A near-UV reconnaissance of metal-poor massive stars

We use synthetic model spectra to investigate the potential of near-ultraviolet (3000-4050 \r{A}) observations of massive O-type stars. We highlight the He I $\lambda$3188 and He II $\lambda$3203 pair as a potential temperature diagnostic in this range, supported by estimates of gravity using the high Balmer series lines. The near-ultraviolet also contains important metallic lines for determinations of chemical abundances (oxygen in particular) and estimates of projected rotational velocities for O-type spectra. Using the model spectra we present performance estimates for observations of extragalactic massive stars with the Cassegrain U-Band Efficient Spectrograph (CUBES) now in construction for the Very Large Telescope. The high efficiency of CUBES will open-up exciting new possibilities in the study of massive stars in external galaxies. For instance, CUBES will provide new insights into the physical properties of O-type stars, including oxygen abundances, in metal-poor irregular galaxies at ~1 Mpc from integrations of just 2-3 hrs. Moreover, CUBES will bring quantitative spectroscopy of more distant targets within reach for the first time, such as the O-type star (V~21.5 mag) in Leo P (at 1.6 Mpc) in only half a night of observations.Comment: Published in Experimental Astronomy as part of the CUBES Special Issu

Massive stars at low metallicity: Evolution and surface abundances of O dwarfs in the SMC

We study the evolution, rotation, and surface abundances of O-type dwarfs in the Small Magellanic Cloud. We analyzed the UV and optical spectra of twenty-three objects and derived photospheric and wind properties. The observed binary fraction of the sample is ~ 26%, which is compatible with more systematic studies, if one considers that the actual binary fraction is potentially larger owing to low-luminosity companions and that the sample excluded obvious spectroscopic binaries. The location of the fastest rotators in the H-R diagram indicates that these could be several Myr old. The offset in the position of these fast rotators compared with the other stars confirms the predictions of evolutionary models that fast-rotating stars tend to evolve more vertically in the H-R diagram. Only one star of luminosity-class Vz, expected to best characterize extreme youth, is located on the ZAMS, the other two stars are more evolved. The distribution of nitrogen abundance of O and B stars suggests that the mechanisms responsible for the chemical enrichment of slowly rotating massive stars depends only weakly on the star's mass. We confirm that the group of slowly rotating N-rich stars is not reproduced by the evolutionary tracks. Our results call for stronger mixing in the models to explain the range of observed N abundances. All stars have an N/C ratio as a function of stellar luminosity that matches the predictions of the stellar evolution models well. More massive stars have a higher N/C ratio than the less massive stars. Faster rotators show on average a higher N/C ratio than slower rotators. The N/O versus N/C ratios agree qualitatively well with those of stellar evolution models. The only discrepant behavior is observed for the youngest two stars of the sample, which both show very strong signs of mixing, which is unexpected for their evolutionary status.Comment: Accepted for publication in A&A (43 pages, 60 figures

Clumping and X-Rays in cooler B supergiant stars

B supergiants (BSGs) are evolved stars with effective temperatures between 10 to 30 kK and are important to understand massive star evolution. Located on the edge of the line-driven wind regime, the study of their atmospheres is helpful to understand phenomena such as the bi-stability jump. Key UV features of their spectra have so far not been reproduced by models for types later than B1. Here, we aim to remedy this situation via spectral analysis that accounts for wind clumping and X-rays. In addition, we investigate the evolutionary status of our sample stars based on the obtained stellar parameters. We determined parameters via quantitative spectroscopy using CMFGEN and PoWR codes. The models were compared to UV and optical data of four BSGs: HD206165, HD198478, HD53138, and HD164353. We also study the evolutionary status of our sample using GENEC and MESA tracks. When including clumping and X-rays, we find good agreements between synthetic and observed spectra for our sample stars. For the first time, we reproduced key lines in the UV. For that, we require a moderately clumped wind (f_infty > ~0.5). We also infer relative X-ray luminosities of ~10^-7.5 to 10^-8 -- lower than the typical ratio of 10^-7. Moreover, we find a possible mismatch between evolutionary and spectroscopic masses, which could be related to the mass-discrepancy problem present in other OB stars. Our results provide evidence that X-rays and clumping are needed to describe the winds of cool BSGs. However, their winds seem less structured than in earlier type stars. This aligns with observational X-rays and clumping constraints as well as recent hydrodynamical simulations. The BSGs' evolutionary status appears diverse: some objects are potentially post-red supergiants or merger products. The wind parameters provide evidence for a moderate mass-loss rate increase around the bi-stability jump. Abstract abridgedComment: 27 pages, 22 figures, accepted for publication in A&

