Publications of Damien Hutsemekers
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See detailPhotometry and high-resolution spectroscopy of comet 21P/Giacobini-Zinner during its 2018 apparition
Moulane, Y.; Jehin, Emmanuel ULiege; Rousselot, P. et al

E-print/Working paper (2020)

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 ... [more ▼]

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$\rho$) 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$\rho$ 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. [less ▲]

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See detailTracking the state transitions in changing-look active galactic nuclei through their polarized-light echoes
Marin, Frédéric; Hutsemekers, Damien ULiege

in Astronomy and Astrophysics (2020), 636

Context. Variations in the mass accretion rate appear to be responsible for the rapid transitions in spectral type that are observed in increasingly more active galactic nuclei (AGNs). These objects are ... [more ▼]

Context. Variations in the mass accretion rate appear to be responsible for the rapid transitions in spectral type that are observed in increasingly more active galactic nuclei (AGNs). These objects are now labeled "changing-look" AGNs and are key objects for understanding the physics of accretion onto supermassive black holes. <BR /> Aims: We aim to complement the analysis and interpretation of changing-look AGNs by modeling the polarization variations that can be observed, in particular, polarized-light echoes. <BR /> Methods: We built a complex and representative model of an AGN and its host galaxy and ran radiative transfer simulations to obtain realistic time-dependent polarization signatures of changing-look objects. Based on actual data, we allowed the system to become several times fainter or brighter within a few years, assuming a rapid change in accretion rate. <BR /> Results: We obtain time-dependent polarization signatures of distant high-luminosity (quasars) and nearby low-luminosity (Seyferts) changing-look AGNs for a representative set of inclinations. We predict the evolution of the continuum polarization for future polarimetric campaigns with the goal to better understand the physics at work in these objects. We also investigate highly inclined AGNs that experience strong accretion rate variations without appearing to change state. We apply our modeling to Mrk 1018, the best-documented case of a changing-look AGN, and predict a variation in its polarization after the recent dimming of its continuum. <BR /> Conclusions: We demonstrate that polarization monitoring campaigns that cover the transitions that are observed in changing-look AGNs might bring crucial information on the geometry and composition of all the reprocessing regions within the nucleus. In particular, specific features in the time variation of the polarization position angle can provide a new and efficient method for determining AGN inclinations. [less ▲]

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See detailForbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)
Raghuram, S.; Hutsemekers, Damien ULiege; Opitom, C. et al

in Astronomy and Astrophysics (2020), 635

Context. The N[SUB]2[/SUB] and CO-rich and water-depleted comet C/2016 R2 (Pan-STARRS) - hereafter "C/2016 R2" - is a unique comet for detailed spectroscopic analysis. <BR /> Aims: We aim to explore the ... [more ▼]

Context. The N[SUB]2[/SUB] and CO-rich and water-depleted comet C/2016 R2 (Pan-STARRS) - hereafter "C/2016 R2" - is a unique comet for detailed spectroscopic analysis. <BR /> Aims: We aim to explore the associated photochemistry of parent species, which produces different metastable states and forbidden emissions, in this cometary coma of peculiar composition. <BR /> Methods: We reanalyzed the high-resolution spectra of comet C/2016 R2 obtained in February 2018 using the UVES spectrograph of the European Southern Observatory Very Large Telescope. Various forbidden atomic emission lines of [CI], [NI], and [OI] were observed in the optical spectrum of this comet when it was at 2.8 au from the Sun. The observed forbidden emission intensity ratios are studied in the framework of a couple-chemistry emission model. <BR /> Results: The model calculations show that CO[SUB]2[/SUB] is the major source of both atomic oxygen green and red doublet emissions in the coma of C/2016 R2 (while for most comets it is generally H[SUB]2[/SUB]O), whereas, CO and N[SUB]2[/SUB] govern the atomic carbon and nitrogen emissions, respectively. Our modeled oxygen green-to-red-doublet and carbon-to- nitrogen emission ratios are higher by a factor of three than what is found from observations. These discrepancies could be due to uncertainties associated with photon cross sections or unknown production and/or loss sources. Our modeled oxygen green-to-red-doublet emission ratio is close to what is seen in observations when we consider an O[SUB]2[/SUB] abundance with a production rate of 30% relative to the CO production rate. We constrained the mean photodissociation yield of CO, producing C([SUP]1[/SUP]S) at about 1%, a quantity which has not been measured in the laboratory. The collisional quenching is not a significant loss process for N([SUP]2[/SUP]D) though its radiative lifetime is significant (~10 h). Hence, the observed [NI] doublet- emission ratio ([NI] 5198/5200) of 1.22, which is smaller than the terrestrial measurement by a factor 1.4, is mainly due to the characteristic radiative decay of N([SUP]2[/SUP]D). [less ▲]

