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The Infrared Imaging System (IRIS) is a 0.8m telescope and a 1024×1024 pixels camera (IRISCAM) with a HAWAII-1 detector array. IRIS is located at the Cerro Armazones Observatory in Chile that is operated by the Ruhr University Bochum jointly with the Universidad Catolica del Norte in Antofagasta. It will be used primarily to survey star-forming regions for variability. Our goal is to discover young stellar objects undergoing accretion instabilities or rotational modulation of star spots, eclipsing binaries, and variable reflection nebulae. The telescope and the infrared camera are completed and first light was achieved in May of 2010. IRIS is currently being tested and characterized, before the longterm monitoring project will commence.

We explore the origin of a population of distant companions (~1000 - 5000 AU) to Class I protostellar sources recently found by Connelley and co-workers, who noted that the companion fraction diminished as the sources evolved. Here we present N-body simulations of unstable triple systems embedded in dense cloud cores. Many companions are ejected into unbound orbits and quickly escape, but others are ejected with insufficient momentum to climb out of the potential well of the cloud core and associated binary. These loosely bound companions reach distances of many thousands of AU before falling back and eventually being ejected into escapes as the cloud cores gradually disappear. We use the term orphans to denote protostellar objects that are dynamically ejected from their placental cloud cores, either escaping or for a time being tenuously bound at large separations. Half of all triple systems are found to disintegrate during the protostellar stage, so if multiple systems are a frequent outcome of the collapse of a cloud core, then orphans should be common. Bound orphans are associated with embedded close protostellar binaries, but escaping orphans can travel as far as ~0.2 pc during the protostellar phase. The steep climb out of a potential well ensures that orphans are not kinematically distinct from young stars born with a less violent pre-history. The identification of orphans outside their heavily extincted cloud cores will allow the detailed study of protostars high up on their Hayashi tracks at near-infrared and in some cases even at optical wavelengths., Comment: 16 pages, 5 figures

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 have observed a complete sample of southern Hα emission line pre-main sequence (PMS) stars associated with the nearby dark clouds Chamaeleon and Ophiuchus in an effort to detect binary companions. We have used the high resolution CCD camera SUSI (0.13″ /pixel) at the ESO 3.5m New Technology Telescope (NTT). The observations were carried out in the Gunn z filter (0.95μ). As a result, we have discovered 28 companions out of 160 objects surveyed in the range of angular separation 0″.5 – 10″. We present histograms of semi-major axes and of brightness ratios of these PMS binaries. Taking small incompleteness corrections into account, the frequency of PMS binaries with semi-major axes in the range 100-1500 AU is 20%, while it is only 15% in same range for solar-type Main Sequence (MS) stars. Extrapolation yields a total PMS binary frequency of 70%. Evolutionary and environmental implications are briefly discussed.

We present Hubble Space Telescope Hα and [S II] images of HH 29. The proximity of HH 29 (140 pc) and the high resolution of the Planetary Camera has resulted in the most detailed images obtained so far of any Herbig-Haro object. The most prominent feature is a linear Hα ridge leading the working surface of a bow shock with a chaotic trailing [S II] bright region. The high-excitation ridge is perpendicular to a line extending toward the class 0 protostar L1551-NE, supporting its recent identification as the driving source. Previous studies have identified several low-velocity features within the working surface. Our images reveal them to be miniature bow shocks facing upstream. Evidently a cluster of dense quasi-stationary clumps have been overrun by a faster, lower density flow. The shock front impacted the front of the largest clump several decades ago, and during the 1990s, a prominent gap appeared in the advancing bow shock in the wake of the obstacle. The Hubble Space Telescope images show that by 1998 the shock front had wrapped around the back of the clump, closing the shock shadow it produced.

In order to test the hypothesis that Class I protostellar binary stars are a product of ejections during the dynamical decay of non-hierarchical multiple systems, we combined the results of new adaptive optics (AO) observations of Class I protostars with our previously published AO data to investigate whether Class I protostars with a widely separated companion (r>200 AU) are more likely to also have a close companion (r, 22 pages, 3 figures, 2 tables

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 the result of a survey for Herbig-Haro objects in the “Gulf of Mexico”, a part of the dark cloud L935. Images taken through Hα and [S ii] filters at SUBARU Suprime Cam covering an area of 34 ′ ×27 ′ reveal 35 new Herbig-Haro objects in this active star forming region, little studied so far.

