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The Monoceros R2 cloud - Near-infrared and molecular observations of a rotating collapsing cloud
Abstract
The velocity structure of the CO line profiles of the Mon R2 molecular cloud is studied for the combined effects of rotation and collapse motions. The CO line broadening shows that the collapse velocity (km s/sup -1/) is related to radial distance (pc) by V (r) =4.7r/sup -1/2/, if one assumes pure collapse, i.e., no turbulence. The numerical constant is in reality much less since turbulence probably makes a substantial contribution. The cloud rotates about a NW-SE axis with a projected angular velocity of 0.4 km s/sup -1/ pc/sup -1/. The rotation is a minor effect compared to the collapse. Free-fall collapse, along the rotational axis, can be observed as spatially extended high-velocity CO emission on the SE side and low-velocity emission on the opposite NW side of the dense core of the cloud. The comparison of the self-reversed CO and /sup 13/CO line profiles allows the sense of the large velocity gradient flow to be determined. This shows that the cloud is collapsing, not expanding. A comparison of CO, 6 cm and 2 mm H/sub 2/CO, and recombination line profiles also shows that a self-consistent collapse model can be constructed with one continuum source, a compact H II region, more » located on the far side of the dense molecular cloud. A second larger H II region may lie in front of the dense core.A survey of the cloud at 2..mu..m detected five sources; two sources located near the dense molecular core, including the brightest, have large infrared color indices, indicating A/sub v/approx.40 mag toward the center of the cloud. The dense nature of the core is confirmed by detection of the 3..mu..m ice feature with tau/sub ice/ approximately 1 toward the brightest infrared source. A comparison of the A/sub v/ toward all the infrared sources, with the CO column densities, indicates that only 12% of the carbon is found in CO molecules. « less
... This UC H ii region ) powered by a B0-type star (Downes et al. 1975) is associated with the infrared source Mon R2 IRS1 (Massi et al. 1985;Henning et al. 1992). The UCH ii region drives a large bipolar outflow (Richardson et al. 1988;Meyers-Rice & Lada 1991;Xie & Goldsmith 1994) oriented NW-SE approximately aligned with the rotation axis of the full cloud found by Loren (1977). The molecular cloud is part of a CO shell with ∼26 pc size whose border encompasses the UCH ii region (Wilson et al. 2005). ...
... Fig. 12). Loren (1977) also observed CO motions he interpreted as tracing the global collapse of the molecular cloud, with an infall speed of a few km s −1 . This typical line profile of infall has also been seen locally in CO and marginally in 13 CO near the central UCH ii region thanks to higher resolution observations (Tang et al. 2013). ...
... Differences in size between the northern and the western H ii regions, which harbour an exciting star of the same spectral type, tend to suggest that the northern H ii region is more evolved than the western one (see Fig. 2a and Table 1). The larger number of striations observed in the visible image of the northern reflection nebula also indicate an older stage of evolution (Thronson et al. 1980;Loren 1977). ...
The surroundings of HII regions can have a profound influence on their
development, morphology, and evolution. This paper explores the effect of the
environment on H II regions in the MonR2 molecular cloud. We aim to investigate
the density structure of envelopes surrounding HII regions and to determine
their collapse and ionisation expansion ages. The Mon R2 molecular cloud is an
ideal target since it hosts an H II region association. Column density and
temperature images derived from Herschel data were used together to model the
structure of HII bubbles and their surrounding envelopes. The resulting
observational constraints were used to follow the development of the Mon R2
ionised regions with analytical calculations and numerical simulations. The
four hot bubbles associated with H II regions are surrounded by dense, cold,
and neutral gas envelopes. The radial density profiles are reminiscent of those
of low-mass protostellar envelopes. The inner parts of envelopes of all four
HII regions could be free-falling because they display shallow density
profiles. As for their outer parts, the two compact HII regions show a density
profile, which is typical of the equilibrium structure of an isothermal sphere.
In contrast, the central UCHii region shows a steeper outer profile, that could
be interpreted as material being forced to collapse. The size of the heated
bubbles, the spectral type of the irradiating stars, and the mean initial
neutral gas density are used to estimate the ionisation expansion time, texp,
0.1Myr,for the dense UCHII and compact HII regions and 0.35 Myr for the
extended HII region. The envelope transition radii between the shallow and
steeper density profiles are used to estimate the time elapsed since the
formation of the first proto stellar embryo, Tinf : 1Myr, for the
ultra-compact, 1.5 / 3Myr for the compact, and greater than 6Myr for the
extended HII regions.
... CCCs are not very rare in the Milky Way. Early observations of CCCs include studies of NGC 133, where the CO spectra revealed two velocity components (Loren 1976(Loren , 1977. The first cluster to be identified as a possible collision object was Westerland 2, which harbors two associated clouds with a velocity difference of approximately 20 km s −1 (Furukawa et al. 2009;Ohama et al. 2010). ...
... In most cases, the relative velocity between the two colliding clouds leads to two different velocity peaks or components in the resulting molecular spectrum (Loren 1976(Loren , 1977Dickel et al. 1978). Because the mixing of the colliding clouds continues for a long time after the instance of the collision, a collision-front of shocked materials having intermediate velocities also forms in between the colliding clouds. ...
We report the results obtained from a multiwavelength study of the H ii region G18.148−0.283 using the upgraded Giant Metrewave Radio Telescope at 1350 MHz, along with other archival data. In addition to the radio continuum emission, we have detected the H169 α and H170 α radio recombination lines toward G18.148−0.283 using a correlator bandwidth of 100 MHz. The moment-1 map of the ionized gas reveals a velocity gradient of approximately 10 km s ⁻¹ across the radio continuum peaks. The ¹² CO ( J = 3−2) molecular line data from the CO High-Resolution Survey (COHRS) also show the presence of two velocity components that are very close to the velocities detected in the ionized gas. The spectrum and position–velocity diagram from CO emission reveal molecular gas at an intermediate-velocity range bridging the velocity components. We see mid-infrared absorption and far-infrared emission establishing the presence of a filamentary infrared dark cloud, the extent of which includes the targeted H ii region. The magnetic field inferred from dust polarization is perpendicular to the filament within the H ii region. We have also identified two O9 stars and 30 young stellar objects toward the target using data from the Two Micron All Sky Survey (2MASS), UKIRT Infrared Deep Sky Survey (UKIDSS), and Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE). Cumulatively, this suggests that the region is the site of a cloud–cloud collision that has triggered massive star formation and subsequent formation of an H ii region.
... Although most of the previous literature on Mon R2 region has presented data and analysis on the gas (e.g., Loren 1977;Ginard et al. 2012;Pilleri et al. 2012;Treviño-Morales et al. 2016;Keown et al. 2019) or the dust (e.g., Pokhrel et al. 2016;Sokol et al. 2019;Hwang et al. 2022), our focus is on the young stellar population. ...
