The physics and modes of star cluster formation: simulations

We review progress in numerical simulations of star cluster formation. These simulations involve the bottom-up assembly of clusters through hierarchical mergers, which produces a fractal stellar distribution at young (~0.5 Myr) ages. The resulting clusters are predicted to be mildly aspherical and highly mass-segregated, except in the immediate aftermath of mergers. The upper initial mass function within individual clusters is generally somewhat flatter than for the aggregate population. Recent work has begun to clarify the factors that control the mean stellar mass in a star-forming cloud and also the efficiency of star formation. The former is sensitive to the thermal properties of the gas while the latter depends both on the magnetic field and the initial degree of gravitational boundedness of the natal cloud. Unmagnetized clouds that are initially bound undergo rapid collapse, which is difficult to reverse by ionization feedback or stellar winds.Comment: 21 pages, 10 figures. To appear as invited review article in a special issue of the Phil. Trans. Royal Soc. A: Ch. 3 "Star clusters as tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed. LaTeX, requires rspublic.cls style fil

Spectroscopic Detection of a Stellar-like Photosphere in an Accreting Protostar

We present the first spectrum of a highly veiled, strongly accreting protostar which shows photospheric absorption features and demonstrates the stellar nature of its central core. We find the spectrum of the luminous (L_bol = 10 L_sun) protostellar source, YLW 15, to be stellar-like with numerous atomic and molecular absorption features, indicative of a K5 IV/V spectral type and a continuum veiling r_k = 3.0. Its derived stellar luminosity (3 L_sun) and stellar radius (3.1 R_sun) are consistent with those of a 0.5 M_sun pre-main-sequence star. However, 70% of its bolometric luminosity is due to mass accretion, whose rate we estimate to be 1.6 E-6 M_sun / yr onto the protostellar core. We determine that excess infrared emission produced by the circumstellar accretion disk, the inner infalling envelope, and accretion shocks at the surface of the stellar core of YLW 15 all contribute signifi- cantly to its near-IR continuum veiling. Its projected rotation velocity v sin i = 50 km / s is comparable to those of flat-spectrum protostars but considerably higher than those of classical T Tauri stars in the rho Oph cloud. The protostar may be magnetically coupled to its circumstellar disk at a radius of 2 R_*. It is also plausible that this protostar can shed over half its angular momentum and evolve into a more slowly rotating classical T Tauri star by remaining coupled to its circumstellar disk (at increasing radius) as its accretion rate drops by an order of magnitude during the rapid transition between the Class I and Class II phases of evolution. The spectrum of WL 6 does not show any photospheric absorption features, and we estimate that its continuum veiling is r_k >= 4.6. Together with its low bolometric luminosity (2 L_sun), this dictates that its central core is very low mass, ~0.1 M_sun.Comment: 14 pages including 9 figures (3 figures of 3 panels each, all as separate files). AASTeX LaTex macros version 5.0. To be published in The Astronomical Journal (tentatively Oct 2002

On the fidelity of the core mass functions derived from dust column density data

Aims: We examine the recoverability and completeness limits of the dense core mass functions (CMFs) derived for a molecular cloud using extinction data and a core identification scheme based on two-dimensional thresholding. Methods: We performed simulations where a population of artificial cores was embedded into the variable background extinction field of the Pipe nebula. We extracted the cores from the simulated extinction maps, constructed the CMFs, and compared them to the input CMFs. The simulations were repeated using a variety of extraction parameters and several core populations with differing input mass functions and differing degrees of crowding. Results: The fidelity of the observed CMF depends on the parameters selected for the core extraction algorithm for our background. More importantly, it depends on how crowded the core population is. We find that the observed CMF recovers the true CMF reliably when the mean separation of cores is larger than their mean diameter (f>1). If this condition holds, the derived CMF is accurate and complete above M > 0.8-1.5 Msun, depending on the parameters used for the core extraction. In the simulations, the best fidelity was achieved with the detection threshold of 1 or 2 times the rms-noise of the extinction data, and with the contour level spacings of 3 times the rms-noise. Choosing larger threshold and wider level spacings increases the limiting mass. The simulations show that when f>1.5, the masses of individual cores are recovered with a typical uncertainty of 25-30 %. When f=1 the uncertainty is ~60 %. In very crowded cases where f<1 the core identification algorithm is unable to recover the masses of the cores adequately. For the cores of the Pipe nebula f~2.0 and therefore the use of the method in that region is justified.Comment: 9 pages, 6 figures, accepted for publication in A&

