arXiv subjects

M. Kuffmeier

Publications and source records attributed to M. Kuffmeier.

At least 19 recordsLinked to original sources

arXiv2024

PRODIGE -- envelope to disk with NOEMA. IV. An infalling gas bridge surrounding two Class 0/I systems in L1448N

Context. The formation of stars has been subject to extensive studies in the past decades from molecular cloud to protoplanetary disk scales. It is still not fully understood how the surrounding material in a protostellar system, that often shows asymmetric structures with complex kinematic properties, feeds the central protostar(s) and their disk(s). Aims. We study the spatial morphology and kinematic properties of the molecular gas surrounding the IRS3A and IRS3B protostellar systems in the L1448N region located in the Perseus molecular cloud. Methods. We present 1 mm NOEMA observations of the PRODIGE large program and analyze the kinematic properties of molecular lines. Given the complexity of the spectral profiles, the lines are fitted with up to three Gaussian velocity components. The clustering algorithm DBSCAN is used to disentangle the velocity components into the underlying physical structure. Results. We discover an extended gas bridge (~3000 au) surrounding both the IRS3A and IRS3B systems in six molecular line tracers (C18O, SO, DCN, H2CO, HC3N, and CH3OH). This gas bridge is oriented along the northeast-southwest direction and shows clear velocity gradients on the order of 100 km/s/pc towards the IRS3A system. We find that the observed velocity profile is consistent with analytical streamline models of gravitational infall towards IRS3A. The high-velocity C18O (2-1) emission towards IRS3A indicates a protostellar mass of ~1.2 Msun. Conclusions. While high angular resolution continuum data often show IRS3A and IRS3B in isolation, molecular gas observations reveal that these systems are still embedded within a large-scale mass reservoir with a complex spatial morphology as well as velocity profiles. The kinematic properties of the extended gas bridge are consistent with gravitational infall toward the IRS3A protostar.

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arXiv2024

Magnetohydrodynamical modeling of star-disk formation: from isolated spherical collapse towards incorporation of external dynamics

The formation of protostars and their disks has been understood as the result of the gravitational collapse phase of an accumulation of dense gas that determines the mass reservoir of the star-disk system. Against this background, the broadly applied scenario of considering the formation of disks has been to model the collapse of a dense core assuming spherical spherical symmetry. Our understanding of the formation of star-disk systems is currently undergoing a reformation though. The picture evolves from interpreting disks as the sole outcome of the collapse of an isolated prestellar core to a more dynamic picture where disks are affected by the molecular cloud environment in which they form. In this review, we provide a status report of the state-of-the-art of spherical collapse models that are highly advanced in terms of the incorporated physics together with constraints from models that account for the possibility of infall onto star-disk systems in simplified test setups, as well as in multi-scale simulations that cover a dynamical range from the Giant Molecular Cloud environment down to the disk. Considering the observational constraints that favor a more dynamical picture of star formation, we finally discuss the challenges and prospects in linking the efforts of tackle the problem of star-disk formation in combined multi-scale, multi-physics simulations.

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arXiv2024

Constraints on the (re-)orientation of star-disk systems through infall

It has been consensus that star-disk systems accrete most of their mass and angular momentum during the collapse of a prestellar core, such that the rotational direction of a system is equivalent to the net rotation of the core. Recent results, however, indicate that stars experience post-collapse or late infall, during which the star and its disk is refreshed with material from the protostellar environment through accretion streamers. Apart from adding mass to the star-disk system, infall potentially supplies a substantial amount of angular momentum as the infalling material is initially not bound to the collapsing prestellar core. We investigate the orientation of infall on star-disk systems by analyzing the properties of accreting tracer particles in 3D magnetohydrodynamical simulations of a molecular cloud that is (4 pc)$^3$ in volume. In contrast to the traditional picture, where the rotational axis is inherited from the collapse of a coherent pre-stellar core, the orientation of star-disk systems can change substantially during the accretion process. In agreement with previous results that show larger contributions of late infall for increasing stellar masses, infall is more likely to lead to a prolonged change in orientation for stars of higher final mass. On average, brown dwarfs and very low mass stars are more likely to form and accrete all of their mass as part of a multiple system, while stars with final masses above a few 0.1 M$_{\odot}$ are more likely to accrete part of their mass as single stars. Finally, we find an overall trend: the post-collapse accretion phase is more anisotropic than the early collapse phase. This result is consistent with a scenario, where mass accretion from infall occurs via infalling streamers along a preferred direction, while the initial collapse is less anisotropic albeit the fact that material is funneled through accretion channels.

