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Magakian et al. called attention to the current bright state of LkHα 225 South, a well-known highly embedded, intermediate-mass young stellar object that over the past two decades has brightened visually from >20 m to m . We present recent optical photometric monitoring showing colorless, nonsinusoidal, periodic brightness oscillations occurring every 43 days with amplitude ∼0.7 mag. We also present new flux-calibrated optical and near-infrared spectroscopy, which we model in terms of a Keplerian accretion disk, and high-dispersion spectra that demonstrate similarity to some categories of “mixed-temperature” accretion-outburst objects. At blue wavelengths, LkHα 225 South has a pure absorption spectrum and is a good spectral match to the FU Ori stars V1515 Cyg and V1057 Cyg. At red optical and infrared wavelengths, however, the spectrum is more similar to Gaia 19ajj, showing emission in TiO, CO, and metals. Sr ii absorption indicates a low-surface-gravity atmosphere. There are also signatures of a strong wind/outflow. LkHα 225 South was moderately bright in the early 1950s as well as in the late 1980s, with evidence for deep fades during intervening epochs. The body of evidence suggests that LkHα 225 South is another case of a source with episodically enhanced accretion that causes brightening by orders of magnitude, and development of a hot absorption spectrum and warm wind. It is similar to Gaia 19ajj, but also reminiscent in its long brightening time and brightness oscillation near peak to the embedded sources L1634 IRS7 and ESO Ha 99.

Magnetic fields and mass accretion processes create dark and bright spots on the surface of young stars. These spots manifest as surface thermal inhomogeneities, which alter the global temperature measured on the stars. To understand the effects and implications of these starspots, we conducted a large iSHELL high-resolution infrared spectroscopic survey of T Tauri stars in Taurus-Auriga and Ophiuchus star-forming regions. From the K band spectra, we measured stellar temperatures and magnetic field strengths using a magnetic radiative transfer code. We compared our infrared-derived parameters against literature optical temperatures and found a) a systematic temperature difference between optical and infrared observations, and b) a positive correlation between the magnetic field strengths and the temperature differences. The discrepant temperature measurements imply significant differences in the inferred stellar masses from stellar evolutionary models. To discern which temperature better predicts the mass of the star, we compared our model-derived masses against dynamical masses measured from ALMA and PdBI for a sub-sample of our sources. From this comparison we conclude that, in the range of stellar masses from 0.3 to 1.3 $\rm M_\odot$, neither infrared nor optical temperatures perfectly reproduce the stellar dynamical masses. But, on average, infrared temperatures produce more precise and accurate stellar masses than optical ones., Accepted for publication in ApJ. The manuscript has 20 pages, 11 Figures (+2 figure sets in the online version of ApJ), and 2 Tables

We present mid infrared imaging of two young clusters, the Coronet in the CrA cloud core and B59 in the Pipe Nebula, using the FORCAST camera on the Stratospheric Observatory for Infrared Astronomy. We also analyze Herschel Space Observatory PACS and SPIRE images of the associated clouds. The two clusters are at similar, and very close, distances. Star formation is ongoing in the Coronet, which hosts at least one Class 0 source and several pre-stellar cores, which may collapse and form stars. The B59 cluster is older, although it still has a few Class I sources, and is less compact. The CrA cloud has a diameter of about 0.16 pc, and we determine a dust temperature of 15.7 K and a star formation efficiency of about 27 %, while the B59 core is approximately twice as large, has a dust temperature of about 11.4 K and a star formation efficiency of about 14 %. We infer that the gas densities are much higher in the Coronet, which has also formed intermediate mass stars, while B59 has only formed low-mass stars., 15 pages, 6 figures, Accepted for publication in The Astrophysical Journal

We present a (previously unpublished) 1.5µm archival HST image of the HH 34 Herbig-Haro jet, in which the northern counterjet is seen at an unprecedented angular resolution of ≈ 0.1′′ (this counterjet had only been imaged previously at lower resolution with Spitzer). The jet/counterjet structure observed in this image shows evidence of low-amplitude, point-symmetric deviations from the outflow axis, indicating the presence of a precession in the ejection direction. We use the ratios between the 1.5 and 4.5 µm intensities of the emitting knots (from the HST image and from a previously published 4.5 µm Spitzer image) to obtain an estimate of the spatial dependence of the optical extinction to the HH 34 jet/counterjet system. We find evidence for extinction from a central, dense core surrounding the outflow source and from a more extended region in the foreground of the HH 34 counterjet.

We present new results on the young star population of the Lynds~1188 molecular cloud, associated with the Cepheus Bubble, a giant interstellar shell around the association Cep~OB\,2. In order to reveal the star-forming scenario of the molecular cloud located on the supershell, and understand the history of star formation in the region, we identified and characterized young star candidates based on an H$\alpha$ emission survey and various published photometric datasets. Using Gaia DR2 astrometry we studied the spatial distribution of the young star candidates and isolated three groups based on their distances. We constructed spectral energy distributions of our target stars, based on Pan-STARRS, 2MASS, {\it Spitzer} and {\it WISE} photometric data, estimating their spectral types, extinctions, and luminosities. We estimated masses by means of pre-main-sequence evolutionary models, and derived accretion rates from the equivalent width of the H$\alpha$ line. We studied the structure of the cloud by constructing a new extinction map, based on Pan-STARRS data. Our results show that the distribution of low-mass young stars in L1188 is well correlated with that of the dust and mole\-cular gas. We identified two small, compact clusters and a loose aggregate of young stars. We found that star formation in L1188 started about 5 million years ago. The apparent age gradient of young stars across the cloud and the ammonia cores located to the east of the optically visible young stellar groups support the scenario of star formation propagating away from the centre of the Cepheus Bubble.

