Aggregates of clusters in the Gaia data

The precision of the parallax measurements by Gaia is unprecedented. As of Gaia Data Release 2, the number of known nearby open clusters has increased. Some of the clusters appear to be relatively close to each other and form aggregates, which makes them interesting objects to study. We study the aggregates of clusters which share several of the assigned member stars in relatively narrow volumes of the phase space. Using the most recent list of open clusters, we compare the cited central parallaxes with the histograms of parallax distributions of cluster aggregates. The aggregates were chosen based on the member stars which are shared by multiple clusters. Many of the clusters in the aggregates have been assigned parallaxes which coincide with the histograms. However, clusters that share a large number of members in a small volume of the phase space display parallax distributions which do not coincide with the values from the literature. This is the result of ignoring a possibility of assigning multiple probabilities to a single star. We propose that this small number of clusters should be analysed anew.Comment: 14 pages, 7 figures, 2 tables, published in A&

Science with a small two-band UV-photometry mission II: Observations of stars and stellar systems

We outline the impact of a small two-band UV-photometry satellite mission on the field of stellar physics, magnetospheres of stars, binaries, stellar clusters, interstellar matter, and exoplanets. On specific examples of different types of stars and stellar systems, we discuss particular requirements for such satellite missions in terms of specific mission parameters such as bandpass, precision, cadence, and mission duration. We show that such a mission may provide crucial data not only for hot stars that emit most of their light in UV, but also for cool stars, where UV traces their activity. This is important, for instance, for exoplanetary studies, because the level of stellar activity influences habitability. While the main asset of the two-band UV mission rests in time-domain astronomy, an example of open clusters proves that such a mission would be important also for the study of stellar populations. Properties of the interstellar dust are best explored when combining optical and IR information with observations in UV. It is well known that dust absorbs UV radiation efficiently. Consequently, we outline how such a UV mission can be used to detect eclipses of sufficiently hot stars by various dusty objects and study disks, rings, clouds, disintegrating exoplanets or exoasteroids. Furthermore, UV radiation can be used to study the cooling of neutron stars providing information about the extreme states of matter in the interiors of neutron stars and used for mapping heated spots on their surfaces.Comment: Submitted to Space Science Review

Quick Ultra-VIolet Kilonova surveyor (QUVIK)

We present a near-UV space telescope on a ~70kg micro-satellite with a moderately fast repointing capability and a near real-time alert communication system that has been proposed in response to a call for an ambitious Czech national mission. The mission, which has recently been approved for Phase 0, A, and B1 study shall measure the brightness evolution of kilonovae, resulting from mergers of neutron stars in the near-UV band and thus it shall distinguish between different explosion scenarios. Between the observations of transient sources, the satellite shall perform observations of other targets of interest, a large part of which will be chosen in open competition.Comment: SPIE Astronomical Telescopes and Instrumentatio

VISIONS: the VISTA Star Formation Atlas: I. Survey overview

Galaxie

White dwarf-open cluster associations based on

Context. Fundamental parameters and physical processes leading to the formation of white dwarfs (WDs) may be constrained and refined by discovering WDs in open clusters (OCs). Cluster membership can be utilized to establish the precise distances, luminosities, ages, and progenitor masses of such WDs. Aims. We compile a list of probable WDs that are OC members in order to facilitate WD studies that are impractical or difficult to conduct for Galactic field WDs. Methods. We use recent catalogs of WDs and OCs that are based on the second data release of the Gaia satellite mission (GDR2) to identify WDs that are OC members. This crossmatch is facilitated by the astrometric and photometric data contained in GDR2 and the derived catalogs. Assuming that most of the WD members are of the DA type, we estimate the WD masses, cooling ages, and progenitor masses. Results. We have detected several new likely WD members and reassessed the membership of the literature WDs that had been previously associated with the studied OCs. Several of the recovered WDs fall into the recently reported discontinuity in the initial-final mass relation (IFMR) around Mi ∼ 2.0 M⊙, which allows for tighter constrains on the IFMR in this regime

Understanding the power of luminescence ratiometric thermal history indicators driven by phase transitions: the case of Eu3+ doped LaVO4

Finding thermal history phosphors with high sensitivity and a consistent readout is required for reliable thermal history determination with high temperature resolution. This work presents a new thermal history phosphor based on the luminescence of Eu3+ ions in LaVO4 to meet these requirements. As demonstrated, raising the annealing temperature causes a structural phase transition from a low-temperature tetragonal phase to a high-temperature single-stranded phase. The associated change in the local point symmetry of the crystallographic site occupied by Eu3+ ions result in a significant decrease in the emission intensity ratio of the 5D0 → 7F2 band relative to the 5D0 → 7F1 band, which enables the development of the ratiometric thermal history phosphor with the relative sensitivity of 0.38% °C−1 at 800 °C. Its applicative potential for thermal history readout was proved in the proof-of-concept experiment