arXiv subjects

R. Claes

Publications and source records attributed to R. Claes.

8 recordsLinked to original sources

arXiv2026

X-Shooter survey of disk accretion in Upper Scorpius II. A lack of correlation between accretion rates and disk properties

The evolution of protoplanetary discs is intertwined with the process of planet formation, growth and migration. Studies of nearby star forming regions of different ages and properties provide the necessary information needed to understand the processes dictating their evolution. This paper presents the results of a spectroscopic study of the stellar and accretion properties of a large sample of 127 stars with protoplanetary discs in the Upper Scorpius region with disc dust masses inferred from ALMA continuum measurements. The accretion luminosity is derived from the excess UV continuum emission with respect to the photospheric and chromospheric one self-consistently with the stellar spectral types, extinction and luminosity, using FRAPPE. We apply a new method to evaluate upper limits to the accretion luminosity. In ~50% of cases we evaluate upper limits on the accretion luminosity, either because the S/N of the data is insufficient or because the measured value of the accretion luminosity is below the estimate of the emission due to chromospheric activity. The results show that the mass accretion rate has a weak correlation with the stellar mass, while no correlation is observed with disc properties such as dust mass or gas disc radius. The dispersion is larger than what is found in younger star forming regions such as Lupus and Cham. I, and suggests a fading of the correlations with age. We find no evidence that membership to Upper Scorpius sub-groups, nor the properties of known binary or transition discs can explain the origin of this dispersion. The lack of correlation and large dispersion of accretion rates challenge the current expectations of evolutionary models. The observed properties point to a decoupling of the inner and outer disc by the age of Upper Scorpius and a fading of the relations observed in younger star forming regions.

astro-ph.SR↗

arXiv2026

PENELLOPE: IX. Lithium, iron and barium elemental abundances in eight nearby young clusters

We conducted a homogeneous chemical analysis of pre-main sequence stars with effective temperatures ranging from $\sim$ 3000 K to $\sim$ 5500 K in eight nearby star-forming regions (SFRs): Chamaeleon I, $\eta$ Chamaeleonis, Lupus, Orion OB1a, Orion OB1b, $\sigma$Orionis, Taurus, and Corona-Australis. Our study aims to: 1) derive the lithium abundance (A(Li)) and highlight the impact of veiling correction on both A(Li) and age determination; 2) perform the iron (Fe) and barium (Ba) abundance analysis in regions with scarce previous measurements; 3) investigate the possible Ba enhancement. The analyzed data were obtained as part of the PENELLOPE Large Program using the ESPRESSO, UVES, and X-Shooter instruments. We measured the equivalent width of the lithium line (EWLi) at $\lambda$ = 6707.8 Angstrom, from which A(Li) is derived using the curves of growth method. The Fe and Ba abundances have been measured through spectral synthesis analysis. Using the EAGLES code, we derived an upper limit on the age of the eight SFRs. Our findings underscore the necessity of veiling corrections on EWLi, which can shift A(Li) and age estimates by up to $\sim$ 0.7 dex and $\sim$ 20 Myr, respectively. Accounting for veiling, the A(Li) distributions peak in a range between 3.3 and 3.8 dex for most clusters, and the upper age limit is approximately 5 Myr for all SFRs. We successfully measured the mean iron and barium abundances in Lupus, Taurus, Cha I, and $\eta$ Cha, showing slightly sub-solar iron abundance, and a clear Ba overabundance, with [Ba/H] values reaching up to 0.75 dex.