We report photometry and spectroscopy of the outburst of the young stellar object ESO-Halpha 99. The outburst was first noticed in Gaia alert Gaia18dvc and later by ATLAS. We have established the outburst light curve with archival ATLAS ``Orange'' filter photometry, Gaia data, new V-band photometry, and J, H, and K_s photometry from IRIS and UKIRT. The brightness has fluctuated several times near the light curve maximum. The TESS satellite observed ESO-Halpha 99 with high cadence during one of these minor minima and found brightness fluctuations on timescales of days and hours. Imaging with UKIRT shows the outline of an outflow cavity, and we find one knot of H_2 1-0 S(1) emission, now named MHO 1520, on the symmetry axis of this nebula, indicating recent collimated outflow activity from ESO-Halpha 99. Its pre-outburst SED shows a flat FIR spectrum, confirming its early evolutionary state and its similarity to other deeply embedded objects in the broader EXor class. The pre-outburst luminosity is 34 L_sun, a much higher luminosity than typical EXors, indicating that ESO-Halpha 99 may be a star of intermediate mass. Infrared and optical spectroscopy show a rich emission line spectrum, including HI lines, strong red CaII emission, as well as infrared CO bandhead emission, all characteristic EXors in the broadest sense. Comparison of the present spectra with an optical spectrum obtained in 1993, presumably in the quiescent state of the object, shows that during the present outburst the continuum component of the spectrum has increased notably more than the emission lines. The Halpha equivalent width during the outburst is down to one half of its 1993 level, and shock-excited emission lines are much less prominent., Accepted by The Astronomical Journal

We present the results of a J, H, and K photometric variability survey of the central 0.78 square degrees of the young OB association Cygnus OB2. We used data observed with the Wide-Field CAMera at the United Kingdom Infrared Telescope in 2007 (spanning 217 days) to investigate the light curves of 5083 low mass candidate members in the association and explore the occurrence and main characteristics of their near-infrared variability. We identified 2529 stars ($\sim$50$\%$ of the sample) with significant variability with time-scales ranging from days to months. We classified the variable stars into the following three groups according to their light curve morphology: periodic variability (1697 stars), occultation variability (124 stars), and other types of variability (726 stars). We verified that the disk-bearing stars in our sample are significantly more variable in the near-infrared than diskless stars, with a steep increase in the disk-fraction among stars with higher variability amplitude. We investigated the trajectories described by variable stars in the color-space and measured slopes for 335 stars describing linear trajectories. Based on the trajectories in the color-space, we inferred that the sample analyzed is composed of a mix of young stars presenting variability due to hot and cold spots, extinction by circumstellar material, and changes in the disk emission in the near-infrared. We contemplated using the use of near-infrared variability to identify disk-bearing stars and verified that 53.4$\%$ of the known disk-bearing stars in our sample could have been identified as such based solely on their variability. We present 18 newly identified disk-bearing stars and 14 eclipsing binary candidates among CygOB2 lower-mass members., 19 pages, 21 Figures

We present an analysis of four epochs of H$\alpha$ and [S II] $\lambda\lambda$ 6716/6731 HST images of HH 1. For determining proper motions we explore a new method based on analysis of spatially degraded images obtained convolving the images with wavelet functions of chosen widths. With this procedure we are able to generate maps of proper motion velocities along and across the outflow axis, as well as (angularly integrated) proper motion velocity distributions. From the four available epochs, we find the time evolution of the velocities, intensities and spatial distribution of the line emission. We find that over the last two decades HH 1 shows a clear acceleration. Also, the H$\alpha$ and [S II] intensities have first dropped, and then recovered in the more recent (2014) images. Finally, we show a comparison between the two available HST epochs of [O III] $\lambda$ 5007 (1994 and 2014), in which we see a clear drop in the value of the [O III]/H$\alpha$ ratio., Comment: 28 pages, 11 figures, accepted for publication in The Astronomical Journal

A population of 11 faint, collimated jets has been discovered in the northern part of the L1641 cloud in the region of HH 1/2, HH 34, and the L1641-N cluster. These jets were missed in previous imaging surveys on account of their weak emission, and they were discovered only on deep exposures with the Subaru 8 m telescope. With these new faint jets, the number of HH flows within the area surveyed has doubled. This suggests that collimated jets from young stars may be more common than previously assumed. It is noteworthy that all of the jets are associated with optically visible stars with r magnitudes ranging from 13.8 to 22.0. The driving sources of jets in regions flooded by ultraviolet radiation from nearby OB stars are known to be excavated by photoionization, and in three cases remnant Hα emission envelopes are found associated with the sources, although the more benign environment in the region observed here, about 10 pc distant from the Orion Nebula Cluster, makes the optical visibility of all these sources rather surprising. Such faint jets from visible stars represent either the final vestiges of the outflow phenomenon, or they are triggered by disturbances of the remnant disks, possibly initiated by the orbital evolution of binaries that spiral in to form close binaries. Among the known Hα emission stars within the region surveyed, 8% are found to be associated with jets.

