Content uploaded by Craig Smith
Author content
All content in this area was uploaded by Craig Smith on Jul 18, 2014
Content may be subject to copyright.
Infrared Spectropolarimetry of AFGL 2591: Evidence for an Annealed Grain Component
Abstract
We present spectropolarimetric observations of AFGL 2591 in the 10 and 20 μm regions. These confirm that the polarization at these wavelengths is due to dichroic absorption by aligned grains and infer
that this is also the polarization mechanism in the near infrared. The aligning field is normal to the bipolar axis of molecular
flow and also to the larger scale field in the local interstellar medium. As well as the polarization feature usually seen
and attributed to amorphous silicates (e.g. the BN object in Orion) an additional narrow feature is observed at 11.2 μm. It is considered that this is due to a more structured silicate, similar to olivine, which could have been produced in
an annealing episode in which grains reach a temperature in excess of 1300 K. If the annealment has been produced by the central
source, the material must now be expanding.
The 20 μm data rule out metal oxides as significant contributors to the extinction.
... The peak in absorption which arises due to Si-O bond stretching near MIR 10 µm has been widely studied and this feature, visible clearly in the linear polarization profile of interstellar dust grains, is attributed to nonspherical and aligned silicate dust grains [Whittet (2003); Min et al. (2007)]. While amorphous silicates present in the ISM are mostly responsible for the 10 µm peak, Aitken et al. (1988) observed an additional feature around 11 µm in the polarization profile towards a Class I Young Stellar Object (YSO), AFGL 2591, which was considered to have formed due to crystalline silicates similar to olivine. Separate studies have placed an upper limit on the abundance of silicates found in crystalline form in the ISM: 2.2% by Kemper et al. (2004Kemper et al. ( , 2005 and 1% by Min et al. (2007). ...
... Such an occurrence of polarization due to absorption beyond the typical 10 µm feature was observed by Aitken et al. (1988) which was considered to be due to the presence of crystalline silicates peaking at 11.2 µm. Min et al. (2007) state that SiC grains that are irregular in shape show a broad feature in the spectra which peaks near 11.25 µm. ...
The presence of crystalline silicates has been detected in the circumstellar environment of several young stars in the recent past and there is evidence of silicon carbide (SiC) detection in the envelope of pre-main sequence star SVS13. In this work, we have attempted to probe the presence of SiC in the dust around protoplanetary discs in a sample of young stars. We have modelled the linear polarization of composite dust grains in the mid-infrared (MIR: 8–13 μm) using silicates as the host with various inclusions of SiC and graphites using the Discrete Dipole Approximation (DDA) and the Effective Medium Approximation (EMA) T-Matrix methods. We have then compared our modelling results with polarimetric observations made in the protoplanetary discs surrounding two Herbig Be stars and one T-Tauri star with particular emphasis towards the 10 μm silicate feature using CanariCam mounted over the Gran Telescopio Canarias (GTC). We report the possible existence of SiC in the outer disc/envelope around one star in our sample which has been interpreted based on the shape, size, composition, and fraction of inclusions by volume in our dust grain models.
... The peak in absorption which arises due to Si-O bond stretching near MIR 10 µm has been widely studied and this feature, visible clearly in the linear polarization profile of interstellar dust grains, is attributed to nonspherical and aligned silicate dust grains [Whittet (2003); Min et al. (2007)]. While amorphous silicates present in the ISM are mostly responsible for the 10 µm peak, Aitken et al. (1988) observed an additional feature around 11 µm in the polarization profile towards a Class I Young Stellar Object (YSO), AFGL 2591, which was considered to have formed due to crystalline silicates similar to olivine. Separate studies have placed an upper limit on the abundance of silicates found in crystalline form in the ISM: 2.2% by Kemper et al. (2004Kemper et al. ( , 2005 and 1% by Min et al. (2007). ...
... Such an occurrence of polarization due to absorption beyond the typical 10 µm feature was observed by Aitken et al. (1988) which was considered to be due to the presence of crystalline silicates peaking at 11.2 µm. Min et al. (2007) state that SiC grains that are irregular in shape show a broad feature in the spectra which peaks near 11.25 µm. ...
