Center Of Clumps Research Articles

Neutron star mass in dark matter clumps

ABSTRACT This paper investigates a hypothesis proposed in previous research relating neutron star (NS) mass and its dark matter (DM) accumulation. As DM accumulates, NS mass decreases, predicting lower NS masses toward the Galactic centre. Due to limited NSs data near the Galactic centre, we examine NSs located within DM clumps. Using the CLUMPY code simulations, we determine the DM clumps distribution, with masses from 10 to 108 M⊙ and scales from 10−3 to 10 kpc. These clumps’ DM exhibit a peak at the centre, tapering toward the outskirts, resembling our Galaxy’s DM distribution. We analyse these DM clumps’ NS mass variations, considering diverse DM particle masses and galaxy types. We find relatively stable NS mass within 0.01 – 5 kpc from the clump centre. This stability supports the initial hypothesis, particularly for NSs located beyond 0.01 kpc from the clump centre, where NS mass reaches a plateau around 0.1 kpc. Nevertheless, NS mass near the clump’s periphery reveals spatial dependence: NS position within DM clumps influences its mass in Milky Way-type galaxies. Moreover, this dependence varies with the DM model considered. In summary, our study investigates the proposed link between NS mass and DM accumulation by examining NSs within DM clumps. While NS mass remains stable at certain distances from the clump centre, spatial dependencies arise near the clump’s outer regions, contingent on the specific DM model.

FacetClumps: A Facet-based Molecular Clump Detection Algorithm

A comprehensive understanding of molecular clumps is essential for investigating star formation. We present an algorithm for molecular clump detection, called FacetClumps. This algorithm uses a morphological approach to extract signal regions from the original data. The Gaussian facet model is employed to fit the signal regions, which enhances the resistance to noise and the stability of the algorithm in diverse overlapping areas. The introduction of the extremum determination theorem of multivariate functions offers theoretical guidance for automatically locating clump centers. To guarantee that each clump is continuous, the signal regions are segmented into local regions based on gradient, and then the local regions are clustered into the clump centers based on connectivity and minimum distance to identify the regional information of each clump. The experiments conducted with both simulated and synthetic data demonstrate that FacetClumps exhibits great recall and precision rates, small location error and flux loss, and a high consistency between the region of detected clump and that of simulated clump, and the experiments demonstrate that FacetClumps is generally stable in various environments. Notably, the recall rate of FacetClumps in the synthetic data, which comprises 13CO (J = 1−0) emission line of the MWISP within 11.°7 ≤ l ≤ 13.°4, 0.°22 ≤ b ≤ 1.°05, and 5 km s−1 ≤ v ≤ 35 km s−1 and simulated clumps, reaches 90.2%. Additionally, FacetClumps demonstrates satisfactory performance when applied to observational data.

A New Parameterization of the Star Formation Rate Dense Gas Mass Relation: Embracing Gas Density Gradients

Abstract It is well established that a gas density gradient inside molecular clouds and clumps raises their star formation rate (SFR) compared with what they would experience from a gas reservoir of uniform density. This effect should be observed in the relation between dense-gas mass M dg and SFR of molecular clouds and clumps, with steeper gas density gradients yielding higher SFR/M dg ratios. The content of this paper is twofold. First, we build on the notion of a magnification factor introduced by Parmentier to redefine the dense-gas relation (i.e., the relation between M dg and SFR). Not only does the SFR/M dg ratio depend on the mean freefall time of the gas and on its (intrinsic) star formation efficiency per freefall time, but it also depends on the clump density profile (e.g., the logarithmic slope of the gas density profile and on the relative extent of the constant-density region at the clump center). Second, we show that nearby molecular clouds follow the newly defined dense-gas relation, provided that their dense-gas mass is defined based on a volume density criterion. We also find the same trend for the dense molecular clouds of the Central Molecular Zone (CMZ) of the Galaxy, although this one is scaled down by a factor of 10 compared with nearby clouds, even when including the Sgr C and Sgr B2 clouds. The respective locii of both nearby and CMZ clouds in the parameter space is discussed.