In early 2008, the young low-mass star EX Lupi, the prototype of the EXor class of eruptive variables, optically brightened by over five magnitudes for a period of 7 months. The previous time a change of such amplitude had been observed in EX Lup was over 50 years ago. In this Letter we present new optical and near-IR high resolution spectroscopy of EX~Lup during the 2008 outburst. We investigate the physical characteristics of the outburst both soon after it began and some four months later, and consider the energetics and kinematics observed. Emission line strengths, widths, and profiles changed significantly between the two observations. Also, modeling of the 2.2935 um CO overtone bandhead emission suggests that an inner gap in the circumstellar gas disk around the star may be present and it is from the inner edge of the gas disk that the CO overtone emission probably arises. We derive a mass accretion luminosity and rate during the extreme outburst of ~2+-0.5~Lsun and ~2+-0.5x10^-7 Msun yr^-1, respectively, which suggests that this outburst was indeed one of the strongest witnessed in EX Lup, yet not as strong as those observed in FU Orionis stars., Comment: 5 pages, 4 figures

Using the United Kingdom Infrared Telescope on Mauna Kea, we have carried out a new near-infrared J, H, K monitoring survey of almost a square degree of the star-forming Orion Nebula Cluster with observations on 120 nights over three observing seasons, spanning a total of 894 days. We monitored ~15,000 stars down to J=20 using the WFCAM instrument, and have extracted 1203 significantly variable stars from our data. By studying variability in young stellar objects (YSOs) in the H-K, K color-magnitude diagram, we are able to distinguish between physical mechanisms of variability. Many variables show color behavior indicating either dust-extinction or disk/accretion activity, but we find that when monitored for longer periods of time, a number of stars shift between these two variability mechanisms. Further, we show that the intrinsic timescale of disk/accretion variability in young stars is longer than that of dust-extinction variability. We confirm that variability amplitude is statistically correlated with evolutionary class in all bands and colors. Our investigations of these 1203 variables have revealed 73 periodic AA Tau type variables, many large-amplitude and long-period (P > 15 day) YSOs, including three stars showing widely-spaced periodic brightening events consistent with circumbinary disk activity, and four new eclipsing binaries. These phenomena and others indicate the activity of long-term disk/accretion variability processes taking place in young stars. We have made the light curves and associated data for these 1203 variables available online., Corrected typo in author name

We report the discovery of a small bipolar Herbig-Haro jet, HH 872, powered by the low-luminosity source ESO-H α 574 in Chamaeleon I. The end-to-end projected size of the jet in [SII] images is only 3150 AU. Infrared images marginally resolve the central source showing a structure elongated perpendicularly to the jet axis, possibly a disk seen at a large angle with respect to the plane of the sky. The brightest feature of the jet, peaking at a distance of $1''6$ from the central source, is likely to result from enhanced outflow activity in the last decade.

Commission 34 covers diffuse matter in space on scales ranging from the circumstellar to the galactic and intergalactic. As such it has enormous scope and because of this, it alone forms Division VI. Key aspects include star formation, matter around evolved stars, astrochemistry, nebulae, galactic and intergalactic clouds and the multitude of effects of the interaction of stars with their surroundings. Associated with these areas are a huge range of physical and chemical processes including hydrodynamics and magnetohydrodynamics, radiative processes, molecular physics and chemistry, plasma processes and others too numerous to name. These are complemented by an equally huge range of observational studies using practically all space and ground-based instrumentation at nearly all observable wavelengths. A glance at any data-base of publications over the past few years attests to the vigorous state of these studies. The current membership of the Division is around 800. It also has three separate working groups.

We present a deep survey of H? emission-line stars in the NGC 2264 region using wide-field objective prism films. We find 357 H? emission-line stars, of which 113 were previously detected, within an area of 3? ? 3? centered on the Cone Nebula, with a majority of stars being concentrated in a dense cluster at the center of the region. We present a large-scale CO map of NGC 2264 and find a strong correlation between the cluster of H? emission-line stars and the most massive core in the cloud complex. A more extended halo of stars poorly correlated with the gas may represent stars that have drifted away from their birth place, suggesting that star formation has taken place in NGC 2264 for several million years.

We present new Chandra X-Ray Observatory observations of the L1551 molecular cloud in Taurus. We find a compact, but slightly resolved, X-ray source, displaced westward from the IRS 5 binary protostar by about 50-100 AU, which coincides with the base of the HH 154 protostellar jet. The column density of material lying in front of the X-ray source is more than an order of magnitude lower than the column density toward L1551 IRS 5. Thus, it is highly improbable that the X-rays come directly from the embedded protobinary. It is possible, however, that X-rays produced by one or both members of the IRS 5 binary escape through the outflow cavity and are scattered into the line of sight by a dense infalling envelope or material in the outflow. Constraints on the physical properties of the scattering medium are discussed. It is also possible that the X-rays are produced in situ by fast shocks at the base of the HH 154 jet. We consider several possible geometries for such shocks. The radiating plasma may be located behind standing shocks, where a wide-angle wind from one member of the IRS 5 binary is collimated into a jet, or behind shocks formed on the axis of the outflow, where winds from each member of the binary collide or where these winds impact a tilted circumbinary disk. To produce the observed X-ray luminosity and plasma temperature, shock velocities larger than 350 km s-1 are required, with a preshock hydrogen density of (1–10) × 103 cm-3. The implied postshock cooling length is around 800 AU, close to the observed length of the bright near-infrared [Fe II] emission in the inner portion of the HH 154 jet.

We discuss a scenario in which brown dwarfs are formed like stars, except that their full collapse phases are interrupted through dynamical interactions in small multiple systems, leading to the ejection of the lightest member. This disintegration is a stochastic process, often resulting in the expulsion of newborn low mass stars, but when it occurs early enough the ejected stellar embryo will be a substellar object. This process may be so common at early ages that a large fraction of the ubiquitous brown dwarfs could have formed in this manner. Detailed gas dynamical simulations are required in order to better understand the details of the decay of small newborn multiple systems. We discuss the observational consequences of the ejection hypothesis, noting especially the importance of binaries with brown dwarf components as an observational test. Finally, we note that brown dwarfs that have recently been ejected may be so disturbed, by infall from the collapsing core and also by heavy accretion from perturbed circumstellar disks, that traditional spectral and luminosity criteria may fail to identify their substellar nature.

