Browsing by Department "O.A. Capodimonte"

We have started a search for supernovae as a collaboration between the University of Chile and the Cerro Tololo Inter-American Observatory, with the aim of producing a moderately distant (0.01 < z <0.10) sample of Type Ia and Type II supernovae suitable for cosmological studies. The project began in mid-1990 and continues to the present. This paper reports on the Calan/Tololo discoveries in the course of 1990, and on the spectroscopic and photometric observations gathered for these objects. All of these observations were obtained with CCDs, with the extensive collaboration of visiting astronomers. Great care was exercised in the reduction of the light curves in order to properly correct for the background light of the host galaxy of each supernova. Of the four supernovae found in 1990, one proved to be a SN II-n; the remaining three were members of the Type Ia class at redshifts that ranged between z = 0.04-0.05. One of the Type Ia events, SN 1990af, was found in the elusive premaximum phase at a redshift of z = 0.0503, and was observed through maximum light. Peak magnitudes for the other two SNe Ia, which were not observed at maximum light, were derived using a χ^2^ minimization technique to fit the data with various template curves that represent a broad range of SNe Ia light curves. In future papers we will make use of these estimates in order to discuss the Hubble diagram of SNe Ia.

On 2016 Feb 19, nine Rosetta instruments serendipitously observed an outburst of gas and dust from the nucleus of comet 67P/Churyumov-Gerasimenko. Among these instruments were cameras and spectrometers ranging from UV over visible to microwave wavelengths, in situ gas, dust and plasma instruments, and one dust collector. At 09:40 a dust cloud developed at the edge of an image in the shadowed region of the nucleus. Over the next two hours the instruments recorded a signature of the outburst that significantly exceeded the background. The enhancement ranged from 50 per cent of the neutral gas density at Rosetta to factors >100 of the brightness of the coma near the nucleus. Dust related phenomena (dust counts or brightness due to illuminated dust) showed the strongest enhancements (factors >10). However, even the electron density at Rosetta increased by a factor 3 and consequently the spacecraft potential changed from ∼-16 V to -20 V during the outburst. A clear sequence of events was observed at the distance of Rosetta (34 km from the nucleus): within 15 min the Star Tracker camera detected fast particles (∼25 m s^-1) while 100 μm radius particles were detected by the GIADA dust instrument ∼1 h later at a speed of 6 m s^-1. The slowest were individual mm to cm sized grains observed by the OSIRIS cameras. Although the outburst originated just outside the FOV of the instruments, the source region and the magnitude of the outburst could be determined.

Substellar companions at wide separation around stars hosting planets or brown dwarfs (BDs) yet close enough for their formation in the circumstellar disc are of special interest. In this Letter we report the discovery of a wide (projected separation ∼16.0", or 2400 AU, and position angle 114.61°) companion of the GQ Lup A-B system, most likely gravitationally bound to it. A VLT/X-shooter spectrum shows that this star, 2MASS J15491331-3539118, is a bonafide low-mass (∼0.15 M_⊙) young stellar object (YSO) with stellar and accretion/ejection properties typical of Lupus YSOs of similar mass, and with kinematics consistent with that of the GQ Lup A-B system. A possible scenario for the formation of the triple system is that GQ Lup A and 2MASS J15491331-3539118 formed by fragmentation of a turbulent core in the Lup I filament, while GQ Lup B, the BD companion of GQ Lup A at 0.7", formed in situ by the fragmentation of the circumprimary disc. The recent discoveries that stars form along cloud filaments would favour the scenario of turbulent fragmentation for the formation of GQ Lup A and 2MASS J15491331-3539118.

Based on observations collected at the European Southern Observatory at Paranal, under program 103.C-0200(A), and archive data from 074.C-0037(A) and 082.C-0390(A).

