Small Angular Diameter Research Articles

Editorial

Editorial

Antony Hewish. 11 May 1924—13 September 2021

Antony (Tony) Hewish was a pioneer radio astronomer who will always be remembered as the leader of the team in 1967 that discovered the pulsars, which proved to be rapidly rotating, magnetized neutron stars. The discovery resulted from Tony's programme of systematic all-sky surveys to detect the scintillation, or flickering, of small angular diameter radio sources due to electron density fluctuations in the solar wind flowing out from the Sun. The large low-frequency 4.5-acre array was designed by Tony to find radio quasars, which often display radio scintillations, to estimate the angular sizes of the sources and to study the physics of the interplanetary medium. In the course of commissioning the telescope, his research student, Jocelyn Bell (Jocelyn Bell Burnell, FRS 2003), noted a strange 100% scintillating source unlike anything seen before. Tony and the team soon established that this source was a pulsating radio source, Jocelyn first observing the pulsations with period 1.33 s in November 1967. The discovery paved the way for the rapid development of high energy astrophysics and an appreciation that general relativity plays a key role in the stability of neutron stars. Tony's contributions spanned a very wide range of pioneering studies in the new discipline of radio astronomy, including telescope and electronic design, cosmological studies of distant radio sources and the physics of the ionospheric, interplanetary and interstellar plasmas. He was awarded the 1974 Nobel Prize in physics for ‘his decisive role in the discovery of pulsars’.

Integrated spectral study of small angular diameter galactic open clusters

This paper presents flux-calibrated integrated spectra obtained at Complejo Astronómico El Leoncito (CASLEO, Argentina) for a sample of 9 Galactic open clusters of small angular diameter. The spectra cover the optical range (3800–6800 Å), with a resolution of ∼14 Å. With one exception (Ruprecht 158), the selected clusters are projected into the fourth Galactic quadrant (282o < l < 345o) near the Galactic plane (∣b∣ ≤ 9o). We performed simultaneous estimates of foreground interstellar reddening and age by comparing the continuum distribution and line strenghts of the cluster spectra with those of template cluster spectra with known parameters. We thus provide spectroscopic information independent from that derived through color-magnitude diagram studies. We found three clusters (Collinder 249, NGC 4463 and Ruprecht 122) younger than ∼40 Myr, four moderately young ones (BH 92, Harvard 5, Hogg 14 and Pismis 23) with ages within 200–400 Myr, and two intermediate-age ones (Ruprecht 158 and ESO 065-SC07) with ages within 1.0–2.2 Gyr. The derived foreground E(B−V) color excesses vary from around 0.0 in Ruprecht 158 to ∼1.1 in Pismis 23. In general terms, the results obtained show good agreement with previous photometric results. In Ruprecht 158 and BH 92, however, some differences are found between the parameters here obtained and previous values in the literature. Individual spectra of some comparatively bright stars located in the fields of 5 out of the 9 clusters here studied, allowed us to evaluate their membership status. The current cluster sample complements that of 46 open clusters previously studied by our group in an effort to gather a spectral library with several clusters per age bin. The cluster spectral library that we have been building is an important tool to tie studies of resolved and unresolved stellar content.

Fast Integrated Spectra Analyzer: A New Computational Tool for Age and Reddening Determination of Small Angular Diameter Open Clusters

ABSTRACTWe present a new algorithm called “Fast Integrated Spectra Analyzer” (FISA) that permits fast and reasonably accurate age and reddening determinations for small angular diameter open clusters by using their integrated spectra in the (3600–7400) Å range and currently available template spectrum libraries. This algorithm and its implementation help to achieve astrophysical results in shorter times than from other methods. A brief review is given of the integrated spectroscopic technique applied to the study of open clusters, as well as the basic assumptions that justify its use. We describe the numerical algorithm employed in detail, show examples of its application, and provide a link to the code. Our method has successfully been applied to integrated spectroscopy of open clusters, both in the Galaxy and in the Magellanic Clouds, to determine ages and reddenings.

