arXiv subjects

Jochen Eislöffel

Publications and source records attributed to Jochen Eislöffel.

At least 37 records · Page 2Linked to original sources

arXiv2020

A survey for variable young stars with small telescopes: III -- Warm spots on the active star V1598Cyg

Magnetic spots on low-mass stars can be traced and characterised using multi-band photometric light curves. Here we analyse an extensive data set for one active star, V1598Cyg, a known variable K dwarf which is either pre-main sequence and/or in a close binary system. Our light curve contains 2854 photometric data points, mostly in $V$, $R_c$, $I_c$, but also in $U$, $B$ and $Hα$, with a total baseline of about 4yr, obtained with small telescopes as part of the HOYS project. We find that V1598Cyg is a very fast rotator with a period of 0.8246 days and varying amplitudes in all filters, best explained as a signature of strong magnetic activity and spots. We fit the photometric amplitudes in $V$, $R_c$, $I_c$ and use them to estimate spot properties, using a grid-based method that is also propagating uncertainties. We verify the method on a partial data set with high cadence and all five broad-band filters. The method yields spot temperatures and fractional spot coverage with typical uncertainties of 100K and 3-4%, respectively. V1598Cyg consistently exhibits spots that are a few hundred degrees warmer than the photosphere, most likely indicating that the light curve is dominated by chromospheric plage. The spot activity varies over our observing baseline, with a typical time scale of 0.5-1yr, which we interpret as the typical spot lifetime. Combining our light curve with archival data, we find a six year cycle in the average brightness, that is probably a sign of a magnetic activity cycle.

astro-ph.SR↗

arXiv2020

The one that got away: A unique eclipse in the young brown dwarf Roque 12

We report the discovery of a deep, singular eclipse of the bona fide brown dwarf Roque 12, a substellar member of the Pleiades. The eclipse was 0.65mag deep, lasted 1.3h, and was observed with two telescopes simultaneously in October 2002. No further eclipse was recorded, despite continuous monitoring with Kepler/K2 over 70d in 2015. There is tentative (2sigma) evidence for radial velocity variations of 5km/s, over timescales of three months. The best explanation for the eclipse is the presence of a companion on an eccentric orbit. The observations constrain the eccentricity to e>0.5, the period to P>70d, and the mass of the companion to ~0.001-0.04Msol. In principle it is also possible that the eclipse is caused by circum-sub-stellar material. Future data releases by Gaia and later LSST as well as improved radial velocity constraints may be able to unambiguously confirm the presence of the companion. This would turn the system into one of the very few known eclipsing binary brown dwarfs with known age.

astro-ph.SR↗

arXiv2020

A LOFAR Observation of Ionospheric Scintillation from Two Simultaneous Travelling Ionospheric Disturbances

This paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cas A, taken overnight on 18-19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10-80MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR "core" reveals two different velocities in the scintillation pattern: a primary velocity of ~30m/s with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of ~110m/s with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller--scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported.

astro-ph.EP↗

arXiv2020

A survey for variable young stars with small telescopes: II -- Mapping a protoplanetary disk with stable structures at 0.15 AU

The HOYS citizen science project conducts long term, multifilter, high cadence monitoring of large YSO samples with a wide variety of professional and amateur telescopes. We present the analysis of the light curve of V1490Cyg in the Pelican Nebula. We show that colour terms in the diverse photometric data can be calibrated out to achieve a median photometric accuracy of 0.02mag in broadband filters, allowing detailed investigations into a variety of variability amplitudes over timescales from hours to several years. Using GaiaDR2 we estimate the distance to the Pelican Nebula to be 870$^{+70}_{-55}$pc. V1490Cyg is a quasi-periodic dipper with a period of 31.447$\pm$0.011d. The obscuring dust has homogeneous properties, and grains larger than those typical in the ISM. Larger variability on short timescales is observed in U and R$_c-$H$α$, with U-amplitudes reaching 3mag on timescales of hours, indicating the source is accreting. The H$α$ equivalent width and NIR/MIR colours place V1490Cyg between CTTS/WTTS and transition disk objects. The material responsible for the dipping is located in a warped inner disk, about 0.15AU from the star. This mass reservoir can be filled and emptied on time scales shorter than the period at a rate of up to 10$^{-10}$M$_\odot$/yr, consistent with low levels of accretion in other T Tauri stars. Most likely the warp at this separation from the star is induced by a protoplanet in the inner accretion disk. However, we cannot fully rule out the possibility of an AA Tau-like warp, or occultations by the Hill sphere around a forming planet.