The hard X-ray source 2PBC J0658.0-1746 was proposed as an eclipsing magnetic cataclysmic variable of the polar type, based on optical follow-ups. We present the first spectral and timing analysis at X-ray energies with XMM-Newton, complemented with archival X-ray, optical, infrared (IR) photometry, and spectroscopy. The X-ray emission shows bright and faint phases and total eclipses recurring every 2.38 h, consistent with optical properties. This firmly identifies 2PBC J0658.0-1746 as an eclipsing polar, the second hard X-ray selected in the orbital period gap. The X-ray orbital modulation changes from cycle-to-cycle and the X-ray flux is strongly variable over the years, implying a non-stationary mass accretion rate both on short and long time-scales. The X-ray eclipses allow to refine the orbital ephemeris with period 0.09913398(4) d, and to constrain the binary inclination 79^{circ}≲ i ≲ 90^{circ} and the mass ratio 0.18< M_2/M_{ WD}< 0.40. A companion mass M₂=0.2-0.25 M_{\odot } with a radius R₂=0.24-0.26 R_{\odot } and spectral type ∼M4, at D=209^{+3}_{-2} pc, is derived. A lower limit to the white dwarf mass of ∼ 0.6 M_{\odot } is obtained from the X-ray spectrum. An upper limit to the magnetic colatitude, β ≲ 50^{circ}, and a shift in azimuth, ψ ∼ 14^{circ}, of the main accreting pole are also estimated. The optical/IR spectral energy distribution shows large excess in the mid-IR due to lower harmonics of cyclotron emission. A high-state mass accretion rate ∼ 0.4-1× 10^{-10} M_{\odot } yr^{-1}, lower than that of cataclysmic variables above the gap and close to that of systems below it, is estimated. With 2PBC J0658.0-1746, the number of hard X-ray-selected polars increases to 13 members, suggesting that they are not as rare as previously believed.

We present the results from a 500 ks Chandra observation of the z = 6.31 QSO SDSS J1030 + 0524. This is the deepest X-ray observation to date of a z 6 QSO. The QSO is detected with a total of 125 net counts in the full (0.500A0-7 keV) band and its spectrum can be modeled by a single power-law model with photon index of Γ = 1.81 ± 0.18 and full band flux of f = 3.95 × 10^-15 erg s^-1 cm^-2. When compared with the data obtained by XMM-Newton in 2003, our Chandra observation in 2017 shows a harder (∆Γ ≈ -0.6) spectrum and a 2.5 times fainter flux. Such a variation, in a timespan of 2 yr rest-frame, is unexpected for such a luminous QSO powered by a > 10⁹M_☉ black hole. The observed source hardening and weakening could be related to an intrinsic variation in the accretion rate. However, the limited photon statistics does not allow us to discriminate between an intrinsic luminosity and spectral change, and an absorption event produced by an intervening gas cloud along the line of sight. We also report the discovery of diffuse X-ray emission that extends for 30″ × 20″ southward of the QSO with a signal-to-noise ratio (S/N) of approximately six, hardness ratio of HR = 0.03^+0.20_-0.25, and soft band flux of f_{0.5- keV} = 1.1^+0.3_-0.3 × 10^-15 erg s^-1 cm^-2 , that is not associated to a group or cluster of galaxies. We discuss two possible explanations for the extended emission, which may be either associated with the radio lobe of a nearby, foreground radio galaxy (at z ≈ 1 - 2), or ascribed to the feedback from the QSO itself acting on its surrounding environment, as proposed by simulations of early black hole formation.

GIADA (Grain Impact Analyzer and Dust Accumulator) on-board the Rosetta space probe is designed to measure the momentum, mass and speed of individual dust particles escaping the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P). From 2014 August to 2016 June, Rosetta escorted comet 67P during its journey around the Sun. Here, we focus on GIADA data taken between 2015 January and 2016 February which included 67P's perihelion passage. To better understand cometary activity and more specifically the presence of dust structures in cometary comae, we mapped the spatial distribution of dust density in 67P's coma. In this manner, we could track the evolution of high-density regions of coma dust and their connections with nucleus illumination conditions, namely tracking 67P's seasons. We also studied the link between dust particle speeds and their masses with respect to heliocentric distance, I.e. the level of cometary activity. This allowed us to derive a global and a local correlation of the dust particles' speed distribution with respect to the H₂O production rate.

We characterized 67P/Churyumov-Gerasimenko's cometary activity during its inbound arc before perihelion (2014 August-2015 January). We focused on the geomorphological regions of the Northern hemisphere observed by the ESA/Rosetta space probe during this time period. The GIADA dust detector characterized the physical properties of the fluffy and compact particles ejected from the nucleus; the VIRTIS imaging spectrometer detected exposed water ice.

