New Young Star Candidates in CG4 and Sa101

The CG4 and Sa101 regions together cover a region of ~0.5 square degree in the vicinity of a "cometary globule" that is part of the Gum Nebula. There are seven previously identified young stars in this region; we have searched for new young stars using mid- and far-infrared data (3.6 to 70 microns) from the Spitzer Space Telescope, combined with ground-based optical data and near-infrared data from the Two-Micron All-Sky Survey (2MASS). We find infrared excesses in all 6 of the previously identified young stars in our maps, and we identify 16 more candidate young stars based on apparent infrared excesses. Most (73%) of the new young stars are Class II objects. There is a tighter grouping of young stars and young star candidates in the Sa101 region, in contrast to the CG4 region, where there are fewer young stars and young star candidates, and they are more dispersed. Few likely young objects are found in the "fingers" of the dust being disturbed by the ionization front from the heart of the Gum Nebula.Comment: Accepted for publication in A

Spitzer Observations of IC 2118

IC 2118, also known as the Witch Head Nebula, is a wispy, roughly cometary, ~5 degree long reflection nebula, and is thought to be a site of triggered star formation. In order to search for new young stellar objects (YSOs), we have observed this region in 7 mid- and far-infrared bands using the Spitzer Space Telescope and in 4 bands in the optical using the U. S. Naval Observatory 40-inch telescope. We find infrared excesses in 4 of the 6 previously-known T Tauri stars in our combined infrared maps, and we find 6 entirely new candidate YSOs, one of which may be an edge-on disk. Most of the YSOs seen in the infrared are Class II objects, and they are all in the "head" of the nebula, within the most massive molecular cloud of the region.Comment: Accepted to Ap

La stratégie de A à Z, 350 mots pour comprendre

L\u27ouvrage présente et illustre les 350 notions les plus représentatives du champ de la stratégie dans un style à la fois accessible et rigoureux. Il traite des différentes facettes de la stratégie : diagnostic, management stratégique, manoeuvres concurrentielles, innovation, changement, pilotage stratégique.. Chaque entrée propose la traduction anglaise du terme proposé, une présentation simple du concept et une illustration concrète. Une attention particulière est portée aux renvois entre les entrées et à leur complémentarité, afin de veiller à donner de la cohérence à l\u27ouvrage. L\u27équipe de quatre auteurs dispose de compétences et de champ d\u27intérêt très complémentaires

A U-band survey of brown dwarfs in the Taurus Molecular Cloud with the XMM-Newton Optical/UV Monitor

We aim to characterize the U-band variability of young brown dwarfs in the Taurus Molecular Cloud and discuss its origin. We used the XMM-Newton Extended Survey of the Taurus Molecular Cloud, where a sample of 11 young bona fide brown dwarfs (spectral type later than M6) were observed simultaneously in X-rays with XMM-Newton and in the U-band with the XMM-Newton Optical/UV Monitor (OM). We obtained upper limits to the U-band emission of 10 brown dwarfs (U>19.6-20.6 mag), whereas 2MASSJ04141188+2811535 was detected in the U-band. Remarkably, the magnitude of this brown dwarf increased regularly from U~19.5 mag at the beginning of the observation, peaked 6h later at U~18.4 mag, and then decreased to U~18.65 mag in the next 2h. The first OM U-band measurement is consistent with the quiescent level observed about one year later thanks to ground follow-up observations. This brown dwarf was not detected in X-rays by XMM-Newton during the OM observation. We discuss the possible sources of U-band variability for this young brown dwarf, namely a magnetic flare, non-steady accretion onto the substellar surface, and rotational modulation of a hot spot. We conclude that this event is related to accretion from a circumsubstellar disk, where the mass accretion rate was about a factor of 3 higher than during the quiescent level.Comment: 6 pages and 4 Figures. Accepted by A&A, to appear in a special section/issue dedicated to the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST

