arXiv subjects

Paweł Zieliński

Publications and source records attributed to Paweł Zieliński.

11 recordsLinked to original sources

arXiv2026

Computational Complexity of the Recoverable Robust Shortest Path Problem with Discrete Recourse

In this paper the recoverable robust shortest path problem is investigated. Discrete budgeted interval uncertainty representation is used to model uncertain second-stage arc costs. The known complexity results for this problem are strengthened. It is shown that it is Sigma_3^p-hard for the arc exclusion and the arc symmetric difference neighborhoods. Furthermore, it is also proven that the inner adversarial problem for these neighborhoods is Pi_2^p-hard.

cs.CC↗

arXiv2026

An extension of Ordered Weighted Averaging over intervals with application to optimization under risk

The Ordered Weighted Averaging (OWA) operator is a traditional and commonly used criterion for aggregating discrete values of uncertain quantities. In this paper, it is shown that the discrete OWA naturally extends to the continuous case by using the concept of a distortion risk measure. It is shown how to apply the distortion risk measure to optimization problems with a linear objective function, whose coefficients are random variables with continuous distribution functions supported on intervals. The case where these coefficients are independent, uniformly distributed random variables is explored in more detail. The computational complexity of the resulting optimization problem is analyzed, and solution methods with approximation guarantees are proposed. These methods are also verified through computational experiments.

math.OC↗

arXiv2025

ASASSN-24fw: Candidate circumplanetary disk occultation of a main-sequence star

Dusty disks around planetary and substellar companions in outer reaches of exo-planetary systems can be detected as long-lasting occultations, provided the observer is close to the secondary's orbital plane. Here we report optical spectroscopy with KOSMOS (APO), MagE (Magellan) and GHOST (Gemini-S) of ASASSN-24fw (Gaia 07:05:18.97+06:12:19.4), a 4-magnitude dimming event of a main-sequence star which lasted 8.5 months. We discover multiple low-ionization metal emission lines with velocity dispersion $\lesssim 10$ km/s blue-shifted by 27 km/s with respect to the star, as well as kinematically complex Na D absorption. If associated with the occulter, these detections suggest that the occulter is gas-rich. Further, we detect blue-shifted and broad ($\sim 200$ km/s) H$α$ line, which likely originates in the inner circumstellar disk. We confirm the previously reported occultations in 1981 and 1937 seen in historic data, yielding a semi-major axis of the occulter's orbital motion around the star of 14 AU. If the occulter is a circumsecondary disk filling 30-100% of the Hill radius, we estimate the minimum mass of the secondary to be a few Jupiter masses and a disk mass of 1% of the mass of the Moon. Given the age of the star ($>2$ Gyr), the disk is unlikely to be a survivor of the planet formation stage and may be a result of a planetary collision. If Na D absorption and/or metal emission lines originate in the disk, the observations presented here are the first discovery of a circumsecondary disk wind or rotation.

astro-ph.EP↗

arXiv2025

Is the Symbiotic Recurrent Nova T CrB Late? Recent Photometric Evolution and Comparison with Past Pre-Outburst Behaviour

T CrB is a symbiotic recurrent nova that last erupted in 1946. Given its recurrence timescale of approximately 80 years, the next outburst is eagerly anticipated by the astronomical community. In this work, we analyse the optical light curves of T CrB, comparing recent photometric evolution with historical data to evaluate potential predictive indicators of nova eruptions. Although the "super-active" phases preceding both the 1946 and anticipated eruptions are strikingly similar, the subsequent photometric behaviour differs. We find that the decline in brightness observed in 2023, interpreted by some as a "pre-eruption dip", deviates from the deep minimum recorded prior to the 1946 event and does not reliably predict the eruption timing. Recent photometric and spectroscopic observations indicate that the system is returning to a high-accretion state. Given this, an eruption may be imminent, even without distinct precursors. While the next eruption of T CrB will be a major scientific event, its expected peak brightness of $V \sim 2$ mag highlights the importance of setting realistic public expectations for what will be a visually modest, yet astrophysically very significant, celestial event.

astro-ph.SR↗

arXiv2025

The 2023 outburst of the Gaia alerted EXor Gaia23bab

Episodic accretion is a fundamental process in the build-up of the stellar mass. EX Lupi-type eruptive young stars (EXors) represent one of the main types of episodic accretion. We study the recently discovered EXor Gaia23bab during its 2023 outburst. We obtained optical and near-infrared photometry and spectroscopy to probe the variation of the physical properties of Gaia23bab during its recent outburst. We also collected archival photometry to study a previous outburst of the star. We used several accretion tracers, including the Ca II triplet, He I, and various hydrogen lines from the Paschen and Brackett series, to measure the accretion rate during the outburst. The accretion rate is consistent with $\sim 2.0 \times 10^{-7} M_\odot$ $\rm{yr}^{-1}$. Comparing the line fluxes of the hydrogen Brackett series to predictions of Case B theory suggests excitation temperatures of 5000 - 10000 K and electron densities of $10^9$-$10^{10}$ cm$^{-3}$. Comparison to the predictions of a model for T Tauri stars revealed that the fluxes of the Balmer series are consistent with temperatures of 5000 - 12500 K and a hydrogen density of $10^8$ cm$^{-3}$, while the fluxes of the Paschen series are consistent with temperatures in the range between 10000 and 12500 K and a hydrogen density of $10^{11}$ cm$^{-3}$. The derived temperatures and densities confirm that Gaia23bab is a prototypical EXor, not only due to its accretion rate, but also based on the best fit temperatures and densities revealed by the detected hydrogen lines.

