inter-Cloud Region Research Articles

Decoding the Nature of Coherent Radio Emission in Pulsars I: Observational Constraints

Radio observations from normal pulsars indicate that the coherent radio emission is excited by curvature radiation from charge bunches. In this review, we provide a systematic description of the various observational constraints on the radio emission mechanism. We have discussed the presence of highly polarized time samples where the polarization position angle follow two orthogonal well-defined tracks across the profile that closely match the rotating vector model in an identical manner. The observations also show the presence of circular polarization, with both the right and left handed circular polarization seen across the profile. Other constraints on the emission mechanism are provided by the detailed measurements of the spectral index variation across the profile window, where the central part of the profile, corresponding to the core component, has a steeper spectrum than the surrounding cones. Finally, the detailed measurements of the subpulse drifting behaviour can be explained by considering the presence of non-dipolar field on the stellar surface and the formation of the partially screened gap (PSG) above the polar cap region. The PSG gives rise to a non-stationary plasma flow that has a multi-component nature, consisting of highly energetic primary particles, secondary pair plasma, and iron ions discharged from the surface, with large fragmentation resulting in dense plasma clouds and lower-density inter-cloud regions. The physical properties of the outflowing plasma and the observational constraints lead us to consider coherent curvature radiation as the most viable explanation for the emission mechanism in normal pulsars, where propagation effects due to adiabatic walking and refraction are largely inconsequential.

Meterwavelength Single Pulse Polarimetric Emission Survey. VI. Toward Understanding the Phenomenon of Pulsar Polarization in Partially Screened Vacuum Gap Model

We have observed 123 pulsars with periods longer than 0.1 s in the Meterwavelength Single Pulse Polarimetric Emission Survey. In this work, a detailed study of the polarization behavior of these pulsars has been carried out. We were able to fit the rotating vector model to the polarization position angle sweeps in 68 pulsars, and in 34 pulsars the emission heights could be measured. In all cases the radio emission was constrained to arise below 10% of the light cylinder radius. In pulsars with low spin-down energy loss, erg s−1, we found the mean fractional linear polarization of the individual time samples in single pulses to be around 0.57 (57%), which is significantly larger than the fractional linear polarization of 0.29 (29%) obtained from the average profiles. On the other hand, the mean fractional circular polarization of the individual time samples in single pulses is around 0.08 (8%), similar to the measurements from the average profiles. To explain the observed polarization features, we invoke the partially screened vacuum gap model of pulsars, where dense spark-associated plasma clouds exist with high pair plasma multiplicity, with significant decrease of density in the regions between the clouds that are dominated by iron ions. The coherent radio emission is excited by curvature radiation from charge bunches in these dense plasma clouds and escape as linearly polarized waves near cloud boundaries. We suggest that the circular polarization arises due to propagation of waves in the low-pair-multiplicity, ion-dominated inter-cloud regions.

Substructures and Tidal Distortions in the Magellanic Stellar Periphery

Abstract We use a new panoramic imaging survey conducted with the Dark Energy Camera to map the stellar fringes of the Large and Small Magellanic Clouds (LMC/SMC) to extremely low surface brightness V ≈ 32 mag arcsec−2. Our results starkly illustrate the closely interacting nature of the LMC–SMC pair. We show that the outer LMC disk is strongly distorted, exhibiting an irregular shape, evidence for warping, and significant truncation on the side facing the SMC. Large diffuse stellar substructures are present both to the north and south of the LMC, and in the inter-Cloud region. At least one of these features appears as co-spatial with the bridge of RR Lyrae stars that connects the Clouds. The SMC is highly disturbed; we confirm the presence of tidal tails, as well as a large line-of-sight depth on the side closest to the LMC. Young, intermediate-age, and ancient stellar populations in the SMC exhibit strikingly different spatial distributions. In particular, those with ages ∼1.5–4 Gyr exhibit a spheroidal distribution with a centroid offset from that of the oldest stars by several degrees toward the LMC. We speculate that the gravitational influence of the LMC may already have been perturbing the gaseous component of the SMC several Gyr ago. With careful modeling, the variety of substructures and tidal distortions evident in the Magellanic periphery should tightly constrain the interaction history of the Clouds.