The presence of crystalline silicates has been detected in the circumstellar environment of several young stars in the recent past and there is evidence of silicon carbide (SiC) detection in the envelope of pre-main sequence star SVS13. In this work, we have attempted to probe the presence of SiC in the dust around protoplanetary disks in a sample of young stars. We have modelled the linear polarization of composite dust grains in the mid-infrared (MIR: 8--13 m) using silicates as the host with various inclusions of SiC and graphites using the Discrete Dipole Approximation (DDA) and the Effective Medium Approximation (EMA) T-Matrix methods. We have then compared our modelling results with polarimetric observations made in the protoplanetary disks surrounding two Herbig Be stars and one T-Tauri star with particular emphasis towards the 10 m silicate feature using CanariCam mounted over the Gran Telescopio Canarias (GTC). We report the possible existence of SiC in the outer disk/envelope around one star in our sample which has been interpreted based on the shape, size, composition and fraction of inclusions by volume in our dust grain models.
... Carbonate dust may also provide an explanation for a narrow polarization feature at 11.3 m in the N-band polarization spectrum of AFGL 2591 which was tentatively attributed to an annealed (i.e. crystalline) silicate component (Aitken et al. 1988) before the infraredspace-observatory revealed that crystalline silicates were reasonably common in circumstellar environments. The feature is known to be persistent over time due to the use of the source as a position-angle standard (170 • ) . ...
A pair of 6.0 and 6.9 μm absorption features are frequently observed in Milky-Way (MW) molecular-clouds and YSOs; they also occur in the z = 0.886 rest-frame of a molecule-rich spiral galaxy obscuring blazar PKS 1830–211. I calibrate χ2-fitting methods which match observations with two or three laboratory spectra. The 6.0-μm component is dominated by H2O ice, as expected. Included MW sources were selected using opacity criteria which limit the range of explored H2O-ice column densities to 1.6–2.4 × 1018 molecules cm−2, while the H2O-ice density in the galaxy absorber is (2.7 ± 0.5) × 1018 molecules cm−2. CH3OH ice and / or small (< 0.1-μm-sized) Ca- and Mg-bearing carbonates contribute at 6.9 μm. The 41 per cent CH3OH : H2O molecular ratio in the PKS 1830–211 absorber is significantly higher than in the molecular cloud towards Taurus-Elias 16 (<7.5 per cent) and similar to the highest value in MW YSOs (35 per cent in AFGL 989). Fitted carbonate (-CO3) : H2O ratios in the galaxy absorber of 0.091 per cent are low in comparison to most of the ratios detected in the MW sample (0.2–0.4 % ∼0 % in AFGL 989). Inorganic carbonates could explain the increased oxygen depletion at the diffuse-medium-to-molecular-cloud transition which Jones & Ysard associated with unobserved organic carbonates or materials with a C:O ratio of 1:3.
... This interstellar material had been thought to be amorphous due to the lack of spectral structure on the band. However, recent higher resolution spectroscopic and polarization measurements imply that crystalline material is also present [35,36]. ...
Our fundamental knowledge of interstellar grain composition has grown substantially during the past two decades thanks to significant advances in two areas: astronomical infrared spectroscopy and laboratory astrophysics. The opening of the mid-infrared, the spectral range from 4000-400 cm ⁻¹ (2.5-25 μm), to spectroscopic study has been critical to this progress because spectroscopy in this region reveals more about a material's molecular composition and structure than any other physical property.
... Polarimetric observations are particularly useful for differentiating the dust models. Observations found that the 3.4 μm C-H absorption feature is negligibly polarized (Adamson et al. 1999;Chiar et al. 2006), whereas the 9.7 μm Si-O feature is strongly polarized (Aitken et al. 1988) for the light of sights toward the Galactic Center. It is suggested that carbonaceous grains must be a separate component and these grains should be not aligned (Chiar et al. 2006). ...
Interstellar dust is an essential component of the interstellar medium (ISM) and plays critical roles in astrophysics. Achieving an accurate model of interstellar dust is therefore of great importance. Interstellar dust models are usually built based on observational constraints such as starlight extinction and polarization, but dynamical constraints such as grain rotation are not considered. In this paper, we show that a newly discovered effect by Hoang et al., so-called RAdiative Torque Disruption (RATD), can act as an important dynamical constraint for dust models. Using this dynamical constraint, we derive the maximum size of grains that survive in the ISM for different dust models, including contact binary, composite, silicate core and amorphous carbon mantle, and compact grain model for the different radiation fields. We find that the different dust models have different maximum sizes due to their different tensile strengths, and the largest maximum size corresponds to the compact grains with the highest tensile strength. We show that the composite grain model cannot be ruled out if constituent particles are very small with radius a p ≤ 25 nm, but large composite grains would be destroyed if the particles are large with a p ≥ 50 nm. We suggest that grain internal structures can be constrained with observations using the dynamical RATD constraint for strong radiation fields such as supernova, nova, or star-forming regions. Finally, our obtained results suggest that micron-sized grains perhaps have compact/core–mantle structures or have composite structures but are located in regions with slightly higher gas density and weaker radiation intensity than the average ISM.
