arXiv subjects

C. Crociati

Publications and source records attributed to C. Crociati.

7 recordsLinked to original sources

arXiv2026

The multi-age stellar populations of Terzan 5 as revealed by JWST

The James Webb Space Telescope provides an exciting opportunity to investigate stellar systems located in heavily obscured regions like the Galactic bulge. Possibly, the most enigmatic among them is Terzan 5: long classified as a globular cluster, it is now known to host distinct stellar populations with different iron abundances (ranging approximately from [Fe/H]=-$0.8$ to [Fe/H]=$+0.3$ dex). Indeed the chemical and structural properties collected so far suggest that it is the remnant of one of the primordial clumps that contributed to the early assembly of the bulge, a so-called "Bulge Fossil Fragment". Here we present a new photometric analysis of Terzan 5 based on JWST/NIRCam observations in the F115W and F200W filters, as well as archival HST/ACS optical (F606W and F814W) data. The dataset overcomes the severe and spatially variable extinction along the line of sight and yields the deepest color-magnitude diagram ever obtained for Terzan 5. Proper motion selections and high-resolution differential reddening corrections allow us to isolate bona fide cluster members and to provide an unprecedented view of the main-sequence turn-off region. We clearly identify two main components and determine their respective ages: the old, sub-solar component has an age of 12.5 $\pm$ 0.5 Gyr, while the super-solar component is significantly younger with an age of 4.7 $\pm$ 0.5 Gyr. Interestingly, we also find hints of an even younger main sequence turn-off and sub-giant branch, consistent with the presence of a further stellar component with an age of 3.8 $\pm$ 0.5 Gyr. There is also evidence of a blue plume populated by stars as bright as $m_{\rm F115W}\sim 17.4$, suggesting a prolonged period of star formation extending up to 2.5 Gyr ago.

astro-ph.GA↗

arXiv2026

Revealing the stellar population of the ultra-obscured Galactic globular cluster Glimpse-C02

In this paper, we present the results of a detailed photometric analysis of Glimpse-C02, one of the most extincted globular clusters of the Milky Way. We built a deep color magnitude diagram spanning $\approx$ 10 magnitudes, enabling the first identification of the cluster's main sequence turnoff. Due to the extreme reddening affecting the region, a differential reddening correction was necessary. The resulting reddening map reveals variations up to $\delta E(B-V) \approx 2.5$ mag. From isochrone-fitting of the differential reddening corrected color-magnitude diagram, we derived a mean color excess $E(B-V)=6.33^{+0.05}_{-0.04}$, and a distance modulus $(m-M)_0=14.00^{+0.26}_{-0.11}$, corresponding to a distance of $d=6.3^{+0.8}_{-0.3}$ kpc from the Sun, and a Galactocentric distance of $2.6^{+0.6}_{-0.7}$ kpc. This distance value, within the associated uncertainties, suggests that the cluster may be located closer to the Galactic Center compared to previous estimates, possibly supporting its classification as a bulge globular cluster. We obtained a photometric metallicity estimate of [Fe/H]$=-0.30^{+0.10}_{-0.08}$ and the first absolute age determination for Glimpse-C02, resulting in $t=11.9^{+0.7}_{-0.6}$ Gyr, as typically measured for Galactic globular clusters at this metallicity. We also derived a new estimate of the center of gravity of the cluster and determined its projected density profile from resolved star counts, finding a high King concentration parameter ($c = 1.97_{-0.67}^{+0.51}$) and a core radius $r_c =8.72^{+0.40}_{-0.35}$ arcsec. Finally, from the surface brightness profile of the system, we derived an integrated $H$-band magnitude $M_{\rm H}=-7.9$, corresponding to a mass of $M=3.57^{+0.22}_{-0.19}\times 10^4 M_{\odot}$. Thus, our work classifies Glimpse-C02 as an old and metal-rich globular cluster that is in an advanced stage of its dynamical evolution.

astro-ph.GA↗

arXiv2025

The Bulge Cluster Origin (BulCO) survey at the ESO-VLT: probing the early history of the Milky Way assembling. Design and first results in Liller1

We present the scientific goals and the very first results of the Bulge Cluster Origin (BulCO) survey. This survey has been specifically designed to perform an unprecedented chemical screening of stellar systems orbiting the Milky Way bulge, with the aim to unveil their true origin. It takes advantage of the improved performances of the spectrograph CRIRES+ operating at the ESO Very Large telescope, in the near-infrared domain. Due to the complex evolutionary history of the Milky Way, a variety of relics tracing different phenomena is expected to populate the Bulge: globular clusters formed in-situ or accreted from outside the Galaxy, nuclear star clusters of cannibalized structures, and possibly a few remnants of the proto-bulge formation process (the so-called ``bulge fossil fragments"). The signatures of the different origins are imprinted in the chemical properties of these stellar systems because specific abundance patterns provide authentic "chemical DNA" tests univocally tracing the enrichment process and, therefore, the environment where the stellar population formed. Thus, each system can provide a new piece of information on the bulge formation and evolutionary history. As first results of the survey, here we discuss the alpha-element and iron abundances of a sample of stars observed in the stellar system Liller 1, which is proposed to be a bulge fossil fragment. By combining this dataset with a recently published sample of high/mid-resolution spectra, we discuss the overall chemical properties of the stellar populations in Liller1, proving its link with the Galactic bulge and providing new constraints on its star formation history.