We present a long-slit spectrum of the Herbig-Haro object HH 111, obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. This spectrum has a spectral resolution of ≈50 km s-1, so it gives a good picture of the kinematical properties of the observed object at very high angular resolution. We find that some of the knots along the jet are associated with sudden drops in the radial velocity (modulus), confirming that the emission from the knots is formed in shocks. We interpret the observed position-velocity diagrams in terms of a model of a jet from a variable source, and we attempt to carry out a reconstruction of the ejection velocity variability necessary for reproducing the observed kinematical structure.

We report the discovery of a dozen Herbig-Haro jets illuminated by the Lyman continuum and/or softer far-ultraviolet radiation fields of nearby high mass stars. Five irradiated outflows lie in the outer parts of the Orion Nebula and seven lie near the reflection nebula NGC 1333 in the Perseus molecular cloud. We propose that the UV radiation field has eroded residual material left over from the formation of these young source stars. Many of the irradiated jets exhibit unusual C-shaped bending. In the outskirts of the Orion Nebula, most irradiated jets bend away from the core of the nebula. On the other hand, in NGC 1333, the C-shaped jets bend back towards the cluster center. Jet bending in the Orion Nebula may be dominated by either the outflow of material from the nebular core or by the rocket effect pushing on the irradiated portion of a mostly neutral jet beam. But in NGC 1333, jet bending may indicate that the source stars have been ejected from the cluster core. Many irradiated jets are asymmetric with one beam much brighter than the other. When fully photo-ionized, irradiated jets may provide unique insights into the physical conditions within outflows powered by young stars, permitting the determination of the density and location of stellar ejecta even in the absence of shocks. We present a model for the photo-ionization of these outflows by external radiation fields and discuss possible mechanisms for producing the observed asymmetries. We demonstrate that the UV radiation field may alter the amount of cloud material entrained by the jet. We also report the discovery of some large scale bow shocks which face the core of the Orion Nebula and which surround visible young stars. These wind-wind collision fronts provide further evidence for a large scale mass flow originating near the nebular core., Comment: 36 pages, 15 figures

Analysis of a sample of giant Herbig-Haro flow sources shows that 79%–86% are binaries or higher order multiples. This represents the youngest sample of stars studied so far for binarity. A stellar dynamics jet hypothesis is proposed in which the dynamical decay of triple or multiple systems leads to giant outflow activity. Close triple approaches will cause serious perturbations and probably direct collisions among individual circumstellar disks, with a consequent burst of outflow activity, which can produce giant HH bow shocks. As one component is ejected, the two remaining stars and their small truncated disks form a closer bound pair with high eccentricity. Gas streams from a circumbinary disk feed the stars and this as well as other dynamical effects cause the binary orbit to shrink. As the stellar components gradually spiral towards each other, accretion and outflow becomes cyclic, modulated on an orbital time scale. The resulting HH flows can be read as a fossil record of the evolution of orbital motions of the newly formed binary as it shrinks from a typical separation of 100 AU or more to 10 AU or less. After a triple disintegration event, both components (star and close binary) leave their nascent envelope, and while one component becomes visible as a T Tauri star, the other will be obscured for a while by the envelope and will appear as a bright near-infrared object, thus explaining the socalled IRC binaries which are infrequently found in star forming regions.

An analysis of the multiplicity of 14 sources driving giant Herbig-Haro flows has revealed an observed binary frequency between 79% and 86%, of which half are higher order multiples. These sources represent the hitherto youngest sample of stars examined for binarity. I postulate that the dynamical decay of triple or multiple systems leads to strong outflow activity. It is well known that a large fraction of nonhierarchical triple systems rapidly break up and eject the lightest member. At the same time a closer binary in a highly eccentric orbit is formed. Massive disk truncation results, accompanied by large-scale accretion, with a consequent burst of outflow activity, which produces the observed giant HH bow shocks. Some of the material culled from the individual circumstellar disks may settle into a circumbinary disk around the newly bound stellar pair. The small remaining and truncated circumstellar disks are fed from the circumbinary disk through gas streams, and this as well as other dynamical effects cause the binary orbit to shrink. Gas streams together with disk interactions at periastron drive cyclic accretion modulated on an orbital timescale. As the stellar components gradually spiral toward each other, the increasingly frequent mass-loss events form chains of HH objects until eventually the binary has a semimajor axis of only 9–12 AU, at which point the closely spaced shocked ejecta appear as a finely collimated jet. Thus, such HH flows can be read as a fossil record of the evolution of orbital motions of a binary, newly formed in a triple disintegration event, as it shrinks from a typical separation of 100 AU or more to 10 AU or less. When the triple system disintegrates and a single star is ejected, the newly formed binary recoils, and as a result both components (star and close binary) leave their nascent envelope. While one component becomes visible as a T Tauri star, the other will be obscured for a while by the envelope and will appear as a bright near-infrared object. For typical parameters, this geometry persists for only 5000 yr or so. If the ejected star does not escape, cyclic motion of a hierarchical triple begins. This explains the so-called IRC binaries that are infrequently found in star-forming regions. The standard model of early stellar evolution states that young stars gradually and smoothly make the transitions from Class 0 through Class I and II objects to eventually become Class III objects. In contrast, stars born in multiple systems can abruptly transit from a Class 0 or I object to a visible T Tauri star. The main accretion phase may be terminated by the stochastic process of triple decay. Depending on the moment of triple disintegration, the ejected objects can range from stellar embryos, which will emerge as very low mass stars or even brown dwarfs, to essentially fully built-up stars. In this picture, the initial mass function toward its low-mass end has an important stochastic component that can only be described by the half-life of the decay processes. Because the ejected stars can take only limited circumstellar material with them, they will soon lose their classical T Tauri characteristics and join the halo of weak-line T Tauri stars that surround star-forming clouds. Differences in ejection may explain why two apparently similar T Tauri stars of about the same age can have major differences in the size of their circumstellar disks.

