142 research outputs found
Tracking ALMA System Temperature with Water Vapor Data at High Frequency
The ALMA observatory is now putting more focus on high-frequency observations
(frequencies from 275-950 GHz). However, high-frequency observations often
suffer from rapid variations in atmospheric opacity that directly affect the
system temperature . Current observations perform discrete atmospheric
calibrations (Atm-cals) every few minutes, with typically 10-20 occurring per
hour for high frequency observation and each taking 30-40 seconds. In order to
obtain more accurate flux measurements and reduce the number of atmospheric
calibrations (Atm-cals), a new method to monitor continuously is
proposed using existing data in the measurement set. In this work, we
demonstrate the viability of using water vapor radiometer (WVR) data to track
the continuously. We find a tight linear correlation between
measured using the traditional method and extrapolated
based on WVR data with scatter of 0.5-3%. Although the exact form of the linear
relation varies among different data sets and spectral windows, we can use a
small number of discrete measurements to fit the linear relation and
use this heuristic relationship to derive every 10 seconds.
Furthermore, we successfully reproduce the observed correlation using
atmospheric transmission at microwave (ATM) modeling and demonstrate the
viability of a more general method to directly derive the from the
modeling. We apply the semi-continuous from heuristic fitting on a
few data sets from Band 7 to Band 10 and compare the flux measured using these
methods. We find the discrete and continuous methods give us
consistent flux measurements with differences up to 5%. Furthermore, this
method has significantly reduced the flux uncertainty due to
variability for one dataset, which has large precipitable water vapor (PWV)
fluctuation, from 10% to 0.7%.Comment: 24 pages, 18 figures, accepted to PAS
Dust and Gas in the disc of HL Tauri: Surface density, dust settling, and dust-to-gas ratio
The recent ALMA observations of the disc surrounding HL Tau reveal a very
complex dust spatial distribution. We present a radiative transfer model
accounting for the observed gaps and bright rings as well as radial changes of
the emissivity index. We find that the dust density is depleted by at least a
factor 10 in the main gaps compared to the surrounding rings. Ring masses range
from 10-100 M in dust, and, we find that each of the deepest gaps is
consistent with the removal of up to 40 M of dust. If this material
has accumulated into rocky bodies, these would be close to the point of runaway
gas accretion. Our model indicates that the outermost ring is depleted in
millimetre grains compared to the central rings. This suggests faster grain
growth in the central regions and/or radial migration of the larger grains. The
morphology of the gaps observed by ALMA - well separated and showing a high
degree of contrast with the bright rings over all azimuths - indicates that the
millimetre dust disc is geometrically thin (scale height 1 au at 100
au) and that a large amount of settling of large grains has already occurred.
Assuming a standard dust settling model, we find that the observations are
consistent with a turbulent viscosity coefficient of a few . We
estimate the gas/dust ratio in this thin layer to be of the order of 5 if the
initial ratio is 100. The HCO and CO emission is consistent with gas in
Keplerian motion around a 1.7 star at radii from au.Comment: 12 pages, 8 figures, published in ApJ, same version as before but
with slightly extended discussion on temperature and masses to account for
literature published since initial submissio
New sub-millimeter limits on dust in the 55 Cancri planetary system
We present new, high-sensitivity sub-millimeter observations towards 55
Cancri, a nearby G8 star with one, or possibly two, known planetary
companion(s). Our 850 m map, obtained with the SCUBA instrument on the
James Clerk Maxwell Telescope, shows three peaks of emission at the 2.5 mJy
level in the vicinity of the star's position. However, the observed peaks are
25\arcsec--40\arcsec away from the star and a deep -band optical image
reveals faint point sources that coincide with two of the sub-millimeter peaks.
