Bright Rim Research Articles

X-RAY EMISSION FROM STRONGLY ASYMMETRIC CIRCUMSTELLAR MATERIAL IN THE REMNANT OF KEPLER'S SUPERNOVA

Kepler's supernova remnant resulted from a thermonuclear explosion, but is interacting with circumstellar material (CSM) lost from the progenitor system. We describe a statistical technique for isolating X-ray emission due to CSM from that due to shocked ejecta. Shocked CSM coincides well in position with 24 $\mu$m emission seen by {\sl Spitzer}. We find most CSM to be distributed along the bright north rim, but substantial concentrations are also found projected against the center of the remnant, roughly along a diameter with position angle $\sim 100^\circ$. We interpret this as evidence for a disk distribution of CSM before the SN, with the line of sight to the observer roughly in the disk plane. We present 2-D hydrodynamic simulations of this scenario, in qualitative agreement with the observed CSM morphology. Our observations require Kepler to have originated in a close binary system with an AGB star companion.

Spectroscopy of diffuse light in dust clouds

The optical surface brightness of dark nebulae is mainly due to scattering of integrated starlight by classical dust grains. It contains information on the impinging interstellar radiation field, cloud structure, and grain scattering properties. We have obtained spectra of the scattered light from 3500 to 9000 Angstrom in two globules, the Thumbprint Nebula and DC303.8-14.2. We use observations of the scattered light to study the impinging integrated starlight spectrum as well as the scattered H-alpha and other line emissions from all over the sky. We search also for the presence of other than scattered light in the two globules. We obtained long-slit spectra encompassing the whole globule plus adjacent sky in a one-slit setting, thus enabling efficient elimination of airglow and other foreground sky components. We calculated synthetic integrated starlight spectra for the solar neighbourhood using HIPPARCOS-based stellar distributions and the spectral library of Pickles. Spectra are presented separately for the bright rims and dark cores of the globules. The continuum spectral energy distributions and absorption line spectra can be well modelled with the synthetic integrated starlight spectra. Emission lines of H-alpha + NII, H-beta, and SII are detected and are interpreted in terms of scattered light plus an in situ warm ionized medium component behind the globules. We detected an excess of emission over the wavelength range 5200-8000 Angstrom in DC303.8-14.2 but the nature of this emission remains open.

TESTING TRIGGERED STAR FORMATION IN SIX H II REGIONS

We investigated six H II regions with infrared, bright rimmed bubble or cometary morphology, in search of quantitative evidence for triggered star formation, both collect and collapse and radiatively driven implosion. We identified and classified 458 Young Stellar Objects (YSOs) in and around the H II regions. YSOs were determined by fitting a collection of radiative transfer model spectral energy distributions (SEDs) to infrared photometry for a large sample of point sources. We determined areas where there exist enhanced populations of relatively unevolved YSOs on the bright rims of these regions, suggesting that star formation has been triggered there. We further investigated the physical properties of the regions by using radio continuum emission as a proxy for ionizing flux powering the H II regions, and 13CO (1-0) observations to measure masses and gravitational stability of molecular clumps. We used an analytical model of collect and collapse triggered star formation, as well as a simulation of radiatively driven implosion, and thus we compare the observed properties of the molecular gas with those predicted in the triggering scenarios. Notably, those regions in our sample that show evidence of cometary, or "blister," morphology are more likely to show evidence of triggering. Keywords: HII regions -- ISM: bubbles -- Stars: formation -- Stars: protostars

A compact steep spectrum radio source in NGC 1977

Context. A compact steep spectrum radio source (J0535−0452) is located in the sky coincident with a bright optical rim in the H ii region NGC 1977. J0535−0452 is observed to be ≤100 mas in angular size at 8.44 GHz. The spectrum for the radio source is steep and straight with a spectral index of −1.3 between 330 and 8440 MHz. No 2 μm IR counter part for the source is detected. These characteristics indicate that the source may be either a rare high redshift radio galaxy or a millisecond pulsar (MSP). Aims. We investigate whether the steep spectrum source is a millisecond pulsar. The optical rim is believed to be the interface between the H ii region and the adjacent molecular cloud. If the compact source is a millisecond pulsar, it would have eluded detection in previous pulsar surveys because of the extreme scattering due to the H ii region-molecular cloud interface. Methods. The limits obtained on the angular broadening along with the distance to the scattering screen are used to estimate the pulse broadening. The pulse broadening is shown to be less than a few msec at frequencies >∼5 GHz. We therefore searched for pulsed emission from J0535−0452 at 14.8 and 4.8 GHz with the Green Bank Telescope (GBT). Results. No pulsed emission is detected to 55 and 30 μJy level at 4.8 and 14.8 GHz. Based on the parameter space explored by our pulsar search algorithm, we conclude that, if J0535−0452 is a pulsar, then it could only be a binary MSP of orbital period <∼5 h.