We have observed seven regions surrounding the driving sources of Herbig-Haro flows using the NICMOS infrared camera on board the Hubble Space Telescope. These Herbig-Haro energy sources, which power the HH 34, 47, 83, 111, 199, 300, and 454 flows, have all been previously detected in the centimeter-wavelength radio continuum. The regions were imaged in two broadband filters, F160W and F205W, which, in addition to stellar continuum, principally transmit the [Fe II] 1.644 μm and H2 2.122 μm lines, respectively, of shocks present in the images. Highly collimated infrared jets emerge from the sources of HH 34, 111, 300, and 454. In all of these cases, the [Fe II] emission transmitted in the F160W filter is much stronger than the H2 emission in the F205W band. Combined with previously published NICMOS images of the bright [Fe II] HH 1 jet, these data suggest that [Fe II] emission may be as important a tracer of shocked jets in the infrared as [S II] emission is in the optical. [Fe II] emission may be enhanced near the driving sources relative to the H2 emission by a combination of high jet density and strong far-ultraviolet (7.6–13.6 eV) radiation from the forming star. The HH 454 [Fe II] jet, which emanates from the L1551 NE source, is pointed directly at the bright shock HH 29, providing further evidence that this young star, rather than L1551 IRS 5, is the source of the brightest Herbig-Haro object in the L1551 outflow complex. In five regions, the driving sources are visible at 2 μm. Among these, the sources powering HH 47 and HH 300 are found to be binary stars. One other young stellar system, the source of the HH 111 protostellar jet, is triple. When combined with other studies, these results indicate that 36% of 14 Herbig-Haro energy sources observed with NICMOS are multiple at near-infrared wavelengths with component separations greater than 015–020. Therefore, considering incompleteness, obscuration, and faintness of possible companions in the observed bands, the binary frequency is likely to be considerably higher among the selected systems than among low-mass main-sequence dwarf stars. There is no obvious connection between source multiplicity and jet morphology observed on large scales. For example, the beam of the HH 47 jet exhibits quasi-periodic wiggles that might be one indication of jet modulation at the binary orbital period. However, the total mass of the observed central binary would have to be unrealistically large to make the orbital period comparable to the ejection time interval of these jet features. Finally, the properties of the near-infrared reflection nebulae associated with the observed sources are investigated. The shapes of the cavity walls traced by the observed reflection nebulae indicate the presence of large-scale and highly flattened circumstellar structures that are opaque at near-infrared wavelengths. Their flaring shapes either are the result of preferential infall of the protostellar envelope along the outflow axis or were carved by wide-angle winds emerging from the inner disk and the forming star. The inclination angles of several circumstellar structures are estimated.

We present new Hubble Space Telescope (HST ) NICMOS images of the source region of the HH 1/2 —ow. The HH 1 jet is traced in the (Fe II) 1.64 km and the 2.12 km lines to about of the deeply H 2 2A.5 embedded VLA source. In general, the structure of the jet is similar in and (Fe II), with all the H 2 features having comparable brightness in both species. However, there is a gradient in the II) H 2 /(Fe ratio which increases with distance from the source. We also compare our infrared images with almost contemporary HST WFPC2 images in Ha and (S II). The ratio of (Fe II )t o (SII), both low-excitation tracers of weak shocks, is almost constant along the visible part of the jet but then increases by an order of magnitude where the optical jet disappears. If this is due to reddening, then the extinction, A V , increases by at least 4 mag in a space of corresponding to 420 AU. Beyond this obscuring ridge, we 0A.9, detect —ve more infrared knots. Less than 2A now separate the infrared jet and the tip of the compact 3.6 cm radio continuum jet centered on the VLA source. The heavy extinction which obscures the base of the IR jet is probably due to the compact cloud core recently detected in HCO'. The jet is observed to steadily increase in width, and we argue that this is either due to sideways ejection of shocked material from internal working surfaces or a sonic expansion of the hot jet beam. Surprisingly, the width of the jet is the same to within measurement errors in and (Fe II), contrary to the expectations of some H 2 models. Two small, previously detected oU-axis knots appear to form an independent HH —ow, which we call HH 501. An infrared cometary nebula (dubbed the X nebula) is found next to the HH 1 jet, along a line through the HH 501 knots, possibly supporting earlier speculations that yet one more source exists in the region. However, new proper motions of the HH 501 —ow suggest an origin of these knots near VLA 1, and it is therefore possible that the VLA 1 source itself may be a close binary, thus forming a triple system with the more distant VLA 2. Subject headings: infrared: ISM: lines and bandsISM: jets and out—owsstars: formation ¨ stars: mass loss