We characterized the 67P/Churyumov-Gerasimenko's dust activity, by analysing individual dust particle velocity and momentum measurements of Grain Impact Analyser and Dust Accumulator (GIADA), the dust detector onboard the ESA/Rosetta spacecraft, collecting dust from tens to hundreds of kilometres from the nucleus. Specifically, we developed a procedure to trace back the motion of dust particles down to the nucleus, identifying the surface's region ejecting each dust particle. This procedure has been developed and validated for the first part of the mission by Longobardo et al. and was extended to the entire GIADA data set in this work. The results based on this technique allowed us to investigate the link between the dust porosity (fluffy/compact) and the morphology of the ejecting surface (rough/smooth). We found that fluffy and compact particles, despite the lack of correlation in their coma spatial distribution (at large nucleocentric distances) induced by their different velocities, have common ejection regions. In particular, the correlation between the distributions of fluffy and compact particles is maintained up to an altitude of about 10 km. Fluffy particles are more abundant in rough terrains. This could be the result of past cometary activity that resurfaced the smooth terrains and/or of the comet formation process that stored the fluffy particles inside the voids between the pebbles. The variation of fluffy particle concentration between rough and smooth terrains agrees with predictions of comet formation models. Finally, no correlation between dust distribution on the nucleus and surface thermal properties was found.

We report the search for 7Be isotope in the outbursts of the classical nova V6595 Sgr by means of high resolution UVES observations taken at the ESO VLT in April 2021, about two weeks after discovery and under difficult circumstances due to the pandemic. Narrow absorption components with velocities at about -2620 and -2820 km/s, superposed on broader and shallow absorption, are observed in the outburst spectra for the 7BeII 313.0583, 313.1228 nm doublet resonance lines, as well as in several other elements such as CaII, FeI, MgI, NaI, HI but LiI. Using CaII K line as a reference element, we infer N(7Be)/N(H) ~ 7.4 x 10^{-6}, or ~ 9.8 x 10^{-6} when the 7Be decay is taken into account. The 7Be abundance is about half of the value most frequently measured in novae. The possible presence of over-ionization in the layers where 7Be is detected is also discussed. Observations taken at the Telescopio Nazionale Galileo (TNG) in La Palma 91 days after discovery showed prominent emission lines of Oxygen and Neon which allow to classify the nova as ONe type. Therefore, although 7Be is expected to be higher in CO novae, it is found at comparable levels in both nova types.

Multimessenger astronomy received a great boost following the discovery of kilonova (KN) AT2017gfo, the optical counterpart of the gravitational wave source GW170817 associated with the short gamma-ray burst GRB 170817A. AT2017gfo was the first KN that could be extensively monitored in time using both photometry and spectroscopy. Previously, only few candidates have been observed against the glare of short GRB afterglows. In this work, we aim to search the fingerprints of AT2017gfo-like KN emissions in the optical/NIR light curves of 39 short GRBs with known redshift. For the first time, our results allow us to study separately the range of luminosity of the blue and red components of AT2017gfo-like kilonovae in short GRBs. In particular, the red component is similar in luminosity to AT2017gfo, while the blue KN can be more than 10 times brighter. Finally, we exclude a KN as luminous as AT2017gfo in GRBs 050509B and 061201.

Photometric redshifts (photo-z) are fundamental in galaxy surveys to address different topics, from gravitational lensing and dark matter distribution to galaxy evolution. The Kilo Degree Survey (KiDS), I.e. the European Southern Observatory (ESO) public survey on the VLT Survey Telescope (VST), provides the unprecedented opportunity to exploit a large galaxy data set with an exceptional image quality and depth in the optical wavebands. Using a KiDS subset of about 25000 galaxies with measured spectroscopic redshifts, we have derived photo-z using (I) three different empirical methods based on supervised machine learning; (II) the Bayesian photometric redshift model (or BPZ); and (III) a classical spectral energy distribution (SED) template fitting procedure (LE PHARE). We confirm that, in the regions of the photometric parameter space properly sampled by the spectroscopic templates, machine learning methods provide better redshift estimates, with a lower scatter and a smaller fraction of outliers. SED fitting techniques, however, provide useful information on the galaxy spectral type, which can be effectively used to constrain systematic errors and to better characterize potential catastrophic outliers. Such classification is then used to specialize the training of regression machine learning models, by demonstrating that a hybrid approach, involving SED fitting and machine learning in a single collaborative framework, can be effectively used to improve the accuracy of photo-z estimates.

