The earliest stages of low-mass star formation Mapping the dust ring in Coalsack's Globule 2
Coordinator: D. Muders, R. Klessen, C. Lada
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Low-mass stars are formed from dense cores in molecular clouds or from dark (Bok) globules. These cores areusually centrally condensed and studying their radial density profiles allows to determine their evolutionarystage. Some cores are in a hydrostatic equilibrium or Bonnor-Ebert sphere phase (e.g. B68) while others showsignatures of current or previous collapse phases (e.g. B335). Quite a number of those sources have been studiedusing mm, submm and IR techniques.
While most of the known cores are already on the way to form stars, very little is known about how theywere formed from the surrounding molecular gas. It is important to explore this phase because it can help todistinguish between different star formation theories.
Only few sources are known to be in such an early evolutionary stage. The most promising areas to search forthose objects are dark, starless clouds. One very interesting object in this regard is the Coalsack region (Tapia1973) in the southern sky. Its linear dimension is almost 6o which corresponds to about 15 pc at the distanceof 150 pc determined for the cloud. It contains about 3500 solar masses of material distributed in a highlyfilamentary structure as seen from a 12 CO survey (Nyman 1989).
A recent deep infrared extinction study (Lada 2004) of the most prominent dense core (Tapia's Globule 2 (G2))in the Coalsack revealed a ring structure (see figure 1) rather than the usual central condensation. The ring ismost likely a transient feature on the way to a central condensation. The collapse time scale would be about2 × 105 yr. The outer part of the G2 core shows a structure that is similar to a Bonnor-Ebert sphere. G2 isthus likely in an extremely early stage of star formation before forming a Bok globule.
A C18 O 2-1 spectrum (figure 2) taken with the SEST telescope towards the southern part of the ring showsturbulent, non-thermal structure and indicates that the object is gravitationally bound. The narrow line widthsof about 0.7 km/s in the two components are similar to those of other globules. The two component structuremay originate from specific velocity field within the source but this can only be explored in more detail with amap.
We propose to investigate the question of whether G2 really is in the assumed early stage of star formation atthe beginning of collapse by mapping its velocity field using submillimeter spectroscopy. We first want to getan overview of the overall spatial and velocity structure of the source by mapping the main CO isotope becausethere are no maps of the molecular gas yet. Given that the source is very cold (around 10 K), we aim at usingthe CO 3-2 line at 345.795 GHz.
We intend to use the CO data to derive velocity gradients across the source that will then be compared to themodels presented by Klessen et al. (Klessen 2005). According to those models, cores are formed by large scaleturbulence. Depending on the location of the condensations within the turbulent flow they may be transient andget dissolved later on. There are however, also quiescent, sometimes even subsonic zones, where the gravitationalforce may become dominant and initiate a collapse towards a Bok globule. The goal is to find out whether G2is such a candidate.
Given its low declination of about -64o , only the new class of submm telescopes like APEX are able to observethe G2 globule. Furthermore its unusual ring structure makes it a prime target for the science verificationperiod to show the capabilities of the APEX telescope using the "On-The-Fly" (OTF) observing technique.
We would like to reach a signal to noise ratio of about 10 in the individual spectra in order to be able to mapthe ring's contrast to the background medium properly. Judging from the C18 O 21 data we can expect a fewK line strengths in CO 32. The lines will be about 0.7 km/s wide. To examine the velocity structure we needat least 10 spectral channels across this width.
Using the facility correlator at 128 MHz bandwidth with 2048 spectral channels delivers a velocity resolutionof about 0.055 km/s. The system temperature at 345 GHz is typically 250 K. We need an RMS of about 0.3 Kwhich leads to on-source times of around 10 seconds per beam.
For this first overview, we intend to map the inner 4.5 × 4.5 ' of G2 to cover the ring area. A fully sampledmap of this region using the 15 beam at 345 GHz requires 37 × 37 points @ 10 seconds each, i.e. 3.8 hourson source. We will need about one sky reference for every 3 beams, i.e. another 1.3 hours off source. The totalintegration time is therefore about 5 hours. The overhead for calibration, pointing and system overheads atAPEX is currently about a factor of 3. We therefore ask for a total of 15 hours of telescope time during thescience verification.
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