arXiv subjects

Jochen Eislöffel

Publications and source records attributed to Jochen Eislöffel.

At least 19 recordsLinked to original sources

arXiv2026

A Pre-Main Sequence Binary Viewed Through its Circumbinary Disk: A New Phase for KH 15D

The binary T Tauri system, KH 15D, is poised to provide unprecedented detail of gas and ices located between $\sim$3-5 AU of its circumbinary disk over the next few years. We analyze for the first time a complete set of ground-based time series photometry of the KH 15D system for which the 2022 October - 2025 April data have never been presented. By combining the latest $I$-band data with historical measurements, we show that the era of large amplitude photometric variability has ended. We define a revised photometric period of $P$ = 48.365 $\pm$ 0.005 days, which accurately phases data over a time span of seventy-five years. We adopt magnitudes and colors for the stars that define the epochs when either star A (1995-2008) or star B (2010-2012) is the primary source of the out-of-eclipse brightness of the system. We determine for the first time the effective optical depth for the system between 2002 and 2025 assuming gray extinction. The current out-of-eclipse system color is consistent with a combination of the light from both stars, supporting the previously suggested model in which the trailing edge of the circumbinary disk is more transparent than the leading edge. We continue to see evidence for ``clumps'' and variable transparency along ingress and egress when the stars are probing the occulting material and suggest that observations made during these phases should permit studies of the physical and chemical nature of the planet-forming zone with the circumbinary disk.

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arXiv2026

A survey for variable young stars with small telescopes - XI. Spot Lifetimes and Coverage Distributions

We present a homogeneous analysis of rotational variability and spot properties in young stellar objects across multiple star-forming regions observed by the Hunting Outbursting Young Stars (HOYS) project. From over 2000 candidate members, we identify 144 YSOs with robust periodic signals and well-constrained multi-band amplitudes. The sample has a median age of $\sim$1~Myr, effective temperatures of 3500--6500~K (masses $\sim$0.6--2~M$_\odot$), and is dominated by Class~2 objects, one third of which exhibit inner disc dust emission. The rotation period distribution is strongly bimodal, with 55 percent fast rotators ($P<5.5$~d) and 45 percent slow rotators. Fast rotators are predominantly inner disc-less, whereas slow rotators include both disc-bearing and disc-free systems, indicating that disc braking alone cannot explain the observed rotational states. We derive spot properties from multi-band amplitudes and find that, after correcting for observational biases, the intrinsic cold-spot coverage distribution of fast rotators is well described by an exponential function. This implies that small spot coverages are intrinsically much more common than large ones, consistent with stochastic magnetic flux emergence governing spot formation. In contrast, slow rotators show a pronounced deficit of small cold spots. After considering observational biases and alternative physical explanations, we conclude that small spots on slowly rotating YSOs have significantly shorter lifetimes. These results provide new evidence that magnetic surface structure and its evolution depend on stellar rotation, placing new empirical constraints on models of magnetic activity and angular momentum evolution in young stars.

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arXiv2026

From Fragments to Flares: Migration, Tidal Disruption, and Observable Bursts in Massive Protostellar Disks

We investigate how resolving the inner few astronomical units of a massive protostellar disk affects the migration, disruption, and accretion signatures of an inward-moving fragment. In particular, we aim to determine whether the predicted burst strength and duration depend on the adopted sink cell size. We present a new three-dimensional radiation-hydrodynamic simulation of a $\sim$5$M_{\odot}$ protostar surrounded by a self-gravitating disk, comparing the original 30 AU sink model to a refined model with a 1 AU sink that resolves the inner disk. The resulting gas structures are post-processed with radiative transfer calculations to derive synthetic photometry and multi-band images. Both simulations produce a major accretion burst as a migrating fragment is tidally disrupted, but their detailed behavior differs markedly. The refined model shows faster migration, a complete tidal disruption of the fragment, and a shorter, sharper outburst (more consistent with observations) with nearly the same peak accretion rate as the 30 AU model, which yields a broader, smoother event. The refined run produces much stronger near- and mid-infrared emission, reflecting the formation of a compact, hot inner disk. Resolving the inner few AU qualitatively changes the dynamics and observable appearance of fragment-driven bursts. Diffuse fragment disruption can reproduce decade-long events, but the much shorter ($<$3 yr) bursts observed in some massive protostars likely require the tidal disruption of more compact objects such as second Larson cores. Our trajectory analysis indicates that second Larson cores can migrate sufficiently close to the star to be tidally destroyed, offering a plausible mechanism for the fastest FU-Ori-like bursts observed in massive protostars.

