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A Massive Star Odyssey, from Main Sequence to Supernova Proceedings IAU Symposium No. 212, c 2002 IAU K.A. van der Hucht, A. Herrero & C. Esteban, eds.

The ionized gas in and around the Wolf-Rayet galaxy NGC 1741
´ Angel R. L´pez-S´nchez, C´sar Esteban o a e Instituto de Astrof´ ica de Canarias, 38200 - La Laguna, Tenerife, Spain is M´nica Rodr´ uez o ig Instituto Nacional de Astronom´ , Optica y Electronica, Apdo. Postal ia ´ ´ 51 y 216, 72000 Puebla, Pue., Mexico
Abstract. Interactions can be the main mechanism that triggers star formation in Wolf-Rayet galaxies. The compact group HCG 31 has one of this ob jects, NGC 1741. We present new observations of this compact interacting group in order to analyse the physical conditions and chemical composition of the ionized gas as well as to estimate the ages of the starbursts.

1.

Discussion

The Hickson Compact Group 31 (HCG 31) lies at 51 Mpc (Vacca & Conti 1992). Galaxies A and C form NGC 1741, which is one of the most luminous WolfRayet galaxies known (MB = -20.3). Both ob jects are in interaction (Johnson et al, 1999), and they host a burst of star formation of around 5 Myr (Johnson & Conti 2000). Ob ject D is a background galaxy (Rubin et al. 1990). E and F may be Tidal Dwarf Galaxies (TDGs), and F shows a starburst of around 4 Myr (Johnson & Conti 2000). This last ob ject does not possess an old population. We have distinguished the F1 and F2 zones inside F and the new member H. Images showing the location of F1 , F2 and H will be published elsewhere. We have obtained long-slit intermediate-resolution spectroscopy. We observe the [OIII] 4363 ° emission line in most of the members, and we have been A able to derive the electron temperature. Electron densities are always below the low density limit (< 100 cm-3 ), except in member C. For A and H we have estimated the electron temperature making use of empirical methods. We have also determined the radial velocity of each member. H belongs to HCG 31 because it has a radial velocity similar to the system. We have determined O+ , O++ and N+ abundances, as well the O/H ratio. We checked the obtained results with empirical calibrations. According to the O/H ratio of each ob ject, we can see that there are important differences. Member C shows the highest abundance (8.55) and F1 and F2, the youngest bursts (see below), show the lowest values (8.06 and 8.05). This difference makes difficult that F can be considered as a TDG produced from material stripped from C or A. In fact, the O/H ratio of F is more similar to that of G, or even B. 1


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L´pez-S´nchez, Esteban & Rodr´ uez o a ig

° Another interesting result is the detection of C II 4267 A in the spectrum of C. This is the first time this recombination line is reported in an H II galaxy.
Table 1. Abundance ratios and age estimations of HCG 31. For A and H abundances were calculated by empirical calibrations. All the ages are in Myr.
Member 12+log(O/H) log(N+ /O+ ) Age indicator EW(H ) SL96 EW(H ) SV98 EW([OIII]) SL96 H and U SSCs Photometry A 8.45 -1.29 4-8 5-6 5-8 3 - 10 5 B 8.31 -1.45 6-8 5.5 - 7 5-8 10 5 - 10 C 8.55 -1.62 4-6 3 - 4.5 3-5 3 - 10 5 E 8.29 -1.36 6-8 5-6 5-8 10 1-3 F1 8.06 -1.27 2223< 3 3 3 10 4 F2 8.05 -1.46 2-3 2-3 2-3 G 8.26 -1.38 4-7 4.5 - 6 3-7 5 5 H 8.40 -1.36 4-6 4-5 3-7 Ref. This W. This W. This W. This W. This W. IV97 JC00

We have used the models of Stasinska & Leitherer (1996) -SL96 in Table ´ 1- to estimate the age of the bursts. Our observations fit well with a Z/Z 0.25 and M/M 1000 burst. We can obtain a good estimation of the age of the bursts plotting the observed EW(H ). We observed that F1 and F2 are the youngest members, and we find that they have an age between 2 and 3 Myr. The rest of the bursts are more evolved, and they have an age between 4 and 8 Myr. We have also used the Schaerer & Vacca (1998) models -SV98 in Table 1to estimate ages, and they provided similar results. We show them in Table 1, as well as other estimations of the age of the burst in the literature. A We can observe clearly the WR Bump between 4650 and 4698 ° in member A C. Furthermore, we detect the nebular He II 4686 ° emission in F1. Galaxies A A and B could also show a faint WR Bump. In addition, faint He II 4686 ° emission could be present in F2. 2. Conclusions

Deep intermediate-resolution spectra of the different members of HCG 31 show that there are important differences in their chemical content. Member C shows the highest O/H ratio and the youngest burst, F (age between 2 and 3 Myr), shows the lowest one. This suggests that F is not a tidal dwarf galaxy (TDG) produced from material stripped from C, instead it can be originated from material from G or B. We have detected He II 4686 ° emission in F, indicating the A presence of Wolf-Rayet stars in this interesting and extremely young ob ject. References
Iglesias-P´ramo, J., & V´ chez, J. M., 1997, AJ 479, 190 a il Johnson, K. E., Vacca, W. D., Leitherer, C., Conti, P. S., Lipscy, S., 1999, AJ 117, 1708 Johnson, K. E., & Conti, P. S. 2000, AJ 119, 2146 Schaerer, D., & Vacca, W. D., 1998, AJ 497, 618 Stasinska & Leitherer 1996, Ap&SS 107, 661 ´ Rubin, V. C., Hunter, D. A., Ford, W. K. Jr., 1990, ApJ 365, 86 Vacca, W. D., & Conti, P. S., 1992, ApJ 401, 543