Showing papers on "Star formation published in 1979"
TL;DR: In this article, the authors obtained optical scanner data from 4270 to 6710 A with a resolution of 7 A for some 600 H..cap alpha.. emission stars listed in the compilation of T Tauri stars by Herbig and Rao and in the original H..Cap alpha.. surveys.
Abstract: Optical scanner data from 4270 to 6710 A with a resolution of 7 A have been obtained for some 600 H..cap alpha.. emission stars listed in the compilation of T Tauri stars by Herbig and Rao and in the original H..cap alpha.. surveys. These are used to obtain spectral types, reddening corrections, and emission-line strengths and are combined with infrared broad-band measurements out to 3.5 ..mu..m to provide bolometric luminosities and effective temperatures. H-R diagrams are presented for the Taurus-Auriga complex, Orion, NGC 2264, NGC 7000/IC 5070, and the rho Ophiuchi association. The following conclusions are reached: Most H..cap alpha.. emission stars lie on convective tracks in the H-R diagram, and their distribution does not correspond to any published dynamical evolutionary tracks. Most H..cap alpha.. emission stars are of spectra type late K and luminosity class between III and V. Radii and masses derived from the H-R diagram lie between 1 and 5 R/sub sun/ and between 0.2 and 3 M/sub sun/, respectively. The value of d (log N)/d (log M) ranges from -1.35 to -2.9 and for the three best-observed groups is consistent with the -1.35 of the Salpeter initial mass function. Star formation is not coeval on amore » time scale of less than a10/sup 4/to approx.6 x 10/sup 6/ years. The overall efficiency of star formation is approx.10%.6.. The bulk of the extinction occurs within the dark clouds rather than in circumstellar envelopes. High-mass stars may therefore form preferentially in the dense cores of clouds, and low-mass stars may form throughout the cloud. It is very likely that all close pairs of T Tauri stars are binaries.« less
819 citations
TL;DR: In this paper, an O star near the surface of an extended molecular cloud ionizes a H II region which is radiationbounded on the inner side (towards the cloud centre) and density-bounded in the outer side; consequently the ionized gas can stream away into space on outer side, and this in turn enables the ionization front to advance faster into the cloud, thus eroding a large cavity around the O star.
Abstract: We evaluate the efficiency with which O stars disperse the molecular clouds from which they form. An O star near the surface of an extended molecular cloud ionizes a H II region which is radiation-bounded on the inner side (towards the cloud centre) and density-bounded on the outer side; consequently the ionized gas can stream away into space on the outer side, and this in turn enables the ionization front on the inner side to advance faster into the cloud, thus eroding a large cavity around the O star. This process may be very effective in destroying molecular clouds; if ∼ 4 per cent of a cloud&s material were converted into new stars with a Salpeter Initial Mass Function, the remaining 96 per cent could be broken up and dispersed by the ionizing radiation from the O stars
217 citations
TL;DR: In this article, a detailed study of the peculiar elliptical galaxy NGC 5128 (Cen A), based on computer video analysis of several photographic plates of exceptional quality reduced to the standard UBV system, is presented.
Abstract: Results are presented for a detailed study of the peculiar elliptical galaxy NGC 5128 (Cen A), based on computer video analysis of several photographic plates of exceptional quality reduced to the standard UBV system. The picture-processing results and the measured properties of the elliptical and gaseous-disk components of NGC 5128 are examined, along with the distribution, spectral characteristics, and chemical composition of the H II regions in the disk. The data show that NGC 5128 consists of a giant E2 galaxy containing a significant amount of gas and dust situated predominantly in an equatorial disk where vigorous star formation is occurring. Reasons why NGC 5128 is so different from giant ellipticals in clusters are considered.
102 citations
TL;DR: In this paper, the authors used a sample of 178 supernovae in external galaxies, and found three pieces of evidence that Type I supernova are associated with a young stellar population: (1) the number of Type Is per unit luminosity is much higher in I0 galaxies than in any other galaxy type; this result cannot be ascribed to small number statistics, and it implies that the supernova are related to the peculiarities defining the I0 class.
