Continuum Polarization Research Articles

Spectropolarimetry of O supergiants

We present medium-resolution spectropolarimetry at high signal-to-noise ratio of the Hα emission line of 20 O-type supergiants. Five stars (25 per cent) of the sample show a statistically significant change in polarization through the line. We combine our Hα data with new K-band spectropolarimetry and archival low-resolution optical spectropolarimetry to determine the polarigenic mechanism in the stars that show a line effect. We show that the line polarization change in the binary systems is caused by the classical ‘dilution’ mechanism, in which the Hα emission is essentially unpolarized and the continuum polarization is caused by intrabinary scattering. We find that the line effect in HD 108 is also well modelled by pure dilution, but suggest that the continuum polarization is the result of stochastic wind clumping. A similar description applies to the continuum polarization of HD 188001, although the line effect cannot be reproduced by pure dilution. We use low-resolution spectropolarimetry to determine the interstellar polarization vector to λ Cephei, and confirm that the intrinsic polarization of the object is very low ( 200 km s −1 ), and that the system is similar polarimetrically to the O4 supergiant ζ Puppis. Finally, we note that the precision of current photo- and spectro-polarimetric observations is insufficient to test structured wind models, which predict a continuum polarization of ∼0.1 per cent.

The Twice-Overlooked, Second Fanaroff-Riley II Broad Absorption Line Quasar LBQS 1138−0126

We report the correct classification of an overlooked Fanaroff-Riley class II radio-loud quasar with broad absorption lines, only the second such object so identified. The rare properties of this quasar, LBQS 1138-0126, are twice overlooked. First, LBQS 1138-0126 was found in the Large Bright Quasar Survey but only noted as a possible broad absorption line quasar without additional follow-up. Later, LBQS 1138-0126 was rediscovered and classified as a radio-loud broad absorption line quasar but not recognized as an FR II radio source. We describe the radio, absorption line, and optical polarization properties of LBQS 1138-0126 and place it in context with respect to related quasars. In particular, spectropolarimetry shows that LBQS 1138-0126 has high continuum polarization increasing from 3% in the red (rest-frame 2400 A) to over 4% in the blue (rest-frame 1650 A), essentially confirming the intrinsic nature of the absorption. The polarization position angle rotates from ~-30° in the red to ~0° in the blue; the radio lobe position angle is ~52° for comparison. LBQS 1138-0126 is additionally notable for being one of the most radio-loud broad absorption line quasars and for having low-ionization broad absorption lines as well.

Evidence for Extremely High Dust Polarization Efficiency in NGC 3184

Recent studies have found the Type II-plateau supernova (SN) 1999gi to be highly polarized (p_max = 5.8%, where p_max is the highest degree of polarization measured in the optical bandpass; Leonard & Filippenko 2001) and minimally reddened (E[B-V] = 0.21 +/- 0.09 mag; Leonard et al. 2002). From multiple lines of evidence, including the convincing fit of a ``Serkowski'' interstellar polarization (ISP) curve to the continuum polarization shape, we conclude that the bulk of the observed polarization is likely due to dust along the line of sight (l-o-s), and is not intrinsic to SN 1999gi. We present new spectropolarimetric observations of four distant Galactic stars close to the l-o-s to SN 1999gi (two are within 0.02 degrees), and find that all are null to within 0.2%, effectively eliminating Galactic dust as the cause of the high polarization. The high ISP coupled with the low reddening implies an extraordinarily high polarization efficiency for the dust along this l-o-s in NGC 3184: ISP / E(B-V) = 31^{+22}_{-9} % mag^{-1}. This is inconsistent with the empirical Galactic limit (ISP / E[B-V] < 9% mag^{-1}), and represents the highest polarization efficiency yet confirmed for a single sight line in either the Milky Way or an external galaxy.