The coming decades will establish the exploration of the gravitational wave (GW) Universe over a broad frequency range by ground and space interferometers. Meanwhile, wide-field, high-cadence and sensitive surveys will span the electromagnetic spectrum from radio all the way up to TeV, as well as the high-energy neutrino window. Among the numerous classes of transients, γ-ray bursts (GRBs) have direct links with most of the hot topics that will be addressed, such as the strong gravity regime, relativistic shocks, particle acceleration processes, equation of state of matter at nuclear density, and nucleosynthesis of heavy elements, just to mention a few. Other recently discovered classes of transients that are observed throughout cosmological distances include fast radio bursts (FRBs), fast blue optical transients (FBOTs), and other unidentified high-energy transients. Here we discuss how these topics can be addressed by a mission called ASTENA (Advanced Surveyor of Transient Events and Nuclear Astrophysics, see Frontera et al. 18). Its payload combines two instruments: (i) an array of wide-field monitors with imaging, spectroscopic, and polarimetric capabilities (WFM-IS); (ii) a narrow field telescope (NFT) based on a Laue lens operating in the 50-600 keV range with unprecedented angular resolution, polarimetric capabilities, and sensitivity.

Context. The majority of young stars form in multiple systems, the properties of which can significantly impact the evolution of any circumstellar disks. Aims. We investigate the physical and chemical properties of the equal-mass, small-separation (∼66 milliarcsecond, ∼9 au) binary system DF Tau. Previous spatially resolved observations indicate that only DF Tau A has a circumstellar disk, while DF Tau B does not, as concluded by a lack of accretion signatures and a near-infrared excess. Methods. We present JWST-MIRI MRS observations of DF Tau. The MIRI spectrum shows emission from a forest of H2O lines and emission from CO, C2H2, HCN, CO2, and OH. Local thermodynamic equilibrium slab models were used to determine the properties of the gas. The binary system is not spatially or spectrally resolved in the MIRI observations; therefore, we analyzed high spatial and spectral resolution observations from ALMA, VLTI-GRAVITY, and IRTF-iSHELL to aid in the interpretation of the molecular emission observed with JWST. Results. The 1.3 mm ALMA observations show two equal-brightness sources of compact (R ≲ 3 au) continuum emission that are detected at high significance, with separations consistent with astrometry from VLTI-GRAVITY and movement consistent with the known orbital parameters of the system. We interpret this as a robust detection of the disk around DF Tau B, which we suggest may host a small (∼1 au) cavity; such a cavity would reconcile all of the observations of this source. In contrast, the disk around DF Tau A is expected to be a full disk, and spatially and spectrally resolved dust and gas emission traced by ground-based infrared observations point to hot, close-in (≲0.2 au) material around this star. High-temperature emission (∼500–1000 K) from H2O, HCN, and potentially C2H2 in the MIRI data likely originates in the disk around DF Tau A, while a cold H2O component (≲200 K) with an extended emitting area is consistent with an origin from both disks. Conclusions. Given the unique characteristics of this binary pair, complementary observations are critical for constraining the properties of these disks. Despite the very compact outer disk properties, the inner disk composition and the conditions of the DF Tau disks are remarkably similar to those of isolated systems, suggesting that neither the outer disk evolution nor the close binary nature are driving factors in setting the inner disk chemistry in this system. However, constraining the geometry of the disk around DF Tau B, via higher angular resolution ALMA observations for instance, would provide additional insight into the properties of the mid-infrared gas emission observed with MIRI. JWST observations of spatially resolved binaries, at a range of separations, will be important for understanding the impact of binarity on inner disk chemistry more generally.