Abstract: Using a sample of 178 supernovae in external galaxies, we find three pieces of evidence that Type I supernovae are associated with a young stellar population: (1) The number of Type I supernovae per unit luminosity is much higher in I0 galaxies than in any other galaxy type; this result cannot be ascribed to small-number statistics, and it implies that the supernovae are related to the peculiarities defining the I0 class, namely signs of intense star formation in an otherwise old population. (2) The number of Type I supernovae per unit luminosity increases from early through late Hubble types; if such supernovae arose in the very old (bulge or old-disk) population, the opposite trend would be expected. (3) The Type I supernova rates in spiral galaxies are proportional to their present star formation rates, as estimated from colors; this result again implies that fairly short-lived stars become Type I supernovae. We discuss the star formation rate that is implied for elliptical galaxies if their supernovae have the same origin as those in spirals and irregulars; signs of such star formation could have escaped attention so far, but not by a wide margin, so it is not clear whether a distinctmore » type of supernova need be postulated. In any case, most Type I supernovae must come from short-lived stars.« less
97 citations
TL;DR: In this paper, three regions which illustrate different stages of star formation have been identified in the Cepheus OB3 cloud, and all are situated at one edge of the molecular cloud, close to the association stars.
Abstract: Molecular clouds of mass approx.10/sup 4/ M/sub sun/ and linear extent approx.60 pc have been detected in the vicinity of the young OB associations Cepheus OB3 and Perseus OB2. These clouds have been mapped in the J=1..-->..0 transition of /sup 12/CO, and observations of /sup 13/CO and 2 mm H/sub 2/CO have been made in selected regions.Three regions which illustrate different stages of star formation have been identified in the Cepheus OB3 cloud. An embedded star may be present in one region. A new subgroup of the association is being created in another. The third is probably the precursor of a star-forming region. All are situated at one edge of the molecular cloud, close to the association stars and the H II region S155. The site of the currently active star-forming region adjacent to the birth sites of the older and younger subgroups of the Cepheus OB3 association suggests that each burst of star formation is produced as a result of the birth of a previous group and thus continues sequentially. Although the way in which star formation is initiated in this association is not established, it is clearly very different from the manner in which it proceeds and maymore » well have resulted from the passage of the galactic density wave.The Perseus OB2 association and molecular cloud are found to deviate considerably from the pattern frequently observed in OB associations and typified by Cepheus OB3. The two known stellar birth sites, IC 348 and NGC 1333, are spatially very far apart, and no other star-forming regions have been discovered. There is no evidence that the birth of stars in one region triggers star formation in an adjacent area. This may be due to the orientation of the cloud with respect to the local magnetic field. It is clear, however, that no unique mechanism produces OB associations.« less
97 citations
TL;DR: In this article, the relative drift velocity between the neutral and ionized matter in a cylindrically symmetric, magnetic cloud, which contracted gravitationally to an equilibrium state from an initially uniform state, was calculated as a function of position inside the cloud.
Abstract: We present a solution for the relative drift velocity between the neutral and ionized matter in a cylindrically symmetric, magnetic cloud, which contracted gravitationally to an equilibrium state from an initially uniform state. The cloud is bounded by a hot and tenuous external medium. The characteristic time (tau/sub D/) for ambipolar diffusion is calculated as a function of position inside the cloud and is compared with the only other known solution, which refers to a uniform density. It becomes necessary to define a second characteristic time, tau/sub B/, which is correct locally and which is the aapropriate one for deciding at which position, if anywhere, in the cloud ambipolar diffusion can resolve ''the magnetic flux problem'' during star formation. The two time scales differ by many orders of magnitude.It is likely that, while ambipolar diffusion is in progress in a cloud, the total magnetic energy of the cloud will not decrease. In such a case, gravitational energy is converted into heat directly through ion-neutral collisions, rather than through dissipation of stored magnetic energy.
85 citations
TL;DR: The NGC 7538 region appears to contain examples of different stages in the formation of massive stars; it is suggested that the center of star formation is moving systematically to the southeast in this region as discussed by the authors.
Abstract: Infrared observations of the NGC 7538 region at wavelengths from 1 micron to 1 mm are presented and analyzed with the aim of understanding both the large-scale structure of this region of current star formation and the properties of the individual compact objects within it. At far-infrared wavelengths (25-130 microns), emission is seen from the visible H II region, from the vicinity of the previously known maser sources and dust-embedded compact HII regions, and from a new region called NGC 7538(E). Coincident with NGC 7538(E) are a point-like 1-25 micron infrared source, NGC 7538-IRS9, which probably provides the power for the far-infrared emission, and an extended source of 2.2 micron emission which appears to be an infrared reflection nebula. The compact H II regions, the maser sources and IRS9 are located within a dense molecular cloud at the edge of the optical H II region. This cloud, which has a mass of approximately 9000 solar masses, is detected in emission at 1 mm. The NGC 7538 region appears to contain examples of different stages in the formation of massive stars; it is suggested that the center of star formation is moving systematically to the southeast in this region.