Mass-loss rate determination for the massive binary V444 Cygni using 3-D Monte-Carlo simulations of line and polarization variability

A newly developed 3-D Monte Carlo model is used, in conjunction with a multi-line non-LTE radiative transfer model, to determine the mass-loss rate of the Wolf-Rayet (W-R) star in the massive binary \object{V444 Cyg} (WN5+O6). This independent estimate of mass-loss rate is attained by fitting the observed \HeI (5876) \AA and \HeII (5412) \AA line profiles, and the continuum light curves of three Stokes parameters ((I, Q, U)) in the (V) band simultaneously. The high accuracy of our determination arises from the use of many observational constraints, and the sensitivity of the continuum polarization to the mass-loss rate. Our best fit model suggests that the mass-loss rate of the system is (\dot{M}_{\WR}=0.6(\pm 0.2) \times 10^{-5} M_{\sun} \mathrm{yr}^{-1} ), and is independent of the assumed distance to \object{V444 Cyg}. The fits did not allow a unique value for the radius of the W-R star to be derived. The range of the volume filling factor for the W-R star atmosphere is estimated to be in the range of 0.050 (for $R_{\WR}=5.0 R_{\sun}$) to 0.075 (for $R_{\WR}=2.5 R_{\sun}$). We also found that the blue-side of \HeI (5876 ) \AA and \HeII (5412) \AA lines at phase 0.8 is relatively unaffected by the emission from the wind-wind interaction zone and the absorption by the O-star atmosphere; hence, the profiles at this phase are suitable for spectral line fittings using a spherical radiative transfer model.

Investigating the nature of the $\mathsf {\vec z}\simeq\mathsf {2.8}$ submillimeter selected galaxy SMM J02399-0136 with VLT spectropolarimetry

We present deep optical spectropolarimety of SMM J02399-0136 (z=2.8) done with the VLT Antu 8.2 m telescope equipped with FORS1. Moderate continuum and emission line polarization are measured (P~5%). We do not detect broad lines in scattered flux as would be expected for a type-2 object but rather a polarization behaviour similar to BAL quasars. This classification is confirmed by the detection of both high and low ionization broad absorption troughs and a very red continuum. We argue that this object shares several properties with local ULIGs such as Mrk 231 and other ultraluminous infrared Lo-BAL quasars. However, the fact that the ultraviolet spectrum is dominated by non-stellar radiation does not prove that the dust that is thermally radiating in the far infrared is predominantly heated by the AGN. Since the energy that we get in the far-infrared is precisely that which is removed from the ultraviolet spectrum, this could mean that the starburst is more dust-enshrouded than the AGN due to a peculiar dust distribution. The limits we place on the putative starburst contribution to the restframe ultraviolet continuum together with constraints on the amount of extinction provides an upper limit to the star formation rate of about 2000 Msun/yr, consistent with previously claimed high star formation rate in this object.

Tree-structured grid model of line and polarization variability from massive binaries

We have developed a 3-D Monte Carlo radiative transfer model which computes line and continuum polarization variability for a binary system with an optically thick non-axisymmetric envelope. This allows us to investigate the complex (phase-locked) line and continuum polarization variability features displayed by many massive binaries: W-R+O, O+O, etc. An 8-way tree data structure constructed via a ``cell-splitting'' method allows for high precision with efficient use of computer resources. The model is not restricted to binary systems; it can easily be adapted to a system with an arbitrary density distribution and large density gradients. As an application to a real system, the phase dependent Stokes parameters (I, Q, U) and the phase dependent He I (5876) profiles of the massive binary system V444 Cyg (WN5+O6 III-V) are computed.