Testing diagnostics of triggered star formation

We produce synthetic images and SEDs from radiation hydrodynamical simulations of radiatively driven implosion. The synthetically imaged bright rimmed clouds (BRCs) are morphologically similar to those observed in star forming regions. Using nebular diagnostic line-ratios, simulated Very Large Array (VLA) radio images, H{\alpha} imaging and SED fitting we compute the neutral cloud and ionized boundary layer gas densities and temperatures and perform a virial stability analysis for each model cloud. We determine that the neutral cloud temperatures derived by SED fitting are hotter than the dominant neutral cloud temperature by 1 - 2 K due to emission from warm dust. This translates into a change in the calculated cloud mass by 8-35 %. Using a constant mass conversion factor (C{\nu}) for BRCs of different class is found to give rise to errors in the cloud mass of up to a factor of 3.6. The ionized boundary layer (IBL) electron temperature calculated using diagnostic line ratios is more accurate than assuming the canonical value adopted for radio diagnostics of 10^4 K. Both radio diagnostics and diagnostic line ratios are found to underestimate the electron density in the IBL. Each system is qualitatively correctly found to be in a state in which the pressure in the ionized boundary layer is greater than the supporting cloud pressure, implying that the objects are being compressed. We find that observationally derived mass loss estimates agree with those on the simulation grid and introduce the concept of using the mass loss flux to give an indication of the relative strength of photo-evaporative flow between clouds. The effect of beam size on these diagnostics in radio observations is found to be a mixing of the bright rim and ambient cloud and HII region fluxes, which leads to an underestimate of the cloud properties relative to a control diagnostic.

Microscale mineralogical characterization of As, Fe, and Ni in uranium mine tailings

Uranium (U) ores can contain high concentrations of elements of concern (EOCs), such as arsenic (As) and nickel (Ni) present in sulfide and arsenide minerals. The U in these ores is often solubilized by adding H2SO4 to attain a pH ∼1 under oxic conditions. This process releases some EOCs from the primary minerals into solution. The barren raffinate (solution remaining after U extraction) is subsequently neutralized with Ca(OH)2 to a terminal pH of ∼10.5, resulting in a reduction in the aqueous concentrations of the EOCs. These neutralized raffinates are mixed with the non-reacted primary minerals and discharged as tailing into tailings management facilities (TMFs). To aid in the accurate characterization and quantification of the mineralogical controls on the concentrations of EOCs in the tailings porewater, their spatial distribution and speciation were studied at the micron scale in tailings samples collected from the Deilmann U Tailings Management Facility (DTMF), northern Saskatchewan, Canada. Backscattered electron images of the tailings samples generated using an electron microprobe show the presence of nodules (10–200μm size) surrounded by bright rims. Wavelength dispersive spectrometric (WDS) and synchrotron-based micro-X-ray fluorescence (μ-XRF) elemental mapping show that the nodules are dominated by Ca and S (as gypsum) and the bright rims are dominated by Fe, As, and Ni. Micro-X-ray absorption near-edge structure (μ-XANES) spectra collected within and near the rims indicate that the Fe and Ni are present mainly in the +3 and +2 oxidation states, respectively; for As, the +5 oxidation state dominates but significant amounts of the +3 oxidation state are present in some areas. Linear combination fit analyses of the K-edges for the Fe, As, and Ni μ-XANES spectra to reference compounds suggest the Fe in the rims is present as ferrihydrite with As and Ni are adsorbed to it. Energy dispersive spectrometric (EDS) data indicate that isolated, highly reflective particles distributed throughout the tailings matrix are primary As-, Cu-, Fe-, and Ni-bearing minerals. Geochemical modeling of the neutralization process shows that the nodules (gypsum) formed at pH ∼1 and acted as a substrate for the precipitation of ferrihydrite at pH ∼3.4. The As and Ni subsequently adsorb to the ferrihydrite. Overall, the microscale data suggest that the As and Ni adsorbed onto the ferrihydrite should remain stable for many years and continue to the control the Fe, As, and Ni concentrations in the tailings porewater.

