On the master equation approach to diffusive grain-surface chemistry: the H, O, CO system

We have used the master equation approach to study a moderately complex network of diffusive reactions occurring on the surfaces of interstellar dust particles. This network is meant to apply to dense clouds in which a large portion of the gas-phase carbon has already been converted to carbon monoxide. Hydrogen atoms, oxygen atoms, and CO molecules are allowed to accrete onto dust particles and their chemistry is followed. The stable molecules produced are oxygen, hydrogen, water, carbon dioxide (CO2), formaldehyde (H2CO), and methanol (CH3OH). The surface abundances calculated via the master equation approach are in good agreement with those obtained via a Monte Carlo method but can differ considerably from those obtained with standard rate equations.Comment: 13 pages, 5 figure

The Enigmatic Young Object: Walker 90/V590 Monocerotis

We assess the evolutionary status of the intriguing object Walker 90/V590 Mon, which is located about 20 arcminutes northwest of the Cone Nebula near the center of the open cluster NGC 2264. This object, according to its most recent optical spectral type determination (B7), which we confirmed, is at least 3 magnitudes too faint in V for the cluster distance, but it shows the classical signs of a young pre-main sequence object, such as highly variable Halpha emission, Mg II emission, IR excess, UV continuum, and optical variability. We analyzed a collection of archival and original data on Walker 90, covering 45 years including photometry, imaging, and spectroscopic data ranging from ultraviolet to near-infrared wavelengths. According to star formation processes, it is expected that, as this object clears its primordial surroundings, it should become optically brighter, show a weakening of its IR excess and present decreasing line emissions. This behavior is supported by our observations and analysis, but timescales are expected to be longer than the one observed here. Based on photometric data secured in 2007, we find Walker 90 at its brightest recorded optical magnitude. We document an evolution in spectral type over the past five decades (from A2/A3 to currently B7 and as early as B4), along with a decrease in the near-infrared K fluxes. From near-infrared images secured in 2004, Walker 90 appears as a point source placing an upper limit of 0.1 arcsec for its diameter. We conclude that many observational features are explained if W90 is a flared disk system, surrounded by an inclined optically thick accretion disk.Comment: 13 pages, 19 figure

Effects of Initial Condition and Cloud Density on the Composition of the Grain Mantle

Evolution of grain mantles in various interstellar environment is studied. We concentrate mainly on water, methanol, carbon di-oxide, which constitute nearly 90% of the grain mantle. We investigate how the production rates of these molecules depend on the relative gas phase abundances of oxygen and carbon monoxide and constrain the relevant parameter space which reproduces these molecules closed to the observed abundances. Allowing to accrete only H, O and CO on the grains and using the Monte-Carlo method we follow the chemical processes for a few million years. We allow formation of multi-layers on the grains and incorporate the freeze-out effects of accreting O and CO. We find that the formation of these molecules depends on the initial conditions as well as the average cloud density. Specifically, when the number density of accreting O is less than 3 times more than that of CO, methanol is always over-produced. Using available reaction pathways it appears to be difficult to match the exact observed abundances of all the three molecules simultaneously. Only in a narrow region of parameter space all these three molecules are produced within the observed limit. In addition to this, we found that the incorporation of the freeze-outs of O and CO leads to almost steady state on the grain surface. The mantle thickness grows anywhere between 60 to 500 layers in a period of two million years. In addition, we consider a case where the gas number density changes with time due to gradual collapse of the molecular cloud and present the evolution of composition of different species as a function of radius of the collapsing cloud.Comment: 30 pages, 9 figure

Ab Initio Calculation of Crystalline Electric Fields and Kondo Temperatures in Ce-Compounds

We have calculated the band-$f$ hybridizations for Ce$_x$La$_{1-x}$M$_3$ compounds ($x=1$ and $x\rightarrow 0$; M=Pb, In, Sn, Pd) within the local density approximation and fed this into a non-crossing approximation for the Anderson impurity model applied to both dilute and concentrated limits. Our calculations produce crystalline electric field splittings and Kondo temperatures with trends in good agreement with experiment and demonstrate the need for detailed electronic structure information on hybridization to describe the diverse behaviors of these Ce compounds.Comment: 13 pages(RevTeX), 3 Postscript figure

Bruguiera Species in Hawai'i: Systematic Considerations and Ecological Implications

