arXiv subjects

M. Griggio

Publications and source records attributed to M. Griggio.

29 records · Page 2Linked to original sources

arXiv2024

HST astrometry of the closest Brown Dwarfs -- II. Improved parameters and constraints on a third body

Located at less than 2pc away, Luhman16AB (WISE.J104915.57-531906.1) is the closest pair of brown dwarfs and third closest `stellar' system to Earth. An exoplanet candidate in the Luhman16 binary system was reported in 2017 based on a weak astrometric signature in the analysis of 12 HST epochs. An additional epoch collected in 2018 and re-analysis of the data with more advanced methods further increased the significance level of the candidate, consistent with a Neptune-mass exoplanet orbiting one of the Luhman16 brown dwarf components. We report the joint analysis of these previous data together with two new astrometric HST epochs we obtained to confirm or disprove this astrometric signature. Our new analysis rules out presence of a planet orbiting one component of the Luhman16AB system for masses M > 1.5 M_Nep (Neptune masses) and periods between 400 and 5000 days. However, the presence of third bodies with masses M < 3 M_Nep and periods between 2 and 400 days (~1.1yrs) can not be excluded. Our measurements make significant improvements to the characterization of this sub-stellar binary, including its mass-ratio 0.8305+/-0.0006, individual component masses 35.4+/-0.2 M_Jup and 29.4+/-0.2 M_Jup (Jupiter masses), and parallax distance 1.9960pc +/- 50AU. Comparison of the masses and luminosities of Luhman16AB to several evolutionary models shows persistent discrepancies in the ages of the two components, but strengthens the case that this system is a member of the 510+/-95 Myr Oceanus Moving Group.

astro-ph.EP↗

arXiv2023

Photometry and astrometry with JWST -- III. A NIRCam-Gaia DR3 analysis of the open cluster NGC 2506

In the third paper of this series aimed at developing the tools for analysing resolved stellar populations using the cameras on board of the James Webb Space Telescope (JWST), we present a detailed multi-band study of the 2 Gyr Galactic open cluster NGC 2506. We employ public calibration data-sets collected in multiple filters to: (i) derive improved effective Point Spread Functions (ePSFs) for ten NIRCam filters; (ii) extract high-precision photometry and astrometry for stars in the cluster, approaching the main-sequence (MS) lower mass of ~0.1 Msun; and (iii) take advantage of the synergy between JWST and Gaia DR3 to perform a comprehensive analysis of the cluster's global and local properties. We derived a MS binary fraction of ~57.5 %, extending the Gaia limit (~0.8 Msun) to lower masses (~0.4 Msun) with JWST. We conducted a study on the mass functions (MFs) of NGC 2506, mapping the mass segregation with Gaia data, and extending MFs to lower masses with the JWST field. We also combined information on the derived MFs to infer an estimate of the cluster present-day total mass. Lastly, we investigated the presence of white dwarfs (WDs) and identified a strong candidate. However, to firmly establish its cluster membership, as well as that of four other WD candidates and of the majority of faint low-mass MS stars, further JWST equally deep observations will be required. We make publicly available catalogues, atlases, and the improved ePSFs.

astro-ph.SR↗

arXiv2023

Exploring the origin of the extended main sequence turn off in M37 through the white dwarf cooling sequence

We use new observations from the Canada-France-Hawaii Telescope to study the white dwarf cooling sequence of the open cluster M37, a cluster that displays an extended main sequence turn-off and, according to a recent photometric analysis, also a spread of initial chemical composition. By taking advantage of a first epoch collected in 1999 with the same telescope, we have been able to calculate proper motions for sources as faint as g ~ 26 (about ~ 6 magnitudes fainter than the Gaia limit), allowing us to separate cluster members from field stars. This has enabled us to isolate a sample of the white dwarf population of M37, reaching the end of the cooling sequence (at g ~ 23.5). The here-derived atlas and calibrated catalogue of the sources in the field of view is publicly released as supplementary on-line material. Finally, we present an exhaustive comparison of the white dwarf luminosity function with theoretical models, which has allowed us to exclude the age-spread scenario as the main responsible for the extended turnoff seen in the cluster colour-magnitude-diagram.