We have mapped the central region of the HH 111 protostellar system in 1.33 mm continuum, C18O(J=2-1), 13CO (J=2-1), and SO (N_J=5_6-4_5) emission at ~3" resolution with the Submillimeter Array. There are two sources, VLA 1 (=IRAS 05491+0247) and VLA 2, with the VLA 1 source driving the HH 111 jet. Thermal emission is seen in 1.33 mm continuum tracing the dust in the envelope and the putative disks around the sources. A flattened, torus-like envelope is seen in C18O and 13CO around the VLA 1 source surrounding the dust lane perpendicular to the jet axis, with an inner radius of ~ 400 AU (1"), an outer radius of ~ 3200 AU (8"), and a thickness of ~ 1000 AU (2.5"). It seems to be infalling toward the center with conservation of specific angular momentum rather than with a Keplerian rotation as assumed by Yang et al. 1997. An inner envelope is seen in SO, with a radius of ~ 500 AU (1.3"). The inner part of this inner envelope, which is spatially coincident with the dust lane, seems to have a differential rotation and thus may have formed a rotationally supported disk. The outer part of this inner envelope, however, may have a rotation velocity decreasing toward the center and thus represent a region where an infalling envelope is in transition to a rotationally supported disk. A brief comparison with a collapsing model suggests that the flattened, torus-like envelope seen in C18O and 13CO could result from a collapse of a magnetized rotating toroid., 17 pages, 6 figures. Accepted by the ApJ

We present new Keck II NIRSPEC high-spectral resolution 2 um echelle observations of the young eruptive variable star V1647 Orionis. This star went into outburst in late 2003 and faded to its pre-outburst brightness after approximately 26 months. V1647 Orionis is the illuminating star of McNeil's Nebula and is located near M 78 in the Lynds 1630 dark cloud. Our spectra have a resolving power of approximately 18,000 and allow us to study in detail the weak absorption features present on the strong near-IR veiled continuum. An analysis of the echelle orders containing Mg I (2.1066 um) and Al I (2.1099 um), Br-gamma (2.1661 um), the Na I doublet (2.206 and 2.209 um), and the CO overtone bandhead (2.2935 um) gives us considerable information on the physical and geometric characteristics of the regions producing these spectral features. We find that, at high-spectral resolution, V1647 Orionis in quiescence resembles a significant number of FU Orionis type eruptive variables and does not appear similar to the quiescent EX Lupi variables observed. This correspondence is discussed and implications for the evolutionary state of the star are considered., 37 pages, 3 Tables, 17 Figures

The Infrared Imaging System (IRIS) is a 0.8 m telescope equipped with a 1024×1024 pixels near-infrared camera using a HAWAII-1 detector array. IRIS will be located at the Cerro Armazones Observatory in Chile that is operated by the Ruhr University Bochum jointly with the Universidad Catolica del Norte in Antofagasta. The system is specifically designed to survey star forming regions and to search for deeply embedded variable young stars.

During the first observing period with SEST we have looked for molecular outflows around a number of prominent southern Herbig Haro objects. We present here preliminary observational results for three of them, HH46/47, HH56/57 and HH120. A more detailed analysis will be presented in forthcoming papers. High spatial resolution radio data at millimetre wavelengths are essential to complete our picture of the physical conditions in the observed regions which so far has been based exclusively on optical and infrared data obtained at ESO and elsewhere.

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 the observational results of a near-infrared survey of a large sample of Class I protostars designed to determine the Class I binary separation distribution from ~100 AU to ~5000 AU. We have selected targets from a new sample of 267 nearby candidate Class I objects. This sample is well understood, consists of mostly Class I young stellar objects (YSOs) within 1 kpc, has targets selected from the whole sky, and is not biased by previous studies of star formation. We have observed 189 Class I YSOs north of -40 degrees declination at H, K and L'-bands, with a median angular resolution of 0.33" at L'. We determine our detection limit for close binary companions by observing artificial binaries. We choose a contrast limit and an outer detection limit to minimize contamination and to ensure that a candidate companion is gravitationally bound. Our survey uses observations at L' rather than K-band for the detection of binary companions since there is less scattered light and better seeing at L'. This paper presents the positions of our targets, the near-IR photometry of sources detected in our fields at L', as well as the observed properties of the 89 detected companions (73 of which are newly discovered). Although we have chosen contrast and separation limits to minimize contamination, we expect that there are ~6 stars identified as binary companions that are due to contamination. Finder charts at L' for each field are shown to facilitate future studies of these objects., Comment: 52 pages, 8 figures, 6 tables, accepted by the Astronomical Journal

