arXiv subjects

Willem-Jan de Wit

Publications and source records attributed to Willem-Jan de Wit.

At least 19 recordsLinked to original sources

arXiv2023

Effects of the Hunga Tonga-Hunga Ha'apai Volcanic Eruption on Observations at Paranal Observatory

The Hunga Tonga-Hunga Ha'apai volcano erupted on 15 January 2022 with an energy equivalent to around 61 megatons of TNT. The explosion was bigger than any other volcanic eruption so far in the 21st century. Huge quantities of particles, including dust and water vapour, were released into the atmosphere. We present the results of a preliminary study of the effects of the explosion on observations taken at Paranal Observatory using a range of instruments. These effects were not immediately transitory in nature, and a year later stunning sunsets are still being seen at Paranal.

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arXiv2020

Towards a physical understanding of the thermal background in large ground-based telescopes

Ground-based thermal-infrared observations have a unique scientific potential, but are also extremely challenging due to the need to accurately subtract the high thermal background. Since the established techniques of chopping and nodding need to be modified for observations with the future mid-infrared ELT imager and spectrograph (METIS), we investigate the sources of thermal background subtraction residuals. Our aim is to either remove or at least minimise the need for nodding in order to increase the observing efficiency for METIS. To this end we need to improve our knowledge about the origin of chop residuals and devise observing methods to remove them most efficiently, i.e. with the slowest possible nodding frequency. Thanks to dedicated observations with VLT/VISIR and GranTeCan/CanariCam, we have successfully traced the origin of three kinds of chopping residuals to (1) the entrance window, (2) the spiders and (3) other warm emitters in the pupil, in particular the VLT M3 mirror cell in its parking position. We conclude that, in order to keep chopping residuals stable over a long time (and therefore allow for slower nodding cycles), the pupil illumination needs to be kept constant, i.e. (imaging) observations should be performed in pupil-stabilised, rather than field-stabilised mode, with image de-rotation in the post-processing pipeline. This is now foreseen as the default observing concept for all METIS imaging modes.

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arXiv2018

Multiple Star Systems in the Orion Nebula

This work presents an interferometric study of the massive-binary fraction in the Orion Trapezium Cluster with the recently comissioned GRAVITY instrument. We observe a total of 16 stars of mainly OB spectral type. We find three previously unknown companions for $θ^1$ Ori B, $θ^2$ Ori B, and $θ^2$ Ori C. We determine a separation for the previously suspected companion of NU Ori. We confirm four companions for $θ^1$ Ori A, $θ^1$ Ori C, $θ^1$ Ori D, and $θ^2$ Ori A, all with substantially improved astrometry and photometric mass estimates. We refine the orbit of the eccentric high-mass binary $θ^1$ Ori C and we are able to derive a new orbit for $θ^1$ Ori D. We find a system mass of 21.7 $M_{\odot}$ and a period of $53$ days. Together with other previously detected companions seen in spectroscopy or direct imaging, eleven of the 16 high-mass stars are multiple systems. We obtain a total number of 22 companions with separations up to 600 AU. The companion fraction of the early B and O stars in our sample is about 2, significantly higher than in earlier studies of mostly OB associations. The separation distribution hints towards a bimodality. Such a bimodality has been previously found in A stars, but rarely in OB binaries, which up to this point have been assumed to be mostly compact with a tail of wider companions. We also do not find a substantial population of equal-mass binaries. The observed distribution of mass ratios declines steeply with mass, and like the direct star counts, indicates that our companions follow a standard power law initial mass function. Again, this is in contrast to earlier findings of flat mass ratio distributions in OB associations. We exclude collision as a dominant formation mechanism but find no clear preference for core accretion or competitive accretion.

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arXiv2018

A sextet of clusters in the Vela OB2 region revealed by Gaia

Using Gaia DR2 data, combined with OmegaCAM ground-based optical photometry from the AD-HOC survey, and detailed Radial Velocity measurements from ESO-Gaia, we analyse in detail a 10x5 deg region around the Wolf-Rayet star $γ^2$ Vel, including the previously known clusters Gamma Vel and NGC2547. Using clustering analysis that considers positions, proper motions and parallax, we discover 6 clusters or associations -- 4 of which appear new. Analysis of the colour-magnitude diagram for these clusters show that 4 of them formed coevally from the same molecular clouds 10 Myr ago, while NGC 2547 formed together with a newly discovered cluster 30 Myr ago. This study shows the incredible wealth of data provided by Gaia for the study of young stellar clusters.

