Unbiased Spectral Survey of the Low-Mass Protostar IRAS16293-2422
Coordinator: E. Caux, A. Castets, V. Wakelam, P. Schilke, C. Comito, C. Ceccarelli, C. Kahane, X. Tielens, F. Helmich ,E. van Dishoeck, S. Cazaux,B. Parise, A. Walters,
With APEX, we propose to complete the unbiased spectral survey towards the low-mass protostar IRAS16293 started in January 2004 at the IRAM 30 m telescope in the 80 - 280 GHz range and at the JCMT telescope in the 328 - 366.5 GHz range. This spectral survey provides a wealth of molecular rotational transitions. Most of these originate in the warm, dense gas surrounding the protostar. This set of data provides a unique view of the molecular content of this gas and hence the chemistry in regions of low mass star formation. In addition, the multitude of rotational lines provide a detailed probe of the physical conditions in the emitting gas. APEX observations in the 366.5 - 372.5 GHz range will allow to observe important transitions of a number of key species, but above all they will provide a scientific validation of an important observing modefor an heterodyne instrument, a spectral survey.
Program is available and data products can be downloaded
IRAS16293-2422, RA = 16:32:22.7, DEC = -24:28:33.0 (J2000), Vlsr = 3.8 km/s
Summary of the observations already performed
So far, about 75% of the spectral survey has been achieved at the IRAM 30 m telescope and 100% at JCMT, with a rms of 5 - 10 mK from 80 to 250 GHz, and of 10 - 15 mK from 250 to 366.5 GHz.
The analysis of the data is in progress. This include data reduction (completed) and line identification (in progress). From this it results that the line density is more than 20 / GHz (see Figure 1), regardless of the frequency range observed. Some species have been observed for the first time in a star forming region (high or low mass). About 75 species are already detected (not taking into account isotopic species).
Here we propose to carry on this survey at APEX, focusing the observations on the upper end of the 350 GHz band (366.5 - 372.5 GHz) to complete the JCMT observations. This frequency range harbour an important transition of H2 D+ , as well as transitions of HDS and HCNH+ , species detected in the survey, but with too few transitions to allow an analysis of their physical parameters.
But the more important expected return of this mini spectral survey would be a validation of the ability of APEX to perform spectral surveys, and its performances in such an observing mode would be directly compared to those of JCMT in the 350 GHz window.
Unbiased line surveys are a unique way of determining the molecular content of astrophysical objects. The study of excitation and line shapes of individual species can tell about the conditions in the regions where these species exist, allowing to distinguish sub-structures even in the absence of sufficient spatial resolution. In particular hot cores in low (Cazaux et al. 2003) and high (e.g. Wyrowski et al. 1999) mass star forming regions show a spectrum rich in molecular lines. In hot cores, the central object provides sufficient energy input to heat the gas to temperatures higher than 100 K (Walmsley et al. 1992). The richness of the spectrum is partly due to the high temperatures and densities which excite many transitions which are not visible from cooler regions. However a significant factor is also the molecular richness of the sources themselves, which is due both to the evaporation of molecules trapped in ice mantles, and to gas phase chemical reactions, which can proceed faster at higher densities. Also the high temperatures open many chemical pathways which, because they are endothermic, are inefficient at lower temperatures (Charnley et al. 1992; Caselli et al. 1993).
The 80 - 380 GHz band is of particular relevance to detect complex, large organic molecules in hot core regions. For example, spectral surveys in the direction of the SgrB2 complex have detected a plethora of lines providing a complete census of the most abundant complex molecules in the region (Nummelin et al. 2000). Similar spectral surveys have been obtained towards several high mass star forming regions (Schilke et al. 1997; Helmich & van Dishoeck 1997; Hatchell et al. 1998; Schilke et al. 2001).
In contrast, only one partial unbiased line survey has been obtained so far towards a solar type protostar, IRAS16293-2422 (hereinafter IRAS16293, Blake et al. 1994; van Dishoeck et al. 1995). This survey covers -partially- only two windows in the 200 GHz and 350 GHz bands, and has been obtained with the JCMT and CSO telescopes. In this survey (down to the 40 mK level) only the most abundant molecules could be observed for sensitivity reasons. However, the more sensitive, spectrally targeted (down to the 5-10 mK level) observations obtained with IRAM towards IRAS16293 by Cazaux et al. (2003) detected plenty of complex molecules. Because the lines of these molecules originate from levels with very different excitation energiesand critical densities, they will provide a detailed view of the physical conditions and the kinematics of the emitting gas. Furthermore, they will allow a complete study of the chemistry in the envelope of IRAS16293, where previous studies have already shown abundance jumps for various molecules (Ceccarelli et al. 2000b; Schoier et al. 2002). Present `hot core' models, appropriate for high-mass YSOs, require a few 104 yr to produce these species with the abundances observed, consistent with their size and crossing times. But the dynamical time scale of the low-mass YSOs `hot cores' is supposed to be only a few 102 yr, too short for the hot core chemistry to develop (Schoier et al. 2002). The presence of complex molecules in IRAS16293 provokes some questions : does this mean that all molecules are first generation species directly evaporated off the grains? Or is something "halting" the infall in the inner few hundred AU? The requested observations will be therefore particularly valuable for a study of the warm, dense gas directly surrounding the protostar which is difficult to observe otherwise.
