Using data from the core of 47 Tuc we have identified stars in different evolutionary stages in the colour-magnitude diagram, and used the effects of mass segregation on their radial distribution to study the evolution and origin of blue stragglers (BSS). We separate the BSS into 2 samples by their magnitude and find considerable differences in their distribution. Bright BSS are more centrally concentrated with mass estimates over twice the turn-off mass suggesting an origin involving a triple or multiple star system. The distribution of the faint BSS is close to that of the main-sequence (MS) binaries pointing to these stars as their likely progenitors. Using MESA models, we calculate the expected number of stars in each evolutionary stage and compare it with the observed number of stars. Results indicate that BSS have a post-MS evolution comparable to that of a normal star of the same mass and a MS-BSS lifetime of about 200 – 300 Myr.

It is thought that protoplanets formed in protoplanetary disks excite the orbital motion of the surrounding planetesimals, and the bow shocks caused by the highly excited planetesimals heat their icy component evaporating into gas. We have performed model calculations to study the evolution of molecular abundances of the evaporated icy component, which suggests sulfur-bearing molecules can be good tracers of icy planetesimal evaporation. Here we report the result of our ALMA observations of sulfur-bearing molecules towards protoplanetary disks. The lines were undetected but the obtained upper limits of the line fluxes and our model calculations give upper limits of the fractional abundances of x(H2S) < 10−11 and x(SO) < 10−10 in the outer disk. These results are consistent with the molecular abundances in comets in our Solar system.

Herschel PACS imaging observations of carbon stars show well-resolved spherically symmetric detached shells around several objects. In the case of U Hya the shell is additionally detected in scattered visible light and in the far UV. The remarkable spherical symmetry justifies a straightforward application of 1D models to constrain the properties of the dust envelope, whose modulation in density is a consequence of short epochs of highly increased mass loss and/or wind-wind interaction between outflows of different velocity. We perform dust radiative transfer calculations, first based on a parametrised density distribution, and in a more sophisticated approach on a combination of stationary wind models. The impact of dust properties, particularly grain geometry, on the results is highlighted.

This study has been published in Sánchez-Menguiano et al. (2018). We encourage the reader to that article for more details on the study and the results.

High spatial resolution observations with ALMA and VLT/SPHERE show gaps and rings in continuum emission of protoplanetary disks, possibly indicating ongoing planet formation. However, it is still unclear if the gas follows the dust distribution. We present radiation thermo-chemical models for the disk of HD 163296 to study the impact of dust and gas gaps on the chemistry and molecular line emission. We compare a model with only dust gaps to a model that also has gas gaps. In both models, rings and gaps are visible in (sub)mm molecular line emission. Due to chemistry, certain molecules are sensitive to dust gaps where others are more sensitive to gas depletion. Observations of multiple molecules might allow to accurately determine the degree of gas depletion within the dust gaps, information crucial to discriminate between gap formation theories (e.g. planets, ice lines).

Young star clusters are a promising environment for forming binary black holes. Such binaries may form dynamically or via binary star evolution or through the interplay of these two channels. To study these formation pathways, we have performed high precision direct N-body simulations of low-mass (M < 1000 M⊙) young star clusters. The simulations were carried out with the code Nbody6++GPU coupled with the population synthesis code MOBSE. Our results highlight the importance of dynamics to form massive black hole binaries even in low-mass young star clusters.

Aiming at investigating the roles of rotation and magnetic fields on AGB stars, the rotating version of the ATON stellar evolution code is being extended in order to account for intermediate--mass stars and their later evolutionary stages. Here we report some preliminary results on the effects of rotation and of a large-scale magnetic field on the structure and evolution of 3 and 5 M⊙ stellar models from the pre-main sequence up to the AGB.

The building blocks of planets in planet-forming (“protoplanetary”) disks are assembled early in the lifetime of a young star. The gas disks are relatively short-lived, with a half-life of about 3 million years, as chemical reactions modify the reservoir of material from the natal molecular cloud. Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of protoplanetary disks around T Tauri stars show emission from H2O and absorption from other gases, sometimes consistent with formaldehyde, H2CO , and other times consistent with formic acid, HCOOH, in the 5-7.5 μm region. SOFIA-EXES spectra of YSOs that follow up on these Spitzer-IRS studies are presented. How the gaseous features observed between 5-7.5 μm relate to those at other wavelengths is discussed. This work suggests that water and organic molecules, which are crucial for life as we know it, are present in the habitable zones of stars at a very early age [of 1-3 million years].

We have developed a new version of the SD model for type Ia supernovae (SNe Ia) in which a common envelope (CE) is assumed to form if the mass-transfer rate between a carbon/oxygen white dwarf (CO WD) and its companion exceeds a critical accretion rate. Based on this model, we found that both SN 2002cx-like and SN Ia-CSM objects may share a similar origin, i.e. these peculiar objects may originate from the explosion of hybrid carbon/oxygen/neon white dwarfs (CONe WDs) in SD systems, where SNe Ia-CSM explode in systems with a massive CE of ∼1 M⊙, while SN 2002cx-like events correspond to events without a massive CE.

