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Astron. Astrophys. 318, L35-L38 (1997)

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1. Introduction

In the standard big bang model, the primordial abundance of deuterium is a sensitive function of the baryon to photon ratio, (e.g. Walker et al. 1991) making it a quantity of great cosmological interest. Further, since all known astrophysical processes (apart from the big bang itself, of course) result in a net destruction of deuterium, the currently observed value of the deuterium abundance is a strict lower limit to its primordial abundance.

HST observations of the DI Lyman [FORMULA] line in the local solar neighborhood (Linsky et al. 1993) yield a deuterium abundance of [FORMULA]. Conversion from this local current abundance to the primordial abundance depends on less well understood details of the history of the stellar reprocessing of matter in the local ISM. In order to circumvent this problem a number of groups have been attempting to measure the deuterium abundance in high-redshift Lyman-limit systems. Since these systems are less chemically evolved than the local ISM, conversion from the measured to the primordial abundance should be more straightforward. However, the results of these observations are conflicting, with different groups (Carswell et al. 1994, Songalia et al. 1994, Tytler et al. 1996) measuring abundances which differ by more than an order of magnitude. Part of the problem is that the DI Lyman [FORMULA] line is separated from the HI Lyman [FORMULA] line by only 82 km s-1, making the chance of contamination of the DI line by absorption from a small parcel of HI at a slightly different velocity from that of the main Lyman-limit system non-negligible.

There have also been several attempts to observe the hyperfine transition of DI at radio frequencies. Since the frequency of this line is more than a factor of 4 lower than the frequency of the corresponding transition in HI, the question of HI contamination does not arise. Two kinds of lines-of-sight have been favored in the past, the first towards bright radio sources (Sgr A & Cas A; Weinreb 1962, Anantharamiah & Radhakrishnan 1979, Heiles et al. 1993), where the hydrogen column density is known to be high. The disadvantages of these lines of sight are that: (i) the bright radio sources contribute significantly to the system temperature, making detection more difficult; (ii) any measurement refers only to the thin pencil beam subtended by the absorber; and (iii) the molecular column density is also high, making it likely that most of the deuterium is in molecular rather than atomic form (Heiles et al. 1993). Anantharamiah & Radhakrishnan (1979) placed an upper limit of [FORMULA] on the DI abundance towards Sgr A. Heiles et al. (1993) reached similar limits towards Sgr A as well as Cas A.

The other promising direction for a search for the radio emission from DI is that towards the galactic anticenter, where one expects the line to be in emission. The advantages of this direction are that (i) the high optical depth of HI is due to velocity crowding along a long pathlength rather than a high volume density; (ii) the molecular column density and metallicity are low; and (iii) the observations are sensitive to the DI abundance within the entire telescope beam, and not just a narrow cone towards the background source as in the case of absorption observations.

The results of a long integration in the direction [FORMULA] using the Hat Creek telescope were presented by Blitz & Heiles (1987), who found an upper limit ([FORMULA]) of [FORMULA] for [FORMULA]. Here we report on a long integration towards a partially overlapping line-of-sight, [FORMULA], with the Westerbork synthesis array. Using the 14 WSRT telescopes as independent single dishes allowed us to significantly increase the effective integration time.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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