Molecules in Space & Laboratory, Paris, 2007
J.L. Lemaire & F. Combes (eds)
SOLID STATE ASTROPHYSICS AND -CHEMISTRY
FOUR QUESTIONS - FOUR ANSWERS
H. Linnartz, K. Acharyya, Z. Awad, S.E. Bisschop, S. Bottinelli,
J. Bouwman, H.M. Cuppen, G.W. Fuchs, S. Ioppolo, K.I. Öberg and
E.F. van Dishoeck 1
Abstract. Recent progress in ultra high vacuum surface experiments allows
detailed investigations of the physical and chemical parameters governing astro-
nomically relevant solid state processes on icy dust grains. In this proceeding
four questions are shortly addressed that are related to the infrared signature,
the thermal and photodesorption behavior and the chemical reactivity of inter-
stellar ice analogues.
1 Question 1: How to determine interstellar ice compositions ?
Infrared spectroscopy towards dense molecular clouds and young stellar objects often
reveals prominent bands that can be attributed to H2 O ice. It has been a long
standing problem in the astronomical community that the observed intensity ratio of
the 3 µm stretching mode and the 6 µm bending mode differs as much as a factor two
compared to laboratory spectra recorded for pure water ice. Recently, it has been
suggested (Knez et al. 2005) that this discrepancy may be due to substantial amounts
of other species mixed into the H2 O ice matrix. Two likely pollutants are CO2 and
CO. Incorporation in the ice perturbs the spectroscopic signature of the fundamental
vibrations reflection molecular interactions: intensities and intensity ratios change,
peak positions shift and band widths are affected. These values are depending on
mixing ratio, temperature, morphology and deposition conditions of the ice. I.e. in
order to relate an observed astronomical ice spectrum to a specific ice composition
it is necessary to determine spectroscopic parameters in systematic dependence of
a number of physical parameters. In the last year such systematic spectroscopic
searches have been performed for H2 O:CO2 ices (Öberg et al. 2007a), H2 O:CO ices
(Bouwman et al. 2007), HCOOH containing ices, mixed with CH3 OH, HCOOH and
H2 O (Bisschop et al 2007a) and NH3 and CH3 OH containing ices (Bottinelli et al.
2008) using Fourier transform transmission spectroscopy. As an example conclusions
for H2 O:CO2 ices are summarized in a correlation diagram (Fig. 1). The 6:3 µm
integrated intensity ratios are shown for values of 0.2, 0.5, 1, 2 and 5 as function of
both temperature and mixing ratio.
1Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, P.O. Box 9513,
NL 2300 RA Leiden, the Netherlands
c Observatoire de Paris & Université de Cergy-Pontoise
, 2 Molecules in Space & Laboratory
Fig. 1. Correlation diagram showing the 6:3 µm intensity ratio distribution of H2 O ice in
dependence of CO2 concentration and temperature.
In combination with peak position and band width it becomes possible to de-
rive rather specific ice compositions and conditions as recently discussed by Bisschop
et al. (2007a) where ISO HCOOH features are reproduced by tertiary laboratory
mixtures of formic acid, methanol and water. These spectra together with data
available from previous studies are accessible online via the Leiden Ice Database at:
http://www.laboratory-astrophysics.eu.
2 Question 2: Is molecular oxygen hiding in interstellar ice ?
During the first stages of star formation virtually all species accrete onto grains in
dense cold clouds. Later on in the star formation process, grains are warmed to tem-
peratures where molecules can desorb again. Detailed ultra high vacuum experiments
applying TPD (temperature programmed desorption) provide accurate information
on the involved thermal desorption behavior of astrophysically relevant species and
yielding values for the desorption temperature, i.e. binding energy. Typically Polanyi-
Wigner type of equations are used to describe the thermal desorption mechanism in
terms of an empirical kinetic model.
In the last year special attention has been given in our laboratory to oxygen bearing
ices. The reason is that a substantial amount of interstellar oxygen may well freeze
out onto grains in the form of molecular oxygen, potentially explaining the very low
O2 -abundances observed in space (Bergin et al. 2000). Recent ODIN/SWAS cam-
paigns (Larson et al. 2007) put upper limits on the O2 gas abundance in cold dark
clouds in the range of 10−7 to 10−8 with respect to H2 . This low abundance raises se-
rious questions about the total oxygen budget when compared with the well observed
atomic oxygen abundance of 3.10−4 in diffuse clouds (Ehrenfreund & van Dishoeck,
1998). In order to investigate this hypothesis, it is interesting to ask to what extent
O2 differs from CO, since CO is readily observed in the gas phase and in solid form,
and how O2 compares to N2 that is observed in the gas phase through the detection
J.L. Lemaire & F. Combes (eds)
SOLID STATE ASTROPHYSICS AND -CHEMISTRY
FOUR QUESTIONS - FOUR ANSWERS
H. Linnartz, K. Acharyya, Z. Awad, S.E. Bisschop, S. Bottinelli,
J. Bouwman, H.M. Cuppen, G.W. Fuchs, S. Ioppolo, K.I. Öberg and
E.F. van Dishoeck 1
Abstract. Recent progress in ultra high vacuum surface experiments allows
detailed investigations of the physical and chemical parameters governing astro-
nomically relevant solid state processes on icy dust grains. In this proceeding
four questions are shortly addressed that are related to the infrared signature,
the thermal and photodesorption behavior and the chemical reactivity of inter-
stellar ice analogues.