On the detectability of H i 21 cm in Mg ii absorption systems

We investigate the effect of two important, but oft neglected, factors which can affect the detectability of HI 21-cm absorption in MgII absorption systems: The effect of line-of-sight geometry of the coverage of the background radio flux and any possible correlation between the 21-cm line strength and the rest frame equivalent width of the MgII line. Regarding the former, while the observed detection rate at small angular diameter distance ratios is a near certainty, for an unbiased sample, where either a detection or a non-detection are equally likely, at ratios > 0.8 the observed detection rate has an 8 sigma significance, suggesting that the mix of ratios values at z < 1 is correlated with the mix of detections and non-detections at low redshift, while the exclusively high values of the ratio at z > 1 contribute to the low detection rates at high redshift. In DLAs, the correlation between the 21-cm line strength and the MgII equivalent width is dominated by the velocity spread of the 21-cm line. This has recently been shown not to hold for MgII systems in general. However, we do find the significance of the correlation to increase when the MgII absorbers with MgI equivalent widths of >0.5 A are added to the DLA sample. Large values of the angular diameter distance ratio may explain why the absorbers which have similar equivalent widths to the detections remain undetected. We do, however, also find the neutral hydrogen column densities of the non-detections to be significantly lower. Applying the 21-cm line strength/equivalent width correlation to yield column densities for the MgII absorbers in which this is unmeasured, we find no evidence of a cosmological evolution in the neutral hydrogen column density.

AMI observations of Lynds dark nebulae: further evidence for anomalous cm-wave emission

Observations at 14.2 to 17.9 GHz made with the AMI Small Array towards fourteen Lynds Dark Nebulae with a resolution of 2' are reported. These sources are selected from the SCUBA observations of Visser et al. (2001) as small angular diameter clouds well matched to the synthesized beam of the AMI Small Array. Comparison of the AMI observations with radio observations at lower frequencies with matched uv-plane coverage is made, in order to search for any anomalous excess emission which can be attributed to spinning dust. Possible emission from spinning dust is identified as a source within a 2' radius of the Scuba position of the Lynds dark nebula, exhibiting an excess with respect to lower frequency radio emission. We find five sources which show a possible spinning dust component in their spectra. These sources have rising spectral indices in the frequency range 14.2--17.9 GHz. Of these five one has already been reported, L1111, we report one new definite detection, L675, and three new probable detections (L944, L1103 and L1246). The relative certainty of these detections is assessed on the basis of three criteria: the extent of the emission, the coincidence of the emission with the Scuba position and the likelihood of alternative explanations for the excess. Extended microwave emission makes the likelihood of the anomalous emission arising as a consequence of a radio counterpart to a protostar or a proto-planetary disk unlikely. We use a 2' radius in order to be consistent with the IRAS identifications of dark nebulae (Parker 1988), and our third criterion is used in the case of L1103 where a high flux density at 850 microns relative to the FIR data suggests a more complicated emission spectrum.

New young planetary nebulae in IPHAS

We search for very small-diameter galactic planetary nebulae (PNe) representing the earliest phases of PN evolution. A recently published IPHAS catalogue of Ha-emitting stars provides a useful base for this study as all sources present in this catalogue must be of small angular diameter. The PN candidates are selected based on their location in two colour-colour diagrams: IPHAS (r' - Ha) vs. (r' - i'), and 2MASS (J - H) vs. (H - Ks). Spectroscopic follow-up has been carried out on a sample of candidates in order to confirm their nature. We present a total of 83 PN candidates. We were able to obtain spectra or find the classification from the literature for 35 candidates. Five of these objects are likely to be new PNe, including one large bipolar PN discovered serendipitously near an emission-line star. PN distances deduced from extinction-distance relations based on IPHAS field-star photometry are presented for the first time. These yield distance estimates for our objects in the range from 2 kpc to 6 kpc. From the data to hand, we conclude that four of the discovered objects are very probably young PNe.

