arXiv subjects

D. Hutsemékers

Publications and source records attributed to D. Hutsemékers.

At least 19 recordsLinked to original sources

arXiv2026

Optical spectropolarimetry of extreme H$α$ line profiles in seven active galactic nuclei

Some active galactic nuclei (AGNs) are known to show extremely asymmetric broad Balmer lines, with their main peak redshifted or blueshifted by thousands of km s$^{-1}$, severely challenging our understanding of the standard AGN paradigm. We wanted to explore the causes of such asymmetric features by carrying out a spectropolarimetric study of a sample of bright AGNs with known extreme Balmer line profiles. We present optical spectropolarimetry of seven bright (V $<$ 17.5) Seyfert-1 galaxies obtained with the Very Large Telescope (VLT) FOcal Reducer/low dispersion Spectrograph 2 (FORS2) instrument from March 2012 to January 2013. The main broad Balmer lines (H$α$ and H$β$, and H$γ$ occasionally) were sampled with a resolving power ranging from 350 to 500. We performed a detailed spectral decomposition of each H$α$ profile to isolate kinematic substructures within the broad line region (BLR) and measured the polarization degree and angle of each component, providing constraints on the inner geometry and scattering properties. The broad H$α$ components exhibit various scattering geometries. Their polarization as a function of full width at half maximum (FWHM) reveals two distinct behaviors, corresponding to either a BLR of roughly constant scale height or a stratified structure with decreasing height at higher velocities. This supports a disk-wind geometry linking equatorial and polar regions.

astro-ph.GA↗

arXiv2026

High-resolution optical spectroscopy reveals the distinctive volatile composition of the interstellar comet 3I/ATLAS

Comets preserve volatile material from the early stages of planetary-system formation. The discovery of the interstellar comet 3I/ATLAS enables a direct comparison between a comet formed around another star and comets in the Solar System. We characterise the coma composition of 3I/ATLAS across perihelion using homogeneous high-resolution near-UV and optical spectroscopy with UVES on the ESO Very Large Telescope, from August 2025 (r$_h$=3.14 au) pre-perihelion to February 2026 (r$_h$=4.34 au) post-perihelion. From flux-calibrated spectra, we measured OH, NH, CN, C$_3$, CH and C$_2$ emissions, derived production rates, examined abundance ratios and heliocentric-distance trends, constrained NH$_2$, and compared OH-based water production with estimates from forbidden oxygen lines. Gas production rates show broadly symmetric pre- and post-perihelion behaviour, with steeper heliocentric-distance dependences than typically observed in Solar System comets. NH and NH$_2$ are strongly depleted, indicating a severe deficiency of ammonia-related volatiles. The C$_2$/CN ratio indicates carbon-chain depletion and evolves with heliocentric distance, consistent with coma-driven effects such as hyperactivity rather than intrinsic compositional changes. Water production from forbidden oxygen lines exceeds OH-based estimates except near perihelion, suggesting varying contributions from H$_2$O and CO$_2$ to coma chemistry. Relative abundance correlations place 3I/ATLAS outside the canonical Solar System comet cluster and closest to strongly carbon-chain depleted comets, including G-Z-type comets, while showing strong depletion of NH- and C$_3$-bearing species relative to CN. 3I/ATLAS therefore extends the known diversity of cometary compositions, demonstrating that volatile reservoirs in other planetary systems can differ substantially from those sampled by Solar System comets.

astro-ph.EP↗

arXiv2026

Radiation-pressure instability is an artifact of constant-$α$ closure. Implications for AGN disk tensions

