- Email: [email protected]
Ion irradiation of astrophysical ices
Pergamon
Planet. Space Sci., Vol. 43, Nos. lO/ll, pp. 1247-1251, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-0633/95 $9.50+0.00 0032-0633(95)00040-2
Ion irradiation of astrophysical ices G. Strazzulla, A. C. Castorina and M. E. Palumbo Istituto di Astronomia, Cittk Universitaria, I-95125 Catania, Italy Received 14 November 1994; revised and accepted 13 February 1995
Absbact. We report recent experimental studies on the
physical-chemical effects induced by fast ions colliding with frozen gases (CO, C&OH, CW,, CO,) and mixtures with water simulating icy targets in space (frosts on external planets and satellites, comets and interstellar ices). We have studied, by IR spectroscopy, the formation of new molecules, the alteration of band profile, the spectral changes of the origina compounds and the formation of an organic refractory residue left over after ion irradiation. In particular, we present here the results of the measurements .of the CO/CO2 ratio resulting from the irradiation of several ice mixtures.
1. Introduction
A fast ion impinging on solid targets produces many effects that have been studied in several laboratories around the world because of their astrophysical relevance (Johnson, 1990; Strazzulla and Johnson, 1991; Roessler, 1992; Moore and Hudson, 1994). Our group has, in particular, studied the formation and some properties of organic residues resulting from ion irradiation of carboncontaining frozen ices. When further irradiated, the residue evolves towards what we call IPHAC (Ion Produced Hydrogenated Amorphous Carbon). Both fast ions (low energy cosmic rays, galactic protons, solar protons, solar wind particles) and carbon-containing solid targets (interstellar grains, comet mantles, interplanetary dust particles etc.) are present in space, thus under certain circumstances, material similar to that produced in the laboratory may form in space (Strazzulla and Baratta, 1992 ; Jenniskens et al., 1993). In interstellar molecular clouds, icy grain mantles are believed to be an important component of dust. Their presence is indicated by IR absorption features, e.g. at 3.1, 4.67 and 6.0 pm, observed in the spectra of objects embedded in or located behind dark clouds. These feaCorrespondence to : G. Strazzulla
tures are generally attributed to simple molecules, such as H,O, CO and CH,OH, frozen in icy grain mantles (Tielens and Allamandola, 1987a). These icy mantles are thought to form after accretion and reaction of gas phase molecules onto grain surfaces (Tielens and Hagen, 1982 ; d’Hendecourt et al., 1985; Brown et al., 1988). During their lifetime, volatile icy grain mantles may suffer UV and cosmic ray irradiation and be transformed into organic refractory mantles, consisting of quite complex molecules which will probably survive in the diffuse interstellar medium (Greenberg, 1982; Strazzulla and Baratta, 1992). In this paper we present some results obtained from recent experimental studies on the physical-chemical effects induced by fast ions colliding with frozen gases (CO, CH,OH, CH4, C02) and mixtures with water simulating icy targets in space.
2. Experimental apparatus
The in situ analyses were performed in a stainless steel scattering chamber facing an FTIR spectrometer (PerkinElmer model 1710) through KBr windows. A stream of gas from an inlet tube was used to form frost on a silicon crystal placed into thermal contact with the tail section of a close-cycle helium cryostat (lo-300 K). Samples can be bombarded by 3 keV helium ions. The beam produces a 2 x 2 cm2 spot on the target and currents in the range of 0.2-2 PA cm-‘. The directions of the IR and ion beam are mutually perpendicular. The substrate is at an angle of 45” with respect to the ion beam direction. Hence, before, during and after irradiation, spectra can be obtained without tilting the sample (Spinella et al., 199 1). For calibration purposes many spectra have been taken at different rates of ice deposition. The sample thickness was determined by previously obtained calibration data. The latter have been obtained by depositing a quantity of ice whose thickness was checked by counting the interference fringes of a reflected laser light (632.8 nm). Thus, when a target was simultaneously deposited and irradiated we knew both the number of molecules deposited (on
at a dose of 44 eV/l6 amu. Because of Irradiation, new species are formed; in fact, we observe in the irradiated spectrum the presence of new’ bands. Among these, the 2136, 1308 and 2344 cm-’ bands are due, respectively, to CO, CH, and COZ ; II,0 is also formed. The formation of H,O is, however, better evidenced by irradiation of pure CH30H (Baratta et al., 1994). Furthermore, the relative intensity of CH,OH bands changes as a function of the dose. The abundances of new species were estimated from the peak absorption coefficients that were measured previously, together with the integrated band strengths, in calibration experiments described by Palumbo and Strazzulla (1993). In Fig. 1 there is a band at 1720 cm-‘. This could be associated with formaldehyde (H,CO). Nevertheless, IR