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arXiv2024

Feasibility of detecting shadows in disks induced by infall

Observations performed with high-resolution imaging techniques revealed the existence of shadows in circumstellar disks that can be explained by the misalignment of an inner with respect to an outer disk. The cause of misalignment, however, is still debated. In this study, we investigate the feasibility of observing shadows induced by one prominent scenario that may lead to misalignment, which involves the late infall of material onto a protostellar system. In particular, we use previously performed hydrodynamical simulations of such events, and generate flux maps in the visible, near-infrared, submillimeter, and millimeter wavelength range using Monte Carlo radiative transfer. Based on that, we derive synthetic observations of these systems performed with the instruments SPHERE/VLT and ALMA, which we use as a basis for our subsequent analysis. We find that near-infrared observations with SPHERE are particularly well suited for detecting shadows via direct imaging alongside other features such as gaps, arcs, and streamers. On the contrary, performing a shadow detection based on reconstructed ALMA observations is very challenging due to the high sensitivity that is required for this task. Thus, in cases that allow for a detection, sophisticated analyses may be needed, for instance by the utilization of carefully constructed azimuthal profiles, aiding the search for potentially shallow shadows. Lastly, we conclude that late infall-induced disk misalignment offers a plausible explanation for the emergence of shadows that are observed in various systems.

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arXiv2023

A dusty streamer infalling onto the disk of a class I protostar. ALMA dual-band constraints on grain properties and mass infall rate

Observations of interstellar material infalling onto star- and planet-forming systems have become increasingly common thanks to recent advancements in radio interferometry. These structures replenish disks with fresh material, have the potential to significantly alter their dynamics, trigger the formation of substructures, induce shocks, and modify their physical and chemical properties. In this study, we combine new ALMA band 3 and archival band 6 observations to characterize the dust content and mass infall rate of a 4,000 au arc-like structure infalling onto M512, a class I young stellar object located in the Lynds 1641 region of the Orion A molecular cloud. We measure for the first time spectral index maps and derive a dust opacity index profile along a streamer, constraining grain properties and its dust mass. We measure a spectral index $\alpha \sim$ 3.2 across the entire structure, and a dust opacity index $\beta \sim$ 1.6. Given grain properties consistent with the measured $\beta$, the structure can host up to 245 M$_{\oplus}$ of dust, being comparable or even exceeding the mass of the inner, unresolved 600 au, which contains the protoplanetary disk of M512. Such a massive streamer can strongly affect the evolution of the star- and planet-forming inner system. Assuming typical ISM dust-to-gas ratio of 1%, free-fall timescales (50 kyr) imply total mass infall rates up to 1.5 $\cdot$ 10$^{-6}$ M$_{\odot}$/yr. M512 has been classified as an outbursting source with multi-epoch photometry, thus representing an interesting case study to explore the possible connection between infalling streamers and accretion outbursts.

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arXiv2023

Rejuvenating infall: a crucial yet overlooked source of mass and angular momentum