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arXiv2018

Radiative transfer modelling of W33A MM1: 3-D structure and dynamics of a complex massive star forming region

We present a composite model and radiative transfer simulations of the massive star forming core W33A MM1. The model was tailored to reproduce the complex features observed with ALMA at $\approx 0.2$ arcsec resolution in CH$_3$CN and dust emission. The MM1 core is fragmented into six compact sources coexisting within $\sim 1000$ au. In our models, three of these compact sources are better represented as disc-envelope systems around a central (proto)star, two as envelopes with a central object, and one as a pure envelope. The model of the most prominent object (Main) contains the most massive (proto)star ($M_\star\approx7~M_\odot$) and disc+envelope ($M_\mathrm{gas}\approx0.4~M_\odot$), and is the most luminous ($L_\mathrm{Main} \sim 10^4~L_\odot$). The model discs are small (a few hundred au) for all sources. The composite model shows that the elongated spiral-like feature converging to the MM1 core can be convincingly interpreted as a filamentary accretion flow that feeds the rising stellar system. The kinematics of this filament is reproduced by a parabolic trajectory with focus at the center of mass of the region. Radial collapse and fragmentation within this filament, as well as smaller filamentary flows between pairs of sources are proposed to exist. Our modelling supports an interpretation where what was once considered as a single massive star with a $\sim 10^3$ au disc and envelope, is instead a forming stellar association which appears to be virialized and to form several low-mass stars per high-mass object.

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arXiv2017

Science cases for a visible interferometer

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized on the 15th and 16th of January, 2015 in Nice with the support of the Action Specific in Haute Resolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la Cote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41.

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arXiv2016

A high-mass protobinary system with spatially resolved circumstellar accretion disks and circumbinary disk

High-mass multiples might form via fragmentation of self-gravitational disks or alternative scenarios such as disk-assisted capture. However, only few observational constraints exist on the architecture and disk structure of high-mass protobinaries and their accretion properties. Here we report the discovery of a close ($57.9\pm0.2$mas=170au) high-mass protobinary, IRAS17216-3801, where our VLTI/GRAVITY+AMBER near-infrared interferometry allows us to image the circumstellar disks around the individual components with 3 milliarcsecond resolution. We estimate the component masses to $\sim20$ and $\sim18 M_{\odot}$ and find that the radial intensity profiles can be reproduced with an irradiated disk model, where the inner regions are excavated of dust, likely tracing the dust sublimation region in these disks. The circumstellar disks are strongly misaligned with respect to the binary separation vector, which indicates that the tidal forces did not have time to realign the disks, pointing towards a young dynamical age of the system. We constrain the distribution of the Br$γ$ and CO-emitting gas using VLTI/GRAVITY spectro-interferometry and VLT/CRIRES spectro-astrometry and find that the secondary is accreting at a higher rate than the primary. VLT/NACO imaging shows $L'$-band emission on 3-4 times larger scales than the binary separation, matching the expected dynamical truncation radius for the circumbinary disk. The IRAS17216-3801 system is $\sim3\times$ more massive and $\sim5\times$ more compact than other high-mass multiplies imaged at infrared wavelengths and the first high-mass protobinary system where circumstellar and circumbinary dust disks could be spatially resolved. This opens exciting new opportunities for studying star-disk interactions and the role of multiplicity in high-mass star formation.

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arXiv2016

The 2nd Generation VLTI path to performance

The upgrade of the VLTI infrastructure for the 2nd generation instruments is now complete with the transformation of the laboratory, and installation of star separators on both the 1.8-m Auxiliary Telescopes (ATs) and the 8-m Unit Telescopes (UTs). The Gravity fringe tracker has had a full semester of commissioning on the ATs, and a first look at the UTs. The CIAO infrared wavefront sensor is about to demonstrate its performance relative to the visible wavefront sensor MACAO. First astrometric measurements on the ATs and astrometric qualification of the UTs are on-going. Now is a good time to revisit the performance roadmap for VLTI that was initiated in 2014, which aimed at coherently driving the developments of the interferometer, and especially its performance, in support to the new generation of instruments: Gravity and MATISSE.

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arXiv2016

Planet Formation Imager (PFI): science vision and key requirements

The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to about 100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs.