spectra of H&O (Moore and Khanna, 1991) also show a strong band at about 1500 cm-‘, and several bands of lower intensity at 1249, 1178, 2891, 2829 and 2725 cm-’ which do not appear in our spectra. Other compounds, such as acetone [(CH,),CO] (Baratta et al., 1994) also show a band at 1720 cm-‘; thus, we can infer that this band, formed after irradiation of the CH30H-H,O mixture, may be due to more than one species [e.g. H&O, (CH,),COJ and it cannot be used to estimate the abundance of any species. We compared our results with those obtained by Allamandola et aE. (198&), who irradiated H,O-CH30H mixtures (2 : 1) with UV photons. The CH,/CO and CO/CO:! ratios value N 0.1 and E 1.25 respectively, at the end of UV irradiation. These values are more consistent with those we find irradiating pure CH,OH than those obtained from the irradiation of a CH,OH-H,O (1 : 2) mixture (Baratta et aZ., 1994). A possible explanation is that, in contrast with what happens upon ion irradiation, H,O is almost unaltered by UV photons. Thus, the products from Hz0 radiolysis, mainly H, HZ, 0 and OH, do not participate in the UV-induced radiation chemistry. Another step of our work has been the study of the effects produced after warm-up of our samples. ‘The range of temperatures was lo-213 K. In Fig. 2 we present a comparison between the spectra of an unirradiated H,OCH,OH mixture (1 : 1) and those of the irradiated mixture at different temperatures. At 157 K in the spectrum of the unirradiated mixture we observe that : helium ‘Ions
0 3000
2000
1000
Wavenumber (cm-‘)
iR spectra of H,O-CH,OH (1 : l), deposited on a cold (10 K) silicon substrate, before and after irradiation with 3 keV helium ions (44 eV/16 amu) the silicon wafer) per time unit and the number of ions (current) impinging in the time unit, i.e. we were able to evaluate the irradiation dose of that experiment. Moreover, taking in situ spectra we could stop the experiment at any time when the irradiated sample showed well defined features. Here, we give doses in eV per molecule of 16 amu because this is a convenient way to characterize chemical changes (Strazzulla and Johnson, 1991) and compare them with experiments on other targets and, with some precautions, with other ions or other ion energies. 3. Laboratory studies of solid methanol and watermethanol mixtures
Besides H,O and CO, methanol (CH,OH) has also been detected (Tielens and Allamandola, 1987a; Grim et al., 1991; Schutte et al., 1991; Skinner et al., 1992) in icy grain mantles. In fact, a large number of IR spectra of sources obscured by dense clouds show different absorption features which, according to laboratory experiments, have been attributed to solid CH,OH. However, the abundance of solid CH,OH is quite controversial. Its abundance is estimated to be 5-50% of that of H20, depending on the absorption features used to estimate it. In general the C-H deformation mode at 6.85 pm gives an abundance of about 50%, while the C-H stretching mode at 3.5 pm and the C-O stretching mode at 9.7 ,um indicate an abundance of about 10% (Grim et al., 1991 ; Schutte et al., 1991; Skinner et al., 1992). We have performed several laboratory experiments in order to study the properties of solid CH,OH and mixtures with water (H,O-CH,OH). We have also irradiated our samples in order to analyze, by IR spectroscopy,
the
effects induced by impinging ions during and after the bombardment. The IR spectrum of H,O-CH30H (1: 1) mixture deposited on a cold (10 K) silicon substrate is compared (Fig. 1) with that obtained after irradiation with 3 keV
CH,OH is still present (see 1034 cm-l band) with an abundance of about 10% with respect to water ; the shape of the 3260 cm-] band is that of a crystal ice, showing that the amorphous to crystal transition has occurred. The unirradiated sample sublimes at about 200 K. When we analyze the spectra of irradiated mixtures, we observe that : some structures are still evident at 2 13 K ; the sample thus shows refractory properties ; the amorphous to crystal transition has not occurred at 157 K, but takes place at about 200 K, as we infer from the shape of 3260 cm-’ band ; volatile species, such as CO and COZ, are still present at
1249
G. Strazzulla et al. : Ion irradiation of astrophysical ices H20:CH80H W
C
(1: 1) ”
+ 44 d/16 amu “
“
“
“
“
“
”
4 1
3
1
4000
3000 2000 1000 Wavenumber (cm-‘)
4000
3000 2000 1000 Wavenumber (a~~‘)
Fig. 2. IR spectra of an unirradiated H,O-CH,OH mixture (1 : 1) are compared, at different temperatures, with those of the irradiated mixture 157 K (pure CO sublimes at - 30 K). This suggests that those species have been trapped into the refractory structure produced by ion irradiation. Finally, the band at 1100 cm-‘, evident in these spectra (downwards), is due to the silicon substrate on which the sample has been deposited. In fact, the profile of the silicon band changes as the temperature increases. The main implication of these results in astrophysical scenarios is that one has to consider that the sublimation properties of irradiated ice are greatly different from those of unirradiated ice mixtures. These differences should be taken into serious account.
