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ISO-SWS observations of interstellar solid 13CO2: heated ice and the Galactic 12C/13C abundance ratio

We present observations of the stretching mode of 13CO2 ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory. Remarkable variations are seen in the absorption band profile in the different lines of sight. The main feature is attributed to 13CO2 mixed with polar molecules such as H2O, and CH3OH. Several high-mass protostars show an additional narrow substructure at 4.382 um, which we attribute to a polar CO2 ice, that experienced heating. This heating effect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar CO2 ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing. Furthermore, we determine for the first time the Galactic 12C/13C ratio from the solid state. The 12CO2/13CO2 ratio for the local ISM (69+/-15), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase (C18O, H2CO) studies. For the few individual objects for which gas phase values are available, the 12C/13C ratios derived from CO2 tend to be higher compared to CO studies (albeit with 2.5 sigma significance only). We discuss the implications of this possible difference for the chemical origin of interstellar CO2.


Introduction
Space based observations have revealed that the CO 2 molecule is an important constituent of quiescent and star forming molecular clouds (d 'Hendecourt & de Muizon 1989;de Graauw et al. 1996;d'Hendecourt et al. 1996;Gürtler et al. 1996;Strazzulla et al. 1998;Whittet et al. 1998;Gerakines et al. 1999). It is primarily present in the solid state , with an abundance of typically 20% of water ice, consuming ∼3% of the carbon budget. The main focus of research has been on the high quality observations of the solid 12 CO 2 stretching and bending modes at 4.27 and 15.2 µm, which are prominently present in the spectra obtained with ISO-SWS Boogert 1999).
The detection of the 13 CO 2 isotope, with a two orders of magnitude lower abundance, has been reported as well d'Hendecourt et al. 1996). Its stretching mode frequency is well separated from that of 12 CO 2 (4.38 µm vs. 4.27 µm). The analysis of this weak band is very attractive and has some specific advantages over 12 CO 2 studies. It is an independent and very sensitive probe of the ice mantle composition. 13 CO 2 is always a trace constituent. In contrast to 12 CO 2 , interactions of 13 CO 2 molecules with themselves, and consequent effects on the band profile, are unimportant. For the same reason, the polarizability of the ice at the 13 CO 2 resonance frequency can be neglected, and the absorption profile is insensitive to the shape and thickness of the ice mantle, and composition of the grain core, contrary to the 12 CO 2 bands. Thus the laboratory spectrum can be compared directly to the interstellar spectrum, and uncertainties resulting from corrections induced by the grain shape or uncertain optical constants are avoided (Ehrenfreund et al. 1996;1997).
The main motivation for our study is to determine the physical and chemical history of interstellar CO 2 , and interstellar ices in general. Once interstellar ices are formed, by a combination of direct accretion from the gas phase and chemical reactions on grain surfaces, they can be exposed to a variety of violent processes. Among these are cosmic ray bombardment, ultraviolet irradiation, heating by visible and infrared photons, and disruption in shocks. What is the importance of these processes in various environments, such as protostars of low and high mass, and quiescent clouds? Can the ices around low mass protostars survive the various destruction mechanisms, and be included in comets? If so, how much do its composition and structure still resemble the interstellar ices? In order to answer these questions, we analyze the absorption band profile of the stretching mode of interstellar 13 CO 2 . We make use of a large database of spectra of CO 2 ices, with a range of compositions and temperatures, obtained in the Leiden Observatory Laboratory (Ehrenfreund et al. 1996;1997;. In a separate study, the analysis of the absorption bands of 12 CO 2 will be presented .
Another important motivation for this study is to determine the 13 CO 2 abundance, and derive the Galactic 12 C/ 13 C abundance ratio. It is the first time that the 12 C/ 13 C ratio is determined from the solid state. An important advantage over gas phase studies is that the column density can be straightforwardly derived from the observed integrated optical depth and the intrinsic band strength determined in the laboratory. In contrast, for gas phase species excitation, and radiative transfer models are required.
Previous (gas phase C 18 O, H 2 CO) studies have shown that the 12 C/ 13 C ratio increases with Galacto-centric radius, with 12 C/ 13 C=25 in the Galactic Center, and 12 C/ 13 C = 77 in the local interstellar medium (see Wilson & Rood 1994 for an overview). Recent observations of atomic C and C + yield 12 C/ 13 C=60 toward the Orion Bar (Keene et al. 1998). Determination of the 12 C/ 13 C ratio is an important input for evolutionary models of our Galaxy, since 12 C is produced by Helium burning by massive stars, which can be converted to 13 C in the CNO cycle of lowand intermediate-mass stars at later times.
Furthermore, comparison of the 12 C/ 13 C ratios derived from various species, will allow to determine the importance of chemical fractionation ( 13 C preferentially incor-porated in CO) and isotope-selective destruction ( 13 CO preferentially destroyed). Models of photo-dissociation regions (PDRs), including chemical fractionation and isotope-selective destruction, show that the 12 C/ 13 C ratio for gaseous C 18 O can decrease by ∼50%. Recent observations of C and C + , however, indicate that chemical fractionation is not an important effect, or is compensated for by isotope-selective photo dissociation, in these PDRs (Keene et al. 1998). We do not expect that these effects play an important role for the CO in our lines of sight, since they mainly trace dense molecular cloud material with low atomic C abundances. However, species that are formed from atomic C or C + rather than CO, may have very different 12 C/ 13 C ratios (e.g. H 2 CO; Tielens 1997). Thus, the determination of the solid 12 CO 2 / 13 CO 2 ratio is a potentially powerful tool to trace the chemical history of interstellar CO 2 ; does it originate from CO, as is generally assumed, or perhaps from C (+) ?
In Sect. 2 we present our ISO-SWS observations and data reduction techniques. The laboratory results of the 13 CO 2 stretching mode are summarized in Sect. 3. The analysis of the interstellar 13 CO 2 band profile, and derivation of the column densities is presented in Sect. 4. We discuss the main results in Sect. 5. First, we correlate the depth of the detected narrow 2282 cm −1 absorption feature with known physical parameters along the observed lines of sight. Then we compare the 12 C/ 13 C ratios, derived from solid CO 2 and gas phase molecules, and discuss the importance of chemical fractionation, and the reaction pathway to form CO 2 . The summary and conclusions are given in Sect. 6.