Molecular Clumps Disguising Their Star Formation Efficiency per Free-fall Time: What We Can Do Not to Be Fooled

Abstract The presence of a volume-density gradient in molecular clumps allows them to raise their star formation rate compared with what they would experience if their gas were uniform in density. This higher value for the star formation rate yields in turn a higher star formation efficiency per free-fall time that we measure. The measured star formation efficiency per free-fall time, , of clumps is therefore plagued by a degeneracy, as two factors contribute to it: one is the density gradient of the clump gas, the other is the intrinsic star formation efficiency per free-fall time, , with which the clump would form stars should there be no gas-density gradient. This paper presents a method allowing one to recover the intrinsic efficiency of a centrally concentrated clump. It hinges on the relation between the surface densities in stars and gas measured locally from clump center to clump edge. Knowledge of the initial density profile of the clump gas is not required. A step-by-step description of the method is provided as a tool in hand for observers. Once has been estimated, it can be compared with its measured, clump-averaged, counterpart, , to quantify the impact that the initial gas-density profile of a clump has had on its star formation history.

Gravitoviscous protoplanetary disks with a dust component

Aims. Spatial distribution and growth of dust in a clumpy protoplanetary disk subject to vigorous gravitational instability and fragmentation is studied numerically with sub-au resolution using the FEOSAD code. Methods. Hydrodynamics equations describing the evolution of self-gravitating and viscous protoplanetary disks in the thin-disk limit were modified to include a dust component consisting of two parts: sub-micron-sized dust and grown dust with a variable maximum radius. The conversion of small to grown dust, dust growth, friction of dust with gas, and dust self-gravity were also considered. Results. We found that the disk appearance is notably time-variable with spiral arms, dusty rings, and clumps, constantly forming, evolving, and decaying. As a consequence, the total dust-to-gas mass ratio is highly non-homogeneous throughout the disk extent, showing order-of-magnitude local deviations from the canonical 1:100 value. Gravitationally bound clumps formed through gravitational fragmentation have a velocity pattern that deviates notably from the Keplerian rotation. Small dust is efficiently converted into grown dust in the clump interiors, reaching a maximum radius of several decimeters. Concurrently, grown dust drifts towards the clump center forming a massive compact central condensation (70–100 M⊕). We argue that protoplanets may form in the interiors of inward-migrating clumps before they disperse through the action of tidal torques. We foresee the formation of protoplanets at orbital distances of several tens of au with initial masses of gas and dust in the protoplanetary seed in the (0.25–1.6) MJup and (1.0–5.5) M⊕ limits, respectively. The final masses of gas and dust in the protoplanets may however be much higher due to accretion from surrounding massive metal-rich disks/envelopes. Conclusions. Dusty rings formed through tidal dispersal of inward-migrating clumps may have a connection to ring-like structures found in youngest and massive protoplanetary disks. Numerical disk models with a dust component that can follow the evolution of gravitationally bound clumps through their collapse phase to the formation of protoplanets are needed to make firm conclusions on the characteristics of planets forming through gravitational fragmentation.

Fixed and variable components of evapotranspiration in a Mediterranean wild-olive - grass landscape mosaic

