Lara Piscarreta

Aims. We present the largest homogeneous VLT/X-shooter study of accretion in young stellar objects spanning the entire Orion A complex. Our sample includes 91 pre-main-sequence stars hosting a protoplanetary disk according to Spitzer photometry, selected to span a wide range of optical colors, 34 of which have complementary ALMA dust mass measurements. Methods. We derived the stellar and accretion properties using a self-consistent multicomponent fitting procedure. Our sample encompasses spectral types from K3 down to M5, corresponding to stellar masses within ∼0.8−0.1 M _⊙ . Results. The accretion-stellar luminosity ( L _acc − L _★ ) and the scaling relations of the mass-accretion rate to stellar mass ( Ṁ _acc − M _★ ) in Orion A are consistent with those of other star-forming regions that span a range of stellar densities, far-ultraviolet (FUV) irradiation fields, and ages, with all regions occupying the same locus in parameter space. For the 34 sources with complementary dust mass measurements, we present the first investigation of the Ṁ _acc and disk mass ( M _disk ) correlation in Orion A, recovering a spread consistent with that reported for other regions. Although our sample spans nearly five orders of magnitude in local FUV field strength, we found no statistically significant correlation between Ṁ _acc and FUV irradiation. However, by combining our sample with literature measurements in Lupus, the Orion nebula cluster, and σ Orionis, we found a tentative population-level decrease in the inferred disk lifetime t _disk = M _disk / Ṁ _acc toward stronger FUV environments, but the large intrinsic scatter and limited sample sizes at intermediate- and high-FUV fields prevent firm conclusions. Conclusions. The homogeneous Ṁ _acc measurements reported here provide a solid foundation for future studies of accretion and disk evolution in the diverse environments of Orion A and other star-forming regions.

Context . Orion is the closest region hosting active star formation and young OBA stars. Accurately determining the far-ultraviolet (FUV) flux at its stellar population is essential to connect stellar and protoplanetary disc properties to the environment. Aims . We (1) accurately estimated the FUV flux and its distribution at a numerous stellar population of Orion by statistically accounting for the uncertainty in parallax measurements, and (2) investigated the relation between stellar accretion and external FUV radiation field by comparing observations and disc evolution models. Methods . We selected a large stellar population in Orion (within a 6° radius of the Orion Nebula Cluster core), assigned sub-cluster memberships, and used the two-dimensional sub-cluster geometry to infer three-dimensional separations from OBA stars and compute the FUV flux (and its uncertainty) at each stellar position. We studied the accretion luminosities ( L _acc ) inferred from H _α emission in Gaia XP spectra of Orion sources and determined their detection fraction as a function of age and FUV flux. We compared the results with population synthesis models of viscous discs experiencing external photoevaporation. Results . We provided a publicly available table of FUV fluxes at ~8600 stars in Orion. Most of this stellar population is weakly FUV irradiated, 10 ⁴ G ₀ . Gaia -based L _acc decreases with age, and H _α detection fraction declines more rapidly in regions with strong FUV fluxes (≳10 ² G ₀ ) than in regions exposed to weaker FUV fluxes (≲10 ² G ₀ ), broadly consistent with the model. This result may suggest that external photoevaporation efficiently depletes strongly FUV irradiated accretion discs, but it is not sufficient to reliably confirm this conclusion. Conclusions . The tools we provided for accurately computing FUV fluxes at the Orion stellar population will be essential for targeting sources in future observations aimed at assessing the role of external photoevaporation on protoplanetary disc. Our study highlights the need for additional measurements of stellar and disc properties across the Orion population, covering the FUV flux range 1–10 ⁵ G ₀ .