Monoceros R2 (Mon R2) is one of the closest large active star-forming regions. This extremely young and partially embedded region provides an excellent laboratory for studying star formation and the early evolution of young stellar objects (YSOs). In this paper, we conduct an optical study of the greater Mon R2 region. Beginning with 1690 previously identified candidate YSOs, we used 496 sources with good proper motions and parallaxes from Gaia Data Release (DR) 3 to determine the astrometric properties for likely members of Mon R2. We then used both astrometric and photometric (isochronal and variability) criteria to determine that 308 of these stars are highly probable members. Using the same criteria, we considered a broad area search around Mon R2 in Gaia DR3 and separated candidate members from field stars. In total, we selected 651 likely new cluster members that had been missed in the previous X-ray and infrared excess selection techniques used in the past to establish cluster membership. Revised astrometric properties of the cluster were found using the combined sample of ∼959 highly probable member stars. For the literature plus the new candidate member list, optical light curves were compiled from the Zwicky Transient Facility. For 470 identified variable sources, we attempted classification based on the flux asymmetry ( M ) and quasiperiodicity ( Q ) metrics. We find that Mon R2 is dominated by quasiperiodic symmetric variables, with aperiodic sources also a significant population. A few tens of large-amplitude variables are identified that may be of interest for further study.
Context. The mass segregation of stellar clusters could be primordial rather than dynamical. Despite the abundance of studies of mass segregation for stellar clusters, those for stellar progenitors are still scarce, so the question concerning the origin and evolution of mass segregation is still open.
Aims. Our goal is to characterize the structure of the NGC 2264 molecular cloud and compare the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation.
Methods. We separated the Herschel column density map of NGC 2264 into three subregions and compared their cloud power spectra using a multiscale segmentation technique. We extracted compact cloud fragments from the column density image, measured their basic properties, and studied their spatial and mass distributions.
Results. In the whole NGC 2264 cloud, we identified a population of 256 clumps with typical sizes of ~0.1 pc and masses ranging from 0.08 M ⊙ to 53 M ⊙ . Although clumps have been detected all over the cloud, most of the massive, bound clumps are concentrated in the central subregion of NGC 2264. The local surface density and the mass segregation ratio indicate a strong degree of mass segregation for the 15 most massive clumps, with a median Σ 6 three times that of the whole clumps population and Λ MSR ≃ 8. We show that this cluster of massive clumps is forming within a high-density cloud ridge, which is formed and probably still fed by the high concentration of gas observed on larger scales in the central subregion. The time sequence obtained from the combined study of the clump and YSO populations in NGC 2264 suggests that the star formation started in the northern subregion, that it is now actively developing at the center, and will soon start in the southern subregion.
Conclusions. Taken together, the cloud structure and the clump and YSO populations in NGC 2264 argue for a dynamical scenario of star formation. The cloud could first undergo global collapse, driving most clumps to centrally concentrated ridges. After their main accretion phase, some YSOs, and probably the most massive, would stay clustered while others would be dispersed from their birth sites. We propose that the mass segregation observed in some star clusters is inherited from that of clumps, originating from the mass assembly phase of molecular clouds.
This review examines the state-of-the-art knowledge of high-mass star and massive cluster formation, gained from ambitious observational surveys, which acknowledge the multi-scale characteristics of these processes. After a brief overview of theoretical models and main open issues, we present observational searches for the evolutionary phases of high-mass star formation, first among high-luminosity sources and more recently among young massive protostars and the elusive high-mass prestellar cores. We then introduce the most likely evolutionary scenario for high-mass star formation, which emphasizes the link of high-mass star formation to massive cloud and cluster formation. Finally, we introduce the first attempts to search for variations of the star formation activity and cluster formation in molecular cloud complexes, in the most extreme star-forming sites, and across the Milky Way. The combination of Galactic plane surveys and high-angular resolution images with submillimeter facilities such as Atacama Large Millimeter Array (ALMA) are prerequisites to make significant progresses in the forthcoming decade.
The interstellar medium may be characterized by several physically rather distinct regimes: coronal gas, intercloud gas, diffuse clouds, isolated dark clouds and globules (of small to modest mass), more massive molecular clouds containing OB (and later) stars, and giant molecular clouds. Molecules first appear in the denser diffuse clouds, and occur everywhere that A V ≳ 1 m . Values of temperature, density, ionization fraction, mass, size, and velocity field are discussed for each regime. Heating and cooling mechanisms are reviewed. Nearly all molecular clouds exceed the Jeans criteria for gravitational instability, yet detailed models reveal no cases where observations can be interpreted unambiguously in terms of rapid collapse. The possibility that clouds are supported by turbulence, rotation, or magnetic fields is discussed, and it is concluded that none of these agencies suffice. Comments are made about fragmentation and star formation in molecular clouds, with possible explanations for why only low mass stars form in low mass clouds, why early-type stars form only in clouds with masses ≳ 10 3 M ⊙ , and why O-stars seem to form near edges of clouds. Finally, large-scale interactions between molecular clouds and the galactic disk stellar population are discussed.
A 1.2-meter millimeter-wave telescope has been used to survey CO in the constellations of Orion and Monoceros. Many new molecular clouds have been found. The distribution of molecular material shows two striking characteristics: 1) Most of the molecular clouds in this region appear to be connected by continuous extensions and filaments. To judge from continuity in radial velocity, most of these connections appear to be real, and are not merely the result of projection along the line of sight. 2) There are at least two slender filamentary features longer than 10° in angular extent. These filaments may connect the molecular clouds lying well out of the Galactic plane to clouds lying in the plane. Their shape and orientation suggest that magnetic fields may play a role in their evolution. The observed velocity gradients may be explained by accelerated gas flow along the filament.
Observations of molecular clouds show evidence of rotation and of fragmentation of subregions of the clouds into multiple stellar or protostellar systems. This review concentrates on the effects that rotation and pressure gradients have in a self-gravitating cloud to cause it to undergo the crucial process of fragmentation. Recent two-dimensional and three-dimensional numerical hydrodynamic calculations have made progress in determining these effects. In most cases the calculations are performed with modest spatial resolution and are limited to isothermal clouds with neglect of viscous and magnetic effects. The combined results of several calculations strongly suggest that rotating clouds that are unstable to collapse are also unstable to fragmentation.
Emission from interstellar dust in L 1641 and Mon R2 was mapped with three arcminute resolution at 400 microns wavelength from White Mountain, California at 3.9 km elevation. Dust brightness distributions and 13 CO antenna temperatures are shown below with equal linear scales, emphasizing that the core of Mon R2 is considerably larger and more massive than the core of the Orion region containing OMC-1 and OMC-2. Dust and 13 CO brightness distributions for each object are similar, but some of the differences may be real. The differences become accentuated when mass distributions are compared.
The one particle probability distribution function, f(
r, p, t), is defined so that (3–1)
is the probability that, at the time, t, a particle has momentum, p, in the volume element dV
p
at p and position, r, in the volume element dV
r
at r. Similarly, the distribution function f(
r,v, t) is defined so that for an average particle density, N, (3–2)
gives the probable number of particles in the six dimensional phase space dV
r
dV
dV
around position, r, and velocity, v. Boltzmann’s equation for f(
r,p,
t) may be written as (Boltzmann, 1872) (3–3)
where φ(r) is the potential energy acting on every particle, p is the momentum, m is the particle mass, ∇r is the gradient in position space, ∇p is the gradient in momentum space, and (df/dt)
coll is the rate of change in f due to collisions. Noting that p
̇ = -∇rφ, we may write Eq. (3–3) in Cartesian coordinates as
where denotes the first derivative with respect to time.
