The Serpens cloud has received considerable attention in the last years, in particular the small region known as the Serpens cloud core where a plethora of star formation related phenomena are found. This review summarizes our current observational knowledge of the cloud, with emphasis on the core. Recent results are converging to a distance for the cloud of ~ 230 +- 20 pc, an issue which has been controversial over the years. We present the gas and dust properties of the cloud core and describe its structure and appearance at different wavelengths. The core contains a dense, very young, low mass stellar cluster with more than 300 objects in all evolutionary phases, from collapsing gaseous condensations to pre-main sequence stars. We describe the behaviour and spatial distribution of the different stellar populations (mm cores, Classes 0, I and II sources). The spatial concentration and the fraction number of Class 0/Class I/Class II sources is considerably larger in the Serpens core than in any other low mass star formation region, e.g. Taurus, Ophiuchus or Chamaeleon, as also stated in different works. Appropriate references for coordinates and fluxes of all Serpens objects are given. However, we provide for the first time a unified list of all near-IR sources which have up to now been identified as members of the Serpens core cluster; this list includes some members identified in this review. A cross-reference table of the near-IR objects with optical, mid-IR, submillimeter, radio continuum and X-ray surces is also provided. A simple analysis has allowed us to identify a sample of ~ 60 brown dwarf candidates among the 252 near-IR objects; some of them show near-IR excesses and, therefore, they constitute an attractive sample to study very young substellar objects. (abridged)
We present VLA 3.5 cm continuum observations of the Serpens cloud core. 22 radio continuum sources are detected. 16 out of the 22 cm sources are suggested to be associated with young stellar objects (Class 0, Class I, flat-spectrum, and Class II) of the young Serpens cluster. The rest of the VLA sources plausibly are background objects. Most of the Serpens cm sources likely represent thermal radio jets; on the other hand, the radio continuum emission of some sources could be due to a gyrosynchroton mechanism arising from coronally active young stars. The Serpens VLA sources are spatially distr…
We have undertaken an unprecedentedly large 1.1 millimeter continuum survey of three nearby star forming clouds using Bolocam at the Caltech Submillimeter Observatory. We mapped the largest areas in each cloud at millimeter or submillimeter wavelengths to date: 7.5 sq. deg in Perseus (Paper I), 10.8 sq. deg in Ophiuchus (Paper II), and 1.5 sq. deg in Serpens with a resolution of 31", detecting 122, 44, and 35 cores, respectively. Here we report on results of the Serpens survey and compare the three clouds. Average measured angular core sizes and their dependence on resolution suggest that many…
We present maps of 1.5 square degrees of the Serpens dark cloud at 24, 70, and 160\micron observed with the Spitzer Space Telescope MIPS Camera. More than 2400 compact sources have been extracted at 24um, nearly 100 at 70um, and 4 at 160um. We estimate completeness limits for our 24um survey from Monte Carlo tests with artificial sources inserted into the Spitzer maps. We compare source counts, colors, and magnitudes in the Serpens cloud to two reference data sets, a 0.50 deg^2 set on a low-extinction region near the dark cloud, and a 5.3 deg^2 subset of the SWIRE ELAIS N1 data that was proces…
The young stellar population data of the Perseus, Ophiuchus and Serpens molecular clouds are obtained from the Spitzer c2d legacy survey in order to investigate the spatial structure of embedded clusters using the nearest neighbour and minimum spanning tree method. We identify the embedded clusters in these clouds as density enhancements and analyse the clustering parameter Q with respect to source luminosity and evolutionary stage. This analysis shows that the older Class 2/3 objects are more centrally condensed than the younger Class 0/1 protostars, indicating that clusters evolve from an in…
