H2D+ as a tracer of massive, very cold cores
Coordinator: J. Harju, O. Miettinen, M. Juvela, K. Mattila
The collapse of massive star forming cores should be preceded by a verycold, quiescent phase in their evolution. However, few high-mass coldcores have been discovered so far. To our knowledge the only massive 10K core with nearly thermal linewidths found to date is located in theOri B9 cloud in Orion B. This object is likely to represent an elusive,very early stage of high-mass star formation. Depending on the timespent in the pre-collapse phase, the high density and low temperaturemay have resulted in a high degree of depletion and an increased H2D+abundance.
We propose the measurement of the H2D+ (1_10-->1_11 ) line at 372GHz with APEXtowards three positions representing local density maxima in this core.One of the condensations has an embedded low-luminosity IRAS source(IRAS 05405-0117) whereas the others are starless. The H2D+ abundancedetermined from these observations will be used together with otherobservational data for estimating the degree of depletion and thechemical age of the core, and thereby the time scale of thepre-collapse phase. Furthermore, if detected, H2D+ lines will be usedto model the structure and kinematics of the interior parts of thiscore.
The total time requested for the project is 3.5 hours. The source (R.A.=5 h 40m ) is observable in the small hours in October.
Program is available and data products can be downloaded
The study of the inital conditions for the formation of massivestars is hampered by their rapid evolution and strong interaction withthe surroundings. Almost all dense cores identified in Giant MolecularClouds show signs of star formation: mid- or far-infrared sources,molecular masers and outflows, and have elevated kinetic temperatures(about 30 K or more). The few detected cores at 20 K have beencharacterised as "cold" (e.g. Hill et al. 2005, MNRAS, in press).Because compression leads to an intensified cooling by molecules anddust, much colder regions than 20 K should, however, exist in GMCs.Very cold GMC cores may have remained indiscernible because of theirshort life-time or the fact that large-scale surveys are usually biasedtowards the presence of a certain molecular species which might bedepleted in the coldest regions. Indirect evidence for the existence ofcold gas is provided by the discoveries of low-mass stars (Naylor &Fabian 1999, MNRAS 302, 714) and low-mass clumps (Beuther & Schilke2004, Sci 303, 1167) in regions of massive star formation. Thefragmentation of a cloud cannot proceed to one solar mass clumps unlessthe kinetic temperature decreases close to 10K.
Ori B9 - the missing link?
The dense core associated withthe low-luminosity far-infrared source IRAS 05405-0117 in the Ori B9cloud is a very exceptional massive core. The average kinetictemperature derived from the NH3 (1,1) and (2,2) lines is 10.2 ±1.5 K and the average ammonia linewidth is 0.29 ± 0.06 km.s-1(Harju, Walmsley & Wouterloot 1993, A&AS 98, 51). The numbersafter the ± signs are the sample standard deviations in themapped region. The core has been mapped also in N2H+ (1 - 0) by Caselli& Myers (1994, in Clouds, Cores, and Low Mass Stars, ASP CS 65,52). In the latter mapping the core is resolved into threecondensations, one encircling theIRAS point source, another lying 40" west, and a third one some 100" NEof it. The latter condensations have no infrared sources. The massestimated from ammonia is about 400 M_sun . The location of the core onthe large-scale CO map of Caselli & Myers (1995, ApJ 446,665) isindicated in Fig. 1. Also shown is the N2H+ map of the core by Caselli& Myers (1994) with the outline of the NH3 map.
The core associated with IRAS 05405-0017 is likely to represent anearly stage of massive star formation in which newly born stars havenot yet disturbed their surroundings. The subsidiary cores may be in astill earlier, pre-collapse phase. Detailed investigation of the corestructure, kinematics and chemical composition seems thereforewarranted. To our knowledge the object is unique among the GMC coresstudied so far, but represents an inevitable phase which has notreceived much attention yet because of observational limitations.
The goals of the proposed observations are 1) to estimate the H2D+abundance in the interiors of the three condensations of the IRAS05405-0117 core; and 2) use the H2D+ profile along with other molecularlines for modelling the density and temperature structure andkinematics of these regions.
The H2D+ abundance together with the H2D+/H3+ abundance ratio andthe degree of CO depletion derived from available other observationswill be used to estimate the chemical age of the core, and the durationof the quiescent phase. The H2D+ /H3+ ratio can be estimated, e.g.,from DCO+/HCO+ or N2D+/N2H+ observations. The H3+ abundance towards theIRAS source can possibly be estimated later with the aid of inraredabsorption line measurements.
H2D+ may be the only available tracer of very dense interiors (n> 106 cm-3 ) of a pre-stellar core just before or during itscollapse (Bergin et al. 2002). Also other lines are needed, however, toconstrain the physical core model, and these are preferably of specieswhich can resist depletion up to high densities, e.g. NH3 and N2 H+ andtheir deuterated isotopomers. The line profiles of H2D+ and otherspecies will be analysed using our Monte Carlo radiative transfer code(Juvela 1997). Line profiles predicted from hydrostatic, quasi-staticcontraction and dynamical collapse models will be compared with theobserved ones.
When combining chemical age estimates with the estimates ofdynamical state, the results of this study will be useful for theunderstanding of the earliest phases of star formation in massivecores.
The ground-state transition of H2D+ at 372 GHz lies betweenatmospheric O2 and H2O absorption lines and requires the best possibleobserving conditions. The atmospheric transmission at this frequency atAPEX is about 0.7. The line is observable at CSO on Mauna Kea, but thetypical observing conditions are not as good as at APEX. No othertransitions of H2D+ than (1_10-->1_11) can be observed from theground. To demonstrate the feasibility of the proposed observatons, weshow in Fig. 2 an H2D+ spectrum observed with APEX during the firstScience Verification period. An rms noise of about 0.08 K in the TAscale was reached after 9 minutes ON-time.Observing plan and time estimate
We propose single pointing observations towards the three N2H+peaks representing the centres of separate condensations in the IRAS05405-0117 core. The J2000 coordinates of these positions are given inthe attached table. The 'visibility' plot of the source is shown inFig. 3.
The expected line intensites are in the range TA = 0.5 - 1.5 K,and we wish to reach the rms noise level 0.05 K at the spectralresolution 50 m/s (62.5 kHz, bandwidth 128 MHz). Based on theexperience from the SV1 observations (assuming that TSYS = 180 K) weestimate that the average ON-integration required per position is 20minutes. In the position switching mode the total observing time withthe OFF-position and 50 % overheads will be 60 minutes per position.Allocating 0.5 hours for tuning, pointing and focusing, the total timerequested for the complete programme is 3.5 hours.
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