Our general understanding of multiple star and planet formation is primarily based on observations of young multiple systems in low density regions like Tau-Aur and Oph. Since many, if not most, of the stars are born in clusters, observational constraints from young binaries in those environments are fundamental for understanding both the formation of multiple systems and planets in multiple systems throughout the Galaxy. We build upon the largest survey for young binaries in the Orion Nebula Cluster (ONC) which is based on Hubble Space Telescope observations to derive both stellar and circumstellar properties of newborn binary systems in this cluster environment. We present Adaptive Optics spatially-resolved JHKL'-band photometry and K-band R$\sim$\,5000 spectra for a sample of 8 ONC binary systems from this database. We characterize the stellar properties of binary components and obtain a census of protoplanetary disks through K-L' color excess. For a combined sample of ONC binaries including 7 additional systems with NIR spectroscopy from the literature, we derive mass ratio and relative age distributions. We compare the stellar and circumstellar properties of binaries in ONC with those in Tau-Aur and Oph from samples of binaries with stellar properties derived for each component from spectra and/or visual photometry and with a disk census obtained through K-L color excess. The mass ratio distribution of ONC binaries is found to be indistinguishable from that of Tau-Aur and, to some extent, to that of Oph in the separation range 85-560\,AU and for primary mass in the range 0.15 to 0.8\,M$_{\sun}$.A trend toward a lower mass ratio with larger separation is suggested in ONC binaries which is not seen in Tau-Aur binaries.The components of ONC binaries are found to be significantly more coeval than the overall ONC population and as coeval as components of binaries in Tau-Aur and Oph[...], Comment: Accepted for publication in Astronomy & Astrophysics

The young stellar cluster illuminating the W40 H II region, one of the nearest massive star forming regions, has been observed with the ACIS detector on board the Chandra X-ray Observatory. Due to its high obscuration, this is a poorly-studied stellar cluster with only a handful of bright stars visible in the optical band, including three OB stars identified as primary excitation sources. We detect 225 X-ray sources, of which 85% are confidently identified as young stellar members of the region. Two potential distances of the cluster, 260 pc and 600 pc, are used in the paper. Supposing the X-ray luminosity function to be universal, it supports a 600 pc distance as a lower limit for W40 and a total population of at least 600 stars down to 0.1 Mo under the assumption of a coeval population with a uniform obscuration. In fact, there is strong spatial variation in Ks-band-excess disk fraction and non-uniform obscuration due to a dust lane that is identified in absorption in optical, infrared and X-ray. The dust lane is likely part of a ring of material which includes the molecular core within W40. In contrast to the likely ongoing star formation in the dust lane, the molecular core is inactive. The star cluster has a spherical morphology, an isothermal sphere density profile, and mass segregation down to 1.5 Mo. However, other cluster properties, including a \leq{1} Myr age estimate and ongoing star formation, indicate that the cluster is not dynamically relaxed. X-ray diffuse emission and a powerful flare from a young stellar object are also reported., Comment: Accepted for publication in The Astrophysical Journal. 60 pages, 16 figures

Two groups of Herbig-Haro objects, designated HH 124 and HH 125, have been discovered in the NGC 2264 complex. Direct plates, narrow-band CCD images, low-dispersion spectra and high-resolution line profiles of the sources have been obtained. HH 124, which lies to the north of NGC 2 264, has a typical linear structure with HH knots on both sides of an IRAS source which is situated close to a central emission knot. The knots of HH 124 show spectra typical of low-velocity shocks and the knots on opposite sides of the central one show blue- or redshifted emission, characteristic of a bipolar outflow tilted to the line of sight

We present results from an optical imaging and spectroscopic monitoring campaign on the young, low-mass eruptive variable star V1647 Orionis. The star and associated nebulosity (McNeil's Nebula) were observed over the period February 2004 to February 2006 with observations commencing a few months after the original outburst event occurred. Using the Gemini North telescope, we obtained multi-band optical imaging photometry and medium-resolution long-slit spectroscopy of V1647 Ori on an approximately monthly interval. During this period, V1647 Ori remained at, or close to, peak brightness and then faded by 5 magnitudes to close to its pre-outburst brightness. This implies an outburst timescale of around 27 months. Spectral features seen in both emission and absorption varied considerably during the monitoring period. For example, the Halpha line changed significantly in both intensity and profile. We present and discuss the observed photometric and spectroscopic changes and consider how this eruptive event relates to the early formative stages of low-mass stars., Comment: 59 pages, 22 figures

We present sensitive, high angular resolution ($0\rlap.{''}05$) VLA continuum observations made at 7 mm of the core of the HH 111/121 quadrupolar outflow. We estimate that at this wavelength the continuum emission is dominated by dust, although a significant free-free contribution ($\sim$30%) is still present. The observed structure is formed by two overlapping, elongated sources approximately perpendicular to each other as viewed from Earth. We interpret this structure as either tracing two circumstellar disks that exist around each of the protostars of the close binary source at the core of this quadrupolar outflow or a disk and a jet perpendicular to it. Both interpretations have advantages and disadvantages, and future high angular resolution spectroscopic millimeter observations are required to favor one of them in a more conclusive way., Comment: 12 pages, 3 figures