astro-ph.SR↗

arXiv2023

The broadening of the main sequence in the open cluster M38

Our recent multi-band photometric study of the colour width of the lower main sequence of the open cluster M37 has revealed the presence of a sizeable initial chemical composition spread in the cluster. If initial chemical composition spreads are common amongst open clusters, this would have major implications for cluster formation models and the foundation of the chemical tagging technique. Here we present a study of the unevolved main sequence of the open cluster M38, employing Gaia DR3 photometry and astrometry, together with newly acquired Sloan photometry. We have analysed the distribution of the cluster's lower main sequence stars with a differential colour-colour diagram made of combinations of Gaia and Sloan magnitudes, like in the study of M37. We employed synthetic stellar populations to reproduce the observed trend of M38 stars in this diagram, and found that the observed colour spreads can be explained simply by the combined effect of differential reddening across the face of the cluster and the presence of unresolved binaries. There is no need to include in the synthetic sample a spread of initial chemical composition as instead necessary to explain the main sequence of M37. Further photometric investigations like ours, as well as accurate differential spectroscopic analyses on large samples of open clusters, are necessary to understand whether chemical abundance spreads are common among the open cluster population.

astro-ph.SR↗

arXiv2023

Photometry and astrometry with JWST -- II. NIRCam distortion correction

In preparation to make the most of our own planned James Webb Space Telescope investigations, we take advantage of publicly available calibration and early-science observations to independently derive and test a geometric-distortion solution for NIRCam detectors. Our solution is able to correct the distortion to better than ~0.2 mas. Current data indicate that the solution is stable and constant over the investigated filters, temporal coverage, and even over the available filter combinations. We successfully tested our geometric-distortion solution in three cases: (i) field-object decontamination of M 92 field; (ii) estimate of internal proper motions of M 92; and (iii) measurement of the internal proper motions of the Large Magellanic Cloud system. To our knowledge, the here-derived geometric-distortion solution for NIRCam is the best available and we publicly release it, as many other investigations could potentially benefit from it. Along with our geometric-distortion solution, we also release a Python tool to convert the raw-pixels coordinates of each detector into distortion-free positions, and also to put all the ten detectors of NIRCam into a common reference system.

astro-ph.IM↗

arXiv2023

JWST unveils the brown dwarf sequence of 47 Tucanae

We have developed a technique to restore scientific usage in compromised (publicly-available) images collected with the James Webb Space Telescope (JWST) of the Galactic globular cluster NGC 104 (47 Tucanae). In spite of the degradation and limited data, we were able to recover photometry and astrometry for the coolest stellar objects ever observed within a globular cluster, possibly unveiling the brightest part of the brown dwarf (BD) sequence. This is supported by: (i) proper motion membership, derived by the comparison with positions obtained from Hubble Space Telescope archival early epochs; (ii) the predicted location of the BD sequence; and (iii) the mass function for low-mass stars derived from models. Future JWST observations will provide the necessary deep and precise proper motions to confirm the nature of the here-identified BD candidates belonging to this globular cluster.

astro-ph.SR↗

arXiv2022

The HST large programme on NGC 6752 -- IV. The White Dwarf Sequence

We present our final study of the white dwarf cooling sequence (WD CS) in the globular cluster NGC 6752. The investigation is the main goal of a dedicated Hubble Space Telescope large Program, for which all the observations are now collected. The WD CS luminosity function (LF) is confirmed to peak at m_F606W = 29.3+/-0.1, consistent within uncertainties with what has been previously reported, and is now complete down to m_F606W~29.7. We have performed robust and conclusive comparisons with model predictions that show how the theoretical LF for hydrogen envelope WD models closely follow the shape of the empirical LF. The magnitude of the peak of the observed LF is matched with ages between 12.7 and 13.5 Gyr, consistent with the cluster age derived from the main sequence turn off and subgiant branch. We also find that the impact of multiple populations within the cluster on the WD LF for m_F606W below 27.3 is negligible, and that the presence of a small fraction of helium envelope objects is consistent with the data. Our analysis reveals a possible hint of an underestimate of the cooling timescales of models in the magnitude range 28.1 < m_F606W < 28.9. Finally, we find that hydrogen envelope models calculated with a new tabulation of electron conduction opacities in the transition between moderate and strong degeneracy provide WD ages that are too small in comparison to the Main Sequence turnoff age.

astro-ph.SR↗

arXiv2022

Photometry and astrometry with JWST -- I. NIRCam Point Spread Functions and the first JWST colour-magnitude diagrams of a globular cluster