Integrated spectral properties of 22 small angular diameter galactic open clusters

Aims. Flux-calibrated integrated spectra of a sample of 22 Galactic open clusters of small angular diameter are presented. With one exception (ESO 429-SC2), all objects have Galactic longitudes in the range 208 ◦ < l < 33 ◦ . The spectra cover the range ≈3600−6800 A, with a resolution of ≈14 A. The properties of the present cluster sample are compared with those of well-studied clusters located in two 90 ◦ sectors, centred at l = 257 ◦ and l = 347 ◦ . The dissolution rate of Galactic open clusters in these two sectors is examined. Methods. Using the equivalent widths of the Balmer lines and comparing line intensities and continuum distribution of the cluster spectra with those of template cluster spectra with known properties, we derive both foreground reddening values and ages. Thus, we provide information independent of that determined through colour–magnitude diagrams. Results. The derived E(B − V) values for the whole sample vary from 0.0 in ESO 445-SC74 to 1.90 in Pismis 24, while the ages range from ∼3 Myr (NGC 6604 and BH 151) to ∼3.5 Gyr (Ruprecht 2). For six clusters (Dolidze 34, ESO 429-SC2, ESO 445-SC74, Ruprecht 2, BH 151 and Hogg 9) the foreground E(B − V) colour excesses and ages are determined for the first time. The results obtained for the remaining clusters show, in general terms, good agreement with previous photometric results. Conclusions. The age and reddening distributions of the present sample match those of known clusters in the two selected Galactic sectors. The present results would favour a major dissolution rate of star clusters in these two sectors. Two new solar-metallicity templates are defined corresponding to the age groups of (4−5) Myr and 30 Myr among those of Piatti et al. (2002, MNRAS, 335, 233). The Piatti et al. templates of 20 Myr and (3−4) Gyr are here redefined.

Mid-Infrared (8.1–12.5 μm) Imaging of Mercury

The first images of Mercury to be obtained in thermal emission are discussed. Images at 22 wavelengths between 8.1 and 13.25 μm obtained with MIRAC (Mid-Infrared Array Camera) have been reduced and analyzed to obtain new information regarding the thermal and chemical makeup of Mercury's surface. The region observed is centered at ∼210–250° mercurian longitude, encompassing the location of a ∼1000-km-diameter radar-bright region on Mercury's surface that was not imaged by Mariner 10. We found some evidence supporting ground-based spectroscopy of alkali-rich feldspathoids. However, the large pixel size (at the sub-Earth point the area per square pixel is 28,000 km 2—8 μm; 63,000 km 2—12 μm), the diffraction limit of the telescope, and the small angular diameter of Mercury all conspired to prevent any compositional spatial discrimination on the Earth-facing disk. Our four results are: (1) data fits to rough surface thermal models indicate the surface is smoother at this location (Thetabar ∼8°–11°) than at previous locations (10–75° and 100–160° longitude) measured in the mid-infrared (Thetabar ∼20°); (2) with the spatial resolution of these data, color ratios of images do not reveal any locations of enhanced or depressed flux that can be associated with regions of different rock composition across the disk of Mercury; (3) 18-channel spectra generated from Mercury whole-disk averages exhibit emissivity features over part of the spectral range (8.1–10.5 μm) indicative of bronzite, a pyroxene composed of ∼2% CaSiO 3, 86% MgSiO 3, and 12% FeSiO 3. In addition, the overall spectrum was grossly matched by a spectrum of picrite (an ultra-mafic rock) with 25% MgO, 8% CaO, and 11% FeO, and other minor components including plagioclase feldspar (4). TIMS six-channel simulations have a best match to sodalite (Na 8Al 6Si 6O 24C 12), a mineral associated with nephaline syenite rocks. On Earth, all of these materials are found in association in the Teschenite Sills of Shiant Isles, Scotland.

