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087_1. Introduction_ H__ regions are considered as the most reliable targets for the determination of the present-day abundances in the Interstellar Medium (ISM)

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087_1. Introduction_ H__ regions are considered as the most reliable targets for the determination of the present-day abundances in the Interstellar Medium (ISM)

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     Astronomy & Astrophysics manuscript no. Hl121 (DOI: will be inserted by hand later)

     February 5, 2008

     Chemical Self-Enrichment of H?? Regions by the Wolf-Rayet Phase of an 85M?Ñ star

     D. Kr?? ger1 , G. Hensler2 , and T. Freyer1 o

     arXiv:astro-ph/0603189v1 8 Mar 2006

     1

     2

     Institut f?? r Theoretische Physik und Astrophysik der Universit?? t Kiel, D-24098 Kiel, Germany u a e-mail:

    danica@astrophysik.uni-kiel.de Institute of Astronomy, University of Vienna, T?? rkenschanzstr. 17, A-1180 Vienna, Austria u e-mail: hensler@astro.univie.ac.at

     Received / Accepted

     Abstract. It is clear from stellar evolution and from observations of WR stars that massive stars are releasing metal-enriched gas through their stellar winds in the Wolf-Rayet phase. Although H?? region spectra serve as diagnostics to determine the present-day chemical composition of the interstellar medium, it is far from being understood to what extent the H?? gas is already contaminated by chemically processed stellar wind. Therefore, we analyzed our models of radiative and wind bubbles of an isolated 85M?Ñ star with solar metallicity (Kr?? ger et al. 2006) with respect to the chemical enrichment of the circumstellar o H?? region. Plausibly, the hot stellar wind bubble (SWB) is enriched with 14 N during the WN phase and even much higher with 12 C and 16 O during the WC phase of the star. During the short period that the 85M?Ñ star spends in the WC stage enriched SWB material mixes with warm H?? gas of solar abundances and thus enhances the metallicity in the H?? region. However, at the end of the stellar lifetime the mass ratios of the traced elements N and O in the warm ionized gas are insigni?cantly higher than solar, whereas an enrichment of 22% above solar is found for C. Important issues from the presented study comprise a steeper radial gradient of C than O and a decreasing e?ect of self-enrichment for metal-poor galaxies. Key words. galaxies: evolution ?C H?? regions ?C hydrodynamics ?C instabilities ?C ISM: bubbles ?C ISM: structure

     1. Introduction

     H?? regions are considered as the most reliable targets for the determination of the present-day abundances in the Interstellar Medium

    (ISM). Nevertheless, heavy elements are released by massive stars into their surrounding H?? regions. The knowledge of its amount is of particular interest for the observation of very metal-poor galaxies. Kunth & Sargent (1986) e.g. discussed the problem of determining the heavy-element abundance of very metal-poor blue compact dwarf galaxies from emission lines of H?? regions in the light of local selfenrichment by massive stars. This can happen during two stages in their evolutionary course: In the Wolf-Rayet (WR) stage, when the stellar wind has peeled o? the outermost stellar layers, and by supernovae of type II (SNeII). Because of their energetics, stellar winds and SNeII contribute their gas to the hot phase. Therefore, it is of high relevance for observations and the addressed question of the presented exploration to study the amount to what it can be mixed into the diagnosed H?? regions. As discussed by Chiosi & Maeder (1986), massive stars with an initial mass greater than 25 M?Ñ evolve into WR stars

