The background to and conduct of the "Must-Do" process was provided in the invitation to participate that was circulated by a Helpdesk mailing on 25 May 2012 with a reminder sent on 12 June 2012 before the 14 June 2012 deadline.

In summary - on the initiative of the Project Scientist with the backing of the Herschel Science Team and the Herschel Users' Group - the Herschel Mission Scientists undertook the task to review the current approved to-be-executed observing programmes with the objective to identify potential "gaps" that could and should be filled by conducting "Must-Do" observations. In parallel, suggestions from the the general community were solicited.

A total of 58 suggestions from the community were received, and initial technical checking was performed by the Herschel Science Centre. A meeting was held on 25-26 June 2012 to discuss the community input together with the output from the review conducted by the Mission Scientists. A shortlist of potential "Must-Do" observation programmes was produced, later reduced to seven programmes for a total of about 106 hr. This list has been formally submitted as a request for Director's Discretionary Time, and it has been approved.

Following approval, the seven programmes were subjected to a more thorough technical checking and optimisation. For a couple of programmes this has taken longer than initially foreseen. The selected programmes are now released for scheduling, and in fact various "Must-Do" observations have been scheduled from OD#1236 onwards.

The final list of seven programmes comprising a total consolidated observing time of 107.6 hr is:

• DDT_mustdo_1 "Sagittarius B2: FTS mapping of a starburst in the Milky Way's Centre" - 4.5 hr

  Large area SPIRE FTS mapping of 10x10 arcmin² along SgrB2, the closest starburst, in the Galactic Centre. The observations cover the main three star forming cores in SgrB2 (M, N and S), and importantly also the entire extended envelope with the IR sources and HII regions in the area.

• DDT_mustdo_2 "Superthermal H₃O⁺, a key probe of molecule formation" - 4.0 hr

  HIFI observations of two excited metastable H₃O⁺ lines in order to determine definitively whether the superthermal molecules seen in absorption toward the SgrB2 envelope are specific to that environment or the result of more general chemical processes.

• DDT_mustdo_3 "The thermal lightcurve of Centaur (2060) 95P/Chiron" - 7.8 hr

  Chiron is an unusual body in the Solar System. It was discovered in 1977, and is a member of what now is called the Centaur family, a class of objects whose orbits cross those of the Giant Planets. Centaurs are on unstable orbits and thought to be transition objects between trans-neptunian objects and short period comets. Chiron's diameter is about 200 km, typical of an asteroid, however, its physical behaviour is rather cometary-like. The main goal is, by obtaining a PACS 70/160 µm lightcurve, to determine for the first time the surface albedo and thermal inertia of the surface of an active Centaur.

• DDT_mustdo_4 "Herschel observations of a benchmark sample of nearby powerful radio galaxies" - 28.9 hr

  PACS photometry of the complete, unbiased 2 Jy sample of 46 powerful, southern radio galaxies (0.05 > z > 0.7), and SPIRE photometry of a subsample. This sample is currently the best observed of any sample of radio-loud AGN, and is unique in terms of the completeness of its data at optical, mid- to far-IR and radio wavelengths, and is available for future ALMA observations. These observations will provide the link for comparison with existing Herschel observations of samples of both high-z radio galaxies and nearby radio-quiet AGN.

• DDT_mustdo_5 "The Herschel & Planck All-Sky Source Snapshot Legacy Survey" - 46.3 hr

  SPIRE scanmaps of a sample of 200 new Planck sources found all over the sky. The sample includes: 75 Planck Catalogue of Compact Sources high-z SEDs (50 dusty; 25 synchrotron); 100 Planck faint high-z candidates; and 25 SZ clusters. With 14 minutes per AOR to reach the confusion limit on the central ~10'x10' of a ~20'x20' field, the identification of the sources will be straightforward.

• DDT_mustdo_6 "A unique PACS spectral map of a complete bipolar outflow" - 11.3 hr

  A full PACS spectral mapping of the HH 211 outflow in Perseus, providing a unique datacube of a unique source. HH 211 stands out among the best-studied outflows due to its pristine appearance, which has made it become a true prototype of its class. It combines a relatively small size and a very well-defined geometry consisting of a jet, and inner cavity near the YSO, and two bright bow-shocks at the opposite ends of the outflow, making it ideal to use it as template to investigate in a self-consistent manner the different processes operating in the bipolar outflow of a YSO.

• DDT_mustdo_7 "SPIRE imaging and spectroscopy of the Helix Nebula" - 4.8 hr

  The Helix Nebula is one of the most spectacular Planetary Nebulae in the sky. Due to its size it is particularly well adapted to study the physical conditions of the gas and dust submitted to a very strong UV field. In order to obtain the physical conditions of the dust along the different shells of the nebula a deep photometer map of the whole nebula will be made with SPIRE, revealing the "blobby" structure and constraining the dust emission. The gas properties will be traced by observing four selected positions along the shells with SPIRE/FTS in order to trace the change of physical conditions (temperature, density, molecular abundances) as a function of the distance to the central star.

  Data access

  All "Must-Do" data have no proprietary time, they are publicly available from the HSA as they are observed and pipeline processed. In practise this means the data are typically available for retrieval 1-2 days after execution.

  As with all other programmes you can inspect AORs using the HSpot "View Accepted Proposal" function. You can follow scheduling on the Observing Schedule web page, and the execution on the Observing Log. In addition, as described in the HSC e-News#22, you can subscribe to a daily SMS service alerting you every time there are new observations executed for any observing programme (including, but not restricted to, "Must-Do" programmes) provided you are a Herschel registered user. Not registered? To register go to User Registration.

  Thanks!

  Warm thanks to each and every one who spent time and effort providing suggestions and helping to create this programme one way or another. Thank you!