Hipparcos distances of Ophiuchus and Lupus cloud complexes

We combine extinction maps from the Two Micron All Sky Survey (2MASS) with Hipparcos and Tycho parallaxes to obtain reliable and high-precision estimates of the distance to the Ophiuchus and Lupus dark complexes. Our analysis, based on a rigorous maximum-likelihood approach, shows that the rho-Ophiuchi cloud is located at (119 +/- 6) pc and the Lupus complex is located at (155 +/- 8) pc; in addition, we are able to put constraints on the thickness of the clouds and on their orientation on the sky (both these effects are not included in the error estimate quoted above). For Ophiuchus, we find some evidence that the streamers are closer to us than the core. The method applied in this paper is currently limited to nearby molecular clouds, but it will find many natural applications in the GAIA-era, when it will be possible to pin down the distance and three-dimensional structure of virtually every molecular cloud in the Galaxy.Comment: A&A in press - Corrected typo (Lupus distance) in the electronic abstrac

Larson's third law and the universality of molecular cloud structure

Larson (1981) first noted a scaling relation between masses and sizes in molecular clouds that implies that these objects have approximately constant column densities. This original claim, based upon millimeter observations of carbon monoxide lines, has been challenged by many theorists, arguing that the apparent constant column density observed is merely the result of the limited dynamic range of observations, and that in reality clouds have column density variations over two orders of magnitudes. In this letter we investigate a set of nearby molecular clouds with near-infrared excess methods, which guarantee very large dynamic ranges and robust column density measurements, to test the validity of Larson's third law. We verify that different clouds have almost identical average column densities above a given extinction threshold; this holds regardless of the extinction threshold, but the actual average surface mass density is a function of the specific threshold used. We show that a second version of Larson's third law, involving the mass-radius relation for single clouds and cores, does not hold in our sample, indicating that individual clouds are not objects that can be described by constant column density. Our results instead indicate that molecular clouds are characterized by a universal structure. Finally we point out that this universal structure can be linked to the log-normal nature of cloud column density distributions.Comment: 5 pages, 4 figures, A&A in press (letter

The mid-infrared extinction law in the darkest cores of the Pipe Nebula

Context. The properties of dust grains, in particular their size distribution, are expected to differ from the interstellar medium to the high-density regions within molecular clouds. Aims. We measure the mid-infrared extinction law produced by dense material in molecular cloud cores. Since the extinction at these wavelengths is caused by dust, the extinction law in cores should depart from that found in low-density environments if the dust grains have different properties. Methods. We use the unbiased LINES method to measure the slope of the reddening vectors in color-color diagrams. We derive the mid-infrared extinction law toward the dense cores B59 and FeSt 1-457 in the Pipe Nebula over a range of visual extinction between 10 and 50 magnitudes, using a combination of Spitzer/IRAC, and ESO NTT/VLT data. Results. The mid-infrared extinction law in both cores departs significantly from a power-law between 3.6 and 8 micron, suggesting that these cores contain dust with a considerable fraction of large dust grains. We find no evidence for a dependence of the extinction law with column density up to 50 magnitudes of visual extinction in these cores, and no evidence for a variation between our result and those for other clouds at lower column densities reported elsewhere in the literature. This suggests that either large grains are present even in low column density regions, or that the existing dust models need to be revised at mid-infrared wavelengths. We find a small but significant difference in the extinction law of the two cores, that we tentatively associate with the onset of star formation in B59.Comment: 8 pages, 6 figures. Accepted to A&

