arXiv subjects

P. Aschenbrenner

Publications and source records attributed to P. Aschenbrenner.

6 recordsLinked to original sources

arXiv2026

Discovery of the $\lambda$ Bootis phenomenon in an early B-type star

Chemically peculiar A-type stars of the $\lambda$ Boo class are characterised by depletions of refractory elements and normal volatile species. Their formation channel(s) have not yet been fully established, but they provide insight into star-disc-planet interactions during star formation, star-planet interactions in later system evolution, or interactions between stars and the interstellar medium. We report the first detection of the $\lambda$ Boo phenomenon in a massive early B-type star, HD 37356, a member of Ori OB1c. We analysed a high-quality spectrum using a hybrid non-LTE model-atmosphere technique. We determined atmospheric parameters and elemental abundances using neural networks to emulate synthetic spectra, coupled with a Markov chain Monte Carlo algorithm. We also derived the fundamental parameters of the star using Gaia data and stellar evolution models. HD 37356 closely resembles the Morgan-Keenan standard $\gamma$ Peg, closely agreeing in spectral signatures except for the lines of refractory elements, which are systematically weaker. The quantitative analysis yields similar stellar parameters. The abundances of volatile elements are compatible with cosmic standard abundances, while refractory species are depleted by 0.25 to 0.45dex. In particular, the lower iron abundance explains the different pulsational behaviour of HD 37356 compared to $\gamma$ Peg. Of the different formation channels for $\lambda$ Boo stars, only the accretion of gas depleted in refractory elements from the circumstellar disc during star formation appears to be feasible in this case. This scenario requires HD 37356 to be a true slow rotator in order to prevent the abundance pattern from being erased by internal mixing processes such as meridional circulation.

astro-ph.SR↗

arXiv2026

Quantitative spectroscopy of single and multiple OB-type stars. Non-LTE spectrum analysis with machine learning

The plethora of spectra of OB-type stars in observatory archives and the much larger numbers to come from the WEAVE and 4MOST spectroscopic facilities require efficient, but also accurate and precise methods for (semi)automatic quantitative analyses. Neural networks were used to emulate the spectra of single- and multi-star systems, trained on hybrid non-local thermodynamic equilibrium (non-LTE) models that cover a wide range of atmospheric parameters and chemical compositions. To derive the full set of stellar atmospheric parameters and uncertainties, a Markov chain Monte Carlo algorithm was implemented to fit high-resolution spectra within 3000A-10500A. The neural networks and fitting algorithm were bundled into a programme called Spectral Analysis Tool Using Restricted Neural networks (SATURN). In its current implementation, SATURN facilitates the emulation of synthetic spectra for spectral types O7 to B9, which differ only negligibly from computed models. SATURN was tested on a number of benchmark stars that have been studied before, including single OB stars and a detached eclipsing binary (DEB) system. Excellent agreement of atmospheric parameters and elemental abundances for up to ten metal species is found with respect to the data in the literature, often with reduced uncertainties. For DEB components, the uncertainties are larger, in particular for the fainter secondaries when only a single-epoch spectrum is considered. Uncertainties of elemental abundances are typically <0.10dex. Some first applications of SATURN for analyses of new targets are shown to demonstrate its capabilities, such as fast rotators, including HD149757 (Zeta Ophiuchi). Consistent results are also found at reduced spectral resolutions relevant for observations with WEAVE and 4MOST.

astro-ph.SR↗

arXiv2025

The SRG/eROSITA All-Sky Survey $J$-Band Follow-Up Observations for Selected High-Redshift Galaxy Cluster Candidates

We select galaxy cluster candidates from the high-redshift (BEST_Z > 0.9) end of the first SRG/eROSITA All-Sky Survey (eRASS1) galaxy cluster catalogue, for which we obtain moderately deep J-band imaging data with the OMEGA2000 camera at the 3.5m telescope of the Calar Alto Observatory. We include J-band data of four additional targets obtained with the three-channel camera at the 2m Fraunhofer telescope at the Wendelstein Observatory. We complement the new J-band photometric catalogue with forced photometry in the i- and z-bands of the tenth data release of the Legacy Survey (LSDR10) to derive the radial colour distribution around the eRASS1 clusters. Without assuming a priori to find a cluster red sequence at a specific colour, we try to find a radially weighted colour over-density to confirm the presence of high-redshift optical counterparts for the X-ray emission. We compare our confirmation with optical properties derived in earlier works based on LSDR10 data to refine the existing high-redshift cluster confirmation of eROSITA-selected clusters. We attempt to calibrate the colour-redshift-relation including the new J-band data by comparing our obtained photometric redshift estimate with the spectroscopic redshift of a confirmed, optically selected, high-redshift galaxy cluster. We confirm 9 out of 18 of the selected galaxy cluster candidates with a radial over-density of similar coloured galaxies for which we provide a photometric redshift estimate. We can report an increase in the relative colour measurement precision from 8% to 4% when including J-band data. In conclusion, our findings indicate a not insignificant spurious contaminant fraction at the high-redshift end (BEST_Z > 0.9) of the eROSITA/eRASS1 galaxy cluster catalogue, as well as it underlines the necessity for wide and deep near infrared imaging data for confirmation and characterisation of high-$z$ galaxy clusters.

