Physical properties of_molecular_clouds_in_the_inner_disk_of_m31

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Sérgio Sacani

Artigo clássico sobre um dos grandes mistérios da galáxia M31.

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Results from telescope_array_experiment

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Alma observations of_feeding_and_feedback_in_nearby_seyfert_galaxies_outflow_...

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Ecography of young_star_reveals_their_evolution

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Detonating failed deflagration_model_of_themonuclear_supernovae_explosion_dyn...

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Galactic bridges and_tails

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Xray and protostars_in_the_trifid_nebula

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The birth of_a_galaxy_propelling_reionisation_with_the_faintest_galaxies

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Rapid growth of_seed_black_holes_in_the_early_universe_by_supra_exponential_a...

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Using the milk_way_satellites_to_study_interactions_between_cold_dark_matter_...

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Stellar tidal streams_in_spiral_galaxies_of_the_local_volume

Sérgio Sacani

Rapid formation of_large_dust_grains_in_the_luminous_supernova_sn2010jl

Sérgio Sacani

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Gj 832c a_super_earth_in_the_habitable_zone

Sérgio Sacani

Destacado (19)

Results from telescope_array_experiment

Alma observations of_feeding_and_feedback_in_nearby_seyfert_galaxies_outflow_...

Ecography of young_star_reveals_their_evolution

An unindetified line_in_xray_spectra_of_the_adromeda_galaxy_and_perseus_galax...

Propagation of highly_efficient_star_formation_in_ngc7000

Non xrays from_the_very_nearby_typeia_sn_2014j_constraints_on_its_environment

Detonating failed deflagration_model_of_themonuclear_supernovae_explosion_dyn...

Widespread mixing and_burrial_of_earth_hadean_crust_by_asteroid_impacts

The three dimensional_structure_of_the_eta_carinae_homunculus

Galactic bridges and_tails

Xray and protostars_in_the_trifid_nebula

The birth of_a_galaxy_propelling_reionisation_with_the_faintest_galaxies

Rapid growth of_seed_black_holes_in_the_early_universe_by_supra_exponential_a...

Detection of an_unindentified_emission_line_in_the_stacked_x_ray_spectrum_of_...

Using the milk_way_satellites_to_study_interactions_between_cold_dark_matter_...

Stellar tidal streams_in_spiral_galaxies_of_the_local_volume

Rapid formation of_large_dust_grains_in_the_luminous_supernova_sn2010jl

Kinematics and simulations_of_the_stellar_stream_in_the_halo_of_the_umbrella_...

Gj 832c a_super_earth_in_the_habitable_zone

Más de Sérgio Sacani

Theoretical predictions and observational data indicate a class of sub-Neptune exoplanets may have water-rich interiors covered by hydrogen-dominated atmospheres. Provided suitable climate conditions, such planets could host surface liquid oceans. Motivated by recent JWST observations of K2-18 b, we self-consistently model the photochemistry and potential detectability of biogenic sulfur gases in the atmospheres of temperate sub-Neptune waterworlds for the first time. On Earth today, organic sulfur compounds produced by marine biota are rapidly destroyed by photochemical processes before they can accumulate to significant levels. Domagal-Goldman et al. suggest that detectable biogenic sulfur signatures could emerge in Archean-like atmospheres with higher biological production or low UV flux. In this study, we explore biogenic sulfur across a wide range of biological fluxes and stellar UV environments. Critically, the main photochemical sinks are absent on the nightside of tidally locked planets. To address this, we further perform experiments with a 3D general circulation model and a 2D photochemical model (VULCAN 2D) to simulate the global distribution of biogenic gases to investigate their terminator concentrations as seen via transmission spectroscopy. Our models indicate that biogenic sulfur gases can rise to potentially detectable levels on hydrogen-rich water worlds, but only for enhanced global biosulfur flux (20 times modern Earth’s flux). We find that it is challenging to identify DMS at 3.4 μm where it strongly overlaps with CH4, whereas it is more plausible to detect DMS and companion byproducts, ethylene (C2H4) and ethane (C2H6), in the mid-infrared between 9 and 13 μm. Unified Astronomy Thesaurus concepts: Exoplanet atmospheres (487); Exoplanet