S 111 and the polarization of the B[e] supergiants in the Magellanic Clouds

We have obtained linear polarization measurements of the Large Magellanic Cloud B(e) supergiant S 111 using optical imaging polarimetry. The intrinsic polarization found is consistent with the presence of an axisymmetric circumstellar envelope. We have additionally estimated the electron density for S 111 using data from the literature and revisited the correlation between polarization and envelope parameters of the B(e) supergiant stars using more recent IR calibration color data. The data suggest that the polarization can be indeed explained by electron scattering. We have used Monte Carlo codes to model the continuum polarization of the Magellanic B(e) supergiants. The results indicate that the electron density distribution in their envelopes is closer to a homogeneous distribution rather than an r 2 dependence. At the same time, the data are best tted by a spherical distribution with density contrast than a cylindrical distribution. The data and the model results support the idea of the presence of an equatorial disk and of the two-component wind model for the envelopes of the B(e) supergiants. Spectropolarimetry would help further our knowledge of these envelopes.

Theoretical Models of Polarized Dust Emission from Protostellar Cores

We model the polarized thermal dust emission from protostellar cores that are assembled by supersonic turbulent flows in molecular clouds. Self-gravitating cores are selected from a three-dimensional simulation of supersonic and super-Alfvenic magnetohydrodynamic (MHD) turbulence. The polarization is computed in two ways. In model A it is assumed that dust properties and grain alignment efficiency are uniform; in model B it is assumed that grains are not aligned at visual extinction larger than AV,0 = 3 mag, consistent with theoretical expectations for grain alignment mechanisms. Instead of using a specific set of grain properties, we adopt a maximum degree of polarization Pmax = 15%. Results are therefore sensitive mainly to the topology of the magnetic field (model A) and to the gas distribution that determines the distribution of AV (model B). Furthermore, the radiative transfer in the MHD model is solved with a non-LTE Monte Carlo method, to compute spectral maps of the J = 1-0 transition of CS. The CS spectral maps are used to estimate the turbulent velocity, as in the observations. The main results of this work are the following: (1) Values of P between 1% and 10% (up to almost Pmax) are typical, despite the super-Alfvenic nature of the turbulence. (2) A steep decrease of P with increasing values of the submillimeter dust continuum intensity I is always found in self-gravitating cores selected from the MHD simulations if grains are not aligned above a certain value of visual extinction AV,0 (model B). (3) The same behavior is hard to reproduce if grains are aligned independently of AV (model A). (4) The Chandrasekhar-Fermi formula, corrected by a factor f ≈ 0.4, provides an approximate estimate of the average magnetic field strength in the cores. Submillimeter dust continuum polarization maps of quiescent protostellar cores and Bok globules have recently been obtained. They always show a decrease in P with increasing value of I consistent with the predictions of our model B. We therefore conclude that submillimeter polarization maps of quiescent cores do not map the magnetic field inside the cores at visual extinction larger than AV,0 ≈ 3 mag. The use of such maps to constrain models of protostellar core formation and evolution is questionable. This conclusion suggests that there is no inconsistency between the results from optical and near-IR polarized absorption of background stars and the observed polarization of submillimeter dust continuum from quiescent cores. In both cases, grains at large visual extinction appear to be virtually unaligned.

Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi

ABSTRACTWe present single‐epoch spectropolarimetry of the Type II supernovae (SNe II) 1997ds, 1998A, and 1999gi. SN 1997ds and SN 1998A were both observed during the early photospheric phase, less than 50 days after explosion, while spectropolarimetry of SN 1999gi was obtained near the start of the transition to the nebular phase, about 110 days after explosion. Uncorrected for interstellar polarization (ISP), SN 1997ds is characterized by pV = 0.85% ± 0.02%, SN 1998A has pV = 0.24% ± 0.05%, and SN 1999gi is polarized at pV = 5.72% ± 0.01%. SN 1997ds and SN 1999gi exhibit distinct polarization modulations (up to Δptot = 1.6% in SN 1997ds and Δptot = 1.0% in SN 1999gi) at the wavelengths of the strongest spectral line features. While no spectral polarization features were observed in SN 1998A, the data are insensitive to polarization features at the levels confirmed in the other two objects.The low continuum polarization inferred for SN 1997ds and SN 1998A and the amplitude of (or limits on) the polarization modulations are consistent with those measured at similar epochs for SN 1987A and the Type II plateau SN 1999em and supports the growing consensus that core‐collapse events with hydrogen envelopes substantially intact at the time of explosion are not significantly aspherical during the early photospheric phase. The spectral shape of the high continuum polarization of SN 1999gi closely resembles a “Serkowski” ISP curve (characterized by pmax = 5.8%, θ = 154°, and λmax = 5300 Å) and is inconsistent with the wavelength‐independent nature of electron scattering expected for an aspherical SN atmosphere. Since Galactic reddening is minimal along this line of sight, the majority of the observed polarization in SN 1999gi is believed to be due to ISP of the host galaxy, although significant (up to p ≈ 2%) intrinsic polarization cannot be ruled out. The potential power of SN spectropolarimetry to study the properties of interstellar dust in external galaxies is described and applied to the SN 1999gi data, where it is shown that if the polarization is indeed predominantly interstellar in origin, then RV = 3.0 ± 0.2 for the dust along this line of sight in NGC 3184.