We present new high resolution (R=18,000) near-infrared spectroscopic observations of a sample of classical FU Orionis stars (FUors) and other young stars with FUor characteristics that are sources of Herbig-Haro flows. Spectra are presented for the region 2.203 - 2.236 microns which is rich in absorption lines sensitive to both effective temperatures and surface gravities of stars. Both FUors and FUor-like stars show numerous broad and weak unidentified spectral features in this region. Spectra of the 2.280 - 2.300 micron region are also presented, with the 2.2935 micron v=2-0 CO absorption bandhead being clearly the strongest feature seen in the spectra all FUors and Fuor-like stars. A cross-correlation analysis shows that FUor and FUor-like spectra in the 2.203 - 2.236 micron region are not consistent with late-type dwarfs, giants, nor embedded protostars. The cross-correlations also show that the observed FUor-like Herbig-Haro energy sources have spectra that are substantively similar to those of FUors. Both object groups also have similar near-infrared colors. The large line widths and double-peaked nature of the spectra of the FUor-like stars are consistent with the established accretion disk model for FUors, also consistent with their near-infrared colors. It appears that young stars with FUor-like characteristics may be more common than projected from the relatively few known classical FUors., Comment: 21 pages, 4 figures, accepted by The Astronomical Journal

The FU Orionis phenomenon has attracted increasing attention in recent years, and is now accepted as a crucial element in the early evolution of low mass stars. The general characteristics of FUors are outlined and individual members of the class are discussed. The discovery of a new FUor, BBW 76, is presented, together with a discussion of photometric and spectroscopic observations of the star. The evidence for circumstellar disks around T Tauri stars is briefly outlined, and the FUor phenomenon is discussed in the context of a disk accretion model. A large increase in the accretion rate through a circumstellar disk makes the disk self-luminous with a luminosity two or more orders larger than that of the star. Massive cool winds rise from FUors, and it is conceivable that they are related to the initiation of Herbig-Haro flows. The FUor phenomenon appears to be repetitive, and newborn low-mass stars may be cycling between the FUor state and the T Tauri state.

We present new optical, near-IR, and mid-IR observations of the young eruptive variable star V1647 Orionis that went into outburst in late 2004 for approximately two years. Our observations were taken one year after the star had faded to its pre-outburst optical brightness and show that V1647Ori is still actively accreting circumstellar material. We compare and contrast these data with existing observations of the source from both pre-outburst and outburst phases. From near-IR spectroscopy we identify photospheric absorption features for the first time that allow us to constrain the classification of the young star itself. Our best fit spectral type is M0+-2 sub-classes with a visual extinction of 19+-2 magnitudes and a K-band veiling of rK~1.5+-0.2. We estimate that V1647Ori has a quiescent bolometric luminosity of ~9.5Lsun and a mass accretion rate of ~1.10^-6Msun yr^-1. Our derived mass and age, from comparison with evolutionary models, are 0.8+-0.2 Msun and ~0.5Myrs, respectively. The presence towards the star of shock excited optical [S II] and [Fe II] emission as well as near-IR H2 and [Fe II] emission perhaps suggests that a new Herbig-Haro flow is becoming visible close to the star., 22 pages, 19 Figures, accepted AJ 13 October 2007

We have carried out a major survey for visual binaries towards the Orion Nebula Cluster using HST images obtained with an H-alpha filter. Among 781 likely ONC members more than 60" from theta-1 Ori C, we find 78 multiple systems (75 binaries and 3 triples), of which 55 are new discoveries, in the range from 0.1" to 1.5". About 9 binaries are likely line-of-sight associations. We find a binary fraction of 8.8%+-1.1% within the limited separation range from 67.5 to 675 AU. The field binary fraction in the same range is a factor 1.5 higher. Within the range 150 AU to 675 AU we find that T Tauri associations have a factor 2.2 more binaries than the ONC. The binary separation distribution function of the ONC shows unusual structure, with a sudden steep decrease in the number of binaries as the separation increases beyond 0.5", corresponding to 225 AU. We have measured the ratio of binaries wider than 0.5" to binaries closer than 0.5" as a function of distance from the Trapezium, and find that this ratio is significantly depressed in the inner region of the ONC. The deficit of wide binaries in the central part of the cluster is likely due to dissolution or orbital change during their passage through the potential well of the inner cluster region. Many of the companions are likely to be brown dwarfs., 27 pages, 10 figures, 2 tables, accepted by the Astronomical Journal

