848 research outputs found
The lower mass function of the young open cluster Blanco 1: from 30 Mjup to 3 Mo
18 pages, 15 figures and 5 tables accepted in A&AWe performed a deep wide field optical survey of the young (~100-150 Myr) open cluster Blanco1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range.The survey covers 2.3 square degrees in the I and z-bands down to I~z~24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I,I-z) CMD relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy. We find that our survey should contain about 57% of the cluster members in the 0.03-0.6 Mo mass range, including 30-40 brown dwarfs. The candidate's radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03Mo and 0.6Mo, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index alpha=0.69 +/- 0.15. Over the whole mass domain, from 0.03Mo to 3Mo, the mass function is better fitted by a log-normal function with m0=0.36 +/- 0.07Mo and sigma=0.58 +/- 0.06. Comparison between the Blanco1 mass function, other young open clusters' MF, and the galactic disc MF suggests that the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution
The substellar population of the young cluster lambda Orionis
By collecting optical and infrared photometry and low resolution
spectroscopy, we have identified a large number of low mass stars and brown
dwarf candidates belonging to the young cluster (~5 Myr) associated with the
binary star lambda Orionis. The lowest mass object found is a M8.5 with an
estimated mass of 0.02 Msun (~0.01 Msun for objects without spectroscopic
confirmation).
For those objects with spectroscopy, the measured strength of the Halpha
emission line follows a distribution similar to other clusters with the same
age range, with larger equivalent widths for cooler spectral types. Three of
the brown dwarfs have Halpha emission equivalent widths of order 100 \AA,
suggestive that they may have accretion disks and thus are the substellar
equivalent of Classical T Tauri stars. We have derived the Initial Mass
Function for the cluster. For the substellar regime, the index of the mass
spectrum is alpha=0.60$+-0.06, very similar to other young associations.Comment: Astrophysica Journal, accepted April 2, 200
Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry
We aim at characterizing the accretion properties of several hundred members
of the star-forming cluster NGC 2264 (3 Myr). We performed a deep u,g,r,i
mapping and a simultaneous u+r monitoring of the region with CFHT/MegaCam in
order to directly probe the accretion process from UV excess measurements.
Photometric properties and stellar parameters are determined homogeneously for
about 750 monitored young objects, spanning the mass range 0.1-2 Mo. About 40%
are classical (accreting) T Tauri stars, based on various diagnostics (H_alpha,
UV and IR excesses). The remaining non-accreting members define the
(photospheric+chromospheric) reference UV emission level over which flux excess
is detected and measured. We revise the membership status of cluster members
based on UV accretion signatures and report a new population of 50 CTTS
candidates. A large range of UV excess is measured for the CTTS population,
varying from a few 0.1 to 3 mag. We convert these values to accretion
luminosities and obtain mass accretion rates ranging from 1e-10 to 1e-7 Mo/yr.
Taking into account a mass-dependent detection threshold for weakly accreting
objects, we find a >6sigma correlation between mass accretion rate and stellar
mass. A power-law fit, properly accounting for upper limits, yields M_acc
M^{1.4+/-0.3}. At any given stellar mass, we find a large spread of
accretion rates, extending over about 2 orders of magnitude. The monitoring of
the UV excess on a timescale of a couple of weeks indicates that its
variability typically amounts to 0.5 dex, much smaller than the observed
spread. We suggest that a non-negligible age spread across the cluster may
effectively contribute to the observed spread in accretion rates at a given
mass. In addition, different accretion mechanisms (like, e.g., short-lived
accretion bursts vs. more stable funnel-flow accretion) may be associated to
different M_acc regimes.Comment: 24 pages, 21 figures, accepted for publication in Astronomy &
Astrophysic
Interaction of Climate Change with Effects of Conspecific and Heterospecific Density on Reproduction
We studied the relationship between temperature and the coexistence of great titParus majorand blue titCyanistes caeruleus, breeding in 75 study plots across Europe and North Africa. We expected an advance in laying date and a reduction in clutch size during warmer springs as a general response to climate warming and a delay in laying date and a reduction in clutch size during warmer winters due to density-dependent effects. As expected, as spring temperature increases laying date advances and as winter temperature increases clutch size is reduced in both species. Density of great tit affected the relationship between winter temperature and laying date in great and blue tit. Specifically, as density of great tit increased and temperature in winter increased both species