Triggered Star Formation Inside the Shell of a Wolf-Rayet Bubble As The
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Accepted to the Astrophysical Journal A Preprint typeset using L TEX style AASTeX6 v. 1.0 TRIGGERED STAR FORMATION INSIDE THE SHELL OF A WOLF-RAYET BUBBLE AS THE ORIGIN OF THE SOLAR SYSTEM Vikram V. Dwarkadas Astronomy and Astrophysics, University of Chicago, 5640 S Ellis Ave, ERC 569, Chicago, IL 60637 Nicolas Dauphas Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago IL 60637 Bradley Meyer Department of Physics and Astronomy, Clemson University, Clemson, SC, 29634-0978, USA. Peter Boyajian Astronomy and Astrophysics, University of Chicago, 5640 S Ellis Ave, ERC 569, Chicago, IL 60637 Michael Bojazi Department of Physics and Astronomy, Clemson University, Clemson, SC, 29634-0978, USA. ABSTRACT A critical constraint on solar system formation is the high 26Al /27Al abundance ratio of 5 ×10−5 at the time of formation, which was about 17 times higher than the average Galactic ratio, while the 60Fe/56Fe value was about 2 × 10−8, lower than the Galactic value. This challenges the assumption that a nearby supernova was responsible for the injection of these short-lived radionuclides into the early solar system. We show that this conundrum can be resolved if the Solar System was formed by triggered star formation at the edge of a Wolf-Rayet (W-R) bubble. Aluminium-26 is produced during the evolution of the massive star, released in the wind during the W-R phase, and condenses into dust grains that are seen around W-R stars. The dust grains survive passage through the reverse shock and the low density shocked wind, reach the dense shell swept-up by the bubble, detach from the decelerated wind and are injected into the shell. Some portions of this shell subsequently collapses to form the dense cores that give rise to solar-type systems. The subsequent aspherical supernova does not inject appreciable amounts of 60Fe into the proto-solar-system, thus accounting for the observed arXiv:1712.10053v1 [astro-ph.SR] 28 Dec 2017 low abundance of 60Fe. We discuss the details of various processes within the model and conclude that it is a viable model that can explain the initial abundances of 26Al and 60Fe. We estimate that 1-16% of all Sun-like stars could have formed in such a setting of triggered star formation in the shell of a WR bubble. Keywords: Astrochemistry, Meteoroids, Solar System: Formation [email protected] [email protected] [email protected] [email protected] [email protected] 2 1. INTRODUCTION to work, but this requires that hot bottom burning The discovery and subsequent characterization of ex- in these stars was stronger than was assumed in their trasolar planetary systems has shed new light on the calculations, and/or an additional neutron source was origin and peculiarities of the solar system. Astronomi- present. cal observations offer clues about the grand architecture One way to test whether supernovae or stellar winds of planetary systems. They cannot provide, however, ac- from massive stars are the source is to examine 60 cess to the intricate details that the study of meteorites Fe (t1/2=2.6 Myr) (Wasserburg et al. 1998). This ra- offers. One important constraint on models of solar sys- dioactive nuclide is produced mostly by neutron capture 26 tem formation comes from measurements of the abun- in the inner part of massive stars, whereas Al is pro- dances of now extinct short-lived radionuclides, whose duced in the external regions (Limongi & Chieffi 2006). 26 past presence is inferred from isotopic variations in their If a supernova injected Al , one would expect copious 60 decay products. Isotopic abundances in meteorites pro- amounts of Fe to also be present. vide insight into the makeup of the cloud material from The formation of refractory Ca, Al-rich inclusions which the solar system formed. They can be used as (CAIs) in meteorites marks time zero in early so- tracers of the stellar processes that were involved in the lar system chronology (Dauphas & Chaussidon 2011). 60 56 formation of the solar system, and of galactic chemical To better constrain the Fe/ Fe ratio at CAI for- evolution up until the time of solar system formation. mation, many studies in the early 2000’s analyzed More than 60 years ago, Urey (1955) speculated chondrites using Secondary Ion Mass Spectrometry 60 56 about the possible role of 26Al as a heat source in (SIMS) techniques, and reported high Fe/ Fe ra- 60 56 −7 planetary bodies. It was not until 1976, however, tios of ∼ Fe/ Fe = 6 × 10 . For reference, the ex- 60 56 that Lee et al. (1976) demonstrated the presence of pected Fe/ Fe ratio in the interstellar medium (ISM) this radioactive nuclide in meteorites at a high level. 