Radio emission of the NGC 1499 nebula at decametric wavelengths

Observations of the ionized hydrogen region NGC 1499 have been carried out with the radio telescope UTR-2 at frequencies 12.6, 14.7, 16.7, 20 and 25 MHz. The half-power resolution of the instrument to zenith is 28×34 at 25 MHz. The average volume density of the non-thermal radio emission between the Sun and the nebula (1.75×10^–40 W m^–3 Hz^–1 ster^–1 at 25 MHz), the electron temperature of the HII nebula (T _e =4400 K), the measure of emission (ME=1500 cm^–6 pc) and other parameters have been obtained. Maps of brightness distribution over the source are presented for each observation frequency. The results are compared with previously obtained data.     

Evidence for supernova-induced star formation in the monoceros OB2 association

The Monoceros ring, a circular optical nebulosity 3°.5 in diameter and centred at R.A.=6^h37^m, Dec.=6°30 (l ⁱⁱ =205°.5,b ⁱⁱ =0°.2) is in good structural agreement with radio observations. A neutral hydrogen shell is also accurately projected on the ring. These observations are consistent with the Monoceros ring being a supernova remnant 90–100 pc in diameter expanding at about 45 km s^–1 and having an age of the order of a million years. Bright Hii regions containing early-type stars (e.g., galactic cluster NGC 2244 in the Rosette nebula) and extremely young stars of the OB association Mon OB2 lie at the edges of the ring. The positional and temporal coincidence of the Mon OB2 association with a supernova remnant suggests that probably the star formation in this region is induced or speeded up by the passage of a supernova shock wave through the clumpy interstellar medium.     

Excitation class of nebulae—An evolution criterion?

A principally new, quantitative system of the classification of the spectra of planetary nebulae is proposed. Spectral class of excitation class of the nebulap is determined according to the relative intensities of emission lines (N ₁+N ₂) [OIII]/4686 HeII and (N ₁+N ₂) [OIII]/H (Table I, Figure 1). The excitation classes are obtained for 142 planetary nebulae of all classes—low (p=1–3), middle (p=4–8), and high (p=9–12⁺) (Tables II, III, and IV). An empirical relationship between excitation classp and mean radius of nebulae is discovered (Figure 2). This relationship as well as excitation classp, as an independend parameter, admit an evolutionary interpretation. It is shown that after reaching the highest class of excitationp=12⁺ the nebulae decrease their class of excitation with the further increases of sizes. The diagram of this relationship has two nearly-symmetric branches — rising and descending with the apogee onp=12⁺ (Figure 2).     

The measurement of magnetic fields in the solar atmosphere above sunspots using gyroresonance emission

An analysis of the local sources (LS) structure of the S-component of solar radio emission confirms the presence of a core component which is characterized by strong circular polarization and a steep growing spectrum at shorter centimeter wavelengths. These details coincide in position with the sunspots' umbra and their height above the photosphere does not generally exceed about 2000 km. Gyroresonance emission of thermal electrons of the corona is generally accepted as being responsible for this type of emission. The spectral and polarization observations of LS made with RATAN-600 using high resolution in the wavelength range 2.0–4.0 cm, allow us to measure the maximum magnetic fields of the corresponding sunspots at the height of the chromosphere-corona transition region (CCTR). This method is based on determining the short wavelength limit of gyroresonance emission of the LS and relating it to the third harmonic of gyrofrequency.An analysis of a large number of sunspots and their LS (core component) has shown a good correlation between radio magnetic fields near the CCTR and optical photospheric ones. The magnetic field in CCTR above a sunspot is found only 10 to 20% lower than in the photosphere. The resulting gradient of the field strength is not less than 0.25 G km^–1. This result seems to contradict the lower values of magnetic fields generally found above sunspots using the chromospheric H line. Some possible ways of overcoming this difficulty are proposed.     