Biogenic Sulfur Gases as Biosignatures on Temperate Sub-Neptune Waterworlds

Sérgio Sacani

Context. WASP-76 b has been a recurrent subject of study since the detection of a signature in high-resolution transit spectroscopy data indicating an asymmetry between the two limbs of the planet. The existence of this asymmetric signature has been confirmed by multiple studies, but its physical origin is still under debate. In addition, it contrasts with the absence of asymmetry reported in the infrared (IR) phase curve. Aims. We provide a more comprehensive dataset of WASP-76 b with the goal of drawing a complete view of the physical processes at work in this atmosphere. In particular, we attempt to reconcile visible high-resolution transit spectroscopy data and IR broadband phase curves. Methods. We gathered 3 phase curves, 20 occultations, and 6 transits for WASP-76 b in the visible with the CHEOPS space telescope. We also report the analysis of three unpublished sectors observed by the TESS space telescope (also in the visible), which represents 34 phase curves. Results. WASP-76 b displays an occultation of 260±11 and 152±10 ppm in TESS and CHEOPS bandpasses respectively. Depending on the composition assumed for the atmosphere and the data reduction used for the IR data, we derived geometric albedo estimates that range from 0.05 ± 0.023 to 0.146 ± 0.013 and from <0.13 to 0.189 ± 0.017 in the CHEOPS and TESS bandpasses, respectively. As expected from the IR phase curves, a low-order model of the phase curves does not yield any detectable asymmetry in the visible either. However, an empirical model allowing for sharper phase curve variations offers a hint of a flux excess before the occultation, with an amplitude of ∼40 ppm, an orbital offset of ∼−30◦ , and a width of ∼20◦ . We also constrained the orbital eccentricity of WASP-76 b to a value lower than 0.0067, with a 99.7% confidence level. This result contradicts earlier proposed scenarios aimed at explaining the asymmetry observed in high-resolution transit spectroscopy. Conclusions. In light of these findings, we hypothesise that WASP-76 b could have night-side clouds that extend predominantly towards its eastern limb. At this limb, the clouds would be associated with spherical droplets or spherically shaped aerosols of an unknown species, which would be responsible for a glory effect in the visible phase curves.

Asymmetry in the atmosphere of the ultra-hot Jupiter WASP-76 b

Sérgio Sacani

Understanding circumstellar disks is of prime importance in astrophysics, however, their birth process remains poorly constrained due to observational and numerical challenges. Recent numerical works have shown that the small-scale physics, often wrapped into a sub-grid model, play a crucial role in disk formation and evolution. This calls for a combined approach in which both the protostar and circumstellar disk are studied in concert. Aims. We aim to elucidate the small scale physics and constrain sub-grid parameters commonly chosen in the literature by resolving the star-disk interaction. Methods. We carry out a set of very high resolution 3D radiative-hydrodynamics simulations that self-consistently describe the collapse of a turbulent dense molecular cloud core to stellar densities. We study the birth of the protostar, the circumstellar disk, and its early evolution (< 6 yr after protostellar formation). Results. Following the second gravitational collapse, the nascent protostar quickly reaches breakup velocity and sheds its surface material, thus forming a hot (∼ 103 K), dense, and highly flared circumstellar disk. The protostar is embedded within the disk, such that material can flow without crossing any shock fronts. The circumstellar disk mass quickly exceeds that of the protostar, and its kinematics are dominated by self-gravity. Accretion onto the disk is highly anisotropic, and accretion onto the protostar mainly occurs through material that slides on the disk surface. The polar mass flux is negligible in comparison. The radiative behavior also displays a strong anisotropy, as the polar accretion shock is shown to be supercritical whereas its equatorial counterpart is subcritical. We also f ind a remarkable convergence of our results with respect to initial conditions. Conclusions. These results reveal the structure and kinematics in the smallest spatial scales relevant to protostellar and circumstellar disk evolution. They can be used to describe accretion onto regions commonly described by sub-grid models in simulations studying larger scale physics.