We have mapped the large-scale structure of the Serpens cloud core using moderately optically thick (13CO(1--0) and CS(2--1)) and optically thin tracers (C18O(1--0), C34S(2--1), and N2H+(1--0)), using the 16-element focal plane array operating at a wavelength of 3mm at the Five College Radio Astronomy Observatory. Our main goal was to study the large-scale distribution of the molecular gas in the Serpens region and to understand its relation with the denser gas in the cloud cores, previously studied at high angular resolution. All our molecular tracers show two main gas condensations, or sub-c…
We report the discovery of a nearby, embedded cluster of young stellar objects, associated filamentary infrared dark cloud, and 4.5 micron shock emission knots from outflows detected in Spitzer/IRAC mid-infrared imaging of the Serpens-Aquila Rift obtained as part of the Spitzer Gould Belt Legacy Survey. We also present radial velocity measurements of the region from molecular line observations obtained with the Submillimeter Array (SMA) that suggest the cluster is co-moving with the Serpens Main embedded cluster 3 degrees to the north. We therefore assign it the same distance, 260 pc. The core…
We present Spitzer and Chandra observations of the nearby (~260 pc) embedded stellar cluster in the Serpens Cloud Core. We observed, using Spitzer's IRAC and MIPS instruments, in six wavelength bands from 3 to 70 $μm$, to detect thermal emission from circumstellar disks and protostellar envelopes, and to classify stars using color-color diagrams and spectral energy distributions (SEDs). These data are combined with Chandra observations to examine the effects of circumstellar disks on stellar X-ray properties. Young diskless stars were also identified from their increased X-ray emission. We hav…
We present new near- and mid-infrared observations of 19 Class I/flat-spectrum young stellar objects in the nearby Rho Oph (d=125pc) and Serpens (d=310pc) dark clouds. These observations are part of a larger systematic infrared multiplicity survey of Class I/flat-spectrum objects in the nearest dark clouds. We find 7/19 (37% +/- 14%) of the sources surveyed to be multiple systems over a separation range of ~150 - 1800 AU. This is consistent with the fraction of multiple systems found among older pre-main-sequence stars in each of the Taurus, Rho Oph, Chamaeleon, Lupus, and Corona Australis sta…
We report on the results of an optical spectroscopic survey designed to confirm the youth and determine the spectral types among a sample of young stellar object (YSO) candidates in the Serpens Molecular Cloud. We observed 150 infrared excess objects, previously discovered by the Spitzer Legacy Program "From Molecular Cores to Planet-Forming Disks" (c2d), bright enough for subsequent Spitzer/IRS spectroscopy. We obtained 78 optical spectra of sufficient S/N for analysis. Extinctions, effective temperatures and luminosities are estimated for this sample, and used to construct H-R diagrams for t…
Aperture synthesis and single-dish (sub) millimeter molecular lines and continuum observations reveal in great detail the envelope structure of deeply embedded young stellar objects (SMM1, SMM2, SMM3, SMM4) in the densely star-forming Serpens Molecular Cloud. Resolved millimeter continuum emission constrains the density structure to a radial power law with index -2.0 +/- 0.5, and envelope masses of 8.7, 3.0, and 5.3 M_sol for SMM1, SMM3, and SMM4. The core SMM2 does not seem to have a central condensation and may not have formed a star yet. The molecular line observations can be described by t…
We present the detection of kinematic infall signatures towards the Class 0 protostellar system SMM4 in the Serpens cloud core. We have observed the dense molecular gas towards the embedded source using millimeter and submillimeter line transitions of density sensitive molecular tracers. High signal-to-noise ratio maps obtained in HCO+ J=1-0, J=3-2, and J=4-3, and CS J=2-1 show the blue-bulge infall signature. The blue-bulge infall signature can be observed in the centroid velocity maps of protostellar objects when infall dominates over rotation. The line profiles of HCO+ and CS exhibit the ch…
We present high resolution interferometric and single dish observations of molecular gas in the Serpens cluster-forming core. Star formation does not appear to be homogeneous throughout the core, but is localised in spatially- and kinematically-separated sub-clusters. The stellar (or proto-stellar) density in each of the sub-clusters is much higher than the mean for the entire Serpens cluster. This is the first observational evidence for the hierarchical fragmentation of proto-cluster cores suggested by cluster formation models.