The standard $α$-disk formalism parametrizes turbulent angular momentum transport through a dimensionless coefficient $α$, assumed to be spatially and thermodynamically invariant. While analytically convenient, this assumption leads to the well-known thermal and viscous instabilities in radiation-pressure dominated (RPD) regions. We show that this instability is not the consequence of radiation pressure, but is due to enforcing a constant $α$ across distinct thermodynamic regimes. Requiring the steady thin-disk (TD) to remain thermally stable and single-valued in the $\dot{M}$--$Σ$ plane yields a necessary condition on the stress response, expressed as $η_{\rm x} \equiv d\lnα_{\rm x}\,/\,d\ln X > 4/7$, where $X \equiv P_{\rm gas}/P_{\rm rad}$. The resulting viscosity law $α_{\rm x} \equiv α(X)$ emerges directly from the internal consistency of TD equations, without modifying the stress law or invoking any additional physics. $α_{\rm x}$ removes the RPD unstable branch. The disk structure becomes smooth and globally single-valued, with higher $Σ$ and $τ$ in the inner RPD disk, while preserving the standard effective-temperature profile. This increases thermal and inflow timescales, offering a natural route to accretion-state dependent variability without large-amplitude radiation-pressure limit cycles. It also motivates revisiting AGN disk tensions, including microlensing sizes and continuum reverberation lags with improved radiative-transfer modeling. The results show that the RPD instability, and possibly some associated AGN disk tensions, reflect an inconsistent viscosity closure.

astro-ph.HE↗

arXiv2026

Spectropolarimetry of the changing-look active galactic nucleus NGC 1566 and its potential link to supermassive black hole binaries

The AGN NGC~1566 is known to present dramatic and regular spectral shape changes, associated with the appearance and disappearance of broad emission lines. The underlying mechanism responsible for such changes is yet to be identified, but occultation, eccentric accretion disks, turbulent disk-dominated broad line regions (BLRs) or binary supermassive black holes have been hypothesized. Because the scenarios used to explain the variable spectral shapes of NGC~1566 each have a specific geometric configuration, we used the VLT/FORS2 instrument to obtain nine 3500-10\,000~Å\, polarized spectra of the source between August 2 and September 21, 2025. We caught the AGN in a type-2 state, i.e., without any broad component in total nor polarized fluxes. Its low and wavelength-independent polarization degree (and angle) above 4000~Å\, argues against occultation of the BLR and is consistent with a significant weakening or disappearance of the BLR. The polarized spectrum reveals a strong rise of polarization in the blue band, likely echoing the 2018 outburst of the AGN. The temporal variability of the total flux continuum but the steadiness of the line profiles demonstrate that the object is viewed close to pole-on, irrespective of its spectral type at the time of observation. Relative to archival data, NGC 1566 shows significant variability in polarization degree, angle, and wavelength dependence. Even more surprisingly, NGC~1566 behaves opposite to the basic predictions of the unified model: its polarization angle is perpendicular to the AGN polar axis and its polarization degree is higher when in a brighter, type-1 phase. The results reported above contradict occultation and binary supermassive black hole hypotheses, rather supporting accretion-driven photoionization/structural changes in the internal accretion flow and the BLR.

astro-ph.GA↗

arXiv2026

High nitrogen and carbon isotopic ratios in the interstellar comet 3I/ATLAS

Interstellar objects provide a unique opportunity to further our understanding of the planetary formation process by studying in detail material formed around another star. Their ices contain precious clues about the environment and conditions prevailing in their home system. As fractionation processes can be sensitive to the temperature and radiation environment, isotopic ratios are powerful tracers of the origin and evolution of different species. While isotopic ratios have been measured in solar system comets, previously detected interstellar objects have been too faint to measure isotopic ratios. Here we report the measurement of two ratios in 3I/ATLAS from observations of the CN molecule: $^{12}$C/$^{13}$C and $^{14}$N/$^{15}$N. We report $^{12}$C/$^{13}$C=$147^{+87}_{-40}$ and $^{14}$N/$^{15}$N=$343^{+454}_{-124}$. The $^{14}$N/$^{15}$N is higher than the value of $\sim$~150 usually measured for solar system comets, close to the values measured in the interstellar medium, pre-stellar phases or the outside of protoplanetary discs. The $^{12}$C/$^{13}$C is marginally higher than the values usually measured for solar system comets and in the interstellar medium. These measurements could indicate an origin of 3I in the outer disc around an older low-metallicity star.

astro-ph.EP↗

arXiv2026

Polarization properties of changing-look active galactic nuclei: NGC 1365 and NGC 2992

