C18O Cores Research Articles

Probing fragmentation with ALMA continuum and spectral line observations of the dense clumps in the ℓ = 224° region

ABSTRACT We report observations, performed with the Atacama Large Millimeter/submillimeter Array (ALMA), of 1 mm dust continuum emission and molecular line emission in 13CO(2–1) and C18O(2–1), towards a sample of starless and protostellar clumps selected from a region, towards the ℓ = 224° field, of the Herschel Infrared GALactic Plane Survey (Hi-GAL). Using the ALMA images and a source extraction algorithm we have analysed the small-scale (∼1000 AU) structure of the clumps and their population of cores (or fragments). We find in general multiple cores in each Hi-GAL clump, both in the continuum and spectral lines, but we do not find a dominant fragmentation mode and the morphologies are very different among the various sources. Our results suggest that during the transition phase from clump to core, those sources with a higher core formation efficiency are also associated with parent clumps that are more likely to convert a higher fraction of their initial mass into a single or a few cores. We were able to obtain a core mass function, or CoMF, covering masses in the range ∼2 × 10−3 to ∼1 M⊙ for the C18O cores, and ∼4 × 10−2 to ∼10 M⊙ for the continuum cores. We find that the CoMF in our sample is much shallower than the higher mass ($\gtrsim 1$ M⊙) IMF, thus indicating that while approaching the final phase of fragmentation the mass function does not resemble the IMF more closely.

A Wide-field CO Survey toward the California Molecular Filament

Abstract We present the survey of 12CO/13CO/C18O (J = 1 − 0) toward the California molecular cloud (CMC) within the region of 161.°75 ≤ l ≤ 167.°75, −9.°5 ≤ b ≤ −7.°5 using the Purple Mountain Observatory (PMO) 13.7 m millimeter telescope. Adopting a distance of 470 pc, the mass of the observed molecular cloud estimated from 12CO, 13CO, and C18O is about 2.59 × 104, 0.85 × 104, and 0.09 × 104 M ⊙, respectively. A large-scale continuous filament extending about 72 pc is revealed from the 13CO images. A systematic velocity gradient perpendicular to the major axis appears and is measured to be ∼0.82 km s−1 pc−1. The kinematics along the filament shows an oscillation pattern with a fragmentation wavelength of ∼2.3 pc and velocity amplitude of ∼0.92 km s−1, which may be related to core-forming flows. Furthermore, assuming an inclination angle to the plane of the sky of 45°, the estimated average accretion rate is ∼101 M ⊙ Myr−1 for the cluster LkHα 101 and ∼21 M ⊙ Myr−1 for the other regions. In the C18O observations, the large-scale filament could be resolved into multiple substructures, and their dynamics are consistent with the scenario of filament formation from converging flows. Approximately 225 C18O cores are extracted, of which 181 are starless cores. Roughly 37% (67/181) of the starless cores have α vir less than 1. Twenty outflow candidates are identified along the filament. Our results indicate active early-phase star formation along the large-scale filament in the CMC region.

The C18O core mass function toward Orion A: Single-dish observations

Abstract We have performed an unbiased dense core survey toward the Orion A Giant Molecular Cloud in the C18O (J = 1–0) emission line taken with the Nobeyama Radio Observatory (NRO) 45 m telescope. The effective angular resolution of the map is 26″, which corresponds to ∼0.05 pc at a distance of 414 pc. By using the Herschel–Planck H2 column density map, we calculate the C18O fractional abundance and find that it is roughly constant over the column density range of ≲5 × 1022 cm−3, although a trend of C18O depletion is determined toward higher column density. Therefore, C18O intensity can follow the cloud structure reasonably well. The mean C18O abundance in Orion A is estimated to be 5.7 × 10−7, which is about three times larger than the fiducial value. We identified 746 C18O cores with astrodendro and classified 709 cores as starless cores. We compute the core masses by decomposing the Herschel–Planck dust column density using the relative proportions of the C18O integrated intensities of line-of-sight components. Applying this procedure, we attempt to remove the contribution of the background emission, i.e., the ambient gas outside the cores. Then, we derived mass function for starless cores and found that it resembles the stellar initial mass function (IMF). The CMF for starless cores, dN/dM, is fitted with a power-law relation of Mα with a power index of α = −2.25 ± 0.16 at the high-mass slope (≳0.44 M⊙). We also found that the ratio of each core mass to the total mass integrated along the line of sight is significantly large. Therefore, in the previous studies, the core masses derived from the dust image are likely to be overestimated by at least a factor of a few. Accordingly, such previous studies may underestimate the star formation efficiency of individual cores.