At least two mangrove tree species in the genus Bruguiera were introduced into Hawai'i from the Philippines in 1922. The two are described in the most current manual on the flora of Hawai'i as B. gymnorrhiza (L.) Lamk. and B. parviflora (Roxb.) W. & A. ex. Griff. There has, however, been some confusion since its introduction as to the identity of what is currently known as B. gymnorrhiza. Early Hawaiian flora manuals (1948 and earlier) and ecological research reports up until at least 1972 referred to the species as B. sexangula (Lour.) Poir. Flora manuals published after 1948 and recent ecological papers describe the species as B. gymnorrhiza. The reason for the change appears to have been based strictly on an assessment of flower color. In this study we collected specimens of Bruguiera from Hawai'i and known samples of B. sexangula, B. gymnorrhiza, and B. exaristata C. G. Rogers from Australia or Micronesia. Based on a multivariate comparison of flower and hypocotyl morphology of this material, an assessment of other diagnostic attributes, and amplified fragment length polymorphism (AFLP) mapping, we conclude that the primary, and perhaps only, Bruguiera species present in Hawai'i is B. sexangula. We argue that the current distribution of Bruguiera in Hawai'i fits the pattern that might be expected of B. sexangula, which is less salt tolerant than B. gymnorrhiza. We also conclude that sufficient regional variation occurs to warrant morphological and genetic comparisons of the three species across their whole geographic range

The Initial Mass Function and Disk Frequency of the Rho Ophiuchi Cloud: An Extinction-Limited Sample

We have completed an optical spectroscopic survey of an unbiased, extinction-limited sample of candidate young stars covering 1.3 square degrees of the Rho Ophiuchi star forming region. While infrared, X-ray, and optical surveys of the cloud have identified many young stellar objects (YSOs), these surveys are biased towards particular stages of stellar evolution and are not optimal for studies of the disk frequency and initial mass function.We have obtained over 300 optical spectra to help identify 135 association members based on the presence of H-alpha in emission, lithium absorption, X-ray emission, a mid-infrared excess, a common proper motion, reflection nebulosity, and/or extinction considerations. Spectral types along with R and I band photometry were used to derive effective temperatures and bolometric luminosities for association members to compare with theoretical tracks and isochrones for pre-main-sequence stars. An average age of 3.1 Myr is derived for this population which is intermediate between that of objects embedded in the cloud core of Rho Ophiuchi and low mass stars in the Upper Scorpius subgroup. Consistent with this age we find a circumstellar disk frequency of 27% plus or minus 5%. We also constructed an initial mass function for an extinction-limited sample of 123 YSOs (A_v less than or equal to 8 mag), which is consistent with the field star initial mass function for YSOs with masses > 0.2 M_sun. There may be a deficit of brown dwarfs but this result relies on completeness corrections and requires confirmation.Comment: 46 pages, 7 figures, 4 table

Topological Defects in Nematic Droplets of Hard Spherocylinders

Using computer simulations we investigate the microscopic structure of the singular director field within a nematic droplet. As a theoretical model for nematic liquid crystals we take hard spherocylinders. To induce an overall topological charge, the particles are either confined to a two-dimensional circular cavity with homeotropic boundary or to the surface of a three-dimensional sphere. Both systems exhibit half-integer topological point defects. The isotropic defect core has a radius of the order of one particle length and is surrounded by free-standing density oscillations. The effective interaction between two defects is investigated. All results should be experimentally observable in thin sheets of colloidal liquid crystals.Comment: 13 pages, 16 figures, Phys. Rev.

Near-Infrared Imaging Polarimetry of the Serpens Cloud Core: Magnetic Field Structure, Outflows, and Inflows in A Cluster Forming Clump

We made deep NIR imaging polarimetry toward the Serpens cloud core. The polarization vector maps enable us to newly detect 24 small IR reflection nebulae with YSOs. Polarization measurements of NIR point sources indicate an hourglass-shaped magnetic field, of which symmetry axis is nearly perpendicular to the elongation of the C18O (J=1-0) or submillimeter continuum emission. The bright part of C18O (J=1-0), submillimeter continuum cores as well as many class 0/I objects are located just toward the constriction region of the hourglass-shaped magnetic field. Applying the CF method, the magnetic field strength was estimated to be ~100 muG, suggesting that the ambient region of the Serpens cloud core is moderately magnetically supercritical. These suggest that the Serpens cloud core first contracted along the magnetic field to be an elongated cloud, which is perpendicular to the magnetic field, and that then the central part contracted cross the magnetic field due to the high density in the central region of the cloud core, where star formation is actively continuing. Comparison of this magnetic field with the previous observations of molecular gas and large-scale outflows suggests a possibility that the cloud dynamics is controlled by the magnetic field, protostellar outflows and gravitational inflows. This appears to be in good agreement with the outflow-driven turbulence model and implies the importance of the magnetic field to continuous star formation in the center region of the cluster forming region.Comment: 36 pages, 11 figures, 2 tables, accepted for publication in the Astrophysical Journa