Structured star formation in the Magellanic inter-Cloud region

We use a new contiguous imaging survey conducted using the Dark Energy Camera to investigate the distribution and properties of young stellar populations in the Magellanic inter-Cloud region. These young stars are strongly spatially clustered, forming a narrow chain of low-mass associations that trace the densest HI gas in the Magellanic Bridge and extend, in projection, from the SMC to the outer disk of the LMC. The associations in our survey footprint have ages $\lesssim 30$ Myr, masses in the range $\sim 100-1200\,{\rm M}_\odot$, and very diffuse structures with half-light radii of up to $\sim 100$ pc. The two most populous are strongly elliptical, and aligned to $\approx 10^{{\rm o}}$ with the axis joining the centres of the LMC and SMC. These observations strongly suggest that the young inter-Cloud populations formed in situ, likely due to the compression of gas stripped during the most recent close LMC-SMC encounter. The associations lie at distances intermediate between the two Clouds, and we find no evidence for a substantial distance gradient across the imaged area. Finally, we identify a vast shell of young stars surrounding a central association, that is spatially coincident with a low column density bubble in the HI distribution. The properties of this structure are consistent with a scenario where stellar winds and supernova explosions from massive stars in the central cluster swept up the ambient gas into a shell, triggering a new burst of star formation. This is a prime location for studying stellar feedback in a relatively isolated environment.

GMC evolution in a barred spiral galaxy with star formation and thermal feedback

We explore the impact of star formation and thermal stellar feedback on the giant molecular cloud (GMC) population forming in a M83-type barred spiral galaxy. We compare three high-resolution simulations (1.5 pc cell size) with different star formation/feedback models: one with no star formation, one with star formation but no feedback, and one with star formation and thermal energy injection. We analyze the resulting population of clouds, finding that we can identify the same population of massive, virialized clouds and transient, low-surface density clouds found in our previous work (that did not include star formation or feedback). Star formation and feedback can affect the mix of clouds we identify. In particular, star formation alone simply converts dense cloud gas into stars with only a small change to the cloud populations, principally resulting in a slight decrease in the transient population. Feedback, however, has a stronger impact: while it is not generally sufficient to entirely destroy the clouds, it does eject gas out of them, increasing the gas density in the inter-cloud region. This decreases the number of massive clouds, but substantially increases the transient cloud population. We also find that feedback tends to drive a net radial inflow of massive clouds, leading to an increase in the star formation rate in the bar region. We examine a number of possible reasons for this and conclude that it is possible that the drag force from the enhanced intercloud density could be responsible.

The MAGellanic Inter-Cloud (MAGIC) project – II. Slicing up the Bridge

The origin of the gas in between the Magellanic Clouds (MCs), known as the Magellanic Bridge, has always been the subject of controversy. To shed light into this, we present the results from the MAGellanic Inter-Cloud II (MAGIC II) project aimed at probing the stellar populations in 10 large fields located perpendicular to the main ridge-line of H i in the Inter-Cloud region. We secured these observations of the stellar populations in between the MCs using the WFI (Wide Field Imager) camera on the 2.2 m telescope in La Silla. Using colour–magnitude diagrams, we trace stellar populations across the Inter-Cloud region. In good agreement with MAGIC I, we find significant intermediate-age stars in the Inter-Cloud region as well as young stars of a similar age to the last pericentre passage in between the MCs (∼200 Myr ago). We show here that the young, intermediate-age and old stars have distinct spatial distributions. The young stars correlate well with the H i gas suggesting that they were either recently stripped from the Small Magellanic Cloud (SMC) or formed in situ. The bulk of intermediate-age stars are located mainly in the Bridge region where the H i column density is higher, but they are more spread out than the young stars. They have very similar properties to stars located ∼2 kpc from the SMC centre, suggesting that they were tidally stripped from this region. Finally, the old stars extend to some 8 kpc from the SMC supporting the idea that all galaxies have a large extended metal-poor stellar halo.

