107 research outputs found
Book Reviews
THE LAW SCHOOL OF TOMORROW: THE PROJECTION OF AN IDEAL. Edited by David Haber and Julius Cohen.
SHOULD CHURCHES BE TAXED? By D. B. Robertson.
CONSTITUTIONAL LAW FOR POLICE (Police Text Series). By John C Klotter and Jacqueline R. Kanovitz
The Solar Neighborhood. XXXIV. A Search for Planets Orbiting Nearby M Dwarfs using Astrometry
Astrometric measurements are presented for seven nearby stars with previously
detected planets: six M dwarfs (GJ 317, GJ 667C, GJ 581, GJ 849, GJ 876, and GJ
1214) and one K dwarf (BD 10 3166). Measurements are also presented for six
additional nearby M dwarfs without known planets, but which are more favorable
to astrometric detections of low mass companions, as well as three binary
systems for which we provide astrometric orbit solutions. Observations have
baselines of three to thirteen years, and were made as part of the RECONS
long-term astrometry and photometry program at the CTIO/SMARTS 0.9m telescope.
We provide trigonometric parallaxes and proper motions for all 16 systems, and
perform an extensive analysis of the astrometric residuals to determine the
minimum detectable companion mass for the 12 M dwarfs not having close stellar
secondaries. For the six M dwarfs with known planets, we are not sensitive to
planets, but can rule out the presence of all but the least massive brown
dwarfs at periods of 2 - 12 years. For the six more astrometrically favorable M
dwarfs, we conclude that none have brown dwarf companions, and are sensitive to
companions with masses as low as 1 for periods longer than two years.
In particular, we conclude that Proxima Centauri has no Jovian companions at
orbital periods of 2 - 12 years. These results complement previously published
M dwarf planet occurrence rates by providing astrometrically determined upper
mass limits on potential super-Jupiter companions at orbits of two years and
longer. As part of a continuing survey, these results are consistent with the
paucity of super-Jupiter and brown dwarf companions we find among the over 250
red dwarfs within 25 pc observed longer than five years in our astrometric
program.Comment: 18 pages, 5 figures, 4 tables, accepted for publication in A
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VIII. Serpens Observed with MIPS
We present maps of 1.5 deg^2 of the Serpens dark cloud at 24, 70, and 160 Îźm observed with the Spitzer Space Telescope MIPS camera. We describe the observations and briefly discuss the data processing carried out by the c2d team on these data. More than 2400 compact sources have been extracted at 24 Îźm, nearly 100 at 70 Îźm, and four at 160 Îźm. We estimate completeness limits for our 24 Îźm survey from Monte Carlo tests with artificial sources inserted into the Spitzer maps. We compare source counts, colors, and magnitudes in the Serpens cloud to two reference data sets: a 0.50 deg^2 set on a low-extinction region near the dark cloud, and a 5.3 deg^2 subset of the SWIRE ELAIS N1 data that was processed through our pipeline. These results show that there is an easily identifiable population of young stellar object candidates in the Serpens cloud that is not present in either of the reference data sets. We also show a comparison of visual extinction and cool dust emission illustrating a close correlation between the two and find that the most embedded YSO candidates are located in the areas of highest visual extinction
From Molecular Cores to Planet-forming Disks: An SIRTF Legacy Program
Crucial steps in the formation of stars and planets can be studied only at midâ to farâinfrared wavelengths, where the Space Infrared Telescope (SIRTF) provides an unprecedented improvement in sensitivity. We will use all three SIRTF instruments (Infrared Array Camera [IRAC], Multiband Imaging Photometer for SIRTF [MIPS], and Infrared Spectrograph [IRS]) to observe sources that span the evolutionary sequence from molecular cores to protoplanetary disks, encompassing a wide range of cloud masses, stellar masses, and starâforming environments. In addition to targeting about 150 known compact cores, we will survey with IRAC and MIPS (3.6â70 Îźm) the entire areas of five of the nearest large molecular clouds for new candidate protostars and substellar objects as faint as 0.001 solar luminosities. We will also observe with IRAC and MIPS about 190 systems likely to be in the early stages of planetary system formation (ages up to about 10 Myr), probing the evolution of the circumstellar dust, the raw material for planetary cores. Candidate planetâforming disks as small as 0.1 lunar masses will be detectable. Spectroscopy with IRS of new objects found in the surveys and of a select group of known objects will add vital information on the changing chemical and physical conditions in the disks and envelopes. The resulting data products will include catalogs of thousands of previously unknown sources, multiwavelength maps of about 20 deg^2 of molecular clouds, photometry of about 190 known young stars, spectra of at least 170 sources, ancillary data from groundâbased telescopes, and new tools for analysis and modeling. These products will constitute the foundations for many followâup studies with groundâbased telescopes, as well as with SIRTF itself and other space missions such as SIM, JWST, Herschel, and TPF/Darwin
The Spitzer c2d survey of Large, Nearby, Interstellar Clouds. V. Chamaeleon II Observed with IRAC
We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the
Chamaeleon II molecular cloud. The observed area covers about 1 square degree
defined by . Analysis of the data in the 2005 c2d catalogs reveals a
small number of sources (40) with properties similar to those of young
stellaror substellar objects (YSOs). The surface density of these YSO
candidates is low, and contamination by background galaxies appears to be
substantial, especially for sources classified as Class I or flat SED. We
discuss this problem in some detail and conclude that very few of the candidate
YSOs in early evolutionary stages are actually in the Cha II cloud. Using a
refined set of criteria, we define a smaller, but more reliable, set of 24 YSO
candidates.Comment: 19 pages, 10 figures, in press Ap
Dust filtration at gap edges: Implications for the spectral energy distributions of discs with embedded planets
The spectral energy distributions (SEDs) of some T Tauri stars display a
deficit of near-IR flux that could be a consequence of an embedded Jupiter-mass
planet partially clearing an inner hole in the circumstellar disc. Here, we use
two-dimensional numerical simulations of the planet-disc interaction, in
concert with simple models for the dust dynamics, to quantify how a planet
influences the dust at different radii within the disc. We show that pressure
gradients at the outer edge of the gap cleared by the planet act as a filter -
letting particles smaller than a critical size through to the inner disc while
holding back larger particles in the outer disc. The critical particle size
depends upon the disc properties, but is typically of the order of 10 microns.