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arXiv2025

Sub-millimeter wavelength protostellar accretion rate monitoring with AtLAST

How a star forms is a fundamental question in astrophysics. In the earliest stages of protostellar evolution high extinction prevents a direct study of the accretion processes and their temporal evolution. Monitoring the variations of the accretion luminosity in a large protostar sample over decades is needed to reveal how protostars accrete -- in major bursts or in a quasi-steady fashion. We here argue that a large ground based sub-millimeter single-dish facility with a wide FoV is required to fulfill this task.

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arXiv2024

A Multi-wavelength, Multi-epoch Monitoring Campaign of Accretion Variability in T Tauri Stars from the ODYSSEUS Survey. I. HST FUV and NUV Spectra

The Classical T Tauri Star (CTTS) stage is a critical phase of the star and planet formation process. In an effort to better understand the mass accretion process, which can dictate further stellar evolution and planet formation, a multi-epoch, multi-wavelength photometric and spectroscopic monitoring campaign of four CTTSs (TW Hya, RU Lup, BP Tau, and GM Aur) was carried out in 2021 and 2022/2023 as part of the Outflows and Disks Around Young Stars: Synergies for the Exploration of ULYSSES Spectra (ODYSSEUS) program. Here we focus on the HST UV spectra obtained by the HST Director's Discretionary Time UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. Using accretion shock modeling, we find that all targets exhibit accretion variability, varying from short increases in accretion rate by up to a factor of 3 within 48 hours, to longer decreases in accretion rate by a factor of 2.5 over the course of 1 year. This is despite the generally consistent accretion morphology within each target. Additionally, we test empirical relationships between accretion rate and UV luminosity and find stark differences, showing that these relationships should not be used to estimate the accretion rate for individual target. Our work reinforces that future multi-epoch and simultaneous multi-wavelength studies are critical in our understanding of the accretion process in low-mass star formation.

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arXiv2024

A Multi-wavelength, Multi-epoch Monitoring Campaign of Accretion Variability in T Tauri Stars from the ODYSSEUS Survey. II. Photometric Light Curves

Classical T Tauri Stars (CTTSs) are young, low-mass stars which accrete material from their surrounding protoplanetary disk. To better understand accretion variability, we conducted a multi-epoch, multi-wavelength photometric monitoring campaign of four CTTSs: TW Hya, RU Lup, BP Tau, and GM Aur, in 2021 and 2022, contemporaneous with HST UV and optical spectra. We find that all four targets display significant variability in their light curves, generally on days-long timescales (but in some cases year-to-year) often due to periodicity associated with stellar rotation and to stochastic accretion variability. Their is a strong connection between mass accretion and photometric variability in all bands, but the relationship varies per target and epoch. Thus, photometry should be used with caution as a direct measure of accretion in CTTSs.

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arXiv2024

A survey for variable stars with small telescopes: IX -- Evolution of Spot Properties on YSOs in IC5070

We present spot properties on 32 periodic young stellar objects in IC 5070. Long term, $\sim$5 yr, light curves in the $V$, $R$, and $I$-bands are obtained through the HOYS (Hunting Outbursting Young Stars) citizen science project. These are dissected into six months long slices, with 3 months oversampling, to measure 234 sets of amplitudes in all filters. We fit 180 of these with reliable spot solutions. Two thirds of spot solutions are cold spots, the lowest is 2150 K below the stellar temperature. One third are warm spots that are above the stellar temperature by less than $\sim$2000 K. Cold and warm spots have maximum surface coverage values of 40 percent, although only 16 percent of warm spots are above 20 percent surface coverage as opposed to 60 percent of the cold spots. Warm spots are most likely caused by a combination of plages and low density accretion columns, most common on objects without inner disc excess emission in $K-W2$. Five small hot spot solutions have $<3$ percent coverage and are 3000 - 5000 K above the stellar temperature. These are attributed to accretion, and four of them occur on the same object. The majority of our objects are likely to be accreting. However, we observe very few accretion hot spots as either the accretion is not stable on our timescale or the photometry is dominated by other features. We do not identify cyclical spot behaviour on the targets. We additionally identify and discuss a number of objects that have interesting amplitudes, phase changes, or spot properties.