CATALOG OF DENSE CORES IN THE ORION A GIANT MOLECULAR CLOUD

We present Orion A giant molecular cloud core catalogs, which are based on 1.1 mm map with an angular resolution of 36 arcsec (sim 0.07 pc) and C18O (1-0) data with an angular resolution of 26.4 arcsec (sim 0.05 pc). We have cataloged 619 dust cores in the 1.1 mm map using the Clumpfind method. The ranges of the radius, mass, and density of these cores are estimated to be 0.01 - 0.20 pc, 0.6 - 1.2 times 10^2 Msun, and 0.3 times 10^4 - 9.2 times 10^6 cm^{-3}, respectively. We have identified 235 cores from the C18O data. The ranges of the radius, velocity width, LTE mass, and density are 0.13 -- 0.34 pc, 0.31 - 1.31 km s^{-1}, 1.0 - 61.8 Msun, and (0.8 - 17.5) times 10^3 cm^{-3}, respectively. From the comparison of the spatial distributions between the dust and C18O cores, four types of spatial relations were revealed: (1) the peak positions of the dust and C18O cores agree with each other (32.4% of the C18O cores), (2) two or more C18O cores are distributed around the peak position of one dust core (10.8% of the C18O cores), (3) 56.8% of the C18O cores are not associated with any dust cores, and (4) 69.3% of the dust cores are not associated with any C18O cores. The data sets and analysis are public.

A Study on the CO Isotopic Lines of the Star Forming Region AFGL 5157

By the mapping observations simultaneously at the 12CO (J=1-0), 13CO (J=1-0), and C18O (J=1-0) lines on the area of 24’×24’ (12 pc×12 pc) of the star forming region AFGL 5157, we have obtained the distribution and averaged physical parameters for the respective 13CO and C18O cores of this molecu- lar cloud. At the edge of the molecular cloud, the isotopic abundance ratio is X [(13CO)/(C18O)] ≈ 10, close to the ratio of a giant molecular cloud. The viral masses of the 13CO and C18O cores are less than the masses of the molecu-lar cloud cores, so the molecular cloud cores are gravitationally unstable, and the C18O molecular cloud core is more easy to collapse. The column density distributions of the C18O molecular cloud core in the northeast and southwest directions are, respectively, 1.1 × 1023× z−0.43 and 4.6 × 1025× z−0.58, where z is the distance from the center of the molecular cloud core. The high velocity molecular outflow has been confirmed from our 12CO spectra, the mass loss rate of the outflow has been estimated, and the mass-velocity relation of the outflow is fitted by a power-law function of m ∝ v−1.8. The star formation rate of the 13CO molecular cloud core is as high as 23%, probably, under the influence ofthe reflection nebula NGC 1985, this region is forming medium and large mass stars or clusters.

TRIGGERED STAR FORMATION IN A BRIGHT-RIMMED CLOUD (BRC 5) OF IC 1805

We report recent optical, near-infrared (NIR), and millimeter observations which have revealed some new features of the bright-rimmed cloud BRC 5 associated with W4. With slitless spectroscopy, we detected 17 Hα emission stars around the cloud; 4 are near the surface of the cloud, and 1 is toward IRAS 02252+6120. NIR photometry shows that the central Hα emission star, together with one bright infrared source, has large NIR excesses and Class I spectral energy distributions. These two Class I objects are associated with the 2.9 mm continuum peaks and with a bipolar outflow, and are in between two separate, elongated C18O(J = 1-0) cores. The C18O cores and the two Class I sources are aligned along a line at position angle ~240°, somewhat less than perpendicular to the direction of UV radiation from the OB stars. Most of the detected Hα emission stars, all T Tauri candidates, are located within ~3' of the cloud on the exciting star side. An estimate of the age of the stars based on a color-magnitude diagram suggests that these T Tauri candidates have ages of ~1 Myr or less, but are more evolved objects than the central young stellar objects. This age sequence suggests sequential star formation within the BRC 5 cloud. The 13CO(J = 1-0) emission shows three elongated structures, which indicates the asymmetric structure toward the UV incident axis. We present our exploratory simulation results by using a smoothed particle hydrodynamic code that suggests that the asymmetrical BRC 5 structure could possibly result from the evolution of a preexisting prolate molecular cloud subject to radiation-driven implosion (RDI). Our best-fit prolate cloud has an initial mass of ~400 M ☉, an axial ratio of ~1.7, and a semi-major axis of ~1.6 pc, pointing away from the ionization flux by an angle of 15°. The simulated cloud structure not only closely matches the observed asymmetric morphological structure of BRC 5, but also reveals the possibility of the development of two major cores at the head of BRC 5. For the first time, the possibility of forming two stars by an RDI mechanism in a BRC is investigated.