Is It Round? Spectropolarimetry of the Type II‐P Supernova 1999em

We present the first multiepoch spectropolarimetry of a Type II plateau supernova (SN II-P), with optical observations of SN 1999em on days 7, 40, 49, 159, and 163 after discovery. These data are used to probe the geometry of the electron scattering atmosphere before, during, and after the plateau phase, which ended roughly 90 days after discovery. Weak continuum polarization with an unchanging polarization angle (θ ≈ 160°) is detected at all epochs, with p ≈ 0.2% on day 7, p ≈ 0.3% on days 40 and 49, and p ≈ 0.5% in the final observations. Distinct polarization modulations across strong line features are present on days 40, 49, 159, and 163. Uncorrected for interstellar polarization (which is believed to be quite small), polarization peaks are associated with strong P Cygni absorption troughs, and nearly complete depolarization is seen across the Hα emission profile. The temporal evolution of the continuum polarization and sharp changes across lines indicate polarization intrinsic to SN 1999em. When modeled in terms of the oblate, electron scattering atmospheres of Hoflich, the observed polarization suggests an asphericity of at least 7% during the period studied. The temporal polarization increase may indicate greater asphericity deeper into the ejecta. We discuss the implications of asphericity on the use of Type II-P supernovae as primary extragalactic distance indicators through the expanding photosphere method (EPM). If asphericity produces directionally dependent flux and peculiar galaxy motions are characterized by σ = 300 km s-1, it is shown that the agreement between previous EPM measurements of SNe II and distances to the host galaxies predicted by a linear Hubble law restricts mean SN II asphericity to values less than 30% (3 σ) during the photospheric phase.

A Useful Approximation for Computing the Continuum Polarization of Be Stars

This paper describes a practical model for the polarization of Be stars which can be used to estimate roughly the physical parameters for optically thin circumstellar envelopes from broadband UBVRI photopolarimetry data. Analysis of long-term variability in terms of these parameters is a promising approach toward understanding the Be phenomenon. An interesting result from fitting the model to observations of eight Be stars is that all of them may have geometrically thin disks, with opening half-angles on the order of ten degrees or less. This contributes to the growing evidence that most Be disks are geometrically thin.

Ab Initio Quantum Chemical Studies of Reactions in Astrophysical Ices: 2. Reactions in H 2CO/HCN/HNC/H 2O Ices