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See detailSpatially separated continuum sources revealed by microlensing in the gravitationally lensed broad absorption line quasar SDSS J081830.46+060138.0⋆
Hutsemekers, Damien ULiege; Sluse, Dominique ULiege; Kumar, P.

in Astronomy and Astrophysics (2020), 633

Gravitational microlensing is a powerful tool for probing the inner structure of distant quasars. In this context, we have obtained spectropolarimetric observations of the two images of the broad ... [more ▼]

Gravitational microlensing is a powerful tool for probing the inner structure of distant quasars. In this context, we have obtained spectropolarimetric observations of the two images of the broad absorption line (BAL) quasar SDSS J081830.46+060138.0 (J0818+0601) at redshift z ≃ 2.35. We first show that J0818+0601 is actually gravitationally lensed, and not a binary quasar. A strong absorption system detected at z = 1.0065 ± 0.0002 is possibly due to the lensing galaxy. Microlensing is observed in one image and it magnifies the emission lines, the continuum, and the BALs differently. By disentangling the part of the spectrum that is microlensed from the part that is not microlensed, we unveil two sources of continuum that must be spatially separated: a compact one, which is microlensed, and an extended one, which is not microlensed and contributes to two thirds of the total continuum emission. J0818+0601 is the second BAL quasar in which an extended source of rest-frame ultraviolet continuum is found. We also find that the images are differently polarized, suggesting that the two continua might be differently polarized. Our analysis provides constraints on the BAL flow. In particular, we find that the outflow is seen with a nonzero onset velocity, and stratified according to ionization. <P />The reduced spectra are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/633/A101">http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/633/A101</A> <P />Based on observations made with ESO Very Large Telescope at the Paranal Observatory under program ID 100.B-0590. [less ▲]

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See detailChanging-look Seyfert galaxies with optical linear polarization measurements
Marin, F.; Hutsemekers, Damien ULiege; Agis-Gonzalez, Beatriz ULiege

in SF2A-2019: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics (2019, December 01)

In this lecture note, we make the case for new (spectro)polarimetric measurements of ``changing-look'' AGNs (CLAGNs), a subclass of the AGN family tree that shows long-term (months to years) large flux ... [more ▼]

In this lecture note, we make the case for new (spectro)polarimetric measurements of ``changing-look'' AGNs (CLAGNs), a subclass of the AGN family tree that shows long-term (months to years) large flux variability associated with the appearance or disappearance of optical broad emission lines. We discuss how polarization measurements could help to distinguish which of the several scenarios proposed to explain such variations is/are the most likely. We collected all the past polarization measurements of nearby, Seyfert-like CLAGNs and take stock that almost all polarimetric information we have on those fascinating objects dates from the 80's and 90's. We thus explain how polarization could help us understanding the physical processes happening in the first parsecs of CLAGNs and why new polarization monitoring campaigns are strongly needed. [less ▲]

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See detailVizieR Online Data Catalog: SDSS J081830.46+060138.0 spectropolarimetry (Hutsemekers+, 2020)
Hutsemekers, Damien ULiege; Sluse, Dominique ULiege; Kumar, P.

