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The evolutionary history of ultra-compact accreting binaries
No full textContext. AM Canum Venaticorum (AM CVn) stars are ultra-compact binary systems composed of a white dwarf primary accreting from a hydrogen-deficient donor. They play a crucial role in astrophysics as potential progenitors of Type Ia supernovae and as laboratories for gravitational wave studies. However, their formation and evolutionary history remain incomplete. Three formation channels have been discussed in the literature: the white dwarf, He-star, and cataclysmic variable channels.
Aims. The chemical composition of the accretor atmosphere reflects the material transferred from the donor. In this work we took the first accurate measurements of the fundamental parameters of the accreting white dwarf in ZTF J225237.05−051917.4, including the abundances of key elements such as carbon, nitrogen, and silicon, by analysing ultraviolet spectra obtained with the Hubble Space Telescope (HST). These measurements provide new insight into the evolutionary history of the system and, together with existing optical observations, establish it as a benchmark to develop our pipeline, paving the way for its application to a larger sample of AM CVn systems.
Methods. We determined the binary parameters through photometric analysis and constrained the atmospheric parameters of the white dwarf accretor, including its effective temperature, surface gravity, and chemical abundances, by fitting the HST ultraviolet spectrum with synthetic spectral models. We then inferred the system’s formation channel by comparing the results with theoretical evolutionary models.
Results. According to our measurements, the accretor’s effective temperature (Teff) is 23 300 ± 600 K and the surface gravity (log g) is 8.4 ± 0.3, which imply an accretor mass (MWD) of 0.86 ± 0.16 M⊙. We find a high nitrogen-to-carbon abundance ratio by mass of > 153.
Conclusions. The accretor is significantly hotter than previous estimates based on simplified blackbody fits to the spectral energy distribution, underscoring the importance of detailed spectral modelling for accurately determining system parameters. Our results show that ultraviolet spectroscopy is well suited to constraining the formation channels of AM CVn systems. Of the three proposed formation channels, the He-star channel can be excluded given the high nitrogen-to-carbon ratio. Our results are consistent with both the white dwarf and cataclysmic variable channels
Spatially resolved polarization swings in the supermassive binary black hole candidate OJ 287 with first Event Horizon Telescope observations
No full textWe present the first Event Horizon Telescope 1.3 mm observations of the supermassive binary black hole candidate OJ 287. The observations achieved an unprecedented angular resolution of 18 μas and reveal significant structural and polarization variability over just five days, marking the shortest timescale on which such changes have been directly imaged in this source. The inner jet exhibits a twisted ridgeline structure, with features displaying apparent superluminal motions up to about 22 c. The linear polarization maps reveal three main polarized features whose electric-vector position angles (EVPAs) change substantially over the time span of our observations, including a component with a radial polarization consistent with being produced by a recollimation shock. Most notably, we directly resolved two innermost jet components whose EVPAs rotate in opposite directions. The faster component, moving at 2.4 ± 0.9 μas/day (17.4 ± 6.5 c), exhibits counterclockwise EVPA swings of roughly 3.7° per day, while the slower component, with a proper motion of 1.4 ± 0.3 μas/day (10.2 ± 2.2 c), rotates clockwise at approximately 2.5° per day. Previous studies inferred helical magnetic fields in AGN jets from time-resolved or integrated polarization variability but lacked the angular resolution to directly image this effect. Our results provide spatially resolved evidence that a helical magnetic field threads the jet’s collimation and acceleration zone, ruling out models based on the superposition of unresolved components. Our analysis suggests that propagating shocks interact with a Kelvin–Helmholtz plasma instability, illuminating different phases of the helical magnetic field and producing the observed polarization spatial and temporal variability. Moreover, our model naturally accounts for the more rapid polarization rotation observed in the faster moving component. Our model predicts even more rapid swings in polarization, which could be tested with future observations featuring a more densely sampled time coverage
The making of robust and highly performing imaging spectropolarimeters for large solar telescopes
No full textWe discuss the requirements, concepts, simulations, implementation, and calibration of two dual Fabry-Perot (FPI)-based imaging spectropolarimeters, CRISP and CHROMIS, at the Swedish 1-m Solar Telescope, and CRISP2, which is under construction. These instruments are optimised for a large field of view and high cadence at the expense of a moderate spectral resolution, and use a combination of a high-resolution and a low-resolution etalon together with an order-sorting prefilter to define the bandpass. The overall design is made robust and stable by tailoring the low-resolution etalon reflectivity to accommodate expected cavity errors from both etalons, by using a compact optical design that eliminates the need for folding mirrors, and enclosing the entire system within a single container sealed by lenses. By using a telecentric design based on lenses rather than mirrors, image degradation by the FPI system is negligible, as shown in a previous publication, and the throughput of the system is maximised. The initial alignment, and maintaining that alignment over time, is greatly simplified. Moreover, the telecentric design allows the full calibration and/or modelling of essential system parameters to be carried out without interfering with the optical set-up or the cameras. We also discuss briefly the polarimeters developed for CRISP and CHROMIS. The high performance of CRISP and CHROMIS has been demonstrated in an earlier publication through measurements of the granulation contrast and comparisons with similar measurements simultaneously made through broadband continuum filters. Here, we focus on the aspects of the design that are central to enabling high performance and robustness, but also discuss the calibration and processing of the data, and use a few examples of processed data to demonstrate the achievable image and data quality. We put forward a proposal for a similar conceptual design for the European Solar Telescope and conclude by discussing potential problems of the proposed approach to designs of this type. Some aspects of these FPI systems may also be of interest outside the solar community
