Astronomy and Cosmogony

We have derived the disruption times of star clusters from cluster samples of four galaxies, M51, M33, the Small Magellanic Cloud (SMC) and the solar neighbourhood. If the disruption time of clusters in a galaxy depends only on their initial mass as t d i s (yr) = t d i s 4 (M c l /10 4 M O .) γ , and if the cluster formation rate is constant, then the mass and age distributions of the observed clusters will each be given by double power-law relations. For clusters of low mass or young age the power law depends on the fading of the clusters below the detection limit due to the evolution of the stars. For clusters of high mass and old age the power law depends on the disruption time of the clusters. The samples of clusters in M51 and M33, observed with HST-WFPC2, indeed show the predicted double power-law relations in both their mass and age distributions. The values of t d i s 4 and y can be derived from these relations. For the cluster samples of the SMC and the solar neighbourhood, taken from the literature, only the age distribution is known. This also shows the characteristic double power-law behaviour, which allows the determination of t d i s 4 and y in these galaxies. The values of y are the same in the four galaxies within the uncertainty, and the mean value is y = 0.62 ′ 0.06. However, the disruption time t d i s 4 of a cluster of 10 4 M O . is very different in the different galaxies. The clusters in the SMC have the longest disruption time, t d i s 4 ≃ 8 x 10 9 yr, and the clusters at 1-3 kpc from the nucleus of M51 have the shortest disruption time of t d i s 4 ≃ 4 x 10 7 yr. The disruption time of clusters 1-5 kpc from the nucleus of M33 is t d i s 4 ≃ 1.3 x 10 8 yr and for clusters within 1 kpc from the Sun we find t d i s 4 ≃ 1.0 × 10 9 yr.

Star cluster formation and disruption time-scales — I. An empirical determination of the disruption time of star clusters in four galaxies

In this paper we study the evolution of the core and corona of nine open clusters using the projected radial density profiles derived from homogeneous CCD photometric data obtained with the 105 cm Kiso Schmidt telescope The age and galactocentric distance of the target clusters vary from 16 to 2000 Myr and 9 to 108 kpc, respectively Barring Be 62, which is a young open cluster, other clusters show a uniform reddening across the cluster region The reddening in Be 62 varies from E(B - V)min = 070 mag to E(B - V)max = 100 mag The coronae of six of the clusters in the present sample are found to be elongated; however, on the basis of the present sample it is not possible to establish any correlation between the age and shape of the core The elongated core in the case of the young cluster Be 62 may reflect the initial conditions in the parental molecular cloud The other results of the present study are as follows: (1) Core radius rc and corona size rcn/cluster radius rcl are linearly correlated (2) The rc, rcn, and rcl are linearly correlated with the number of stars in that region (3) In the age range 10-1000 Myr, the core and corona shrink with age (4) We find that in the galactocentric distance range 9-10 kpc, the core and corona/cluster extent of the clusters increase with the galactocentric distance

https://iopscience.iop.org/article/10.1086/507094/pdf

Wide-Field CCD Photometry around Nine Open Clusters

We continue our study of rich Galactic clusters by presenting deep CCD observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22, and NGC 2323 contains ~2100 stars brighter than our photometric limit of V ~ 23) and young (NGC 2168 age = 180 Myr and for NGC 2323 age = 130 Myr). The color-magnitude diagrams for the clusters exhibit clear main sequences stretching over 14 mag in the (V, B-V)-plane. Comparing these long main sequences with those of earlier clusters in the survey, as well as with the Hyades, has allowed for accurate distances to be established for each cluster (for NGC 2168 d = 912 pc and for NGC 2323 d = 1000 pc). Analysis of the luminosity and mass functions suggests that, despite their young ages, both clusters are somewhat dynamically relaxed, exhibiting signs of mass segregation. This is especially interesting in the case of NGC 2323, which has an age of only 1.3 times the dynamical relaxation time. The present photometry is also deep enough to detect all of the white dwarfs in both clusters. We discuss some interesting candidates that may be the remnants of quite massive (M ≥ 5 M⊙) progenitor stars. The white dwarf cooling age of NGC 2168 is found to be in good agreement with the main-sequence turnoff age. These objects are potentially very important for setting constraints on the white dwarf initial-final mass relationship and the upper mass limit for white dwarf production.

http://iopscience.iop.org/article/10.1086/377320/pdf

The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)

We present the 1.1 mm Bolocam Galactic Plane Survey (BGPS) observations of the Gemini OB1 molecular cloud complex, and targeted NH{sub 3} observations of the BGPS sources. When paired with molecular spectroscopy of a dense gas tracer, millimeter observations yield physical properties such as masses, radii, mean densities, kinetic temperatures, and line widths. We detect 34 distinct BGPS sources above 5{sigma} = 0.37 Jy beam{sup -1} with corresponding 5{sigma} detections in the NH{sub 3}(1,1) transition. Eight of the objects show water maser emission (20%). We find a mean millimeter source FWHM of 1.12 pc and a mean gas kinetic temperature of 20 K for the sample of 34 BGPS sources with detections in the NH{sub 3}(1,1) line. The observed NH{sub 3} line widths are dominated by non-thermal motions, typically found to be a few times the thermal sound speed expected for the derived kinetic temperature. We calculate the mass for each source from the millimeter flux assuming the sources are isothermal and find a mean isothermal mass within a 120'' aperture of 230 {+-} 180 M{sub sun}. We find a total mass of 8400 M{sub sun} for all BGPS sources in the Gemini OB1 molecular cloud, representing 6.5% of themore » cloud mass. By comparing the millimeter isothermal mass to the virial mass calculated from the NH{sub 3} line widths within a radius equal to the millimeter source size, we find a mean virial parameter (M{sub vir}/M {sub iso}) of 1.0 {+-} 0.9 for the sample. We find mean values for the distributions of column densities of 1.0 x 10{sup 22} cm{sup -2} for H{sub 2}, and 3.0 x 10{sup 14} cm{sup -2} for NH{sub 3}, giving a mean NH{sub 3} abundance of 3.0 x 10{sup -8} relative to H{sub 2}. We find volume-averaged densities on the order of 10{sup 3}-10{sup 4} cm{sup -3}. The sizes and densities suggest that in the Gem OB1 region the BGPS is detecting the clumps from which stellar clusters form, rather than smaller, higher density cores where single stars or small multiple systems form.« less