MHD models and the observation of accretion streamers confirmed that protostars can undergo late accretion events after the initial collapse phase. To provide better constraints, we study the evolution of stellar masses in MHD simulations of a (4 pc)^3 molecular cloud. Tracer particles allow us to accurately follow the trajectory of accreting material for all protostars and thereby constrain the accretion reservoir of the stars. The diversity of the accretion process implies that stars in the solar mass regime can have vastly different accretion histories. Some stars accrete most of their mass during the initial collapse phase, while others gain >50 % of their final mass from late infall. The angular momentum budget of stars that experience substantial infall, so-called late accretors, is significantly higher than for stars without or with only little late accretion. As the probability of late infall increases with increasing final stellar mass, the specific angular momentum budget of higher mass stars is on average higher. The hypothetical centrifugal radius computed from the accreting particles at the time of formation is orders of magnitude higher than observed disk sizes, which emphasizes the importance of angular momentum transport during disk formation. Nevertheless, we find a correlation that the centrifugal radius is highest for stars with substantial infall, which suggests that very large disks are the result of recent infall events. There are also indications for a subtle trend of increasing centrifugal radius with increasing final stellar mass, which is in agreement with an observed marginal correlation of disk size and stellar mass. Finally, we show that late accretors become embedded again during infall. As a consequence, late accretors are (apparently) rejuvenated and would be classified as Class 0 objects according to their bolometric temperature despite being 1 Myr old.

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arXiv2021

Misaligned disks induced by infall

Arc- and tail-like structures associated with disks around Herbig stars can be a consequence of infall events occurring after the initial collapse phase of a forming star. An encounter event of gas with a star can lead to the formation of a second-generation disk after the initial protostellar collapse phase. Additionally, observations of shadows in disks can be well described by a configuration of misaligned inner and outer disk, such that the inner disk casts a shadow on the outer disk. Carrying out altogether eleven 3D hydrodynamical models with the moving mesh code AREPO, we test whether a late encounter of an existing star-disk system with a cloudlet of gas can lead to the formation of an outer disk that is misaligned with respect to the primordial inner disk. Our models demonstrate that a second-generation disk with large misalignment with respect to an existing primordial disk can easily form if the infall angle is large. The second-generation outer disk is more eccentric, though the asymmetric infall also triggers eccentricity of the inner disk of $e\approx 0.05$ to $0.1$. Retrograde infall can lead to the formation of counter-rotating disks and enhanced accretion. As the angular momentum of the inner disk is reduced, the inner disk shrinks and a gap forms between the two disks. The resulting misaligned disk system can survive for $\sim 100$ kyr or longer without aligning each other even for low primordial disk masses given an infall mass of $\sim 10^{-4}$ M$_{\odot}$. A synthetic image reveals shadows in the outer disk similar to the ones observed in multiple transition disks that are caused by the misaligned inner disk. We conclude that late inclined infall onto a star-disk system leads to the formation of a misaligned outer disk. Infall might therefore be responsible for observations of shadows in at least some transition disks.

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arXiv2020

Linear dust polarization during the embedded phase of protostar formation

Measuring polarization from thermal dust emission can provide constraints on the magnetic field structure around embedded protostars. However, interpreting the observations is challenging without models that consistently account for both the complexity of the protostellar birth environment and polarization mechanisms. We aim to provide a better understanding with a focus on bridge-like structures such as that observed towards the protostellar multiple IRAS 16293--2422 by comparing synthetic polarization maps of thermal reemission with observations. We analyze the magnetic field properties associated with the formation of a protostellar multiple based on ideal MHD 3D zoom-in simulations carried out with the RAMSES code. To compare with observations, we post-process a snapshot of a bridge-like structure that is associated with a forming triple star system with the radiative transfer code POLARIS and produce multi-wavelength dust polarization maps. In the most prominent bridge of our sample, the typical density is about 10^(-16) g cm^(-3), and the magnetic field strength is about 1 to 2 mG. The magnetic field structure has an elongated toroidal morphology and the dust polarization maps trace the complex morphology. In contrast, the magnetic field strength associated with the launching of asymmetric bipolar outflows is significantly more magnetized (~100 mG). At {\lambda}=1.3 mm, the orientation of grains in the bridge is similar for the case accounting for radiative alignment torques (RATs) compared to perfect alignment with magnetic field lines. However, the polarization fraction in the bridge is three times smaller for the RAT scenario compared to assuming perfect alignment. At shorter wavelengths ({\lambda} < 200 {\mu}m), dust polarization does not trace the magnetic field because other effects such as self-scattering and dichroic extinction dominate the orientation of the polarization.

