A Chemical and Dynamical Study of the B68 Prestellar Core with APEX
Coordinator: S. Maret, T. Bergin , C. Lada, and P. Teixeira
In the past few years, we have used IRAM-30m molecular linesobservations in combination with near-IR extinction maps to derive thechemical abundance profiles in the cold dark cloud B68.With these observations, we have obtained a detailed picture of thechemical structure of B68: an outer layer dominated byphotodissociation, a deeper layer with undepleted abundances, and acentral region where selective freeze-out of molecules occurs. We alsohave demonstrated that thechemistry can be a powerful tool to contrain the dynamics of the cloud:the line profile of optically thick tracers indicate that the outerlayer of the cloud is infalling, while the central region of the coreis outflowing. Here we propose to continue our study of the chemistryand the dynamics of B68 at higher frequency with the APEX telescope.
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Over the past decade, maps of dust continuum emission (Andre et al.2000) and dust absorption (Alves et al. 2001) have expanded ourcapability to study the chemistry and the dynamics associated with starformation. In these studies, prestellar cores are unique laboratoriesbecause they present pristine and well described environments. Whencombined with dust measurements that constrain the physical structure,millimeter molecular line observations have provided detailed chemicalabundances profiles within these objects (Tafalla et al. 2002; Berginet al. 2002). Overall, these observations and modeling provide acomprehensive understanding of both the chemistry and the dynamics ofthese cores.
The B68 cold dark core is a representative example of thosestudies. Using dust extinctionmaps, Alves et al. (2001) have determined the density profile of thecore. In the past years our team has used the IRAM-30 to mapextensively the core in different molecular lines (13 CO, C18 O, HCO+ ,H13CO+ , DCO+ , N2H+ , CS and C34S). Fig. 1 shows three of these maps,overlaid with the distribution of visual extinction. In these maps, onecan see that the N2 H+ emission peaks in a shell partially surroundingthe peak of dust emission. Moreover, the N2 H+ emission peaks insidethe much larger C18O hole, which lies itself inside the CS emission.These molecules represent an increasing sequence of moleculardepletion: CS is depleted in almost all the cloud, with the exceptionof the outermost layers. C18 O depletes deeper in the cloud, creatingthe "hole" in the map. In the denser part of the cloud, where the dustemission peaks, even N2 H+ is depleted. Using a chemistry modelincluding molecular depletion (Bergin & Langer 1997), coupled witha radiative transfer Monte Carlo code, we have determined theabundances profiles of these molecules (Bergin et al. 2002, Maret etal. in prep). Fig. 2 shows the integrated intensity of the C18O, N2H+and H13 CO+ lines as a function of Av . On this figure, we see that theC18 O emission decreases for high Av as a result of freeze out ontograin surfaces. The N2 H+ abundance decreases as well at high Av .H13CO+ is an intermediate case between these two molecules. Throughthese observations and modeling, we have obtained a detailed picture ofthe B68 chemical structure: an outer layer of the cloud is dominated byphotodissociation, followed by a deeper region with undepletedabundances, and a central region dominated by the selective freeze-outof various molecular species.
The understanding of the chemistry is important on its own, but alsobecause the chemistrycan be used as a tool to understand gas physical and dynamicalstructure. Because of selective desorption and excitation, differentmolecules will probe different layers of the cloud; as seen on Fig.2,C18O probes mostly the outer layers of the cloud, while N2H+ , which isless depleted, probes the inner regions. Moreover, if the lines areoptically thick, the line profile can be used to determine the velocityat a given radius in the cloud, where tau ~ 1 (see Evans et al. 2001,for a discussion of using molecular emission to trace motions). Withseveral molecules probing different regions, it is possible toreconstruct the velocity profile as a function of the radius. This isillustrated on Fig. 3, which shows spectra of different moleculesobtained towards the center of B68. The outer edges of the cloud aretraced by CS and HCO+ , and the lines profiles indicate infallingmotions. H13CO+ and DCO+ are tracking a layer where the velocity shiftsfrom infall to outflow. Finally, N2H+ tracks an outflowing center. Weare on the process of quantifying the velocity structure by a detailedchemical analysis. Preliminary results are shown in Fig. 2 (Lada et al.2003, Maret et al., in prep.). Besides the dynamics, the gastemperature has been examined in a similar multi-molecular study(Bergin et al. 2004).
A chemical and dynamical study of B68 with APEX
Ourmillimeter IRAM-30m observations of B68 and modeling have demonstratedhow the chemistrycan be used as a tool to understand the dynamics of pre-stellar cores.Here, we propose to extend these study to the submillimeter range withthe APEX telescope. Our project is to map the core in the N2H+ (3-2)and HCO+ (4-3) transitions. These two molecules are of peculiarinterest for the following reasons: N2H+ (1-0) observations have shownthat this molecule depletes only in the densest past of the core (SeeFig. 2). Therefore, it is a good tracor of the inner parts of B68.Moreover, the N2H+ (1-0) line has a blue shifted line profile (Fig. 3),which seems to indicate outflowing motion of the center region of thecore. The opacity of N2 H+ (3-2) is likely to be lower than the N2H+(1-0), and will therefore probe more inner regions than the N2 H+ (1-0)line. If the line is optically thick, we expect the line to have ablue-shifted profile, which would confirm that the inner part of thecore is outflowing. Moreover, the comparison of the line fluxes of theN2H+ (3-2) with the N2H+ (1-0) can be use to estimate the temperatureof the central region, and confirm the results of the gas temperaturemodeling by Bergin et al. (2004).
On the other hand, our chemical modeling shows that the HCO+ abundancepeaks in the outerpart of the core, at an Av of 3. Consequently, lines of this moleculecan be use to probe outer parts of the core. Furthermore, the lineprofile of the HCO+ (1-0) observed at IRAM-30m is red shifted, andindicates that the outer part of the core is inflowing. The H13CO+(1-0)line has a lower opacity, and probe an innermost part and static partof the core. We expect the HCO+(4-3) to have an intermediate opacitybetween the HCO+ (1-0) and the H13CO+(1-0) lines. It will thereforeprobe an intermediate region between the infalling and the static partsof the core. Beside dynamics, HCO+ and its isotopes will be used todetermine the ionization fraction, which plays a key role in thechemical and dynamical evolution of prestellar cores.
We propose to map the B68 core in these two lines. We will use theAPEX-2a receiver, withthe backend set to a 64 MHz bandwidth. For these two lines, we set thetypical RMS in the peak emission to be 20 mK. Due to the relatively lowtemperature of the core ( 10 K), we expect both N2H+ (3-2) and HCO+(4-3) lines to be weak. However, they should be easily detectable atthe requested level. Therefore, we think that this project would be agood opportunity to validate the sensitivity of the telescope.
For good Chajnantor weather conditions (0 = 0.10), a 31.25 kHzresolution and a 60 averagesource elevation, the APEX time estimator gives a observing time of 10minutes per position (in frequency switching mode) for these settings.
We will make a 2 × 2 raster map with a 20" sampling. Thisrequires a total of 36 pointsper map, i.e. about 6 hours of observing time for each settings.Assuming 30% of overheads for receiver tuning, pointing andcalibration, about 16 hours will be needed to complete the project. Therequired observing time can be scheduled in 2 nights of 8 hours.
|Source||RA(2000)||DEC(2000)||LSR Velocity (km/s)|
|Line||Frequency(GHz)||RMS(20mK)||Integration time per position|
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