Radiative Transfer in Prestellar Cores: A Monte Carlo Approach

We use our Monte Carlo radiative transfer code to study non-embedded prestellar cores and cores that are embedded at the centre of a molecular cloud. Our study indicates that the temperature inside embedded cores is lower than in isolated non-embedded cores, and generally less than 12 K, even when the cores are surrounded by an ambient cloud of small visual extinction (Av~5). Our study shows that the best wavelength region to observe embedded cores is between 400 and 500 microns, where the core is quite distinct from the background. We also predict that very sensitive observations (~1-3 MJy/sr) at 170-200 microns can be used to estimate how deeply a core is embedded in its parent molecular cloud. Finally, we present preliminary results of asymmetric models of non-embedded cores.Comment: 8 pages, 15 figures, to appear in the conference proceedings of "Open Issues in Local Star Formation and Early Stellar Evolution", held in Ouro Preto (Brazil), April 5-10, 200

The Intrinsic Shapes of Molecular Cloud Fragments over a Range of Length Scales

We decipher intrinsic three-dimensional shape distributions of molecular clouds, cloud cores, Bok globules, and condensations using recently compiled catalogues of observed axis ratios for these objects mapped in carbon monoxide, ammonia, through optical selection, or in continuum dust emission. We apply statistical techniques to compare assumed intrinsic axis ratio distributions with observed projected axis ratio distributions. Intrinsically triaxial shapes produce projected distributions which agree with observations. Molecular clouds mapped in $^{12}$CO are intrinsically triaxial but more nearly prolate than oblate, while the smaller cloud cores, Bok globules, and condensations are also intrinsically triaxial but more nearly oblate than prolate.Comment: 12 pages, 11 figures. Version with color figures can be found at http://www.astro.uwo.ca/~cjones/ or http://www.astro.uwo.ca/~basu/. To appear in ApJ, 10 April 2002, v. 569, no.

Sensitive Limits on the Water Abundance in Cold Low Mass Molecular Cores

We present SWAS observations of water vapor in two cold star-less clouds, B68 and Core D in rho Ophiuchus. Sensitive non-detections of the 1(10)-1(01) transition of o-H2O are reported for each source. Both molecular cores have been previously examined by detailed observations that have characterized the physical structure. Using these rather well defined physical properties and a Monte-Carlo radiation transfer model we have removed one of the largest uncertainties from the abundance calculation and set the lowest water abundance limit to date in cold low-mass molecular cores. These limits are < 3 x 10^{-8} (relative to H2) and < 8 x 10^{-9} in B68 and rho Oph D, respectively. Such low abundances confirm the general lack of ortho-water vapor in cold (T < 20 K) cores. Provided that the ortho/para ratio of water is not near zero, these limits are well below theoretical predictions and appear to support the suggestion that most of the water in dense low-mass cores is frozen onto the surfaces of cold dust grains.Comment: 12 pages, 3 figures, accepted by Astrophysical Journal Letter

The Earliest Phases Of High-Mass Star Formation, As Seen In Ngc 6334 By Herschel -Hobys

Aims. To constrain models of high-mass star formation, the Herschel-HOBYS key program aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes

The "Mysterious" Origin of Brown Dwarfs

Hundreds of brown dwarfs (BDs) have been discovered in the last few years in stellar clusters and among field stars. BDs are almost as numerous as hydrogen burning stars and so a theory of star formation should also explain their origin. The ``mystery'' of the origin of BDs is that their mass is two orders of magnitude smaller than the average Jeans' mass in star--forming clouds, and yet they are so common. In this work we investigate the possibility that gravitationally unstable protostellar cores of BD mass are formed directly by the process of turbulent fragmentation. Supersonic turbulence in molecular clouds generates a complex density field with a very large density contrast. As a result, a fraction of BD mass cores formed by the turbulent flow are dense enough to be gravitationally unstable. We find that with density, temperature and rms Mach number typical of cluster--forming regions, turbulent fragmentation can account for the observed BD abundance.Comment: 11 pages, 3 figures, ApJ submitted Error in equation 1 has been corrected. Improved figure

Search for starless clumps in the ATLASGAL survey

In this study, we present an unbiased sample of the earliest stages of massive star formation across 20 square-degree of the sky. Within the region 10deg < l < 20deg and |b| < 1deg, we search the ATLASGAL survey at 870 micron for dense gas condensations. These clumps are carefully examined for indications of ongoing star formation using YSOs from the GLIMPSE source catalog as well as sources in the 24 micron MIPSGAL images, to search for starless clumps. We calculate the column densities as well as the kinematic distances and masses for sources where the v_lsr is known from spectroscopic observations. Within the given region, we identify 210 starless clumps with peak column densities > 1 x 10e23 cm^(-2). In particular, we identify potential starless clumps on the other side of the Galaxy. The sizes of the clumps range between 0.1 pc and 3 pc with masses between a few tens of solar masses up to several ten thousands of solar masses. Most of them may form massive stars, but in the 20 square-degree we only find 14 regions massive enough to form stars more massive than 20 solar masses and 3 regions with the potential to form stars more massive than 40 40 solar masses. The slope of the high-mass tail of the clump mass function for clumps on the near side of the Galaxy is 2.2 and, therefore, Salpeter-like. We estimate the lifetime of the most massive starless clumps to be 60000 yr. The sample offers a uniform selection of starless clumps. In the large area surveyed, we only find a few potential precursors of stars in the excess of 40 solar masses. It appears that the lifetime of these clumps is somewhat shorter than their free-fall times, although both values agree within the errors. In addition, these are ideal objects for detailed studies and follow-up observations.Comment: 15 pages plus appendix, in total 44 pages, accepted for publication in Astronomy & Astrophysics, full tables will be added soo

Molecular Tracers of Embedded Star Formation in Ophiuchus

In this paper we analyze nine SCUBA cores in Ophiuchus using the second-lowest rotational transitions of four molecular species (12CO, 13CO, C18O, and C17O) to search for clues to the evolutionary state and star-formation activity within each core. Specifically, we look for evidence of outflows, infall, and CO depletion. The line wings in the CO spectra are used to detect outflows, spectral asymmetries in 13CO are used to determine infall characteristics, and a comparison of the dust emission (from SCUBA observations) and gas emission (from C18O) is used to determine the fractional CO freeze-out. Through comparison with Spitzer observations of protostellar sources in Ophiuchus, we discuss the usefulness of CO and its isotopologues as the sole indicators of the evolutionary state of each core. This study is an important pilot project for the JCMT Legacy Survey of the Gould Belt (GBS) and the Galactic Plane (JPS), which intend to complement the SCUBA-2 dust continuum observations with HARP observations of 12CO, 13CO, C18O, and C17O J = 3 - 2 in order to determine whether or not the cold dust clumps detected by SCUBA-2 are protostellar or starless objects. Our classification of the evolutionary state of the cores (based on molecular line maps and SCUBA observations) is in agreement with the Spitzer designation for six or seven of the nine SCUBA cores. However, several important caveats exist in the interpretation of these results, many of which large mapping surveys like the GBS may be able to overcome to provide a clearer picture of activity in crowded fields.Comment: 43 pages including 19 postscript figures. Accepted for publication in the PAS