A&A 394, L27-L30 (2002)
DOI: 10.1051/0004-6361:20021353
W. F. Thi1,2 - K. M. Pontoppidan 2 - E. F. van Dishoeck2 - E. Dartois3 - L. d'Hendecourt3
1 - Department of Physics and Astronomy, University College
London, Gower Street, London WC1E 6BT, UK
2 -
Leiden Observatory, PO Box 9513, 2300 RA, Leiden, The Netherlands
3 -
Astrochimie Expérimentale, Institut d'Astrophysique Spatiale,
Université Paris-Sud, Bât. 121, 91405 Orsay, France
Received 15 August 2002 / Accepted 17 September 2002
Abstract
We present direct evidence for CO freeze-out in a
circumstellar disk around the edge-on class I object CRBR 2422.8-3423, observed in the M band with VLT-ISAAC at a
resolving power
.
The spectrum shows strong solid
CO absorption, with a lower limit on the column density of
cm-2. The solid CO column is the highest
observed so far, including high-mass protostars and background field
stars. Absorption by foreground cloud material likely accounts for
only a small fraction of the total solid CO, based on the weakness
of solid CO absorption toward nearby sources and the absence of
gaseous C18O
emission 30
south.
Gas-phase ro-vibrational CO absorption lines are also detected with
a mean temperature of
K. The average gas/solid CO ratio
is
1 along the line of sight. For an estimated inclination
of 20
,
the solid CO absorption originates mostly
in the cold, shielded outer part of the flaring disk, consistent
with the predominance of apolar solid CO in the spectrum and the
non-detection of solid OCN-, an indicator of thermal/ultraviolet
processing of the ice mantle. By contrast, the warm gaseous CO
likely originates closer to the star.
Key words: star formation - ISM: dust, extinction - molecules - abundances - infrared: ISM: lines and bands
Interstellar gas and dust form the basic ingredients from which
planetary systems are built (e.g., van Dishoeck & Blake 1998;
Ehrenfreund & Charnley 2000). In particular, the icy
grains can agglomerate in the cold midplane of circumstellar disks to
form planetesimals such as comets. In the cold (T< 20 K) and dense
(
-109 cm-3) regions of disks, all chemical
models predict a strong freeze-out of molecules onto grain surfaces
(e.g., Aikawa et al. 2002). The low molecular abundances
in disks compared to those in dense clouds as derived from
(sub)millimeter lines is widely considered to be indirect evidence for
freeze-out (Dutrey et al. 1997; Thi et al. 2001).
Observations of gaseous and solid CO have been performed for a few
transitional objects from class I to class II that are known to posses
a disk. Boogert et al. (2002a) observed L 1489
in Taurus - a large 2000 AU rotating disk -, but the amount of solid
CO is not exceptionally high (7% of gaseous CO). This may stem
from the fact that these systems are still far from edge-on
(inclination
)
so that the line of sight
does not intersect the midplane, the largest reservoir of solid CO.
Shuping et al. (2001) found strong CO depletion toward
Elias 18 in Taurus, but both the disk stucture and its
viewing angle are not well constrained. More promising targets are
pre-main-sequence stars for which near-infrared images have revealed
nebulosities separated by a dark lane (e.g. Padgett et al.
1999). The lane is interpreted as the cold midplane of a
disk seen close to edge-on where visible and even near-infrared light
are extremely extinct. Among such dark-lane objects, CRBR
2422.8-3423 is a red (H-K=4.7) low luminosity (
,
Bontemps et al. 2001) object surrounded by a near
edge-on disk, discovered in images with the ESO Very Large
Telescope (VLT) at 2
m (Brandner et al. 2000).
Its spectral energy distribution (SED) is consistent with that of a
class I object or an edge-on class II object with strong silicate
absorption at 9.6
m. It is located at the edge of the
Oph cloud complex in the core F,
30
west
of the infrared source IRS 43 and a few arcmin south-east of
Elias 29 (Motte et al. 1998).
