To date, nearly two hundred planet-forming disks have been imaged at high resolution. Our propensity to study bright and extended objects does, however, bias our view of the disk demography. In this ...work, we aim to help alleviate this bias by analyzing fifteen disks targeted with VLT/SPHERE that look faint in scattered light. Sources were selected based on a low far-infrared excess from the spectral energy distribution. The comparison with the ALMA images available for a few sources shows that the scattered light surveyed by these datasets is only detected from a small portion of the disk extent. The mild anticorrelation between the disk brightness and the near-infrared excess demonstrates that these disks are self-shadowed: the inner disk rim intercepts much starlight and leaves the outer disk in penumbra. Based on the uniform distribution of the disk brightness in scattered light across all spectral types, self-shadowing would act similarly for inner rims at a different distance from the star. We discuss how the illumination pattern of the outer disk may evolve with time. Some objects in the sample are proposed to be at an intermediate stage toward bright disks from the literature, with either no shadow or with signs of azimuthally confined shadows.
We derived the atmospheric parameters and elemental abundances of Fe, O, and Na for about 120 red giant stars in the Galactic globular cluster NGC 2808. Our results are based on the analysis of ...medium-high resolution (R = 22 000-24 000) GIRAFFE spectra acquired with the FLAMES spectrograph at VLT-UT2 as a part of a project aimed at studying the Na-O anticorrelation as a function of physical parameters in globular clusters. We present the anticorrelation of Na and O abundances in NGC 2808 here, and discuss the distribution function of stars along this relation. Besides a bulk of O-normal stars with the typical composition of field halo stars, NGC 2808 seems to host two other groups of O-poor and super O-poor stars. In this regard, NGC 2808 is similar to M 13, the template cluster for the Na-O anticorrelation. However, in contrast to M 13, most stars in NGC 2808 are O-rich. This might be related to the horizontal branch morphologies that are very different in these two clusters. The average metallicity we found for NGC 2808 is Fe/H = -1.10 (rms = 0.065 dex, from 123 stars). We also found some evidence of a small intrinsic spread in metallicity, but more definitive conclusions are hampered by the presence of a small differential reddening.
Homogeneous abundances of light elements, alpha -elements, and Fe-group elements from high-resolution FLAMES spectra are presented for 76 red giant stars in NGC 6715 (M 54), a massive globular ...cluster (GC) lying in the nucleus of the Sagittarius dwarf galaxy. We also derived detailed abundances for 27 red giants belonging to the Sgr nucleus. Our abundances measure the intrinsic metallicity dispersion (~0.19 dex, rms scatter) of M 54, with the bulk of stars peaking at Fe/H ~ -1.6 and a long tail extending to higher metallicities, similar to omega Cen. The spread in these probable nuclear star clusters exceeds those of most GCs: these massive clusters are located in a region intermediate between normal GCs and dwarf galaxies. The GC M 54 exibits a Na-O anticorrelation, a typical signature of GCs, which is instead absent for the Sgr nucleus. The light elements (Mg, Al, Si) participating in the high temperature Mg-Al cycle show that the entire pattern of (anti)correlations produced by proton-capture reactions in H-burning is clearly different between the most metal-rich and most metal-poor components in the two most massive GCs in the Galaxy, confirming early results based on the Na-O anticorrelation. As in omega Cen, stars affected by most extreme processing, i.e. showing the signature of more massive polluters, are those of the metal-rich component. These observations can be understood if the burst of star formation giving birth to the metal-rich component was delayed by as much as 10-30 Myr with respect to the metal-poor one. The evolution of these massive GCs can be easily reconciled in the general scenario for the formation of GCs sketched previously by ourselves, taking into account that omega Cen may have already incorporated the surrounding nucleus of its progenitor and lost the remainder of the hosting galaxy while the two are still observable as distinct components in M 54 and the surrounding field.
