Reinhard Mundt

Hubble Space Telescope/STIS Spectroscopy of the Optical Outflow from DG Tauri: Indications for Rotation in the Initial Jet Channel*

We have carried out a kinematical, high angular resolution (~01) study of the optical blueshifted flow from DG Tau within 05 from the source (i.e., 110 AU when deprojected along this flow). We analyzed optical emission line profiles extracted from a set of seven long-slit spectra taken with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope, obtained by maintaining the slit parallel to the outflow axis while at the same time moving it transversely in steps of 007. For the spatially resolved flow of moderate velocity (peaking at -70 km s-1), we have found systematic differences in the radial velocities of lines from opposing slit positions, i.e., on alternate sides of the jet axis. The results, obtained using two independent techniques, are corrected for the spurious wavelength shift due to the uneven illumination of the STIS slit. Other instrumental effects are shown to be either absent or unimportant. The derived relative Doppler shifts range from 5 to 20 km s-1. Assuming that the flow is axially symmetric, the velocity shifts are consistent with the southeastern side of the flow moving toward the observer faster than the corresponding northwestern side. If this finding is interpreted as rotation, the flow is then rotating clockwise looking from the jet toward the source and the derived toroidal velocities are in the range 6-15 km s-1, depending on position. Combining these values with recent estimates of the mass-loss rate, one would obtain an angular momentum flux, for the low- to moderate-velocity regime of the flow, of w,lm ~ 3.8 × 10-5 M☉ yr-1 AU km s-1. Our findings may constitute the first detection of rotation in the initial channel of a jet flow. The derived values appear to be consistent with the predictions of popular magnetocentrifugal jet-launching models, although we cannot exclude the possibility that the observed velocity differences are due to some transverse outflow asymmetry other than rotation.

The Substellar Mass Function in σ Orionis

We combine results from imaging searches for substellar objects in the σ Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin2. We identify 64 very low mass cluster member candidates in this region. We have available three-color (I, Z, and J) photometry for all of them, spectra for 24 objects, and K photometry for 27% of our sample. These data provide a well-defined sequence in the I versus I-J and I versus I-K color-magnitude diagrams and indicate that the cluster exhibits little reddening despite its young age (~5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 M☉) across the complete brown dwarf domain (0.075 to 0.013 M☉) and into the realm of free-floating planetary-mass objects (≤0.013 M☉). We find that the mass spectrum (dN/dm) m-α increases toward lower masses, with an exponent α = 0.8 ± 0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in σ Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 M☉ should be abundant in the solar vicinity, with a density similar to M-type stars.

Stellar rotation and variability in the Orion Nebula Cluster

A wide field imager attached to the MPG/ESO 2.2 m telescope on La Silla has been used to monitor the Orion Nebula Cluster on 45 nights between 25 Dec. 1998 and 28 Feb. 1999. Ninety-two images were obtained during this period through an intermediate band filter centered at 815.9 nm. More than 1500 sources with I magnitudes between 12.5 and 20 were monitored. We find that essentially every star brighter than 16th mag (where the precision is 0:1 yielded periods. Our work confirms the existence of a bimodal period distribution, with peaks near 2 and 8 days, for stars with M> 0:25 M and a unimodal distribution peaked near 2 days, for lower mass stars. We show that a statistically significant correlation exists between infrared excess emission and rotation in the sense that slower rotators are more likely to show evidence of circumstellar disks. Slowly rotating stars, with angular velocities,! 2r adians/d, corresponding to rotation periods shorter than 3.14 d, have a much smaller mean of 0:17 0:05. This supports the hypothesis that disks are involved in regulating stellar rotation during the pre-main sequence phase. We explore a simple and commonly adopted model of rotational evolution in which stars conserve angular velocity while locked to a disk and conserve angular momentum once released. If these assumptions are valid, and if the locking period is 8 days, we find that more than half of the stars in the ONC are no longer locked to disks and that an exponential decay model with a disk-locking half-life of about 0.5-1 My fits the observations well. Assuming that the mean ages of the higher and lower mass stars are the same, the faster rotation of the lower mass stars can be understood as either a consequence of a shorter disk-locking time or a shorter initial disk-locking period, or both.

Erratum Variability in ultra cool dwarfs: Evidence for the evolution of surface features

We present photometric light curves for a sample of 21 ultra cool M and L dwarfs in the field and in the young open clusters σ Orionis and the Pleiades. The list of targets includes both low mass hydrogen burning stars and brown dwarfs. Evidence for variability with rms amplitudes (in the I band) of 0.01 to 0.055 magnitudes on timescales of 0.4 to 100 hours is discovered in half of these objects. Power spectral analysis using the CLEAN algorithm was performed to search for evidence of periodic variability. Some objects show strong periodicities at around a few hours, which could be due to rotational modulation of the light curve by surface features. However, several objects do not have any significant periodicities to explain their variability. The

A Search for Hot Massive Extrasolar Planets around Nearby Young Stars with the Adaptive Optics System NACO

v \sin i

A Methane isolated planetary mass object in orion

values of a similar population of objects makes it very likely that our time sampling was sensitive to the expected range of rotation periods, and simulations show that we would have detected these if they were caused by long-lived surface features. We argue that this absence of periodicity is due to the evolution of the brightness, and presumably also the physical size, of surface features on timescales of a few to a few tens of hours. This is supported in the case of 2M1145 for which two light curves have been obtained one year apart and show no common periodicity. The surface features could plausibly be photospheric dust clouds or magnetically-induced spots. The recently observed decline in chromospheric activity for late type M and L dwarfs hints towards the former explanation for at least our later-type objects. Furthermore, our sample suggests that variability is more common in objects later than M9, indicating that the variability may be related to dust formation. One light curve shows a brief, but significant, dip, which could be a short-lived feature or possibly an eclipse by a companion.