Time is ripe to obtain a survey as sensitive as possible in the whole frequency range attainable from the ground to perform a complete census of the complex molecules in this Sun-like protostar. It is probably needless to emphasize the importance of having this census in a progenitor of our Sun, our best chance to know what the primordial solar nebula looked like. This almost complete census of the molecular content of IRAS16293 will provide a precious observational study of how the abundance ratios of all the detected species vary in this low mass protostar. Its molecular composition will be compared to what we already know for massive hot-core regions (Schilke et al. 1997; Helmich & van Dishoeck 1997; Hatchell et al. 1998; Wyrowski et al. 1999; Nummelin et al. 2000; Schilke et al. 2001). Will this low mass hot core have a similar, or a qualitatively different chemistry than the massive hot cores? In any case, line surveys of different sources in the age-mass plane are needed to establish useful chemical fingerprints and therefore this first complete survey of a low mass core will be very valuable. We will compare the observed molecular abundances to those predicted by chemical models in regions of low-mass and high-mass star formation ranging from those for cold, dark-cloud-like envelopes to those specifically developed for hot core high-mass protostar regions. As some molecular ratios can be used as chemical clocks, these observations will set very stringent constrains for the chemical models, and, consequently, they will form a essential database for complementary or follow-up studies with other instruments at higher frequencies like Herschel-HIFI, a first priority of the HIFI Guaranteed Time Program, and ALMA.
Source selection: why IRAS16293-2422
To date the object that presents by far the richest and brightest line spectrum among Class 0 low-mass protostars is IRAS16293, in the Ophiuchus complex. This is probably due to its relatively large envelope as well as to its proximity (120 pc). It is not by chance that the only unbiased high resolution spectral survey in the 250 and 350 GHz bands has been obtained towards this source. Its 250 - 350 GHz spectrum is rich in molecular transitions from relatively light molecules like H2 S to heavier molecules like SO2 and to complex molecule like CH3 OH (Blake et al. 1994; van Dishoeck et al. 1995). Targeted deep IRAM observations have allowed the detection of many complex molecules in the 3 mm and 1.3 mm bands (Cazaux et al. 2003): CH3 OCHO, CH3 OCH3 , CH3 CHO, HCOOH, CH3 COOH, CH3 CN and C2 H3 CN. Indeed, the line richness of this source is so large (more than 10 lines / GHz at any frequency observed, that it certainly deserve a full spectral survey. IRAS16293 has also the richest observed line spectrum in the ISO-LWS band, showing up many transitions from H2 O, CO , OH, and other molecules (Ceccarelli et al. 1998a). Actually the spectrum is so rich that many lines are blended in the ISO low resolution ( 200) spectrum and about 40 % remain unidentified. IRAS16293 is the source in which some molecules such as H2 D+ (Stark et al. 1999), CO+ (Ceccarelli et al. 1998b) and D2 CO (Ceccarelli et al. 1998c) have been observed for the first time in this class of objects.
In addition, many studies have been carried out towards this source (Wootten 1989; Mundy et al. 1990, 1992; Looney et al. 2000), which is a prime example of an object surrounded by a collapsing envelope (Walker et al. 1986; Zhou 1995; Narayanan et al. 1998). All these studies have been able to reconstruct the structure of its infalling envelope (Ceccarelli et al. 2000a, 2000b, 2000c; Sch�ier et al. 2002), making IRAS16293 a o promising target for the proposed unbiased spectral survey. The observation of its line spectrum in a wide frequency range will fill in our gap in the knowledge of the important complex molecular transitions in Class 0 low-mass protostars.
The 366-373 GHz band
The 366-373 GHz band contains the ground state 111 - 110 transition of ortho H2 D+ at 372.465 GHz. This species is the cornerstone molecule in gas phase ion molecule reaction networks. The small zero-point energy difference between this species and H+3 leads to enormously enhanced H2 D+ / H+ 3ratios. In gas phase chemistry schemes, this deuterium fractionation is then quickly passed on to other species. High deuterium fraction has been observed towards IRAS 16293, including the doubly deuterated species D2 CO, and CD2 HOH and the triply deuterated species CD3 OH at abundances approaching that of H2 CO and CH3 OH (Ceccarelli et al 1998c; Parise et al. 2004). In addition, this frequency range contains transitions due to HDS and HCNH+ . The former likely results from the reactions involving H2 D+ . The latter, protonated prussic acid, is the key intermediary, linking HCN and HNC (through proton transfer followed by dissociative electron recombination). In combination with low energy transitions of these species detected in the IRAM and the JCMT, this data will allow us to determine an accurate abundance of these species and hence provides direct tests of the chemical networks involved.
Observing strategy at APEX
We plan to complete the survey performed at JCMT in the 350 GHz band, with about the same sensitivity (15 mK at 1.5 km/s spectral resolution). Because of the line crowding expected at any frequency for this source (see Figure 1), we will use a Wobbling Switching Observing Mode if available, otherwise a standard Position Switching Observing Mode. The estimation of the integration time has been evaluated with the JCMT Time Estimator at 370 GHz, upper limit of the frequency range attainable at JCMT. For that, we assumed that the weather at Chajnantor will be at least as good as Mauna-Kea weather grade 1 (tau225 0.05). We also assumed that the Tsys will be better than that of JCMT receivers as the APEX receiver is much better (Trec = 50 K wrt 170 K at JCMT). The estimated integration time needed is about 3 hours per 1 GHz band, and assuming the telescope overheads will add about 33%, we need 4 hours of telescope time per setting. To perform the 366.5 - 372.5 GHz survey, we therefore ask for 24 h of telescope time. To cross calibrate the different spectra, we will ensure a small frequency overlap from one observed setting to the other.
The observations can be easily split into one block of 240 min or two blocks of 120 min for each setting of telescope to achieve the aimed RMS. This allow flexible observing schedule that can be done in service mode.
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