Theoretical electronic structure calculations were used to investigate reactions between formaldehyde (H 2CO) and both hydrogen cyanide (HCN) and isocyanide (HNC) in search of other favorable reactions such as ammonia–formaldehyde addition, which was found in a recent theoretical study to be strongly enhanced when it occurs within cold ices (D. E. Woon 1999, Icarus 142, 550–556). Reaction components were characterized in clusters composed of the reactants and up to two explicit, catalytic waters and then embedded in a continuum polarization field to incorporate the bulk solvation effects of ice. Intriguingly, reactions between H 2CO and HCN or HNC exhibit isomerization during the reaction: H 2CO+HCN yields HOCH 2NC (isocyanomethanol), while H 2CO+HNC yields HOCH 2CN (glyconitrile). As a direct consequence of the greater stability of the -CN bond over the -NC bond, H 2CO+HNC has a lower reaction barrier and is substantially more exothermic. However, the barrier for isomerization of HOCH 2NC to HOCH 2CN is comparable with the initial barrier and may yield the more stable nitrile if conditions are favorable. Although both reactions are enhanced by active and passive interactions with water in the ice, neither barrier is reduced to the point where the reaction is likely to proceed at very cold temperatures without another source of energy. If ammonia were also present in the ice, heat from its reaction with formaldehyde is predicted to be sufficient to initiate H 2CO+HNC reactions and may also drive less favorable H 2CO+HCN reactions. Three-body reactions that yield very small polyoxymethylene polymers terminated with -CN and -NC groups were also studied, as well as reactions between HCN and ammonia or water.

Radio galaxies at $z\sim$ 2.5: Results from Keck spectropolarimetry

In classifying the ensemble of powerful extragalactic radio sources, considerable evidence has accumulated that radio galaxies and quasars are orientation-dependent manifestations of the same parent population: massive spheroidal galaxies containing correspondingly massive black holes. One of the key factors in establishing this unification has been the signature of a hidden quasar detected in some radio galaxies in polarized light. The obscuration of our direct view of the active nucleus usually, but not necessarily exclusively, by a thick nuclear disk or torus can act conveniently as a natural coronograph that allows a much clearer view of the host of a radio galaxy than of a quasar. In this study, we exploit the opportunity to eliminate the quasar glare by performing sensitive spectropolarimetry with the Keck II telescope of a sample of radio galaxies with redshifts around 2.5. This represents the epoch when quasars were many times more common that they are now and is likely to be the period during which their host galaxies were being assembled into what become the most massive galaxies in the Universe today. We show that dust-reflected quasar light generally dominates the restframe ultraviolet continuum of these sources and that a highly clumped scattering medium results in almost grey scattering of the active galactic nucleus photons. The observations, however, do not exclude a substantial star formation rate averaged over a Gyr of evolution. The sub-mm reradiation from the scattering dust is likely to represent only a small fraction (∼10% ) of the total far infrared luminosity. An analysis of the emission lines excited in the interstellar medium of the host galaxy by the hard quasar radiation field reveals evidence of a dramatic chemical evolution within the spheroid during this epoch. Secondary nitrogen production in intermediate mass stars produces a characteristic signature in the Nv/Civ and Nv/Heii line ratios which has been seen previously in the broad line region of quasars at similar redshifts. We find intriguing correlations between the strengths of the Lyα and N v emission lines and the degree of ultraviolet continuum polarization which may represent the dispersal of dust associated with the chemical enrichment of the spheroid.

Halpha spectropolarimetry of the Herbig Ae star AB Aurigae

We present spectropolarimetric observations, obtained at Hα, of the Herbig Ae star AB Aurigae. Changes in linear polarization across the Hα line probe structure in the immediate circumstellar environment of the central star, down to scales of the order of one to a few stellar radii. In the case of AB Aurigae the observed polarimetric signature is complex. After applying a correction for foreground continuum polarization, we find that there is a linear-polarized Hα emission component intrinsic to the source. Rotation of the angle of polarization through the emission-line profile suggests scattering in a rotating circumstellar disc. The magnetic accretor model commonly applied to T Tauri stars shows promise of explaining these data.

