In this paper, we give a brief description of the Polstar instrument, an ultraviolet (UV) spectropolarimetric, Midex-class mission proposed to NASA to study the winds of massive stars as well as interstellar medium and protoplanetary discs topics.

One significant difficulty in reliable quantification of the rates of mass-loss from hot, massive stars lies in uncertainties associated with quantifying temporal and spatial variability within stellar winds. The consequences of low-metallicity conditions for wind structure also merit continued investigation. We present initial results from ULLYSES data with the aim of identifying structure within the stellar winds of early B type supergiants with sub-solar metallicities in the Large and Small Magellanic Clouds. We demonstrate how single-epoch ULLYSES data can be used to investigate significant wind structure for these stars.

Super star cluster (SSC) A1 in starburst galaxy NGC 3125 has the strongest broad He II λ1640 emission line ever observed in the nearby Universe and constitutes an important template for interpreting observations of galaxies that are located out to a redshift of z∼3. We use observations of SSC A1 obtained with the Cosmic Origins Spectrograph (COS) on board of the Hubble Space Telescope (HST) in order to check if there is a contribution of nebular emission to the He II line. In addition, we compare the COS G130M + G160M observations of A1 (1150 – 1750∘A) to the latest Charlot & Bruzual population synthesis models, which account for Very Massive Stars (VMS) of up to 300 Mȯ. A model with Z = 0.008 and age = 2.4 Myr provides a very reasonable fit to the C III λ1175, N V λ1240, C IV λ1550, He II λ1640, and N IV λ1718 stellar-wind features, although the O V λ1371 line is not well reproduced. Overall, our results show the great improvement of stellar evolution and population synthesis models over the past decade, and in particular, the improved formulation of stellar mass loss rates.

B supergiants (BSGs) are evolved objects on the cool end of the line-driven wind regime. Studying their atmospheres provides important insights on the stellar wind physics of these objects and their evolutionary status. So far important features of their spectra, especially in the UV region, could not be reproduced consistently with atmosphere models. This translates directly into problems of our understanding of their wind properties. Here, we present new insights about the BSGs on the cooler side of the Bi-Stability Jump, corresponding to spectral types later than B1. Using UV and optical data, we analysed a sample of Galactic cool BSGs. Including for the first time X-rays and clumping the wind models, we show that the spectra of cool BSGs cannot be explained without X-rays, despite any clear detection of the target stars.

Stellar magnetic field is the driver of activity in stars and can trigger spots, energetic flares, coronal plasma ejections and ionized winds. These phenomena play a crucial role in understanding the internal mechanisms of the star, but can also have potential effects in orbiting planets. During the transit of a planet, spots can be occulted producing features imprinted in the transit light curve. Here, we modelled these features to characterize the physical properties of the spots (radius, intensity, and location). In addition, we monitor spots signatures on multiple transits to estimate magnetic cycles length of Kepler stars. Flares have also been observed during transits in active stars. We derive the properties of the flares and analyse their UV impact on possible living organisms in planets orbiting in the habitable zone.

The high energy X-ray and UV radiation fields of host stars play a crucial role in determining the atmospheric conditions and habitability of potentially-habitable exoplanets. This paper focuses on the major surveys of the UV/X-ray emissions of M- and K-type exoplanet hosts that have been undertaken by the MUSCLES and MegaMUSCLES Hubble Space Telescope (HST) Treasury programs and associated contemporaneous X-ray and ground-based observations. The quiescent and flaring radiation (both photons and implied particles) were observed from this extensive sample of relatively old, low mass, exoplanet host stars and show that, from the viewpoint of a habitable-zone exoplanet, there is no such thing as an “inactive” M dwarf star. The resulting implications are significant for planetary habitability. Extensive monitoring of the X-ray/UV emission from a representative younger M dwarf is also presented and the direct stellar effects that influence exoplanets during the earlier phases of their formation and evolution discussed.