Textual, factual or bibliographical database (2019)

This Table contains the spectropolarimetric data obtained for images A and B of the gravitationally lensed quasar SDSS J081830.46+060138 on December 24 and 25, 2017, with FORS2 at the ESO VLT. <P />(1 ... [more ▼]

This Table contains the spectropolarimetric data obtained for images A and B of the gravitationally lensed quasar SDSS J081830.46+060138 on December 24 and 25, 2017, with FORS2 at the ESO VLT. <P />(1 data file). [less ▲]

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See detailComet 66P/du Toit: not a near-Earth main belt comet
Yang, Bin; Jehin, Emmanuel ULiege; Pozuelos, Francisco J. et al

in Astronomy and Astrophysics (2019), 631

Context. Main belt comets (MBCs) are a peculiar class of volatile- containing objects with comet-like morphology and asteroid-like orbits. However, MBCs are challenging targets to study remotely due to ... [more ▼]

Context. Main belt comets (MBCs) are a peculiar class of volatile- containing objects with comet-like morphology and asteroid-like orbits. However, MBCs are challenging targets to study remotely due to their small sizes and the relatively large distance they are from the Sun and the Earth. Recently, a number of weakly active short-period comets have been identified that might originate in the asteroid main belt. Among all of the known candidates, comet 66P/du Toit has been suggested to have one of the highest probabilities of coming from the main belt. <BR /> Aims: The main goal of this study is to investigate the physical properties of 66P via spectroscopic and imaging observations to constrain its formation conditions. In particular, the isotopic abundance ratio and the ortho-to-para ratio (OPR) of gaseous species can be derived via high-resolution spectroscopy, which is sensitive to the formation temperature of the nucleus. <BR /> Methods: We obtained medium and high-resolution spectra of 66P from 300-2500 nm with the X-shooter and the UVES instruments at the Very Large Telescope in July 2018. We also obtained a series of narrow-band images of 66P to monitor the gas and dust activity between May and July 2018 with TRAPPIST-South. In addition, we applied a dust model to characterize the dust coma of 66P and performed dynamical simulations to study the orbital evolution of 66P. <BR /> Results: We derive the OPR of ammonia (NH[SUB]3[/SUB]) in 66P to be 1.08 ± 0.06, which corresponds to a nuclear spin temperature of 34 K. We compute the production rates of OH, NH, CN, C[SUB]3[/SUB], and C[SUB]2[/SUB] radicals and measure the dust proxy, Afρ. The dust analysis reveals that the coma can be best-fit with an anisotropic model and the peak dust production rate is about 55 kg s[SUP]-1[/SUP] at the perihelion distance of 1.29 au. Dynamical simulations show that 66P is moderately asteroidal with the capture time, t[SUB]cap[/SUB] 10[SUP]4[/SUP] yr. <BR /> Conclusions: Our observations demonstrate that the measured physical properties of 66P are consistent with typical short-period comets and differ significantly from other MBCs. Therefore, 66P is unlikely to have a main belt origin. [less ▲]

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See detailConstraining the geometry and kinematics of the quasar broad emission line region using gravitational microlensing. II. Comparing models with observations in the lensed quasar HE0435-1223
Hutsemekers, Damien ULiege; Braibant, L.; Sluse, Dominique ULiege et al

in Astronomy and Astrophysics (2019), 629

The quadruply lensed quasar HE0435-1223 shows a clear microlensing effect that affects differently the blue and red wings of the Hα line profile in its image D. To interpret these observations, and ... [more ▼]

The quadruply lensed quasar HE0435-1223 shows a clear microlensing effect that affects differently the blue and red wings of the Hα line profile in its image D. To interpret these observations, and constrain the broad emission line region (BLR) properties, the effect of gravitational microlensing on quasar broad emission line profiles and their underlying continuum has been simulated considering representative BLR models and microlensing magnification maps. The amplification and distortion of the Hα line profile, characterized by a set of four indices, can be reproduced by the simulations. Although the constraints on the BLR models set by the observed single-epoch microlensing signal are not very robust, we found that flattened geometries (Keplerian disk and equatorial wind) can more easily reproduce the observed line profile deformations than a biconical polar wind. With an additional independent constraint on the size of the continuum source, the Keplerian disk model of the Hα BLR is slightly favored. [less ▲]