Observations
The 13 CO 2 spectra were obtained with ISO-SWS Kessler et al. 1996). Most spectra were observed in the high resolution grating mode (R = λ/∆λ=1500; "SWS06" mode). Only the source GL 989 was observed in the fast scanning mode ("SWS01 speed 3") at an effective resolving power R ∼400.
The spectra were reduced with version 6 of the SWS pipeline during January-April 1998 at SRON Groningen, using the latest calibration files available at that time. For some low flux spectra we applied a "pulse shape" correction on ERD level  to linearize the slopes, resulting in ∼10% better signal-to-noise. All detector scans were checked for excessive noise levels, deviating flux levels and continuum slopes, and dark current jumps. Bad scans were taken out off the data, and then the scans were "flat-fielded" to the median spectrum using a first order polynomial. Cosmic ray hits were removed by clipping all points deviating more than 3 σ from the median, and subsequently the scans were averaged and rebinned per scan direction to a resolving power of R = 1500 (R = 400 for GL 989). The effective "signal-to-noise" values, as determined from the systematic differences be-3 tween the SWS up and down scans, are given in Table 1. We note that due to the weakness of the interstellar 13 CO 2 band, ∼ 10% of the continuum, our analysis (column densities, band profiles) is not significantly affected by detector memory effects, known to exist at this wavelength. Also, the detector response function is very smooth over the narrow 13 CO 2 band, and thus will not induce spurious features due to small wavelength calibration errors or dark current uncertainties.
An accurate wavelength calibration is important for our study. The standard SWS wavelength calibration was applied (Valentijn et al. 1996). The satellite velocity was converted to the heliocentric reference frame, using standard ISO-SWS pipeline routines. Then we corrected for the source velocity in the heliocentric frame, using published radial velocities for each source (Table 1). Recent millimeter observations of C 17 O emission lines show velocity shifts that are in excellent agreement with the values that we use (Van der Tak et al., in prep.). To check the accuracy of the wavelength calibration and velocity corrections, we compared the position of ro-vibrational 12 CO gas phase lines ( 13 CO lines are too weak), when present in the same spectrum, with peak positions from the HI-TRAN database (Rothman et al. 1992). For S 140 : IRS1, and W 3 : IRS5 the lines are blue-shifted by 10% of a resolution element (∼0.15 cm −1 ; ∼20 km s −1 ) compared to the HITRAN wavelengths, for GL 2591 by 20%, and for GL 2136 by 30%. Part of this blue-shift may be real. Mitchell et al. (1991) find that, whereas the velocity shift of 13 CO lines is in good agreement with millimeter studies, the 12 CO absorption lines show significant blue shifted shoulders and sub-peaks, originating from the outflow associated with the protostar. At the much lower resolution of our observations (∼160 km s −1 ), this may result in a blue shift of the 12 CO absorption line. However, the main absorption is present at the cloud velocity, and at our resolution the peak would shift by not more than −15 km s −1 (GL 2591), −10 km s −1 (W 3 : IRS5) or much less than that for the other sources. Thus, we attribute the CO line shifts in the ISO-SWS spectra primarily to uncertainties in the wavelength calibration and small pointing errors. In particular, GL 2136 was observed at the end of the ISO mission for which no updated wavelength calibration files are available yet. We applied the small shift derived from the CO lines to further improve the wavelength calibration of the 13 CO 2 band. For one object, GC 3, we found no radial velocity observation in the literature, and determined it from the CO lines to be v helio = −90 km s −1 . The fully reduced spectra are given in Fig. 1.

Laboratory CO 2 studies
The absorption band profiles of the stretching and bending modes of 12 CO 2 and 13 CO 2 in apolar ices (CO, N 2 , O 2 ) were extensively studied in the laboratory by Ehrenfreund et al. (1996;1997). Additional experiments of CO 2 mixed with polar molecules (H 2 O, CH 3 OH) were performed in the Leiden Observatory Laboratory as well (Ehrenfreund et al., 1998;. Figure 2a summarizes the peak shifts and broadenings of the stretching mode of 13 CO 2 in these ices at a temperature T =10 K. For a pure CO 2 ice, i.e. 12 CO 2 : 13 CO 2 =90:1, the absorption band of 13 CO 2 is centered on 2283.0 cm −1 and is Fig. 1. The ISO-SWS spectra of 13 CO 2 on optical depth scale toward massive protostars (left two panels). The right panel shows low-mass protostars, a background star of the Taurus Molecular Cloud (Elias 16) and a Galactic Center source (GC 3). The spectra were normalized to the peak optical depth by multiplication with the factors given in square brackets. The error bars were determined from the difference between the ISO-SWS up and down scans, and thus include systematic errors. The vertical dashed lines are given to facilitate comparison of the spectra in the different panels.
very narrow (FWHM∼2.3 cm −1 ). A large broadening and shift to lower frequencies is observed when H 2 O is added. An even larger peak shift, up to 2274 cm −1 , but a small narrowing, occurs in CO 2 :CH 3 OH ices. This difference in spectroscopic behavior of H 2 O:CO 2 and CH 3 OH:CO 2 ices is also particularly evident in the 12 CO 2 bending mode (Ehrenfreund et al. 1998;. It is ascribed to the formation of stable CH 3 OH.CO 2 complexes. Thus, at low temperatures, H 2 O and CH 3 OH-rich CO 2 ices have very distinct 13 CO 2 peak positions. Mixtures of CO 2 with both H 2 O and CH 3 OH lie in between these extremes, as indicated by the "weak" (H 2 O:CH 3 OH:CO 2 =7:0.6:1) and "strong" (H 2 O: CH 3 OH: CO 2 =1.7:0.6:1) interstellar mixtures in Fig. 2a. Apolar molecules, such as CO and O 2 , induce much smaller broadenings and peak shifts to the 13 CO 2 ice band. Thus, the laboratory simulations show that the 13 CO 2 stretching mode is a sensitive probe to discriminate between interstellar polar and apolar ices.
The peak position and width depend strongly on the temperature of the ice. Upon warm-up, the peak position of 13 CO 2 in H 2 O-, and CH 3 OH-rich ices (with less than ∼20% of CO 2 ) shifts significantly to lower frequencies, and at the same time becomes narrower (Fig. 2b). For example, in the mixture H 2 O:CO 2 =10:1, the 13 CO 2 band shifts by 2.5 cm −1 , and the FWHM decreases with 2.5 cm −1 , when heated from T =10 to 140 K in the laboratory. Unfortunately, heating of the polar, CO 2 -poor ice has the same effect on the band profile as increasing the H 2 O/CO 2 and CH 3 OH/CO 2 mixing ratios (Figs. 2a and 2b). Thus, the 13 CO 2 stretching mode is less suited to determine an accurate H 2 O/CH 3 OH/CO 2 mixing ratio, or the precise temperature of these particular ices. However, for H 2 O:CO 2 and CH 3 OH:CO 2 ices with CO 2 concentrations between 20-90%, heating induces a very specific spectroscopic signature. The 13 CO 2 profile becomes asymmetric at a laboratory temperature of T ∼50 K, and ∼105 K respectively (Figs. 2 and 3; see Sect. 5.1 for the corresponding temperatures in interstellar space). A second peak develops at the blue side of the band, which is reflected in Fig. 2b as a "broadening" of the overall profile. At higher temperatures, this new peak dominates the 13 CO 2 spectrum. Its peak position of ∼2282-2283 cm −1 , and narrow width (∼3 cm −1 ) are close to the values for a pure CO 2 ice, although, in particular for CH 3 OH ices, this correspondence is not exact. This spectroscopic behavior is attributed to the destruction of bonds between the polar molecules and CO 2 , and the formation of new, stronger, hydrogen bonds between the polar molecules . The CO 2 molecules now interact primarily with themselves. This re-arrangement of bonds ("segregation") is also particularly well traced in the 12 CO 2 bending mode, which shows, after heating, the typical double peaked structure of a pure CO 2 ice . In accordance with heating experi- Laboratory spectroscopy of the stretching mode of 13 CO 2 ice at T =10 K (a), and at higher temperatures (b). The average Gaussian parameters of the interstellar spectra are given by crosses. In panel a, the peak position versus width variations are shown for CO 2 in series of increasing H 2 O, CH 3 OH, and CO abundances (connected with a line). "W" and "S" indicate the weak and strong interstellar mixtures (see text). Panel b clearly illustrates the large effect of temperature on width and peak position of the 13 CO 2 stretching mode, due to the segregation of CO 2 . ments of H 2 O:CH 3 OH ices (Blake et al. 1991), it seems likely that also a spatial segregation exists between the CO 2 molecules and polar species. Crystalline phases of H 2 O ice enclosing 0.1 µm size pockets of CH 3 OH were observed in these heated H 2 O:CH 3 OH ices. Such microscopic observations are essential to determine the structure of heated polar CO 2 ices as well.
Finally, for apolar ices containing CO, O 2 , and N 2 , the 13 CO 2 band shifts to higher frequencies, and becomes narrower upon warm-up. This can be attributed to evaporation of the host molecule. These laboratory results will be used to derive the composition of interstellar ice, i.e. polar versus apolar, and possibly its temperature history.

