Showing papers on "Star formation published in 1984"

Cool stars, stellar systems and the Sun

Abstract: These conference proceedings focus on cool stars, stellar systems and the Sun. Individual papers deal with star clusters, star evolution, and star models. Particular attention is paid to binary stars.

The nature of the embedded population in the rho Ophiuchi dark cloud : mid-infrared observations.

Abstract: In combination with previous IR and optical data, the present 10-20 micron observations of previously identified members of the embedded population of the Rho Ophiuchi dark cloud allow determinations to be made of the broadband energy distributions for 32 of the 44 sources. The majority of the sources are found to emit the bulk of their luminosity in the 1-20 micron range, and to be surrounded by dust shells. Because they are, in light of these characteristics, probably premain-sequence in nature, relatively accurate bolometric luminosities for these objects can be obtained through integration of their energy distributions. It is found that 44 percent of the sources are less luminous than the sun, and are among the lowest luminosity premain-sequence/protostellar objects observed to date.

The formation and early dynamical evolution of bound stellar systems.

Abstract: We present the results of numerical N-body calculations which simulate the dynamical evolution of young clusters as they emerge from molecular clouds. We follow the evolution of initially virialized stellar systems of 50 and, in some cases, 100 stars from the point in time immediately after the stars have formed in a cloud until a time long after all the residual star-forming gas has been dispersed from the system. By varying the star formation efficiency and the gas dispersal time for each model, we determined the combination of these parameters which result in the production of bound stellar groups after the gas not used in star formation is completely dispersed.

Cooling flows in clusters of galaxies

Abstract: Cooling gas in the centre of many clusters of galaxies is compressed and pushed inward by the thermal pressure of the hot, uncooled outer gas. Gravity acts as a focusing agent until the gas has cooled to galactic temperatures. The cooling process is thermally unstable, giving rise to optical filamentation. Once the gas cools below 104 K it presumably forms stars which add to the mass of a central galaxy. Accretion rates of up to 400 M⊙ yr−1 are inferred from X-ray measurements so that the mass of such central galaxies may be due to cooling flows. Their optical and UV appearance emphasize that the gas predominantly turns into low-mass stars with an initial mass function unlike that of the solar neighbourhood. Optical and X-ray observations of cooling flows can be used to investigate the continuing formation of the largest known galaxies.

Dissipative formation of an elliptical galaxy

Abstract: Dissipative collapse and star formation within a protogalaxy are modeled using a simple scheme of gas cloud collisions within an N-body code. The gas physics model incorporates pressure, dissipation, and star formation, with the associated ejection of metal-enriched gas back into the cloud medium. The gas and stars collapse under their self-gravity alone; adding a static potential corresponding to a massive dark halo suppresses both velocity anisotropies and mass loss in low-mass ellipticals. A set of models in general accord with the kinematic and chemical properties of individual ellipticals is found. These models are slowly rotating and flattened by anisotropic velocity dispersions, with the radial profiles of surface density having a logarithmic gradient near -2. The chemical enrichment of the gas is calculated with a local instantaneous recycling approximation. The logarithmic abundance gradient within a massive model galaxy is -0.5, and it flattens toward zero with increasing mass loss in lower mass galaxies. The projected contours of constant metallicity are usually only slightly more flattened than the constant surface density contours. The dependence of the results on the assumed description of the gas physics is fairly small for high-mass galaxies, where the gas processes are overwhelmed by the gravitational collapse.more » However, lower mass model galaxies have significant pressure support during collapse which makes the results more dependent on the gas physics description.« less

Infrared observations of interacting/merging galaxies

Abstract: The present sample of 20 galaxy systems, selected on the basis of morphological evidence for the tidal interaction or merger of two galaxies and observed at 1-10 microns, is noted to include 11 systems, detected at 10 microns, which have on average a significantly higher IR luminosity than noninteracting galaxies. The enhanced IR radiation is due to star formation bursts. On the basis of IR Astronomical Satellite results for a sample of galaxies, as much as 30 percent of all the far-IR emission observed arises in bursts of star formation that are triggered by interactions, and massive stars account for most of the luminosity in these bursts. It is suggested, in view of a massive star formation rate in the interacting and merging galaxies that is about 3 times higher than in noninteracting systems, that much of this star formation occurred in either nuclear regions or merger remnants.