SIMILARITY BETWEEN THE C18O (J= 1-0) CORE MASS FUNCTION AND THE INITIAL MASS FUNCTION (IMF) IN THE S140 REGION

We present the results of C18O(J=1-0) mapping observations of a 20'x18' area in the Lynds 1204 molecular cloud associated with the Sharpless 2-140 (S140) H II region. The C18O cube (alpha-delta-vLSR) data shows that there are three clumps with sizes of \sim 1 pc in the region. Two of them have peculiar red shifted velocity components at their edges, which can be interpreted as the results of the interaction between the cloud and the Cepheus Bubble. From the C18O cube data, the clumpfind identified 123 C18O cores, which have mean radius, velocity width in FWHM, and LTE mass of 0.36\pm0.07 pc, 0.37\pm0.09 km s-1, and 41\pm29 Msun, respectively. All the cores in S140 are most likely to be gravitationally bound by considering the uncertainty in the C18O abundance. We derived a C18O core mass function (CMF), which shows a power-law-like behavior above a turnover at 30 Msun. The best-fit power-law index of -2.1\pm0.2 is quite consistent with those of the IMF and the C18O CMF in the OMC-1 region by Ikeda & Kitamura (2009). Kramer et al. (1998) estimated the power-law index of -1.65 in S140 from the C18O(J=2-1) data, which is inconsistent with this study. However, the C18O(J=2-1) data are spatially limited to the central part of the cloud and are likely to be biased toward high-mass cores, leading to the flatter CMF. Consequently, this study and our previous one strongly support that the power-law form of the IMF has been already determined at the density of \sim 10^{3-4} cm^{-3}, traced by the C18O(J=1-0) line.

CO observations of massive star forming regions at different evolutionary phases

The 12CO (J=1-0), 13CO (J=1-0) and C18O (J=1-0) emissions in 9 massive star forming regions, which are believed to be at different stages of massive star formation, were mapped with the 13.7 m millimeter wave telescope at Qinghai Station of Purple Mountain Observatory. Of the observed 9 sources, 13CO cores were detected in seven of them, and C18O cores in five of them. And only two sources associated with C18O cores and H2O masers showed the extended structures and strong outflows. This is the first detection of outflow associated with IRAS 22566+5828 in the observing field of S152/S153. The physical parameters of cores and outflows for these sources, derived from Local Thermal Equilibrium (LTE) analysis, are presented. These observing results suggest that the C18O cores will only appear when the gas density is high enough and the probability to have an outflow is very high when the clumps show the C18O and H2O maser simultaneously.