This letter reports the detection of a large quantity of solid CO and the presence of gaseous CO in the line of sight of CRBR 2422.8-3423 using the ESO-VLT (Sects. 2 and 3). Possible contamination by foreground cloud material is considered in Sect. 4, followed by a discussion on the location and origin of the CO gas and dust in the disk (Sect. 5).
CRBR 2422.8-3423 was observed with the ESO VLT-ANTU mounted
with the Infrared Spectrometer And Array Camera (ISAAC) on May
6, 2002. A spectrum at
was obtained in the M band
using a slit-width of 0
3, which matched the excellent seeing
(
0
3 at 4
m). The integration time was 36 min, resulting in a continuum S/N of
20. The spectra
of BS 6084 (B1III) and BS 6378 (A2.5Va), observed
immediately before and after CRBR 2422.8-3423, were used to
remove the atmospheric features and to calibrate the spectrum in flux
and wavelength. All the reduction steps were carried out using an
in-house data reduction package written in IDL.
Observations of gaseous CO (sub)millimeter lines on source and at an
offset position 30
south were obtained at the James Clerk
Maxwell Telescope (JCMT)
in
June 2002. The dual-polarization receiver B3 was tuned to observe the
12CO
(345.796 GHz, beam 13
8) and
13CO
(330.587 GHz, beam 14
4) lines, and
receiver A3 to obtain C18O
(219.560 GHz, beam
22
2). The backend was the Digital Autocorrelator
Spectrometer, set at a resolution of
0.15 km s-1. The beam
efficiencies are
(345 GHz) and
(230 GHz). The fluxes were calibrated against the bright
nearby source IRAS 16293-2422. The observations were acquired
in position switching with a throw of 1800
south and reduced
using the SPECX software.
A saturated absorption feature centered at 4.67
m and
assigned to solid CO is seen in the M-band spectrum displayed in the
upper panel of Fig. 1. No broad absorption at 4.62
m, usually attributed to solid OCN- and a signpost of thermal
and/or ultraviolet processing, is present, although the limit of
is not strong. Unresolved gas-phase CO lines are also
detected. A continuum which takes into account absorption by silicate
is fitted to the data. The continuum substracted spectrum is shown in
the lower panel of Fig. 1 in transmission scale.
![]() |
Figure 1:
VLT-ISAAC M band spectrum toward CRBR 2422.8-3423 (upper
panel). The insert shows the VLT Ks image taken by Brandner et al.
(2000). The upper curve of the lower panel shows the
gas-phase model, convolved to the observed spectral resolution and
shifted in wavelength to account for a source heliocentric velocity
of 10 km s-1. The middle curve displays laboratorium
absorption spectra of apolar (dotted) and polar (dashed) CO ice
mixtures. The sum of the two models including Pfund ![]() ![]() |
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The best fit to the saturated solid CO band is
,
corresponding to N(CO
cm-2 using the integrated band strength of Gerakines et al.
(1995). This is the highest solid CO column density
found to date, even compared with ice-rich high-mass sources such as
NGC 7538 IRS9 (e.g. Chiar et al. 1998,
1995). The solid CO profile toward CRBR 2422.8-3423
can be decomposed into a narrow saturated component, usually ascribed
to apolar CO in a H2O-poor matrix, and a broad red wing at 4.685
m, indicative of polar CO in an H2O-rich matrix. Using
optical constants from the Leiden database including grain shape
effects (Ehrenfreund et al. 1997), a
search
for the best-fitting laboratory mixtures was performed. The
saturation of the line prevents any unique fit, but the apolar CO is
best matched by a mixture CO:N2:CO2=100:50:20 at 10 K with
cm-2. The red-wing is fitted by
H2O:CO = 4:1 at 40 K with
cm-2. The
latter column density is comparable to that found toward L 1489, which
has a luminosity of 3.7
.
