702 research outputs found
Orion Revisited - I. The massive cluster in front of the Orion Nebula Cluster
The aim of this work is to characterize the stellar population between Earth
and the Orion A molecular cloud where the well known star formation benchmark
Orion Nebula Cluster (ONC) is embedded. We use the denser regions the Orion A
cloud to block optical background light, effectively isolating the stellar
population in front of it. We then use a multi-wavelength observational
approach to characterize the cloud's foreground stellar population. We find
that there is a rich stellar population in front of the Orion A cloud, from
B-stars to M-stars, with a distinct 1) spatial distribution, 2) luminosity
function, and 3) velocity dispersion from the reddened population inside the
Orion A cloud. The spatial distribution of this population peaks strongly
around NGC 1980 (iota Ori) and is, in all likelihood, the extended stellar
content of this poorly studied cluster. We infer an age of ~4-5 Myr for NGC
1980 and estimate a cluster population of the order of 2000 stars, which makes
it one of the most massive clusters in the entire Orion complex. What is
currently taken in the literature as the ONC is then a mix of several
intrinsically different populations, namely: 1) the youngest population,
including the Trapezium cluster and ongoing star formation in the dense gas
inside the nebula, 2) the foreground population, dominated by the NGC 1980
cluster, and 3) the poorly constrained population of foreground and background
Galactic field stars. Our results support a scenario where the ONC and L1641N
are not directly associated with NGC 1980, i.e., they are not the same
population emerging from its parental cloud, but are instead distinct
overlapping populations. This result calls for a revision of most of the
observables in the benchmark ONC region (e.g., ages, age spread, cluster size,
mass function, disk frequency, etc.). (abridged)Comment: Version 2 includes comments and clarifications from John Tobin,
Nicola Da Rio, and Lynne Hillenbrand (minor clarifying changes were made to
Figures 1, 8, and 10). A&A accepted (15 pages, 10 figures). Higher resolution
figures available upon reques
High-resolution imaging of planet host candidates. A comprehensive comparison of different techniques
The Kepler mission has discovered thousands of planet candidates. Currently,
some of them have already been discarded; more than 200 have been confirmed by
follow-up observations, and several hundreds have been validated. However, most
of them are still awaiting for confirmation. Thus, priorities (in terms of the
probability of the candidate being a real planet) must be established for
subsequent observations. The motivation of this work is to provide a set of
isolated (good) host candidates to be further tested by other techniques. We
identify close companions of the candidates that could have contaminated the
light curve of the planet host. We used the AstraLux North instrument located
at the 2.2 m telescope in the Calar Alto Observatory to obtain
diffraction-limited images of 174 Kepler objects of interest. The lucky-imaging
technique used in this work is compared to other AO and speckle imaging
observations of Kepler planet host candidates. We define a new parameter, the
blended source confidence level (BSC), to assess the probability of an object
to have blended non-detected eclipsing binaries capable of producing the
detected transit. We find that 67.2% of the observed Kepler hosts are isolated
within our detectability limits, and 32.8% have at least one visual companion
at angular separations below 6 arcsec. We find close companions (below 3
arcsec) for the 17.2% of the sample. The planet properties of this sample of
non-isolated hosts are revised. We report one possible S-type binary
(KOI-3158). We also report three possible false positives (KOIs 1230.01,
3649.01, and 3886.01) due to the presence of close companions. The BSC
parameter is calculated for all the isolated targets and compared to both the
value prior to any high-resolution image and, when possible, to observations
from previous high-spatial resolution surveys in the Kepler sample.Comment: Accepted for publication in A&A on April 29, 2014; 32 pages, 11
figures, 11 table
Protoplanetary disk lifetimes vs stellar mass and possible implications for giant planet populations
We study the dependence of protoplanetary disk evolution on stellar mass
using a large sample of young stellar objects in nearby young star-forming
regions. We update the protoplanetary disk fractions presented in our recent
work (paper I of this series) derived for 22 nearby (< 500 pc) associations
between 1 and 100 Myr. We use a subsample of 1 428 spectroscopically confirmed
members to study the impact of stellar mass on protoplanetary disk evolution.
We divide this sample into two stellar mass bins (2 M boundary) and
two age bins (3 Myr boundary), and use infrared excesses over the photospheric
emission to classify objects in three groups: protoplanetary disks, evolved
disks, and diskless. The homogeneous analysis and bias corrections allow for a
statistically significant inter-comparison of the obtained results. We find
robust statistical evidence of disk evolution dependence with stellar mass. Our
results, combined with previous studies on disk evolution, confirm that
protoplanetary disks evolve faster and/or earlier around high-mass (> 2
M) stars. We also find a roughly constant level of evolved disks
throughout the whole age and stellar mass spectra. We conclude that
protoplanetary disk evolution depends on stellar mass. Such a dependence could
have important implications for gas giant planet formation and migration, and
could contribute to explaining the apparent paucity of hot Jupiters around
high-mass stars.Comment: Accepted for publication in A&A. 13 pages, 8 figures, 5 table
Multiplicity of very low-mass objects in the Upper Scorpius OB association: a possible wide binary population
We report the initial results of a VLT/NACO high spatial resolution imaging
survey for multiple systems among 58 M-type members of the nearby Upper
Scorpius OB association. Nine pairs with separations below 100 have been
resolved. Their small angular separations and the similarity in the brightness
of the components (DMagK <1 for all of them), indicate there is a reasonable
likelihood several of them are true binaries rather than chance projections.