ISO-LWS mapping observations of the Serpens molecular cloud core are presented. The spectral range is 50 - 200 micron and the map size is 8' X 8'. These observations suffer from severe source confusion at FIR wavelengths and we employ a Maximum Likelihood Method for the spectro-spatial deconvolution. The strong and fairly isolated source SMM1 FIRS1 presented a test case, whose modelled spectral energy distribution (SED), within observational errors, is identical to the observed one. The model results for the other infrared and submillimetre sources are therefore likely to represent their corre…
We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy observations of the Serpens star-forming region. In particular we present a set of criteria for isolating bona fide young stellar objects, YSO's, from the extensive background contamination by extra-galactic objects. We then discuss the properties of the resulting high confidence set of YSO's. We find 235 such objects in the 0.85 deg^2 field that was covered with both IRAC and MIPS. An additional set of 51 lower confidence YSO's outside this area is identified from the MIPS data combined with 2MASS photometry. We describ…
This article reports first results of a long-term observational program aimed to study the earliest evolution of jet/disk systems in low-mass YSOs by means of VLBI observations of the 22.2 GHz water masers. We report here data for the cluster of low-mass YSOs in the Serpens molecular core and for the single object RNO~15-FIR. Towards Serpens SMM1, the most luminous sub-mm source of the Serpens cluster, the water maser emission comes from two small (< 5 AU in size) clusters of features separated by ~25 AU, having line of sight velocities strongly red-shifted (by more than 10 km/s) with respect …
We present results from an ISOCAM survey in the two broad band filters LW2 (5-8.5 mu) and LW3 (12-18 mu) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 mu band and 139 in the 14.3 mu band to a limiting sensitivity of ~ 2 mJy. Only about 50% of the mid-IR excess sources show excesses in the near-IR J-H/H-K diagram. In the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as rho Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spect…
We present combined single-dish and interferometric CS(2--1) and N2H+(1--0) observations of a compact core in the NW region of the Serpens molecular cloud. The core is starless according to observations from optical to millimeter wavelengths and its lines have turbulent widths and ``infall asymmetry''. Line profile modeling indicates supersonic inward motions v_in>0.34 km/s over an extended region L>12000AU. The high infall speed and large extent exceeds the predictions of most thermal ambipolar diffusion models and points to a more dynamical process for core formation. A short (dynamic) times…
We compute the mid-infrared extinction law from 3.6-24 microns in three molecular clouds: Ophiuchus, Perseus, and Serpens, by combining data from the "Cores to Disks" Spitzer Legacy Science program with deep JHKs imaging. Using a new technique, we are able to calculate the line-of-sight extinction law towards each background star in our fields. With these line-of-sight measurements, we create, for the first time, maps of the chi-squared deviation of the data from two extinction law models. Because our chi-squared maps have the same spatial resolution as our extinction maps, we can directly obs…
This poster presents single-dish and aperture-synthesis observations of the J=1-0 (lambda~3 mm) transitions of HCO+, HCN, and N2H+ towards the Serpens star-forming region. Jets driven by young stars affect the structure and the chemistry of their surrounding cloud, and this work aims to assess the extent to which the emission of these three molecular lines is dominated by such processes. In Serpens I find that N2H+ 1-0 traces the total amount of material, except in two regions slightly ahead of shocks. In contrast, the HCO+ and, especially, HCN emission is dominated by regions impacted by outf…
We report the results of a millimeter interferometric survey of compact 3 mm continuum sources in the inner 5.5'x5.5' region of the Serpens core. We detect 32 discrete sources above 4.0 mJy/beam, 21 of which are new detections at millimeter wavelengths. By comparing our data with published infrared surveys, we estimate that 26 sources are probably protostellar condensations and derive their mass assuming optically thin thermal emission from dust grains. The mass spectrum of the clumps, dN/dM~M^(-2.1), is consistent with the stellar initial mass function, supporting the idea that the stellar ma…