Ionized winds from late-type main-sequence stars are important for stellar spin-down and therefore the evolution of stellar activity; winds blow an “astrosphere” into the interstellar medium that absorbs a large part of galactic cosmic rays; and the winds play a key role in shaping planetary environments, in particular their upper atmospheres. These issues have been well studied for the solar wind but little is known about winds escaping from other solar-type stars. Several methods have been devised to either detect winds directly or to infer the presence of such winds from features that are shaped by the winds. This paper summarizes these methods and discusses exemplary findings. There is need for more studies using multiple methods for the same stars.

We investigate the old open cluster M67 using ultraviolet photometric data of Ultra-Violet Imaging Telescope in multi-filter far-UV bands. M67, well known for the presence of several blue straggler stars (BSS), has been put to detailed tests to understand their formation pathways. Currently, there are three accepted formation channels: mass transfer due to Roche-lobe overflow in binary systems, stellar mergers either due to dynamical collisions or through coalescence of close binaries. So far, there had not been any confirmed detection of a white dwarf (WD) companion to any of the BSSs in this cluster. Here, we present the detection of WD companions to 5 bright BSSs in M67. The multiwavelength spectral energy distributions covering 0.12 -11.5 μm range, were found to require binary spectral fits for 5 BSSs, consisting of a cool (BSS) and a hot companion. The parameters (Luminosity, Temperature, Radius and Mass) of the hot companions suggest them to be WDs with mass in the range 0.2 - 0.35 M⊙ with Teff ~11000 –24000 K.

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.

Accurate atomic data for line wavelengths, energy levels, line broadening such as hyperfine structure and isotope structure, and f-values, particularly for the line rich iron group elements, are needed for stellar astrophysics applications, and examples of recent measurements are given. These atomic data are essential for determination of elemental abundances in astronomical objects. With modern facilities, telescopes and spectrographs, access to underexplored regions (IR, UV, VUV), and improved stellar atmosphere models (3D, NLTE), and extremely large datasets, astronomers are tackling problems ranging from studying Galactic chemical evolution, to low mass stars and exoplanets. Such advances require improved accuracy and completeness of the atomic database for analyses of astrophysical spectra.

The primary objectives of the ExoplANETS-A project are to: establish new knowledge on exoplanet atmospheres; establish new insight on influence of the host star on the planet atmosphere; disseminate knowledge, using online, web-based platforms. The project, funded under the EU’s Horizon-2020 programme, started in January 2018 and has a duration ∼3 years. We present an overview of the project, the activities concerning the host stars and some early results on the host stars.

UV radiation plays a critical role in the chemistry of circumstellar envelopes (CSEs) around evolved stars on the asymptotic giant branch (AGB). However, the effects of all potential sources of UV radiation have not been included in models. We present preliminary results of adding an internal source of UV to the CSE chemistry and predict large enhancements of atomic and ionic species arising from photo-destruction of parent species. Observations of atomic carbon towards the UV-bright AGB star o Cet are consistent with the modelling. In addition, we calculate the precise depth dependence of the CO photodissociation rate in an expanding CSE. We incorporate this within a chemical network active in the outflows of AGB stars, which includes 933 species and 15182 reactions. Our results show that the CO envelope size is about 30% smaller at half abundance than the most commonly used radius reported by Mamon et al. (1988).

The stellar ultraviolet radiation (UVR) has been studied in the last decade and has been found to be an important factor to determine the habitability of planetary surfaces. It is known that UVR can be a constraint for life. However, most of the studies of UVR and habitability have missed some fundamental aspects: i) Accurate estimation of the planetary atmospheric attenuation, ii) The biological inferences used to represent the impact of the stellar UVR on life are theoretical and based on the action spectrum (for DNA or microorganisms) or considering parameters as the “lethal dose” obtained from non-astrobiological experiments. Therefore, the conclusions reached by previous studies about the UVR habitability of planetary bodies may be inaccurate. In this work, we propose how to address these studies in a more accurate way through an interdisciplinary approach that combines astrophysics, microbiology, and photobiology and by the use of specially designed laboratory experiments.