Changing-look active galactic nuclei (CLAGNs) represent a rare class of AGNs that undergo transitions from type 1 (characterized by the presence of broad emission lines in their spectra) to type 2 (absence of broad emission lines) or vice versa, over timescales ranging from months to years. Since normal type 1 and type 2 AGNs are known to show different polarization properties, detailed investigations of the CLAGN polarization can shed light on the underlying mechanisms responsible for the changing-look phenomenon. We present new (spectro)polarimetric observations of two changing-look AGNs located in the core of the inclined spiral galaxies NGC 1365 and NGC 2992. Both AGNs are radio emitters, thereby enabling a comparison of their polarization to the radio jet axis, which defines the accretion disk geometry. In the case of NGC 1365, the AGN shows polarization characteristics consistent with those observed in type 1 Seyferts, in particular polarization parallel to the radio jet. This intrinsic polarization is modified by the wavelength-dependent dichroic extinction that occurs in the galaxy bar and that rotates the polarization angle at the shortest wavelengths. NGC 2992, on the other hand, is so inclined that dichroic dust extinction in the disk completely dominates the polarization of the AGN, thus overwhelming any polarization due to scattering. Consequently, the polarization properties remain essentially constant between the different AGN states, and the faint broad lines observed in the polarized flux are most likely not scattered light. Differential dilution between the continuum and the narrow-line polarizations can explain the unusually high polarization measured in the emission lines.

astro-ph.GA↗

arXiv2026

Polarization echoes from past nuclear activity in the quasi-periodic eruption source GSN 069

Context. X-ray quasi-periodic eruptions (QPEs) are repeating, high-amplitude, soft X-ray bursts observed from the nuclei of a dozen nearby low-mass galaxies. Their origin remains a puzzle in the physics of accretion variability. Observational data indicate that X-ray and/or optical tidal disruption events (TDEs) may precede QPE detections. Although both kinds of outburst are driven by supermassive black holes, they are more frequently detected in faded active galactic nuclei (AGNs), when the TDE is not happening in a dormant galaxy. In the case of the QPE discovery source, GSN 069, it remains debated whether its past activity arose from a previous AGN phase or from an enhanced TDE rate. Aims. We investigated the origin of the past nuclear activity in GSN 069. Methods. Past AGN activity imprints detectable polarization in optical light, due to the expected delay between direct and scattered light. On 6 September 2019, we targeted GSN 069 with VLT/FORS2 in both imaging polarimetry and spectropolarimetry modes so that its optical polarization could be investigated while the first detected QPE phase was still active. Results. We measured a rising polarization, from ~0% to ~1.5%, as moving away from the nucleus of GSN 069. This rise is probed to be intrinsic to the central engine, confirming the already detected extended emission line region (EELR) by integral field unit data. Conclusions. The increasing radial polarization demonstrates a switched-off nucleus. The polarization angle traces an axis aligned with elongated [OIII], [NII], and Hα gas distributions, revealing an EELR that may be consistent with relic polarization cones, therefore suggesting the presence of a torus-like structure in the past. Thus, optical polarization echoes geometrically favor a faded AGN as the origin of the EELR rather than a past elevated TDE rate, although the latter cannot be excluded.

astro-ph.GA↗

arXiv2025

Spectropolarimetry of NGC 1275 reveals a narrow-line radio galaxy with polarization parallel to its radio jet axis

Concomitant with the Imaging X-ray Polarimetry Explorer (IXPE) observation of the Perseus cluster, we obtained optical spectropolarimetry of its central active galactic nucleus, NGC 1275, using the Alhambra Faint Object Spectrograph and Camera (ALFOSC) on the Nordic Optical Telescope (NOT). While the total-light spectrum confirms its edge-on, core obscured (type-2) classification, the polarized spectrum shows a polarization angle aligned with the arcsecond radio jet axis -- an exceptional behavior for type-2 objects. Our polarization analysis also reveals wavelength-dependent linear polarization at level 2-3% in the continuum, likely rising from a combination of variable syn

astro-ph.GA↗

arXiv2025

Quasar Main Sequence unfolded by 2.5D FRADO (Natural expression of Eddington ratio, black hole mass, and inclination)