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arXiv2016

The LBV HR Car has a partner: Discovery of a companion with the VLTI

Luminous Blue Variables (LBVs) are massive stars caught in a post-main sequence phase, during which they are losing a significant amount of mass. As, on one hand, it is thought that the majority of massive stars are close binaries that will interact during their lifetime, and on the other, the most dramatic example of an LBV, Eta Car, is a binary, it would be useful to find other binary LBVs. We present here interferometric observations of the LBV HR Car done with the AMBER and PIONIER instruments attached to ESO's Very Large Telescope Interferometer (VLTI). Our observations, spanning two years, clearly reveal that HR Car is a binary star. It is not yet possible to constrain fully the orbit, and the orbital period may lie between a few years and several hundred years. We derive a radius for the primary in the system and possibly resolve as well the companion. The luminosity ratio in the H-band between the two components is changing with time, going from about 6 to 9. We also tentatively detect the presence of some background flux which remained at the 2% level until January 2016, but then increased to 6% in April 2016. Our AMBER results show that the emission line forming region of Br gamma is more extended than the continuum emitting region as seen by PIONIER and may indicate some wind-wind interaction. Most importantly, we constrain the total masses of both components, with the most likely range being 33.6 and 45 solar masses. Our results show that the LBV HR Car is possibly an Eta Car analog binary system with smaller masses, with variable components, and further monitoring of this object is definitively called for.

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arXiv2016

Training telescope operators and support astronomers at Paranal

The operations model of the Paranal Observatory relies on the work of efficient staff to carry out all the daytime and nighttime tasks. This is highly dependent on adequate training. The Paranal Science Operations department (PSO) has a training group that devises a well-defined and continuously evolving training plan for new staff, in addition to broadening and reinforcing courses for the whole department. This paper presents the training activities for and by PSO, including recent astronomical and quality control training for operators, as well as adaptive optics and interferometry training of all staff. We also present some future plans.

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arXiv2015

Star Formation at milli-arcsecond resolution

This chapter discusses the use and possibilities of optical and infrared interferometry to study star formation. The chapter starts with a brief overview of the star formation process and highlights the open questions from an observational point of view. These are found at the smallest scales, as this is, inevitably, where all the action such as accretion and outflows, occurs. We then use basic astrophysical concepts to assess which scales and conditions can be probed with existing interferometric set-ups for which we use the ESO/VLTI instrument suite as example. We will concentrate on the more massive stars observed at high resolution with continuum interferometry. Throughout, some of the most recent interferometric results are used as examples of the various processes discussed.

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arXiv2015

A multi-wavelength interferometric study of the massive young stellar object IRAS 13481-6124

We present new mid-infrared interferometric observations of the massive young stellar object IRAS 13481-6124, using VLTI/MIDI for spectrally-resolved, long-baseline measurements (projected baselines up to $\sim120$ m) and GSO/T-ReCS for aperture-masking interferometry in five narrow-band filters (projected baselines of $\sim1.8-6.4$ m) in the wavelength range of $7.5-13$ $μ$m. We combine these measurements with previously-published interferometric observations in the $K$ and $N$ bands in order to assemble the largest collection of infrared interferometric observations for a massive YSO to date. Using a combination of geometric and radiative-transfer models, we confirm the detection at mid-infrared wavelengths of the disk previously inferred from near-infrared observations. We show that the outflow cavity is also detected at both near- and mid-infrared wavelengths, and in fact dominates the mid-infrared emission in terms of total flux. For the disk, we derive the inner radius ($\sim1.8$ mas or $\sim6.5$ AU at 3.6 kpc), temperature at the inner rim ($\sim1760$ K), inclination ($\sim48$ deg) and position angle ($\sim107$ deg). We determine that the mass of the disk cannot be constrained without high-resolution observations in the (sub-)millimeter regime or observations of the disk kinematics, and could be anywhere from $\sim10^{-3}$ to $20$ M$_\odot$. Finally, we discuss the prospects of interpreting the spectral energy distributions of deeply-embedded massive YSOs, and warn against attempting to infer disk properties from the SED.

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arXiv2014

VLTI status update: a decade of operations and beyond

We present the latest update of the European Southern Observatory's Very Large Telescope interferometer (VLTI). The operations of VLTI have greatly improved in the past years: reduction of the execution time; better offering of telescopes configurations; improvements on AMBER limiting magnitudes; study of polarization effects and control for single mode fibres; fringe tracking real time data, etc. We present some of these improvements and also quantify the operational improvements using a performance metric. We take the opportunity of the first decade of operations to reflect on the VLTI community which is analyzed quantitatively and qualitatively. Finally, we present briefly the preparatory work for the arrival of the second generation instruments GRAVITY and MATISSE.