Magnetic Resonance Imaging Features of Osteochondral Lesions of the Talus

Osteochondral lesions of the talus (OLT) traditionally have been thought to occur anterolaterally or posteromedially. Recent studies utilizing magnetic resonance imaging (MRI) have questioned this teaching. The purpose of this study was to use MRI to describe the location, frequency, and morphology of OLT and determine if any correlations exist between lesion location and other data points. The location, frequency, and size of OLT based on a nine-zone grid were recorded on 77 MRI examinations. Lateral ligaments were inspected for evidence of injury. Stability of the lesions was assessed based on four MRI criteria: presence or absence of cartilage defects, edema-like signal abnormality, T2 bright rim, and/or subchondral cysts. Demographic data including patient age, injury mechanism, and chronicity were recorded. An ANOVA model was used to determine if statistical differences existed between lesion size and location. Pearson correlation coefficients were used to examine any association between lesion location and demographic data. Most of the lesions were located medially and centrally on the talar dome (54.5%), with the second highest frequency found laterally and centrally (31.2%). With the numbers available there was no statistical difference between the size of the lesions based on location. No strong correlations were found between lesion location and demographic data. This study refutes traditional teachings regarding the location of OLT and supports recent studies showing that most lesions are located medially and centrally on the talar dome.

ACHANDRA-VLA INVESTIGATION OF THE X-RAY CAVITY SYSTEM AND RADIO MINI-HALO IN THE GALAXY CLUSTER RBS 797

We present a study of the cavity system in the galaxy cluster RBS 797 based on Chandra and VLA data. RBS 797 (z = 0.35), is one of the most distant galaxy clusters in which two pronounced X-ray cavities have been discovered. The Chandra data confirm the presence of a cool core and indicate an higher metallicity along the cavity directions. This is likely due to the AGN outburst, which lifts cool metal-rich gas from the center along the cavities, as seen in other systems. We find indications that the cavities are hotter than the surrounding gas. Moreover, the new Chandra images show bright rims contrasting with the deep, X-ray deficient cavities. The likely cause is that the expanding 1.4 GHz radio lobes have displaced the gas, compressing it into a shell that appears as bright cool arms. Finally we show that the large-scale radio emission detected with our VLA observations may be classified as a radio mini-halo, powered by the cooling flow (CF), as it nicely follows the trend P_radio vs. P_CF predicted by the re-acceleration model.

Near-infrared interferometric observation of the Herbig Ae star HD 144432 with VLTI/AMBER

We study the sub-AU-scale circumstellar environment of the Herbig Ae star HD144432 with near-infrared (NIR) VLTI/AMBER observations to investigate the structure of its inner dust disk. The interferometric observations were carried out with the AMBER instrument in the H and K band. We interpret the measured H- and K-band visibilities, the near- and mid-infrared visibilities from the literature, and the SED of HD144432 by using geometric ring models and ring-shaped temperature-gradient disk models with power-law temperature distributions. We derived a K-band ring-fit radius of 0.17 \pm 0.01 AU and an H-band radius of 0.18 \pm 0.01 AU (for a distance of 145 pc). This measured K-band radius of \sim0.17 AU lies in the range between the dust sublimation radius of \sim0.13 AU (predicted for a dust sublimation temperature of 1500 K and gray dust) and the prediction of models including backwarming (\sim0.27 AU). We found that an additional extended halo component is required in both the geometric and temperature-gradient modeling. In the best temperature- gradient model, the disk consists of two components. The inner part of the disk is a thin ring with an inner radius of \sim0.21 AU, a temperature of \sim1600 K, and a ring thickness \sim0.02 AU. The outer part extends from \sim1 AU to \sim10 AU with an inner temperature of \sim400 K. We find that the disk is nearly face-on with an inclination angle of < 28 degree. Our temperature-gradient modeling suggests that the NIR excess is dominated by emission from a narrow, bright rim located at the dust sublimation radius, while an extended halo component contributes \sim6% to the total flux at 2 {\mu}m. The MIR model emission has a two-component structure with \sim20% flux from the inner ring and the rest from the outer part. This two-component structure suggests a disk gap, which is possibly caused by the shadow of a puffed-up inner rim.

