C18O (3-2) observations of CG12, APEX Facility Science Verification

Coordinator: L. Haikala, M. Juvela, J. Harju, K. Lehtinen, K. Mattila

APEX telescope will provide the possibility to observe the C18O (3-2) line. The line at 329.33GHz lies in frequency in the wing of the strong H2O atmospheric line which separates the 200 GHz and the 300GHz atmospheric windows. This C18O line is difficult to observe from other observatories and very few observations have been published. Using the three lowest C18O transitions in modeling the molecular clouds instead of only two will provide more reliable cloud models. Cometary globule CG12 (NGC 5367) has two compact molecular cores which are also detected in the 1.2mm dust continuum. Even though the lines are relatively narrow the cores contain two (CG12South) and three (CG12North) velocity components. The C18O(1-0) to C18O(2-1) line ratio differs between the cores and between the velocity components within each core. Apex high spectral resolution (62.5 KHz) C18O (3-2) observations will show the feasibility of C18O observations with the telescope and in addition will provide means to more accurately model the Cometary globule.

Program is available and data products can be downloaded

Scientific justification:

Circumstellar disks contain the material necessary for planet formation, con- sisting mainly of H2 gas. Thus good knowledge of its mass, distribution and physical conditions are true imperatives towards a complete understanding of the formation and evolution of planetary systems. The ongoing detection of large planets around other stars, with the implied large H2 gas reservoir neces- sary for their build-up, brings these matters to the fore with renewed focus and timeliness. In the hereby proposed observations we will utilize APEX to study the molecular content of planet-forming disks surrounding a sample of Southern Herbig Ae stars.
The material around Herbig Ae stars is most commonly studied in dust continuum emission. However, molecular gas - traced by the low-J lines of CO - has also been detected in a small number of objects (e.g. Zuckerman et al., 1995; Coulson et al., 1998; Greaves et al., 2000; Thi et al., 2001). In many cases the signal:noise has been too low to obtain details of the line profiles, although some show a double-peaked line.

Technical aspect
The C180(3-2) transition at 329.33GHz lies well inside the atmospheric H2O absorption line. Therefore, even though the transition has been observed in intestellar molecular couds, the observations are few. APEX offers an excellent possibility to observe the C180(3-2) line. The high altitude and the low water wapor content of the atmosphere at the site make the transparency at 329GHz superior to other sites like eg. Mauna Kea. The measured submillimeter atmospheric transmission is shown in Figure 1. The coloured lines indicate the fitted atmospheric H2O (red) and oxygen (blue) absorption lines. Besides the atmospheric water content the absorption line is sensitive to pressure broadening. The latter aspect provides a clear advantage to APEX as compared with other observatories when observing the C18O(2-1) as this line lies in the H2O absorption line wing.
Published C18O(3-2) data of dark clouds practically do not exist. Jessop and Ward-Thompson 2001, MNRAS.323, present low spectral resolution (3-2) data of the pre-stellar core in L1689B obtained with JCMT at Mauna Kea. The DSB system temperatures reported in the paper are 2000 to 12000 K for these observations.
The feasibility of observing the C18O(2-1) line at APEX should be tested. Compared to the more abundant CO isotopes the C18O lines are likely to be optically thin in most astronomical sources. Therefore observing this transition in addition to the lower C18O transitions would provide essential information when modelling the clouds.
To optimize the system temperature the C18O(3-2) line should be placed in the lower side band. If the observations are made using position switching the off position should be as near as possible and at the same elevation as the observed point. For the head of the Cometary globule CG12 an off position can be chosen within 5 arcminutes of the observed position at the same elevation as the on source position if the observations are done within +/- 3 hours of the meridian.
The expected C18O (2-1) line antenna temperature is appox. 1K. Assuming the observations are done at high elevations (the source culminates at 75 degrees) the SSB equivalent system temperature is estimated to be 300K or less. For S/N of 10, channel width 62.5 KHz, position switching with 30s integration time and 30% dead time, the time needed for one position is 8 minutes.
For scientific return a minimum of three positions are needed (two in CG12S and one in CG12N). A 3 by 3 map in both cores would require 2.5 hours plus 0.5 hours for calibration purposes, in total this is 3 hours. If the line is stronger than expected or the system temperature is better, a larger map can be observed.