     Send o?print requests to: D. Kr?? ger o

     with enhanced mass-loss rates and chemically enriched stellar winds. Abbott & Conti (1987) concluded from stellar evolution models that WR winds strongly contribute to the Galactic enrichment of 4 He, 12 C, 17 O, and 22 Ne, while contributing only moderately to the enrichment of 14 N, 26 Mg, 25 Mg, and 16 O. For metallicities less than solar two e?ects reduce the heavy element release by WR stars: First, the lower the metallicity the more massive a star has to be to evolve through the WR stages. Therefore, the number of WR stars decreases with decreasing metallicity. Schaller et al. (1992, hereafter SSMM) found that with metallicity Z = 0.001 the minimal initial H-ZAMS mass for a WR star is > 80 M?Ñ . At Z = 0.02 the minimal initial mass is > 25 M?Ñ . Second, the lower the metallicity the shorter are the WR lifetimes, and not all WR stages are reached. At solar metallicity WR stars enter all three WR stages (WNL, WNE, WC), whereas at Z = 0.001 only the WNL phase is reached (SSMM). The WR lifetime of an 85 M?Ñ star, e.g., is tWR = 0.204 ?Á 105 yr at Z = 0.001 and tWR = 4.008 ?Á 105 yr at Z = 0.02 (SSMM). That WR stars play an important role for carbon enrichment of the ISM at solar metallicity was shown by Dray et al. (2003). Their models predict that the C enrichment by WR stars is at least comparable to that by AGB stars while the enrich-

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     D. Kr?? ger et al.: Chemical Enrichment of H?? Regions by WR stars o

     ment of N is dominated by AGB stars and the O enrichment is dominated by SNeII. At other metallicities Dray & Tout (2003) found an increase in carbon enrichment with increasing metallicity but a decrease in oxygen enrichment. The N enrichment by WR stars is negligible compared to that by other sources. Additionally, the total amount of N ejected

    by a WR star generally decreases with decreasing metallicity. Hirschi et al. (2004, 2005) showed that the consideration of stellar rotation in the Geneva stellar evolution code increases the yields for heavy elements by a factor of 1.5 ? 2.5 for stars between 15 and 30 M?Ñ . For the more massive stars rotation raises the yields of 4 He and other H-burning products like 14 N, but the yields of He-burning products like 12 C are smaller. Additionally, for stars with M 60 M?Ñ , the evolution di?ers from that of non-rotating stars by the following manner: Rotating stars enter the WR regime already in the course of their main-sequence. As an extent to stellar evolution models mentioned above hydrodynamical simulations of radiation and wind-driven H?? regions can trace the spatial distribution of the elements ejected by the star, and, therefore, we can study the mixing of the hot chemically enriched stellar wind with the warm photoionized gas.

     3. Results

     The spatial distribution of the tracer elements is shown as log(?Ñ (element)/?Ñ (element, solar)) at di?erent stages of stellar evolution, where ??element?? means 12 C (Fig. 1, middle column), 14 N (Fig. 1, left column), or 16 O (Fig. 1, right column), respectively. All plots cover the whole computational domain of 60 pc ?Á 60 pc. The gas density is overlaid as a contour plot. Before the onset of the WN phase, these density ratios are unity by de?nition. With the onset of the WN phase the combined SWB/H?? region is chemically enriched by the WR wind. We study the temporal evolution of 12 C, 14 N, and 16 O, respectively, normalized to solar values in the two di?erent temperature regimes of the combined SWB/H?? region: For the hot gas of the SWB the mass ratios are depicted in Figure 2. Figure 3 shows it for the ??warm?? H?? gas (see ?ì 2).

     3.1. Chemical enrichment of the stellar wind bubble

     In the hot gas of the SWB the abundance of 14 N at t = 2.99 Myr (close to the end of the WN phase, Fig. 1, upper left plot) that lasts only for ?Ö 0.17 Myr, reaches values in the range of 0.1 log 14 N/solar 0.98. 14 N is distributed in the whole SWB except in the H?? material enveloped by the SWB. 12 C and 16 O can be found at the same places, but their concentration is as low as log 12 C/solar ?Ö ?1.4 and log 16 O/solar ?Ö ?1.5. During its WN phase the star releases 0.143 M?Ñ of 14 N, which is more than half of its total 14 N release, but almost no extra 12 C or 16 O is supplied. With the onset of the WC phase at t = 3.005 Myr the 14 N release ends and the main enrichment of the SWB with 12 C and 16 O starts. Due to the beginning WC phase with its high mass-loss rates and high terminal wind velocity the previously ejected material in the SWB including the emitted 14 N is compressed into a shell-like structure. Therefore, after t = 3.005 Myr 12 C and 16 O are distributed in the whole SWB and 14 N can mostly be found in an expanding shell (see lower

