Low mass T Tauri and young brown dwarf candidates in the Chamaeleon II dark cloud found by DENIS

We define a sample designed to select low-mass T Tauri stars and young brown dwarfs using DENIS data in the Chamaeleon II molecular cloud. We use a star count method to construct an extinction map of the Chamaeleon II cloud. We select our low-mass T Tauri star and young brown dwarf candidates by their strong infrared colour excess in the I-J/J-K_s colour-colour dereddened diagram. We retain only objects with colours I-J>2, and spatially distributed in groups around the cloud cores. This provides a sample of 70 stars of which 4 are previously known T Tauri stars. We have carefully checked the reliability of all these objects by visual inspection on the DENIS images. Thanks to the association of the optical I-band to the infra-red J and K_s bands in DENIS, we can apply this selection method to all star formation regions observed in the southern hemisphere. We also identify six DENIS sources with X-ray sources detected by ROSAT. Assuming that they are reliable low-mass candidates and using the evolutionary models for low-mass stars, we estimate the age of these sources between 1 Myr and < 10 Myr.Comment: 7 Pages, including 3 PostScript figures. Accepted for publication in Astronomy & Astrophysic

Large-scale variations of the dust optical properties in the Galaxy

We present an analysis of the dust optical properties at large scale, for the whole galactic anticenter hemisphere. We used the 2MASS Extended Source Catalog to obtain the total reddening on each galaxy line of sight and we compared this value to the IRAS 100 microns surface brightness converted to extinction by Schlegel et al (1998). We performed a careful examination and correction of the possible systematic effects resulting from foreground star contamination, redshift contribution and galaxy selection bias. We also evaluated the contribution of dust temperature variations and interstellar clumpiness to our method. The correlation of the near-infrared extinction to the far-infrared optical depth shows a discrepancy for visual extinction greater than 1 mag with a ratio A_V(FIR) / A_V(gal) = 1.31 +- 0.06. We attribute this result to the presence of fluffy/composite grains characterized by an enhanced far--infrared emissivity. Our analysis, applied to half of the sky, provides new insights on the dust grains nature suggesting fluffy grains are found not only in some very specific regions but in all directions for which the visual extinction reaches about 1 mag.Comment: 10 pages, 11 figures, accepted for publication in A&

The Cosmic Infrared Background at 1.25 and 2.2 Microns Using DIRBE and 2MASS: A Contribution Not Due to Galaxies?

Using the 2MASS second incremental data release and the zodiacal subtracted mission average maps of COBE/DIRBE, we estimate the cosmic background in the J (1.25 μm) and K (2.2 μm) bands using selected areas representing ~550 deg^2 of sky. We find a J background of 22.9 ± 7.0 kJy sr^(-1) (54.0 ± 16.8 nW m^(-2) sr^(-1)) and a K background of 20.4 ± 4.9 kJy sr^(-1) (27.8 ± 6.7 nW m^(-2) sr^(-1)). This large-scale study shows that the main uncertainty comes from the residual zodiacal emission. The cosmic background we obtain is significantly higher than integrated galaxy counts (3.6 ± 0.8 kJy sr^(-1) and 5.3 ± 1.2 kJy sr^(-1) for J and K, respectively), suggesting either an increase of the galaxy luminosity function for magnitudes fainter than 30 mag or the existence of another contribution to the cosmic background from primeval stars, black holes, or relic particle decay

Surface density of the young cluster IC 348 in the Perseus molecular cloud

The IC 348 young star cluster contains more than 300 confirmed members. It is embedded in the Perseus molecular cloud, making any clustering analysis subject to an extinction bias. In this work, we derive the extinction map of the cloud and revisit the content of IC 348 through a statistical approach that uses the 2MASS data. Our goal was to address the question of the completeness of IC 348 and of young clusters in general. We performed a combined analysis of the star color and density in this region, in order to establish the surface density map of the cluster. We reached the conclusion that IC 348 has structures up to 25' from the cluster center, and we estimate that about 40 members brighter than Ks=13 mag are still unidentified. Although we cannot use our statistical method to identify these new members individually, the surface density map gives a strong indication of their actual location. They are distributed in the outer regions of the cluster, where very few dedicated observations have been made so far, which is probably why they escaped previous identification. In addition, we propose the existence of a new embedded cluster associated to the infrared source MSX6C G160.2784-18.4216, about 38' south of IC 348.Comment: 5 pages, 5 figures, accepted for publication in A&