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arXiv2020

Ionization: a possible explanation for the difference of mean disk sizes in star-forming regions

Surveys of protoplanetary disks in star-forming regions of similar age revealed significant variations in average disk mass between some regions. For instance, disks in the Orion Nebular Cluster (ONC) and Corona Australis (CrA) are on average smaller than disks observed in Lupus, Taurus, Chamaeleon I or Ophiuchus. In contrast to previous models that study truncation of disks at a late stage of their evolution, we investigate whether disks may already be born with systematically smaller disk sizes in more massive star-forming regions as a consequence of enhanced ionization rates. Assuming various cosmic-ray ionization rates, we compute the resistivities for ambipolar diffusion and Ohmic dissipation with a chemical network, and perform 2D non-ideal magnetohydrodynamical protostellar collapse simulations. A higher ionization rate leads to stronger magnetic braking, and hence to the formation of smaller disks. Accounting for recent findings that protostars act as forges of cosmic rays and considering only mild attenuation during the collapse phase, we show that a high average cosmic-ray ionization rate in star-forming regions like the ONC or CrA can explain the detection of smaller disks in these regions. Our results show that on average a higher ionization rate leads to the formation of smaller disks. Therefore, smaller disks in regions of similar age can be the consequence of different levels of ionization, and may not exclusively be caused by disk truncation via external photoevaporation. We strongly encourage observations that allow measuring the cosmic-ray ionization degrees in different star-forming regions to test our hypothesis.

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arXiv2019

Late encounter-events as a source of disks and spiral structures -- Forming second generation disks

Both observations of arc-like structures and luminosity bursts of stars > 1 Myr in age indicate that at least some stars undergo late infall events. We investigate scenarios of replenishing the mass reservoir around a star via capturing and infalling events of cloudlets. We carry out altogether 24 three-dimensional hydrodynamical simulations of cloudlet encounters with a Herbig star of mass 2.5 solar mass using the moving mesh code AREPO. To account for the two possibilities of a star or a cloudlet traveling through the interstellar medium (ISM), we put either the star or the cloudlet at rest with respect to the background gas. For absent cooling in the adiabatic runs, almost none of the cloudlet gas is captured due to high thermal pressure. However, second-generation disks easily form when accounting for cooling of the gas. The disk radii range from several 100 au to about 1000 au and associated arc-like structures up to 10 000 au in length form around the star for runs with and without stellar irradiation. Consistent with angular momentum conservation, the arcs and disks are larger for larger impact parameters. Accounting for turbulence in the cloudlet only mildly changes the model outcome. In the case of the star being at rest with the background gas, the disk formation and mass replenishment process is more pronounced and the associated arc-shaped streamers are longer-lived. The results of our models confirm that late encounter events lead to the formation of transitional disks associated with arc-shaped structures such as observed for AB Aurigae or HD 100546. In addition, we find that second-generation disks and their associated filamentary arms are longer lived (>100 000 yrs) in infall events, when the star is at rest with the background gas.

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arXiv2019

An analysis of binary microlensing event OGLE-2015-BLG-0060

We present the analysis of stellar binary microlensing event OGLE-2015-BLG-0060 based on observations obtained from 13 different telescopes. Intensive coverage of the anomalous parts of the light curve was achieved by automated follow-up observations from the robotic telescopes of the Las Cumbres Observatory. We show that, for the first time, all main features of an anomalous microlensing event are well covered by follow-up data, allowing us to estimate the physical parameters of the lens. The strong detection of second-order effects in the event light curve necessitates the inclusion of longer-baseline survey data in order to constrain the parallax vector. We find that the event was most likely caused by a stellar binary-lens with masses $M_{\star1} = 0.87 \pm 0.12 M_{\odot}$ and $M_{\star2} = 0.77 \pm 0.11 M_{\odot}$. The distance to the lensing system is 6.41 $\pm 0.14$ kpc and the projected separation between the two components is 13.85 $\pm 0.16$ AU. Alternative interpretations are also considered.