Dry regions are typically characterized by heterogeneous ecosystems where trees are competing with the surrounding grasses for limited amount of water. In these regions, evapotranspiration (ET) is the leading loss term in the soil water budget, and its estimate, and the dynamic contribution of each ET component (i.e. tree and grass transpiration, and dry bare soil and wet surface evaporation), are still poorly quantified. In a typical heterogeneous Mediterranean ecosystem in Sardinia, we combined eddy-covariance estimates of ET with sap flux and energy balance estimates of wild-olive tree transpiration, a common tree species of the region, and with modeled evapotranspiration from the seasonal grass. Trees located in the southern edge of clumps, thus receiving more radiation, transpired more and showed a greater sensitivity to increasing vapor pressure deficit and soil moisture than trees located in clump centers or northern edges. Transpiration of the tree clumps in the footprint (Et), summed up with the modeled evapotranspiration components of the surrounding grass (mostly transpiration during the wet season and evaporation during the dry season), matched latent heat flux measurements, lending confidence in the estimates. Proper accounting for heterogeneity of sources within the eddy covariance footprint seems to have overcome potential errors from not preserving an important assumption of the method, the land-surface homogeneity, highlighting the methods reliability in such inhomogeneous ecosystem. Compared to ET, Et of wild olives was nearly constant over the hydrologic year, insensitive to variation in soil moisture and atmospheric conditions. In contrast, under favorable spring environmental conditions (radiation, vapor pressure deficit, and soil moisture), the pasture leaf area transpires at high rates, contributing to, and dominating the high ET during that season. Conversely, in dry periods, when evapotranspiration from the grass cover is dominated by low evaporation from the, principally, bare soil, Et dominants ecosystem ET.

Compressed Magnetic Field in the Magnetically Regulated Global Collapsing Clump of G9.62+0.19

Abstract How stellar feedback from high-mass stars (e.g., H ii regions) influences the surrounding interstellar medium and regulates new star formation is still unclear. To address this question, we observed the G9.62+0.19 complex in 850 μm continuum with the James Clerk Maxwell Telescope/POL-2 polarimeter. An ordered magnetic field has been discovered in its youngest clump, the G9.62 clump. The magnetic field strength is determined to be ∼1 mG. Magnetic field plays a larger role than turbulence in supporting the clump. However, the G9.62 clump is still unstable against gravitational collapse even if thermal, turbulent, and magnetic field support are taken into account together. The magnetic field segments in the outskirts of the G9.62 clump seem to point toward the clump center, resembling a dragged-in morphology, indicating that the clump is likely undergoing magnetically regulated global collapse. However, the magnetic field in its central region is aligned with the shells of the photodissociation regions and is approximately parallel to the ionization (or shock) front, indicating that the magnetic field therein is likely compressed by the expanding H ii regions that formed in the same complex.

A Statistical Study of Massive Cluster-forming Clumps

Abstract We report results of the observations of 15 regions in several molecular lines for a statistical study of massive cluster-forming clumps. We identified 24 clumps based on the C18O (J = 1–0) data obtained by the NRO 45 m telescope, and found that 16 of them are associated with young clusters. The clumps associated with clusters have a typical mass, radius, and molecular density of ∼1 × 103 M ☉, ∼0.5 pc, ∼1 × 105 cm−3, respectively. We categorized the clumps and clusters into four types according to the spatial coincidence of gas and star density, and discussed their evolutions: Clumps without clusters (Type 1), clumps showing good correlations with clusters (Type 2), clumps showing poor correlations with clusters (Type 3), and clusters with no associated clumps (Type 4). Analyses of the velocity structures and the chemical compositions imply that the clump + cluster systems should evolve from Type 1 to Type 4. We found that some of the Type 2 clumps are infalling on the clump-scale to form clusters at the clump center, which should commonly occur in the beginning of cluster formation. Interestingly, all of the identified Type 1 clumps are likely to be older than the Type 2 clumps in terms of chemical compositions, suggesting that they have been gravitationally stable for a long time, possibly being supported by the strong magnetic field of ≳1 mG. Type 1 clumps younger than the observed Type 2 clumps should be very rare to find because of their short lifetime.

DISCOVERY OF INFALLING MOTION WITH ROTATION OF THE CLUSTER-FORMING CLUMP S235AB AND ITS IMPLICATION TO THE CLUMP STRUCTURES

ABSTRACT We report the discovery of infalling motion with the rotation of S235AB, a massive cluster-forming clump ( ) in the S235 region. Our C18O observations with the 45 m telescope at the Nobeyama Radio Observatory have revealed an elliptical shape of the clump. A position–velocity diagram taken along its major axis exhibits two well-defined peaks symmetrically located, with respect to the clump center. This is similar to that found for a dynamically infalling envelope with rotation around a single protostar, modeled by N. Ohashi et al., indicating that the cluster-forming clump is also collapsing by the self-gravity toward the clump center. With analogue to Ohashi et al.'s model, we made a simple model of an infalling, rotating clump to fit the observed data. Based on the inferred model parameters, as well as results of earlier observations and simulations in the literature, we discuss the structures of the clump such as the relation among the global mass infall rate ( yr−1), formation of a compact core (with a mass and size of ∼4 and pc) at the center, and a massive star (∼11 ) forming in the core.