The surroundings of HII regions can have a profound influence on their development, morphology, and evolution. This paper explores the effect of the environment on H II regions in the MonR2 molecular cloud. We aim to investigate the density structure of envelopes surrounding HII regions and to determine their collapse and ionisation expansion ages. The Mon R2 molecular cloud is an ideal target since it hosts an H II region association. Column density and temperature images derived from Herschel data were used together to model the structure of HII bubbles and their surrounding envelopes. The resulting observational constraints were used to follow the development of the Mon R2 ionised regions with analytical calculations and numerical simulations. The four hot bubbles associated with H II regions are surrounded by dense, cold, and neutral gas envelopes. The radial density profiles are reminiscent of those of low-mass protostellar envelopes. The inner parts of envelopes of all four HII regions could be free-falling because they display shallow density profiles. As for their outer parts, the two compact HII regions show a density profile, which is typical of the equilibrium structure of an isothermal sphere. In contrast, the central UCHii region shows a steeper outer profile, that could be interpreted as material being forced to collapse. The size of the heated bubbles, the spectral type of the irradiating stars, and the mean initial neutral gas density are used to estimate the ionisation expansion time, texp, 0.1Myr,for the dense UCHII and compact HII regions and 0.35 Myr for the extended HII region. The envelope transition radii between the shallow and steeper density profiles are used to estimate the time elapsed since the formation of the first proto stellar embryo, Tinf : 1Myr, for the ultra-compact, 1.5 / 3Myr for the compact, and greater than 6Myr for the extended HII regions.
Observational studies of interstellar Carbon Monoxide have enabled astronomers to gather a wealth of information on the structure, energetics and kinematics of galactic molecular clouds. The majority of this information has been obtained from observations of the CO J = 1 → O rotational transition at 115.271 GHz (λ = 2.6 mm). Studies of higher rotational transitions have awaited the development of low-noise receiver systems for frequencies above 200 GHz. In this paper we report new data obtained using the 3.8 m United Kingdom Infrared Telescope (UKIRT) with the QMC and UKIRT submillimeter heterodyne receivers. These cryogenic heterodyne systems have system noise temperatures varying between 270 K at 220 GHz and 400 K at 370 GHz.
Context. Monoceros R2 (Mon R2), at a distance of 830 pc, is the only ultra-compact H ii region (UC H ii) where its associated photon-dominated region (PDR) can be resolved with the Herschel Space Observatory.
Aims. Our aim is to investigate observationally the kinematical patterns in the interface regions (i.e., the transition from atomic to molecular gas) associated with Mon R2. Methods. We used the HIFI instrument onboard Herschel to observe the line profiles of the reactive ions CH+, OH+ and H2O+ toward different positions in Mon R2. We derive the column density of these molecules and compare them with gas-phase chemistry models.
Results. The reactive ion CH+ is detected both in emission (at central and red-shifted velocities) and in absorption (at blue-shifted velocities). OH+ is detected in absorption at both blue- and red-shifted velocities, with similar column densities. H2O+ is not detected at any of the positions, down to a rms of 40 mK toward the molecular peak. At this position, we find that the OH+ absorption originates in a mainly atomic medium, and therefore is associated with the most exposed layers of the PDR. These results are consistent with the predictions from photo-chemical models. The line profiles are consistent with the atomic gas being entrained in the ionized gas flow along the walls of the cavity of the H ii region. Based on this evidence, we are able to propose a new geometrical model for this region.
Conclusions. The kinematical patterns of the OH+ and CH+ absorption indicate the existence of a layer of mainly atomic gas for which we have derived, for the first time, some physical parameters and its dynamics.
Consideration is given to the turbulence properties of molecular clouds and their implications for star formation. Data for 54 molecular clouds and condensations is presented which reveals cloud velocity dispersion and region size to follow a power-law relation, similar to the Kolmogoroff law for subsonic turbulence. Examination of the dynamics of the molecular clouds for which mass determinations are available reveals essentially all of them to be gravitationally bound, and to approximately satisfy the virial theorem. The observation of moderate scatter in the dispersion-size relation is noted to imply that most regions have not collapsed much since formation, suggesting that processes of turbulent hydrodynamics have played an important role in producing the observed substructures. A lower limit to the size of subcondensations at which their internal motions are no longer supersonic is shown to predict a minimum protostellar mass on the order of a few tenths of a solar mass, while massive protostellar clumps are found to develop complex internal structures, probably leading to the formation of prestellar condensation nuclei. The observed turbulence of molecular clouds is noted to imply lifetimes of less than 10 million years.
Observations of the J = 2-1 CO line in the galactic source Mon R2 were obtained at 230 and 220 GHz. Antenna measurements indicate that (C-13)O is optically thick with an opacity greater than 1.1. A close similarity was detected between line intensities and spectral shapes in the (C-12)O J = 1-0 and 2-1 transitions. Evidence that (C-12)O is optically thick with both transitions in thermal equilibrium is considered.
A similarity exists between the shapes of HCO(+) self-reversals observed toward a number of molecular clouds where the CO line profiles are self-reversed. The HCO(+) is not thermalized as shown by its line intensities whose strengths are only one-tenth of the CO intensities; the self-reversals tend to be deeper in HCO(+). The self-reversals and broad line wings in the HCO(+) show the significance of radiative trapping resulting from high optical depth in HCO(+) excitation. It is predicted that the J = 3-2HCO(+) line profiles portions of the clouds where the HCO(+) abundance is high but density is too low to thermalize the molecule will not have the prominent self-reversals shown in the J = 1-0 HCO(+) lines.
We present the results of more than 10 years of monitoring of the water vapor maser emission in 14 star forming regions obtained with the Medicina 32-m radiotelescope. The sample of objects covers a large range of luminosities of the associated FIR sources. In order to present in a compact and representative way the large amount of data available, we give for each source: a time-velocity-flux density plot, the time dependent integrated flux, the lower and upper envelopes of the maser emission, the frequency of the maser occurrence as a function of velocity, and the mean velocity averaged over the observing period. A brief morphological description of the environment of the maser source is also given. The present data form the basis for a discussion of the main properties of the water vapor maser emission to be presented in a forthcoming paper.
We report on the hard X-ray detection from the core of the Monoceros R2 giant molecular cloud. The temperature is extremely high of about 5.5 keV in quiescent state. The strongest source among hard sources is associated with molecular core and suffered deep absorption. A hard X-ray flare near the core is also observed. The temperature is also extremely high. Strong Ca and Fe lines are observed in flare spectrum, and the energies are significantly lower than those of usual plasma model. The flare spectrum show some differences from those of low-mass young stars. It might be evidence of the X-rays from a high-mass protostar.
We study the chemistry of small hydrocarbons in the photon-dominated regions
(PDRs) associated with the ultra-compact HII region Mon R2. Our goal is to
determine the variations of the abundance of small hydrocarbons in a high-UV
irradiated PDR and investigate their chemistry. We present an observational
study of CH, CCH and c-CH in Mon R2 combining data obtained with the
IRAM 30m telescope and Herschel. We determine the column densities of these
species, and compare their spatial distributions with that of polycyclic
aromatic hydrocarbon (PAH). We compare the observational results with different
chemical models to explore the relative importance of gas-phase, grain-surface
and time-dependent chemistry in these environments. The emission of the small
hydrocarbons show different patterns. The CCH emission is extended while CH and
c-CH are concentrated towards the more illuminated layers of the PDR.