Hyper-hemispherical lenses are optical objective lenses with an ultra-wide field of view. Due to their extreme field of view, the geometric calibration introduces several setup difficulties. Various methods have been developed for simplifying the geometric calibration of a camera in the close-range field by defining intrinsic and extrinsic calibration parameters in a limited ambience of a clean room and taking advantage of simple calibration chessboards. A well-known example in this regard is Zhang’s approach for pinhole cameras. A similar approach was pursued by Scaramuzza to model more extensive cameras that include fisheye and panoramic lenses. However, despite this toolbox’s efficiency, it is ineffectual in the case of a hyper-hemispherical lens because of model limitations in the approximation of the corresponding projection system. In this study, we define and validate what we believe to be a novel a-central model extension (AME) for the description of hyper-hemispherical lenses that is able to overcome these limitations and to describe more accurately the physical behavior introduced by the design of these lenses. For this apparently new a-central model, we obtain significantly improved calibration results for one of the cameras designed in our laboratory and planned to become a planetary payload of an ESA mission.

Deep observations of the dwarf elliptical (dE) galaxy NGC 1396 (M_V = -16.60, Mass ∼4 × 10⁸ M_☉), located in the Fornax cluster, have been performed with the Very Large Telescope/Multi Unit Spectroscopic Explorer spectrograph in the wavelength region from 4750 to 9350 Å. In this paper, we present a stellar population analysis studying chemical abundances, the star formation history (SFH) and the stellar initial mass function (IMF) as a function of galactocentric distance. Different, independent ways to analyse the stellar populations result in a luminosity-weighted age of ∼6 Gyr and a metallicity [Fe/H]∼ -0.4, similar to other dEs of similar mass. We find unusually overabundant values of [Ca/Fe] ∼+ 0.1, and underabundant Sodium, with [Na/Fe] values around -0.1, while [Mg/Fe] is overabundant at all radii, increasing from ∼+ 0.1 in the centre to ∼+ 0.2 dex. We notice a significant metallicity and age gradient within this dwarf galaxy. To constrain the stellar IMF of NGC 1396, we find that the IMF of NGC 1396 is consistent with either a Kroupa-like or a top-heavy distribution, while a bottom-heavy IMF is firmly ruled out. An analysis of the abundance ratios, and a comparison with galaxies in the Local Group, shows that the chemical enrichment history of NGC 1396 is similar to the Galactic disc, with an extended SFH. This would be the case if the galaxy originated from a Large Magellanic Cloud-sized dwarf galaxy progenitor, which would lose its gas while falling into the Fornax cluster.

Very-low-mass stars (those less than 0.3 solar masses) host orbiting terrestrial planets more frequently than other types of stars. The compositions of those planets are largely unknown but are expected to relate to the protoplanetary disk in which they form. We used James Webb Space Telescope mid-infrared spectroscopy to investigate the chemical composition of the planet-forming disk around ISO-ChaI 147, a 0.11-solar-mass star. The inner disk has a carbon-rich chemistry; we identified emission from 13 carbon-bearing molecules, including ethane and benzene. The high column densities of hydrocarbons indicate that the observations probe deep into the disk. The high carbon-to-oxygen ratio indicates radial transport of material within the disk, which we predict would affect the bulk composition of any planets forming in the disk.

Accreting white dwarfs are often found in close binary systems with orbital periods ranging from tens of minutes to several hours. In most cases, the accretion process is relatively steady, with significant modulations only occurring on timescales of ~days or longer^1,2. Here we report the discovery of abrupt drops in the optical luminosity of the accreting white dwarf binary system TW Pictoris by factors up to 3.5 on timescales as short as 30 minutes. The optical light curve of this binary system obtained by the Transiting Exoplanet Survey Satellite (TESS) clearly displays fast switches between two distinct intensity modes that probably track the changing mass accretion rate onto the white dwarf. In the low mode, the system also displays magnetically gated accretion bursts^3-5, which implies that a weak magnetic field of the white dwarf truncates the inner disc at the co-rotation radius in this mode. The properties of the mode switching observed in TW Pictoris appear analogous to those observed in transitional millisecond pulsars^6-10, where similar transitions occur, although on timescales of ~tens of seconds. Our discovery establishes a previously unrecognized phenomenon in accreting white dwarfs and suggests a tight link to the physics governing magnetic accretion onto neutron stars.