Follow-up imaging observations with WHT/LIRIS of the two widest binaries
confirm that their near-infrared colours are consistent with physical very low
mass binaries. For one of these two binaries, WHT/LIRIS spectra of each
component were obtained. We find that the two components have similar M6-M7
spectral types and signatures of low-gravity, as expected for a young brown
dwarf binary in this association. Our preliminary results indicate a possible
population of very low-mass binaries with semimajor axis in the range 100 AU
150 AU, which has not been seen in the Pleiades open cluster. If these
candidates are confirmed (one is confirmed by this work), these results would
indicate that the binary properties of very low-mass stars and brown dwarfs may
depend on the environment where they form.Comment: 11 pages, 1 table, 7 figures, request high resolution copies to
[email protected]
Resolving the L/T transition binary SDSS J2052-1609 AB
Binaries provide empirical key constraints for star formation theories, like
the overall binary fraction, mass ratio distribution and the separation
distribution. They play a crucial role to calibrate the output of theoretical
models, like absolute magnitudes, colors and effective temperature depending on
mass, metallicity and age. We present first results of our on-going
high-resolution imaging survey of late type brown dwarfs. The survey aims at
resolving tight brown dwarf binary systems to better constrain the T dwarf
binary fraction. We intent to follow-up the individual binaries to determine
orbital parameters. Using NACO at the VLT we performed AO-assisted
near-infrared observations of SDSS J2052-1609. High-spatial resolution images
of the T1 dwarf were obtained in H and Ks filters. We resolved SDSS J2052-1609
into a binary system with a separation of 0.101" \pm 0.001". Archival data from
HST/NICMOS taken one year previous to our observations proves the components to
be co-moving. Using the flux ratio between the components we infer J, H and Ks
magnitudes for the resolved system. From the near-IR colors we estimate
spectral types of T1 +1 -4 and T2.5 \pm 1 for component A and B, respectively.
A first estimate of the total system mass yields Mtot > 78 Mjup, assuming a
circular orbit.Comment: 5 pages, 3 figures, 3 tables, accepted for publication by A&
Mass spectrometric study of anomalous water
Considerable interest has developed recently in a material called polywater or anomalous water . This material has properties quite different from those normally associated with liquid water. The purpose of this study is to give some mass spectrometric observations of polywater . The polywater samples were produced within silica capillaries. These capillaries were heated by means of a helical coil to vaporize the polywater . The vapor was directed into the source of a mass spectrometer. The results of this study show that a significant portion of the anomalous water samples is composed of a polymer of dimethylsiloxane. To date, the observations are not complete enough to determine if this polymer is the main constituent of anomalous water. The question is still open as to whether the polymer of dimethylsiloxane is a contaminant or synthesized in the silica capillaries --Abstract, page ii
Orion revisited. II. The foreground population to Orion A
Following the recent discovery of a large population of young stars in front
of the Orion Nebula, we carried out an observational campaign with the DECam
wide-field camera covering ~10~deg^2 centered on NGC 1980 to confirm, probe the
extent of, and characterize this foreground population of pre-main-sequence
stars. We confirm the presence of a large foreground population towards the
Orion A cloud. This population contains several distinct subgroups, including
NGC1980 and NGC1981, and stretches across several degrees in front of the Orion
A cloud. By comparing the location of their sequence in various color-magnitude
diagrams with other clusters, we found a distance and an age of 380pc and
5~10Myr, in good agreement with previous estimates. Our final sample includes
2123 candidate members and is complete from below the hydrogen-burning limit to
about 0.3Msun, where the data start to be limited by saturation. Extrapolating
the mass function to the high masses, we estimate a total number of ~2600
members in the surveyed region. We confirm the presence of a rich, contiguous,
and essentially coeval population of about 2600 foreground stars in front of
the Orion A cloud, loosely clustered around NGC1980, NGC1981, and a new group
in the foreground of the OMC-2/3. For the area of the cloud surveyed, this
result implies that there are more young stars in the foreground population
than young stars inside the cloud. Assuming a normal initial mass function, we
estimate that between one to a few supernovae must have exploded in the
foreground population in the past few million years, close to the surface of
Orion A, which might be responsible, together with stellar winds, for the
structure and star formation activity in these clouds. This long-overlooked
foreground stellar population is of great significance, calling for a revision
of the star formation history in this region of the Galaxy.Comment: Accepted for publication in A&
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