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See detailVLT multi-instrument observations of comet 46P/Wirtanen
Opitom, Cyrielle; Hutsemekers, Damien ULiege; Manfroid, Jean ULiege et al

Conference (2019, September 01)

We present observations of the Jupiter family comet 46P/Wirtanen performed at the ESO/VLT in December 2018 with three different instruments: the high-resolution spectrographs UVES and ESPRESSO, and the ... [more ▼]

We present observations of the Jupiter family comet 46P/Wirtanen performed at the ESO/VLT in December 2018 with three different instruments: the high-resolution spectrographs UVES and ESPRESSO, and the MUSE integral field spectrograph. Combining those observations, we investigate the coma composition and main activity drivers of 46P, as well as species parentage and jets in its coma. [less ▲]

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See detailModeling of the CO+ emission spectrum in comet C/2016 R2 (PanSTARRS)
Rousselot, Philippe; Jehin, Emmanuel ULiege; Curti, Mathieu et al

Conference (2019, September 01)

The long period comet C/2016 R2 (PanSTARRS), discovered on 2016 September 7 by the PansSTARRS telescope, presented an unusual composition. Most of the emission lines due to radicals usually abundant in ... [more ▼]

The long period comet C/2016 R2 (PanSTARRS), discovered on 2016 September 7 by the PansSTARRS telescope, presented an unusual composition. Most of the emission lines due to radicals usually abundant in comets (such as C2, CN, C3 ) appeared to be faint while unexpected strong emission lines due to N2+ and CO+ ions were clearly apparent in the spectra obtained in the optical range (some fainter CO2+ emission lines were also present). It is the first comet with such bright N2+ emission lines but also the brightness and abundance of CO+ lines allow to study this ion in great details. We obtained in February 2018 high resolution spectra with the ESO 8-m Very Large Telescope and the UVES spectrograph, that allowed us to obtain a CO+ spectrum with an unprecedented signal-to-noise ratio. We also developed a fluorescence model for CO+ to model these spectra. We will present these observations, their modeling and a first estimate of the 12C/13C isotopic ratio for CO+ ions in comet C/2016 R2. It is the first measurement of such a ratio with ground-based telescopes, the only related measurement published so far being done in situ in the coma of comet 67P/Churyumov-Gerasimenko by the ROSINA instrument on-board the Rosetta spacecraft for CO and CO2 molecules. [less ▲]

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See detailThe unique spectrum of comet C/2016 R2 (PanSTARRS)
Jehin, Emmanuel ULiege; Opitom, Cyrielle; Hutsemekers, Damien ULiege et al

Conference (2019, September 01)

We present optical observations of the long period comet C/2016 R2 (PanSTARRS) performed with the high resolution UVES spectrograph at the ESO VLT (Paranal), complemented by low resolution spectra at INT ... [more ▼]

We present optical observations of the long period comet C/2016 R2 (PanSTARRS) performed with the high resolution UVES spectrograph at the ESO VLT (Paranal), complemented by low resolution spectra at INT (La Palma) and narrow-band imaging obtained with the TRAPPIST telescopes. We show that this comet has a unique chemical composition, with strong N2+ and CO+ emission lines in the optical, several well detected CO2+ bands while the usual radicals CN, C3, C2, are very faint. OH, OH+ and H2O+ as well as NH and NH2, are not detected, suggesting that C/2016 R2 is a comet rich in N2 and CO ices but poor in water and ammonia. We will present new results obtained from these observations and discuss the origin of this type of comets. [less ▲]

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See detailSearch for water outgassing of (1) Ceres near perihelion
Rousselot, P.; Opitom, C.; Jehin, Emmanuel ULiege et al

in Astronomy and Astrophysics (2019), 628

Context. (1) Ceres is the largest body in the main asteroid belt and one of the most intriguing objects in the solar system, in part because of the discovery of water outgassing by the Herschel Space ... [more ▼]