Spatial behavior of the coherence tensors for blackbody radiation emission

The paper discusses the tensor coherence properties of thermal radiation emitted into space by a blackbody source and presents numerical results for observation points restricted to a volume of space with linear dimensions small compared with the distance to the source, from which the source subtends a solid angle of circular cross section. For the first time, to my knowledge, surface and contour plots are given, showing the magnitude of the electric, magnetic, and mixed cross-spectral tensor components on selected planes as a function of the spatial separation between the observation points. Also, it is shown how these distributions change as the angular diameter of the source is varied. Small angular diameter sources are well described by the van Cittert–Zernike theorem of scalar coherence theory. For large angular diameter sources all nonzero components are of the same order of magnitude and are not properly described by the scalar theory. However, the trace of the electric tensor is in agreement with scalar theory.

The use of interplanetary scintillation maps in the prediction of geomagnetic activity

Abstract. Interplanetary scintillation (IPS), the twinkling of small angular diameter radio sources, arises from the interaction of the signal with small-scale plasma irregularities in the solar wind. The technique may be used to sense remotely the near-Earth heliosphere and has potential for tracking large-scale interplanetary disturbances from close to the Sun to the Earth. Such observations might be useful within routine geomagnetic forecasts, and we use data from the Mullard Radio Astronomy Observatory to test this suggestion. A forecast was based on the visual evaluation of each daily map. If an IPS event was observed then we proposed that any associated geomagnetic activity would occur either on that day, or during the following two days. We consider the success of these forecasts in predicting days when either an SSC/SI or an Ap value exceeding 30 were recorded. The identification of IPS events is necessarily subjective and so two observers compiled independent events lists, and the results were compared. Approximately half of the IPS events in each list were followed by a geomagnetic signature but comparison of the two lists showed that different days were being chosen. We also found that the forecasts had very high false alarm rates. Since IPS is sensitive to a volume we did not expect all events to be associated with a geomagnetic signature. However, the technique failed to forecast a large proportion of geomagnetic events and the association between IPS events and geomagnetic activity is not much better than would be expected by chance. Comparing the IPS forecasts with forecasts of Ap released by the Space Environment Services Center (SESC) we found that SESC correctly predicted a similar proportion of days when Ap\\geq30, but that the performance was significantly better than would be expected by chance, and had a much lower false alarm rate. We conclude that these IPS data cannot be used alone to produce reliable geomagnetic activity forecasts.

The use of various interplanetary scintillation indices within geomagnetic forecasts

Abstract. Interplanetary scintillation (IPS), the twinkling of small angular diameter radio sources, is caused by the interaction of the signal with small-scale plasma irregularities in the solar wind. The technique may be used to sense remotely the near-Earth heliosphere and observations of a sufficiently large number of sources may be used to track large-scale disturbances as they propagate from close to the Sun to the Earth. Therefore, such observations have potential for use within geomagnetic forecasts. We use daily data from the Mullard Radio Astronomy Observatory, made available through the World Data Centre, to test the success of geomagnetic forecasts based on IPS observations. The approach discussed here was based on the reduction of the information in a map to a single number or series of numbers. The advantages of an index of this nature are that it may be produced routinely and that it could ideally forecast both the occurrence and intensity of geomagnetic activity. We start from an index that has already been described in the literature, INDEX35. On the basis of visual examination of the data in a full skymap format modifications were made to the way in which the index was calculated. It was hoped that these would lead to an improvement in its forecasting ability. Here we assess the forecasting potential of the index using the value of the correlation coefficient between daily Ap and the IPS index, with IPS leading by 1 day. We also compare the forecast based on the IPS index with forecasts of Ap currently released by the Space Environment Services Center (SESC). Although we find that the maximum improvement achieved is small, and does not represent a significant advance in forecasting ability, the IPS forecasts at this phase of the solar cycle are of a similar quality to those made by SESC.