Formation of low mass protostars and their circumstellar disks

Sérgio Sacani

HotJupiters are amongthebest-studied exoplanets, but it is still poorly understood how their chemical composition and cloud properties vary with longitude. Theoretical models predict that clouds may condense on the nightside and that molecular abundances can be driven out of equilibrium by zonal winds. Here we report a phase-resolved emission spectrum of the hot Jupiter WASP-43b measured from 5–12µ 5–12µ 5–12µm with JWST’s Mid-Infrared Instrument (MIRI). 1524 ±35 1524 ±35 and 863±23 The spectra reveal a large day–night temperature contrast (with average brightness temperatures of 1524 ± 35 863 ±23 863 ±23Kelvin, respectively) and evidence for water absorption at all orbital phases. Comparisons with three-dimensional atmospheric models show that both the phase curve shape and emission spectra strongly suggest the presence of nightside clouds which become optically thick to thermal emission at pressures greater than ∼100mbar. The dayside is consistent with a cloudless atmosphere above the mid-infrared photosphere. Con3trary to expectations from equilibrium chemistry but consistent with disequilibrium kinetics models, methane is not detected on the nightside (2σ upper limit of 1–6 parts per million, depending on model assumptions).

Nightside clouds and disequilibrium chemistry on the hot Jupiter WASP-43b

Sérgio Sacani

Wepresent Atacama Large Millimeter/submillimeter Array 12-m, 7-m, and Total Power Array observations of the FUOrionis outbursting system, covering spatial scales ranging from 160 to 25,000 au. The high-resolution interferometric data reveal an elongated 12CO(2–1) feature previously observed at lower resolution in 12CO(3–2). Kinematic modeling indicates that this feature can be interpreted as an accretion streamer feeding the binary system. The mass infall rate provided by the streamer is significantly lower than the typical stellar accretion rates (even in quiescent states), suggesting that this streamer alone is not massive enough to sustain the enhanced accretion rates characteristic of the outbursting class prototype. The observed streamer may not be directly linked to the current outburst, but rather a remnant of a previous, more massive streamer that may have contributed enough to the disk mass to render it unstable and trigger the FU Orionis outburst. The new data detect, for the first time, a vast, slow-moving carbon monoxide molecular outflow emerging from this object. To accurately assess the outflow properties (mass, momentum, and kinetic energy), we employ 13CO(2–1) data to correct for optical depth effects. The analysis indicates that the outflow corresponds to swept-up material not associated with the current outburst, similar to the slow molecular outflows observed around other FUor and Class I protostellar objects.

Discovery of an Accretion Streamer and a Slow Wide-angle Outflow around FUOri...

Sérgio Sacani

We explore different scenarios to explain the chemical difference found in the remarkable giant-giant binary system HD138202+CD−3012303. For the first time, we suggest how to distinguish these scenarios by taking advantage of the extensive convective envelopes of giant stars. Methods. We carried out a high-precision determination of stellar parameters and abundances by applying a full line-by-line differential analysis on GHOST high-resolution spectra. We used the FUNDPAR program with ATLAS12 model atmospheres and specific opacities calculated for an arbitrary composition through a doubly iterated method. Physical parameters were estimated with the isochrones package and evolutionary tracks were calculated via MIST models. Results. We found a significant chemical difference between the two stars (∆[Fe/H]∼0.08dex), which is largely unexpected considering the insensitivity of giant stars to planetary ingestion and diffusion effects. We tested the possibility of engulfment events by using several different combinations of stellar mass, ingested mass, metallicity of the engulfed object and different convective envelopes. However, the planetary ingestion scenario does not seem to explain the observed differences. For the first time, we distinguished the source of chemical differences using a giant-giant binary system. By ruling out other possible scenarios such as planet formation and evolutionary effects between the two stars, we suggest that primordial inhomogeneities might explain the observed differences. This remarkable result implies that the metallicity differences that were observed in at least some main-sequence binary systems might be related to primordial inhomogeneities rather than engulfment events. We also discuss the important implications of finding primordial inhomogeneities, which affect chemical tagging and other fields such as planet formation. We strongly encourage the use of giantgiant pairs. They are a relevant complement to main-sequence pairs for determining the origin of the observed chemical differences in multiple systems.

