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 ![]() ![]() |
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
![]() |
Copyright ESO 2002