astro-ph.GA↗

arXiv2024

Multi-iron subpopulations in Liller 1 from high resolution H-band spectroscopy

We present a high resolution chemical study of a representative sample of 21 luminous giant stars of Liller~1, a complex stellar system in the Galactic bulge, based on H band spectra acquired with the Near InfraRed Spectrograph at KeckII. 15 stars turn out to have a subsolar iron abundance and enhanced [$\alpha$/Fe] and [Al/Fe], likely old that formed early and quickly from gas mainly enriched by type~II supernovae, and 6 stars with supersolar iron and roughly solar-scaled [$\alpha$/Fe] and [Al/Fe], likely younger, thus formed at later epochs from gas also enriched by type~Ia supernovae. Moreover, both subpopulations show enhanced [N/Fe], as in the bulge field, about solar-scaled [V/Fe], and depletion of [C/Fe] and $^{12}$C/$^{13}$C with respect to the solar values, indicating the occurrence of significant mixing in the stellar interiors of these evolved stars. The current study has also made evident that the sub-solar subpopulation shows some structuring, and the presence of a third subcomponent with iron content and [$\alpha$/Fe] enhancement somewhat intermediate between the metal-poor and metal-rich main subpopulations, has been statistically assessed, providing the chemical signature of an extended star formation with multiple bursts and of some self-enrichment.

astro-ph.GA↗

arXiv2024

X-shooter spectroscopy of Liller1 giant stars

We present the first comprehensive chemical study of a representative sample of 27 luminous red giant branch (RGB) stars belonging to Liller 1, a complex stellar system in the Galactic bulge. This study is based on medium-resolution near-infrared spectra acquired with X-shooter at the Very Large Telescope. We found a subpopulation counting 22 stars with subsolar metallicity ($<$[Fe/H]$>=-0.31\pm0.02$ and 1$\sigma$ dispersion of 0.08 dex) and with enhanced [$\alpha$/Fe], [Al/Fe], and [K/Fe] that likely formed early and quickly from gas that was mainly enriched by type II supernovae, and a metal-rich population counting 5 stars with supersolar metallicity ($<$[Fe/H]$>$=+0.22$\pm$0.03 and 1$\sigma$ dispersion of 0.06 dex) and roughly solar-scaled [$\alpha$/Fe], [Al/Fe], and [K/Fe] that formed at later epochs from gas that was also enriched by type Ia supernovae. Moreover, both subpopulations show enhanced [Na/Fe], as in the bulge field, about solar-scaled [V/Fe], and depletion of [C/Fe] and $^{12}$C/$^{13}$C with respect to the solar values. This indicates that mixing and extra-mixing processes during the RGB evolution also occur at very high metallicities. Notably, no evidence of a Na-O anticorrelation, which is considered the fingerprint of genuine globular clusters, has been found. This challenges any formation scenarios that invoke the accretion of a molecular cloud or an additional stellar system onto a genuine globular cluster. The results of this study underline the strong chemical similarity between Liller 1 and Terzan 5 and support the hypothesis that these complex stellar systems might be fossil fragments of the epoch of Galactic bulge formation.

astro-ph.GA↗

arXiv2022

Clues to the formation of Liller 1 from modeling its complex star formation history

Liller 1 and Terzan 5 are two massive systems in the Milky-Way bulge hosting populations characterized by significantly different ages ($\Delta t>7-8$ Gyr) and metallicities ($\Delta$[Fe/H]$\sim1$ dex). Their origin is still strongly debated in the literature and all formation scenarios proposed so far require some level of fine-tuning. The detailed star formation histories (SFHs) of these systems may represent an important piece of information to assess their origin. Here we present the first attempt to perform such an analysis for Liller 1. The first key result we find is that Liller 1 has been forming stars over its entire lifetime. More specifically, three broad SF episodes are clearly detected: 1) a dominant one, occurred some 12-13 Gyr ago with a tail extending for up to $\sim3$ Gyr, 2) an intermediate burst, between 6 and 9 Gyr ago, 3) and a recent one, occurred between 1 and 3 Gyr ago. The old population contributes to about $70\%$ of the total stellar mass and the remaining fraction is almost equally split between the intermediate and young populations. If we take these results at a face value, they would suggest that this system unlikely formed through the merger between an old globular cluster and a Giant Molecular Cloud, as recently proposed. On the contrary, our findings provide further support to the idea that Liller 1 is the surviving relic of a massive primordial structure that contributed to the Galactic bulge formation, similarly to the giant clumps observed in star-forming high-redshift galaxies.

astro-ph.GA↗

arXiv2020

A new class of fossil fragments from the hierarchical assembly of the Galactic bulge

The formation and evolutionary processes of galaxy bulges are still unclear, and the presence of young stars in the bulge of the Milky Way is largely debated. We recently demonstrated that Terzan 5, in the Galactic bulge, is a complex stellar system hosting stars with very different ages and a striking chemical similarity to the field population. This indicates that its progenitor was likely one of the giant structures that are thought to generate bulges through coalescence. Here we show that another globular cluster-like system in the bulge (Liller 1) hosts two distinct stellar populations with remarkably different ages: only 1-3 Gyr for the youngest, 12 Gyr for the oldest, which is impressively similar to the old component of Terzan 5. This discovery classifies Liller 1 and Terzan 5 as sites of recent star formation in the Galactic bulge and provides clear observational proof that the hierarchical assembly of primordial massive structures contributed to the formation of the Milky Way spheroid.

astro-ph.GA↗