Neutral hydrogen filaments at high galactic latitudes

Small angular scale twists in Hi filaments identified at high galactic latitudes are associated with the location of enhanced emission features (EEFs), structures that have traditionally been referred to as clouds. It is shown that in these directions the column density of gas is enhanced through geometrical effects because the line-of-sight intersects a greater pathlength when viewing along a segment of filament axis (flux tube) than when the tline-of-sight is more normal to the filament axis. By interpreting the EEFs as isolated entities (clouds) we derive an incorrect impression as regards the properties of interstellar Hi. For example, EEFs are typically a factor of five to ten times deeper than they are wide and, hence, the derived properties of Hi structures that have traditionally been taken to be as deep as they are wide are incorrect. This study leads to questions about the way observations of 21 cm, molecular, and 100 emission are currently being interpreted. It is concluded that much of what is observed to be cloud structure in the interstellar medium (except in regions directly associated with star formation) is telling us about geometry of filaments and not about the physics of clouds, The very notion of an interstellar cloud may have outlived its usefulness and previous work that has attempted to account for these structures in terms of gravitational stability or pressure equilibrium has to be reconsidered in the light of the existence of complex patterns of filamentary structure, not only in the Hi distribution but also of interstellar cirrus defined by 100 emission.     

Neutral hydrogen filaments at high galactic latitudes

Large-scale morphological waves with wavelengths of order 30° and amplitude=1/8 exist in several long Hi filaments in a 540 square degrees of sky aroundl=230°,b=+40°. The extent of the longest filament is greater than 72°, the limit set by the boundaries of the area surveyed. TheHi gas appears to be controlled by magnetic fields and the motion within flux tubes, determined by analysis of velocity along the filament axes, shows the presence of wave patterns with amplitude 5 to 6 km s^–1 on an angular scale similar to that seen in the spatial structure projected on the sky.     

Studies of blue objects at high galactic latitudes

Continuing the survey for faint blue objects at high galactic latitudes, a field of 25 square degrees centered at the Coma Cluster of Galaxies has been examined on two-color plates taken with the 90-67 cm Schmidt telescope of Asiago. 487 objects have been identified. A selected list of those with the largest negative color index is given in Table I. Identification charts, notes and comments follow.     

The distribution of pulsar periods at different galactic latitudes

Pulsars at different galactic latitudes have different periods. It seems possible that there are two different types of pulsars.     

Reddenings of quasars VS PSA counts at high galactic latitudes

The correlation betweenE(B-V) reddenings of high latitude radio quasars (from Teerikorpi, 1981) and star/galaxy counts from the PSA prints was investigated. With the resolution down to 5×5, the reddenings and the counts show the relationship expected if it is assumed that most of the high latitude stars (B<21 mag) are situated above the dust clouds. The dust causing the reddenings is probably connected with the population of molecular clouds observed at high galactic latitudes. The present study underlines the importance of simple star/galaxy counts (all images counted) for the study of high latitude variable extinction. The existence of variable extinction shows that also the average extinction must be significant.     

Night-time geomagnetic variations at low latitudes

Using hourly values of the magnetic elements H, D and Z for 1964, 1965, their variation during night-time hours is examined from both their monthly means and from a previously used harmonic analysis method. The data set used represents quiet magnetic conditions. Consistent changes during the night are often found. Seasonal changes are also examined and it seems necessary to modify the Malin—Isikara hypothesis of a moving ring current by including a seasonal modulation of ring current strength with equinoctial maximum and a local time-varying component or partial ring current. The night-time D component shows considerable asymmetry between North and South hemispheres and this might be due to field-aligned current structure. There is a small amount of evidence for a night-time westward equatorial electrojet enhancement.     

Interstellar neutral hydrogen filaments at high galactic latitudes and the bennett pinch

Observed properties of interstellar neutral hydrogen filaments suggest the presence of the Bennett pinch as described by the Carlqvist relationship with rotation around the filament axes included. A brief summary is first given of three ways in which a filament model for interstellar cloud structure was tested. Preliminary results from highresolution HI mapping of gas and dust in an apparent HI cloud indicate that the neutral gas and dust within and around its boundary is itself highly filamentary. An attempt to detect magnetic fields in this and similar features using the Zeeman effect technique at the 21-cm wavelength of interstellar neutral hydrogen set upper limits of a fewµG. In contrast, the strength of the toroidal magnetic field expected from the examination of the Carlqvist relationship is of order 5µG, which would be produced by a current of 1.4 · 10¹³ A. Zeeman effect technology is at present not able to detect toroidal magnetic fields of this order at the edge of barely resolved HI filaments. Nevertheless, currently available high-resolution HI data suggest that interstellar filament physics should take into account the role of currents and pinches for creating and stabilizing the structures.     