Context. 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 information needed to understand the processes governing their evolution. Aims. 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, a relatively old (5–10 Myr), nearby (∼145 pc) star-forming region, with disc dust masses inferred from ALMA continuum measurements. Methods. We derived the accretion luminosity from the excess UV continuum emission with respect to the photospheric and chromospheric emission self-consistently with the stellar spectral types, extinction, and luminosity, using the FitteR for Accretion ProPErties of T Tauri stars (FRAPPE) code. We applied a new method to evaluate upper limits on the accretion luminosity. In ∼50% of cases, we could only evaluate upper limits on the accretion luminosity, either because the signal-to-noise ratio of the data was insufficient or because the measured value of the accretion luminosity was below the statistical estimate of the emission due to chromospheric activity. Results. The mass accretion rate shows a weak correlation with stellar mass, while we find no correlation with disc properties such as dust mass or gaseous disc radius. The dispersion is larger than that found in younger star-forming regions such as Lupus and Chamaeleon I, and suggests fading of the correlations with age. We find no evidence that the observed dispersion can be explained by membership in Upper Scorpius sub-groups, or by the properties of known binary systems or transition discs. Conclusions. The lack of correlation and the large dispersion of accretion rates challenge the current expectations of evolutionary models. The observed properties point to a decoupling of the inner and outer discs by the age of Upper Scorpius and a fading of the relations observed in younger star-forming regions, which calls for further development of current theoretical frameworks to be explained.

Context . Direct observations of exoplanet and brown dwarf companions with near-infrared interferometry, first enabled by the dualfield mode of VLTI/GRAVITY, provide unique measurements of the objects’ orbital motions and atmospheric compositions. Aims . Here we compile a homogeneous library of all exoplanet and brown dwarf K -band spectra observed by GRAVITY thus far. This ExoGRAVITY Spectral Library is made publicly available online. Methods . We re-reduced all the available GRAVITY dual-field high-contrast data in a uniform and highly automated way and, where companions were detected, extracted their ~2.0-2.4 μm K -band contrast spectra. We then derived stellar model atmospheres for all the employed flux references (either the host star or the swap calibrator), which we used to convert the companion contrast into companion flux spectra. Solely from the resulting GRAVITY K -band flux spectra, we extracted spectral types, spectral indices, and bulk physical properties for all the companions. Finally, and with the help of age constraints from the literature, we also derived isochronal masses for most of the companions using evolutionary models. Results . The resulting library contains R ~ 500 GRAVITY K -band spectra of 39 substellar companions from late M to late T spectral types, including the entire L-T transition. Throughout this transition, a shift from CO-dominated late M- and L-type dwarfs to CH ₄ -dominated T-type dwarfs can be observed in the K -band. The GRAVITY spectra alone constrain the objects’ bolometric luminosity to typically within ±0.15 dex. The derived isochronal masses agree with dynamical masses from the literature where available, except for HD 4113 c for which we confirm its previously reported potential underluminosity. Conclusions . Medium-resolution spectroscopy of substellar companions with GRAVITY provides insight into the carbon chemistry and the cloudiness of these objects’ atmospheres. It also constrains these objects’ bolometric luminosities, which can yield measurements of their formation entropy if combined with dynamical masses, for instance from Gaia and GRAVITY astrometry.

Context. Multiple photometric studies have reported the presence of seemingly older accreting pre-main-sequence (PMS) stars in optical colour-magnitude diagrams (CMDs). These sources appear bluer than the majority of cluster members, leading to older isochronal age estimates. Aims. We investigated this phenomenon in the Orion Nebula, which harbours a subset of stars that show infrared excess detected by Spitzer (which indicates the presence of protoplanetary discs) and H α excess emission (which traces ongoing mass accretion), yet seem to have significantly older isochronal ages (≳10 Myr) than the bulk population (∼1−3 Myr) in the r , ( r − i ) CMD. This raises the question of whether these stars are truly older or whether their photometric properties are affected by observational biases or other physical processes. Methods. We performed a detailed spectroscopic analysis of 40 Orion Nebula stars using VLT/X-Shooter, covering CMD-based isochronal ages from 1 to over 30 Myr. We derived extinction values, stellar properties, and accretion parameters by modelling the ultraviolet excess emission via a multi-component fitting procedure. The sample spans spectral types from M4.5 up to K6, and masses in the range ∼0.1−0.8 M _⊙ . Results. We demonstrate that when extinction and, more importantly, accretion effects are accurately constrained, the stellar luminosity and effective temperature of the majority of the seemingly old stars become consistent with a younger population (∼1−5 Myr). This is supported by strong lithium absorption (EW _Li ≳ 400 mÅ), which corroborates their youth, and by the accretion-to-stellar luminosity ratios ( L _acc / L _⋆ ) typical for young, accreting stars. Three of these sources, however, remain old even after our analysis, despite showing signatures consistent with ongoing accretion from a protoplanetary disc. More generally, our analysis indicates that excess continuum emission from accretion shocks affects the placement of PMS stars in the CMD, displacing sources towards bluer optical colours. Conclusions. This study highlights the critical role of accretion in shaping stellar property estimates (including age) derived from optical CMDs and emphasises the need to carefully account for accretion effects when interpreting age distributions in star-forming regions. Understanding these biases is essential for accurately constraining the early evolution of PMS stars.