astro-ph.GA↗

arXiv2019

The first detection of a low-frequency turnover in nonthermal emission from the jet of a young star

Radio emission in jets from young stellar objects (YSOs) in the form of nonthermal emission has been seen toward several YSOs. Thought to be synchrotron emission from strong shocks in the jet, it could provide valuable information about the magnetic field in the jet. Here we report on the detection of synchrotron emission in two emission knots in the jet of the low-mass YSO DG Tau A at 152 MHz using the Low-Frequency Array (LOFAR), the first time nonthermal emission has been observed in a YSO jet at such low frequencies. In one of the knots, a low-frequency turnover in its spectrum is clearly seen compared to higher frequencies. This is the first time such a turnover has been seen in nonthermal emission in a YSO jet. We consider several possible mechanisms for the turnover and fit models for each of these to the spectrum. Based on the physical parameters predicted by each model, the Razin effect appears to be the most likely explanation for the turnover. From the Razin effect fit, we can obtain an estimate for the magnetic field strength within the emission knot of $\sim 20\ μ\mathrm{G}$. If the Razin effect is the correct mechanism, this is the first time the magnetic field strength along a YSO jet has been measured based on a low-frequency turnover in nonthermal emission.

astro-ph.SR↗

arXiv2019

Identifying transient and variable sources in radio images

With the arrival of a number of wide-field snapshot image-plane radio transient surveys, there will be a huge influx of images in the coming years making it impossible to manually analyse the datasets. Automated pipelines to process the information stored in the images are being developed, such as the LOFAR Transients Pipeline, outputting light curves and various transient parameters. These pipelines have a number of tuneable parameters that require training to meet the survey requirements. This paper utilises both observed and simulated datasets to demonstrate different machine learning strategies that can be used to train these parameters. The datasets used are from LOFAR observations and we process the data using the LOFAR Transients Pipeline; however the strategies developed are applicable to any light curve datasets at different frequencies and can be adapted to different automated pipelines. These machine learning strategies are publicly available as Python tools that can be downloaded and adapted to different datasets (https://github.com/AntoniaR/TraP_ML_tools).

astro-ph.IM↗

arXiv2019

Clumpy dust rings around non-accreting young stars

We investigate four young, but non-accreting, very low mass stars in Orion, which show irregular eclipses by circumstellar dust. The eclipses are not recurring periodically, are variable in depth, lack a flat bottom, and their duration is comparable to the typical timescale between eclipses. The dimming is associated with reddening consistent with dust extinction. Taken together this implies the presence of rings around these four stars, with radii ranging from 0.01 to 40 AU, comprised of optically thin dust clouds. The stars also show IR excess indicating the presence of evolved circumstellar disks, with orders of magnitude more material than needed for the eclipses. However, the rings need to cover an opening angle of about 20 degrees to explain how common these variable stars are in the coeval population in the same region, which is more extended than a typical disk. Thus, we propose that the rings may not be part of the disks, but instead separate structures with larger scale heights. To be sustained over years, the rings need to be replenished by dust from the disk or gravitationally bound to an object (e.g., planets or planetesimals). These four stars belong to a growing and diverse class of post-T Tauri stars with dips or eclipses in their lightcurves. Dusty rings with scale heights exceeding those of disks may be a common phenomenon at stellar ages between 5 and 10 Myr, in the transition from accretion disks to debris disks. These structures could be caused by migrating planets and may be signposts for the presence of young planetary systems.

