Showing papers on "Star formation published in 1981"
2,529 citations
TL;DR: In this article, conditions for the existence of a hydrodynamic instability by which galaxy formation leads to more galaxy formation due to the propagation of the energy released at the death of massive stars are examined, and it is shown that such an explosive amplification is possible at redshifts less than about 5 and stellar system masses between 10 to the 8th and 10 to 12th solar masses.
Abstract: The evolution of galaxies in an intergalactic medium dominated by explosions of star systems is considered analogously to star formation by nonlinearly interacting processes in the interstellar medium. Conditions for the existence of a hydrodynamic instability by which galaxy formation leads to more galaxy formation due to the propagation of the energy released at the death of massive stars are examined, and it is shown that such an explosive amplification is possible at redshifts less than about 5 and stellar system masses between 10 to the 8th and 10 to the 12th solar masses. Explosions before a redshift of about 5 are found to lead primarily to the formation of massive stars rather than galaxies, while those at a redshift close to 5 will result in objects of normal galactic scale. The model also predicts a dusty interstellar medium preventing the detection of objects of redshift greater than 3, numbers and luminosities of protogalaxies comparable to present observations, unvirialized groups of galaxies lying on two-dimensional surfaces, and a significant number of black holes in the mass range 1000-10,000 solar masses.
264 citations
TL;DR: In this paper, an infrared survey of a dense molecular cloud obscuring a part of the optical nebulosity associated with the supernova remnant W28 has been obtained, revealing a warm dense core in a region of substantial line broadening near a maximum of nonthermal radio emission from the remnant.
Abstract: Molecular spectra and an infrared survey of a dense molecular cloud obscuring a part of the optical nebulosity associated with the supernova remnant W28 have been obtained. The spectra reveal a warm dense core in a region of substantial line broadening near a maximum of nonthermal radio emission from the remnant. A small (approx.2 km s/sup -1/) shift in the velocity of peak molecular emission also occurs in this region. No embedded infrared source capable of heating the cloud appears to be present. The cloud appears to have been impacted by the expanding supernova remnant. The cloud appears to be an ambient cloud only recently compressed and heated by the nearby remnant; no evidence for star formation has been found. The remarkably broad HCO/sup +/ lines found near the cloud core originate in a region of enhanced ionization in the cloud, quite possibly resulting from penetration of the cloud by energetic radiation from the remnant.
80 citations
TL;DR: In this paper, three extragalactic regions of rapid star formation with redshifts great enough to separate the Lyman alpha region from geocoronal Lyman-alpha have been observed with the IUE satellite.
Abstract: Three extragalactic regions of rapid star formation with redshifts great enough to separate the Lyman-alpha region from geocoronal Lyman-alpha have been observed with the IUE satellite. Only the low metal abundance object had detectable Lyman-alpha emission with Lyman-alpha/H-alpha of about 6; the other two have Lyman-alpha/H-alpha less than 0.6 (2 sigma). In the normal metallicity objects, there is some evidence for Lyman-alpha extinction in addition to that computed from the Balmer decrement. Lyman-alpha is therefore expected to be weak or absent in collapsed primeval galaxies because of their predicted moderate to high metallicity.
64 citations
TL;DR: In this article, the expansion law of a supernova remnant, which is formed by sequential explosions of supernovae, was studied and the superbubbles and supershells with the radii 200∼1000 pc were naturally explained by this model.
Abstract: From the standpoint of view that the early type stars are formed sequentially at an OB association, it is expected that the supernova explosions will also occur sequentially. We study the expansion law of a supernova remnant, which is formed by sequential explosions of supernovae. The superbubbles and supershells with the radii 200∼1000 pc are naturally explained by this model. Assuming that the sequential explosion of supernovae occurs at every OB association, we deduce the star formation rate in our Galaxy.
62 citations
TL;DR: In this article, the authors considered the response of a gas to a forcing potential in the tight-winding density wave theory, and treated the cloud fluid in the hydrodynamic limit with an equation of state which softens at high densities.
Abstract: With regard to the galactodynamics of the cloudy interstellar medium, the paper considers the response of such a gas to a forcing potential in the tight-winding density wave theory. The cloud fluid is treated in the hydrodynamic limit with an equation of state which softens at high densities. It is shown that in the inner regions of the galaxy, cooling of the cloud fluid in the arms can result in gravitational instability and the formation of large bound complexes of clouds which are identified with the giant molecular clouds (GMCs). Masses, dimensions, distributions, and scale heights of the GMCs are predicted by the theory. It is suggested that the interstellar gas density in the disk is regulated by the gravitational instability mechanism in the arms which siphons material into star formation. Implications for the evolution of individual GMCs and for galactic morphology are discussed.
58 citations
TL;DR: In this article, the general equations of chemical evolution are revised to separate living stars explicitly from dead remnants; it is then easier to incorporate constraints based on star counts, etc, consistently into models.