This filtration process will lead to discontinuous grain populations across the
planet's orbital radius, with small grains in the inner disc and an outer
population of larger grains. We show that this type of dust population is
qualitatively consistent with SED modelling of systems that have optically thin
inner holes in their circumstellar discs. This process can also produce a very
large gas-to-dust ratio in the inner disc, potentially explaining those systems
with optically thin inner cavities that still have relatively high accretion
rates.Comment: 9 pages, 7 figures, Accepted fir publication in MNRA
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VII. Ophiuchus Observed with MIPS
We present maps of 14.4 deg^2 of the Ophiuchus dark clouds observed by the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). These high-quality maps depict both numerous point sources and extended dust emission within the star-forming and nonâstar-forming portions of these clouds. Using PSF-fitting photometry, we detect 5779 sources at 24 Îźm and 81 sources at 70 Îźm at the 10 Ď level of significance. Three hundred twenty-three candidate young stellar objects (YSOs) were identified according to their positions on the MIPS/2MASS K versus color-magnitude diagrams, as compared to 24 Îźm detections in the SWIRE extragalactic survey. We find that more than half of the YSO candidates, and almost all those with protostellar Class I spectral energy distributions, are confined to the known cluster and aggregates
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
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. IV. Lupus Observed with MIPS
We present maps of 7.78 square degrees of the Lupus molecular cloud complex
at 24, 70, and m. They were made with the Spitzer Space Telescope's
Multiband Imaging Photometer for Spitzer (MIPS) instrument as part of the
Spitzer Legacy Program, ``From Molecular Cores to Planet-Forming Disks'' (c2d).
The maps cover three separate regions in Lupus, denoted I, III, and IV. We
discuss the c2d pipeline and how our data processing differs from it. We
compare source counts in the three regions with two other data sets and
predicted star counts from the Wainscoat model. This comparison shows the
contribution from background galaxies in Lupus I. We also create two color
magnitude diagrams using the 2MASS and MIPS data. From these results, we can
identify background galaxies and distinguish them from probable young stellar
objects. The sources in our catalogs are classified based on their spectral
energy distribution (SED) from 2MASS and Spitzer wavelengths to create a sample
of young stellar object candidates. From 2MASS data, we create extinction maps
for each region and note a strong corresponence between the extinction and the
m emission. The masses we derived in each Lupus cloud from our
extinction maps are compared to masses estimated from CO and CO
and found to be similar to our extinction masses in some regions, but
significantly different in others. Finally, based on our color-magnitude
diagrams, we selected 12 of our reddest candidate young stellar objects for
individual discussion. Five of the 12 appear to be newly-discovered YSOs.Comment: 15 pages, 17 figures, uses emulateapj.cls. Accepted for publication
in ApJ. A version with high-quality figures can be found at
http://peggysue.as.utexas.edu/SIRTF
The Spitzer c2d Survey of Weak-Line T Tauri Stars. III. The Transition from Primordial Disks to Debris Disks
We present 3.6 to 70 {\mu}m Spitzer photometry of 154 weak-line T Tauri stars
(WTTS) in the Chamaeleon, Lupus, Ophiuchus and Taurus star formation regions,
all of which are within 200 pc of the Sun. For a comparative study, we also
include 33 classical T Tauri stars (CTTS) which are located in the same star
forming regions. Spitzer sensitivities allow us to robustly detect the
photosphere in the IRAC bands (3.6 to 8 {\mu}m) and the 24 {\mu}m MIPS band. In
the 70 {\mu}m MIPS band, we are able to detect dust emission brighter than
roughly 40 times the photosphere. These observations represent the most
sensitive WTTS survey in the mid to far infrared to date, and reveal the
frequency of outer disks (r = 3-50 AU) around WTTS. The 70 {\mu}m photometry
for half the c2d WTTS sample (the on-cloud objects), which were not included in
the earlier papers in this series, Padgett et al. (2006) and Cieza et al.
(2007), are presented here for the first time. We find a disk frequency of 19%
for on-cloud WTTS, but just 5% for off- cloud WTTS, similar to the value
reported in the earlier works. WTTS exhibit spectral energy distributions
(SEDs) that are quite diverse, spanning the range from optically thick to
optically thin disks. Most disks become more tenuous than Ldisk/L* = 2 x 10^-3
in 2 Myr, and more tenuous than Ldisk/L* = 5 x 10^-4 in 4 Myr.Comment: 40 pages, 13 figures, 4 tables. Accepted for publication in ApJ on
September 20, 201
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