Properties of dust and detection of Halpha emission in LDN 1780

We present ISOPHOT observations between 60 and 200 microns and a near-infrared extinction map of LDN 1780. We have used the ISOPHOT data together with the 25, 60 and 100 microns IRIS maps to disentangle the warm and cold components of large dust grains that are observed in translucent and dense clouds. The warm and cold components in LDN 1780 have different properties and spatial distributions, with the warm component surrounding the cold component. The cold component is associated with molecular gas at densities of ~1000 cm^-3. The warm component has a uniform colour temperature of 25 K, and the colour temperature of the cold component slightly varies between 15.8 and 17.3 K (beta=2, Delta(T)=0.5 K). The ratio between the 200 microns emission of the cold component and Av is Ic(200)/Av=12.1 MJy/sr/mag and the average ratio tau(200)/Av=2.0x10^-4 mag^-1. The far-infrared emissivity of the warm component is significantly lower than that of the cold component. The Halpha emission and Av correlate very well; a ratio I(Halpha)/Av=2.2 Rayleigh/mag is observed. This correlation indicates the presence of a source of ionisation that can penetrate deeply into the cloud. Using the ratio I(Halpha)/Av we have estimated a ionisation rate for LDN 1780 that results to be ~10^-16 photons/s. We interpret this relatively high value as due to an enhanced cosmic ray flux of ~10 times the standard value. The origin of the cosmic rays could be from supernovae in the Scorpio-Centaurus OB association and/or the runaway zeta Ophiuchus.Comment: 19 pages, 12 figures, accepted for publications in MNRA

Modeling and predicting the shape of the far-infrared to submillimeter emission in ultra-compact HII regions and cold clumps

Dust properties are very likely affected by the environment in which dust grains evolve. For instance, some analyses of cold clumps (7 K- 17 K) indicate that the aggregation process is favored in dense environments. However, studying warm (30 K-40 K) dust emission at long wavelength ($\lambda$$>$300 $\mu$m) has been limited because it is difficult to combine far infared-to-millimeter (FIR-to-mm) spectral coverage and high angular resolution for observations of warm dust grains. Using Herschel data from 70 to 500 $\mu$m, which are part of the Herschel infrared Galactic (Hi-GAL) survey combined with 1.1 mm data from the Bolocam Galactic Plane Survey (BGPS), we compared emission in two types of environments: ultra-compact HII (UCHII) regions, and cold molecular clumps (denoted as cold clumps). With this comparison we tested dust emission models in the FIR-to-mm domain that reproduce emission in the diffuse medium, in these two environments (UCHII regions and cold clumps). We also investigated their ability to predict the dust emission in our Galaxy. We determined the emission spectra in twelve UCHII regions and twelve cold clumps, and derived the dust temperature (T) using the recent two-level system (TLS) model with three sets of parameters and the so-called T-$\beta$ (temperature-dust emissvity index) phenomenological models, with $\beta$ set to 1.5, 2 and 2.5. We tested the applicability of the TLS model in warm regions for the first time. This analysis indicates distinct trends in the dust emission between cold and warm environments that are visible through changes in the dust emissivity index. However, with the use of standard parameters, the TLS model is able to reproduce the spectral behavior observed in cold and warm regions, from the change of the dust temperature alone, whereas a T-$\beta$ model requires $\beta$ to be known.Comment: Accepted for publication in A&A. 19 pages, 8 figures, 7 table

Ultraviolet-Selected Field and Pre-Main-Sequence Stars Towards Taurus and Upper Scorpius

We have carried out a Galaxy Evolution Explorer (GALEX) Cycle 1 guest investigator program covering 56 square degrees near the Taurus T association and 12 square degrees along the northern edge of the Upper Scorpius OB association. We combined photometry in the GALEX FUV and NUV bands with data from the Two Micron All Sky Survey to identify candidate young (<100 Myr old) stars as those with an ultraviolet excess relative to older main sequence stars. Follow-up spectroscopy of a partial sample of these candidates suggest 5 new members of Taurus, with 8-20 expected from additional observations, and 5 new members of Upper Scorpius, with 3-6 expected from additional observations. These candidate new members appear to represent a distributed, non-clustered population in either region, although our sample statistics are as of yet too poor to constrain the nature or extent of this population. Rather, our study demonstrates the ability of GALEX observations to identify young stellar populations distributed over a wide area of the sky. We also highlight the necessity of a better understanding of the Galactic ultraviolet source population to support similar investigations. In particular, we report a large population of stars with an ultraviolet excess but no optical indicators of stellar activity or accretion, and briefly argue against several interpretations of these sources.Comment: 46 pages, 16 figures, 13 tables; Accepted to the Astronomical Journa

