arXiv subjects

Jochen Eisloeffel

Publications and source records attributed to Jochen Eisloeffel.

29 records · Page 2Linked to original sources

arXiv2004

An S-shaped outflow from IRAS 03256+3055 in NGC 1333

The IRAS source 03256+3055 in the NGC 1333 star forming region is associated with extended sub-millimeter emission of complex morphology, showing multiple clumps. One of these is found to coincide with the driving source of a bipolar jet of S-shaped morphology seen in the emission lines of H_alpha and [SII] as well as in the H2 emission lines in the K-band. Detailed images of the driving source at the wavelengths of H_alpha and [SII] and in the I, J, H, and K bands as well as a K-band spectrum and polarimetry are discussed. The near-infrared morphology is characterized by a combination of line emission from the jet and scattered light from a source with a steep continuum spectrum. The morphology and proper motion of the jet are discussed in the context of a binary system with a precessing disk. We conclude that the molecular core associated with IRAS 03256+3055 consists of several clumps, only one of which shows evidence of recent star formation at optical and near-infrared wavelengths.We also briefly discuss a second, newly found near-infrared source associated with a compact sub-millimeter continuum source near IRAS 03256+3055, and conclude that this source may be physically unrelated the cluster of molecular clumps.

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arXiv2004

Rotation and Disc Accretion in Very Low Mass Stars and Brown Dwarfs

The regulation of angular momentum is one of the key processes for our understanding of stellar evolution. In contrast to solar-mass stars, very low mass (VLM) objects and brown dwarfs are believed to be fully convective. This may lead to major differences of rotation and activity, since fully convective objects may not host a solar-type dynamo. Here, we report on our observational efforts to understand the rotational evolution of VLM objects.

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arXiv2004

Jet rotation: launching region, angular momentum balance and magnetic properties in the bipolar outflow from RW Aur

Using STIS on board the HST we have obtained a spectroscopic map of the bipolar jet from RW Aur. We find signatures of rotation within the first 300 AU of the jet (1.''5 from RW Aur). Both lobes rotate in the same direction (i.e. with different helicities). Toroidal velocities are in the range 5 - 30 km/s at 20 (30) AU from the symmetry axis in the blueshifted (redshifted) lobe, in line with other STIS observations and with theoretical simulations. The sense of rotation is anti-clockwise looking from the tip of the blue lobe down to the star. Rotation is more evident in the [OI] and [NII] lines and at the largest sampled distance from the axis. We derive that the mass ejected in the observed part of the outflow is accelerated from a region in the disk within about 0.5 (1.6) AU from the star for the blue (red) lobe. We estimate that the angular momentum transport rate of the jet can be two thirds or more of the estimated rate transported through the relevant portion of the disk. The magnetic lever arm (ratio rA/r0 between the Alfv`en and footpoint radii) is between 3.5 and 4.6 (accuracy 20-25%), or, the ejection index $ξ$ = d ln($\dot{M}_acc$) / d r is in the range 0.025 - 0.046 (same accuracy).These values are in the range predicted by the models, and suggest that some heating must be provided at the base of the flow. We also derive the ratio B_phi / B_p of the toroidal and poloidal components of the magnetic field at the observed location. B_phi / B_p = 3.8 +- 1.1 at 30 AU from the axis in the red lobe and -8.9 +- 2.7 at 20 AU from the axis in the blue lobe. The toroidal component is dominant, as predicted by magnetic collimation models.

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arXiv2004

Submillimetre continuum emission from Class 0 sources: Theory, Observations, and Modelling

We report on a study of the thermal dust emission of the circumstellar envelopes of a sample of Class 0 sources. The physical structure (geometry, radial intensity profile, spatial temperature and spectral energy distribution) and properties (mass, size, bolometric luminosity (L_bol) and temperature (T_ bol), and age) of Class 0 sources are derived here in an evolutionary context. This is done by combining SCUBA imaging at 450 and 850 microm of the thermal dust emission of envelopes of Class 0 sources in the Perseus and Orion molecular cloud complexes with a model of the envelope, with the implementation of techniques like the blackbody fitting and radiative transfer calculations of dusty envelopes, and with the Smith evolutionary model for protostars. The modelling results obtained here confirm the validity of a simple spherical symmetric model envelope, and the assumptions about density and dust distributions following the standard envelope model. The spherically model reproduces reasonably well the observed SEDs and the radial profiles of the sources. The implications of the derived properties for protostellar evolution are illustrated by analysis of the L_bol, the T_bol, and the power-law index p of the density distribution for a sample of Class 0 sources.

