Molecular Clumps Research Articles

The Orientation of Eta Carinae and the Powering Mechanism of Intermediate-luminosity Optical Transients (ILOTS)

Abstract Contrary to recent claims, we argue that the orientation of the massive binary system Eta Carinae is such that the secondary star is closer to us at periastron passage, and it is on the far side during most of the time of the eccentric orbit. The binary orientation we dispute is based on problematic interpretations of recent observations. Among these are the radial velocity of the absorption component of He i P-Cyg lines, of the He ii λ4686 emission line, and of the Br γ line emitted by clumps close to the binary system. We also base our orientation on observations of asymmetric molecular clumps that were recently observed by ALMA around the binary system, and were claimed to compose a torus with a missing segment. The orientation has implications for the modeling of the binary interaction during the nineteenth century Great Eruption (GE) of Eta Carinae that occurred close to periastron passage. The orientation where the secondary is closer to us at periastron leads us to suggest that the mass-missing side of the molecular clumps is a result of accretion onto the secondary star during periastron passage when the clumps were ejected, probably during the GE. The secondary star accreted a few solar masses during the GE and the energy from the accretion process consists of the majority of the GE energy. This in turn strengthens the more general model according to which many intermediate-luminosity optical transients (ILOTS) are powered by accretion onto a secondary star.

Stellar mass spectrum within massive collapsing clumps

Context. Stars constitute the building blocks of our Universe, and their formation is an astrophysical problem of great importance.Aim. We aim to understand the fragmentation of massive molecular star-forming clumps and the effect of initial conditions, namely the density and the level of turbulence, on the resulting distribution of stars. For this purpose, we conduct numerical experiments in which we systematically vary the initial density over four orders of magnitude and the turbulent velocity over a factor ten. In a companion paper, we investigate the dependence of this distribution on the gas thermodynamics.Methods. We performed a series of hydrodynamical numerical simulations using adaptive mesh refinement, with special attention to numerical convergence. We also adapted an existing analytical model to the case of collapsing clouds by employing a density probability distribution function (PDF) ∝ρ−1.5 instead of a lognormal distribution.Results. Simulations and analytical model both show two support regimes, a thermally dominated regime and a turbulence-dominated regime. For the first regime, we infer that dN∕d logM ∝ M0, while for the second regime, we obtain dN∕d logM ∝ M−3∕4. This is valid up to about ten times the mass of the first Larson core, as explained in the companion paper, leading to a peak of the mass spectrum at ~0.2 M⊙. From this point, the mass spectrum decreases with decreasing mass except for the most diffuse clouds, where disk fragmentation leads to the formation of objects down to the mass of the first Larson core, that is, to a few 10−2 M⊙.Conclusions. Although the mass spectra we obtain for the most compact clouds qualitatively resemble the observed initial mass function, the distribution exponent is shallower than the expected Salpeter exponent of − 1.35. Nonetheless, we observe a possible transition toward a slightly steeper value that is broadly compatible with the Salpeter exponent for masses above a few solar masses. This change in behavior is associated with the change in density PDF, which switches from a power-law to a lognormal distribution. Our results suggest that while gravitationally induced fragmentation could play an important role for low masses, it is likely the turbulently induced fragmentation that leads to the Salpeter exponent.