4.5 Ga from γ-ray observations (Wang et al. 2007) is −7 The high abundance of 26Al (26Al/27Al ∼ 5 × 10−5) at ∼ 3 × 10 without accounting for isolation from solar system birth (Lee et al. 1976; MacPherson et al. fresh nucleosynthetic input before solar system forma- 1995; Jacobsen et al. 2008) can be compared to tion (Tang & Dauphas 2012). The SIMS results were expectations derived from modeling the chemical later shown to suffer from statistical artifacts, lead- evolution of the Galaxy (Meyer & Clayton 2000; ing Telus et al. (2012) to revise downward the initial 60 56 Wasserburg et al. 2006; Huss et al. 2009), or γ-ray Fe/ Fe ratios that had been previously reported. 60 56 observations (Diehl et al. 2006), which give a maximum The high initial Fe/ Fe ratios inferred from in situ 26Al/27Al ratio of ∼ 3 × 10−6 (Tang & Dauphas 2012). measurements of chondritic components contradicted Aluminium-26 in meteorites is in too high abundance lower estimates obtained from measurements of bulk to be accounted for by long-term chemical evolution of rocks and components of achondrites. This led to specu- 60 the Galaxy or early solar system particle irradiation lations that Fe was heterogeneously distributed in the (Marhas et al. 2002; Duprat & Tatischeff 2007). In- protoplanetary disk, with chondrites and achondrites 60 56 stead, 26Al must have been directly injected by a nearby characterized by high and low Fe/ Fe ratios, respec- source (see §2). Such sources can include supernovae tively (Sugiura et al. 2006; Quitt´eet al. 2010, 2011). 60 (Cameron & Truran 1977; Meyer & Clayton 2000), The issue of the abundance and distribution of Fe stellar winds from massive stars (Arnould et al. 1997; in meteorites was addressed by Tang and Dauphas Diehl et al. 2006; Arnould et al. 2006; Gaidos et al. (Tang & Dauphas 2012, 2015). Using Multi Collector 2009; Gounelle & Meynet 2012; Young 2014; Inductively Coupled Plasma Mass Spectrometry (MC- Tang & Dauphas 2015; Young 2016), or winds from ICPMS), these authors measured various components an AGB-star (Wasserburg et al. 2006). The latter is from chondrites Semarkona (LL3.0), NWA 5717 (un- unlikely, because of the remote probability of finding grouped 3.05) and Gujba (CBa), as well as bulk rocks an evolved star at the time and place of solar system and mineral separates from HED and angrite achon- formation (Kastner & Myers 1994). Recent calculations drites. They showed that in these objects, the ini- 60 56 by Wasserburg et al. (2017) have shown that it is un- tial Fe/ Fe ratio was low, corresponding to an ini- likely that an AGB star could simultaneously account tial value at the formation of CAIs of 11.57 ± 2.6 −9 58 56 for the abundances of 26Al, 60Fe, 182Hf and 107Pd. ×10 . They also measured the Fe/ Fe ratio and Aluminium-26 is mainly produced by hot bottom found that it was constant between chondrites and 182 achondrites, thus ruling out a heterogeneous distribution burning in stars & 5M⊙, which produce too little Hf 60 58 and 107Pd. The latter are mainly products of neutron of Fe as collateral isotopic anomalies on Fe would be expected. Some SIMS studies have continued to capture processes in stars . 5 M⊙. A small window report higher ratios but the data do not define clear of AGB star masses between 4-5.5 M⊙ could be made isochrones (Mishra et al. 2010; Mishra & Chaussidon 3 2012; Marhas & Mishra 2012; Mishra & Chaussidon Young 2014; Tang & Dauphas 2015; Young 2016). The 2014; Mishra & Goswami 2014; Telus et al. 2016). Fur- last scenario is the most appealing because it could be a thermore, recent measurements using the technique natural outcome of the presence of one or several Wolf- of Resonant Ionization Mass Spectrometry (RIMS; Rayet (W-R) stars, which shed their masses through (Trappitsch et al. 2017; Boehnke et al. 2017) ) have winds rich in 26Al, whereas 60Fe is ejected at a later time called into question the existence of the 60Ni excesses following supernova explosion. The W-R winds expand- measured by SIMS. The weight of evidence at the present ing out into the surrounding medium would have carved time thus favors a low uniform initial 60Fe/56Fe ratio at wind-blown bubbles in molecular cloud material, and solar system formation (Tang & Dauphas 2012, 2015). would have enriched the bubbles in 26Al, which would The low initial 60Fe/56Fe ratio may be consistent with have later been incorporated in the molecular cloud core derivation from background abundances in the Galaxy that formed the solar system. with no compelling need to invoke late injection from a In this contribution, we take this idea a step further nearby star (Tang & Dauphas 2012). Iron-60 is a sec- and suggest that our solar system was born inside the ondary radioactive isotope and 56Fe is a primary isotope shell of a Wolf-Rayet wind bubble.