Bolometric luminosity variations in the Luminous Blue Variable AFGL2298

We characterise the variability in the physical properties of the luminous blue variable AFGL2298 between 1989-2008. In conjunction with published data from 1989-2001, we have undertaken a long term (2001-2008) near-IR spectroscopic and photometric observational campaign for this star and utilise a non-LTE model atmosphere code to interpret these data. We find AFGL2298 to have been highly variable during the two decades covered by the observational datasets. Photometric variations of >1.6 mag have been observed in the JHK wavebands; however, these are not accompanied by correlated changes in near-IR colour. Non-LTE model atmosphere analysis of 4 epochs of K band spectroscopy obtained between 2001-7 suggests that the photometric changes were driven by expansion and contraction of the stellar photosphere accompanied by comparatively small changes in the stellar temperature. Unclumped mass loss rates throughout this period were modest and directly comparable to those of other highly luminous LBVs. However, the bolometric luminosity of AFGL2298 appears to have varied by at least a factor of ~2 between 1989-2008, with it being one of the most luminous stars in the Galaxy during maximum. Comparison to other LBVs that have undergone non bolometric luminosity conserving `eruptions' shows such events to be heterogeneous, with AFGL2298 the least extreme example. These results - and the diverse nature of both the quiescent LBVs and associated ejecta - may offer support to the suggestion that more than one physical mechanism is responsible for such behaviour. [ABRIDGED]Comment: 12 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

Genome of the Avirulent Human-Infective Trypanosome—Trypanosoma rangeli

Background: Trypanosoma rangeli is a hemoflagellate protozoan parasite infecting humans and other wild and domestic mammals across Central and South America. It does not cause human disease, but it can be mistaken for the etiologic agent of Chagas disease, Trypanosoma cruzi. We have sequenced the T. rangeli genome to provide new tools for elucidating the distinct and intriguing biology of this species and the key pathways related to interaction with its arthropod and mammalian hosts.  Methodology/Principal Findings: The T. rangeli haploid genome is ,24 Mb in length, and is the smallest and least repetitive trypanosomatid genome sequenced thus far. This parasite genome has shorter subtelomeric sequences compared to those of T. cruzi and T. brucei; displays intraspecific karyotype variability and lacks minichromosomes. Of the predicted 7,613 protein coding sequences, functional annotations could be determined for 2,415, while 5,043 are hypothetical proteins, some with evidence of protein expression. 7,101 genes (93%) are shared with other trypanosomatids that infect humans. An ortholog of the dcl2 gene involved in the T. brucei RNAi pathway was found in T. rangeli, but the RNAi machinery is non-functional since the other genes in this pathway are pseudogenized. T. rangeli is highly susceptible to oxidative stress, a phenotype that may be explained by a smaller number of anti-oxidant defense enzymes and heatshock proteins.  Conclusions/Significance: Phylogenetic comparison of nuclear and mitochondrial genes indicates that T. rangeli and T. cruzi are equidistant from T. brucei. In addition to revealing new aspects of trypanosome co-evolution within the vertebrate and invertebrate hosts, comparative genomic analysis with pathogenic trypanosomatids provides valuable new information that can be further explored with the aim of developing better diagnostic tools and/or therapeutic targets

