Speaker
Description
The MAGIC + CTAO LST-1 Stellar Intensity Interferometer (SII) located in La Palma (Spain) has been operational since January 2024. The MAGIC SII is capable of resolving stars between ~ 0.4 and 1.4 mas angular diameter and magnitude 3 in Blue band with 10% uncertainty in 2.5 hours. Instead, the addition of CTAO LST-1 allows to measure with that same precision and conditions stars of magnitude 4. Not only that, but the inclusion of one more telescope to the array allows for a better 2D coverage of the visibility distribution (the so-called UV plane). Broad UV coverage is needed to identify non-radially symmetric features within stellar surfaces. A case study that benefits from this fact is the one of fast-rotating stars, whose photosphere become oblate due to high rotational velocity. A specific type of fast-rotating stars are Be stars for which rotational velocities are close to critical and often result in decretion disks, observed via detection of characteristic hydrogen emission lines through spectroscopy. However, spectroscopy alone can not break the velocity-inclination degeneracy. This is where the combination between this technique and Intensity Interferometry becomes powerful, which provides geometrical constraints such as size and orientation, which break such degeneracy and allow for the characterization of the stellar surface. For the purpose of studying such a case, MAGIC SII and later MAGIC + CTAO LST-1 SII have observed the star gamma Cassiopeiae since February 2022 and since January 2024 respectively. We present the combined analysis of such observations together with high-resolution spectroscopy. We consider two scenarios: opaque and transparent decretion disk, and compare the obtained physical parameters with other SII and Michelson Stellar Interferometer results. Finally, we also discuss the expected improvement in sensitivity with the future inclusion of three more CTAO LSTs, as well as the one-order-of-magnitude increase in the number of resolvable fast-rotating stars.