WEAK AND COMPACT RADIO EMISSION IN EARLY HIGH-MASS STAR-FORMING REGIONS. I. VLA OBSERVATIONS

ABSTRACT We present a high-sensitivity radio continuum survey at 6 and 1.3 cm using the Karl G. Jansky Very Large Array toward a sample of 58 high-mass star-forming regions. Our sample was chosen from dust clumps within infrared dark clouds with and without IR sources (CMC–IRs and CMCs, respectively), and hot molecular cores (HMCs), with no previous, or relatively weak radio continuum detection at the 1 mJy level. Due to the improvement in the continuum sensitivity of the Very Large Array, this survey achieved map rms levels of ∼3–10 μJy beam−1 at sub-arcsecond angular resolution. We extracted 70 continuum sources associated with 1.2 mm dust clumps. Most sources are weak, compact, and prime candidates for high-mass protostars. Detection rates of radio sources associated with the millimeter dust clumps for CMCs, CMC–IRs, and HMCs are 6%, 53%, and 100%, respectively. This result is consistent with increasing high-mass star formation activity from CMCs to HMCs. The radio sources located within HMCs and CMC–IRs occur close to the dust clump centers, with a median offset from it of 12,000 au and 4000 au, respectively. We calculated 5–25 GHz spectral indices using power-law fits and obtained a median value of 0.5 (i.e., flux increasing with frequency), suggestive of thermal emission from ionized jets. In this paper we describe the sample, observations, and detections. The analysis and discussion will be presented in Paper II.

Kinematic and thermal structure at the onset of high-mass star formation

We want to understand the kinematic and thermal properties of young massive gas clumps prior to and at the earliest evolutionary stages of high-mass star formation. Do we find signatures of gravitational collapse? Do we find temperature gradients in the vicinity or absence of infrared emission sources? Do we find coherent velocity structures toward the center of the dense and cold gas clumps? To determine kinematics and gas temperatures, we used ammonia, because it is known to be a good tracer and thermometer of dense gas. We observed the NH$_3$(1,1) and (2,2) lines within seven very young high-mass star-forming regions with the VLA and the Effelsberg 100m telescope. This allows us to study velocity structures, linewidths, and gas temperatures at high spatial resolution of 3-5$"$, corresponding to $\sim$0.05 pc. We find on average cold gas clumps with temperatures in the range between 10 K and 30 K. The observations do not reveal a clear correlation between infrared emission peaks and ammonia temperature peaks. We report an upper limit for the linewidth of $\sim$1.3 km s$^{-1}$, at the spectral resolution limit of our VLA observation. This indicates a relatively low level of turbulence on the scale of the observations. Velocity gradients are present in almost all regions with typical velocity differences of 1 to 2 km s$^{-1}$ and gradients of 5 to 10 km s$^{-1}$ pc$^{-1}$. These velocity gradients are smooth in most cases, but there is one exceptional source (ISOSS23053), for which we find several velocity components with a steep velocity gradient toward the clump centers that is larger than 30 km s$^{-1}$ pc$^{-1}$. This steep velocity gradient is consistent with recent models of cloud collapse. Furthermore, we report a spatial correlation of ammonia and cold dust, but we also find decreasing ammonia emission close to infrared emission sources.

