Abstract James Webb Space Telescope (JWST) observations have been demonstrated to be efficient in detecting multiple stellar populations in globular clusters (GCs) in the low-mass regime of M dwarfs. ...We present an overview, and first results, of different projects that can be explored by using the JWST observations gathered under program GO2560 for 47 Tucanae, the first program entirely devoted to the investigation of multiple populations in very-low-mass stars, which includes spectroscopic data for the faintest GC stars for which spectra are available. Our color–magnitude diagram (CMD) shows some substructures for ultracool stars, including gaps and breaks in slope. In particular, we observe both a gap and a minimum in the F322W2 luminosity function less than 1 mag apart, and discuss which it could be associated with the H-burning limit. We detect stars fainter than this minimum, very likely brown dwarfs. We corroborate the ubiquity of the multiple populations across different masses, from ∼0.1 M ⊙ up to red giants (∼0.8 M ⊙ ). The oxygen range inferred for the M dwarfs, both from the CMD and from the spectra of two M dwarfs associated with different populations, is similar to that observed for giants. We have not detected any difference between the fractions of stars in distinct populations across stellar masses ≳ 0.1 M ⊙ . This work demonstrates the JWST's capability in uncovering multiple populations within M dwarfs and illustrates the possibility to analyze very-low-mass stars in GCs approaching the H-burning limit and the brown-dwarf sequence.
Abstract
The “chromosome maps” (ChMs) of globular clusters (GCs) have revealed that these ancient structures are inhomogeneous in metallicity in various ways and in different natures. Type II GCs ...generally display larger variations, sometimes coupled with slow neutron-capture (
s
) element enrichments on the ChMs redder sequences, which have been interpreted as due to multiple generations of stars. On the other hand, most GCs have inhomogeneous first populations (1P) in the form of large ranges in the Δ
F275W,F814W
values, pointing toward a not fully mixed pristine molecular cloud. We analyze the chemical composition of GC 47 Tucanae, which shows both inhomogeneous 1P stars and, although not formally a Type II GC, hosts a small number of stars distributed on the red side of the main stream of ChM stars. Our results suggest that 1P stars are inhomogeneous in the overall metallicity, with variations on the order of ∼0.10 dex in all the chemical species. The
anomalous
stars distributed on a redder sequence of the ChM are further enriched in metals but have no evidence for a significant enrichment in the
s
elements. Our three second population stars located on the
normal
component of the map have metallicities similar to those of the metal-richer 1P oup, suggesting that this population formed from these stars. Although three stars is a too-small sample to draw strong conclusions, the low spread in metals of these objects might point toward formation in a fully mixed medium, possibly after a cooling flow phase.
ABSTRACT
Disentangling distinct stellar populations along the red-giant branches (RGBs) of globular clusters (GCs) is possible by using the pseudo-two-colour diagram dubbed chromosome map (ChM). One ...of the most intriguing findings is that the so-called first-generation (1G) stars, characterized by the same chemical composition of their natal cloud, exhibit extended sequences in the ChM. Unresolved binaries and internal variations in helium or metallicity have been suggested to explain this phenomenon. Here, we derive high-precision Hubble Space Telescope photometry of the GCs NGC 6362 and NGC 6838 and build their ChMs. We find that both 1G RGB and main-sequence (MS) stars exhibit wider ChM sequences than those of second-generation (2G). The evidence of this feature even among unevolved 1G MS stars indicates that chemical inhomogeneities are imprinted in the original gas. We introduce a pseudo-two-magnitude diagram to distinguish between helium and metallicity, and demonstrate that star-to-star metallicity variations are responsible for the extended 1G sequence. Conversely, binaries provide a minor contribution to the phenomenon. We estimate that the metallicity variations within 1G stars of 55 GCs range from less than Fe/H∼0.05 to ∼0.30 and mildly correlate with cluster mass. We exploit these findings to constrain the formation scenarios of multiple populations showing that they are qualitatively consistent with the occurrence of multiple generations. In contrast, the fact that 2G stars have more homogeneous iron content than the 1G challenges the scenarios based on accretion of material processed in massive 1G stars on to existing protostars.