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arXiv2024

A survey for variable young stars with small telescopes: VIII -- Properties of 1687 Gaia selected members in 21 nearby clusters

The Hunting Outbursting Young Stars (HOYS) project performs long-term, optical, multi-filter, high cadence monitoring of 25 nearby young clusters and star forming regions. Utilising Gaia DR3 data we have identified about 17000 potential young stellar members in 45 coherent astrometric groups in these fields. Twenty one of them are clear young groups or clusters of stars within one kiloparsec and they contain 9143 Gaia selected potential members. The cluster distances, proper motions and membership numbers are determined. We analyse long term (about 7yr) V, R, and I-band light curves from HOYS for 1687 of the potential cluster members. One quarter of the stars are variable in all three optical filters, and two thirds of these have light curves that are symmetric around the mean. Light curves affected by obscuration from circumstellar materials are more common than those affected by accretion bursts, by a factor of 2-4. The variability fraction in the clusters ranges from 10 to almost 100 percent, and correlates positively with the fraction of stars with detectable inner disks, indicating that a lot of variability is driven by the disk. About one in six variables shows detectable periodicity, mostly caused by magnetic spots. Two thirds of the periodic variables with disk excess emission are slow rotators, and amongst the stars without disk excess two thirds are fast rotators - in agreement with rotation being slowed down by the presence of a disk.

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arXiv2023

A survey for variable young stars with small telescopes: VI -- Analysis of the outbursting Be stars NSW284, Gaia19eyy, and VES263

This paper is one in a series reporting results from small telescope observations of variable young stars. Here, we study the repeating outbursts of three likely Be stars based on long-term optical, near-infrared, and mid-infrared photometry for all three objects, along with follow-up spectra for two of the three. The sources are characterised as rare, truly regularly outbursting Be stars. We interpret the photometric data within a framework for modelling light curve morphology, and find that the models correctly predict the burst shapes, including their larger amplitudes and later peaks towards longer wavelengths. We are thus able to infer the start and end times of mass loading into the circumstellar disks of these stars. The disk sizes are typically 3-6 times the areas of the central star. The disk temperatures are ~40%, and the disk luminosities are ~10% of those of the central Be star, respectively. The available spectroscopy is consistent with inside-out evolution of the disk. Higher excitation lines have larger velocity widths in their double-horned shaped emission profiles. Our observations and analysis support the decretion disk model for outbursting Be stars.

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arXiv2023

Lyman-alpha Scattering Models Trace Accretion and Outflow Kinematics in T Tauri Systems

T Tauri stars produce broad Lyman-alpha emission lines that contribute $\sim$88% of the total UV flux incident on the inner circumstellar disks. Lyman-alpha photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Lyman-alpha emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high velocity emission line wings, providing estimates of the average velocities within the bulk intervening H I. The model velocities are significantly correlated with the K band veiling, indicating a turnover from Lyman-alpha profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with red-shifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Lyman-alpha photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Lyman-alpha irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.

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arXiv2022

Towards a comprehensive view of accretion, inner disks, and extinction in classical T Tauri stars: an ODYSSEUS study of the Orion OB1b association

The coevolution of T Tauri stars and their surrounding protoplanetary disks dictates the timescales of planet formation. In this paper, we present magnetospheric accretion and inner disk wall model fits to NUV-NIR spectra of nine classical T Tauri stars in Orion OB1b as part of the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) Survey. Using NUV-optical spectra from the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director's Discretionary Program and optical-NIR spectra from the PENELLOPE VLT Large Programme, we find that the accretion rates of these targets are relatively high for the region's intermediate age of 5.0 Myr; rates range from $0.5-17.2 \times 10^{-8}$ M$_{\odot}$/yr, with a median value of $1.2\times 10^{-8}$ M$_{\odot}$/yr. The NIR excesses can be fit with 1200-1800 K inner disk walls located at 0.05-0.10 AU from the host stars. We discuss the significance of the choice in extinction law, as the measured accretion rate depends strongly on the adopted extinction value. This analysis will be extended to the complete sample of T Tauri stars being observed through ULLYSES to characterize accretion and inner disks in star-forming regions of different ages and stellar populations.