Fig. 3.
Effect of heating on the 13 CO 2 stretching mode in the ice mixture H 2 O:CH 3 OH:CO 2 =1:0.92:1. This shows clearly the segregation of CO 2 at high temperatures. The peak at ∼2282 cm −1 is close to the peak position for a pure CO 2 ice (Fig. 2). We note that the indicated temperature was measured in the laboratory. At the much longer time scales in interstellar space, the segregation temperature is much lower (∼78 K; Sect. 5.1).

Contamination by gas phase lines
The spectral region of the 13 CO 2 ice band is contaminated by narrow absorption lines of gas phase CO in the lines of sight of GL 2136, GL 2591, and possibly S 140 : IRS1 ( Fig. 4). They originate from CO in the ground vibrational state (v=0-1), at high rotational levels, up to perhaps J=45. The presence of hot CO gas in these lines of sight was shown in other studies as well (e.g. Mitchell et al. 1990; Table 4). The main component of the 13 CO 2 ice band is much broader than the unresolved gas phase CO lines, and our conclusions are not influenced by this contamination. However, the narrow line at 2282 cm −1 detected in these sources (Sect. 4.2) is at the same frequency as the CO J=46→47 R-branch line. It is unlikely that this line can be attributed to gaseous CO, given its large depth relative to neighboring R-branch lines, in particular for S 140 : IRS1. Furthermore, we checked for the presence of features from vibrationally excited 12 CO. The band head of the v=1-2 transition at ∼2298 cm −1 (Goorvitch 1994) may be present toward GL 2136, and GL 2591 (Fig. 4). This feature is well separated from the 13 CO 2 ice band. There is no indication for absorption at the frequency of the v=2-3 band head (∼2268 cm −1 ). Given the weakness of these 12 CO band heads, the contribution of the v=0-1 band head of 13 CO (∼2276 cm −1 ) to the 13 CO 2 ice band can probably be neglected.
One object in our sample, Elias 16, is an evolved star located behind the Taurus molecular cloud. It is a K 1 giant (Elias 1978), and the 2-5 µm spectral region shows many narrow absorption lines, most likely associated with the photosphere of this object ). In the spectral region for this study (∼2260-2300 cm −1 ), we detect narrow absorption lines at 2268, and 2297 cm −1 (Fig. 1). These lines are well separated from the region where 13 CO 2 ice absorption is expected, and are attributed to the v=2-3 and v=1-2 band heads of 12 CO (Goorvitch 1994). However, another narrow line at 2276 cm −1 , probably the v=0-1 band head of 13 CO, might be blended with the 13 CO 2 feature. We compared the spectrum of Elias 16 with the K 1.5 giant α Bootis (Fig. 5). The spectral type of these stars is very similar, and indeed α Bootis does show absorption lines at 2268, 2276, and 2297 cm −1 . However, the lines in α Bootis have rather prominent wings on the red side, which are not recognized in Elias 16. Perhaps the spectral type or luminosity class of Elias 16 deviates from a K 1 giant. To correct for photospheric absorption, we subtracted the optical depth spectrum of α Bootis from Elias 16 (Fig. 5). The narrow lines at 2268, and 2297 cm −1 are nicely removed, giving confidence in the correctness of this method. A significant absorption remains at ∼2278 cm −1 , which we attribute to solid 13 CO 2 in the Taurus molecular cloud. In our further analysis, we used this corrected spectrum of Elias 16.

Interstellar solid 13 CO 2 band profile analysis
The 13 CO 2 band profile varies remarkably between the observed lines of sight ( Fig. 1). Two independent components are present. The high-mass protostars GL 2136, S 140 : IRS1, W 3 : IRS5, and GL 2591 show both a "broad" absorption extending over 2270-2287 cm −1 , and a very narrow feature at ∼2282 cm −1 . Two lines of sight do not show the narrow component (Elias 16, GC 3), while in others it may be blended with the blue edge of the broader feature. In NGC 2024 : IRS2 and GC 3 the profile analysis is limited by a low depth and poor signal-to-noise, while in GL 989 the resolving power is too low to resolve the narrow feature, if present.
We followed two different approaches to analyze the interstellar 13 CO 2 band profile. First, analogous to the solid CO band (e.g. Tielens et al. 1991), the narrow and broad component may originate from different ices along the line of sight. The ice composition and structure depend on the chemical (e.g. the H/CO ratio) and physical (e.g. temperature) history, which may have varied along the line of sight. Therefore, we decomposed the absorption band by fitting Gaussians to the observed broad and narrow components, and we compared the peak positions and widths with the laboratory results (Figs. 2, and 6). In our second approach, we investigate whether the 13 CO 2 ice band profile can be explained by a single ice. Here, we determine the observed peak position and width (not fitting Gaussians) and again compare these with the laboratory results in Figs. 2, and 6. Both methods provide good insight in the characteristics of the observed band profile, and the variations between the various lines of sight.