Disentangling the origin of chemical differences using GHOST

Sérgio Sacani

On 26 January 2023, a military pilot reported four potential unidentified anomalous phenomena (UAP) while operating in the Eglin Air Force Base training range off the coast of Florida. Through the onboard radar system, the pilot initially observed that the four objects were aloft between 16,000 – 18,000 feet and appeared to be flying in formation. However the pilot observed only one of the four objects visually and captured two images of the single object via the aircraft’s electro-optical/infrared (EO/IR) sensor (see Figures 1A and 1B). The pilot could not record video of the event because the aircraft’s video recording equipment was inoperable prior to and during the aircraft’s flight. The pilot observed this single object aloft at 16,000 feet. The following case information is based on the pilot’s initial report and a subsequent discussion that AARO held with the pilot to seek additional details about the pilot’s observation of the reported object.

All-domain Anomaly Resolution Office U.S. Department of Defense (U) Case: “Eg...

Sérgio Sacani

RecoveringancientrecordsofEarth'smagneticfieldisessentialfordeterminingtheroleofthemagnetosphereinprotectingearlyEarthfromcosmicradiationandatmosphericescape.WepresentpaleomagneticfieldtestshintingthatarecordofEarth's3.7‐billion‐year(Ga)oldmagneticfieldmaybepreservedinthenortheasternIsuaSupracrustalBeltasachemicalremanentmagnetizationacquiredduringamphibolite‐grademetamorphisminthebandedironformation.MultiplepetrologicalandgeochronologicallinesofevidenceindicatethatthenorthernmostpartofIsuahasnotexperiencedmetamorphictemperaturesexceeding380°CsincetheEoarchean,suggestingtherockshavenotbeensignificantlyheatedsincemagnetizationwasacquired.Weuse“pseudo”bakedcontacttests(intrusionsemplaced3.26–3.5Gaago)andafoldtest(folding3.6Gaago)todemonstratethatsomesamplespreserveaca.3.7Garecordofthemagneticfield.Werecoverafieldstrengthof>15μT.ThissuggeststhatEarth'smagneticfieldmayhavebeenweakenoughtoenhanceatmosphericescapeduringtheArchean

PossibleEoarcheanRecordsoftheGeomagneticFieldPreservedintheIsuaSupracrustalBe...

Sérgio Sacani

Jupiter’s moon Io hosts extensive volcanism, driven by tidal heating. The isotopic composition of Io's inventory of volatile chemical elements, including sulfur and chlorine, reflects its outgassing and mass loss history, and thus records information about its evolution. We used millimeter observations of Io’s atmosphere to measure sulfur isotopes in gaseous SO2 and SO, and chlorine isotopes in gaseous NaCl and KCl. We find 34S/32S = 0.0595 ± 0.0038 (equivalent to δ34S = +347 ± 86‰), which is highly enriched compared to average Solar System values and indicates that Io has lost 94 to 99% of its available sulfur. Our measurement of 37Cl/35Cl = 0.403 ± 0.028 (δ37Cl = +263 ± 88‰) shows that chlorine is similarly enriched. These results indicate that Io has been volcanically active for most (or all) of its history, with potentially higher outgassing and mass-loss rates at earlier times.

Isotopic evidence of long-lived volcanism on Io

Sérgio Sacani

Context. Determining the size distribution of asteroids is key to understanding the collisional history and evolution of the inner Solar System. Aims. We aim to improve our knowledge of the size distribution of small asteroids in the main belt by determining the parallaxes of newly detected asteroids in the Hubble Space Telescope (HST) archive and subsequently their absolute magnitudes and sizes. Methods. Asteroids appear as curved trails in HST images because of the parallax induced by the fast orbital motion of the spacecraft. Taking into account the trajectory of this latter, the parallax effect can be computed to obtain the distance to the asteroids by fitting simulated trajectories to the observed trails. Using distance, we can obtain the absolute magnitude of an object and an estimation of its size assuming an albedo value, along with some boundaries for its orbital parameters. Results. In this work, we analyse a set of 632 serendipitously imaged asteroids found in the ESA HST archive. Images were captured with the ACS/WFC and WFC3/UVIS instruments. A machine learning algorithm (trained with the results of a citizen science project) was used to detect objects in these images as part of a previous study. Our raw data consist of 1031 asteroid trails from unknown objects, not matching any entries in the Minor Planet Center (MPC) database using their coordinates and imaging time. We also found 670 trails from known objects (objects featuring matching entries in the MPC). After an accuracy assessment and filtering process, our analysed HST asteroid set consists of 454 unknown objects and 178 known objects. We obtain a sample dominated by potential main belt objects featuring absolute magnitudes (H) mostly between 15 and 22 mag. The absolute magnitude cumulative distribution logN(H > H0) ∝ αlog(H0) confirms the previously reported slope change for 15 < H < 18, from α ≈ 0.56 to α ≈ 0.26, maintained in our case down to absolute magnitudes of around H ≈ 20, and therefore expanding the previous result by approximately two magnitudes. Conclusions. HST archival observations can be used as an asteroid survey because the telescope pointings are statistically randomly oriented in the sky and cover long periods of time. They allow us to expand the current best samples of astronomical objects at no extra cost in regard to telescope time.

