13 research outputs found
Young LMC clusters: the role of red supergiants and multiple stellar populations in their integrated light and CMDs
The optical integrated spectra of three LMC young stellar clusters (NGC 1984,
NGC 1994 and NGC 2011) exhibit concave continua and prominent molecular bands
which deviate significantly from the predictions of single stellar population
(SSP) models. In order to understand the appearance of these spectra, we create
a set of young stellar population (MILES) models, which we make available to
the community. We use archival International Ultraviolet Explorer integrated UV
spectra to independently constrain the cluster masses and extinction, and rule
out strong stochastic effects in the optical spectra. In addition, we also
analyze deep colour-magnitude diagrams of the clusters to provide independent
age determinations based on isochrone fitting. We explore hypotheses including
age-spreads in the clusters, a top-heavy initial mass function, different SSP
models and the role of red supergiant stars (RSG). We find that the strong
molecular features in the optical spectra can only be reproduced by modeling an
increased fraction of about 20 per cent by luminosity of RSG above what is
predicted by canonical stellar evolution models. Given the uncertainties in
stellar evolution at Myr ages, we cannot presently rule-out the presence of Myr
age-spreads in these clusters. Our work combines different wavelengths as well
as different approaches (resolved data as well as integrated spectra for the
same sample) in order to reveal the complete picture. We show that each
approach provides important information but in combination can we better
understand the cluster stellar populations.Comment: Accepted for publication in MNRA
Searching for Multiple Populations in the Integrated Light of the Young and Extremely Massive Clusters in the Merger Remnant NGC~7252
Recent work has shown that the properties of multiple populations within
massive stellar clusters (i.e., in the extent of their abundance variations as
well as the fraction of stars that show the anomalous chemistry) depend on the
mass as well as the age of the host cluster. Such correlations are largely
unexpected in current models for the formation of multiple populations and
hence provide essential insight into their origin. Here we extend or previous
study into the presence or absence of multiple populations using integrated
light spectroscopy of the ~Myr, massive (~\msun)
clusters, W3 and W30, in the galactic merger remnant, NGC 7252. Due to the
extreme mass of both clusters, the expectation is that they should host rather
extreme abundance spreads, manifested through, e.g., high mean [Na/Fe]
abundances. However, we do not find evidence for a strong [Na/Fe] enhancement,
with the observations being consistent with the solar value. This suggests that
age is playing a key role, or alternatively that multiple populations only
manifest below a certain stellar mass, as the integrated light at all ages
above ~Myr is dominated by stars near or above the main sequence
turn-off.Comment: 7 pages, 7 figures, accepted for publication in MNRA
An Investigation of the Use of Synthetic Spectra to Find the Ages of Stellar Clusters
The utility of stellar cluster aging methods based on integrated spectra is clearly demonstrated by its extensive use by researchers for decades. However, few studies have determined the accuracy of such methods using a significant sample of age-calibrated stellar clusters with recent models. We have amassed a sample of 27 stellar clusters, 20 with new spectra we obtained with the SOAR and Blanco 4-m Telescopes, with previously determined ages using color-magnitude diagrams (CMDs). We compared the CMD ages to ages obtained from the clusters’ integrated spectra. We find that the integrated spectra in the wavelength range 3626–6230 Ã…, when compared with high resolution computational models, provide very good age predictions. In particular, the spectral method is more robust in resolving the age-extinction degeneracy that plagues broad-band photometric aging methods. We observe no significant difference in the ability of the synthetic spectra to fit and properly age stellar clusters based on the intrinsic mass of the stellar clusters in our sample.Fil: Asa'd, Randa S.. American University of Sharjah; Emiratos Arabes Unidos. University Of Cincinnati; Estados UnidosFil: Hanson, M. M.. University Of Cincinnati; Estados UnidosFil: Ahumada, Andrea Veronica. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentin
Young LMC clusters: the role of red supergiants and multiple stellar populations in their integrated light and CMDs
The optical integrated spectra of three Large Magellanic Cloud young stellar clusters (NGC 1984, NGC 1994 and NGC 2011) exhibit concave continua and prominent molecular bands which deviate significantly from the predictions of single stellar population (SSP) models. In order to understand the appearance of these spectra, we create a set of young stellar population (MILES) models, which we make available to the community. We use archival International Ultraviolet Explorer integrated UV spectra to independently constrain the cluster masses and extinction, and rule out strong stochastic effects in the optical spectra. In addition, we also analyse deep colour–magnitude diagrams of the clusters to provide independent age determinations based on isochrone fitting. We explore hypotheses, including age spreads in the clusters, a top-heavy initial mass function, different SSP models and the role of red supergiant stars (RSG). We find that the strong molecular features in the optical spectra can be only reproduced by modelling an increased fraction of about ∼20 per cent by luminosity of RSG above what is predicted by canonical stellar evolution models. Given the uncertainties in stellar evolution at Myr ages, we cannot presently rule out the presence of Myr age spreads in these clusters. Our work combines different wavelengths as well as different approaches (resolved data as well as integrated spectra for the same sample) in order to reveal the complete picture. We show that each approach provides important information but in combination we can better understand the cluster stellar populations
To Be or Not to Be: Alicante-8, a Cluster or Not?
Recent surveys have uncovered new young massive clusters that host dozens of red supergiants (RSGs) near the inner Galaxy. However, many of them have still not been fully studied. Using Very Large Telescope/X-shooter near-infrared spectra, we present the first radial velocity analysis for the putative members of the candidate RSG cluster Alicante-8. Our results show a large dispersion of radial velocities among the candidate member stars, indicating that Alicante-8 does not seem to be a real cluster, unlike Alicante-7 and Alicante-10, which are confirmed by the distribution of the radial velocities of their RSG members. Measuring the spectral indices reveals that the assumption that the candidate stars are RSGs was incorrect, leading to the misclassification of Alicante-8 as a candidate RSG cluster. Our results imply that spectral classification based on the widely used CO band at 2.3 μm alone is not a sufficient criterion, because both red giants and RSGs can attain similar CO equivalent widths, and that spectroscopic radial velocities are needed in order to confirm unambiguously the cluster membership.This research is supported in part by the FRG Grant P.I., R. Asa'd and the Open Access Program from American University of Sharjah. It is also partially supported by the Spanish Government Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (MCIN/AEI/10.13039/501100011033/FEDER, UE) under grants PGC2018-093741-B-C21/C22 and PID2021-122397NB-C21/C22, and by the Generalitat Valenciana under grants PROMETEO/2019/041 and ASFAE/2022/017