Thus, we do not find evidence for dust emission spatially associated with 55
Cancri. The excess 60 m emission detected with ISO may originate from one
or more of the 850 m peaks that we attribute to background sources. Our
new results, together with the HST/NICMOS coronographic images in the
near-infrared, place stringent limits on the amount of dust in this planetary
system, and argue against the existence of a detectable circumstellar dust disk
around 55 Cnc.Comment: 11 pages, 2 PostScript figures, to appear in The Astrophysical
Journal Letter
Dust in the 55 Cancri planetary system
The presence of debris disks around 1-Gyr-old main sequence stars
suggests that an appreciable amount of dust may persist even in mature
planetary systems. Here we report the detection of dust emission from 55
Cancri, a star with one, or possibly two, planetary companions detected through
radial velocity measurements. Our observations at 850m and 450m imply
a dust mass of 0.0008-0.005 Earth masses, somewhat higher than that in the the
Kuiper Belt of our solar system. The estimated temperature of the dust grains
and a simple model fit both indicate a central disk hole of at least 10 AU in
radius. Thus, the region where the planets are detected is likely to be
significantly depleted of dust. Our results suggest that far-infrared and
sub-millimeter observations are powerful tools for probing the outer regions of
extrasolar planetary systems.Comment: 8 pages and 2 figures, to appear in the Astrophysical Journa
Imaging the Inner and Outer Gaps of the Pre-Transitional Disk of HD 169142 at 7 mm
We present Very Large Array observations at 7 mm that trace the thermal
emission of large dust grains in the HD 169142 protoplanetary disk. Our images
show a ring of enhanced emission of radius ~25-30 AU, whose inner region is
devoid of detectable 7 mm emission. We interpret this ring as tracing the rim
of an inner cavity or gap, possibly created by a planet or a substellar
companion. The ring appears asymmetric, with the western part significantly
brighter than the eastern one. This azimuthal asymmetry is reminiscent of the
lopsided structures that are expected to be produced as a consequence of
trapping of large dust grains. Our observations also reveal an outer annular
gap at radii from ~40 to ~70 AU. Unlike other sources, the radii of the inner
cavity, the ring, and the outer gap observed in the 7 mm images, which trace
preferentially the distribution of large (mm/cm sized) dust grains, coincide
with those obtained from a previous near-infrared polarimetric image, which
traces scattered light from small (micron- sized) dust grains. We model the
broad-band spectral energy distribution and the 7 mm images to constrain the
disk physical structure. From this modeling we infer the presence of a small
(radius ~0.6 AU) residual disk inside the central cavity, indicating that the
HD 169142 disk is a pre-transitional disk. The distribution of dust in three
annuli with gaps in between them suggests that the disk in HD 169142 is being
disrupted by at least two planets or substellar objects.Comment: Accepted by ApJ Letters, 16 pages, 3 figures, ApJ Letters 201
ALMA 1.3 Millimeter Map of the HD 95086 System
Planets and minor bodies such as asteroids, Kuiper-belt objects and comets
are integral components of a planetary system. Interactions among them leave
clues about the formation process of a planetary system. The signature of such
interactions is most prominent through observations of its debris disk at
millimeter wavelengths where emission is dominated by the population of large
grains that stay close to their parent bodies. Here we present ALMA 1.3 mm
observations of HD 95086, a young early-type star that hosts a directly imaged
giant planet b and a massive debris disk with both asteroid- and Kuiper-belt
analogs. The location of the Kuiper-belt analog is resolved for the first time.
The system can be depicted as a broad (0.84), inclined
(30\arcdeg3\arcdeg) ring with millimeter emission peaked at 2006 au
from the star. The 1.3 mm disk emission is consistent with a broad disk with
sharp boundaries from 1066 to 32020 au with a surface density
distribution described by a power law with an index of --0.50.2. Our deep
ALMA map also reveals a bright source located near the edge of the ring, whose
brightness at 1.3 mm and potential spectral energy distribution are consistent
with it being a luminous star-forming galaxy at high redshift. We set
constraints on the orbital properties of planet b assuming co-planarity with
the observed disk.Comment: accepted for publication in A
Solving grain size inconsistency between ALMA polarization and VLA continuum in the Ophiuchus IRS 48 protoplanetary disk
The protoplanetary disk around Ophiuchus IRS 48 shows an azimuthally
asymmetric dust distribution in (sub-)millimeter observations, which is
interpreted as a vortex, where millimeter/centimeter-sized particles are
trapped at the location of the continuum peak. In this paper, we present 860
m ALMA observations of polarized dust emission of this disk. The polarized
emission was detected toward a part of the disk. The polarization vectors are
parallel to the disk minor axis, and the polarization fraction was derived to