Comparative study of CH+ and SH+ absorption lines observed towards distant star-forming regions

Aims. The HIFI instrument onboard Herschel has allowed high spectral resolution and sensitive observations of ground-state transi- tions of three molecular ions: the methylidyne cation CH+, its isotopologue 13CH+, and sulfanylium SH+. Because of their unique chemical properties, a comparative analysis of these cations provides essential clues to the link between the chemistry and dynamics of the diffuse interstellar medium. Methods. The CH+, 13CH+, and SH+ lines are observed in absorption towards the distant high-mass star-forming regions (SFRs) DR21(OH), G34.3+0.1, W31C, W33A, W49N, and W51, and towards two sources close to the Galactic centre, SgrB2(N) and SgrA*+50. All sight lines sample the diffuse interstellar matter along pathlengths of several kiloparsecs across the Galactic Plane. In order to compare the velocity structure of each species, the observed line profiles were deconvolved from the hyperfine structure of the SH+ transition and the CH+, 13CH+, and SH+ spectra were independently decomposed into Gaussian velocity components. To analyse the chemical composition of the foreground gas, all spectra were divided, in a second step, into velocity intervals over which the CH+, 13CH+, and SH+ column densities and abundances were derived. Results. SH+ is detected along all observed lines of sight, with a velocity structure close to that of CH+ and 13CH+. The linewidth distributions of the CH+, SH+, and 13CH+ Gaussian components are found to be similar. These distributions have the same mean ( ~ 4.2 km s-1) and standard deviation (\sigma(\delta\u{psion}) ~ 1.5 km s-1). This mean value is also close to that of the linewidth distribution of the CH+ visible transitions detected in the solar neighbourhood. We show that the lack of absorption components narrower than 2 km s-1 is not an artefact caused by noise: the CH+, 13CH+, and SH+ line profiles are therefore statistically broader than those of most species detected in absorption in diffuse interstellar gas (e. g. HCO+, CH, or CN). The SH+/CH+ column density ratio observed in the components located away from the Galactic centre spans two orders of magnitude and correlates with the CH+ abundance. Conversely, the ratio observed in the components close to the Galactic centre varies over less than one order of magnitude with no apparent correlation with the CH+ abundance. The observed dynamical and chemical properties of SH+ and CH+ are proposed to trace the ubiquitous process of turbulent dissipation, in shocks or shears, in the diffuse ISM and the specific environment of the Galactic centre regions

On the lifetime of discs around late type stars

We address the question of whether protoplanetary discs around low mass stars (e.g. M-dwarfs) may be longer lived than their solar-type counterparts. This question is particularly relevant to assess the planet-making potential of these lower mass discs. Given the uncertainties inherent to age-dating young stars, we propose an alternative approach that is to analyse the spatial distribution of disc-bearing low-mass stars and compare it to that of disc-bearing solar-type stars in the same cluster. A significant age difference between the two populations should be reflected in their average nearest neighbour distance (normalised to the number of sources), where the older population should appear more spread out. To this aim, we perform a Minimum Spanning Tree (MST) analysis on the spatial distribution of disc-bearing young stellar objects (YSOs) in six nearby low mass star forming regions. We find no evidence for significant age differences between the disc-bearing low-mass (later than M2) and 'solar-type' (earlier than M2) stars in these regions. We model our results by constructing and analysing synthetic fractal distributions that we evolve for typical values of the velocity dispersions. A comparison of simple models to our MST analysis suggests that the lifetime of discs around M-stars is similar to that of discs around solar-type stars. Furthermore, a model-independent spatial analysis of the observations robustly shows that any age differences between the two samples must be smaller than the average age difference between disc-bearing classical T-Tauri stars and disc-less Weak-Lined T-Tauri stars.Comment: 8 pages, 3 Figures, 3Tables; Accepted MNRA