Molecular Cloud Structures and Massive Star Formation in N159

Abstract The N159 star-forming region is one of the most massive giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC). We show the 12CO, 13CO, CS molecular gas lines observed with ALMA in N159 west (N159W) and N159 east (N159E). We relate the structure of the gas clumps to the properties of 24 massive young stellar objects (YSOs) that include 10 newly identified YSOs based on our search. We use dendrogram analysis to identify properties of the molecular clumps, such as flux, mass, linewidth, size, and virial parameter. We relate the YSO properties to the molecular gas properties. We find that the CS gas clumps have a steeper size–linewidth relation than the 12CO or 13CO gas clumps. This larger slope could potentially occur if the CS gas is tracing shocks. The virial parameters of the 13CO gas clumps in N159W and N159E are low (<1). The threshold for massive star formation in N159W is 501 M ⊙ pc−2, and the threshold for massive star formation in N159E is 794 M ⊙ pc−2. We find that 13CO is more photodissociated in N159E than N159W. The most massive YSO in N159E has cleared out a molecular gas hole in its vicinity. All the massive YSO candidates in N159E have a more evolved spectral energy distribution type in comparison to the YSO candidates in N159W. These differences lead us to conclude that the giant molecular cloud complex in N159E is more evolved than the giant molecular cloud complex in N159W.

ATLASGAL – Ammonia observations towards the southern Galactic plane

Context. The initial conditions of molecular clumps in which high-mass stars form are poorly understood. In particular, a more detailed study of the earliest evolutionary phases is needed. The APEX Telescope Large Area Survey of the whole inner Galactic disk at 870 μm, ATLASGAL, has therefore been conducted to discover high-mass star-forming regions at different evolutionary phases. Aims. We derive properties such as velocities, rotational temperatures, column densities, and abundances of a large sample of southern ATLASGAL clumps in the fourth quadrant. Methods. Using the Parkes telescope, we observed the NH3 (1, 1) to (3, 3) inversion transitions towards 354 dust clumps detected by ATLASGAL within a Galactic longitude range between 300° and 359° and a latitude within ± 1.5°. For a subsample of 289 sources, the N2H+ (1–0) line was measured with the Mopra telescope. Results. We measured a median NH3 (1, 1) line width of ~ 2 km s-1, rotational temperatures from 12 to 28 K with a mean of 18 K, and source-averaged NH3 abundances from 1.6 × 10-6 to 10-8. For a subsample with detected NH3 (2, 2) hyperfine components, we found that the commonly used method to compute the (2, 2) optical depth from the (1, 1) optical depth and the (2, 2) to (1, 1) main beam brightness temperature ratio leads to an underestimation of the rotational temperature and column density. A larger median virial parameter of ~ 1 is determined using the broader N2H+ line width than is estimated from the NH3 line width of ~ 0.5 with a general trend of a decreasing virial parameter with increasing gas mass. We obtain a rising NH3 (1, 1)/N2H+ line-width ratio with increasing rotational temperature. Conclusions. A comparison of NH3 line parameters of ATLASGAL clumps to cores in nearby molecular clouds reveals smaller velocity dispersions in low-mass than high-mass star-forming regions and a warmer surrounding of ATLASGAL clumps than the surrounding of low-mass cores. The NH3 (1, 1) inversion transition of 49% of the sources shows hyperfine structure anomalies. The intensity ratio of the outer hyperfine structure lines with a median of 1.27 ± 0.03 and a standard deviation of 0.45 is significantly higher than 1, while the intensity ratios of the inner satellites with a median of 0.9 ± 0.02 and standard deviation of 0.3 and the sum of the inner and outer hyperfine components with a median of 1.06 ± 0.02 and standard deviation of 0.37 are closer to 1.