The quasar main sequence (QMS), characterized by the Eigenvector 1 (EV1), serves as a unifying framework for classifying type-1 active galactic nuclei (AGNs) based on their diverse spectral properties. Although a fully self-consistent physical interpretation has long been lacking, our physically motivated 2.5D FRADO (Failed Radiatively Accelerated Dusty Outflow) model naturally predicts that the Eddington ratio ($\dot{m}$) is the primary physical driver of the QMS, with black hole mass ($M_{\rm BH}$) and inclination ($i$) acting as secondary contributors. We employed a dense grid of FRADO simulations of the geometry and dynamics of the broad-line region (BLR), covering a representative range of $M_{\rm BH}$ and $\dot{m}$. For each simulation, we computed the full width at half maximum (FWHM) of the H$β$ line under different $i$. The resulting FWHM--$\dot{m}$ diagram closely resembles the characteristic trend observed in the EV1 parameter space. This establishes the role of $\dot{m}$ as the true proxy for the Fe II strength parameter ($R_{\rm Fe}$), and vice versa. Our results suggest that $\dot{m}$ can be regarded as the sole underlying physical tracer of $R_{\rm Fe}$ and should therefore scale directly with it. The $M_{\rm BH}$ accounts for the virial mass-related scatter in FWHM, while $i$ acts as a secondary driver modulating $R_{\rm Fe}$ and FWHM for a given $\dot{m}$ and $M_{\rm BH}$.

astro-ph.GA↗

arXiv2025

Variations in Volatile-Driven Activity of Comet C/2017 K2 (PanSTARRS) Revealed by Long-Term Multi-Wavelength Observations

Context. A comprehensive study of comets over a wide heliocentric distance range helps us understand the physical processes driving their activity and reveals compositional differences across dynamical groups. C/2017 K2 (PANSTARRS) is a Dynamically New Oort Cloud comet (DNC) that showed activity as far as 23.75 au and displayed a CO-rich coma at 6.72 au, making it a key object to investigate pre- and post-perihelion behavior. Aims. We aim to study the long-term activity evolution and chemical composition of C/2017 K2 using photometry and spectroscopy, from October 2017 (r$_h$ = 15.18 au) pre-perihelion to April 2025 (r$_h$ = 8.46 au) post-perihelion. Methods. Broad-band and narrow-band imaging from both TRAPPIST telescopes enabled us to produce an 8-year light curve, color analysis, and derivation of activity slopes. Production rates of OH, NH, CN, C$_3$, and C$_2$ were computed using a Haser model, along with the dust proxy A(0)f$ρ$. High-resolution spectra from CRIRES$^+$ and UVES at three epochs (May - September 2022) provided simultaneous observations of parent and daughter species as the comet crossed the water sublimation zone. Results. The light curve of C/2017 K2 shows a complex evolution with varying slopes and a brightness plateau around perihelion, indicating multiple active species. Coma colors remained constant, suggesting uniform dust properties and similarity to other active long-period comets. Gas production rates indicate a typical C$_2$/CN composition with a high dust-to-gas ratio. Analysis of forbidden oxygen lines shows a transition from CO and CO$_2$-driven activity to water-driven sublimation inside 3 au. Infrared spectra reveal C/2017 K2 as a typical-to-enriched comet, with HCN identified as the main parent of CN, and C$_2$ likely originating from C$_2$H$_2$ rather than C$_2$H$_6$.

astro-ph.EP↗

arXiv2025

A new type-1 transition in NGC 1346 revealed by (spectro)polarimetry

We present the first photo- and spectropolarimetric observations of the changing-look active galactic nucleus NGC~1346 taken with the Perkins telescope (November 2022, March 2025) and VLT/FORS2 (August and September 2024), which have revealed a new spectral transition. We find that NGC~1346 has reverted to its former (2001) type-1 spectral state after having been a type-2 active galactic nucleus from 2004 to 2022. More and more intense broad H$β$ (3720 $\pm$ 310 km.s$^{-1}$ full width at half maximum) and H$α$ (2985 $\pm$ 230 km.s$^{-1}$) emission lines appeared between August and September 2024, suggesting that the broad-line region (BLR) is increasingly irradiated. The strong blueshift of the Balmer lines ($\sim$ 1600 km.s$^{-1}$) indicates that the gas is moving toward the observer, possibly due to a radiation-driven outflow. The strong difference between 2022's ($\sim$ 3\%) and 2024-2025's de-biased polarization ($\le$ 0.5\%), the blueing of the spectra between August and September 2024, and the slow rise in the integrated flux between 2022 and 2025 argue against asymmetric or temporary obscuration by a cloud passing in front of the line of sight. Two options, either a re-illumination of the BLR or a tidal disruption event, can explain the observed properties of NGC~1346. Timely follow-up observations taken while the phenomenon is still ongoing are needed to determine which of the two solutions is the correct one.