The large amount of apolar CO
indicates low temperatures along the line of sight, since it
evaporates around
20 K.
The VLT spectrum shows the presence of narrow gas-phase 12CO and
13CO ro-vibrational lines originating from levels up to J=9(250 K above ground). Synthetic LTE model spectra were fitted to the
observed spectrum using data from the HITRAN database (Rothman et al.
1992). The fit parameters are the gas temperature
,
the column density
(CO) and the velocity
broadening
,
assumed to be smaller than 2 km s-1 from
the JCMT data. The limited number and the high optical depth of the
12CO lines prevent a unique fit, but the best result (see Fig. 1)
is obtained for
K, which is probably a mean between
the cold and warm components along the line of sight. The fit includes
the 13CO lines. The optical depth of the 12CO lines is
likely underestimated so that the gas-phase CO column density can
range from 1 to
cm-2. Adopting a mean value
of
cm-2, the line of sight average gas/solid
CO ratio is
1. Comparing the column of gas-phase CO with that
of polar solid CO only, the ratio drops to
0.1. The latter value
is comparable to that found for L 1489 (0.07, see Boogert et al. 2002a) but still higher than for Elias 18
(0.01, Shuping et al. 2001). If the lines were
significantly wider than 2 km s-1 as found for L 1489
(
20 km s-1), the gaseous CO column density would drop by
an order of magnitude.
The C18O
emission observed with the JCMT is
shown in the upper panel of Fig. 2. Strong
12CO
(
12 K peak temperature) and
13CO
(
7 K peak temperature) lines are
also detected, but are not displayed here since their profiles are
similar to that of C18O. C18O
is
detected on source only. Compared with single-dish CO 3-2 and 2-1
lines from disks around older isolated pre-main-sequence stars in
Taurus (e.g., Thi et al. 2001), the lines toward CRBR
2422.8-3423 are more than an order of magnitude stronger and do not
show the double-peak profile resulting from the projection of a disk
in Keplerian rotation. The CRBR 2422.8-3423 spectrum shows
three peaks, two of which have velocities similar to those seen toward
Elias 29 and interpreted as arising from the cloud ridge in
which the source is embedded (
cm-2, Boogert et al.
2002b). The total integrated C18O 2-1 intensity on
source is
K km s-1. The corresponding 12CO
column density is (0.5-1
cm-2 assuming
K (cf. Motte et al. 1998 for
Oph
clump F) and 18O/16O = 560 (Wilson & Rood 1994).
For
K, the column densities are lowered by 30%. The
non-detection of C18O
at the offset position
suggests a 12CO column density less than
cm-2 for
K, around 20 times lower.
![]() |
Figure 2:
C18O
![]() |
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Absorption studies toward young stellar objects located in the
Oph cloud complex can be dominated by foreground
cloud(s) (Boogert et al. 2002b). The SCUBA 850
m map
of
Oph by Johnstone et al. (2000) shows that
CRBR 2422.8-3423 is indeed at the edge of the ridge which
contains both Elias 29 and the nearby class I object IRS 43.
The lack of CO emission at the offset position indicates however that
the bulk of the gas-phase CO is located within 3000 AU from
CRBR 2422.8-3423. The similarity of the gaseous CO column
densities derived from the on-source C18O 2-1 emission and the
VLT-ISAAC infrared absorption may be fortuitous since the VLT-ISAAC
data do not probe very cold CO with small (
1 km s-1)
line widths. We cannot exclude that part of the gas-phase CO seen in
the infrared arises in a more extended envelope or cloud, but the fact
that the CO excitation temperature is significantly above 10 K
indicates that at least some fraction must originate close to the
young star in a warm part of the disk.