We present spectrophotometric ISO imaging with the LWS and the CAM-CVF of the Serpens molecular cloud core. The LWS map is centred on the far infrared and submillimetre source SMM1 and its size is 8' x 8'. The fine structure line emission in [OI] and [CII] is extended and can be successfully modelled to originate in a PDR with G_0 = 15 and n(H2) about 10^4 - 10^5 cm^-3. Extended emission is also observed in the rotational line emission of H2O and high-J CO. However, lack of sufficient angular resolution prevents us from excluding the possibility that the emssion regions of these lines are poin…
Aims. The aim of this paper is to characterise the star formation activity in the poorly studied embedded cluster Serpens/G3-G6, located ~ 45' (3 pc) to the south of the Serpens Cloud Core, and to determine the luminosity and mass functions of its population of Young Stellar Objects (YSOs). Methods. Multi-wavelength broadband photometry was obtained to sample the near and mid-IR spectral energy distributions to separate YSOs from field stars and classify the YSO evolutionary stage. ISOCAM mapping in the two filters LW2 (5-8.5 um) and LW3 (12-18 um) of a 19' x 16' field was combined with JHKs d…
We present an unbiased census of deeply embedded protostars in Perseus, Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam continuum and Spitzer Legacy surveys. We identify protostellar candidates based on their mid-infrared properties, correlate their positions with 1.1 mm core positions, and construct well-sampled SEDs using our extensive wavelength coverage (lam=1.25-1100 micron). Source classification based on the bolometric temperature yields a total of 39 Class 0 and 89 Class I sources in the three cloud sample. We compare to protostellar evolutionary models using …
AIMS: To study the properties of X-ray emissions from young stellar objects (YSOs), through their evolution from Class I to Class III and determine whether Class 0 protostars emit in X-rays. METHODS: A deep Chandra X-ray observation of the Serpens star-forming region was obtained. The Serpens Cloud Core is ideally suited for this type of investigation, being populated by a dense and extremely young cluster whose members are found in all different evolutionary stages, including six well studied Class 0 sources. RESULTS: None of the six Class 0 protostars is detected in our observations, excludi…
We have mapped the dense gas distribution and dynamics in the NW region of the Serpens molecular cloud in the CS(2-1) and N2H+(1-0) lines and 3 mm continuum using the FCRAO telescope and BIMA interferometer. 7 continuum sources are found. The N2H+ spectra are optically thin and fits to the 7 hyperfine components are used to determine the distribution of velocity dispersion. 8 cores, 2 with continuum sources, 6 without, lie at a local linewidth minimum and optical depth maximum. The CS spectra are optically thick and generally self-absorbed over the full 0.2 pc extent of the map. We use the lin…
We present an unbiased census of starless cores in Perseus, Serpens, and Ophiuchus, assembled by comparing large-scale Bolocam 1.1 mm continuum emission maps with Spitzer c2d surveys. We use the c2d catalogs to separate 108 starless from 92 protostellar cores in the 1.1 mm core samples from Enoch et al. (2006), Young et al. (2006), and Enoch et al. (2007). A comparison of these populations reveals the initial conditions of the starless cores. Starless cores in Perseus have similar masses but larger sizes and lower densities on average than protostellar cores, with sizes that suggest density pr…
(abridged) Our project endeavors to obtain a robust view of multiplicity among embedded Class I and Flat Spectrum protostars in a wide array of nearby molecular clouds to disentangle ``universal'' from cloud-dependent processes. We have used near-infrared adaptive optics observations at the VLT through the H, Ks and L' filters to search for tight companions to 45 Class I and Flat Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga, Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with published high-resolution surveys of 13 additional objects in…
We present VLT-ISAAC L-band spectroscopy toward 10 stars in SVS 4, a 30"x45" dense cluster of pre-main sequence stars deeply embedded in the Serpens star forming cloud. The ISAAC spectra are combined with archival imaging from UKIRT and ISOCAM to derive accurate extinctions toward the SVS 4 stars. The data are then used to construct a spatial map of the distribution of ice in front of the cluster stars with an average angular resolution of 6" or 1500 AU, three orders of magnitude better than previous maps. We show that water ice is present throughout the region and confirm the presence of meth…