Water-vapour maser emission from galactic nuclei

Abstract: We report here the progress of a survey of galactic nuclei for water-vapour maser emission at 22.235 GHz. We observed 29 late-type galaxies in November 1982, January 1983, and September 1983 using the 40-m radio telescope of the Owens Valley Radio Observatory (OVRO) equipped with a travelling-wave maser receiver1. We have detected maser emission from four nuclei: NGC4258 (M106), NGC1068 (M77), NGC3034 (M82), and NGC6946. The masers in NGC4258 and NGC1068 are extremely luminous; NGC1068 has the most luminous water-vapour maser yet reported—350 L⊙ assuming isotropic emission. We suggest that the extremely bright nuclear masers indicate an ongoing burst of star formation, and therefore provide a useful probe of the physical conditions of the starburst phenomenon.

New light on faint stars. VI: Structure and evolution of the Galactic spheroid

Abstract: Analyse d'observations recentes de la structure du spheroide galactique. On confirme l'existence d'une composante tres applatie du spheroide. Cette composante contient environ 2% des etoiles dans le voisinage solaire. On interprete les observations dans un modele

A luminous 3 kiloparsec infrared disk in NGC 1068

Abstract: A 10 micron map of the Seyfert galaxy NGC 1068 and airborne measurements of its angular extent in the far-infrared are presented. It is shown that the infrared emission originates primarily from two physically distinct regions; approximately half of the total infrared luminosity of 3 x 10 to the 11th solar luminosities is associated with the Seyfert nucleus and half with a 3 kpc (35 arc sec) diameter disk surrounding it. It is argued that the disk component of infrared emission originates from an extended but heavily obscured burst of star formation which resembles those seen in some non-Seyfert galaxies. This high-luminosity disk is distinguished more by its large size than by its high surface brightness. On the basis of current evidence it cannot be concluded that the high disk luminosity in NGC 1068 is causally related to its Seyfert activity.

Infrared observations of dust cloud structure in young R associations - NGC 1333, S68, and NGC 7129

Abstract: Infrared photometry is presented from 1 to 100 microns of the major sources of luminosity in three regions of star formation which contain young B and A, but not O, stars. The reflection nebulae in all three regions are sources of 100-micron emission similar to previously observed optical reflection nebulae. All three regions also contain highly obscured sources which are often associated with radio maser activity, high-velocity gas outflows, and unusual radio continuum emission. There is a strong correlation between the occurrence of these radio phenomena and the presence of dust close enough to the central objects to radiate strongly in the 3-20 micron spectral region. These obscured sources are likely to be the pre-main-sequence counterparts to the nearby visible main-sequence stars; the obscured objects may still be undergoing mass accretion.

Spiral structure and star formation. I. Formation mechanisms and mean free paths

Abstract: The wide variety of optical appearances observed in spiral galaxies has encouraged the growth of two theoretical approaches to explain the spiral patterns exhibited by such young objects as OB associations, giant H II regions, and dark dust lanes. These approaches are related to the density wave theory of spiral structure and 'stochastic, self-propagating star formation'. Levinson and Roberts (1981) tried to reconcile these two theoretical approaches, and considered a disk filled with discrete gas clouds. The present investigation is concerned with refinements and extensions of the model of Levinson and Roberts. Attention is given to gravitational forces and dynamical propagation, cloud-cloud collisions, supernova interactions, computational models, a theory concerning the interactions of the gas cloud and stellar association systems, the time evolution of the gas cloud-stellar association systems, and aspects of collisionally triggered star formation.

