We calculated pre-supernova evolution models of single rotating massive
stars. These models reproduce observations during the early stages of the
evolution very well, in particular Wolf--Rayet (WR) populations and ratio
between type II and type Ib,c supernovae at different metallicities (Z). Using
these models we found the following results concerning long and soft gamma--ray
burst (GRB) progenitors:
  - GRBs coming from WO--type (SNIc) WR stars are only produced at low Z (LMC
or lower).
  - The upper metallicity limit for GRBs is reduced to Z ~ 0.004 (SMC) when the
effects of magnetic fields are included.
  - GRBs are predicted from the second (and probably the first) stellar
generation onwards.Comment: 5 pages, 1 figure, to appear in the proceedings of "Swift and GRBs:
  Unveiling the Relativistic Universe", San Servolo, Venice, 5-9 June 200

Hirschi, R.

Maeder, A.

Meynet, G.

English

arXiv

We calculated pre-supernova evolution models of single rotating massive stars. These models reproduce observations during the early stages of the evolution very well, in particular Wolf--Rayet (WR) populations and ratio between type II and type Ib,c supernovae at different metallicities (Z). Using these models we found the following results concerning long and soft gamma--ray burst (GRB) progenitors:  - GRBs coming from WO--type (SNIc) WR stars are only produced at low Z (LMC or lower).  - The upper metallicity limit for GRBs is reduced to Z ~ 0.004 (SMC) when the effects of magnetic fields are included.  - GRBs are predicted from the second (and probably the first) stellar generation onwards