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arXiv2019

Physical properties and transmission spectrum of the WASP-74 planetary system from multi-band photometry

We present broad-band photometry of eleven planetary transits of the hot Jupiter WASP-74b, using three medium-class telescopes and employing the telescope-defocussing technique. Most of the transits were monitored through I filters and one was simultaneously observed in five optical (U, g', r', i', z') and three near infrared (J, H, K) passbands, for a total of 18 light curves. We also obtained new high-resolution spectra of the host star. We used these new data to review the orbital and physical properties of the WASP-74 planetary system. We were able to better constrain the main system characteristics, measuring smaller radius and mass for both the hot Jupiter and its host star than previously reported in the literature. Joining our optical data with those taken with the HST in the near infrared, we built up an observational transmission spectrum of the planet, which suggests the presence of strong optical absorbers, as TiO and VO gases, in its atmosphere.

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arXiv2017

High-resolution Imaging of Transiting Extrasolar Planetary systems (HITEP). II. Lucky Imaging results from 2015 and 2016

The formation and dynamical history of hot Jupiters is currently debated, with wide stellar binaries having been suggested as a potential formation pathway. Additionally, contaminating light from both binary companions and unassociated stars can significantly bias the results of planet characterisation studies, but can be corrected for if the properties of the contaminating star are known. We search for binary companions to known transiting exoplanet host stars, in order to determine the multiplicity properties of hot Jupiter host stars. We also characterise unassociated stars along the line of sight, allowing photometric and spectroscopic observations of the planetary system to be corrected for contaminating light. We analyse lucky imaging observations of 97 Southern hemisphere exoplanet host stars, using the Two Colour Instrument on the Danish 1.54m telescope. For each detected companion star, we determine flux ratios relative to the planet host star in two passbands, and measure the relative position of the companion. The probability of each companion being physically associated was determined using our two-colour photometry. A catalogue of close companion stars is presented, including flux ratios, position measurements, and estimated companion star temperature. For companions that are potential binary companions, we review archival and catalogue data for further evidence. For WASP-77AB and WASP-85AB, we combine our data with historical measurements to determine the binary orbits, showing them to be moderately eccentric and inclined to the line of sight and planetary orbital axis. Combining our survey with the similar Friends of Hot Jupiters survey, we conclude that known hot Jupiter host stars show a deficit of high mass stellar companions compared to the field star population; however, this may be a result of the biases in detection and target selection by ground-based surveys.

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arXiv2017

DISPATCH: A Numerical Simulation Framework for the Exa-scale Era. I. Fundamentals

We introduce a high-performance simulation framework that permits the semi-independent, task-based solution of sets of partial differential equations, typically manifesting as updates to a collection of `patches' in space-time. A hybrid MPI/OpenMP execution model is adopted, where work tasks are controlled by a rank-local `dispatcher' which selects, from a set of tasks generally much larger than the number of physical cores (or hardware threads), tasks that are ready for updating. The definition of a task can vary, for example, with some solving the equations of ideal magnetohydrodynamics (MHD), others non-ideal MHD, radiative transfer, or particle motion, and yet others applying particle-in-cell (PIC) methods. Tasks do not have to be grid-based, while tasks that are, may use either Cartesian or orthogonal curvilinear meshes. Patches may be stationary or moving. Mesh refinement can be static or dynamic. A feature of decisive importance for the overall performance of the framework is that time steps are determined and applied locally; this allows potentially large reductions in the total number of updates required in cases when the signal speed varies greatly across the computational domain, and therefore a corresponding reduction in computing time. Another feature is a load balancing algorithm that operates `locally' and aims to simultaneously minimise load and communication imbalance. The framework generally relies on already existing solvers, whose performance is augmented when run under the framework, due to more efficient cache usage, vectorisation, local time-stepping, plus near-linear and, in principle, unlimited OpenMP and MPI scaling.