The distance dependence of interstellar extinction in the direction of the Aquila Rift is investigated using 473 stars observed in the Vilnius photometric system. Front edge of the dark clouds in the area is found to be at 225$\pm$55 pc and the thickness of the cloud system can be about 80 pc. The maximum extinction A_V in the clouds is close to 3.0 mag. Two stars with larger extinction are found and discussed. Since the new distance of the clouds is larger than the previously accepted distance, the cloud system mass should be increased up to 2.7 10^5 M_sun which is close to the virial mass es…
We study numerically the ambipolar diffusion-driven evolution of non-rotating, magnetically subcritical, disk-like molecular clouds, assuming axisymmetry. Previous similar studies have concentrated on the formation of single magnetically supercritical cores at the cloud center, which collapse to form isolated stars. We show that, for a cloud with many Jeans masses and a relatively flat mass distribution near the center, a magnetically supercritical ring is produced instead. The supercritical ring contains a mass well above the Jeans limit. It is expected to break up, through both gravitational…
By using dark nebulae as opaque outer screens, the luminosity function of white dwarfs and M dwarfs has been studied. High-extinction areas towards the Orion A, Serpens and rho Oph cloud complexes were observed, covering a volume corresponding to 464 pc3 in the solar neighbourhood, complete to Mv=16.5. Foreground stars were selected by VRI photometry and photometric parallaxes. The resulting foreground sample consists of 21 M dwarfs and 7 white dwarfs. The derived M-dwarf luminosity function is consistent with previous studies, showing no substantial upturn beyond Mv=16. The 7 white dwarfs imp…
This paper presents large scale extinction maps of most nearby Giant Molecular Clouds of the Galaxy (Lupus, rho-Ophiuchus, Scorpius, Coalsack, Taurus, Chamaeleon, Musca, Corona Australis, Serpens, IC 5146, Vela, Orion, Monoceros R1 and R2, Rosette, Carina) derived from a star count method using an adaptive grid and a wavelet decomposition applied to the optical data provided by the USNO-Precision Measuring Machine. The distribution of the extinction in the clouds leads to estimate their total individual masses M and their maximum of extinction. I show that the relation between the mass contain…
The high Galactic longitude end of the Aquila Rift comprises the large Aquila molecular cloud complex, however, few young stars are known to be located in the area, and only one is directly associated with the Rift. In contrast, the Serpens star-forming region at the low Galactic longitude end of the Rift contains hundreds of young stars. We review studies of the raw molecular material and describe searches for young objects in the Aquila clouds. The characteristics of the known young stars and associated jets and outflows are also provided. Finally, we suggest some possible explanations for t…
To study the interaction of star-formation and turbulent molecular cloud structuring, we analyse numerical models and observations of self-gravitating clouds using the Delta-variance as statistical measure for structural characteristics. In the models we resolve the transition from purely hydrodynamic turbulence to gravitational collapse associated with the formation and mass growth of protostellar cores. We compare models of driven and freely decaying turbulence with and without magnetic fields. Self-gravitating supersonic turbulence always produces a density structure that contains most powe…
The Carina Nebula is an extremely bright southern HII region embedded in a giant molecular cloud and contains some of the most massive stars known in our Galaxy. We are undertaking a multi-wavelength study of the Carina Nebula in order to examine the detailed kinematics and distribution of the molecular and ionised gas, and to look for further evidence of ongoing star formation. Here we present the results of the initial molecular cloud observations which were made by observing the 12CO(1-0) emission with the Mopra antenna. The observations reveal the clumpy morphology of the molecular gas, an…
We have used an antenna temperature thresholding algorithm on the Bell Laboratories 13CO Milky Way Survey to create a catalog of 1,400 molecular clouds. Of these, 281 clouds were selected for having well-determined kinematic distances. The scaleheight, luminosity, internal velocity dispersion, and size of the cloud sample are analyzed to show that clouds smaller than ~200,000 solar masses have a scaleheight which is about 35 pc, roughly independent of cloud mass, while larger clouds, the Giant Molecular Clouds, have a reduced scaleheight which declines with increasing cloud mass.
