42 research outputs found
The Neon Abundance in the Ejecta of QU Vul From Late-Epoch IR Spectra
We present ground-based SpectroCam-10 mid-infrared, MMT optical, and Spitzer
Space Telescope IRS mid-infrared spectra taken 7.62, 18.75, and 19.38 years
respectively after the outburst of the old classical nova QU Vulpeculae (Nova
Vul 1984 #2). The spectra of the ejecta are dominated by forbidden line
emission from neon and oxygen. Our analysis shows that neon was, at the first
and last epochs respectively, more than 76 and 168 times overabundant by number
with respect to hydrogen compared to the solar value. These high lower limits
to the neon abundance confirm that QU Vul involved a thermonuclear runaway on
an ONeMg white dwarf and approach the yields predicted by models of the
nucleosynthesis in such events.Comment: ApJ 2007 accepted, 18 pages, including 5 figures, 1 tabl
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
Five Years of Mid-Infrared Evolution of the Remnant of SN 1987A: The Encounter Between the Blast Wave and the Dusty Equatorial Ring
We have used the Spitzer satellite to monitor the mid-IR evolution of SN
1987A over a 5 year period spanning the epochs between days 6000 and 8000 since
the explosion. The supernova (SN) has evolved into a supernova remnant (SNR)
and its radiative output is dominated by the interaction of the SN blast wave
with the pre-existing equatorial ring (ER). The mid-IR spectrum is dominated by
emission from ~180 K silicate dust, collisionally-heated by the hot X-ray
emitting gas with a temperature and density of ~5x10^6 K and 3x10^4 cm-3,
respectively. The mass of the radiating dust is ~1.2x10^(-6) Msun on day 7554,
and scales linearly with IR flux. The infrared to soft-X-ray flux ratio is
roughly constant with a value of 2.5. Gas-grain collisions therefore dominate
the cooling of the shocked gas. The constancy of of this ratio suggests that
very little grain processing or gas cooling have occurred throughout this
epoch. The shape of the dust spectrum remained unchanged during the
observations while the total flux increased with a time dependence of t^(0.87),
t being the time since the first encounter between the blast wave and the ER.
These observations are consistent with the transitioning of the blast wave from
free expansion to a Sedov phase as it propagates into the main body of the ER.Comment: Accepted for publication in the ApJ, 11 pages, 11 figure
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 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
Spitzer IRAC Observations of Star Formation in N159 in the LMC
We present observations of the giant HII region complex N159 in the LMC using
IRAC on the {\it Spitzer Space Telescope}. One of the two objects previously
identified as protostars in N159 has an SED consistent with classification as a
Class I young stellar object (YSO) and the other is probably a Class I YSO as
well, making these two stars the youngest stars known outside the Milky Way. We
identify two other sources that may also be Class I YSOs. One component,
N159AN, is completely hidden at optical wavelengths, but is very prominent in
the infrared. The integrated luminosity of the entire complex is L L, consistent with the observed radio emission assuming a
normal Galactic initial mass function (IMF). There is no evidence for a red
supergiant population indicative of an older burst of star formation. The N159
complex is 50 pc in diameter, larger in physical size than typical HII regions
in the Milky Way with comparable luminosity. We argue that all of the
individual components are related in their star formation history. The
morphology of the region is consistent with a wind blown bubble $\approx
1-2Myr-old that has initiated star formation now taking place at the rim. Other
than its large physical size, star formation in N159 appears to be
indistinguishable from star formation in the Milky Way.Comment: 14 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
Keck spectroscopy and Spitzer Space Telescope analysis of the outer disk of the Triangulum Spiral Galaxy M33
In an earlier study of the spiral galaxy M33, we photometrically identified
arcs or outer spiral arms of intermediate age (0.6 Gyr - 2 Gyr) carbon stars
precisely at the commencement of the HI-warp. Stars in the arcs were
unresolved, but were likely thermally-pulsing asymptotic giant branch carbon
stars. Here we present Keck I spectroscopy of seven intrinsically bright and
red target stars in the outer, northern arc in M33. The target stars have
estimated visual magnitudes as faint as V \sim 25 mag. Absorption bands of CN
are seen in all seven spectra reported here, confirming their carbon star
status. In addition, we present Keck II spectra of a small area 0.5 degree away
from the centre of M33; the target stars there are also identified as carbon
stars. We also study the non-stellar PAH dust morphology of M33 secured using
IRAC on board the Spitzer Space Telescope. The Spitzer 8 micron image attests
to a change of spiral phase at the start of the HI warp. The Keck spectra
confirm that carbon stars may safely be identified on the basis of their red
J-K_s colours in the outer, low metallicity disk of M33. We propose that the
enhanced number of carbon stars in the outer arms are an indicator of recent
star formation, fueled by gas accretion from the HI-warp reservoir.Comment: 9 pages, 5 figures, accepted in A&