Angular momentum loss during pre-main sequence contraction

Abstract: Current ideas on the early stages of star formation suggest strongly that proto-stars enter the pre-main sequence phase rotating rapidly. The subsequent rotational evolution of both single and binary stars is largely determined by angular momentum transport by a magnetically-controlled stellar wind. A massive star with a cool corona but with a modest relic magnetic field may be prevented from fragmenting into a binary system through the braking action of a centrifugally-driven wind. A late-type pre-main sequence star will generate by dynamo action a magnetic field of surface strength Bo that depends on the angular velocity Ω. The consequent braking can be powerful enough to cause spin-down in spite of the contraction. The predicted Ω(t) law depends on the Bo (Ω) relation, on whether the wind is thermal, thermo-centrifugal, or magneto-centrifugal, and on the fraction of the magnetosphere with open field-line. Angular momentum loss from a tidally locked binary pair can lead to the formation of late-type contact binaries with rapidly rotating members, and ultimately to their coalescence into one rapid rotator.

Dissipative models for the sequence of elliptical galaxies

Abstract: Protogalaxies with a range of masses and sizes are numerically collapsed with a dissipative model for the formation of elliptical galaxies discussed in a companion paper (Carlberg 1984). The rates of dissipation and star formation in the model are calculated with a simple cloud description of the gas physics. The correlations between the final mass, metal abundance, and kinematics in the models are compared with the empirical correlations of these quantities in an attempt to constrain the size-mass relation of the protogalaxies. To reproduce the variation of central velocity dispersion and metallicity with mass requires that the protogalaxies and sizes such that the average initial surface density was approximately mass independent. The second-parameter sizes such that the average initial surface density was approximately mass independent. The second-parameter effect in ellipticals-at a fixed mass the scatter in central velocity dispersion is correlated with the scatter in metallicity-is possibly explained as the consequence of an initial scatter of protogalaxy sizes. These models can be placed in a cosmological context with the results that the ratio of the initial to final half-mass radii is near 10 for the favored models, and that the gas in the models needs to be largely self-gravitating tomore » produce strong anisotropy, suggesting formation at a redshift near 10 from sheets of gas gathered up with respect to the dark material.« less

Molecular hydrogen jets from the Orion nebula

Abstract: The environs of interstellar space in which star formation takes place involve high-velocity gas flows. Channelled by embryonic dust toroids, these gas flows culminate in energy-losing shock processes at the interface with the ambient interstellar cloud. The excitation of molecular hydrogen transitions in this gas provides one means of observing these shocked regions and their relation to the well-known indicators of star formation such as the Herbig Haro objects and T Tauri stars. In an attempt to understand the relationship of the recently discovered complex of Herbig Haro objects in Orion1 to the IR sources in this region, we have carried out a survey of the molecular hydrogen S(1) line distribution. These observations have led to the discovery of a previously unsuspected structure of finger-like filaments of H2 emission extending radially outwards from a common centre at IRC9.

The ultraluminous interacting galaxy NGC6240

Abstract: In the course of checking preliminary lists of IRAS (Infrared Astronomical Satellite) sources to see whether any of the interacting galaxies that we have been studying1 were included, we discovered that the position of the IRAS source 1650+024PO4 (ref. 2) is within a few arc seconds of one of our programme interacting galaxies, NGC6240. Examination of the Palomar sky survey prints shows that NGC6240 is the only non-stellar object within the 1 arc min IRAS error box for this source. Photometry at 10 and 20 µm from the United Kingdom IR Telescope (UKIRT) confirms that this galaxy is among the most luminous IR galaxies yet discovered. An interaction-induced burst of star formation is the most likely source of the strong IR emission.