be \%. These characteristics are consistent with models of self-scattering
of submillimeter-wave emission, which indicate a maximum grain size of
m. However, this is inconsistent with the previous
interpretation of millimeter/centimeter dust particles being trapped by a
vortex. To explain both, ALMA polarization and previous ALMA and VLA
observations, we suggest that the thermal emission at 860 m wavelength is
optically thick () at the dust trap with the maximum
observable grain size of m rather than an optically thin case
with cm dust grains. We note that we cannot rule out that larger dust
grains are accumulated near the midplane if the 860 m thermal emission is
optically thick.Comment: 22 pages, 17 figures, accepted for publication in Ap
Observations of gas flows inside a protoplanetary gap
Gaseous giant planet formation is thought to occur in the first few million
years following stellar birth. Models predict that giant planet formation
carves a deep gap in the dust component (shallower in the gas). Infrared
observations of the disk around the young star HD142527, at ~140pc, found an
inner disk ~10AU in radius, surrounded by a particularly large gap, with a
disrupted outer disk beyond 140AU, indicative of a perturbing planetary-mass
body at ~90 AU. From radio observations, the bulk mass is molecular and lies in
the outer disk, whose continuum emission has a horseshoe morphology. The
vigorous stellar accretion rate would deplete the inner disk in less than a
year, so in order to sustain the observed accretion, matter must flow from the
outer-disk into the cavity and cross the gap. In dynamical models, the putative
protoplanets channel outer-disk material into gap-crossing bridges that feed
stellar accretion through the inner disk. Here we report observations with the
Atacama Large Millimetre Array (ALMA) that reveal diffuse CO gas inside the
gap, with denser HCO+ gas along gap-crossing filaments, and that confirm the
horseshoe morphology of the outer disk. The estimated flow rate of the gas is
in the range 7E-9 to 2E-7 Msun/yr, which is sufficient to maintain accretion
onto the star at the present rate
Hints for Small Disks around Very Low Mass Stars and Brown Dwarfs
The properties of disks around brown dwarfs and very low mass stars (hereafter VLMOs) provide important boundary conditions on the process of planet formation and inform us about the numbers and masses of planets than can form in this regime. We use the Herschel Space Observatory PACS spectrometer to measure the continuum and [O I] 63 μm line emission toward 11 VLMOs with known disks in the Taurus and Chamaeleon I star-forming regions. We fit radiative transfer models to the spectral energy distributions of these sources. Additionally, we carry out a grid of radiative transfer models run in a regime that connects the luminosity of our sources with brighter T Tauri stars. We find that VLMO disks with sizes 1.3-78 au, smaller than typical T Tauri disks, fit well the spectral energy distributions assuming that disk geometry and dust properties are stellar mass independent. Reducing the disk size increases the disk temperature, and we show that VLMOs do not follow previously derived disk temperature-stellar luminosity relationships if the disk outer radius scales with stellar mass. Only 2 out of 11 sources are detected in [O I] despite a better sensitivity than was achieved for T Tauri stars, suggesting that VLMO disks are underluminous. Using thermochemical models, we show that smaller disks can lead to the unexpected [O I] 63 μm nondetections in our sample. The disk outer radius is an important factor in determining the gas and dust observables. Hence, spatially resolved observations with ALMA—to establish if and how disk radii scale with stellar mass—should be pursued further. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA
The surprisingly low carbon mass in the debris disk around HD 32297
Gas has been detected in a number of debris disks. It is likely secondary,
i.e. produced by colliding solids. Here, we report ALMA Band 8 observations of
neutral carbon in the CO-rich debris disk around the 15--30 Myr old A-type star
HD 32297. We find that C is located in a ring at 110 au with a FWHM
of 80 au, and has a mass of M.
Naively, such a surprisingly small mass can be accumulated from CO
photo-dissociation in a time as short as 10 yr. We develop a simple
model for gas production and destruction in this system, properly accounting
for CO self-shielding and shielding by neutral carbon, and introducing a
removal mechanism for carbon gas. We find that the most likely scenario to
explain both C and CO observations, is one where the carbon gas is rapidly
removed on a timescale of order a thousand years and the system maintains a
very high CO production rate of 15 M Myr, much higher
than the rate of dust grind-down. We propose a possible scenario to meet these
peculiar conditions: the capture of carbon onto dust grains, followed by rapid
CO re-formation and re-release. In steady state, CO would continuously be
recycled, producing a CO-rich gas ring that shows no appreciable spreading over
time. This picture might be extended to explain other gas-rich debris disks.Comment: accepted for publication in the Ap
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