APEX SV projects

A Chemical and Dynamical Study of the B68 Prestellar Core with APEX

Coordinator: S. Maret, T. Bergin , C. Lada, and P. Teixeira

Abstract:
In the past few years, we have used IRAM-30m molecular lines observations in combination with near-IR extinction maps to derive the chemical abundance profiles in the cold dark cloud B68. With these observations, we have obtained a detailed picture of the chemical structure of B68: an outer layer dominated by photodissociation, a deeper layer with undepleted abundances, and a central region where selective freeze-out of molecules occurs. We also have demonstrated that the chemistry can be a powerful tool to contrain the dynamics of the cloud: the line profile of optically thick tracers indicate that the outer layer of the cloud is infalling, while the central region of the core is outflowing. Here we propose to continue our study of the chemistry and the dynamics of B68 at higher frequency with the APEX telescope.

Data:
Program is available and data products can be downloaded

Introduction

Over the past decade, maps of dust continuum emission (Andre et al. 2000) and dust absorption (Alves et al. 2001) have expanded our capability to study the chemistry and the dynamics associated with star formation. In these studies, prestellar cores are unique laboratories because they present pristine and well described environments. When combined with dust measurements that constrain the physical structure, millimeter molecular line observations have provided detailed chemical abundances profiles within these objects (Tafalla et al. 2002; Bergin et al. 2002). Overall, these observations and modeling provide a comprehensive understanding of both the chemistry and the dynamics of these cores.
The B68 cold dark core is a representative example of those studies. Using dust extinction maps, Alves et al. (2001) have determined the density profile of the core. In the past years our team has used the IRAM-30 to map extensively 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, one can see that the N2 H+ emission peaks in a shell partially surrounding the peak of dust emission. Moreover, the N2 H+ emission peaks inside the much larger C18O hole, which lies itself inside the CS emission. These molecules represent an increasing sequence of molecular depletion: CS is depleted in almost all the cloud, with the exception of the outermost layers. C18 O depletes deeper in the cloud, creating the "hole" in the map. In the denser part of the cloud, where the dust emission peaks, even N2 H+ is depleted. Using a chemistry model including molecular depletion (Bergin & Langer 1997), coupled with a radiative transfer Monte Carlo code, we have determined the abundances profiles of these molecules (Bergin et al. 2002, Maret et al. 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 the C18 O emission decreases for high Av as a result of freeze out onto grain surfaces. The N2 H+ abundance decreases as well at high Av . H13CO+ is an intermediate case between these two molecules. Through these observations and modeling, we have obtained a detailed picture of the B68 chemical structure: an outer layer of the cloud is dominated by photodissociation, followed by a deeper region with undepleted abundances, and a central region dominated by the selective freeze-out of various molecular species.
The understanding of the chemistry is important on its own, but also because the chemistry can be used as a tool to understand gas physical and dynamical structure. Because of selective desorption and excitation, different molecules will probe different layers of the cloud; as seen on Fig.2, C18O probes mostly the outer layers of the cloud, while N2H+ , which is less depleted, probes the inner regions. Moreover, if the lines are optically thick, the line profile can be used to determine the velocity at a given radius in the cloud, where tau ~ 1 (see Evans et al. 2001, for a discussion of using molecular emission to trace motions). With several molecules probing different regions, it is possible to reconstruct the velocity profile as a function of the radius. This is illustrated on Fig. 3, which shows spectra of different molecules obtained towards the center of B68. The outer edges of the cloud are traced by CS and HCO+ , and the lines profiles indicate infalling motions. H13CO+ and DCO+ are tracking a layer where the velocity shifts from infall to outflow. Finally, N2H+ tracks an outflowing center. We are on the process of quantifying the velocity structure by a detailed chemical analysis. Preliminary results are shown in Fig. 2 (Lada et al. 2003, Maret et al., in prep.). Besides the dynamics, the gas temperature has been examined in a similar multi-molecular study (Bergin et al. 2004).

A chemical and dynamical study of B68 with APEX
Our millimeter IRAM-30m observations of B68 and modeling have demonstrated how the chemistry can be used as a tool to understand the dynamics of pre-stellar cores. Here, we propose to extend these study to the submillimeter range with the APEX telescope. Our project is to map the core in the N2H+ (3-2) and HCO+ (4-3) transitions. These two molecules are of peculiar interest for the following reasons: N2H+ (1-0) observations have shown that this molecule depletes only in the densest past of the core (See Fig. 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 the core. 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 a blue-shifted profile, which would confirm that the inner part of the core is outflowing. Moreover, the comparison of the line fluxes of the N2H+ (3-2) with the N2H+ (1-0) can be use to estimate the temperature of the central region, and confirm the results of the gas temperature modeling by Bergin et al. (2004).
On the other hand, our chemical modeling shows that the HCO+ abundance peaks in the outer part of the core, at an Av of 3. Consequently, lines of this molecule can be use to probe outer parts of the core. Furthermore, the line profile of the HCO+ (1-0) observed at IRAM-30m is red shifted, and indicates 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 part of the core. We expect the HCO+(4-3) to have an intermediate opacity between the HCO+ (1-0) and the H13CO+(1-0) lines. It will therefore probe an intermediate region between the infalling and the static parts of the core. Beside dynamics, HCO+ and its isotopes will be used to determine the ionization fraction, which plays a key role in the chemical and dynamical evolution of prestellar cores.
We propose to map the B68 core in these two lines. We will use the APEX-2a receiver, with the backend set to a 64 MHz bandwidth. For these two lines, we set the typical RMS in the peak emission to be 20 mK. Due to the relatively low temperature 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 at the requested level. Therefore, we think that this project would be a good opportunity to validate the sensitivity of the telescope.
For good Chajnantor weather conditions (0 = 0.10), a 31.25 kHz resolution and a 60 average source elevation, the APEX time estimator gives a observing time of 10 minutes per position (in frequency switching mode) for these settings.
We will make a 2 × 2 raster map with a 20" sampling. This requires a total of 36 points per map, i.e. about 6 hours of observing time for each settings. Assuming 30% of overheads for receiver tuning, pointing and calibration, about 16 hours will be needed to complete the project. The required observing time can be scheduled in 2 nights of 8 hours.

*Source coordinates*
Source	RA(2000)	DEC(2000)	LSR Velocity (km/s)
B68	17:22:38.201	-23:49:34.0	+0

*Frequency settings*
Line	Frequency(GHz)	RMS(20mK)	Integration time per position
N2H+ (3-2)	279.511	20	10
HCO+ (4-3)	356.734	20	10

References

Alves, J. F., Lada, C. J., & Lada, E. A. 2001, Nature, 409, 159
Andre, P., Ward-Thompson, D., & Barsony, M. 2000, Protostars and Planets IV, 59+
Bergin, E., Maret, S., & van der Tak, F. 2004, in The Dusty and Molecular Universe: A Prelude to Herschel and ALMA, 139+
Bergin, E. A., Alves, J. ., Huard, T., & Lada, C. J. 2002, ApJ, 570, L101
Bergin, E. A. & Langer, W. D. 1997, ApJ, 486, 316
Evans, N. J., Rawlings, J. M. C., Shirley, Y. L., & Mundy, L. G. 2001, ApJ, 557, 193
Lada, C. J., Bergin, E. A., Alves, J. F., & Huard, T. L. 2003, ApJ, 586, 286
Tafalla, M., Myers, P. C., Caselli, P., Walmsley, C. M., & Comito, C. 2002, ApJ, 569, 815