Tracking down R Coronae Borealis stars from their mid-infrared WISE colours

R Coronae Borealis stars (RCBs) are hydrogen-deficient and carbon-rich supergiant stars. They are very rare, as only $\sim50$ are actually known in our Galaxy. Interestingly, RCBs are strongly suspected to be the evolved merger product of two white dwarfs and could therefore be an important tool to understand Supernovae type Ia in the double degenerate scenario. Constraints on the spatial distribution and the formation rate of such stars are needed to picture their origin and test it in the context of actual population synthesis results. To do so, it is crucial to increase significantly the number of known RCBs. With an absolute magnitude $\mathrm{M_V\sim-5}$ and a bright/hot circumstellar shell made of amorphous carbon grains, RCBs are really distinctive stars. Mono-epoch mid-infrared data can help us to discriminate RCBs among other dust-producing stars. The aim is to produce from the WISE and 2MASS infrared catalogues a new catalogue of reasonable size, enriched with RCB stars. Colour-Colour cuts used on all stars detected are the main selection criteria. The selection efficiency was monitored using the 52 known RCBs. It has been found that selection cuts in mid-infrared colour-colour diagrams are a very efficient method of discriminating RCBs from other stars. An RCB enriched catalogue made of only 1602 stars, with a high detection efficiency of about 77%, was produced. Spectral energy distributions of 49 known RCBs and 5 known HdCs are also presented with estimates of their photosphere and circumstellar shell temperatures. The newly released WISE all sky catalogue has proven to be a valuable resource in finding RCB stars. Actual scenarios predict that between 100 and 500 RCBs exist in our Galaxy. The newly created RCB enriched catalogue is an important step forward to significantly increase the number of known RCB stars and therefore better understand their origin.Comment: 16 pages, 12 figures, Accepted in A&

Peculiar Hydrogen-deficient Carbon Stars: Strontium-Rich Stars and the s-Process

Context: R Coronae Borealis (RCB) variables and their non-variable counterparts, the dustless Hydrogen-Deficient Carbon (dLHdC) stars have been known to exhibit enhanced s-processed material on their surfaces, especially Sr, Y, and Ba. No comprehensive work has been done to explore the s-process in these types of stars, however one particular RCB star, U Aqr, has been under scrutiny for its extraordinary Sr enhancement. Aims: We aim to identify RCB and dLHdC stars that have significantly enhanced Sr abundances, such as U Aqr, and use stellar evolution models to begin to estimate the type of neutron exposure that occurs in a typical HdC star. Methods: We compare the strength of the Sr II 4077 $\AA$ spectral line to Ca II H to identify the new subclass of Sr-rich HdCs. We additionally use the structural and abundance information from existing RCB MESA models to calculate the neutron exposure parameter, $\tau$ Results: We identify six stars in the Sr-rich class. Two are RCBs, and four are dLHdCs. We additionally find that the preferred RCB MESA model has a neutron exposure $\tau$ ~ 0.1 mb$^{-1}$, which is lower than the estimated $\tau$ between 0.15 and 0.6 mb$^{-1}$ for the Sr-rich star U Aqr found in the literature. We find trends in the neutron exposure corresponding to He-burning shell temperature, metallicity, and assumed s-processing site. Conclusions: We have found a sub-class of 6 HdCs known as the Sr-rich class, which tend to lie in the halo, outside the typical distribution of RCBs and dLHdCs. We find that dLHdC stars are more likely to be Sr-rich than RCBs, with an occurrence rate of ~13\% for dLHdCs and ~2\% for RCBs. This is one of the first potential spectroscopic differences between RCBs and dLHdCs, along with dLHdCs having stronger surface abundances of $^{18}$O.Comment: 8 pages, submitted to A&