Spectroscopic study of the supergiant star 6 Cassiopeiae

Twenty-five coudé spectrograms (22 with dispersion 12 Å mm^–1 and three 7 Å mm^–1) of 6 Cassiopeiae (A3 Ia) have been studied. The observations were made at the Haute Provence Observatory. The results of the analysis suggest a correlation between the variations of the equivalent widths, the microturbulence and the radial velocity. The radial velocity and turbulent velocity present a rapid variation with time, even in intervals as short as about an hour. The hydrogen lines are slightly asymmetric but the strongest Feii lines are clearly asymmetric. We found that the amount of asymmetry of the strongest Feii lines (I>6) correlates with the loggf value, with the estimated laboratory intensityI, and with the equivalent widthW .The observations have been made at the Astronomical Observatory of Haute Provence (CNRS). This work has been supported by TUBITAK (Scientific and Technical Research Council of Turkey), and partially by CNR (Consiglio Nazionale delle Ricerche) of Italy.     

Estimates of the physical parameters of supernova remnants (SNR)

The various physical parameters of a SNR can easily be worked out from graphs established on the basis of the recalibrated relation between the radio surface brightness , and the linear diameterD of a SNR (Ilovaisky and Lequeux, 1972). These graphs lead to the estimation of the distancer (kpc), linear diameterD (pc), monochromatic power at 1 GHz,P ₁ GHz (W Hz^–1); and total powerP _tot (a) (erg. s^–1) of a SNR, given its mean angular diameter <> (arc min), flux density at 1 GHz, S₁ GHz (f.u.) and spectral indexa. Three SNR (W28A₁, Monoceros SNR, W49B) are used to illustrate the case. The radio spectrum of one of these (W49B), curved at low frequencies, is explained in terms of absorption by the diffuse interstellar medium. Various cases are discussed and some physical parameters of the absorbing matter are established.     

On the continuum and the absolute magnitude of eta Carinae

The excitation temperature, colour excess and continuum of Car in the visible and near ultraviolet have been determined from the study of the excitation of the ionized iron emission lines. The excitation temperatures of Feii and [Feii] are about 8000K, value which is very much lower than the mean ionization temperature of the elements in Car.E _B-V is about 1^m.1; the absolute visual magnitude is presently –10^m.5±1^m. From the equivalent widths of the emission lines the true continuum between 1.5 and 3.0 ^–1 has been derived. The correspondingB-V is –0^m.14, while the Balmer jump is less than 0^m.5. The continuum appears mainly nebular in origin with a strong contribution of the two-photon emission from the 2s-level of hydrogen in the blue.     

Local interstellar medium

This paper reviews and analyses various observational data about the local interstellar medium (LISM)-a volume with a radius of about 200 pc near the Sun. There are collected radio, IR, optical, UV, and X-ray observations of the ISM and data on the Sco-Cen association. All available information confirms Weaver's (1979) conclusions that the Sun is located near an edge of a giant cavern with a radius of about 180 pc and the cavern center coincides with the Sco-Cen associated center. The outer rim of the cavern is observed as numerous, very longHi flaments, filaments of the interstellar polarization, and soft X-rays radiated by coronal gas with a temperature of about 10⁶K. Close environment (from 10^–4 to 2–5 pc) of the Sun is filled by warm (about 10⁴ K)Hi with the number density 0.1–0.2 cm^–3, which is a corona of the local cloud of the ISM. The central part of the cloud is observed to the galactical center direction at a distance of 10–20 pc as Sancini and van Woerden's (1970)Hi filament. The cloud blown round by stellar winds has a horseshoe-like shape, bordering the Sun. Tinbergen's (1982) patch of polarization is observational evidence of the shape.Several arguments are given to show that the bright spots of soft X-rays (130–284 eV) near the galactic poles are produced by an interaction of stellar winds with outer edge of the local cloud near the ends of the patch of polarization. Lyman continuum radiation from Sco-Cen stars was shown to be probably the main source of ionization of extendedHii regions of low density in the LISM. Various data evidence that the North Polar Spur is a SNR in the local cavern with the age of about 10⁵ years. Interaction of the local cavern with an interstellar absorption-free tunnel stretched for more than 1 kpc along the galactical longitudel=240° is discussed. In conclusion several actual problems of investigation of the LISM were formulated.     