Context. (1) Ceres is the largest body in the main asteroid belt and one of the most intriguing objects in the solar system, in part because of the discovery of water outgassing by the Herschel Space Observatory (HSO) and its still-debated origin. Ceres was the target of NASA's Dawn spacecraft for 3.5 yr, which achieved a detailed characterization of the dwarf planet. The possible influence of the local flux of solar energetic particles (SEP) on the production of a Cerean exosphere and water vapor has been suggested, in addition to the sublimation of water ice that depends on the temperature, meaning the heliocentric distance. <BR /> Aims: We used the opportunity of both the perihelion passage of (1) Ceres in April 2018, and the presence of Dawn in its vicinity (for measuring the SEP flux in real time) to check the influence of heliocentric distance and SEP flux on water outgassing. <BR /> Methods: We searched for OH emission lines near the limb of Ceres in the near-UV with the UVES spectrograph mounted on the 8-m ESO Very Large Telescope. Two spectra were recorded when Ceres was close to its perihelion, in February 2018, and with Dawn spacecraft orbiting Ceres. It was possible to simultaneously measure energetic particles around Ceres at the time of our observations. <BR /> Results: Our observations did not permit detection of OH emission lines to a very high sensitivity level. This level is estimated to correspond to a global water production rate of Q[SUB]H[SUB]2[/SUB]O[/SUB] ∽ 2 × 10[SUP]26[/SUP] molecules s[SUP]-1[/SUP], similar to the water production rate derived from HSO observations. The solar energetic particles flux measured around Ceres was negligible at the time of these observations. <BR /> Conclusions: Our observations support the idea that heliocentric distance (i.e., the sublimation of water ice) does not play a major role in the water emission from Ceres. This production rate could be either related to SEP events or to other mechanisms, possibly of endogenic origin. Based on observations collected at the European Southern Observatory under ESO program 2100.C-5038(A). [less ▲]

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See detailPolarization of changing-look quasars
Hutsemekers, Damien ULiege; Agis-Gonzalez, Beatriz ULiege; Marin, F. et al

in Astronomy and Astrophysics (2019), 625

If the disappearance of the broad emission lines observed in changing- look quasars originates from the obscuration of the quasar core by dusty clouds moving in the torus, high linear optical polarization ... [more ▼]

If the disappearance of the broad emission lines observed in changing- look quasars originates from the obscuration of the quasar core by dusty clouds moving in the torus, high linear optical polarization would be expected in those objects. We then measured the rest-frame UV-blue linear polarization of a sample of 13 changing-look quasars, 7 of them being in a type 1.9-2 state. For all quasars but one the polarization degree is lower than 1%. This suggests that the disappearance of the broad emission lines cannot be attributed to dust obscuration, and supports the scenario in which changes of look are caused by a change in the rate of accretion onto the supermassive black hole. Such low polarization degrees also indicate that these quasars are seen under inclinations close to the system axis. One type 1.9-2 quasar in our sample shows a high polarization degree of 6.8%. While this polarization could be ascribed to obscuration by a moving dusty cloud, we argue that this is unlikely given the very long time needed for a cloud from the torus to eclipse the broad emission line region of that object. We propose that the high polarization is due to the echo of a past bright phase seen in polar-scattered light. This interpretation raises the possibility that broad emission lines observed in the polarized light of some type 2 active galactic nuclei can be echoes of past type 1 phases and not evidence of hidden broad emission line regions. Based on observations made with the William Herschel telescope operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias and observations made with ESO Very Large Telescope at the Paranal Observatory under program ID 101.B-0209. [less ▲]

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See detailHigh resolution optical spectroscopy of the N2-rich comet C/2016 R2 (PanSTARRS)
Opitom, C.; Hutsemekers, Damien ULiege; Jehin, Emmanuel ULiege et al

in Astronomy and Astrophysics (2019), 624

Context. Early observations of comet C/2016 R2 (PanSTARRS) have shown that the composition of this comet is very peculiar. Radio observations have revealed a CO-rich and HCN-poor comet and an optical coma ... [more ▼]

