Schutte, W.A.

Leiden University Scholary Publications

A&A 386, 1103–1105 (2002)DOI: 10.1051/0004-6361:20020333c© ESO 2002Astronomy&AstrophysicsResearch NoteThe low temperature crystallization effect reevaluatedW. A. Schutte?Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, PO Box 9513,2300 RA Leiden, The NetherlandsReceived 7 February 2002 / Accepted 4 March 2002Abstract. We reevaluate the Low Temperature Crystallization (LTC) effect reported by Moore et al. (1994) forices condensed on a layer of amorphous silicate smoke on an aluminum substrate at <20 K. We consider thepossibility that the observations put forward by Moore et al. in support of the LTC phenomenon could resultfrom poor thermal conductivity in the fluffy smoke layer causing the temperature of the smoke particles to bewell above that of the aluminum substrate. We conclude that there is currently no persuasive evidence for LTCin the Moore et al. experiments.Key words. ISM: dust, extinction – infrared: ISM – ISM: general1. IntroductionThe presence of crystalline ices in interstellar environ-ments is generally considered exclusive evidence for ther-mal annealing. Indeed, when gases such as H2O andCH3OH are deposited at 10–20 K, temperatures repre-sentative for the dust in the interstellar medium, this re-sults in the formation of amorphous ices (e.g., Hudginset al. 1993). Crystallization only occurs after warm-upto >∼150 K in the laboratory. However, Moore et al.(1994) observed the formation of crystalline ice when H2O,CH3OH or NH3 ices were condensed on a layer of amor-phous silicate smoke which was deposited on a <20 K alu-minum substrate. The authors named this phenomenonLow Temperature Crystallization (LTC). They hypothe-sized that active defect sites on the grains which constitutethe smoke layer could cause the crystallization.It is clear that the occurrence of LTC would have farreaching implications for the interpretation of interstellarice observations. Generally, the OH stretching mode ab-sorption feature of H2O ice observed towards high andlow mass protostars indicates that the ice is predom-inantly amorphous (Smith et al. 1988; Whittet et al.1988). However, the emission from the disk surroundingthe young star HD 100546 shows a feature at 60 µm,clearly indicating the presence of crystalline ice (Malfaitet al. 1998). So far these observations have been inter-preted in terms of the thermal history of the ices, but ifLTC occurs, this would imply that crystallization of the? e-mail: schutte@strw.Leidenuniv.nlice relates to the nature of the silicate dust onto which itis condensed.Which physical mechanism could cause the LTC effectis unclear. As noted by Moore et al., it is possible thatthe interaction between the smoke particle surface andthe condensing gases could cause the first 1 or 2 mono-layers to crystallize at low temperature, but it is hard tounderstand why any such surface reactivity would extendfurther into the ice sample. Thus the possibility that theLTC was caused by an experimental effect deserves closeconsideration. In particular, because of the fluffy natureof the smoke and because of the close correspondence ofthe composition of the smoke with quartz glass, a materialknown for its low thermal conductivity, the cooling of thesmoke layer by the transportation of heat to the under-lying aluminum substrate may have been very inefficient.The top of the layer is constantly heated by thermal radi-ation emanating from the room temperature set-up walls.This may have caused the smoke layer to have a temper-ature well in excess of the 20 K aluminum surface.In this short communication we investigate whetherthe various results put forward by Moore et al. in supportof the LTC phenomenon can also be explained by the ef-fects of a steep temperature gradient in the smoke layerresulting from its low thermal conductivity.2. EvaluationTo exclude the possibility of crystalline condensation dueto elevated temperatures in the smoke layer, Moore et al.relied on a number of experimental results. First, it was1104 W. A. Schutte: The low temperature crystallization effect reevaluatednoted that crystalline ices form on all smoke layers, irre-spective of the layer thickness or the nature of the gas.Second, they found that CO and CH4 also condense, indi-cating temperatures below 50 K. Third, the temperaturedependent sublimation behaviour of the ices deposited onthe smoke layer “appeared to be the same” as that ofice films that were directly deposited on the aluminumsubstrate.We now consider the possibility that each of these ob-servations could be explained in terms of condensation ofgases on a smoke layer with a steep vertical temperaturegradient. Such a temperature gradient could result froma low thermal conductivity of the smoke layer. The topof the layer is constantly heated by the absorption of ra-diation from the substrate walls, while at the bottom thelayer is cooled by its contact with the cold (<20 K) fin-ger. If the thermal conductivity of the smoke layer is low,a steep temperature gradient would be required to allowtransportation of the absorbed heat to the aluminum sub-strate. (It must be noted that at the deposition rates usedby Moore et al. (∼1020 molecules hr−1), the heat of con-densation is small relative to the energy absorbed fromthe thermal radiation by the ∼290 K walls of the set-up).Considering this possibility, the first observation by Mooreet al. can be understood in terms of inward diffusion of thegases through the upper warm regions of the smoke layer,until they reach a region where the temperature is just lowenough for them to condense. Since in this picture the con-densation will always take place at the temperature whichis just sufficient for condensation, irrespective of smokelayer thickness and the nature of the gas, this process willalways result in the formation of crystalline ice. The con-densation of very volatile gases like CO can be understoodif these gases diffuse through the warm smoke layer untilthey