Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters

As a first step for studying star formation in the extreme outer Galaxy (EOG), we obtained deep near-infrared images of two embedded clusters at the northern and southern CO peaks of Cloud 2, which is one of the most distant star forming regions in the outer Galaxy (galactic radius R_g ~ 19 kpc). With high spatial resolution (FWHM ~ 0".35) and deep imaging (K ~ 21 mag) with the IRCS imager at the Subaru telescope, we detected cluster members with a mass detection limit of < 0.1 M_{sun}, which is well into the substellar regime. These high quality data enables a comparison of EOG to those in the solar neighborhood on the same basis for the first time. Before interpreting the photometric result, we have first constructed the NIR color-color diagram (dwarf star track, classical T Tauri star (CTTS) locus, reddening law) in the Mauna Kea Observatory filter system and also for the low metallicity environment since the metallicity in EOG is much lower than those in the solar neighborhood. The estimated stellar density suggests that an ``isolated type'' star formation is ongoing in Cloud 2-N, while a ``cluster type'' star formation is ongoing in Cloud 2-S. Despite the difference of the star formation mode, other characteristics of the two clusters are found to be almost identical: (1) K-band luminosity function (KLF) of the two clusters are quite similar, as is the estimated IMF and ages (~ 0.5--1 Myr) from the KLF fitting, (2) the estimated star formation efficiencies (SFEs) for both clusters are typical compared to those of embedded clusters in the solar neighborhood (~ 10 %). The similarity of two independent clusters with a large separation (~ 25 pc) strongly suggest that their star formation activities were triggered by the same mechanism, probably the supernova remnant (GSH 138-01-94).Comment: 14pages, 11 figures; Accepted for publication in Ap

Spitzer Observations of NGC 1333: A Study of Structure and Evolution in a Nearby Embedded Cluster

We present a comprehensive analysis of structure in the young, embedded cluster, NGC 1333 using members identified with Spitzer and 2MASS photometry based on their IR-excess emission. In total, 137 members are identified in this way, composed of 39 protostars and 98 more evolved pre-main sequence stars with disks. Of the latter class, four are transition/debris disk candidates. The fraction of exposed pre-main sequence stars with disks is 83% +/- 11%, showing that there is a measurable diskless pre-main sequence population. The sources in each of the Class I and Class II evolutionary states are shown to have very different spatial distributions relative to the distribution of the dense gas in their natal cloud. However, the distribution of nearest neighbor spacings among these two groups of sources are found to be quite similar, with a strong peak at spacings of 0.045 pc. Radial and azimuthal density profiles and surface density maps computed from the identified YSOs show that NGC 1333 is elongated and not strongly centrally concentrated, confirming previous claims in the literature. We interpret these new results as signs of a low velocity dispersion, extremely young cluster that is not in virial equilibrium.Comment: 59 pages, 20 figures, accepted to ApJ, verion with full resolution figures available at http://www.cfa.harvard.edu/~rgutermuth/preprints/gutermuth_ngc1333.pdf . Updated to fix astro-ph figure garblin

Can Protostellar Jets Drive Supersonic Turbulence in Molecular Clouds?

Jets and outflows from young stellar objects are proposed candidates to drive supersonic turbulence in molecular clouds. Here, we present the results from multi-dimensional jet simulations where we investigate in detail the energy and momentum deposition from jets into their surrounding environment and quantify the character of the excited turbulence with velocity probability density functions. Our study include jet--clump interaction, transient jets, and magnetised jets. We find that collimated supersonic jets do not excite supersonic motions far from the vicinity of the jet. Supersonic fluctuations are damped quickly and do not spread into the parent cloud. Instead subsonic, non-compressional modes occupy most of the excited volume. This is a generic feature which can not be fully circumvented by overdense jets or magnetic fields. Nevertheless, jets are able to leave strong imprints in their cloud structure and can disrupt dense clumps. Our results question the ability of collimated jets to sustain supersonic turbulence in molecular clouds.Comment: 33 pages, 18 figures, accepted by ApJ, version with high resolution figures at: http://www.ita.uni-heidelberg.de/~banerjee/publications/jet_paper.pd

Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood

We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the ^12CO and ^13CO isotopologues from their ultraviolet A--X absorption bands detected in archival Space Telescope Imaging Spectrograph data with lambda/Deltalambda geq 46,000. Five more targets with Goddard High-Resolution Spectrograph data are added to the sample that more than doubles the number of sight lines with published Hubble Space Telescope observations of ^13CO. Most sight lines have 12-to-13 isotopic ratios that are not significantly different from the local value of 70 for ^12C/^13C, which is based on mm-wave observations of rotational lines in emission from CO and H_2CO inside dense molecular clouds, as well as on results from optical measurements of CH^+. Five of the 25 sight lines are found to be fractionated toward lower 12-to-13 values, while three sight lines in the sample are fractionated toward higher ratios, signaling the predominance of either isotopic charge exchange or selective photodissociation, respectively. There are no obvious trends of the ^12CO-to-^13CO ratio with physical conditions such as gas temperature or density, yet ^12CO/^13CO does vary in a complicated manner with the column density of either CO isotopologue, owing to varying levels of competition between isotopic charge exchange and selective photodissociation in the fractionation of CO. Finally, rotational temperatures of H_2 show that all sight lines with detected amounts of ^13CO pass through gas that is on average colder by 20 K than the gas without ^13CO. This colder gas is also sampled by CN and C_2 molecules, the latter indicating gas kinetic temperatures of only 28 K, enough to facilitate an efficient charge exchange reaction that lowers the value of ^12CO/^13CO.Comment: 1-column emulateapj, 23 pages, 9 figure

Near-Infrared Spectroscopy of Molecular Hydrogen Emission in Four Reflection Nebulae: NGC 1333, NGC 2023, NGC 2068, and NGC 7023

We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2) emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derive the physical properties of the molecular material in these reflection nebulae. Our observations of NGC 2023 and NGC 7023 and the physical parameters we derive for these nebulae are in good agreement with previous studies. Both NGC 1333 and NGC 2068 have no previously-published analysis of near-infrared spectra. Our study reveals that the rotational-vibrational states of molecular hydrogen in NGC 1333 are populated quite differently from NGC 2023 and NGC 7023. We determine that the relatively weak UV field illuminating NGC 1333 is the primary cause of the difference. Further, we find that the density of the emitting material in NGC 1333 is of much lower density, with n ~ 10^2 - 10^4 cm^-3. NGC 2068 has molecular hydrogen line ratios more similar to those of NGC 7023 and NGC 2023. Our model fits to this nebula show that the bright, H_2-emitting material may have a density as high as n ~ 10^5 cm^-3, similar to what we find for NGC 2023 and NGC 7023. Our spectra of NGC 2023 and NGC 7023 show significant changes in both the near-infrared continuum and H_2 intensity along the slit and offsets between the peaks of the H_2 and continuum emission. We find that these brightness changes may correspond to real changes in the density and temperatures of the emitting region, although uncertainties in the total column of emitting material along a given line of sight complicates the interpretation. The spatial difference in the peak of the H_2 and near-infrared continuum peaks in NGC 2023 and NGC 7023 shows that the near-infrared continuum is due to a material which can survive closer to the star than H_2 can.Comment: Submitted for publication in ApJ. 34 pages including 12 embedded postscript figures. Also available at http://www.astronomy.ohio-state.edu/~martini/pub

Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide

Ultraviolet absorption from interstellar 12CO and 13CO was detected toward rho Oph A and chi Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column density ratios, N(12CO)/N(13CO), of 125 \pm 23 and 117 \pm 35 were derived for the sight lines toward rho Oph A and chi Oph, respectively. A value of 1100 \pm 600 for the ratio N(12C16O)/N(12C18O) toward rho Oph A was also obtained. Absorption from vibrationally excited H_2 (v" = 3) was clearly seen toward this star as well. The ratios are larger than the isotopic ratios for carbon and oxygen appropriate for ambient interstellar material. Since for both carbon and oxygen the more abundant isotopomer is enhanced, selective isotopic photodissociation plays the key role in the fractionation process for these directions. The enhancement arises because the more abundant isotopomer has lines that are more optically thick, resulting in more self shielding from dissociating radiation. A simple argument involving the amount of self shielding [from N(12CO)] and the strength of the ultraviolet radiation field premeating the gas (from the amount of vibrationally excited H_2) shows that selective isotopic photodissociation controls the fractionation seen in these two sight lines, as well as the sight line to zeta Oph.Comment: 40 pages, 8 figures, to appear in 10 July 2003 issue of Ap

Building productive relationships with young people with SEBD in transition: the role of identity

This article reports a study of the experiences of school leavers with social, emotional and behavioural difficulties (SEBD), which identified supportive relationships as key elements in young people demonstrating resilience through this transitional period. Almost all the young people involved in the study had access to potential helpers, but few managed to establish productive relationships with them. Analysis of interviews, conducted over a 15 month period with a group of 15 school leavers, their parents and those who worked with them, suggested that barriers and facilitators to relationship development existed at two levels: institutional and individual. This article focuses on the individual level, in which identity processes appear to play a key role. These processes are used to explain why some school leavers built productive relationships and thrived, whilst many failed to do so, and struggled. These findings have implications for policy, practice and theory

The gas temperature in flaring disks around pre-main sequence stars

A model is presented which calculates the gas temperature and chemistry in the surface layers of flaring circumstellar disks using a code developed for photon-dominated regions. Special attention is given to the influence of dust settling. It is found that the gas temperature exceeds the dust temperature by up to several hundreds of Kelvins in the part of the disk that is optically thin to ultraviolet radiation, indicating that the common assumption that Tgas=Tdust is not valid throughout the disk. In the optically thick part, gas and dust are strongly coupled and the gas temperature equals the dust temperature. Dust settling has little effect on the chemistry in the disk, but increases the amount of hot gas deeper in the disk. The effects of the higher gas temperature on several emission lines arising in the surface layer are examined. The higher gas temperatures increase the intensities of molecular and fine-structure lines by up to an order of magnitude, and can also have an important effect on the line shapes.Comment: 14 pages, 10 figures, accepted for publication in A&

Unveiling the Circumstellar Envelope and Disk: A Sub-Arcsecond Survey of Circumstellar Structures

We present the results of a 2.7 mm continuum interferometric survey of 24 young stellar objects in 11 fields. The target objects range from deeply embedded Class 0 sources to optical T Tauri sources. This is the first sub-arcsecond survey of the 2.7 mm dust continuum emission from young, embedded stellar systems. The images show a diversity of structure and complexity. The optically visible T Tauri stars (DG Tauri, HL Tauri, GG Tauri,and GM Aurigae) have continuum emission dominated by compact, less than 1", circumstellar disks. The more embedded near-infrared sources (SVS13 and L1551 IRS5) have continuum emission that is extended and compact. The embedded sources (L1448 IRS3, NGC1333 IRAS2, NGC1333 IRAS4, VLA1623, and IRAS 16293-2422) have continuum emission dominated by the extended envelope, typically more than 85%. In fact, in many of the deeply embedded systems it is difficult to uniquely isolate the disk emission component from the envelope extending inward to AU size scales. All of the target embedded objects are in multiple systems with separations on scales of 30" or less. Based on the system separation, we place the objects into three categories: separate envelope (separation > 6500 AU), common envelope (separation 150-3000 AU), and common disk (separation < 100 AU). These three groups can be linked with fragmentation events during the star formation process: separate envelopes from prompt initial fragmentation and the separate collapse of a loosely condensed cloud, common envelopes from fragmentation of a moderately centrally condensed spherical system, and common disk from fragmentation of a high angular momentum circumstellar disk.Comment: 47 Pages, 18 Figures, ApJ accepte