The ratio of the column densities of c-CH and CCH shows spatial
variations up to a factor of a few, increasing from
_3H_2 in the envelope to a maximum of
towards the 8m emission peak. Comparing these results
with other galactic PDRs, we find that the abundance of CCH is quite constant
over a wide range of G, whereas the abundance of c-CH is higher in
low-UV PDRs. In Mon R2, the gas-phase steady-state chemistry can account
relatively well for the abundances of CH and CCH in the most exposed layers of
the PDR, but falls short by a factor of 10 to reproduce c-CH.
In the molecular envelope, time-dependent effects and grain surface chemistry
play a dominant role in determining the hydrocarbons abundances. Our study
shows that CCH and c-CH present a complex chemistry in which UV
photons, grain-surface chemistry and time dependent effects contribute to
determine their abundances.
A good correlation between interstellar polarization and interstellar
extinction is found at 2.2 microns for almost all lines of sight. This
correlation has a slope less than unity, holds true over a range in
optical depth of a factor of 100, and is independent of the physical
path length. A model involving equal contributions from a uniform
component and a random component to the interstellar magnetic field fits
the data well. These results imply that the two components of the
interstellar magnetic field maintain roughly the same proportions and
strength in most phases of the ISM.
A new numerical method for three-dimensional protostellar hydrodynamics
is presented. Its compatibility with existing numerical codes is
demonstrated by test calculations in two dimensions (axial symmetry).
Shortcomings and advantages of the method are discussed. The compactness
of the present code is emphasized: many interesting three-dimensional
calculations can be performed on a medium-size computer (300 K bytes of
central storage). Several tests of the stability of rotating
interstellar clouds against non-axisymmetric perturbations are reported.
In three cases fragmentation leading to the formation of a binary system
is observed. Some general conclusions concerning the stability of
rotating clouds are drawn.
Millimeter-wavelength molecular line observations of CO, (C-13)O,
C(O-18), H2CO, HCN, HCO(+), CS, and SO toward a dust cloud in CrA that
contains several young Herbig Be/Ae stars are analyzed. The internal and
external heating sources are identified on the basis of the CO
corrected-antenna-temperature distribution and compared with the
column-density distribution as determined by additional (C-13)O
corrected-antenna-temperature mapping. The hybrid ratio of (C-13)O to
C(O-18) corrected antenna temperature is examined as a function of both
velocity and spatial variation across the cloud core. Differences in the
distributions of the other molecules are discussed, CO line-broadening
observations are presented, and the data are used to determine the
velocity-radial distance law. It is concluded that star formation in the
CrA cloud has resulted from self-gravitational collapse guided primarily
by the cloud's magnetic field, that slow gravitational contraction has
maintained the cloud's ellipticity up to the highest densities observed,
and that the velocity field follows from a nonhomologous collapse with
velocity varying as the -0.5 + or - 0.2 power of radial distance.
Observations of the high-velocity (FWZP of about 44km/s) HCO(+) source
in the J = 1-0 and J = 3-2 transitions toward Cep A are presented. The
line frequency, beam size, and antenna efficiency for the observations
are given in a table. A statistical analysis of the intensity ratio in
HCO(+) and 13-CO(+) lines showed that the emission originates from
high-density clumps of about 10 to the 6th per cu cm within a
high-velocity outflow. The highest HCO(+) emission was confined to an
unresolved region near the IR cluster and H2O masers. At intermediate
velocities the HCO(+) emission was spatially extended with redshifted
and blueshifted lobes on opposite sides of the dense core. It is shown
that the HCO(+) abundance of 3-6 x 10 to the -9th, and the electron
abundance of up to 10 to the -8th in the high velocity flow are similar
to those observed in more quiescent molecular clouds. The spatial
distribution of the integrated blueshifted and redshifted HCO(+)
emission in Cep A is illustrated in a series of contour maps.
A fast-mapping technique, developed for use on the Wyoming Infrared
Telescope, has been used to make infrared images of regions of star
formation. We present 10 μm and 20 μm images of Mon R2, W51 IRS
1 and IRS 2, S140, and NGC 7538; and 5 μm images of W51 IRS 2 and
NGC 7538. The maps have angular resolutions approaching the Rayleigh
limit of the 2.3 m telescope and have a sensitivity to extended
sources which is as good as or better than previous observations of
the same regions.
The most luminous infrared sources are associated with radio continuum
sources and, for these objects, the radio and 20 μm contours bear a
remarkable similarity to one another. We also find qualitative support
in the cases of W51 IRS 1, and Mon R2 IRS 1, for the blister model of
H II regions, in which the exciting source is relatively well
separated from the peak radio and infrared emission.
The low-luminosity infrared sources are either not coincident with
radio emission or are weak radio sources. This is either because the
embedded exciting sources are relatively low luminosity late-type
stars which emit relatively few ionizing photons or are surrounded by
thick dust shells which block the ionizing flux from the interstellar
medium.
The observational technique also provides highly accurate positional
information for the objects observed, and we make positional
comparisons between near-infrared objects, radio continuum sources, OH
and H2O masers, and far-infrared sources.
We conclude that, in several cases, the OH and H2O masers
may be at relatively large distances (1017 -
1018 cm) from their primary sources.
Dense gas associated with seven regions of recent star formation and
outflow activity has been studied using primarily observations of CS J =
2-1 emission. It is found that the distribution of the dense gas is
often elongated, yet shows no preferred orientation with respect to the
local magnetic field, rotational axis of the core, or outflow direction.
Thus, it is unlikely that large (10 to the 17th cm) interstellar objects
can be responsible for the focusing of stellar winds into collimated
jets. The position of the embedded stellar object in all seven sources
is at, or near, the maximum of CS emission. If stars acquire a peculiar
velocity with respect to the cloud, the observed positional coincidence
suggests that embedded stars are very young, and that outflow activity
occurs within the first 100 kyr of the star's lifetime.
Luminosities are determined, for embedded sources found in the course of
mapping areas on the order of 20 x 30 arcmin in the molecular clouds
associated with NGC 2071, Mon R2, ON 1, and IC 5146 at 85 and 150
microns. The energetics of each cloud are also examined, and it is found
that external sources may contribute to the extended far-IR emission
from these clouds. The observations indicate that radio-quiet,
protostellar sources have much less effect on the observed temperature
structure of star-forming clouds than do objects of comparable
luminosity which have already formed compact H II regions, and that the
radio-quiet sources appear to be more deeply embedded than those now
associated with ionized gas. It is proposed that massive stars may form
deep within molecular clouds rather than only at their surfaces.
Upper limits have been placed on the B-gamma flux from three objects
(S140 IR, Mon R2 IRS 3, and R Mon). In each case these observations
result in upper limits on the Ne-squared V value for the object and on
the spectral type of the zero-age main-sequence star that may be
contained in the object. These limits appear consistent with the total
luminosity of the objects. Upper limits are also placed on the molecular
hydrogen emission from the objects.
Lines of CS(J = 1 to 0) and CS(J = 2 to 1) have been observed with
nearly equal beam sizes and high velocity resolution in 32 sources
including both giant molecular clouds and dark clouds; with a few
exceptions, these two transitions show nearly equal intensities and line
widths. Hydrogen densities and CS fractional abundances derived from the
data using a velocity gradient radiative transfer (RT) model are in good
agreement with the same quantities obtained from the microturbulent
model of Liszt and Leung. In addition, values for the (J = 2 to 1)
optical depths obtained from RT modeling and other independent arguments
fall in the range 0.3 to 3.0, indicating that mold saturation effects
can be expected for some CS lines.
