The infrared band strengths of H O CO and CO in lab oratory simulations of astrophysical ice mixtures P A Gerakines W A Schutte J M Greenb erg and E F van Disho eck Leiden Observatory PO Box RA Leiden The Netherlands Department of Physics Rensselaer Polytechnic Institute Troy NY USA Accepted by Astronomy Astrophysics Abstract Infrared sp ectroscopic observations toward ob jects obscured by dense cloud material show that H O CO and likely CO are imp ortant constituents of interstellar ice mantles In order to accurately calculate the column densities of these molecules it is imp ortant to have go o d measurements of their infrared band strengths in astrophysical ice analogs We present the results of lab oratory exp eriments to determine these band strengths Improved exp erimental metho ds relying on simultaneous indep endent dep o sitions of the molecule to b e studied and of the dominating ice comp onent have led to accuracies b etter than a few p ercent Furthermore the temp erature b ehavior of the in frared band strengths of CO and H O are studied In contrast with previous work the strengths of the CO CO and H O infrared features are found to dep end only weakly on the comp osition of the ice matrix and the reversible temp erature dep endence of the CO band is found to b e weaker than previously measured for a mixture of CO in H O astro-ph/9409076 14 Mar 1995 1. Introduction Infrared astronomy has led to the identication of various sp ecies existing within icy grain mantles in dense clouds The two most abundant molecules in grain mantles identied to date are water H O and carb on monoxide CO eg Willner et al Smith et al Whittet et al Whittet et al Tielens et al Chiar et al Limited observational evidence as well as theoretical and lab oratory mo deling indicate that carb on dioxide CO should also b e an imp ortant comp onent of interstellar ices dHendecourt Jourdain de Muizon Whittet et al Breukers The abundances of these molecules in dierent phases of ice mantles provide imp ortant clues to the chemical pro cesses in dense interstellar clouds and therefore it is of imp ortance to accurately measure the band strengths of the infrared features of these molecules The goal of this pap er is to provide new as well as more accurate measurements of the band strengths of H O CO and CO contained within astrophysical ice analogs so that their abundances in the ices in dense interstellar clouds may b e determined with more condence Such measurements are particularly imp ortant for CO a molecule whose principle absorption band is totally obscured by Earths atmosphere and which will b e widely searched for with the Infrared Space Observatory ISO Astrophysical ices consist of complex mixtures of molecules eg Whittet and references therein At least two distinct phases app ear to exist one which is dominated by p olar molecules of which the most abundant is H O and one dominated by ap olar molecules such as CO eg Sandford et al Tielens et al It is not yet clear which sp ecies dominate the ap olar phase although CO and p ossibly O and CO seem to b e abundant Furthermore observations towards emb edded sources indicate a wide range of ice temp eratures from less than K up to K Smith et al Observations of the m cm CO stretching band indicate that CO re sides in b oth the p olar and ap olar phases of interstellar ice Eiroa Ho dapp Kerr et al Chiar et al CO could b e present in b oth phases as well Although H O app ears to b e the most abundant molecule in the p olar phase the presence of ap olar sp ecies like CO and CO could inuence the strength of its infrared bands Sp ecically the resulting breakup of the hydrogen b onding network would weaken the intensity of the OH stretching feature of fully Hb onded H O at m cm Hagen Tielens As of yet H O has not b een observed in the ap olar ice Previous metho ds of measuring band strengths of molecules in a mixed ice involved the preparation of several gases within one gas container by adding the comp onents one after another and using the ideal gas law to convert pressures to abundances This mixture is then dep osited onto a cold substrate and it is assumed that the molecular abundances in the ice sample equal the gas abundances in the container The ice abundances are then used to convert the measured integrated optical depths to infrared band strengths either by measuring the thickness of the ice from the interference fringes pro duced by a laser directed at the sample and assuming a value for the ice density thus obtaining molecular column densities dHendecourt Allamandola Hudgins et al or by assuming that the band strengths of the dominant comp onent are equal to those in a pure sample and using these to calibrate the other