THE MAGELLANIC INTER-CLOUD PROJECT (MAGIC). I. EVIDENCE FOR INTERMEDIATE-AGE STELLAR POPULATIONS IN BETWEEN THE MAGELLANIC CLOUDS

The origin of the gas in between the Magellanic Clouds (MCs) -- known as the `Magellanic Bridge' (MB) -- is puzzling. Numerical simulations suggest that the MB formed from tidally stripped gas and stars in a recent interaction between the MCs. However, the apparent lack of stripped intermediate- or old-age stars associated with the MB is at odds with this picture. In this paper, we present the first results from the MAGellanic Inter-Cloud program (MAGIC) aimed at probing the stellar populations in the inter-cloud region. We present observations of the stellar populations in two large fields located in between the Large and Small Magellanic Clouds (LMC/SMC), secured using the WFI camera on the 2.2 m telescope in La Silla. Using a synthetic color-magnitude diagram (CMD) technique, we present the first quantitative evidence for the presence of intermediate-age and old stars in the inter-Cloud region. The intermediate-age stars -- that make up ~28% of all stars in the region -- are not present in fields at a similar distance from the SMC in a direction pointing away from the LMC. This provides potential evidence that these intermediate-age stars could have been tidally stripped from the SMC. However, spectroscopic studies will be needed to confirm or rule out the tidal origin for the inter-Cloud gas and stars.

Nanobacteria in clouds can spread oral pathologic calcifications around the world

Introduction: Nanobacteria (calcifying nanoparticles, nanobes) are one of the most controversial issues in contemporary biology. Studies show accumulating evidence on association of nanobacteria with oral pathologic calcifications such as calculus, pulp stone, and salivary gland stones. The Hypothesis: Experiments have shown that nanobacteria are excreted from the body in urine and saliva, lifted from the ground by winds into the cloud, and transit between the high humidity region of the clouds and the relatively dry inter-cloud regions. Remnants of a sticky protein coating that nanobacteria make it act as an extremely efficient cloud condensation nuclei. Following condensation of cloud, nanobacteria return to the earth via rain and snow. Evaluation of the Hypothesis: Transmission of nanobacteria via clouds is not surprising when compared with cosmic transmission of nanobacteria. The apatite mineral layer around the organism serves as a primary defence shield against various seriously life-threatening conditions. A double defence with the apatite layer and an impermeable membrane combined with a very slow metabolism is a likely explanation for the resistance of nanobacteria.

Triggered star formation in the Magellanic Clouds

AbstractWe discuss how tidal interaction between the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), and the Galaxy triggers galaxy-wide star formation in the Clouds for the last ~0.2 Gyr based on our chemodynamical simulations on the Clouds. Our simulations demonstrate that the tidal interaction induces the formation of asymmetric spiral arms with high gas densities and consequently triggers star formation within the arms in the LMC. Star formation rate in the present LMC is significantly enhanced just above the eastern edge of the LMC's stellar bar owing to the tidal interaction. The location of the enhanced star formation is very similar to the observed location of 30 Doradus, which suggests that the formation of 30 Doradus is closely associated with the last Magellanic collision about 0.2 Gyr ago. The tidal interaction can dramatically compress gas initially within the outer part of the SMC so that new stars can be formed from the gas to become intergalactic young stars in the inter-Cloud region (e.g., the Magellanic Bridge). The metallicity distribution function of the newly formed stars in the Magellanic Bridge has a peak of [Fe/H] ~−0.8, which is significantly lower than the stellar metallicity of the SMC.