The use of various interplanetary scintillation indices within geomagnetic forecasts

Interplanetary scintillation (IPS), the twinkling of small angular diameter radio sources, is caused by the interaction of the signal with small-scale plasma irregularities in the solar wind. The technique may be used to sense remotely the near-Earth heliosphere and observations of a sufficiently large number of sources may be used to track large-scale disturbances as they propagate from close to the Sun to the Earth. Therefore, such observations have potential for use within geomagnetic forecasts. We use daily data from the Mullard Radio Astronomy Observatory, made available through the World Data Centre, to test the success of geomagnetic forecasts based on IPS observations. The approach discussed here was based on the reduction of the information in a map to a single number or series of numbers. The advantages of an index of this nature are that it may be produced routinely and that it could ideally forecast both the occurrence and intensity of geomagnetic activity. We start from an index that has already been described in the literature, INDEX35. On the basis of visual examination of the data in a full skymap format modifications were made to the way in which the index was calculated. It was hoped that these would lead to an improvement in its forecasting ability. Here we assess the forecasting potential of the index using the value of the correlation coefficient between daily Ap and the IPS index, with IPS leading by 1 day. We also compare the forecast based on the IPS index with forecasts of Ap currently released by the Space Environment Services Center (SESC). Although we find that the maximum improvement achieved is small, and does not represent a significant advance in forecasting ability, the IPS forecasts at this phase of the solar cycle are of a similar quality to those made by SESC.

Large scale anisotropies and polarization of the microwave background radiation in homogeneous cosmologies

The polarization and anisotropy of the cosmological microwave background radiation on large angular scales are discussed. A quadrupole anisotropy in the expansion of the universe (shear) is considered in realistic cosmological models and the resulting anisotropies and polarization of the radiation are calculated. The role of spatial curvature is considered separately, and it is found to have two profound effects: first, in closed models only, the direction of polarization of the radiation will appear at observation to be twisted relative to the anisotropy; the existence of this twist implies that the closed universe has a handedness property. Second, in open models a quadrupole anisotropy may be distorted by the spatial curvature so that it resembles a dipole; in the extreme case all the aniso­tropy is confined to a region of small angular diameter (a ‘spot’). Following previous work by Dautcourt and Rose, a transfer equation for polarized radiation in a general curved space-time is derived. The effect of Thomson scattering by free electrons is included, and the equation is separated into those for the multipoles up to quadrupole by expanding in polynomials formed from spin-weighted spherical harmonics. A numerical integration of the equations is described, and the results are presented for the twist angle, the dipole and quadrupole anisotropies, the degree of polarization in the quadrupole mode and the ratio of polarization to quadrupole aniso­tropy in all models considered. The twist of polarization in closed models is large and should be easily observable if the polarization could be. This result suggests an important observational test of the spatial curvature of the standard models. The dipole produced in open models, due to distortion by the spatial curvature, is prominent; it appears unlikely that an intrinsic dipole to quadrupole ratio of less than unity arises in any open models in which the effect occurs when the density is below one-half the critical density. Finally, the ratio of polarization to anisotropy is expected to be a good indicator of the presence of shear, and is sensitive to the ionization history of the matter.

Microwave Holographic Surface Measurement of the Tidbinbilla 64-m Antenna

The ‘holographic’ technique for accurately measuring the surface figure of large reflector antennas, described by Bennet et al, (1976) and Scott and Ryle (1977), has many advantages over older conventional survey methods. These include high speed, low cost, and the absence of any need for additional complex mechanical or optical survey devices. In essence, the technique consists of measuring the complex far-field response of the antenna at a single frequency using a terrestrial, satellite-borne or celestial radiation source of small angular diameter. This two-dimensional pattern is then Fourier-transformed to yield the complex illumination function across the antenna aperture. Antenna surface deviations are manifested as phase fluctuations in this function. In practice, a second antenna is needed to provide a phase reference.