astro-ph.GA↗

arXiv2025

Scattered synchrotron emission and a giant torus revealed in polarized light in the nearest radio galaxy Centaurus A

Centaurus A (Cen A) is the closest radio galaxy and a prime example of a low-luminosity active galactic nucleus (AGN), exhibiting complex emissions across the electromagnetic spectrum. The nature of its continuum emission, particularly the mechanisms powering it, has been a subject of considerable debate due to the fact that the AGN is deeply buried in dust. This study aims to elucidate the origin of the continuum emission in Cen A and determine the geometrical arrangement of matter in the nuclear region by the mean of optical and near-infrared spectropolarimetry. We obtained spectropolarimetric data of Cen A using the VLT/FORS2. The analysis revealed a region showing strong and narrow emission lines associated with AGN activity. After correction for interstellar polarization in the dust lane (but not for starlight), the intrinsic polarization of the scattered AGN light exhibits a polarization degree of 2-4%, decreasing from optical to near-infrared, associated with a polarization position angle perpendicular to the radio jet axis. We exclude the presence of hidden broad line in our polarized flux spectrum at more than 99% probability. Narrow emission lines are found to be strongly polarized and orthogonal to the jet position angle. We demonstrate that a beamed synchrotron jet, scattering onto the narrow line region (NLR) best fits all the observational properties reported in this paper and the literature. In this model, the base of the NLR is obscured by a giant circumnuclear region and can only become visible through perpendicular scattering onto the outermost part of the NLR, naturally producing high polarization degrees and polarization angles perpendicular to the radio structure. This study provides strong evidence that Cen A defines a new class of hidden-NLR AGNs and supports old predictions that beamed synchrotron jets can be observed in reflection.

astro-ph.GA↗

arXiv2024

Probing the broad line region geometry and size of the gravitationally lensed quasar Q2237+0305 with microlensing time series

Lensed quasars are powerful cosmic laboratories; they are used to simultaneously probe various astrophysical phenomena. Microlensing by stars within distant galaxies acts as strong gravitational lenses of multiply imaged quasars, and provides a unique and direct measurement of the lensed quasar internal structure. Microlensing of the continuum emitting region as well as the broad-line region (BLR) is well characterized by four observable indices, $μ^{cont}$, $μ^{BLR}$, $WCI$ (wing-core), and $RBI$ (red-blue), measured directly from the spectra. During the 2004-2007 monitoring period, image A of the quadruply lensed system Q2237+0305 underwent a strong microlensing amplification, while image D remained unaffected. We used 35 epochs of archival spectrophotometric data of Q2237+0305 obtained with the Very Large Telescope of the European Southern Observatory to develop an independent microlensing method for estimating the geometry and size of the BLR. We measured the index time series for the CIV line and the continuum emission at $1450\,\unicode{x212B}$. We built a library of the simulated microlensing index time series that reproduce the observed times series based on three representative BLR models: Keplerian disk (KD), polar wind (PW), and equatorial wind (EW). After sampling the model parameter space, we find that KD is the predominant model, while PW and EW are less likely. We infer that the system is viewed at an intermediate viewing angle $i\sim 35^\circ$, and we estimate the most likely CIV BLR half-light radius $r_\mathrm{1/2}=51\pm 23$ light days. Our results are in good agreement with previous findings in the literature and extend the validity of the index-based approach to a temporal domain.