Abstract We present the first spectroscopic estimates of the chemical abundance of M dwarf stars in a globular cluster (GC), namely 47 Tucanae. By exploiting NIRSpec on board the James Webb Space ...Telescope, we gathered low-resolution spectra for 28 stars with masses in the range ∼0.4–0.5 M ⊙ . The spectra are strongly affected by the H 2 O water vapor bands, which can be used as indicators of oxygen abundance. The spectral analysis reveals that the target stars feature a different O abundance, with a difference of ∼0.40 dex between the first and the most polluted second population. The observed range is similar to that observed among red giant stars. This result reinforces previous findings based on the analysis of photometric diagrams, including the “chromosome maps,” providing a first, and more direct, evidence of light element variations in the M dwarfs’ mass regime. The observation that the multiple populations, with their variations in light elements, exhibit the same patterns from the lower main sequence all the way to the red giant branch further strengthens the notion that multiple stellar populations in GCs formed in a series of bursts of star formation.
ABSTRACT
We use images collected with the near-infrared camera (NIRCam) onboard the JWST and with the Hubble Space Telescope (HST) to investigate multiple populations at the bottom of the main ...sequence (MS) of 47 Tucanae. The mF115W versus mF115W − mF322W2 colour–magnitude diagram (CMD) from NIRCam shows that, below the knee, the MS stars span a wide colour range, where the majority of M-dwarfs exhibit blue colours, and a tail of stars are distributed towards the red. A similar pattern is observed from the mF160W versus mF110W − mF160W CMD from HST, and multiple populations of M-dwarfs are also visible in the optical mF606W versus mF606W − mF814W CMD. The NIRCam CMD shows a narrow sequence of faint MS stars with masses smaller than $0.1\, \mathcal {M}_{\odot }$. We introduce a chromosome map of M-dwarfs that reveals an extended first population and three main groups of second-population stars. By combining isochrones and synthetic spectra with appropriate chemical composition, we simulate colours and magnitudes of different stellar populations in the NIRCam filters (at metallicities Fe/H = −1.5 and Fe/H = −0.75) and identify the photometric bands that provide the most efficient diagrams to investigate the multiple populations in globular clusters. Models are compared with the observed CMDs of 47 Tucanae to constrain M-dwarfs’ chemical composition. Our analysis suggests that the oxygen range needed to reproduce the colours of first- and second-population M-dwarfs is similar to that inferred from spectroscopy of red giants, constraining the proposal that the chemical variations are due to mass transfer phenomena in proto-clusters.
ABSTRACT
The presence of differential reddening in the direction of Galactic globular clusters (GCs) has proven to be a serious limitation in the traditional colour–magnitude diagram (CMD) analysis. ...Here, we estimate local reddening variations in the direction of 56 Galactic GCs. To do that, we use the public catalogues derived as part of the Hubble Space Telescope UV Legacy Survey of Galactic GCs, which include photometry in the F275W, F336W, F438W, F606W, and F814W filters. We corrected photometry for differential reddening and found that for 21 out of 56 GCs the adopted correction procedure significantly improved the CMDs. Moreover, we measure the reddening law in the direction of these clusters, finding that RV exhibits a high level of variability within the Galaxy, ranging from ∼2.0 to ∼4.0. The updated values of RV have been used to improve the determination of local reddening variations and derive high-resolution reddening maps in the direction of the 21 highly-reddened targets within our sample. To compare the results of the different clusters, we compute the 68th percentile of the differential-reddening distribution, $\sigma _{\Delta A_{\rm F814W}}$. This quantity ranges from 0.003 to 0.030 mag and exhibits a significant anticorrelation with the absolute module of the Galactic latitude and a strong correlation with the average reddening in the direction of each cluster. Therefore, highly-reddened GCs located in the proximity of the Galactic plane typically show higher differential-reddening variations across their field of view.
ABSTRACT
Pseudo two-colour diagrams or Chromosome maps (ChM) indicate that NGC 2808 host five different stellar populations. The existing ChMs have been derived by the Hubble Space Telescope ...photometry, and comprise of stars in a small field of view around the cluster centre. To overcome these limitations, we built a ChM with U, B, I photometry from ground-based facilities that disentangle the multiple stellar populations of NGC 2808 over a wider field of view. We used spectra collected by GIRAFFE@VLT in a sample of 70 red giant branch and seven asymptotic giant branch (AGB) stars to infer the abundances of C, N, O, Al, Fe, and Ni, which combined with literature data for other elements (Li, Na, Mg, Si, Ca, Sc, Ti, Cr, and Mn), and together with both the classical and the new ground-based ChMs, provide the most complete chemical characterization of the stellar populations in NGC 2808 available to date. As typical of the multiple population phenomenon in globular clusters, the light elements vary from one stellar population to another; whereas the iron peak elements show negligible variation between the different populations (at a level of ≲0.10 dex). Our AGB stars are also characterized by the chemical variations associated with the presence of multiple populations, confirming that this phase of stellar evolution is affected by the phenomenon as well. Intriguingly, we detected one extreme O-poor AGB star (consistent with a high He abundance), challenging stellar evolution models that suggest that highly He-enriched stars should avoid the AGB phase and evolve as AGB-manqué star.
