[Show abstract][Hide abstract] ABSTRACT: We explore the variability and cross-frequency correlation of the flux density and polarization of the blazar OJ287, using imaging at 43 GHz with the Very Long Baseline Array, as well as optical and near-infrared (near-IR) polarimetry. The polarization and flux density in both the optical waveband and the 43 GHz compact core increased by a small amount in late 2005, and increased significantly along with the near-IR polarization and flux density over the course of 10 days in early 2006. Furthermore, the values of the electric vector position angle (EVPA) at the three wavebands are similar. At 43 GHz, the EVPA of the blazar core is perpendicular to the flow of the jet, while the EVPAs of emerging superluminal knots are aligned parallel to the jet axis. The core polarization is that expected if shear aligns the magnetic field at the boundary between flows of disparate velocities within the jet. Using variations in flux density, percentage polarization, and EVPA, we model the inner jet as a spine-sheath system. The model jet contains a turbulent spine of half-width 12 and maximum Lorentz factor of 16.5, a turbulent sheath with Lorentz factor of 5, and a boundary region of sheared field between the spine and sheath. Transverse shocks propagating along the fast, turbulent spine can explain the superluminal knots. The observed flux density and polarization variations are then compatible with changes in the direction of the inner jet caused by a temporary change in the position of the core if the spine contains wiggles owing to an instability. In addition, we can explain a stable offset of optical and near-IR percentage polarization by a steepening of spectral index with frequency, as supported by the data.
The Astrophysical Journal 05/2009; 697(2):985. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe IRAC 3.6-8 μm observations and ground-based near-IR JHKs photometry from Mimir and 2MASS of the massive double cluster h and χ Persei complete to J = 15.5 (M ~ 1.3 M☉). Within 25' of the cluster centers we detect ~11,000 sources with J ≤ 15.5, ~7000 sources with [4.5] ≤ 15, and ~5000 sources with  ≤ 14.5. In both clusters the surface density profiles derived from the 2MASS data decline with distance from the cluster centers as expected for a bound cluster. Within 15' of the cluster centers, ~50% of the stars lie on a reddened ~13 Myr isochrone; at 15'-25' from the cluster centers, ~40% lie on this isochrone. Thus, the optical/2MASS color-magnitude diagrams indicate that h and χ Per are accompanied by a halo population with roughly the same age and distance as the two dense clusters. The double cluster lacks any clear IR excess sources for J ≤ 13.5 (~2.7 M☉). Therefore, disks around high-mass stars disperse prior to ~107 yr. At least 2%-3% of the fainter cluster stars have strong IR excess at both [5.8] and . About 4%-8% of sources slightly more massive than the Sun (~1.4 M☉) have IR excesses at . Combined with the lack of detectable excesses for brighter stars, this result suggests that disks around lower mass stars have longer lifetimes. The IR excess population also appears to be larger at longer IRAC bands ([5.8], ) than at shorter IRAC/2MASS bands (Ks, [4.5]), a result consistent with an inside-out clearing of disks.
The Astrophysical Journal 12/2008; 659(1):599. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Infrared source counts at wavelengths 3 μm < λ < 10 μm cover more than 10 mag in source brightness, reach 4 orders of magnitude in surface density, and reach an integrated surface density of 105 sources deg-2. At m < 14 mag, most of the sources are Galactic stars, in agreement with models. After removal of Galactic stars, galaxy counts are consistent with what few measurements exist at nearby wavelengths. At 3.6 and 4.5 μm, the galaxy counts follow the expectations of a Euclidean world model down to ~16 mag and drop below the Euclidean curve for fainter magnitudes. Counts at these wavelengths begin to show decreasing completeness around 19.5 mag. At 5.8 and 8 μm, the counts relative to a Euclidean world model show a large excess at bright magnitudes. This is probably because local galaxies emit strongly in the aromatic dust ("polycyclic aromatic hydrocarbon") features. The counts at 3.6 μm resolve less than 50% of the cosmic infrared background at that wavelength.
