157 research outputs found
The Search for Other Planets and Life
This Les Houches School offers students a wide ranging view of the field of exoplanets and the search for life beyond the solar system. Observational and theoretical opportunities abound in a new field of astronomy that will be growing for decades to come. I give a brief introduction and overview to the many detailed talks that will be presented in this volume
Observational Constraints on the Formation and Evolution of Binary Stars
We present a high spatial resolution UV to NIR survey of 44 young binary
stars in Taurus with separations of 10-1000 AU. The primary results include:
(1) The relative ages of binary star components are more similar than the
relative ages of randomly paired single stars, supporting coeval formation. (2)
Only one of the companion masses is substellar, and hence the apparent
overabundance of T Tauri star companions relative to main-sequence star
companions can not be explained by a wealth of substellar secondaries that
would have been missed in main-sequence surveys. (3) Roughly 10% of T Tauri
binary star components have very red NIR colors (K-L > 1.4) and unusually high
mass accretion rates. This phenomenon does not appear to be restricted to
binary systems, however, since a comparable fraction of single T Tauri stars
exhibit the same properties. (4) Although the disk lifetimes of single stars
are roughly equal to their stellar ages, the disk lifetimes of binary stars are
an order of magnitude less than their ages. (5) The accretion rates for both
single and binary T Tauri stars appear to be moderately mass dependent. (6)
Although most classical T Tauri star binaries retain both a circumprimary and a
circumsecondary disk, there are several systems with only a circumprimary disk.
Together with the relative accretion rates, this suggests that circumprimary
disks survive longer, on average, than circumsecondary disks. (7) The disk
lifetimes, mass ratios, and relative accretion signatures of the closest
binaries (10-100 AU) suggest that they are being replenished from a
circumbinary reservoir with low angular momentum. Overall, these results
support fragmentation as the dominant binary star formation mechanism.Comment: 67 pages including 11 figures, LaTeX2e, accepted for publication in
Ap
Unveiling the Circumstellar Envelope and Disk: A Sub-Arcsecond Survey of Circumstellar Structures
We present the results of a 2.7 mm continuum interferometric survey of 24
young stellar objects in 11 fields. The target objects range from deeply
embedded Class 0 sources to optical T Tauri sources. This is the first
sub-arcsecond survey of the 2.7 mm dust continuum emission from young, embedded
stellar systems. The images show a diversity of structure and complexity. The
optically visible T Tauri stars (DG Tauri, HL Tauri, GG Tauri,and GM Aurigae)
have continuum emission dominated by compact, less than 1", circumstellar
disks. The more embedded near-infrared sources (SVS13 and L1551 IRS5) have
continuum emission that is extended and compact. The embedded sources (L1448
IRS3, NGC1333 IRAS2, NGC1333 IRAS4, VLA1623, and IRAS 16293-2422) have
continuum emission dominated by the extended envelope, typically more than 85%.
In fact, in many of the deeply embedded systems it is difficult to uniquely
isolate the disk emission component from the envelope extending inward to AU
size scales. All of the target embedded objects are in multiple systems with
separations on scales of 30" or less. Based on the system separation, we place
the objects into three categories: separate envelope (separation > 6500 AU),
common envelope (separation 150-3000 AU), and common disk (separation < 100
AU). These three groups can be linked with fragmentation events during the star
formation process: separate envelopes from prompt initial fragmentation and the
separate collapse of a loosely condensed cloud, common envelopes from
fragmentation of a moderately centrally condensed spherical system, and common
disk from fragmentation of a high angular momentum circumstellar disk.Comment: 47 Pages, 18 Figures, ApJ accepte
Direct evidence of a sub-stellar companion around CT Cha
In our ongoing search for close and faint companions around T Tauri stars, we
found a very faint (Ks=14.9mag, Ks_0=14.4mag) object, just ~2.67" northwest of
the Chamaeleon star-forming region member CT Cha corresponding to a projected
separation of ~440AU at 165+/-30 pc. We show that CT Cha A and this faint
object form a common proper motion pair from data of the VLT Adaptive Optics
(AO) instrument NACO taken in February 2006 and March 2007 and that the
companion is by >=4 sigma significance not a stationary background object. Our
AO integral field spectroscopy with SINFONI in J, and H+K bands yields a