The Bright Rim Sign on MRI for Anterior Talofibular Ligament Injury With Arthroscopic Correlation

The purpose of this article is to determine whether bright rim lesions on MRI are a marker for anterior talofibular ligament injury. The study included 34 patients who had an ankle injury and underwent arthroscopic surgery. All patients underwent 3-T MRI for the diagnosis of anterior talofibular ligament injury. If MRI revealed nonvisualization of the ligament, ligament discontinuity, and unusual ligament thickening (criterion 1) or the bright rim sign (criterion 2), the injury was considered to be a ligament disruption. After MRI, ankle arthroscopy was performed in all patients for a definitive diagnosis. Arthroscopy showed anterior talofibular ligament disruption in 33 patients. When the MRI diagnosis was based on criterion 1, anterior talofibular ligament disruption was diagnosed with a sensitivity of 60.6-66.7% and an accuracy of 58.8-67.6%. When the MRI diagnosis was based on both criteria 1 and 2, anterior talofibular ligament disruption was diagnosed with a sensitivity of 90.9-97.0% and an accuracy of 88.2-94.1%. By adding criterion 2 to the diagnosis, the sensitivity for anterior talofibular ligament injury was increased significantly (p < 0.01), and 8-12 additional patients with anterior talofibular ligament injury were diagnosed, most of whom exhibited a partial tear of the anterior talofibular ligament on arthroscopy. The interobserver agreement rate for the presence of anterior talofibular ligament disruption using criterion 1, both criteria, and the bright rim sign was fair to excellent. A cortical defect with bright dotlike or curvilinear high-signal-intensity lesions on T2-weighted MRI may be an additional morphologic feature to increase the diagnostic performance of detecting anterior talofibular ligament injuries, including those with partial tears.

Inner dusty regions of protoplanetary discs - I. High-resolution temperature structure

Our current understanding of the physical conditions in the inner regions of protoplanetary discs is becoming increasingly challenged by the more detailed observational and theoretical explorations. Calculation of dust temperature is one of the key features we strive to understand and a necessary step in image and flux reconstruction. We explore coexistence of small (0.1mic radius) and big (2mic radius) dust grains can coexist at distances from the star where small grains would not survive without big grains shielding them from the direct starlight. The study required a high resolution radiative transfer calculation capable of resolving large temperature gradients and disc surface curvatures caused by dust sublimation. The calculation was also capable of resolving temperature inversion effect in big grains, where the maximum dust temperature is at visual optical depth of tau_V~1.5. We also show disc images and spectra, with disentangled contributions from small and big grains. Big grains dominate the near IR flux, mainly because of the bright hot inner disc rim. Small grains populate almost the entire inner disc interior, but appear in the disc surface at distances 2.2 times larger than the closest distance of big grains from the star. Nevertheless, small grains can contribute to the image surface brightness at smaller radii because they are visible below the optically thin surface defined by stellar heating. Our calculations demonstrate that the sublimation temperature does not provide a unique boundary condition for radiative transfer models of optically thick discs. The source of this problem is the temperature inversion effect, which allows survival of optically thin configurations of big grains closer to the star than the inner radius of optically thick disc. Future attempts to derive more realistic multigrain inner disc models will require the numerical resolution ...

SHRIMP zircon chronology of HP-UHP rocks of the Makbal metamorphic complex in the Northern Tien Shan, Kyrgyzstan

In this paper we present the first results of sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating obtained for the ultra-high pressure (UHP) metamorphic rocks of the Makbal metamorphic complex in the Northern Tien Shan, Kyrgyzstan. Zircon grains from two HP eclogite samples and one sample of UHP garnet-talk-chloritoid schist have similar bright rims identified on cathodoluminescence (CL) images. The rims are interpreted as zircon overgrowth during UHP metamorphism. Dating of zircon rims from the three samples yielded similar ages of 509±7Ma, 502±10Ma and 498±7Ma. The time span of 510–500Ma is considered as the “minimum” age of UHP metamorphism of the Makbal complex. This age is in accordance with Middle Cambrian faunal age of the Kara-Archa ophiolite in the Makbal area. It is suggested that the Makbal rocks were metamorphosed in the Middle Cambrian subduction zone dipping north under the Northern Tien Shan microcontinent. A deformed granodiorite associated with the Makbal metamorphic rocks yielded an age of 514±5Ma, indicating development of a magmatic arc simultaneously with the UHP metamorphism. Cores of zircon grains from the garnet-talk-chloritoid schist yielded a wide spectrum of ages (1850–800Ma) characteristic for detrital zircons of continental origin. A depositional age <800Ma points to a possible derivation from a Riphean passive margin of the Terskey Ocean. Analyses of zircon grains from two eclogite samples fit well-defined discordia lines with upper intercepts at 1446±25Ma and 1447±29Ma, indicating a homogeneous Mezoproterozoic protolith of these rocks. The possible presence of such protoliths in Makbal is illustrated by a similar age of inherited zircon recovered from the granodiorite (1443±25Ma) and by Mesoproterozoic detrital zircons from the garnet-talk-chloritoid schist. Ages of Kokchetav, Makbal, Aktyuz and Anrakhai HP-UHP complexes in the Kokchetav–Northern Tien Shan belt vary from 537 to 475Ma and register subduction–accretion events during the Caledonian orogeny which finally formed the Paleo-Kazakhstan continent prior to Middle Ordovician.