astro-ph.SR↗

arXiv2024

Robust Min-Max (Regret) Optimization using Ordered Weighted Averaging

In decision-making under uncertainty, several criteria have been studied to aggregate the performance of a solution over multiple possible scenarios. This paper introduces a novel variant of ordered weighted averaging (OWA) for optimization problems. It generalizes the classic OWA approach, which includes robust min-max optimization as a special case, as well as min-max regret optimization. We derive new complexity results for this setting, including insights into the inapproximability and approximability of this problem. In particular, we provide stronger positive approximation results that asymptotically improve the previously best-known bounds for the classic OWA approach. In computational experiments, we evaluate the quality of the proposed methods and compare the proposed setting with classic OWA and min-max regret approaches.

math.OC↗

arXiv2023

The Gaia alerted fading of the FUor-type star Gaia21elv

FU Orionis objects (FUors) are eruptive young stars, which exhibit outbursts that last from decades to a century. Due to the duration of their outbursts, and to the fact that only about two dozens of such sources are known, information on the end of their outbursts is limited. Here we analyse follow-up photometry and spectroscopy of Gaia21elv, a young stellar object, which had a several decades long outburst. It was reported as a Gaia science alert due to its recent fading by more than a magnitude. To study the fading of the source and look for signatures characteristic of FUors, we have obtained follow-up near infrared (NIR) spectra using Gemini South/IGRINS, and both optical and NIR spectra using VLT/X-SHOOTER. The spectra at both epochs show typical FUor signatures, such as a triangular shaped $H$-band continuum, absorption-line dominated spectrum, and P Cygni profiles. In addition to the typical FUor signatures, [OI], [FeII], and [SII] were detected, suggesting the presence of a jet or disk wind. Fitting the spectral energy distributions with an accretion disc model suggests a decrease of the accretion rate between the brightest and faintest states. The rapid fading of the source in 2021 was most likely dominated by an increase of circumstellar extinction. The spectroscopy presented here confirms that Gaia21elv is a classical FUor, the third such object discovered among the Gaia science alerts.

astro-ph.SR↗

arXiv2023

Robust optimization with belief functions

In this paper, an optimization problem with uncertain objective function coefficients is considered. The uncertainty is specified by providing a discrete scenario set, containing possible realizations of the objective function coefficients. The concept of belief function in the traditional and possibilistic setting is applied to define a set of admissible probability distributions over the scenario set. The generalized Hurwicz criterion is then used to compute a solution. In this paper, the complexity of the resulting problem is explored. Some exact and approximation methods of solving it are proposed.

cs.DS↗

arXiv2021

Recurrent strong outbursts of an EXor-like young eruptive star Gaia20eae

We present follow-up photometric and spectroscopic observations, and subsequent analysis of Gaia20eae. This source triggered photometric alerts during 2020 after showing a $\sim$3 mag increase in its brightness. Its Gaia Alert light curve showed the shape of a typical eruptive young star. We carried out observations to confirm Gaia20eae as an eruptive young star and classify it. Its pre-outburst spectral energy distribution shows that Gaia20eae is a moderately embedded Class II object with $L_\mathrm{bol} = 7.22$ L$_\odot$. The color-color and color-magnitude diagrams indicate that the evolution in the light curve is mostly gray. Multiple epochs of the H$α$ line profile suggest an evolution of the accretion rate and winds. The near-infrared spectra display several emission lines, a feature typical of EXor-type eruptive young stars. We estimated the mass accretion rate during the dimming phase to be $\dot{M} = 3-8 \times 10^{-7}$ M$_\odot$ yr$^{-1}$, higher than typical T Tauri stars of similar mass and comparable to other EXors. We conclude Gaia20eae is a new EXor-type candidate.

astro-ph.SR↗

arXiv2018

Approximating multiobjective combinatorial optimization problems with the OWA criterion

The paper deals with a multiobjective combinatorial optimization problem with $K$ linear cost functions. The popular Ordered Weighted Averaging (OWA) criterion is used to aggregate the cost functions and compute a solution. It is well known that minimizing OWA for most basic combinatorial problems is weakly NP-hard even if the number of objectives $K$ equals two, and strongly NP-hard when $K$ is a part of the input. In this paper, the problem with nonincreasing weights in the OWA criterion and a large $K$ is considered. A method of reducing the number of objectives by appropriately aggregating the objective costs before solving the problem is proposed. It is shown that an optimal solution to the reduced problem has a guaranteed worst-case approximation ratio. Some new approximation results for the Hurwicz criterion, which is a special case of OWA, are also presented.

cs.DS↗

arXiv2017

On Recoverable and Two-Stage Robust Selection Problems with Budgeted Uncertainty

In this paper the problem of selecting $p$ out of $n$ available items is discussed, such that their total cost is minimized. We assume that costs are not known exactly, but stem from a set of possible outcomes. Robust recoverable and two-stage models of this selection problem are analyzed. In the two-stage problem, up to $p$ items is chosen in the first stage, and the solution is completed once the scenario becomes revealed in the second stage. In the recoverable problem, a set of $p$ items is selected in the first stage, and can be modified by exchanging up to $k$ items in the second stage, after a scenario reveals. We assume that uncertain costs are modeled through bounded uncertainty sets, i.e., the interval uncertainty sets with an additional linear (budget) constraint, in their discrete and continuous variants. Polynomial algorithms for recoverable and two-stage selection problems with continuous bounded uncertainty, and compact mixed integer formulations in the case of discrete bounded uncertainty are constructed.

math.OC↗