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

We mapped the kinetic temperature structure of the Orion molecular cloud 1 (OMC-1) with para-H2CO (JKaKc = 303–202, 322–221, and 321–220) using the APEX 12 m telescope. This is compared with the temperatures derived from the ratio of the NH3 (2, 2)/(1, 1) inversion lines and the dust emission. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured averaged line ratios of para-H2CO 322–221/303–202 and 321–220/303–202. The gas kinetic temperatures derived from the para-H2CO line ratios are warm, ranging from 30 to >200 K with an average of 62 ± 2 K at a spatial density of 105 cm-3. These temperatures are higher than those obtained from NH3 (2, 2)/(1, 1) and CH3CCH (6–5) in the OMC-1 region. The gas kinetic temperatures derived from para-H2CO agree with those obtained from warm dust components measured in the mid infrared (MIR), which indicates that the para-H2CO (3–2) ratios trace dense and warm gas. The cold dust components measured in the far infrared (FIR) are consistent with those measured with NH3 (2, 2)/(1, 1) and the CH3CCH (6–5) line series. With dust at MIR wavelengths and para-H2CO (3–2) on one side, and dust at FIR wavelengths, NH3 (2, 2)/(1, 1), and CH3CCH (6–5) on the other, dust and gas temperatures appear to be equivalent in the dense gas (n(H2) ≳ 104 cm-3) of the OMC-1 region, but provide a bimodal distribution, one more directly related to star formation than the other. The non-thermal velocity dispersions of para-H2CO are positively correlated with the gas kinetic temperatures in regions of strong non-thermal motion (Mach number ≳ 2.5) of the OMC-1, implying that the higher temperature traced by para-H2CO is related to turbulence on a ~0.06 pc scale. Combining the temperature measurements with para-H2CO and NH3 (2, 2)/(1, 1) line ratios, we find direct evidence for the dense gas along the northern part of the OMC-1 10 km s-1 filament heated by radiation from the central Orion nebula.

A multi-wavelength analysis of the diffuse H ii region G25.8700+0.1350

We present a multiwavelength investigation of the HII region G25.8700+0.1350, located in the inner part of the Galaxy. In radio continuum emission, the region is seen as a bright arc-shaped structure. An analysis of the HI line suggests that G25.8700+0.1350 lies at a distance of 6.5 kpc. The ionized gas is bordered by a photodissociation region which is encircled by a molecular structure where four molecular clumps are detected. At infrared wavelengths, the region is also very conspicuous. Given the high level of visual absorption in the region, the exciting stars should be searched for in the infrared band. In this context, we found in the literature one Wolf-Rayet and one red supergiant which, together with 37 2MASS sources candidates to be O-type stars, could be related to the origin of G25.8700+0.1350. Finally, as expanding HII regions are hypothesized to trigger star formation, we used different infrared point source catalogues to search for young stellar object candidates (cYSOs). A total of 45 cYSOs were identified projected onto the molecular clouds.

Dynamical equivalence, the origin of the Galactic field stellar and binary population, and the initial radius–mass relation of embedded clusters

Abstract In order to allow a better understanding of the origin of Galactic field populations, dynamical equivalence of stellar-dynamical systems has been postulated by Kroupa and Belloni et al. to allow mapping of solutions of the initial conditions of embedded clusters such that they yield, after a period of dynamical processing, the Galactic field population. Dynamically equivalent systems are defined to initially and finally have the same distribution functions of periods, mass ratios and eccentricities of binary stars. Here, we search for dynamically equivalent clusters using the mocca code. The simulations confirm that dynamically equivalent solutions indeed exist. The result is that the solution space is next to identical to the radius–mass relation of Marks & Kroupa, $\left( r_{\rm h}/{\rm pc} \right)= 0.1^{+0.07}_{-0.04}{\, } \left( M_{\rm ecl}/{\rm M}_{\odot } \right)^{0.13\pm 0.04}$. This relation is in good agreement with the observed density of molecular cloud clumps. According to the solutions, the time-scale to reach dynamical equivalence is about 0.5 Myr which is, interestingly, consistent with the lifetime of ultra-compact H ii regions and the time-scale needed for gas expulsion to be active in observed very young clusters as based on their dynamical modelling.