astro-ph.SR↗

arXiv2017

The binary fraction of planetary nebula central stars: the promise of VPHAS+

The majority of planetary nebulae (PNe) are not spherical, and current single-star models cannot adequately explain all the morphologies we observe. This has led to the Binary Hypothesis, which states that PNe are preferentially formed by binary systems. This hypothesis can be corroborated or disproved by comparing the estimated binary fraction of all PNe central stars (CS) to that of the supposed progenitor population. One way to quantify the rate of CS binarity is to detect near infra-red (IR) excess indicative of a low-mass main sequence companion. In this paper, a sample of known PNe within data release 2 of the ongoing VPHAS+ are investigated. We give details of the method used to calibrate VPHAS+ photometry, and present the expected colours of CS and main sequence stars within the survey. Objects were scrutinized to remove PN mimics from our sample and identify true CS. Within our final sample of 7 CS, 6 had previously either not been identified or confirmed. We detected an $i$ band excess indicative of a low-mass companion star in 3 CS, including one known binary, leading us to to conclude that VPHAS+ provides the precise photometry required for the IR excess method presented here, and will likely improve as the survey completes and the calibration process finalised. Given the promising results from this trial sample, the entire VPHAS+ catalogue should be used to study PNe and extend the IR excess-tested CS sample.

astro-ph.SR↗

arXiv2016

A LOFAR detection of the low mass young star T Tau at 149 MHz

Radio observations of young stellar objects (YSOs) enable the study of ionised plasma outflows from young protostars via their free-free radiation. Previous studies of the low-mass young system T Tau have used radio observations to model the spectrum and estimate important physical properties of the associated ionised plasma (local electron density, ionised gas content and emission measure). However, without an indication of the low-frequency turnover in the free-free spectrum, these properties remain difficult to constrain. This paper presents the detection of T Tau at 149 MHz with the Low Frequency Array (LOFAR) - the first time a YSO has been observed at such low frequencies. The recovered total flux indicates that the free-free spectrum may be turning over near 149 MHz. The spectral energy distribution is fitted and yields improved constraints on local electron density ($(7.2 \pm 2.1)\times10^{3}$ cm$^{-3}$), ionised gas mass ($(1.0 \pm 1.8)\times10^{-6}$ M$_{\odot}$) and emission measure ($(1.67 \pm 0.14)\times10^5$ pc cm$^{-6}$).

astro-ph.SR↗

arXiv2015

The LOFAR Transients Pipeline

Current and future astronomical survey facilities provide a remarkably rich opportunity for transient astronomy, combining unprecedented fields of view with high sensitivity and the ability to access previously unexplored wavelength regimes. This is particularly true of LOFAR, a recently-commissioned, low-frequency radio interferometer, based in the Netherlands and with stations across Europe. The identification of and response to transients is one of LOFAR's key science goals. However, the large data volumes which LOFAR produces, combined with the scientific requirement for rapid response, make automation essential. To support this, we have developed the LOFAR Transients Pipeline, or TraP. The TraP ingests multi-frequency image data from LOFAR or other instruments and searches it for transients and variables, providing automatic alerts of significant detections and populating a lightcurve database for further analysis by astronomers. Here, we discuss the scientific goals of the TraP and how it has been designed to meet them. We describe its implementation, including both the algorithms adopted to maximize performance as well as the development methodology used to ensure it is robust and reliable, particularly in the presence of artefacts typical of radio astronomy imaging. Finally, we report on a series of tests of the pipeline carried out using simulated LOFAR observations with a known population of transients.

astro-ph.IM↗

arXiv2012

The LOFAR Magnetism Key Science Project

Measuring radio waves at low frequencies offers a new window to study cosmic magnetism, and LOFAR is the ideal radio telescope to open this window widely. The LOFAR Magnetism Key Science Project (MKSP) draws together expertise from multiple fields of magnetism science and intends to use LOFAR to tackle fundamental questions on cosmic magnetism by exploiting a variety of observational techniques. Surveys will provide diffuse emission from the Milky Way and from nearby galaxies, tracking the propagation of long-lived cosmic-ray electrons through magnetic field structures, to search for radio halos around spiral and dwarf galaxies and for magnetic fields in intergalactic space. Targeted deep-field observations of selected nearby galaxies and suspected intergalactic filaments allow sensitive mapping of weak magnetic fields through Rotation Measure (RM) grids. High-resolution observations of protostellar jets and giant radio galaxies reveal structures on small physical scales and at high redshifts, whilst pulsar RMs map large-scale magnetic structures of the Galactic disk and halo in revolutionary detail. The MKSP is responsible for the development of polarization calibration and processing, thus widening the scientific power of LOFAR.

