203 research outputs found
Hierarchical fragmentation and differential star formation in the Galactic "Snake": infrared dark cloud G11.11-0.12
We present Submillimeter Array (SMA) 0.88 and 1.3 mm broad band
observations, and the Jansky Very Large Array (VLA) observations in
up to , and maser lines toward
the two most massive molecular clumps in infrared dark cloud (IRDC)
G11.11-0.12. Sensitive high-resolution images reveal hierarchical fragmentation
in dense molecular gas from the pc clump scale down to pc
condensation scale. At each scale, the mass of the fragments is orders of
magnitude larger than the Jeans mass. This is common to all four IRDC clumps we
studied, suggesting that turbulence plays an important role in the early stages
of clustered star formation. Masers, shock heated gas, and outflows
indicate intense ongoing star formation in some cores while no such signatures
are found in others. Furthermore, chemical differentiation may reflect the
difference in evolutionary stages among these star formation seeds. We find
ortho/para ratios of , , and
associated with three outflows, and the ratio tends to increase along the
outflows downstream. Our combined SMA and VLA observations of several IRDC
clumps present the most in depth view so far of the early stages prior to the
hot core phase, revealing snapshots of physical and chemical properties at
various stages along an apparent evolutionary sequence.Comment: 21 pages, 11 figures, 8 tables, accepted to MNRAS; this version
includes minor typo corrections from proo
The essential value of long-term experimental data for hydrology and water management
We would like to thank the European Research Council ERC for funding the VeWa project and most of Tetzlaff's time (project GA 335910 VeWa). No data were used in producing this manuscript.Peer reviewedPublisher PD
Spitzer Infrared Array Camera (IRAC) Pipeline: final modifications and lessons learned
In more than ten years of operations, the Spitzer Space Telescope has conducted a wide range of investigations from observing nearby asteroids to probing atmospheric properties of exoplanets to measuring masses of the most distance galaxies. Observations using the Infrared Array Camera (IRAC) at 3.6 and 4.5um will continue through mid-2019 when the James Webb Space Telescope will succeed Spitzer. In anticipation of the eventual end of the mission, the basic calibrated data reduction pipeline designed to produce flux-calibrated images has been finalized and used to reprocess all the data taken during the Spitzer warm mission. We discuss all final modifications made to the pipeline
Calibration trending in the Spitzer beyond era
The Spitzer Space Telescope currently operates in the "Beyond Era", over nine years past an original cryogenic mission. As the astronomy community continues to advance scientific boundaries and push beyond original specifications, the stability of the Infrared Array Camera (IRAC) instrument is paramount. The Instrument Team (IST) monitors the pointing accuracy, temperature, and calibration and provides the information in a timely manner to observers. The IRAC IST created a calibration trending web page, available to the general astronomy community, where the team posts updates of three most pertinent scientific stability measures of the IRAC data: calibration, bias, and bad pixels. In addition, photometry and telescope properties from all the staring observations (>1500 as of April 2018) are trended to examine correlations with changes in the age or thermal properties of the telescope. A long, well-sampled baseline established by consistent monitoring outside anomalies and space weather events allows even the smallest changes to be detected
Intra-pixel gain variations and high-precision photometry with the Infrared Array Camera (IRAC)
The Infrared Array Camera (IRAC) on the Spitzer Space Telescope has been used to measure < 10^(-4) temporal variations in point sources (such as transiting extrasolar planets) at 3.6 and 4.5 Ī¼m. Due to the under-sampled nature of the PSF, the warm IRAC arrays show variations of as much as 8% in sensitivity as the center of the PSF moves across a pixel due to normal spacecraft pointing wobble and drift. These intra-pixel gain variations are the largest source of correlated noise in IRAC photometry. Usually this effect is removed by fitting a model to the science data themselves (self-calibration), which could result in the removal of astrophysically interesting signals. We describe a new technique for significantly reducing the gain variations and improving photometric precision in a given observation, without using the data to be corrected. This comprises: (1) an adaptive centroiding and repositioning method ("Peak-Up") that uses the Spitzer Pointing Control Reference Sensor (PCRS) to repeatedly position a target to within 0.1 IRAC pixels of an area of minimal gain variation; and (2) the high-precision, high-resolution measurement of the pixel gain structure using non-variable stars. We show that the technique currently allows the reduction of correlated noise by almost an order of magnitude over raw data, which is comparable to the improvement due to self-calibration. We discuss other possible sources of correlated noise, and proposals for reducing their impact on photometric precision
Photometry using the Infrared Array Camera on the Spitzer Space Telescope
We present several corrections for point source photometry to be applied to
data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope.
