275 research outputs found
Adaptive Real Time Imaging Synthesis Telescopes
The digital revolution is transforming astronomy from a data-starved to a
data-submerged science. Instruments such as the Atacama Large Millimeter Array
(ALMA), the Large Synoptic Survey Telescope (LSST), and the Square Kilometer
Array (SKA) will measure their accumulated data in petabytes. The capacity to
produce enormous volumes of data must be matched with the computing power to
process that data and produce meaningful results. In addition to handling huge
data rates, we need adaptive calibration and beamforming to handle atmospheric
fluctuations and radio frequency interference, and to provide a user
environment which makes the full power of large telescope arrays accessible to
both expert and non-expert users. Delayed calibration and analysis limit the
science which can be done. To make the best use of both telescope and human
resources we must reduce the burden of data reduction.
Our instrumentation comprises of a flexible correlator, beam former and
imager with digital signal processing closely coupled with a computing cluster.
This instrumentation will be highly accessible to scientists, engineers, and
students for research and development of real-time processing algorithms, and
will tap into the pool of talented and innovative students and visiting
scientists from engineering, computing, and astronomy backgrounds.
Adaptive real-time imaging will transform radio astronomy by providing
real-time feedback to observers. Calibration of the data is made in close to
real time using a model of the sky brightness distribution. The derived
calibration parameters are fed back into the imagers and beam formers. The
regions imaged are used to update and improve the a-priori model, which becomes
the final calibrated image by the time the observations are complete
A Keplerian disk around Orion Source I, a ~15 Msun YSO
We report ALMA long-baseline observations of Orion Source I (SrcI) with
resolution 0.03-0.06" (12-24 AU) at 1.3 and 3.2 mm. We detect both continuum
and spectral line emission from SrcI's disk. We also detect a central weakly
resolved source that we interpret as a hot spot in the inner disk, which may
indicate the presence of a binary system. The high angular resolution and
sensitivity of these observations allow us to measure the outer envelope of the
rotation curve of the HO line, which gives a mass
Msun. We detected several other lines that more closely
trace the disk, but were unable to identify their parent species. Using
centroid-of-channel methods on these other lines, we infer a similar mass.
These measurements solidify SrcI as a genuine high-mass protostar system and
support the theory that SrcI and the Becklin Neugebauer Object were ejected
from the dynamical decay of a multiple star system 500 years ago, an
event that also launched the explosive molecular outflow in Orion.Comment: Accepted to ApJ. Data at https://zenodo.org/record/1213350, source
repository at https://github.com/keflavich/Orion_ALMA_2016.1.00165.
Orion Source I's disk is salty
We report the detection of NaCl, KCl, and their Cl and K
isotopologues toward the disk around Orion SrcI. About 60 transitions of these
molecules were identified. This is the first detection of these molecules in
the interstellar medium not associated with the ejecta of evolved stars. It is
also the first ever detection of the vibrationally excited states of these
lines in the ISM above v = 1, with firm detections up to v = 6. The salt
emission traces the region just above the continuum disk, possibly forming the
base of the outflow. The emission from the vibrationally excited transitions is
inconsistent with a single temperature, implying the lines are not in LTE. We
examine several possible explanations of the observed high excitation lines,
concluding that the vibrational states are most likely to be radiatively
excited via rovibrational transitions in the 25-35 {\mu}m (NaCl) and 35-45
{\mu}m (KCl) range. We suggest that the molecules are produced by destruction
of dust particles. Because these molecules are so rare, they are potentially
unique tools for identifying high-mass protostellar disks and measuring the
radiation environment around accreting young stars.Comment: Accepted to ApJ. Analysis code at
https://github.com/keflavich/Orion_ALMA_2016.1.00165.S, paper source at
https://github.com/keflavich/SaltyDisk, and data at
https://zenodo.org/record/121335
Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow
We report the discovery of variability in the linear polarization from the
Galactic Center black hole source, Sagittarius A*. New polarimetry obtained
with the Berkeley-Illinois-Maryland Association array at a wavelength of 1.3 mm
shows a position angle that differs by 28 +/- 5 degrees from observations 6
months prior and then remains stable for 15 months. This difference may be due
to a change in the source emission region on a scale of 10 Schwarzschild radii
or due to a change of 3 x 10^5 rad m^-2 in the rotation measure. We consider a
change in the source physics unlikely, however, since we see no corresponding