The Stellar Content of the Infalling Molecular Clump G286.21+0.17

Abstract The early evolution during massive star cluster formation is still uncertain. Observing embedded clusters at their earliest stages of formation can provide insight into the spatial and temporal distribution of the stars and thus probe different star cluster formation models. We present near-infrared imaging of an 8′ × 13′ (5.4 pc × 8.7 pc) region around the massive infalling clump G286.21+0.17 (also known as BYF73). The stellar content across the field is determined and photometry is derived in order to obtain stellar parameters for the cluster members. We find evidence for some sub-structure (on scales less than a parsec in diameter) within the region with apparently at least three different sub-clusters associated with the molecular clump based on differences in extinction and disk fractions. At the center of the clump we identify a deeply embedded sub-cluster. Near-infrared excess is detected for 39%–44% in the two sub-clusters associated with molecular material and 27% for the exposed cluster. Using the disk excess as a proxy for age, this suggests the clusters are very young. The current total stellar mass is estimated to be at least 200 M ⊙. The molecular core hosts a rich population of pre-main-sequence stars. There is evidence for multiple events of star formation both in terms of the spatial distribution within the star-forming region and possibly from the disk frequency.

Study of the molecular and ionized gas in a possible precursor of an ultra-compact H II region

We study the molecular and the ionized gas in a possible precursor of an UC HII region to contribute to the understanding of how high-mass stars build-up their masses once they have reached the zero-age main secuence. We carried out molecular observations toward the position of the Red MSX source G052.9221-00.4892, using ASTE in the 12CO, 13CO, and C18O J=3-2, and HCO+ J=4-3 lines. We also present radio continuum observations at 6 GHz carried out with the JVLA interferometer. Combining these observations with public infrared data allowed us to inquire about the nature of the source. The analysis of the molecular observations reveals the presence of a kinetic temperature and H2 column density gradients across the molecular clump in which the source is embedded, with the hotter and less dense gas in the inner region. The 12CO J=3-2 emission shows evidence of misaligned massive molecular outflows, with the blue lobe in positional coincidence with a jet-like feature seen at 8 um. The radio continuum emission shows a slightly elongated compact radio source in positional coincidence with the Red MSX source. The polar-like morphology of this compact radio source perfectly matches the hourglass-like morphology exhibited by the source in the Ks-band. The axes of symmetry of the radio source and the near-IR nebula are perfectly aligned. Based on the multiwavelength analysis, we suggest that the analyzed source could be transiting a HC HII region phase, in which the young central star emits winds and ionizing radiation through the poles. By the other hand, according to a comparison between the Br-gamma and the radio flux density, the source would be in a more evolved evolutionary stage of an optically thin UC HII region in photoionization equilibrium. If this is the case, from the radio continuum emission, we can conjecture upon the spectral type of its exciting star which would be a B0.5V.

Complex organics in IRAS 4A revisited with ALMA and PdBI: Striking contrast between two neighbouring protostellar cores

We used the Atacama Large (sub-)Millimeter Array (ALMA) and the IRAM Plateau de Bure Interferometer (PdBI) to image, with an angular resolution of 0.5$''$ (120 au) and 1$''$ (235 au), respectively, the emission from 11 different organic molecules in the protostellar binary NGC1333 IRAS 4A. We clearly disentangled A1 and A2, the two protostellar cores present. For the first time, we were able to derive the column densities and fractional abundances simultaneously for the two objects, allowing us to analyse the chemical differences between them. Molecular emission from organic molecules is concentrated exclusively in A2 even though A1 is the strongest continuum emitter. The protostellar core A2 displays typical hot corino abundances and its deconvolved size is 70 au. In contrast, the upper limits we placed on molecular abundances for A1 are extremely low, lying about one order of magnitude below prestellar values. The difference in the amount of organic molecules present in A1 and A2 ranges between one and two orders of magnitude. Our results suggest that the optical depth of dust emission at these wavelengths is unlikely to be sufficiently high to completely hide a hot corino in A1 similar in size to that in A2. Thus, the significant contrast in molecular richness found between the two sources is most probably real. We estimate that the size of a hypothetical hot corino in A1 should be less than 12 au. Our results favour a scenario in which the protostar in A2 is either more massive and/or subject to a higher accretion rate than A1, as a result of inhomogeneous fragmentation of the parental molecular clump. This naturally explains the smaller current envelope mass in A2 with respect to A1 along with its molecular richness.