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arXiv2004

Rotation and variability of very low mass stars and brown dwarfs near Epsilon Ori

We explore the rotation and activity of very low mass (VLM) objects by means of a photometric variability study. Our targets in the vicinity of Epsilon Ori belong to the OriOB1b population in the Orion star-forming complex. In this region we selected 143 VLM stars and brown dwarfs (BDs), whose photometry in RIJHK is consistent with membership of the young population. The variability of these objects was investigated using a densely sampled I-band time series covering four consecutive nights with altogether 129 data points per object. Our targets show three types of variability: Thirty objects, including nine BDs, show significant photometric periods, ranging from 4h up to 100h, which we interpret as the rotation periods. Five objects, including two BDs, exhibit variability with high amplitudes up to 1 mag which is at least partly irregular. This behaviour is most likely caused by ongoing accretion and confirms that VLM objects undergo a T Tauri phase similar to solar-mass stars. Finally, one VLM star shows a strong flare event of 0.3 mag amplitude. The rotation periods show dependence on mass, i.e. the average period decreases with decreasing object mass, consistent with previously found mass-period relationships in younger and older clusters. The period distribution of BDs extends down to the breakup period, where centrifugal and gravitational forces are balanced. Combining our BD periods with literature data, we found that the lower period limit for substellar objects lies between 2h and 4h, more or less independent of age. Contrary to stars, these fast rotating BDs seem to evolve at constant rotation period from ages of 3 Myr to 1 Gyr, in spite of the contraction process. Thus, they should experience strong rotational braking.

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arXiv2004

Formation and Evolution of Very Low Mass Stars and Brown Dwarfs

The formation and evolution of brown dwarfs are currently "hot topics" in cool star research. Latest observations and modeling efforts on disks, accretion, outflows, spatial distribution, and binarity in the context of the formation by ejection model and alternative scenarios were presented and discussed vigorously in this splinter session. A second major topic was rotation and activity of these objects. This part included observations of rotation periods and amplitudes, variability, and X-ray activity, and their consequences for the generation of magnetic fields and the cool atmospheres.

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arXiv2004

Rotational evolution of very low mass objects and brown dwarfs

The regulation of angular momentum is one of the key processes for our understanding of stellar evolution. The rotational evolution of solar-mass stars is mainly determined by the magnetic interaction with their circumstellar disk and angular momentum loss through stellar winds, and In contrast to solar-mass stars, very low mass (VLM) objects and brown dwarfs are believed to be fully convective. This may lead to major differences of rotation and activity, since fully convective objects may not host a solar-type dynamo. Here, we report on our observational efforts to understand the rotational evolution of VLM objects. By means of photometric monitoring, we determined 62 rotation periods for targets in three clusters, which form an age sequence from 3 to 125 Myr. We find that VLM objects rotate faster than their solar-mass siblings in all evolutionary stages. Their rotational evolution seems to be determined by hydrostatic contraction and exponential angular momentum loss. The photometric amplitudes of the light curves are much lower than for solar-mass stars. This may be explained as a consequence of smaller spot coverage, more symmetric spot distributions, or lower contrast between spots and their environment. Most of these results can be explained with a change of the magnetic field properties with decreasing mass. VLM objects possibly possess only small-scale, turbulent magnetic fields.

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arXiv2004

Rotation periods for very low mass stars in the Pleiades

We present the results of a photometric monitoring campaign for very low mass (VLM) members of the Pleiades. Periodic photometric variability was detected for nine VLM stars with masses between 0.08 and 0.25 MS. These variations are most likely caused by co-rotating, magnetically induced spots. In comparison with solar-mass stars, the photometric amplitudes are very low (<0.04 mag), implying that either the fraction of the spot-covered area, the asymmetry of the spot distribution, or the contrast between spots and photospheric environment decreases with mass. From our lightcurves, there is evidence for temporal evolution of the spot patterns on timescales of about two weeks. The rotation periods range from 2.9 h to 40 h and tend to increase linearly with mass. Compared with more massive stars, we clearly see a lack of slow rotators among VLM objects. The rotational evolution of VLM stars is investigated by evolving the previously published periods for very young objects (Scholz & Eisloeffel 2004) forward in time, and comparing them with those observed here in the Pleiades. We find that the combination of spin-up by pre-main sequence contraction and exponential angular momentum loss through stellar winds is able to reproduce the observed period distribution in the Pleiades. This result may be explained as a consequence of convective, small-scale magnetic fields.

