The formation of condensed carbon particles in the atmospheres of white dwarfs

The existence of condensed carbon in the form of liquid droplets and graphite grains is found in white dwarf atmospheres with parametersg=10⁸ cm s^–2, H/He10^–3, andT _eff6000 K on the basis of model atmospheres techniques. It is shown that the condensation layers are dynamically stable and, consequently, that white dwarfs cannot supply the condensed particles to the interstellar medium. Possible observable effects are considered.     

Quasi-stellar objects as rotating magnetic superstars

In this paper the magnetic superstar model is used to discuss QSO luminosity and density evolution. Our main hypotheses are that (i) mass loss from old stars in massive galaxies cools and then falls into the centre to form a nuclear disc (Bailey, 1980); and (ii) magnetic superstars in galactic nuclei condense out of gaseous material at the centre of a supermassive-magnetised disc (Kundt, 1979). On this generalised model we find that the non-thermal (synchrotron) optical luminosity scales asL _opt L ³ t ^–7/3, whereL is the total blue luminosity of old stars in the galaxy and t is cosmic time. In addition we show that QSO co-moving density follows the lawD(t)exp-(t/t _Evol)^16/15 with an evolution timescalet _Evol = 1.95 × 10⁹ yr. The model as a whole is in good agreement with observations.     

Far-ultraviolet photometry of orion stars

Far-ultraviolet photometric data for early type stars in Orion, in the 1050–1180 and 1230–1350 Å wavelength ranges, were obtained in an Aerobee rocket flight on 30 January 1969. The results corrected for interstellar extinction, appear in good agreement with model atmospheres in the case of main-sequence stars. Bright giant and supergiant stars, however, appear to be up to one magnitude fainter than main-sequence stars of similar spectral class in the 1050–1180 Å range.The present results indicate that the detectors used in a previous flight were lower in sensitivity than was thought, so the results from that flight have been corrected using the present data for stars in common with the previous flight.The far-ultraviolet extinction law for Orionis is shown to be abnormal, with the observed extinction far less than predicted on the basis of ultraviolet extinction measurements of other stars, and the observedE(B-V).     

Infrared emission of dust grains in the clusters of galaxies

The observations of the reddening of the distant galaxies and the weak diffuse radiation in the clusters of galaxies can be interpreted as a consequence of the presence of dust grains in the intergalactic medium. When allowance is made for the destruction of the grains in collision with particles of the hot gas, its lifetime is about 10⁷–10⁸ yr at a gas concentrationn _g 10^–3 cm^–3. The detection of the infrared (IR) emission from the galaxy clusters might be the test for the proof of the presence of dust grains in the intergalactic medium. In this paper the estimates of the expected intensities and fluxes of IR emission for the spectral region 50–300 are presented for two galaxy clusters in Coma and Perseus. The parameters of the hot gas spatial distribution are chosen from X-ray observations. Having assumed that intergalactic dust can be ejected only from the galaxies, we used such a model for intergalactic dust grains which explains very well the interstellar dust effects. It is shown that the dust temperature, which is determined from the general energetic balance of the dust grains, can achieve some scores of degrees of Kelvin. Two models of the dust spatial distribution are considered. It is found that the maximum of IR flux for the Coma cluster lies near =100 and the same for the Perseus cluster near 50–70. The total fluxes of IR emission from these clusters are about 10⁵–10⁶ Jy and can be detected by modern observational methods.     

Mass outflow and its consequences in the circumnuclear zones of galaxies

The distribution of hydrogen gas, atomic plus molecular, in the discs of spiral galaxies, takes two characteristic extreme alternative forms. In one, the density peaks at the nucleus, and falls radially monotonically and roughly exponentially with radius. In the other there is a hole in the gas distribution in the circumnuclear region. In this paper we examine the distributions of gas, and the kinematics in the central zones of a number of spirals which have been observed both spectroscopically and with photometric mapping. We find in addition to a ring structure in the gas, there is often measurable expansion with higher radial velocities occurring near the nucleus. Associated with the more expanded of these ring structures there appear annuli of younger stars with enhanced metallicities; and inside the gaseous ring older, less metallic populations. A strong correlation exists between the absence of central gas and the size of the nuclear bulge of a galaxy: Sa's and Sb's have lower ratios of gas to stars than Sc's and Sd's. We show that radially progressive bursts of star formation can account for a wide range of these observed phenomena and could be related to the presence ofliners in the interstellar medium close to the nucleus. The energetics and dynamical balance within the burst are considered in terms of three sources of outflow: supernovae, stellar winds, and radiation from massive stars, with the probability that all these mechanisms contribute to the collective phenomenon.     