started to reproduce later. Density of blue tit affected the relationship between spring temperature and blue and great tit laying date. Thus, both species start to reproduce earlier with increasing spring temperature as density of blue tit increases, which was not an expected outcome, since we expected that increasing spring temperature should advance laying date, while increasing density should delay it cancelling each other out. Climate warming and its interaction with density affects clutch size of great tits but not of blue tits. As predicted, great tit clutch size is reduced more with density of blue tits as temperature in winter increases. The relationship between spring temperature and density on clutch size of great tits depends on whether the increase is in density of great tit or blue tit. Therefore, an increase in temperature negatively affected the coexistence of blue and great tits differently in both species. Thus, blue tit clutch size was unaffected by the interaction effect of density with temperature, while great tit clutch size was affected in multiple ways by these interactions terms.Institute of Biology of Karelian Research Centre 0218-2019-0080Academy of Finland 265859Ministerio de EconomĂa y Competitividad CGL-2016-79568-C3-3-
The Spitzer c2d Survey of Weak-line T Tauri Stars II: New Constraints on the Timescale for Planet Building
One of the central goals of the Spitzer Legacy Project ``From Molecular Cores
to Planet-forming Disks'' (c2d) is to determine the frequency of remnant
circumstellar disks around weak-line T Tauri stars (wTTs) and to study the
properties and evolutionary status of these disks. Here we present a census of
disks for a sample of over 230 spectroscopically identified wTTs located in the
c2d IRAC (3.6, 4.5, 4.8, and 8.0 um) and MIPS (24 um) maps of the Ophiuchus,
Lupus, and Perseus Molecular Clouds. We find that ~20% of the wTTs in a
magnitude limited subsample have noticeable IR-excesses at IRAC wavelengths
indicating the presence of a circumstellar disk. The disk frequencies we find
in these 3 regions are ~3-6 times larger than that recently found for a sample
of 83 relatively isolated wTTs located, for the most part, outside the highest
extinction regions covered by the c2d IRAC and MIPS maps. The disk fractions we
find are more consistent with those obtained in recent Spitzer studies of wTTs
in young clusters such as IC 348 and Tr 37. From their location in the H-R
diagram, we find that, in our sample, the wTTs with excesses are among the
younger part of the age distribution. Still, up to ~50% of the apparently
youngest stars in the sample show no evidence of IR excess, suggesting that the
circumstellar disks of a sizable fraction of pre-main-sequence stars dissipate
in a timescale of ~1 Myr. We also find that none of the stars in our sample
apparently older than ~10 Myrs have detectable circumstellar disks at
wavelengths < 24 um. Also, we find that the wTTs disks in our sample exhibit a
wide range of properties (SED morphology, inner radius, L_DISK/L*, etc) which
bridge the gaps observed between the cTTs and the debris disk regimes.Comment: 54 pages, 13 figures, Accepted by Ap
C2D Spitzer-IRS spectra of disks around T Tauri stars: I. Silicate emission and grain growth
Infrared ~5--35 um spectra for 40 solar-mass T Tauri stars and 7
intermediate-mass Herbig Ae stars with circumstellar disks were obtained using
the Spitzer Space Telescope as part of the c2d IRS survey. This work
complements prior spectroscopic studies of silicate infrared emission from
disks, which were focused on intermediate-mass stars, with observations of
solar-mass stars limited primarily to the 10 um region. The observed 10 and 20
um silicate feature strengths/shapes are consistent with source-to-source
variations in grain size. A large fraction of the features are weak and flat,
consistent with um-sized grains indicating fast grain growth (from 0.1--1.0 um
in radius). In addition, approximately half of the T Tauri star spectra show
crystalline silicate features near 28 and 33 um indicating significant
processing when compared to interstellar grains. A few sources show large
10-to-20 um ratios and require even larger grains emitting at 20 um than at 10
um. This size difference may arise from the difference in the depth into the
disk probed by the two silicate emission bands in disks where dust settling has
occurred. The 10 um feature strength vs. shape trend is not correlated with age
or Halpha equivalent width, suggesting that some amount of turbulent mixing and
regeneration of small grains is occurring. The strength vs. shape trend is
related to spectral type, however, with M stars showing significantly flatter
10 um features (larger grain sizes) than A/B stars. The connection between
spectral type and grain size is interpreted in terms of the variation in the
silicate emission radius as a function of stellar luminosity, but could also be
indicative of other spectral-type dependent factors (e.g, X-rays, UV radiation,
stellar/disk winds, etc.).Comment: 17 pages, 13 figures, accepted for publication by ApJ, formatted with
emulateapj using revtex4 v4.