Scientific case
Observations of the (1-0) and (2-1) transitions of CO and its isotopes are used as a standard tool to investigate the physics and the structure of dark clouds and low mass star forming regions. Due to high opacities in the line centre the 12CO and 13CO lines can mainly only be used to study outflow phenomena. C18O emission can be expected to be optically thin or only moderately thick in most clouds and is therefore well suited to probe the molecular cloud all the way to the centre of the cloud. Physical parameters are usually extracted from the C18O data either by making an educated guess of the excitation temperature and assuming LTE conditions or by modeling the cloud and trying to reproduce the observed line ratios and profiles. In the latter approach measurements of other molecules can also be used. Besides assuming LTE the first approach assumes that all the emission is coming from a single clump which has a constant temperature and density, not a very likely situation. The basic drawback of cloud modelling is that there is rarely ever a single solution but rather a multitude of geometries, temperatures and densities which can reproduce the observed lines. This is not suprising as only two transitions have been observed. Adding a third observed transition (C18O (3-2)) will immediately exclude a multitude of models. Possible subthermal excitation will also be apparent if present. The only reason why the C18O(3-2) transition is not used as input in cloud modeling is the observational difficulty of this transition because of atmospheric opacity at 329 GHz.
Cometary globule 12 (CG12, NGC 5367) lies at a distance of about 500pc and is 20 degrees above the galactic plane. The cloud has an associated young stellar cluster and is therefore an example of starformation 200pc out of the galactic plane.
The globule has been observed extensively at SEST. It has been mapped in C18O(1-0) and (2-1) transitions. Two compact cores, CG12N(orth) and CG12S(outh) were found. The cores have been further mapped in DCO+, H13CO+ and CS. Pointed observations in various other molecules have also been done. The two cores and an extended halo of dust continuum emission were also detected at 1.2mm with SIMBA at SEST. The C18O(2-1) and the SIMBA maps are shown in Figure 2. The white contours delineate the DCO+(2-1) emission. The CG12S DCO+ and continuum cores are spatially offset from the C18O (2-1) (and also the (1-0)) core.
Observed spectral line profiles in the direction of the two cores are shown in Figure 3. Position -20,-20 corresponds to CG12S and -20,160 to CG12N. There is a notable difference in the C18O(1-0) to (2-1) line ratio in the cores. In CG12S the (2-1) emission is stronger than the (1-0) emission whereas in CG12N the opposite is the case. Even though the CS(2-1) line is aligned at the same velocity as the C18O lines in CG12N the ions DCO+ and N2H+ are situated in the C18O line wing.
The data have been analysed in Haikala et al 2005 (submitted). Even though the lines are relatively narrow two and three velocity components are needed to explain the observed line profiles in CG12S and N, respectively. CG12N consist of three components, one warm, a cooler one and a very cold component, all separated in velocity by about 0.4 km/s. The molecular material associated with the coldest core seen in mm dust emission, seems to be heavily depleted as practically no C18O emission is observed at the velocities of eg. DCO+ and N2H+. CG12S consists of two components, a cold one associated with the mm dust emission core and a warmer one. These two components are also separated in velocity.

Data reduction and analysis
Reduction of the spectra can be done with standard spectral line reduction packages like eg. Class. Analysis of the data will be done using three dimensional Monte Carlo simulation (Juvela 1997, A&A 322, 943, Juvela & Padoan 2005, ApJ 618, 744). The new data will be analysed with the already existing C18O data, other molecular line data and the mm continuum data, all obtained at SEST by us.