We present new Hubble Space Telescope (HST) narrow band images of the bright Herbig–Haro (HH) objects HH 1 and 2 in the light of the Hα, Hβ, [O i] 6300, [O ii] 3726+28, [O iii] 5007 and [S ii] 6716+30 emission lines. The resulting emission and line ratio maps give an improved picture of the physical structure of these HH objects, showing the presence of spatially limited, high excitation/ionization ridges. We find that HH 1 has a morphology that could be interpreted in terms of a single, asymmetric bow shock, and that many of the clumps of HH 2 fall in two bow-shaped structures of different excitations. We also construct two-line ratio plots showing clear trends, which are much simpler than the highly complex spatial distributions of the emission, and are therefore interesting for testing shock models of HH objects (we only present a comparison with previously published, steady plane-parallel shock models). We have also used the temperature-sensitive [O i]/[S ii] line ratio to evaluate the temperature range and to obtain temperature maps of HH 1 and 2. We find that this line ratio picks out emitting regions with temperatures ≈104 K, except along the leading edges of the HH 1 and 2 bow shocks (in which temperatures of ~3 → 5 × 104 K are obtained).

The little-studied Herbig Be star V1818 Ori is located in the direction of the southern L1641 cloud and the Mon R2 star-forming complex, and is most likely associated with the latter at a distance of ~900 pc. A high-resolution spectrum is consistent with a spectral type around B7 V, with lines of Hα, the red Ca ii triplet, and several forbidden lines in emission. An All Sky Automated Survey V-band light curve spanning 9 yr reveals major variability with deep absorption episodes reminiscent of the UX Orionis stars. We have searched for additional young stars clustering around V1818 Ori using grism images and the 2MASS and Wide-field Infrared Survey Explorer catalogs, and have found almost two dozen fainter stars with evidence of youth. Direct images show that the bright star IRAS 05510–1025, only about 3 arcmin from V1818 Ori, is surrounded by a reflection nebula, indicating its association with a molecular cloud. A spectrum of the star shows no emission-lines, and it is found to be a close binary with late B and early G type components. Its radial velocity indicates that it is an interloper, accidentally passing through the cloud and not physically associated with V1818 Ori.

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.

Using optical (Hα, [S II], and i'), near-IR (H2, J, H, and KS), mid-IR (Spitzer Space Telescope IRAC 4.5 μm), and submillimeter (850 and 450 μm) data, we have examined the region surrounding the IC 348 cluster and the neighboring "Flying Ghost Nebula" (FGN) and found a multitude of shocks from protostellar outflows including HH 211, which had previously not been detected in visible wavelength images. We have identified 13 protostars in the region that drive protostellar outflows. The region surrounding the FGN is rich in ongoing star formation, with a number of outflows similar to those found in other sites of moderate star formation in Perseus (e.g., L1448, L1455, and Barnard 1). We have also found a candidate bent jet in this region. The axis defined by the bending angle suggests that this source may have been ejected from a multiple star system near the IC 348 IR source.

We present a proper motion study of the shock waves within the classic stellar jet HH 47 based on Hubble Space Telescope H-alpha and [S II] images of the region taken over two epochs. Individual knots within the jet and in the bow shock/Mach disk working surface of HH 47A move significantly in the five years that separate the images, and the excellent spatial resolution of HST makes it possible to measure the proper motions with enough precision to easily observe differential motions throughout the flow. The bright portion of the jet emerges at 37.5 +/- 2.5 degrees from the plane of the sky with an average velocity of 300 km/s. Dynamical ages of the shock waves in the jet range from a few decades for knots recently ejected by the source to ~ 1300 years for the faint extended bow shock HH 47D. The jet curves, but motions of knots in the jet are directed radially away from the exciting source, and velocity variability in the flow drives the shock waves that heat the jet internally. The jet orientation angle varies with time by about 15 degrees, and currently points to the northwestern portion of a cavity outlined by a reflection nebula, where a quasi-stationary shock deflects the jet. The major working surface HH 47A is more complex than a simple bow shock/Mach disk, and contains numerous clumps that move relative to one another with velocities of ~ +/- 40 km/s. Small clumps or instabilities affect the Mach disk, and dense clumps may move all the way through the working surface to cause the bumpy morphology seen at the bow shock. A localized area between the bow shock and Mach disk varies significantly between the two sets of images., full resolution figures available at http://sparky.rice.edu/~hartigan/pub.html; Accepted for publication in the Astronomical Journal