4. The CO/CO2 ratio in irradiated ices The observation of solid CO, on dust grains in dense clouds, until now lacking, will be one of the major tasks of the Infrared Space Observatory (ISO), whose launch is scheduled in the near future. In particular, the measure of the CO/CO, ratio will be very useful to get insight into physical-chemical processes taking place on interstellar grains. The present situation is that many interstellar spectra of field and embedded stars show an absorption band centred at about 2140 cm-’ which is attributed to solid CO in grain mantles. CO abundance in interstellar grains relative to that of HZ0 is deduced from the strength of solid CO absorption relative to that of the water-ice feature at 3.1 pm and it has values of 040% in different molecular clouds (Eiroa and Hodapp, 1989; Whittet et
al., 1989; Tielens et al., 1991 ; Kerr et al., 1993; Chiar et al., 1994). The asymmetric profile of this band is believed to derive from two independent components (Lacy et al., 1984) : a narrow component (FWHM 5-8 cm-‘) centred at 2140 cm-’ and a broader one (FWHM N 10 cm-‘) centred at 2136 cm-‘. The identification of the species which give rise to these different components is not yet fully understood. On the basis of our experimental results we proposed some comparisons between interstellar and laboratory spectra, which take into account the interaction of grain mantles with cosmic ions and the presence of CH30H in grain mantles [see Palumbo and Strazzulla (1993)]. Our best fits, in the spectral range 2120-2155 cm-‘, relative to field stars were obtained using the 2140 cm-’ band of irradiated CO ice at 10 K (dose = 12 eV/16 amu) and the 2136 cm-’ band due to CO produced after ion irradiation of an HPO-CH,OH (2 : 1) mixture at 10 K (dose = 41 eV/16 amu). Fits relative to embedded stars were obtained using the 2140 cm-’ band of irradiated CO ice at 10 K (dose = 12 eV/16 amu) and the 2136 cm-’ band due to CO produced by irradiation of an H,O-CH30H (2: 1) mixture (dose = 41 eV/16 amu) warmed up to 67 K. The astrophysical situation is, of course, more complex : doses, temperatures and composition of irradiated samples are to be considered as rough averages. It is interesting to note that, as H,O-rich samples are more stable than COrich ones, the findings that the latter are “on average” four times less irradiated than the former is quite plausible. From estimates of low energy cosmic rays flux in the interstellar medium (Jenniskens et al., 1993) and grain mantles’ lifetime (Greenberg, 1982) we deduced that the energy deposited on a grain (dose) has values of 0.05-50 eV/atom (C, N, 0, Si). [Note that the dose scales roughly according to molecular weight.] As concerns those sources embedded in their placental clouds, our best fits are obtained for the same combination as above, but with the H,O-CH,OH irradiated samples warmed up to 67 K. Even in this case such a temperature has to be considered as an average. We can surmise that around these stars the radiation field increases the grain temperature : near the star all icy mantles sublimate ; at a certain temperature (140-l 60 K) only the H,O-dominated mantles survive ; at greater distance (lower r) the grains are unheated impertubated, like those observed towards field stars. In a series of laboratory experiments we have studied the formation of CO, after ion irradiation of frozen CO, CH,OH, CH, and mixtures with H20. CO, is not predicted to have appreciable abundance in the gas phase in dense clouds, so that its condensation on interstellar grains can be neglected, while its presence may be due to surface reactions between 0 and CO as well as to surface processing, such as UV and cosmic ray irradiation (Tielens and Allamandola, 1987b; Whittet and Walker, 1991). In Fig. 3 the ratios measured for several irradiated ices and mixtures are shown as a function of the irradiation dose. It is interesting to note that after a dose of the order of 30 eV/16 amu, the ratio has values of 0.3-l for all of the considered targets, except for CO, for which it is about 4. Using these data and the grain model described above, we can give a rough estimation of the amount of CO, we
S. lStrazzuila 21* ai. ion trraaration 31 asirophysicai ices
A co q
CH,OH
x H,O:CH,OH=Z:l . . A
H,O:CO=lO:l _ H,O:CO,=l:l
0 co, * H,O:CH,=l:l O.Olk
3
j
20
’
2
’
dose (e”,l@%)
’
80
*
I
’ 2400
Fig. 3. CO/CO2 ratios are given as a function of energy deposited by 3 keV helium ions on different types of ice targets
expect in the line of sight of several sources. In fact, our fits of the CO bands give us an estimate of CO abundance in grain mantles as well as of the predicted CO/CO, ratio. Our results are shown in Table 1. These results have to be carefully interpreted because of the uncertainty in the initial composition of icy mixtures, in the doses suffered by icy mantles by ions and/or UV photons and temperatures. However, it turns out that CO2 abundance is comparable with that of CO and hence it should be observable by the 1SO. Figures 4 and 5 show a comparison between the CO band observed towards Elias 16 and Tamura 8, and laboratory spectra in the 2400-2100 cm-’ range. The band at about 2340 cm-’ is due to the sum of two contributions : CO, produced after ion irradiation of CO (12 eV/16 amu) and that produced from H,O-CH30H (2: 1; after 41 eV/16 amu) ices. The astrophysical plausibility of such a two-population model has been discussed in detail by Palumbo and Strazzulla (1993). Different CO/CO2 ratios, in the two sources, are due to different contributions of the narrow and broad components in the fit of the CO band.
