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A&A
Volume 644, December 2020
Article Number L1
Number of page(s) 4
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202039334
Published online 30 November 2020

© ESO 2020

1. Introduction

Recent work from Lichtenberg et al. (2019) suggests that the bulk water fraction and radius of terrestrial exoplanets are anticorrelated with the abundance of 26Al. Heat released by the radioactive decay of 26Al (half-life ≈0.72 Myr) provides an additional heat source that aids planetesimal differentiation (e.g., Hevey & Sanders 2006) and dehydrates planetesimals (e.g., Grimm & McSween 1993), leading to a rockier composition of the final planet.

Whether rocky planets are more conducive to life and detectable bio-signatures is an active area of research (e.g., Kaltenegger 2017). There is particular interest in whether water worlds – those with a significantly higher bulk water fraction than Earth – can support life (e.g., Noack et al. 2016; Kite & Ford 2018; Olson et al. 2020). Until observations clarify terrestrial exoplanet compositions (e.g., Teske et al. 2018; Rice et al. 2019), models provide the best constraints on the most likely outcomes of planet formation.

The short half-life of 26Al requires production near planet formation in both time and space. Meteoritic evidence indicates that 26Al was abundant in the early Solar System and most enrichment models aim to reproduce this specific case. Early models considered direct supernova (SN) injection into the star-forming cloud, perhaps stimulating its collapse (e.g., Cameron & Truran 1977) and direct enrichment of planet-forming disks (e.g., Chevalier 2000). Recently, attention has turned to pre-SN mass loss, particularly from Wolf-Rayet (W-R) stars, as a pathway to earlier enrichment (e.g., Gounelle & Meibom 2008; Gaidos et al. 2009; Dwarkadas et al. 2017). Scenarios involving either SNe or W-R winds require finely-tuned conditions to place young planet-forming systems in close proximity, suggesting that few systems will be formed under their influence.

Extrapolating from the Solar System has led some authors to suggest that the Earth represents a minority (∼1%) of the outcomes of planet formation (see e.g., Adams 2010; Gounelle 2015; Portegies Zwart 2019). However, the specific conditions inferred for the Solar System may not apply to exoplanets in general. Abundant 26Al in many star- and planet-forming systems would challenge suggestions that water-rich planets may be the dominant family of terrestrial planet analogs (e.g., Alibert & Benz 2017; Miguel et al. 2020). The importance of 26Al in planet composition clearly warrants a broader assessment of the likelihood of 26Al-enrichment.

In this Letter, I compare observations of 26Al in the Galaxy with typical star- and planet-forming conditions to estimate the likelihood of 26Al enrichment. I focus on 26Al because of its critical role in determining the water budget of terrestrial exoplanets. My goal is not to explain the formation of the Earth, so I do not consider other elemental abundances (i.e., 60Fe) specific to the Solar System. I conclude with a discussion of how models of 26Al-enrichment can be improved in light of recent observations.

2. Observational constraints

2.1. Observations of 26Al and its production by high-mass stars

Most of our knowledge of the production and distribution of 26Al in the Galaxy comes from observations of the 1.809 MeV γ-ray photons produced (along with positrons) when it undergoes radioactive decay (Endt 1990). Emission is bright along the Galactic plane corresponding to an average abundance ∼3−25 lower than the early Solar System (Lugaro et al. 2018). However, the emission is predominately associated with young high-mass star-forming regions (e.g., Knödlseder et al. 1999; Diehl et al. 2006). An observed close correlation between 1.809 MeV emission and ionized gas in the interstellar medium (ISM) can only be explained if high-mass stars (> 10 ) dominate 26Al production (Knödlseder et al. 1999).

Classical W-R stars are the late evolutionary phase of high-mass stars (Minitial ≳ 25 M⊙ in the Galaxy), and multiple studies argue that their winds contribute ≳40% of the observed 26Al (e.g., Knödlseder et al. 1999). This enhances the ISM abundance of 26Al prior to the onset of supernovae (SNe). Diehl et al. (2010) propose a time-evolution of the 26Al abundance for a given star-forming event as follows: 26Al is initially enhanced in the ISM after ∼3 Myr by the onset of mass loss from W-R winds. Further enhancement comes over the next 1−2 Myr from core-collapse SNe, with the peak value at ∼5 Myr. The abundance then decreases as lower-mass stars do not reach a W-R phase, making their most significant contribution later from their core-collapse SNe explosions. Beyond 7−8 Myr, the 26Al abundance continues to decrease, with a tail extending to ∼20 Myr.

