The He abundance in NGC 1850 A and B: are we observing the early stage of formation of multiple populations in a stellar cluster?

Abstract

We present the result of a sample of B-stars in the Large Magellanic Cloud young double stellar cluster NGC 1850 A and NGC 1850 B, observed with the integral-field spectrograph at the Very Large Telescope, the Multi Unit Spectroscopic Explorer. We compare the observed equivalent widths (EWs) of four He lines (4922 $\mathring{\mathrm{A}}$, 5015 $\mathring{\mathrm{A}}$, 6678 $\mathring{\mathrm{A}}$, and 7065 $\mathring{\mathrm{A}}$) with the ones determined from synthetic spectra computed with different He mass fraction (Y=0.25, 0.27, 0.30 and 0.35) with the code SYNSPEC, that takes into account the non-LTE effect. From this comparison, we determined the He mass fraction of the B stars, finding a not homogeneous distribution. The stars can be divided in three groups, He-weak (Y $<$ 0.24) and the He-normal (0.24 $⩽$ Y $⩽$ 0.26) belonging to the MS of NGC 1850 A, and the He-rich stars (0.33 $⩽$ Y $⩽$ 0.38) situated in the MS associated to NGC 1850 B. We have analyzed the stellar rotation as possible responsible of the anomalous features of the He lines in the He-rich stars. We provide a simple analysis of the differences between the observed EWs and the ones obtained from the theoretical models with different rotation velocity (V$\sin i$ = 0 and 250 Km/s). The resolution of the MUSE spectra do not allow to get a conclusive result, however our analysis support the He-enhanced hypothesis.

arXiv commentAccepted for publication by MNRAS, 10 pages, 8 figures

References in corpus (46)

• Evolution of asymptotic giant branch stars II. Optical to far-infrared isochrones with improved TP-AGB models
• WFPC2 Stellar Photometry with HSTphot
• The MUSE second-generation VLT instrument
• Multiple Stellar Populations in Globular Clusters
• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• A catalogue of masses, structural parameters and velocity dispersion profiles of 112 Milky Way globular clusters
• New Grids of ATLAS9 Model Atmospheres
• Effects of gravitational darkening on the determination of fundamental parameters in fast rotating B-type stars
• What is a globular cluster? An observational perspective
• The early evolution of Globular Clusters: the case of NGC 2808
• The VLT-FLAMES survey of massive stars: Surface chemical compositions of B-type stars in the Magellanic Clouds
• The Effect of Stellar Rotation on Colour-Magnitude Diagrams: On the apparent presence of multiple populations in intermediate age stellar clusters
• PARSEC V2.0: Stellar tracks and isochrones of low and intermediate mass stars with rotation
• Multiple stellar populations in three rich Large Magellanic Cloud star clusters
• Helium enhanced stars and multiple populations along the horizontal branch of NGC2808: direct spectroscopic measurements
• Chemical abundances in LMC stellar populations. II. The bar sample
• Multiple stellar populations in Magellanic Cloud clusters. VI. A survey of multiple sequences and Be stars in young clusters
• Super and massive AGB stars - III. Nucleosynthesis in metal-poor and very metal-poor stars - Z=0.001 and 0.0001
• Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars
• The extended Main Sequence Turn Off cluster NGC1856: rotational evolution in a coeval stellar ensemble
• A high fraction of Be stars in young massive clusters: evidence for a large population of near-critically rotating stars
• A brief introductory guide to TLUSTY and SYNSPEC
• No Evidence for Significant Age Spreads in Young Massive LMC Clusters
• Helium and Multiple Populations in the Massive Globular Cluster NGC6266 (M62)
• Multiple stellar populations in Magellanic Cloud clusters. V. The split main sequence of the young cluster NGC1866
• No evidence of chemical anomalies in the bimodal turnoff cluster NGC 1806 in the LMC
• Hubble Space Telescope survey of Magellanic Cloud star clusters. Photometry and astrometry of 113 clusters and early results
• Stellar Evolution with Rotation V: Changes in all the Outputs of Massive Star Models
• The Extended Main-Sequence Turn-off Clusters of the Large Magellanic Cloud - Missing links in Globular Cluster Evolution
• Empirical calibration of the reddening maps in the Magellanic Clouds
• Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters: Stellar Rotation diminishes, but does not eliminate, Age Spreads
• A Young Cluster With an Extended Main Sequence Turnoff: Confirmation of a Prediction of the Stellar Rotation Scenario
• Multiple stellar populations in NGC 1866. New clues from Cepheids and Colour-Magnitude Diagram
• Evolution of long-lived globular cluster stars III. Effect of the initial helium spread on the position of stars in a synthetic Hertzsprung-Russell diagram
• The effects of stellar rotation along the main sequence of the 100 Myr old massive cluster NGC 1850
• Dissecting the Extended Main Sequence Turn-off of the Young Star Cluster NGC1850
• Dynamical masses and mass-to-light ratios of resolved massive star clusters -- II. Results for 26 star clusters in the Magellanic Clouds
• Extreme helium stars: non-LTE matters Helium and hydrogen spectra of the unique objects V652 Her and HD144941
• Exploring the role of binarity in the origin of the bimodal rotational velocity distribution in stellar clusters
• Dynamics of the Young Binary LMC Cluster NGC 1850
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XXI. Binaries among multiple stellar populations
• On the determination of the He abundance distribution in globular clusters from the width of the main sequence
• TLUSTY and SYNSPEC Users's Guide IV: Upgraded Versions 208 and 54
• MUSE observations of NGC330 in the Small Magellanic Cloud. Helium abundance of bright main sequence stars
• MUSE spectroscopic observations of the Young Massive Cluster NGC1850
• Multiple stellar populations at less evolved stages-III: a possible helium spread in NGC 2210