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The Bolocam Galactic Plane Survey. III. Characterizing Physical Properties of Massive Star-forming Regions in the Gemini OB1 Molecular Cloud

Brosche (1970) has proposed a theory in which the energy loss due to collisions among gas clouds contained in a galaxy constitutes the driving mechanism for its evolution, through virial equilibrium states which, from an initial spherical shape, makes it to contract towards an elongated form; moreover, the value of the total angular momentum, assumed as given by uniform rotation, is assumed to determine the galaxy type on the Hubble sequence and to strongly influence the contraction time from the initial spherical to the final flat configuration.

A dynamical interpretation of the Hubble sequence of galaxies

A statistical research on evolved stars beyond hydrogen exhaustion is performed by comparing the H-R diagrams of about 60 open clusters with a set of isochronous curves without mass loss derived from Iben’s evolutionary tracks and time scales for Population I stars. Interpreting the difference in magnitude between the theoretical positions thus calculated and the observed ones as due to mass loss, when negative, the results indicate that this loss may be conspicuous only for very massive and red stars. However, a comparison with an analogous work of Lindoff reveals that the uncertainties connected with the bolometric and colour corrections may invalidate by a large amount the conclusions which might be drawn from such research.

Possibility of Mass Loss in the Red-Giant Stage from H-R Diagrams of Galactic Clusters

In a previous work by two of us, the main difficulties of the theory of galaxy formation from universal primordial cosmic turbulence were focussed; it was mainly found that the decay of the turbulence was so strong during the plasma era that turbulence should have been almost completely washed out before recombination, unless, and only for low-density universes, unlikely large turbulent velocities were postulated at the largest scale of the turbulence at the end of the radiation era. It was suggested that a possibility to turn around such a difficulty should consist in finding en adequate physical energy source feeding the turbulence during at least the whole plasma phase. The present work is intended to test whether matter-antimatter annihilation theory at the origin of the universe, as it is developed by Omnes and his collaborators, may be considered to yield the required energy source of the turbulence. To this aim, the main results of the work of Stecker and Puget concerning the behaviour of the most important quantities of the matter- antimatter annihilation theory are used, in order to describe the properties of the energy source for the turbulence down to recombination time, and extended to take a somewhat more detailed account of theirz and mean matter density dependences. The energy feeding source thus obtained is then introduced into the general framework of the galaxy formation theory from cosmic turbulence taking account of energy dissipation as developed in our previous works. The results obtained concerning the comparison of the main parameters of galaxies derived from the theory with the experimental data are much more satisfactory than in the previous approach, and show that, inside the errors of the present day values, sufficiently good fits are possible in a range of mean universe densities between 0.4 to 1 times the critical density.

Primeval turbulence from matter-antimatter annihilation

Il Principio Antropico in fisica ed in cosmologia.

A quantitative re-formulation of Hoyle's theory on the formation of the solar system is attempted, according to a three-dimensional scheme based on the assumption that the original magnetic field of the star is a dipole field. This allows us to obtain analytic expressions for the main paraeters describing the different phases of the process. The protostar is assumed to evolve in gravitational contraction along the Hayashi track, along which, for a given value of the radiusR0 depending on the total angular momentum, matter begins to be shed at the equator as a consequence of centrifugal instability. However, owing to the geometry of the dipole field and to Hoyle's assumption that, for a star with a convective envelope, the sign of the inward magnetic pressure determines whether the magnetic lines do wind up or not, it turns out that the magnetic coupling between the star and the disk formed at the equator starts only when the radius of the contracting star has reached the value ofRs=4/5R0; and that the shedding of matter stops for a radius valueRd, depending on the strength of the magnetic field. One is thus able to calculate the total quantity of matter emitted at the solar equator and the distance reached by the rings thus formed as functions of the radius of the star, of the initial values of the magnetic field, of the total angular momentum and of the structural factors of the star. The quantitative results are discussed in order to see whether it is possible to deduce the main characteristics of the disk, from which the solar system should have originated, for reasonable values of these main parameters.

N. Dallaporta

Papers

A dynamical interpretation of the Hubble sequence of galaxies

Possibility of Mass Loss in the Red-Giant Stage from H-R Diagrams of Galactic Clusters

Primeval turbulence from matter-antimatter annihilation

Il Principio Antropico in fisica ed in cosmologia.

On Hoyle's Theory of the Origin of the Solar System