Population-synthesis models also point to significant early, pre-supernova enrichment (e.g., Voss et al. 2009). Voss et al. (2010, 2012) modeled the combined high-mass star content of the Orion and Carina star-forming complexes and argue that the current high observed abundance of 26Al has persisted for ≳5 Myr and will be maintained for another ∼5 Myr with an additional enhancement from SNe.

Doppler shifts of the 1.809 MeV line indicate that 26Al-producing sources corotate with the Galaxy (Diehl et al. 2006). However, the bulk velocity of 26Al-enriched material is higher than that of the molecular gas in the Galaxy, with a systematic offset of ∼200 km s−1 in the direction of Galactic rotation (Kretschmer et al. 2013). In the specific case of Sco OB2, 26Al was detected with slightly blueshifted energies, suggesting a bulk velocity of 137 ± 75 km s−1 (Diehl et al. 2010). These velocities are roughly an order of magnitude slower than the expected ejection velocities (> 1000 km s−1) for winds and SNe.

2.2. Typical star-forming conditions

Observations in the Milky Way and in other galaxies suggest a cluster mass function of the form dN/dM ∼ M−2 (e.g., Mok et al. 2020). I refer to clustering in the statistical sense that stars form near other stars (see discussion in Lee & Hopkins 2020), noting that many extra-galactic studies do not apply additional criteria to determine whether stellar over-densities are gravitationally bound. As Dukes & Krumholz (2012) point out, this cluster mass function implies that ≳50% of stars are born in high-mass star-forming regions with as many or more high-mass stars than the Orion Nebula Cluster (ONC).

The Orion star-forming complex is nevertheless observed to have 26Al (Diehl et al. 2003) at a level similar to the early Solar System (Jura et al. 2013). The same is true of Carina (Reiter & Parker 2019). Jura et al. (2013) also make a more general argument by comparing the Galactic 26Al abundance with molecular (star-forming) gas to show that star- and planet-formation regions have abundances on the order of the early Solar System, unlike the average ISM.

3. Discussion

The majority of stars (≳50%) form in high-mass regions where 26Al is observed to be abundant. While this suggests that many systems are exposed to 26Al, two key uncertainties affect the fraction of systems that are ultimately enriched: (1) when and how long 26Al is highly abundant and (2) how and whether the 26Al mixes with the star- and planet-forming material. I discuss these in turn below.

3.1. The timescale of 26Al availability

Observations of 26Al indicate a fundamental discrepancy with typical model assumptions: high 26Al abundances appear to persist for a few million years, which is much longer than the 0.7 Myr half-life assumed for an instantaneous production event. The longer timescale reflects production by a stellar population rather than a single high-mass source.

For models that assume a single-age stellar population, disks must survive long enough for direct pollution from a nearby SN or W-R star. Prior to these late evolutionary stages, feedback from those same high-mass stars may accelerate disk destruction, limiting the number of remaining systems to enrich (e.g., Winter et al. 2018; Concha-Ramírez et al. 2019).

Other models start with a SN or W-R winds polluting nearby cold gas that will subsequently form a second (or third) generation of stars (e.g., Gaidos et al. 2009; Gounelle & Meynet 2012; Kuffmeier et al. 2016; Dwarkadas et al. 2017). However, several studies show that most of the gas is cleared from star-forming regions by pre-SN feedback (e.g., Hollyhead et al. 2015; Chevance et al. 2019).

Reality likely lies between these scenarios. Observations consistently suggest small age spreads in star-forming regions (e.g., Getman et al. 2014; Beccari et al. 2017; Venuti et al. 2018; Jerabkova et al. 2019; Prisinzano et al. 2019). Age differences – where younger and older populations are not necessarily spatially coincident – are also common within OB associations (e.g., Wright et al. 2010; Pecaut & Mamajek 2016; Getman et al. 2018). In both cases, age differences are a few million years, which is much smaller than would be produced by triggered sequential star formation (Parker & Dale 2016).

Age differences affect 26Al-enrichment in two ways: (1) they extend the time that disks are in the region (as in the proplyd lifetime solution proposed by Winter et al. 2019); and (2) they provide regular replenishment to maintain the high abundance of 26Al for several million years. This pragmatic approach is more akin to the hierarchical star formation models from Fujimoto et al. (2018), which show that galactic-scale correlations in star formation strongly affect enrichment.