A detailed model of centrally condensed clouds containing regions of
active star formation is derived from observations. The model has nearly
equal turbulent and systematic collapse velocities and indicates that
such clouds are not clumpy. Alfvenic waves can explain the turbulent
velocities if the magnetic field varies as nH/2 to the 1/2 power. The
model indicates the importance of chemical isotopic fractionation in CO,
and suggests that metals play an important role in the ionization
balance.
The paper analyzes CO spectra of NGC 2071 which exhibit broad wings and
prominent self-reversed line profiles. The wings provide evidence for
high-velocity bipolar gas flow powered by a strong stellar wind along
the axis of a slowly rotating disklike structure. Model calculations
indicate that there is systematic radial mass motion in the main body of
the cloud and that the hydrogen density is 1000-10,000 per cu cm. The
self-reversals arise in part from transfer effects in collapsing or
expanding material. A significant amount of self-absorption also
originates in cooler foreground gas at the cloud periphery
Using the 37-m antenna of Haystack Observatory nine regions of molecular
outflow (S187, AFGL 490, L1455, R Mon, AFGL 961, GGD 27-28, L723, AFGL
2591, and PV Cep) are observed in the (1,1) inversion transition line of
NH3. NH3 is detected in six of these regions. Most of the observed
ammonia condensations are unresolved with 1.4-arcmin resolution, so that
neither the morphology nor the orientation with respect to the outflows
could be determined. The only exception is the NH3 condensation in
L1455, which is resolved and appears to straddle one of the blue CO
lobes and the red CO lobe. There also appears to be some extended weaker
NH3 emission that may be related to the CO outflow. The NH3 condensation
in L723 shows a velocity gradient, which would be consistent with a
rotating structure with its major axis aligned perpendicular to the
outflow axis. High-angular-resolution observations of ammonia are
required to test this suggestion. An H2O maser is detected that may be
associated with the outflow from PV Cep. A statistical study of the
sources with known molecular outflows shows that about 90 percent of the
bipolar outflows are associated with high-density gaseous condensations.
However, the association with the nonbipolar outflows is only about 50
percent. This result suggests an important role for the surrounding
high-density gas on the collimation of the bipolar outflows.
The mean properties of hot-centered and cold dust/molecular clouds are
estimated, and their relationship to each other and contribution to the
mass of the Galaxy are discussed. Reference is made to the properties of
strong, confirmed AFGL 20-micron sources identified with H II regions
having delta greater than -30 deg, and to some weaker sources. A 20
micron-100 micron-1 mm color-color diagram for hot-centered clouds
suggests a mass-weighted mean dust temperature in the 40-60 K range.
From a comparison of the density of the clouds derived from far IR and
from CO observations as well as from dynamical arguments, a (C-13)O/H2
of about 5 x 10 to the -7th power is deduced for hot-centered clouds.
Attention is given to the local surface density of Lynds clouds, as well
as to the equilibrium of the more massive cold clouds and the means of
their support against gravitational collapse. It is indicated that there
are about 2 million cold clouds in the Galaxy, with a total mass of
about 2 billion solar masses.
CO emission has been mapped over a 5 x 5 deg area in the Canis Major
OB1/R1 region; most of the emission is confined to an elliptical region
of approximately 90 x 60 pc. While most of the emission falls in the LSR
velocity range 10-20 km/s, some material is found over the full velocity
range covered (-30 to +45 km/s). There is no simple pattern that would
indicate a single expanding shell, but the observations are consistent
with the idea that some energetic process, which occurred in an
initially inhomogeneous cloudy medium, was responsible for the observed
morphology of the region. Simple arguments suggest that a supernova
explosion is the most likely candidate for the energetic process.
Observations are recounted addressing the nature of the focusing agent
in the phenomenon of bipolar mass outflow in regions of star formation.
The (J, K)=(1, 1) inversion transition of ammonia was searched in 10
sources of bipolar outflow, and the emission was detected and mapped in
seven of them. The ammonia data show the correlation between internal
velocity dispersion and cloud size and mass previously obtained from
carbon monoxide observations. The molecular condensations associated
with the bipolar outflows are generally elongated and engulf the compact
object suspected to be the source of energy of the system. For nine
sources where clear orientations could be assigned to both the major
axis of the condensation and the direction of the outflow, seven cases
were found where the axes were nearly perpendicular. This suggests that
these elongated clouds may be toroidal with dimensions of tenths of
parsecs and may provide the focusing mechanism in bipolar outflows. In
this model, the central object's stellar wind has created a bipolar
cavity in an originally pancake-shaped cloud. The wind, which could be
isotropic near the star, is stopped in the plane of the dense toroid but
escapes along the poles, accelerating the surrounding lower density
molecular gas.
In order to determine if newly formed early-type stars are present in
the large molecular cloud associated with the Mon R2 cluster of
reflection nebulae, the whole cloud has been scanned at 3.2 and 10.55
GHz using the 46-m radio telescope of the Algonquin Radio Observatory.
The limiting flux density was 40 mJy at both frequencies, corresponding
to the flux from an H II region excited by a B1 star at the 830 pc
distance of Mon R2. The total solid angle covered was 0.005 sr, or 16 sq
deg. Only three H II regions were definitely detected, all of which are
located in the central core region, and each has been previously
observed. Young stars, reflection nebulae, IR objects, and OH and H2O
masers have been seen by others near the core region, but no other
optical identifications of young stellar objects were made outside the
core. The majority of all these objects lie close together along a line
which is almost perpendicular to the galactic plane, implying that the
formation of stars with spectral types ranging from B1 to B9 has only
occurred in an annulus, plane, or line through the center of the cloud
and nowhere else in the cloud. It appears that star formation was
triggered 10 to the 7th yr ago, and Mon R2 could be an older and larger
scale version of the Cep A cloud.
Observations of molecular clouds show evidence of rotation and of
fragmentation of subregions of the clouds into multiple stellar or
protostellar systems. This review concentrates on the effects that
rotation and pressure gradients have in a self-gravitating cloud to
cause it to undergo the crucial process of fragmentation. Recent
two-dimensional and three-dimensional numerical hydrodynamic
calculations have made progress in determining these effects. In most
cases the calculations are performed with modest spatial resolution and
are limited to isothermal clouds with neglect of viscous and magnetic
effects. The combined results of several calculations strongly suggest
that rotating clouds that are unstable to collapse are also unstable to
fragmentation.
A large, massive outflow associated with the Mon R2 infrared cluster has
been identified and mapped. The outflow line components are found to be
well separated in velocity from the ambient cloud line component. These
'detached wings' suggest high optical depths and filling factors that
may be characteristic of old, evolved flows. The blueshifted lobe shows
evidence of shell structure similar to that seen in the L1551 outflow.
The redshifted lobe is spatially more confined in the plane of the sky
and its structure is uncertain. A dense, flattened, rotating core is
found near the origin of the bipolar outflow. The physical properties of
this core are consistent with magnetized collapsing cloud models. The
derived dynamical mass of the core, roughly 1013 solar masses, is
supercritical by a factor of about 10. The large-scale velocity
structure of the cloud do not appear to be consistent with such angular
momentum conservation.