Hubble Asteroid Hunter III. Physical properties of newly found asteroids

Sérgio Sacani

Massive stars (those ≥8 solar masses at birth) have radiative envelopes that cannot sustain the dynamos that produce magnetic fields in lower mass stars. Despite this, ∼7% of massive stars have observed magnetic fields. We use multi-epoch interferometric and spectroscopic observations to characterise a magnetic binary system formed of two massive stars. We find that only one star of the binary is magnetic. Using the non-magnetic star as an independent reference clock to estimate the age of the system, we show that the magnetic star appears younger than its companion. The system properties, and a surrounding bipolar nebula, can be reproduced by a model in which this system was originally a triple within which two of the stars merged, producing the magnetic massive star. Thus, our results provide observational evidence that magnetic fields form in at least some massive stars through stellar mergers.

Observational constraints on mergers creating magnetism in massive stars

Sérgio Sacani

We report the observation of a coalescing compact binary with component masses 2.5–4.5 M⊙ and 1.2–2.0 M⊙ (all measurements quoted at the 90% credible level). The gravitational-wave signal GW230529 181500 was observed during the fourth observing run of the LIGO–Virgo–KAGRA detector network on 2023 May 29 by the LIGO Livingston observatory. The primary component of the source has a mass less than 5 M⊙ at 99% credibility. We cannot definitively determine from gravitational-wave data alone whether either component of the source is a neutron star or a black hole. However, given existing estimates of the maximum neutron star mass, we find the most probable interpretation of the source to be the coalescence of a neutron star with a black hole that has a mass between the most massive neutron stars and the least massive black holes observed in the Galaxy. We estimate a merger rate density of 55+127 −47 Gpc−3yr−1 for compact binary coalescences with properties similar to the source of GW230529 181500; assuming that the source is a neutron star–black hole merger, GW230529 181500-like sources constitute about 60% of the total merger rate inferred for neutron star–black hole coalescences. The discovery of this system implies an increase in the expected rate of neutron star–black hole mergers with electromagnetic counterparts and provides further evidence for compact objects existing within the purported lower mass gap.

Observation of Gravitational Waves from the Coalescence of a 2.5–4.5 M⊙ Compa...

Sérgio Sacani

We use the SAMI Galaxy Surv e y to examine the drivers of galaxy spin, λR e , in a multidimensional parameter space including stellar mass, stellar population age (or specific star formation rate), and various environmental metrics (local density, halo mass, satellite versus central). Using a partial correlation analysis, we consistently find that age or specific star formation rate is the primary parameter correlating with spin. Light-weighted age and specific star formation rate are more strongly correlated with spin than mass-weighted age. In fact, across our sample, once the relation between light-weighted age and spin is accounted for, there is no significant residual correlation between spin and mass, or spin and environment. This result is strongly suggestive that the present-day environment only indirectly influences spin, via the removal of gas and star formation quenching. That is, environment affects age, then age affects spin. Older galaxies then have lower spin, either due to stars being born dynamically hotter at high redshift, or due to secular heating. Our results appear to rule out environmentally dependent dynamical heating (e.g. g alaxy–g alaxy interactions) being important, at least within 1 R e where our kinematic measurements are made. The picture is more complex when we only consider high-mass galaxies ( M ∗ ≳ 10 11 M ). While the age-spin relation is still strong for these high-mass galaxies, there is a residual environmental trend with central galaxies preferentially having lower spin, compared to satellites of the same age and mass. We argue that this trend is likely due to central galaxies being a preferred location for mergers.