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arXiv2014

Dusty Blue Supergiants: News from High-Angular Resolution Observations

An overview is presented of the recent advances in understanding the B[e] phenomenon among blue supergiant stars in light of high-angular resolution observations and with an emphasis on the results obtained by means of long baseline optical stellar interferometry. The focus of the review is on the circumstellar material and evolutionary phase of B[e] supergiants, but recent results on dust production in regular blue supergiants are also highlighted.

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arXiv2014

Speckle Imaging with VLT/NACO No-AO Mode

Long-exposure stellar images recorded with large ground-based telescopes are blurred due to the turbulent nature of the atmosphere. The VLT employs active and adaptive optics (AO) systems to compensate for the deleterious effects of the atmosphere in real time. The speckle imaging technique provides an alternative way to achieve diffraction-limited imaging by post-processing a series of short-exposure images. The use of speckle imaging with the no-AO mode of NACO at the VLT is demonstrated. Application of this technique is particularly suited to the J-band and it provides versatile high angular resolution imaging under mediocre conditions and/or in imaging extended objects. The implementation of this mode underlines the continuing attractiveness of NACO at the VLT.

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arXiv2014

Near-Infrared Evidence for a Sudden Temperature Increase in Eta Carinae

Aims. Eta Car's ultra-violet, optical, and X-ray light curves and its spectrum suggest a physical change in its stellar wind over the last decade. It was proposed that the mass-loss rate decreased by a factor of about 2 in the last 15 years. We complement these recent results by investigating the past evolution and the current state of eta Car in the near-infrared (IR). Methods. We present JHKL photometry of eta Car obtained at SAAO Sutherland from 2004-2013 with the Mk II photometer at the 0.75-m telescope and JHKs photometry with SIRIUS at the 1.4-m IRSF telescope from 2012-2013. The near-IR light curves since 1972 are analyzed. Results. The long-term brightening trends in eta Car's JHKL light curves were discontinuous around the 1998 periastron passage. After 1998, the star shows excess emission above the extrapolated trend from earlier dates, foremost in J and H, and the blueward, cyclical progression in its near-IR colors is accelerated. The near-IR color evolution is strongly correlated with the periastron passages. After correcting for the secular trend we find that the color evolution matches an apparent increase in blackbody temperature of an optically thick near-IR emitting plasma component from about 3500 to 6000 K over the last 20 years. Conclusions. We suggest that the changing near-IR emission may be caused by variability in optically thick bremsstrahlung emission. Periastron passages play a key role in the observed excess near-IR emission after 1998 and the long-term color evolution. We thus propose as a hypothesis that angular momentum transfer (via tidal acceleration) during periastron passages leads to sudden changes in eta Car's atmosphere resulting in a long-term decrease in the mass-loss rate.

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arXiv2012

Neutral and ionized gas around the post-Red Supergiant IRC+10420 at au size scales

IRC +10420 is one of the few known massive stars in rapid transition from the Red Supergiant phase to the Wolf-Rayet or Luminous Blue Variable phase. The star has an ionised wind and using the Br gamma hydrogen recombination emission we assess the mass-loss on spatial scales of order 1 au. We present new VLT Interferometer AMBER data which are combined with all other AMBER data in the literature. The final dataset covers a position angle range of 180 degrees and baselines up to 110 meters. The spectrally dispersed visibilities, differential phases and line flux are conjointly analyzed and modelled. We also present AMBER/FINITO observations which cover a larger wavelength range and allow us to observe the Na I doublet at 2.2 micron. The data are complemented by X-Shooter data, which provide a higher spectral resolution view. The Brackett gamma line and the Na I doublet are both spatially resolved. After correcting the AMBER data for the fact that the lines are not spectrally resolved, we find that Br gamma traces a ring with a diameter of 4.18 milli-arcseconds. We consider a geometric model in which the Br gamma emission emerges from the top and bottom rings of an hour-glass shaped structure, viewed almost pole-on. It provides satisfactory fits to most visibilities and differential phases. The fact that we detect line emission from a neutral metal like Na I within the ionized region, a very unusual occurrence, suggests the presence of a dense pseudo-photosphere. The ionized wind can be reproduced with a polar wind, which could well have the shape of an hour-glass. The resolved Na I emission is found to occur on scales barely larger than the continuum. This fact and that many Yellow Hypergiants exhibit this comparatively rare emission hints at the presence of a "Yellow" or even "White Wall" in the Hertzsprung-Russell diagram, preventing them from visibly evolving to the blue.

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