Abstract: Three main points are made in this paper: (1) It is shown that, contrary to common belief, extrapolation of standard data suggests that ''stars'' below 01 M/sub sun/ are most unlikely to add significantly to the local surface density (2) The general equations of chemical evolution are revised to separate living stars explicitly from dead remnants; it is then easier to incorporate constraints based on star counts, etc, consistently into models (3) A schematic, analytic model is proposed for the solar neighborhood: there is an initial burst of (halo) star formation, followed by a lull with no star formation, and then by evolution of the disk itself with constant rates of star formation and infall This model crudely represents the outer regions of some dynamical models for disk formation, and it is related to two-era models by many authors, and to a recent disk model by Twarog A new specific model is proposed, with empirical constraints based on point (1) and on Twarog's stellar ages and metallicities Predictions of the model agree with nucleochronological ages of the elements and with the stellar age-metallicity relation
36 citations
TL;DR: In this article, a method for the construction of color variations and surface brightness across a galactic spiral arm, using the concepts and results of the density-wave theory, is developed, where the color variation measured by the index Q varies with the ratio of the light of young stars to that of the old stars, the initial mass function, and the pattern speed.
Abstract: A method for the construction of color variations and surface brightness across a galactic spiral arm, using the concepts and results of the density-wave theory, is developed. The color variation measured by the index Q varies with the ratio of the light of young stars to that of the old stars, the initial mass function, and the pattern speed. The fact that the variation of brightness and color across a spiral arm is relatively smooth and symmetrical and not sharp as in the galactic shock picture is due to the smearing-out effects of star formation inside a cloud in the regions behind the galactic shock, and the tendency of new stars to fall back to small galactocentric distances after their formation because of their lower angular velocity at birth. Both effects arise naturally in the context of the density-wave theory.
33 citations
31 citations
TL;DR: In this article, five-band UV photometry is used to investigate the character of interstellar extinction toward 1367 stars of spectral type B3 and earlier, with observations producing a galactic average of 5.11, 4.78, 6.52,4.10 and 1.95 at 1550, 1800, 2200, 2500, and 3300 A, respectively.
Abstract: Five-band UV photometry is used to investigate the character of interstellar extinction toward 1367 stars of spectral type B3 and earlier, with observations producing a galactic average of 5.11, 4.78, 6.52, 4.10 and 1.95 at 1550, 1800, 2200, 2500, and 3300 A, respectively. Differences in the strengths of the UV extinction features of these stars appear to be independent, since objects with either strong or weak 2200 A extinction can have strong, normal or weak far-UV extinction. Further investigation reveals that some of the deviant stars are embedded in localized regions whose average extinction curves diverge dramatically from the galactic mean. The anomalous extinction of these regions attests to localized inhomogeneities in the UV characteristics of interstellar dust. Because the evaluation of UV extinction toward a particular star would be more accurate if based on the average curve of the region immediately surrounding that star rather than that of the entire Galaxy, it is recommended that the galactic average presented be used with caution to correct astronomical data of individual objects.
31 citations
TL;DR: In this article, a near infrared search of the H II region/molecular cloud complex N159 in the Large Magellanic Cloud has revealed a very red (H-K = 2.1, K-L prime = 0.7) compact object.
Abstract: A near infrared search of the H II region/molecular cloud complex N159 in the Large Magellanic Cloud has revealed a very red (H-K = 2.1, K-L-prime = 2.7) compact object. The location, brightness, color and 2.1-2.4 micron spectrum of this source suggest that it is very young, and similar to the galactic infrared protostars. This is the first identification of an infrared protostar in an external galaxy. Its discovery provides direct evidence of current star formation in the Large Magellanic Cloud, and suggests that regions of star formation in external galaxies will appear similar to those in the Milky Way.
Journal Article•
TL;DR: In this article, the effect of stellar-induced star formation is explored in the context of a one-zone model galaxy, and it is shown that under a variety of assumptions, it is possible to obtain bursts of star formation for two and three-phase models (stars, diffuse gas, and clouds).
Abstract: The effect of stellar-induced star formation is explored in the context of a one-zone model galaxy. The formalism is similar to that employed in standard models for the chemical evolution of such a system. It is shown that under a variety of assumptions, it is possible to obtain bursts of star formation for two- and three-phase models (stars, diffuse gas, and clouds). One of the controlling factors during such bursts is the rate of infall of halo and extragalactic material. The model can be generalized to include also the effects of density wave-induced collapse of clouds and formation of clouds from diffuse gas due to spiral arm shocks. A comparison is drawn with the behavior observed in stochastic models for flat rotation curve galaxies.
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TL;DR: In this paper, strong interstellar absorption lines of C I, arising from the two excited fine-structure levels, are found in IUE observations of HD 72350 (type B4 III).