For solid CO, there are strong arguments that most of the absorption
must arise in the disk. The bright nearby (30
east)
source IRS 43 was observed simultaneously with CRBR 2422.8-3423 with VLT-ISAAC and has a CO ice optical depth of only
,
corresponding to N(CO
cm-2 (Pontoppidan et al., in
prep.). This is at least a factor of 3 lower than toward CRBR 2422.8-3423 even though its 850
m flux is a factor of 1.5
higher. Toward Elias 29, solid CO has an optical depth of
only 0.25, most of which is believed to be located in the foreground
clouds. Compared to the optical depth found toward CRBR 2422.8-3423 (
(CO
), this amount of
foreground material can account for only an insignificant fraction of
the observed solid CO. Finally, if clouds happen to lie in front of
CRBR 2422.8-3423, the moderate extinction (
)
of those clouds probably prevents them to harbour
significant amounts of solid CO (Shuping et al. 2000).
Indeed, in a M-band survey of more than 30 young stellar objects,
this is the deepest CO ice band observed.
![]() |
Figure 3: Fraction of CO molecules in the solid phase at different temperatures as function of density. Adsorption onto H2O and CO ice produce different results. The desorption energies are from Sandford & Allamandola (1988) for CO-H2O and Sandford et al. (1988) for CO-CO. |
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A simple disk model cf. Chiang & Goldreich (1999) with
T*=3500 K and a disk radius of 250 AU has been adopted to
investigate the location of the solid and gaseous CO seen in infrared
absorption. Because of its higher temperature, the gaseous CO is
likely not co-located with the bulk of the apolar solid CO, which
evaporates at 20 K. The polar solid CO can however reside in
the same region of the disk as the CO gas at 40-60 K. Assuming that
CO is frozen out at <20 K (apolar CO) and <40 K (polar CO), the
best fit to the column densities, gas/solid CO and polar ice/apolar
ice ratio is obtained for
.
This inclination is
consistent with the flux asymmetry seen in the near-infrared image of
Brandner et al. (2000). For such line of sight, most of
the CO ice is located above the midplane in the outer disk, whereas
the CO gas is found in the warm inner disk. Thus, the overall CO
depletion could be significantly higher than the ratio of
1
found here.
Several time-dependent chemical models were run to quantify the
gas/solid CO ratios in different density and temperature regimes. The
models simulate gas-phase chemistry, freeze-out onto grain surfaces
and thermal as well as non-thermal evaporation. Cosmic-ray induced
desorption is modeled as in Hasegawa & Herbst (1993),
which may be an overestimate for large grains in disks (Shen et al.,
in prep.). The sticking coefficient was set at 0.3 to account for
other non-thermal mechanisms (e.g., Schutte & Greenberg
1997) and photodesorption is assumed ineffective. At
thermal desorption dominates, whereas at
cosmic-ray desorption prevails. In the model,
is 20 K for apolar CO ice (
K) and 40 K (
K) for polar CO. To illustrate the effects of thermal
and cosmic-ray induced desorptions, Fig. 3 shows
the gas/solid CO ratio at chemical equilibrium. In cold (T< 20 K)
but moderately dense (104-105 cm-3) regions of the disk
around CRBR 2422.8-3423, only a small amount of CO is
depleted onto grains in the apolar form, but much larger fractions
>50% can occur at higher densities (
106-108 cm-3).
In summary, we detected a large amount of solid CO in the line of sight toward CRBR 2422.8-3423. The majority of this ice is likely located in the flaring outer regions of the edge-on circumstellar disk. Very high resolution (R>105) near-infrared spectroscopy is needed to reveal the gaseous CO line profiles and thus their origin and the gas dynamics in the inner disk. Future submillimeter interferometer data can probe the velocity pattern and excitation conditions of the gas as functions of disk radius, whereas mid-infrared spectroscopy with, e.g., the Space Infrared Telescope Facility (SIRTF) will allow searches for other ice components, in particular solid CO2.
Acknowledgements
WFT thanks PPARC for a postdoctoral grant to UCL. Astrochemistry in Leiden is supported by a Spinoza grant from the Netherlands Organization for Scientific Research (NWO) and a PhD grant from the Netherlands Research School for Astronomy (NOVA). We thank the ESO staff for their help during the observations, P. Papadopoulos for performing the JCMT observations and D. Johnstone for a blow-up of the SCUBA map ofOph.