The Resolved Near-Infrared Extragalactic Background

We present a current best estimate of the integrated near-infrared (NIR) extragalactic background light (EBL) attributable to resolved galaxies in J, H, and Ks. Our results in units of nW m-2 sr-1 are 11.7+5.6 -2.6 in J, 11.5+4.5 -1.5 in H and 10.0+2.8 -0.8 in Ks. We derive these new limits by combining our deep wide-field NIR photometry from five widely separated fields with other studies from the literature to create a galaxy counts sample that is highly complete and has good counting statistics out to JHKs ~ 27-28. As part of this effort we present new ultradeep Ks-band galaxy counts from 22 hours of observations with the Multi Object Infrared Camera and Spectrograph (MOIRCS) instrument on the Subaru Telescope. We use this MOIRCS Ks-band mosaic to estimate the total missing flux from sources beyond our detection limits. Our new limits to the NIR EBL are in basic agreement with, but 10 - 20% higher than previous estimates, bringing them into better agreement with estimates of the total NIR EBL (resolved + unresolved sources) obtained from TeV gamma-ray opacity measurements and recent direct measurements of the total NIR EBL. We examine field to field variations in our photometry to show that the integrated light from galaxies is isotropic to within uncertainties, consistent with the expected large-scale isotropy of the EBL. Our data also allow for a robust estimate of the NIR light from Galactic stars, which we find to be 14.7 +/- 2.4 in J, 10.1 +/- 1.9 in H and 7.6 +/- 1.8 in Ks in units of nW m-2 sr-1.Comment: Accepted to Ap

Carbon isotope fractionation and depletion in TMC1

12C/13C isotopologue abundance anomalies have long been predicted for gas-phase chemistry in molecules other than CO and have recently been observed in the Taurus molecular cloud in several species hosting more than one carbon atom, i.e. CCH, CCS, CCCS and HC$_3$N. Here we work to ascertain whether these isotopologic anomalies actually result from the predicted depletion of the 13C+ ion in an oxygen-rich optically-shielded dense gas, or from some other more particular mechanism or mechanisms. We observed $\lambda$3mm emission from carbon, sulfur and nitrogen-bearing isotopologues of HNC, CS and \HH CS at three positions in Taurus(TMC1, L1527 and the ammonia peak) using the ARO 12m telescope. We saw no evidence of 12C/13C anomalies in our observations. Although the pool of C+ is likely to be depleted in 13C 13C is not depleted in the general pool of carbon outside CO, which probably exists mostly in the form of C^0. The observed isotopologic abundance anomalies are peculiar to those species in which they are found.Comment: Accepted for publication in The Astrophysical Journal (mail journal

Cosmic Background dipole measurements with Planck-High Frequency Instrument

This paper discusses the Cosmic Background (CB) dipoles observations in the framework of the Planck mission. Dipoles observations can be used in three ways: (i) It gives a measurement of the peculiar velocity of our Galaxy which is an important observation in large scale structures formation model. (ii) Measuring the dipole can give unprecedent information on the monopole (that can be in some cases hard to obtain due to large foreground contaminations). (iii) The dipole can be an ideal absolute calibrator, easily detectable in cosmological experiments. Following the last two objectives, the main goal of the work presented here is twofold. First, we study the accuracy of the Planck-HFI calibration using the Cosmic Microwave Background (CMB) dipole measured by COBE as well as the Earth orbital motion dipole. We show that we can reach for HFI, a relative calibration between rings of about 1% and an absolute calibration better than 0.4% for the CMB channels (in the end, the absolute calibration will be limited by the uncertainties on the CMB temperature). We also show that Planck will be able to measure the CMB dipole direction at better than 1.7 arcmin and improve on the amplitude. Second, we investigate the detection of the Cosmic Far-Infrared Background (FIRB) dipole. Measuring this dipole could give a new and independent determination of the FIRB for which a direct determination is quite difficult due to Galactic dust emission contamination. We show that such a detection would require a Galactic dust emission removal at better than 1%, which will be very hard to achieve.Comment: 10 pages, 13 figures, submitted to A&A, uses aa.sty V5.