change in the total intensity or polarized intensity fraction. On the other
hand, turbulence in the accretion region at a radius ~ 10 to 1000 R_s could
readily account for the magnitude and time scale of the position angle change.Comment: accepted for publication in ApJ
The Rotation Measure and 3.5mm Polarization of Sgr A*
We report the detection of variable linear polarization from Sgr A* at a
wavelength of 3.5mm, the longest wavelength yet at which a detection has been
made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 deg
with rms scatters of 0.4% and 9 deg over the five epochs. We also detect
polarization variability on a timescale of days. Combined with previous
detections over the range 150-400GHz (750-2000 microns), the average
polarization position angles are all found to be consistent with a rotation
measure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotation
occurs external to the polarized source at all wavelengths. This implies an
accretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profiles
expected of ADAF, jet and CDAF models and assuming that the region at which
electrons in the accretion flow become relativistic is within 10 R_S. The
inferred accretion rate is inconsistent with ADAF/Bondi accretion. The
stability of the mean polarization position angle between disparate
polarization observations over the frequency range limits fluctuations in the
accretion rate to less than 5%. The flat frequency dependence of the inter-day
polarization position angle variations also makes them difficult to attribute
to rotation measure fluctuations, and suggests that both the magnitude and
position angle variations are intrinsic to the emission.Comment: Ap.J.Lett. accepte
The EGNoG Survey: Gas Excitation in Normal Galaxies at z~0.3
As observations of molecular gas in galaxies are pushed to lower star
formation rate galaxies at higher redshifts, it is becoming increasingly
important to understand the conditions of the gas in these systems to properly
infer their molecular gas content. The rotational transitions of the carbon
monoxide (CO) molecule provide an excellent probe of the gas excitation
conditions in these galaxies. In this paper we present the results from the gas
excitation sample of the Evolution of molecular Gas in Normal Galaxies (EGNoG)
survey at the Combined Array for Research in Millimeter-wave Astronomy (CARMA).
This subset of the full EGNoG sample consists of four galaxies at z~0.3 with
star formation rates of 40-65 M_Sun yr^-1 and stellar masses of ~2x10^11 M_Sun.
Using the 3 mm and 1 mm bands at CARMA, we observe both the CO(1-0) and CO(3-2)
transitions in these four galaxies in order to probe the excitation of the
molecular gas. We report robust detections of both lines in three galaxies (and
an upper limit on the fourth), with an average line ratio, r_31 = L'_CO(3-2) /
L'_CO(1-0), of 0.46 \pm 0.07 (with systematic errors \lesssim 40%), which
implies sub-thermal excitation of the CO(3-2) line. We conclude that the
excitation of the gas in these massive, highly star-forming galaxies is
consistent with normal star-forming galaxies such as local spirals, not
starbursting systems like local ultra-luminous infrared galaxies. Since the
EGNoG gas excitation sample galaxies are selected from the main sequence of
star-forming galaxies, we suggest that this result is applicable to studies of
main sequence galaxies at intermediate and high redshifts, supporting the
assumptions made in studies that find molecular gas fractions in star forming
galaxies at z~1-2 to be an order of magnitude larger than what is observed
locally.Comment: Accepted for publication in the Astrophysical Journal, to appear
January 2013; 18 pages, 10 figures, 6 table
Sub-arcsec Observations of NGC 7538 IRS 1: Continuum Distribution and Dynamics of Molecular Gas
We report new results based on the analysis of the SMA and CARMA observations
of NGC 7538\,IRS\,1 at 1.3 and 3.4 mm with sub-arcsec resolutions. With angular
resolutions 0\farcs7, the SMA and CARMA observations show that the
continuum emission at 1.3 and 3.4 mm from the hyper-compact \ion{H}{2} region
IRS\,1 is dominated by a compact source with a tail-like extended structure to
the southwest of IRS\,1. With a CARMA B-array image at 1.3 mm convolved to
0\farcs1, we resolve the hyper-compact \ion{H}{2} region into two components:
an unresolved hyper-compact core, and a north-south extension with linear sizes
of AU and 2000 AU, respectively. The fine structure observed with
CARMA is in good agreement with the previous VLA results at centimeter
wavelengths, suggesting that the hyper-compact \ion{H}{2} region at the center
of IRS\,1 is associated with an ionized bipolar outflow. We image the molecular
lines OCS(19-18) and CHCN(12-11) as well as CO(2-1) surrounding
IRS\,1, showing a velocity gradient along the southwest-northeast direction.
The spectral line profiles in CO(2-1), CO(2-1), and HCN(1-0) observed
toward IRS\,1 show broad redshifted absorption, providing evidence for gas
infall with rates in the range of M yr
inferred from our observations.Comment: 19 pages, 14 figure
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