Detection of molecular hydrogen in two merging galaxies

Abstract: NGC6240 and Arp 220 (IC4553) are two of the most luminous IR galaxies known1,2. They are also both thought to be examples of galaxy–galaxy mergers. As part of our study of interacting and merging galaxies1,3,4 we have obtained IR spectra in the 2-µm region of NGC6240 and Arp 220 and detected the ν = 1–0 S(1) quadrupole rotation–vibration emission line of H2 (rest wavelength 2.122 µm) in both galaxies. (This line in NGC6240 has also recently been detected by E. E. Becklin et al., personal communication.) These detections, which double the number of previously published measurements of extragalactic H2 (refs 5, 6) suggest that the merger of two galaxies results in the production of massive quantities of shocked molecular gas. This shocked gas must cool and collapse, leading to an enormous burst of star formation. These measurements of shocked H2 thus strengthen our earlier interpretation1 of the extremely large IR luminosity of NGC6240 in terms of a ‘super-starburst’. Arp 220 may be undergoing similar activity. We suggest that merging galaxies are a unique new class of ultra-luminous IR galaxies.

Iras observations near young objects with bipolar outflows - l1551 and hh 46-47

Abstract: A 6-solar luminosity dust-embedded precursor of a low-mass (about 1 solar mass) pre-main-sequence star has been discovered with IRAS near the northeast lobe of the bipolar outflow region in the dust cloud L1551 and designated L1551 NE. Star formation is proceeding in at least two locations in L1551, reminiscent of the situation in regions of more massive star formation. If the position of NE in the flow from IRS 5 is indicative of the flow having initiated star formation in NE, then the object can be only about 24,000 years old. Alternatively, NE could appear by chance to lie in the flow from IRS 5. In the globule ES 0210-6A, a 12 solar luminosity dust-embedded precursor of a low-mass (about 1 solar mass) pre-main-sequence star is found which drives the bipolar flow responsible for the string of Herbig-Haro objects HH 46-47 A-D. In this globule, there is only one region of active star formation.

On the formation of stars from disk accretion.

Abstract: We have constructed models of stars which form by accretion from a disk. We have calculated accretion and pre-main-sequence evolutionary tracks for a range of initial parameters which spans the physically reasonable modes for accretion. We find that these models agree with the observed positions of T Tauri stars in the H-R diagram and avoid some of the problems which were endemic to traditional pre-main-sequence evolution. We find no evidence to indiccate that the formation of high- and low-mass stars in young open clusters is noncoeval. We point out a problem that arises when one attempts to use an H-R diagram of T Tauri stars to distinguish between various models of star formation.

Star formation in the NGC 7538 molecular cloud: near-infrared and radio spectroscopy

Abstract: We present new CO millimeter-wave observations of the star-formation region NGC 7538 centered in the region around IRS 1--3, in which a large (rroughly-equal1.3 pc) and massive (75 M/sub sun/) high-velocity molecular outflow is identified. The correspondence of far-infrared emission with the extent of the molecular outflow leads to the proposal of an in situ mechanism for radiative acceleration of the gas, operable in cases where tau (dust) is insufficient to permit the ''snowplow'' outflow effect. We also synthesize new and previously published near infrared emission line observations of IRS 1--3, finding an apparent deficit in ionizing flux relative to the total far-infrared luminosity. We identify IRS 1 as the most luminous of the three sources, in which dust absorption within the ultracompact H II region has sharply reduced the ionizing flux. On this basis we argue that IRS 1 may also be the primary if not sole cuase of the molecular outflow.

Massive star formation in NGC 6946

Abstract: We present H..cap alpha.. measurements of the late-type spiral galaxy NGC 6946. Assuming an initial mass function, we use the H..cap alpha.. data to calculate the total star formation rate and efficiency of massive star formation as a function of galactocentric radius. We then attempt to determine the factors influencing these quantities by comparing them to the surface densities of H I and H/sub 2/ and the degree of compression suffered by the gas in spiral density wave theory. The rate of star formation is very closely correlated with the first power of the surface density of H/sub 2/, and somewhat less correlated with the degree of compression of the gas predicted by density wave theory. In contrast, the efficiency of massive star formation is roughly constant across the face of the galaxy. These results are consistent with results from previous studies of our own and other galaxies.