astro-ph.IM↗

arXiv2012

SOFIA observations of CO(12-11) emission along the L1157 bipolar outflow

Carbon monoxide is an excellent tracer of the physical conditions of gas in molecular outflows from young stars. To understand the outflow mechanism we need to investigate the origin of the molecular emission and the structure and interaction of the outflowing molecular gas. Deriving the physical parameters of the gas will help us to trace and understand the various gas components in the flow. We observed CO(12-11) line emission at various positions along the L1157 bipolar outflow with GREAT aboard SOFIA. Comparing these new data with CO(2-1), we find basically constant line ratios along the outflow and even at the position of the source. These line ratios lead us to estimates of 10^5 to 10^6 cm^-3 for the gas density and 60 to 100 K for the gas temperature of the outflowing gas. The constrained density and temperature values indicate that we are mostly tracing a low-velocity gas component everywhere along the outflow, which is intermediate between the already known cold gas component, which gets entrained into the flow, and the hot gas, which gets shocked in the outflow.

astro-ph.SR↗

arXiv2012

Jet rotation investigated in the near-ultraviolet with HST/STIS

We present results of the second phase of our near-ultraviolet investigation into protostellar jet rotation using HST/STIS. We obtain long-slit spectra at the base of five T Tauri jets to determine if there is a difference in radial velocity between the jet borders which may be interpreted as a rotation signature. These observations are extremely challenging and push the limits of current instrumentation, but have the potential to provide long-awaited observational support for the magneto-centrifugal mechanism of jet launching in which jets remove angular momentum from protostellar systems. We successfully detect all five jet targets (from RW Aur, HN Tau, DP Tau and CW Tau) in several near-ultraviolet emission lines, including the strong Mg II doublet. However, only RW Aur's bipolar jet presents sufficient signal-to-noise for analysis. The approaching jet lobe shows a difference of 10 km/s in a direction which agrees with the disk rotation sense, but is opposite to previously published optical measurements for the receding jet. The near-ultraviolet difference is not found six months later, nor is it found in the fainter receding jet. Overall, in the case of RW Aur, differences are not consistent with a simple jet rotation interpretation. Indeed, given the renowned complexity and variability of this system, it now seems likely that any rotation signature is confused by other influences, with the inevitable conclusion that RW Aur is not suited to a jet rotation study.

astro-ph.GA↗

arXiv2011

Magnetic activity and accretion on FU Tau A: Clues from variability

FU Tau A is a young very low mass object in the Taurus star forming region which was previously found to have strong X-ray emission and to be anomalously bright for its spectral type. In this study we discuss these characteristics using new information from quasi-simultaneous photometric and spectroscopic monitoring. From photometric time series obtained with the 2.2m telescope on Calar Alto we measure a period of ~4d for FU Tau A, most likely the rotation period. The short-term variations over a few days are consistent with the rotational modulation of the flux by cool, magnetically induced spots. In contrast, the photometric variability on timescales of weeks and years can only be explained by the presence of hot spots, presumably caused by accretion. The hot spot properties are thus variable on timescales exceeding the rotation period, maybe due to long-term changes in the accretion rate or geometry. The new constraints from the analysis of the variability confirm that FU Tau A is affected by magnetically induced spots and excess luminosity from accretion. However, accretion is not sufficient to explain its anomalous position in the HR diagram. In addition, suppressed convection due to magnetic activity and/or an early evolutionary stage need to be invoked to fully account for the observed properties. These factors cause considerable problems in estimating the mass of FU Tau A and other objects in this mass/age regime, to the extent that it appears questionable if it is feasible to derive the Initial Mass Function for young low-mass stars and brown dwarfs.