Two component ices
For five objects in our sample, good fits are obtained by fitting two Gaussians: NGC 7538 : IRS9, GL 2136, S 140 : IRS1, GL 2591, and W 3 : IRS5 (Table 2). For the other lines of sight, no significant improvement is achieved, when using two instead of one Gaussian. Nevertheless, for some sources the narrow 2282 cm −1 feature might be blended with the blue edge of the broader component, and we do perform a fit with two Gaussians as well.
For the broad component we find a Gaussian FWHM of typically 6-9 cm −1 , with a peak position varying between 2276-2280 cm −1 for the different objects. Thus, this feature can be ascribed to absorption by 13 CO 2 ice mixed with molecules with a large dipole moment, i.e. H 2 O and CH 3 OH (Figs. 2 and 6b; Table 2). To derive the chemical history of interstellar CO 2 it is important to determine the relative amounts of H 2 O, CH 3 OH, and CO 2 mixed in the ice. At high CH 3 OH concentrations, both CH 3 OH:CO 2 and H 2 O:CH 3 OH:CO 2 laboratory ices have a 13 CO 2 band  (a), but here the observed interstellar 13 CO 2 band is decomposed with Gaussians. This Figure shows that most interstellar 13 CO 2 bands can be explained by a single mixture at different temperatures (a) or by a strongly heated "segregated" ice component together with a non-segregated (perhaps cold) component (b).

Fig. 5.
Comparison of the spectra of Elias 16 and the K giant α Bootis on optical depth scale. The bottom spectrum is the subtraction of α Bootis from Elias 16, to correct for photospheric lines. The * symbols on top indicate the expected frequencies of the gas phase CO band heads (see text). The residual feature at ∼2278 cm −1 is attributed to 13 CO 2 ice in the Taurus molecular cloud. that peaks at frequencies less than ∼2276.0 cm −1 . Thus, we can exclude that the ice is CH 3 OH-rich, i.e. we conclude that CH 3 OH/CO 2 ≤ 1, which is in good agreement with the observed column densities (Sect. 4.4).
For several reasons, the broad component of the 13 CO 2 absorption band does not provide further constraints on the CH 3 OH/CO 2 mixing ratio, nor the H 2 O/CO 2 ratio.
First, although the width of the absorption band is a good discriminator between polar and and apolar ices (Sect. 3), it depends only weakly on the relative abundance of H 2 O and CH 3 OH in the ice (Fig. 2). Second, for peak positions in the range 2277.5-2279.5 cm −1 there is an ambiguity between H 2 O, and CH 3 OH ices. Good fits to the interstellar spectra can be obtained with H 2 O:CO 2 , CH 3 OH:CO 2 , as well as H 2 O:CH 3 OH:CO 2 ices at a range of temperatures. Thus, although we can put an upper limit of CH 3 OH/ CO 2 ≤ 1, no lower limit to the CH 3 OH concentration can be set from the broad 13 CO 2 component. Furthermore, for most objects good fits are obtained for a wide range of H 2 O concentrations. In some lines of sight we can put a significant lower limit H 2 O/CO 2 ≥ 1, when fitting H 2 O:CO 2 ices (e.g. Elias 29, NGC 7538 : IRS9, W 33A). However, when adding CH 3 OH to the ice, equally good fits with even lower H 2 O concentrations can be obtained.
The peak position of the narrow component is remarkably constant with a weighted mean of 2282.5±0.2 cm −1 , while the width is typically FWHM=2.5 cm −1 . This small width can be ascribed to 13 CO 2 interacting with apolar molecules (Figs. 2 and 6b). The accurate peak position provides further constraints to the origin of this feature. For some sources (GL 2136, W 3 : IRS5), both pure CO 2 and heated "segregated" polar CO 2 ices provide good fits. However, for the sources with the strongest narrow peaks (S 140 : IRS1, and GL 2591), the best fits are only obtained by heated polar CO 2 ices, showing segregation behavior. These ices must contain CH 3 OH, and may contain H 2 O (but not necessarily). The initial CO 2 concentration in these heated CH 3 OH-containing ices must be in between 20-90%. At higher CO 2 concentrations, the peak position and width are not well matched (Fig. 7). At very low CO 2 concentrations, no segregation takes place upon heating (see Sect. 3 and the CH 3 OH:CO 2 =1:10 ice in Fig. 2). Table 2. The observed peak position (ν), width (FWHM), and peak optical depth (τ ) of the interstellar solid 13 CO 2 band. For sources with high quality spectra we give in each second line the results of a decomposition of the profile, using 2 Gaussians. The standard deviation σ is given in parentheses.   7. Laboratory fits to the narrow 13 CO 2 component of S 140 : IRS1, indicating that a heated "segregated" CH 3 OH containing ice, rather than a pure CO 2 ice is needed to fit the exact peak position.
For completeness, we mention that satisfactory fits to the narrow features in GL 2591, and S 140 : IRS1 are also obtained by CO-rich ices, peaking at 2281.7 cm −1 . However, in these lines of sight no CO ice was detected Gerakines et al. 1999;Table 4), and these mixtures can be excluded. Concluding, the detected narrow 2282 cm −1 component can be well fitted with a heated polar CO 2 ice showing segregation. For at least two sources in our sample, this ice must contain CH 3 OH (CH 3 OH/CO 2 >0.10) to fit the exact peak position. For the other sources, a pure CO 2 gives reasonable fits as well, but mixtures with significant amounts of other apolar species (CO, O 2 ) can be excluded.

Single component ices
When comparing the observed peak frequencies and overall widths (not fitting Gaussians) with the laboratory results, we find that toward most lines of sight the complete 13 CO 2 ice band profile can be fitted with one single ice at a specific laboratory temperature (Figs. 6a and 8). Good fits are obtained with an ice in which H 2 O, CH 3 OH, and CO 2 are about equally abundant, which is in good agreement with studies of the 12 CO 2 bending mode (Gerakines et al. 1999). However, equally good fits are obtained 9 with other mixing ratios as well. For objects showing evidence for a separate 2282 cm −1 component (e.g. S 140, GL 2591), the ice must contain at least some CH 3 OH to fit the peak position (CH 3 OH/CO 2 >0.10) and the H 2 O abundance must not be too large (H 2 O/CO 2 <3). To fit W 33A with a single ice, the CH 3 OH abundance must be at least 30% of CO 2 . In contrast, the object R CrA : IRS2 must have an CH 3 OH/CO 2 ratio less than ∼50% (note the offset from the other sources in Fig. 6a). For other objects, such as HH 100, the ice composition can not be significantly constrained.
If we do assume that the 13 CO 2 band profile originates from a single ice, and it contains CH 3 OH and H 2 O with comparable concentrations, the temperatures of the best fitting laboratory spectra to the sources showing the 2282 cm −1 substructure are well constrained within the range T = 115 − 118 K (Figs. 6, and 8;Gerakines et al. 1999). At the long time scales in interstellar space, the segregation takes place at a much lower temperature of ∼77 K (Sect. 5.1).