Context. Early observations of comet C/2016 R2 (PanSTARRS) have shown that the composition of this comet is very peculiar. Radio observations have revealed a CO-rich and HCN-poor comet and an optical coma dominated by strong emission bands of CO[SUP]+[/SUP] and, more surprisingly, N[SUB]2[/SUB][SUP]+[/SUP]. <BR /> Aims: The strong detection of N[SUB]2[/SUB][SUP]+[/SUP] in the coma of C/2016 R2 provided an ideal opportunity to measure the [SUP]14[/SUP]N/[SUP]15[/SUP]N isotopic ratio directly from N[SUB]2[/SUB][SUP]+[/SUP] for the first time, and to estimate the N[SUB]2[/SUB]/CO ratio, which is an important diagnostic to constrain formation models of planetesimals, in addition to the more general study of coma composition. <BR /> Methods: We obtained high resolution spectra of the comet in February 2018 when it was at 2.8 au from the Sun. We used the UVES spectrograph of the European Southern Observatory Very Large Telescope, complemented with narrowband images obtained with the TRAPPIST telescopes. <BR /> Results: We detect strong emissions from the N[SUB]2[/SUB][SUP]+[/SUP] and CO[SUP]+[/SUP] ions, but also CO[SUB]2[/SUB][SUP]+[/SUP], emission lines from the CH radical, and much fainter emissions of the CN, C[SUB]2[/SUB], and C[SUB]3[/SUB] radicals that were not detected in previous observations of this comet. We do not detect OH or H[SUB]2[/SUB]O[SUP]+[/SUP], and we derive an upper limit of the H[SUB]2[/SUB]O[SUP]+[/SUP]/CO[SUP]+[/SUP] ratio of 0.4, implying that the comet has a low water abundance. We measure a N[SUB]2[/SUB][SUP]+[/SUP]/CO[SUP]+[/SUP] ratio of 0.06 ± 0.01. The non-detection of NH[SUB]2[/SUB] indicates that most of the nitrogen content of the comet is in N[SUB]2[/SUB]. Together with the high N[SUB]2[/SUB][SUP]+[/SUP]/CO[SUP]+[/SUP] ratio, this could indicate a low formation temperature of the comet or that the comet is a fragment of a large differentiated Kuiper Belt object. The CO[SUB]2[/SUB][SUP]+[/SUP]/CO[SUP]+[/SUP] ratio is 1.1 ± 0.3. We do not detect [SUP]14[/SUP]N[SUP]15[/SUP]N[SUP]+[/SUP] lines and can only put a lower limit on the [SUP]14[/SUP]N/[SUP]15[/SUP]N ratio (measured from N[SUB]2[/SUB][SUP]+[/SUP]) of about 100, which is compatible with measurements of the same isotopic ratio for NH[SUB]2[/SUB] and CN in other comets. Finally, in addition to the [OI] and [CI] forbidden lines, we detect for the first time the forbidden nitrogen lines [NI] doublet at 519.79 and 520.03 nm in the coma of a comet. [less ▲]

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See detailQuasar Microlensing: Revolutionizing our Understanding of Quasar Structure and Dynamics
Moustakas, Leonidas; O'Dowd, Matthew; Anguita, Timo et al

in arXiv e-prints (2019)

Microlensing by stars within distant galaxies acting as strong gravitational lenses of multiply-imaged quasars, provides a unique and direct measurement of the internal structure of the lensed quasar on ... [more ▼]

Microlensing by stars within distant galaxies acting as strong gravitational lenses of multiply-imaged quasars, provides a unique and direct measurement of the internal structure of the lensed quasar on nano-arcsecond scales. The measurement relies on the temporal variation of high-magnification caustic crossings which vary on timescales of days to years. Multiwavelength observations provide information from distinct emission regions in the quasar. Through monitoring of these strong gravitational lenses, a full tomographic view can emerge with Astronomical-Unit scale resolution. Work to date has demonstrated the potential of this technique in about a dozen systems. In the 2020s there will be orders of magnitude more systems to work with. Monitoring of lens systems for caustic-crossing events to enable triggering of multi- platform, multi-wavelength observations in the 2020s will fulfill the potential of quasar microlensing as a unique and comprehensive probe of active black hole structure and dynamics. [less ▲]