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arXiv:astro-ph/0610276v1  10 Oct 2006IL NUOVO CIMENTO Vol. ?, N. ? ?GRB progenitors at low metallicitiesR. Hirschi(1), G. Meynet(2), A. Maeder(2)(1) Physics and Astronomy, University of Basel, Klingelberstr. 82, 4056 Basel, Switzerland(2) Geneva Observatory, Ch. des Maillettes 51, 1290 Sauverny, SwitzerlandSummary. — We calculated pre–supernova evolution models of single rotatingmassive stars. These models reproduce observations during the early stages of theevolution very well, in particular Wolf–Rayet (WR) populations and ratio betweentype II and type Ib,c supernovae at different metallicities (Z). Using these modelswe found the following results concerning long and soft gamma–ray burst (GRB)progenitors:-GRBs coming from WO–type (SNIc) WR stars are only produced at low Z(ZLMC or lower).-The upper metallicity limit for GRBs is reduced to Z ∼ 0.004 (SMC) whenthe effects of magnetic fields are included.-GRBs are predicted from the second (and probably the first) stellar generationonwards.PACS 00.00 – By the way, which PACS is it, the 00.00? GOK..PACS —.— – . . . .1. – IntroductionGamma–Ray bursts (GRBs) can be divided into two main types: a) short and hardand b) long and soft (see 13). The long soft GRBs have now been firmly connected tosupernova (SN) explosions of the type Ic (GRB 980425–SN 98bw, GRB 030329–SN 03dh,GRB 031203–SN 03lw, GRB 060218–SN 06aj; see for example 11; 18). The leading modelfor GRB–SN explosions is the collapsar model (17; 9). In this model, the progenitor starmust first lose its hydrogen rich envelope during its lifetime but retain enough angularmomentum in its core. The star must also form a black hole when it dies. The largeamount of angular momentum induces the formation of an accretion disk around the blackhole. Then accretion from the disk onto the black hole powers bipolar jets. These jetscan only pierce through the outer part of the star and be detected if the star is compact.This explain why the progenitor star has to lose its hydrogen rich envelope during itslifetime. Additional constraints come from the fact that close-by SN associated GRBsc© Società Italiana di Fisica 12 R. HIRSCHI ETC.are found (8; 16; 1) in low metallicity (Z) environments, corresponding to the metallicityof the Magellanic Clouds (Z ∼ 0.004 − 0.008). Low metallicity environments are alsofavoured from the theoretical point of view because lower metallicity means weaker massloss and therefore smaller angular momentum loss (9).Several studies were devoted to finding which stars can lead to GRBs (14; 3; 7; 6; 12;2; 19; 20). The key ingredients of the progenitor models in the context of GRBs are thetreatment of rotation, magnetic fields, binarity and metallicity. In this work, we lookat GRB progenitors coming from single stars with models, which include the treatmentof differential rotation and its transport in the interior. We focus on the metallicitydependence of the GRB rates and discuss the impact of magnetic fields on our results.2. – Computer model & calculationsThe stellar evolution code used for this work is described in (5). Convective stabilityis determined by the Schwarzschild criterion. Overshooting is only considered for H–and He–burning cores with an overshooting parameter, αover, of 0.1 HP. The mass lossis proportional to the square root of the metallicity. It is also dependent on the surfacerotation velocity. The centrifugal force is included in the structure equations. Last butnot least, the processes taken into account, which induce transport and mixing of angularmomentum and matter, are meridional circulation and dynamical and secular shears.Magnetic fields are included only in the models discussed in the section on magneticfields. For these models, the treatment of magnetic fields (15) is included according to(10). Models were calculated at various metallicities: Z = 0.04 (GC), 0.02 (Z⊙), 0.008(LMC), 0.004 (SMC), and 10−8 (second stellar generation) (see 6; 4, for more details).3. – Theoretical GRB rates around solar metallicitiesOur models predict the production of fast rotating BHs. Models with large initialmasses or high metallicity end with less angular momentum in their central remnantwith respect to the break–up limit for the remnant. Many WR star models satisfy thethree main criteria (BH formation, loss of hydrogen–rich envelope and enough angularmomentum to form an accretion disk around the black hole) for long and soft gamma–ray bursts (GRB) production via the collapsar model (17). Considering all types ofWR stars as GRB progenitors, we predict too many GRBs compared to observation. Ifwe consider only WO–type WR stars (type Ic supernovae as is the case for the GRBrelated SNe discovered so far, see the list in the introduction) as GRB progenitors, theGRB production rates are in much better agreement with observations (see Table I).In particular, WO stars are found only at low metallicities (Z.ZLMC) and have masses,M & 50 M⊙.4. – Do the first stars produce GRBs?We calculated a series of models at Z = 10−8 (which corresponds to the second stellargeneration). In this series, the 85 M⊙ model becomes a WO–type WR star (see Fig. 1left). The core of the 85 M⊙ model retains enough angular momentum to produce aGRB via the collapsar model (see Fig. 1 right). SNe of type Ib,c and GRBs are thereforepredicted to ensue from the death of single massive