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arXiv2016

Many new variable stars discovered in the core of the globular cluster NGC 6715 (M54) with EMCCD observations

We show the benefits of using Electron-Multiplying CCDs and the shift-and-add technique as a tool to minimise the effects of the atmospheric turbulence such as blending between stars in crowded fields and to avoid saturated stars in the fields observed. We intend to complete, or improve, the census of the variable star population in globular cluster NGC~6715. Our aim is to obtain high-precision time-series photometry of the very crowded central region of this stellar system via the collection of better angular resolution images than has been previously achieved with conventional CCDs on ground-based telescopes. Observations were carried out using the Danish 1.54-m Telescope at the ESO La Silla observatory in Chile. The telescope is equipped with an Electron-Multiplying CCD that allowed to obtain short-exposure-time images (ten images per second) that were stacked using the shift-and-add technique to produce the normal-exposure-time images (minutes). The high precision photometry was performed via difference image analysis employing the DanDIA pipeline. We attempted automatic detection of variable stars in the field. We statistically analysed the light curves of 1405 stars in the crowded central region of NGC~6715 to automatically identify the variable stars present in this cluster. We found light curves for 17 previously known variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67 new variables (30 RR Lyrae, 21 long-period irregular, 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11 unclassified). Photometric measurements for these stars are available in electronic form through the Strasbourg Astronomical Data Centre.

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arXiv2016

High-resolution Imaging of Transiting Extrasolar Planetary systems (HITEP). I. Lucky imaging observations of 101 systems in the southern hemisphere

(abridged) Context. Wide binaries are a potential pathway for the formation of hot Jupiters. The binary fraction among host stars is an important discriminator between competing formation theories, but has not been well characterised. Additionally, contaminating light from unresolved stars can significantly affect the accuracy of photometric and spectroscopic measurements in studies of transiting exoplanets. Aims. We observed 101 transiting exoplanet host systems in the Southern hemisphere in order to create a homogeneous catalogue of both bound companion stars and contaminating background stars. We investigate the binary fraction among the host stars in order to test theories for the formation of hot Jupiters, in an area of the sky where transiting exoplanetary systems have not been systematically searched for stellar companions. Methods. Lucky imaging observations from the Two Colour Instrument on the Danish 1.54m telescope at La Silla were used to search for previously unresolved stars at small angular separations. The separations and relative magnitudes of all detected stars were measured. For 12 candidate companions to 10 host stars, previous astrometric measurements were used to evaluate how likely the companions are to be physically associated. Results. We provide measurements of 499 candidate companions within 20 arcseconds of our sample of 101 planet host stars. 51 candidates are located within 5 arcseconds of a host star, and we provide the first published measurements for 27 of these. Calibrations for the plate scale and colour performance of the Two Colour Instrument are presented. Conclusions. We find that the overall multiplicity rate of the host stars is 38 +17 -13%, consistent with the rate among solar-type stars in our sensitivity range, suggesting that planet formation does not preferentially occur in long period binaries compared to a random sample of field stars.

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arXiv2016

Spitzer Observations of OGLE-2015-BLG-1212 Reveal a New Path to Breaking Strong Microlens Degeneracies

Spitzer microlensing parallax observations of OGLE-2015-BLG-1212 decisively breaks a degeneracy between planetary and binary solutions that is somewhat ambiguous when only ground-based data are considered. Only eight viable models survive out of an initial set of 32 local minima in the parameter space. These models clearly indicate that the lens is a stellar binary system possibly located within the bulge of our Galaxy, ruling out the planetary alternative. We argue that several types of discrete degeneracies can be broken via such space-based parallax observations.

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arXiv2015

Spitzer Microlensing Program as a Probe for Globular Cluster Planets. Analysis of OGLE-2015-BLG-0448

The microlensing event OGLE-2015-BLG-0448 was observed by Spitzer and lay within the tidal radius of the globular cluster NGC 6558. The event had moderate magnification and was intensively observed, hence it had the potential to probe the distribution of planets in globular clusters. We measure the proper motion of NGC 6558 ($\mu_{\rm cl}$(N,E) = (+0.36+-0.10, +1.42+-0.10) mas/yr) as well as the source and show that the lens is not a cluster member. Even though this particular event does not probe the distribution of planets in globular clusters, other potential cluster lens events can be verified using our methodology. Additionally, we find that microlens parallax measured using OGLE photometry is consistent with the value found based on the light curve displacement between Earth and Spitzer.

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