67 research outputs found
Radiative Transfer in a Translucent Cloud Illuminated by an Extended Background Source
We discuss the radiative transfer theory for translucent clouds illuminated
by an extended background source. First we derive a rigorous solution based on
the assumption that multiple scattering produce an isotropic flux. Then we
derive a more manageable analytic approximation showing that it nicely matches
the results of the rigorous approach. To validate our model, we compare our
predictions with accurate laboratory measurements for various types of well
characterized grains, including purely dielectric and strongly absorbing
materials representative of astronomical icy and metallic grains, respectively,
finding excellent agreement without the need of adding free parameters. We use
our model to explore the behavior of an astrophysical cloud illuminated by a
diffuse source with dust grains having parameters typical of the classic ISM
grains of Draine & Lee (1984) and protoplanetary disks, with an application to
the dark silhouette disk 114-426 in Orion Nebula. We find that the scattering
term modifies the transmitted radiation, both in terms of intensity
(extinction) and shape (reddening) of the spectral distribution. In particular,
for small optical thickness our results show that scattering makes reddening
almost negligible at visible wavelengths. Once the optical thickness increases
enough and the probability of scattering events become close to or larger than
1, reddening becomes present but appreciably modified with respect to the
standard expression for line-of-sight absorption. Moreover, variations of the
grain refractive index, in particular the amount of absorption, also play an
important role changing the shape of the spectral transmission curve, with
dielectric grain showing the minimum amount of reddening.Comment: 19 pages, 11 figures, accepted for publication on The Astrophysical
Journa
A Multi-Color Optical Survey of the Orion Nebula Cluster. Part I: the Catalog
We present U, B, V, I broad-band, 6200A TiO medium-band and Halpha photometry
of the Orion Nebula Cluster obtained with the WFI imager at the ESO/MPI 2.2
telescope. The nearly-simultaneous observations cover the entire ONC in a field
of about 34x34 arcmin. They enable us to determine stellar colors avoiding the
additional scatter in the photometry induced by stellar variability typical of
pre-main sequence stars. We identify 2,612 point-like sources in I band, 58%,
43% and 17% of them detected also in V, B and U, respectively. 1040 sources are
identified in Halpha band. In this paper we present the observations, the
calibration techniques, and the resulting catalog. We show the derived CMD of
the population and discuss the completeness of our photometry. We define a
spectro-photometric TiO index from the fluxes in V, I, and TiO-band. We find a
correlation between the index and the spectral type valid for M-type stars,
that is accurate to better than 1 spectral sub-class for M3-M6 types and better
than 2 spectral subclasses for M0-M2 types. This allows us to newly classify
217 stars. We subtract from our Halpha photometry the photospheric continuum at
its wavelength, deriving calibrated line excess for the full sample. This
represents the largest Halpha star catalog obtained to date on the ONC. This
data set enables a full re-analysis of the properties of the Pre-Main Sequence
population in the Orion Nebula Cluster to be presented, in an accompanying
paper.Comment: 15 pages, 15 figures. To be published in The Astrophysical Journal
Supplement Serie
HST astrometry in the Orion Nebula Cluster: census of low-mass runaways
We present a catalog of high-precision proper motions in the Orion Nebula
Cluster (ONC), based on Treasury Program observations with the Hubble Space
Telescope's (HST) ACS/WFC camera. Our catalog contains 2,454 objects in the
magnitude range of , thus probing the stellar masses
of the ONC from 0.4 down to 0.02 over an area
of 550 arcmin. We provide a number of internal velocity dispersion
estimates for the ONC that indicate a weak dependence on the stellar location
and mass. There is good agreement with the published velocity dispersion
estimates, although nearly all of them (including ours at
and mas yr) might be biased by the overlapping young
stellar populations of Orion A. We identified 4 new ONC candidate runaways
based on HST and the Gaia DR2 data, all with masses less than 1
. The total census of known candidate runaway sources is 10 -- one of
the largest samples ever found in any Milky Way open star cluster.
Surprisingly, none of them has the tangential velocity exceeding 20 km
s. If most of them indeed originated in the ONC, it may compel
re-examination of dynamical processes in very young star clusters. It appears
that the mass function of the ONC is not significantly affected by the lost
runaways.Comment: 16 pages, 10 figures, 5 tables. Accepted for publication in A
Photoevaporation and spatial variation of grain sizes in Orion 114-426
Deep HST broad-band images taken with ACS and WFPC2 of the giant (
AU diameter) dark silhouette proplyd 114-426 in the Orion Nebula show that this
system is tilted, asymmetric, warped and photoevaporated. The exquisite angular
resolution of ACS allows us to map the distribution of dust grains at the
northern translucent edge of the disk, dominated by the photoevaporative flow.
Using the Mie theory for standard circumstellar disk grains, we find evidence
for a spatial gradient in grain size. The typical dust radius, m (less than what reported by previous studies) becomes smaller as the
distance from the disk center increases, consistent with the expectations for
the dynamic of dust entrained in a gaseous photoevaporative wind. Our analysis
of the disk morphology and location within the nebula indicates that this
system is photoevaporated by the diffuse radiation field of the Orion Nebula,
while being shielded from the radiation coming directly from the central
Trapezium stars. We estimate the mass-loss rate from the disk surface and the
time-scale for total disk dissipation, which turns out to be of the order of
yr. Such a short time, of the order of 1/100 of the cluster age,
indicates that this system is seen on the verge of destruction. This is
compatible with the exceptional nature of the disk, namely its combination of
huge size and low mass. Finally, we briefly discuss the viability of possible
mechanisms that may lead to the peculiar morphology of this system: external UV
flux, binary star and past close encounter.Comment: Ap.J.87356; accepted Aug.1, 2012; Scheduled for the September 20,
2012, V757 issu
Direct Imaging Discovery of a Jovian Exoplanet Within a Triple Star System
Direct imaging allows for the detection and characterization of exoplanets
via their thermal emission. We report the discovery via imaging of a young
Jovian planet in a triple star system and characterize its atmospheric
properties through near-infrared spectroscopy. The semi-major axis of the
planet is closer relative to that of its hierarchical triple star system than
for any known exoplanet within a stellar binary or triple, making HD 131399
dynamically unlike any other known system. The location of HD 131399Ab on a
wide orbit in a triple system demonstrates that massive planets may be found on
long and possibly unstable orbits in multi-star systems. HD 131399Ab is one of
the lowest mass (4+/-1 MJup) and coldest (850+/-50 K) exoplanets to have been
directly imaged
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