CUBES : the Cassegrain U-band Efficient Spectrograph

In the era of Extremely Large Telescopes, the current generation of 8-10m facilities are likely to remain competitive at ground-UV wavelengths for the foreseeable future. The Cassegrain U-Band Efficient Spectrograph (CUBES) has been designed to provide high-efficiency (> 40%) observations in the near UV (305-400 nm requirement, 300-420 nm goal) at a spectral resolving power of R >20, 000 (with a lower-resolution, sky-limited mode of R ~7, 000). With the design focusing on maximizing the instrument throughput (ensuring a Signal to Noise Ratio (SNR) ~20 per high-resolution element at 313 nm for U ~18.5 mag objects in 1h of observations), it will offer new possibilities in many fields of astrophysics, providing access to key lines of stellar spectra: a tremendous diversity of iron-peak and heavy elements, lighter elements (in particular Beryllium) and light-element molecules (CO, CN, OH), as well as Balmer lines and the Balmer jump (particularly important for young stellar objects). The UV range is also critical in extragalactic studies: the circumgalactic medium of distant galaxies, the contribution of different types of sources to the cosmic UV background, the measurement of H2 and primordial Deuterium in a regime of relatively transparent intergalactic medium, and follow-up of explosive transients. The CUBES project completed a Phase A conceptual design in June 2021 and has now entered the detailed design and construction phase. First science operations are planned for 2028

τ Sco: The Discovery of the Clones

The B0.2 V magnetic star τ Sco stands out from the larger population of massive magnetic OB stars due to its remarkable, superionized wind, apparently related to its peculiar magnetic field - a field which is far more complex than the mostly-dipolar fields usually observed in magnetic OB stars. τ Sco is therefore a puzzling outlier in the larger picture of stellar magnetism - a star that still defies interpretation in terms of a physically coherent model. Recently, two early B-type stars were discovered as τ Sco analogues, identified by the striking similarity of their UV spectra to that of τ Sco, which was - until now - unique among OB stars. We present the recent detection of their magnetic fields by the MiMeS collaboration, reinforcing the connection between the presence of a magnetic field and a superionized wind. We will also present ongoing observational efforts undertaken to establish the precise magnetic topology, in order to provide additional constrains for existing models attempting to reproduce the unique wind structure of τ Sco-like stars

Mid-infrared observations of O-type stars: spectral morphology

16 pages, 9 figures, accepted for publication in MNRASInternational audienceWe present mid-infrared observations for a sample of 16 O-type stars. The data were acquired with the NASA Spitzer Space Telescope, using the IRS instrument at moderate resolution (R $\sim$ 600), covering the range $\sim 10-37$ microns. Our sample includes early, mid and late O supergiants and dwarfs. We explore for the first time their mid-IR spectral morphology in a quantitative way. We use NLTE expanding atmosphere models to help with line identifications, analyze profile contributions and line-formation regions. The O supergiants present a rich emission line spectra. The most intense features are from hydrogen - $6\alpha$, 7$\alpha$, and $8\alpha$ - which have non-negligible contributions of HeI or HeII lines, depending on the spectral type. The spectrum of early O supergiants is a composite of HI and HeII lines, HeI lines being absent. On the other hand, late O supergiants present features composed mainly by HI and HeI lines. All emission lines are formed throughout the stellar wind. We found that O dwarfs exhibit a featureless mid-IR spectrum. Two stars of our sample exhibit very similar mid-IR features, despite having a very different optical spectral classification. The analysis of O-type stars based on mid-IR spectra alone to infer spectral classes or to estimate physical parameters may thus be prone to substantial errors. Our results may therefore inform spectroscopic observations of massive stars located in heavily obscured regions and help establish an initial framework for observations of massive stars using the Mid-Infrared Instrument on the James Webb Space Telescope