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HCl and HF are expected to be the main reservoirs of fluorine and chlorinewherever hydrogen is predominantly molecular. They are found to be stronglydepleted in dense molecular clouds, suggesting freeze-out onto grains in suchcold environments. We can then expect that HCl and HF were also the majorcarriers of Cl and F in the gas and icy phases of the outer solar nebula, andwere incorporated into comets. We aimed to measure the HCl and HF abundances incometary ices as they can provide insights on the halogen chemistry in theearly solar nebula. We searched for the J(1-0) lines of HCl and HF at 626 and1232 GHz, respectively, using the HIFI instrument on board the Herschel SpaceObservatory. HCl was searched for in comets 103P/Hartley 2 and C/2009 P1(Garradd), whereas observations of HF were conducted in comet C/2009 P1. Inaddition, observations of H$_2$O and H$_2^{18}$O lines were performed in C/2009P1 to measure the H$_2$O production rate. Three lines of CH$_3$OH wereserendipitously observed in the HCl receiver setting. HCl is not detected,whereas a marginal (3.6-$\sigma$) detection of HF is obtained. The upper limitsfor the HCl abundance relative to water are 0.011% and 0.022%, for 103P andC/2009 P1, respectively, showing that HCl is depleted with respect to the solarCl/O abundance by a factor more than 6$^{+6}_{-3}$ in 103P, where the error isrelated to the uncertainty in the chlorine solar abundance. The marginal HFdetection obtained in C/2009 P1 corresponds to an HF abundance relative towater of (1.8$\pm$0.5) $\times$ 10$^{-4}$, which is approximately consistentwith a solar photospheric F/O abundance. The observed depletion of HCl suggeststhat HCl was not the main reservoir of chlorine in the regions of the solarnebula where these comets formed. HF was possibly the main fluorine compound inthe gas phase of the outer solar nebula.

Searches for HCl and HF in comets 103P/Hartley 2 and C/2009 P1 (Garradd) with the Herschel space observatory