The presence of multiple populations in globular clusters has been well established thanks to high-resolution spectroscopy. It is widely accepted that distinct populations are a consequence of ...different stellar generations: intracluster pollution episodes are required to produce the peculiar chemistry observed in almost all clusters. Unfortunately, the progenitors responsible have left an ambiguous signature and their nature remains unresolved. To constrain the candidate polluters, we have measured lithium and aluminium abundances in more than 180 giants across three systems: NGC 1904, NGC 2808, and NGC 362. The present investigation along with our previous analysis of M12 and M5 affords us the largest data base of simultaneous determinations of Li and Al abundances. Our results indicate that Li production has occurred in each of the three clusters. In NGC 362, we detected an M12-like behaviour, with first- and second-generation stars sharing very similar Li abundances favouring a progenitor that is able to produce Li, such as asymptotic giant branches stars. Multiple progenitor types are possible in NGC 1904 and NGC 2808, as they possess both an intermediate population comparable in lithium to the first generation stars and also an extreme population, that is enriched in Al but depleted in Li. A simple dilution model fails in reproducing this complex pattern. Finally, the internal Li variation seems to suggest that the production efficiency of this element is a function of the cluster's mass and metallicity – low-mass or relatively metal-rich clusters are more adept at producing Li.
Abundances of C and N are derived from features due to the CH G-band and to the UV CN band measured on high resolution (R less than or approximate to 40 000) UVES spectra of more than 40 dwarfs and ...subgiants in NGC 6397, NGC 6752 and 47 Tuc. Oxygen abundances (or upper limits) are available for all stars in the sample. Isotopic ratios super(12)C/ super(13)C were derived from the CH molecular band. This is the first determination of this ratio in unevolved dwarf stars in globular clusters. By enlarging the sample of subgiants in NGC 6397 studied in Gratton et al. (2001), we uncovered, for the first time, large variations in both Na and O in this cluster, too. The origin of the chemical inhomogeneities must be searched for outside the stars under scrutiny. Our data indicate that C abundances are low but not zero in unevolved or slightly evolved stars in these clusters, including in stars with large N-enhancements and O-depletions. The isotopic ratios super(12)C/ super(13)C are low, but never reach the equilibrium value of the CN-cycle. When coupled to the run of O and Na abundances, these findings may require that, in addition to CNO burning and p-captures, some triple- alpha process is also involved: previously evolved intermediate-mass AGB stars are then the most likely polluters.
We present Li and Fe abundances for 87 stars in the globular cluster M4, obtained by using high-resolution spectra collected with GIRAFFE at the Very Large Telescope. The targets range from the ...turn-off up to the red giant branch bump. The Li abundance in the turn-off stars is uniform, with an average value equal to A(Li)= 2.30 ± 0.02 dex (σ= 0.10 dex), consistent with the upper envelope of Li content measured in other globular clusters and in the halo field stars, confirming also for M4 the discrepancy with the primordial Li abundance predicted by Wilkinson Microwave Anisotropy Probe+ big bang nucleosynthesis (WMAP+BBNS). The global behaviour of A(Li) as a function of the effective temperature allows us to identify the two main drops in the Li evolution due to the first dredge-up and to the extra-mixing episode after the red giant branch bump. The measured iron content of M4 results to Fe/H=−1.10 ± 0.01 dex (σ= 0.07 dex), with no systematic offsets between dwarf and giant stars.
The behaviour of the Li and Fe abundances along the entire evolutionary path is incompatible with theoretical models including pure atomic diffusion, pointing out that an additional turbulent mixing below the convective region needs to be taken into account, able to inhibit the atomic diffusion. The measured value of A(Li) and its homogeneity in the turn-off stars allow us to put strong constraints on the shape of the Li profile inside the M4 turn-off stars. The global behaviour of A(Li) with the effective temperature can be reproduced with different pristine Li abundances, depending on the kind of adopted turbulent mixing. One cannot reproduce the global trend that starts from the WMAP+BBNS A(Li) and adopts the turbulent mixing described by Richard, Michaud & Richer with the same efficiency as that used by Korn et al. to explain the Li content in NGC 6397. In fact, such a solution is not able to well reproduce simultaneously the Li abundance observed in turn-off and red giant branch stars. However, the WMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing able to reach deeper stellar regions where the Li is burned.