CO 2 column densities
Column densities of solid 13 CO 2 are given in Table 3 and were derived by integrating the optical depth in the frequency range 2269-2288 cm −1 and dividing this by the integrated band strength A( 13 CO 2 ). Laboratory experiments indicate that A( 13 CO 2 ) depends on the ice composition, but only weakly on temperature . The band strength is 7.8 10 −17 cm molecule −1 for pure CO 2 ice, and similar for the mixture H 2 O:CO 2 =1.6:1, but 15% lower for H 2 O:CO 2 =24:1. Since in the interstellar medium typically H 2 O/CO 2 < 5 (Sect. 4.4), we take A( 13 CO 2 )=7.8 10 −17 cm molecule −1 . Note that the variation of A( 13 CO 2 ) seen in apolar CO and O 2 containing ices  is not applicable, since our study shows that interstellar CO 2 is absent in these ices. The error bars were determined from the average signal-to-noise values given in Table 1.
Using these 13 CO 2 column densities, we are for the first time able to calculate the 12 CO 2 / 13 CO 2 abundance ratio from the solid state, in a large number of sight-lines. The 12 CO 2 column densities were derived from the 12 CO 2 bending and stretching modes (∼4.27 and 15.2 µm; Gerakines et al. 1999). The column densities derived from both modes agree very well within the error bars given in Gerakines et al. (1999). These errors include ISO-SWS calibration uncertainties (mainly applicable to the stretching mode) and uncertainties due to the continuum determination, which is particularly difficult for the 12 CO 2 bending mode, since it is blended with the bending mode of silicates Boogert 1999).
The 12 CO 2 / 13 CO 2 column density ratio has a relative error less than 16% in 6 lines of sight (Table 3). However, laboratory experiments have shown that the 12 CO 2 / 13 CO 2 ratio can not be determined with an ac- Fig. 8. Laboratory fits to the interstellar 13 CO 2 band. For each source, the left panel gives a two component fit with the thin solid line representing a mixture of CO 2 with H 2 O or CH 3 OH. The dashed line is the mixture H 2 O:CH 3 OH:CO 2 =1:1:1, heated to T =125 K. Note the significant variations in the relative contribution of these components. The thick gray line is the sum of both laboratory spectra. The right panel shows for each source the best fit with H 2 O:CH 3 OH:CO 2 =1:0.92:1 at the indicated temperature. For sources that do not give good fits (e.g. HH 100), changing the mixing ratio somewhat will improve the fit at the indicated temperature. The luminous protostars in the left panels are ordered with a decreasing strength of the narrow 2282 cm −1 peak. curacy better than 20% (Sandford & Allamandola 1990;Ehrenfreund et al. 1997), unless the ice composition is accurately known. The 12 CO 2 / 13 CO 2 ratio of integrated optical depths is 19% lower in an H 2 O:CO 2 =1:10 ice compared to pure CO 2 , and 14% higher for H 2 O: CO 2 = 5:1. For 3 objects with low statistical errors (GL 2136, GL 2591, and S 140), we find that the CO 2 is heated and segregated, closely resembling the band profile of pure CO 2 . Thus, the band strengths are likely similar to pure CO 2 . However, for Elias 29, W 33A, and NGC 7538 : IRS9 we can not exclude that a large fraction of the CO 2 is embedded in an H 2 O-rich ice, and we conservatively assume a 14% error in the 12 CO 2 / 13 CO 2 column density ratio for these sources. Interesting variations in the 12 CO 2 / 13 CO 2 ratio are evident. Most objects have ratios in between 70-110, but some are significantly lower, e.g. W 33A, GC 3 and HH 100. We will discuss these results in Sect. 5.3.

Further constraints on the solid H 2 O/CO 2 and CH 3 OH/CO 2 ratios
The composition of interstellar CO 2 ices can be further constrained by the CH 3 OH, H 2 O, and CO 2 column densities along the observed lines of sight. The H 2 O/CO 2 ice column density ratio is typically ∼ 5, but can be as high as 8 (GL 2136, GL 2591) and as low as 1.5 (GC 3; Gerakines et al. 1999 lamandola et al. 1992;Chiar et al. 1996), resulting in typically CH 3 OH/CO 2 ≤0.50. For some individual sources, the observed CH 3 OH/CO 2 ratio is higher (W 33A: CH 3 OH/ CO 2 = 1.90), or lower (Elias 16: CH 3 OH/CO 2 <0.20). This is consistent with our conclusion that the interstellar 13 CO 2 ice band for most sources is best fitted with laboratory ices with a low CH 3 OH concentration, i.e. for which 0.1< CH 3 OH/CO 2 ≤ 1. To further limit the CH 3 OH/CO 2 ratio in the ice, it is, in particular for the low mass protostars, important to obtain very sensitive observations of the C-H stretching mode of CH 3 OH.