Abstract: Strong interstellar absorption lines of C I, arising from the two excited fine-structure levels, are found in IUE observations of HD 72350 (type B4 III). An analysis of the excited-level populations of C I gives local temperature and pressure limits, and auxiliary data on the limit of column density for excited O I and the carbon ionization help to establish that (1) the local temperature is within the limits of 25-100 K, and (2) the pressure/Boltzmann's constant ratio is at least 10 to the 4.3/cu cm K, despite its small size. This high-pressure cloud is discussed in terms of shock compression by the Vela supernova blast wave, along with the relationship of this kind of cloud compression to star formation and to the origin of the characteristic filamentary emission arcs seen in Vela and other supernova remnants
Journal Article•
TL;DR: The Imaging Proportional Counter on the Einstein X-ray Observatory has observed an approximately 1.0 deg square region in Orion centered near right ascension (1950) = 5 hours 34 min, declination = -6 deg 47 arcmin this paper.
Abstract: The Imaging Proportional Counter on the Einstein X-ray Observatory has observed an approximately 1.0 deg square region in Orion centered near right ascension (1950) = 5 hours 34 min, declination = -6 deg 47 arcmin. This is an area containing a dark cloud with evidence for recent star formation and several Herbig-Haro objects. Some 25 stellar X-ray sources are found, and several are identified with stars which have X-ray luminosities at the upper end of the distribution of X-ray luminosity versus stellar type. In addition there is evidence for X-ray emission associated with HH 1 which can provide support for models which include strong stellar winds interacting with the interstellar medium.
TL;DR: In this article, it was found that the central, optically bright region of the spiral galaxy NGC 1097 is a strong 10 micron source and a striking correlation is seen between the optical and infrared surface brightnesses.
Abstract: It is found that the central, optically bright region of the spiral galaxy NGC 1097 is a strong 10 micron source. A striking correlation is seen between the optical and infrared surface brightnesses. The properties of these sources imply that young stars power the infrared emission. It is inferred that the central 30 arcsec of NGC 1097 has a large bolometric luminosity of about 10 to the 11th solar luminosities, most of which originates in a ring approximately 2 kpc in diameter. These observations strongly support the hypothesis that the infrared emission from galaxies is often associated with bursts of star formation.
TL;DR: A numerical hydrodynamics code has been used to calculate the collapse of rotating, adiabatic clouds as discussed by the authors, which allows the clouds to fragment in the dynamic collapse phase, implying that the low thermal energy fragments produced in isothermal collapse calculations may undergo a subsequent dynamic fragmentation in the nonisothermal regime.
Abstract: A numerical hydrodynamics code has been used to calculate the collapse of rotating, adiabatic clouds. The three-dimensional nature of the calculation allows the clouds to fragment in the dynamic collapse phase. Clouds with adiabatic exponent of 7/5 and initial cos(2 phi) density variations fragment into binary systems if the initial ratio of thermal to gravitational energy is small (about 0.05). Clouds with higher thermal energy, however, damp the density variation and form near-equilibrium ellipsoids, with ratios of rotational to gravitational energy less than the critical value for dynamic growth of nonaxisymmetry in Maclaurin spheroids. Even with an adiabatic pressure law, dynamic fragmentation of a collapsing cloud is possible, implying for star formation theory that the low thermal energy fragments produced in isothermal collapse calculations may undergo a subsequent dynamic fragmentation in the nonisothermal regime.
TL;DR: In this article, collapse and star formation processes in rotating turbulent interstellar gas clouds have been studied and a physical mechanism for angular momentum transport by turbulent viscosity has been proposed and incorporated in new collapse calculations.
Abstract: Collapse and star formation processes in rotating turbulent interstellar gas clouds have been studied For this purpose numerical collapse calculations have been performed for a number of representative cases These calculations have been carried out by a two-dimensional hydrodynamical computer code, which solves the equations of hydrodynamics explicitly, coupled to the Poisson equation The computer code has been written especially for this work and has been thoroughly tested The calculations in this work have been performed with an effort to obtain physically reliable results (by repeating the same calculations with different numerical spatial resolutions) A physical mechanism for angular momentum transport by turbulent viscosity has been proposed and incorporated in new collapse calculations The main results can be summerized as follows:
TL;DR: In this paper, a two-armed spiral shock (TASS) wave was fitted to observations of the H I gas in M81 by Visser, giving initial conditions for ballistic particles launched from the sprial arms.