81 citations
TL;DR: In this article, a 3.5 hour observation on 1979 March 6 with the imaging proportional counter aboard the Einstein Observatory has revealed the presence of numerous low-luminosity (10 to the 31st ergs/s) X-ray sources in a region centered near the Trapezium in the Orion Nebula.
Abstract: A single 3.5 hour observation on 1979 March 6 with the imaging proportional counter aboard the Einstein Observatory has revealed the presence of numerous low-luminosity (10 to the 31st ergs/s) X-ray sources in a region centered near the Trapezium in the Orion Nebula. In addition to the Trapezium, 22 discrete sources have been detected in a 1 x 1 deg field at or above the 4 sigma level. These sources are clearly associated with star formation, as indicated by the similarity of their spatial distribution with that of the nebular variables, a class of pre-main-sequence stars. Because the field is quite crowded, however, individual identifications of these sources are difficult. In addition to the discrete sources, there is evidence for diffuse or unresolved emission from two ridges extending 16 deg to the north and 15 deg to the west of the Trapezium. The nature of the discrete sources and of the diffuse emission is discussed.
65 citations
TL;DR: In this paper, the authors examined the properties of model galaxies produced by the stochastic, self-propagating, star-formation model and compared them with those observed in real galaxies.
Abstract: We have examined the properties of model galaxies produced by the stochastic, self-propagating, star-formation model and compared them with those observed in real galaxies. Observations have shown that a good correlation exists between the morphological type of a spiral galaxy and its rotational velocity, color, and gas fraction. We have shown that the stochastic, self-propagating, star-formation model directly correlates the morphological type with the rotation curve, the most important parameter being the maximum rotational velocity. The relative star-formation rates given by the model lead qualitatively to the observed variation of integrated colors and gas fraction as a function of morphological type, as well as to the observed decrease in the density of regions of recent star formation with radius. We show that this model is also capable of giving spiral arms in red plates and discuss the color profiles across spiral arms. Inclusion of the interaction of star formation and the interstellar gas shows that this model is also capable of inducing spiral ordering in the gas.
57 citations
TL;DR: In this article, the internal structure and dynamics of a giant molecular cloud complex was studied in an attempt to understand the internal structures and the dynamics of star formation in the cloud complex.
Abstract: Carbon monoxide emission at ..nu../sub LSR/ = 20 +- 2 km s/sup -1/ is found to extend 4/sup 0/ (approx.170 pc) southwest of M17, and is studied in an attempt to understand the internal structure and dynamics of a giant molecular cloud complex. The region contains two primary clouds. The first has at least 2 x 10/sup 5/ M/sub sun/ of molecular gas and extends for 1./sup 0/8 (72 pc) parallel to, but below the galactic plane southwest of M17. The second, located above the plane approximately 2./sup 0/5 southwest of M17, is about 1./sup 0/7 in extent, but contains considerably less molecular mass (> or approx. =3 x 10/sup 4/ M/sub sun/). Between these two clouds is a 1/sup 0/ long region of relatively low intensity, clumpy CO emission which appears to bridge the two main clouds. The molecular mass within this bridge is estimated to be 2 x 10/sup 4/ M/sub sun/. The cloud associated with M17 is itself divided into four discrete fragments of approximately equal mass (4 x 10/sup 4/ M/sub sun/). The /sup 12/CO and /sup 13/CO line widths are higher in these four fragments than they are between the fragments. OB star formation ismore » active only in the northeastern two of these fragments. The /sup 13/CO line widths in the discrete fragments satisfy the virial theorem for the derived masses. (b) The /sup 13/CO velocity structure in the large complex containing M17 shows a gradual change from regularity in the northeast to irregularity and occasionally multipeaked profiles in the southwest. This change corresponds to a gradient in the degree of compactness and intensity of star formation in the four fragments. A massive (10/sup 5/ M/sub sun/) molecular cloud complex associated with M16, 2/sup 0/ north of M17, and the two clouds southwest of M17, form a pattern of equally spaced star-forming clouds whose positions alternate above and below the galactic plane. Patchy CO emission is found between these three objects. The entire region of molecular emission is approx.250 pc long.« less
54 citations
TL;DR: In this article, a specific model for the formation of disk galaxies by dissipation was proposed to account for their rotation properties in a natural way, which can explain the low rotation velocities of bright elliptical galaxies.