astro-ph.SR↗

arXiv2025

The dance of dust: Investigating young stellar object dipper variability

The dipper subclass of YSOs are characterised by frequent dips in their light curves. Irregular dippers do not show periodic signatures and have dips accounting for significant proportions of their photospheric flux. Given the short timescales on which these dips occur, their driving mechanisms are linked to the inner circumstellar disc dynamics. We present the first multi-epoch analysis of 16 irregular dippers observed with X-Shooter. Investigating the properties of their dips, and in particular the analysis of the dust characteristics, we aim to understand the root of their variability, and get a glimpse of the inner disc behaviour. We employed a novel approach to measure the properties of the dips, by combining class III templates with Gaia photometry to construct the intrinsic photospheres. We measured several dip properties including the depth of the dips, near-infrared (NIR) excesses, and their optical depths as a function of wavelength. We record 20 significant dips that range in their dip properties and show no relation to one another. In almost all cases, the low optical depths and small NIR excesses are observed. Comparison of their optical depths with grain opacity models show that the dips can be explained by the presence of dust substructures containing processed grains obscuring their photospheres and/or their discs. These grain distributions can have maximum sizes as large as 20$\mu m$ and in many cases have almost grey-like extinction, while some require a strong scattering component. The findings highlight the extent of the irregularity of dippers, but also link it to the dust dynamics in the inner regions of circumstellar discs. The dust substructures causing the variability require processed dust grains to be lifted above the disc into the line of sight. Possible lifting mechanisms including disc winds, unstable accretion columns, and disc warps are discussed.

astro-ph.SR↗

arXiv2025

PENELLOPE VII: Revisiting empirical relations to measure accretion luminosity

The accretion luminosity (Lacc) in young, low-mass stars is crucial for understanding stellar formation, but direct measurements are often hindered by limited spectral coverage and challenges in UV-excess modeling. Empirical relations linking Lacc to various accretion tracers are widely used to overcome these limitations. This work revisits these empirical relations using the PENELLOPE dataset, evaluating their applicability across different star-forming regions and to accreting young objects other than Classical T Tauri Stars (CTTSs). We analyzed the PENELLOPE VLT/X-Shooter dataset of 64 CTTSs, measuring fluxes of several accretion tracers and adopting the stellar and accretion parameters derived from PENELLOPE works. We supplemented our analysis with the ODYSSEUS HST data set, which covers a wider spectral range in NUV bands. We compared the Lacc values obtained in the PENELLOPE and ODYSSEUS surveys finding statistically consistent results. Our analysis confirms that existing empirical relations, previously derived for the Lupus sample, provide reliable Lacc estimates for CTTSs in several other star-forming regions. We revisit empirical relations for accretion tracers in our dataset, based on HST-fit, with coefficients which are consistent within 1sigma with XS-fit results for most lines. We also propose a method to estimate extinction using these relations and investigate the empirical relations for Brackett lines (Br8 to Br21). The Lacc vs Lline empirical relations can be successfully used for statistical studies of accretion on young forming objects in different star-forming regions. These relations also offer a promising approach to independently estimate extinction in CTTSs. We confirm that near-infrared lines (PaB and BrG) reliably trace Lacc in high accretors, making them valuable tools for probing accretion properties of high accreting young stars not accessible in the UVB.

astro-ph.SR↗

arXiv2025

The accretion luminosity of Class I protostars

The value of the accretion luminosity during the early phases of star formation is a crucial information which helps us understand how stars form, yet it is still very difficult to obtain. We develop a new methodology to measure accretion luminosity using mid-infrared hydrogen recombination lines, and apply it to a limited sample of Class~I protostars in the Taurus and Ophiuchus star forming regions. We adopt the commonly used assumption that the properties of disk-protostar accretion in Class I objects is similar to the disk-star accretion in Class II objects. Using simultaneous observations of three hydrogen recombination lines Brg, Pfg, and Bra, we derive the mean intrinsic line ratios, and we verified that these are constant across the probed range of photospheric and accretion properties. We establish correlations between the line luminosities and accretion luminosity. We measure the extinction towards the line emission regions in Class I protostars comparing the observed line ratios to the Class II mean values. We then derive the Class I accretion luminosities from the established Class II correlations. We find that the accretion luminosity dominates the bolometric luminosity for the more embedded protostars, corresponding to lower values of the bolometric temperature. As the bolometric temperature increases above ~700K, there is a sharp drop of the contribution of the accretion from the bolometric luminosity. Our finding are in qualitative agreement with numerical simulations of star formation. We suggest that this methodology should be applied to larger and more statistically significant samples of Class I objects, for a more detailed comparison. Our results also suggest that by combining multiple infrared line ratios, it will be possible to derive a more detailed description of the dust extinction law in protostellar envelopes.

