Leveraging <i>HST</i> with MUSE: II. Na-abundance variations in intermediate age star clusters

Martocchia, S.; Kamann, Sebastian; Saracino, S.; Usher, Christopher; Bastian, Nate; Rejkuba, M.; Latour, M.; Lardo, C.; Cabrera-Ziri, I.; Kacharov, N.; Kozhurina-Platais, V.; Larsen, Soeren S.; Mancino, Sara; Platais, I.; Salaris, Maurizio

doi:10.1093/mnras/staa2929

Cited by 9 publications

(5 citation statements)

References 76 publications

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“…In the ancient clusters, the complex CMDs are caused mainly by star-to-star chemical abundance variations (see reviews by Bastian & Lardo 2018;Gratton et al 2019). Similar variations have been observed in massive Magellanic Cloud clusters with ages of ∼ 2 Gyr or above (e.g., Saracino et al 2020;Martocchia et al 2020;Cadelano et al 2022), while they appear to be absent in the stars observed in even younger clusters (e.g., Mucciarelli et al 2014). Instead, stellar rotation has been suggested as the dominant cause of the unexpected features in the young clusters (e.g., Bastian & de Mink 2009;D'Antona et al 2015).…”

supporting

confidence: 71%

The effects of stellar rotation along the main sequence of the 100 Myr old massive cluster NGC 1850

Kamann¹,

Saracino²,

Bastian³

et al. 2022

Preprint

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Young star clusters enable us to study the effects of stellar rotation on an ensemble of stars of the same age and across a wide range in stellar mass and are therefore ideal targets for understanding the consequences of rotation on stellar evolution. We combine MUSE spectroscopy with HST photometry to measure the projected rotational velocities (𝑉 sin 𝑖) of 2 184 stars along the split main sequence and on the main sequence turn-off (MSTO) of the 100 Myr-old massive (10 5 M ) star cluster NGC 1850 in the Large Magellanic Cloud. At fixed magnitude, we observe a clear correlation between 𝑉 sin 𝑖 and colour, in the sense that fast rotators appear redder. The average 𝑉 sin 𝑖 values for stars on the blue and red branches of the split main sequence are ∼ 100 km s −1 and ∼ 200 km s −1 , respectively. The values correspond to about 25 − 30% and 50 − 60% of the critical rotation velocity and imply that rotation rates comparable to those observed in field stars of similar masses can explain the split main sequence. Our spectroscopic sample contains a rich population of ∼200 fast rotating Be stars. The presence of shell features suggests that 23% of them are observed through their decretion disks, corresponding to a disk opening angle of 15 degrees. These shell stars can significantly alter the shape of the MSTO, hence care should be taken when interpreting this photometric feature. Overall, our findings impact our understanding of the evolution of young massive clusters and provide new observational constraints for testing stellar evolutionary models.

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supporting

confidence: 71%

The effects of stellar rotation along the main sequence of the 100 Myr old massive cluster NGC 1850

Kamann¹,

Saracino²,

Bastian³

et al. 2022

Preprint

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“…Na variations have also been found, by first using integrated light techniques in quite young clusters (∼2-3 Gyr, Bastian et al 2019). Subsequently, Saracino et al (2020b) and Martocchia et al (2020b) showed that Na variations are present in massive star clusters at different ages (∼ 2 − 7.5 Gyr), by combining the power of HST and VLT/MUSE, a technique introduced by Latour et al (2019).…”

Section: Introductionmentioning

confidence: 99%

On the Nitrogen variation in ∼2 Gyr old massive star clusters in the large Magellanic Cloud

Martocchia

Lardo

Rejkuba

et al. 2021

Monthly Notices of the Royal Astronomical Society

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We present ESO/VLT FORS2 low resolution spectroscopy of red giant branch stars in three massive, intermediate age (∼1.7 − 2.3 Gyr) star clusters in the Large Magellanic Cloud. We measure CH and CN index bands at 4300Å and 3883Å, as well as [C/Fe] and [N/Fe] abundance ratios for 24, 21 and 12 member stars of NGC 1978, NGC 1651, NGC 1783, respectively. We find a significant intrinsic spread in CN in NGC 1978 and NGC 1651, a signal of multiple stellar populations (MPs) within the clusters. On the contrary, we report a null CN spread in NGC 1783 within our measurement precision. For NGC 1978, we separated the two populations in the CN distribution and we translated the CN spread into an internal N variation Δ[N/Fe]=0.63 ± 0.49 dex. For NGC 1651 and NGC 1783, we put upper limits on the N abundance variations of Δ[N/Fe]≤0.2, 0.4 dex, respectively. The spectroscopic analysis confirms previous results from HST photometry, where NGC 1978 was found to host MPs in the form of N spreads, while slightly younger clusters (e.g. NGC 1783, < 2 Gyr old) were not, within the limits of the uncertainties. It also confirms that intermediate age massive clusters show lower N abundance variations with respect to the ancient globular clusters, although this is in part due to the effect of the first dredge up at these stellar masses, as recently reported in the literature. We stress the importance of future studies to estimate the initial N abundance variations, free of stellar evolutionary mixing processes, by observing unevolved stars in young clusters.