Multiple photometric studies have reported the presence of seemingly older accreting pre-main sequence stars (PMS) in optical colour-magnitude diagrams (CMDs). We investigate this phenomenon in the Orion Nebula, which harbors a subset of stars that show infrared excess detected by Spitzer and Halpha excess emission, yet display significantly older isochronal ages (>10 Myr) compared to the bulk population (~1-3 Myr) in the r, (r-i) CMD. We perform a detailed spectroscopic analysis of 40 Orion Nebula stars using VLT/X-Shooter, covering CMD-based isochronal ages from 1 to over 30 Myr. We derive extinction values, stellar properties, and accretion parameters by modeling the ultraviolet excess emission through a multicomponent fitting procedure. The sample spans spectral types from M4.5 up to K6, and masses in the range ~0.1-0.8 Msun. We demonstrate that, when extinction and, most importantly, accretion effects are accurately constrained, the stellar luminosity and effective temperature of the majority of the seemingly old stars become consistent with a younger population (~1-5 Myr). This is supported by strong lithium absorption, which corroborates their youth, and by the accretion-to-stellar luminosity ratios typical for young, accreting stars. Three of these sources, however, remain old even after our analysis, despite showing signatures consistent with ongoing accretion from a protoplanetary disc. More generally, our analysis indicates that excess continuum emission from accretion shocks affects the placement of PMS stars in the CMD, displacing sources towards bluer optical colours. This study highlights the critical role of accretion in shaping the stellar properties estimates (including age) derived from optical CMDs and emphasizes the need to carefully account for accretion effects when interpreting age distributions in star-forming regions.

The first few hours of a supernova (SN) contain significant information about the progenitor system. The most modern wide-field surveys that scan the sky repeatedly every few days can discover all kinds of transients in those early epochs. At such times, some progenitor footprints may be visible, elucidating critical explosion parameters and helping to distinguish between leading explosion models. A dedicated spectroscopic classification programme using the optical spectrograph OSIRIS mounted on the Gran Telescopio de Canarias was set up to try to obtain observations of supernovae (SNe) at those early epochs. With the time awarded, we obtained spectra of 10 SN candidates, which we present here. Half of them were thermonuclear SNe, while the other half were core-collapse SNe. Most (70%) were observed within the first six days of the estimated explosion, with two being captured within the first 48 hours. We present a characterization of the spectra, together with other public ancillary photometry from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS). This project shows the need for an accompanying rapid-response spectroscopic programme for existing and future deep photometric wide-field surveys located at the right longitude to be able to trigger observations in a few hours after the discovery of the SN candidate. Both the future La Silla Southern Supernova Survey (LS4) and the Legacy Survey of Space and Time (LSST), both located in Chile, will be providing discovery and follow-up of most of the transients in the southern hemisphere. This paper demonstrates that with a rapid spectroscopic programme and stringent triggering criteria, obtaining a sample of SN with spectra within a day of the explosion is possible.