Molecular abundance variations near our galactic center

Millimeter-wave observations of CO, HCO+, HNCO, and SiO molecular lines toward Sgr A show that the prominent gas condensations in this region differ significantly in their chemical properties, which may be a result of gradients in the physical conditions as a function of location. HNCO and SiO observations suggest that M−0.13−0.08 (the +20kms−1 cloud) appears to consist of two different components, M−0.13−0.08a and M−0.13−0.08b, with LSR velocities of about +25 and +5kms−1, respectively. The clump M−0.13−0.08a (the +25kms−1 cloud) seems to be located very close to Sgr A East and the observed enhancement of HNCO and SiO may result from the interaction with Sgr A East. The molecular gas clump M−0.02−0.07 (the +50kms−1 cloud) shows the strongest 13CO emission among the identified clouds in this region, and is the most massive condensation (⩾3×105M⊙), but may be located closer to us than other clouds, since it appears to be chemically quiescent and shade the “inter-cloud region” where the HCO+/13CO ratio is enhanced. The −30kms−1 component shows a similar morphology to the HCO+ enhanced “inter-cloud” region. M+0.07−0.08 and M+0.11−0.08 show strong HCO+ 1–0 emission, and appear to consist of one cloud (the HCO+ cloud) in this observation. The HCO+/13CO abundance ratio is enhanced by a factor of 2–3 toward this HCO+ cloud and the inter-cloud region between l=0.1° and 0° near to Sgr A East, compared to other clouds observed in this region. This relative enhancement of HCO+ is thought to result from interaction with Sgr A East.

Chemical compositions of four B-type supergiants in the SMC wing

High-resolution UCLES/AAT spectra of four B-type supergiants in the SMC South East Wing have been analysed using non-LTE model atmosphere techniques to determine their atmospheric parameters and chemical compositions. The prin- ciple aim of this analysis was to determine whether the very low metal abundances (−1.1 dex compared with Galactic value) previously found in the Magellanic Inter Cloud region (ICR) were also present in the SMC Wing. The chemical compositions of the four targets are similar to those found in other SMC objects and appear to be incompatible with those deduced previously for the ICR. Given the close proximity of the Wing to the ICR, this is difficult to understand and some possible explanations are briefly discussed.

The dynamical evolution and star formation history of the Small Magellanic Cloud: effects of interactions with the Galaxy and the Large Magellanic Cloud

In order to better understand the dynamical evolution and star formation history of the Magellanic system, realistic N-body simulations of the tidal distortion of the Small Magellanic Cloud (SMC) as a result of the Galaxy and the Large Magellanic Cloud have been carried out, taking into account gas dynamics and star formation processes explicitly. The best model succeeds in reproducing the observed structural, kinematic and star formation properties of the SMC, including other related tidal features in the Magellanic system, without resorting to the ram pressure model. The best-fitting simulation reproduced a gas stream with almost no stars and the observed H I gas fraction, for which the morphology and velocity field agree quite well with those of the Magellanic stream, a result of adopting an initial SMC model that has a compact stellar disc embedded in an extended gaseous disc. This implies that the existence of a purely gaseous Magellanic stream does not poses serious problems to a tidal model of formation. Also, in this best model, the central and south-east side (wing region) of the SMC contained an excess of young stars, as is observed. Comparison with a reference simulation of isolated evolution demonstrated that the acceleration of star formation activity in these regions may be a direct result of the last interaction between the Magellanic Clouds roughly 0.2 Gyr ago, which formed the inter-cloud region. The large extent in depth of the SMC implied by the spatial distribution of Cepheids, and the line-of-sight velocity pattern in H I around the SMC is also reproduced. Finally, the dependences of these results on the numerical parameters that specify the SMC mass model and interstellar gas processes are discussed.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Observations of Dust and Star-forming Regions in the Ocular Galaxy IC 2163 and Its Spiral Companion NGC 2207

Hubble Space Telescope observations in U, B, V, and I passbands of the interacting spiral galaxies IC 2163 and NGC 2207 are used to measure extinctions in the cloud and intercloud regions and ages and luminosities of the star-forming regions. The extinction in the part of NGC 2207 seen in projection against IC 2163 was determined by using the method of White & Keel. The extinctions there and elsewhere were also determined from radiative transfer models of the magnitude differences between clouds and their surroundings. The intercloud extinction in V band ranges from 0.5 to 1 mag on the line of sight, and the cloud extinction ranges from 1 to 2 mag. The measured star-forming regions in these galaxies have a power-law relation between size and luminosity and a power-law luminosity distribution function. These power laws are consistent with a fractal dimension for the star formation that is the same as that for interstellar gas, D ~ 2.2, extending over scales ranging from 20 to 1000 pc. Fifteen compact massive star clusters that are analogous to super–star clusters found in starburst regions are in the spiral arms of NGC 2207. Nothing is peculiar about these regions except for a high H I velocity dispersion (~50 km s-1). Two more super–star clusters are in the tidally compressed oval of IC 2163. These clusters have masses ranging from ~10^4 to 2 × 10^5 M⊙ and ages of a few times 10^6 yr.