Heating of CO 2 ices
The 13 CO 2 ice band profile varies remarkably between the observed lines of sight. Most significant is the detection of a narrow substructure at ∼2282 cm −1 toward the massive protostellar objects S 140 : IRS1, GL 2136, GL 2591, and W 3 : IRS5. None of the low-mass protostars show this component, nor do some high-mass objects (W 33A, NGC 2024 : IRS2), the Galactic Center object GC 3, and quiescent cloud material traced by Elias 16. Laboratory simulations indicate that this distinct profile can be attributed to a heated, "segregated" polar CO 2 ice. Here, we further investigate this hypothesis by comparing the strength of the narrow 13 CO 2 peak, normalized to the Gaussian depth of the broad component, to a number of established temperature tracers (Table 4). Although only sources with a bolometric luminosity L bol > 10 4 L ⊙ have detected narrow 13 CO 2 peaks (Table 4; Fig. 9), a high luminosity can not be the only necessary condition. There is no, or very weak, sign of the 13 CO 2 substructure in the luminous objects W 33A and NGC 7538 : IRS9. The dust temperature in these sight lines may be too low to cause the ice to segregate. The interstellar CO 2 ice affected by segregation must have a temperature of T = 50−90 K (or ∼100-180 K in the laboratory), depending on whether the ice is H 2 O, or CH 3 OHrich. Assuming the dust radiates as a blackbody, modified by a power law emissivity with index −1, this corresponds to emission peaking at wavelengths of ∼25-50 µm. Non-heated ice, at temperatures of ∼20 K, radiates near Fig. 9. a-c The strength of the narrow 13 CO 2 peak plotted against various physical parameters of the protostars. High and low luminosity objects are indicated by bullets and triangles respectively. Panel a plots the bolometric luminosity, showing a poor correlation. The ratio of hot over cold dust emission in panel b gives a much better correlation. The far infrared flux ratio for W 3 : IRS5, indicated with an open symbol, is unreliable due to contamination in the large beam. Panel c shows that there is also a fairly good anti-correlation with the solid CO 2 abundance. 100 µm. Hence, a useful parameter for comparison is the flux ratio where the F (45 µm), and F (100 µm) fluxes are determined from ISO-Long Wavelength Spectrometer (ISO-LWS) spectra. We checked for contamination by extended emission in the 80" ISO-LWS beam, by comparing with the ISO-SWS flux at 45 µm in a 20" beam. For most sources there is good agreement within the instruments cross calibration uncertainty of ∼ 30%. An exception is W 3 : IRS5, with a difference of more than a factor of 2, and we consider the flux ratio unreliable. We find that sources with a large narrow 13 CO 2 peak strength have a high hot/cold dust flux ratio (Table 4; Fig. 9). And indeed, sources with low upper limits to the 13 CO 2 substructure, such as W 33A, NGC 7538 : IRS9, and Elias 29, are surrounded by much larger fractions of cold dust.
A similar picture is seen in the gas phase characteristics. Near-infrared CO observations indicate that a large fraction (∼50%) of the gas toward the objects with the narrow 13 CO 2 ice substructure is hot, T = 400 − 1000 K (Table 4). It must be noted that in all objects the temperature of the hot CO gas is well above the ice sublimation temperature T = 90 K, and thus the observed heated CO 2 ice originates from a region outside the hot core. On the other hand, the temperature of the cold CO gas component is in some lines of sight too low to cause the CO 2 ice to segregate (e.g. T = 28 K for S 140 : IRS1; Mitchell et al. 1990). Perhaps the observed hot CO 2 ice originates from the interface between the hot core and the cool surrounding medium. Indeed, other gas phase temperature tracers (e.g. CH 3 CN, CH 3 OH; van der Tak et al. 1999) reveal temperatures that are in between the cold and hot components found by CO studies.
Finally, the abundance of volatiles, such as ices, is expected to be a good tracer of the dust temperature along the line of sight. Indeed, sources with low CO 2 abundances have the strongest substructure in the 13 CO 2 ice band (Table 4; Fig. 9). For solid CO, a similar trend is observed. The objects with the strongest narrow 13 CO 2 peaks, S 140 : IRS1 and GL 2591, have no detected CO ice along the line of sight. This contrasts with W 33A and NGC 7538 : IRS9, which even have detections of highly volatile apolar CO detections (Smith et al. 1989;Tielens et al. 1991). All low-mass objects have a high fraction of apolar CO, and at the same time show no sign of heated CO 2 ice along the line of sight.
Thus, we find that the strength of the narrow 13 CO 2 component at 2282 cm −1 correlates with the dust and CO gas temperature along the line of sight, and anti-correlates with the ice abundances. This is good evidence that the observed feature can be attributed to a heated polar CO 2 ice, and it confirms our results from the laboratory fits. Thermal processing is thus an important process in interstellar space, at least toward luminous protostars. The differences in the various lines of sight are intriguing, and may indicate an evolutionary sequence with increasing thermal processing. Perhaps several distinct layers exist around massive protostars, in a way sketched in Ehrenfreund et al. (1998): a hot core where the ices have evaporated, a warm region surrounding it, where ice crystallizes, and further out a colder region. With time, these temperature regions expand outward. This would correspond to the two component ice model, where the narrow peak and broad components arise in physically different regions (Sect. 4.2.1). a narrow over broad Gaussian 13 CO2 peak depth ratio b hot CO gas column density in percentage of total N (CO gas); Mitchell et al. 1990 c column density of CO in apolar ice in percentage of total N (CO ice). Entries with '0' indicate that no CO ice was detected, with a significant upper limit. d calculated as F (45µm)/F (100µm) from ISO-LWS spectra e line profile unresolved f observed FIR flux heavily contaminated by nearby objects Alternatively, the very good fits that we obtain with single ices, would suggest that all the ice has one specific temperature, and thus is present in a specific region around the protostar. The temperatures of the least (W 33A; T =115 K) and most (S 140; T =118 K) evolved luminous objects are extremely well constrained in an interval of only 3 K. At the long time scales in interstellar space, this small temperature difference can be translated to a difference in heating time at the amorphouscrystalline phase transformation (segregation) temperature. Amorphous ice has a highly disordered structure, characterized by a broad distribution of bond angles between neighboring H 2 O molecules (Madden et al. 1978). In contrast, crystalline ice has well defined bond angles. This broad distribution implies a range of activation energies associated with the amorphous-crystalline phase transition. At a relatively low temperature in the laboratory (T = 115 K), only the lowest barriers can be surmounted. As the temperature increases (to T = 118 K), the higher barriers can be scaled as well. Hence, this temperature range implies a variation in barrier height by about 3%, with a typical height of E segr =4600 K (calculated from a 5500 K barrier at 140 K for pure ice; Tielens & Allamandola 1987). In interstellar space, rather than temperature, time is the essential parameter. The time scale τ segr needed to surmount an energy barrier E segr at temperature T is given by: with ν 0 = 5 10 13 s −1 the O-H bending mode vibration frequency (Tielens & Allamandola 1987). As the protostar is formed, it heats the surrounding molecular cloud, creating a hot core region around it where the ice evaporates. Immediately surrounding this region is a zone where the ice has been warmed sufficiently to start the amorphous-crystalline phase transformation, corresponding to the T = 115 K case in the laboratory. As time progresses, this phase transformation progresses as well, and in essence, although the temperature does not change, the interstellar ice corresponds to warmer and warmer (i.e. 115-118 K) laboratory experiments. At the typical time scale of evolution for hot core regions of τ ∼3 10 4 years (Charnley et al. 1992), the laboratory temperature range of 115-118 K effectively corresponds to a segregation temperature of ∼77 K (Eq. 2). Thus, at this temperature, W 33A and NGC 7538 : IRS9 would have the youngest and least evolved hot cores, while S 140 and GL 2591 are ∼ 3 10 4 years older and have succeeded to heat the surrounding gas and ice significantly. However, the derivation of a heating time scale is complicated by the presence of temperature gradients, and possible intrinsic differences 13 between the sources. It seems thus likely that both time and temperature effects play a role in the evolution of interstellar CO 2 ices. Detailed modeling of the temperature and density structure around massive protostars is needed to derive evolutionary time scales (e.g. Van der Tak et al. 1999).

