HST astrometry in the Orion Nebula Cluster: census of low-mass runaways

We present a catalog of high-precision proper motions in the Orion Nebula Cluster (ONC), based on Treasury Program observations with the Hubble Space Telescope's (HST) ACS/WFC camera. Our catalog contains 2,454 objects in the magnitude range of $14.2<m_{\rm F775W}<24.7$, thus probing the stellar masses of the ONC from $\sim$0.4 $M_\odot$ down to $\sim$0.02 $M_\odot$ over an area of $\sim$550 arcmin$^2$. We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on the stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at $\sigma_{v,x}=0.94$ and $\sigma_{v,y}=1.25$ mas yr$^{-1}$) might be biased by the overlapping young stellar populations of Orion A. We identified 4 new ONC candidate runaways based on HST and the Gaia DR2 data, all with masses less than $\sim$1 $M_\odot$. The total census of known candidate runaway sources is 10 -- one of the largest samples ever found in any Milky Way open star cluster. Surprisingly, none of them has the tangential velocity exceeding 20 km s$^{-1}$. If most of them indeed originated in the ONC, it may compel re-examination of dynamical processes in very young star clusters. It appears that the mass function of the ONC is not significantly affected by the lost runaways.Comment: 16 pages, 10 figures, 5 tables. Accepted for publication in A

Who Uses Community-Based Youth Shelters? An Inter-Group And Intra-Group Analysis

Community-based youth shelters are the primary method of intervention designed to meet the complex needs of at-risk youth who leave home before they have developed the skills to live independently. This research examines shelter users\u27 patterns of cross-sector service use to better understand the needs and resources of shelter residents. The aims of this study are 1) to perform an inter-group analysis, comparing sheltered youth with status offense runaways and foster care runaways, and 2) to explore the population of emergency shelter residents using an intra-group analysis, determining whether distinct profiles of sheltered youth exist, based on individual characteristics and service use patterns over time. The study samples were drawn from a larger longitudinal study of services and outcomes. The samples included subjects born between 1981 and 1992 who were reported for child maltreatment and/or lived in families receiving Aid to Families with Dependent Children during childhood. For the first Aim, the sample included subjects identified as runaway by the court system: status offense runaways), foster care runaways, and sheltered youth. The sample for the second aim included only sheltered youth. Bivariate analyses found differences between runaway groups in terms of poverty, maltreatment history, school-identified disability, report of neglect, receipt of family services, delinquent offenses, truancy, ethnicity, and parent mental health or substance abuse treatment. Controlling for covariates in the multinomial logistic regression, only age discriminated between all three groups. Just 20% of the sheltered youth ran away from their previous residence or spent time living on the street. Latent class analysis suggests sheltered youth fall into four clearly distinct categories, clearly defined by connection to school and family. These include: 1) a parent time-out group: attending school and living with family), 2) a school/behavior problem group: not attending school and living with family), 3) youth in DFS custody placed at the shelter: disconnected from family but attending school), and 4) multi-problem youth: disconnected from both school and family)

On the absence of wind bow-shocks around OB-runaway stars: probing the physical conditions of the interstellar medium

High-resolution IRAS maps are used to search for the presence of stellar-wind bow-shocks around high-mass X-ray binaries (HMXBs). Their high space velocities, recently confirmed with Hipparcos observations, combined with their strong stellar winds should result in the formation of wind bow-shocks. Except for the already known bow-shock around Vela X-1 (Kaper et al. 1997), we do not find convincing evidence for a bow-shock around any of the other HMXBs. Also in the case of (supposedly single) OB-runaway stars, only a minority appears to be associated with a bow-shock (Van Buren et al. 1995). We investigate why wind bow-shocks are not detected for the majority of these OB-runaway systems: is this due to the IRAS sensitivity, the system's space velocity, the stellar-wind properties, or the height above the galactic plane? It turns out that none of these suggested causes can explain the low detection rate (~40%). We propose that the conditions of the interstellar medium mainly determine whether a wind bow-shock is formed or not. In hot, tenuous media (like inside galactic superbubbles) the sound speed is high (~100 km/s), such that many runaways move at subsonic velocity through a low-density medium, thus preventing the formation of an observable bow-shock. Superbubbles are expected (and observed) around OB associations, where the OB-runaway stars were once born. Turning the argument around, we use the absence (or presence) of wind bow-shocks around OB runaways to probe the physical conditions of the interstellar medium in the solar neighbourhood.Comment: 12 pages, 13 postscript figures included, accepted for publications in Astronomy & Astrophysic

IT governance in SMEs: trust or control?