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“…Instead of a simple population of coeval stars formed from a chemically homogeneous protocluster cloud, individual GCs associated with the Milky Way and to several galaxies of the Local Group host stars formed with large variations of their content in C, N, O, Na, and Al, with Mg also varying in the most massive and/or most metal-poor Galactic GCs (Cohen 1978;Peterson 1980;Carretta et al 2009Carretta et al , 2010Mészáros et al 2015Mészáros et al , 2020Pancino et al 2017;Schiavon et al 2017;Wang et al 2017;O'Malley & Chaboyer 2018;Mucciarelli et al 2009;Schiavon et al 2013;Dalessandro et al 2016;Carretta et al 2017;Niederhofer et al 2017;Gilligan et al 2019;Milone et al 2020;Vanaraj et al 2021;Leath et al 2022;Larsen et al 2022). Hints for the presence of MSP with similar characteristics are also found in intermediate-age (down to ∼ 2 Gyr) massive star clusters in the Magellanic Clouds (Hollyhead et al 2017(Hollyhead et al , 2019Saracino et al 2020;Martocchia et al 2020Martocchia et al , 2021Asa'd et al 2022;Cadelano et al 2022;Salgado et al 2022). While the epoch when the clusters formed is thus not critical for MSP to be present, the mass of the cluster is.…”

Section: Introductionmentioning

confidence: 93%

Search strategies for supermassive stars in young clusters and application to nearby galaxies

A.¹,

D.²,

Messa³

et al. 2023

A&A

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Context. Supermassive stars (SMS) with masses M 10 3 − 10 4 M , formed by runaway collisions in young, massive, and dense star clusters have been invoked as a possible solution to the puzzles raised by the presence of multiple stellar populations and peculiar abundance patterns observed in globular clusters. However, such objects have not been observed so far. Aims. We developed observational strategies to search for SMS hosted within young massive clusters (thought to be the precursors of globular clusters, GCs), which could be applicable in a relatively general fashion, using both photometric and spectroscopic observations. Methods. We used theoretical predictions of spectra of SMS and SMS-hosting clusters, together with predictions from standard simple stellar populations to examine their impact on color-color diagrams and on individual optical spectral lines (primarily Hydrogen emission and absorption lines). As a first step, we apply our search strategies to a sample of ∼ 3000 young star clusters (YSC) from two nearby galaxies with multi-band observations from the HST and optical integral-field spectroscopy obtained with MUSE on the Very Large Telescope. Results. We focus on models for SMS with large radii (corresponding to T eff 7000 K), which predict strong Balmer breaks, and construct proper color-color diagrams to select the corresponding SMS-hosting cluster candidates. We show that their spectrophotometric properties are similar to that of normal clusters with ages of a few hundred Myr, which would, however, show signs of composite stellar populations, in particular the presence of nebular lines ( Hα and others). Examining the photometry, overall SEDs, and the spectra of ∼ 100 clusters with strong Balmer breaks, we have found several objects with peculiar SEDs, the presence of emission lines, or other peculiar signatures. After careful inspection of the available data, we do not find good candidates of SMS-hosting clusters. In most cases, the composite spectra can be explained by multiple clusters or H ii regions inside the aperture covered by the spectra, by contamination from a Planetary Nebula or diffuse gas, or by improper background subtraction. Furthermore, most of our candidate clusters are too faint to host SMS. Conclusions. We demonstrate a search strategy for SMS by applying it to a sample of YSCs in two nearby galaxies. Our method can be applied to larger samples and also extended to higher redshift with existing and upcoming telescopes. It should thus provide an important test for GC formation scenarios invoking such extreme stars.

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Leveraging HST with MUSE: II. Na-abundance variations in intermediate age star clusters

Cited by 9 publications

References 76 publications

The effects of stellar rotation along the main sequence of the 100 Myr old massive cluster NGC 1850

The effects of stellar rotation along the main sequence of the 100 Myr old massive cluster NGC 1850

On the Nitrogen variation in ∼2 Gyr old massive star clusters in the large Magellanic Cloud

Search strategies for supermassive stars in young clusters and application to nearby galaxies

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