1 Question 1: How to determine interstellar ice compositions ?
Infrared spectroscopy towards dense molecular clouds and young stellar objects often
reveals prominent bands that can be attributed to H2 O ice. It has been a long
standing problem in the astronomical community that the observed intensity ratio of
the 3 µm stretching mode and the 6 µm bending mode differs as much as a factor two
compared to laboratory spectra recorded for pure water ice. Recently, it has been
suggested (Knez et al. 2005) that this discrepancy may be due to substantial amounts
of other species mixed into the H2 O ice matrix. Two likely pollutants are CO2 and
CO. Incorporation in the ice perturbs the spectroscopic signature of the fundamental
vibrations reflection molecular interactions: intensities and intensity ratios change,
peak positions shift and band widths are affected. These values are depending on
mixing ratio, temperature, morphology and deposition conditions of the ice. I.e. in
order to relate an observed astronomical ice spectrum to a specific ice composition
it is necessary to determine spectroscopic parameters in systematic dependence of
a number of physical parameters. In the last year such systematic spectroscopic
searches have been performed for H2 O:CO2 ices (Öberg et al. 2007a), H2 O:CO ices
(Bouwman et al. 2007), HCOOH containing ices, mixed with CH3 OH, HCOOH and
H2 O (Bisschop et al 2007a) and NH3 and CH3 OH containing ices (Bottinelli et al.
2008) using Fourier transform transmission spectroscopy. As an example conclusions
for H2 O:CO2 ices are summarized in a correlation diagram (Fig. 1). The 6:3 µm
integrated intensity ratios are shown for values of 0.2, 0.5, 1, 2 and 5 as function of
both temperature and mixing ratio.
1Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, P.O. Box 9513,
NL 2300 RA Leiden, the Netherlands
c Observatoire de Paris & Université de Cergy-Pontoise
, 2 Molecules in Space & Laboratory
Fig. 1. Correlation diagram showing the 6:3 µm intensity ratio distribution of H2 O ice in
dependence of CO2 concentration and temperature.
In combination with peak position and band width it becomes possible to de-
rive rather specific ice compositions and conditions as recently discussed by Bisschop
et al. (2007a) where ISO HCOOH features are reproduced by tertiary laboratory
mixtures of formic acid, methanol and water. These spectra together with data
available from previous studies are accessible online via the Leiden Ice Database at:
http://www.laboratory-astrophysics.eu.
2 Question 2: Is molecular oxygen hiding in interstellar ice ?
During the first stages of star formation virtually all species accrete onto grains in
dense cold clouds. Later on in the star formation process, grains are warmed to tem-
peratures where molecules can desorb again. Detailed ultra high vacuum experiments
applying TPD (temperature programmed desorption) provide accurate information
on the involved thermal desorption behavior of astrophysically relevant species and
yielding values for the desorption temperature, i.e. binding energy. Typically Polanyi-
Wigner type of equations are used to describe the thermal desorption mechanism in
terms of an empirical kinetic model.
In the last year special attention has been given in our laboratory to oxygen bearing
ices. The reason is that a substantial amount of interstellar oxygen may well freeze
out onto grains in the form of molecular oxygen, potentially explaining the very low
O2 -abundances observed in space (Bergin et al. 2000). Recent ODIN/SWAS cam-
paigns (Larson et al. 2007) put upper limits on the O2 gas abundance in cold dark
clouds in the range of 10−7 to 10−8 with respect to H2 . This low abundance raises se-
rious questions about the total oxygen budget when compared with the well observed
atomic oxygen abundance of 3.10−4 in diffuse clouds (Ehrenfreund & van Dishoeck,
1998). In order to investigate this hypothesis, it is interesting to ask to what extent
O2 differs from CO, since CO is readily observed in the gas phase and in solid form,
and how O2 compares to N2 that is observed in the gas phase through the detection
