228 research outputs found
The unusual hydrocarbon emission from the early carbon star HD 100764: The connection between aromatics and aliphatics
We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope
to obtain spectra of HD 100764, an apparently single carbon star with a
circumstellar disk. The spectrum shows emission features from polycyclic
aromatic hydrocarbons (PAHs) that are shifted to longer wavelengths than
normally seen, as characteristic of ``class C'' systems in the classification
scheme of Peeters et al. All seven of the known class C PAH sources are
illuminated by radiation fields that are cooler than those which typically
excite PAH emission features. The observed wavelength shifts are consistent
with hydrocarbon mixtures containing both aromatic and aliphatic bonds. We
propose that the class C PAH spectra are distinctive because the carbonaceous
material has not been subjected to a strong ultraviolet radiation field,
allowing relatively fragile aliphatic materials to survive.Comment: 11 pages (in emulateapj), 5 tables, 7 figures. Accepted for
publication in Ap
The circumstellar disc around the Herbig AeBe star HD169142
We present 7 mm and 3.5 cm wavelength continuum observations toward the
Herbig AeBe star HD169142 performed with the Very Large Array (VLA) with an
angular resolution of ~1". We find that this object exhibits strong (~4.4 mJy),
unresolved (~1") 7 mm continuum emission, being one of the brightest isolated
Herbig AeBe stars ever detected with the VLA at this wavelength. No emission is
detected at 3.5 cm continuum, with a 3 sigma upper limit of ~0.08 mJy. From
these values, we obtain a spectral index of ~2.5 in the 3.5 cm to 7 mm
wavelength range, indicating that the observed flux density at 7mm is most
likely dominated by thermal dust emission coming from a circumstellar disc. We
use available photometric data from the literature to model the spectral energy
distribution (SED) of this object from radio to near-ultraviolet frequencies.
The observed SED can be understood in terms of an irradiated accretion disc
with low mass accretion rate, 10^{-8} solar masses per year, surrounding a star
with an age of ~10 Myr. We infer that the mass of the disc is ~0.04 solar
masses, and is populated by dust grains that have grown to a maximum size of 1
mm everywhere, consistent with the lack of silicate emission at 10 microns.
These features, as well as indications of settling in the wall at the dust
destruction radius, led us to speculate the disc of HD169142 is in an advanced
stage of dust evolution, particularly in its inner regions.Comment: 13 pages, 2 figures. Accepted by MNRA
Atomic and molecular interstellar absorption lines toward the high galactic latitude stars HD~141569 and HD~157841 at ultra-high resolution
We present ultra-high resolution (0.32 km/s) spectra obtained with the 3.9m
Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF), of
interstellar NaI D1, D2, Ca II K, K I and CH absorption toward two high
galactic latitude stars HD141569 and HD157841. We have compared our data with
21-cm observations obtained from the Leiden/Dwingeloo HI survey. We derive the
velocity structure, column densities of the clouds represented by the various
components and identify the clouds with ISM structures seen in the region at
other wavelengths. We further derive abundances, linear depletions and H2
fractional abundances for these clouds, wherever possible. Toward HD141569, we
detect two components in our UHRF spectra : a weak, broad component at - 15
km/s, seen only in CaII K absorption and another component at 0 km/s, seen in
NaI D1, D2, Ca II K, KI and CH absorption. In the case of the HD157841
sightline, a total of 6 components are seen on our UHRF spectra in NaI D1, D2
Ca II K, K I and CH absorption. 2 of these 6 components are seen only in a
single species.Comment: 16 pages, Latex, 4 figures, ps files Astrophysical Journal (in press
PAH emission from Herbig AeBe stars
We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained
with the Infrared Spectrograph on the Spitzer Space Telescope. All but one of
the Herbig stars show emission from polycyclic aromatic hydrocarbons (PAHs) and
seven of the spectra show PAH emission, but no silicate emission at 10 microns.
The central wavelengths of the 6.2, 7.7--8.2, and 11.3 micron emission features
decrease with stellar temperature, indicating that the PAHs are less
photo-processed in cooler radiation fields. The apparent low level of photo
processing in HAeBe stars, relative to other PAH emission sources, implies that
the PAHs are newly exposed to the UV-optical radiation fields from their host
stars. HAeBe stars show a variety of PAH emission intensities and ionization
fractions, but a narrow range of PAH spectral classifications based on
positions of major PAH feature centers. This may indicate that, regardless of
their locations relative to the stars, the PAH molecules are altered by the
same physical processes in the proto-planetary disks of intermediate-mass
stars. Analysis of the mid-IR spectral energy distributions indicates that our
sample likely includes both radially flared and more flattened/settled disk
systems, but we do not see the expected correlation of overall PAH emission
with disk geometry. We suggest that the strength of PAH emission from HAeBe
stars may depend not only on the degree of radial flaring, but also on the
abundance of PAHs in illuminated regions of the disks and possibly on the
vertical structure of the inner disk as well.Comment: 52 pages, 12 figure
The near-infrared size-luminosity relations for Herbig Ae/Be disks
We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes
using the 85-m baseline Keck Interferometer. Targets were chosen to span the
maximum range of stellar properties to probe the disk size dependence on
luminosity and effective temperature. For most targets, the measured
near-infrared sizes (ranging from 0.2 to 4 AU) support a simple disk model
possessing a central optically-thin (dust-free) cavity, ringed by hot dust
emitting at the expected sublimation temperatures (T_sub~1000-1500K).