High‐Mass Cloud Cores in the η Carinae Giant Molecular Cloud

We carried out an unbiased survey for massive dense cores in the giant molecular cloud associated with η Carinae with the NANTEN telescope in the 12CO, 13CO, and C18O J = 1-0 emission lines. We identified 15 C18O cores, whose typical line width ΔVcomp, radius r, mass M, column density N(H2), and average number density n(H2) were 3.3 km s-1, 2.2 pc, 2.6 × 103 M☉, 1.3 × 1022 cm-2, and 1.2 × 103 cm-3, respectively. Two of the 15 cores are associated with IRAS point sources whose luminosities are larger than 104 L☉, which indicates that massive star formation is occurring within these cores. Five cores, including the two with IRAS sources, are associated with MSX point sources. We detected H13CO+ (J = 1-0) emission toward four C18O cores, two of which are associated with IRAS and MSX point sources; another one is associated only with an MSX point source, and the other is associated with neither IRAS nor MSX point sources. The core with neither IRAS nor MSX point sources shows the presence of a bipolar molecular outflow in 12CO (J = 2-1), which indicates that star formation is also occurring in the core, and the other three of the four H13CO+ detections show winglike emission. In total, 6 C18O cores out of 15 (=40%) have experienced star formation, and at least 2 of 15 (=13%) are massive star-forming cores in the η Car GMC. We found that massive star formation occurs preferentially in cores with larger N(H2), M, and n(H2) and a smaller ratio of Mvir/M. We also found that the cores in the η Car GMC are characterized by large ΔV and Mvir/M on average compared to the cores in other GMCs observed with the same telescope. These properties of the cores may account for the fact that as much as 60%-87% of the cores do not show any signs of massive star formation. We investigated the origin of a large amount of turbulence in the η Car GMC. We found that turbulence injection from stellar winds, molecular outflows, and supernova remnants that originated from stars formed within the GMC are not enough to explain the existing turbulence. We propose the possibility that the large turbulence was preexisting when the GMC was formed and is now dissipating. Mechanisms such as multiple supernova explosions in the Carina flare supershell may have contributed to form a GMC with a large amount of turbulence.

IRAS 18317−0757: A Cluster of Embedded Massive Stars and Protostars

We present high-resolution, multiwavelength continuum and molecular-line images of the massive star forming region IRAS 18317-0757. The IR through mm spectral energy distribution can be approximated by a two-temperature model (25 and 63 K) with a total luminosity of approximately log(L/Lsun)=5.2. Previous submm imaging resolved this region into a cluster of 5 dust cores, one of which is associated with the UCHII region G23.96+0.15, and another with an H2O maser. In our new 2.7 mm continuum image, only the UCHII region is detected, with total flux and morphology in good agreement with the free-free emission in VLA cm-wave maps. For the other four objects, the nondetections at 2.7 mm and in the MSX mid-IR bands are consistent with cool dust emission with a temperature of 13-40K and luminosity of 1000-40000 Lsun. By combining single-dish and interferometric data, we have identified over two dozen virialized C18O cores in this region that contain ~40% of the total molecular gas mass present. While the overall extent of the C18O and dust emission is similar, the emission peaks do not correlate well in detail. At least 11 of the 123 stars identified by 2MASS in this region are likely to be within the star-forming cluster. Two stars (both associated with the UCHII region) were previously identified as O stars via IR spectroscopy. Most of the rest of the reddened stars have no obvious correlation with the C18O cores or the dust cores. In summary, our observations indicate that considerable fragmentation of the molecular cloud has taken place during the time required for the UCHII region to form and the O stars to become detectable at IR wavelengths. Additional star formation appears to be ongoing on the periphery of the central region, where up to four B-type (proto)stars have formed among a substantial number of C18O molecular cores.

Linear Sequences of Starless Cores and Young Stellar Objects in the Eagle Nebula

We observed heads of two molecular pillars in the Eagle Nebula using the Nobeyama Millimeter Array with a spatial resolution of 3'', 5'', and 3'' in the 13CO (J = 1-0) line, C18O (J = 1-0) line, and 2.7 mm continuum, respectively. We found six 13CO subclumps and four C18O cores. The 13CO clouds are elongated so as to have a head-tail structure, with the heads orientated toward the O star exciting M16. The elongation is likely to be due to radiation or wind from the O star. The cloud surface appears to be compressed, as indicated by strong 13CO emission at the cloud rim. The shapes of the 13CO clouds are quite similar to those of the dark cloud observed in the near-infrared. Three out of the four C18O cores are located within one of the 13CO clouds. One of the three cores, located near the tip of the 13CO cloud, is associated with a 2.7 mm continuum emission peak and is most likely a protostar. It is not associated with a known near-infrared source. The other two cores are located farther from the O star and are most likely starless cores. Thus, these C18O cores are aligned in order of age, with the more evolved objects closer to the O star. This linear sequence suggests the propagation of star formation activity, i.e., sequential star formation, driven by the O star. A similar sequence of a young stellar object and a C18O core was found in the other head of the molecular pillar.