Gradients of excitation in the nuclei of starburst galaxies

This article forms a part of a wider study of the nuclear and circum-nuclear zones of moderately active galaxies. The use of long-slit spectroscopy at high spectral and spatial resolution has enabled us to measure gradients of gaseous excitation and also velocity fields in a sample of four bright nuclear starburst galaxies selected from the sample of Balzano (1983). We find variations not only in the intensity but also in the quality of the emission spectra (line ratios and line widths) of the interstellar gas between regions separated by relatively short distances. We stress the need for studies with at least the present degree of angular and spectral resolution if physical sense is to be made of the interstellar excitation regimes in external galaxies, as well as to investigate a possible evolutionary link between nuclei of intermediate activity.     

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.     

Galactic longitude dependence of the intensity ratio of the diffuse interstellar absorption bands at λλ6180 and 4430

Equivalent widths, as published by Seddon, for the diffuse interstellar absorption band at 6180 are compared with photoelectric 4430 indices for ten stars in the northern Milky Way. The intensity ratio is found to depend on galactic longitude in a way similar to that found by Johnson for the ratioA _V /E _B–V of total to selective interstellar absorption.The dependence found, combined with deductions from a tentative identification of the bands as due to negative hydrogen, points to a direction for the galactic magnetic field that is in better agreement with the direction derived from rotation measures of extragalactic sources than with that based on the dust-hypothesis interpretation of interstellar optical polarization.     

Starbursts at space ultraviolet wavelengths

Starbursts are systems with very high star formation rate per unit area. They are the preferred place where massive stars form; the main source of thermal and mechanical heating in the interstellar medium, and the factory where the heavy elements form. Thus, starbursts play an important role in the origin and evolution of galaxies. The similarities between the physical properties of local starbursts and high-z star-forming galaxies, highlight the cosmological relevance of starbursts. On the other hand, nearby starbursts are laboratories where to study violent star formation processes and their interaction with the interstellar and intergalactic media, in detail and deeply. Starbursts are bright at ultraviolet (UV) wavelengths, as they are in the far-infrared, due to the ‘picket-fence’ interstellar dust distribution. After the pioneering IUE program, high spatial and spectral resolution UV observations of local starburst galaxies, mainly taken with HST and FUSE, have made relevant contributions to the following issues:

The determination of the initial mass function (IMF) in violent star forming systems in low and high metallicity environments, and in dense (e.g. in stellar clusters) and diffuse environments: A Salpeter IMF with high-mass stars constrains well the UV properties.

The modes of star formation: Starburst clusters are an important mode of star formation. Super-stellar clusters have properties similar to globular clusters.

The role of starbursts in AGN: Nuclear starbursts can dominate the UV light in Seyfert 2 galaxies, having bolometric luminosities similar to the estimated bolometric luminosities of the obscured AGN.

The interaction between massive stars and the interstellar and intergalactic media: Outflows in cold, warm and coronal phases leave their imprints on the UV interstellar lines. Outflows of a few hundred km s^?1 are ubiquitous phenomena in starbursts. These metal-rich outflows and the ionizing radiation can travel to the halo of galaxies and reach the intergalactic medium.

The contribution of starbursts to the reionization of the universe: In the local universe, the fraction of ionizing photons that escape from galaxies and reach the intergalactic medium is of a few percent. However, in high-z star-forming galaxies, the results are more controversial.

Despite the very significant progress over the past two decades in our understanding of the starburst phenomenon through the study of the physical processes revealed at satellite UV wavelengths, there are important problems that still need to be solved. High-spatial resolution UV observations of nearby starbursts are crucial to further progress in understanding the violent star formation processes in galaxies, the interaction between the stellar clusters and the interstellar medium, and the variation of the IMF. High-spatial resolution spectra are also needed to isolate the light from the center to the disk in UV luminous galaxies at z = 0.1–0.3 found by GALEX. Thus, a new UV mission furnished with an intermediate spectral resolution long-slit spectrograph with high spatial resolution and high UV sensitivity is required to further progress in the study of starburst galaxies and their impact on the evolution of galaxies.     