The science case on studies of accretion and outflows in low-mass (<1.5M⊙) young stellar objects (YSOs) with the new CUBES instrument is presented. We show the need for a high-sensitivity, near-ultraviolet (NUV) spectrograph like CUBES, with a resolving power at least four times that of X-Shooter and combined with UVES via a fibrelink for simultaneous observations. Simulations with the CUBES exposure time calculator and the end-to-end software show that a significant gain in signal-to-noise can be achieved compared to current instruments, for both the spectral continuum and emission lines, including for relatively embedded YSOs. Our simulations also show that the low-resolution mode of CUBES will be able to observe much fainter YSOs (V ∼ 22 mag) in the NUV than we can today, allowing us extend studies to YSOs with background-limited magnitudes. The performance of CUBES in terms of sensitivity in the NUV will provide important new insights into the evolution of circumstellar disks, by studying the accretion, jets/winds and photo-evaporation processes, down to the low-mass brown dwarf regime. CUBES will also open-up new science as it will be able to observe targets that are several magnitudes fainter than those reachable with current instruments, facilitating studies of YSOs at distances of ∼ kpc scale. This means a step-change in the field of low-mass star formation, as it will be possible to expand the science case from relatively local star-forming regions to a large swathe of distances within the Milky Way.

Accretion bursts from low-mass young stellar objects (YSOs) are known for many decades. In recent years, the first accretion bursts of massive YSOs (MYSOs) have been observed. These phases of intense protostellar growth are of particular importance for studying massive star formation. Bursts of MYSOs are accompanied by flares of Class II methanol masers (hereafter masers), caused by an increase in exciting mid-infrared (MIR) emission. The G323.46$-$0.08 (hereafter G323) event extends the small sample of known MYSO bursts. Maser observations of the MYSO G323 show evidence of a flare, which was presumed to be caused by an accretion burst. This should be verified with IR data. We used time-dependent radiative transfer (TDRT) to characterize the heating and cooling timescales for eruptive MYSOs and to infer the main burst parameters. The G323 accretion burst is confirmed. It reached its peak in late 2013/early 2014 with a Ks-band increase of 2.5mag. TDRT indicates that the duration of the thermal afterglow in the far-infrared (FIR) can exceed the burst duration by years. The latter was proved by SOFIA observations, which indicate a flux increase of $(14.2\pm4.6)$% at $70\, \rm \mu m$ and $(8.5\pm6.1)$% at $160\, \mu$m in 2022 (2 years after the burst end). A one-sided light echo emerged that was propagating into the interstellar medium. The G323 burst is probably the most energetic MYSO burst observed so far. Within $8.4 \rm \, yrs$, an energy of $(0.9\pm_{0.8}^{2.5}) \times 10^{47}\,\rm erg$ was released. The short timescale points to the accretion of a compact body, while the burst energy corresponds to an accumulated mass of at least $(7\pm_{6}^{20})\,M_{Jup}$ and possibly even more if the protostar is bloated. In this case, the accretion event might have triggered protostellar pulsations, which give rise to the observed maser periodicity.

The process of accretion in classical T Tauri stars (CTTSs) has been observed to vary on different timescales. Studying this variability is vital to understanding a star's evolution and provides insight into the complex processes at work within. Understanding the dichotomy between continuum veiling and emission line veiling is integral to accurately measuring the amount of veiling present in stellar spectra. Here, 15 roughly consecutive nights of optical spectroscopic data from the spectropolarimeter ESPaDOnS are utilised to characterise the short-term accretion activity in the CTTS, RU Lup, and investigate its relationship with the veiling in the LiI 6707A absorption line. The accretion-tracing HI Balmer series emission lines were studied and used to obtain the accretion luminosity (Lacc) and mass accretion rate (Macc) for each night, which vary by a factor of ~2 between the brightest and dimmest nights. We also measured the veiling using multiple photospheric absorption lines (NaI 5688A, MnI 6021A, and LiI 6707A) for each night. We find the LiI 6707A line provides measurements of veiling that produce a strong, positive correlation with Lacc in the star. When corrected for Li depletion, the average veiling measured in the LiI 6707A line is r_LiI(avg)~3.25+/-0.20, which is consistent with the other photospheric lines studied (r_avg~3.28+/-0.65). We measured short timescale variability in the Lacc and Macc that are intrinsic and not due to geometric effects. Upon comparing the changes in veiling and Lacc, we find a strong, positive correlation. This study provides an example of how this correlation can be used as a tool to determine whether a measured variability is due to extinction or an intrinsic change in accretion. As the determination of veiling is an independent process from measuring Lacc, their relationship allows further exploration of accretion phenomena in young stars.