Far-infrared observations of galactic nuclei.

Abstract: Measurements of far-infrared continuum emission from the central regions of 30 late-type galaxies, 24 of which were previously unreported, are given. Photometry covering wavelengths from 40 to 160 microns has been obtained, yielding information on luminosities, temperatures, and dust masses for most. Good correlations of the far-infrared luminosity with blue colors, reddening, and molecular line radiation are found, supporting the identification of the sources with the molecular cloud component in these galaxies, and strengthening the identification of the energy sources as the young massive stars formed from those clouds. Comparisons with 10-micron data indicate that such data are useful for statistical studies, but have too much scatter to use directly in the analysis of individual sources. Strong correlation with nonthermal radio continuum emission and radio-to-optical flux ratios suggest that massive star formation underlies a wide range of galactic activity.

Protostellar formation in rotating interstellar clouds. IV: Nonisothermal collapse

Abstract: Radiative transfer in the Eddington approximation is included in a multidimensional, self-gravitational, hydrodynamical computer code. Details of the numerical solution and thermodynamic relations are given. Comparison calculations with previous spherically symmetrical models of protostellar collapse are used to validate the basic approach and the artifices which allow the explicit hydrodynamics code to follow the accretion of gas onto a quasi-equilibrium core. A series of axisymmetric models is used to investigate the importance of rotation in collapsing clouds, as the initial amount of angular momentum is lowered, with an emphasis on the possible formation of rings. Rings readily form even in the nonisothermal regime except for very low initial angular momenta; even these clouds may experience ring formation prior to reaching stellar densities. The models imply that other effects (such as gravitational torques or turbulent viscosity) may be necesary to avoid binary formation and thus result in a presolar nebula consistent with the assumptions of either Safronov or Cameron.

Young Supernovae in the Starburst Galaxy M82

Abstract: MERLIN observations at 1666 MHz detect 18 compact (< 0.25 arcsec) radio sources within the central few hundred parsecs of M82. M82 is well known as a ‘starburst’ galaxy, and it is probable that these objects are very young supernova remnants. The distribution of radio sources appears asymmetric with respect to the centre of the galaxy, and we suggest that this is due to the current region of star formation lying within a molecular ring.

The population structure of the Large Magellanic cloud bar.

Abstract: We have used a combination of profile-deconvolution and automatized techniques of photographic photometry in crowded fields to construct a rich and deep color-magnitude diagram near the NW end of the Large Magellanic Cloud (LMC) bar. The morphology of the diagram and its analysis through the use of luminosity functions and evolutionary models for the core-helium-burning ''clump'' red giants indicate that (1) stellar formation continues in the bar areas; (2) an old Population II in the Galaxy sense is not a significant contributor to the observed diagram, although stars of all ages are probably present in LMC disk; (3) the bulk of star formation, which continues to date, started earlier than about one billion years ago but later than about three billion years ago, although the precise rate of star formation, including the possible presence of bursts, inside such an age range is not known from our data. The observed population is considerably younger and more metal deficient than that of the solar neighborhood.

Spiral structure and star formation. II - Stellar lifetimes and cloud kinematics

Abstract: The reliability studies using continuum gas dynamical calculations becomes questionable in connection with the apparent clumpiness of the Galaxy's interstellar medium (ISM). Roberts and Hausman (1984) have, therefore, presented a detailed model of a disk galaxy in which the ISM consists entirely of 'cloud particles', which orbit ballistically in the galaxy's gravitational field, collide inelastically with one another, and give birth to and subsequently interact with young star associations. The effects of changing the clouds's collisional mean free path have been examined, and the variations in the young star system's spiral morphology have been explored. The present investigation is concerned with a further study of this clumpy, cloudy ISM model, taking into account longer mean free path models likely to be appropriate for systems of molecular clouds. Attention is also given to the kinematics of clouds as they orbit under the influence of galactic gravity, collisions, and supernova remnants.