Adaptive optics imaging of the bright visual T Tauri binary AS 353 with the Subaru Telescope shows that it is a hierarchical triple system. The secondary component, located 5.6" south of AS 353A, is resolved into a subarcsecond binary, AS 353Ba and Bb, separated by 0.24". Resolved spectroscopy of the two close components shows that both have nearly identical spectral types of about M1.5. Whereas AS 353A and Ba show clear evidence for an infrared excess, AS 353Bb does not. We discuss the possible role of multiplicity in launching the large Herbig-Haro flow associated with AS 353A., AASTeXv5.0, 21 pages, 5 figures, Astronomical Journal, in press

We have obtained a Halpha position-velocity cube from Fabry-Perot interferometric observations of the HH 110 flow. We analyze the results in terms of anisotropic wavelet transforms, from which we derive the spatial distribution of the knots as well as their characteristic sizes (along and across the outflow axis). We then study the spatial behaviour of the line width and the central radial velocity. The results are interpreted in terms of a simple ``mean flow+turbulent eddy'' jet/wake model. We find that most of the observed kinematics appear to be a direct result of the mean flow, on which are superposed low amplitude (35 km/s) turbulent velocities., 27 pages, 8 Postscript figures. Astronomical Journal (accepted)

We present deep 3.6 cm radio continuum observations of the H II region NGC 2024 in Orion B obtained using the Very Large Array in its A-configuration, with $0\rlap.{''}2$ angular resolution. We detect a total of 25 compact radio sources in a region of $4' \times 4'$. We discuss the nature of these sources and its relation with the infrared and X-ray objects in the region. At least two of the radio sources are obscured proplyds whose morphology can be used to restrict the location of the main ionizing source of the region. This cluster of radio sources is compared with others that have been found in regions of recent star formation., Comment: 21 pages, 7 figures

The discovery of close pairs of stars where both are FU Orionis objects poses important constraints on the triggering mechanism for these dramatic eruptive events among pre-main sequence stars. It is argued here that FU or eruptions are signposts of the last stages of orbital evolution as a close binary is being formed. In this view, FU ors represent an evolutionary phase immediately following the formation of Herbig-Haro jets.

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.

Star Clusters: Basic Galactic Building Blocks Throughout Time and Space, IAU S266, edited by Richard de Grijs and Jacques Lepine, Cambridge, Cambridge University Press, 2010, 594 pp., £70.00 (hardb...

NGC 1333, a highly active star formation region within the Perseus molecular cloud complex, has been observed with the ACIS-I detector on board the Chandra X-ray Observatory. In our image with a sensitivity limit of ~ 1e28 erg/s, we detect 127 X-ray sources, of which, 95 are known cluster members. The X-ray luminosity function of the discovered YSO population spans a range of 1e28-3.2e31 erg/s in the 0.5-8 keV band. Comparison with K-band source counts indicates that we detect all of the known cluster members with K < 12, and about half of members with K > 12. We detect seven X-ray emitting YSOs which drive optically visible jets as well as one deeply embedded object that has not been detected in near-IR observations. The presence or absence of an outflows does not appear to produce any difference in X-ray properties of YSOs. We also find no systematic differences in X-ray luminosity distributions between two complete subsamples of CTTs and WTTs. That suggests that there is no difference in the astrophysical mechanism of WTT and CTT X-ray emission production. Additional results include: the X-ray emission from two late-B stars which illuminate the reflection nebula originates from unresolved late-type companions; two T Tauri stars are discovered as previously unknown components of visual binaries; and the X-ray counterpart of SVS 16 has the column density much lower than that expected from near-IR photometry and thus its X-ray luminosity is not anomalously high, as has been previously suggested., 53 pages, 14 figures, 3 tables. To appear in the Astrophysical Journal; scheduled for 2002, ApJ, 575 (August 10th). High quality copy available at ftp://ftp.astro.psu.edu/pub/gkosta/ngc1333/

