APEX SV projects

H2D+ as a tracer of massive, very cold cores

Coordinator: J. Harju, O. Miettinen, M. Juvela, K. Mattila

Abstract:

The collapse of massive star forming cores should be preceded by a very cold, quiescent phase in their evolution. However, few high-mass cold cores have been discovered so far. To our knowledge the only massive 10 K core with nearly thermal linewidths found to date is located in the Ori B9 cloud in Orion B. This object is likely to represent an elusive, very early stage of high-mass star formation. Depending on the time spent in the pre-collapse phase, the high density and low temperature may 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 372 GHz with APEX towards 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+ abundance determined from these observations will be used together with other observational data for estimating the degree of depletion and the chemical age of the core, and thereby the time scale of the pre-collapse phase. Furthermore, if detected, H2D+ lines will be used to model the structure and kinematics of the interior parts of this core.
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.

Data:
Program is available and data products can be downloaded

Background

The study of the inital conditions for the formation of massive stars is hampered by their rapid evolution and strong interaction with the surroundings. Almost all dense cores identified in Giant Molecular Clouds 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 been characterised as "cold" (e.g. Hill et al. 2005, MNRAS, in press). Because compression leads to an intensified cooling by molecules and dust, much colder regions than 20 K should, however, exist in GMCs. Very cold GMC cores may have remained indiscernible because of their short life-time or the fact that large-scale surveys are usually biased towards the presence of a certain molecular species which might be depleted in the coldest regions. Indirect evidence for the existence of cold gas is provided by the discoveries of low-mass stars (Naylor & Fabian 1999, MNRAS 302, 714) and low-mass clumps (Beuther & Schilke 2004, Sci 303, 1167) in regions of massive star formation. The fragmentation of a cloud cannot proceed to one solar mass clumps unless the kinetic temperature decreases close to 10K.

Ori B9 - the missing link?
The dense core associated with the low-luminosity far-infrared source IRAS 05405-0117 in the Ori B9 cloud is a very exceptional massive core. The average kinetic temperature derived from the NH3 (1,1) and (2,2) lines is 10.2 ą 1.5 K and the average ammonia linewidth is 0.29 ą 0.06 km.s-1 (Harju, Walmsley & Wouterloot 1993, A&AS 98, 51). The numbers after the ą signs are the sample standard deviations in the mapped 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 three condensations, one encircling the IRAS point source, another lying 40" west, and a third one some 100" NE of it. The latter condensations have no infrared sources. The mass estimated from ammonia is about 400 M_sun . The location of the core on the large-scale CO map of Caselli & Myers (1995, ApJ 446,665) is indicated 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 an early stage of massive star formation in which newly born stars have not yet disturbed their surroundings. The subsidiary cores may be in a still earlier, pre-collapse phase. Detailed investigation of the core structure, kinematics and chemical composition seems therefore warranted. To our knowledge the object is unique among the GMC cores studied so far, but represents an inevitable phase which has not received much attention yet because of observational limitations.

Scientific aims

The goals of the proposed observations are 1) to estimate the H2D+ abundance in the interiors of the three condensations of the IRAS 05405-0117 core; and 2) use the H2D+ profile along with other molecular lines for modelling the density and temperature structure and kinematics of these regions.
The H2D+ abundance together with the H2D+/H3+ abundance ratio and the degree of CO depletion derived from available other observations will be used to estimate the chemical age of the core, and the duration of the quiescent phase. The H2D+ /H3+ ratio can be estimated, e.g., from DCO+/HCO+ or N2D+/N2H+ observations. The H3+ abundance towards the IRAS source can possibly be estimated later with the aid of inrared absorption 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 its collapse (Bergin et al. 2002). Also other lines are needed, however, to constrain the physical core model, and these are preferably of species which can resist depletion up to high densities, e.g. NH3 and N2 H+ and their deuterated isotopomers. The line profiles of H2D+ and other species will be analysed using our Monte Carlo radiative transfer code (Juvela 1997). Line profiles predicted from hydrostatic, quasi-static contraction and dynamical collapse models will be compared with the observed ones.
When combining chemical age estimates with the estimates of dynamical state, the results of this study will be useful for the understanding of the earliest phases of star formation in massive cores.

Telescope justification

The ground-state transition of H2D+ at 372 GHz lies between atmospheric O2 and H2O absorption lines and requires the best possible observing conditions. The atmospheric transmission at this frequency at APEX is about 0.7. The line is observable at CSO on Mauna Kea, but the typical observing conditions are not as good as at APEX. No other transitions of H2D+ than (1_10-->1_11) can be observed from the ground. To demonstrate the feasibility of the proposed observatons, we show in Fig. 2 an H2D+ spectrum observed with APEX during the first Science Verification period. An rms noise of about 0.08 K in the TA scale 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 IRAS 05405-0117 core. The J2000 coordinates of these positions are given in the attached table. The 'visibility' plot of the source is shown in Fig. 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 spectral resolution 50 m/s (62.5 kHz, bandwidth 128 MHz). Based on the experience from the SV1 observations (assuming that TSYS = 180 K) we estimate that the average ON-integration required per position is 20 minutes. In the position switching mode the total observing time with the OFF-position and 50 % overheads will be 60 minutes per position. Allocating 0.5 hours for tuning, pointing and focusing, the total time requested for the complete programme is 3.5 hours.

Object	R.A. (2000)	Dec. (2000)	V_LSR	OFF
IRAS 05405-0117	05 43 02.5	-01 16 23	9.3 km.s-1	30' west in azimuth
OriB9 E	05 43 05.2	-01 16 23	9.3 km.s-1	30' west in azimuth
OriB9 N	05 43 07.8	-01 15 03	9.3 km.s-1	30' west in azimuth