PENETRATING THE HOMUNCULUS—NEAR-INFRARED ADAPTIVE OPTICS IMAGES OF ETA CARINAE

Near-infrared adaptive optics imaging with NICI and NaCO reveal what appears to be a three-winged or lobed pattern, the "butterfly nebula", outlined by bright Br$\gamma$ and H$_{2}$ emission and light scattered by dust. In contrast, the [Fe II] emission does not follow the outline of the wings, but shows an extended bipolar distribution which is tracing the Little Homunculus ejected in $\eta$ Car's second or lesser eruption in the 1890's. Proper motions measured from the combined NICI and NaCO images together with radial velocities show that the knots and filaments that define the bright rims of the butterfly were ejected at two different epochs corresponding approximately to the great eruption and the second eruption. Most of the material is spatially distributed 10$\arcdeg$ to 20$\arcdeg$ above and below the equatorial plane apparently behind the Little Homunculus and the larger SE lobe. The equatorial debris either has a wide opening angle or the clumps were ejected at different latitudes relative to the plane. The butterfly is not a coherent physical structure or equatorial torus but spatially separate clumps and filaments ejected at different times, and now 2000 to 4000 AU from the star.

OBSERVING FLUX ROPE FORMATION DURING THE IMPULSIVE PHASE OF A SOLAR ERUPTION

Magnetic flux rope is believed to be an important structural component of coronal mass ejections (CMEs). While there exist much observational evidence of the flux rope after the eruption, e.g., as seen in remote-sensing coronagraph images or in-situ solar wind data, the direct observation of flux ropes during CME impulsive phase has been rare. In this Letter, we present an unambiguous observation of a flux rope still in the formation phase in the low corona. The CME of interest occurred above the east limb on 2010 November 03 with footpoints partially blocked. The flux rope was seen as a bright blob of hot plasma in AIA 131 \AA\ passband (peak temperature ~11 MK) rising from the core of the source active region, rapidly moving outward and stretching upward the surrounding background magnetic field. The stretched magnetic field seemed to curve-in behind the core, similar to the classical magnetic reconnection scenario in eruptive flares. On the other hand, the flux rope appeared as a dark cavity in AIA 211 \AA\ passpand (2.0 MK) and 171 \AA\ passband (0.6 MK); in these relatively cool temperature bands, a bright rim clearly enclosed the dark cavity. The bright rim likely represents the pile-up of the surrounding coronal plasma compressed by the expanding flux rope. The composite structure seen in AIA multiple temperature bands is very similar to that in the corresponding coronagraph images, which consists of a bright leading edge and a dark cavity, commonly believed to be a flux rope.

Triggered Star Formation

Triggered star formation in bright rims and shells is reviewed. Shells are commonly observed in the Milky Way and other galaxies, but most diffuse shells seen in HI or the infrared do not have obvious triggered star formation. Dense molecular shells and pillars around HII regions often do have such triggering, although sometimes it is difficult to see what is triggered and what stars formed in the gas before the pressure disturbances. Pillar regions without clear age gradients could have their stars scattered by the gravity of the heads. Criteria and timescales for triggering are reviewed. The insensitivity of the average star formation rate in a galaxy to anything but the molecular mass suggests that triggering is one of many processes that lead to gravitational collapse and star formation.

DETECTION OF VORTEX TUBES IN SOLAR GRANULATION FROM OBSERVATIONS WITH SUNRISE

We have investigated a time series of continuum intensity maps and corresponding Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory Sunrise. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. From cross sections through the computational domain of the simulation, we conclude that these `granular lanes' are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. We propose that the observed vortex tubes may represent only the large-scale end of a hierarchy of vortex tubes existing near the solar surface.