Accretion and electrostatic interaction of interstellar dust grains; Interstellar grit

This work is divided into 13 sections and 2 appendices, and aims to elucidate the accretion mechanism, which operates via image-theory forces, whenever two interstellar dust grains come close together. Section 1 is an introduction. Section 2 proposes that the distribution of interstellar grains be taken asn(r) r ^–4 to avoid distortion of the 3K microwave background by radiation from spinning grains. Section 3 examines each of three types of image force accretion processes, finding them to be dominant compared to radiation or gravitational forces by at least a factor of 10¹⁹. Section 4 states that only grains made of conducting material (e.g., graphite, ice, iron) are involved in image theory. Section 5 presents reasons for believing that two grains should coalesce on impact. Section 6 examines the motion of charged interstellar grains in Hi and Hii regions. Section 7 demonstrates, by way of four examples involving dust grains ofr=10^–7 cm up tor=10^–4 cm, that the image effects on conducting grains are not trivial, and that the dynamics involved is not to be compared at all with elementary Coulomb interaction of two changes. Section 8 concludes that accretion with not take place in Hi clouds if thermal (equipartition) velocities prevail among the dust particles. section 9 examines grain interactions in Hii regions: here, following an argument due to Spitzer, consideration is given to the case of a population of dust grains all streaming in the direction of the local magnetic field B at velocities of order 0.1 km s^–1. It is shown that accretion takes place effectively, leading to the formation of interstellar grit, meaning grains of mass 10^–8 to 10^–7 gm, radius 0.1 mm; and leaving also a population ofr10^–6 cm grains, which are observed in polarization and extinction measurements. The existence of the latter is now a deduction and not an ad hoc postulate, as previously, and implies a distribution of the general formn(r) r _mean ^–3 , in approximate agreement with that of Section 2. Section 10 considers the accretion mechanism as a cascade process. Section 11 shows that the existence of grains in space ofr 10^–6 cm rules out an origin in supernova or galactic explosions, and supports a passive origin, perhaps in red giants or Mira variables. Section 12 discusses the implications of the results found for polarization observations and cosmogony, the latter being given a new foundation in which planets of different composition form automatically from a solar nebula. Section 13 is a conclusion.     

Coronal survey in X-rays of O vii and Ne ix

We report some results of a rocket experiment flown on 29 April, 1971. A survey of the solar corona was carried out with a pair of collimated Bragg spectrometers to study the resonance, intersystem and forbidden line emission from the helium-like ions O vii (22 Å) and Ne ix (13 Å). In the direction of dispersion the collimator provided a field of view of 1.7. Also, the continuum radiation near 3 Å was monitored by a collimated proportional counter within a view angle of 4.2. The observed X-ray emission came from the general corona, seven plage regions, and one dynamic feature- the late stage of a small flare. From the intensity of the O vii and Ne ix resonance lines the electron temperature and emission measure of the individual emitting regions are derived on the basis of two models, one (a) in which the region is assumed to be isothermal and another (b) in which the emission measure decreases exponentially with increasing temperature. The latter model, which is the most adequate of the two, yields for the electron temperature of the time-varying feature 2–3 × 10⁶ K, for the other active regions 1.5–2.5 × 10⁶ K, and for the general corona 1.3–1.7 × 10⁶ K. The Ne ix emitting regions are about 1.5 times as hot as the O vii regions. The emission measure ranges from 0.4–2.3 × 10⁴⁸ cm^–3 for all active regions and is about 2 × 10⁴⁹ cm^–3 for one hemisphere of the general corona above 10⁶ K. From an analysis of the ratio, R, of the forbidden and intersystem lines of O vii we conclude that none of the regions producing these lines at the time of the rocket flight had electron densities exceeding about 3 × 10⁹ cm^–3. Our data demonstrate a dependence of R upon temperature in agreement with the theory of Blumenthal et al. (1971). The wavelengths for the intersystem, the 1s ²2s ² S ^e–1s2p2s ² P ⁰ satellite, and the forbidden transition show in the case of Ne ix improved agreement with predictions. The observed strength of the satellite lines for both O vii and Ne ix agrees with the predictions of Gabriel's (1972) theory, which attributes their formation to dielectronic recombination.We are saddened to report the death of A. J. Meyerott on 13 November, 1971.     