Extending the cosmic distance ladder two orders of magnitude with strongly lensed Cepheids, carbon AGB stars, and RGB stars
No full textGravitational lensing by galaxy clusters can create extreme magnification (μ > 1000) near the cluster caustics, thereby enabling detections of individual luminous stars in high-redshift background galaxies. These stars can include nonexplosive standard candles such as Cepheid variables, carbon stars in the asymptotic giant branch (AGB), and stars at the tip of the red giant branch (TRGB) out to z ≲ 1. A large number of such detections, combined with modeling of the magnification affecting these stars (including microlensing), opens the door to extending the distance range of these standard candles by two orders of magnitude, thereby providing a check on the distances derived from supernovae. Practical measurement of a distance modulus depends on measuring the apparent magnitude of a “knee feature” in the lensed luminosity function. The feature comes from the great abundance of red giant branch stars just below the luminosity of the TRGB. This feature is still present even when microlenses smooth out the sharp jump in the luminosity function at the TRGB. The apparent magnitude at which the knee is observed depends on the value of the Hubble constant, H0, and the surface mass density of microlenses, Σ* (with a weak dependence on the macromodel magnification). Therefore, a precise measurement of Σ* is needed in order to use the TRGB knee as a distance indicator. As a bonus, strongly lensed stars detected in deep exposures also provide a robust method of mapping small dark matter substructures, detections of which will cluster around the critical curves of small-scale dark matter halos. The sensitivity of the TRGB knee to Σ* also allows novel avenues to constrain the abundance of compact dark matter such as primordial black holes. Cepheids will also be detectable, but because microlenses modify their apparent luminosity by unknown magnification factors, the main value of Cepheids will be improving cluster lens models
A Bayesian catalog of 100 high-significance voids in the Local Universe
No full textContext. While cosmic voids are now recognized as a valuable cosmological probe, it is challenging to identify them in a galaxy catalog for multiple reasons: Observational effects such as holes in the mask or magnitude selection hinder the detection process; galaxies are biased tracers of the underlying dark matter distribution; and it is nontrivial to estimate the detection significance and parameter uncertainties for individual voids.
Aims. Our goal is to extract a catalog of voids from constrained simulations of the large-scale structure that are consistent with the observed galaxy positions and effectively represent statistically independent realizations of the probability distribution of the cosmic web. This allows us to carry out a full Bayesian analysis of the structures emerging in the Universe.
Methods. We used 50 posterior realizations of the large-scale structure in the Manticore-Loca
The observed total star formation rate function up to
No full textAims. We investigated how the obscured IR-derived and dust-corrected UV star formation rate functions (SFRFs) compare with each other and with predictions from state-of-the-art theoretical models of galaxy formation and evolution.
Methods. We derived the IR SFRF from the ALMA A3COSMOS survey by converting the IR luminosity functions (IR LFs) into SFRFs after correcting for the active galactic nucleus (AGN) contribution. Similarly, we obtained the UV SFRFs from UV LFs in the literature, corrected for dust-extinction. First, we fit the two SFRFs independently via a Markov chain Monte Carlo (MCMC) approach, then we combined them to obtain the first estimate of the “total” SFRF out to z ∼ 6. Finally, we compared this SFRF with predictions of a set of theoretical models.
Results. We derive the UV and IR SFRFs at 0.5 100 M⊙yr−1 for the IR). From the comparison of the total SFRF with model predictions, we find overall good agreement at z < 2.5, with increasing difference at higher redshifts; all models miss the galaxies that form stars with the highest SFRs. Finally, we finally obtain the UV (dust-corrected), IR and total SFR densities (SFRDs), finding that there are no redshift ranges where UV and IR alone are able to reproduce the total SFRD
The origin of gas stripping of galaxies in group environments
No full textWe investigate how low-mass group environments (Mvir ∼ 1012 − 13 M⊙) influence the gas content of their satellite galaxies with M* > 107 M⊙ using the NE
MINDS: Strong oxygen depletion in the inner regions of a very low-mass star disk?
No full textContext. Thanks to JWST, a plethora of species in planet-forming disks around very low mass stars such as C2H2, C6H6, C4H2, CH3 etc. are being discovered. The column densities of these species retrieved from 0D slab models are very large (e.g. of the order of 1020 cm−2). This indicates a carbon-dominated chemistry in a gas with a high C/O ratio. The disk around 2MASS-J1605321-1993159 (M4.5) is one such source showing a molecular pseudo-continuum of C2H2. Notably, two oxygen-bearing molecules, CO and CO2, are also detected in this source.