The interaction of supernova remnant G357.7+0.3 with the interstellar medium

The supernova remnant (SNR) G357.7+0.3 appears to have caused considerable shredding of the local interstellar medium (ISM), leading to the formation of multiple cloud fragments having bright rims and cometary structures. We investigate five of these regions using mid-infrared (MIR) imaging and photometry deriving from the Spitzer Space Telescope (SST), as well as photometry deriving from the 2MASS near-infrared all sky survey, the Mid-Course Science Experiment (MSX), and the Multiband Imaging Photometer for Spitzer (MIPSGAL) survey of the Galactic plane. It is noted that two of the rims show evidence for emission by shock excited H2 transitions, whilst the centres of the clouds also show evidence for dark extinction cores, observed in silhouette against the bright emission rims. Levels of extinction for these cores are determined to be of order AV ~ 17-26 mag, whilst densities n(HI) are of order ~ 10^4 cm^(-3), and masses in the region of ~40-100 Msun. It is shown that the wavelength dependence of extinction is probably similar to that of Cardelli et al. and Martin & Whittet, but differs from the MIR extinction trends of Indebetouw et al. The distributions of Class I young stellar objects (YSOs) implies that many of them are physically associated with the clouds, and were likely formed as a result of interaction between the clouds and SN winds. A determination of the spectral energy distributions (SEDs) of these stars, together with 2-D radiative transfer modelling of their continua is used to place constraints upon their properties.

Radiation-driven implosion in the Cepheus B molecular cloud

We analyze large scale mapping observations of the molecular lines in the 12CO (J = 2 – 1), 12CO (J = 3 – 2), 13CO (J = 2 – 1), and 13CO (J = 3 – 2) transition emissions toward the Cepheus B molecular cloud with the KOSMA 3m-telescope. The integrated intensity map of the 12CO (J = 2 – 1) transition has shown a structure with a compact core and a compact ridge extended to the north-west of the core. The cloud is surrounded by an optically bright rim, where the radiation-driven implosion (RDI) may greatly change the gas properties. The intensities of the CO (J = 3 – 2) transition are higher than those of the CO (J = 2 – 1) transition along the rim area. We find characteristic RDI structure in position-velocity diagrams. Non-LTE large velocity gradient (LVG) model analysis shows that the density and temperature at the edge are higher than that in the center. Our results provide evidences that an RDI is taking place in the Cepheus B molecular cloud.

TRIGGERED STAR FORMATION AND YOUNG STELLAR POPULATION IN BRIGHT-RIMMED CLOUD SFO 38

We have investigated the young stellar population in and around SFO 38, one of the massive globules located in the northern part of the Galactic HII region IC 1396, using the Spitzer IRAC and MIPS observations (3.6 to 24 micron) and followed up with ground based optical photometric and spectroscopic observations. Based on the IRAC and MIPS colors and H-alpha emission we identify ~45 Young Stellar Objects (Classes 0/I/II) and 13 probable Pre Main Sequence candidates. We derive the spectral types (mostly K- and M-type stars), effective temperatures and individual extinction of the relatively bright and optically visible Class II objects. Based on optical photometry and theoretical isochrones, we estimate the spread in stellar ages to be between 1--8 Myr with a median age of 3 Myr and a mass distribution of 0.3--2.2 Msun with a median value around 0.5 Msun. Using the width of the H-alpha emission line measured at 10% peak intensity, we derive the mass accretion rates of individual objects to be between 10^{-10} to 10^{-8} Msun/yr. From the continuum-subtracted H-alpha line image, we find that the H-alpha emission of the globule is not spatially symmetric with respect to the O type ionizing star HD 206267. We clearly detect an enhanced concentration of YSOs closer to the southern rim of SFO~38 and identify an evolutionary sequence of YSOs from the rim to the dense core of the cloud, with most of the Class II objects located at the bright rim. The YSOs appear to be aligned along two different directions towards the O6.5V type star HD 206267 and the B0V type star HD 206773. This is consistent with the Radiation Driven Implosion (RDI) model for triggered star formation. (Abridged)

AN INVESTIGATION OF THE TYPE M MORPHOLOGICAL STRUCTURE OF IC59: A NEW MODEL FOR BRIGHT RIM CLOUDS?

We report the results from a smoothed particle hydrodynamics simulation designed to model recent observational data on the nebula and Bright Rim Cloud IC59. We further examine, in the context of radiative-driven implosion (RDI) models, the possible formation mechanisms of the morphological structure of IC59. The results of the simulation reveal the existence of a new, fourth morphological state for Bright Rim Clouds (BRCs)—which we propose to call a Type M BRC morphology. We discuss the necessary conditions for the appearance of Type M BRCs, based on analytical and numerical simulations. The simulated physical properties from our model are consistent with the available observations of IC59. We further show that the prospect of RDI triggered star formation in all Type M BRCs is not supported by the simulations.