It is argued that if cosmic rays penetrate into molecular clouds, the total energy they lose can exceed the energy from galactic supernovae shocks. It is shown that most likely galactic cosmic rays interacting with the surface layers of molecular clouds are efficiently reflected and do not penetrate into the cloud interior. Low-energy cosmic rays ($E<1$ GeV) that provide the primary ionization of the molecular cloud gas can be generated inside such clouds by multiple shocks arising due to supersonic turbulence.
A population of molecular clouds with a significantly greater scale height than that of Giant Molecular Clouds has been identified by examining maps of the latitude distribution of the $^{12}CO(1-0)$ emission in the first quadrant of the Galaxy. These clouds are found by identifying emission more than 2.6 times the scale-height away from the galactic midplane (centroid of CO emission) at the tangent points. Since the distance to the tangent points is known, we know the height and the sizes of these clouds. They are smaller and fainter than the GMCs and do not seem to be gravitationally bound. …
The Monoceros R2 region was first recognized as a chain of reflection nebulae illuminated by A- and B-type stars. These nebulae are associated with a giant molecular cloud that is one of the closest massive star forming regions to the Sun. This chapter reviews the properties of the Mon R2 region, including the namesake reflection nebulae, the large scale molecular cloud, global star formation activity, and properties of prominent star forming regions in the cloud.
In a recent paper, Elmegreen (2000) has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical timescales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular we consider the hypothesis that molecular clouds may form not by {\it in situ} cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium (ISM), much, if not most, of which is already in molecular form.
Observations of both star-forming regions and young, gas-free stellar associations indicate that most nearby molecular clouds form stars only over a short time span before dispersal; large-scale flows in the diffuse interstellar medium have the potential for forming clouds sufficiently rapidly, and for producing stellar populations with ages much less than the lateral crossing times of their host molecular clouds. We identify four important factors for understanding rapid star formation and short cloud lifetimes. First, much of the accumulation and dispersal of clouds near the solar circle mig…
This book chapter reviews the current state of observations of magnetic fields in diffuse H I and in dense molecular clouds. It covers techniques for measuring magnetic fields, the analysis of the observations, and the role of magnetic fields in the physics of interstellar clouds and in the star formation process.
The second survey of the molecular clouds in 12CO (J = 1-0) was carried out in the Large Magellanic Cloud by NANTEN. The sensitivity of this survey is twice as high as that of the previous NANTEN survey, leading to a detection of molecular clouds with M_CO > 2 x 10^4 M_sun. We identified 272 molecular clouds, 230 of which are detected at three or more observed positions. We derived the physical properties, such as size, line width, virial mass, of the 164 GMCs which have an extent more than the beam size of NANTEN in both the major and minor axes. The CO luminosity and virial mass of the cloud…
The nearby dwarf elliptical galaxy NGC 205 offers a unique opportunity for high resolution studies of the interstellar medium in an elliptical galaxy. This paper investigates the distribution of molecular gas, molecular line ratios, and the relationships between atomic gas, molecular gas, and dust in NGC 205. The line ratios 12CO(2-1)/(1-0) and 12CO(1-0)/13CO(1-0) in one of the molecular clouds in NGC 205 are consistent with the ratios found in other elliptical galaxies and in Galactic giant molecular clouds; they suggest that the CO in this cloud is probably subthermally excited. Atomic gas, …
Polytropic models are reasonably successful in acounting for the observed features of molecular clouds. Multi-pressure polytropes include the various pressure components that are important in molecular clouds, whereas composite polytropes provide a representation for the core halo structure. Small, very dense (n~10^{11} cm^{-3}) molecular clouds have been proposed as models for both dark matter and for extreme scattering events. Insofar as the equation of state in these clouds can be represented by a single polytropic relation (pressure varies as a power of the density), such models conflict w…
We discuss an extension to the Chandrasekhar-Fermi method for the evaluation of the mean magnetic field strength in molecular clouds to cases where the spatial orientation of the field is known. We apply the results to M17, using previously published data.