Abstract
Evidence that multiple populations (MPs) are common properties of globular clusters (GCs) has accumulated over the past decades from clusters in the Milky Way and in its satellites. This ...finding has revived research into GCs, and suggested that their formation at high redshift must have been a much more complex phenomenon than imagined before. However, most information on MPs is limited to nearby GCs. The main limitation is that most studies of MPs rely on resolved stars, posing a major challenge to the investigation of the MP phenomenon in distant galaxies. Here we search for integrated colors of old GCs that are sensitive to the MP phenomenon. To do this, we exploit integrated magnitudes of simulated GCs with MPs, and multiband Hubble Space Telescope photometry of 56 Galactic GCs, where MPs are widely studied, and characterized as part of the UV Legacy Survey of Galactic GCs. We find that both integrated
C
F275W,F336W,F438W
and
m
F275W
−
m
F814W
colors strongly correlate with the iron abundance of the host GC. To second order, the pseudo two-color diagram built with these integrated colors is sensitive to the MP phenomenon. In particular, once the dependence on cluster metallicity is removed, the color residuals depend on the maximum internal helium variation within GCs and on the fraction of second-generation stars. This diagram, which we define here for Galactic GCs, has the potential to detect and characterize MPs from integrated photometry of old GCs, thus providing the possibility to extend their investigation outside the Local Group.
Abstract
Recent work has shown that near-infrared (NIR) Hubble Space Telescope (HST) photometry allows us to disentangle multiple populations (MPs) among M dwarfs of globular clusters (GCs) and to ...investigate this phenomenon in very-low-mass (VLM) stars. Here, we present the color–magnitude diagrams of nine GCs and the open cluster NGC 6791 in the F110W and F160W bands of HST, showing that the main sequences (MSs) below the knee are either broadened or split, thus providing evidence of MPs among VLM stars. In contrast, the MS of NGC 6791 is consistent with a single population. The color distribution of M dwarfs dramatically changes between different GCs, and the color width correlates with the cluster mass. We conclude that the MP ubiquity, variety, and dependence on GC mass are properties common to VLM and more-massive stars. We combined UV, optical, and NIR observations of NGC 2808 and NGC 6121 (M4) to identify MPs along with a wide range of stellar masses (∼0.2–0.8
⊙
), from the MS turnoff to the VLM regime, and measured, for the first time, their mass functions (MFs). We find that the fraction of MPs does not depend on the stellar mass and that their MFs have similar slopes. These findings indicate that the properties of MPs do not depend on stellar mass. In a scenario where the second generations formed in higher-density environments than the first generations, the possibility that the MPs formed with the same initial MF would suggest that it does not depend on the environment.
ABSTRACT
Young and intermediate-age star clusters of both Magellanic Clouds exhibit complex colour–magnitude diagrams. In addition to the extended main-sequence turn-offs (eMSTOs), commonly observed ...in star clusters younger than ∼2 Gyr, the clusters younger than ∼800 Myr exhibit split main sequences (MSs). These comprise a blue MS, composed of stars with low rotation rates, and a red MS, which hosts fast-rotating stars. While it is widely accepted that stellar populations with different rotation rates are responsible for the eMSTOs and split MSs, their formation and evolution are still debated. A recent investigation of the ∼1.7-Gyr-old cluster NGC 1783 detected a group of eMSTO stars extremely dim in ultraviolet (UV) bands. Here, we use multiband Hubble Space Telescope photometry to investigate five star clusters younger than ∼200 Myr, including NGC 1805, NGC 1818, NGC 1850, and NGC 2164 in the Large Magellanic Cloud, and the Small Magellanic Cloud cluster NGC 330. We discover a group of bright MS stars in each cluster that are significantly dim in the F225W and F275W bands, similar to what is observed in NGC 1783. Our result suggests that UV-dim stars are common in young clusters. The evidence that most of them populate the blue MS indicates that they are slow rotators. As a by-product, we show that the star clusters NGC 1850 and BHRT 5b exhibit different proper motions, thus corroborating the evidence that they are not gravitationally bound.
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