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Discrepancies in the Ages of Young Star Clusters Evidence for Mergers.Pdf LJMU Research Online Beasor, ER, Davies, B, Smith, N and Bastian, N Discrepancies in the ages of young star clusters; evidence for mergers? http://researchonline.ljmu.ac.uk/id/eprint/10361/ Article Citation (please note it is advisable to refer to the publisher’s version if you intend to cite from this work) Beasor, ER, Davies, B, Smith, N and Bastian, N (2019) Discrepancies in the ages of young star clusters; evidence for mergers? Monthly Notices of the Royal Astronomical Society, 486 (1). pp. 266-273. ISSN 0035-8711 LJMU has developed LJMU Research Online for users to access the research output of the University more effectively. Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Users may download and/or print one copy of any article(s) in LJMU Research Online to facilitate their private study or for non-commercial research. You may not engage in further distribution of the material or use it for any profit-making activities or any commercial gain. The version presented here may differ from the published version or from the version of the record. Please see the repository URL above for details on accessing the published version and note that access may require a subscription. For more information please contact [email protected] http://researchonline.ljmu.ac.uk/ MNRAS 486, 266–273 (2019) doi:10.1093/mnras/stz732 Advance Access publication 2019 April 3 Discrepancies in the ages of young star clusters; evidence for mergers? Emma R. Beasor,1‹ Ben Davies,1 Nathan Smith2 and Nate Bastian1 1Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK 2Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA Downloaded from https://academic.oup.com/mnras/article-abstract/486/1/266/5426824 by Liverpool John Moores University user on 23 July 2020 Accepted 2019 March 11. Received 2019 March 11; in original form 2018 November 27 ABSTRACT There is growing evidence that star clusters can no longer be considered simple stellar populations. Intermediate- and old-age clusters are often found to have extended main- sequence turn-offs (eMSTOs) which are difficult to explain with single-age isochrones, an effect attributed to rotation. In this paper, we provide the first characterization of this effect in young (<20 Myr) clusters. We determine ages for four young massive clusters (2 Large Magellanic Cloud and 2 Galactic) by three different methods: using the brightest single turn- off (TO) star; using the luminosity function (LF) of the TO; and by using the lowest Lbol red supergiant (RSG). The age found using the cluster TO is consistently younger than the age found using the lowest RSG Lbol. Under the assumption that the lowest luminosity RSG age is the ‘true’ age, we argue that the eMSTOs of these clusters cannot be explained solely by rotation or unresolved binaries. We speculate that the most luminous stars above the TO are massive blue straggler stars formed via binary interaction, either as mass gainers or merger products. Therefore, using the cluster TO method to infer ages and initial masses of post-MS stars, such as Wolf−Rayet stars, luminous blue variables, and RSGs, will result in ages inferred being too young and masses too high. Key words: blue stragglers – stars: evolution – stars: massive. massive stars. Indeed, eMSTOs have been observed in clusters as 1 INTRODUCTION young as 20−30 Myr (Li et al. 2017). Historically, star clusters have been used as a benchmark for stellar One other potential explanation for stars above the MSTO is evolution models, since it was assumed they were simple stellar rejuvenation, either via mergers or mass transfers (e.g. Schneider populations (SSPs) of a single age and metallicity. However, modern et al. 2014; Gosnell et al. 2015). It is now commonly believed observations of intermediate-age and old globular clusters reveal that most massive stars exist in binaries (≥70 per cent, Sana features not easily explained by SSPs, for example, the presence of et al. 2012) and the merger rate of these could be high (de Mink multiple main sequences (MSs, e.g. Marino et al. 2008; Piotto et al. et al. 2014). For stars undergoing binary interaction, mass can be 2015), extended MS turn-offs (eMSTOs, e.g. Keller, Mackey & Da transferred from a secondary on to a primary (mass gainer) or the Costa 2011; Bastian et al. 2016), and abundance anticorrelations two objects can merge entirely. Not only does the merger product (e.g. Gratton, Sneden & Carretta 2004; Gratton et al. 2012). Within gain mass and hence become more luminous, but its MS lifetime is the literature, there are a number of explanations for