2300
wavenumber
2200
2100
(cm-‘)
Fig. 4. Comparison, in the spectral range 2400-2100 cm-‘, between the observed spectrum of the source Elias 16 (boxes) and the sum of laboratory spectra obtained after ion irradiation of CO and H,O-CH,OH samples (solid line ; see text)
3i 4
j
2400
2300
2200
2100
Acknowledgements. We are grateful to the Italian Space Agency
wavenumber
(ASI) for support of this research. We wish also to acknowledge
Fig. 5. Comparison, in the spectral range 2400-2100 cm-‘, between the observed spectrum of the source Tamura 8 (boxes) and the sum of laboratory spectra obtained after ion irradiation of CO and H,O-CH,OH samples (solid line ; see text)
Table I. Estimates of CO abundance and CO/COZ interstellar icy grain mantles Source
N(C0) (lOI mol cm-‘)
Elias 3
1.06 2.01 3.71 2.6 1.58 1.5 1.7 1.62 4.5
Elias 15 Elias 16 Tamura 8 AFGL 989 Elias 18 NGC 2024 Mon R2 W33A
ratios in
2.12 0.65 1.24 0.68 0.99 0.71 1.06 0.74 0.31
(cm-‘)
Max Bernstein and an anonymous referee for their careful reading of the manuscript and helpful suggestions.
References Allamandola, I.. J., Sandford, S. A. and Valero, 6. J., Icurus 76, 225, 1988. Baratta, 6. A., Gastorina, A. CL, Leto, G., Palumbo, M. F., Spinella, F. and Strazzulla, G., Planet. Spuce Sci. 42, 759, 1994.
G. Strazzulla et al. : Ion irradiation of astrophysical ices Brown, P. D., Charnley, S. B. and Millar, T. J., Mon. Not. R. ustron. Sot. 231,409, 1988. Chiar, J. E., Adamson, A. J., Kerr, T. H. and Whittet, D. C. B., Astrophys. J. 426, 240, 1994. Eiroa, C. and Hodapp, K. W., Astron. Astrophys. 210,345, 1989. Greenberg, M. J., What are comets made of? A model based on interstellar dust, in Comets (edited by L. L. Wilkening), p. 13. The University of Arizona Press, Tucson, 1982. Grim, M. J. A., Baas, F., Geballe, T. R., Greenberg, M. J. and Schutte, W., Astron. Astrophys. 243,473, 1991. d’Hendecourt, L. B., Allamandola, L. J. and Greenberg, J. M., Astron. Astvophys. 152, 130, 1985. Jenniskens, P., Baratta, G. A., Kouchi, A., de Groot, M., Greenberg, J. M. and Strazzulla, G., Astron. Astrophys. 273, 583, 1993. Johnson, R. E., Energetic Charged-particle Interactions with Atmospheres and Surfaces. Springer, Berlin, 1990. Kerr, T. H., Adamson, A. J. and Whittet, D. C. B., Mon. Not. R. astron. Sot. 262, 1047, 1993. Lacy, J. H., Baas, F., Allamandola, L. J., Persson, S. E., McGregor, P. J., Lonsdale, C. L., Geballe, T. R. and van de Bult, C. E. P., Astrophys. J. 276, 533, 1984. Moore, M. H. and Hudson, R. L., Astron. Astrophys. Suppl. 103, 45, 1994. Moore, M. H. and Khanna, R. K., Spectrochim. Acta 47, 255, 1991. Palumbo, M. E. and Strazzulla, G., Astron. Astrophys. 269, 568, 1993. Roessler, K., Non-equilibrium chemistry in space. NIMS Phys. Res. B65, 55, 1992.
1251 Schutte, W., Tielens, A. G. G. M. and Sandford, S. A., Astrophys. J. 382, 523, 1991. Skinner, C. J., Tielens, A. G. G. M., Barlow, M. J. and Justtanont, K., Astrophys. J. 399, L79, 1992. Spinella, F., Baratta, G. A. and Strazzulla, G., Rev. Sci. Instrum. 62, 1743, 1991. Strazzulla, G. and Baratta, G. A., Astron. Astrophys. 266, 434, 1992. Strazzulla, G. and Johnson, R. E., Irradiation effects on comets and cometary debris, in Comets in the Post-Halley Era (edited by R. Newburn, Jr., M. Neugebauer and J. Rahe), p. 243. Kluwer, Dordrecht, 1991. Tielens, A. G. G. M. and Allamandola, L. J., Evolution of interstellar dust, in Physical Processes in Interstellar Clouds (edited by G. E. Morfill and M. Scholer), p. 333. Reidel, Dordrecht, 1987a. Tieleus, A. G. G. M. and Allamandola, L. J., in Interstellar Processes (edited by D. J. Hollenbach and H. A. Thronson). Reidel, Dordrecht, 1987b. Tielens, A. G. G. M. and Hagen W., Astron. Astrophys. 114,245, 1982. Tielens, A. G. G. M., Tokunaga, A. T., Geballe, T. R. and Baas, F., Astrophys. J. 381, 181, 1991. Whittet, D. C. B. and Walker, H. J., Solid CO and CO, in grain mantles, in Molecular Clouds (edited by R. A. James and T. J. Millar), p. 309. Cambridge University Press, Cambridge, 1991. Whittet, D. C. B., Adamson, A. J., Duley, W. W., Geballe, T. R. and McFadzean, A. D., Mon. Not. R. astron. Sot. 241, 707, 1989.