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arXiv2022

Resolving the collimation zone of an intermediate-mass protostar

We report new VLA and e-MERLIN high resolution and sensitivity images of the Triple Radio continuum Source in the Serpens star forming region. These observations allowed us to perform a deep multi-frequency, multi-epoch study by exploring the innermost regions (<~100 au) of an intermediate-mass YSO for the first time, with a physical resolution of ~15 au. The kinematic analysis of knots recently ejected by the protostar indicates that the jet is undergoing episodic variations in velocity. In addition, our multi-frequency images reveal striking characteristics, e.g., a highly collimated ionized stream that would be launched at a radial distance of ~0.4 au from the protostar, and a narrow (~28 au wide) ionized cavity that would be excited by the interaction of a wide-angle component with the surrounding toroid of infalling material. In light of these results, we propose the scenario in which both a highly-collimated jet and a wide-angle wind coexist to be the most plausible to explain our observations, either launched by the X-wind or X- plus Disk-wind mechanism.

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arXiv2021

A survey for variable young stars with small telescopes: V - Analysis of TXOri, V505Ori, and V510Ori, the HST ULLYSES targets in the $σ$Ori cluster

Investigations of the formation of young stellar objects (YSOs) and planets require the detailed analysis of individual sources as well as statistical analysis of a larger number of objects. The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) project provides such a unique opportunity by establishing a UV spectroscopic library of young high- and low-mass stars in the local universe. Here we analyse optical photometry of the three ULLYSES targets (TXOri, V505Ori, V510Ori) and other YSOs in the $σ$Ori cluster taken at the time of the HST observations to provide a reference for those spectra. We identify three populations of YSOs along the line of sight to $σ$Ori, separated in parallax and proper motion space. The ULLYSES targets show typical YSO behaviour with pronounced variability and mass accretion rates of the order of 10$^{-8}$M$_\odot$/yr. Optical colours do not agree with standard interstellar reddening and suggest a significant contribution of scattered light. They are also amongst the most variable and strongest accretors in the cluster. V505\,Ori shows variability with a seven day period, indicating an inner disk warp at the co-rotation radius. Uncovering the exact nature of the ULLYSES targets will require improved detailed modelling of the HST spectra in the context of the available photometry, including scattered light contributions as well as non-standard reddening.

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arXiv2021

A survey for variable young stars with small telescopes: IV -- Rotation Periods of YSOs in IC5070

Studying rotational variability of young stars is enabling us to investigate a multitude of properties of young star-disk systems. We utilise high cadence, multi-wavelength optical time series data from the Hunting Outbursting Young Stars citizen science project to identify periodic variables in the Pelican Nebula (IC5070). A double blind study using nine different period-finding algorithms was conducted and a sample of 59 periodic variables was identified. We find that a combination of four period finding algorithms can achieve a completeness of 85% and a contamination of 30% in identifying periods in inhomogeneous data sets. The best performing methods are periodograms that rely on fitting a sine curve. Utilising GaiaEDR3 data, we have identified an unbiased sample of 40 periodic YSOs, without using any colour or magnitude selections. With a 98.9% probability we can exclude a homogeneous YSO period distribution. Instead we find a bi-modal distribution with peaks at three and eight days. The sample has a disk fraction of 50%, and its statistical properties are in agreement with other similarly aged YSOs populations. In particular, we confirm that the presence of the disk is linked to predominantly slow rotation and find a probability of 4.8$\times$10$^{-3}$ that the observed relation between period and presence of a disk has occurred by chance. In our sample of periodic variables, we also find pulsating giants, an eclipsing binary, and potential YSOs in the foreground of IC5070.