    left plot of Fig. 1, at t = 3.22 Myr). The density contour line at a radius of 28 pc in the upper middle and upper right plots of Fig. 1 (t = 3.06 Myr) marks the position of the reverse shock which borders the free-?owing wind zone. At 3.06 Myr the radius of the reverse shock has reached its biggest extent. During this evolutionary phase (see upper middle and upper right plots in Fig. 1, at 3.06 Myr) the star ejects large amounts of 12 C and 16 O: The log 12 C/solar value reaches ?Ö 1.86 inside the highly enriched SWB and the log 16 O/solar maximum value is ?Ö 1.35. At the end of its lifetime at t = 3.22 Myr the 85 M?Ñ star has supplied 0.28 M?Ñ of 14 N, 13.76 M?Ñ of 12 C, and 11.12 M?Ñ of 16 O, which are contained in the combined SWB/H?? region. This situation is shown in the lower plots of Fig. 1. The reverse shock is now located at a radius of 9 pc, the hot gas of the SWB ?lls the area up to a radius of 39 pc. Almost all of the ejected 12 C, 14 N, and 16 O is located in the SWB??s volume. In the outer zones of the SWB the highest abundance of 12 C (see lower middle plot of Fig. 1) is locally reached with log 12 C/solar ?Ö 1.85. For radii r 15 pc the value is ?Ö 1.74.

     2. The model

     We analyze the model of an 85M?Ñ star in a series of radiation and wind-driven H?? regions around massive stars (Freyer et al. 2003, 2006; Kr?? ger et al. 2006, hereafter: Paper I, Paper II, o Paper III) as described in Paper III. A description of the numerical method and further references are given in Paper I. The time-dependent parameters of the 85 M?Ñ star with ??standard?? mass-loss and solar metallicity (Z = 0.02) during its H-MS and its subsequent evolution are taken from SSMM, who used the relative abundance ratios of Anders & Grevesse (1989) for the heavy elements. An 85 M?Ñ star with solar abundance enters its WN stage at an age of t = 2.83 Myr. The WR star enriches the combined SWB/H?? region with 12 C, 14 N, and 16 O. In our 2D simulations we study the spatial distribution of these tracer elements inside the SWB/H?? region as well as their time-dependent mass ratios as M12 C /Mtotal , M14 N /Mtotal , and M16 O /Mtotal in two di?erent temperature regimes: ??warm?? gas 6.0 ?Á 103 K ?Ü T < 5.0 ?Á 104 K accounts for the gas of the H?? region, and ??hot?? gas, whose temperatures T ?Ý 5 ?Á 104 K are reached inside the SWB. Additionally, for the H?? region diagnostic a degree of ionization of ?Ý 0.95 must be reached. The undisturbed ambient medium outside the H?? region as well as the material swept-up by the expanding H?? region are not counted since they are not ionized. The observable abundances of H?? regions are derived for the warm phase only. The mass fractions of 12 C, 14 N, and 16 O in the circumstellar gas as well as of the star itself are initially set to the solar values, taken from Anders & Grevesse (1989), 4.466 ?Á 10?3 , 1.397 ?Á 10?3 , and 1.061 ?Á 10?2 , respectively, in relation to H.