We have obtained optical spectrophotometry of red objects near the
emission peaks of molecular clouds that may be optical counterparts to
heavily obscured infrared objects. Our aim has been to investigate the
stellar population that these embedded objects represent. Six late O or
B-type stars have been recognized.
We present H (1.65 micron), K (2.23 micron), and Ldouble
prime(3.81 micron) broad-band images as well as Br(gamma) (n = 7
approaches 4, 2.166 micron) and Br(alpha) (n = 5 approaches 4, 4.052
micron) hydrogen recombination line images, and 3.29 micron and 3.4
micron unidentified feature emission images of the Monoceros R2 star
formation region at a plate scale of approximately 0.9 sec/pixel. The
Brackett line images ar combined with 5 GHz data to map the
line-of-sight dust extinction to the compact H II region on a small
spatial scale. This extinction map is then used to deredden regions of
the H and K imges interior to the H II region. IRS 1SW, the
ionizing source, is found to be consistent with a B0 star. Comparison of
dereddened H and K images with the Brackett images and recent
high-resolution HCO(+) measurements leads to the development of a torus
model for the dense molecular gas surrounding the H II region. The 3.29
micron emission is found to be coincident with the ring of scattered
light at 2.2 micron and just outside the Br(alpha) and Br(gamma)
emission. The 3.4 micron image is of too low a signal-to-noise ratio to
determine if any variation in the 3.29 to 3.4 micron emission ratio with
distance from the ionizing source is seen; however, 3.4 micron emission
is detected in a ring coincident with the 3.29 micron emission.
A technique for large-scale mapping of the magnetic fields in molecular
clouds using high-sensitivity CCD images of the light from background
stars has been developed. Fractional polarization and its orientation
have been mapped in the Horsehead Nebula and Monoceros R2. In each
cloud, the magnetic field is aligned with the general magnetic field of
the surrounding region, and the percentage polarization does not
increase systematically with extinction at visual wavelengths. These
results suggest that the dust grains may not be aligned with the
magnetic fields in high-density regions. The results also disagree with
a previous model for ablation off the surface of the Horsehead Nebula.
The paper presents 2.6 mm wavelength CO and (C-13)O observations of 130
molecular clouds associated with reflection nebulae. Enhanced CO
emission was found in the vicinity of the illuminating star in about
half the objects studied. There is a tendency for the CO peak to be
slightly displaced from the star. Many examples of peaks that appear to
result from heating of the cloud by the nearby star are found, while
others appear to be associated with independent concentrations of
material.
Observations of HCO(plus), HCN, HNC, and CCH emission lines towards 9
molecular clouds are reported. The HCO(plus), HCN, and HNC lines appear
to have similar spatial distributions. The hyperfine structure of the J
equals 1 to 0 transition of HCN reveals complex excitation conditions.
The CCH radical was detected in three new sources.
A compilation of observed interstellar gas properties is presented based
on over 200 observations of coronal gas, intercloud gas, diffuse clouds,
dark clouds, Bok globules, H II regions, and molecular clouds associated
with H II regions. The mean and standard deviation in particle number
density (n), gas temperature (T), and pressure (P/k) are computed for
each gas type, while the mean and standard deviation in scale length and
mass are calculated for each discrete cloud type. A plot of log T vs log
n is graphed for the compiled data, and four portions of the resulting
diagram are discussed in terms of the reliability of apparent trends,
implications of pressure equilibrium, and constraints on cloud-heating
mechanisms.
An infrared map and infrared polarimetric data on the Mon R2
star-forming region are presented. The extended flux at 2.2 μm in
this complex is found to be a shell-like reflection nebula illuminated
by IRS 2. This shell shows indications of clumpiness. It consists of
molecular material swept up by the expanding H II region ionized by IRS
1. The bright source IRS 3 is not physically near to the reflection
nebula, since it does not contribute noticeably to its illumination.
I-band CCD polarization measurements of stars in the background of the
Mon R2 cloud show a well ordered, bent structure of the magnetic field
that can be understood in a model of a collapsing, magnetized rotating
molecular cloud.
The H2CO molecule has been studied toward 25 regions containing
signposts of recent star formation, either Herbig Be/Ae stars, LkH alpha
emission-line stars, or stars with unusual circumstellar nebulosity. The
2-cm and 2-mm lines of H2CO have been detected toward many of these
regions, allowing a determination of cloud density. Core densities from
20,000 per cu cm to 150,000 per cu cm are found to be associated with
more than half of the regions studied. The cloud L 43 is unusual in
being cold (kinetic temperature approximately 15 K) yet having a high
density (150,000 per cm) more characteristic of the hotter regions of
star formation. The H2CO abundances for these clouds with embedded young
stars are the same as the abundances of other clouds of similar density
that do not have embedded stars.
The results of an optical spectroscopic and photometric study of the
star formation region Lynds 1551 and the young stellar object IRS 5 and
a high-resolution spectroscopic study of the Herbig-Haro (HH) objects
associated with the outflow from this region are reported. The results
further establish IRS 5 as the source of the reflected light in the
region. Spectral studies suggest that the spectrum of IRS 5 is dominated
by the light from an accretion disk analogous to those characterizing
objects of the rare class of FU Orionis objects. Analysis of high and
medium resolution spectra of the optical 'jet' emanating from IRS 5
strongly suggest that the 'jet' HH objects are the bow shock interfaces
between two winds coming from the IRS 5 region. The high-velocity wind
is inferred to have a velocity of 440 km/s, and is identified as the
stellar wind. High-density clumps in the stellar wind shock a
lower-density, but much more pervasive 'second wind' which has an
inferred velocity of about 160 km/s.
We present high-resolution CS and (13)CO observations of the central
part of the Mon R2 outflow. We find that the accelerated gas is
distributed in two opposed, highly fragmented, limb-brightened shells.
The fragments in these shells correspond to clumps of gas that, although
partaking in the outflow motion, have their own peculiar velocities. The
clumps appear in the spectra as secondary components, well separated in
velocity from the ambient line. They are as dense as the ambient cloud
(few 105/cu cm), and each of them contains several solar mass
of gas. From the clumps we directly observe, and from those whose
presence we infer by the sudden changes in the shape of the spectra from
one position to the other, we conclude that very likely most of -- if
not all -- the gas in the Mon R2 outflow is in the form of dense clumps.
The line emission of the (J,K) = (1,1) and (2,2) transition of ammonia,
NH3, has been mapped across the Mon R2 molecular cloud with an angular
resolution of 1.5 min and a velocity resolution of 0.1 km/sec. The
observations show that the NH3 emission is contained within an
ellipsoidal region whose major axis lies nearly along the galactic
plane. The line velocities vary systematically along the major axis,
suggesting that the cloud is rotating along the galactic plane with a
period of about 8 million yr. No systematic variations of the linewidths
across the cloud were observed, indicating that the denser interior of
the cloud outlined by the NH3 emission is not collapsing or expanding.
Kinetic temperatures of about 15 K, hydrogen densities of about 3,000/cu
cm, and a cloud mass of 300 solar masses are inferred from the
observations.