Abstract: The two-armed spiral shock (TASS) wave, fitted to observations of the H I gas in M81 by Visser, is used to give initial conditions for ballistic particles launched from the sprial arms. The model predicts the location of molecular clouds which agrees with the one detection of CO in M81. In addition, our model agrees with the observed distribution of H II regions near the eastern arm. The model also predicts the radial velocities of, and velocity differenses between, H II regions and molecular clouds near the major and minor axes. When the model's disk linear density wave amplitude, spiral arm star mass function, and star formation rate are fitted to observations of the surface brightness, the predicted arm colors agree best with the observed ones when we use a linear density wave amplitude very near the observed one. The spiral arm stellar mass function required for this agreement suggests that spiral arms essentially make only O and B stars or that low-mass stars are hidden or not yet formed while the O and B stars are shining. The model also produces a galaxy which resembles a photograph of M81
TL;DR: The H II region S128 was mapped using the VLA and the WSRT at 6 and 20 cm wavelengths in this article, where a compact (approx.3'') HII region was observed approx.60'' north of the more extended H II regions whose radio structure corresponds to the optical emission.
Abstract: The H II region S128 was mapped using the VLA and the WSRT at 6 and 20 cm wavelengths. A compact (approx.3'') H II region was observed approx.60'' north of the more extended H II region whose radio structure corresponds to the optical emission. An H/sub 2/O maser source was detected offset from the compact source by approx.12''. This indicates three separate sites of star formation in this region. Various schemes of initiating star formation are discussed with reference to this distant H II region.
TL;DR: In this article, it was shown that the H/sub 2/ lines are formed in molecular clouds whose kinematics are determined by the gravitational potential of the nucleus of a star.
Abstract: A 2.0-2.4 ..mu..m spectrum taken at a resolution of 5 cm/sup -1/ (350 km s/sup -1/) with a small aperture (3''.8) centered on the nucleus of NGC 1068 exhibits both stellar and interstellar features. Stars are evident from the presence of first overtone CO absorption bands, the strengths of which suggest that late-type giant stars contribute about 25% of the 2.3 ..mu..m continuum within 2 '' (200 pc) of the galactic center. The estimated velocity dispersion of the stars is or approx. =3 ..mu..m and is inconsistent with extrapolation of the ..nu../sup -1.85/ power law ultraviolet source. Measured Brackett ..gamma.. (n = 7..-->..4) and quadrupole H/sub 2/ emission line fluxes are in agreement with previous, larger aperture measurements, implying that both the atomic and molecular emissions are predominantly confined to within 200 pc of the nucleus. The observed H/sub 2/ lines are much narrower than atomic emission lines, exhibiting widths similar to those of stellar absorption features.more » We argue that the H/sub 2/ lines are formed in molecular clouds whose kinematics are determined by the gravitational potential of the nucleus. Comparison of infrared luminosity, B2lt. slash luminosity, total H/sub 2/ line luminosity, and cloud mass supports the hypothesis that the infrared luminosity of NGC 1068 is determined by the level of star formation activity.« less
TL;DR: In this article, the distances of 284 galactic cepheids with photoelectric observations have been derived, with 111 additional ones without photo-electric observations, in spite of the strong influence of the absorption matter.
Abstract: On the basis of the PLC relation (1) or the PL relation by Van den Bergh (2) and the PC relation by Deanet al. (1978), the distances of 284 galactic cepheids with photoelectric observations have been derived. The space distribution of these cepheids with 111 additional ones without photoelectric observations, is studied. In spite of the strong influence of the absorption matter, which makes a great number of distant cepheids unknown (Figure 4), a conclusion is drawn that the cepheids do not trace spiral arms with only one possible exception: the Carina arm. The cepheidz-coordinate distribution confirms the finding of Fernie (1968) that the cepheid layer is inclined towards the formal galactic plane. On the basis of cepheid space density, a number of vast star complexes (Table I) are identified in which other young objects, together with cepheids fall. The existence of these complexes is explained by star formation in giant molecular clouds. The cepheid mean period increase towards the galactic centre is most probably connected with the existence of a ring between the Sun and the centre of the Galaxy, with the highest density of hydrogen and the highest rate of star formation.
Journal Article•
TL;DR: In this article, it was shown that the pattern of massive star formation in the Large Magellanic Cloud (LMC) is partially ordered. But the mean rate of chemical evolution of the LMC is smaller than that of the Milky Way, which is explained by the difference in shear for these galaxies.
Abstract: The considered investigation provides a unifying theoretical basis for an understanding of a variety of observations on the Large Magellanic Cloud (LMC). It is demonstrated that an intrinsically statistical theory can supply a model for individual objects. With the aid of the model of stochastic self-propagating star formation described by Gerola and Seiden (1978), many structural features, both in the stars and in the gas are related to the process of star formation and the evolution of the LMC. It is found that the pattern of massive star formation in the LMC is partially ordered. The stars occur in large clumps and their global pattern is basically irregular and asymmetric with respect to the rotation center. The mean rate of chemical evolution of the LMC is smaller than that of the Milky Way. This fact is largely explained by the model in terms of the difference in shear for these galaxies. The formation of large, dense regions of massive stars has a great effect on the gas.