The SAMI Galaxy Sur v ey: galaxy spin is more strongly correlated with stella...

Sérgio Sacani

The evolved stages of massive stars are poorly understood, but invaluable constraints can be derived from spatially resolved observations of nearby red supergiants, such as Betelgeuse. Atacama Large Millimeter/submillimeter Array (ALMA) observations of Betelgeuse showing a dipolar velocity field have been interpreted as evidence for a projected rotation rate of about 5 km s−1. This is 2 orders of magnitude larger than predicted by single-star evolution, which led to suggestions that Betelgeuse is a binary merger. We propose instead that large-scale convective motions can mimic rotation, especially if they are only partially resolved. We support this claim with 3D CO5BOLDsimulations of nonrotating red supergiants that we postprocessed to predict ALMA images and SiO spectra. We show that our synthetic radial velocity maps have a 90% chance of being falsely interpreted as evidence for a projected rotation rate of 2 km s−1 or larger for our fiducial simulation. We conclude that we need at least another ALMA observation to firmly establish whether Betelgeuse is indeed rapidly rotating. Such observations would also provide insight into the role of angular momentum and binary interaction in the late evolutionary stages. The data will further probe the structure and complex physical processes in the atmospheres of red supergiants, which are immediate progenitors of supernovae and are believed to be essential in the formation of gravitational-wave sources.

Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-scale Con...

Sérgio Sacani

We present a comprehensive analysis aimed at proving the hypothesis that a train of small-scale features observed by the Wide-field Imager (WISPR) onboard the Parker Solar Probe (PSP) are the signature of a Kelvin–Helmholtz instability (KHI). These features were seen near the flank of a Coronal Mass Ejection (CME) wake between 7.5 Re and 9.5Re, lasting for about 30 minutes. The CME was a slow event, associated with a streamer blowout. We analyzed the size of the eddies and found growth during their evolution while maintaining separation distances and alignment typical of Kelvin–Helmholtz vortexes. We then assessed the magnetic field conditions that would make the observation of such an instability plausible. Two methods were used to cross-check our findings. The measured thickness of the boundary layer supports KHI candidacy, and the estimated linear growth rate suggests nonlinear saturation within the expected timescale. We conclude that a KHI is a plausible explanation for the observed features, and therefore that such instabilities might exist in the low and middle solar corona (within ∼15 Re) and can be detected in white light observations. Their observation, however, might be rare due to stringent conditions like the observer’s proximity, suitable viewing circumstances, magnetic field topology, and flow properties. This study highlights the unique capability of PSP/WISPR in observing such phenomena, especially as PSP perihelia reach closer distances to the Sun.

First Direct Imaging of a Kelvin–Helmholtz Instability by PSP/WISPR

Sérgio Sacani

Rapidly rotating fluids have a rotation profile that depends only on the distance from the rotation axis, in accor-dance with the Taylor- Proudman theorem. Although the Sun was expected to be such a body, helioseismology showed that the rotation rate in the convection zone is closer to constant on radii. It has been postulated that this deviation is due to the poles being warmer than the equator by a few degrees. Using numerical simulations, we show that the pole- to-equator temperature difference cannot exceed 7 kelvin as a result of the back- reaction of the high- latitude baroclinically unstable inertial modes. The observed amplitudes of the modes further indicate that this maximum temperature difference is reached in the Sun. We conclude that the Sun’s latitudinal differential rotation reaches its maximum allowed value

The Sun’s differential rotation is controlled by high- latitude baroclinicall...