reach the cold aluminum substrate itself. Finally, thesimilar vaporization behaviour during warm-up of a sam-ple deposited on smoke on top of aluminum or depositeddirectly on the aluminum substrate can be understood interms of a sublimation and recondensation sequence. Aftertheir initial sublimation, the molecules can, through a ran-dom walk, diffuse down to slightly colder regions, wherethey recondense. Going through this cycle a number oftimes, the gases could eventually diffuse all the way downto the aluminum substrate, and will only be finally lostwhen the substrate itself becomes sufficiently warm. Weconclude that all the observations by Moore et al. in sup-port of the LTC phenomenon may be explained by thepresence of a temperature gradient in the smoke layer.To investigate the possible influence of the thermalconductivity of the substrate material on the physicalstate of a condensed ice layer, we have performed an ex-periment in which H2O ice was deposited on a quartz win-dow. For details on the experimental procedure we referto Gerakines et al. (1995). We note that the compositionof quartz is very similar to that of the silica smokes in theMoore et al. experiments. It is expected that for a ma-terial with a low thermal conductivity such as quartz athermal gradient could arise from the center to the edge3800 3600 3400 3200 3000 28000.00.10.20.30.40.50.60.7Absorbance (arb. units)ν (cm-1)54321Fig. 1. The OH stretching mode of a sample of pure waterice as deposited on the quartz substrate with the cold fingerat 12 K (dashed line), compared with the spectra of waterice deposited on a CsI substrate: 1) after deposition at 12 K;2) after warm-up to 50 K; 3) to 80 K; 4) to 120 K, 5) to 160 K.of the substrate. At the edge the substrate is in intimatecontact with the 12 K coldfinger through the use of anindium seal, and therefore will be at 12 K as well. Thepositive thermal gradient towards the center, where thesubstrate temperature may be considerably higher, allowsthe transportation to the cold finger of the heat which re-sults from the absorption by the substrate of the radiationfrom the set-up walls.Figure 1 shows the spectrum of a sample of pureH2O deposited on a quartz substrate mounted on a12 K cold finger. We used a low deposition rate(102 monolayers hr−1), analogous to the deposition ratesreported by Moore et al. By comparison to an H2O sampledeposited on a standard CsI sample, it can be seen that thesample on the quartz window is somewhat annealed, anal-ogous to an ice sample after warm-up to 60–70 K on theCsI substrate. This clearly shows that low thermal conduc-tivity causes the temperature on the quartz window to beconsiderably higher than that of the cold finger itself. Toverify that the annealing is caused by the temperature andnot by some kind of LTC effect, we also deposited CO onthe substrate. It was observed that, following deposition,the CO ice gradually evaporates. The sublimation rate ob-tained from the decrease of the 2140 cm−1 CO stretchingmode was equal to 1.6×1013 molecules s−1, correspondingto a temperature of 30 K (Sandford & Allamandola 1988).We note that the difference in the temperature obtainedfrom the CO sublimation rate and the degree of annealingof the H2O ice can be easily understood when consideringthe temperature gradient over the substrate. Since CO isa highly volatile species, it will not be able to condense onthe warmest regions of the substrate, i.e., near the center.Therefore the CO ice will, on the average, have a lowertemperature than the H2O ice, which can condense alsoat the center of the substrate where the temperatures arehighest.W. A. Schutte: The low temperature crystallization effect reevaluated 1105The experimental results emphasize that low thermalconductivity is a major concern when attempting to ob-tain cryogenic temperatures with materials of chemicalcomposition similar to quartz. We note that these prob-lems will be greatly enhanced when using a loose agglom-erate of particles like the fluffy smokes in the Moore et al.experiments.In summary, all experimental results by Moore et al.can be rationalized in terms of poor thermal conductivitythrough the smoke layer and the resulting high tempera-ture of the particles. To exclude this possibility, a carefulmeasurement would need to be made of the thermalconductivity of the smoke layer in the Moore et al. exper-iments, followed by modelling of the equilibrium betweenabsorption of thermal radiation and cooling by heattransport to the cold finger to determine the temperaturestructure of the smoke layer. Only after such an analysishas clearly shown that the temperature throughout thesmoke layer is below the crystallization temperature ofH2O ice (∼130 K), low temperature crystallization can beconsidered a viable explanation. We conclude that thereis currently no persuasive experimental evidence that lowtemperature crystallization occurs in the Moore et al.experiments.Acknowledgements. This research would not have taken placewithout the encouragement of Rens Waters. We thank ThomasHenning for sharing his knowledge on the nature of quartzglass. A thorough review of the manuscript by Helen Fraserprovided some important new insights into the subject.ReferencesGerakines, P. A., Schutte, W. A., Greenberg, J. M., &van Dishoeck, E. F. 1995, A&A, 296, 810Hudgins, D. M., Sandford, S. A., Allamandola, L. J., & Tielens,A. G. G. M. 1993, ApJS, 86, 713Malfait, K., Waelkens, C., Waters, L. B. F. M., et al. 1998,A&A, 332, L25Moore, M. H., Ferrante, F. R., Hudson, R. L., Nuth III, J. A.,& Donn, B. 1994, ApJ, 428, L81Sandford, S. A., & Allamandola, L. J. 1988, Icarus, 76, 201Smith, R. G., Sellgren, K., & Tokunaga, A. T. 1989, ApJ, 344,413Whittet, D. C. B., Bode, M. F., Longmore, A. J., et al. 1988,MNRAS, 233, 321

The low temperature crystallization effect reevaluated

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The low temperature crystallization effect reevaluated

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