supported by the Russian Foundation for Basic Research (project no. 07­02­ 00227) and is based on observational material obtained with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences funded by the Ministry of Science of the Russian Federation (registration number 01-43).


References.
1. 2. Lozi nskaya T.A., M oi seev A.V., M NRAS, 381, 26L (2007) I zotov Y. I., Stas inska G., Meynet G., Guseva N.G., Thuan T.X., Ast ron.Ast rophys.,448, No. 3, 955-970 (2006) . 3. 4. 5. 6. 7. Pi lyugin L.S., Thuan T.X., Ast roph.J . 631, 231 (2005). Char lot S., Longett i M., M NRAS 323,887 ( 2001) Hodge P., Le e M.G., PASP, 102, 26 (1990). Lozi nskaya T.A., Egorov O .V., Moi seev A.V., Biz ya ev D.V., As tron. Let t. Vol 35, №10, 730- 747, in pre ss ( 2009) All en M.G., Gr ovs B.A., Dopi ta M .A., Suthe rla nd R.S., Kewle y L.J., ASTRO-PH 0805.0204 (2008). 8. Lozi nskaya T.A., M oi seev A.V., Podorvan yu k N.Yu., Bur enkov A.N., Ast ron. Le tt. 34, No. 4, 217-230 ( 2008).