Chemistry of interstellar CO 2
Interstellar CO 2 may form on grain surfaces, through the reaction of atomic O with CO, atomic C, or C + , accreted from the gas phase. Although CO, rather than C or C + , is generally assumed to be the origin species, solid and gas phase isotope 12 C/ 13 C ratios provide a good test for this (Sect. 5.3). In this grain surface scenario, the ice composition provides a measure for the gas phase abundances during accretion, convolved with reaction efficiencies. The ratio of CH 3 OH/CO 2 is a measure for hydrogenation over oxidation reactions of CO. The 13 CO 2 profile provides an upper limit CH 3 OH/CO 2 ≤1 in the ice, and thus oxidation of CO is at least as efficient as hydrogenation, despite the much larger atomic H abundance compared to atomic O. At the same time, the solid H 2 O/CO 2 ratio measures the gas phase (O+O 2 )/CO ratio at accretion, since H 2 O is formed from O 2 or O 3 . The 13 CO 2 profile does not significantly constrain the solid H 2 O/CO 2 ratio, it could be as high as the ratio of column densities, i.e. H 2 O/CO 2 ≤5 . Further constraints to the chemical conditions during CO 2 formation on interstellar grains can be obtained from the 12 CO 2 bending and stretching modes Boogert 1999). In particular, the prominent red wing in the bending mode is a signature for the presence of CO 2 :CH 3 OH clusters in interstellar ices.
An alternative way to form interstellar CO 2 is through irradiation of CO containing ices by ultraviolet (UV) photons. Even in "cold" sight-lines such as Elias 16, the UV flux from the ISRF or induced by cosmic rays may be high enough to explain the observed CO 2 abundance (see Whittet et al. 1998 for a quantitative discussion). The oxygen atoms are liberated from H 2 O in polar ices, or O 2 in apolar ices. The interstellar 13 CO 2 profile does not show the narrow signatures of CO 2 mixed with apolar molecules such as CO and O 2 . Hence, if UV processing is the dominant mechanism, CO 2 most likely originates from polar CO ices. Indeed, interstellar CO is present in significant quantities in the polar phase Chiar et al. 1998).

Galactic 12 C/ 13 C abundance ratio
The derived 13 CO 2 , and published 12 CO 2 , column densities were used to calculate 12 C/ 13 C abundance ratios (Table 3). It is the first time that the 12 C/ 13 C ratio is determined from the solid state. Previously, gas phase molecules were used for this purpose, using UV, optical, and radio observations (see Wilson & Rood 1994 for an overview). C 18 O and H 2 CO observations yield an increasing 12 C/ 13 C ratio with Galacto-centric radius (Langer & Penzias 1990). This can be understood by the different origin of 12 C and 13 C atoms. The 12 C is rapidly produced by Helium burning in massive stars, and injected into the interstellar medium by supernovae. In later stellar generations, 12 C seeds are converted to 13 C in the CNO cycle of low-and intermediate-mass stars, during their red giant phase. This is a much slower process. Thus, the 12 C/ 13 C ratio is a measure for the enrichment of the interstellar medium by primary to secondary stellar processes. The observed gradient in the Galactic 12 C/ 13 C ratio can then be understood by a higher star formation rate in the inner parts of the Galaxy (Tosi 1982).
The 12 C/ 13 C ratio derived from solid CO 2 indicate the same trend as found in gas phase studies (Table 3; Fig. 10). Low values are found near the Galactic Center ( 12 CO 2 / 13 CO 2 =52±11; GC 3) and in the molecular ring, at ∼4 kpc from the Galactic Center (W 33A; 53±8). Objects at larger Galactic radii mostly have higher 12 CO 2 / 13 CO 2 ratios. We have fitted a linear relation to the 12 CO 2 / 13 CO 2 ratio as a function of Galacto-centric radius, where, as in the gas phase studies, we exclude the Galactic Center from the fit. We find the following relation, using the errors as statistical weights: If instead we take an unweighted fit, like in some gas phase studies (Wilson & Rood 1994), we find a gradient of 6.5±4.5 and absisca 33±21. Thus, the evidence for a gradient in the 12 C/ 13 C ratio derived from solid CO 2 is weak. Within the (rather large) error bars, this gradient is comparable to the H 2 CO (6.6±2.0) and CO data (7.5±1.2). It must be mentioned that a limitation of our analysis is the lack of solid CO 2 observations at low Galactic radii (3-6 kpc). We note that also in gas phase studies (Wilson & Rood 1994), there is considerable scatter in the 12 C/ 13 C ratio, and no clear trend at Galactic radii > 6 kpc is present either (Fig. 10).
Although the gradients agree well for CO 2 , CO, and H 2 CO, we find that 12 C/ 13 C ratios derived from CO (abscissa −2.9±7.5 in Eq. 2) are systematically somewhat smaller. A comparison of the gas and solid state 12 C/ 13 C ratios for individual objects is possible for only 4 sources. For all of these, our solid CO 2 observations yield higher 12 C/ 13 C ratios compared to gas phase CO observations (Table 3). However, for both the overall linear fit (Eq. 2) and for these individual sources, the differences are statistically small (< 3σ). For a proper interpretation of these possible differences it is important to note that the gas phase observations were done several arcminutes away from our CO 2 observations (Table 3). In the gas phase, considerable variations of the 12 C/ 13 C ratio were reported within the same region, such as within the W 33 cloud Fig. 10. The ratio of solid 13 CO 2 and 12 CO 2 column densities, versus the Galacto-centric radius of the observed sources (left panel). The solid line is a linear weighted fit to the data. The right panel shows the 12 C/ 13 C ratio from gas phase studies (bullets: C 18 O, open circles: H 2 CO) as taken from Wilson & Rood (1994). The dotted and dashed lines are linear fits to these gas data. (Langer & Penzias 1990) and toward the supernova remnant Cas A (Wilson & Rood 1994). In particular, comparison of our 12 CO 2 / 13 CO 2 ratio toward the Galactic Center object GC 3 with the gas phase determination toward the Galactic Center supernova remnant Sgr B2 is complicated by the quite different stellar evolution history that these regions may have.
Finally, we determined that 12 CO 2 / 13 CO 2 =69±15 (1 σ) for the local ISM, by taking an unweighted mean for the three objects located in the ρ Ophiuchi, and Corona Australis clouds. We excluded Elias 16 from this mean, since it has a much larger error bar than the other sources (Table 3). This value is comparable to the 12 C/ 13 C ratio in the local ISM, as determined from atomic C (58±12; Keene et al. 1998) and C + (58±6; Boreiko & Betz 1996) observations, as well as from C 18 O and H 2 CO observations ( 12 C/ 13 C=77±7; Wilson & Rood 1994).
In summary, we find that the gradient of the 12 C/ 13 C ratio with Galacto-centric radius is the same for solid CO 2 and gaseous CO and H 2 CO. In this relation, the values derived from CO, however, tend to be smaller compared to CO 2 and H 2 CO. The value for the local ISM ( 12 CO 2 / 13 CO 2 = 69±15) is in reasonable agreement with gas phase studies. These results could have important implications for our knowledge of interstellar chemistry. Both gaseous CO and H 2 CO may have been affected by chemical fractionation and isotope selective destruction (e.g. Langer et al. 1984;Tielens 1997). If fractionation is an important mechanism, preferential incorporation of 13 C + in CO, leads to low 12 CO/ 13 CO and high 12 C + / 13 C + ra-tios. On the other hand, if isotope selective destruction dominates over fractionation, 13 CO is preferentially destroyed, leading to high 12 CO/ 13 CO and low 12 C/ 13 C ratios. Carbonaceous molecules derived from C + or C would then have high or low 12 C/ 13 C ratios, depending on the dominant process and chemical pathway. Our lines of sight trace dense molecular clouds, and CO is the main reservoir of C. Then, fractionation or selective destruction will highly influence the 12 C (+) / 13 C (+) ratio, but will hardly affect the 12 CO/ 13 CO ratio. The generally higher H 12 2 CO/ H 13 2 CO ratios compared to CO thus suggest that H 2 CO originates from C + (Tielens 1997). Similarly, if indeed the 12 CO 2 / 13 CO 2 ratios are higher compared to CO, this would imply that CO 2 is formed from C + rather than C or, as is generally assumed, from CO (van Dishoeck et al. 1996). However, we must emphasize that with the present large uncertainties in the 12 C/ 13 C ratios we can not make strong statements about the chemical pathway to form interstellar CO 2 . We expect that with future improvements of the ISO-SWS data reduction, the large error bars in the CO 2 abundances for some sources (GL 989, NGC 2024 : IRS2, and W 3 : IRS5) can be reduced. Furthermore, it is important to obtain 12 CO/ 13 CO ratios in the same line of sight as our CO 2 observations, preferably by absorption line studies along a pencil beam. Observations of the solid 12 CO/ 13 CO ratio would be particularly useful. This will also shed light on the origin of the large scatter seen in the 12 C/ 13 C ratio as a function of Galactocentric radius for both solid CO 2 (i.e. note the particularly high value for GL 2136 in Fig. 10) and gas phase species. It has been suggested that perhaps most of this scatter is real, and can be attributed to local variations in the star formation history (Wilson & Rood 1994).