AllBRICQS: the All-sky BRIght, Complete Quasar Survey

We describe the first results from the All-sky BRIght, Complete Quasar Survey (AllBRICQS), which aims to discover the last remaining optically bright quasars. We present 116 quasars (105 newly identified) having |b| > 10deg and Gaia magnitudes brighter than B_P =16.5 or R_P =16 mag (plus another four at slightly fainter magnitudes), which span a redshift range of z = 0.07 - 2.45. The AllBRICQS sources have been selected by combining data from the Gaia and WISE all-sky satellite missions, and we successfully identify quasars not flagged as candidates by Gaia Data Release 3. We expect the completeness to be approximately 96% within our magnitude and latitude limits, while the preliminary results indicate a selection purity of approximately 97%. The optical spectroscopy used for source classification will also enable detailed quasar characterisation, including black hole mass measurements and identification of foreground absorption systems. The AllBRICQS sources will greatly enhance the number of quasars available for high-signal-to-noise follow-up with present and future facilities.Comment: 23 pages. Submitted to PAS

A Spectral Classification System for Hydrogen-deficient Carbon Stars

Stellar spectral classification, and especially the Yerkes system, has been highly useful in the study of stars. While there is a currently accepted classification system for carbon stars, the subset of Hydrogen-deficient Carbon (HdC) stars has not been well described by such a system, due in part to their rarity and their variability. Here we present a new system for the classification of HdCs based on their spectra, which is made wholly on their observable appearance. We use a combination of dimensionality reduction and clustering algorithms with human classification to create such a system. We classify over half of the known sample of HdC stars using this, and roughly calibrate the temperatures of each class using their colors. Additionally, we express trends in the occurrence of certain spectral peculiarities such as the presence of Hydrogen and Lithium lines. We also present three previously unpublished spectra, and report the discovery of three new Galactic dustless HdC (dLHdC) stars and additionally discuss one especially unique star that appears to border between the hottest HdCs and the coolest Extreme Helium (EHe) stars.Comment: 30 pages, 18 figures, submitted to MNRA

The accretion of a solar mass per day by a 17-billion solar mass black hole

Around a million quasars have been catalogued in the Universe by probing deeper and using new methods for discovery. However, the hardest ones to find seem to be the rarest and brightest specimen. In this work, we study the properties of the most luminous of all quasars found so far. It has been overlooked until recently, which demonstrates that modern all-sky surveys have much to reveal. The black hole in this quasar accretes around one solar mass per day onto an existing mass of $\sim$17 billion solar masses. In this process its accretion disc alone releases a radiative energy of $2\times 10^{41}$ Watts. If the quasar is not strongly gravitationally lensed, then its broad line region (BLR) is expected to have the largest physical and angular diameter occurring in the Universe, and will allow the Very Large Telescope Interferometer to image its rotation and measure its black hole mass directly. This will be an important test for BLR size-luminosity relations, whose extrapolation has underpinned common black-hole mass estimates at high redshift.Comment: This version of the article has been accepted for publication after peer review and is subject to Springer Nature's AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at http://dx.doi.org/10.1038/s41550-024-02195-

High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates from the SkyMapper Survey

The SkyMapper Southern Sky Survey is carrying out a search for the most metal-poor stars in the Galaxy. It identifies candidates by way of its unique filter set that allows for estimation of stellar atmospheric parameters. The set includes a narrow filter centered on the Ca II K 3933A line, enabling a robust estimate of stellar metallicity. Promising candidates are then confirmed with spectroscopy. We present the analysis of Magellan-MIKE high-resolution spectroscopy of 122 metal-poor stars found by SkyMapper in the first two years of commissioning observations. 41 stars have [Fe/H] <= -3.0. Nine have [Fe/H] <= -3.5, with three at [Fe/H] ~ -4. A 1D LTE abundance analysis of the elements Li, C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, Ba and Eu shows these stars have [X/Fe] ratios typical of other halo stars. One star with low [X/Fe] [X/Fe values appears to be "Fe-enhanced," while another star has an extremely large [Sr/Ba] ratio: >2. Only one other star is known to have a comparable value. Seven stars are "CEMP-no" stars ([C/Fe] > 0.7, [Ba/Fe] < 0). 21 stars exhibit mild r-process element enhancements (0.3 <=[Eu/Fe] < 1.0), while four stars have [Eu/Fe] >= 1.0. These results demonstrate the ability to identify extremely metal-poor stars from SkyMapper photometry, pointing to increased sample sizes and a better characterization of the metal-poor tail of the halo metallicity distribution function in the future.Comment: Minor corrections to text, missing data added to Tables 3 and 4; updated to match published version. Complete tables included in sourc