The Carina star-forming complex offers an illustrative example. Several studies suggest age differences between the primary clusters Tr14, Tr15, and Tr16 (see, e.g., the discussion in Townsley et al. 2011 for a review). Recent work from Povich et al. (2019) quantifies variations in the duration of star formation in Carina. They conclude that star formation has been ongoing for ∼10 Myr, with a peak in the star-formation rate ∼3 Myr ago with the birth of the famous central clusters, Tr14 and Tr16.

These multiple episodes of star formation are all contained within the ≳1° beamsize of the γ-ray observations. Voss et al. (2012) performed population synthesis modeling of the combined population of all clusters to estimate that the current high abundance has been maintained for ∼5 Myr and will persist for another ∼5 Myr.

If 26Al abundances have been maintained at the current high levels for ∼5 Myr while star formation peaked within the last ∼3 Myr, then the majority of systems were likely exposed to 26Al. Similar arguments can be made for the Orion star-forming complex. If ≳50% of all stars are born in high-mass regions with (conservatively) half of those exposed to significant 26Al, then we may crudely estimate that as many as ∼25% of systems may be enriched with 26Al, possibly at a level adequate to make rocky planets. This is an order of magnitude larger than the few percent estimated for Solar-System-specific conditions, but is consistent with model results from Fujimoto et al. (2018) that short-lived radioactive isotopes, including 26Al, are abundant in newly formed stars.

This order-of-magnitude estimate does not account for uncertainties in the cluster mass function and ignores possible variations with location in a galaxy. No provision is made for differences in 26Al production with stellar mass, although not all high-mass stars undergo a W-R phase and SN yields depend on the mass of the progenitor. More quantitative analysis is clearly needed. I outline some suggested directions in Sect. 3.4.

3.2. The incorporation of 26Al into planet-forming material

Exposure to 26Al does not imply that it mixes with planet-forming material. Here too, observations invite the reconsideration of typical model assumptions. The observed bulk velocity of the 26Al is ∼200 km s−1, which is an order of magnitude slower than the ∼1000 km s−1 expected from W-R winds or SNe (see Sect. 2). Slower 26Al may mix more readily with planet-forming material. Existing parameter studies of disk enrichment investigate only a narrow range of velocities that are much faster than the observed bulk 26Al velocity (e.g., Ouellette et al. 2007).

Other models for the enrichment of star- and planet-forming gas enforce mixing by confining 26Al-enriched ejecta (e.g., Vasileiadis et al. 2013; Gounelle 2015; Kuffmeier et al. 2016). This would preclude 26Al-enrichment in regions such as Carina that have developed into superbubbles that are unable to contain 26Al-enriched material. As pointed out by Fujimoto et al. (2018), these scenarios are at odds with both the short observed time for young high-mass star-forming regions to clear their gas and the large observed scale height of 26Al in the Galaxy (Wang et al. 2009).

While the physical pathway by which 26Al is incorporated into planet-forming material remains unclear, composition studies of extrasolar asteroids suggest that mixing, nevertheless, does occur. Jura et al. (2013) estimated the 26Al required to enable the observed differentiation and find that it is comparable to the 26Al abundance in Orion, implying that Solar System levels of enrichment are common.

Additional evidence may come from the observed size distribution of exoplanets. The models of Lichtenberg et al. (2019) predict that the planet radius is anticorrelated with the bulk water fraction. Observations of small, close-in exoplanets represent a bimodal radius distribution with current data suggesting the same intrinsic frequency for the two regimes (Fulton et al. 2017).

3.3. Some environments are more likely to form rocky planets

The points in the previous sections suggest that some star-forming environments will be more conducive than others to 26Al-enrichment, and thus rocky planet formation. The most massive member of Taurus, the local template of a low-mass star-forming region, is ∼3.5 M⊙ (Table 1, De Marchi et al. 2010). Such a star will end its life as an AGB star that will not sustain temperatures sufficient to synthesize 26Al (e.g., Abia et al. 2017). If 26Al regulates the water budget, as in the models of Lichtenberg et al. (2019), this suggests that terrestrial planets that form in low-mass regions such as Taurus are more likely to be water worlds.

3.4. Future directions

In the following sections, I highlight key areas for future work to quantify the fraction of systems that are enriched with 26Al and, therefore, able to form rocky planets.