Synthetic line profiles of rotationally distorted hot-star winds

ABSTRA C T A new Monte Carlo stellar wind radiative-transfer code is presented. The code employs a three-dimensional opacity grid, and fully treats polarization and multiple scattering. Either Mie or Rayleigh scattering phase matrices may be used, and the line-transfer is treated by means of the Solobolev approximation. Variance reduction techniques are employed to increase computational efficiency. The results of several tests of the code are reported. It is confirmed that no continuum polarization is produced in the spherically symmetric wind case, and that the line profiles computed match those computed using established radiative-transfer codes. The continuum polarization produced by a latitudinally structured low-density wind is found to be in good agreement with that predicted by the single-scattering analytical treatment of Fox, while in the higher density regime the polarizations are consistent with the multiple-scattering code given by Hillier. Two illustrative applications of the code are described, using the wind parameters of z Puppis [O4 I(n)f] as the base model. In the first the effect on the line profile of a corotating spiral density enhancement is examined. It is found that the spiral gives line profile variations on the order of 5 per cent, and that it produces an S-wave-like pattern as a function of rotational phase. It is noted that the accelerations described by the spiral wave may mimic those produced by tangentially accelerating wind clumps. The variable polarization produced by the spiral is found to have an amplitude of 0.1 per cent, with two maxima per rotational period in phase with the line emission modulation. The second application investigates the profiles and polarization produced in a clumped wind. Although the parameters of the discrete wind clumps are necessarily arbitrary, it is found that a clumpedwind model reproduces the level of spectroscopic variability found by Eversberg et al. It is shown that the wind emission ‘bumps’ produced in the synthetic spectra often arise from the superposition in velocity space of flux from several spatially discrete wind blobs. Although the two example models may, in combination, reproduce the observed spectroscopic variability of OB supergiants, it appears that the predicted polarimetric variability of these models is too weak to explain the polarimetric observations of OB stars by Hayes and Lupie & Nordsieck. Finally, a new line polarization effect is described, resulting from line absorption of continuum photons in a rotating wind. The effect has a striking resemblance to the observations of z Puppis presented by Harries & Howarth (1996), and it is possible that the Ha polarization structure observed arises from this effect, rather than by line dilution of a continuum polarization.

Evidence for Asphericity in the Type IIn Supernova SN 1998S

We present optical spectropolarimetry obtained at the Keck II 10 m telescope on 1998 March 7 UT along with total flux spectra spanning the first 494 days after discovery (1998 March 2 UT) of the peculiar Type IIn supernova (SN) 1998S. The SN is found to exhibit a high degree of linear polarization, implying significant asphericity for its continuum-scattering environment. Prior to the removal of interstellar polarization, the polarization spectrum is characterized by a flat continuum (at p ≈ 2%) with distinct changes in polarization associated with both the broad (symmetric, half-width near zero intensity 10,000 km s-1) and narrow (unresolved, full width at half-maximum less than 300 km s-1) line emission seen in the total flux spectrum. When analyzed in terms of a polarized continuum with unpolarized broad-line recombination emission, an intrinsic continuum polarization of p ≈ 3% results, suggesting a global asphericity of 45% from the oblate, electron-scattering dominated models of Hoflich. The smooth, blue continuum evident at early times is shown to be inconsistent with a reddened, single-temperature blackbody, instead having a color temperature that increases with decreasing wavelength. Broad emission-line profiles with distinct blue and red peaks are seen in the total flux spectra at later times, suggesting a disklike or ringlike morphology for the dense (ne ≈ 107 cm-3) circumstellar medium, generically similar to what is seen directly in SN 1987A, although much denser and closer to the progenitor in SN 1998S. Implications of the circumstellar scattering environment on the spectropolarimetry are discussed, as are the effects of uncertainty in the removal of interstellar polarization; the importance of obtaining multiple spectropolarimetric epochs in future studies to help better constrain the interstellar polarization value is particularly stressed. Using information derived from the spectropolarimetry and the total flux spectra, an evolutionary scenario for SN 1998S and its progenitor are presented.