astro-ph.GA↗

arXiv2023

Polarimetry of the potential binary supermassive black hole system in J1430+2303

The growth of supermassive black holes (SMBHs) through merging has long been predicted but its detection remains elusive. However, a promising target has been discovered in the Seyfert-1 galaxy J1430+2303. If a binary system truly lies at the center of J1430+2303, the usual symmetry expected from pole-on views in active galactic nuclei (AGNs) responsible for the observed low ($\le$ 1\%) optical linear polarization in the continuum of these objects is expected to be broken. This should lead to higher-than-usual polarization degrees, together with time-dependent variations of the polarization signal. We used the specialized photopolarimeters RoboPol mounted on the 1.3m telescope at the Skinakas Observatory and the Alhambra Faint Object Spectrograph and Camera (ALFOSC) mounted on the 2.56m Nordic Optical Telescope (NOT) at the "Roque de los Muchachos" Observatory to measure the B-, V-, R-, and I-band polarization of J1430+2303. Observations were complemented using the FORS2 spectropolarimeter mounted on the VLT to acquire 3500 -- 8650 Angs polarized spectra. We compared our set of observations to Monte Carlo radiative-transfer predictions to look for the presence of a SMBH binary. The observed linear continuum polarization of J1430+2303 in the V and R bands is $\sim$ 0.4\% with an associated polarization angle of slightly larger than 0$^\circ$. We detected no significant changes in polarization or photometry between May, June, and July of 2022. In addition, there is no significant difference between the polarization of H$α$ and the polarization of the continuum. A single SMBH at the center of an AGN model is able to reproduce the observed spectrum and polarization, while the binary hypothesis is rejected with a probability of $\sim$85\%.

astro-ph.GA↗

arXiv2023

Activity and composition of the hyperactive comet 46P/Wirtanen during its close approach in 2018

Hyperactive comets are a small group of comets whose activity are higher than expected. They seem to emit more water than they should based on the size of their nucleus and comet 46P/Wirtanen is one of them. Investigating its activity and composition evolution could provide clues about its origins and formation region in the Solar nebulae. Given the exceptional close approach in 2018 of comet 46P to the Earth, we aim to study the evolution of its activity and composition as a function of heliocentric distances before and after perihelion. We used both TRAPPIST telescopes to monitor the comet for almost a year with broad-band and narrow-band filters. We derived the production rates of five gaseous species, e.g. OH, NH, CN, C$_3$ and C$_2$, using a Haser model as well as the A($θ$)f$ρ$, dust proxy parameter. The comet was also observed with two optical high resolution spectrographs UVES and ESPRESSO mounted on the 8-m ESO VLT to measure the isotopic ratios of C and N, the oxygen forbidden lines ratios and the NH$_2$ ortho-to-para ratios. We followed during almost a year the rise and decline of the production rates of different species as well as the dust activity of 46P on both pre- and post-perihelion. Relative abundances with respect to CN and OH along the orbit of the comet show constant and symmetric abundance ratios and a typical coma composition. We determined the rotation period of the nucleus using high cadence observations and long series of CN images on several nights, and we obtained a value of (9.18$\pm$0.05) hr at perihelion. Using high resolution spectra of 46P coma, we derived C and N isotopic ratios of 100$\pm$20 and 150$\pm$30 and a green-to-red forbidden oxygen [OI] lines ratio of 0.23$\pm$0.02. We measured a NH$_2$ ortho-to-para ratio of 3.31$\pm$0.03 and derived an ammonia ratio of 1.19$\pm$0.03 corresponding to a spin temperature of 27$\pm$1 K.

astro-ph.EP↗

arXiv2021

The similarity of the interstellar comet 2I/Borisov to solar system comets from high resolution optical spectroscopy

2I/Borisov - hereafter 2I - is the first visibly active interstellar comet observed in the solar system, allowing us for the first time to sample the composition of a building block from another system. We report on the monitoring of 2I with UVES, the high resolution optical spectrograph of the ESO Very Large Telescope at Paranal, during four months from November 15, 2019 to March 16, 2020. Our goal is to characterize the activity and composition of 2I with respect to solar system comets. We collected high resolution spectra at 12 different epochs from 2.1 au pre-perihelion to 2.6 au post perihelion. On December 24 and 26, 2019, close to perihelion, we detected several OH lines of the 309 nm (0-0) band and derived a water production rate of $2.2\pm0.2 \times 10^{26}$ molecules/s. The three [OI] forbidden oxygen lines were detected at different epochs and we derive a green-to-red doublet intensity ratio (G/R) of $0.31\pm0.05$ close to perihelion. NH$_2$ ortho and para lines from various bands were measured and allowed us to derive an ortho-to-para ratio (OPR) of $3.21\pm0.15$, corresponding to an OPR and spin temperature of ammonia of $1.11\pm0.08$ and $31^{+10}_{-5}$ K, respectively. These values are consistent with the values usually measured for solar system comets. Emission lines of the radicals NH (336 nm), CN (388 nm), CH (431 nm), and C$_2$ (517 nm) were also detected. Several FeI and NiI lines were identified and their intensities were measured to provide a ratio of log (NiI/FeI) = $0.21\pm0.18$ in agreement with the value recently found in solar system comets. Our high spectral resolution observations of 2I/Borisov and the associated measurements of the NH$_2$ OPR and the Ni/Fe abundance ratio are remarkably similar to solar system comets. Only the G/R ratio is unusually high but consistent with the high abundance ratio of CO/H$_2$O found by other investigators.