Cloud Collision–induced Star Formation in Sagittarius B2. I. Large‐Scale Kinematics

We present maps of a ~ 14 pc × 20 pc region covering the Sgr B2 molecular cloud complex in the 13CO (1-0) and CS (1-0) lines with high angular resolution. A more restricted central area was also mapped in the C18O (1-0) line. The 13CO cloud consists of two components: a compact core of ~ 3 pc in diameter roughly centered on Sgr B2(M) and an extended plateau of ~ 17 pc × 7.5 pc area containing the maser sources and most of the H II regions. Compact, massive (~ 3 × 105 M☉) C18O cores are associated with Sgr B2(M) and Sgr B2(N). The obtained intensity distributions are compared with those of molecular masers and compact H II regions reported in the literature. Low-velocity (40-65 km s-1) masers, with positions roughly aligned in the north-south line connecting the three major compact H II region complexes, are located near the eastern margin of a large hole in the low-velocity molecular cloud. In contrast, most of the high-velocity (65-80 km s-1) masers are distributed near the center of a compact molecular cloud at higher velocity, where a shock seems to have occurred. These results provide further evidence supporting the cloud-cloud collision hypothesis based on limited 13CO (1-0) data. In addition, this hypothesis appears to be consistent with a wide range of new observations that have appeared in the literature.

C18O Observations of the Dense Cloud Cores and Star Formation in Ophiuchus

In order to reveal the distribution of the dense gas of ≥104 cm3, C18O (J = 1-0) observations have been made toward the molecular clouds in the Ophiuchus region of ~6.4 deg2 with the two 4 m telescopes of Nagoya University. Forty dense cores have been identified, providing the first complete sample of such dense cores in Ophiuchus. The C18O dense cores are distributed not only in the active star-forming region, ρ Oph cloud core, but also in the North region where star formation is less active. The typical core mass, MLTE, radius, R, and average number density, n(H2), of the cores are 90 M☉, 0.24 pc, and 1.7 × 104 cm-3 in the ρ Oph region, respectively, and 14 M☉, 0.19 pc, and 7.6 × 103 cm-3 in the North region, respectively. Nine of the 40 cores are associated with young stellar objects, and most of the C18O cores are starless. An analysis of the physical parameters of the C18O cores show that star-forming cores tend to have larger N(H2) than the rest by a factor of ~3, although there is no significant trend in the other physical parameters between star forming and starless cores. We have compared the present C18O data with the 13CO data (Nozawa et al.) and with the associated YSOs, in order to understand better the condensing process from molecular gas with density of ~103 cm-3 to protostars. It is found that 55% of the 13CO cores are associated with C18O cores and that the C18O cores are typically less massive, smaller and denser by ~34%, ~32%, and a factor of ~3, respectively, than the 13CO cores. It is also found that the C18O cores have steeper density profiles than the 13CO cores; when we fit the density profile by a power law as ρ ∝ r-β, the values of β for C18O and 13CO are estimated as ~1.5 and 1.2, respectively. This suggests that the C18O cores are gravitationally more relaxed than the 13CO cores. In order to investigate the energetics of the cores, the virial mass, MVIR, has been calculated for each core. It is found that most of the 13CO cores have MVIR larger than MLTE. On the other hand, 22 of the 40 C18O cores have MVIR smaller than MLTE, suggesting that the C18O cores are more deeply gravitationally bound than the 13CO cores. Further, we have found a correlation between the ratio MVIR/MLTE and star formation activity: (1) For 13CO cores, the fraction of the 13CO cores associated with the C18O cores tends to increase with decreasing MVIR/MLTE, and (2) for the C18O cores, the fraction of the C18O cores associated with stars tends to increase with decreasing of MVIR/MLTE. We interpret this to indicate that the gradual dissipation of the internal turbulence leads to formation of denser cores and subsequent star formation. Through the evolution from the 13CO cores to the C18O cores, they should lose the turbulence energy of ~1044 ergs. The supersonic gas motion with the magnetic fields produces shocks, and the radiation from the small shocked region may significantly contribute to the cooling. We suggest that the cores have continuous collisions between turbulent eddies to produce the C-shocks. Also, the Alfvenic energy loss may be viable as the dissipation mechanism.