Abstract: The distinction between elliptical and disk galaxies is often explained by invoking different rates of dissipation and star formation in the proto-galactic gas clouds from which they are presumed to have formed1–3. Another possibility is raised by the suggestion that many ellipticals are the remnants of galaxies that merged while suffering the tidal distortion and violent relaxation of a slow encounter4,5. Various N-body simulations indicate that merging can account for many of the observed properties of bright elliptical galaxies6–9, although some reservations have been expressed about the possibility of accounting for their low rotation velocities10. My aim here is to consider a specific model for the formation of disk galaxies by dissipation and the formation of bright ellipticals by merging and to suggest that it can account for their rotation properties in a natural way.
TL;DR: In this article, a model of the thermal emission from cool circumstellar dust (30-70 K) shows that these stars are surrounded by material left over from the star formation process; this result confirms the extreme youth of these stars.
Abstract: Far-infrared observations (40-160 microns) of eight optical emission-line stars are presented. Six of these stars, LkH-alpha 198, T Tau, LkH-alpha 101, V380 Ori, R Mon, and MWC 1080, show substantially more far-infrared emission than would be expected on the basis of a blackbody extrapolation of their 10-20-micron fluxes. Additionally, in three cases, the far-infrared emission is shown to be spatially extended (greater than 40 arcsec). A simple model of the thermal emission from cool circumstellar dust (30-70 K) shows that these stars are surrounded by material left over from the star formation process; this result confirms the extreme youth of these stars. MWC 349 was detected at a level consistent with the expected free-free flux from its surrounding H II region, and RY Tau was not detected in the far-infrared; there is little circumstellar dust with temperatures of 20-100 K in these objects.
TL;DR: In this article, infrared polarimetry has been obtained of 16 objects in the proposed region of star formation in the rho Oph dark cloud, which is used along with visual polarimeters, infrared photometry, and molecular line studies to deduce the cloud morphology around the very young stellar cluster.
Abstract: Infrared polarimetry has been obtained of 16 objects in the proposed region of star formation in the rho Oph dark cloud. This study is used along with visual polarimetry, infrared photometry, and molecular line studies to deduce the cloud morphology around the very young stellar cluster.
TL;DR: In this article, the authors extend the application of stochastic self-propagating star formation to account for the diversity of morphological types and the evolution of galaxies, and show that the rate of star formation exhibits the critical behavior of a phase transition, which is not strongly dependent on the details of the star-interstellar gas interaction.
Abstract: The mechanism of stochastic self-propagating star formation has previously been invoked to explain the origin of spiral arms in galaxies. In this paper we extend the application of this mechanism to account for the diversity of morphological types and the evolution of galaxies. The new property that arises from consideration of this mechanism is that the rate of star formation exhibits the critical behavior of a phase transition. This is a general property of the system and is not strongly dependent on the details of the star--interstellar gas interaction. Examination of the properties of this phase transition provides a general scenario for the evolution of galaxies and the origin of the various morphological types.
TL;DR: In this article, the authors compute the UV luminosity produced by a distribution of supernova remnants resulting from a burst of star formation in primeval galaxies at large redshift, and the implications for the detection of line emission from young galaxies and for the diffuse background radiation are discussed.
Abstract: The paper computes models of the UV luminosity produced by a distribution of supernova remnants resulting from a burst of star formation in primeval galaxies at large redshift. The evolution of low metal abundance remnants is described analytically, and their UV emission is found from a velocity grid of radiative shock models. The implications of these results for the detection of line emission from young galaxies and for the diffuse background radiation are discussed.
TL;DR: In this paper, the authors used millimeter-wavelength molecular emission observations for a dust cloud in CrA in which several young Herbig Be/Ae stars are found.