Extreme ultraviolet spectra of solar active regions and their analysis

Extreme ultraviolet spectra of several active regions are presented and analyzed. Spectral intensities of 3 active regions observed with the NRL Skylab XUV spectroheliograph (170–630 Å) are derived. From this data density sensitive line ratios of Mg viii, Si x, S xii, Fe ix, Fe x, Fe xi, Fe xii, Fe xiii, Fe xiv, and Fe xv are examined and typically yield, to within a factor of 2, electron pressures of 1 dyne cm^–2 (n _e T = 6 × 10¹⁵ cm^–3 K). The differential emission measure of the brightest 35 × 35 portion of an active region is obtained between 1.4 × 10⁴ K and 5 × 10⁶ K from HCO OSO-VI XUV (280–1370 Å) spectra published by Dupree et al. (1973). Stigmatic EUV spectra (1170–1710 Å) obtained by the NRL High Resolution Telescope and Spectrograph (HRTS) are also presented. Doppler velocities as a function of position along the slit are derived in an active region plage and sunspot. The velocities are based on an absolute wavelength scale derived from neutral chromospheric lines and are accurate to ±2 km s^–1. Downflows at 10⁵ K are found throughout the plage with typical velocities of 10 km s^–1. In the sunspot, downflows are typically 5 to 20 km s^–1 over the umbra and zero over the penumbra. In addition localized 90 and 150 km s^–1 downflows are found in the umbra in the same 1 × 1 resolution elements which contain the lower velocity downflows. Spectral intensities and velocities in a typical plage 1 resolution element are derived. The velocities are greatest ( 10 km s^–1) at 10⁵ K with lower velocities at higher and lower temperatures. The differential emission measure between 1.3 × 10⁴ K and 2 × 10⁶ K is derived and is found to be comparable to that derived from the OSO-VI data. An electron pressure of 1.4 dynes cm^–2 (n _e T = 1.0 × 10¹⁶ cm^–3 K) is determined from pressure sensitive line ratios of Si iii, O iv, and N iv. From the data presented it is shown that convection plays a major role in determining the structure and dynamics of the active region transition zone and corona.     

The study of multi-band radio observations on ultraviolet flares of chromospheric active binary V711 Tauri

The chromospherically-active binary, V711 Tau, had been observed by using the American Very Large Array (VLA) at five bands from 1.4 to 15 GHz. During the observation, the source was undergoing an intense flare, its radio luminosity up to 1.8 × 10¹⁸ erg s^–1 Hz^–1. The degree of circular polarization in the phase of the most intense flare was very small. With the decaying of the flare the flux density decreased, spectral index became smaller, spectra steeper and reversal frequency lower; the degree of circular polarization increased and its direction was dependent on frequency. These observational facts support the conclusion that the emission during intense flare is synchrotron (or synchro-cyclotron) mechanism. The magnetic intensity is about 10 G near = 1, the average electron energy, 4 MeV, the electron density with larger than 10 keV, 3 × 10⁴–9 × 10⁴ cm^–3 and the electronic energy spectrum index in power-law distribution 1.3.     

An estimate of the energy spectrum of gamma rays from the central region of the Galaxy and some implications

High resolution surveys of the galactic centre suggest the existence of an extended nonthermal source (Bulge) with an intensity much larger than the total background radiation in that direction. In this paper, we have first evaluated the physical conditions existing in this restricted region of space from an analysis of the radio spectrum and shown that if the distribution of matter, magnetic fieldB(r) and cosmic ray densityk(r) in the plane of the Galaxy is of gaussian type then at the centreB (0)=25–30 G andk(0)=25–35 times that in the near interstellar space. It is also found that most of the absorption in the Sagittarius A spectrum at low frequencies takes place in the Bulge and one requires a small additional absorption to take place in the line of sight corresponding to n _e ²10 cm^–6 pc at a temperature typically of clouds 100 K. The gamma ray spectra from the Bulge arising from interactions of cosmic rays with matter and radiation are then calculated in detail. A comparison made with the estimated background gamma ray spectra from the disk reveals that a detector with angular resolution 6° having a threshold of a few times 10^–6 photons cm^–2 s^–1 can detect this source; this bulge is not found to be a good X-ray source for detection. From a comparison of these calculations with the observed flux above 100 MeV, the following inferences have been deduced: (i) the lower limit to the magnetic field strength at the centre is 12 G, (ii) the observed gamma ray flux towards the Anti-centre can be well explained as due to interactions of cosmic rays with matter alone and a similar explanation towards the center reveals that cloud complexes could be more in the inner parts of the Galaxy than in the outer parts, and (iii) the observed flux values are found to be inconsistent with the existence of submillimeter radiation in the galactic scale.     

