The clumpy distribution of warm gas in the Rosette molecular cloud complex

Coordinator: C. Kramer and V. Ossenkopf

We propose to conduct a study of the bright photon dominated regions(PDRs) in the Rosette molecular cloud complex (RMC). The aim is toresolve the temperature, chemical, and excitation structure in theclumpy transition from the HII region to the dense molecular cloud. Wesuggest to map the warm gas in the CO 3-2 transition and to scan thechemical structure of the warm and dense gas across the PDR conductingcuts in HCN, HCO+ , CS, and CN. To study the detailed structure, thecombination of a high spatial resolution with receivers suitable forthe higher transitions of the species, as provided by APEX, isessential.

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Scientific justification:

In preparation of a HIFI key project containing a study of PDRs, wehave set up a comprehensive data base on PDR observations covering abroad range of physical parameters. Here, we propose to study a PDRexposed to a moderate FUV radiation field of 100 times the ISRF at adistance of 1.6 kpc. The structure and excitation conditions of thecold gas of the Rosette MC have been studied with the IRAM-30mtelescope at resolutions of 10 to 22 by Schneider et al. (1998a). KOSMACO 3-2 low resolution observations show the large-scale distribution ofthe warm gas. KAO observations of the CII FIR line at resolutions ofabout ~1' (Schneider et al. 1998b) show extended emission of ionizedcarbon. The good correlation of CO and CII emission can be explained bya high clump/interclump density contrast. This scenario is supported bydeep optical imaging (Block et al. 1992). However, there exist nohigh-resolution observations of this clumpy structure in tracers of thewarm gas. Thus, we propose to observe with APEX higher-lying rotationallines of CO, HCN, HCO+ , CN, and CS. The combination of CO 3-2observations with the existing CO data of lower transitions allows totrace the excitation conditions of the dense, warm gas at the surfacesof PDRs inside the Monocerus ridge to the south of the OB clusterNGC2244 (see Figs.1,5,15 in Schneider et al. 1998a). From theobservations of higher HCN, HCO + , CN, and CS transitions we cansimultaneously derive the chemical structure of the warm gas. With thehigh resolution of APEX at 0.8 mm of 18 we can better separate theeffect of the stratification of the PDR from the influence of aninherent clumpiness of the medium, thus resolving the physicalstructure of the PDR.
CO 3-2 is selected for the following reasons:
- Tracing the extended warm molecular gas, we expect brightemission over a relatively large area to be studied, from the boundaryof the PDR deep into the dense molecular cloud.
-Using the high spatial and spectral resolution of APEX we expectto resolve at least part of the clumpy structure of the PDRs allowingto derive an estimate for the penetration of the UV field into thecloud.
- Comparing the CO 3-2 data with existing, lower resolution 1-0and 2-1 IRAM data allows to estimate the temperature structure in thestratification region of the PDRs. In contrast to CO 1-0, the proposedCO 3-2 observations allow to sample the dense, warm gas, heated by theFUV radiation of the surrounding young OB stars. This helps toconstrain the energy balance in the gas as a function of depth into thecloud.
HCN, HCO+ , CS, and CN are selected for the following reasons:
-The 4-3 transitions of HCN and HCO+ are thought to trace very highdensities because of their high critical density. However, HCN 4-3 mapsof the MonR2 molecular cloud show a surprisingly good correlation withCO 3-2 (Giannakopoulou et al. 1997; Choi et al. 2000). Thiscontradiction needs to be resolved by observations with higherresolution. We plan to conduct short cuts centered at the corepositions in the rarer species.
- HCN and HCO+ are sensitive tracers of the chemistry and theionization fraction in photon dominated regions (Boger & Sternberg2005).
- The CN/HCN line ratio has been identified as an importantindicator of the physics of dense PDRs (Boger & Sternberg 2005).High ratios are observed in PDRs near the illuminated surfaces (e.g.Fuente et al. 2003). However, different scenarios are suggested toexplain the observations. New observations of higher excited lines andof more species are needed to improve on this analysis.
- Chemical stratification of CN and CS submillimeter emission havebeen observed in Orion Bar (Simon et al. 1997). Such observations areyet missing for other edge-on PDRs subject to weaker radiation fields.

We propose to measure the physical and chemical structure inan elongated map across the Monocerus Ridge of the Rosette MC where weexpect to trace the stratified structure of the PDR as a function ofthe distance from the illuminating sources. To focus on the effect ofthe combination of density structure and radiation field we omit clumpswith known star-formation activity, but propose to map a regioncontaining the northern and south-western core of the Monocerus Ridgeshown in Fig. 1.

Observing strategy
We propose to map a 80" × 160"region in CO 3-2 by conducting 9 parallel cuts at constant declinationbetween RA=6:33:09 and 6:33:20. Declinations range between 4:38:00 and4:36:40 (eq2000). This results in 128 positions on a fully-sampled 10grid. The cuts are suited for OTF observations, but as theimplementation of the APEX OTF mode is still open, they can be observedin a normal raster map mode as well.
For the other weaker lines, we propose to conduct only small cutsof 100" length at constant declination centered on the two cores seenin the 13CO 2-1 map.
Cut 1 starts at RA/DEC(2000)=6:33:09, 4:38:00.
Cut 2 starts at RA/DEC(2000)=6:33:18, 4:37:10.
This results in 20 positions.
To resolve details of the clumpy structure in velocity space we aimat a spectral resolution of 0.2 kms-1 . At 345 GHz this corresponds toa required resolution of about 0.25 MHz which can be translated intothe 256 MHz bandwidth configuration of the correlator with a channelspacing of 125 kHz.

Time estimates
In October 2005, Rosette is observableduring the late night at an average elevation of about 55deg, during 4hours in the LST interval 3:00 to 7:00. The LSR center velocity of theMonocerus Ridge is ~15km.s-1 .
KOSMA CO 3-2 observations ( 90" resolution) of the Extended Ridgeand the Monocerus Ridge in the RMC show peak main-beam temperatures of8 to 10 K. We expect significantly higher peak temperatures at 18"resolution. In order to also trace the weak emission outside the peakregions, we aim at a noise rms of 0.25 K (T*_A ). Assuming a systemtemperature of 250 K as suggested in the call for proposal, thistranslates into about 24s integration time for the ON-OFF cycle at eachpoint to reach the noise limit. Neglecting any overheads (calibration,pointing, tuning), we derive at a total on+off observing time of 1 hourfor CO 3-2 at 128 positions.
The CO 3-2 is found to be a factor ~8 stronger than the HCN 4-3line in the MonR2 molecular cloud (Giannakopoulou et al. 1997; Choi etal. 2000). We thus expect HCN 4-3 peak temperatures of 1 K and morewith APEX. Aiming at an rms of 100 mK, we need 2.5 min per position. Wethus need ~1 hour for the 20 positions in HCN 4-3.
HCO+ 4-3, CN 3-2, and CS 7-6 are expected to have similar line strengths.
In total, we thus ask for 5 hours of on+off time.


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