NANTEN Observations of the Pipe Nebula; A Filamentary Massive Dark Cloud with Very Low Star-Formation Activity

We present molecular line observations toward the “Pipe Nebula” in the J = 1−0 lines of 12CO, 13CO, and C18O by using “NANTEN” telescope. An area of ∼ 27 deg2 was covered at a 4′ grid spacing with a 2′.7 beam in 12CO. The 12CO velocity-integrated intensity map and channel maps show a filamentary distribution. The total mass of the 12CO- and 13CO-emitting gas is estimated to be ∼ 10000 M⊙ and ∼ 3000 M⊙ , respectively. We have identified 14 C18O cores whose mass is typically ∼ 30 M⊙ . Star formation is active only in the B 59 region. This activity is best demonstrated by a newly detected CO outflow toward the center of B 59. We suggest that the dynamical effects of tau Sco may be responsible for triggering star formation only in the B 59 region. The C18O column density toward B 59 is extremely high compared with the rest of the cloud. This confirms that high C18O column density is a necessary condition of star formation as previously suggested. Although the star-formation efficiency is estimated to be quite low, ≲ 0.1%, except for B 59, the existence of the C18O cores suggests that there is molecular gas that is massive and dense enough to form stars, and that star formation is likely to occur in the near future.

A Study of Dense Cloud Cores and Star Formation in Lupus: C18O J = 1−0 Observations with NANTEN

We have made an extensive study of dense cloud cores and star formation in the Lupus dark-cloud complex. Millimeter-wave observations in the C18O (J = 1−0) emission at 2.6 mm wavelength were made with the “NANTEN,” a 4-m mm-wave telescope of Nagoya University at Las Campanas Observatory. Thirty-six dense cores were identified and mapped in the C18O emission at 2′ grid spacing with a 2′.7 beam. The typical mass, radius, H2 column density, and line width of the C18O cores are 9.7 M⊙ , 0.17 pc, 4.1 × 1021 cm-2, 0.90 km s-1, respectively. The mass spectrum of the C18O cores was fitted by a single power-law index of γ = −1.7± 0.6 for Mcore ≥ 3 M⊙ . The physical parameters of the dense C18O cores were derived and compared with those in Taurus, Ophiuchus North, and L 1333. The average characteristics of the C18O cores in Lupus are similar to those in Ophiuchus North and L 1333, although those in Taurus are found to be different, especially in the line width, virial ratio, and mass spectrum index among the four regions. By comparing the physical properties of the C18O cores with the distribution of YSOs and H13CO+ condensations in Lupus, we have confirmed a trend suggested in previous studies, that star formation preferentially occurs in cores having a small virial ratio, Mvir/Mcore, and a large H2 column density.

Dense Cores and Star-Formation Activities in the Chamaeleon Dark-Cloud Complex

We carried out a C18O (J = 1−0) survey for dense molecular cores in the Chamaeleon (Cha) I, II, and III dark clouds with the NANTEN millimeter-wave telescope. The present survey covered ∼ 93% of the area where the molecular column density is greater than ∼ 4 × 1021 cm-2, estimated from preceding 13CO(J = 1−0) observations. We identified 23 C18O cores, whose typical mass, radius, peak column density, line width are 22 M⊙ , 0.22 pc, 9.7 × 1021 cm-2, and 0.82 km s-1, respectively. The surface density of classical T Tauri stars abruptly increases for the area whose column density is greater than 1022 cm-2, suggesting that this value gives a certain threshold value for star formation. The star-formation efficiency varies over a wide range among the three clouds: 13%, 1%, and 0% in Cha I, II, and III, respectively. The C18O cores in Cha I are characterized by (1) a high column density, (2) being almost in virial equilibrium, and (3) a high Mcore/Mcloud ratio. The cores in Cha III show the opposite trend, and those in Cha II are in between. Such trends suggest that Cha I is a well-evolved or well gravitationally relaxed cloud-core system, which is probably related to the very high star-formation activity in the cloud.

NANTEN Observations of Dense Cores in the Corona Australis Molecular Cloud

We carried out a C18O survey for dense molecular cores in the Corona Australis (CrA) molecular cloud with the NANTEN telescope. We observed 2.2 deg2 at a 2′ grid spacing with a 2′.7 beam, and 1980 positions were observed. We identified 8 C18O cores, whose typical line width, average column density, radius, mass, and average number density were 0.66 km s-1, 1.1×1022 cm-2, 0.13 pc, 18 M⊙ , and 1.4 × 104 cm-3, respectively. We found that Δ Vcomp, 〈 N(H2)〉, R, M, and n(H2) become larger along with an increase in the star-formation activity, whereas the ratio Mvir/M becomes smaller. A comparison of the present cores with those in Chamaeleon, Lupus, Ophiuchus, Taurus, and L 1333 indicates that star-forming cores tend to have a high column density, as well as a smaller Mvir/M ratio.