Summary and Conclusions
We have presented ISO-SWS observations of the stretching mode of 13 CO 2 ice in the spectral range 2255-2300 cm −1 (4.34-4.43 µm) in 13 Galactic lines of sight. All sight-lines show an absorption feature with a peak position in the range 2276-2280 cm −1 , and a Gaussian FWHM of typically 6-9 cm −1 . Additionally, the four high-mass protostars GL 2136, S 140 : IRS1, GL 2591, and W 3 : IRS5, show a narrow (FWHM∼3 cm −1 ) absorption line at ∼ 2282.3 cm −1 . This feature is much weaker or absent toward other high-mass objects (W 33A, NGC 7538 : IRS9), toward low-mass protostars, the Galactic Center (GC 3), and quiescent molecular cloud material (Elias 16).
These observational results are compared to laboratory experiments of the stretching mode of 13 CO 2 ice. We conclude that this band is a sensitive probe of the heating history and composition of interstellar ice mantles. The profile of the interstellar band can be fitted in two different ways. First, the detected broad and narrow components could originate from ices with different heating histories and perhaps composition along the line of sight.
In this scenario, the detected broad component is ascribed to a non-heated mixture of CO 2 with polar molecules such as most likely H 2 O, and CH 3 OH. The mixing ratio of CH 3 OH/CO 2 in the ice can generally be constrained to an upper limit of CH 3 OH/CO 2 ≤1, in agreement with the observed column densities along the lines of sight. The narrow 2282 cm −1 absorption feature detected toward several high-mass protostars is ascribed to 13 CO 2 in a polar CO 2 ice that has been heated to temperatures of at least 50 K (in interstellar space). In such an ice, the bondings of the CO 2 , and polar molecules are segregated and the CO 2 band profiles resemble those in a pure CO 2 ice. To fit the exact peak position of the 2282 cm −1 feature, the CO 2 ice must contain at least some CH 3 OH (CH 3 OH/CO 2 > 0.1).
A second way to fit the 13 CO 2 band, is with a single ice at a specific temperature. This would locate the ice in a well confined region around the protostar, rather than in regions of different temperature as for the two component model. For the luminous protostars showing evidence for this narrow component, the best fitting laboratory ices have temperatures within a very small interval of T = 115 − 118 K (using H 2 O:CH 3 OH:CO 2 =1:1:1). At the long time scales in interstellar space, this laboratory temperature interval corresponds to a heating time difference between the luminous protostars comparable to the lifetime of hot cores (∼3 10 4 years) at an amorphouscrystalline phase transition temperature of ∼77 K. However, the derivation of heating time scales likely is complicated by the presence of temperature gradients, and possible intrinsic differences between the sources, such as different temperature and density structure.
To further test the hypothesis that interstellar CO 2 ice is affected by heating, we calculate the ratio, and upper limits, of the narrow to broad 13 CO 2 component peak depths. This quantity is compared to known physical parameters of all objects. We conclude that the strength of the narrow 13 CO 2 substructure correlates with the dust and CO gas temperature along the line of sight, and anticorrelates with the ice abundances. This is further good evidence that the structure of ices around luminous protostars is affected by thermal processing, as concluded from the laboratory fits. This effect appears to be not important for low mass protostars and the quiescent medium. Although we have to keep in mind that our selection of low mass objects is small, this might imply that unaltered interstellar ices are included in comets.
Finally, we have determined solid 13 CO 2 column densities. These were used to derive 12 C/ 13 C ratios in all 13 sight-lines. It is the first time that the Galactic 12 C/ 13 C ratio is determined from the solid state. Solid state determinations of the 12 C/ 13 C ratio are more reliable than gas phase studies, because the column density can be determined without uncertain radiative transfer effects. Like in gas phase studies, we find low values for the Galactic Center (GC 3; 12 CO 2 / 13 CO 2 = 52±11) and the Galactic molecular ring (W 33A; 53±8), and higher val-ues at larger Galacto-centric radii. The gradient of the 12 CO 2 / 13 CO 2 ratio as a function of Galacto-centric radius (4.5±2.2 kpc −1 ) agrees with gas phase studies. The scatter in this relation is however large, and may perhaps indicate local differences in star formation history, since 12 C is converted to 13 C in low and intermediate mass stars. Comparison of solid and gas phase isotope ratios can be used to trace the chemical history of molecules. In general, the 12 CO 2 / 13 CO 2 ratio, as for H 2 CO, tends to be larger compared to CO studies (∼ 2.5 σ difference). In the dense molecular clouds that we have observed, where CO is likely unaffected by fractionation or isotope-selective destruction, this would imply that interstellar CO 2 is formed from C + , rather than CO or C. However, to draw definite conclusions on interstellar chemistry and on global and local variations in the Galactic star formation history, we emphasize that a systematic determination of the 12 C/ 13 C ratio from atomic C (+) , CO, and solid CO 2 in individual lines of sight is needed. At present, there is little overlap between our solid CO 2 observations and published gas phase observations.