Young Stellar Objects and Triggered Star Formation in the Vulpecula OB Association

The Vulpecula OB association, VulOB1, is a region of active star formation located in the Galactic plane at 2.3 kpc from the Sun. Previous studies suggest that sequential star formation is propagating along this 100 pc long molecular complex. In this paper, we use Spitzer MIPSGAL and GLIMPSE data to reconstruct the star formation history of VulOB1, and search for signatures of past triggering events. We make a census of Young Stellar Objects (YSO) in VulOB1 based on IR color and magnitude criteria, and we rely on the properties and nature of these YSOs to trace recent episodes of massive star formation. We find 856 YSO candidates, and show that the evolutionary stage of the YSO population in VulOB1 is rather homogeneous - ruling out the scenario of propagating star formation. We estimate the current star formation efficiency to be ~8 %. We also report the discovery of a dozen pillar-like structures, which are confirmed to be sites of small scale triggered star formation.Comment: 30 pages, 11 figures, accepted for publication in Ap

Masses of the components of SB2 binaries observed with Gaia. II. Masses derived from PIONIER interferometric observations for Gaia validation

In anticipation of the Gaia astrometric mission, a sample of spectroscopic binaries is being observed since 2010 with the Sophie spectrograph at the Haute--Provence Observatory. Our aim is to derive the orbital elements of double-lined spectroscopic binaries (SB2s) with an accuracy sufficient to finally obtain the masses of the components with relative errors as small as 1 % when combined with Gaia astrometric measurements. In order to validate the masses derived from Gaia, interferometric observations are obtained for three SB2s in our sample with F-K components: HIP 14157, HIP 20601 and HIP 117186. The masses of the six stellar components are derived. Due to its edge-on orientation, HIP 14157 is probably an eclipsing binary. We note that almost all the derived masses are a few percent larger than the expectations from the standard spectral-type-mass calibration and mass-luminosity relation. Our calculation also leads to accurate parallaxes for the three binaries, and the Hipparcos parallaxes are confirmed.Comment: 10 pages, 3 figures, accepted by MNRA

The low-mass Initial Mass Function in the Orion Nebula cluster based on HST/NICMOS III imaging

We present deep HST/NICMOS Camera 3 F110W and F160W imaging of a 26'x33', corresponding to 3.1pcx3.8pc, non-contiguous field towards the Orion Nebula Cluster (ONC). The main aim is to determine the ratio of low--mass stars to brown dwarfs for the cluster as a function of radius out to a radial distance of 1.5pc. The sensitivity of the data outside the nebulous central region is F160W=21.0 mag, significantly deeper than previous studies of the region over a comparable area. We create an extinction limited sample and determine the ratio of low-mass stars (0.08-1Msun) to brown dwarfs (0.02-0.08Msun and 0.03-0.08Msun) for the cluster as a whole and for several annuli. The ratio found for the cluster within a radius of 1.5pc is R(02)=N(0.08-1Msun)/N(0.02-0.08Msun)=1.7+-0.2, and R(03)=N(0.08-1Msun)/N(0.03-0.08Msun)=2.4+-0.2, after correcting for field stars. The ratio for the central 0.3pcx0.3pc region down to 0.03Msun was previously found to be R(03)=3.3+0.8-0.7, suggesting the low-mass content of the cluster is mass segregated. We discuss the implications of a gradient in the ratio of stars to brown dwarfs in the ONC in the context of previous measurements of the cluster and for other nearby star forming regions. We further discuss the current evidence for variations in the low-mass IMF and primordial mass segregation.Comment: Accepted to A&

