A system of OB stars in the Carina nebula

Byurakan Astrophysical Observatory and Geneva Observatory. Translated from Astrofizika, Vol. 36, No. 1, pp. 35–54, January–Feburary, 1993.     

Temperature fluctuations in interstellar dust grains

Temperature fluctuations in interstellar dust grains caused by absorption of ultraviolet photons are discussed. Temperature probability distributions are presented for various assumptions about the grain specific heat and far infrared emissivity. Grains smaller than or about 0.01 may suffer large temperature fluctuations and would spend most of their time at very low temperatures. The equilibrium temperature is not a good measure for most average temperatures of such grains. It is shown that, although small grains spend only a small fraction of their time at the higher temperatures, the emitted far infrared spectrum is significantly shifted towards shorter wavelengths than predicted for grains assumed to be at the equilibrium temperature.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

The infrared spectrum of interstellar dust

The observed spectrum of GC-IRS 7 over the waveband 2.9–13 m can be explained on the basis of interstellar grains which are spectroscopically similar to dried-out diatomaceous organisms.     

Cross-linked hetero aromatic polymers in interstellar dust

The discovery of cross-linked hetero-aromatic polymersin interstellar dust by instruments aboard theStardust spacecraft would confirm the validity of thebiological grain model that was suggested fromspectroscopic studies over 20 years ago. Suchstructures could represent fragments of cell wallsthat survive 30 km s^-1 impacts onto detector surfaces.     

Shattering and coagulation of dust grains in interstellar turbulence

We investigate shattering and coagulation of dust grains in turbulent interstellar medium (ISM). The typical velocity of dust grain as a function of grain size has been calculated for various ISM phases based on a theory of grain dynamics in compressible magnetohydrodynamic turbulence. In this paper, we develop a scheme of grain shattering and coagulation and apply it to turbulent ISM by using the grain velocities predicted by the above turbulence theory. Since large grains tend to acquire large velocity dispersions as shown by earlier studies, large grains tend to be shattered. Large shattering effects are indeed seen in warm ionized medium within a few Myr for grains with radius   a ≳ 10⁻⁶  cm. We also show that shattering in warm neutral medium can limit the largest grain size in ISM  ( a ∼ 2 × 10⁻⁵ cm)  . On the other hand, coagulation tends to modify small grains since it only occurs when the grain velocity is small enough. Coagulation significantly modifies the grain size distribution in dense clouds (DC), where a large fraction of the grains with   a < 10⁻⁶ cm  coagulate in 10 Myr. In fact, the correlation among   R_V   , the carbon bump strength and the ultraviolet slope in the observed Milky Way extinction curves can be explained by the coagulation in DC. It is possible that the grain size distribution in the Milky Way is determined by a combination of all the above effects of shattering and coagulation. Considering that shattering and coagulation in turbulence are effective if dust-to-gas ratio is typically more than ∼1/10 of the Galactic value, the regulation mechanism of grain size distribution should be different between metal-poor and metal-rich environments.     

A new model for interstellar dust

Some physical, chemical and optical properties of mixed oxide grain material are discussed and the formation of such dust is examined. Surface chemistry on these grains may be responsible for the selective depletion of elements and for the formation of molecules such as H₂CO in diffuse clouds. Spectra of mixed oxide grains yield features at 9.6 and 18 m together with a broad band at 4.6 m^–1.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

A proto-planetary contribution to interstellar dust

It is argued that a significant mass fraction of interstellar dust might be formed in protoplanetary particle streams. Some astrophysical and cosmogonic consequences are discussed.     

Three-component dust models for interstellar extinction

Interstellar extinction curves obtained from the ‘extinction without standard’ method were used to constrain the dust characteristics in the mean ISM (R _V = 3.1), along the lines of sight through a high latitude diffuse molecular cloud towards HD 210121 (R _V = 2.1) and in a dense interstellar environment towards the cluster NGC 1977 (R _V = 6.42). We have used three-component dust models comprising silicate, graphite and very small carbonaceous grains (polycyclic aromatic hydrocarbons) following the grain size distributions introduced by Li & Draine in 2001. It is shown that oxygen, carbon and silicon abundances derived from our models are closer with the available elemental abundances for the dust grains in the ISM if F & G type stars atmospheric abundances are taken for the ISM than the solar. The importance of very small grains in modelling the variation of interstellar extinction curves has been investigated. Grain size distributions and elemental abundances locked up in dust are studied and compared at different interstellar environments using these three extinction curves. We present the albedo and the scattering asymmetry parameter evaluated from optical to extreme-UV wavelengths for the proposed dust models.     