SUBMILLIMETER ARRAY HIGH-ANGULAR RESOLUTION OBSERVATIONS OF THE MONOCEROS R2 STAR-FORMING CLUSTER

We present the first high-angular resolution study of the MonR2 star-forming complex carried out with the Submillimeter Array at (sub-)millimeter wavelengths. We image the continuum and molecular line emission toward the young stellar objects in MonR2 at 0.85mm and 1.3mm, with resolutions ranging from 0.5" to ~3". While free-free emission dominates the IRS1 and IRS2 continuum, dust thermal emission prevails for IRS3 and IRS5, giving envelope masses of ~0.1-0.3 M_Sun. IRS5 splits into at least two sub-arcsecond scale sources, IRS5B and the more massive IRS5A. Our 12CO(2-1) images reveal 11 previously unknown molecular outflows in the MonR2 clump. Comparing these outflows with known IR sources in the IRS5 and IRS3 subclusters allows for tentative identification of driving stars. Line images of molecular species such as CH3CN or CH3OH show that, besides IRS3 (a well-known hot molecular core), IRS5 is also a chemically active source in the region. The gas excitation temperature derived from CH3CN lines toward IRS5 is 144 \pm 15 K, indicating a deeply embedded protostar at the hot-core evolutionary stage. SED fitting of IRS5 gives a mass of ~7 M_Sun and a luminosity of 300 L_Sun for the central source. The derived physical properties of the CO outflows suggest that they contribute to the turbulent support of the MonR2 complex and to the gas velocity dispersion in the clump's center. The detection of a large number of CO outflows widespread across the region supports the competitive accretion scenario as origin of the MonR2 star cluster.

DESICCATION PROTECTION AND DISRUPTION: A TRADE-OFF FOR AN INTERTIDAL MARINE ALGA(1).

For marine algae, the benefits of drying out are often overshadowed by the stresses involved. Here we used laboratory and field experiments to examine both the costs and benefits of desiccation in the intertidal turf alga Endocladia muricata (Endlichter) J. Agardh. Laboratory experiments showed that when Endocladia is dry, photosynthesis stops, but thermotolerance increases to the point that the alga is protected from heat-induced mortality. Drying rates measured in a wind tunnel, combined with tidal data and measured wave splash, indicate that a substantial fraction of the year is spent "drying out" (∼30% of the total time available for photosynthesis). During these periods, the rate of drying determines how much time is spent hydrated and potentially engaged in photosynthesis, but also vulnerable to high temperatures. Turf algae such as Endocladia dry from the edge of a clump inward. Consequently, the clump center remains hydrated longer than the clump edge. The resulting regionalization of a clump results in notable patterns of frond mortality ("fairy rings," and zoned patterns of frond bleaching) within the Endocladia zone.

Observations on Changes in Soil Properties in Grazed and Nongrazed Areas of Alxa Desert Steppe, Inner Mongolia

Soil chemical properties in different locations around and away from the perennial shrub Zygophyllum xanthoxylum in a west Inner-Mongolian desert of China were investigated in a freely grazed area (FG) and areas excluded from grazing for two (2EX) and six years (6EX). The results showed that concentrations of soil organic carbon (SOC) and total N (Nt) were highest at the clump center, followed by the canopy periphery, and lowest in the intershrub space in both grazed and non-grazed areas. Accumulation of SOC and Nt in the clump center were significantly greater in nongrazed compared to grazed areas. The nutrient enrichment ratios under the shrub canopy in 2EX and 6EX were greater than that in the grazed area, especially in the 0–10 cm soil. Soils under the shrub canopy had a lower pH as compared to soils in the intershrub space, especially in the 0–10 cm soil. Shrub cover, biomass, and the mound height around shrub crowns were higher in the nongrazed than the grazed area. Significant correlation between soil chemical concentration and shrub biomass suggests that fertile islands rely on the stability of the shrub communities. Grazing exclusion favors further development of fertile islands around the shrub, but did not improve the soil nutrient status in the intershrub space. Although shrub invasion into the grassland is generally considered as a desertification process, preservation of shrubs from overgrazing is of vital importance in preventing the rangeland from further desertification.