Observational manifestations of early mixing in B- and O-type stars

The helium and nitrogen enrichment of the atmospheres of early B-type stars during the main sequence (MS) evolutionary phase is re-analysed. It is confirmed that the effect depends on both the aget and the stellar massM. For example, the helium abundanceHe/H increases by 0.04 (60–70% of initial value) for stars withM=8–13M and by 0.025 (about 30%) for stars withM=6M . The nitrogen abundance rises by three times forM=14M and by, two times forM=10M . According to the latest theoretical computations, the observed appearance of CNO-cycled material in surface layers of the stars can be a result of the rotationally induced mixing, in particular, of the turbulent diffusion. Carbon is in deficiency in B stars, but unexpectedly does not show any correlation with the stellar age. However it is shown that the total C+N abundance derived for early B stars conflicts with the theory.Basing on modern data the helium enrichment is first examined in O-type MS stars, as well as in components of binaries. As compared with early B stars, the He abundance for more massive O stars and for components of binaries show a different relation with the relative aget/t _MS . Namely during short time betweent/t _MS 0.5 and 0.7 a sharp jump is observed up toHe/H=0.2 and more. In particular, such a jump is typical for fast rotating O stars (v sini200 km s^–1),. Therefore the effect of mixing depends on massM, relative aget/t _MS , rotational velocityv and duplicity.The mass problem (the discrepancy betweenM _ev andM _sp ) is also analysed, because some authors consider it as a possible evidence of early mixing, too. It is shown that the accurate data for components of binaries lead to the conclusion that the discrepancy is less than 30%. Such a difference can be removed at the expense of theM _ev lowering, if the displacement of evolutionary tracks, owing to the rotationally induced mixing is taken into consideration.     

Evolution of the M31 disk population

An evolutionary model for the M31 inner disk population is described, which at age 12 billion years agrees closely with the narrow-band colors and line indices recently measured by Spinradet al. (1971), and with the broad-band colors from 0.36 to 3.4 . Assuming that gE galaxies have the same stellar population as the M31 inner disk, this model is used to derive evolutionary effects in cosmology. Interpretation of the magnitude-redshift relation is substantially affected, since the evolutionary correction to V magnitudes at redshift 0.2 may be 0.09 mag. if H₀=100 km s^–1 Mpc^–1, or 0.25 mag. ifH ₀=50. Changes in some line strengths may be observable at redshift 0.46, showing the younger stellar population at earlier times. Our results differ from estimates by other authors, also based on the assumption that the gE and disk populations are identical, that the photometry of Spinradet al. (1971) precludes a significant rate of evolution. One reason for the difference is that our model has an unexpectedly shallow luminosity function just below the main sequence turnoff; another reason is that authors in the past have used the change in absolute V magnitude as the evolutionary correction, whereas the appropriate quantity is the change in absolute magnitude at theemitted wavelengths, and evolution is more rapid at these shorter wavelengths.     

Desorption of molecules from interstellar grains and the heating of dust clouds

Temperature fluctuations induced in very small (r10^–3 ) interstellar grains by the absorption of photons from the ultra-violet radiation field or by energy released on molecule formation are shown to lead to significant gas heating due to thermal desorption of condensed atoms or molecules. For clouds with N(H)=1–10×10²⁰ cm^–2, heating rates due to this process are comparable to direct heating by cosmic rays or grain photoelectrons.     

The interstellar band at λ 4430 Å and the abundance of interstellar iron,titanium, and molecular hydrogen

For the direction to a number of stars the depletion of interstellar gaseous iron and titanium as well as the relative abundance of molecular hydrogen and the strength of the interstellar band at 4430 Å were determined by different authors and can be found in the literature. In this paper it is shown that the difference (A _c-A_co) is a more reliable measurement of the column density of 4430 Å absorbers than the usually used central depth,A _c, because the positive valueA _co ofA _c forE(B-V)=0, i.e. the intercept with theA _c axis of a least-squares fit to the observedA _c vsE(B-V) data, is with a high probability not caused by an interstellar effect.There was no correlation found between the interstellar depletion of iron and titanium, respectively, and (A _c-A_co), whereas a tendency exists that with increasing relative molecular hydrogen abundance the number of 4430 Å absorbers per hydrogen atom decreases. If the carriers of the 4430 Å absorbers are interstellar grains, then these grains must be altered during the same process in which molecular hydrogen is built. The found correlation is also compatible with the assumption that the 4430 Å absorber is related to an interstellar gaseous species.     