     D. Kr?? ger et al.: Chemical Enrichment of H?? Regions by WR stars o

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     Fig. 1. Concentration of 14 N (left plots), 12 C (middle), and 16 O (right plots). Upper row: t = 2.99 Myr (top left), near the end of the WN phase, and at t = 3.06 Myr (middle and right), after the onset of the WC phase. Lower row: At the end of the stellar lifetime (t = 3.22 Myr). The log 14 N/solar ratio (Fig. 1, lower left) reaches ?Ö 0.55 in the outer zones of the SWB, where the material ejected during the WN phase is located. In the inner parts of the SWB only 12 C and 16 O can be found. The maximum of the log 16 O/solar value (Fig. 1, lower right) of ?Ö 1.65 is reached at radii r 15 pc. Within the outer parts of the SWB the values range from 0.5 to 1.4, highly depending on the location, since the abundance within the embedded clumps di?ers from that of the hot SWB material. The temporal evolution of the 12 C abundance in the hot gas phase is plotted in Fig. 2 as dash-dotted line. The plot starts at t = 2.83 Myr, when the WN phases are reached. Additionally, the onset of the WC phase at t = 3.005 Myr is indicated. During the WN phase the stellar parameters (SSMM) specify the 12 C abundance of the WR wind to about 1/14 solar. Due to the mixing of the WR wind with the main-sequence wind the averaged mass ratio in the hot gas of the SWB is only 0.7 times solar during the WN and early WC phase. The 12 C enrichment starts with the onset of the WC phase. After the newly ejected material passed the reverse shock the 12 C abundance in the hot gas reaches 2.1 times the solar value at t = 3.02 Myr. This value increases further to 38 times solar at the end of the stellar lifetime. The 14 N abundance is depicted in Fig. 2 as solid line. It rises as the enriched free-?oating stellar wind passes the reverse shock, where the material is heated to several 107 K. Since in the WC stage the stellar wind contains no 14 N, the 14 N abundance in the hot SWB decreases strongly after it reached its maximum value of 3.4 times solar at t = 3.0 Myr. At the end of the stellar lifetime the 14 N abundance in the hot gas phase is only 0.8 times solar. Because the enrichment with 16 O starts together with 12 C at the onset of the WC phase and due to the high mass-loss rate in the WC phase the O-abundance in the hot gas phase rises steeply after the material passed the reverse shock. After t = 3.02 Myr the 16O mass ratio in the hot gas becomes supersolar and increases until the end of the stellar lifetime up to 15.7 times solar.

     3.2. Chemical enrichment of the H?? region

     During the WR phases of the 85M?Ñ star the SWB expands into the surrounding H?? region without being bordered by a shock front. Therefore, the hot SWB gas structures the H?? region and encloses H?? gas which is thus compressed into clumps. This embedded H?? material

    contributes to the warm gas phase as well as the H?? layer around the SWB. At the end of the stellar lifetime the 12 C abundance (lower middle plot in Fig. 1) in the H?? layer reaches a local maximum of log 12 C/solar ?Ö 0.17. It contains a small fraction of 14 N, the log

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     N/solar values there are up to 0.01 (Fig. 1, lower

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     left). The log 0 to 0.03.

     O/solar values (Fig. 1, lower right) range from

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     D. Kr?? ger et al.: Chemical Enrichment of H?? Regions by WR stars o

     WN

     12

     WC

     14 16

     ( Melement / Mtotal ) /( solar )

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     C N O

     t = 3.02 Myr. Until the end of the stellar lifetime it increases slightly but reaches only 1.06 times the solar value at t = 3.22 Myr.

     4. Discussion

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     10

     2.9

     3.0 time / Myr

     3.1

     3.2

     Fig. 2. Time-dependent abundance of 12 C, 14 N, and 16 O in the hot gas phase. For details see text.

     1.25 WN

     12

     WC

     14 16

     ( Melement / Mtotal ) /( solar )

     1.20 1.15 1.10 1.05 1.00 2.9

     C N O

     3.0 time / Myr

     3.1

     3.2

     Fig. 3. Same as Fig. 2, for the warm H?? gas phase. For details see text. The temporal evolution of the 12 C abundance is plotted as dash-dotted line in Fig. 3. As in Fig. 2, the plot starts with the beginning of the WN phases, the vertical dotted line indicates the onset