Planet. Space Sci., Vol. 43, Nos. lO/ll, pp. 1247-1251, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-0633/95 $9.50+0.00 0032-0633(95)00040-2
Ion irradiation of astrophysical ices G. Strazzulla, A. C. Castorina and M. E. Palumbo Istituto di Astronomia, Cittk Universitaria, I-95125 Catania, Italy Received 14 November 1994; revised and accepted 13 February 1995
Absbact. We report recent experimental studies on the
physical-chemical effects induced by fast ions colliding with frozen gases (CO, C&OH, CW,, CO,) and mixtures with water simulating icy targets in space (frosts on external planets and satellites, comets and interstellar ices). We have studied, by IR spectroscopy, the formation of new molecules, the alteration of band profile, the spectral changes of the origina compounds and the formation of an organic refractory residue left over after ion irradiation. In particular, we present here the results of the measurements .of the CO/CO2 ratio resulting from the irradiation of several ice mixtures.
1. Introduction
A fast ion impinging on solid targets produces many effects that have been studied in several laboratories around the world because of their astrophysical relevance (Johnson, 1990; Strazzulla and Johnson, 1991; Roessler, 1992; Moore and Hudson, 1994). Our group has, in particular, studied the formation and some properties of organic residues resulting from ion irradiation of carboncontaining frozen ices. When further irradiated, the residue evolves towards what we call IPHAC (Ion Produced Hydrogenated Amorphous Carbon). Both fast ions (low energy cosmic rays, galactic protons, solar protons, solar wind particles) and carbon-containing solid targets (interstellar grains, comet mantles, interplanetary dust particles etc.) are present in space, thus under certain circumstances, material similar to that produced in the laboratory may form in space (Strazzulla and Baratta, 1992 ; Jenniskens et al., 1993). In interstellar molecular clouds, icy grain mantles are believed to be an important component of dust. Their presence is indicated by IR absorption features, e.g. at 3.1, 4.67 and 6.0 pm, observed in the spectra of objects embedded in or located behind dark clouds. These feaCorrespondence to : G. Strazzulla
tures are generally attributed to simple molecules, such as H,O, CO and CH,OH, frozen in icy grain mantles (Tielens and Allamandola, 1987a). These icy mantles are thought to form after accretion and reaction of gas phase molecules onto grain surfaces (Tielens and Hagen, 1982 ; d’Hendecourt et al., 1985; Brown et al., 1988). During their lifetime, volatile icy grain mantles may suffer UV and cosmic ray irradiation and be transformed into organic refractory mantles, consisting of quite complex molecules which will probably survive in the diffuse interstellar medium (Greenberg, 1982; Strazzulla and Baratta, 1992). In this paper we present some results obtained from recent experimental studies on the physical-chemical effects induced by fast ions colliding with frozen gases (CO, CH,OH, CH4, C02) and mixtures with water simulating icy targets in space.
2. Experimental apparatus
The in situ analyses were performed in a stainless steel scattering chamber facing an FTIR spectrometer (PerkinElmer model 1710) through KBr windows. A stream of gas from an inlet tube was used to form frost on a silicon crystal placed into thermal contact with the tail section of a close-cycle helium cryostat (lo-300 K). Samples can be bombarded by 3 keV helium ions. The beam produces a 2 x 2 cm2 spot on the target and currents in the range of 0.2-2 PA cm-‘. The directions of the IR and ion beam are mutually perpendicular. The substrate is at an angle of 45” with respect to the ion beam direction. Hence, before, during and after irradiation, spectra can be obtained without tilting the sample (Spinella et al., 199 1). For calibration purposes many spectra have been taken at different rates of ice deposition. The sample thickness was determined by previously obtained calibration data. The latter have been obtained by depositing a quantity of ice whose thickness was checked by counting the interference fringes of a reflected laser light (632.8 nm). Thus, when a target was simultaneously deposited and irradiated we knew both the number of molecules deposited (on
at a dose of 44 eV/l6 amu. Because of Irradiation, new species are formed; in fact, we observe in the irradiated spectrum the presence of new’ bands. Among these, the 2136, 1308 and 2344 cm-’ bands are due, respectively, to CO, CH, and COZ ; II,0 is also formed. The formation of H,O is, however, better evidenced by irradiation of pure CH30H (Baratta et al., 1994). Furthermore, the relative intensity of CH,OH bands changes as a function of the dose. The abundances of new species were estimated from the peak absorption coefficients that were measured previously, together with the integrated band strengths, in calibration experiments described by Palumbo and Strazzulla (1993). In Fig. 1 there is a band at 1720 cm-‘. This could be associated with formaldehyde (H,CO). Nevertheless, IR spectra of H&O (Moore and Khanna, 