Sérgio Sacani

We present the analysis of multiepoch observations of a set of 12 variable, Compton-thin, local (z<0.1) active galactic nuclei (AGN) selected from the 100-month BAT catalog. We analyze all available X-ray data from Chandra, XMMNewton, and NuSTAR, adding up to a total of 53 individual observations. This corresponds to between 3 and 7 observations per source, probing variability timescales between a few days and ∼ 20 yr. All sources have at least one NuSTAR observation, ensuring high-energy coverage, which allows us to disentangle the line-of-sight and reflection components in the X-ray spectra. For each source, we model all available spectra simultaneously, using the physical torus models MYTorus, borus02, and UXCLUMPY. The simultaneous fitting, along with the high-energy coverage, allows us to place tight constraints on torus parameters such as the torus covering factor, inclination angle, and torus average column density. We also estimate the line-of-sight column density (NH) for each individual observation. Within the 12 sources, we detect clear line-of-sight NH variability in 5, non-variability in 5, and for 2 of them it is not possible to fully disentangle intrinsic-luminosity and NH variability. We observe large differences between the average values of line-ofsight NH (or NH of the obscurer) and the average NH of the torus (or NH of the reflector), for each source, by a factor between ∼ 2 to > 100. This behavior, which suggests a physical disconnect between the absorber and the reflector, is more extreme in sources that present NH variability. NH-variable AGN also tend to present larger obscuration and broader cloud distributions than their non-variable counterparts. We observe that large changes in obscuration only occur at long timescales, and use this to place tentative lower limits on torus cloud sizes.

Hydrogen Column Density Variability in a Sample of Local Compton-Thin AGN

Sérgio Sacani

Titã, que é a maior lua de Saturno e envolta em mistério, possui uma atmosfera densa repleta de vários compostos orgânicos, alguns dos quais poderiam servir como indicadores de vida se fossem encontrados em nosso próprio planeta. Surge a questão intrigante sobre os mecanismos subjacentes à formação desses compostos em Titã e se eles são a chave para desvendar os mistérios que cercam as origens da vida na Terra, o que leva a uma busca por respostas que entrelaçam os dois corpos celestes. A sonda Huygens da Agência Espacial Européia (ESA), que fez sua chegada histórica a Titã no ano de 2005, é um instrumento fundamental pronto para esclarecer esses processos enigmáticos. Ao mesmo tempo, observatórios terrestres equipados com capacidades telescópicas avançadas contribuem significativamente para esse empreendimento científico, auxiliando no planejamento estratégico para determinar o local preciso de pouso da Huygens, contemplando a dicotomia entre solo sólido e um oceano envolto em metano como possíveis zonas de pouso.

Huygens - Exploring Titan A Mysterious World

Sérgio Sacani

Our Sun lies within 300 parsecs of the 2.7-kiloparsecs-long sinusoidal chain of dense gas clouds known as the Radclife Wave1 . The structure’s wave-like shape was discovered using three-dimensional dust mapping, but initial kinematic searches for oscillatory motion were inconclusive2–7 . Here we present evidence that the Radclife Wave is oscillating through the Galactic plane while also drifting radially away from the Galactic Centre. We use measurements of line-of-sight velocity8 for 12CO and threedimensional velocities of young stellar clusters to show that the most massive star-forming regions spatially associated with the Radclife Wave (including Orion, Cepheus, North America and Cygnus X) move as though they are part of an oscillating wave driven by the gravitational acceleration of the Galactic potential. By treating the Radclife Wave as a coherently oscillating structure, we can derive its motion independently of the local Galactic mass distribution, and directly measure local properties of the Galactic potential as well as the Sun’s vertical oscillation period. In addition, the measured drift of the Radclife Wave radially outwards from the Galactic Centre suggests that the cluster whose supernovae ultimately created today’s expanding Local Bubble9 may have been born in the Radclife Wave.

The Radcliffe Wave Of Milk Way is oscillating

Sérgio Sacani

Relativistic jets are observed from accreting and cataclysmic transients throughout the Universe, and have a profound impact on their surroundings1,2 . Despite their importance, their launch mechanism is not known. For accreting neutron stars, the speed of their compact jets can reveal whether the jets are powered by magnetic felds anchored in the accretion fow3 or in the star itself 4,5 , but so far no such measurements exist. These objects can show bright explosions on their surface due to unstable thermonuclear burning of recently accreted material, called type-I X-ray bursts6 , during which the mass-accretion rate increases7–9 . Here, we report on bright fares in the jet emission for a few minutes after each X-ray burst, attributed to the increased accretion rate. With these fares, we measure the speed of a neutron star compact jet to be v c = 0.38−0.08 +0.11 , much slower than those from black holes at similar luminosities. This discovery provides a powerful new tool in which we can determine the role that individual system properties have on the jet speed, revealing the dominant jet launching mechanism.

Thermonuclear explosions on neutron stars reveal the speed of their jets

Sérgio Sacani

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