Recommended from our members
The catastrophic flash-flood event of 8â9 September 2002 in the Gard region, France: a first case study for the CĂ©vennesâVivarais Mediterranean Hydrometeorological Observatory
The CĂ©vennesâVivarais Mediterranean Hydrometeorological Observatory (OHM-CV) is a research initiative aimed at improving the understanding and modeling of the Mediterranean intense rain events that frequently result in devastating flash floods in southern France. A primary objective is to bring together the skills of meteorologists and hydrologists, modelers and instrumentalists, researchers and practitioners, to cope with these rather unpredictable events. In line with previously published flash-flood monographs, the present paper aims at documenting the 8â9 September 2002 catastrophic event, which resulted in 24 casualties and an economic damage evaluated at 1.2 billion euros (i.e., about 1 billion U.S. dollars) in the Gard region, France. A description of the synoptic meteorological situation is first given and shows that no particular precursor indicated the imminence of such an extreme event. Then, radar and rain gauge analyses are used to assess the magnitude of the rain event, which was particularly remarkable for its spatial extent with rain amounts greater than 200 mm in 24 h over 5500 km2. The maximum values of 600â700 mm observed locally are among the highest daily records in the region. The preliminary results of the postevent hydrological investigation show that the hydrologic response of the upstream watersheds of the Gard and Vidourle Rivers is consistent with the marked spaceâtime structure of the rain event. It is noteworthy that peak specific discharges were very high over most of the affected areas (5â10 m3 sâ1 kmâ2) and reached locally extraordinary values of more than 20 m3 sâ1 kmâ2. A preliminary analysis indicates contrasting hydrological behaviors that seem to be related to geomorphological factors, notably the influence of karst in part of the region. An overview of the ongoing meteorological and hydrological research projects devoted to this case study within the OHM-CV is finally presented
Le projet SEBIOPAG-PHYTO : déterminants agricoles parcellaires et paysagers des variations de niveaux de régulation biologique
Ce sĂ©minaire est une restitution des principales avancĂ©es obtenues dans le cadre des projets ANR PEERLESS «viabilitĂ© dâune gestion Ă©cologique renforcĂ©e de la santĂ© des plantes dans les paysages agricoles » (2013-2017) et FRB SEBIOPAG-PHYTO «dĂ©terminants agricoles parcellaires et paysagers des variations de niveaux de rĂ©gulation biologique » (2014-2017). Le sĂ©minaire a rassemblĂ© 60 scientifiques, pour moitiĂ© extĂ©rieure aux unitĂ©s INRA partenaires de ces projets. Il s'est dĂ©roulĂ© Ă Paris Paris les 27-28 novembre 2017
A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
Globular clusters with their large populations of millisecond pulsars (MSPs)
are believed to be potential emitters of high-energy gamma-ray emission. Our
goal is to constrain the millisecond pulsar populations in globular clusters
from analysis of gamma-ray observations. We use 546 days of continuous
sky-survey observations obtained with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular
clusters. Steady point-like high-energy gamma-ray emission has been
significantly detected towards 8 globular clusters. Five of them (47 Tucanae,
Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices and clear evidence for an exponential cut-off in the range
1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission
from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral
indices , however the presence of an exponential cut-off
can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC
6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral
properties. From the observed gamma-ray luminosities, we estimate the total
number of MSPs that is expected to be present in these globular clusters. We
show that our estimates of the MSP population correlate with the stellar
encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters,
commensurate with previous estimates. The observation of high-energy gamma-ray
emission from a globular cluster thus provides a reliable independent method to
assess their millisecond pulsar populations that can be used to make
constraints on the original neutron star X-ray binary population, essential for
understanding the importance of binary systems in slowing the inevitable core
collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J.
Kn\"odlseder, N. Webb, B. Pancraz
Les effets conjugués de la gestion parcellaire et du contexte paysager et de sa dynamique sur les bioagresseurs et les niveaux de régulation biologique
Ce sĂ©minaire est une restitution des principales avancĂ©es obtenues dans le cadre des projets ANR PEERLESS «viabilitĂ© dâune gestion Ă©cologique renforcĂ©e de la santĂ© des plantes dans les paysages agricoles » (2013-2017) et FRB SEBIOPAG-PHYTO «dĂ©terminants agricoles parcellaires et paysagers des variations de niveaux de rĂ©gulation biologique » (2014-2017). Le sĂ©minaire a rassemblĂ© 60 scientifiques, pour moitiĂ© extĂ©rieure aux unitĂ©s INRA partenaires de ces projets. Il s'est dĂ©roulĂ© Ă Paris Paris les 27-28 novembre 2017
- âŠ