these peculiar extended by additional hydrogen being mixed into the core. Merger features. Recent work suggests that the eMSTO is an effect of stellar products would therefore appear as younger, brighter stars above rotation, not age spreads as previously thought (see Bastian & Lardo the TO, much like blue straggler (BS) stars seen in globular clusters 2018, for a recent review). This can be understood as rotational (Sandage 1984; Knigge, Leigh & Sills 2009; Gosnell et al. 2015). mixing lengthens the MS lifetime by injecting more hydrogen into BSs have been tentatively observed in the young cluster NGC 330 the core, also causing the terminal age MS stars to appear more (Lennon et al. 1993; Evans et al. 2006) and in Westerlund 1 (Clark luminous. et al. 2019), but there has been no systematic study on whether So far, eMSTOs are found to exist in intermediate- and old-age they routinely exist in young clusters. If ubiquitous, the eMSTOs in clusters. If the eMSTO phenomenon is caused by stellar rotation, young clusters would impact their age estimates in the literature. then young (<50 Myr) clusters should also exhibit this feature, since In previous papers, we have determined the ages to young clusters rotation is thought to have a greater influence the evolution of more using two different methods. The first, for NGC 2100, involved fitting the full observed luminosity range of the red supergiants (RSGs, Beasor & Davies 2016) with predictions from various stellar evolution models. In Beasor & Davies (2018), we used the previ- E-mail: [email protected] ously determined ages for χ Per and NGC 7419, in both cases found C 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Discrepancies in the ages of young star clusters 267 by fitting isochrones to MS stars by eye (14 Myr for both, Currie base for OB stars in the plane of each cluster. Next, we obtain et al. 2010; Marco & Negueruela 2013). For both of these methods, Gaia DR2 parallaxes and proper motions for all stars. Following we assumed only single stellar evolution (no binaries, no mergers). Aghakhanloo et al. (in preparation),√ the error on the parallax σ i of Each age estimation technique has its potential weaknesses. If the each star i is defined to be σi = (1.086σω + i)wherei is the merger rate in young clusters is high (as suggested by de Mink et al. excess astrometric noise on the Gaia parallax solution. We then 2014), post-merger objects (i.e. an object resulting from the merging iteratively sigma-clip the sample by discarding those stars with of two stars in a binary system) could affect the age determined in proper motions outside 3σ of the mean for the whole cluster. Of each method significantly, as they would appear as bright objects the N remaining stars (140 for χ Per, and 20 for NGC 7419), Downloaded from https://academic.oup.com/mnras/article-abstract/486/1/266/5426824 by Liverpool John Moores University user on 23 July 2020 above the TO. Once these stars leave the MS, they could evolve to we determine the sigma-weighted mean of the parallaxes π¯ and become RSGs which are anomalously bright compared to the single the standard√ deviation σπ¯ , and define the formal error onπ ¯ to be star population, again causing the observer to infer a younger age dπ¯ = σπ¯ / (N). for the cluster. The posterior probability distribution function Pr on the distance 2 In this paper, we compare ages for four young clusters using r to the cluster is determined from Pr ∝ exp (− 0.5z ), where three different methods and attempt to reconcile these differences z = (π¯ − πZP − 1/r)/dπ¯ ,andπ ZP is the zero-point parallax offset 1 in terms of non-simple stellar evolution. In Section 3, we describe in Gaia DR2. Various independent measurements of π ZP have each of the three age determination methods in detail as well as the been made, with a global average seeming to converge on π ZP = method for determining extinction towards the cluster. In Section 4, −0.05 mas (see Aghakhanloo et al. in preparation, for a discussion). we discuss the results of this work, and finally, in Section 5, we However, Lindegren et al. (2018) find that this value varies across discuss the implications of our results with respect to possible the sky by ±0.03 mas on spatial scales of about a degree (i.e. larger evidence for mergers. than the cluster field of view). We therefore adopt a value of π ZP = −0.05 ± 0.03 mas, where this error on the zero-point offset must be added in quadrature to that of π¯ . 2 OBSERVATIONS For χ Per and NGC 7419, we find average parallaxes of 0.448 ± 0.003 and 0.333 ± 0.009 mas, respectively, not including 2.1 Sample the uncertainty on π ZP.
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