Extinction trends at high galactic latitudes as shown by extragalactic extinction indicators

In the galactic latitude range 40°<|b|<80° several extinction indicators of extragalactic type show features with similar longitude dependence. Amplitudes of these variations correspond toB-extinction variations of about 0 _. ^m 3–0 _. ^m 4. The results favour the view that instead of nearly zero extinction, there are significant amounts of dust at high galactic latitudes.     

Disentangling the Galaxy at low Galactic latitudes

We have used the field stars from the open cluster survey BOCCE (Bologna Open Clusters Chemical Evolution), to study three low-latitude fields imaged with the Canada–France–Hawaii telescope (CFHT), with the aim of better understanding the Galactic structure in those directions. Because of the deep and accurate photometry in these fields, they provide a powerful discriminant among Galactic structure models. In the present paper we discuss if a canonical star count model, expressed in terms of thin and thick disc radial scales, thick disc normalization and reddening distribution, can explain the observed colour–magnitude diagrams (CMDs). Disc and thick disc are described with double exponentials, the spheroid is represented with a De Vaucouleurs density law. In order to assess the fitting quality of a particular set of parameters, the colour distribution and luminosity function of synthetic photometry is compared to that of target stars selected from the blue sequence of the observed CMDs. Through a Kolmogorov–Smirnov test, we find that the classical decomposition halo-thin/thick disc is sufficient to reproduce the observations – no additional population is strictly necessary. In terms of solutions common to all three fields, we have found a thick disc scalelength that is equal to (or slightly longer than) the thin disc scale.     

Observation of mesospheric ozone at low latitudes

Stellar ultraviolet light near 2500 Å is attenuated in the Earth's upper atmosphere due to strong absorption in the Hartley continuum of ozone. The intensity of stars in the Hartley continuum region has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the Earth's atmosphere. These data have been used to determine the ozone number density profile at the occultation tangent point. The results of approximately 12 stellar occultations, obtained in low latitudes, are presented, giving the nighttime vertical number density profile of ozone in the 60- to 100-km region. The nighttime ozone number density has a bulge in its vertical profile with a peak of 1 to 2×10⁸ cm^?3 at approximately 83 km and a minimum near 75 km. The shape of the bulge in the ozone number density profile shows considerable variability with no apparent seasonal or solar cycle change. The ozone profiles obtained during a geomagnetic storm showed little variation at low latitudes.     

Seasonal surface frost at low latitudes on Mars

During the past 4 Mars years, Mars Orbiter Camera imaging capabilities have been used to document occurrence of seasonal patches of frost at latitudes as low as 33° S, and even 24° S. Monitoring reveals bright patches on pole-facing slopes; these appear in early southern winter and disappear in mid winter. The frost forms annually. Thermal Emission Spectrometer and daytime Thermal Emission Imaging System observations show surface temperatures on and near pole facing slopes reach the condensation temperature of CO₂, indicating the patches consist of carbon dioxide rather than water frost. For several months, temperatures on pole-facing crater walls are so low that even carbon dioxide condenses on them, although the slopes are illuminated by the Sun every day. Thermal model calculations show slopes accumulate a several centimeter thick layer of CO₂ frost. The frost becomes visible only months after it has begun to form, and has an orientational preference which is due to illumination bias at the time of observation. H₂O condenses at higher temperatures and water frost must therefore also be present. Potential opportunities to observe seasonal water frost at low latitudes are also described.     

Propagational features of higher harmonic tweeks at low latitudes

Higher harmonic tweeks observed for the first time at the low latitude station Varanasi (geomag. lat. 14 55 N) are reported. The analysis of data shows that higher harmonic tweeks are usually not associated with whistlers and occur when the ionization in the lower ionosphere would not increases with height. The Earth-ionosphere waveguide dispersion features play an important role in the propagation of broad band lightning generated signals and their occasional observation as higher harmonic tweeks. It is shown that the conductivity of ground and sea mixed path, forming the lower surface of the waveguide, provide an estimate of the travelled distances of higher harmonic tweeks in the waveguide. The attenuation factors are computed which shows that as the harmonic number increases their probability of observation decreases. The attenuation increases as the frequencies approach the cut-off frequencies and also as the layer height falls. The tweek activity is found to increase during periods of magnetic disturbances.     