The system of old clusters in the magellanic clouds

IntegratedUBV colours have been computed for synthetic clusters older than one billion years and for two chemical composition: (a)Y=0.30;Z=10^–4 and (b)Y=0.30;Z=10^–2, taking into account the contribution to the integrated light of Main Sequence, subgiant, red giant and horizontal branch stars. It has been found that integrated colours depend onZ and allow an estimate of the metal content, however not generally. Horizontal branch stars contribute to the integrated colours of clusters not significantly and the contribution of stars in more advanced phases (e.g., asymptotic branch stars) is almost negligible.Old clusters in LMC and SMC have been studied in terms of colour calibrations and this analysis has been supplemented, when possible, by photometric and spectroscopic data of individual stars. It was found that in the LMC clusters withZ=10^–2 andt>5×10⁹ yr are lacking, clusters with relatively blue colours are similar, both in age and chemical composition, to the halo galactic globular clusters. Moreover, there is a group of clusters with 1×10⁹t5×10⁹. In the SMC clusters withZ=10^–2 andt>5×10⁹ yr are lacking and clusters with 1×10⁹t5×10⁹ are rare. Clusters with relatively blue colours are interpreted with the following parameters:t=5×10⁹ yr, 10^–4Z10^–3 andY=0.20.The implication of these results on the chemical history of the two galaxies is discussed.     

The distribution of interstellar matter in NGC 654

The young cluster NGC 654 is studied using UBV photographic photometry with a view to determining the distribution of interstellar matter in a region where star formation recently occurred.NGC 654 is found to be enclosed in a shell of interstellar matter of mass 1500M . The mass of all stars in the cluster is 4000M .     

Circumstellar clouds derived from the 2800 Mgii data

As a result of the analysis of the observed interstellar 2800 Mgii absorption line data, an empirical relationship — a positive correlation — between the equivalent widthW(2800) and the effective temperature of the starT was discovered (Figure 1). However, in the case when this doublet is of stellar (photospheric) origin, only a negative correlation betweenW(2800) andT exists. Hence, the existence itself of such a positive correlation betweenW(2800) andT may be viewed as incomprehensible for the present influence of the star on the strength of the absorption line 2800 Mgii of nonstellar origin.On the other hand, we have evidence that the ionizing radiation of hot stars cannot provide for the observed very high degree of ionization of the interstellar magnesium. In particular, the observations give for interstellar magnesium the ratioN ⁺/N ₁ 1000, while in the case of ionization under the action of stellar radiation only we haveN ⁺/N ₁ 10.The assumption that circumstellar clouds surround hot stars can naturally explain these and other similar facts. A method for the determination of the general parameters-size, concentration, mass etc. — of the circumstellar clouds is developed. The main results of the application of this method to the relation of more than 20 hot stars are:(1) The circumstellar clouds surround almost (70%) all hot giants and subgiants. In the remaining (30%) cases, the absence of circumstellar envelopes requires additional evidence. (2) The linear sizes of circumstellar clouds vary within wide ranges — from 0.002 pc up to 1 pc. Most frequent are clouds with size of 0.1 pc. (3) The main concentration of hydrogen atoms (electrons) in circumstellar clouds is of the order of 100 cm^–3; the minimum value is 20–30 cm^–3, the maximum 10⁴ cm^–3. In one case (Deneb) the electron concentration rises up to 10⁵ cm^–3 for the size of the cloud 0.001 pc=3×10¹⁵ cm. (4) Stars of the same spectral and luminosity classes may possess circumstellar clouds characterized by quite different parameters. (5) Hydrogen in circumstellar clouds is completely ionized; for these clouds the optical depth _c 1; on the average,T _c 0.005. (6) The integrated brightness of circumstellar clouds is substantially fainter (by 8–10^m) than that of the central star. This is the reason why these clouds cannot be detected by ground-based observations. (7) The masses of individual circumstellar clouds vary from 1 down to 10^–4 . This gives for the mass ejection rate from 10^–10 to 10^–6 per year in case if these clouds are formed by the braking and accumulation of the ejected mass.The method of 2800 Mgii seems very convenient, fruitful and promising for the detection and study of circumstellar envelopes. Also, this method is very sensitive for a determination of the general parameters of such clouds, and concerns practically all their geometric, physical, kinematic and other properties.     