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arXiv2025

Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): Evidence of planet-disk interaction in the 2MASSJ16120668-3010270 system

The architectures of exoplanet systems are likely set during the initial planet-formation phase in the circumstellar disk. To understand this process, we have to study the earliest phases of planet formation. Complex sub-structures, believed to be driven by embedded planets, have been detected in a significant portion of disks observed at high angular resolution. We aim to extend the sample of such disks to low stellar masses and to connect the disk morphology to the expected proto-planet properties. We resolve the disk in the 2MASSJ16120668-3010270 system for the first time in scattered near-infrared light on scales of 10 au using VLT/SPHERE and reveal an exceptionally structured disk. We find an inner disk (inside 40 au) with two spiral arms, separated by a gap from an outer ring. By comparison with hydrodynamic models, we find that these structures are consistent with the presence of an embedded gas giant with a mass range between 0.1 and 5 MJup depending on the employed model. Our SPHERE observations find a tentative candidate point source within the disk gap, which may be consistent with this mass range if it indeed traces thermal emission by an embedded planet. This interpretation is somewhat strengthened by the proximity of this signal to compact mm continuum emission in the disk gap, which may trace circumplanetary material. It is, however, unclear if this tentative companion candidate could be responsible for the observed disk gap size, given its close proximity to the inner disk. The 2MASSJ16120668-3010270 system is one of only a few systems that shows this exceptional morphology of spiral arms located inside a scattered light gap and ring. We speculate that this may have to do with a higher disk viscosity compared with other systems such as PDS 70.

astro-ph.EP↗

arXiv2023

Testing external photoevaporation in the $\sigma$-Orionis cluster with spectroscopy and disk mass measurements

The evolution of protoplanetary disks is regulated by an interplay of several processes, either internal to the system or related to the environment. As most of the stars and planets have formed in massive stellar clusters, studying the effects of UV radiation on disk evolution is of paramount importance. Here we test the impact of external photoevaporation on the evolution of disks in the $\sigma$ Orionis cluster by conducting the first combined large-scale UV to IR spectroscopic and mm-continuum survey of this region. We study a sample of 50 targets located at increasing distances from the central, OB system $\sigma$ Ori. We combine new VLT/X-Shooter spectra with new and previously published ALMA measurements of disk dust and gas fluxes and masses. We confirm the previously found decrease of $M_{\rm dust}$ in the inner $\sim$0.5 pc of the cluster. This is particularly evident when considering the disks around the more massive stars ($\ge$ 0.4 $M_{\odot}$), where those located in the inner part ($<$ 0.5 pc) have $M_{\rm dust}$ about an order of magnitude lower than the more distant ones. About half of the sample is located in the region of the $\dot{M}_{\rm acc}$ vs $M_{\rm disk}$ expected by models of external photoevaporation, namely showing shorter disk lifetimes. These are observed for all targets with projected separation from $\sigma$ Ori $<$ 0.5 pc, proving that the presence of a massive stellar system affects disk evolution. External photoevaporation is a viable mechanism to explain the observed shorter disk lifetimes and lower $M_{\rm dust}$ in the inner $\sim$0.5 pc of the cluster. Follow-up observations of the low stellar mass targets are crucial to confirm the dependence of the external photoevaporation process with stellar host mass. This work confirms that the effects of external photoevaporation are significant down to impinging radiation as low as $\sim 10^{4}$ G$_0$.

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arXiv2020

Neural network based image reconstruction with astrophysical priors

With the advent of interferometric instruments with 4 telescopes at the VLTI and 6 telescopes at CHARA, the scientific possibility arose to routinely obtain milli-arcsecond scale images of the observed targets. Such an image reconstruction process is typically performed in a Bayesian framework where the function to minimize is made of two terms: the datalikelihood and the Bayesian prior. This prior should be based on our prior knowledge of the observed source. Up to now,this prior was chosen from a set of generic and arbitrary functions, such as total variation for example. Here, we present an image reconstruction framework using generative adversarial networks where the Bayesian prior is defined using state-of-the-art radiative transfer models of the targeted objects. We validate this new image reconstruction algorithm on synthetic data with added noise. The generated images display a drastic reduction of artefacts and allow a more straight forward astrophysical interpretation. The results can be seen as a first illustration of how neural networks can provide significant improvements to the image reconstruction of a variety of astrophysical sources.

astro-ph.IM↗