Early science with the Large Millimetre Telescope: fragmentation of molecular clumps in the Galaxy

Sensitive, imaging observations of the 1.1 mm dust continuum emission from a 1 deg^2 area collected with the AzTEC bolometer camera on the Large Millimeter Telescope are presented. A catalog of 1545 compact sources is constructed based on a Wiener-optimization filter. These sources are linked to larger clump structures identified in the Bolocam Galactic Plane Survey. Hydrogen column densities are calculated for all sources and mass and mean volume densities are derived for the subset of sources for which kinematic distances can be assigned. The AzTEC sources are localized, high density peaks within the massive clumps of molecular clouds and comprise 5-15% of the clump mass. We examine the role of the gravitational instability in generating these fragments by comparing the mass of embedded AzTEC sources to the Jeans' mass of the parent BGPS object. For sources with distances less than 6 kpc the fragment masses are comparable to the clump Jeans' mass, despite having isothermal Mach numbers between 1.6 and 7.2. AzTEC sources linked to ultra-compact HII regions have mass surface densities greater than the critical value implied by the mass-size relationship of infrared dark clouds with high mass star formation while AzTEC sources associated with Class II methanol masers have mass surface densities greater than 0.7 g cm^{-2} that approaches the proposed threshold required to form massive stars.

Triggered massive star formation associated with the bubble Hii region Sh2-39 (N5)

Aiming at studying the physical properties of Galactic IR bubbles and to explore their impact in triggering massive star formation, we perform a multiwavelength analysis of the bubble HII region Sh2-39 (N5) and its environs. To analyze the molecular gas we use CO(3-2) and HCO+(4-3) line data obtained with the ASTE telescope. To study the physical characteristics of the dust, we make use of archival data from ATLASGAL, Herschel, and MSX, while the ionized gas was studied making use of NVSS data. We use public IR point source catalogs to search for candidate YSOs in the region. To investigate the stellar cluster [BDS2003]6 we use IR spectroscopic data obtained with the ARCoIRIS spectrograph, and new available IR Ks band observations from VVVX survey. The new ASTE observations allowed the molecular gas component in the velocity range from 30 km/s to 46 km/s, associated with Sh2-39, to be studied in detail. We have identified four molecular clumps, that were likely formed by the expansion of the ionization front, and determined some of their physical and dynamical properties. Clumps having HCO+ and 870 micron counterparts show evidence of gravitational collapse. We identified several candidate YSOs across the molecular component. Their spatial distribution, as well as the fragmentation time derived for the collected layers of the molecular gas, suggest that massive star formation might have been triggered by the expansion of the nebula via the collect and collapse mechanism. The spectroscopical distance obtained for the stellar cluster [BDS2003]6, placed over one of the collapsing clumps, reveals that this cluster is physically associated with the neabula and gives more support to the triggered massive star formation scenario. A radio continuum data analysis indicates that the nebula is older and expands at lower velocity than typical Galactic IR bubbles.

MALT90 Kinematic Distances to Dense Molecular Clumps

Abstract Using molecular-line data from the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90), we have estimated kinematic distances to 1905 molecular clumps identified in the ATLASGAL 870 μm continuum survey over the longitude range 295° < l < 350°. The clump velocities were determined using a flux-weighted average of the velocities obtained from Gaussian fits to the HCO+, HNC, and N2H+ (1–0) transitions. The near/far kinematic distance ambiguity was addressed by searching for the presence or absence of absorption or self-absorption features in 21 cm atomic hydrogen spectra from the Southern Galactic Plane Survey. Our algorithm provides an estimation of the reliability of the ambiguity resolution. The Galactic distribution of the clumps indicates positions where the clumps are bunched together, and these locations probably trace the locations of spiral arms. Several clumps fall at the predicted location of the far side of the Scutum–Centaurus arm. Moreover, a number of clumps with positive radial velocities are unambiguously located on the far side of the Milky Way at galactocentric radii beyond the solar circle. The measurement of these kinematic distances, in combination with continuum or molecular-line data, now enables the determination of fundamental parameters such as mass, size, and luminosity for each clump.