These corrections are necessary because of characteristics of the IRAC arrays
and optics and the way the instrument is calibrated in-flight. When these
corrections are applied, it is possible to achieve a ~2% relative photometric
accuracy for sources of adequate signal to noise in an IRAC image.Comment: 16 pages, 13 figures. Accepted for publication in the Publications of
the Astronomical Society of the Pacifi
Spitzer/IRAC precision photometry: a machine learning approach
The largest source of noise in exoplanet and brown dwarf photometric time series made with Spitzer/IRAC is the coupling between intra-pixel gain variations and spacecraft pointing fluctuations. Observers typically correct for this systematic in science data by deriving an instrumental noise model simultaneously with the astrophysical light curve and removing the noise model. Such techniques for self-calibrating Spitzer photometric datasets have been extremely successful, and in many cases enabled near-photon-limited precision on exoplanet transit and eclipse depths. Self-calibration, however, can suffer from certain limitations: (1) temporal astrophysical signals can become aliased as part of the instrument model; (2) for some techniques adequate model estimation often requires a high degree of intra-pixel positional redundancy (multiple samples with nearby centroids) over long time spans; (3) many techniques do not account for sporadic high frequency telescope vibrations that smear out the point spread function. We have begun to build independent general-purpose intra-pixel systematics removal algorithms using three machine learning techniques: K-Nearest Neighbors (with kernel regression), Random Decision Forests, and Artificial Neural Networks. These methods remove many of the limitations of self-calibration: (1) they operate on a dedicated calibration database of approximately one million measurements per IRAC waveband (3.6 and 4.5 microns) of non-variable stars, and thus are independent of the time series science data to be corrected; (2) the database covers a large area of the "Sweet Spot, so the methods do not require positional redundancy in the science data; (3) machine learning techniques in general allow for flexibility in training with multiple, sometimes unorthodox, variables, including those that trace PSF smear. We focus in this report on the K-Nearest Neighbors with Kernel Regression technique. (Additional communications are in preparation describing Decision Forests and Neural Networks.
A Spitzer IRAC Measure of the Zodiacal Light
The dominant non-instrumental background source for spaceābased infrared observatories is the zodiacal light
(ZL). We present Spitzer Infrared Array Camera (IRAC) measurements of the ZL at 3.6, 4.5, 5.8, and 8.0 Ī¼m,
taken as part of the instrument calibrations. We measure the changing surface brightness levels in approximately
weekly IRAC observations near the north ecliptic pole over the period of roughly 8.5 years. This long time
baseline is crucial for measuring the annual sinusoidal variation in the signal levels due to the tilt of the dust disk
with respect to the ecliptic, which is the true signal of the ZL. This is compared to both Cosmic Background
Explorer Diffuse Infrared Background Experiment data and a ZL model based thereon. Our data show a few
percent discrepancy from the Kelsall et al.(1998) model including a potential warping of the interplanetary dust
disk and a previously detected overdensity in the dust cloud directly behind the Earth in its orbit. Accurate
knowledge of the ZL is important for both extragalactic and Galactic astronomy including measurements of the
cosmic infrared background, absolute measures of extended sources, and comparison to extrasolar interplanetary
dust models. IRAC data can be used to further inform and test future ZL models
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