Magellanic Cloud Periphery Carbon Stars. IV. The SMC

The kinematics of 150 carbon stars observed at moderate dispersion on the periphery of the Small Magellanic Cloud are compared with the motions of neutral hydrogen and early-type stars in the intercloud region. The distribution of radial velocities implies a configuration of these stars as a sheet inclined at 73° ± 4° to the plane of the sky. The near side, to the south, is dominated by a stellar component; to the north, the far side contains fewer carbon stars and is dominated by the neutral gas. The upper velocity envelope of the stars is closely the same as that of the gas. This configuration is shown to be consistent with the known extension of the SMC along the line of sight and is attributed to a tidally induced disruption of the SMC that originated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago. The dearth of gas on the near side of the sheet is attributed to ablation processes akin to those inferred in 1996 by Weiner & Williams to collisional excitation of the leading edges of Magellanic Stream clouds. Comparison with the 1989 kinematic data of Hardy, Suntzeff, & Azzopardi and Maurice, Martin, & Bouchet and the 1986 and 1988 data of Mathewson et al. leaves little doubt that forces other than gravity play a role in the dynamics of the H I.

A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud

A survey of extended objects in the Large Magellanic Cloud was carried out on the ESO/SERC R and J Sky Survey Atlases, checking entries in previous catalogs and searching for new objects. The census provided 6659 objects including star clusters, emission-free associations, and objects related to emission nebulae. Each of these classes contains three subclasses with intermediate properties, which are used to infer total populations. The survey includes cross identifications among catalogs, and we present 3246 new objects. We provide accurate positions, classification, and homogeneous measurements of sizes and position angles, as well as information on cluster pairs and hierarchical relation for superimposed objects. This unification and enlargement of catalogs is important for future searches of fainter and smaller new objects. We discuss the angular and size distributions of the objects of the different classes. The angular distributions show two off-centered systems with different inclinations, suggesting that the LMC disk is warped. The present catalog together with its previous counterpart for the SMC and the inter-Cloud region provide a total population of 7847 extended objects in the Magellanic System. The angular distribution of the ensemble reveals important clues on the interaction between the LMC and SMC.

Carbon stars in the halo of the Magellanic Clouds: Identification and radial velocity data

We present the current status of our ongo- ing cool carbon star survey in the halo of the Magellanic Clouds. Candidate cool carbon stars were identied from APM measures of pairs of UK Schmidt Telescope BJ and R survey plates. Intermediate resolution spectroscopy on the duPont Telescope, Las Campanas, was used to simul- taneously verify the nature of the candidates, late M-type giants or AGB carbon stars, and to derive their radial velocity. Coordinates, nding charts and radial velocity data for 392 spectroscopically identied cool carbon stars distributed out to angular distances of 10 degrees from the Cloud optical centres are given. Radial velocities were also obtained for 133 known carbon stars in the Large Magellanic Cloud, in the inter-Cloud region, in the wing of the Small Magellanic Cloud and in a few SMC star clusters. These intermediate-age carbon stars dene ideal kinematic test particles to investigate recent dynamical interactions between the Galaxy-LMC-SMC system and in particular the origin of the morphological disturbances seen in the SMC and parts of the LMC, the origin of the Magellanic Stream and the total mass of the LMC.