IDENTIFYING AGGREGATION AND ASSOCIATION IN FULLY MAPPED SPATIAL DATA

We describe a clump recognition process that may be used to analyze fully mapped spatial data. Any given spatial pattern can be made less aggregated by replacing the closest-together pair of plants by a single individual at their centroid position. By repeatedly amalgamating pairs of individuals in this way, an initially aggregated pattern can be reduced to one indistinguishable from complete spatial randomness (i.e. a two-dimensional Poisson pattern). The clump recognition process provides information on the size structure of aggregates within a population. Randomizing the position of clump centers can be used to generate patterns that have similar aggregation characteristic to the original pattern. This property is used to develop Monte Carlo simulations for testing interspecific associations. We also discuss tests of association that are based on measuring segregation between clump centers. We illustrate the methods with a series of patterns from (1) simple, stochastic processes, (2) a spatially explicit population model, and (3) a dune annual community.

The NGC 281 west cluster. I. Star formation in photoevaporating clumps.

view Abstract Citations (34) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The NGC 281 west cluster. I. Star formation in photoevaporating clumps. Megeath, S. T. ; Wilson, T. L. Abstract The NGC281 West molecular cloud is an excellent test case for studying star formation in the clumpy interface between a \hii region and a giant molecular cloud. We present here a study based on new high resolution radio and near-infrared data. Using the IRAM 30-meter telescope, we have mapped the interface in the \cotwo, \coone, and \cs transitions with FWHP beamwidths <= 22''. We have imaged the same region with the VLA in the 20, 6 and 2 cm continuum bands to obtain complementary maps of the ionized gas distribution with angular resolutions <= 13''. In addition, we have obtained near-infrared J and K'-band images to detect young stars in the interface. The 30-meter data shows the molecular gas is concentrated into three clumps with masses of 570, > 210, and 300 \msun and average volume densities of 1.4, >1, and 2 x 10(4) \cm. We detect \cs emission in two of the clumps, indicating peak densities in excess of 5x 10(5) \cm are attained in the clumps. A comparison of the \co line data with the 20 cm continuum image suggests that the molecular clumps are being photoevaporated through their direct exposure to the UV radiation from neighboring OB stars. The luminosity and extent of the observed 20 cm emission is in good agreement with theoretical predictions. We use models of photoevaporative flows to estimate the pressure exerted on the clumps by the ionized gas and find that it exceeds the internal, turbulent pressure of the clumps by a factor of a 2.5. Although a pressure equilibrium is not excluded given the uncertainties inherent in determining the pressures of the ionized and molecular gases, our best estimates of the clumps and flow parameters favor the the existence of low velocity shocks (1.5 \kms) in the clumps. The clumps exhibit broad, non-gaussian lineshapes and complex kinematical structures suggestive of shocks. Further evidence for shocks is found in a comparison of position-velocity diagrams with published numerical simulations of imploding spherical clumps. We discuss the possibility that the knots of \cs emission may trace gas compressed by converging shock waves. The K'-band observations show a rich cluster of primarily low mass stars in the \hii/molecular interface, which we argue is divided into two distinct sub-clusters. We associate one sub-cluster with the two clumps nearest the OB stars, and the second sub-cluster with the third clump. The two clumps nearest the OB stars contain an embedded population, suggesting that star formation is ongoing. We discuss the impact photoevaporation is having on star formation in these two clumps. We find that photoevaporation is dispersing the molecular gas from which the cluster is forming and estimate that the molecular gas will be completely evaporated in 5 Myr. Deep K'-band imaging of the two clumps show that the stars are detected primarily on the sides of the clumps facing the OB stars and in the adjoining \hii region. We examine three explanations for this asymmetry: displacement of the clump centers from the cluster center by the acceleration of the molecular gas through photoevaporation (i.e. the rocket effect), unveiling of young, embedded stars by ionization-shock fronts, and the triggered formation of stars by shocks advancing into the clumps. If shock compression is indeed ongoing in the clumps, then we argue that there is a good case for shock triggered star formation. Publication: The Astronomical Journal Pub Date: September 1997 DOI: 10.1086/118541 Bibcode: 1997AJ....114.1106M Keywords: ISM: INDIVIDUAL: NGC 281; ISM: CLOUDS; ISM: STRUCTURE; STARS: FORMATION full text sources ADS | data products SIMBAD (2)