A study of non-thermal radio emission features using fine spectral BAO and high-sensitivity RATAN observations of a solar active region

Based on spectral observations of active region NOAA 8545 on 19 May 1999, we describe the processes responsible for non-thermal long-lasting radio emission and for narrow-band non-drifting bursts observed at the same time. Non-thermal long-lasting radio emission consisted of two components: short-duration (1–2 s) microbursts with fluxes about 0.001 s.f.u. and continuum emission with growing spectrum in the range of 1000–2000 MHz. Energetic electrons continuously existed in the active region for more than 2.2 hours. The nature and parameters of microbursts were discussed by Bogod, Mercier, and Yasnov (2001). Here we consider the continuum source nature. It is shown that the model, taking into account the cyclotron loss-cone instability of hot electrons and the generation of plasma waves at the upper hybrid frequency, may explain the observed continuum source parameters. For the narrow-band non-drifting bursts we consider two models: the first taking into account an excitation of weak shock waves across the magnetic field and the second with an excitation of the upper hybrid waves under the double plasma resonance. Continuum source parameters are close to the last model. Our estimations for the magnetic field strength are as follows: H=120–126 G, which is valid for the region where the electron density of background plasmas n=(1.4–1.9) ×10⁹ cm^–3; H=180–190 G for the region where n=(3.0–4.3) ×10⁹ cm^–3; H=290 G for the region where n _e=2.5×10¹⁰ cm^–3; and H=350 G for the region where n _e=3.5×10¹⁰ cm^–3. The speed of the fast electrons is about 0.10–0.14 c.     

A proposed correction to the solar abundances of carbon and oxygen utilizing new and accurate theoretical forbidden transition probabilities

Previously published solar abundances of oxygen and carbon can be corrected to be logN(O) = 8.93 and logN(C) = 8.60 on the hydrogen log-scale when new accurate forbidden electric quadrupole transition probabilities A _Q(s^–1) are used. Such A _Q's, based on the new atomic structure and electron correlation theory, developed recently by Sinanolu and coworkers, are reported for the (¹ S ₀-¹ D ₂) lines of [C i], [N ii, [O i] and [O iii] and the (² P-² D) lines of [N i] and [O ii]. The available experimental values are also given for comparison.Work supported by Grant No. GP-29471 from the U.S. National Science Foundation.     

Extreme ultraviolet flashes of solar flares observed via sudden frequency deviations: Experimental results

Properties of solar-flare EUV flashes measured via a type of ionospheric event, called a sudden frequency deviation (SFD), are presented. SFD's are sensitive to bursts of radiation in the 1–1030 Å wavelength range. He ii 303.8 Å, O v 629.7 Å, HL 972.5 Å and C iii 977.0 Å have essentially the same impulsive time dependence as the 1–1030 Å flash responsible for SFD's. Soft X-rays (2–20 Å) and certain EUV lines have a much slower time dependence than the 1–1030 Å flash. Most SFD's have some fine structure, but marked quasi-periodicity in EUV flashes is quite rare. EUV flashes are closely associated with hard X-ray bursts, white-light emission, microwave radio bursts and small bright impulsive kernels in the H flare. The intensity of EUV flashes depends on the central meridian distance of the H flare location; the intensity decreases at the limb. The total energy radiated in the 10–1030 Å flash for the largest events observed is about 10³¹ ergs.     