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See detailChanging-look quasars: a challenge for the AGN unification model
Hutsemekers, Damien ULiege

Scientific conference (2019, March 05)

Type 1 active galactic nuclei (AGNs) are characterized by both broad and narrow emission lines in their optical spectrum while type 2 AGNs only show narrow emission lines. The “unification model” suggests ... [more ▼]

Type 1 active galactic nuclei (AGNs) are characterized by both broad and narrow emission lines in their optical spectrum while type 2 AGNs only show narrow emission lines. The “unification model” suggests that type 1 and type 2 AGNs are the same objects viewed under different inclinations, the AGN core being obscured by an equatorial dusty torus in type 2 objects. A key argument in favor of the unification model was the discovery of hidden broad line regions in type 2 AGNs using spectropolarimetry. A handful of high luminosity AGNs (quasars) changing from type 1 to type 2 or vice-versa on timescales of a few years have been recently uncovered, thus challenging the unification model. To explain these spectral changes, two main scenarios have been proposed: a variation of the rate of accretion onto the supermassive black hole, or variable dust absorption due to the motion of individual clouds in a clumpy torus. Observations constraining these scenarios will be discussed, with particular emphasis on polarization measurements. [less ▲]

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See detailThe role of Active Galactic Nuclei in galaxy evolution: insights from space ultraviolet spectropolarimetry
Marin, F.; Charlot, S.; Agis-Gonzalez, Beatriz ULiege et al

in arXiv e-prints (2019)

This Astro2020 white paper summarizes the unknowns of active galactic nuclei (AGN) physics that could be unveiled thanks to a new, space-born, ultraviolet spectropolarimeter. The unique capabilities of ... [more ▼]

This Astro2020 white paper summarizes the unknowns of active galactic nuclei (AGN) physics that could be unveiled thanks to a new, space-born, ultraviolet spectropolarimeter. The unique capabilities of high energy polarimetry would help us to understand the precise mechanisms of matter and energy transfer and supermassive black holes growth, together with the impact of AGN feedback on galaxy evolution. [less ▲]

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See detailTRUE2: Establishing a detectability limit on hidden broad line regions
Agis-Gonzalez, Beatriz ULiege; Hutsemekers, Damien ULiege; Acosta Pulido, Jose et al

in "Highlights on Spanish Astrophysics X, Proceedings of the XIII Scientific Meeting of the Spanish Astronomical Society" (ISBN 978-84-09-09331-1) (2019, March)

True Seyfert 2 candidates are those Seyferts galaxies whose optical spectral do not show broad lines, nevertheless in the X-ray domain, they exhibit some characteristic behavior of Seyferts 1 such as lack ... [more ▼]

True Seyfert 2 candidates are those Seyferts galaxies whose optical spectral do not show broad lines, nevertheless in the X-ray domain, they exhibit some characteristic behavior of Seyferts 1 such as lack of X-ray obscuration and/or short timescale variability. A true 2 candidate will be confirmed as a true Seyfert 2 if the lack of its broad line region (BLR) is not only observational but physical. These kind of objects are thought to accrete at low Eddington rates, in agreement with theoretical models that predict that the BLR disappears below a certain critical value of accretion rate and/or luminosity. In the last decade, a significant number of true Seyfert 2s with low accretion rates has been claimed in the literature. However, some exceptions as GNS 069 or 2XMM J1231+1106 show high accretion rates, which seem to contradict the generally accepted explanation. A limit on the detection of hidden broad line regions (HBLRs) must be established in order to make sure that BLRs are not present intrinsically. Since true Seyfert 2 candidates are selected by the absence of X-ray obscuration, the most plausible explanation to cause the non-detection of a physically present HBLR would be the absence of an adequate scattering medium. Polarimetry can play a key role to answer this question. The presence of an efficient scattering region would imply a high continuum of polarization. We propose to assess what degrees of polarization are high enough to indicate the presence of a scattering medium able to act as a mirror and thus providing us with the indirect view of the HBLRs. We got new imaging polarimetry data from ISIS@WHT of 10 true 2 candidates which had not been checked in polarized light. If scattering regions are present, undeniable degrees of polarization around 1−3% should be measured. Comparing the measured continuum of polarization with simulations we will be able to estimate a decidability limit on HBLRs. Specifically, we will apply STOKES, a Monte Carlo radiative transfer code which can be used to model, predict, fit and interpret the polarization of AGN [less ▲]