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 present new Hα and Hβ images of the HH 1/2 system, and we find that the Hα/Hβ ratio has high values in ridges along the leading edges of the HH 1 bow shock and of the brighter condensations of HH 2. These ridges have Hα/Hβ = 4 → 6, which is consistent with collisional excitation from the n = 1 to the n = 3 and 4 levels of hydrogen in a gas of temperatures T = 1.5 → 10 × 10{sup 4} K. This is therefore the first direct evidence that the collisional excitation/ionization region of hydrogen just behind Herbig-Haro shock fronts is detected.

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 narrowband filter and high-resolution, velocity-resolved Fabry-Perot images of outflows in the southern portion of the Orion Nebula. HH 400 is a giant, low-velocity, redshifted bow shock located about 10' south of the core of the Orion Nebula. Its axis of symmetry passes close to the Trapezium cluster 1.5 pc to the north. The most likely point of origin is a young stellar object in the OMC-1 cloud core. HH 400 may be the remnant of a parsec-scale bipolar outflow powered by one of the young stars forming within this region. The radial velocity of gas in the limb-brightened rim of HH 400 is low, with redshifted speeds ranging from 8 to 20 km s-1 with respect to the emission from the Orion Nebula. The shape of the bow indicates that it lies close to the plane of the sky. For an inclination angle of 30° to the plane of the sky and assuming that the plasma is flowing mostly along the axis of symmetry, the visible gas at the rim of HH 400 has a mean velocity of about 30 km s-1, a mass of about 3 × 10-2 M⊙, and a dynamical age of about 5 × 104 yr, assuming the source lies in the OMC-1 cloud core. The estimated mass flux in the HH 400 bow is about 10-6 M⊙ yr-1. The bent bipolar irradiated jet HH 502 is superposed on the western rim of HH 400. We resolve the spatial and velocity structure of the jet and its multiple bow shocks. The jet consists of a chain of photoionized segments separated by wide gaps; bow shocks lie at the leading edges of these jet segments. The mean radial velocities of the jet segments decrease with increasing distance from the source. The large radial velocity dispersions of the gas at the tips of the HH 502 internal working surfaces and the small spreading angles of the HH 502 jet segments, combined with their low radial velocities, indicate that this flow lies close to the plane of the sky. Assuming that the jet is fully ionized, that it spreads at the Mach angle, and that the internal sound speed in the photoionized gas is about 10 km s-1, the jet must have a space velocity of about 400 km s-1. Finally, we present velocity-resolved images of the bow shocks in HH 540, a flow that may originate from the large protoplanetary disk 181-826. Several additional high-velocity features identified in the Fabry-Perot data trace additional jets and outflows in this portion of the Orion Nebula.

In the course of objective prism surveys of dark clouds, Herbig (1950, 1951, 1952) and Haro (1952, 1953) discovered a number of small objects, which display characteristic emission line spectra of hydrogen and forbidden low excitation lines of mainly [OI], [NII] and [SII], with at most very weak continua. On direct photographic plates, Herbig and Haro found these objects to be tiny, almost semi-stellar nebulae, with a tendency to be located in groups, often aligned in strings of objects. Ambartsumian (1954) subsequently coined the term Herbig-Haro objects for these tiny nebulae. Prom their location in dark clouds often containing nebulous stars or the newly recognized T Tauri stars (cf. Joy 1942, Ambartsumian 1947), early researchers concluded that the HH objects were somehow related to the process of star birth.

We present a millimeter wave study of a cluster of bipolar CO outflows embedded in the western end of the Circinus molecular cloud complex, G317-4, that is traced by very high optical extinction. For an assumed distance of 700 pc, the entire Circinus cloud is estimated to have a mass of about 5E4 solar masses. The opaque western portion that was mapped in this study has a mass of about 10E3 solar masses, contains a number of embedded infrared sources and various compact 1.3 mm continuum sources, and has a remarkable filamentary structure with numerous cavities which appears to be the fossil remnants of past star formation activity. The profusion of outflows in this region are disentangled and linked to driving IRAS sources. Thus, the mapped portion of Circinus contains at least 10 CO emitting molecular outflows. Assuming that star formation has continued at a steady rate for the last several hundred thousand years, the Circinus cloud is expected to have produced dozens of young stars. Their outflows have severely altered the structure and kinematics of this cloud as evidenced by the multitude of prominent cavities and dust filaments that outline their boundaries. This level of star formation activity is consistent with the numerous post-outflow phase H-alpha emission line stars that have been found in this region. The Circinus cloud complex is an archetypical case where star formation activity may have profoundly affected the structure of a molecular cloud, producing its strikingly filamentary and cavitated appearance and providing further evidence that star formation may be a self regulated process., 30 pages of text including tables, 17 figures, to appear in the Astronomical Journal