As the James Webb Space Telescope (JWST) has become fully operational, early-release data are now available to begin building the tools and calibrations for precision point-source photometry and astrometry in crowded cluster environments. Here, we present our independent reduction of NIRCam imaging of the metal-poor globular cluster M92, which were collected under Director's Discretionary Early Release Science programme ERS-1334. We derived empirical models of the Point Spread Function (PSF) for filters F090W, F150W, F277W, and F444W, and find that these PSFs: (i) are generally under-sampled (FWHM~2 pixel) in F150W and F444W and severely under-sampled (FWHM~1 pixel) in F090W and F277W; (ii) have significant variation across the field of view, up to ~15-20 %; and (iii) have temporal variations of ~3-4 % across multi-epoch exposures. We deployed our PSFs to determine the photometric precision of NIRCam for stars in the crowded, central regions of M92, measured to be at the ~0.01 mag level. We use these data to construct the first JWST colour-magnitude diagrams of a globular cluster. Employing existing stellar models, we find that the data reach almost the bottom of the M92 main sequence (~0.1 M$_{\odot}$), and reveal 24 white dwarf candidate members of M92 in the brightest portion of the white dwarf cooling sequence. The latter are confirmed through a cross-match with archival HST UV and optical data. We also detect the presence of multiple stellar populations along the low-mass main sequence of M92.

astro-ph.SR↗

arXiv2022

Astrometric star-cluster membership probability: application to the case of M 37 with Gaia EDR3

In this work, starting from the well-accepted relations in literature, we introduce a new formalism to compute the astrometric membership probabilities for sources in star clusters, and we provide an application to the case of the open cluster M 37. The novelty of our approach is a refined -- and magnitude-dependent -- modelling of the parallax distribution of the field stars. We employ the here-derived list of members to estimate the cluster's mean systemic astrometric parameters, which are based on the most recent Gaia's catalog (EDR3).

astro-ph.GA↗

arXiv2022

Signature of a chemical spread in the open cluster M37

Recent Gaia photometry of the open cluster M37 have disclosed the existence of an extended main-sequence turn off -- like in Magellanic clusters younger than about 2 Gyr -- and a main sequence that is broadened in colour beyond what is expected from the photometric errors, at magnitudes well below the region of the extended turn off, where neither age differences nor rotation rates (the candidates to explain the extended turn off phenomenon) are expected to play a role. Moreover, not even the contribution of unresolved binaries can fully explain the observed broadening. We investigated the reasons behind this broadening by making use of synthetic stellar populations and differential colour-colour diagrams using a combination of Gaia and Sloan filters. From our analysis we have concluded that the observed colour spread in the Gaia colour-magnitude diagram can be reproduced by a combination of either a metallicity spread Delta[Fe/H] ~ 0.15 plus a differential reddening across the face of the cluster spanning a total range DeltaE (B - V) ~ 0.06, or a spread of the initial helium mass fraction DeltaY ~ 0.10 plus a smaller range of reddening DeltaE (B - V) ~ 0.03. High-resolution differential abundance determinations of a sizeable sample of cluster stars are necessary to confirm or exclude the presence of a metal abundance spread. Our results raise the possibility that also individual open clusters, like globular clusters and massive star clusters, host stars born with different initial chemical compositions.

astro-ph.SR↗

arXiv2022

Astro-photometric study of M37 with Gaia and wide-field ugi-imaging

We present an astrometric and photometric wide-field study of the Galactic open star cluster M37 (NGC 2099). The studied field was observed with ground-based images covering a region of about four square degrees in the Sloan-like filters ugi. We exploited the Gaia catalogue to calibrate the geometric distortion of the large field mosaics, developing software routines that can be also applied to other wide-field instruments. The data are used to identify the hottest white dwarf (WD) member candidates of M37. Thanks to the Gaia EDR3 exquisite astrometry we identified seven such WD candidates, one of which, besides being a high-probability astrometric member, is the putative central star of a planetary nebula. To our knowledge, this is a unique object in an open cluster, and we have obtained follow-up low-resolution spectra that are used for a qualitative characterisation of this young WD. Finally, we publicly release a three-colour atlas and a catalogue of the sources in the field of view, which represents a complement of existing material.

astro-ph.SR↗