Abstract: Millimeter-wavelength molecular emission observations are presented for a dust cloud in CrA in which several young Herbig Be/Ae stars are found. The dominant heating source in the cloud is near the B star TY CrA. Local heating of the dust surrounding the young star R CrA also occurs. Other regions of enhanced T/sub A/* (CO) are not associated with known embedded infrared sources and may possibly be due to surface heating of the cloud by a pair of B stars that lie outside the dust cloud. CO isotope observations indicate a terrestrial /sup 12/C//sup 13/C ratio. The position of peak antenna temperature, T/sub A/*, of molecules such as CS, H/sub 2/CO, HCN, and HCO/sup +/, which require high densities for excitation, is coincident with the position of the Herbig star R CrA. On the other hand, the optically thin /sup 12/C/sup 18/O has peak T/sub A/* at a position 5' away from R CrA, indicating a higher column density that is the result of cloud geometry rather than higher density.Because this cloud is so close, high spatial resolution allows a determination of the radial variation of line broadening. No abrupt transition from broad to narrow lines occurs, as might bemore » expected for an embedded source of turbulence such as a compact H II region. The observed line broadening is matched by a r/sup -1/2/ power law suggesting nonhomologous collapse of the core of the molecular cloud. While the line broadening of the CO indicates that the cloud is probably in a state of gravitational collapse, only a small rotation is indicated in the main body of the cloud. Elongation of the cloud is seen at all densities up to at least 4 x 10/sup 5/ cm/sup -3/. This elongation most likely arises because of the dominance of the magnetic field in the cloud evolution compared to dynamical effects such as shock waves or cloud-cloud collisions.« less
TL;DR: In this article, it was shown that the presence of initial velocity fields in the interstellar medium can alter Jeans's criterion in a fundamental way, given plausible temperatures, and velocity fields conforming to idealized symmetries but of reasonable amplitudes.
Abstract: Utilizing a virial approach and assuming homologous compression, it is shown that the presence of initial velocity fields in the interstellar medium can alter Jeans's criterion in a fundamental way. Given plausible temperatures, and velocity fields conforming to idealized symmetries but of reasonable amplitudes, the formation of self-gravitating clouds of extraordinarily low mass is, in principle, possible. For cases of collapse with radial symmetry, a comparison of the virial predictions with the results of numerical modeling shows these predictions to be roughly correct.
TL;DR: In this article, a model for the galactic orbits of molecular clouds is devised, where the molecular clouds are assumed to be launched from the two-armed spiral-shock wave, to orbit in the Galaxy like ballistic particles with gravitational perturbations due to the density-wave spiral potential, and each cloud is assumed to produce a cluster of stars.
Abstract: A model for the galactic orbits of molecular clouds has been devised. The molecular clouds are assumed to be launched from the two-armed spiral-shock wave, to orbit in the Galaxy like ballistic particles with gravitational perturbations due to the density-wave spiral-potential, and each cloud is assumed to produce a cluster of stars. Each cloud radiates detectable /sup 12/C/sup 16/O (J=0..-->..1) spectral line radiation from birth for 40 million years. Stars are seen in the cloud about 25 million years after birth, and the star cluster is assumed to continue in ballistic orbit around the Galaxy.The model has been tested by comparing its predicted velocity-longitude diagram for CO against that observed for the Galaxy and by comparing the model's predicted distribution of light in the UBV photometric bands against observed surface photometry for Sb and SC galaxies. The interpolation of the initial velocities in the model was corrected, and the model was examined to see whether preshock or postshock initial velocities better fit the observations. The model gives very good general agreement and reproduces many of the features observed in the CO velocity-longitude diagram.
01 Jan 1979
TL;DR: In this article, an extensive survey of the galactic distribution, size, mass and age of molecular clouds is presented, which leads to a new picture of the interstellar medium dominated by very stable stable long-lived clouds which are referred to as giant molecular clouds.
Abstract: Millimeter wave observations of emission from the CO molecule have become, over the past eight years, the dominant method for determining the physical properties of dense interstellar clouds, composed primarily of molecular hydrogen and for exploring the structure and kinematics of the galactic disk. In this paper we briefly review the CO survey results in the literature (Section 2) and then present new results (Section 3-7) of an extensive -^CO and -^CO survey of the galactic distribution, size, mass and age of molecular clouds. The interpretation of this survey leads to a new picture of the interstellar medium dominated by very mas sive stable long-lived clouds which we refer to as Giant Molecular Clouds. We find that Giant Molecular Clouds (GMC's) with M = 10 5 3 x 1 0 ^ are a major constituent of the galactic disk, the dominant component of the interstellar medium in the galaxy interior to the sun and the most mas sive objects in the galaxy. We find that the interstellar medium and star formation are dominated by massive gravitationally bound clouds in which stars and associations are forming but at a very low rate in com parison to the free fall time. The galactic distribution of the mole cules as traced by CO emission is interpreted as the distribution of GMC's. As the most massive objects in the galaxy they are also basic to the dynamics of the disk.
TL;DR: In this article, four new water masers have been found in the vicinity of three OB associations and discussed in the context of star formation and CO line self-reversal within the individual sources.
Abstract: Four new water masers have been found in the vicinity of three OB associations. One of these, in Cep OB3, is among the most intense sources yet discovered and may be related to the most recent epoch of star formation in the OB association. Two others, one in CMa OB1, the other in M16, have large radial velocities relative to their associated molecular clouds. These results are discussed in the context of star formation and CO line self-reversal within the individual sources.