features Sandford et al Sandford Allamandola These metho ds rely heavily on the assumed equality of the comp ositions of the ice sample and the gas mixture However several problems may arise in this assumption First if a bulb is made with H O close to its vap or pressure a small change in temp erature could signicantly inuence the amount of H O in the gas phase inside the bulb due to the strong temp erature dep endence of the vap or pressure For example b etween and C it varies from to mbar Second mixing of the dierent gases which are sequentially allowed to enter the bulb may b e incomplete Finally the dep osition rate of a molecule will b e prop ortional to its thermal velo city which dep ends on the molecular mass 1 2 This eect could give rise to a signicant dierence in the comp osition of ie v m th the gas mixture and the ice sample if molecules with very dierent molecular masses are involved For example H O and CO have thermal velo cities that are dierent by a factor of In the new pro cedure that we have implemented we have avoided these pitfalls by pro ducing ice mixtures using simultaneous dep ositions of pure gases through separate dep osition tub es We then have the ability to measure the ratio of the band strengths of molecules in a binary ice to those in pure ice AA since the column densities in each pure case pure and mixed can b e kept the same The band strengths in the mixed ice can then b e calculated using the strengths of the pure ice bands which are accurately known and present in the literature This pap er is organized as follows In x we review our exp erimental metho ds In x we summarize our measurements of the band strengths of H O CO and CO in various binary mixtures with p olar and ap olar molecules Finally in x we discuss our results and their astrophysical implications 2. Experimental In this section we will describ e the exp erimental equipment and pro cedures used for pro ducing and analyzing astrophysical ice analogs 2.1 Sample Chamber The vacuum system used to prepare the ice samples and to obtain infrared transmis sion sp ectra is similar to those previously used to study ice analogs Hagen et al Allamandola et al Hudgins et al with some signicant mo dications The chamb er is situated within the sample compartment of an infrared sp ectrometer BioRad FTS A An infrared transmitting substrate CsI is mounted in the vacuum chamb er and can b e co oled by a closedcycle helium refrigerator expander Air Pro ducts Displex DE compressor mo dule Air Pro ducts RWSL to a temp erature of K The temp erature of the substrate is continuously adjustable by a resistive typ e heater element up to ro om temp erature The temp erature is monitored by a chromelAu thermo couple with an accuracy of K The chamb er has four p orts Two of these consist of KBr al lowing transmission of the infrared b eam of the sp ectrometer One of the p orts consists of MgF to enable ultraviolet UV irradiation of the ice samples this option was not used for the exp eriments describ ed in this pap er while the fourth p ort consists of glass and is used for visual monitoring Additionally the chamb er is equipp ed with two dep osition Figure Diagram of the substrate holder showing the cm oaxis mounting of the substrate a so that it may b e turned completely out of the infrared b eam b the b eam is represented by the dotted line and the cross denotes the rotation axis tub es which are directed at the center of the substrate from a distance of cm For each dep osition system the gas ow from a storage bulb see x to the vacuum sys tem is regulated by a variable leak valve Leyb oldHeraeus The leak valve is equipp ed with a shuto valve and a regulation valve that function indep endently The stainlesssteel chamb er is sealed with Viton Orings and is evacuated by a turb omolecular pump PfeierBalzers TPH backed by a rotary pump PfeierBalzers DUOB A liquid nitrogen trap b etween the two pumps prevents any rotary pump oil from back streaming to the system Ion and thermo couple pressure gauges placed b etween the sample chamb er and the turb o pump monitor the internal pressure The vacuum system can b e externally heated using heating tap e After ab out two days of pumping and externally heating to C a vacuum of mbar was obtained The residual gases at this pressure were analyzed by collecting them on the substrate after co oling to K and sub sequent infrared sp ectroscopy It was found to consist mainly of H O accreting at a rate of molec cm s m hr while organic contaminants were found to accrete at ab out half this rate The substrate holder can b e rotated without breaking the vacuum During an exp eri ment the substrate is rotated b etween two p ositions which are degrees apart In p os an