Jets from young stars - CCD imaging, long-slit spectroscopy, and interpretation of existing data

We report on a survey devoted to the search of exoplanets around young and nearby stars carried out with NACO at the VLT. The detection limit for 28 among the best available targets versus the angular separation from the star is presented. The nondetection of any planetary mass companion in our survey is used to derive, for the first time, the frequency of the upper limit of the projected planet-star separation. In particular, we find that in 50% of the cases, no 5MJ (or more massive) planet has been detected at projected separations larger than 14 AU, and no 10MJ (or more massive) planet has been detected at projected separations larger than 8.5 AU. In 100% of the cases, these values increase to 36 and 65 AU, respectively. The excellent sensitivity reached by our study leads to a much lower upper limit of the projected planet-star separation compared with previous studies. For example, for the β Pictoris group (~12 Myr), we did not detect any 10MJ planet at distances larger than 15 AU. A previous study carried out with 4 m class telescopes put an upper limit for 10MJ planets at ~60 AU. For our closest target (V2306 Oph; d = 4.3 pc), it is shown that it would be possible to detect a 10MJ planet at a minimum projected separation from the star of 1 AU and a 5MJ planet at a minimum projected separation of 3.7 AU. Our results are discussed with respect to mechanisms explaining planet formation and migration and forthcoming observational strategies and future planet-finder observations from the ground.

A rotational and variability study of a large sample of PMS stars in NGC 2264

We report on the discovery of a free-floating methane dwarf toward the direction of the young star cluster σ Ori. Based on the objects far-red, optical, and near-infrared photometry and spectroscopy, we conclude that it is a possible member of this association. We have named it S Ori J053810.1-023626 (S Ori 70 is the abridged name). If it is a true member of σ Ori, the comparison of the photometric and spectroscopic properties of S Ori 70 with state-of-the-art evolutionary models yields a mass of 3 Jupiter mass for ages between 1 and 8 Myr. The presence of such a low-mass object in our small search area (55.4 arcmin2) would indicate a rising substellar initial mass function in the σ Ori cluster, even for planetary masses.

The afterglow of the short/intermediate-duration gamma-ray burst GRB 000301C: A jet at z = 2:04 ?;??;???

High-velocity jets and collimated outflows are now recognized as phenomena commonly associated with young stars. New CCD imaging of five objects, and in particular spatially resolved spectroscopy of eight highly-collimated flows, are discussed. Through the CCD imaging, three new jets have been discovered. It is shown that several previously known Herbig-Haro objects have extended bow-shock-like structures. In most of the latter cases, a jet is pointing from the star toward the bow-shock apex. The concave side of the bow-shock structure is in all cases oriented toward the outflow source. Using a data base of about 20 known jets, a detailed list of observational criteria describing these phenomena is compiled. A physical description and interpretation is presented which discusses the origin of knots in these jets and other structures, the relationship between jets and Herbig-Haro objects, the dissipation of energy along the jet due to internal shocks, the physical parameters, the relationship of optical jets to molecular outflows, and time scales for outflow activity. A short discussion of the driving sources for these jets is also included. 116 references.

The substellar mass function in σ Orionis II. Optical, near-infrared and IRAC/Spitzer photometry of young cluster brown dwarfs and planetary-mass objects ,

We present the results of an extensive search for periodic and irregular variable pre-main sequence (PMS) stars in the young (2-4 Myr) open cluster NGC 2264, based on photometric monitoring using the Wide Field Imager (WFI) on the 2.2 m telescope on La Silla (Chile). In total, about 10 600 stars with IC magnitudes between 9.8 mag and 21 mag have been monitored in our 34 � × 33 � field. Time series data were obtained in the IC band in 44 nights between Dec. 2000 and March 2001; altogether we obtained 88 data points per star. Using two different time series analysis techniques (Scargle periodogram and CLEAN) we found 543 periodic variable stars with periods between 0.2 days and 15 days. Also, 484 irregular variable stars were identified using a χ 2 -test. In addition we have carried out nearly simultaneous observations in V, RC and a narrow-band Hα filter. The photometric data enable us to reject background and foreground stars from our sample of variable stars according to their location in the IC vs. (RC − IC) colour-magnitude and (RC − Hα )v s. (RC − IC) colour-colour diagrams. We identified 405 periodic variable and 184 irregular variable PMS stars as cluster members using these two different tests. In addition 35 PMS stars for which no significant variability were detected could be identified as members using an Hα emission index criterion. This yields a total of 624 PMS stars in NGC 2264, of which only 182 were previously known. Most of the newly found PMS stars are fainter than IC � 15 mag and of late spectral type (>M 2). We find that the periodic variables, as a group, have a smaller degree of variability and smaller Hα index than the irregular variables. This suggests that the sample of periodic variables is biased towards weak-line T Tauri stars (WTTSs) while most of the irregular variables are probably classical T Tauri stars (CTTSs). We have quantified this bias and estimated that the expected fraction of WTTSs among PMS stars in the cluster is 77%. This is relatively close to the fraction of WTTSs among the periodic variables which is 85%. We also estimated the total fraction of variables in the cluster using only two well selected concentrations of PMS stars called NGC 2264 N & S in which we can easily estimate the total number of PMS stars. We find that at least 74% of the PMS stars in the cluster with IC ≤ 18.0 mag were found to be variable (either periodic or irregular) by our study. This number shows that our search for PMS stars in NGC 2264 through extensive and accurate photometric monitoring is very efficient in detecting most PMS stars down to at least IC = 18. 0m ag.