We report the discovery of molecular hydrogen absorption in a Galactic high-velocity cloud (HVC) in the direction of the Large Magellanic Cloud. For the same HVC we derive an iron abundance which is half of the solar value. Thus, all evidence points to a Galactic origin for high-velocity cloud complex in front of the LMC.
Recent high resolution numerical simulations have suggested that the interstellar atomic hydrogen clouds have a complex two-phase structure. Since molecular clouds form through the contraction of HI gas, the question arises as to whether this structure is maintained in the molecular phase or not. Here we investigate whether the warm neutral atomic hydrogen (WNM) can exist in molecular clouds. We calculate how far a piece of WNM which is not heated by the UV photons could penetrate into the cloud, and find that in the absence of any heating it is unlikely that large fraction of WNM survives ins…
In this paper, we review some of the properties of dense molecular cloud cores. The results presented here rely on three-dimensional numerical simulations of isothermal, magnetized, turbulent, and self-gravitating molecular clouds (MCs) in which dense core form as a consequence of the gravo-turbulent fragmentation of the clouds. In particular we discuss issues related to the mass spectrum of the cores, their lifetimes and their virial balance.
The universality of interstellar turbulence is examined from observed structure functions of 27 giant molecular clouds and Monte Carlo modeling. We show that the structure functions, dv=v0 l^gamma, derived from wide field imaging of CO J=1-0 emission from individual clouds are described by a narrow range in the scaling exponent, gamma, and the scaling coefficient, v0. The similarity of turbulent structure functions emphasizes the universality of turbulence in the molecular interstellar medium and accounts for the cloud-to-cloud size-line width relationship initially identified by Larson (1981)…
Molecular hydrogen clouds can contribute substantially to the galactic halo< dark matter and may lead to the birth of massive halo objects (MHOs) observed indirectly by microlensing. We present a method to detect these molecular clouds in the halo of M31 using the Doppler shift effect. We also consider the possibility to directly observe MHOs in the halo of M31 via their infrared emission.
We re-examine critically the estimates of the duration of different phases of star formation and the lifetimes of molecular clouds, based on the ages of T-Tauri stars, age spreads of stars in clusters, and statistics of pre-stellar cores. We show that all available observational data are consistent with lifetimes of molecular clouds comparable to 10 Myr, as well as with the predictions of the theory of self-initiated, ambipolar-diffusion--controlled star formation. We conclude that there exists no observational support for either "young" molecular clouds or "rapid" star formation.
The data from the Nobeyama Radio Observatory 45 m telescope Galactic Center CO survey have been analyzed to generate a compilation of molecular clouds with intense CO emission in this region. Clouds are identified in an automated manner throughout the main part of the survey data for all CO emission peaks exceeding 10 K ($T_R^*$). Correlations between the size, velocity dispersion, virial mass, and the CO luminosity, for the molecular clouds in the Galactic center were shown. We diagnosed gravitational stabilities of identified clouds assuming that the disk clouds are nearly at the onset of gr…
The interstellar medium (ISM) provides a unique laboratory for highly supersonic, driven hydrodynamics turbulence. We present a theory of such turbulence, confirm it by numerical simulations, and use the results to explain observational properties of interstellar molecular clouds, the regions where stars are born.