It is believed by many scholars that a small and medium-sized enterprise (SME) cannot be seen through the lens of a large firm. Theories which explain IT governance in large organizations and methodologies used by practitioners can therefore not be extrapolated to SMEs, which have a completely different economic, cultural and managerial environment. SMEs suffer from resource poverty, have less IS experience and need more external support. SMEs largely contribute to the failure of many IS projects. We define an out-sourced information system failure (OISF) as a failure of IT governance in an SME environment and propose a structure for stating propositions derived from both agency theory and theory of trust. The theoretical question addressed in this paper is: how and why do OISFs occur in SMEs? We have chosen a qualitative and positivistic IS case study research strategy based on multiple cases. Eight cases of IS projects were selected. We found that trust is more important than control issues like output-based contracts and structured controls for eliminating opportunistic behaviour in SMEs. We conclude that the world of SMEs is significantly different from that of large companies. This necessitates extra care to be taken on the part of researchers and practitioners when designing artefacts for SMEs

Hubble Space Telescope Astrometry in the Orion Nebula Cluster: Census of Low-mass Runaways

We present a catalog of high-precision proper motions in the Orion Nebula Cluster (ONC), based on Treasury Program observations with the Hubble Space Telescope's (HST) ACS/WFC camera. Our catalog contains 2454 objects in the magnitude range of 14.2 < m_(F775W) < 24.7, thus probing the stellar masses of the ONC from ~0.4 M☉ down to ~0.02 M☉ over an area of ~550 arcmin². We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at σ_(v,x) = 0.94 and σ_(v,y) = 1.25 mas yr⁻¹) might be biased by the overlapping young stellar populations of Orion A. We identified four new ONC candidate runaways based on HST and the Gaia DR 2 data, all with masses less than ~1 M☉. The total census of known candidate runaway sources is 10—one of the largest samples ever found in any Milky Way open star cluster. Surprisingly, none of them have tangential velocities exceeding 20 km s⁻¹. If most of them indeed originated in the ONC, it may compel the re-examination of dynamical processes in very young star clusters. It appears that the mass function of the ONC is not significantly affected by the lost runaways

Multiwavelength Studies of Young OB Associations

We discuss how contemporary multiwavelength observations of young OB-dominated clusters address long-standing astrophysical questions: Do clusters form rapidly or slowly with an age spread? When do clusters expand and disperse to constitute the field star population? Do rich clusters form by amalgamation of smaller subclusters? What is the pattern and duration of cluster formation in massive star forming regions (MSFRs)? Past observational difficulties in obtaining good stellar censuses of MSFRs have been alleviated in recent studies that combine X-ray and infrared surveys to obtain rich, though still incomplete, censuses of young stars in MSFRs. We describe here one of these efforts, the MYStIX project, that produced a catalog of 31,784 probable members of 20 MSFRs. We find that age spread within clusters are real in the sense that the stars in the core formed after the cluster halo. Cluster expansion is seen in the ensemble of (sub)clusters, and older dispersing populations are found across MSFRs. Direct evidence for subcluster merging is still unconvincing. Long-lived, asynchronous star formation is pervasive across MSFRs.Comment: 22 pages, 9 figures. To appear in "The Origin of Stellar Clusters", edited by Steven Stahler, Springer, 2017, in pres

Systems Development Tools and the Relationship to Project Design: Cost and Budget Implications

An investigation into current system development trends, including a summary of projects, platforms, and tools. The majority of projects get completed on time and within budget, but 14 percent still present problems. The choice of tools appears to be changing to represent the demand for new projects on new platforms—particularly the Internet. CASE tools are used sparingly, and Java is beginning to replace C++ in many projects. Database management systems are critical to almost all projects