The Lyman Continuum Polarization Rise in the QSO PG 1222+228

ABSTRACTSome QSOs show an abrupt, strong rise in polarization at rest wavelength ∼750 Å. If this arises in the atmosphere of an accretion disk around a supermassive black hole, it may have diagnostic value. In PG 1222+228, the polarization rise occurs at the wavelength of a sharp drop in flux. We examine and reject interpretations of this feature involving a high‐velocity outflow. The observations agree with a model involving several intervening Lyman limit systems, two of which happen to coincide with the Lyman continuum polarization rise. After correction for the Lyman limit absorption, the continuum shortward of 912 Å is consistent with a typical power‐law slope, α ≈ -1.8. This violates the apparent pattern for the Lyman limit polarization rises to occur only in “candidate Lyman edge QSOs.” The corrected, polarized flux rises strongly at the wavelength of the polarization rise, resembling the case of PG 1630+377. The rise in polarized flux places especially stringent requirements on models.

A generic scattering model for AGN

I present a development of the scattering model of Young et al. that can reproduce the velocity resolved polarization of the scattered broad lines, along with the continuum polarization of AGN. Two possible emission models are presented, namely a rotating disc and a radially expanding shell of emission. Using known, or expected, scattering regions present in AGN it is possible to reproduce the polarization observed to be present in type 1 and type 2 active galaxies. The use of the model is illustrated by its application of the spectropolarimetric observations of the Seyfert 1 galaxy, Mrk 509. By a simple alteration of the incident flux upon the scattering regions it is shown that the time variable polarization in this object can be reproduced.

A Connection Between V/R and Polarization in Be Stars

AbstractOkazaki (1991) and Papaloizou, Savonije, &amp; Henrichs (1992) suggested that the quasi-cyclic V/R variability observed in the emission line profiles of many Be stars is caused by a precessing one-armed density wave in the circumstellar disk. It seems likely that the changing aspect of such a non-axisymmetric density pattern might also lead to a related variation of the continuum polarization. We have searched for such an effect in two well-studied Be shell stars, ζ Tau and 48 Lib, based on data compiled from several groups of observers from 1984 to 1998. Using the Monte Carlo radiation transfer code of Wood, Bjorkman, Whitney, &amp; Code (1996), we have calculated the polarization due to electron scattering in Be disks in the presence of one-armed density perturbations. Although the notorious long and short term deviations from strict periodicity present in Be stars make it difficult to rigorously demonstrate the connection between the V/R variability and the polarization variations, we have been able to find specific modes that are consistent with the observed V/R line profile variations together with the suspected polarization cycles.

Ab Initio Quantum Chemical Studies of Reactions in Astrophysical Ices: 1. Aminolysis, Hydrolysis, and Polymerization in H 2CO/NH 3/H 2O Ices

Laboratory studies of astrophysical ices by W. A. Schutte, L. J. Allamandola, and S. A. Sandford (1993, Science 259, 1143–1145; 1993, Icarus 104, 118–137) indicate that ices containing formaldehyde (H 2CO) and at least a trace of ammonia (NH 3) will undergo reactions as the samples are heated from 10 K with no additional irradiation. A residue composed of higher-boiling-point organic species persists to 190 K and beyond. Spectral changes indicate that reactions begin to occur as low as 40 K. The present study employed theoretical electronic structure methods to investigate possible microscopic mechanisms that would account for various aspects of the experimental phenomena. Reaction components were characterized in clusters composed of reactants with up to two explicit catalytically active waters present and then embedded in a continuum polarization field to incorporate the bulk solvation effects of ice. Direct dimerization and trimerization of H 2CO were first considered, but no process was found that could account for the low-temperature reactivity observed in the laboratory. Two ice-bound aminolysis reactions were then identified that are predicted to possess barriers low enough to be viable at 40 K: H 2CO–NH 3 → NH 2CH 2OH and (H 2CO) 2–NH 3 → NH 2CH 2OCH 2OH. The latter yields an amide-terminated polyoxymethylene polymer. Analogous hydrolysis reactions are enhanced in ice, but not sufficiently to occur at cold temperatures on their own, which is consistent with the experimental observation that NH 3 is a critical component and water alone is insufficient to catalyze H 2CO reactions.