Near-infrared sky background fluctuations at mid- and low latitudes

The emission of the upper atmosphere introduces an additional variable component into observations of astronomical objects in the NIR 700–3,000 nm range. The subtraction of this component is not easy because it varies during the night by as much as 100% and it is not homogeneous over the sky. A program aimed at measuring and understanding the main characteristics of the atmospheric NIR emission was undertaken. A 512?×?512 CCD camera equipped with a RG780/2 mm filter is used to obtain images of the sky in a 36°?×?36° field of view. The intensities of a given star and of the nearby region devoid of star in a 439 arcmin² area are monitored during periods of time of several hours. The sky intensity measured in the 754–900 nm bandpass, reduced to zenith and zero airmass is comprised between mag20 and mag18.5 per arcsecond². A diminution by a factor of two during the night is frequently observed. Intensity fluctuations having an amplitude of 15% and periods of 5–40 min are present in the images with a structure of regularly spaced stripes. The fluctuations of the NIR sky background intensity are due to (1) the chemical evolution of the upper atmosphere composition during the night and (2) dynamical processes such as tides with periods of 3–6 h or gravity waves with periods of several tens of minutes. We suggest that a monitoring of the sky background intensity could be set up when quantitative observations of astronomical objects require exposure times longer than ~10 min. The publication is illustrated with several video films accessible on the web site http://www.obs-besancon.fr/nirsky/. Enter username: nirsky and password: skynir.     

The UU Her-type stars: Possible tracers of recent star formation at high galactic latitudes

A general review of the group of UU Her-type stars proposed recently as a possible new type of variables is given. The available observations (spectra and photometry) reveal them as luminous Population I F-type supergiants of normal composition. This fact contradicts their great distances above the galactic plane. As there is no reason so far to distrust the results from the spectroscopic analyses and photometry, the galactic plane should be excluded as a possible birthplace of the UU Her-stars. In this respect they might probably indicate that recent star formation (though occasional) from already enriched material was possible at high galactic latitudes. Moreover, normal Main-Sequence B-stars seem now also to be found there.Much attention is paid to the specific semiregular variability of the UU Her-stars. The type of pulsations remains conjectural, but simple radial pulsations should obviously be ruled out.     

The longitudinal range of Pi2 propagation at low latitudes

The longitudinal range over which Pi2 pulsations are propagated at low latitudes is investigated and found to vary from one substorm to another. The results show that the longitudinal extent over which Pi2's can be detected varies from a narrow range of longitudes either side of the 23 L.T. meridian in some cases to almost all longitudes around the Earth in other cases. The longitudinal range of propagation does not appear to be a function of substorm intensity.     

Stereoscopic imaging of the hydroxyl emissive layer at low latitudes

The hydroxyl nightglow layer is an excellent tracer of the dynamical processes occurring within the mesosphere. A new stereo-imaging method is applied that not only measures the altitude of the airglow layer but also provides a three-dimensional map of the OH-layer centroid heights. A campaign was conducted in July 2006 in Peru to obtain NIR images of the OH nightglow layer which were simultaneously taken for two sites separated by 645 km: Cerro Cosmos (12°09′08.2″S, 75°33′49.3″W, altitude 4630 m) and Cerro Verde Tellolo (16°33′17.6″S, 71°39′59.4″W, altitude 2330 m). Data represented by pairs of images obtained during the nights of July 26-27 and 28-29 are analyzed to yield satellite-type views of the wave field. These are obtained by application of an inversion algorithm. In calculating the normalized cross-correlation parameter for the intensity, three-dimensional maps of the OH nightglow layer surface are retrieved. The mean altitude of the emission profile barycenter is found to be at 87.1 km on July 26 and 89.5 km on July 28. In these two cases the horizontal wavelengths determined are 21.1 and 24.6 km with periods of 18 and 34 min, respectively. A panoramic view of the OH nightglow emission obtained on July 29 at 8 h51-9 h26 UT is presented, in which the overall direction of the waves is found to be N-NW to S-SE, azimuth 150°-330° (counted from South). The wave kinetic energy density at the OH nightglow layer altitude is 3.9×10⁻⁴ W/kg, which is comparable to the values derived from partial reflection radiowave data.     

Active galactic nuclei dust tori at low and high luminosities

A cornerstone of active galactic nuclei (AGN) unification schemes is the presence of an optically and a geometrically thick dust torus. It provides the obscuration to explain the difference between type 1 and type 2 AGN. We investigate the influence of the dust distribution on the Eddington limit of the torus. For smooth dust distributions, the Eddingtion limit on the dust alone is five orders of magnitudes below the limit for electron scattering in a fully ionized plasma, while a clumpy dust torus has an Eddington limit slightly larger than the classical one. We study the behaviour of a clumpy torus at low and high AGN luminosities. For low luminosities of the order of  ∼10⁴² erg s⁻¹  , the torus changes its characteristics and obscuration becomes insufficient. In the high-luminosity regime, the clumpy torus can show a behaviour which is consistent with the 'receding torus' picture. The derived luminosity-dependent fraction of type 2 objects agrees with recent observational results. Moreover, the luminosity-dependent covering factor in a clumpy torus may explain the presence of broad-line AGN with high column densities in X-rays.