138 research outputs found
GRAIN PROPERTIES OF COMET C/1995 O1 (HALE-BOPP)
We present the analysis of 7.6-13.2 μm infrared (IR) spectrophotometry (R 250) of comet C/1995 O1 (Hale-Bopp), in conjunction with concurrent observations that extend the wavelength coverage of the spectral energy distribution from near- to far-infrared wavelengths. The observations include temporal epochs preperihelion (1996 October and 1997 February UT), near perihelion (1997 April UT), and postperihelion (1997 June UT). Through the modeling of the thermal emission from small, amorphous carbon grains and crystalline and amorphous silicate grains in Hale-Bopp's coma, we find that as the comet approached perihelion, the grain size distribution (the Hanner modified power law) steepened (from N = 3.4 preperihelion to N = 3.7 near and postperihelion), along with an increase in the fractal porosity of larger (greater than 1 μm) grains. The peak of the grain size distribution remained constant (ap = 0.2 μm) at each epoch. We attribute the emergence of the 9.3 μm peak near perihelion to crystalline orthopyroxene grains released during epochs of high jet activity. Crystalline silicates (olivine and orthopyroxene) make up about 30% (by mass) of the submicron-sized (≤1 μm) dust grains in Hale-Bopp's coma during each epoch
Spitzer Observations of Comet 67P/Churyumov-Gerasimenko at 5.5-4.3 AU From the Sun
We report Spitzer Space Telescope observations of comet
67P/Churyumov-Gerasimenko at 5.5 and 4.3 AU from the Sun, post-aphelion. Comet
67P is the primary target of the European Space Agency's Rosetta mission. The
Rosetta spacecraft will rendezvous with the nucleus at heliocentric distances
similar to our observations. Rotationally resolved observations at 8 and 24
microns (at a heliocentric distance, rh, of 4.8 AU) that sample the size and
color-temperature of the nucleus are combined with aphelion R-band light curves
observed at the Very Large Telescope (VLT) and yield a mean effective radius of
2.04 +/- 0.11 km, and an R-band geometric albedo of 0.054 +/- 0.006. The
amplitudes of the R-band and mid-infrared light curves agree, which suggests
that the variability is dominated by the shape of the nucleus. We also detect
the dust trail of the comet at 4.8 and 5.5 AU, constrain the grain sizes to be
less than or similar to 6 mm, and estimate the impact hazard to Rosetta. We
find no evidence for recently ejected dust in our images. If the activity of
67P is consistent from orbit to orbit, then we may expect the Rosetta
spacecraft will return images of an inactive or weakly active nucleus as it
rendezvous with the comet at rh = 4 AU in 2014.Comment: 19 pages, 2 tables, 10 figures. Accepted for publication in the
Astronomical Journa
Absorption Efficiencies of Forsterite. I: DDA Explorations in Grain Shape and Size
We compute the absorption efficiency (Qabs) of forsterite using the discrete
dipole approximation (DDA) in order to identify and describe what
characteristics of crystal grain shape and size are important to the shape,
peak location, and relative strength of spectral features in the 8-40 {\mu}m
wavelength range. Using the DDSCAT code, we compute Qabs for non-spherical
polyhedral grain shapes with a_eff = 0.1 {\mu}m. The shape characteristics
identified are: 1) elongation/reduction along one of three crystallographic
axes; 2) asymmetry, such that all three crystallographic axes are of different
lengths; and 3) the presence of crystalline faces that are not parallel to a
specific crystallographic axis, e.g., non-rectangular prisms and (di)pyramids.
Elongation/reduction dominates the locations and shapes of spectral features
near 10, 11, 16, 23.5, 27, and 33.5 {\mu}m, while asymmetry and tips are
secondary shape effects. Increasing grain sizes (0.1-1.0 {\mu}m) shifts the 10,
11 {\mu}m features systematically towards longer wavelengths and relative to
the 11 {\mu}m feature increases the strengths and slightly broadens the longer
wavelength features. Seven spectral shape classes are established for
crystallographic a-, b-, and c-axes and include columnar and platelet shapes
plus non-elongated or equant grain shapes. The spectral shape classes and the
effects of grain size have practical application in identifying or excluding
columnar, platelet or equant forsterite grain shapes in astrophysical environs.
