63 research outputs found
The M33 Variable Star Population Revealed by Spitzer
We analyze five epochs of Spitzer Space Telescope/Infrared Array Camera
(IRAC) observations of the nearby spiral galaxy M33. Each epoch covered nearly
a square degree at 3.6, 4.5, and 8.0 microns. The point source catalog from the
full dataset contains 37,650 stars. The stars have luminosities characteristic
of the asymptotic giant branch and can be separated into oxygen-rich and
carbon-rich populations by their [3.6] - [4.5] colors. The [3.6] - [8.0] colors
indicate that over 80% of the stars detected at 8.0 microns have dust shells.
Photometric comparison of epochs using conservative criteria yields a catalog
of 2,923 variable stars. These variables are most likely long-period variables
amidst an evolved stellar population. At least one-third of the identified
carbon stars are variable.Comment: Accepted for publication in ApJ. See published article for full
resolution figures and electronic table
Spitzer Far-Infrared Detections of Cold Circumstellar Disks
Observations at 70 microns with the Spitzer Space Telescope have detected
several stellar systems within 65 pc of the Sun. Of 18 presumably young systems
detected in this study, as many as 15 have 70-micron emission in excess of that
expected from their stellar photospheres. Five of the systems with excesses are
members of the Tucanae Association. The 70-micron excesses range from a factor
of ~2 to nearly 30 times the expected photospheric emission from these stars.
In contrast to the 70-micron properties of these systems, there is evidence for
an emission excess at 24 microns for only HD 3003, confirming previous results
for this star. The lack of a strong 24-micron excess in most of these systems
suggests that the circumstellar dust producing the IR excesses is relatively
cool (T_dust < 150 K) and that there is little IR-emitting material within the
inner few AU of the primary stars. Many of these systems lie close enough to
Earth that the distribution of the dust producing the IR excesses might be
imaged in scattered light at optical and near-IR wavelengths.Comment: Accepted for publication in The Astrophysical Journal Letters; 5
pages, 2 tables, 2 figure
Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula
We present 3.6, 4.5, 5.8, 8.0, 24, and 70 micron images of the Crab Nebula
obtained with the Spitzer Space Telescope IRAC and MIPS cameras, Low- and
High-resolution Spitzer IRS spectra of selected positions within the nebula,
and a near-infrared ground-based image made in the light of [Fe II]1.644
micron. The 8.0 micron image, made with a bandpass that includes [Ar II]7.0
micron, resembles the general morphology of visible H-alpha and near-IR [Fe II]
line emission, while the 3.6 and 4.5 micron images are dominated by continuum
synchrotron emission. The 24 micron and 70 micron images show enhanced emission
that may be due to line emission or the presence of a small amount of warm dust
in the nebula on the order of less than 1% of a solar mass. The ratio of the
3.6 and 4.5 micron images reveals a spatial variation in the synchrotron power
law index ranging from approximately 0.3 to 0.8 across the nebula. Combining
this information with optical and X-ray synchrotron images, we derive a
broadband spectrum that reflects the superposition of the flatter spectrum jet
and torus with the steeper diffuse nebula, and suggestions of the expected
pileup of relativistic electrons just before the exponential cutoff in the
X-ray. The pulsar, and the associated equatorial toroid and polar jet
structures seen in Chandra and HST images (Hester et al. 2002) can be
identified in all of the IRAC images. We present the IR photometry of the
pulsar. The forbidden lines identified in the high resolution IR spectra are
all double due to Doppler shifts from the front and back of the expanding
nebula and give an expansion velocity of approximately 1264 km/s.Comment: 21 pages, 4 tables, 16 figure
A Spitzer Study of Comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT)
We present infrared images and spectra of comets 2P/Encke,
67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT) as part of a larger
program to observe comets inside of 5 AU from the sun with the Spitzer Space
Telescope. The nucleus of comet 2P/Encke was observed at two vastly different
phase angles (20 degrees and 63 degrees). Model fits to the spectral energy
distributions of the nucleus suggest comet Encke's infrared beaming parameter
derived from the near-Earth asteroid thermal model may have a phase angle
dependence. The observed emission from comet Encke's dust coma is best-modeled
using predominately amorphous carbon grains with a grain size distribution that
peaks near 0.4 microns, and the silicate contribution by mass to the sub-micron
dust coma is constrained to 31%. Comet 67P/Churyumov-Gerasimenko was observed
with distinct coma emission in excess of a model nucleus at a heliocentric
distance of 5.0 AU. The coma detection suggests that sublimation processes are
still active or grains from recent activity remain near the nucleus. Comet
C/2001 HT50 (LINEAR-NEAT) showed evidence for crystalline silicates in the
spectrum obtained at 3.2 AU and we derive a silicate-to-carbon dust ratio of
0.6. The ratio is an order of magnitude lower than that derived for comets