1991) also show a strong band at about 1500 cm-‘, and several bands of lower intensity at 1249, 1178, 2891, 2829 and 2725 cm-’ which do not appear in our spectra. Other compounds, such as acetone [(CH,),CO] (Baratta et al., 1994) also show a band at 1720 cm-‘; thus, we can infer that this band, formed after irradiation of the CH30H-H,O mixture, may be due to more than one species [e.g. H&O, (CH,),COJ and it cannot be used to estimate the abundance of any species. We compared our results with those obtained by Allamandola et aE. (198&), who irradiated H,O-CH30H mixtures (2 : 1) with UV photons. The CH,/CO and CO/CO:! ratios value N 0.1 and E 1.25 respectively, at the end of UV irradiation. These values are more consistent with those we find irradiating pure CH,OH than those obtained from the irradiation of a CH,OH-H,O (1 : 2) mixture (Baratta et aZ., 1994). A possible explanation is that, in contrast with what happens upon ion irradiation, H,O is almost unaltered by UV photons. Thus, the products from Hz0 radiolysis, mainly H, HZ, 0 and OH, do not participate in the UV-induced radiation chemistry. Another step of our work has been the study of the effects produced after warm-up of our samples. ‘The range of temperatures was lo-213 K. In Fig. 2 we present a comparison between the spectra of an unirradiated H,OCH,OH mixture (1 : 1) and those of the irradiated mixture at different temperatures. At 157 K in the spectrum of the unirradiated mixture we observe that : helium ‘Ions
0 3000
2000
1000
Wavenumber (cm-‘)
iR spectra of H,O-CH,OH (1 : l), deposited on a cold (10 K) silicon substrate, before and after irradiation with 3 keV helium ions (44 eV/16 amu) the silicon wafer) per time unit and the number of ions (current) impinging in the time unit, i.e. we were able to evaluate the irradiation dose of that experiment. Moreover, taking in situ spectra we could stop the experiment at any time when the irradiated sample showed well defined features. Here, we give doses in eV per molecule of 16 amu because this is a convenient way to characterize chemical changes (Strazzulla and Johnson, 1991) and compare them with experiments on other targets and, with some precautions, with other ions or other ion energies. 3. Laboratory studies of solid methanol and watermethanol mixtures
Besides H,O and CO, methanol (CH,OH) has also been detected (Tielens and Allamandola, 1987a; Grim et al., 1991; Schutte et al., 1991; Skinner et al., 1992) in icy grain mantles. In fact, a large number of IR spectra of sources obscured by dense clouds show different absorption features which, according to laboratory experiments, have been attributed to solid CH,OH. However, the abundance of solid CH,OH is quite controversial. Its abundance is estimated to be 5-50% of that of H20, depending on the absorption features used to estimate it. In general the C-H deformation mode at 6.85 pm gives an abundance of about 50%, while the C-H stretching mode at 3.5 pm and the C-O stretching mode at 9.7 ,um indicate an abundance of about 10% (Grim et al., 1991 ; Schutte et al., 1991; Skinner et al., 1992). We have performed several laboratory experiments in order to study the properties of solid CH,OH and mixtures with water (H,O-CH,OH). We have also irradiated our samples in order to analyze, by IR spectroscopy,
the
effects induced by impinging ions during and after the bombardment. The IR spectrum of H,O-CH30H (1: 1) mixture deposited on a cold (10 K) silicon substrate is compared (Fig. 1) with that obtained after irradiation with 3 keV
CH,OH is still present (see 1034 cm-l band) with an abundance of about 10% with respect to water ; the shape of the 3260 cm-] band is that of a crystal ice, showing that the amorphous to crystal transition has occurred. The unirradiated sample sublimes at about 200 K. When we analyze the spectra of irradiated mixtures, we observe that : some structures are still evident at 2 13 K ; the sample thus shows refractory properties ; the amorphous to crystal transition has not occurred at 157 K, but takes place at about 200 K, as we infer from the shape of 3260 cm-’ band ; volatile species, such as CO and COZ, are still present at
1249
G. Strazzulla et al. : Ion irradiation of astrophysical ices H20:CH80H W
C
(1: 1) ”
+ 44 d/16 amu “
“
“
“
“
“
”
4 1
3
1
4000
3000 2000 1000 Wavenumber (cm-‘)
4000
3000 2000 1000 Wavenumber (a~~‘)
Fig. 2. IR spectra of an unirradiated H,O-CH,OH mixture (1 : 1) are compared, at different temperatures, with those of the irradiated mixture 157 K (pure CO sublimes at - 30 K). This suggests that those species have been trapped into the refractory structure produced by ion irradiation. Finally, the band at 1100 cm-‘, evident in these spectra (downwards), is due to the silicon substrate on which the sample has been deposited. In fact, the profile of the silicon band changes as the temperature increases. The main implication of these results in astrophysical scenarios is that one has to consider that the sublimation properties of irradiated ice are greatly different from those of unirradiated ice mixtures. These differences should be taken into serious account.