temperature of 2600+/-250K for the companion and an optical extinction of
A_V=5.2+/-0.8mag, when compared to spectra calculated from Drift-Phoenix model
atmospheres. We demonstrate the validity of the model fits by comparison to
several other well-known young sub-stellar objects. Relative flux calibration
of the bands was achieved using photometry from the NACO imaging data. We
conclude that the CT Cha companion is a very low-mass member of Chamaeleon and
very likely a physical companion to CT Cha, as the probability for a by chance
alignment is <=0.01. Due to a prominent Pa-Beta emission in the J-band,
accretion is probably still ongoing onto the CT Cha companion. From temperature
and luminosity (log(Lbol/Lsun)= -2.68+/-0.21), we derive a radius of
R=2.20+0.81-0.60 R_Jup. We find a consistent mass of M=17+/-6 MJup for the CT
Cha companion from both its luminosity and temperature when placed on
evolutionary tracks. Hence, the CT Cha companion is most likely a wide brown
dwarf companion or possibly even a planetary mass object.Comment: 10 pages, 11 figures, accepted for publication in A&
Limits on additional planetary companions to OGLE-2005-BLG-390L
We investigate constraints on additional planets orbiting the distant M-dwarf
star OGLE-2005-BLG-390L, around which photometric microlensing data has
revealed the existence of the sub-Neptune-mass planet OGLE-2005-BLG-390Lb. We
specifically aim to study potential Jovian companions and compare our findings
with predictions from core-accretion and disc-instability models of planet
formation. We also obtain an estimate of the detection probability for
sub-Neptune mass planets similar to OGLE-2005-BLG-390Lb using a simplified
simulation of a microlensing experiment. We compute the efficiency of our
photometric data for detecting additional planets around OGLE-2005-BLG-390L, as
a function of the microlensing model parameters and convert it into a function
of the orbital axis and planet mass by means of an adopted model of the Milky
Way. We find that more than 50 % of potential planets with a mass in excess of
1 M_J between 1.1 and 2.3 AU around OGLE-2005-BLG-390L would have revealed
their existence, whereas for gas giants above 3 M_J in orbits between 1.5 and
2.2 AU, the detection efficiency reaches 70 %; however, no such companion was
observed. Our photometric microlensing data therefore do not contradict the
existence of gas giant planets at any separation orbiting OGLE-2005-BLG-390L.
Furthermore we find a detection probability for an OGLE-2005-BLG-390Lb-like
planet of around 2-5 %. In agreement with current planet formation theories,
this quantitatively supports the prediction that sub-Neptune mass planets are
common around low-mass stars.Comment: 10 pages, 4 figures, accepted by A&
The T Tauri star RY Tau as a case study of the inner regions of circumstellar dust disks
We study the inner region of the circumstellar disk around the TTauri star RY
Tau. Our aim is to find a physical description satisfying the available
interferometric data, obtained with the mid-infrared interferometric instrument
at the Very Large Telescope Interferometer, as well as the spectral energy
distribution. We also compare the findings with the results of similar studies,
including those of intermediate-mass stars. Our analysis is done within the
framework of a passive circumstellar disk, which is optionally supplemented by
the effects of accretion and an added envelope. To achieve a more consistent
and realistic model, we used our continuum transfer code MC3D. In addition, we
studied the shape of the 10um silicate emission feature in terms of the
underlying dust population, both for single-dish and for interferometric
measurements. We show that a modestly flaring disk model with accretion can
explain both the observed spectral energy distribution and the mid-infrared
visibilities obtained with the mid-infrared infrared instrument. We found an
interesting ambiguity: a circumstellar active disk model with an added
envelope, and a lower accretion rate than in the active disk model without
envelope, could represent the observations equally as well. This type of model
with the envelope should be considered a viable alternative in future models of
other TTauri stars. The approach of a disk with a puffed-up inner rim wall and
the influence of a stellar companion is also discussed. From the study of the
silicate emission feature we see evidence for dust evolution in a TTauri star,
with a decreasing fraction of small amorphous and an increasing fraction of
crystalline particles closer to the star.Comment: 19 pages, 23 figures; accepted by Astronomy & Astrophysic
Angular Momentum and the Formation of Stars and Black Holes
The formation of compact objects like stars and black holes is strongly