Furthermore, we find a tight correlation of disk size with source luminosity R
propto L^(1/2) for Ae and late Be systems (valid over more than 2 decades in
luminosity), confirming earlier suggestions based on lower-quality data.
Interestingly, the inferred dust-free inner cavities of the highest luminosity
sources (Herbig B0-B3 stars) are under-sized compared to predictions of the
optically-thin cavity model, likely due to optically-thick gas within the inner
AU.Comment: Accepted by Astrophysical Journal; 24 pages, 4 figures, 4 table
Mid-infrared spectra of PAH emission in Herbig AeBe stars
We present spectra of four Herbig AeBe stars obtained with the Infrared
Spectrograph (IRS). on the Spitzer Space Telescope. All four of the sources
show strong emission from polycyclic aromatic hydrocarbons (PAHs), with the 6.2
um emission feature shifted to 6.3 um and the strongest C-C skeletal-mode
feature occuring at 7.9 um instead of at 7.7 um as is often seen. Remarkably,
none of the four stars have silicate emission. The strength of the 7.9 um
feature varies with respect to the 11.3 um feature among the sources,
indicating that we have observed PAHs with a range of ionization fractions. The
ionization fraction is higher for systems with hotter and brighter central
stars. Two sources, HD 34282 and HD 169142, show emission features from
aliphatic hydrocarbons at 6.85 and 7.25 um. The spectrum of HD 141569 shows a
previously undetected emission feature at 12.4 um which may be related to the
12.7 um PAH feature. The spectrum of HD 135344, the coolest star in our sample,
shows an unusual profile in the 7-9 um region, with the peak emission to the
red of 8.0 um and no 8.6 um PAH feature.Comment: Accepted by ApJ 23 June, 2005, 8 pages (emulateapj), 5 figures (3 in
color
C2D Spitzer-IRS spectra of disks around T Tauri stars: I. Silicate emission and grain growth
Infrared ~5--35 um spectra for 40 solar-mass T Tauri stars and 7
intermediate-mass Herbig Ae stars with circumstellar disks were obtained using
the Spitzer Space Telescope as part of the c2d IRS survey. This work
complements prior spectroscopic studies of silicate infrared emission from
disks, which were focused on intermediate-mass stars, with observations of
solar-mass stars limited primarily to the 10 um region. The observed 10 and 20
um silicate feature strengths/shapes are consistent with source-to-source
variations in grain size. A large fraction of the features are weak and flat,
consistent with um-sized grains indicating fast grain growth (from 0.1--1.0 um
in radius). In addition, approximately half of the T Tauri star spectra show
crystalline silicate features near 28 and 33 um indicating significant
processing when compared to interstellar grains. A few sources show large
10-to-20 um ratios and require even larger grains emitting at 20 um than at 10
um. This size difference may arise from the difference in the depth into the
disk probed by the two silicate emission bands in disks where dust settling has
occurred. The 10 um feature strength vs. shape trend is not correlated with age
or Halpha equivalent width, suggesting that some amount of turbulent mixing and
regeneration of small grains is occurring. The strength vs. shape trend is
related to spectral type, however, with M stars showing significantly flatter
10 um features (larger grain sizes) than A/B stars. The connection between
spectral type and grain size is interpreted in terms of the variation in the
silicate emission radius as a function of stellar luminosity, but could also be
indicative of other spectral-type dependent factors (e.g, X-rays, UV radiation,
stellar/disk winds, etc.).Comment: 17 pages, 13 figures, accepted for publication by ApJ, formatted with
emulateapj using revtex4 v4.
The Interaction between Elastic wave Motion and a Mag-netic Field in Electrical Conductors
Again, in the case when Mr, M2 are elastically homogeneous but magnetic permeabilities are different, i.e., #1 < #2 then/3~ </32, and consequently magnetoelastic SH-waves exist in this case as well. (1 -c2/b~) In T -(42) (ca/b~-1)1t2 âą Equation Numerical Results From the frequency Eq
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