Abstract.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 <0.01 mag) is a variable, with about half having a peak-to-peak variation of ∼0.2 mag or more. A clear correlation is found between the level of variability and infrared excess emission, in the sense that stars with evidence for circumstellar disks have larger amplitudes of variation. A search for periodic variables was carried out and 369 such stars were discovered, most or all of which are rotating, spotted T Tauri stars. Periodic variables are most commonly found among the low amplitude variables. 46% of the stars with magnitudes between 12.5 and 16 and standard deviation, σ < 0.1 mag, were found to be periodic, whereas only 24% of the stars in the same magnitude range with σ > 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, ω < 1 radian/d, corresponding to rotation periods longer than 6.28 d, have a mean infrared excess emission, ∆(I − K) = 0.55 ± 0.05, indicative of the presence of inner disks, while rapid rotators, with ω > 2 radians/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.

We have carried out a kinematical, high angular resolution (∼ 0. ′′ 1) study of the optical blueshifted flow from DG Tau within 0. ′′ 5 from the source (i.e. 110 AU when de-projected along this flow). We analysed 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 (HST), obtained by maintaining the slit parallel to the outflow axis while at the same time moving it transversely in steps of 0. ′′ 07. 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 the flow is axially symmetric, the velocity shifts are consistent with the southeastern side of the flow moving towards 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 towards the source and the derived toroidal velocities are in 1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association -2the range 6 to 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, ofJ 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 magneto-centrifugal jet-launching models, although we cannot exclude the possibility that the observed velocity differences are due to some transverse outflow asymmetry other than rotation.

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 Hα. Here we present high spatial resolution synthetic line images close to the star in distinct radial velocity intervals (from ∼ +50 km s −1 to ∼ -450 km s −1 in four bins, each ∼ 125 km s −1 wide). The lowest velocity emission is also examined in finer detail (from +60 km s −1 to -70 km s −1 in five bins ∼ 25 km s −1 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 not only does the density in the outflow increase longitudinally with proximity to the source but that it also increases 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. Implications of these observations for FEL models will be considered in a future paper (Bacciotti et al. 2000).

Abstract. 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 I C magnitudes between 9.8 mag and 21 mag have been monitored in our 34 × 33 field. Time series data were obtained in the I C 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, R C 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 I C vs. (R C − I C ) colour-magnitude and (R C − Hα) vs. (R C − I C ) 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 I C 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 I C ≤ 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 I C = 18.0 mag.

Abstract. We have carried out a long-slit spectroscopic survey for 38 T Tauri stars (TTSs) to study the spatial and kinematic properties of their forbidden emission line (FEL) regions. With these observations we hope to provide more insight into the complex physical structure of the outflows from young stars on the smallest spatial scales observable by long-slit spectroscopy. Due to the differential nature of the observational method, information on the spatial properties (offset from the stellar continuum and spatial width) on sub-arcsec (sub-seeing) scales can be obtained. . A sub-sample of 9 closeby stars (Haro 6-10, XZ Tau, UZ Tau E, HN Tau, DO Tau, DP Tau, UY Aur, RW Aur and V536 Aql) has been extensively studied and the direction of their outflows has been approximately determined by taking spectra at several slit position angles, if not known from emission-line CCD imaging. The spatial and kinematic properties of the FEL regions of these 9 TTSs are described in detail. Together with 3 additional stars discussed in the literature, a sample of 12 stars provides the basis for the following main results of our survey: The so-called high-velocity component (HVC) of the FELs (or gas of high velocity which presumably represents in many cases a HVC being blended with emission of lower velocity) is generally spatially more extended than the so-called low-velocity component (LVC, or gas near the stellar velocity). In the [SII] λ6731 line the centroid of the high-velocity gas is located typically at distances of 0.6 from the TTS while for the low-velocity gas this value is smaller on average by more than a factor of 3. Comparing the spatial properties of the high-velocity gas among the investigated FELs, it turns out that the largest spatial width and the largest offset of the centroid from the star is usually observed in the [NII] Our data provide additional support for the model of Kwan & Tademaru (1988 according to which the HVC observed in the FELs of many TTSs is formed in a well-collimated jet, while the LVC represents gas from a physically distinct flow component (possibly a disk wind or a disk corona). The larger spatial extent of the highvelocity gas in [SII] and [NII] compared to that in [OI] is most probably the result of a jet decreasing in density with increasing distance from the source combined with an increase in excitation. The decrease of the electron density with distance is rather obvious for a jet with diverging stream lines, but why the electron temperature increases is unclear.