The Astrophysical Journal Supplement Series 12/2008; 154(1):39. · 14.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: New near-infrared galaxy counts in the J and K bands are presented over a total area of 0.70 and 0.97 deg2, respectively. The limiting magnitudes of the deepest regions are 19.5 in J and 18.0 in K. At J > 16 and K > 15, our J- and K-band number counts agree well with existing surveys, provided that all data are corrected to a common magnitude scale. There are real differences from field to field, and the European Large-Area ISO Survey (ELAIS) N1 and N2 fields show an overdensity of J < 16, K < 15 galaxies. The slopes of log N(m)/dm are ~0.40-0.45 at 15 < K < 18 and 16 < J < 19.5. Our counts favor galaxy models with a high normalization of the local luminosity function and without strong evolution.
The Astrophysical Journal 12/2008; 540(2):593. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Mid-Infrared Spectrometer and Imager (MIRSI) is a mid-infrared
camera system built at Boston University for ground-based observing.
MIRSI offers complete spectral coverage over the atmospheric windows at
8-14 and 18-26 μm for both imaging (discrete filters and a circular
variable filter) and spectroscopy (in the 10 and 20 μm windows with
resolutions of and 100, respectively). The optical design was optimized
for use at NASA's Infrared Telescope Facility (IRTF). MIRSI utilizes a
detector array with a plate scale of , covering a field-of-view of at
the IRTF. MIRSI's optics provide diffraction-limited spatial resolution,
and the instrument achieves 1 σ detection limits of 4 and 236 mJy
at 10 and 21 μm, respectively, in 60 s of on-source integration time.
Publications of the Astronomical Society of the Pacific 12/2008; 120:1271-1281. · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Observations of oscillations of temperature and wind in planetary atmospheres provide a means of generalizing models for atmospheric dynamics in a diverse set of planets in the Solar System and elsewhere. An equatorial oscillation similar to one in the Earth's atmosphere has been discovered in Jupiter. Here we report the existence of similar oscillations in Saturn's atmosphere, from an analysis of over two decades of spatially resolved observations of its 7.8-microm methane and 12.2-microm ethane stratospheric emissions, where we compare zonal-mean stratospheric brightness temperatures at planetographic latitudes of 3.6 degrees and 15.5 degrees in both the northern and the southern hemispheres. These results support the interpretation of vertical and meridional variability of temperatures in Saturn's stratosphere as a manifestation of a wave phenomenon similar to that on the Earth and in Jupiter. The period of this oscillation is 14.8 +/- 1.2 terrestrial years, roughly half of Saturn's year, suggesting the influence of seasonal forcing, as is the case with the Earth's semi-annual oscillation.
[Show abstract][Hide abstract] ABSTRACT: White Oval BA, constituted from 3 predecessor vortices (known as
Jupiter's "classical" White Ovals) after successive mergers in 1998 and
2000, became second-largest vortex in the atmosphere of Jupiter (and
possibly the solar system) at the time of its formation. While it
continues in this distinction,it required a name change after a 2005
December through 2006 February transformation which made it appear
visually the same color as the Great Red Spot. Our campaign to
understand the changes involved examination of the detailed color and
wind field using Hubble Space Telescope instrumentation on several
orbits in April. The field of temperatures, ammonia distribution and
clouds were also examined using the mid-infrared VISIR
camera/spectrometer on ESO's 8.2-m Very Large Telescope, the NASA
Infrared telescope with the mid-infrared MIRSI instrument and the
refurbished near-infrared facility camera NSFCam2. High-resolution
images of the Oval were made before the color change with the COMICS
mid-infrared facility on the 8.2-m Subaru telescope.We are using these
images, togther with images acquired at the IRTF and with the
Gemini/North NIRI near-infrared camera between January, 2005, and
August, 2006, to characterize the extent to which changes in storm
strength (vorticity, postive vertical motion) influenced (i) the depth
from which colored cloud particles may have been "dredged up" from depth
or (ii) the altitude to which particles may have been lofted and subject
to high-energy UV radiation which caused a color change, as alternative
explanations for the phenomenon. Clues to this will provide clues to the
chemistry of Jupiter's cloud system and its well-known colors in
general. The behavior of Oval BA, its interaction with the Great Red
Spot in particular,are also being compared with dynamical models run
with the EPIC code.
[Show abstract][Hide abstract] ABSTRACT: We present UBVRI photometry of 44 Type Ia supernovae (SNe Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe Ia to date, nearly doubling the number of well-observed, nearby SNe Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SNe Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to the B band.