TL;DR: In this article, the principle conclusions of a large study of star formation covering several hundred young objects and combining optical and infrared techniques were presented, and a large number of stars were identified.
Abstract: The principle conclusions are presented of a large study of star formation covering several hundred young objects and combining optical and infrared techniques.
TL;DR: In this paper, the authors used a multislit echelle spectrograph to study H-alpha emission line profiles at about 700 points in the Rosette Nebula and found that the highest velocities are more probably inward.
Abstract: Emission-line profiles of H-alpha were studied at about 700 points in the Rosette Nebula by using a multislit echelle spectrograph. Numerical analysis of the profiles indicates that variations in the line-of-sight velocity occur within the nebula, reaching about + or - 20 km/s in the inner regions. Evidence is presented that these highest velocities are more probably inward, favoring a model where the central cavity is due to a depletion resulting from rapid star formation
TL;DR: In this paper, the authors examined the evolution of refractory interstellar grain abundances and corresponding metal deplections in the solar neighborhood and found that in a two-phase model, grain destruction is very efficient and the abundance of refractive grains should be negligible, contrary to observations.
Abstract: We have examined theoretically the evolution of refractory interstellar grain abundances and corresponding metal deplections in the solar neighborhood. The calculations include a self-consistent treatment of red-giant winds, planetary nebulae, protostellar nebulae, and suprnovae as sources of grains and star formation, and of encounters with supernova blast waves as sinks. We find that in the standard two-phase model for the interstellar medium (ISM), grain destruction is very efficient, and the abundance of refractory grains should be negligible, contrary to observations. In a cloudy three-phase ISM most grains reside in the warm and cold phases of the medium. Supernova blast waves expand predominantly in the hot and tenuous phase of the medium and are showed down as they propagate through a cloud. In order to obtain significant (approx.3) depletions of metals presubably locked up in refractory grain cores, the destruction of grains that reside in the clouds must be minimal. This requires that (a) the density contrast between the cloud and intercloud medium be sufficiently high, and (b) the filling factor of the hot and tenuous gas of the interstellar medium, which presumably gives rise to the O VI absorption and soft X-ray emission, be nearly unity. Much larger depletions (>more » or approx. =10) must reflect accretion of mantles within interstellar clouds.« less
TL;DR: In this paper, intermediate infrared and (V-I) photometry is presented for 14 reddened B- and A-type stars in Ori OB1 (Belt) and a color-difference analysis is found to be uniform across the subgroup, and calculated to be 3.1 +- 0.2.
Abstract: Intermediate infrared and (V-I) photometry is presented for 14 reddened B- and A-type stars in Ori OB1 (Belt). From a color-difference analysis, R (the ratio of total-to-selective absorption) is found to be uniform across the subgroup, and calculated to be 3.1 +- 0.2. One star (HD 38087) does exhibit an abnormal reddening law which is probably due to the presence of an optically thin shell of circumstellar material. From UBV observations for 152 B- and A-type stars with no prior available three-color photometry, and existing UBV photometry for 203 additional B- and A-type stars, a variation in distance moduli of 0/sup m/.2 +- 0/sup m/.1 is observed between the stars in the Eastern part and those in the remainder of the Belt. The resultant H--R diagram shows that stars with (B-V)/sub 0/approx. =+0/sup m/.3 still follow the ZAMS, which implies an age of at least 15 x 10/sup 6/ years. As the brighter stars must be much younger, this indicates that star formation in Ori OB1 (Belt) was a continuous process. From the ZAMS fitting method, an intrinsic distance modulus of 8/sup m/.3 +- 0/sup m/.4 is derived.
TL;DR: In this paper, a model for the three most likely precursor events: a supernova blast wave, a shock driven by an expanding H II region, and a stellar wind from an early-type star is discussed.
Abstract: CO observations of the Mon R1 region show a partial ring structure of molecular clouds which is kinematically distinct from the larger complex of clouds associated with Mon OB1. This morphology suggests that a previous energetic event was responsible for ordering the currently observed ring. There is also some evidence for an expanding H I feature in this direction. If the molecular and atomic features have a common origin, an expansion age approx.1--3 x 10/sup 6/ years is derived by considering models for the three most likely precursor events: a supernova blast wave, a shock driven by an expanding H II region, and a stellar wind from an early-type star. The relationship of star formation in this region to these possible events is discussed.