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arXiv2021

Probing jets from young embedded sources: clues from HST near-IR [Fe II] images

We present near-infrared [Fe II] images of four Class 0/I jets (HH 1/2, HH 34, HH 111, HH 46/47) observed with the Hubble Space Telescope Wide Field Camera 3. The unprecedented angular resolution allows us to measure proper motions, jet widths and trajectories, and extinction along the jets. In all cases, we detect the counter-jet which was barely visible or invisible at shorter wavelengths. We measure tangential velocities of a few hundred km/s, consistent with previous HST measurements over 10 years ago. We measure the jet width as close as a few tens of au from the star, revealing high collimations of about 2 degrees for HH 1, HH 34, HH 111 and about 8 degrees for HH 46, all of which are preserved up to large distances. For HH 34, we find evidence of a larger initial opening angle of about 7 degrees. Measurement of knot positions reveals deviations in trajectory of both the jet and counter-jet of all sources. Analysis of asymmetries in the inner knot positions for HH 111 suggests the presence of a low mass stellar companion at separation 20-30 au. Finally, we find extinction values of 15-20 mag near the source which gradually decreases moving downstream along the jet. These observations have allowed us to study the counter-jet at unprecedented high angular resolution, and will be a valuable reference for planning future JWST mid-infrared observations which will peer even closer into the jet engine.

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arXiv2021

Accretion bursts in high-mass protostars: a new testbed for models of episodic accretion

It is well known that low mass young stellar objects (LMYSOs) gain a significant portion of their final mass through episodes of very rapid accretion, with mass accretion rates up to $\dot M_* \sim 10^{-4} M_{\odot}$~yr$^{-1}$. Recent observations of high mass young stellar objects (HMYSO) with masses $M_* \gtrsim 10 M_{\odot}$ uncovered outbursts with accretion rates exceeding $\dot M_*\sim 10^{-3}M_{\odot}$~yr$^{-1}$. Here we examine which scenarios proposed in the literature so far to explain accretion bursts of LMYSOs can apply to the episodic accretion in HMYSOs. We utilise a 1D time dependent models of protoplanetary discs around HMYSOs to study burst properties. We find that discs around HMYSOs are much hotter than those around their low mass cousins. As a result, much more extended regions of the disc are prone to the thermal hydrogen ionisation and MRI activation instabilities. The former in particular is found to be ubiquitous in a very wide range of accretion rates and disc viscosity parameters. The outbursts triggered by these instabilities, however, always have too low $\dot M_*$, and are one to several orders of magnitude too long compared to those observed from HMYSOs so far. On the other hand, bursts generated by tidal disruptions of gaseous giant planets formed by the gravitational instability of the protoplanetary discs yield properties commensurate with observations, provided that the clumps are in the post-collapse configuration with planet radius $R_{\rm p} \gtrsim 10 $ Jupiter radii. Furthermore, if observed bursts are caused by disc ionisation instabilities then they should be periodic phenomena with the duration of the quiescent phase comparable to that of the bursts. This may yield potentially observable burst periodicity signatures in the jets, the outer disc, or the surrounding diffuse material of massive HMYSOs. (abridged)

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arXiv2021

TAUKAM: a new prime-focus camera for the Tautenburg Schmidt Telescope

TAUKAM stands for "TAUtenburg KAMera", which will become the new prime-focus imager for the Tautenburg Schmidt telescope. It employs an e2v 6kx6k CCD and is under manufacture by Spectral Instruments Inc. We describe the design of the instrument and the auxiliary components, its specifications as well as the concept for integrating the device into the telescope infrastructure. First light is foreseen in 2017. TAUKAM will boost the observational capabilities of the telescope for what concerns optical wide-field surveys.

astro-ph.IM↗

arXiv2020

The brown dwarf population in the star forming region NGC2264

The brown dwarf population in the canonical star forming region NGC2264 is so far poorly explored. We present a deep, multi-wavelength, multi-epoch survey of the star forming cluster NGC2264, aimed to identify young brown dwarf candidates in this region. Using criteria including optical/near-infrared colours, variability, Spitzer mid-infrared colour excess, extinction, and Gaia parallax and proper motion (in order of relevance), we select 902 faint red sources with indicators of youth. Within this sample we identify 429 brown dwarf candidates based on their infrared colours. The brown dwarf candidates are estimated to span a mass range from 0.01 to 0.08$M_{\odot}$. We find rotation periods for 44 sources, 15 of which are brown dwarf candidates, ranging from 3.6 hours to 6.5 days. A subset of 38 brown dwarf candidates show high level irregular variability indicative of ongoing disc accretion, similar to the behaviour of young stars.

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