3.4.1. Production of 26Al by high-mass stars

Stellar mass is only one of the relevant variables affecting the timescale and abundance of 26Al production. Rotation is a key parameter that influences the amount of 26Al introduced into the ISM by high-mass stars, primarily via rotational-mixing of material from deeper in the star to the surface (e.g., Voss et al. 2009). By bringing 26Al to the surface earlier than in models without rotation, stars can potentially return 26Al to the ISM via their stellar winds both earlier and for lower initial masses than in nonrotating models. For 26Al-enrichment, the effects of rotation have been explored for initial rotational velocities of 300 km s−1 (e.g., Palacios et al. 2005; Voss et al. 2009).

Surveys such as the VLT-FLAMES Tarantula Survey (Evans et al. 2011) and the IACOB project (Simón-Díaz et al. 2011a,b, 2015) have provided large spectroscopic samples to investigate the rotation rates of high-mass stars (e.g., Ramírez-Agudelo et al. 2013; Holgado et al. 2018; Holgado 2019). Typical rotation rates are less than half of those assumed in population synthesis studies (e.g., Voss et al. 2009). At these velocities, stellar evolution more closely resembles nonrotating models (Evans et al. 2020). Existing population synthesis models matched the observed 26Al, so it is unclear if changes to 26Al production also affect the time-evolution of the abundance.

Another critical development in the last 10 years is evidence that > 70% of high-mass stars are in close-separation binaries that will eventually interact (Sana et al. 2012). Large grids of binary models are only now being computed. Recent work from Brinkman et al. (2019) presents non-rotating models that suggest that binary interactions affect the total 26Al mass ejected by only ∼5−10%. There is a substantial enhancement of 26Al from 10−15 M⊙ primaries in binary systems, but the total amount from a given population is dominated by higher mass stars (> 30 M⊙), where binary effects appear to be less important. Further work is required to investigate the impact of stellar rotation on such models.

3.4.2. Pre-supernova enrichment models

Studies of the γ-ray emission from the radioactive decay of 26Al point to the important role of pre-SN mass loss. Including the pre-SN enrichment in cluster-scale simulations (e.g., Lichtenberg et al. 2016; Nicholson et al. 2019) may significantly change the fraction of disks enriched with 26Al. Models should also consider on-going 26Al production, and perhaps steady accretion instead of a single injection event. Meteoritic evidence suggests this may have happened in the Solar System with 26Al accrued quickly, but not instantaneously (Liu et al. 2012).

4. Summary

Recent models suggest that the 26Al abundance plays a central role in determining the water budget of terrestrial planets. Galactic observations of the short-lived radioactive isotope 26Al provide a constraint on the fraction of star- and planet-forming systems that may be enriched. Observations indicate that 26Al is predominantly produced by high-mass stars. Pre-SN mass loss, especially from W-R stars, is thought to contribute nearly half of the Galactic 26Al budget. Furthermore, 26Al is particularly abundant in high-mass star-forming regions where the majority of stars, and thus planets, form. Mass loss by stellar winds can enrich the ISM in such regions in a few million years. Critically for planet formation, the regular replenishment may sustain high levels of 26Al for millions of years (much longer than its 0.7 Myr half-life). I argue that the majority of star and planet systems in high-mass star-forming regions may be exposed to abundant 26Al. Exposure does not imply enrichment, but observed bulk 26Al velocities are an order of magnitude slower than expected from winds and SNe, which may increase the likelihood of enrichment. If half of the exposed systems are enriched with 26Al, this (rough) estimate suggests that the fraction of such systems is at least an order of magnitude higher than the few percent found by studies that extrapolate from Solar-System-specific conditions. More quantitative estimates that do not assume Solar-System-specific conditions a priori are clearly needed to clarify this fraction.

These conditions describe the environment in which most stars and planets form, but they do not reflect those sampled in low-mass star-forming clouds. This suggests that rocky planets are more likely to form around stars born in high-mass star-forming regions. Low-mass regions such as Taurus do not contain the high-mass stars that dominate the production of 26Al and thus are unlikely to form rocky planets.

Acknowledgments

I would like to thank the referee for a timely and thoughtful report that improved the manuscript. I am deeply grateful to Chris Evans, Richard Parker, Ken Rice, and Tim Lichtenberg for reading the manuscript and providing thoughtful comments. Thanks also to Brandt Gaches and Karen Moran. This project received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skĺodowska-Curie grant agreement No. 665593 awarded to the Science and Technology Facilities Council.

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