We present optical and near-infrared photometry and spectroscopy of SN 2023aew and our findings on its remarkable properties. This event, initially resembling a Type IIb supernova (SN), rebrightens dramatically ∼90 d after the first peak, at which time its spectrum transforms into that of a SN Ic. The slowly evolving spectrum specifically resembles a post-peak SN Ic with relatively low line velocities even during the second rise. The second peak, reached 119 d after the first peak, is both more luminous (M_r = −18.75 ± 0.04 mag) and much broader than those of typical SNe Ic. Blackbody fits to SN 2023aew indicate that the photosphere shrinks almost throughout its observed evolution, and the second peak is caused by an increasing temperature. Bumps in the light curve after the second peak suggest interaction with circumstellar matter (CSM) or possibly accretion. We consider several scenarios for producing the unprecedented behavior of SN 2023aew. Two separate SNe, either unrelated or from the same binary system, require either an incredible coincidence or extreme fine-tuning. A pre-SN eruption followed by a SN requires an extremely powerful, SN-like eruption (consistent with ∼10⁵¹ erg) and is also disfavored. We therefore consider only the first peak a true stellar explosion. The observed evolution is difficult to reproduce if the second peak is dominated by interaction with a distant CSM shell. A delayed internal heating mechanism is more likely, but emerging embedded interaction with a CSM disk should be accompanied by CSM lines in the spectrum, which are not observed, and is difficult to hide long enough. A magnetar central engine requires a delayed onset to explain the long time between the peaks. Delayed fallback accretion onto a black hole may present the most promising scenario, but we cannot definitively establish the power source.

Context. JWST and next-generation facilities are expected to uncover populations of free-floating objects below the deuterium-burning limit in a number of young clusters and star-forming regions. These young planetary-mass brown dwarfs have spectral types L and T; they are shaped by molecular absorption and modified by their low gravity, which makes them distinct from field objects. Aims. We aim to provide a detailed characterization of near-infrared spectra for young LT brown dwarfs, including robust spectral typing, calibrating spectral indices, identifying possible binaries, and selecting suitable spectral standards. Methods. We processed and analyzed archival spectra from VLT/X-shooter for a sample of 56 dwarfs with ages between 10 and 600 Myr and spectral types between late-M and mid-T. We redetermined their spectral types by comparing them with a set of literature templates. We assessed a large range of spectral indices, calibrated using a specifically designed literature sample. Results. We identify 15 spectral indices that are useful for spectral typing for specific spectral ranges discussed here and provide the scaling relations with spectral types. We also identify six spectral indices that can be used to separate young L dwarfs from the field ones. The equivalent-widths of the alkali lines show a correlation with age, increasing toward objects with higher surface gravities. From our sample, we confirm three that are likely to be binaries due to their anomalous spectra that appear to be better fitted by a combination of spectral types. We identify 12 objects as preliminary near-infrared spectral standards for young LT dwarfs. Conclusions. This study represents a significant step toward understanding the spectral sequence and properties of young L and T dwarfs. The relations and standards provided here will be useful for future spectroscopic work on young brown dwarfs and giant planets.

We aim to provide a detailed characterization of near-infrared spectra for young LT brown dwarfs, including robust spectral typing, calibrating spectral indices, identifying possible binaries, and selecting suitable spectral standards. We processed and analyzed archival spectra from VLT/X-shooter for a sample of 56 dwarfs with ages between 10 and 600 Myr and spectral types between late-M and mid-T. We re-determine spectral types by comparing them with a set of literature templates and assess a large range of spectral indices, calibrated using a specifically designed literature sample. We are able to identify 15 spectral indices that are useful for spectral typing for specific spectral ranges discussed here and provide the scaling relations with spectral types. Moreover, we also identify 6 spectral indices which can be used to separate young L dwarfs from the field ones. The equivalent-widths of the alkali lines show a correlation with age, increasing towards the objects with higher surface gravity. From our sample, we confirm 3 that are likely to be binaries by their anomalous spectra that appear to be better fitted by a combination of spectral types. Finally, we identify 12 objects as preliminary near-infrared spectral standards for young LT dwarfs. This paper presents a significant step toward understanding the spectral sequence and properties of young L and T dwarfs. The relations and standards provided here will be useful for future spectroscopic work on young brown dwarfs and giant planets.