A C18O Survey of Dense Cloud Cores in Taurus: Star Formation

This paper discusses star formation in the 40 C18O molecular cloud cores in Taurus by studying relations between the cores and young stellar objects (YSOs). These C18O cores constitute a complete sample of dense gas in Taurus as described in detail by Onishi and coworkers. The YSOs treated in the present work fall into the following four categories: (1) starless H13CO+ compact cores, (2) cold IRAS sources, (3) warm IRAS sources, and (4) T Tauri stars without far-infrared emission. Starless H13CO+ compact cores are assumed to be protostellar condensations that are in a phase just prior to star formation, although they are not yet considered to be YSOs in the usual sense. In the present study, we shall call the objects 1 and 2 cold objects and objects 3 and 4 warm objects. They constitute the most complete sample of YSOs, including compact dense molecular condensations, at present. It is found that C18O cores with cold objects have clearly higher molecular column densities than the rest. These C18O cores also tend to be massive and large in size. If we assume that the cold objects represent the protostellar condensations or protostars in the main accretion phase, the results imply that the actual process of star formation, i.e., collapse and significant mass accretion, takes place when a column density of the molecular cloud core becomes greater than a certain value. This threshold value for star formation is ~8.0 ? 1021 cm-2 in average column density. On the other hand, the C18O cores without cold objects are characterized by a typical column density of 5.5 ? 1021 cm-2, significantly smaller than the threshold value. The fact that all the C18O cores whose average column density is higher than 8.0 ? 1021 cm-2 are associated with cold objects suggests that star formation takes place immediately and without exception when the column density satisfies the above value. Such C18O cores with cold objects typically have more than one YSO. The number of YSOs in a C18O core increases with core mass. The C18O core mass divided by the number of YSOs does not depend strongly on the C18O core mass and is nearly a constant mass of 11 M?, i.e., the number of YSOs within a C18O core is proportional to the core mass. This suggests that formation of a protostellar condensation requires ~11 M? molecular gas of n(H)2 ~ 104 cm-3 indicating a rather uniform star formation efficiency (Mstar/Mgas) of ~6% (the mass of a YSO is taken to be 0.7 M?) at a density of ~104 cm-3. The mean projected separation of the cold objects in a C18O core is estimated to be ~0.3 pc. This separation is roughly in a range explicable by existing theoretical studies of gravitational instability, while the core morphology is not yet exactly taken into account in these studies. The C18O data suggest that the amount of the surrounding gas decreases on a timescale of, ~5 ? 105 yr or larger. Dissipation of surrounding gas occurs not only in the vicinity of a star but also in the scale of C18O cores, 0.1 pc. This dissipation cannot be explained simply by an outflow or mass accretion onto a central star.

Star Formation in the L1333 Molecular Cloud in Cassiopeia

?????Radio and optical observations have been made in order to examine star formation in the dark cloud L1333 [?(1950) = 2h210, ?(1950) = 75?15']. A study of the cumulative distribution of field star distance moduli yields a distance value close to 180 pc for the L1333 molecular cloud. 13CO observations revealed two filamentary molecular clouds with distinct velocities of VLSR ~ -2 and 3 km s-1, respectively. Thirteen C18O cores, characterized by an average molecular mass (MLTE) of ~9 M? and a mean density of ~1.4 ? 104 cm-3, are embedded in the two 13CO clouds. We have detected 18 H? emission-line stars projected within or near the 13CO clouds on objective-prism plates. They can be regarded as candidate pre?main-sequence stars formed in the clouds. Five IRAS sources with flux density distributions characteristic of young stellar objects (YSOs) are found in the area of the cloud. Three of the IRAS sources coincide with H? emission-line stars, the others with faint stars without detected H? emission. One of the latter sources is associated with a C18O core and exhibits a protostar-type infrared spectrum. The IRAS source exhibits a winglike feature of ~1.6 km s-1 (full width at zero intensity) in C18O, which may indicate that the YSO has an outflow. We note that the C18O core has the smallest ratio of virial mass (Mvir) to MLTE among the C18O cores in L1333, suggesting that star formation occurs preferentially in a core whose internal kinetic energy is low compared with the self-gravitational energy. A possible scenario for the past star formation activity is discussed.