We present high spatial resolution optical integral field spectroscopy of a collimated Herbig-Haro jet viewed nearly edge-on. Maps of the line emission, velocity centroid, and velocity dispersion were generated for the H$\alpha$ and [S II] emission features from the inner collimated jet and exciting source region of the HH 34 outflow. The kinematic structure of the jet shows several maxima and minima in both velocity centroid value and velocity dispersion along the jet axis. Perpendicular to the flow direction the velocity decreases outward from the axis to the limb of the jet, but the velocity dispersion increases. Maps of the electron density structure were derived from the line ratio of [S II] 6731/6716 emission. We have found that the jet exhibits a pronounced ``striped'' pattern in electron density; the high $n_e$ regions are at the leading side of each of the emission knots in the collimated jet, and low $n_e$ regions in the down-flow direction. On average, the measured electron density decreases outward from the inner regions of the jet, but the highest $n_e$ found in the outflow is spatially offset from the nominal position of the exciting star. The results of our high spatial resolution optical integral field spectroscopy show very good agreement with the kinematics and electron density structure predicted by the existing internal working surface models of the HH~34 outflow., Comment: Accepted for publication in the Astronomical Journal

We present single-dish and interferometric millimeter line observations of the HH 111 outflow and its driving source. The physical conditions of the core have been determined from the emission of the millimeter line of CO and its isotopomers and CS with the IRAM 30 m telescope, and the CO J=7-->6 line with the Caltech Submillimeter Observatory. The emission reveals a small condensation of cold (T=20-25 K) and dense gas [n(H2)=3x10^5 cm^-3]. The outflow has been mapped with the IRAM Plateau de Bure Interferometer (PdBI). The cold gas is distributed in a hollow cylinder surrounding the optical jet. The formation of this cavity and its kinematics are well accounted for in the frame of outflow gas entrainment by jet bow shocks. Evidence of gas acceleration is found along the cavity walls, correlated with the presence of optical bow shocks. The separation of the inner walls reaches 8"-10", which matches the transverse size of the wings in the bow shock. CSO observations of the J=7-->6 line show evidence of a high-velocity and hot gas component (T=300-1000 K) with a low filling factor. This emission probably arises from shocked gas in the jet. Observations of the 3P2-3P1 [CI] line are consistent with C-type nondissociative shocks. Mapping of the high-velocity molecular bullets B1-B3, located beyond the optical jet, reveals small structures of 3" x 7" flattened perpendicular to the flow direction. They are made of cold (T ~ 30K), moderate density gas [n(H2) = (0.5-1.0)x10^4 cm^-3], expanding into the low-density surrounding medium. Their properties are consistent with their being shocked gas knots resulting from past time-variable ejections in the jet.

We present images of the young stellar object ASR 41 in the NGC 1333 star forming region at the wavelengths of H_alpha and [SII] and in the I, J, H, and K-bands. ASR 41 has the near-infrared morphology of an edge-on disk object, but appears an order of magnitude larger than typical systems of this kind. We also present detailed models of the scattering and radiative transfer in systems consisting of a young star surrounded by a proto-planetary disk, and the whole system being embedded in either an infalling envelope or a uniform molecular cloud. The best fit to the observed morphology can be achieved with a disk of approx. 200 AU diameter, immersed in a low density cloud. The low cloud density is necessary to stay below the sub-mm flux upper limits and to preserve the shadow cast by the disk via single scattering. The results demonstrate that ASR 41 is probably not inherently different from typical edge-on disk objects, and that its large apparent size is due to the shadow of a much smaller disk being projected into the surrounding dusty molecular material.

Numerous brown dwarfs have been found in recent years, re-igniting debate about their origin. A natural way to stunt the growth of a nascent stellar embryo is to eject it out of the infalling gas envelope. This can be achieved by dynamical interactions in newborn multiple systems or small-N clusters of stellar embryos. In this scenario brown dwarfs form like stars, but are sent into distant orbits or completely ejected, thus depriving them of sufficient growth to ever exceed the hydrogen burning limit of 0.08 M⊙. Decay of a multiple system is a stochastic process and, with better luck, brown dwarfs could indeed have become normal stars.

We conjecture that brown dwarfs are substellar objects because they have been ejected from small newborn multiple systems which have decayed in dynamical interactions. In this view, brown dwarfs are stellar embryos for which the star formation process was aborted before the hydrostatic cores could build up enough mass to eventually start hydrogen burning. The disintegration of a small multiple system is a stochastic process, which can be described only in terms of the half-life of the decay. A stellar embryo competes with its siblings in order to accrete infalling matter, and the one that grows slowest is most likely to be ejected. With better luck, a brown dwarf would therefore have become a normal star. This interpretation of brown dwarfs readily explains the rarity of brown dwarfs as companions to normal stars (aka the ``brown dwarf desert''), the absence of wide brown dwarf binaries, and the flattening of the low mass end of the initial mass function. Possible observational tests of this scenario include statistics of brown dwarfs near Class 0 sources, and the kinematics of brown dwarfs in star forming regions while they still retain a kinematic signature of their expulsion. Because the ejection process limits the amount of gas brought along in a disk, it is predicted that substellar equivalents to the classical T Tauri stars should be very rare., Comment: 8 pages, 1 figure, Accepted by the Astronomical Journal

We present high angular resolution observations of the molecular outflow associated with the optical jet and HH objects of the HH111 system. Interferometric observations in the CO J=2-1 and J=1-0 lines of the high velocity bullets associated with HH111 are presented for the first time. The molecular gas in these high velocity clumps has a moderate kinetic temperature and a mass of a few 10-4 M⊙ per bullet. We favor the view that HH jets and CO bullets, which represent different manifestations of the same physical phenomena, are driving the low-velocity molecular outflow.