The Astronomical Journal 01/2006; 131(1):527. · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The new Mimir near-infrared wide-field imager, spectrometer, and polarimeter has been used on the 1.8m Perkins Telescope outside Flagstaff, Arizona to begin a project to systematically survey a large portion of the northern Galactic plane using infrared polarimetry. In this poster, we report the results of our initial observing run, during May and June of 2005, aimed at calibrating the Mimir instrument for polarimetric work and at performing two pilot surveys. During this run, Mimir was operated in the NIR H-band (1.6 microns) with a cold, rotating zero-order half-wave plate, a wire grid analyzer, and 10x10 arcmin wide-field imaging at 0.588 arcsec per pixel. Polarimetric calibration consisted of repeated observations of all available Whittet et al. polarization standards, including a 5x5 raster grid of observations of one highly polarized source across the instrument field of view, and repeated observations of unpolarized high-latitude globular clusters, also across the instrument field of view. The Pilot surveys consisted of targeted polarimetric observations of a number of star clusters located within one degree of latitude of the Galactic plane and area surveys toward L=49.5, the tangent direction to the Sagittarius spiral arm, and L=53, a comparison region located outside the Sagittarius arm. Both regions were observed with 21 10x10 arcmin footprints of Mimir, covering approximately one-half square degree each. Integration times were sufficient to polarimetrically probe to H=12, yielding thousands of newly measured stellar polarizations designed to uncover the projected direction of the Galactic magnetic field in front of the stars.
[Show abstract][Hide abstract] ABSTRACT: The Mimir instrument completed its 5-year development in our Boston University lab and was delivered this past July to Flagstaff, Arizona and the Perkins telescope for commissioning. Mimir is a "facility-class" multi-function near-infrared imager, spectrometer, and polarimeter developed under a joint program by Boston University and Lowell Observatory scientists, staff, and engineers. It fully covers the wavelength range 1-5 microns onto its 1024x1024 Aladdin III InSb array detector. In its wide-field imaging mode, a 10x10 arcmin field is sampled at 0.6 arcsec per pixel. In its narrow-field mode, the field is 3x3 arcmin, sampled at 0.2 arcsec per pixel. A full complement of JHKsL'M' broad-band filters are present in its four filter wheels. Spectroscopy is accomplished using a matched slit-plate and selector system, three grisms, and special spectroscopy filters (for order suppression). Polarimetry is accomplished using rotating half-wave plates and a fixed wire grid. All of these modes were certified in the lab; all have been certified at the Perkins telescope during the August/September commissioning run. Mode switches are accomplished in a matter of only seconds, making Mimir exceedingly versatile. The poster highlights the designs and components of Mimir as well as examples of images, spectra, and polarimetry from the commissioning telescope runs this past fall. Internal, shared-risk observations with Mimir begin this quarter. Mimir design and development has been funded by NASA, NSF, and the W.M. Keck Foundation.
[Show abstract][Hide abstract] ABSTRACT: The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 μm. Two nearly adjacent 52 × 52 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 μm; 4.5 and 8 μm). All four detector arrays in the camera are 256 × 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.
The Astrophysical Journal Supplement Series 09/2004; 154:10-17. · 14.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present J- and K-band near-infrared (near-IR) photometry of a sample
of mid-infrared (mid-IR) sources detected by the Infrared Space
Observatory (ISO) as part of the European Large Area ISO Survey (ELAIS)
and study their classification and star-forming properties. We have used
the Preliminary ELAIS Catalogue for the 6.7-μm (LW2) and 15-μm
(LW3) fluxes. All of the high-reliability LW2 sources and 80 per cent of
the LW3 sources are identified in the near-IR survey reaching K~ 17.5
mag. The near-IR/mid-IR flux ratios can effectively be used to separate
stars from galaxies in mid-IR surveys. The stars detected in our survey
region are used to derive a new accurate calibration for the ELAIS
ISOCAM data in both the LW2 and LW3 filters. We show that near- to
mid-IR colour-colour diagrams can be used to classify galaxies further,
as well as to study star formation. The ELAIS ISOCAM survey is found
mostly to detect strongly star-forming late-type galaxies, possibly
starburst-powered galaxies, and it also picks out obscured active
galactic nuclei. The ELAIS galaxies yield an average mid-IR flux ratio
LW2/LW3 = 0.67 +/- 0.27. We discuss the fν(6.7 μ
m)/fν(15 μ m) ratio as a star formation tracer using
ISO and IRAS data of a local comparison sample. We find that the
fν(2.2 μ m)/fν(15 μ m) ratio is also a
good indicator of activity level in galaxies and conclude that the drop
in the fν(6.7 μ m)/fν(15 μ m) ratio
seen in strongly star-forming galaxies is a result of both an increase
of 15-μm emission and an apparent depletion of 6.7-μm emission.
