336 research outputs found
Properties of grains derived from IRAS observations of dust
The authors used the results of Infrared Astronomy Satellite (IRAS) observations of diffuse medium dust to develop a theoretical model of the infrared properties of grains. Recent models based entirely on traditional observations of extinction and polarization include only particles whose equilibrium temperatures do not exceed 20 K in the diffuse interstellar medium. These classical grains, for which the authors have adopted the multipopulation model developed by Hong and Greenberg (1980), can explain only the emission in the IRAS 100 micron band. The measurements at shorter wavelengths (12, 25 and 60 microns) require two new particle populations. Vibrational fluorescence from aromatic molecules provides the most likely explanation for the emission observed at 12 microns, with polycyclic aeromatic hydrocarbons (PAHs) containing about 10 percent of cosmic carbon. A simplified model of the emission process shows that PAH molecules can also explain most of the emission measured by IRAS at 25 microns. The authors identified the warm particles responsible for the excess 60 microns emission with small (a approx. equals 0.01 microns) iron grains. A compilation of the available data on the optical properties of iron indicates that the diffuse medium temperature of small iron particles should be close to 50 K and implies that a large, possibly dominant, fraction of cosmic iron must be locked up in metallic particles in order to match the observed 60 microns intensities. The model matches the infrared fluxes typically observed by IRAS in the diffuse medium and can also reproduce the infrared surface brightness distribution in individual clouds. In particular, the combination of iron and classical cool grains can explain the surprising observations of the 60/100 microns flux ratio in clouds, which is either constant or increases slightly towards higher opacities. The presence of metallic grains has significant implications for the physics of the interstellar medium, including catalytic H2 formation, for which iron grains could be the main site; differences in depletion patterns between iron and other refractory elements (Mg, Si); and superparamagnetic behavior of large grains with embedded iron clusters giving rise to the observed high degree of alignment by the galactic magnetic field
A multi-wavelength study of the peculiar galaxy NGC 2976
Researchers are currently studying NGC 2976 at many wavelengths to investigate the extent to which an interaction with M81 may have affected the star formation history of this galaxy. Here, researchers present observations of NGC 2976 made at 50 microns with the high resolution (CPC) instrument onboard Infrared Astronomy Satellite (IRAS) at 21-cm (both HI line and radio continuum) with the Westerbork Synthesis Radio Telescope (WSRT) and in the H alpha line with the Kitt Peak National Observatory (KPNO) 36 inch telescope. The far infrared emission is not centrally peaked as in other spirals (e.g., Wainscoat et al. 1987), but has obvious intensity peaks near the ends of the disk. The ionized gas as inferred from the H alpha observations, is largely confined to two large, symmetrically placed emission regions near the ends of the disk. Finally, the HI and 21-cm radio continuum emission also exhibit this strongly double-peaked structure. At all of the above wavelengths the emission peaks are roughly coincident and lie approx. 1.2 minutes to the NW and approx. 1.1 minutes to the SE of the optical center of this galaxy
ISO Spectroscopy of the Young Bipolar Nebulae S106 IR and Cep A East
We present the results of ISO SWS and LWS grating scans towards the embedded
Young Stellar Objects (YSOs) S106 IR and Cep A East. Emission from the pure
rotational lines of H2 and the infrared fine structure lines of [C II], [O I],
[S I], [Si II] and [Fe II], as well as absorption bands due to H2O, CO and CO2
ice were detected toward Cep A. In S106 we detected emission lines of H2, CO, H
I, and a large number of ionized species including Fe, O, N, C, Si, S, Ne and
Ar. S106 also shows many of the infrared PAH bands in emission. Excitation
temperatures and molecular hydrogen masses were derived from the low-lying pure
rotational levels of H2 and are 500 and 730 K and 8 and 3 x 10^{-3} solar
masses for S106 and Cep A, respectively. Since both objects are expected to
have several solar masses of H2 in their environment, we conclude that in both
cases the bulk of the H2 is cooler than a few hundred Kelvins. Excitation
temperatures and line ratios were compared with those predicted by theoretical
models for PDRs and dissociative and non-dissociative shocks. The [S I] 25.2
micron/[Si II] 34.8 micron ratio is a particularly useful shock versus PDR
discriminant and we conclude that S106 IR is dominated by PDR emission while
Cep A East has a large shock component. From an analysis of the ionic lines in
S106 we conclude that the central star must have a temperature around 37,000 K,
corresponding to a spectral type of O8. From its luminosity it is concluded
that the driving source of Cep A must also be a massive early-type star. The
absence of strong high-ionization ionic lines in its ISO spectrum shows that
Cep A has not yet created a significant H II region and must be younger than
S106, illustrating the process of the clearing of the surroundings of a massive
young star.Comment: 15 pages (including 10 figures), to appear in Astronomy &
Astrophysic
Dust emission from high latitude cirrus clouds