astro-ph.EP↗

arXiv2021

Multiplicity of Galactic Luminous Blue Variable stars

Context. Luminous Blue Variables (LBVs) are thought to be in a transitory phase between O stars on the main-sequence and the Wolf-Rayet stage. Recent studies suggest that they might be formed through binary interaction. Only a few are known in binary systems but their multiplicity fraction is uncertain. Aims. This study aims at deriving the binary fraction among the Galactic (confirmed and candidate) LBV population. We combine multi-epoch spectroscopy and long-baseline interferometry. Methods. We use cross-correlation to measure their radial velocities. We identify spectroscopic binaries through significant RV variability (larger than 35 km/s). We investigate the observational biases to establish the intrinsic binary fraction. We use CANDID to detect interferometric companions, derive their parameters and positions. Results. We derive an observed spectroscopic binary fraction of 26 %. Considering period and mass ratio ranges from Porb=1 to 1000 days, and q = 0.1-1.0, and a representative set of orbital parameter distributions, we find a bias-corrected binary fraction of 62%. From interferometry, we detect 14 companions out of 18 objects, providing a binary fraction of 78% at projected separations between 1 and 120 mas. From the derived primary diameters, and the distances of these objects, we measure for the first time the exact radii of Galactic LBVs to be between 100 and 650 Rsun, making unlikely to have short-period systems. Conclusions. This analysis shows that the binary fraction among the Galactic LBV population is large. If they form through single-star evolution, their orbit must be initially large. If they form through binary channel that implies that either massive stars in short binary systems must undergo a phase of fully non-conservative mass transfer to be able to sufficiently widen the orbit or that LBVs form through merging in initially binary or triple systems.

astro-ph.SR↗

arXiv2020

Photometry and high-resolution spectroscopy of comet 21P/Giacobini-Zinner during its 2018 apparition

We report on photometry and high resolution spectroscopy of the chemically peculiar Jupiter-family Comet (hereafter JFC) 21P/Giacobini-Zinner. Comet 21P is a well known member of the carbon-chain depleted family but displays also a depletion of amines. We monitored continuously the comet over more than seven months with the two TRAPPIST telescopes (TN and TS), covering a large heliocentric distance range from 1.60 au inbound to 2.10 au outbound with a perihelion at 1.01 au on September 10, 2018. We computed and followed the evolution of the dust (represented by Af$ρ$) and gas production rates of the daughter species OH, NH, CN, C$_3$, and C$_2$ and their relative abundances to OH and to CN over the comet orbit. We compared them to those measured in the previous apparitions. The activity of the comet and its water production rate reached a maximum of (3.72$\pm$0.07)$\times$10$^{28}$ molec/s on August 17, 2018 (r$_h$=1.07 au), 24 days before perihelion. The peak value of A(0)f$ρ$ was reached on the same date (1646$\pm$13) cm in the red filter. The abundance ratios of the various species are remarkably constant over a large range of heliocentric distances, before and after perihelion, showing a high level of homogeneity of the ices in the surface of the nucleus. The behaviour and level of the activity of the comet is also remarkably similar over the last five orbits. About the coma dust colour, 21P shows reflectively gradients similar to JFCs. We obtained a high resolution spectrum of 21P with UVES at ESO VLT one week after perihelion. Using the CN B-X (0,0) violet band, we measured $^{12}$C/$^{13}$C and $^{14}$N/$^{15}$N isotopic ratios of 100$\pm$10 and 145$\pm$10, respectively, both in very good agreement with what is usually found in comets.

astro-ph.EP↗