Near-IR together with the mid-IR data make it possible to estimate the
relative amount of interstellar matter in the galaxies.
Monthly Notices of the Royal Astronomical Society 12/2002; 337(3):1043-1058. · 5.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent ISO data have allowed, for the first time, observationally based estimates for source confusion in mid-infrared surveys. We use the extragalactic source counts from ISOCAM in conjunction with K-band counts to predict the confusion resulting from galaxies in deep mid-infrared observations. We specifically concentrate on the near-future Space Infrared Telescope Facility (SIRTF) mission, and calculate expected confusion for the Infrared Array Camera (IRAC) on board SIRTF. A defining scientific goal of the IRAC instrument will be the study of high-redshift galaxies using a deep, confusion-limited wide-field survey at 3-10mum. A deep survey can reach 3-muJy sources with reasonable confidence in the shorter wavelength IRAC bands. Truly confusion-limited images with the 8mum will be difficult to obtain because of practical time constraints, unless infrared galaxies exhibit very strong evolution beyond the deepest current observations. We find L* galaxies to be detectable to z=3-3.5 at 8mum, which is slightly more pessimistic than found in 1999 by Simpson & Eisenhardt.
Monthly Notices of the Royal Astronomical Society 08/2001; 325(3):1241-1252. · 5.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Infrared Array Camera (IRAC) on NASA's Space Infrared Telescope
Facility (SIRTF) will provide imaging at 3.6, 4.6, 5.8 and 8 μm. The
four bands are imaged simultaneously in two adjacent 5 arcminute fields
with 1.2^'' pixels. The expected point source sensitivities in the four
bands are 3, 4, 17, and 24 μJy (5σ in 200 seconds) for objects
well away from the ecliptic. SIRTF's angular resolution will be
diffraction limited for λ > 6.5μm; λ/D = 0.2^'' for
λ in μm and SIRTF's 85 cm primary mirror. SIRTF Guaranteed
Time Observers (GTO's) have specified field surveys to be carried out
with IRAC with integration times per position ranging from 90 to over
10,000 seconds. I describe the plans for these surveys and their
application to the study of galaxy formation and evolution. For further
information about SIRTF, see http://sirtf.caltech.edu.
[Show abstract][Hide abstract] ABSTRACT: We present new medium-deep, wide field galaxy counts at J- and K-bands,
performed in the ELAIS fields. The source detection limits are at J=19.5
and K=18.0 magnitudes. The sky coverage is 1 degree2 making
our survey the largest to date at these wavelengths and magnitudes.
Counts in this brightness range are critical for normalization of models
when explaining the deepest galaxy counts. In our survey fields we find
evidence for a higher than expected local galaxy density. If normalized
to our counts, galaxy models do not need strong brightness evolution or
exotic populations to be explained even at the faintest levels. We
measure clearly sub-Euclidean slopes of log N(m) / dm ~ 0.45 to 0.50 at
13 < K < 17.5 and 14.5 < J < 19. Normal passively evolving
stellar populations along with a high normalization of the local
luminosity function and an open cosmological model fit best both the
slope and amplitude of our number counts. We compute the indicated
extragalactic background light in the NIR and compare it to recent
Proceedings of the International Astronomical Union 12/2000; 204:215.