We have analyzed FUSE, COS, GHRS, and Keck/HIRES spectra of the UV-bright star Barnard 29 in M13. Fits to the star’s optical spectrum yield Teff = 20,000 ± 100 K and log g = 3.00 ± 0.01. Using modern stellar-atmosphere models, we are able to reproduce the complex shape of the Balmer H.. feature. We derive photospheric abundances of He, C, N, O, Mg, Al, Si, P, S, Cl, Ar, Ti, Cr, Fe, Ni, and Ge. Barnard 29 exhibits an abundance pattern typical of the first-generation stars in M13, enhanced in oxygen and depleted in aluminum. We see no evidence of significant chemical evolution since the star left the RGB; in particular, it did not undergo third dredge-up. Previous workers found that the star’s FUV spectra yield an iron abundance about 0.5 dex lower than its optical spectrum, but the iron abundances derived from all of our spectra are consistent with one another and with the cluster value. We attribute this difference to our use of model atmospheres without microturbulence. By comparing our best-fit model with the star’s optical magnitudes, we derive a mass M*/M=0.40 − 0.49 and luminosity log L*/L⊙=3.20 − 3.29, depending on the cluster distance. Comparison with stellar-evolution models suggests that Barnard 29 evolved from a ZAHB star of mass M*/M⊙∼0.50, placing it near the boundary between the extreme and blue horizontal branches.

The Ultra-Violet Imaging Telescope (UVIT) is one of the payloads in Astrosat, the first Indian Space Observatory. The UVIT instrument has two 375 mm telescopes: one for the far-ultraviolet (FUV) channel (1300-1800 Å), and the other for the near-ultraviolet (NUV) channel (2000-3000 Å) and the visible (VIS) channel (3200-5500 Å). We shall discuss the issues with standardization in the UV with reference to Astrosat Observations (Cycle A04). I shall discuss the problems faced in data-analysis and how these in turn lead to serious issues dealing with the color-magnitude diagarms, membership and age of the young embedded clusters studied.

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.

Large magnetometric surveys have contributed to the detection of an increasing number of magnetic massive stars, and to the recognition of a population of magnetic massive stellar objects with distinct properties. Among these, NGC 1624-2 possesses the largest magnetic field of any O-type star; such a field confines the stellar wind into a circumstellar magnetosphere, which can be probed using observations at different wavelength regimes. Recent optical and X-ray observations suggest that NGC 1624-2’s magnetosphere is much larger than that of any other magnetic O star. By modeling the variations of UV resonance lines, we can constrain its velocity structure. Furthermore, recent spectropolarimetric observations raise the possibility of a more complex field topology than previously expected. Putting all of these multi-wavelength constraints together will allow us to paint a consistent picture of NGC 1624-2 and its surprising behavior, giving us valuable insight into the very nature of massive star magnetospheres.

Magnetic massive stars comprise approximately 10% of the total OB star population. Modern spectropolarimetry shows these stars host strong, stable, large-scale, often nearly dipolar surface magnetic fields of 1 kG or more. These global magnetic fields trap and deflect outflowing stellar wind material, forming an anisotropic magnetosphere that can be probed with wind-sensitive UV resonance lines. Recent HST UV spectra of NGC 1624-2, the most magnetic O star observed to date, show atypically unsaturated P-Cygni profiles in the Civ resonant doublet, as well as a distinct variation with rotational phase. We examine the effect of non-radial, magnetically-channeled wind outflow on P-Cygni line formation, using a Sobolev Exact Integration (SEI) approach for direct comparison with HST UV spectra of NGC 1624-2. We demonstrate that the addition of a magnetic field desaturates the absorption trough of the P-Cygni profiles, but further efforts are needed to fully account for the observed line profile variation. Our study thus provides a first step toward a broader understanding of how strong magnetic fields affect mass loss diagnostics from UV lines.

The double distance and metallicity frontier marked by the SMC has been finally broken with the aid of powerful multi-object spectrographs installed at 8-10m class telescopes. VLT, GTC and Keck have enabled studies of massive stars in dwarf irregular galaxies of the Local Group with poorer metal-content than the SMC. The community is working to test the predictions of evolutionary models in the low-metallicity regime, set the new standard for the metal-poor high-redshift Universe, and test the extrapolation of the physics of massive stars to environments of decreasing metallicity. In this paper, we review current knowledge on this topic.