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Hunt L.K.,National institute for astrophysics | Garcia-Burillo S.,Observatorio Astronomico Nacional | Casasola V.,National institute for astrophysics | Caselli P.,Max Planck Institute for Extraterrestrial Physics | And 8 more authors.
Astronomy and Astrophysics

Tracing molecular hydrogen content with carbon monoxide in low-metallicity galaxies has been exceedingly difficult. Here we present a new effort, with IRAM 30-m observations of 12CO(1-0) of a sample of 8 dwarf galaxies having oxygen abundances ranging from 12 + log(O/H) ~ 7.7 to 8.4. CO emission is detected in all galaxies, including the most metal-poor galaxy of our sample (0.1 Z⊙); to our knowledge this is the largest number of 12CO(1-0) detections ever reported for galaxies with 12 + log(O/H) ≲ 8 (0.2 Z⊙) outside the Local Group. We calculate stellar masses, Mstar, and star-formation rates (SFRs), and analyze our results in conjunction with galaxy samples from the literature. Extending previous results for a correlation of the molecular gas depletion time, τdep, with Mstar and specific SFR (sSFR), we find a variation in τdep of a factor of 200 or more (from ≲ 50 Myr to ~10 Gyr) over a spread of ~103 in sSFR and Mstar. We exploit the variation of τdep to constrain the CO-to-H2 mass conversion factor αCO at low metallicity, and assuming a power-law variation find αCO∝Z/Z⊙ -2, similar to results based on dust continuum measurements compared with gas mass. By including Hi measurements, we show that the fraction of total gas mass relative to the baryonic mass is higher in galaxies that are metal poor, of low mass, and with high sSFR. Finally, comparisons of the data with star-formation models of the molecular gas phases show that the models are generally quite successful, but at metallicities Z/Z⊙ ≲ 0.2, there are some discrepancies. © 2015 ESO. Source

Fedele D.,Johns Hopkins University | Pascucci I.,Johns Hopkins University | Pascucci I.,US Space Telescope Science Institute | Brittain S.,Clemson University | And 7 more authors.
Astrophysical Journal

We present high-resolution (R ∼ 100,000) L-band spectroscopy of 11 Herbig AeBe stars with circumstellar disks. The observations were obtained with the VLT/CRIRES to detect hot water and hydroxyl radical emission lines previously detected in disks around T Tauri stars. OH emission lines are detected toward four disks. The OH 2Π3/2 P4.5 (1+,1-) doublet is spectrally resolved as well as the velocity profile of each component of the doublet. Its characteristic double-peak profile demonstrates that the gas is in Keplerian rotation and points to an emitting region extending out to ∼15-30AU. The OH emission correlates with disk geometry as it is mostly detected toward flaring disks. None of the Herbig stars analyzed here show evidence of hot water vapor at a sensitivity similar to that of the OH lines. The non-detection of hot water vapor emission indicates that the atmospheres of disks around Herbig AeBe stars are depleted of water molecules. Assuming LTE and optically thin emission we derive a lower limit to the OH/H2O column density ratio >1-25 in contrast to T Tauri disks for which the column density ratio is 0.3-0.4. © 2011. The American Astronomical Society. All rights reserved. Source

Melin J.-B.,CEA Saclay Nuclear Research Center | Aghanim N.,CNRS Paris Institute of Astrophysics | Bartelmann M.,University of Heidelberg | Bartlett J.G.,Paris West University Nanterre La Defense | And 23 more authors.
Astronomy and Astrophysics

We evaluate the construction methodology of an all-sky catalogue of galaxy clusters detected through the Sunyaev-Zel'dovich (SZ) effect. We perform an extensive comparison of twelve algorithms applied to the same detailed simulations of the millimeter and submillimeter sky based on a Planck-like case. We present the results of this "SZ Challenge" in terms of catalogue completeness, purity, astrometric and photometric reconstruction. Our results provide a comparison of a representative sample of SZ detection algorithms and highlight important issues in their application. In our study case, we show that the exact expected number of clusters remains uncertain (about a thousand cluster candidates at |b| > 20 deg with 90% purity) and that it depends on the SZ model and on the detailed sky simulations, and on algorithmic implementation of the detection methods. We also estimate the astrometric precision of the cluster candidates which is found of the order of ~2 arcmin on average, and the photometric uncertainty of about 30%, depending on flux. © ESO, 2012. Source

Matthews B.C.,National Research Council Canada | Matthews B.C.,University of Victoria | Sibthorpe B.,Astronomy Technology Center | Kennedy G.,University of Cambridge | And 35 more authors.
Astronomy and Astrophysics

We present results from the earliest observations of DEBRIS, a Herschel key programme to conduct a volume- and flux-limited survey for debris discs in A-type through M-type stars. PACS images (from chop/nod or scan-mode observations) at 100 and 160 μm are presented toward two A-type stars and one F-type star: β Leo, β UMa and η Corvi. All three stars are known disc hosts. Herschel spatially resolves the dust emission around all three stars (marginally, in the case of β UMa), providing new information about discs as close as 11 pc with sizes comparable to that of the Solar System. We have combined these data with existing flux density measurements of the discs to refine the SEDs and derive estimates of the fractional luminosities, temperatures and radii of the discs. © 2010 ESO. Source

Lestrade J.-F.,Paris Observatory | Matthews B.C.,National Research Council Canada | Matthews B.C.,University of Victoria | Sibthorpe B.,Astronomy Technology Center | And 20 more authors.
Astronomy and Astrophysics

Debris disks have been found primarily around intermediate and solar mass stars (spectral types A-K) but rarely around low mass M-type stars. We have spatially resolved a debris disk around the remarkable M3-type star GJ 581 hosting multiple planets using deep PACS images at 70, 100 and 160 μm as part of the DEBRIS Program on the Herschel Space Observatory. This is the second spatially resolved debris disk found around an M-type star, after the one surrounding the young star AU Mic (12 Myr). However, GJ 581 is much older (2-8 Gyr), and is X-ray quiet in the ROSAT data. We fit an axisymmetric model of the disk to the three PACS images and found that the best fit model is for a disk extending radially from 25 ± 12 AU to more than 60 AU. Such a cold disk is reminiscent of the Kuiper belt but it surrounds a low mass star (0.3 Mθ) and its fractional dust luminosity Ldust/L * of ∼ 10-4 is much higher. The inclination limits of the disk found in our analysis make the masses of the planets small enough to ensure the long-term stability of the system according to some dynamical simulations. The disk is collisionally dominated down to submicron-sized grains and the dust cannot be expelled from the system by radiation or wind pressures because of the low luminosity and low X-ray luminosity of GJ 581. We suggest that the correlation between low-mass planets and debris disks recently found for G-type stars also applies to M-type stars. Finally, the known planets, of low masses and orbiting within 0.3 AU from the star, cannot dynamically perturb the disk over the age of the star, suggesting that an additional planet exists at larger distance that is stirring the disk to replenish the dust. © ESO, 2012. Source

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