astro-ph.SR↗

arXiv2011

The nature of the embedded intermediate-mass T Tauri star DK Cha

Most of our knowledge about star formation is based on studies of low-mass stars, whereas very little is known about the properties of the circumstellar material around young and embedded intermidiate-mass T Tauri stars (IMTTSs). We present an analysis of the excitation and accretion properties of the young IMTTS DK Cha. The nearly face-on configuration of this source allows us to have direct access to the star-disk system through the excavated envelope and outflow cavity. Based on LR optical and IR spectroscopy obtained with SofI/EFOSC2 on the NTT we derive the spectrum of DK Cha from ~0.6 to 2.5 μm. From the detected lines we probe the conditions of the gas that emits the HI IR emission lines. In addition, we derive the mass accretion rate (Macc) from the relationships that connect the luminosity of the Brγ and Paβ lines with the accretion luminosity (Lacc). The observed optical/IR spectrum is extremely rich in forbidden and permitted atomic and molecular emission lines, which makes this source similar to very active low-mass T Tauri stars. Some of the permitted emission lines are identified as being excited by fluorescence. We derive Brackett decrements and compare them with different excitation mechanisms. The Paβ/Brγ ratio is consistent with optically thick emission in LTE at a temperature of ~3500 K, originated from a compact region of ~5 Rsun in size: but the line opacity decreases in the Br lines for high quantum numbers n_{up}. A good fit to the data is obtained assuming an expanding gas in LTE, with an electron density at the wind base of ~10^13 cm-3. In addition, we find that the observed Brackett ratios are very similar to those reported in previous studies of low-mass CTTSs and Class I sources, indicating that these ratios are not dependent on masses and ages. Finally, Lacc~9 Lsun and Macc~3x10-7 Msun/yr values were found.

astro-ph.SR↗

arXiv2010

Resolving debris discs in the far-infrared: early highlights from the DEBRIS survey

We present results from the earliest observations of DEBRIS, a Herschel Key Programme to conduct a volume- and flux-limited survey for debris discs in A-type through M-type stars. PACS images (from chop/nod or scan-mode observations) at 100 and 160 micron are presented toward two A-type stars and one F-type star: beta Leo, beta UMa and eta Corvi. All three stars are known disc hosts. Herschel spatially resolves the dust emission around all three stars (marginally, in the case of beta UMa), providing new information about discs as close as 11 pc with sizes comparable to that of the Solar System. We have combined these data with existing flux density measurements of the discs to refine the SEDs and derive estimates of the fractional luminosities, temperatures and radii of the discs.

astro-ph.SR↗

arXiv2010

The low-mass diskless population of Corona Australis

We combine published optical and near-infrared photometry to identify new low-mass candidate members in an area of about 0.64 deg^2 in Corona Australis, using the S-parameter method. Five new candidate members of the region are selected, with estimated ages between 3 and 15 Myr, and masses between 0.05 and 0.15 M_Sun. Using Spitzer photometry, we confirm that these objects are not surrounded by optically thick disks. However, one of them is found to display excess at 24 micron, thus suggesting it harbours a disk with an inner hole. With an estimated mass of 0.07 M_Sun according to the SED fitting, this is one of the lowest-mass objects reported to possess a transitional disk. Including these new members, the fraction of disks is about 50% among the total Corona Australis population selected by the same criteria, lower than the 70% fraction reported earlier for this region. Even so, we find a ratio of transitional to primordial disks (45%) very similar to the value derived by other authors. This ratio is higher than for solar-type stars (5-10%), suggesting that disk evolution is faster in the latter, and/or that the "transitional disk" stage is not such a short-lived step in the case of very low-mass objects. However, this impression needs to be confirmed with better statistics.

astro-ph.SR↗

arXiv2010

Rotation-disk connection for very low mass and substellar objects in the Orion Nebula Cluster

Angular momentum loss requires magnetic interaction between the forming star and both the circumstellar disk and the magnetically driven outflows. In order to test these predictions many authors have investigated a rotation-disk connection in pre-main sequence objects with masses larger than about 0.4Msun. For brown dwarfs this connection was not investigated as yet because there are very few samples available. We aim to extend this investigation well down into the substellar regime for our large sample of BDs in the Orion Nebula Cluster, for which we have recently measured rotational periods. In order to investigate a rotation-disk correlation, we derived near-infrared (NIR) excesses for a sample of 732 periodic variables in the Orion Nebula Cluster with masses ranging between 1.5-0.02 Msun and whose IJHK colors are available. Circumstellar NIR excesses were derived from the Delta[I-K] index. We performed our analysis in three mass bins.We found a rotation-disk correlation in the high and intermediate mass regime, in which objects with NIR excess tend to rotate slower than objects without NIR excess. Interestingly, we found no correlation in the substellar regime. A tight correlation between the peak-to-peak (ptp) amplitude of the rotational modulation and the NIR excess was found however for all objects with available ptp values. We discuss possible scenarios which may explain the lack of rotation-disk connection in the substellar mass regime. One possible reason could be the strong dependence of the mass accretion rate on stellar mass in the investigated mass range.

astro-ph.SR↗