Recent infrared studies of classical novae are described. The nucleation and growth of dust in nova ejecta was confirmed by optical/infrared photometry of several novae during the past five years including QU Vul, V842 Cen, V827 Her, and QV Vul. These novae provided evidence that dust of every astrophysical chemical and mineral composition can condense in nova ejecta. Infrared forbidden fine structure lines were identified in the recent novae V1819 Cyg, QU Vul, OS And, and V827 Her. The implication of IRAS measurements for nova energetics is discussed.
Molecular ices are a common component of the dust in many molecular clouds and circumstellar shells. The most abundant molecule, H 2 O, has several infrared spectral features which have strongly temperature dependent shapes (FWHM) and peak wavelengths. This paper describes how a study of these features, both astronomically and in the laboratory, can lead to constraints on the temperatures of interstellar dust grains. This is demonstrated in part by comparing several astronomical spectra with laboratory spectra of H 2 O ice.
The historical development of and scientific results from the mid-infrared instruments used on the UK Infrared Telescope from 1980 to 2005 are described. That near- and mid-infrared instruments (and submm photometers) were mounted on the telescope and easily available for use was a particular strength of UKIRT, which encouraged multi-wavelength and long-term monitoring programmes. The mid-infrared spectrometers UCLS and CGS3 were the most sensitive available in the world in the 1980s and 1990s and they established the spectral properties of many classes of astronomical objects. Papers describing mid-infrared measurements accounted for over 10 % of the total output from UKIRT in this period.
Study of the dust in circumstellar disks around young stars is currently an extremely active area in astronomy. There is little doubt that accretion disks are a natural part of protostellar evolution. Much recent observational and theoretical work is giving us a clearer picture of the physical conditions in dust disks and their evolutionary progression. IRAS observations revealed that many main-sequence stars, such as p Pictoris, have circumstellar disks. But whether these disks are related to planetary formation is not yet understood.
A portion of the dust in disks around young stars ultimately may be incorporated into planetary systems. Thus, study of the dust in our own solar system complements the remote sensing of protostellar regions and aids in reconstructing the evolutionary history of the dust. Since comets formed in the cold outer regions of the solar nebula, they may contain intact interstellar grains. As the comets lose material during passage through the warm inner solar system, some of these grains will be released into interplanetary space. Technical advances now allow analysis of individual micrometer or smaller grains in interplanetary dust particles and primitive meteorite samples. Isotopic anomalies and patterns of crystal growth in these particles are yielding tantalizing clues about the interstellar material incorporated into these solar system samples.
An absorption feature is occasionally reported around 11 ?microns in
astronomical spectra, including those of forming stars. Candidate carriers
include water ice, polycyclic aromatic hydrocarbons (PAHs), silicon carbide,
crystalline silicates or even carbonates. All are known constituents of cosmic
dust in one or more types of environments, though not necessarily together. In
this paper we present new ground-based 8-13 ?micron spectra of one evolved
star, several embedded young stellar objects (YSOs) and a background source
lying behind a large column of the interstellar medium (ISM) toward the
Galactic Centre. Our observations, obtained at a spectral resolution of
?approximately 100, are compared with previous lower resolution data, as well
as data obtained with the Infrared Space Observatory (ISO) on these and other
targets. By presenting a subset of a larger sample our aim is to establish the
reality of the feature and subsequently speculate on its carrier. All evidence
points toward crystalline silicate. For instance, the 11 ?micron band profile
is well matched with the emissivity of crystalline olivine. Furthermore, the
apparent association of the absorption feature with a sharp polarisation
signature in the spectrum of two previously reported cases suggests a carrier
with a relatively high band strength compared to amorphous silicates. If true,
this would either set back the evolutionary stage in which silicates are
crystallised, either to the embedded phase or even before within the ISM, or
else the silicates ejected from the outflows of evolved stars retain some of
their crystalline identity during their long residence in the ISM.
The infrared spectra of the type II OH/IR source OH 32.8 - 0.3 are
presented, showing evidence for absorption bands near 12 and 3.1
microns. The 12 micron absorption feature is similar to that seen in OH
231.8 + 4.2 and is attributed to relatively pure water ice formed in the
outflow from an evolved star.
Large circular polarization (approximately 0.5-1.5%) at 2.2 microns has
been observed in a number of protostellar sources exhibiting large
linear polarization. It is argued that the high observed ellipticities
are consistent with a model in which the polarization is produced by a
medium of aligned grains with a twist in the alignment (less than 60
deg, depending upon the model). This model is similar to that used to
account for visual linear and circular interstellar polarization.