Abstract. Our study is based on an extensive photometric monitoring program in the young (2-4 Myr) open cluster NGC 2264 by Lamm et al. (2004a). This program resulted in a sample of 405 periodic variables which are most likely pre-main sequence (PMS) members of the cluster. The periodic variability of these stars results from the rotational modulation of the light by stellar spots. In this paper we investigate the rotation period evolution of young stars. This is done by comparing the period distribution of the older NGC 2264 with that of the younger Orion Nebular Cluster (ONC, age: ∼1 Myr) which is known from the literature. The age ratio between the two clusters was estimated on the basis of PMS models to be about 2 +0.75 −0.5 . We find that the period distribution of NGC 2264 is similar in form to the ONC but shifted to shorter periods. In both clusters the period distribution depends strongly on the mass and it is bimodal for higher mass stars with M > ∼ 0.25 M while it is unimodal for lower mass stars with M < ∼ 0.25 M . In addition the lower mass stars rotate much faster on average than the higher mass stars. Quantitative comparison between the period distributions of both clusters suggests that a large fraction (about 80%) of stars have spun up from the age of the ONC to the age of NGC 2264. Based on this estimate and the estimated age ratio between the two clusters we find that the average spin up by a factor of 1.5−1.8 from the age of the ONC to the age of NGC 2264 is consistent with a decreasing stellar radius and conservation of angular momentum, for most stars. However, within NGC 2264 we did not find any significant spin up from the younger to older stars in the cluster. We also found indications for some ongoing disk-locking in NGC 2264, in particular for the higher mass stars. Our analysis of the period distribution suggests that about 30% of the higher mass stars in NGC 2264 could be magnetically locked into co-rotation with their inner disk. In the case of the lower mass stars, disk-locking seems to be less important for the rotational evolution of the stars. This interpretation is supported by the analysis of the stars' Hα emission. This analysis indicates that the locking period of the higher mass stars is about P = 8 days. For the lower mass stars this analysis indicates a locking period of about 2-3 days. We argue that the latter stars are probably not "completely" locked to their disk and propose an evolution scenario for these stars which we call "moderate angular momentum loss". In this scenario angular momentum is continuously removed from the stars but at a rate too low to lock the stars with a constant rotation period. We have done a detailed comparison with the recently published rotational period study of NGC 2264 of Makidon et al. (2004). Even though their obtained period distribution of their quality 1 data on NGC 2264 is indistinguishable within the statistical errors from ours, we come to quite different conclusions about the interpretation. One major reason for these discrepanci...

Abstract. VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, have been used to establish the membership and the properties of a sample of low-mass candidate members of the σ Orionis cluster with masses spanning from 1 M down to about 0.013 M (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense Hα emission in various cluster members, which, in addition to the previously detected forbidden emision lines and the presence of Li  in absorption at 6708 Å, have allowed us to tentatively classify σ Orionis members as classical or weak-line T Tauri stars and substellar analogs. Variability of the Hα line has been investigated and detected in some objects. Based on the K-band infrared excesses and the intensity of Hα emission, we estimate that the minimum disk frequency of the σ Orionis low-mass population is in the range 5-12%.

We report on an imaging and kinematic study of the jets and outÑows emanating from DG Tau, DG Tau B, FS Tau B, T Tau, and CoKu Tau 1. The kinematic data are based on proper-motion measurements and long-slit spectroscopy, and the imaging data on deep narrowband [S II] jj6716, 6731, Ha, and continuum images. The individual objects and their peculiarities are discussed in detail. The six investigated jets and Ñows di †er from each other in their morphology, spatial extent, and their degree of variations in velocity and in the ratio f between the knot pattern speed and the Ñow speed. Possible reasons for these large di †erences are brieÑy discussed. In DG Tau, FS Tau B, and T Tau strong velocity variations are indicated, while for DG Tau B very small velocity variations, if any, are observed (¹7% in some Ñow sections). The small velocity variations in DG Tau B, together with large variations in the f-values in this and other young stellar object (YSO) jets, pose severe difficulties for any model that explains the observed knots by internal shocks resulting from velocity variations. Our results from DG Tau B and other YSO jets strongly indicate that for a signiÐcant fraction of knots in YSO jets other mechanisms probably excite the internal shocks.The bipolar outÑow from FS Tau B apparently shows indications of a highly collimated and a poorly collimated Ñow in both outÑow lobes. To our knowledge, L1551 IRS 5 is the only other case where such an unusual situation is also indicated. About 60A northeast of FS Tau B we have found an independent faint 150A-long jetlike outÑow oriented approximately perpendicular to the FS Tau B jet, which we have designated FS Tau C (HH 276).