Aims. We aim to take the next step beyond 0D slab models to interpret the spectrum. We examine whether 2D thermo-chemical disk models can produce the large inferred column densities of C2H2 in the inner regions of the disk and produce a pseudo-continuum in the mid-IR spectrum. We also seek to constrain whether the depletion of oxygen or the enrichment of carbon causes the high C/O ratio triggering a carbon-dominated chemistry.
Methods. We utilised the radiative thermo-chemical disk model PRODIMO to identify a disk structure that is capable of producing the observed molecular emission of species such as CO, CO2, C2H2, and H2O simultaneously. The spectrum was generated using the fast line tracer FLiTs. We derived the gas temperature ⟨T⟩, column density ⟨log10N⟩, and the emitting area ⟨r1 − r2⟩ for these molecules from the 2D disk model and compared them to the parameters retrieved originally from 0D slab models. We used the different effect that changing the O or C abundance has on CO and C2H2, respectively to discriminate between O depletion and C enhancement.
Results. We find that a disk structure characterised by the presence of a gap can best explain the observations. The inner disk is strongly depleted in dust, especially small grains (<5 µm), and elemental oxygen, leading to a large C/O ratio. This is required to produce a molecular pseudo-continuum of C2H2 and at the same time a relatively weak CO emission. The P- and R-branch of C2H2 probe deeper layers of the disk whereas the Q-branch probes mostly the surface layers. The combined emission of CO and CO2 puts strong constraints on the gap’s location (0.1–0.5 au) given a disk gas mass. We also report a new detection of the CO ν= 2→1 transition in the JWST spectrum.
Conclusions. Two-dimensional thermo-chemical disk models are able to produce the observed molecular pseudo-continuum of C2H2. We find that the combination of different species emission in the JWST spectra can be used to discriminate between different scenarios such as O-depletion, C-enhancement or both, and offers the potential to extract spatial substructure at scales smaller than ∼1 au
TRIShUL: Technique for Reconstructing magnetic Interstellar Structure Using starLight polarization
No full textWe present a novel technique for the decomposition of line-of-sight (LOS) stellar polarization as a function of distance, aimed at reconstructing 3D plane-of-sky magnetic structures in the interstellar medium. The method is based on the assumption that the observed polarization arises from discrete, thin dust layers located at varying distances along the LOS. Using a simple and intuitive frequentist framework, our method identifies structural changes in the distance-sorted cumulative Mahalanobis distance between Stokes parameters (q and u) to detect the locations of dust layers and estimates their associated physical properties (parallax and Stokes parameters) necessary for constructing 3D maps. We benchmarked the method using mock datasets representative of high-Galactic-latitude regions, incorporating realistic parallax uncertainties from Gaia and expected polarization measurements from the upcoming PASIPHA
CO depletion in infrared dark clouds
No full textContext. Infrared dark clouds (IRDCs) are cold, dense structures that are likely representative of the initial conditions of star formation. Many studies of IRDCs employ CO to investigate cloud dynamics, but CO can be highly depleted from the gas phase in IRDCs, which affects its fidelity as tracer. The CO depletion process is also of great interest in astrochemistry because CO ice in dust grain mantles provides the raw material for the formation of complex organic molecules.
Aims. We study CO depletion towards four IRDCs to investigate its correlation with the H2 number density and dust temperature, calculated from Herschel far-infrared images.
Methods. We used 13CO J = 1 → 0 and 2 → 1 maps to measure the CO depletion factor, fD, across IRDCs G23.46-00.53, G24.49-00.70, G24.94-00.15, and G25.16-00.28. We also considered a normalised CO depletion factor, f′D, which takes a value of unity, that is, no depletion, in the outer lower-density and warmer regions of the clouds. We then investigated the dependence of fD and f′D on the gas density, nH, and dust temperature, Tdust.
Results. The CO depletion rises as the density increases and reaches maximum values of f′D ∼ 10 in some regions with nH ≳ 3 × 105 cm−3, although with significant scatter at a given density. We find a tighter, less scattered relation of f′D with temperature that rapidly rise for temperatures ≲18 K. We propose a functional form f′D = exp(T0/[Tdust − T1]) with T0 ≃ 4 K and T1 ≃ 12 K to reproduce this behaviour.
Conclusions. We conclude that CO is strongly depleted from the gas phase in cold, dense regions of IRDCs. This means that if it is not accounted for, CO depletion can lead to an underestimation of the total cloud masses based on CO line fluxes by factors up to ∼5. These results indicate a dominant role for thermal desorption in setting near equilibrium abundances of gas-phase CO in IRDCs and provide important constraints for astrochemical models and the chemodynamical history of gas in the early stages of star formation