Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Context.About 5% of upper main sequence stars are permeated by a strong magnetic field, the origin of which is still matter of debate. Aims. With this work we provide observational material to study how magnetic fields change with the evolution of stars on the main sequence, and to constrain theory explaining the presence of magnetic fields in A and B-type stars. Methods. Using FORS1 in spectropolarimetric mode at the ESO VLT, we have carried out a survey of magnetic fields in early-type stars belonging to open clusters and associations of various ages. Results. We have measured the magnetic field of 235 early-type stars with a typical uncertainty of G. In our sample, 97 stars are Ap or Bp stars. For these targets, the median error bar of our field measurements was G. A field has been detected in about 41 of these stars, 37 of which were not previously known as magnetic stars. For the 138 normal A and B-type stars, the median error bar was 136 G, and no field was detected in any of them.

The National Science Foundation’sDaniel K. Inouye Solar Telescope(DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute.

Context. A small fraction of upper main sequence stars have strong, highly structured magnetic fields. The origin and evolution of these fields are not adequately understood.Aims. We are carrying out a survey of magnetic fields in Ap stars in open clusters in order to obtain the first sample of magnetic upper main sequence stars with precisely known ages. These data will constrain theories of field evolution in these stars. Methods. A survey of candidate open cluster magnetic Ap stars was carried out using the new ESPaDOnS spectropolarimeter at the CFHT. This instrument provides an alternative to the FORS1 spectropolarimeter used up to now for this survey.Results. We have obtained 44 measurements of the mean longitudinal fields of 23 B6–A2 stars that have been identified as possible Ap stars and that are possible members of open clusters, with a median uncertainty of about 45 G. Of these stars, 10 have definite field detections. Nine stars of our sample are found not to be magnetic Ap stars. These observations significantly increase the information available about low-mass stars near the TAMS compared to our previous sample. Conclusions. We find that ESPaDOnS provides field measurements comparable to those that we have previously obtained with FORS1, and that these data also contain a large amount of useful information not readily obtained from lower resolution spectropolarimetry. With the new data we are able to expand the available data on low-mass, relatively evolved Ap stars, and identify more robustly which observed stars are actually magnetic Ap stars and cluster members. Re-analysis of the enlarged data set of cluster Ap stars indicates that such stars with masses in the range of 2–5 show rms fields larger than about 1 kG only when they are near the ZAMS. The time scale on which these large fields disappear varies strongly with mass, ranging from about 250 Myr for stars of 2–3 to 15 Myr for stars of 4–5 . Our data are consistent either with emergent flux conservation for most (but not all) Ap stars, or with modest decline in flux with age.