We review the events leading to the discovery of Herbig-Haro objects half a century ago, and the early efforts to understand the nature of these enigmatic objects. The recognition in the mid-seventies of the shocked nature of HH objects heralded a burst of observational and theoretical efforts, and further impetus was soon after provided by the discovery of high proper motions, and by detailed optical, infrared and ultraviolet spectroscopic studies. The recognition in the early eighties of HH jets was the starting point for the increasingly intense studies during the last 15 years which we discuss in this Symposium. In the second half of our review, we summarize the insights into the nature of HH jets provided by analyzing high resolution images obtained with the Hubble Space Telescope of two of the finest known HH jets, HH 47 and HH 111.

Following the detection of extended CO wings in Orion A by Kwan & Scoville (1976) and Zuckerman et al. (1976) came the discovery in 1980 of the first bipolar CO outflow from a low luminosity source, L1551 IRS 5 (Snell, Loren, Plambeck 1980). It was soon realized that bipolar outflows are common in star forming regions, as documented in a highly influential paper by Bally & Lada (1983), and a series of papers by Edwards & Snell (1982, 1983, 1984). Many of the presently best studied molecular outflows were already identified in these early papers, and it was established that there is an association between the optically emitting shocks seen as HerbigHaro objects and the large-scale moving CO gas. Further details of the early years of molecular outflows are discussed by Lada (1985).

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.

HH 222 is a giant shocked region in the L1641 cloud, and is popularly known as the Orion Streamers or "the waterfall" on account of its unusual structure. At the center of these streamers are two infrared sources coincident with a nonthermal radio jet aligned along the principal streamer. The unique morphology of HH 222 has long been associated with this radio jet. However, new infrared images show that the two sources are distant elliptical galaxies, indicating that the radio jet is merely an improbable line-of-sight coincidence. Accurate proper motion measurements of HH 222 reveal that the shock structure is a giant bow shock moving directly away from the well-known, very young, Herbig Be star V380 Ori. The already known Herbig-Haro object HH 35 forms part of this flow. A new Herbig-Haro object, HH 1041, is found precisely in the opposite direction of HH 222 and is likely to form part of a counterflow. The total projected extent of this HH complex is 5.3 pc, making it among the largest HH flows known. A second outflow episode from V380 Ori is identified as a pair of HH objects, HH 1031 to the northwest and the already known HH 130 to the southeast, along an axis that deviates from that of HH 222/HH 1041 by only 37. V380 Ori is a hierarchical quadruple system, including a faint companion of spectral type M5 or M6, which at an age of ~1 Myr corresponds to an object straddling the stellar-to-brown dwarf boundary. We suggest that the HH 222 giant bow shock is a direct result of the dynamical interactions that led to the conversion from an initial non-hierarchical multiple system into a hierarchical configuration. This event occurred no more than 28,000 yr ago, as derived from the proper motions of the HH 222 giant bow shock.

We have searched the Lynds 988 dark cloud complex for optical (H{alpha} and [S II]) and near-IR (H{sub 2} 2.12 {mu}m) shocks from protostellar outflows. We find 20 new Herbig-Haro objects and 6 new H{sub 2} shocks (MHO objects), 3 of which are cross detections. Using the morphology in the optical and near-IR, we connect several of these shocks into at least five distinct outflow systems and identify their source protostars from catalogs of infrared sources. Two outflows in the cloud, from IRAS 21014+5001 and IRAS 21007+4951, are in excess of 1 pc in length. The IRAS 21007+4951 outflow has carved a large cavity in the cloud through which background stars can be seen. Also, we have found an optical shock which is the counterflow to the previously discovered ''northwest outflow'' from LkH{alpha} 324SE.

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.