4. The CO/CO2 ratio in irradiated ices The observation of solid CO, on dust grains in dense clouds, until now lacking, will be one of the major tasks of the Infrared Space Observatory (ISO), whose launch is scheduled in the near future. In particular, the measure of the CO/CO, ratio will be very useful to get insight into physical-chemical processes taking place on interstellar grains. The present situation is that many interstellar spectra of field and embedded stars show an absorption band centred at about 2140 cm-’ which is attributed to solid CO in grain mantles. CO abundance in interstellar grains relative to that of HZ0 is deduced from the strength of solid CO absorption relative to that of the water-ice feature at 3.1 pm and it has values of 040% in different molecular clouds (Eiroa and Hodapp, 1989; Whittet et
al., 1989; Tielens et al., 1991 ; Kerr et al., 1993; Chiar et al., 1994). The asymmetric profile of this band is believed to derive from two independent components (Lacy et al., 1984) : a narrow component (FWHM 5-8 cm-‘) centred at 2140 cm-’ and a broader one (FWHM N 10 cm-‘) centred at 2136 cm-‘. The identification of the species which give rise to these different components is not yet fully understood. On the basis of our experimental results we proposed some comparisons between interstellar and laboratory spectra, which take into account the interaction of grain mantles with cosmic ions and the presence of CH30H in grain mantles [see Palumbo and Strazzulla (1993)]. Our best fits, in the spectral range 2120-2155 cm-‘, relative to field stars were obtained using the 2140 cm-’ band of irradiated CO ice at 10 K (dose = 12 eV/16 amu) and the 2136 cm-’ band due to CO produced after ion irradiation of an HPO-CH,OH (2 : 1) mixture at 10 K (dose = 41 eV/16 amu). Fits relative to embedded stars were obtained using the 2140 cm-’ band of irradiated CO ice at 10 K (dose = 12 eV/16 amu) and the 2136 cm-’ band due to CO produced by irradiation of an H,O-CH30H (2: 1) mixture (dose = 41 eV/16 amu) warmed up to 67 K. The astrophysical situation is, of course, more complex : doses, temperatures and composition of irradiated samples are to be considered as rough averages. It is interesting to note that, as H,O-rich samples are more stable than COrich ones, the findings that the latter are “on average” four times less irradiated than the former is quite plausible. From estimates of low energy cosmic rays flux in the interstellar medium (Jenniskens et al., 1993) and grain mantles’ lifetime (Greenberg, 1982) we deduced that the energy deposited on a grain (dose) has values of 0.05-50 eV/atom (C, N, 0, Si). [Note that the dose scales roughly according to molecular weight.] As concerns those sources embedded in their placental clouds, our best fits are obtained for the same combination as above, but with the H,O-CH,OH irradiated samples warmed up to 67 K. Even in this case such a temperature has to be considered as an average. We can surmise that around these stars the radiation field increases the grain temperature : near the star all icy mantles sublimate ; at a certain temperature (140-l 60 K) only the H,O-dominated mantles survive ; at greater distance (lower r) the grains are unheated impertubated, like those observed towards field stars. In a series of laboratory experiments we have studied the formation of CO, after ion irradiation of frozen CO, CH,OH, CH, and mixtures with H20. CO, is not predicted to have appreciable abundance in the gas phase in dense clouds, so that its condensation on interstellar grains can be neglected, while its presence may be due to surface reactions between 0 and CO as well as to surface processing, such as UV and cosmic ray irradiation (Tielens and Allamandola, 1987b; Whittet and Walker, 1991). In Fig. 3 the ratios measured for several irradiated ices and mixtures are shown as a function of the irradiation dose. It is interesting to note that after a dose of the order of 30 eV/16 amu, the ratio has values of 0.3-l for all of the considered targets, except for CO, for which it is about 4. Using these data and the grain model described above, we can give a rough estimation of the amount of CO, we
S. lStrazzuila 21* ai. ion trraaration 31 asirophysicai ices
A co q
CH,OH
x H,O:CH,OH=Z:l . . A
H,O:CO=lO:l _ H,O:CO,=l:l
0 co, * H,O:CH,=l:l O.Olk
3
j
20
’
2
’
dose (e”,l@%)
’
80
*
I
’ 2400
Fig. 3. CO/CO2 ratios are given as a function of energy deposited by 3 keV helium ions on different types of ice targets
expect in the line of sight of several sources. In fact, our fits of the CO bands give us an estimate of CO abundance in grain mantles as well as of the predicted CO/CO, ratio. Our results are shown in Table 1. These results have to be carefully interpreted because of the uncertainty in the initial composition of icy mixtures, in the doses suffered by icy mantles by ions and/or UV photons and temperatures. However, it turns out that CO2 abundance is comparable with that of CO and hence it should be observable by the 1SO. Figures 4 and 5 show a comparison between the CO band observed towards Elias 16 and Tamura 8, and laboratory spectra in the 2400-2100 cm-’ range. The band at about 2340 cm-’ is due to the sum of two contributions : CO, produced after ion irradiation of CO (12 eV/16 amu) and that produced from H,O-CH30H (2: 1; after 41 eV/16 amu) ices. The astrophysical plausibility of such a two-population model has been discussed in detail by Palumbo and Strazzulla (1993). Different CO/CO2 ratios, in the two sources, are due to different contributions of the narrow and broad components in the fit of the CO band.