In order to study dust emission from grains in the interstellar medium, the infrared properties were analyzed in a number of isolated high latitude dust clouds which contain no dominant internal heating sources. The clouds are spatially resolved, have a simple geometry, and are mapped in the IRAS bands at 12, 25, 60, and 100 microns. For a number of these clouds, extinction data (A sub B) were obtained from starcounts. A large part (30 to 50 percent) of the infrared radiation of the clouds in the IRAS wavelength range of 8 to 130 micron is emitted in the short wavelength bands at 12 and 25 micron. The 60/100 micron ratios for the integrated fluxes of the clouds have a typical value of 0.19 + or - 0.05
Infrared properties of dust grains derived from IRAS observations
The analysis of several diffuse interstellar clouds observed by the Infrared Astronomy Satellite (IRAS) is presented. The 60/100 micron flux ratios appear to be nearly constant in clouds with up to 1 sup m visual extinction at the center. Observations of a highly regular cloud in Chamaeleon show that the 12/100 micron ratio peaks at an intermediate radial distance and declines towards the center of the cloud. These observations indicate that nonequilibrium emission accounts only for the 12 and 25 micron bands; strong emission observed at the 60 micron band is probably due to equilibrium thermal radiation. The correlation of the 12 micron emission with a red excess observed for a high latitude cloud, L1780, is shown to be consistent with the assumption that both features are due to fluorescence by the same molecular species
The embedded objects in epsilon Cha I cloud
A study was made of the embedded objects in the epsilon Cha I cloud. General shapes of the spectra were constructed for the members in the cloud. The near infrared data were compiled from the literature and combined with the IRAS Point Source Catalog information. Pointed observations by the IRAS were used in the regions of high source density where the Point Source Catalog is confused. Member objects near the late B star HD 97300 were measured recently in the 3 to 10 micron bands using the ESO 2.2 m telescope in order to study the effects of disks seen in other young stellar objects. A picture is presented of the complete initial luminosity function in the epsilon Cha I cloud. The observations were compared with the theoretical views on low mass star formation
ISO spectroscopy of circumstellar dust in the Herbig Ae systems AB Aur and HD 163296
Using both the Short- and Long-wavelength Spectrometers on board the Infrared
Space Observatory (ISO), we have obtained infrared spectra of the Herbig Ae
systems AB Aur and HD 163296. In addition, we obtained ground-based N band
images of HD 163296. Our results can be summarized as follows: (1) The main
dust components in AB Aur are amorphous silicates, iron oxide and PAHs; (2) The
circumstellar dust in HD 163296 consists of amorphous silicates, iron oxide,
water ice and a small fraction of crystalline silicates; (3) The infrared
fluxes of HD 163296 are dominated by solid state features; (4) The colour
temperature of the underlying continuum is much cooler in HD 163296 than in AB
Aur, pointing to the existence of a population of very large (mm sized) dust
grains in HD 163296; (5) The composition and degree of crystallization of
circumstellar dust are poorly correlated with the age of the central star. The
processes of crystallization and grain growth are also not necessarily coupled.
This means that either the evolution of circumstellar dust in protoplanetary
disks happens very rapidly (within a few Myr), or that this evolution is
governed by factors other than stellar mass and age.Comment: 6 pages, 2 figures, accepted for publication in Astronomy &
Astrophysic
The ISO-SWS spectrum of planetary nebula NGC 7027
We present the infrared spectrum of the planetary nebula NGC7027 observed with the Short Wavelength Spectrometer (SWS), on board the Infrared Space Observatory (ISO). These data allow us to derive the electron density and, together with the IUE and optical spectra, the electron temperature for several ions. The nebular composition has: been determined, the evolutionary status of the central star is discussed and the element depletion in the nebula is given. We conclude that the progenitor was: a C-rich star with a mass: between 3 and 4 M-circle dot
Probing AGB nucleosynthesis via accurate Planetary Nebula abundances
The elemental abundances of ten planetary nebulae, derived with high accuracy
including ISO and IUE spectra, are analysed with the aid of synthetic
evolutionary models for the TP-AGB phase. Model prescriptions are varied until
we achieve the simultaneous reproduction of all elemental features, which
allows placing important constraints on the characteristic masses and
nucleosynthetic processes experienced by the stellar progenitors. First of all,
it is possible to separate the sample into two groups of PNe, one indicating
the occurrence of only the third dredge-up during the TP-AGB phase, and the
other showing also the chemical signature of hot-bottom burning. The former
group is reproduced by stellar models with variable molecular opacities (see
Marigo 2002), adopting initial solar metallicity, and typical efficiency of the
third dredge-up 0.3-0.4. The latter group of PNe, with extremely high He
content 0.15<=He/H<=0.20 and marked oxygen deficiency, is consistent with
original sub-solar metallicity (i.e. LMC composition). Moreover, we are able to
explain quantitatively both the N/H-He/H correlation and the N/H-C/H
anti-correlation, thus solving the discrepancy pointed out long ago by Becker &
Iben (1980). This is obtained only under the hypothesis that intermediate-mass
TP-AGB progenitors (M > 4.5-5.0 Mo) with LMC composition have suffered a number
of very efficient, carbon-poor, dredge-up events. Finally, the neon abundances
of the He-rich PNe can be recovered by invoking a significant production of
22Ne during thermal pulses, which would imply a reduced role of the
22Ne(alpha,n)25Mg reaction as neutron source to the s-process nucleosynthesis
in these stars.Comment: 22 pages, accepted for publication in Astronomy & Astrophysic
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