We have carried out high spatial resolution, 12'', CO J = 2-1 observations at the IRAM 30 m telescope of the molecular outflow in the HH 111 jet complex. The Herbig-Haro jet is found to coincide with a highly collimated CO flow, with two distinct velocities, possibly providing kinematic evidence that the CO flow surrounds the HH jet. A second well-defined bipolar molecular flow, at large angles to the principal flow axis, coincides with the HH 121 infrared flow that emanates from the (presumably binary) VLA driving source; the region thus harbors one of the rare quadrupolar molecular flows. Extremely high velocity CO is found toward the principal HH working surface at the same velocity as the optically emitting gas, whereas this emission is weak toward the Herbig-Haro jet. Since the inclination of the HH jet is known from optical observations to be 10° to the plane of the sky, we conclude that there is CO in the flow with space velocities of up to 500 km s-1! Further out, and precisely along the flow axis, we have discovered three equidistant CO bullets with space velocities of about 240 km s-1, which are not detected in the optical. We interpret these bullets as the result of earlier eruptions from the energy source that are now moving through an ambient medium so tenuous that no observable shock interaction takes place. Finally, we discuss the physical relation between the Herbig-Haro jet, the CO bullets, and the low-velocity molecular outflow. We favor the view that HH jets and CO bullets, which represent different manifestations of the same physical phenomena, are driving the low-velocity molecular outflow.

We present an overview of what can be done in the field of star formation using the VLT and its advanced instrumentation, and especially with the very high angular resolution provided by the VLT in the VLTI mode. Herbig-Haro objects and early stellar evolution, pre-main sequence binaries, circumstellar disks and surface properties of young stars are investigated. We show that the VLT and VISA will be not only very useful to better understand star formation processes, but indeed necessary in order to advance in certain subjects.

We present a study of the HH 110/270 system based on three sets of optical images obtained with the ESO New Technology Telescope, the Subaru Telescope, and the Hubble Space Telescope (HST). The ground-based observations are made in the Hα and [S II] emission lines and the HST observations are made in the Hα line only. Ground-based observations reveal the existence of nine knots, which have not been previously discussed and offer some important insight into the HH 110/270 history. We perform a kinematic study of the HH 110/270 system and an analysis of its emission properties. We measure proper motions of all the knots in the system. Four of the newly identified knots belong to the HH 270 jet. Their positions indicate that the jet's axis changed its direction in the past. We speculate that similar changes may have occurred many times in the past and this could be part of the reason for the unusual structure of the HH 110 jet. The HST observations allow us to resolve individual knots into their substructures and to measure their proper motions. These measurements show that the knots are highly turbulent structures. Finally, we report the discovery of four new Herbig-Haro (HH) objects located near the HH 110/270 system.

Stars are born in dark clouds of molecular gas, which remain shrouded in mystery. Astronomers have found a new way to peer inside these star factories.

Division VI gathers astronomers studying the diffuse matter in space between stars, ranging from primordial intergalactic clouds, via dust and neutral and ionized gas in galaxies, to the densest molecular clouds and the processes by which stars are formed.

Narrow-band CCD images of the HH 111 optical jet complex obtained at the ESO 3.5 m New Technology Telescope under conditions of excellent seeing are presented. These new images reveal the existence of a faint counterjet in the redshifted lobe of this bipolar outflow. The knot at the tip of the main jet is also identified as a further bow shock, in addition to the three already known in this system. By performing a partial deconvolution of the seeing profile, we have been able to resolve the knots in the trunk of the jet, which are found to have widths of about 0.8″-1.1″, and separation-to-width rations of 2-4. The first bright knot at the base of the jet is split in two, perpendicular to the jet. It is suggested that this structure is a ring of enhanced emission, viewed edge-on, such as might result from the passage of a puff of enhanced density through an oblique shock in the jet

Low- and intermediate-mass stars lose significant fractions of their mass during the asymptotic-giant-branch evolution, creating massive circumstellar shells. The deeply enshrouded OH infrared stars are presumably at the end of asymptotic-giant-branch evolution. I report here the discovery of shocked bipolar bubbles expanding supersonically at right angles to a dense dust disk around the red-giant OH infrared star OH231.8+4.2. The relative radial velocity between the northern and southern bubble fronts is over 200 km s-1, and with a total physical extent of 0.42 pc and a known inclination, this gives a dynamical age of about 1,500 yr. Herbig–Haro objects are observed at the front of the bubbles, and are probably formed as instabilities in the bow shocks, where two collimated flows ram into the ambient medium. This new phenomenon represents a brief phase of late stellar evolution, immediately preceding the formation of a planetary nebula.

HERBIG—H ARO jets are shocked beams of gas escaping supersonically from new-born stars. Here I report the discovery of an exceptionally fine jet, HH111, emerging from a young star still embedded in the Orion molecular clouds. The jet system displays four bow-shock-like features, moving in pairs in opposite directions from a 25-L⊙ infrared source. With an extent of more than 0.8 parsec, HH111 is the largest bipolar Herbig—Haro flow known. It is argued that such jets are highly transient phenomena that are related to eruptive accretion events in the build-up of the central star.

The early phases of stellar evolution are frequently characterized by vigorous interaction between the nascent star and its environment, resulting in the formation of molecular outflows, highly collimated jets, Herbig-Haro objects and small emission or reflection nebulae. A deep survey of such small nebulae around young stars in the Orion molecular clouds has recently been completed1. We report here the sudden appearance of a highly variable, conical nebula (Object 50) 1.5 arc min south of a 250L⊙ infrared source in the southern part of the L1641 cloud in Orion. The nebula coincides with the edge of the approaching lobe of a 105-yr-old bipolar molecular outflow. Variability of the infrared source generates light pulses observed to propagate through the surrounding ambient cloud, producing a fluctuating illumination pattern. This emission may represent the first optical radiation to have emerged from this newborn star.