Influence of changing grouse density and forest attributes on the occupancy of a series of potential territories by male ruffed grouse

The history of occupancy of 117 territories established by male ruffed grouse (Bonasa umbellus) on a 700-ha study area in southwestern Alberta was studied over 11 years in relation to changing density of the grouse and attributes of the surrounding forest. The pattern of use of territories varied from continuous occupancy by a series of different birds over as many as 11 years to sporadic occupancy for as little as one season. These two extremes differed in that continuously occupied territories possessed numerous acceptable display sites, were located towards the center of clumps of territories, and hosted long-lived birds. By contrast, sporadically occupied territories possessed few acceptable display sites, tended to be located at the periphery of clumped territories, and hosted short-lived birds. Although the status of the vegetation around the display sites within the territory was a major factor influencing its occupancy, social interactions among the grouse also appeared to influence the history of its use.

New Type of Soredium in the Lichen Family Heppiaceae

A new type of soredium-like granule is described from species of Peltula and compared with soredia. Scanning and transmission electron micros- copy and light microscopy of these new structures reveals that these unique granules lack the outer fungal covering which characterizes soredia but have fungal hyphae and haustoria in the center of clumps of algae. This type of granule is intermediate between hormocysts and soredia and probably represents the most primitive type of soredium. Peltula is the most primitive genus evolu- tionarily to produce soredia. A wider definition of soredia is given to include this primitive type. Soredia are now widely recognized as taxonomically significant structures of the lichen thallus and their production is probably genetically controlled (Hale, 1967). Within the classification of lichen diaspores as modified by Degelius (1945), soredia are one type of composite reproductive structure and contain both the fungus and the algae.

Promotion of conidia aggregation in aspergillus niger by cyclic AMP and 5′-GMP

Conidia of A. niger aggregated into clumps, and then germinated to form masses of mycelia (pellets) when grown in agitated, submerged, liquid cultures. Cyclic AMP and 5′-GMP promoted the aggregation of conidia, while adenosine, AMP, ADP, dibutyryl cyclic AMP, cyclic GMP, guanosine, GDP, GTP and theophylline were ineffective. ATP and 2′, 3′-cyclic AMP elicited a weak response. Conidia in the center of clumps did not germinate. The increase in “adhesiveness” of conidia by cyclic AMP and GMP may have implications in the regulation of germination, as well as the eventual growth and development of hyphae.

Spacing and Competition in Natural Stands of Sand Pine

Spacing studies of many even—aged, pure stands of sand pine of various age classes using nearest neighbor distances show that most stands under 23 years old are either aggregated or essentially randomly distributed. Most stands 23 or more years old show significant to highly significant departures toward regular spacing. Information regarding the severity and duration of maximal intraspecific competition between the trees was obtained by measuring nearest neighbor distances to all pines, dead or alive. departure towards more even spacing was shown in nearly every stand when only live trees of the same quadrat were considered. Only stands showing maximal intraspecific competition–those showing a considerable number of dead trees–were studied. In an aggregate stand which showed marked trunk divergence from the center of clumps, distances were measured in the same sample both at breast height and at ground level. The difference in apparent spacing proved highly significant–the breast height measurements being essentially random, the ground level ones highly aggregate. A marked thinning of the understory herbs and shrubs appeared correlated with longer maximal intraspecific competition in one portion of a stand than in another part of the same stand with a dense understory. Correlations between the sum of the squared diameters of nearest neighbors and the distance between them proved positive at the 1% level in three stands, positive at the 5% level in three, and so weakly positive in three as to have little significance. Correlation coefficients were only weakly consistent with stand age, the older stands tending to have the higher correlations.