The Extreme‐Ultraviolet Emission of the Seyfert Galaxies Markarian 279, Markarian 478, and Ton S180

We present EUV light curves and spectra of three Seyfert galaxies. The data were obtained from EUVE observations of the galaxies Mrk 279, Mrk 478, and Ton S180. The variability in the light curves provides an upper limit to the size of the EUV-emitting region which excludes the broad-line region and diffuse intercloud regions as the source of this emission. The EUV spectra are all characterized by discrete line features; in the case of Mrk 478, we find the lines vary on timescales of less than 1 day. These results allow us to rule out various models proposed for Seyfert galaxies. Comparing these spectra in their rest frames, we find that several of these features appear at the same wavelengths. A composite EUV spectrum is produced from these redshift-corrected spectra. The most striking feature in the composite spectrum is an emission feature at ~79 A, which is also observed in the spectra of each of the individual galaxies. It is difficult to explain this feature by emission from a photoionized or collision-driven plasma in thermal equilibrium. We suggest this line feature may be the fluorescent transition of neutral Fe related to the 6.4 keV Kα emission that has frequently been observed in the X-ray spectra of Seyfert galaxies. If this interpretation is correct, this result imposes strong constraints on the distribution of warm material in these galaxies.

N-body simulations of the Small Magellanic Cloud and the Magellanic Stream

An extensive set of N-body simulations has been carried out on the gravitational interaction of the Small Magellanic Cloud (SMC) with the Galaxy and the Large Magellanic Cloud (LMC). The SMC is assumed to have been a barred galaxy with a disc-to-halo mass ratio of unity before interaction and modelled by a large number of self-gravitating particles, whereas the Galaxy and LMC have been represented by rigid spherical potentials. The best model we have found succeeded in reproducing the Magellanic Stream (MS), as well as a leading counterpart to the Magellanic Stream (the leading arm), on the opposite side of the Magellanic Clouds to the Stream, which mimicks the overall distribution of several neutral hydrogen clumps observed in the corresponding region of the sky. The elongation of the SMC bar along the line-of-sight direction suggested by Cepheid observations has been partially reproduced, alongside its projected appearance on the sky. The model successfully explains some major trends in the kinematics of young populations in the SMC bar and older populations in the `halo' of the SMC, as well as the overall velocity pattern for the gas, young stars, and carbon stars in the inter-Cloud region.

The Study of Stellar Populations and Dynamics in the Small Magellanic Cloud using the UKST

AbstractThe combination of the wide field of Schmidt telescope photographic plates with fast measuring machines such as COSMOS and APM, had a major impact on the study of the global properties of the Magellanic Clouds. Results (based on UKST data) regarding the stellar populations and dynamics of the Small Magellanic Cloud are described here in some detail. The stellar content and kinematics of the intercloud region are also outlined, with particular emphasis on the importance of this region in understanding the interaction between the Magellanic Clouds.

How to Model the Chemical Evolution of Galaxies

For a first interpretation of the comparison of observational data, the crude “Simple Model” of chemical evolution is quite useful. Since it has well been described in the literature (e.g. Pagel and Patchett 1975, Tinsley 1980), let us here just review the assumptions and whether they are satisfied: 1.The galaxy is a closed system, with no exchange of matter with its surroundings: For the solar neighbourhood this probably is not true (the infamous Gdwarf-“problem”, Pagel 1989b). For the Magellanic Clouds this is most certainly wrong, because of the presence of the Inter-Cloud Region and the Magellanic Stream, and evidence for interaction with each other and the Galaxy as well (cf. e.g. Westerlund 1990).2.It initially consists entirely of gas (without loss of generality of primordial composition): This is good approximation also for models with gas infall, as long as the infall occurs with a time scale shorter than the star formation time scale.3.The metal production of the average stellar generation (the yield y) is constant with time: Initially, it is reasonable to make this assumption. For tables of the oxygen yield see Koppen and Arimoto (1991).4.The metal rich gas ejected by the stars is completely mixed with the ambient gas. To neglect the finite stellar life times (“instantaneous recycling approximation”) is appropriate for elements synthesized in stars whose life time is much shorter than the star formation time scale, such as oxygen, neon, sulphur, and argon.5.The gas is well mixed at all times: We don't know. The dispersion of H II region abundances may give an indication. In the Magellanic Clouds Dufour (1984) finds quite a low value (±0.08 dex for oyxgen).