2300
wavenumber
2200
2100
(cm-‘)
Fig. 4. Comparison, in the spectral range 2400-2100 cm-‘, between the observed spectrum of the source Elias 16 (boxes) and the sum of laboratory spectra obtained after ion irradiation of CO and H,O-CH,OH samples (solid line ; see text)
3i 4
j
2400
2300
2200
2100
Acknowledgements. We are grateful to the Italian Space Agency
wavenumber
(ASI) for support of this research. We wish also to acknowledge
Fig. 5. Comparison, in the spectral range 2400-2100 cm-‘, between the observed spectrum of the source Tamura 8 (boxes) and the sum of laboratory spectra obtained after ion irradiation of CO and H,O-CH,OH samples (solid line ; see text)
Table I. Estimates of CO abundance and CO/COZ interstellar icy grain mantles Source
N(C0) (lOI mol cm-‘)
Elias 3
1.06 2.01 3.71 2.6 1.58 1.5 1.7 1.62 4.5
Elias 15 Elias 16 Tamura 8 AFGL 989 Elias 18 NGC 2024 Mon R2 W33A
ratios in
2.12 0.65 1.24 0.68 0.99 0.71 1.06 0.74 0.31
(cm-‘)
Max Bernstein and an anonymous referee for their careful reading of the manuscript and helpful suggestions.
References Allamandola, I.. J., Sandford, S. A. and Valero, 6. J., Icurus 76, 225, 1988. Baratta, 6. A., Gastorina, A. CL, Leto, G., Palumbo, M. F., Spinella, F. and Strazzulla, G., Planet. Spuce Sci. 42, 759, 1994.
G. Strazzulla et al. : Ion irradiation of astrophysical ices Brown, P. D., Charnley, S. B. and Millar, T. J., Mon. Not. R. ustron. Sot. 231,409, 1988. Chiar, J. E., Adamson, A. J., Kerr, T. H. and Whittet, D. C. B., Astrophys. J. 426, 240, 1994. Eiroa, C. and Hodapp, K. W., Astron. Astrophys. 210,345, 1989. Greenberg, M. J., What are comets made of? A model based on interstellar dust, in Comets (edited by L. L. Wilkening), p. 13. The University of Arizona Press, Tucson, 1982. Grim, M. J. A., Baas, F., Geballe, T. R., Greenberg, M. J. and Schutte, W., Astron. Astrophys. 243,473, 1991. d’Hendecourt, L. B., Allamandola, L. J. and Greenberg, J. M., Astron. Astvophys. 152, 130, 1985. Jenniskens, P., Baratta, G. A., Kouchi, A., de Groot, M., Greenberg, J. M. and Strazzulla, G., Astron. Astrophys. 273, 583, 1993. Johnson, R. E., Energetic Charged-particle Interactions with Atmospheres and Surfaces. Springer, Berlin, 1990. Kerr, T. H., Adamson, A. J. and Whittet, D. C. B., Mon. Not. R. astron. Sot. 262, 1047, 1993. Lacy, J. H., Baas, F., Allamandola, L. J., Persson, S. E., McGregor, P. J., Lonsdale, C. L., Geballe, T. R. and van de Bult, C. E. P., Astrophys. J. 276, 533, 1984. Moore, M. H. and Hudson, R. L., Astron. Astrophys. Suppl. 103, 45, 1994. Moore, M. H. and Khanna, R. K., Spectrochim. Acta 47, 255, 1991. Palumbo, M. E. and Strazzulla, G., Astron. Astrophys. 269, 568, 1993. Roessler, K., Non-equilibrium chemistry in space. NIMS Phys. Res. B65, 55, 1992.
1251 Schutte, W., Tielens, A. G. G. M. and Sandford, S. A., Astrophys. J. 382, 523, 1991. Skinner, C. J., Tielens, A. G. G. M., Barlow, M. J. and Justtanont, K., Astrophys. J. 399, L79, 1992. Spinella, F., Baratta, G. A. and Strazzulla, G., Rev. Sci. Instrum. 62, 1743, 1991. Strazzulla, G. and Baratta, G. A., Astron. Astrophys. 266, 434, 1992. Strazzulla, G. and Johnson, R. E., Irradiation effects on comets and cometary debris, in Comets in the Post-Halley Era (edited by R. Newburn, Jr., M. Neugebauer and J. Rahe), p. 243. Kluwer, Dordrecht, 1991. Tielens, A. G. G. M. and Allamandola, L. J., Evolution of interstellar dust, in Physical Processes in Interstellar Clouds (edited by G. E. Morfill and M. Scholer), p. 333. Reidel, Dordrecht, 1987a. Tieleus, A. G. G. M. and Allamandola, L. J., in Interstellar Processes (edited by D. J. Hollenbach and H. A. Thronson). Reidel, Dordrecht, 1987b. Tielens, A. G. G. M. and Hagen W., Astron. Astrophys. 114,245, 1982. Tielens, A. G. G. M., Tokunaga, A. T., Geballe, T. R. and Baas, F., Astrophys. J. 381, 181, 1991. Whittet, D. C. B. and Walker, H. J., Solid CO and CO, in grain mantles, in Molecular Clouds (edited by R. A. James and T. J. Millar), p. 309. Cambridge University Press, Cambridge, 1991. Whittet, D. C. B., Adamson, A. J., Duley, W. W., Geballe, T. R. and McFadzean, A. D., Mon. Not. R. astron. Sot. 241, 707, 1989.