The purpose of this investigation is to show that recourse to
anisotropic compression along a magnetic field is not a necessary
condition for star formation within large collapsing interstellar gas
clouds. We examine angular momentum transfer from magnetically braked,
cool interstellar gas clouds. Magnetic torques acting on a contracting,
rotating cloud, permeated by a frozen-in magnetic field coupling the
cloud to the galactic field of the surrounding interstellar medium,
produce kinks in the galactic field lines which are radiated away in the
form of hydromagnetic waves, thereby rotationally decelerating the
cloud. Initially, the braking constraints the clouds to co-rotate with
the galactic background until just prior to the epoch of magnetic field
uncoupling, when the braking mechanism becomes inefficient and the
clouds contract conserving angular momentum thereafter. Our results are
shown to be consistent with observations of stellar rotational
velocities, and, also, with the angular momentum of the protosun.
Calculations are made for the energy loss rates, brightness
temperatures, and line profiles of carbon monoxide in collapsing
interstellar clouds. The most recent data for the H2-CO collision rates
have been used in the calculations; a useful extrapolation of these data
to high rotational levels is given. The density distribution and
velocity field inside the cloud are varied, with one model applying to a
cloud of uniform density that is collapsing in free fall and a second to
a cloud in isothermal collapse. Analytical relations for total CO
cooling at low and high densities are derived. The effects of varying
the cross sections as well as the density distribution, velocity field,
and geometry of the clouds are discussed. The line profiles are a
sensitive tool for testing the models against the observations. The
observed CO lines can be explained by collapsing models in which both
density and velocity gradients occur. The intensity ratio of the
second-to-first deexcitation transition and the first-to-ground
deexcitation transition CO lines in Orion is consistent with such
models. It is proposed that CO infrared lines from dense interstellar
clouds should be observable with instruments of high spectral resolution
at wavelengths greater than 200 microns.
Maser emission from interstellar water vapor has been found towards four infrared sources: OMC-2, OH231.8-4.2 (also known as OH0739-14), S140-IR and Monoceros R2. Three of these new maser sources appear to be situated in active sites of star formation, while the unusual source OH231.8-4.2 is more likely to be related to a late-type stellar object. The line profiles are relatively simple, showing at most a few velocity components, some of which vary with a time scale as short as a few weeks. The individual sources are discussed in detail.
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The 2.6-mm lines of (C-12)O and (C-13)O have been observed toward the small galactic diffuse nebulae NGC 1579 (S222) and S239. The H92-alpha recombination line has also been detected from NGC 1579. Toward NGC 1579, evidence was observed for self-absorption in the (C-12)O line. For both regions, high-velocity wings and line broadening are observable in the (C-12)O line; the features observed toward S239 suggest ordered free-fall collapse of a localized region of the cloud onto a newborn stellar cluster. Examination of the available data suggests that NGC 1579 is both a reflection nebula (illuminated by LKH-alpha 101) and an obscured H II region excited by a star of spectral type near B1, while S239 is a reflection nebula or a Herbig-Haro object. The present studies of these regions show that line widening in CO is at least as important an indicator of star-formation activity as enhanced line emission.
Equilibrium states were determined for systems of interstellar gas and associated frozen-in magnetic fields. It was concluded that final equilibrium states can be reached after magnetic Rayleigh-Taylor instability develops. Final state properties were found to depend on the horizontal wavelength of the initial perturbation. Equilibrium states were also determined for massive interstellar clouds of high electrical conductivity. Self-gravity and the pressure of the intercloud medium binds them, in general, against the effects of internal and magnetic stresses. Results indicate that a cloud becomes oblate with its major axis normal to the field lines, the flattening depending on the mass, magnetic field, and external pressure. Futhermore, it was concluded that the large line widths in molecular clouds may be explained by oscillations of the cloud about equilibrium states. It is also suggested that the inefficiency of star formation processes can be attributed to the magnetic effects in a contracting cloud.
An experiment is presented which demonstrates that the usual chopper
wheel method for calibrating the intensity of a millimeter spectral line does not
completely account for atmospheric absorption. An improved expression for the
thermal scale determined by this technique is derived and tests confirming its
validity are presented. (auth)
Ongoing star formation in the NGC 1333 molecular cloud is found to be the result of a cloud-cloud collision. Two velocity components at 6.3 and 8.3 km s/sup -1/ are observable in the CO and /sup 13/ CO spectra, with strong self-abosorption occurring only in the 8.3 km s/sup -1/ component. The cloud-cloud collision provides compression and heating of the back side of the 8.3 km s/sup -1/ cloud, while cool, unshocked gas on the front side of this cloud results in the observed self-absorption. With the 6.3 km s/sup -1/ cloud on the far side of the collision interface, no self-absorption occurs at this velocity. One result of the collision is the coalescence of the two velocity components into a single, intermediate velocity component observed at 7.5 km s/sup -1/. Associated with this postcollision gas is a chain of newly formed stars which illuminates and heats the nebulosity of NGC 1333.At one end of this chain of stars is a region of enhanced CO line broadening, indicating a nonhomologous gravitational collapse of this portion of the cloud. The infrared stars closest to the part of the cloud which is collapsing are completely obscured at visual wavelengths, and several are associated with Herbig-Haro (HH) objects. With increasing displacement from the region of collapse, the stars become more visible, are probably older, and the CO self-absorption decreases at these positions in the cloud.The observed region in which the cloud-cloud collision is occurring is located at the intersection of an expanding neutral hydrogen shell and lower-velocity background H I. (AIP)
This paper gives details of 397 radio sources between declina.tions + 200 and +270 which were compiled from a finding survey made at 635 MHz with the 210 ft reflector at the Australian National Radio Astronomy Observatory, Parkes, N.S.W.
Observations of self-reversed CO line profiles in four dense molecular
clouds (Mon R2, NGC 1333, W3, and Rho Oph) are discussed, and it is
shown that these cases are readily explained by self-absorption in the
outer layers of a nonhomologously collapsing cloud. Common features of
the four clouds are reviewed, evidence is presented which indicates that
the clouds are in a state of collapse rather than expansion, and it is
demonstrated that the collapse is most likely nonhomologous with the
collapse velocity varying as the inverse square root of cloud radius. CO
line profiles are calculated with and without small amounts of turbulent
broadening for a spherically symmetric nonhomologous collapsing cloud;
these are compared with observed line-of-sight CO and (C-13)O line
profiles. The general agreement between the line profiles given by the
model with a small amount of turbulence and the observed line profiles
is found to be quite good, with all the common features of the observed
profiles present in the calculated profiles. An observational test of
the model is proposed.
Observations of carbon monoxide lines from massive molecular clouds,
such as those near Orion A and Sgr B2, have recently been interpreted in
terms of large-scale motions, generally collapse or expansion, that
imply rapid evolution of these clouds. An alternative explanation is
suggested - namely, the clouds are not dominated by large-scale
systematic motions and the CO profiles are determined primarily by local
motions. Therefore, it is generally not possible to see through these
clouds in optically thick lines of molecules such as CO and HCN. Some
consequences and tests of this model are briefly discussed.
In the interpretation of emission from turbulent optically thick gas
distributions, distinction must be made between local optical thickness
(which influences emission line ratios) and beam-averaged opacity (which
describes the transparency of the object as seen by a radio telescope).