X-ray emission from the young brown dwarfs of the Taurus Molecular Cloud

The XMM-Newton Extended Survey of the TMC (XEST) is a large program designed to systematically investigate the X-ray properties of young stellar/substellar objects in the TMC. In particular, the area surveyed by 15 XMM-Newton pointings (of which three are archival observations), supplemented with one archival Chandra observation, allows us to study 17 BDs with M spectral types. Half of this sample (9 out of 17 BDs) is detected; 7 BDs are detected here for the first time in X-rays. We observed a flare from one BD. We confirm several previous findings on BD X-ray activity: a log-log relation between X-ray and bolometric luminosity for stars (with L*<10 Lsun) and BDs detected in X-rays; a shallow log-log relation between X-ray fractional luminosity and mass; a log-log relation between X-ray fractional luminosity and effective temperature; a log-log relation between X-ray surface flux and effective temperature. We find no significant log-log correlation between the X-ray fractional luminosity and EW(Halpha). Accreting and nonaccreting BDs have a similar X-ray fractional luminosity. The median X-ray fractional luminosity of nonaccreting BDs is about 4 times lower than the mean saturation value for rapidly rotating low-mass field stars. Our TMC BDs have higher X-ray fractional luminosity than BDs in the Chandra Orion Ultradeep Project. The X-ray fractional luminosity declines from low-mass stars to M-type BDs, and as a sample, the BDs are less efficient X-ray emitters than low-mass stars. We thus conclude that while the BD atmospheres observed here are mostly warm enough to sustain coronal activity, a trend is seen that may indicate its gradual decline due to the drop in photospheric ionization degree (abridged).Comment: 20 pages and 19 Figures. Accepted by A&A, to appear in a special section/issue dedicated to the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST). Preprint with higher resolution figures is available at http://hal.ccsd.cnrs.fr/ccsd-0009049

The North American and Pelican Nebulae I. IRAC Observations

We present a 9 deg^2 map of the North American and Pelican Nebulae regions obtained in all four IRAC channels with the Spitzer Space Telescope. The resulting photometry is merged with that at JHKs from 2MASS and a more spatially limited $BVI$ survey from previous ground-based work. We use a mixture of color- color diagrams to select a minimally contaminated set of more than 1600 objects that we claim are young stellar objects (YSOs) associated with the star forming region. Because our selection technique uses IR excess as a requirement, our sample is strongly biased against inclusion of Class III YSOs. The distribution of IRAC spectral slopes for our YSOs indicates that most of these objects are Class II, with a peak towards steeper spectral slopes but a substantial contribution from a tail of flat spectrum and Class I type objects. By studying the small fraction of the sample that is optically visible, we infer a typical age of a few Myr for the low mass population. The young stars are clustered, with about a third of them located in eight clusters that are located within or near the LDN 935 dark cloud. Half of the YSOs are located in regions with surface densities higher than 1000 YSOs / deg^2. The Class I objects are more clustered than the Class II stars.Comment: 16 pages, 12 figures, ApJ In pres

The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry

We report on the properties of pre-main-sequence objects in the Taurus molecular clouds as observed in 7 mid- and far-infrared bands with the Spitzer Space Telescope. There are 215 previously-identified members of the Taurus star-forming region in our ~44 square degree map; these members exhibit a range of Spitzer colors that we take to define young stars still surrounded by circumstellar dust (noting that ~20% of the bonafide Taurus members exhibit no detectable dust excesses). We looked for new objects in the survey field with similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet imaging, and found 148 candidate new members of Taurus. We have obtained follow-up spectroscopy for about half the candidate sample, thus far confirming 34 new members, 3 probable new members, and 10 possible new members, an increase of 15-20% in Taurus members. Of the objects for which we have spectroscopy, 7 are now confirmed extragalactic objects, and one is a background Be star. The remaining 93 candidate objects await additional analysis and/or data to be confirmed or rejected as Taurus members. Most of the new members are Class II M stars and are located along the same cloud filaments as the previously-identified Taurus members. Among non-members with Spitzer colors similar to young, dusty stars are evolved Be stars, planetary nebulae, carbon stars, galaxies, and AGN.Comment: Accepted to ApJS. Two large online-only figures available with the preprint here: http://web.ipac.caltech.edu/staff/rebull/research.htm