The linear polarization across the 3-micron ice absorption band in the
protostellar source GL 2591 was measured. There is no increase in
polarization in the center of the band relative to the wings as is seen
in the polarization spectrum of the BN source in Orion, implying
variations in the properties of the grains and/or the magnetic fields in
the vicinity of these sources. Assuming that the polarization is caused
by aligned grains, it seems likely that the grains near GL 2591 are
larger than near BN, and have significant albedos at 1 to 4 microns. No
information about the relative location of the ice and silicate grains,
or local magnetic field variations near GL 2591 can be obtained from the
data.
The polarization properties of a class of infrared sources which have apparent 9.7 micron silicate absorption features were investigated using a 10 micron polarimeter. Detailed observations of the Becklin-Neugebauer source (BN), indicate that the linear polarization is produced by absorption in partially aligned, elongate particles. Comparisons of the observations with model calculations demonstrates that the 9.7 micron volume absorption coefficient of the silicate material BN is at least a factor of two smaller than laboratory values of terrestrial silicates. The characteristic size of the particles is much less than 10 microns and the index of refraction has a very small imaginary part near 10 microns. Observations of six other sources imply that the degree of grain alignment around BN is high in comparison to other spectroscopically similar objects.
The 10 µ depression observed in the spectrum of a compact infrared object is usually ascribed to absorption by intervening cold silicate grains, and the underlying source spectrum is taken to be either a blackbody or a blackbody with superposed excess 10 µ emission. We question this assumption of the underlying source spectrum for optically thick compact sources. We find, upon modeling both the objects BN and W3 IRS5, that the source actually emits less at the 10 µ resonance than outside the resonance, so that a depression at 10 µ already exists in the source spectrum. This difference in emission arises because, due to the higher opacity in the resonance, the observed 10 µ radiation is produced further out in the source than is the radiation just outside the resonance. And the lower dust temperature further out gives rise to a weaker emission at 10 µ than in the continuum. An observed 10 µ depression can be largely due to this effect, and little or no intervening extinction is required. This explanation of the 10 µ depression leads to a correlation such that the magnitude of depression will increase with decreasing color temperature of the source. It also predicts no depression at 20 µ for sources with color temperatures greater than 200 K. Observations at 20 µ would then be able to decide on the validity of this explanation.
Narrow-band polarimetry of the BN object and AFGL 2591 has been made
between 2 and 4 m in conjunction with a CVF spectrophotometer. The
results show an ice-band excess in the BN object, but no trace of an
excess larger than 1.6% in AFGL 2591. In the case of the BN object the
polarization maximum shifts to a longer wavelength compared with that of
the extinction peak. Polarization and extinction profiles of elongated
ice particles have been calculated using the perfect Davis-Greenstein
alignment model. They can well explain the observed shift of the
polarization maximum.
The results of a search for high-velocity molecular gas associated with
star forming molecular clouds are reported. Faint, high-velocity
(ΔV ≥ 30 km s-1)12CO emission is
frequently found near buried infrared sources. In most clouds, the
high-velocity gas is observed over an extended area having a typical
radius around 1018 cm and exhibits bipolar structure with the
redshifted emission formed in a different region from the corresponding
blueshifted emission. Analysis of the CO line emission indicates that
typical flows have masses in the range M ≈ 0.3 - 100 M_sun;,
velocities in the range V ≈ 10 - 50 km s-1, and lifetimes
as distinctly recognizable dynamical entities of around t ≈
104 yr. The kinematics of the high-velocity gas are best
explained as outflows from the vicinity of the associated infrared
sources. However, it appears that the ionized stellar winds proposed to
explain infrared line and radio continuum observations toward many of
these sources cannot be driving the energetic molecular flows. The
presence of high-velocity outflow in molecular clouds indicates an
unanticipated phase of energetic mass loss associated with the birth and
early evolution of stars.
Previous studies of suprathermal rotation of interstellar dust grains
are reviewed. Causes and consequences of internal dissipation of
rotational energy are discussed, and consequences of suprathermal
rotation for grain alignment are considered. The statistics of
suprathermal rotation are studied under the assumption that, on a long
time scale, residual torque is a random function of time with a
correlation time that is some measure of the permanence of grain
irregularities. The dependence of the mean energy of suprathermal
rotation and of the mean time between crossovers on correlation time is
determined. The effect of suprathermal rotation on the magnetic
alignment of interstellar grains is also investigated.