Identification of the shape characteristics of forsterite from 8-40 {\mu}m
spectra provides a potential means to probe the temperatures at which
forsterite formed.Comment: 55 pages, 15 figure
Comet Dust After Deep Impact
When the Deep Impact Mission hit Jupiter Family comet 9P/Tempel 1, an ejecta crater was formed and an pocket of volatile gases and ices from 10-30 m below the surface was exposed (A Hearn et aI. 2005). This resulted in a gas geyser that persisted for a few hours (Sugita et al, 2005). The gas geyser pushed dust grains into the coma (Sugita et a1. 2005), as well as ice grains (Schulz et al. 2006). The smaller of the dust grains were submicron in radii (0-25.3 micron), and were primarily composed of highly refractory minerals including amorphous (non-graphitic) carbon, and silicate minerals including amorphous (disordered) olivine (Fe,Mg)2SiO4 and pyroxene (Fe,Mg)SiO3 and crystalline Mg-rich olivine. The smaller grains moved faster, as expected from the size-dependent velocity law produced by gas-drag on grains. The mineralogy evolved with time: progressively larger grains persisted in the near nuclear region, having been imparted with slower velocities, and the mineralogies of these larger grains appeared simpler and without crystals. The smaller 0.2-0.3 micron grains reached the coma in about 1.5 hours (1 arc sec = 740 km), were more diverse in mineralogy than the larger grains and contained crystals, and appeared to travel through the coma together. No smaller grains appeared at larger coma distances later (with slower velocities), implying that if grain fragmentation occurred, it happened within the gas acceleration zone. These results of the high spatial resolution spectroscopy (GEMINI+Michelle: Harker et 4. 2005, 2006; Subaru+COMICS: Sugita et al. 2005) revealed that the grains released from the interior were different from the nominally active areas of this comet by their: (a) crystalline content, (b) smaller size, (c) more diverse mineralogy. The temporal changes in the spectra, recorded by GEMIM+Michelle every 7 minutes, indicated that the dust mineralogy is inhomogeneous and, unexpectedly, the portion of the size distribution dominated by smaller grains has a more diverse mineralogy. The lower spatial resolution, high sensitivity Spitzer IRS data reveal resonances of refractory minerals (those seen by GEMINI+Michelle plus ortho-pyroxene)) as well resonances that can be attributed to phillosilicates (layer lattice silicates such as Montmorillonite) (Lisse et al. 2006). Pre- and post-impact, micron to submicron grains were deciphered to be present in the coma by the modeling the high spatial resolution images to account for nucleus plus inner coma fluxes (Wooden et al. 2005, 2006; Harker et al. 2005, 2006a). Note also that crystalline silicates were released from the interior of 73P-B/SW-3 as it disintegrated (Harker et al. 2006b). From the Deep Impact and the disintegration of 73P-B, we are led to ask the questians: Why is the mineralogy of the dust released from a volatile-rich pocket beneath the surface different from the dust that is released from the nominally active areas? Could the most volatile pockets be exhausted quickly? Why would crystalline silicates be associated with more volatile materials? Perhaps the structure of the comet is so inhomogeneous, e.g., the layered pile mode2 of the nucleus (Belton et al. 2006), that a reservoir of crystalline silicate and submicron grains just happens to not be released by the nominally active areas of comet 9P? Perhaps comets lose matter through their mantles from below their surfaces, thus preserving ancient topographic structures and radiation damaged silicates and carbon? We will discuss and ponder different scenarios. We will discuss future directions for coordinated observations of JF comets
Are There Hidden Supernovae?
Ames Research Center and UCSC have been working on the development of a Mid IR Camera for the KAO in order to search for extra galactic supernovae. The development of the camera and its associated data reduction software have been successfully completed. Spectral Imaging of the Orion Bar at 6.2 and 7.8 microns demonstrates the derotation and data reduction software which was developed
Mid-Infrared Spectrophotometric Observations of Fragments B and C of Comet 73P/Schwassmann-Wachmann 3
We present mid-infrared spectra and images from the GEMINI-N (+Michelle)
observations of fragments SW3-[B] and SW3-[C] of the ecliptic (Jupiter Family)
comet 73P/Schwassmann-Wachmann 3 pre-perihelion. We observed fragment B soon
after an outburst event (between 2006 April 16 - 26 UT) and detected
crystalline silicates. The mineralogy of both fragments was dominated by
amorphous carbon and amorphous pyroxene. The grain size distribution (assuming
a Hanner modified power-law) for fragment SW3-[B] has a peak grain radius of
a_p ~ 0.5 micron, and for fragment SW3-[C], a_p ~ 0.3 micron; both values
larger than the peak grain radius of the size distribution for the dust ejected
from ecliptic comet 9P/Tempel 1 during the Deep Impact event (a_p = 0.2 micron.