9P/Tempel 1 during the Deep Impact encounter and C/1995 O1 (Hale-Bopp).Comment: Accepted for publication in the Astrophysical Journal 48 pages, 15
figures, 10 table
Infrared and X-Ray Evidence for Circumstellar Grain Destruction by the Blast Wave of Supernova 1987A
Multiwavelength observations of supernova remnant (SNR) 1987A show that its morphology and luminosity are rapidly changing at X-ray, optical, infrared, and radio wavelengths as the blast wave from the explosion expands into the circumstellar equatorial ring, produced by mass loss from the progenitor star. The observed infrared (IR) radiation arises from the interaction of dust grains that formed in mass outflow with the soft X-ray emitting plasma component of the shocked gas. Spitzer IRS spectra at 5 - 30 microns taken on day 6190 since the explosion show that the emission arises from approx. 1.1 x 10(exp -6) solar mass of silicate grains radiating at a temperature of approx. 180+/-(15-20) K. Subsequent observations on day 7137 show that the IR flux had increased by a factor of 2 while maintaining an almost identical spectral shape. The observed IR-to-X-ray flux ratio (IRX) is consistent with that of a dusty plasma with standard LMC dust abundances. This flux ratio has decreased by a factor of approx. 2 between days 6190 and 7137, providing the first direct observation of the ongoing destruction of dust in an expanding SN blast wave on dynamic time scales. Detailed models consistent with the observed dust temperature, the ionization fluence of the soft X-ray emission component, and the evolution of IRX suggest that the radiating si1icate grains are immersed in a 3.5 x 10(exp 6) K plasma with a density of (0.3 - 1) x 10(exp 4)/cu cm, and have a size distribution that is confined to a narrow range of radii between 0.02 and 0.2 microns. Smaller grains may have been evaporated by the initial UV flash from the supernova
A Spitzer/IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. I. WLM
We present Spitzer/IRAC observations at 3.6 and 4.5 microns along with
optical data from the Local Group Galaxies Survey to investigate the evolved
stellar population of the Local Group dwarf irregular galaxy WLM. These
observations provide a nearly complete census of the asymptotic giant branch
(AGB) stars. We find 39% of the infrared-detected AGB stars are not detected in
the optical data, even though our 50% completeness limit is three magnitudes
fainter than the red giant branch tip. An additional 4% of the
infrared-detected AGBs are misidentified in the optical, presumably due to
reddening by circumstellar dust. We also compare our results with those of a
narrow-band optical carbon star survey of WLM, and find the latter study
sensitive to only 18% of the total AGB population. We detect objects with
infrared fluxes consistent with them being mass-losing AGB stars, and derive a
present day total mass-loss rate from the AGB stars of 0.7-2.4 x 10^(-3) solar
masses per year. The distribution of mass-loss rates and bolometric
luminosities of AGBs and red supergiants are very similar to those in the LMC
and SMC and the empirical maximum mass-loss rate observed in the LMC and SMC is
in excellent agreement with our WLM data.Comment: Accepted by ApJ, 34 pages, 13 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
High-resolution polarimetry of Parsamian 21: revealing the structure of an edge-on FU Ori disc
We present the first high spatial resolution near-infrared direct and
polarimetric observations of Parsamian 21, obtained with the VLT/NACO
instrument. We complemented these measurements with archival infrared
observations, such as HST/WFPC2 imaging, HST/NICMOS polarimetry, Spitzer IRAC
and MIPS photometry, Spitzer IRS spectroscopy as well as ISO photometry. Our
main conclusions are the following: (1) we argue that Parsamian 21 is probably
an FU Orionis-type object; (2) Parsamian 21 is not associated with any rich
cluster of young stars; (3) our measurements reveal a circumstellar envelope, a
polar cavity and an edge-on disc; the disc seems to be geometrically flat and
extends from approximately 48 to 360 AU from the star; (4) the SED can be
reproduced with a simple model of a circumstellar disc and an envelope; (5)
within the framework of an evolutionary sequence of FUors proposed by Green et
al. (2006) and Quanz et al. (2007), Parsamian 21 can be classified as an
intermediate-aged object.Comment: Accepted for publication in the MNRAS. 16 pages, 18 figures and 5
table
The Spitzer IRS view of V4334 Sgr (Sakurai's Object)
We present an observation of the very late thermal pulse object V4334 Sgr
(Sakurai's Object) with the Infrared Spectrometer (IRS) on the Spitzer Space
Telescope. The emission from 5-38 microns is dominated by the still-cooling
dust shell. A number of features are seen in absorption against the dust shell,
which we attribute to HCN and polyyne molecules. We use these features to
determine the 12C/13C ratio for the absorbing gas to be ~ 3.2 (+3.2,-1.6}; this
implies that, despite the H-content of the molecules, the hydrocarbon-bearing
gas must have originated in material produced in the very late thermal pulse.
We see no evidence of emission lines, despite the recently-reported optical and
radio observations that suggest the effective temperature of the stellar
remnant is rising.Comment: 5 pages, 6 figures. Accepted for publication in MNRA
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