Recent observations of globular clusters imposed major revisions to the previous paradigm, in which they were considered to be isotropic in velocity space and non-rotating. However, the theory of collisionless spheroids with some kinematic richness has seldom been studied. We present here a first step in this direction, owing to new results regarding the linear stability of rotating Plummer spheres, with varying anisotropy in velocity space and total amount of angular momentum. We extend the well-known radial orbit instability to rotating systems, and discover a new regime of instability in fast rotating, tangentially anisotropic systems.

We present an analysis of the stellar and gaseous metallicity gradients in a sample of 260 disc galaxies from the CALIFA survey. The slope of the different components are compared with the main characteristics of the galaxies, such as mass, morphology, presence of a bar, or gas fraction.

We present the results obtained from Near-UV observations of the cluster NGC 5466 taken with UVIT onboard AstroSat to study the radial distribution of Blue Straggler Stars (BSSs), covering the cluster region up to a radius of 14'. Our study confirms that the BSSs are more centrally concentrated than Horizontal Branch (HB) stars in the cluster. We do not find a statistically significant rising trend in the radial distribution of BSSs for the outer regions, as most of the previously catalogued BSSs that are located in the cluster outskirts were found to be non-members, quasars or galaxies. This study highlights the importance of UV imaging combined with membership information to probe the radial distribution of BSSs.

Luminosity outbursts of the FU Ori type stars, which have a magnitude of ∼ 100 L⊙ and last for decades, may affect chemical composition of the surrounding protoplanetary disk. Using astrochemical modelling we analyse the changes induced by the outburst and search for species sensitive to the luminosity rise. Some changes in the disk molecular composition appear not only during the outburst itself but can also retain for decades after the end of the outburst. We analyse main chemical processes responsible for these effects and assess timescales at which chemically inert species return to the pre-outburst abundances.

Interactions and mergers have been known as key scenarios to enhance global star formation rates and to lower the metal content of galaxies. However, little is known on how interactions affect the spatial distribution of gas metallicities. Thanks to the SDSS-IV MaNGA survey we are able to statistically constrain the impact of interactions across the optical distributions of galaxies. In this study, we compare the radial distribution of the ionized gas metallicity from a sample of 329 interacting objects – covering different interaction stages – with a statistical robust control sample. Our results suggest that galaxies close to coalesce tend to have flat, lower metallicities than non-interacting star-forming galaxies.

The long-term SAAO survey of Local Group galaxies in the near-infrared (JHKs) has included five dwarf irregulars (dIrr), namely, NGC 6822, IC 1613, WLM, Sgr dIG and NGC 3109. We have found long-period (Mira) variables in all of them. Most of the Miras, which follow a linear LMC period-luminosity (PL) relation well, are carbon-rich. A small group of oxygen-rich Miras are brighter than the linear PL relation predicts, presumably because they are undergoing hot-bottom burning (HBB).

We analyse the evolution with redshift of the radial gradient of oxygen abundances in spiral disks resulting from our MULCHEM chemical evolution models, computed for galaxies of different sizes or masses, studying the relationships between the gradients and galaxy characteristics as the stellar mass, the size, the gas fraction or the star formation rate for z < 4.

A comprehensive study of UV emission from asymptotic giant branch (AGB) stars with the Galaxy Evolution Explorer (GALEX) revealed that out of the 316 observed AGB stars, 57% were detected in the near-UV (NUV) bandpass and 12% were detected in the far-UV (FUV) bandpass (Montez et al. 2017). A cross-match between our sample and Gaia DR2 results in parallax estimates for 90% of the sample of AGB stars, compared to only 30% from Hipparcos. This increase allowed us to further probe trends and conclusions of our initial study. Specifically, that the detection of UV emission from AGB stars is subject to proximity and favorable lines of sight in our Galaxy. These improved results support the notion that some of the GALEX-detected UV emission is intrinsic to AGB stars, likely due to a combination of photospheric and chromospheric emission.

A long-standing problem of the general paradigm of low-mass star formation is the “luminosity problem”: protostars are less luminous than theoretically predicted. One possible solution is that the accretion process is episodic. FU Orionis-type stars (FUors) are thought to be the visible examples for objects in the high accretion state and it is still debated what physical mechanism triggers the phenomenon. For many of these objects their disk properties are still largely unknown so we conducted a deep, high spatial resolution (down to 20 au) ALMA Band 6 (1.3 mm) dust continuum survey of a sub-sample of known FUors. Here we present preliminary results of our survey, including the mass, size and spectral slope of each disk.

Gaia DR2 catalog provides a unique possibility to study the three-dimensional structure and the three-dimensional velocity field of the nearby open clusters. We can either select stars with a maximum membership probability and the most accurate values for the proper motions, parallaxes, and the radial velocities, or study these clusters statistically using overwhelmingly large areas of sky of tens by tens degrees. The second approach allows us to reveal the extensive outer parts of the clusters - a corona and the tidal tails and to study the luminosity and mass functions of these clusters. We present the first results of the investigation of several nearby open clusters, including Pleiades, Alpha Persei, Ruprecht 147.