On the nature of interstellar grains

Data on interstellar extinction are interpreted to imply an identification of interstellar grains with naturally freeze-dried bacteria and algae. The total mass of such bacterial and algal cells in the galaxy is enormous, 10⁴⁰ g. The identification is based on Mie scattering calculations for an experimentally determined size distribution of bacteria. Agreement between our model calculations and astronomical data is remarkably precise over the wavelength intervals µ^–1 < ;^–2 < 1.94µ^–1 and 2.5µ^–1 < ;^–1 < 3.0 ;^–1. Over the more restricted waveband 4000–5000 Å an excess interstellar absorption is found which is in uncannily close agreement with the absorption properties of phytoplankton pigments. The strongest of the diffuse interstellar bands are provisionally assingned to carotenoid-chlorophyll pigment complexes such as exist in algae and pigmented bacteria. The 2200 Å interstellar absorption feature could be due to degraded cellulose strands which form spherical graphitic particles, but could equally well be due to protein-lipid-nucleic acid complexes in bacteria and viruses. Interstellar extinction at wavelengths <1800 Å could be due to scattering by virus particles.     

Some remarks on the universal X-ray background

It is shown that a universal steady X-ray background with the energy flux 10^–7 erg cm^–2 s^–1 sr^–1 can arise as a superposition of radiation from pulsars (neutron stars) in various galaxies when it is taken into account that supernova outburst occurs in a galaxy at the rate of 10^–2/year.     

The gradient of the brightest star age and density across the spiral arm S4 in the Andromeda galaxy

On a plate obtained with the 2-m RC telescope at the Bulgarian National Observatory about 1400 stars in the spiral arm S4 of the Andromeda galaxy were measured. The limit of completeness is 20 _. ^m 2 (B magnitudes). In the central part of S4 (Figure 3) a pronounced gradient of star luminosity and density is found (Figures 6 and 7a). Here the stars become fainter at about 2 ^m and their surface density decreases tenfold at the distance 1 kpc from the inner edge of the arm. We have interpreted the decline of star maximum brightness from this edge as age gradient and have evaluated from it the velocity of star formation propagation across the arm, which is about 60 km s^–1. If the Andromeda galaxy has trailing spiral arms and the pitch angle of S4 is about 25° in its central part, the pattern velocity p7–14 km s^–1 kpc^–1. This value is close to that obtained earlier with the help of the Cepheids in the same part of S4 (Efremov, 1980). The absence of a pronounced asymmetry in the star distribution across the arm in the OB82 region may be connected with the position of the strongest dust lanes in front of the stellar spiral arm here. We have stressed that in one part of the same spiral arm there may be a pronounced age gradient, and there may be no such gradient in the near-by one. In spite of the known difficulties in understanding the structure of the Andromeda galaxy it is possible to draw some conclusions which are important for the theory of spiral arms. The detailed investigations of the nearest galaxies are, therefore, most useful for understanding the spiral structure nature.     

Optical identification of the IRAS point sources based on the first Byurakan survey

Optical identification of infrared sources from the IRAS Point Source Catalogue (PSC) is made by means of low-dispersion spectra of the First Byurakan Survey (FBS) and Palomar Observatory Sky Survey (POSS) red and blue images. The purpose of this work is to examine the composition of the PSC sample of fainter sources at high galactic latitudes and to reveal QSOs, infrared galaxies, red stars (C and M), planetary nebulae, for their further investigation at the optical range. 100 of 108 unknown IRAS sources in the region with 3^h50^m 7^h40^m and + 69° + 73° are optically identified. Optical coordinates, V magnitudes, color indices, and preliminary classes are determined. According to preliminary classification 3 objects turned out to be QSOs, 36 are galaxies with very interesting morphology, 5 are faint planetary nebulae, 9 are carbon stars, and 47 are late M-type stars.Published in Astrofizika, Vol. 38, No. 4, pp. 625–629, October–December, 1995.