The silicate-to-carbon ratio and the silicate crystalline mass fraction for the
submicron to micron-size portion of the grain size distribution on the nucleus
of fragment SW3-[B] was 1.341 +0.250 -0.253 and 0.335 +0.089 -0.112,
respectively, while on the nucleus of fragment SW3-[C] was 0.671 +0.076 -0.076
and 0.257 +0.039 -0.043, respectively. The similarity in mineralogy and grain
properties between the two fragments implies that 73P/Schwassmann-Wachmann 3 is
homogeneous in composition. The slight differences in grain size distribution
and silicate-to-carbon ratio between the two fragments likely arises because
SW3-[B] was actively fragmenting throughout its passage while the activity in
SW3-[C] was primarily driven by jets. The lack of diverse mineralogy in the
fragments SW3-[B] and SW3-[C] of 73P/Schwassmann-Wachmann 3 along with the
relatively larger peak in the coma grain size distribution suggests the parent
body of this comet may have formed in a region of the solar nebula with
different environmental properties than the natal sites where comet C/1995 O1
(Hale-Bopp) and 9P/Tempel 1 nuclei aggregated.Comment: 31 pages, 5 figure, accepted for publication in A
Dust in Comet C/2007 N3 (Lulin)
We report optical imaging, optical and near-infrared polarimetry, and Spitzer
mid-infrared spectroscopy of comet C/2007 N3 (Lulin). Polarimetric observations
were obtained in R (0.676 micron) at phase angles from 0.44 degrees to 21
degrees with simultaneous observations in H (1.65 micron) at 4.0 degrees,
exploring the negative branch in polarization. Comet C/2007 N3 (Lulin) shows
typical negative polarization in the optical as well as a similar negative
branch near-infrared wavelengths. The 10 micron silicate feature is only weakly
in emission and according to our thermal models, is consistent with emission
from a mixture of silicate and carbon material. We argue that large,
low-porosity (akin to Ballistic Particle Cluster Aggregates) rather absorbing
aggregate dust particles best explain both the polarimetric and the
mid-infrared spectral energy distribution.Comment: 18 pages, 9 figures, 3 table
Crystalline Silicate Emission in the Protostellar Binary Serpens--SVS20
We present spatially resolved mid-infrared spectroscopy of the class
I/flat-spectrum protostellar binary system SVS20 in the Serpens cloud core. The
spectra were obtained with the mid-infrared instrument T-ReCS on Gemini-South.
SVS20-South, the more luminous of the two sources, exhibits a mid-infrared
emission spectrum peaking near 11.3 \micron, while SVS20-North exhibits a
shallow amorphous silicate absorption spectrum with a peak optical depth of
. After removal of the the line-of-sight extinction by the
molecular common envelope, the ``protostar-only'' spectra are found to be
dominated by strong amorphous olivine emission peaking near 10 \micron. We also
find evidence for emission from crystalline forsterite and enstatite associated
with both SVS20-S and SVS20-N. The presence of crystalline silicate in such a
young binary system indicates that the grain processing found in more evolved
HAeBe and T Tauri pre-main sequence stars likely begins at a relatively young
evolutionary stage, while mass accretion is still ongoing.Comment: Accepted for publication by The Astrophysical Journa
Constraining Cometary Crystal Shapes from IR Spectral Features
A major challenge in deriving the silicate mineralogy of comets is ascertaining how the anisotropic nature of forsterite crystals affects the spectral features' wavelength, relative intensity, and asymmetry. Forsterite features are identified in cometary comae near 10, 11.05-11.2, 16, 19, 23.5, 27.5 and 33 microns [1-10], so accurate models for forsterite's absorption efficiency (Qabs) are a primary requirement to compute IR spectral energy distributions (SEDs, lambdaF lambda vs. lambda) and constrain the silicate mineralogy of comets. Forsterite is an anisotropic crystal, with three crystallographic axes with distinct indices of refraction for the a-, b-, and c-axis. The shape of a forsterite crystal significantly affects its spectral features [13-16]. We need models that account for crystal shape. The IR absorption efficiencies of forsterite are computed using the discrete dipole approximation (DDA) code DDSCAT [11,12]. Starting from a fiducial crystal shape of a cube, we systematically elongate/reduce one of the crystallographic axes. Also, we elongate/reduce one axis while the lengths of the other two axes are slightly asymmetric (0.8:1.2). The most significant grain shape characteristic that affects the crystalline spectral features is the relative lengths of the crystallographic axes. The second significant grain shape characteristic is breaking the symmetry of all three axes [17]. Synthetic spectral energy distributions using seven crystal shape classes [17] are fit to the observed SED of comet C/1995 O1 (Hale-Bopp). The Hale-Bopp crystalline residual better matches equant, b-platelets, c-platelets, and b-columns spectral shape classes, while a-platelets, a-columns and c-columns worsen the spectral fits. Forsterite condensation and partial evaporation experiments demonstrate that environmental temperature and grain shape are connected [18-20]. Thus, grain shape is a potential probe for protoplanetary disk temperatures where the cometary crystalline forsterite formed. The forsterite crystal shapes (equant, b-platelets, c-platelets, b-columns - excluding a- and c-columns) derived from our modeling [17] of comet Hale- Bopp, compared to laboratory synthesis experiments [18], suggests that these crystals are high temperature condensates. By observing and modeling the crystalline features in comet ISON, we may constrain forsterite crystal shape(s) and link to their formation temperature(s) and environment(s)
- …