Impact of a star formation efficiency profile on the evolution of open clusters

Aims. We study the effect of the instantaneous expulsion of residual star-forming gas on star clusters in which the residual gas has a density profile that is shallower than that of the embedded cluster. This configuration is expected if star formation proceeds with a given star-formation efficiency per free-fall time in a centrally concentrated molecular gas clump.

Gas kinematics in high-mass star-forming regions from the Perseus spiral arm

We present results of a survey of 14 star-forming regions from the Perseus spiral arm in CS(2-1) and 13CO(1-0) lines with the Onsala Space Observatory 20 m telescope. Maps of 10 sources in both lines were obtained. For the remaining sources a map in just one line or a single-point spectrum were obtained. On the basis of newly obtained and published observational data we consider the relation between velocities of the "quasi-thermal" CS(2-1) line and 6.7 GHz methanol maser line in 24 high-mass star-forming regions in the Perseus arm. We show that, surprisingly, velocity ranges of 6.7 GHz methanol maser emission are predominantly red-shifted with respect to corresponding CS(2-1) line velocity ranges in the Perseus arm. We suggest that the predominance of the "red-shifted masers" in the Perseus arm could be related to the alignment of gas flows caused by the large-scale motions in the Galaxy. Large-scale galactic shock related to the spiral structure is supposed to affect the local kinematics of the star-forming regions. Part of the Perseus arm, between galactic longitudes from 85deg to 124deg, does not contain blue-shifted masers at all. Radial velocities of the sources are the greatest in this particular part of the arm, so the velocity difference is clearly pronounced. 13CO(1-0) and CS(2-1) velocity maps of G183.35-0.58 show gas velocity difference between the center and the periphery of the molecular clump up to 1.2 km/s. Similar situation is likely to occur in G85.40-0.00. This can correspond to the case when the large-scale shock wave entrains the outer parts of a molecular clump in motion while the dense central clump is less affected by the shock.

The CepHeus-A Star formation and proper Motions (CHASM) Survey

AbstractThe “CepHeus-A Star formation and proper Motions” (CHASM) survey is a large project consisting of a combination of astrometric Very Long Baseline Array (VLBA) and Jansky Very Large Array (VLA) observations, to map both the stellar and dense molecular gas components in the star-forming region Cepheus A. With the VLBA, we make use of the CH3OH and H2O maser emission in the vicinity of Cepheus A HW2, in order to measure accurate proper motions and parallax distances to both T Tauri stars and massive young stellar objects (YSOs) belonging to the same star-forming region. With the Jansky VLA, we make use of the interstellar thermometer NH3, in order to image the molecular clump surrounding Cepheus A HW2 and to determine its physical conditions. By combining these informations all together, we can provide, for instance, a direct measurement of the Bondi-Hoyle accretion radius for a massive young star, namely, HW2.

Intermittency and Anisotropy in the Ionized Interstellar Medium

AbstractThe discovery of pulsars was closely followed by the discovery of dispersion and scattering in the interstellar plasma (ionized interstellar medium - IISM). The rich phenomena of scattering and scintillation have since been successfully modelled as propagation through a statistically uniform plasma turbulence with an isotropic Kolmogorov spectrum of density. However, this enticingly simple model fails to explain the many recent observations, that show anisotropic scattering from highly localized regions of the IISM often referred to as phase screens. I summarize the recent evidence from pulsars and also from very compact AGN sources, which can exhibit rapid scintillation and occasionally ESEs. The unknown astrophysical origin of these phenomena includes thin current sheets, the diffuse remnants of old supernova shells, and plasma filaments surrounding ubiquitous molecular clumps near young hot stars.