We conclude that the cosmological Li discrepancy cannot be easily solved with the present, poor understanding of the turbulence in the stellar interiors, and a future effort to well understand the true nature of this non-canonical process is needed.
We present observations of the young multiple system UX Tauri to look for circumstellar disks and for signs of dynamical interactions. We obtained SPHERE/IRDIS deep differential polarization images ...in the
J
and
H
bands. We also used ALMA archival CO data. Large extended spirals are well detected in scattered light coming out of the disk of UX Tau A. The southern spiral forms a bridge between UX Tau A and C. These spirals, including the bridge connecting the two stars, all have a CO (3–2) counterpart seen by ALMA. The disk of UX Tau C is detected in scattered light. It is much smaller than the disk of UX Tau A and has a major axis along a different position angle, suggesting a misalignment. We performed
PHANTOM
SPH hydrodynamical models to interpret the data. The scattered light spirals, CO emission spirals and velocity patterns of the rotating disks, and the compactness of the disk of UX Tau C all point to a scenario in which UX Tau A has been perturbed very recently (∼1000 years) by the close passage of UX Tau C.
Our FLAMES survey of Na-O anticorrelation in globular clusters (GCs) is extended to NGC 4833, a metal-poor GC with a long blue tail on the horizontal branch (HB). We present the abundance analysis ...for a large sample of 78 red giants based on UVES and GIRAFFE spectra acquired at the ESO-VLT. We derived abundances of Na, O, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, La, and Nd. This is the first extensive study of this cluster from high resolution spectroscopy. On the scale of our survey, the metallicity of NGC 4833 is Fe/H = −2.015 ± 0.004 ± 0.084 dex (rms = 0.014 dex) from 12 stars observed with UVES, where the first error is from statistics and the second one refers to the systematic effects. The iron abundance in NGC 4833 is homogeneous at better than 6%. On the other hand, the light elements involved in proton-capture reactions at high temperature show the large star-to-star variations observed in almost all GCs studied so far. The Na-O anticorrelation in NGC 4833 is quite extended, as expected from the high temperatures reached by stars on the HB, and NGC 4833 contains a conspicuous fraction of stars with extreme O/Na ratios. More striking is the finding that large star-to-star variations are also seen for Mg, which spans a range of more than 0.5 dex in this GC. Depletions in Mg are correlated to the abundances of O and anti-correlated with Na, Al, and Si abundances. This pattern suggests the action of nuclear processing at unusually high temperatures, producing the extreme chemistry observed in the stellar generations of NGC 4833. These extreme changes are also seen in giants of the much more massive GCs M 54 and ω Cen, and our conclusion is that NGC 4833 has probably lost a conspicuous fraction of its original mass due to bulge shocking, as also indicated by its orbit.
The most massive star clusters include several generations of stars with a different chemical composition (mainly revealed by an Na-O anti-correlation) while low-mass star clusters appear to be ...chemically homogeneous. We are investigating the chemical composition of several clusters with masses of a few 104 M⊙ to establish the lower mass limit for the multiple stellar population phenomenon. Using VLT/FLAMES spectra we determine abundances of Fe, O, Na, and several other elements (α, Fe-peak, and neutron-capture elements) in the old open cluster Berkeley 39. This is a massive open cluster: M ~ 104 M⊙, approximately at the border between small globular clusters and large open clusters. Our sample size of about 30 stars is one of the largest studied for abundances in any open cluster to date, and will be useful to determine improved cluster parameters, such as age, distance, and reddening when coupled with precise, well-calibrated photometry. We find that Berkeley 39 is slightly metal-poor, ⟨Fe/H⟩ = −0.20, in agreement with previous studies of this cluster. More importantly, we do not detect any star-to-star variation in the abundances of Fe, O, and Na within quite stringent upper limits. The rms scatter is 0.04, 0.10, and 0.05 dex for Fe, O, and Na, respectively. This small spread can be entirely explained by the noise in the spectra and by uncertainties in the atmospheric parameters. We conclude that Berkeley 39 is a single-population cluster.
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