Mixing in massive stellar mergers

The early evolution of dense star clusters is possibly dominated by close interactions between stars, and physical collisions between stars may occur quite frequently. Simulating a stellar collision event can be an intensive numerical task, as detailed calculations of this process require hydrodynamic simulations in three dimensions. We present a computationally inexpensive method in which we approximate the merger process, including shock heating, hydrodynamic mixing and mass loss, with a simple algorithm based on conservation laws and a basic qualitative understanding of the hydrodynamics of stellar mergers. The algorithm relies on Archimedes' principle to dictate the distribution of the fluid in the stable equilibrium situation. We calibrate and apply the method to mergers of massive stars, as these are expected to occur in young and dense star clusters. We find that without the effects of microscopic mixing, the temperature and chemical composition profiles in a collision product can become double-valued functions of enclosed mass. Such an unphysical situation is mended by simulating microscopic mixing as a post-collision effect. In this way we find that head-on collisions between stars of the same spectral type result in substantial mixing, while mergers between stars of different spectral type, such as type B and O stars ($\sim$10 and $\sim$40\msun respectively), are subject to relatively little hydrodynamic mixing.Comment: Accepted by MNRA

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&

2MASS wide field extinction maps: II. The Ophiuchus and the Lupus cloud complexe

We present an extinction map of a ~1,700 deg sq region that encloses the Ophiuchus, the Lupus, and the Pipe dark complexes using 42 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. The use of a robust and optimal near-infrared method to map dust column density (Nicer, described in Lombardi & Alves 2001) allow us to detect extinction as low as A_K = 0.05 mag with a 2-sigma significance, and still to have a resolution of 3 arcmin on our map. We also present a novel, statistically sound method to characterize the small-scale inhomogeneities in molecular clouds. Finally, we investigate the cloud structure function, and show that significant deviations from the results predicted by turbulent models are observed.Comment: 16 pages, A&A in pres

HST/ACS weak lensing analysis of the galaxy cluster RDCS 1252.9-2927 at z=1.24

We present a weak lensing analysis of one of the most distant massive galaxy cluster known, RDCS 1252.9-2927 at z=1.24, using deep images from the Advanced Camera for Survey (ACS) on board the Hubble Space Telescope (HST). By taking advantage of the depth and of the angular resolution of the ACS images, we detect for the first time at z>1 a clear weak lensing signal in both the i (F775W) and z (F850LP) filters. We measure a 5-\sigma signal in the i band and a 3-\sigma signal in the shallower z band image. The two radial mass profiles are found to be in very good agreement with each other, and provide a measurement of the total mass of the cluster inside a 1Mpc radius of M(<1Mpc) = (8.0 +/- 1.3) x 10^14 M_\odot in the current cosmological concordance model h =0.70, \Omega_m=0.3, \Omega_\Lambda=0.7, assuming a redshift distribution of background galaxies as inferred from the Hubble Deep Fields surveys. A weak lensing signal is detected out to the boundary of our field (3' radius, corresponding to 1.5Mpc at the cluster redshift). We detect a small offset between the centroid of the weak lensing mass map and the brightest cluster galaxy, and we discuss the possible origin of this discrepancy. The cumulative weak lensing radial mass profile is found to be in good agreement with the X-ray mass estimate based on Chandr and XMM-Newton observations, at least out to R_500=0.5Mpc.Comment: 38 pages, ApJ in press. Full resolution images available at http://www.eso.org/~prosati/RDCS1252/Lombardi_etal_accepted.pd

Automated classification of stellar spectra - I. Initial results with artificial neural networks

We have initiated a project to classify stellar spectra automatically from high-dispersion objective prism plates. The automated technique presented here is a simple backpropagation neural network, and is based on the visual classification work of Houk. The plate material (Houk's) is currently being digitized, and contains ≈ 105 stars down to V ≈ 11 at ≈ 2-Å resolution from ≈ 3850 to 5150 Å. For this first paper in the series we report on the results of 575 stars digitized from 6 plates. We find that even with the limited data set now in hand we can determine the temperature classification to better than 1.7 spectral subtypes from B3 to M4. Our current sample size provides insufficient training set material to generate luminosity and metallicity classifications. Our eventual aims in this project are (1) to create a large and homogeneous digital stellar spectral library; (2) to create a well-understood and robust automatic classification algorithm which can determine temperatures, luminosities and metallicities for a wide variety of spectral types; (3) to use these data, supplemented by deeper plate material, for the study of Galactic structure and chemical evolution; and (4) to find unusual or new classes of objects