Unreddened stars and the two-phase model of the interstellar medium

We have computed two phase models of the interstellar medium, with cosmic rays and X-rays assumed to be the main ionizing agents, heating due to photoelectron ejection from the interstellar grains. We show that it is possible to have a hot and tenuous intercloud medium in pressure equilibrium with the interstellar clouds for a wide range of physical conditions, possibly existing in the interstellar space. The atomic and ionic line observations towards Sco are shown to be consistent with the origin of these lines in the intercloud medium for a range of values of the ionizing flux. It is suggested that the intercloud medium may be predominantly neutral, with ionization rates consistent with the limits imposed by molecular observations. The mean fractional ionization of the intercloud medium is 1%.On leave from Tata Institute of Fundamental Research, Bombay, India.     

Dust formation in hot stars

It is known that the extremely hot environments of Wolf-Rayet stars and novae support dust formation, although in some selected cases only. The similarities in the luminosities of these objects suggest similar mechanisms of dust formation. The situation is reviewed in terms of the number of ionizing photons available for hydrogen, carbon and nitrogen other than helium. The larger abundance of nitrogen in the ejecta modifies these numbers significantly. Simple calculations for neutral carbon atoms via recombinations show that a critical condition is required to be met with for this purpose. This can be understood as due to the strong UV fields which leave the grains positively charged. Further, the type of dust appears to be decided by the ingredients constituting the ejecta.     

Chemically peculiar hot stars

The upper main sequence is home to a diverse family of chemically peculiar stars, including λ Boo, Am-Fm, Bp-Ap, HgMn, He-weak, and He-rich varieties. This paper presents an informal review of the physical properties of these objects, including their location in the H-R diagram, frequency of incidence, rotation, binarity, magnetic fields, and variability. Part of the discussion is devoted to describing the bizarre surface compositions encountered in chemically peculiar stars, with an emphasis on insights provided by a generation of ultraviolet observations obtained with theIUE satellite. The paper concludes with an overview of the radiative diffusion mechanism which has been proposed as an explanation of the chemically peculiar phenomenon.     

The feedback of massive stars on interstellar astrochemical processes

Astrochemistry is the discipline that studies physico-chemical processes in astrophysical environments. Such environments are characterized by conditions that are substantially different from those existing in usual chemical laboratories. Models which aim to explain the formation of molecular species in interstellar environments must take into account various factors, including many that are directly, or indirectly related to the populations of massive stars in galaxies. The aim of this paper is to review the influence of massive stars, whatever their evolution stage, on the physico-chemical processes at work in interstellar environments. These influences include the ultraviolet radiation field, the production of high energy particles, the synthesis of radionuclides and the formation of shocks that permeate the interstellar medium.     

On the interstellar extinction law toward young stars

We have determined the atomic hydrogen column density N _HI toward all of the young stars from the Taurus-Auriga-Perseus star-forming complex for which the corresponding spectra are available in the Hubble Space Telescope archive (nine stars) by analyzing the Lyα line profile. We show that the stars studied, except DR Tau, lie not far from the edge of the gaseous cloud of the star-forming region closest to us or, more precisely, inside the outer H I shell of the cloud. This shell with a column density of N _HI ? 6 × 10²⁰ cm^?2 surrounds the molecular gas of the cloud composed of a diffuse component (the so-called diffuse screen) in which dense, compact TMC-1 cores are embedded. The properties of the dust grains toward the stars that lie at the front edge of the cloud most likely differ only slightly from those of the interstellar dust outside star-forming regions. This casts doubt on the validity of the hypothesis that the extinction curve toward young stars has an anomalously low amplitude of the 2175 Å bump—such an extinction curve is observed for the field stars HD 29647 and HD 283809 toward which the line of sight passes through the TMC-1 core.     

Diffuse galactic FUV radiation and interstellar dust grains

A re-analysis of the diffuse far UV radiation ( 1350–1480 Å) observed in the sky region ofl ^II180° and 0°b ^II40° is presented, as a revised version of a paper by Hayakawaet al. (1969). In comparison with the previous one, the value of the half optical depth of the Galazxy in our wavelength region is reduced, and the values of the albedo coefficient and the forward phase functiong are not well determined. If, however, we combine our results with the theoretical model of interstellar grains by Gilra, the value of is given by 0.13(5)0.18(5).     

Cold dust around chamaeleon stars

Here we present the results of 1.3 millimetre continuum measurements for intermediatemass stars in the Chamaeleon system of dark clouds. The detected millimetre radiation is most probably thermal emission from cold circumstellar dust grains. The measured millimetre fluxes are combined with infrared observations to model the broad-band spectral energy distribution (SED). In this way the parameters of the emission regions are determined.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.