image Planned targets image Planned targets

Planned targets

To reach Arago's science goals, the observing program will be divided into three parts: statistical surveys (legacy and focused samples), monitoring of targets for 3D mapping, and a Target of Opportunity (ToO) mode. Arago will point at each star individually. Its orbit at L2 will allow the users to observe stars anywhere in the sky.
The survey program: Several of Arago's science goals require observation of a statistical sample of stars to compare their spectral and polarimetric properties as a function of their fundamental properties, such as temperature, rotation rate, or evolutionary stage. Therefore, Arago will observe all types of (magnetic and non-magnetic) stars in a statistical way. First, we propose to observe a large non-biased magnitude-limited legacy sample of thousands of bright stars, as well as more focused survey samples selected through open-call proposals. The legacy sample will include all stars with magnitude B between 3 and 6 (3190 stars, see Fig. 8). This sample will provide a good number of stars of each spectral type. Each target will be observed twice during the Arago mission to provide an indication of stellar Visible and UV variability, with an exposure time varying from 5 minutes to 4 hours depending on the spectral type. In addition, more focused survey samples or individual (snapshot) observations will be selected through a call for proposals. Examples of focused survey samples could be a selection of O stars, of planet hosting stars, of solar twins at various ages, of high- velocity stars for the ISM studies, etc. Individual snapshots targets include solar system objects for example.
The 3D mapping program: The full characterisation of the stellar surface, magnetic field and circumstellar environment requires a complete 3D mapping of the star and its surroundings. This involves temporal monitoring of each star, and should be done for various types of stars: hot stars, PMS stars, solar-type stars, planet-hosting stars, etc. In addition, to follow the magnetic and activity cycles of solar-type stars, it is necessary to carry out their 3D mapping every year. Detailed 3D mapping requires the acquisition of typically 50 spectropolarimetric measurements during one rotation period of the star. For massive stars, for which the rotation period is typically of the order of 1 to a few days, this limits the exposure time to 30 minutes maximum. However, since fossil fields are stable, the phase coverage can be acquired over many years, if necessary. For cooler stars, such as solar-type stars, the rotation period can be up to a few weeks. When the rotation period is long, the exposure time can be long without phase smearing issues, and targets do not have to be observed continuously, but regularly every 1/50 th of the rotation period. However, since dynamo fields are variable, the full dataset must be acquired over one or two stellar rotation periods.
Target of opportunity (ToO): The observation of sudden events, in particular supernovae, novae, and Be outbursts, requires a ToO mode. Other targets in Arago's science objectives would also benefit from this mode, e.g., it would be ground-breaking to follow the dynamics of the accreting material of a bright O outbursting young star (EX Orionis stars, FU Orionis stars) if any such event occurred during Arago's lifetime. These events, however, last several weeks to months, and therefore the time between the triggering of an event (from ground-based or space-based external alerts) and the pointing of Arago to the target can be several days. There is no need for a very quick modification of the observing program and repointing of the satellite. ToO programs will be selected through open-call proposals and observations triggered only when alerts are received from independent programs. Arago will not have its own alert program, only a follow-up for alerts triggered by others.