constrained by the requirement that nearly all of the initial angular momentum
of the diffuse material from which they form must be removed or redistributed
during the formation process. The mechanisms that may be involved and their
implications are discussed for (1) low-mass stars, most of which probably form
in binary or multiple systems; (2) massive stars, which typically form in
clusters; and (3) supermassive black holes that form in galactic nuclei. It is
suggested that in all cases, gravitational interactions with other stars or
mass concentrations in a forming system play an important role in
redistributing angular momentum and thereby enabling the formation of a compact
object. If this is true, the formation of stars and black holes must be a more
complex, dynamic, and chaotic process than in standard models. The
gravitational interactions that redistribute angular momentum tend to couple
the mass of a forming object to the mass of the system, and this may have
important implications for mass ratios in binaries, the upper stellar IMF in
clusters, and the masses of supermassive black holes in galaxies.Comment: Accepted by Reports on Progress in Physic
Towards Omni-Tomography—Grand Fusion of Multiple Modalities for Simultaneous Interior Tomography
We recently elevated interior tomography from its origin in computed tomography (CT) to a general tomographic principle, and proved its validity for other tomographic modalities including SPECT, MRI, and others. Here we propose “omni-tomography”, a novel concept for the grand fusion of multiple tomographic modalities for simultaneous data acquisition in a region of interest (ROI). Omni-tomography can be instrumental when physiological processes under investigation are multi-dimensional, multi-scale, multi-temporal and multi-parametric. Both preclinical and clinical studies now depend on in vivo tomography, often requiring separate evaluations by different imaging modalities. Over the past decade, two approaches have been used for multimodality fusion: Software based image registration and hybrid scanners such as PET-CT, PET-MRI, and SPECT-CT among others. While there are intrinsic limitations with both approaches, the main obstacle to the seamless fusion of multiple imaging modalities has been the bulkiness of each individual imager and the conflict of their physical (especially spatial) requirements. To address this challenge, omni-tomography is now unveiled as an emerging direction for biomedical imaging and systems biomedicine
Mercury dynamics in a San Francisco estuary tidal wetland : assessing dynamics using in situ measurements
© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Estuaries and Coasts 35 (2012): 1036-1048, doi:10.1007/s12237-012-9501-3.We used high-resolution in situ measurements of turbidity and fluorescent dissolved organic matter (FDOM) to quantitatively estimate the tidally driven exchange of mercury (Hg) between the waters of the San Francisco estuary and Browns Island, a tidal wetland. Turbidity and FDOM—representative of particle-associated and filter-passing Hg, respectively—together predicted 94 % of the observed variability in measured total mercury concentration in unfiltered water samples (UTHg) collected during a single tidal cycle in spring, fall, and winter, 2005–2006. Continuous in situ turbidity and FDOM data spanning at least a full spring-neap period were used to generate UTHg concentration time series using this relationship, and then combined with water discharge measurements to calculate Hg fluxes in each season. Wetlands are generally considered to be sinks for sediment and associated mercury. However, during the three periods of monitoring, Browns Island wetland did not appreciably accumulate Hg. Instead, gradual tidally driven export of UTHg from the wetland offset the large episodic on-island fluxes associated with high wind events. Exports were highest during large spring tides, when ebbing waters relatively enriched in FDOM, dissolved organic carbon (DOC), and filter-passing mercury drained from the marsh into the open waters of the estuary. On-island flux of UTHg, which was largely particle-associated, was highest during strong winds coincident with flood tides. Our results demonstrate that processes driving UTHg fluxes in tidal wetlands encompass both the dissolved and particulate phases and multiple timescales, necessitating longer term monitoring to adequately quantify fluxes.This work
was supported by funding from the California Bay Delta Authority
Ecosystem Restoration and Drinking Water Programs (grant ERP-00-
G01) and matching funds from the United States Geological Survey
Cooperative Research Program
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