A simple model, incorporating this distinction, is proposed and is used
to estimate the influence of turbulence on the CO emission lines of
massive molecular clouds. This influence is also verified by computation
of synthetic profiles. The model suggests that large-scale turbulence
can explain the apparent contradiction between the CO-12/CO-13 line
ratio (which indicates high optical depth) and observations indicating
that a CO cloud is transparent in some sense. It is shown that magnetic
pressure appears to support the clouds and that turbulent energy can be
supplied by slow contraction of the cloud, due to neutral particle drift
through the magnetic field.
Theoretical models are constructed to study the distribution of grain temperature (T/sub d/) and infrared emission from molecular clouds associated with H II regions (with embedded O: B stars). The effects of the following parameters on the temperature structure and the emergent spectrum are studied: grain type (graphite, silicate, and core-mantle grains), optical depth, density inhomogeneity, cloud size, anisotropic scattering, radiation field anisotropy, and characteristics of central heat source. T/sub d/ varies from approximately-greater-than100 K to approximately-less-than20 K throughout the major portion of a cloud, and dielectric grains attain lower temperatures. Due to an inward increase in T/sub d/, the radiation field is strongly forward-peaking, thereby producing a pronounced limb-darkening in the surface brightness. Important features of the computed emission spectra from typical models are compared with available observations, and the importance of beam dilution is emphasized. Theoretical surface brightnesses at selected infrared wavelengths are also presented. The outward radiation pressure on the dust grains is found to exceed the self-gravitational force of the gas over a large portion of a cloud, thus possibly causing the gas in the inner region to expand. Assumptions commonly used in the analysis of infrared observations are examined. Finally, observational methods of deriving the temperature structure (from color and brightness temperatures in the far-infrared), density distribution (from surface brightness at lambdaapproximately-greater-than1 mm), and optical depth (from multiaperture photometry) for the dust component in simple sources are discussed. (AIP)
Emission from OH and H2O masers has been detected in the direction of
the Monoceros-R2 molecular cloud. Only 1612- and 1665-MHz masers are
observed, and from the emission characteristics of the sources, we
conclude that they arise in different locations; we are observing both
type I and type IIb OH masers in this region. Two velocity components of
the 1665-MHz source are highly linearly polarized (about 100%). This
result allows us to determine limits on the magnetic-field strength in
the emission region, and to estimate its orientation. The relation of
the maser emission to the embedded H II region and to the infrared
sources in this cloud is discussed. The Mon-R2 region is similar to the
Orion nebula complex, and appears to represent a somewhat earlier stage
of star formation.
Numerical calculations have been made for the early stages of collapse of an axisymmetric cloud, both with and without rotation.
The results show that, in the absence of rotation, deviations from spherical symmetry do not usually grow as the cloud collapses;
instead, pressure forces remain sufficient to maintain rough spherical symmetry and prevent the cloud from fragmenting during
its collapse. Fragmentation can occur, but usually only if the initial configuration is already unstable to fragmentation.
In the presence of rapid rotation, however, the central part of the cloud always appears to condense into a rotating ‘ doughnut
’ or ring with a density minimum at the centre. Such a rotating ring is almost certainly unstable and will presumably fragment
into two or more condensations orbiting around each other. The formation in this way of a binary or multiple system of stars
would largely resolve the classical ‘ angular momentum problem ’ and might account for the fact that most stars are in fact
found in binary or multiple systems; even the single stars might be accounted for as escapers from unstable multiple systems.
Carbon monoxide emission in both the 12C160 and 13C160 J = lines has been observed from the regions near all the Be and Ae stars associated with nebulosity listed by Herbig. The emission intensity is peaked on the position of the star in all but a few cases. Subject headings: molecules, interstellar - pre-main-sequence stars - radio lines - star formation
The paper reports submillimeter (effective wavelength about 400 microns)
observations of three dense molecular clouds: NGC 2024 (Orion B), OMC-2
(in Orion A), and Mon R-2. These objects strongly resemble the
far-infrared source in the Kleinmann-Low nebula in Orion A. An extensive
map of NGC 2024 shows the peak of submillimeter brightness to coincide
with the peak locations of far-infrared continuum and HCN molecular-line
emissions. The submillimeter and far-infrared brightness distributions
differ in spatial detail, suggesting that the submillimeter emission
comes from a cool region with temperature of about 25 K (inferred from
the surface brightness of the optically thick (C-12)(O-16) line).
Arguments based on the column density derived from optically thin
molecular lines yield an effective mass-absorption coefficient of 17 sq
cm/g for the continuum opacity at 400 microns; however, this estimate is
subject to large uncertainties. It is suggested that the star 2024 No. 2
is imbedded in the molecular cloud and supplies the energy of the
submillimeter emission.
Observations of CO emission have been made of the molecular clouds
surrounding the emission-line stars R CrA and LkH-alpha 198 and a cloud
in the reflection nebulosity association Mon R2. In all three clouds the
C-12O-16 J = 1-0 line width shows systematic variations with position in
the cloud, the widest lines occurring at the positions of the central
stars in the R CrA and LkH-alpha 198 clouds and at the position of peak
line intensity in the Mon R2 cloud. For the clouds associated with the
emission-line stars the C-13O-16 lines are narrower than the C-12O-16
lines and show less pronounced line width variation. In the Mon R2
cloud, the C-13O-16 line width variations are easily seen. The line
width behavior cannot be explained in detail by any of the models
currently proposed for molecular clouds. If the clouds are indeed
collapsing, the collapse velocity is inversely proportional to some
power of the distance from the center of collapse.
Observations are reported of the 2.6-mm line of CO from an extended region in Monoceros containing a complex of dust clouds as well as a group of reflection nebulae (Mon R2) and where a source of relatively strong millimeter emission from CS and HCN has been found. Lower limits are obtained for the CO and H2 column densities in this region, and five CO emission peaks are identified, each of which is approximately coincident with at least one reflection nebula. Around the position of the strongest peak, extended emission is observed from CS and HCN, suggesting the existence of a rotating core with an H2 density which corresponds to a mass of 5500 solar masses. The CO profiles in the direction of the strongest peak appear to show self-absorption, which may indicate a collapsing cloud.
Results are reported of a CO and (C-13)O survey of 68 dark clouds from the Lynds catalog. CO was detected in 63 of the 64 sources in which it was searched for, and the (C-13)O line was seen in 52 of 55 clouds. There is a rather narrow distribution of CO peak line radiation temperatures about a mean of 6 K; this may reflect the presence of a roughly uniform kinetic temperature of 9.5 K in the sources. Despite the probably subthermal excitation temperature of the (C-13)O transition observed, derived (C-13)O column densities are most likely good to within a factor of 2. Typical CO column densities for the clouds surveyed are 5 x 10 to the 17-th power per sq cm, assuming a terrestrial carbon isotope ratio. All 68 clouds have previously been studied by Dieter in 6-cm H2CO absorption; a comparison of line widths shows the (C-13)O lines to generally be wider than their formaldehyde counterparts. Possible explanations of this fact in terms of internal cloud motions are discussed.
A cluster of compact infrared sources plus extended emission has been found at the peak of the molecular cloud in Mon R2. The extended emission is probably due to heated dust associated with PKS 0605-06, a compact H II region embedded in the molecular cloud. Several of the compact infrared sources have properties indicative of protostars. For one of these, the associated dust is apparently seen in projection against the H II region.