ALMA Detects CO(3–2) within a Super Star Cluster in NGC 5253

Abstract We present observations of CO(3–2) and 13CO(3–2) emission near the supernebula in the dwarf galaxy NGC 5253, which contains one of the best examples of a potential globular cluster in formation. The 0.″3 resolution images reveal an unusual molecular cloud, “Cloud D1,” that is coincident with the radio-infrared supernebula. The ∼6 pc diameter cloud has a linewidth, Δ v = 21.7 , that reflects only the gravitational potential of the star cluster residing within it. The corresponding virial mass is 2.5 × 105 . The cluster appears to have a top-heavy initial mass function, with M * ≳ 1–2 . Cloud D1 is optically thin in CO(3–2), probably because the gas is hot. Molecular gas mass is very uncertain but constitutes <35% of the dynamical mass within the cloud boundaries. In spite of the presence of an estimated ∼1500–2000 O stars within the small cloud, the CO appears relatively undisturbed. We propose that Cloud D1 consists of molecular clumps or cores, possibly star-forming, orbiting with more evolved stars in the core of the giant cluster.

ALMA Observations of N83C in the Early Stage of Star Formation in the Small Magellanic Cloud

Abstract We have performed Atacama Large Millimeter/submillimeter Array (ALMA) observations in the 12CO( ), 13CO( ), C18O( ), 12CO( ), 13CO( ), and CS( ) lines toward the active star-forming region N83C in the Small Magellanic Cloud (SMC), whose metallicity is about one-fifth of the Milky Way (MW). The ALMA observations first reveal subparsec-scale molecular structures in 12CO( ) and 13CO( ) emissions. We found strong CO peaks associated with young stellar objects (YSOs) identified by the Spitzer Space Telescope, and we also found that overall molecular gas is distributed along the edge of the neighboring ii region. We derived a gas density of cm−3 in molecular clouds associated with YSOs based on the virial mass estimated from the 12CO( ) emission. This high gas density is presumably due to the effect of the ii region under the low-metallicity (and accordingly small-dust content) environment in the SMC; far-UV radiation from the ii region can easily penetrate and photodissociate the outer layer of 12CO molecules in the molecular clouds, and thus only the innermost parts of the molecular clouds are observed even in 12CO emission. We obtained the CO-to-H2 conversion factor of cm−2 (K km s−1)−1 in N83C based on virial masses and CO luminosities, and it is four times larger than that in the MW, 2 cm−2 (K km s−1)−1. We also discuss the difference in the nature between two high-mass YSOs, each of which is associated with a molecular clump with a mass of about a few .

Molecular clumps photoevaporation in ionized regions

We study the photoevaporation of molecular clumps exposed to a UV radiation field including hydrogen-ionizing photons ($h\nu > 13.6$ eV) produced by massive stars or quasars. We follow the propagation and collision of shock waves inside clumps and take into account self-shielding effects, determining the evolution of clump size and density with time. The structure of the ionization-photodissociation region (iPDR) is obtained for different initial clump masses ($M=0.01 - 10^4\,{\rm M}_\odot$) and impinging fluxes ($G_0=10^2 - 10^5$ in units of the Habing flux). The cases of molecular clumps engulfed in the HII region of an OB star and clumps carried within quasar outflows are treated separately. We find that the clump undergoes in both cases an initial shock-contraction phase and a following expansion phase, which lets the radiation penetrate in until the clump is completely evaporated. Typical evaporation time-scales are $\simeq 0.01$ Myr in the stellar case and 0.1 Myr in the quasar case, where the clump mass is 0.1 ${\rm M}_\odot$ and $10^3\,{\rm M}_\odot$ respectively. We find that clump lifetimes in quasar outflows are compatible with their observed extension, suggesting that photoevaporation is the main mechanism regulating the size of molecular outflows.