Modelling solar irradiance variations with an area dependent photometric sunspot index

The He 1083 nm line equivalent width and the 10.7 cm radio flux are employed to model the total solar irradiance corrected for sunspot deficit. A new area dependent photometric sunspot index (APSI) based on sunspot photometry by Steinegger et al. (1990) is used to correct the irradiance data for sunspot deficits. Two periods of time are investigated: firstly, the 1980–1989 period between the maxima of solar cycles 21 and 22; this period is covered by ACRIM I irradiance data. Secondly, the 1978–92 period which includes both maxima; here, the revised Nimbus-7 ERB data are used.For both He 1083 nm and 10.7 cm radio flux irradiance models as well as ACRIM I and ERB irradiance data, the APSI yields an improved fit compared to the one obtained with the standard Photometric Sunspot Index (PSI) which uses a constant bolometric spot contrast. With APSI, the standard deviation calculated from daily values is 0.461 Wm^–2 for the period 1980–89 modelling ACRIM I vs. He 1083 nm, as compared to 0.478 when PSI is used, and to 0.531 for the uncorrected ACRIM series. A similar improvement is obtained for the same period modelling ERB vs. He 1083 nm, while there is almost no improvement for the long period.As a general result the models provide a good fit with the spot-deficit.-corrected irradiance only during the period between the maxima. If both maxima are included (period 1978–92) the He 1083 nm and 10.7 cm radio flux models show appreciably larger discrepancies to the irradiances corrected for PSI or APSI.     

The neutral interface adjoining theHII region DR21

Observations of C100 and C125 atomic carbon recombination lines were made at the Algonquin Radio Observatory, towards the neutral interface separating theHii region DR21 (at RA=20^h37^m14^s, Dec=+42°0900) from its associated molecular cloud. An analysis of the Cn observations in conjunction with a simple model of a neutral interface enabled the derivation of the following parameters: electron density of 300 cm^–3, electron temperature of 30 K, microturbulent velocity of 2.3 km s^–1, and depth of the neutral interface of 0.01 pc. A single,stimulated emission model is sufficient to reproduce the Cn observations in the wavelength range from 4.6 cm (C100) to 21 cm (C166). All the known Cn data do support a pressure equilibrium between the neutral interface and theHii region, after the usual allowance is made for carbon depletion on grains.     

The local interstellar medium: A test-bed for the galactic ISM

We outline a method to explore the column density of the Local Interstellar Medium (LISM) using absorptions in the resonance H and K lines of Mgii. The intrinsic strengths of these lines in the temperature and density conditions prevailing in warm clouds (T _eff<10⁴ K) in the LISM allows them to be used to explore many lines of sight where lines such a NaD and Caii H and K are too weak, but where L is saturated. The number of measurable lines-of-sight is greatly enhanced by using cool stars as the background emitters, but this implies reliable separation of the LISM components from stellar chromospheric selfabsorptions. We explain how to do this, and how to use a combination of column density and radial velocity data to measure the spatial extent and the physical parameters of the single cloud in which the Sun is embedded. This proves to be an oblate spheroid, of characteristic diameter 8 pc, withT _eff 10⁴ K,n(Hi) of 0.1 cm^–3 and a mass <5M , streaming in the LSR from a point 1=4°,B=+16° with velocity equal to 16 km s^–1, and is surrounded by the much hotter lower density ionized gas of the local supernova bubble.     

Observations of the Rosette nebula NGC 2237 at decametric wavelengths

The results of observations of the Rosette emission nebula NGC 2237 with the radio telescope UTR-2 at frequencies 12.6, 14.7, 16.7, 20.0 and 25.0 MHz are given in the shape of contours of constant brightness temperature. The half-power beamwidth of the telescope to zenith at 25.0 MHz was 28×38. Density weighted mean values for the non-thermal radio emissivity between the Sun and the source (7.9×10^–41 W m^–3 Hz^–1 ster^–1 at 25.0 MHz) and the ratio of the intensity of emissivity generated before the area and the intensity of galactic radio emissivity appearing beyond the area equal to 1.3 have been obtained. The electron temperatureT _e=3600 K, the optical depth (about ten at 25 MHz), the measure of emission (ME=3500 cm^–6 pc), the electron densityN _e=8 cm^–3 and the nebular mass 16.6×10⁺³ M have been determined. A comparison with other observation results has been made.