astro-ph.CO↗

arXiv2025

The present-day cosmic phosphorus abundance

The present-day phosphorus abundance in the solar neighbourhood is determined from a sample of OB-type stars. This is in order to constrain the endpoint of the galactochemical evolution of phosphorus in the course of stellar nucleosynthesis over cosmic time and to provide an abundance baseline for the study of the depletion of phosphorus onto dust grains in the interstellar medium. A model atom for P II/III/IV based on a comprehensive new set of ab initio data for line transitions, photoionisations and electron-impact excitation was developed. Non-local thermodynamic equilibrium (non-LTE) line formation calculations with the codes DETAIL and SURFACE were conducted, based on LTE line-blanketed hydrostatic model atmospheres computed with the ATLAS12 code. High-resolution optical spectra for a sample of 42 apparently slowly rotating main-sequence OB-type stars and B-type supergiants in the solar vicinity within ~500 pc and beyond, out to a distance of ~2 kpc, were analysed. The non-LTE effects on the formation of the P II/III/IV lines are discussed. Non-LTE effects on the stellar abundances range from zero to ~0.3 dex. Where available in the spectra, ionisation balance between two phosphorus ionic species is achieved. Accurate and precise abundances are provided for the sample stars, statistical and systematic 1 sigma uncertainties are typically each well below 0.1 dex. The present-day cosmic phosphorus abundance in the solar neighbourhood is constrained to log(P/H)+12 = 5.36 +- 0.14, which is compatible with the solar photospheric abundance, but lower than derived by LTE analyses of neutral phosphorus lines in solar-type stars. The amount of phosphorus depleted onto dust grains is ~0.25 dex.

astro-ph.SR↗

arXiv2023

The blue supergiant Sher 25 revisited in the Gaia era

Aims. The evolutionary status of the blue supergiant Sher 25 and its membership to the massive cluster NGC 3603 are investigated. Methods. A hybrid non-LTE (local thermodynamic equilibrium) spectrum synthesis approach is employed to analyse a high-resolution optical spectrum of Sher 25 and five similar early B-type comparison stars in order to derive atmospheric parameters and elemental abundances. Fundamental stellar parameters are determined by considering stellar evolution tracks, Gaia Data Release 3 (DR3) data and complementary distance information. Interstellar reddening and the reddening law along the sight line towards Sher 25 are constrained employing UV photometry for the first time in addition to optical and infrared data. The distance to NGC 3603 is reevaluated based on Gaia DR3 data of the innermost cluster O-stars. Results. The spectroscopic distance derived from the quantitative analysis implies that Sher 25 lies in the foreground of NGC 3603, which is found to have a distance of $d_\mathrm{NGC 3603}$ = 6250$\pm$150 pc. A cluster membership is also excluded as the hourglass nebula is unaffected by the vigorous stellar winds of the cluster stars and from the different excitation signatures of the hourglass nebula and the nebula around NGC 3603. Sher 25 turns out to have a luminosity of log L/L$_\odot$ = 5.48$\pm$0.14, equivalent to that of a $\sim$27 $M_\odot$ supergiant in a single-star scenario, which is about half of the mass assumed so far, bringing it much closer in its characteristics to Sk-69°202, the progenitor of SN 1987A. Sher 25 is significantly older than NGC 3603. Further arguments for a binary (merger) evolutionary scenario of Sher 25 are discussed.

astro-ph.SR↗

arXiv2017

Candidate exoplanet host HD131399A: a nascent Am star

Direct imaging suggests that there is a Jovian exoplanet around the primary A-star in the triple-star system HD131399. We investigate a high-quality spectrum of the primary component HD131399A obtained with FEROS on the ESO/MPG 2.2m telescope, aiming to characterise the star's atmospheric and fundamental parameters, and to determine elemental abundances at high precision and accuracy. The aim is to constrain the chemical composition of the birth cloud of the system and therefore the bulk composition of the putative planet. A hybrid non-local thermal equilibrium (non-LTE) model atmosphere technique is adopted for the quantitative spectral analysis. Comparison with the most recent stellar evolution models yields the fundamental parameters. The atmospheric and fundamental stellar parameters of HD131399A are constrained to Teff=9200+-100 K, log g=4.37+-0.10, M=1.95+0.08-0.06 Msun, R=1.51+0.13-0.10 Rsun, and log L/Lsun=1.17+-0.07, locating the star on the zero-age main sequence. Non-LTE effects on the derived metal abundances are often smaller than 0.1dex, but can reach up to ~0.8dex for individual lines. The observed lighter elements up to calcium are overall consistent with present-day cosmic abundances, with a C/O ratio of 0.45$\pm$0.07 by number, while the heavier elements show mild overabundances. We conclude that the birth cloud of the system had a standard chemical composition, but we witness the onset of the Am phenomenon in the slowly rotating star. We furthermore show that non-LTE analyses have the potential to solve the remaining discrepancies between observed abundances and predictions by diffusion models for Am stars. Moreover, the present case allows mass loss, not turbulent mixing, to be identified as the main transport process competing with diffusion in very young Am stars.

astro-ph.SR↗