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See detailAnalytic methods for the Abel transform of exponential functions describing planetary and cometary atmospheres.
Hubert, Benoît ULiege; Munhoven, Guy ULiege; Opitom, Cyrielle et al

Poster (2018, December 11)

Remote sensing of planetary and cometary atmosphere is one of the most important source of data and knowledge of the gas layers surrounding the celestial objects of our solar system, including our own ... [more ▼]

Remote sensing of planetary and cometary atmosphere is one of the most important source of data and knowledge of the gas layers surrounding the celestial objects of our solar system, including our own planet. Most of the instruments used up to now and that will be used in a near future study the emission of radiations directly produced by the atmosphere. Under optically thin conditions, this observation method provides the local volume emission rate (VER) originating from the atmosphere, integrated along the full line of sight (l.o.s.) of the instrument. Under a spherical or cylindrical symmetry assumption, the l.o.s. integration of the VER takes the form of the Abel transform of the vertical VER profile. The simplest analytical functions representing VER profiles in real planetary and cometary atmosphere include an exponential function of the altitude (or radial distance), giving the isothermal profile for a planet and the Haser model for a coma. The Abel transform of these functions can be computed analytically using combinations of special functions. Retrieving the vertical (radial) profile of the VER does however require to invert the observed Abel transform to account for possible departures from these idealized analytical expressions, so that indefinite integrals defined from the Abel integral (which we will call indefinite Abel transforms) are needed (or numerical integrations need to be performed). In this study, we present a new method to produce a workable series development allowing accurate computation of the indefinite Abel transforms that appear in the study of optically thin emissions of planetary and cometary atmospheres. Indeed, taking the Taylor series development of the exponential function to reduce the problem to a series of indefinite Abel transforms of polynomial functions (which can be carried analytically) does not work. It leads to the computation of the difference of large, nearly equal numbers, which cannot be done accurately. Our method rather relies on an appropriate series development of the Jacobian of the Abel transform. We show that the computation can be done reliably up to near machine precision, and that accuracy control can be enforced for tailored applications. Possible applications are considered, that include the study of comas and of the upper atmosphere of Mars and the Earth [less ▲]

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See detaillnsight on quasar changing-look physics from optical polarimetry
Hutsemekers, Damien ULiege; Agis-Gonzalez, Beatriz ULiege; Sluse, Dominique ULiege et al

Conference (2018, December 11)

A handful of quasars changing from type 1 (strong broad and narrow emission lines) to type 1.9 (strong narrow lines only and dim continuum) on timescales of a few years have been recently uncovered. If ... [more ▼]

A handful of quasars changing from type 1 (strong broad and narrow emission lines) to type 1.9 (strong narrow lines only and dim continuum) on timescales of a few years have been recently uncovered. If the disappearance of the broad emission lines observed in changing-look quasars were caused by the obscuration of the quasar core through moving dust clouds in the torus, high linear polarization typical of type 2 quasars would be expected. We measured the polarization of the changing-look quasar J1011+5442 in which the broad emission lines have disappeared between 2003 and 2015. We found a polarization degree compatible with null polarization. This measurement suggests that the observed change of look is not due to a change of obscuration in a torus hiding the continuum source and the broad line region. Our results thus support the idea that the vanishing of the broad emission lines in J1011+5442 is due to an intrinsic dimming of the ionizing continuum source that is most likely caused by a rapid decrease in the rate of accretion onto the supermassive black hole. New polarization measurements have been secured for a sample of changing-look quasars. They essentially confirm our previous results. [less ▲]

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