stars at very low metallicities.GRB PROGENITORS AT LOW METALLICITIES 3Table I. – Predicted rates [yr−1] of long soft GRBs coming from WO–type WR stars, as wellas limiting masses at various metallicities.ZSMC ZLMC Z⊙ ZGCMminGRB(WO) 50 45 - -MmaxGRB(WO) 95 95 - -RGRB(WO) 4.74E-04 5.99E-04 0 0Observed rate: ROBSGRB = 310−6− 6 10−4 (depending on the beaming angle of GRBs)Reference total SN rate: ROBSSN ≃ 7 10−3 (14)5. – Models with rotation and magnetic fieldsPetrovic et al (12) and Fryer and Heger (2) studied the evolution of single and binarymassive stars with and without magnetic fields (dynamo theory from 15). They find thatboth single and binary models without magnetic field can produce GRBs and both singleand binary models with magnetic field considered in their studies cannot produce GRBs.Models with magnetic fields nevertheless better reproduce the observed pulsar periods.The most recent studies found a way to produce GRBs in models including magneticfields. Indeed, (20) and (19) find that if a star initially rotates fast enough, it undergoesquasi chemical homogeneous evolution. The key idea is that WR stars are produced byFig. 1. – Left: profiles of the chemical abundances at the pre–SN stage for the 85 M⊙ modelat Z = 10−8. Right: profile of the specific angular momentum (solid line) for the same model.The red dotted line shows the minimum angular momentum necessary in order to form anaccretion disk around a rotating black hole. The blue short dashed and green long dashed linesshow the minimum angular momentum necessary for matter around a maximally rotating anda non–rotating black hole, respectively.4 R. HIRSCHI ETC.mixing and not by mass loss. Since WR mass loss rates are too high at metallicitiesaround solar, GRBs can only be produced via this channel in low metallicity environ-ments. (20) find that GRBs can be produced for Z . 0.004. This is somewhat lowerthan the observed metallicity of close-by GRBs.We calculated 40 M⊙ models at Z = ZSMC and Z = 10−8 with magnetic fields. AtZ = ZSMC, we obtain similar results as (20). At Z = 10−8, we found that 40 M⊙models undergo quasi chemical homogeneous evolution (if the initial velocity is largerthan 600km s−1). With or without magnetic fields, GRBs are therefore expected to followthe death of massive stars from the second stellar generation onwards (and probably fromthe first).The treatment of anisotropic mass loss and the recent downward revision of solarmetallicity are two important physical ingredients that have yet to be included in themodels and that will help GRB progenitors retain more angular momentum.references[1] Fruchter, A. S., Levan, A. J., Strolger, L., et al., Nature, 441 (2006) 463[2] Fryer, C. L. & Heger, A., ApJ, 623 (2005) 302[3] Heger, A., Woosley, S. E., & Spruit, H. C., ApJ, 626 (2005) 350[4] Hirschi, R., 2006, A&A accepted, astro-ph/0608170[5] Hirschi, R., Meynet, G., & Maeder, A., A&A, 425 (2004) 649[6] Hirschi, R., Meynet, G., & Maeder, A., A&A, 443 (2005) 581[7] Izzard, R. G., Ramirez-Ruiz, E., & Tout, C. A., MNRAS, 348 (2004) 1215[8] Le Floc’h, E., Duc, P.-A., Mirabel, I. F., et al., A&A, 400 (2003) 499[9] MacFadyen, A. I. & Woosley, S. E., ApJ, 524 (1999) 262[10] Maeder, A. & Meynet, G., A&A, 440 (2005) 1041[11] Mazzali, P. A., Deng, J., Tominaga, N., et al., ApJ, 599 (2003) L95[12] Petrovic, J., Langer, N., Yoon, S.-C., & Heger, A., A&A, 435 (2005) 247[13] Piran, T., Reviews of Modern Physics, 76 (2005) 1143[14] Podsiadlowski, P., Mazzali, P. A., Nomoto, K., Lazzati, D., & Cappel-laro, E., ApJ, 607 (2004) L17[15] Spruit, H. C., A&A, 381 (2002) 923[16] Stanek, K. Z., Gnedin, O. Y., Beacom, J. F., et al., 2006, astro-ph/0604113[17] Woosley, S. E., ApJ, 405 (1993) 273[18] Woosley, S. E. & Bloom, J. S., ARA&A, 44 (2006) 507[19] Woosley, S. E. & Heger, A., ApJ, 637 (2006) 914[20] Yoon, S. C., Langer, N., & Norman, C., 2006, astro-ph/0606637GRB PROGENITORS AT LOW METALLICITIES 5Appendix A.Questions and answers:Question by A. J. van Marle: In several GRB afterglows, silicon is seen in absorptionat a velocity that indicates its presence in the progenitor wind. Is this possible for lowmetallicity stars?Answer: Yes, pristine silicon is present in the stellar wind. Nearby GRBs probably donot have very low metallicity progenitors, in which case pristine silicon could be detected.Question by D. Fox: You have investigated models of isolated massive stars, but manymassive stars are born in close binaries. Does this affect your results?Answer: The presence of a binary companion may either increase mass loss or induceaccretion (at different times) and spin–up or spin–down a star. This makes it difficult toestimate the final effect for GRBs and obtain quantitative predictions.Question by C. Thompson: Could you summarize how your model of angular momentumtransport is constrained by observations of isotope enhancements in evolved stars?Answer: One important test is comparing the surface enrichment, for example in nitro-gen, between models and observations as a function of the position in the Hertzsprung–Russell diagram.Comment by T. Piran: According to your low metallicity paradigm, we need new es-timates of GRB rates based on the SFR for low metallicity stars.Answer: I agree with this comment. Langer, Norman and collaborators have started tostudy this point.

GRB progenitors at low metallicities

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