[Show abstract][Hide abstract] ABSTRACT: The Infrared Array Camera (IRAC) is one of three focal plane instruments in the Space Infrared Telescope Facility (SIRTF). IRAC is a four-channel camera that obtains simultaneous images at 3.6, 4.5, 5.8, and 8 microns. Two adjacent 5.12 X 5.12 arcmin fields of view in the SIRTF focal plane are viewed by the four channels in pairs (3.6 and 5.8 microns; 4.5 and 8 microns). All four detectors arrays in the camera are 256 X 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. We describe here the results of the instrument functionality and calibration tests completed at Goddard Space Flight Center, and provide estimates of the in-flight sensitivity and performance of IRAC in SIRTF.
Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 11/2000
[Show abstract][Hide abstract] ABSTRACT: The Infrared Array Camera (IRAC) is one of three focal plane instruments in the Space Infrared Telescope Facility (SIRTF). IRAC is a four-channel camera that obtains simultaneous images at 3.6, 4.5, 5.8, and 8 microns. Two adjacent 5.125.12 arcmin fields of view in the SIRTF focal plane are viewed by the four channels in pairs (3.6 and 5.8 microns; 4.5 and 8 microns) . All four detector arrays in the camera are 256256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. We describe here the results of the instrument functionality and calibration tests completed at Goddard Space Flight Center, and provide estimates of the in-flight sensitivity and performance of IRAC in SIRTF.
[Show abstract][Hide abstract] ABSTRACT: We report the results of a sensitive near-infrared JHKL imaging survey of the Trapezium cluster in Orion. We use the JHKL colors to obtain a census of infrared excess stars in the cluster. Of (391) stars brighter than 12th magnitude in the K and L bands, 80 +/- 7% are found to exhibit detectable infrared excess on the J-H, K-L color-color diagram. Examination of a subsample of 285 of these stars with published spectral types yields a slightly higher infrared excess fraction of 85%. We find that 97% of the optical proplyds in the cluster exhibit excess in the JHKL color-color diagram indicating that the most likely origin of the observed infrared excesses is from circumstellar disks. We interpret these results to indicate that the fraction of stars in the cluster with circumstellar disks is between 80-85%. Moreover, we find that the probability of finding an infrared excess/protoplanetary disk around a star is independent of stellar mass over essentially the entire range of the stellar mass function down to the hydrogen burning limit. We identify 78 stars in our sample characterized by K-L colors suggestive of deeply embedded protostellar objects. If even a modest fraction fraction (i.e., ~ 50%) of these objects are protostars, then star formation could be continuing in the molecular ridge at a rate comparable to that which produced the foreground Trapezium cluster. Comment: 33 pages plus 3 separate color figures. For higher resolution color figures and a single file containing the entire paper, figures and tables see http://cfa-www.harvard.edu/~gmuench/thesis/clusters/TRAP/traplband.html Used AASTEX macros v 5.0. Paper will appear in December AJ
The Astronomical Journal 08/2000; · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present optical depth profiles for Saturn's rings at wavelengths of 0.9, 2.1, 3.3, and 3.9 μm, obtained from the occultation of the star 28 Sgr on 3 July 1989 when the rings were near their maximum opening angle. The radial resolution of the data is ∼18 km, as set by the angular diameter of the star. Very few, if any, structural changes are observed in the rings since the Voyager encounters in 1980/1981. Azimuthal variations in optical depth are restricted to the outermost part of the B Ring and to several previously known noncircular features. The mean optical depth in the heretofore unprobed central part of the B Ring is found to be 2.3±0.15 at 3.9 μm. Quantitative comparisons of the optical depths with those measured by the Voyager PPS stellar occultation experiment at 0.27 μm do reveal systematic differences between the major ring regions. In the C Ring, we find τ28 Sgr/τPPS≃1.4, while in the B Ring and outer A Ring τ28 Sgr/τPPS≃1.0. Only in the inner A Ring is τ28 Sgr<τPPS. In agreement with previous studies at microwave and optical wavelengths, we conclude that there is very little evidence in the 28 Sgr data for a significant population of micrometer-sized particles anywhere in the main rings. Most of the observed variation in τ28 Sgr/τPPS can instead be attributed to variations in the particle size distribution in the rings in the centimeter to meter size range, which controls the angular width of the rings' forward-scattering cross section. This in turn controls the effective extinction efficiency for Earth-based occultation experiments, and thus the measured optical depth. In a companion paper we use these data to derive power-law models of the size distribution in each major ring region.