UV Absorption Spectra of Molecules and the Connection with Pumping Mechanisms for Astrophysical Masers

Since interstellar masers were first discovered in 1965, various attempts have been made to account for the variety of behaviour observed. In particular a number of theoretical models have been advanced to account for the pumping mechanisms but none have gained a wide acceptance.Over forty molecules have now been identified in the interstellar medium, of these five exhibit maser action; they are detected by virtue of their small angular diameter and very large brightness temperatures &gt; 1013°K. The molecules in question are H20, OH, SiO, CH3OH and CH, of these the Hydroxyl maser presents the widest range of behaviour and therefore if the physical mechanisms involved could be understood these masers could be used to probe the interstellar environment and obtain more information about this important region where new stars are formed

Rotating neutron stars, pulsars, and cosmic X-ray sources

The purpose of this paper is to discuss the relationship between rotating neutron stars, pulsars, and cosmic X-ray sources. The latter may be divided into at least two classes: the sources with large angular diameters, such as the Crab Nebula, and those with small angular diameter, such as Sco X-1. I submit that a basic model, consisting of a rotating neutron star losing mass in the presence of a large magnetic field, can account for both types of X-ray source. The extended sources represent the case where the energy in the ‘neutron-star wind’ is greater than the magnetic energy. The streaming protons and electrons deposit their energy far out into the nebula in a shock transition region. The relativistic electrons responsible for the extended sources of radio, optical and X-ray emission are produced in the transfer of energy between the protons and electrons in the shock wave, and by magnetic pumping in hydromagnetic waves which are generated by fluctuations in the mass loss rate. The compact sources, such as Sco X-1, represent the other extreme where the magnetic energy dominates, so that no mass loss occurs. The particles are then accelerated and radiate in radiation belts around the neutron star, resulting in a source with a small angular diameter.

Rotating Neutron Stars, Pulsars and Cosmic X-Ray Sources

The purpose of this paper is to discuss the relationship between rotating neutron stars, pulsars, and cosmic X-ray sources. The latter may be divided into at least two classes: the sources with large angular diameters, such as the Crab Nebula, and those with small angular diameter, such as Sco X-1. I submit that a basic model, consisting of a rotating neutron star losing mass in the presence of a large magnetic field, can account for both types of X-ray source. The extended sources represent the case where the energy in the ‘neutron-star wind’ is greater than the magnetic energy. The streaming protons and electrons deposit their energy far out into the nebula in a shock transition region. The relativistic electrons responsible for the extended sources of radio, optical and X-ray emission are produced in the transfer of energy between the protons and electrons in the shock wave, and by magnetic pumping in hydromagnetic waves which are generated by fluctuations in the mass loss rate. The compact sources, such as Sco X-1, represent the other extreme where the magnetic energy dominates, so that no mass loss occurs. The particles are then accelerated and radiate in radiation belts around the neutron star, resulting in a source with a small angular diameter.

Neutral atomic hydrogen in 32 galaxies of small angular diameter.

Neutral atomic hydrogen in 32 galaxies of small angular diameter.

Position of the Low Frequency Radio Source in the Crab Nebula

RECENT radio observations of the Crab Nebula1–3 have demonstrated the existence of a component of small angular diameter which contributes a significant fraction of the total flux density at frequencies of less than about 100 Mc/s. The high brightness temperature deduced from the interplanetary scintillation of this source at 38 Mc/s (ref. 3) makes it difficult to attribute this radiation to the synchrotron process, and coherent radio emission from a dense plasma associated with the star has been suggested as a possible mechanism. The previously reported position for the small diameter source based on a lunar occultation observed at 26.5 Mc/s (ref. 2) lies about 1.2′ arc from the centre of the nebula, however, and is well removed from the centre of the observed expansion. The present observations show that the position of this source in the nebula must be much nearer the centre than the earlier measurement suggests.