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arXiv2004

Rotation and accretion of very low mass objects in the SigmaOri cluster

We report on two photometric monitoring campaigns of Very Low Mass (VLM) objects in the young open cluster around SigmaOrionis. Our targets were pre-selected with multi-filter photometry in a field of 0.36 sqdeg. For 23 of these objects, spanning a mass range from 0.03 to 0.7 MS, we detect periodic variability. Of these, 16 exhibit low-level variability, with amplitudes of less than 0.2 mag in the I-band, which is mostly well-approximated by a sine wave. These periodicities are probably caused by photospheric spots co-rotating with the objects. In contrast, the remaining variable targets show high-level variability with amplitudes ranging from 0.25 to 1.1 mag, consisting of a periodic light variation onto which short-term fluctuations are superimposed. This variability pattern is very similar to the photometric behaviour of solar-mass, classical T Tauri stars. Low-resolution spectra of a few of these objects reveal strong Halpha and Ca-triplet emission, indicative of ongoing accretion processes. This suggests that 5-7% of our targets still possess a circumstellar disk. In combination with previous results for younger objects, this translates into a disk lifetime of 3-4 Myr, significantly shorter than for solar mass stars. The highly variable objects rotate on average slower than the low-amplitude variables, which is expected in terms of a disk-locking scenario. There is a trend towards faster rotation with decreasing mass, which might be caused by shortening of the disk lifetimes or attenuation of magnetic fields.

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arXiv2001

The environment of FS Tau observed with HST WFPC2 in narrow-band emission line filters

We present the results of HST WFPC2 observations of FS Tau and its environment obtained in the narrow-band emission line filters H alpha and [S II] (lambda = 6716,6731 Angstrom). Based on these data the morphology of line emission within this region can be studied on a size scale of 0.1" for the first time. Despite the fact that FS Tau A has strong forbidden emission lines, there is no evidence for extended emission at these wavelengths beyond 0.5" from the components of this close T Tauri binary system. In the FS Tau B outflow interesting morphological properties can be studied at high spatial resolution. In this jet we find a structure where circular or elliptical jet knots are correlated with minima of the jet width. The overall width of this jet decreases with distance from the source. The FS Tau B jet is thus a rare example of a jet which may be recollimated far away from its source. The jet is much more prominent in H alpha than in [S II], while the counterjet shows the opposite behaviour. The line ratio H alpha/[S II] increases with lateral distance from the jet axis which is indicative of entrainment of ambient material.

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arXiv2000

HST/STIS spectroscopy of the optical outflow from DG Tau: structure and kinematics on sub-arcsecond scales

We have carried out a spatio-kinematic study of the outflow from the classical T Tauri star DG Tau using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). A series of seven spatially offset long-slit spectra spaced by 0.07'' were obtained along the axis of the outflow to build up a 3-D intensity-velocity ``cube'' in various forbidden emission lines (FELs) and Ha. Here we present high spatial resolution synthetic line images close to the star in distinct radial velocity intervals (from ~ +50 km/s to ~ -450 km/s in four bins, each ~ 125 km/s wide). The lowest velocity emission is also examined in finer detail (from +60 km/s to -70 km/s in five bins ~ 25 km/s wide). We have found that the highest velocity and most highly collimated component, i.e. the jet, can be traced from DG Tau to a distance D ~ 0.7''. The jet is on the axis of a pear-shaped limb-brightened bubble which extends between 0.4'' and 1.5'' from the source and which we interpret as a bow shock. Other condensations are seen close to the star indicating ongoing temporal variations in the flow. The low-velocity component of the outflow is found to be spatially wide close to the source (~ 0.2'' at D=0.2''), in contrast to the high velocity jet (width <~ 0.1''). We have also found evidence to suggest that the density increases longitudinally with proximity to the source and also laterally towards the flow axis. Thus, at least in the case of DG Tau, the flow becomes gradually denser as it increases in velocity and becomes more collimated. Our observations show a continous bracketing of the higher speed central flow within the lower speed, less collimated, broader flow, down to the lowest velocity scales. This suggests that the low and high velocity FELs in the highly active T Tauri star DG Tau are intimately related.

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