    of the WC phase. In the warm H?? gas the averaged 12 C abundance reaches supersolar values after t = 3.02 Myr, thus the enriched material needs about 1.5 ?Á 104 yr for its way from the stellar surface, through the SWB, until it reaches the H?? region and mixes with the warm H?? gas. It rises steeply until reaching a value of 1.21 times solar at t = 3.15 Myr and is 1.22 times solar at t = 3.22 Myr. The 14 N mass fraction (solid curve in Fig. 3) in the warm gas phase rises to supersolar values after t = 2.88 Myr when the 14 N has passed the SWB and cools down to H?? temperatures. As discussed for Fig. 1, after t = 3.0 Myr the 14 N-enriched material can only be found in the shell-like structure in the outer zones of the SWB. The 14 N abundance in the warm H?? gas reaches its maximum of 1.008 times solar at t = 3.11 Myr and has a value of 1.006 times solar at the end of the stellar lifetime. For the warm gas the 16 O mass ratio is given as dashed line in Fig. 3. Like 12 C it reaches supersolar values after

     This chemical analysis of the wind and radiation-driven H?? region around an 85 M?Ñ star with solar metallicity provides ?rst quantitative conclusions to what extent the C, N, O-enriched WR winds contribute to the observable abundances of the surrounding H?? region. When comparing the C, N, O abundances in our simulation with observed values, one has to take into account that the overall lifetime as well as the WR lifetime of the 85 M?Ñ star are extremely short compared to less massive stars. Also, the choice of the stellar parameters of SSMM in?uences our results, since other evolutionary tracks would provide other WR lifetimes as well as other mass-loss rates (see, e.g., Paper III). At the end of the stellar lifetime, the 12 C abundance in the warm gas phase amounts to 22.3% above solar, whereas we ?nd only 0.6% supersolar for 14 N, and 5.5% supersolar for 16 O. These values measured in the warm gas phase are the quantities which should be compared with observed H?? regions?? emission spectra. On the other hand, the hot gas of the WR wind bubble is highly enriched with 12 C and 16 O since the onset of the WC phase, while it was signi?cantly enriched with 14 N during the preceeding WN phase. From these models we conclude that the enrichment of the circumstellar environment with 14 N and 16 O by WR stars might be negligible if the 85 M?Ñ star is representative for massive stars passing the WR stage. Only for 12 C the enrichment of the H?? gas is signi?cant. Since the occurrence of a WR phase is strongly metal dependent the enrichment with C should also depend on metallicity like e.g. on O. One should therefore expect that the C gradient of H?? regiones in galactic discs is steeper than that of O. And indeed, Esteban et al. (2005) found ? log (C/O) = ?0.058 ?À 0.018 dex kpc?1 for the Galactic disk. In metal-poor galaxies one would expect less chemical selfenrichment because the stellar mass range of the WR occurrence is shrinked and shifted towards higher masses and the WR phases are shorter.

     References

     Abbott, D. C. & Conti, P. S. 1987, ARA&A, 25, 113 Anders, E. & Grevesse, N. 1989, Geochim. Cosmochim. Acta, 53, 197 Chiosi, C. & Maeder, A. 1986, ARA&A, 24, 329 Dray, L. M. & Tout, C. A. 2003, MNRAS, 341, 299 Dray, L. M., Tout, C. A., Karakas, A. I., & Lattanzio, J. C. 2003, MNRAS, 338, 973 Esteban, C., Garc?äa-Rojas, J., Peimbert, M., et al. 2005, ApJ, ? 618, L95 Freyer, T., Hensler, G., & Yorke, H. W. 2003, ApJ, 594, 888 Freyer, T., Hensler, G., & Yorke, H. W. 2006, ApJ, 638, 262 Hirschi, R., Meynet, G., & Maeder, A. 2004, A&A, 425, 649

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     Hirschi, R., Meynet, G., & Maeder, A. 2005, A&A, 433, 1013 Kr?? ger, D., Freyer, T., & Hensler, G. 2006, A&A, in prep. o Kunth, D. & Sargent, W. L. W. 1986, ApJ, 300, 496 Schaller, G., Schaerer, D., Meynet, G., & Maeder, A. 1992, A&AS, 96, 269

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